C:AUCSCAUCSC BIA Edits 2009Advanced Chapter 3 Tables 2009

Advanced Course

Appalachian Underground Corrosion Short CourseWest Virginia UniversityMorgantown, West Virginia

Copyright © 2009

APPALACHIAN UNDERGROUND CORROSION SHORT COURSEADVANCED COURSE

CHAPTER 1 - PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS . . . . . . . . . . 1-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

CORROSION MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

PIPE-TO-SOIL POTENTIAL MEASUREMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

TYPES OF POTENTIAL SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

SINGLE ELECTRODE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

COMPUTERIZED POTENTIAL SURVEY (CLOSE INTERVAL SURVEY) . . . . . . 1-3

TWO ELECTRODE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

SIDE-DRAIN MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

ANALYZING PIPE-TO-SOIL POTENTIAL DATA . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS . . . . . . . . . . . . . . . . . . . 1-5

INTERPRETATION OF POTENTIALS UNDER NON-STRAY CURRENTCONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

COATED CROSS-COUNTRY PIPELINE WITHOUT CATHODICPROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

GAS SERVICE LINE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

GROUNDING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8

STEEL GAS AND WATER LINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8

CRITERIA FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9

WHICH CRITERION? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

POTENTIAL MEASUREMENT OF -850 mV TO A Cu/CuSO4 REFERENCEELECTRODE WITH CATHODIC PROTECTION APPLIED . . . . . . . . . . . . . . . . 1-10

POLARIZED POTENTIAL OF -850 mV MEASURED TO A Cu/CuSO4 REFERENCE ELECTRODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12

100 MILLIVOLT POLARIZATION CRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12

CRITERIA FOR SPECIAL CONDITIONS - NET PROTECTIVE CURRENTCRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-13

OTHER CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14

E LOG I CURVE CRITERION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14

300 MILLIVOLT POTENTIAL SHIFT CRITERION . . . . . . . . . . . . . . . . . . . . . . . 1-15

USE OF CRITERIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-16

CHAPTER 2 - EVALUATION OF UNDERGROUND COATINGS USINGABOVEGROUND TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

SURVEY SEQUENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

REFERENCED PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

SAFETY CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

PIPELINE LOCATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

DIRECT CURRENT VOLTAGE GRADIENT SURVEYS (DCVG) . . . . . . . . . . . . . 2-4

ALTERNATING CURRENT VOLTAGE GRADIENT SURVEYS (ACVG) . . . . . . . 2-7

CLOSE-INTERVAL SURVEYS (CIS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

AC CURRENT ATTENUATION SURVEYS (ELECTROMAGNETIC) . . . . . . . . . 2-10

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

CHAPTER 3 - MATERIALS FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . 3-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

CONSIDERATIONS FOR ALL MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

GALVANIC ANODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Prepared Backfill and Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Wire and Cable Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Magnesium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Zinc Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

IMPRESSED CURRENT CATHODIC PROTECTION SYSTEMS . . . . . . . . . . . . . 3-5

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

High Silicon Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

Graphite Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Aluminum Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Lead Silver Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Magnetite Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Mixed Metal Oxide Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Platinum Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Polymer Conductive Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Scrap Steel Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Backfill for Impressed Current Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

WIRE AND CABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

CABLE CONNECTIONS AND SPLICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

SPLICE ENCAPSULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

POWER SUPPLIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12

JUNCTION AND RESISTOR BOXES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

PERMANENTLY INSTALLED REFERENCE ELECTRODES . . . . . . . . . . . . . . . 3-13

TEST STATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

FLANGE ISOLATORS AND DIELECTRIC UNIONS . . . . . . . . . . . . . . . . . . . . . . 3-15

MONOLITHIC WELD IN ISOLATORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

CASING ISOLATORS AND END SEALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

CHAPTER 4 - DYNAMIC STRAY CURRENT ANALYSIS . . . . . . . . . . . . . . . . . . . . . 4-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

STRAY CURRENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

THE EARTH AS A CONDUCTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

POTENTIAL GRADIENTS IN THE EARTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

DETECTION OF DYNAMIC STRAY CURRENTS . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Interpreting Beta Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

Determining the Point of Maximum Exposure . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

METHODS OF MITIGATING THE EFFECTS OF DYNAMIC STRAY CURRENT 4-4

Controlling Stray Currents at the Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

Mitigation (Drainage) Bonds and Reverse Current Switches . . . . . . . . . . . . . . 4-4

Sizing the Mitigation (Drain) Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5

Use of Sacrificial Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

Use of Impressed Current Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

STRAY CURRENT FROM ALTERNATING CURRENT (AC) SOURCES . . . . . . 4-8

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

CHAPTER 5 - DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION . . 5-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

REVIEW OF IMPRESSED CURRENT SYSTEM FUNDAMENTALS . . . . . . . . . . 5-1

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

INFORMATION USEFUL FOR DESIGN OF AN IMPRESSED CURRENTCATHODIC PROTECTION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

DESIGN OF AN IMPRESSED CURRENT ANODE BED . . . . . . . . . . . . . . . . . . . 5-3

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

Selecting an Anode Bed Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

Selecting Anode Bed Type Based on Site Selection . . . . . . . . . . . . . . . . . . . . 5-4

Distributed Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Deep Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Hybrid Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Tests Required for Anode Bed Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

DESIGN CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Existing Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Current Requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Determining the Number of Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9

Calculating the Anode Resistance-to-Earth . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10

Calculating Total Circuit Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12

Sizing the Rectifier or DC Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

DESIGN OF DISTRIBUTED ANODE BED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14

DESIGN OF DEEP ANODE BED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16

CHAPTER 6 - DESIGN OF GALVANIC ANODE CATHODIC PROTECTION . . . . . . 6-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

GALVANIC ANODE APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

General Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Specific Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

GALVANIC ANODE CATHODIC PROTECTION DESIGN PARAMETERS . . . . . 6-1

Galvanic Anode Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

Anode Current Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Current Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Electrolyte Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2Total Circuit Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Anode Bed Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Resistance of Interconnecting Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Cathode-to-Earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Anode Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

GALVANIC ANODE DESIGN CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

Simplified Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

SPECIFICATION AND MAINTENANCE OF GALVANIC ANODEINSTALLATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

CHAPTER 7 - MIC INSPECTION AND TESTING

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

SITE SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

COATING INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

MIC AND DEPOSIT SAMPLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

MIC SAMPLING AND TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

SUPPORTING ANALYSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3

METALLURGICAL INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

2009 Revision

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PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS1-1

CHAPTER 1

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS

INTRODUCTION

The objective of this chapter is to present oneof the standard corrosion controlmeasurements, the pipe-to-soil potentialmeasurement, and the various types of teststhat can be performed using this measurementtechnique. This chapter will also show how thedata from this particular measurement can beused to identify and evaluate corrosionproblems as well as assist in determining theeffectiveness of a cathodic protection system.

Included in this section is a discussion ofvarious NACE criteria currently in place forcathodic protection and the applications foreach.

All potential values given in this section will bewith respect to a standard copper-coppersulfate (Cu/CuSO4) reference electrode. Unlessotherwise noted, or in an example where lateralmeasurements are being taken, all potentialvalues given will be with the reference electrodeplaced at grade directly over the structure beingtested. Keep in mind that the pipeline potentialis referenced to the electrode location not to thestructure connection point. See Figure 1-1 for atypical pipe-to-soil potential measurement testsetup.

Because the normal pipe-to-soil potential ofsteel measured to a copper-copper sulfatereference electrode is almost always of anegative polarity, this chapter will presentexamples using potentials with negative values.Therefore, negative potentials with greaterabsolute values will be considered “morenegative.” For example, a potential

measurement of -600 millivolts will beconsidered more negative than a measurementof -400 millivolts.

CORROSION MECHANISMS

The corrosion of an underground or submergedmetallic structure is electrochemical in nature.There are basically two different mechanismswhich are responsible for this corrosion andthese are termed electrolytic corrosion andgalvanic corrosion.

Electrolytic corrosion, often called stray currentcorrosion, results from currents which areintroduced into the ground from neighboringsources of direct current (DC) such as electricrailways, DC powered machinery, and foreigncathodic protection systems. Galvanic corrosionis the result of the natural electrochemicalprocess that takes place on a buried metallicstructure due to potential differences whichexist between points on the same structure dueto different surfaces, dissimilar electrolytes, anddifferential aeration or that which result from thecoupling of dissimilar metals. This type ofcorrosion is often classified into categories suchas local actions, long line activity, and bimetallicactivity.

Both of these corrosion mechanisms can bedetected and evaluated through the proper useof the pipe-to-soil potential measurement.

Pipe-to-soil potential measurements canindicate whether a structure is subject todynamic or fluctuating stray current corrosion.Because stray currents that come from DCpowered railways or machinery are rarely if


ever constant, fluctuations in potential on aneffected structure will indicate the presence ofthis type of stray current. In order to determinethe degree of potential fluctuation and theamount of time that the structure is effected, DCvoltage recording instruments can beemployed.

Non-fluctuating stray current or static straycurrent is not as readily apparent as thedynamic type. In this case, the interferenceeffect from a foreign cathodic protection systemwill usually remain constant and unless there isexisting historical potential data or one has theopportunity to participate in cooperativeinterference testing, this type of stray currentinterference may not be immediately detected.However, there are potential measurementtechniques that can detect this type of straycurrent interference such as lateral potentialsurveys, which will be discussed later in thischapter.

PIPE-TO-SOIL POTENTIAL MEASUREMENT

Before the various test methods and evaluationprocesses are discussed, a review of the pipe-to-soil potential measurement is required. Thismeasurement should be obtained using a highresistance (impedance) voltmeter, a calibratedcopper-copper sulfate reference electrode, anelectrolyte present over the pipe where thereference electrode can be placed, and methodby which to contact the structure under test. Ahigh resistance voltmeter will provide the mostaccurate potential reading. The referenceelectrode should be filled with distilled water (orelectrode gel solution) after the copper rod hasbeen cleaned with a nonmetallic abrasive andcopper sulfate crystals have been added. Theelectrolyte for the placement of the referenceelectrode should be soil or water. Concrete andasphalt should be avoided. The connection tothe structure under test can be a valve, riserpipe, test station, etc., any point electricallycontinuous with that portion of pipe beingevaluated.

The preferred polarity convention has thereference electrode connected to the negative

terminal of the voltmeter and the structureconnected to the positive terminal. This polarityconnection will normally result in a negativevalue. It is important to note that consistency iscritical when recording these potential valueswhether it be as a negative value or a positivevalue. The methods and instrumentationemployed for this testing are critical in order toprovide meaningful data that can be properlyevaluated.

TYPES OF POTENTIAL SURVEYS

A normal survey of a pipeline system in order toobtain either static or native potential values orto ascertain the effectiveness of an existingcathodic protection system might consist ofmeasuring pipe-to-soil potentials at all availabletest locations such as those describedpreviously. However, there are occasions whenit is necessary to obtain additional potentialmeasurements between test points. This isaccomplished by typically placing the referenceelectrode at regular intervals over the pipelineand measuring potentials at each referencelocation. The spacing of the intervals willdepend on the type of survey being performedand the type of detail required. This type ofsurvey will indicate the anodic locations alongan unprotected structure which will be thoseareas with the most negative potentials.However, these more negative potentials maycorrespond to a stray current “pickup” area. Inthe case of a pipeline under cathodicprotection, this type of survey will reveal thoselocations where protection might not beachieved based on an evaluation of thepotential values.

In order to insure the accuracy of an over-the-line potential survey, the pipeline should be firstelectronically located and staked out by thesurvey crew. This will help to ensure that thepotential measurements are taken directly overthe pipeline. Just as it is important to use theproper equipment as explained earlier, it isequally important that the reference electrodebe placed in clean soil devoid of rocks, weeds,or grass which could effect the electrodecontact resistance and thereby effect the


accuracy of the measurement. In other words,make sure to establish good cell contact withthe electrolyte.

It is also important to insure that the pipelineyou are testing is electrically continuous for thelength of pipe to be surveyed.

Detailed over-the-line potential surveys can beconducted using various methods and testequipment which will yield a variety of usefulinformation. The following is a description offour commonly used survey methods.

SINGLE ELECTRODE METHOD

The first method to be discussed is the singleelectrode survey, see Figure 1-2. This surveyutilizes one copper-copper sulfate referenceelectrode, a high-resistance voltmeter, and areel of test wire. An additional requirement is apoint of electrical contact to the structure beingtested.

There are two ways to perform this particularsurvey. In the first procedure, the pipeline iscontacted through a test lead or other suitableconnection point which is connected to a testreel. The test reel is connected to the negativeterminal of a voltmeter and the testingpersonnel measure potentials along thepipeline at prescribed intervals, carrying andmoving both the voltmeter and the referenceelectrode together. Once again the referenceelectrode is connected to the negative terminalof the voltmeter. This polarity orientation can bealtered as long as the orientation is consistentand the test personnel are aware of the polarityof the measured pipe-to-soil potential.

In the second procedure the test reel isconnected to the reference electrode and thevoltmeter or recorder is left at the point ofconnection to the pipeline. This methodrequires two testing personnel with one at thevoltmeter logging the data or monitoring therecorder, and the other individual moving theelectrode along the pipeline and contacting thesoil at set intervals. In cases where a longsection of pipeline is to be tested or where the

terrain is such that visual contact cannot bemaintained, some type of communicationsequipment is needed in order to coordinate thetiming of the readings.

It should be noted that potential readings takenusing the one electrode method may beaffected by IR drop in the pipeline andmeasuring circuit or by stray currentinterference.

COMPUTERIZED POTENTIAL SURVEY(CLOSE INTERVAL SURVEY)

The second type of survey method to bediscussed is basically the same as the singleelectrode technique except in this case the testdata is collected on a continuous basis. Thiscan be accomplished using both chartrecorders or computerized test equipment.Figure 1-3 shows the basic components andhookups for a computerized system. In thiscase, the voltage measuring and data loggingequipment can be carried in a compactbackpack arrangement by the operator. Theoperator carries one or two copper-coppersulfate reference electrodes, which may beaffixed to the bottom of extension rod(s), whichare then “walked” forward at some specificinterval, usually two and one-half to three feet.Potential measurements are electronicallyrecorded and stored in the storage module ofthe instrument. The long test lead back to theconnection to the pipeline at the survey startingpoint can be a one-time-use disposable lightgauge wire. During the course of the potentialsurvey the operator is able to electronically notedistances, terrain features and landmarks aswell as other pertinent information.

The electronically collected data package canthen be processed by a portable computer inthe field or at the office. The processingmachines can be programmed to produce apotential profile complete with identifying notesfor the pipeline section surveyed. Someprograms will allow for the plotting of previoussurvey data for comparison.

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TWO ELECTRODE METHOD

The third potential survey method to bediscussed is the two-electrode survey illustratedin Figure 1-4. This survey uses two copper-copper sulfate reference electrodes, a highresistance voltmeter, and a pair of test leads.These leads can be cut equal to the span of thepreselected survey distance. The survey isconducted by first measuring and recording thepipe-to-soil potential with respect to referenceelectrode “A” as shown in the Figure. Electrode“B” is then placed along and directly over thepipeline at some preselected survey intervalfrom “A”. The potential difference between thetwo electrodes is then measured andnumerically added or subtracted from theprevious reading in accordance with themeasured polarity of the forward electrode (“B”in this case). This leap-frogging of the twoelectrodes is continued until the next permanenttest station is reached or any other locationwhere the pipeline can be electricallycontacted.

At this juncture, the cumulative potential up tothis point is compared to the actual potentialmeasured to copper-copper sulfate at thissecond permanent test station and adjusted asnecessary. The survey then continues followingthe above described procedure to the nextcontact point.

The data should be recorded in a permanentlog. See Table 1-1 for a typical recordingformat.

The two electrode method works well. However,it requires that a great deal of care be taken inrecording the data. An error made in thecalculating of potentials or the noting of polarityat any given point will cause all subsequentcalculated potentials to be erroneous. In thecase of an error, the calculated pipe-to-electrode potential will not correspond to thepotential measured at the next contact point.

Note however, that even if there are no dataerrors, a discrepancy could still exist betweenthe last calculated pipe-to-soil potential and the

actual measured potential at the contact point.One possible reason is that any long line directcurrent flowing on the pipe will cause a voltagedrop across the pipeline between the two testpoints which could account for the difference.Another reason for a difference could be theeffect of stray current activity on the pipelinepotential.

SIDE-DRAIN MEASUREMENTS

If more detailed information is desired, thefourth method to be discussed known as side-drain measurements can often be used totroubleshoot problem areas. Side-drainmeasurements are conducted utilizing twocopper-copper sulfate reference electrodes anda high resistance voltmeter, as shown in Figure1-5. The first electrode is placed directly abovethe pipe, in contact with the soil. The secondelectrode is placed in contact with the soil at a90-degree angle to the pipe at a distanceapproximately equal to 2½ times the pipedepth. During testing, the electrode placeddirectly above the pipe should be connected tothe positive terminal of the voltmeter and theother electrode to the negative terminal.Positive side-drain readings indicate thatcurrent is being discharged from the pipe at thispoint, making it an anodic area. Negativeside-drain readings normally indicated thatcurrent is flowing towards or onto the pipe atthis point, making it a cathodic area. Theside-drain measurements should be takentypically at no more than 5-foot intervals. Testsmust be made on both sides of the pipe, untilthe extent of the problem section has beendetermined.

This technique should be used with caution.Under certain conditions a relatively stronglocalized anodic cell could exist on the bottomof the pipe with the top of the pipe serving as acathode and negative side-drain readings couldbe measured while severe corrosion is actuallyoccurring on the bottom of the pipe at thislocation.

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TABLE 1-1

Typical Data Record For Two Electrode Potential SurveyConducted on a Cathodically Protected Pipeline

A B C D Comments

1 -- - -0.860(1) --

2 0.035 + -0.895 --

3 0.021 + -0.916 --

4 0.065 - -0.851 --

5 0.092 - -0.759 Unprotected Area

6 0.045 + -0.804 Unprotected Area

7 0.063 + -0.867 --

8 0.011 + -0.878 --

9 0.020 - -0.858 --

10 0.032 + -0.890 --

Where: A = position or pipeline sectionB = potential drop from electrode at last position (volts)C = polarity of forward electrodeD = pipe to copper sulfate electrode (volts)

Note: (1) Initial value measured via direct pipeline contact at Position 1

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ANALYZING PIPE-TO-SOIL POTENTIALDATA

Upon completion of the pipe-to-soil potentialsurvey, the compiled data is normally plotted ona graph to facilitate interpretation, potential(volts) versus distance (feet).

On cathodically protected pipelines, the pipe-to-soil potential survey is used to determine ifadequate levels of protection are beingprovided to all areas of the pipeline. Adequatelevels of protection are often based on the -0.85volt (immediate “OFF”, or “ON” corrected for IRdrops) versus a copper-copper sulfatereference electrode. However, the 100 millivoltcriterion could be used if a static potentialprofile of the line is available.

Normally after the survey data is plotted, the0.85 volt value is red-lined on the plot. All areasbelow this line are considered to beinadequately protected and in need of remedialmeasures. Figure 1-6 shows a typical pipe-to-soil potential profile of a cathodically protectedpipeline.

On pipelines which are not cathodicallyprotected, the pipe-to-soil potential profile andsurface potential surveys can be used to locatecorroding areas or “hot spots” along thepipeline. Experience has shown that when adifference in pipe-to-soil values exist along apipeline, corrosion occurs at, and for a givendistance on either side of the most negative oranodic points along the pipeline.

Figure 1-7 shows some typical potential andpolarity changes which may be recorded atcorroding or anodic areas on the pipe. Theexact point of “current discharge” can bedetermined by re-surveying the effected areaand successively reducing the electrodespacing by one-half. When the exact point ofmaximum “current discharge” (most negativepotential) has been determined, the pointshould be staked, and all pertinent datarecorded.

Pipe-to-soil potential measurements taken over

the line can be used to indicate whether or nota pipeline is being subjected to dynamic straycurrent corrosion. This is possible however,only if the potentials are fluctuating at the timeof the survey. The two potential profile methodis sometimes used to detect the effects of non-fluctuating stray currents on a section of pipe.The first profile is developed with the referenceelectrode placed directly above the pipe and thesecond by placing the electrode some distancelaterally away from the pipe.

Dual potential profiles, over the pipe andlaterally away from the pipe, are used tointerpret various types of corrosion activity onunprotected pipes. It should be noted, however,that such profiles should be utilized to evaluatethe effects of “long line” corrosion cells. Suchprofiles can sometimes overlook very smalllocalized corrosion cells where the anode andthe cathode are located very close to eachother.

PIPE-TO-SOIL POTENTIAL SURVEYS ANDANALYSIS

Figure 1-8 shows a typical potential profile on apipe on which the corrosion activity is one ofstraight forward galvanic action and is not beinginfluenced by interference currents or bimetalliccorrosion. Figure 1-9 shows a typical potentialprofile on a pipe which is exposed to damageas a result of a rectifier unit on a crossingpipeline. In comparing these two potentialprofiles, it should be noted that irrespective ofthe condition which exists, anodic areas alwaysexist at the locations where the “over the pipe”potentials are more negative than the “off thepipe” potentials. However, in a straightforwardgalvanic situation such as shown in Figure 1-8,the anodic areas occur at locations where the“over the pipe” potentials are of higher negativevalues than those measured in the cathodicareas. In an interference situation such asshown in Figure 1-9, the anodic areas are atthose locations where the potentials (both“over” and “off” the pipe) are of lower negativevalues than those at the cathodic areas.

It should be noted that although a potential

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profile similar to that shown in Figure 1-8 almostalways indicates a galvanic corrosion patternand almost always rules out a stray current orinterference situation, a profile similar to thatshown in Figure 1-9 does not necessarilyindicate a stray current situation. It is possiblethat a bimetallic corrosion condition will yield apotential profile similar to that which isencountered in an interference situation. Figure1-10 shows a potential profile on a pipeline, aportion of which is influenced by a neighboringcopper grounding system to which the pipe isconnected. It can be seen that this potentialprofile is very similar to that shown in Figure 1-9in that the anodic areas occur at locations oflow negative potential. In some situations, itmay be difficult to distinguish between aninterference situation and a bimetallic onemerely by analyzing the potential profiles. Inthat type of situation, a complete investigationof the conditions which exist in a given area willusually yield the information necessary toestablish whether the potentials are attributableto interference or bimetallic activity.

Even without knowledge of the surroundingconditions, there are usually sufficientdifferences between two profiles to distinguishthem. Thus, in an interference situation, theportions of the pipe immediately adjacent to theanodic area are almost always entirelycathodic. In a bimetallic situation, the profile ofthe pipe outside the area of bimetallic influenceis one which is similar to the normal galvanicprofile shown in Figure 1-8.

For the purpose of distinguishing the type ofcorrosion activity that is present, in the absenceof a cathodic protection system, the followinggeneral rules can be given.

1. Anodic areas exist at locations where thelateral measurements are less negative thanthe “over the pipe” measurements.Conversely, cathodic areas exist at locationswhere the lateral measurements are morenegative than the “over the pipe”measurement. This condition always holds,irrespective of whether the corrosion activityis a result of an electrolytic or galvanic

action.

2. When anodic areas coincide with areas ofhigh negative potential, the corrosion activityis one of “straightforward” galvanic action(this does not include bimetallic activity). Ina straightforward galvanic situation, themore negative the potential, the more anodicthe condition will be.

3. When anodic areas coincide with areas oflow negative potentials (or in severe cases,positive potentials), the corrosion activity isone of either electrolytic action or bimetalliccoupling. In an electrolytic or bimetallicsituation, the less negative (or more positive)the potential, the more anodic the area.

INTERPRETATION OF POTENTIALS UNDERNON-STRAY CURRENT CONDITIONS

After it has been determined that stray currentsare not present, potential measurements can beused in analyzing the galvanic corrosion patternwhich may exist. Surface condition of the pipe,chemical composition of the soil, and otherlocal conditions can greatly influence staticpotentials along the length of a pipeline.Therefore, the more readings taken, the betterthe evaluation. A more comprehensiveinterpretation of potential measurements can bederived from the following statements.

1. The potentials of newer pipes are morenegative than those of older pipes.

2. The potentials of coated pipes (organiccoating such a coal tar, asphalt, plastic tape,etc.) are more negative than those of barepipes.

3. The variation in potential with respect todistance is generally greater along bare pipethan along a coated pipe.

4. The normal potentials taken along a barepipe fall in the range of -500 to -600millivolts. Newly installed bare pipe will havehigher negative potentials, and very old barepipe can have much lower negative

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potentials.

5. The normal potentials taken along a coatedline fall in the range of -650 to -750 millivolts.Newly installed coated line can have highernegative potentials, and older pipe or poorlycoated pipe will have less negativepotentials.

6. In highly alkaline soils, the normal potentialswill be less negative than those taken inneutral or acid soils.

7. On bare pipe, there is usually a correlationbetween potential measurements andresistivity measurements, i.e., locations ofhigher resistivity soil correspond withlocations of lower negative potentials.

It should be understood that all of the abovestatements apply primarily to steel pipelinesthat are in non-stray current areas and are notsubject to bimetallic influences or cathodicprotection. Also, where comparisons are made,such as between coated and bare pipe orbetween old and new pipe, it is assumed thatother conditions are equal.

These statements are intended as ageneralized guide for interpretation of potentialmeasurements. They are not to be consideredas scientific principles and they do notnecessarily hold true under all circumstances.They are merely a summation of fieldexperience. They are not derived fromcontrolled experiments or rigid reasoning.Despite the apparently severe qualificationsthat have been applied to these statements, ifthey are used with care and with completeappreciation of the theory of galvanic corrosion,most corrosion problems can be evaluatedsuccessfully.

COATED CROSS-COUNTRY PIPELINEWITHOUT CATHODIC PROTECTION

The potential measurements shown in Table1-2 were recorded during a potential surveyalong a portion of a well-coated, cross-countrypipeline. Examining the data in the table we see

that about one-half of the readings/locations fallin the range as indicated in statement 5,measurements at five of the metering andregulator stations were less negative than thosegiven in statement 5. (The potential of -780millivolts at location 31 is slightly more negativethan the range given in statement 5, but asindicated in that statement, higher negativepotentials are possible on newly installed,well-coated pipes.)

The low negative potentials at the metering andregulator stations prompted a more detailedinvestigation.

This investigation found that the isolators thathad been installed at those stations wereshorted, and as a result, the coated line wasdirectly coupled to older uncoated distributionpiping at those locations.

Statements 1 and 2 indicate that older, barelines are of lower negative potentials thannewer, coated lines. The coupling of the coatedline to the bare line makes the potentials on thecoated line less negative at the shorted meterand regulator stations than along the balance ofthe pipeline. This situation shows theimportance of placing isolating flanges betweennew coated pipe and older bare pipe. Thisisolation is needed whether or not cathodicprotection is provided for the coated line. Ifcathodic protection is not provided, the newcoated pipe will be anodic with respect to theolder, bare pipe. As a result, if there were noisolating flange installed between the twosections, or if the flange were shorted, thenewer coated section of pipe would besubjected to accelerated galvanic corrosionattack. When using isolating flanges or devices,safety precautions must be followed to preventa spark or arc across the isolator during a faultor power surge or static discharge. Devicesexist that will maintain DC isolation while actingas a shunt under the above conditions.

GAS SERVICE LINE

The previous example shows the need for athorough investigation of a situation where

TABLE 1-2

Potential Measurements

Test StationLocation No. Description of Location Pipe-to-Soil

Potential (millivolts)

21 Crestdale Regulator Station -703

22 North side of Blue River on Rt. 95 -700

24 South side of Blue River -735

26 Linden Metering Station -542

26 Valve box approximately 7.8 miles South of Blue River -730

28 Creek, 10.2 miles south of Blue River -674

30 Atlantic Regulator Station -563

31 Glendale Metering Station -780

32 Crossing of railroad near Glendale -506

33 Forest Park Regulator No. 1 -480

35 Forest Park Regulator No. 2 -537


there is an apparent contradiction between themeasurements taken and the statements listed.The next example illustrates this need in aslightly more complicated situation. In thisexample we have gas service lines in a housingdevelopment. The original service lines hadbeen installed without coating, and failuresbegan to appear after five years. The lineswhich failed were replaced with coated steelpipes, and some of the replacement pipedeveloped leaks within only two years ofservice. A potential survey conducted on thesystem found a range of potentials from -250 to-350 millivolts on the new coated replacementlines and potentials ranging from -500 to -600millivolts on the older bare lines.

Examination of these data tends to indicate acontradiction to statements 1, 2, and 5. Uponcloser inspection however, it was discoveredthat the water service lines in the developmentwere electrically continuous with the gasservice lines. Copper, being cathodic withrespect to steel, will when connected to a steelpipe, cause the potential of the steel pipe tobecome less negative than its natural potential.As a result of area relationships (the ratio ofareas of copper to steel is greater when thesteel is coated than when it is bare), the effectof copper on the potential of coated steel pipewill be far greater than that on the potential ofbare pipes.

GROUNDING SYSTEM

The next example involves a somewhat similarsituation as the previous example, but anextensive copper electrical grounding system isinvolved rather than copper pipes.

A potential profile was conducted on a newlyinstalled, underground, coated steel pipe. Theover-the-line potentials were in the range of-600 to -700 millivolts, which seems to be inaccordance with the previous statements. Anoff-the-line potential profile was also conductedon the line. Interestingly, the two profiles werenearly identical.

Potential profiles were also conducted along

another newly installed, well coated pipeline ina similar environment. In this case, however,there was an extensive grounding systeminstalled along the pipeline and electricallyconnected to it. The grounding systemconsisted of copper mats installed at 50 footintervals. The potentials measured along thepipeline were considerably less negative thanexpected along a coated pipe; all of thepotentials were less negative than -500millivolts, and at some locations the potentialswere less negative than -300 millivolts. Inaddition, the off-the-line potentials wereconsiderably less negative than the over-the-pipe potentials.

These low negative potential levels and thelateral potential gradients are directlyattributable to the copper grounding system andindicate that severe bimetallic (galvanic)corrosion activity exists. In fact, the condition atthis plant was so severe that leaks developedwithin the first year of operation despite the factthat the pipeline was extremely well-coated andhad been backfilled with sand. The very goodcoating no doubt contributed to the rapiddevelopment of leaks on the pipeline due to theconcentration of current discharge at holidaysin the coating. The corrosion current wasgenerated by the bimetallic coupling.

STEEL GAS AND WATER LINES

In the examples given thus far, it is possiblethat the bimetallic effect of copper could havebeen anticipated without having a completeknowledge of the potential pattern. The nextexample, however, describes a situation whichis similar to a bimetallic corrosion pattern butwhere there is no copper present. This exampleinvolves a group of school buildings. Leaksoccurred on these pipes within the first year,and these leaks were attributed to the verycorrosive soil in which resistivity was less than100 ohm-centimeters. However, potentialmeasurements taken on the gas and waterlines were all in the range of -350 to -450millivolts. If the corrosion activity was one ofstraightforward galvanic action resulting fromthe corrosiveness of the soil, these potentials


would be considered as very cathodic and,therefore, not indicative of a very corrosivecondition. Therefore, it was difficult to reconcilethe very severe corrosion history with theapparent cathodic potentials.

Because the soil at this location was veryalkaline, somewhat lower potentials thannormal would be expected. However, the verylow negative potentials at this site could not beentirely attributed to the alkaline soil. Moredetailed investigation found that the lowpotentials on the gas and water lines werecaused by the radiant heat piping under each ofthe buildings. Although the radiant heat lineswere also steel, they were installed in concrete,and the normal potential of steel in concrete isapproximately -200 millivolts, versus the normal-500 to -600 millivolt potential of steel in earth.The electrical coupling of the gas and waterlines in the soil to the radiant heat lines in theconcrete (there were common structuralconnections in the boiler room) made thepotential on the gas and water lines lessnegative than normal. These gas and waterlines thus behaved as the anode in amechanism which is similar to a bimetalliccouple. The conditions which made this coupleparticularly severe were the low resistivity soil,the proximity of the gas and water lines to theradiant heat lines, and the fact that the area ofthe gas and water lines was small compared tothe area of the radiant heat lines. This arearelationship was made still worse by the factthat the gas and water lines were coated.

The examples given show the manner in whichpipe-to-soil potential measurement can be usedin analyzing a corrosion problem. Theseexamples further show the need for learning asmuch about the structure under investigation aspossible and about any other structures in thearea. The remaining part of this chapter dealswith the current NACE criteria for cathodicprotection of steel and cast iron structures andsome of their applications.

CRITERIA FOR CATHODIC PROTECTION

The following is a review of the cathodic

protection criteria as they exist in NACEStandard RP0169-2002 “Control of ExternalCorrosion on Underground or SubmergedMetallic Piping Systems”.

In the referenced NACE Standard, three (3)criteria for the cathodic protection of steel andcast iron structures exist. These criteria are asfollows:

1. -850 mV to Cu/CuSO4 with the cathodicprotection current applied

2. A polarized potential of -850 mV toCu/CuSO4

3. 100 millivolt polarization

Section 6 of NACE Standard RP0169-2002“lists criteria and other considerations forcathodic protection that will indicate, when usedeither separately or in combination, whetheradequate cathodic protection of a metallicpiping system has been achieved.”

The standard also states that corrosion controlprograms not be limited to the criteria listedabove and that criteria that have beensuccessfully applied on existing piping systemscan continue to be used. These other criteriaare valid as long as that criteria can achieve thesame degree of corrosion control as that criterialisted above. Some examples of other criteriathat have been used in the past are:

C 300 mV shiftC E log IC Net current flow

The criteria listed for steel and cast iron, as wellas the criteria given for other metals, requirepipe-to-soil potential measurements. Whenthese criteria are to be applied in a particularsituation and potential measurements aretaken, a number of questions that are notalways precisely answerable arise:

1. Which criterion or criteria are applicable tothe particular corrosion mechanism?


2. Where should the reference electrode beplaced to obtain valid readings?

3. How many readings are required to obtainan accurate representation of the entirepipeline or piping network?

WHICH CRITERION?

In order to respond to this question it isnecessary to know the metal underconsideration. RP0169-2002 gives criteria forthe common metals of construction, includingsteel and cast iron, aluminum, and copper. Thisdiscussion will be confined to criteria for steeland cast iron. RP0169-2002 lists the three maincriteria for steel and cast iron as follows:

1. “A negative (cathodic) potential of at least850 mV with the cathodic protection applied.This potential is measured with respect to asaturated copper/copper sulfate referenceelectrode contacting the electrolyte. Voltagedrops other than those across thestructure-to-electrolyte boundary must beconsidered for valid interpretation of thisvoltage measurement.”

2. “A negative polarized potential of at least850 mV relative to a saturated Cu/CuSO4reference electrode.”

3. “A minimum of 100 mV of cathodicpolarization between the structure surfaceand a stable reference electrode contactingthe electrolyte. The formation or decay ofpolarization can be measured to satisfy thiscriterion.”

The selection of the criterion to use should begoverned not merely by what is mostconvenient for the user; it should be recognizedas a function of the particular corrosionmechanism being considered.

POTENTIAL MEASUREMENT OF -850 mV TOA Cu/CuSO4 REFERENCE ELECTRODEWITH CATHODIC PROTECTION APPLIED

Of the three criteria listed above, the -850 mV

criterion with cathodic protection applied isprobably the one most widely used fordetermining if an acceptable degree of cathodicprotection is being attained on a buried orsubmerged metallic structure. In the case of asteel structure, an acceptable degree ofprotection is said to be achieved when at leasta -850 mV potential difference exists betweenthe structure and a copper/copper sulfatereference electrode contacting the soil directlyabove and as close to the pipe as possible.

Some sources indicate that this criterion wasdeveloped from the fact that the most negativenative potential found for coated steel was -800mV. Therefore, the assumption was made thatif sufficient current is applied to raise thepotential of the entire structure to a value morenegative than the open circuit potential, thenthe effects of corrosion will be mitigated. Apotential of -850 mV with respect to acopper/copper sulfate reference electrode wastherefore adopted.

A potential of -850 mV or more negative, withthe protective applied, to a copper/coppersulfate reference electrode placed in contactwith the soil directly over or adjacent to thesteel structure would be an indication thatadequate protection is being achieved at thatpoint on the structure as long as any voltagedrops (IR drops) across the electrolyte orthrough the metallic return path are taken intoaccount. The wording for this particular criteriadoes state that voltage drops other than thoseacross the structure-to-electrolyte boundarymust be considered. Consideration is thendefined as:

C “Measuring or calculating the voltagedrop(s)”

C “Reviewing the historical performance of thecathodic protection system”

C “Evaluating the physical and electricalcharacteristics of the pipe and itsenvironment”

C “Determining whether or not there is physical


evidence of corrosion”

Voltage drops can be reduced by placing thereference electrode as close to the pipe surfaceas possible. The voltage drop can also beeliminated by interrupting all of the sources ofthe cathodic protection system DC currentsupply and measuring the “on” and “off”potentials. The instantaneous “off” potentialshould be free of voltage drop error.Comparison of the two potentials and noting thedifference between the two will indicate theapproximate voltage drop included in thepotential measurement when the measurementis made with protective current applied. This“instant off” or polarized potential will bediscussed in more detail when the second listedcriterion is analyzed.

Voltage drops are more prevalent in the vicinityof an anode bed or in areas where dynamicstray currents are present. Unusually highresistivity soil will also cause high voltagedrops. In areas where dynamic stray currentsare present, an instantaneous reading may beimpossible to accurately record and thereforemeaningless. The use of voltage recordinginstruments is recommended so as to obtainpotential values over a 24-hour period, or atleast during the known period of highest straycurrent activity. Often times it is possible toobtain fairly stable “on” potentials of the pipingsystem once the DC transit system stopsoperating in the early morning hours if recordinginstruments are left in place overnight. Thesepotentials can provide a base-line from which toevaluate other measurements.

The -850 mV criterion with cathodic protectionapplied is the one which is almost always usedin areas with large amounts of dynamic straycurrent activity. It is generally accepted that ifthe potential of the structure to a referenceelectrode remains more negative than -850 mVat all times, even if there are substantialfluctuations in potential with time, then the pipecan be considered protected at that particulartest point. It is possible that the amount of testpoints surveyed and the frequency at whichthose surveys are performed may have to

increase in heavy stray current areas due toever-changing conditions.

Limitations of the -850 mV Potential to aCu/CuSO4 Reference Electrode WithCathodic Protection Applied Criterion

A limitation of this criterion for insuring totalprotection is that potentials can vary widelyfrom one area of the underground structure toanother as a result of coating damage,interference effects, etc. This suggests thepossibility of potentials being less negative than-850 mV in sections of the pipe between twoconsecutive test locations. Also, the voltagedrop component in the potential measurementhas to be considered unless the “instant off” orpolarized potential of -850 mV is used. In manycases, removal of the voltage drop componentor testing for polarized potentials may bedifficult, particularly when it is not possible toremove and/or interrupt all sources of currentfor the cathodic protection system.

More negative potentials than -850 mV can ofcourse be maintained at all test points, whichcould insure a greater degree of protectionbetween test points. However, economics is afactor to consider; higher potentials may resultin a higher power consumption. Additionally, inthis situation, care has to be exercised to insurethat the polarized potential of the pipeline doesnot reach values at which coating damagecould occur due to hydrogen evolution at thesurface of the pipeline.

Although this accepted criterion can be, and is,widely used for all types of structures becauseof its straightforward approach and simplicity,its most economical use is in the case of coatedstructures. Use of this criterion to protect an oldbare structure could require substantially higherprotective current than if one of the othercriterion were used.

Another limitation of this criterion is due to therequirement that potential readings be takenwith the reference electrode contacting theelectrolyte directly over or adjacent to thepipeline to minimize voltage drop. In cases of


river crossings, road crossings, etc., where theelectrode cannot be properly placed, analternative criterion may have to be used.

POLARIZED POTENTIAL OF -850 mVMEASURED TO A Cu/CuSO4 REFERENCEELECTRODE

As discussed in the previous criteria section,one method of considering the voltage (IR) dropacross the electrolyte is to measure thestructure potential with all current sources “off”.The potential that is then measured would bethe polarized potential of the structure at thatparticular test location. A polarized potential isdefined in Section 2 of RP0169-2002 as, “Thepotential across the structure/electrolyteinterface that is the sum of the corrosionpotential and the cathodic polarization”. Thedifference in potential between the static ornative potential and this “instant off” potentialwould be the amount of polarization that hasoccurred due to the operation of the cathodicprotection system. The definition for polarizationis found in the same section as listed aboveand it states, “The deviation from the corrosionpotential of an electrode resulting from the flowof current between the electrode and theelectrolyte.”

Limitations of the Polarized Potential of -850mV Measured to a Cu/CuSO4 ReferenceElectrode Criterion

Polarized potentials well in excess of -850 mVshould be avoided for coated structures in orderto minimize the possibility of cathodicdisbondment of the coating. The NACEStandard points out that, “Polarized potentialsthat result in an excessive generation ofhydrogen should be avoided on all metals,particularly higher strength steel, certain gradesof stainless steel, titanium, aluminum alloys,and prestressed concrete pipe.”

100 MILLIVOLT POLARIZATION CRITERION

The 100 millivolt polarization criterion, like the-850 mV polarized potential, is based on thedevelopment of polarization. This causes the

structure to exhibit a more negative value, withrespect to a copper/copper sulfate referencecell, than in its native state.

Measurement of the polarization shift can bedetermined by either measuring its formation ordecay. Determination of the amount ofpolarization is normally made during thepolarization decay (positive shift) periodsubsequent to de-energizing the cathodicsystem, or in the case of galvanic anodes,when they are disconnected. When cathodicprotection system is de-energized, there is animmediate rapid drop of potential. Thepolarization decay does not include this drop.The potential of the structure after this initialrapid drop be recorded and used as a basis fordetermining the polarization shift. To obtain thetotal polarization shift, the final potential afterpolarization decay has taken place is measuredand subtracted from the potential readimmediately after the cathodic system has beenturned off or disconnected. If this total shift is100 millivolts or more, the structure isconsidered to be cathodically protected.

One drawback of this method is that the timerequired for full depolarization could take fromhours to days for a coated structure to severalweeks for a bare structure. This could make thismethod very time consuming and also mightleave the structure unprotected for an extendedperiod of time. Normally however, the bulk ofthe de-polarization will take place in the initialphase of the polarization decay; therefore itmay not be necessary to wait the full decayperiod except in those cases in which the totalactual polarization shift of the structure isrelatively close to 100 millivolts. If, during theearly phase of polarization decaymeasurement, the potential drops 100 millivoltsor more, there is no real need (unless the actualvalue is desired) to wait for furtherde-polarization. If, on the other hand, thepotential drop in the initial phase of the decayperiod is only on the order of 50 to 60 millivolts,it may be doubtful that the 100 millivolt shift willoccur. In this case, a determination should bemade as to whether a longer wait for totalde-polarization is required and justifiable.


In order to determine the formation ofpolarization on a pipeline/structure, it is firstnecessary to obtain static or native potentials(before the application of cathodic protectioncurrent) on the structure at a sufficient numberof test locations. Once the protection system isenergized and the structure has had time topolarize, these potential measurements arerepeated with the current source interrupted.The amount of polarization formation can thenbe determined by comparing the staticpotentials with the “instant off” potentials.

Applications of the 100 Millivolt PolarizationCriterion

In many cases, the cost of the power requiredto achieve the 100 millivolt polarization decaycriterion may be less than that required to meeteither the -850 mV potential with cathodicprotection applied criterion or the -850 mVpolarized potential. Further, the polarizedpotential corresponding to 100 millivolts ofpolarization may be less negative than -850mV. This criterion takes into account only thepolarization film on the pipe surface and isindependent of any voltage (IR) drops in the soilor at the structure electrolyte interface. Thiscriterion should not be used in areas subject tostray currents because any outside interferencewould tend to break down the polarization at thepoint of discharge. The results obtained underthese conditions could be misleading.Corrosion personnel must determine theeffectiveness of this criterion in areas of straycurrent activity.

The 100 millivolt polarization criterion is mostlyused on poorly coated or bare structures and insome instances could be useful in large pipenetworks as in compressor and regulatingstations where the cost of a cathodic protectionsystem to achieve either a -850 mV “on”potential or a -850 mV polarized potential maybe prohibitive. However, in an economicalanalysis, the additional cost of conductingfuture periodic surveys has to be taken intoaccount; as the use of the 100 millivolt criterionis somewhat more complicated and costly thanthe use of the -850 mV criteria with the cathodic

protection applied.

This criterion can be used on metals other thansteel where there could be some question as towhat specific potential to use as an indication ofprotection. This criterion is often used for thoseinstallations where it is impractical to meeteither of the -850 mV criterion.

In piping networks, where new pipe is coupledto old pipe, it may be good practice to use the-850 mV polarized potential criterion for the newpipes and the 100 millivolt polarization criterionfor the older pipes.

Limitations of The 100 Millivolt PolarizationCriterion

The limitations for the 100 millivolt polarizationcriterion are as follows:

C Interrupting all DC current sources is notalways possible.

C Stray current potential variations can makethis criterion difficult to measure.

C If dissimilar metals are coupled in thesystem, protection may not be complete onthe anodic metal.

C The only additional limitation is related to thetime required for the pipe to de-polarize. Insome cases, adequate time may not beavailable to monitor the polarization decay ofthe pipe to the point where the criterioncould have been met.

CRITERIA FOR SPECIAL CONDITIONS - NETPROTECTIVE CURRENT CRITERION

This criterion is listed in Section 6 of RP0169-2002 under a special conditions section tocover those situations for bare or ineffectivelycoated pipelines where it appears that long linecorrosion activity is the primary concern.

This protection criterion is based on thepremise that if the net current at any point in astructure is flowing from the electrolyte to the


structure, there cannot be any corrosion currentdischarging from the structure to the electrolyteat that point. The principle is based on firstlocating points of active corrosion and thenmeasuring current flow to or from the structure.

Some pipeline companies use the side drainmethod for application of this criterion on thebasis that if the polarity of the voltage readingson each side of the structure indicates currentflow towards the structure, then the structure isreceiving protective current. If the electrodelocated over the pipe is positive with respect tothe other two electrodes, then current isdischarging from the pipe to the electrolyte andcorrosion is taking place.

The above statement is considered correct aslong as there are no outside sources ofinfluence such as other pipelines or othergradient sources such as stray currents. Incases where other gradient sources exist, theresults could be misleading. The results mightalso be questionable in areas with highresistivity surface soil, for deeply buriedpipelines, or where local corrosion cell exists.

Applications of the Net Protective CurrentCriterion

This criterion is normally used on uncoatedstructures. The “side drain’ surface method isnormally used in cases of single isolatedpipelines to obtain an indication of whether thepipeline was receiving cathodic protection. Ifthe electrodes that are placed perpendicular tothe pipeline (remote) are positive in relation tothe electrode over the pipeline, it was assumedthat current is flowing toward the pipe.

In cases where there are sources of outsidegradients, it may be difficult to evaluate theresults of these tests properly.

Limitations of the Net Protective CurrentCriterion

The use of this criterion is to be avoided inareas of stray current activity because potentialvariations could interfere with its use. Also,

extreme care must be taken in commonpipeline corridors because other gradientsources may exist that could result in erroneousor misleading measurements.

Even though the results indicate a net currentflow towards the pipe/structure, that net currentflow is indicative only of what is happening atthe specific point of test and does not representwhat may be happening at other points on thepipeline/structure. Pipeline companies that usethe side drain method for application of thiscriterion, normally require that tests beconducted at close intervals (2 - 20 feet alongthe pipeline).

OTHER CRITERIA

The two criteria listed below have been used inthe past and are presented for information.

E LOG I CURVE CRITERION

The E Log I technique in itself is not so much acriterion of protection as it is a means ofdetermining the correct amount of cathodicprotection current required for a structure.Being complicated in application, the E log Icriterion is limited to specialized applicationswhere other methods may be inadequate.

Applications of the E Log I Curve Criterion

The E Log I Curve criterion is not generallyused by itself to evaluate existing cathodicprotection systems. Its primary use is todetermine the potential value, measured withrespect to a reference electrode, which will givea specific minimum current value required forprotection. This potential value is to be at leastas negative (cathodic) as that originallymeasured at the beginning of the Tafel segmentof the E log I curve.

Once the current value and the potential to aremote electrode have been established, futuresurveys consist of checking the current outputof the cathodic protection system and thepotential of the structure to a remote electrode.It is important that the reference electrode be


located in the same place where it was locatedduring the E Log I tests. Because the testmethod involved is rather elaborate, the use ofthis method is generally limited to structureswhere conventional means of determiningcurrent requirements would be difficult.Examples of such structures are pipeline rivercrossings, well casings, piping networks in aconcentrated area, and in industrial parks.

Limitations of the E Log I Curve Criterion

The main limitation of this technique is thattesting is elaborate, thus making it relativelyslow in comparison to that required for othercriteria. Future tests are relatively simple,however. The remote reference electrode hasto be located exactly at the same locationswhere it was placed during the E Log I tests.Because the E Log I curve is time dependent,there is no guarantee that a repeat E Log Icurve will yield the same minimum potential orcurrent for protection as an earlier curve.

Using this criterion in areas where straycurrents are present could result in erroneousresults as the stray currents may affect thereadings and make obtaining a goodpolarization curve impossible due to thefluctuations in the structure potential.

300 MILLIVOLT POTENTIAL SHIFTCRITERION

As opposed to working toward a certainminimum potential value to a referenceelectrode as discussed in some of the previoussections, the 300 millivolt potential shift criterionis based on changing the potential of thestructure in the negative direction by a specifiedminimum amount. The minimum potential shiftfor steel, as used in the past is 0.300 volt (300millivolts). Any stable reference electrode maybe used with this criterion because the methodconsists of measuring a potential shift and isindependent of the actual potential of theelectrode. Determination of the voltage shift ismade with the protective current applied.

The development of this criterion appears to

have been largely empirical or experimental innature. Although 300 millivolts is the Figurecommonly used at this time, other values suchas 200 or 250 millivolts have also been used inthe past. There were, however, twoconsiderations that supported this criterion,recognizing that corrosion prevention still maynot be 100% complete.

The first consideration was that 300 millivoltsmay be greater than the driving potential ofmost of the galvanic cells on a structure to beprotected. By shifting the structure in thenegative direction by 300 millivolts, the majorityof the cells should be overcome.

The second consideration was based on testsconducted by M. Pourbaix. His work showedthat if a protective potential applied to astructure is made more negative by equalincrements starting from the natural or staticpotential, the first increment would yield thegreatest reduction in corrosion rate andsubsequent increments would result inprogressively smaller reductions in thecorrosion rate per increment. Experienceindicated that the 300 millivolt shiftencompassed sufficient reduction in the rate ofcorrosion to make it a somewhat effectivecriterion for practical applications.

Later evaluation of this criterion by a NACEcommittee concluded that the actualmeasurements or shift was not representativeof what was occurring on the surface of thestructure. This is due to the fact that when acathodic protection system is energized, animmediate shift of potential, due to a voltage(IR) drop, will be seen. In this particularapplication this IR drop value is included in themeasured shift.

Applications of the 300 Millivolt PotentialShift Criterion

This criterion has been used for entirestructures and also for hot-spot protection. It ismainly used for bare steel structures whichhave undergone a slow uniform corrosion ratedue to their age. These structures/pipes


normally have a natural potential range fromabout -0.2 volts to -0.5 volts as a result ofoxidation products developing on the externalsurfaces. This criterion has also been used onsome coated pipelines where soil conditionsalter the natural or static potential of the steel to-0.5 volts or lower. In this case, the 300 millivoltshift may also be easier to attain than achievinga potential reading of -0.85 volt, and could beexpected to stop the majority of corrosion.

The 300 millivolt potential shift criterion isalmost always more applicable to impressedcurrent systems than galvanic anode systems.The reason is that the native state potentialmay be such that a limited number of galvanicanodes will not produce the required change ofpotential on the structure to be protected.

Limitations of the 300 Millivolt PotentialShift Criterion

The 300 millivolt potential shift in someinstances is impractical to use and in othercases, may not correctly indicate the level ofcathodic protection achieved. Some galvaniccells may still be active even after the 300millivolt shift is obtained and there could beareas between adjacent structure to electrolytereadings where the 300 millivolt shift is notbeing attained. Furthermore, a pipeline subjectto stray currents could have positive potentialswith respect to a copper/copper sulfatereference electrode even after the pipe potentialis shifted in the negative direction by 300millivolts. If the potential of the structure isfluctuating more than the shift required, thiscriterion cannot be valid.

If the steel structure is coupled to a more noblemetal, a 300 millivolt shift might indicate thatmuch of the adverse effect produced by thedissimilar metal union has been overcome, butit would not necessarily indicate that completecathodic protection is being provided to thesteel structure/pipeline itself.

USE OF CRITERIA

It must be noted that there will be situations or

conditions where a single criterion cannot beused to evaluate the effectiveness of a cathodicprotection system and it is necessary to employa combination of criteria.

There will also be situations when theapplication of the criteria listed may beinsufficient to achieve protection. Someexamples pointed out in the NACE Standardare situations where the presence of sulfides,bacteria, elevated temperatures, acidenvironments, and/or dissimilar metals increasethe amount of current required for protection. Atother times values less negative than thoselisted may be sufficient. The NACE Standardlists examples such as a pipeline encased inconcrete or dry or aerated high resistivity soil.

EVALUATION OF UNDERGROUND COATINGS USING ABOVEGROUND TECHNIQUES2-1

CHAPTER 2

EVALUATION OF UNDERGROUND COATINGS USINGABOVEGROUND TECHNIQUES

INTRODUCTION

This chapter describes the indirect inspectionmethods intended for use as part of theExternal Corrosion Direct Assessment (ECDA)process for detecting coating flaws anddetermining cathodic protection levels on buriedpipelines. These methods are often morelaborious than surveys completed as part ofnormal daily operating practices as a result ofthe requirement for precise data set alignment.Standard pipeline surveys generally investigatedata trends over time or pipeline distance, whileECDA surveys look for small data variationsover short distances.This chapter describes the methods ofconducting the following surveys for abovegrade indirect inspections. Other inspectionmethods can and should be used as requiredby the unique situations along a pipeline. Thetechniques described herein are not intended toillustrate the only methods by which these toolscan be applied.

• Pipeline Locating is used to establish thelocation and centerline of the pipeline.

• Direct Current Voltage Gradient (DCVG)Surveys are used to locate and size coatingholidays.

• Alternating Current Voltage Gradient(ACVG) Surveys are used to locate andsize coating holidays.

• Close-Interval Surveys (CIS) are used todetermine cathodic protection (CP) levels,electrical shorts to other structures, static

stray current conditions, and large coatingholidays.

• Alternating Current (AC) AttenuationSurveys are used to assess coating qualityand to detect and compare coatinganomalies.

SURVEY SEQUENCE

The sequence in which the surveys areconducted is crucial to optimizing the surveytechniques and data analysis. Coating holidaysurveys (DCVG and ACVG) and AC attenuationsurveys should be completed prior to a CISsurvey. With surveys completed in this manner,pipe to electrolyte potentials can be measureddirectly above the coating holiday indicationsfound using the DCVG or ACVG method.

Some ECDA surveys require existing CPsystems to be turned off, the rectifier outputsincreased, or systems disconnected. Structurepotentials should be measured and recorded atsufficient locations to demonstrate that thepipeline has returned to the original level ofpolarization prior to completion of additionalsurveys that may be influenced by theseactions. The structure potentials should bemeasured before and after these activities havetaken place.

REFERENCED PROCEDURES

Additional information regarding the techniquescited in this chapter can be found in the latestrevisions of the following documents:


• NACE/CEA 54277, Specialized Surveys forBuried Pipelines 1988

• NACE Standard TM0497, MeasurementTechniques Related to Criteria for CathodicProtection on Underground or SubmergedMetallic Piping Systems

• NACE Standard SP0169, Control ofExternal Corrosion on Underground orSubmerged Metallic Piping Systems

• NACE Standard SP0177, Mitigation ofAlternating Current and Lightning Effects onMetallic Structures and Corrosion ControlSystems

DEFINITIONS

a) Anomaly: Any deviation from nominalconditions in the external wall of a pipe, itscoating, or the electromagnetic conditionaround the pipe.

b) Cathodic Protection (CP): A technique toreduce the corrosion of a metal surface bymaking the surface the cathode of anelectrochemical cell.

c) Close-Interval Survey (CIS): A method ofmeasuring the potential between the pipeand earth at regular intervals along thepipeline.

d) Corrosion: The deterioration of a material,usually a metal that results from a reactionwith its environment.

e) Disbonded Coating: Any loss of adhesionbetween the protective coating and a pipesurface as a result of adhesive failure,chemical attack, mechanical damage,hydrogen concentrations, surfacepreparation and application problems, etc.Disbonded coating may or may not beassociated with a coating holiday.

f) ECDA Region: A section or sections ofpipeline that have similar physicalcharacteristics and operating history and in

which the same indirect inspection tools areused.

g) Electrolyte: A chemical substancecontaining ions that migrate in an electricfield. For the purposes of this chapter,electrolyte refers to the soil or liquidadjacent to and in contact with a buried orsubmerged metallic piping system,including the moisture and other chemicalscontained therein.

h) Electromagnetic Inspection Technique: Anaboveground survey technique used tolocate coating defects on buried pipelinesby measuring changes in the magnetic fieldthat are caused by the defects.

i) External Corrosion Direct Assessment(ECDA): A four-step process that combinespre-assessment, indirect inspections, directexaminations, and post assessment toevaluate the impact of external corrosion onthe integrity of a pipeline.

j) Fault: Any anomaly in the coating, includingdisbonded areas and holidays.

k) Holiday: A discontinuity (hole) in aprotective coating that exposes thestructure surface to the environment.

l) Indication: Any deviation from the norm asmeasured by an indirect inspection tool.

m) Indirect Inspection: Equipment andpractices used to take measurements atground surface above or near a pipeline tolocate or characterize corrosion activity,coating holidays, or other anomalies.

n) IR Drop: The voltage difference betweenthe On and Off pipe to soil potential.

o) Microbiologically Influenced Corrosion(MIC): Localized corrosion resulting fromthe presence and activi t ies ofmicroorganisms, including bacteria andfungi.


p) Pipe-to-Electrolyte Potential: SeeStructure-to-Electrolyte Potential.

q) P i p e - t o - S o i l P o t e n t i a l : S e eStructure-to-Electrolyte Potential.

r) Region: See ECDA Region.

s) Structure-to-Electrolyte Potential: Thepotential difference between the surface ofa buried or submerged metallic structureand the electrolyte that is measured withreference to an electrode in contact with theelectrolyte.

SAFETY CONSIDERATIONS

Appropriate safety precautions, including thefollowing, should be observed when makingelectrical measurements.

• Be knowledgeable and qualified in electricalsafety precautions before installing,adjusting, repairing, removing, or testingimpressed current cathodic protectionequipment.

• Use properly insulated test lead clips andterminals to avoid contact with anunanticipated high voltage (HV). Attach testclips one at a time using the single-handtechnique for each connection.

• Use caution when long test leads areextended near overhead high-voltagealternating current (HVAC) power lines,which can induce hazardous voltages ontothe test leads. Refer to NACE StandardSP0177 for additional information aboutelectrical safety.

• Use caution when performing tests atelectrical isolation devices. Beforeproceeding with further tests, useappropriate voltage-detection instrumentsor voltmeters with insulated test leads todetermine whether hazardous voltages,both AC and DC, may exist.

• Avoid testing when thunderstorms are in the

area. Remote lightning strikes can createhazardous voltage surges that travel alongthe pipeline.

• Use caution when stringing test leadsacross streets, roads, and other locationssubject to vehicular and pedestrian traffic.When conditions warrant, use appropriatebarricades, flagging, and/or flag persons.

• Observe appropriate Company safetyprocedures, electrical codes, and applicablesafety regulations.

PIPELINE LOCATING

The pipeline must be located and marked toensure that subsequent measurements aremade directly above the pipeline. An inductiveor conductive pipe locating device can be used.The pipe should be located within six (6) inchesperpendicular of the pipe centerline and surveyflags or paint marks placed directly above thepipeline every 100 feet using a slack chaindistance technique or a measuring wheel. Slackchain stationing error shall be no more than 2%+/-.

The locating flags/paint marks can benumbered using a permanent marker by writingthe flag number directly on the flag or paintingthe flag number on the ground. Numbering ofthe 100 foot stations will ensure that thedifferent indirect survey techniques will beperfectly aligned by using the flag locations asthe point of alignment.

The pipeline should be stationed beginning with0+00 at the initial ECDA region and shouldprogress in increasing station numbers in thedirection of gas/product flow. The pipelinestationing should be reset to 0+00 at eachfollowing ECDA region start. As an alternative,actual pipeline as-built stationing can be used.With this method, the actual pipeline stationingis used for the initial start of the ECDA regionand stationing continued until the end of theECDA region.

PIPELINE LOCATING CREW CLEARING PATH AND MARKING PIPELINE CENTERLINE

FIGURE 2-1

PIPELINE LOCATION STATIONING FLAGS NUMBERED FOR PRECISE DATA ALIGNMENT

FIGURE 2-2


DIRECT CURRENT VOLTAGE GRADIENTSURVEYS (DCVG)

Direct current voltage gradient (DCVG) surveysare used to evaluate the coating condition onburied pipelines. Voltage gradients arise as aresult of current pickup or discharge at coatingholidays. In a DCVG survey, the DC signal iscreated by interrupting the pipeline’s CP currentor a temporary CP current, and the voltagegradient in the soil above the pipeline ismeasured. Voltage gradients are located by achange in the interrupted signal strength atgrade.

DCVG is the only method that can be used toapproximate the size of a coating holiday.DCVG signal strength is not always proportionalto holiday size, as the orientation of the holidayand other factors affect the measured signal.

DCVG surveys are capable of distinguishingbetween isolated and continuous coatingdamage. The shape of the gradient fieldsurrounding a holiday provides this information.Isolated holidays, such as rock damage,produce fairly concentric gradient patterns inthe soil. Continuous coating damage, such asdisbonded coatings or cracking, produceselongated patterns.

The DCVG system consists of a currentinterrupter, an analog or digital voltmeter,connection cables, and two probes withelectrodes filled with water. The interrupter isused to interrupt current from an existingrectifier unit, galvanic anode system, or atemporary CP system installed for the purposeof the DCVG survey.

An analog voltmeter must have a needle withthe ability to deflect in both the positive andnegative directions from the zero point, whichassists in determining the direction the currentis flowing in the soil. Digital voltmeters must besufficiently sensitive to measure 1 mV changesbetween the two reference probes and have theability to indicate the direction of current flow inthe soil. The voltmeter should have the ability toadjust the input impedance for use in high

resistance contact situations.

The current interrupter is installed in series withthe current source and set to cycle at a fast ratewith the “on” period less than the “off” period. Acommon interruption cycle is 0.3 seconds onand 0.7 seconds off. This short cycle allows fora quick deflection by the analog voltmeterneedle.

DCVG surveys can be performed withimpressed current CP systems energized.Sacrificial anodes and bonds that are notdisconnected show up as anomalies. Sacrificialanodes and bonds to other structures areusually disconnected to prevent signal loss andenhance current flow down the pipeline underinvestigation.

The IR drop is measured at the test stations inthe proximity of the DCVG survey. It isdesirable to have a minimum of 100 to 400 mVof IR drop in soil environments and more IRdrop when surveying on asphalt/concrete, in thesection of pipeline to be surveyed. If at theestimated daily survey section limits there is nota 100 to 400 mV IR drop, then the currentoutput of the CP current source should beincreased to achieve the desired result. If theoutput cannot be increased, then the section ofpipe with the 100 to 400 mV IR drop is the onlysection that can be surveyed using that CPcurrent source. Alternative CP current sourcesmay need to be temporarily installed in order toachieve the desired IR drop. It is desirable tohave as large an IR drop value as can beachieved. This will enable the surveyor todetect small holidays distant from the CPcurrent source.

A surveyor walks along the pipeline such thatthe probes can be used in a walking stickfashion. One probe is always kept near thepipeline center line while the other is heldapproximately five (5) feet away perpendicularto the pipe. The voltmeter is read when bothprobes are in contact with the soil.

If possible, the pipeline under investigationshould be electrically isolated from other

ANALOG DCVG METER

FIGURE 2-3

DCVG SURVEYOR MEASURING VOLTAGE GRADIENT ABOVE PIPELINE

FIGURE 2-4

DCVG SURVEY COMPLETED ON WET ASPHALT IN MAJOR CITY

FIGURE 2-5


parallel metallic structures by disconnectingelectrical bonds, negative drains to rectifiers,etc. In pipeline right of ways with multipleelectrically continuous pipelines or metallicconduits/structures, variations in the surveytechnique must be considered. For example, ifparallel pipelines (metallic structures) are lessthan ten (10) feet from the pipeline underinvestigation, then the perpendicular probeshould be placed at half the distance betweenthe two pipelines, however, difficulties may beencountered with current flow to the parallelmetallic structure. The perpendicular probemust be placed on the side of the investigatedpipeline without a parallel pipeline (metallicstructure). If the pipeline under investigationhas pipelines (metallic structures) on eitherside, then the probe should be placedperpendicular, but not above or in closeproximity to the parallel pipeline (metallicstructures).

If the voltmeter indicates a coating holiday,additional measurements should be made toconfirm the coating holiday is on the pipelineunder investigation and not the parallel pipeline.These tests include gradient measurements onboth sides of the pipeline and parallel with thepipeline under investigation to confirm thecoating holiday location.

When parallel metallic structures or right of wayconditions do not allow sufficient room forplacement of the probe to the side of thepipeline, an alternate method of DCVG can beconducted. Place both probes directly abovethe pipe centerline approximately five to ten feetapart from one another and measure thevoltage gradient in this manner. Leap frog therear probe to the forward position and repeatthe process continuing along the right of waydirectly above the pipe. Care must be taken topinpoint the maximum deflection or indicationwhen completing the survey in this manner. Theexact location of the indication requires movingthe probes at closer spacings once the coatingholiday is detected.

When a coating holiday is approached, anoticeable signal swing can be observed on the

voltmeter at the same rate as the interrupterswitching cycle. The amplitude of the swingincreases as the coating holiday is approachedand decreases after it has been passed.Current flow from the interrupted current sourceto the pipeline indicates a possible coatingholiday while current flow away from thepipeline indicates current flow past the pipeline.

When a coating holiday is found, additionalgradient measurements can be beneficial toconfirm its location and that the indication is notcurrent traveling past the pipeline. Thesegradient measurements can be made on bothsides of the pipe and parallel with the pipe oneach side of the assumed coating holiday.

A straight-line attenuation effect is assumedbetween test station locations to calculate thesignal strength at intermediate coating holidaylocations. In order to calculate the coatingholiday size (%IR), the difference between theon and off potentials at each test station, valve,or other above grade appurtenance must bemeasured and recorded.

One reference electrode is placed at the baseof the test station or other electrical contactpoint, in contact with the soil while the secondelectrode porous tip contacts the test stationwire or other properly cleaned electrical contactpoint. The maximum analog needle deflectionis the test station IR drop or difference betweenthe on and off potentials when using a digitalvoltmeter.

The distance between test stations or points ofelectrical contact must be determined from thepipeline stationing and used in the calculationsfor the signal attenuation.

An example of the attenuation calculations canbe seen below:

On potential -1.45 V

Off potential -0.95 V

Signal Strength 1.45 - 0.95 V

= 0.5 V or 500 mV

=

=

=


The estimated signal strength can beexemplified by using the data presented inFigure 2-6.

Estimated signal strength at defect:

Precisely locating a coating holiday is achievedby marking the approximate location of theholiday at the area where the maximumamplitude is indicated. Near the approximatecoating holiday location and offset from the lineby approximately 10 ft, the probes are placedalong the voltage gradient to obtain a null (zero)on the meter. A right-angle line through thecenter of the probe locations passes over thecoating holiday epicenter, as shown in point Ain Figure 2-7. This geometrical procedurerepeated on opposite sides of the pipelinelocates the exact point above the holiday.

A survey flag, wooden stake, paint mark, orlathe is often placed at the indication epicenterand identified by a unique indication numberusing a permanent marker or paint.

After the epicenter of the coating holiday hasbeen detected, a series of continuous lateral(perpendicular) readings are measured movingtoward remote earth. Lateral readings near theholiday yield maximum voltage differenceswhere gradients are at a maximum. Readingsat remote earth are considered to be achievedwhen one has a 1 mV deflection. Thesummation of these readings is commonlyreferred to as the over-the-line-to-remote-earthvoltage. The expression “percentage IR” hasbeen adopted to give a relative indication size.

For instance, if a series of lateral millivoltreadings to remote earth are as follows, 25, 15,6, 4, 3, 1, 1, 0, the percentage IR can becalculated as follows:

The percentage IR is used to develop a coatingcondition classification system to prioritizecoating damage.

Once an indication is located, its size orseverity is estimated by measuring the potentiallost from the holiday epicenter to remote earth.This potential difference is expressed as afraction of the total potential shift on the pipeline(the difference between the “on” and “off”potential, also known as the IR drop) resultingin a value termed % IR. DCVG survey readingscan be broken into four groups based onapproximate size as follows:

Category 1: 1% to 15% IR - Indications in thiscategory are often considered of lowimportance. A properly maintained CP systemgenerally provides effective long-termprotection to these areas of exposed steel.

Category 2: 16% to 35% IR - These indicationsare generally considered of no serious threatand are likely to be adequately protected by aproperly maintained CP system. This type ofindication may be slated for additionalmonitoring. Fluctuations in the levels ofprotection could alter the status at these pointsas the coating further degrades.

Category 3: 36% to 60% IR - The amount ofexposed steel in such an indication indicates itmay be a major consumer of protective CPcurrent and that serious coating damage maybe present. As in Category 2 indications, thistype of possible coating holiday may be slatedfor monitoring as fluctuations in the levels of CPcould alter the status as the coating furtherdegrades.

( )= 200 mV + 1500

500 + 1500300 - 200 mV

= 200 mV + 75 mV

= 275 mV

⎡⎣⎢

⎤⎦⎥

Over the line to remote earth voltages = 25 +15 + 6 + 4 + 3 +1+1 mV

= 55 mv

Percentage IR = Over the line to remote earth voltage * 100%

Signal Strength

= 55 *100

275

= 20%

DCVG SIGNAL STRENGTH

FIGURE 2-6

DEFECT

1500 yds 500 yds

200 mV 375 mV300 mV

DCVG VOLTAGE GRADIENTS

FIGURE 2-7


Category 4: 61% to 100% IR - The amount ofexposed steel indicates that this indication is amajor consumer of protective CP current andthat massive coating damage may be present.Category 4 indications typically indicate thepotential for serious problems with the coating. These example categories are empirical innature and are based on the results of priorexploratory excavations at holiday locationsdetermined by DCVG surveys.

ALTERNATING CURRENT VOLTAGEGRADIENT SURVEYS (ACVG)

Alternating current voltage gradient (ACVG)surveys are used to evaluate the coatingcondition on buried pipelines. Voltage gradientsarise as a result of current pickup or dischargeat coating holidays. In an ACVG survey, the ACsignal is created by a low frequency transmitterconnected to the pipeline and the voltagegradient in the soil above the pipeline ismeasured. Voltage gradients are located by achange in the signal strength at grade.

ACVG signal strength is not always proportionalto holiday size, as the orientation of the holidayand other factors affect the measured signal.

ACVG surveys are capable of distinguishingbetween isolated and continuous coatingdamage. The shape of the gradient fieldsurrounding a holiday provides this information.Isolated holidays, such as rock damage,produce fairly concentric gradient patterns inthe soil. Continuous coating damage, such asdisbonded coatings or cracking, produceselongated patterns.

The ACVG system consists of a commonlyemployed, commercially available battery/ACpowered signal-generating unit that provides a937.5-Hz AC signal with a maximum output of750 mA or a 4-Hz AC signal with a maximumoutput of three amperes. A handheld receiverunit is tuned to detect the signal frequency fromthe transmitter and block other signals. Thereare two probes which connect to the receiverand are in contact with the soil.

An AC current attenuation survey may beperformed with impressed current CP systemsenergized, however by turning off the rectifierand using the positive and negative leads at therectifier station, the signal-generationcapabilities of the equipment can bemaximized. Sacrificial anodes and bonds thatare not disconnected show up as anomalies.Sacrificial anodes and bonds to other structuresare usually disconnected to prevent signal lossand enhance current flow down the pipeline.

The signal generator (transmitter) is connectedto the pipeline and appropriately grounded toearth. A constant AC signal is produced andtransmitted along the pipe. The transmitter isenergized and adjusted to an appropriateoutput. Typically, the largest attainable currentoutput is chosen to maximize the length of pipethat can be surveyed. An impressed currentanode bed or magnesium anode can be used toestablish an electrical ground.

The receiver consists of a handheld,symmetrical, multi-axis antenna array. Theelectromagnetic field radiating from the pipelineis measured by the detector.

The detector is used to measure the attenuationof the signal current that has been applied tothe pipe. An electrical current, when applied toa well coated buried pipeline, graduallydecreases as distance increases from the pointof current application. The electrical resistivityof the coating under test and the surface areain contact with the soil per unit length of pipeare the primary factors affecting the rate ofdecline and the frequency of the signal.

The two probes must be connected to thereceiver unit and plugged in. Survey intervalsshould be no greater than ten (10) feet in thoseareas deemed to have coating holidays.

The operator walks above the pipelinecenterline placing the contact probes into thesoil above the pipe and measuring the voltagegradient and direction. Probes are placedparallel to the pipe. The receiver unit will “point”in the direction of a coating holiday. The

ACVG SURVEY ABOVE PIPELINE LOCATING COATING HOLIDAYS

FIGURE 2-8

AC CURRENT ATTENUATION RECEIVER FACE

FIGURE 2-9


magnitude will increase as the holiday isapproached.

The survey continues until the receiverindicates that the coating holiday has beenpassed (signal magnitude decreases anddirection arrow reverses direction) at whichpoint the operator reverses direction andshortens the interval between readings.

When the holiday is centered between the twoprobes, the magnitude will be zero and thedirection arrows will not indicate a consistentcurrent direction.

The probe assembly can be used to the side ofthe pipe (perpendicular to the pipe) to confirmthe coating indication location. The holidaylocation is indicated by the maximum signalmagnitude with the probes placedperpendicular to the pipe.

Either store the data in the receiver unit orrecord the information in the project field book.

CLOSE-INTERVAL SURVEYS (CIS)

CIS is used to measure the potential differencebetween the pipe and the electrolyte. Data fromclose interval surveys are used to assess theperformance and operation of the CP system.CIS can also be used to detect coatingholidays.

While other indirect inspection tools may bebetter suited to detect coating holidays, CISalso aid in identifying:

1. Interference,

2. Shorted casings,

3. Areas of electrical or geologic currentshielding,

4. Contact with other metallic structures, and

5. Defective electrical isolation joints.

On and off potential surveys are used to

evaluate CP system performance inaccordance with the NACE pipeline CP criteriaas found in SP0169. On and Off surveysmeasure the potential difference between thepipe and the electrolyte as the CP currentsource(s) is switched on and off.

On and Off surveys rely on electronicallysynchronized current interrupters at each CPcurrent source, bond, and other current drainpoint that influences the pipeline potential in thesurvey area. The ratio of the On-to-Offinterruption cycle should be long enough forreadings to be made but short enough to avoidsignificant depolarization. A three second On,one second Off cycle period or similar can beused to maintain pipeline polarization over timeand allow accurate Off potentials to berecorded.

The copper sulfate electrodes (CSE) are placeddirectly over the pipeline, typically at 2.5 to 5foot intervals such that both On and Offpotentials can be measured and recorded ateach reference cell location.

The accuracy of the on and off data can beverified by recording a continuous datalog(waveprint) at test stations or points of electricalcontact such as valves or risers. This data logwill illustrate proper interrupter synchronizationand the effects of pipeline depolarization duringthe survey should it take place. The continuousdata log should measure and store thepotentials at a minimum time period as the totalcurrent interruption cycle. Waveprints can alsobe reviewed to confirm the point at which theOff potential should be recorded by the CISdatalogger. In addition, the waveprints can beanalyzed to determine the affects of:

Inductive potential spiking,

Interrupter synchronization drift,

Stray DC and AC earth currents.

CIS equipment includes, at a minimum, severalhigh input impedance data loggers/voltmeters,sufficient current interrupters for the project,

CLOSE-INTERVAL SURVEY TECHNICIAN WITH DATALOGGER, REFERENCE ELECTRODE, AND WIRE DISPENSERS

FIGURE 2-10


copper sulfate reference electrodes, smallgauge CIS wire (30 AWG), wire dispenser, andpipe/cable locating equipment (See abovesection on pipe locating).

Standard current interrupter units include 30,60, or 100 ampere AC and DC interruptioncapacity, AC or battery-powered units, withelectronic synchronization and Global PositionSatellite (GPS) timing.

Prior to the CIS, a rectifier influence survey maybe completed to determine the CP currentsources which must be interrupted for theaccurate measuring of Off potentials. Theseinclude company rectifiers, galvanic anodesystems, foreign company rectifiers, andelectrical bonds to foreign company structures.Individually, each CP current source should betested. A current interrupter is used tointerrupt the CP current source suspected ofinfluencing the CIS pipeline segment. Typically,a slow interruption cycle is used such as a tensecond On, five second Off period.Pipe-to-electrolyte potentials are measured atthe furthest test points suspected of influencefrom the suspect CP current source.

Test points are monitored moving away fromthe suspect CP current source. Once a 10 mVor less difference between On and Off pipe toelectrolyte potential is observed, the influenceof the CP current source is deemed no longersignificant for analysis of the pipe CP levels.

Several test points beyond the location wherethe influence of the CP current source isdeemed to terminate should be tested toconfirm the end of the influence from thesuspect CP current source.

To start the CIS, a current interrupter isinstalled in series in either the AC or DC circuitsof all of the current sources in the CP currentsources identified. Current interrupters maintaininterruption synchronization. All interruptersmust be synchronized together. Interruptersshould be programmed such that they areinterrupting no longer than the anticipatedsurvey duration each day. When no field

surveying is taking place, the interruptersshould be programmed to turn off in order tominimize the affects of depolarization.

A 30, 32, or 34 AWG gauge insulated wire iselectrically connected to a test station test wire,valve, or other electrically continuous pipelineappurtenance and one terminal of thevoltmeter. The other terminal of the voltmeter isattached to the reference electrode.

The pipeline is located with a pipe locator priorto collecting data to ensure that the referenceelectrode is placed directly over the pipeline(See Pipe Locating Section).

Industry standard copper sulfate referenceelectrodes (CSE) should be used for potentialmeasurements. See NACE Standard TM0497.Reference electrodes should be calibrated withan unused control reference electrode daily, theresults of which should be recorded in theproject field book. The control referenceelectrode should be a recently chargedelectrode not used to gather data in the field.To calibrate the CSE, the ceramic porouselectrode tips are placed tip to tip to measurethe voltage difference between the twoelectrodes or both tips are immersed in acontainer of potable water. A digital voltmeteron the millivolt scale is used to measure thevoltage difference. If the voltage differencebetween the two electrodes exceeds 5 mV, thefield electrode should be emptied, cleanedproperly, and recharged with unused distilledwater and copper sulfate crystals. If the voltagedifference remains greater than 5 mV, the fieldelectrode should be disposed of properly andnot used to collect data.

A datalogger can be placed at the suspectedmidpoint of each day’s CIS progress for lateranalysis of the CIS data. The datalogger shouldmeasure and record the pipe-to-electrolytepotential continuously during each day’s survey.The datalogger should be programmed torecord the pipe potential at a rate of onereading per second or greater. The dataloggershould be installed and turned on prior to thestart of CIS each day and should run until after


the completion of CIS each day. The data canbe retrieved from the datalogger each day andanalyzed for the existence of dynamic straycurrents, de-energizing of a CP source, orimproper current interrupter operation.

On and Off pipe-to-electrolyte potentials arethen measured and recorded typically at 2.5 to5 foot intervals using a high-input impedancevoltmeter/datalogger. The datalogger shouldhave the ability to adjust the time during the Offcycle at which the handheld datalogger storesthe Off potential value due to the possibility ofinductive/capacitive spiking. The dataloggershould be programmed such that the On andOff pipe to electrolyte potentials are measuredand stored from a time period beyond thespiking as determined by the waveprintsdiscussed above. Pipe-to-electrolyte potential measurementsshould be measured and recorded at each teststation and foreign pipeline crossing from eachtest wire within accessible test stations. Nearground (NG), metallic IR Drop (IR), and farground (FG) On and Off pipe-to-electrolytepotential measurements should be made ateach point of pipeline connection.

If the Off metallic IR drop exceeds 5 mV, thesurvey should be halted and an investigationmade to determine the source of the error.Possible sources for this error includeinterrupter malfunction, stray currents, andunknown foreign rectifiers or electrical bonds.

AC pipe to electrolyte potentials can bemeasured at each point of pipeline connectionand recorded in the project field book or withinthe CIS data stream. On and Off casing to electrolyte potentialsshould be measured at each casing, either bymonitoring the test wire attached directly to thecasing pipe or by temporary electricalconnection to the casing vent.

When a stationing flag is encountered, a flagcomment or code is entered into the data loggerfor later computer graphing and stationing

purposes. When a numbered flag isencountered, the flag number can also beentered into the data stream.

All permanent landmarks should be identifiedand entered into the data logger during thesurvey. These include pipeline markers, testpoints, fences, casing vents, creeks, and roadnames.

Upon completion of the survey, all CIS wireshould be retrieved. Flags can be left in placeuntil the final ECDA process/surveys arecompleted and deemed appropriate for finalremoval.

AC CURRENT ATTENUATION SURVEYS(ELECTROMAGNETIC)

AC current attenuation surveys are used toprovide an assessment of the overall quality ofthe pipe coating within a section or as acomparison of several sections. A current isapplied to the pipeline, and coating damage islocated and prioritized according to themagnitude and change of current attenuation.AC current attenuation surveys may beperformed with impressed current CP systemsenergized, however by turning off the rectifierand using the positive and negative leads at therectifier station, signal-generation capabilities ofthe equipment can be maximized. Sacrificialanodes and bonds that are not disconnectedshow up as anomalies. Sacrificial anodes andbonds to other structures are usuallydisconnected to prevent signal loss andenhance current flow down the pipeline.

Commonly employed, commercially availablebattery-powered signal-generating units includeunits that provide a 937.5-Hz AC signal with amaximum output of 750 mA or a 4-Hz AC signalwith a maximum output of three amperes. Ahandheld receiver unit tuned to detect thesignal from the transmitter and block othersignals is used to pick up the signal.

The signal generator is connected to thepipeline and appropriately grounded to earth. Aconstant AC signal is produced and transmitted

AC CURRENT ATTENUATION TRANSMITTER

FIGURE 2-11

TECHNICIAN MEASURING AC CURRENT ATTENUATION WITH RECEIVER

FIGURE 2-12


along the pipe. The transmitter is energized andadjusted to an appropriate output. Typically, thelargest attainable current output is chosen tomaximize the length of pipe that can besurveyed. An impressed current anode bed ormagnesium anode can be used to establish anelectrical ground. If operating rectifiers areinterfering with the signal, then turn the unitsoff.

Signals are measured using the receiver unit.The receiver consists of a handheld,symmetrical, multi-axis antenna array. Theelectromagnetic field radiating from the pipelineis measured by the detector. The detector isused to measure the attenuation of a signalcurrent that has been applied to the pipe. Anelectrical current, when applied to a well-coatedburied pipeline, gradually decreases asdistance increases from the point of currentapplication. The electrical resistivity of thecoating under test and the surface area incontact with the soil per unit length of pipe arethe primary factors affecting the rate of declineand the frequency of the signal.

The logarithmic rate of decline of the current(attenuation), which is effectively independentof the applied current and marginally affectedby seasonal changes in soil resistivity, providesan indication of the average condition of thepipe coating between two given points on thedate of the survey. Changes in attenuationprovide a comparative change in coatingcondition between survey sections. Suchcomparative changes can indicate “better”coating (i.e., fewer anomalies or a small singleanomaly) or “worse” coating (i.e., moreanomalies or a larger single anomaly).

Survey intervals are typically 100 feet at thepipeline locating flags. The measured currentvalue is recorded in the project field book or thetransmitter for later analysis. The receiving unitmust remain upright and perpendicular to thepipe when taking the measurements. Whenreadings are suspect, stay on peak mode andcheck both the peak and null readings andverify pipe depth. Take multiple readings in onelocation if data accuracy is questionable. The

accuracy of the readings may be affected bydistortions in the AC signal caused by otherunderground piping and conduits, traffic controlsignaling, or vibrations due to passing vehicles.

Survey data are analyzed after the survey todetermine which survey intervals exhibitreduced coating quality.

SUMMARY

The indirect above ground inspectiontechniques discussed in this chapter are usedto identify and define coating faults and in turnthose areas where corrosion activity may haveoccurred or may be occurring. Two or more ofthese inspection techniques should be usedwhen conducting this indirect inspection testingso that different types of data can be comparedand analyzed to determine whether there is anycorrelation. The effectiveness of the testingtechniques employed will depend on factorss u c h a s o p e r a t o r e x p e r i e n c e ,pipeline/coating/CP circuit conditions, depth ofpipe, and type of cover at grade.

Should significant coating damage be indicatedfrom these tests, the pipeline should beexcavated and examined for possible corrosiondamage and the appropriate remedial actionsshould be taken.

ACKNOWLEDGMENT

This chapter was written by Kevin T. Parker andwas edited by the AUCSC CurriculumCommittee.

MATERIALS FOR CATHODIC PROTECTION3-1

CHAPTER 3

MATERIALS FOR CATHODIC PROTECTION

INTRODUCTION

This chapter will discuss common materialsused for underground cathodic protectioninstallations. Some of the materials haveestablished track records and some are newwith very little known about the long term lifeeffects in a particular environment. For ourpurposes, we will define “long term” as anymaterial with a successful application record inunderground use of more than 20 years.

Knowledgeable persons in the corrosion controlindustry must have a grasp of the advantagesand limitations of a product and how theproduct relates to any given application.Furthermore, they must be able to convey inwriting, the specifics of how the product is to bemanufactured or supplied to assure that it willconform to predetermined design life criteria.The object of this chapter is to fit the right typeof materials with the structure to be protected atthe lowest possible cost per year ofpredetermined service life.

CONSIDERATIONS FOR ALL MATERIALS

Material must be selected for each applicationwith the following interrelated items considered:

1. Design criteria

2. Life required

3. Capacity or rating of material selected

4. Economics

5. Environment - both underground and abovegrade

Specifying materials, when requisitioning orpurchasing, the following should be checked:

1. Specify materials completely.

2. Make sure that complete specifications areon the purchase order.

3. Check material received to ensure that itconforms to the original specifications.

Usage:

1. Follow the manufacturer’s recommendationsand instructions.

2. Use compatible components.

3. Use the proper tools.

4. When a problem or question arises, ask themanufacturer or distributor for assistance.

GALVANIC ANODES

Galvanic anodes consist primarily ofmagnesium, zinc, and aluminum, with eachtype having different metallurgical compositionswithin each metal group. These subtledifferences in metallurgy can have a profoundeffect on the long term operationalcharacteristics of the system. A successfulgalvanic system must have a more negativepotential than the structure it is designed toprotect. This is referred to as the drivingpotential difference and is controlled by thenegative electrical values of each metal in agiven environment and affected by theresistance of the electrolyte through which thecurrent, generated by the anode, must flow.Anode driving potential is not affected byresistance of electrolyte. Figure 3-1 shows a


schematic diagram of a typical galvanic anodeinstallation.

Table 3-1 shows the metals used for galvanicanodes, the alloys of each metal, the opencircuit potential, and the number of amperehours of capacity per pound of metal for eachtype. Impurities and grain size will cause wideranges in potential, current efficiency values,and consumption rates, which are expressed inpounds per ampere year (Ib/A-y).

These are representative values derived fromliterature provided by various manufacturers. Inmost cases, magnesium is preferred for soilsand fresh water. Zinc is generally limited to usein sea water, brackish water, sea mud, andsoils with resistivities below 1,500 ohm-cm.Aluminum is generally limited to sea water,brackish water, and sea mud environments.

The important point to consider for maximumservice life is the cost per ampere hours ofcurrent capacity, once it has been establishedthat the driving potential is sufficient for thecathode metal and the resistivity of theelectrolyte in the circuit. When working in thehigher resistivity soils, long slender anodeshave a lower resistance to earth than theshorter anodes which are available. This meansthat the circuit resistance for the limited drivingvoltage available from a galvanic anode will beless, and as a result, more current output will beobtained. To say it in another way, the sameamount of anode material (such as a 20 lb.magnesium anode) in a given soil resistivityenvironment will produce more current if it hasbeen cast in a long, slender shape than it will ifcast in a short and more stocky shape. This willresult in the longer shaped anode having ashorter life expectancy - although this “shorter”life expectancy may be fully adequate forapplications in higher resistivity soils. Theanode installation design for a particularsituation will take the resistivity of theenvironment into account and will balancecurrent requirements against desired life todetermine the size, number, and weight ofanodes needed.

Prepared Backfill and Packaging

Most magnesium and zinc anodes used in soilsrequire the use of a prepared backfill aroundthe anode for the following reasons:

C It increases the effective surface area whichlowers the anode to earth contactresistance.

C The bentonite clay absorbs and retainsmoisture.

C The gypsum provides a uniform, lowresistance environment.

C The sodium sulfate (a depolarizing agent)minimizes pitting attack and oxide filmformation on the anode.

C It provides uniform environment directly incontact with anode to assure evenconsumption.

Today, the most commonly used backfill formagnesium and zinc consists of:

75% Hydrated Gypsum (CaSO4@2H20)20% Bentonite Clay5% Sodium Sulfate

When properly combined, these elements willprovide a uniform resistance of 50 ohm-cmbackfill when measured by the ASTM G-58 SoilBox Test Method and corrected for temperaturevariations. Most reputable anode fabricators willtest and document the resistance values foreach batch of backfill. As a historical side note,zinc was formerly thought to perform better in abackfill of 50% gypsum and 50% bentonite.Over the years it has been determined that zincmay passivate if sodium sulfate is not used inthe backfill.

To keep the backfill uniformly around theanode, the anodes with the lead wire attachedare placed in cloth bags or cardboard boxesand the prepared backfill is then added. Clothbagged anodes are usually encased within alayered paper bag for resistance to short

TABLE 3-1

Capabilities and Consumption Rates

Type Potential*(- volts)

Amp Hoursper lb.

Consumption(lb/A-yr)

Magnesium

H-1 AZ-63 Alloy 1.4 - 1.5 250 - 470 19 - 36

High Potential Alloy 1.7 - 1.8 450 - 540 16 - 19

Zinc

ASTM B418-01

Type I (saltwater) 1.1 354 24.8

Type II (soil) 1.1 335 26.2

Aluminum

Mercury Alloys 1.10 1250 - 1290 6.8 - 7.0

Indium Alloys 1.15 1040 - 1180 7.4 - 8.4

* Copper/Copper Sulfate Reference Electrode - Open circuit practical values as shown


periods of inclement weather and handlingdamage. Prior to backfilling, the paper bag isremoved and discarded, permitting the clothbag containing the backfill to absorb moisture,allowing the anode to start putting out currentsoon after installation. During transportationand/or handling the anodes may shift in theprepared backfill. This may result in unevenconsumption of the anode, reduction of currentoutput and premature failure of the anode. Thiscondition should be avoided by carefulspecification of transportation packaging andfield handling precautions. Anodes packaged incardboard boxes or bags with centralizingdevices may tend to reduce anode shifting.

Wire and Cable Attachment

Wire used to connect the anode to the structureis usually a No. 12 AWG solid copper, singleconductor, with TW (30 mils PVC), THW (45mils PVC), or THHN insulation.

THHN is a new insulation for anode use andshort term accelerated testing has found, thatunder normal current flow conditions, it willprovide satisfactory performances. It uses a 15mil PVC insulation jacketed with 4 mils of nylon,but no field testing has been done to determineif the reduced PVC thickness will handle faultcurrent and lightning surges. It is known thatnylon absorbs moisture and eventually breaksdown when subjected to immersion in water.

Proper attachment of the wire or cable to theanode core is critical for two reasons. First, theconnection must be electrically sound tominimize internal resistance; second, it mustalso be strong enough to support the weight ofthe packaged anode if it is lifted by the leadwire. The lead wire is usually silver soldered tothe core with a 50% silver, 50% tin compound.Next, the connection must be primed andcovered with an inert thermoplastic electricalpotting compound to prevent water migration tothe connection.

When anodes are used as grounding cells, andare required to carry more amperes of current,the cable may be larger (i.e., No. 8 AWG, No. 6

AWG, or No. 2 AWG stranded, single conductorwith high molecular weight polyethylene(HMWPE) insulation).

Zinc anodes do not have a recessed core, sothe cable should be crimped and silver solderedto the extended rod core and coated with apiece of heat shrinkable polyethylene tubing orseveral laps of electrical tape to protect theconnection.

Magnesium Alloys

Magnesium anodes are produced in a widevariety of sizes and shapes to fit designparameters. They may be cast in molds orextruded into ribbon or rod shapes, with steelspring, perforated strap or wire cores as shownin Figure 3-2. The core is important because itshould extend 85 percent or more through theanode to reduce the internal circuit resistance.Table 3-2 shows some of the common weightsand corresponding dimensions for bare andpackaged anodes. Chemical composition isshown in Table 3-3.

Several alloys are available and their respectivepotential values will be an important part of thecalculations for a cathodic protection designbased on driving potential and current efficiency(See Table 3-3). Design of galvanic cathodicprotection systems is covered in Chapter 6.

Element Effects*

Aluminum Properties of H-1 (AZ-63) alloysare not affected by variationsbetween 5 - 7%; high potentialalloy current capacity isdecreased if aluminum is>0.01%.

Zinc Current capacity of H-1 andAZ-63 is slightly decreased if>3.0%.

Manganese The detrimental effect of iron oncurrent capacity of al lmagnesium alloys is offset byadding manganese. The

!

TABLE 3-2

Magnesium Anode Dimensions and Weights

Nominal Dimensions (Inches) - See Figure 3-2

Alloy A B C D E F G PackagedWeight (lbs)

1 AZ63 3.2 Rd 2 6 6 - - 3.63 H.P./AZ63 3 3 6 8 6 - - 95 H.P./AZ63 3 3 10 12 5 - - 126 H.P./AZ63 3 3 10 - - 12.5 5 149 H.P./AZ63 3 3 13.5 17 6 - - 2712 AZ63 4 4 12 18 7.5 - - 3217 H.P. 3.5 3.5 25.5 30 6 - - 4217 AZ63 3.5 3.5 28 - - 32 5.5 4520 H.P. 2 2 60 - - 71 4.5 6532 H.P./AZ63 5.5 5.5 21 25 8 - - 7232 H.P./AZ63 5.5 5.5 21 - - 24 7.5 7040 H.P. 3.5 3.5 60 64 6 - - 10548 H.P. 5.5 5.5 32 36 8 - - 10650 AZ63 7 7 15 24 10 - - 110

60 H.P. 4 4 60 64 5.75 - - 130

Magnesium Extruded Ribbon and Rods

Size(Inches)

Weight(lb/ft)

Core(Inches)

d x ¾ 0.24 0.125

0.750 0.36 0.125

0.840 0.45 0.125

1.050 0.68 0.125

1.315 1.06 0.125

1.561 1.50 0.125

2.024 2.50 0.125

* Data were compiled from literature provided by various vendors and may vary slightly.

TABLE 3-3

Composition of Magnesium Alloy

Element AZ63B(H1A)

AZ63C(H1B)

AZ63D(H1C)

M1C(High Potential)

Aluminum (Al) 5.3 - 6.7% 5.3 - 6.7% 5.0 - 7.0% < 0.01%

Zinc (Zn) 2.5 - 3.5% 2.5 - 3.5% 2.0 - 4.0% -

Manganese (Mn) 0.15 - 0.7% 0.15 - 0.7% 0.15 - 0.7% 0.5 - 1.3%

Silicon (Si) < 0.10% < 0.30% < 0.30% < 0.05%

Copper (Cu) < 0.02% < 0.05% < 0.10% < 0.02%

Nickel (Ni) < 0.002% < 0.003% < 0.003% < 0.001%

Iron (Fe) < 0.003% < 0.003% < 0.003% < 0.03%

Others (each) - - - < 0.05%

Others (total) < 0.30% < 0.30% < 0.30% < 0.30%

Magnesium (Mg) Balance Balance Balance Balance

Performance Characteristics*

AZ63B(H1A)

AZ63C(H1B)

AZ63D(H1C)

M1C(HP)

Potential (Cu/CuSO4) -1.60 -1.55 > -1.40 -1.75

Theoretical AH/lb 1000 1000 1000 1000

Actual Amp-hours/lb 450 - 580 300 - 470 250 - 470 400 - 540

Current Efficiency (%) 45 - 58 30 - 47 25 - 47 40 - 54

Actual Consumption 18 - 15 33 - 19 35 - 19 19 - 16

* Using ASTM Standard G97-97 “Standard Test Method for Laboratory Evaluationof Magnesium Sacrificial Anode Test Specimens for Underground Applications”.


manganese surrounds the ironparticles and decreases theirlocal cathodic effect.

Iron Detrimental to current capacity;c o n t r o l l e d b y a d d i n gmanganese in molten state.

Silicon Decreases current capacity ofH-1 (AZ-63) if >0.1%.

Nickel Greatly decreases currentcapacities of all alloys if>0.001%.

Copper Decreases current capacity if>0.02%.

Others Not commonly found in amountsthat are detrimental.

Lead (Pb) Decreases current capacity if>0.04%.

Tin (Sn) Decreases current capacity if>0.005%.

* Data provided by Dow Chemical U.S.A.

The purchaser should request a verifiablespectrographic analysis of any anode purchaseto assure that the alloy falls within thepredetermined range of acceptable impurities.The anodes should be identified by heatnumber and date of manufacture. The chemicalanalysis should detail the heat number, the dateof manufacture, a listing of the impurities withthe percentages of each, and be signed by atechnically qualified person.

Anode current capacity and oxidation potentialmay be measured using ASTM StandardG97-97 “Standard Test Method for LaboratoryEvaluation of Magnesium Sacrificial Anode TestSpecimens for Underground Applications”.

Zinc Alloys

Zinc anodes are used in low resistance soilswhere driving potential is not a major factor in

system design. They are available in a varietyof sizes and shapes including bracelet anodesfor marine pipelines, docks and piers, hullanodes for marine vessels, and in ribbon formfor use in utility ducts and for AC mitigation.Zinc is not recommended in environmentswhere carbonates or bicarbonates are found, orwhere the temperature of the electrolyte is over120º F. Under these situations zinc becomescathodic, rather than anodic, to steel and itsuse should be avoided. Tables 3-4 and 3-5show some common zinc anode sizes andcompositions respectively.

When used strictly as an anode, zinc is wellsuited for low resistivity environments such assea water, salt marshes, and brackish water.Zinc normally becomes impractical forprotecting large bare areas when the resistivityof the electrolyte exceeds 1,500 ohm-cm.

Zinc anodes are also used as grounding cellsfor AC mitigation and electrical protection ofisolators. Figure 3-3 shows two zinc anodesused in a grounding cell installed across anisolating flange.

Zinc anodes are also used as stationaryreference electrodes.

Aluminum Alloys

Aluminum has been generally limited toapplications of sea water and chloride richenvironments such as offshore petroleumplatforms, marine pipelines and onshore oilproduction separation equipment using asaltwater tank. There are two common alloyfamilies. One uses mercury, the other usesindium, to reduce the passivating effect of theoxide film that forms on aluminum.Compositions are shown in Table 3-6.

The mercury alloys are effective in high chlorideenvironments where chloride levels areconsistently greater than 10,000 ppm. Seawater has an average chloride level of 19,000ppm.

Indium alloys are effective in waters containing

TABLE 3-4

Zinc Dimensions and Weights

Weight (lbs) Height Width Length Core (dia.”)

Bare Zinc Anodes

5 1.4 1.4 9 0.250

12 1.4 1.4 24 0.250

18 1.4 1.4 36 0.250

30 1.4 1.4 60 0.250

30-A 2.0 2.0 30 0.250

45 2.0 2.0 45 0.250

60 2.0 2.0 60 0.250

Zinc Ribbons

2.4 1.0 1.250 -- 0.185

1.2 0.625 0.875 -- 0.135

0.6 0.500 0.563 -- 0.130

0.25 0.344 0.469 -- 0.115

TABLE 3-5

Zinc Alloy Compositions

ElementASTM B418-01

Type I(sea water)

ASTM B418-01Type II(soil)

Aluminum 0.1 - 0.5% < 0.005%

Cadmium 0.025 - 0.07% < 0.003%

Iron < 0.005% < 0.0014%

Lead < 0.006% < 0.003%

Copper < 0.005% < 0.002%

Others — 0.1%

Zinc Balance Balance

TABLE 3-6

Aluminum Alloy Composition and Performance

Element Mercury FamilyAI/Hg/Zn

Indium FamilyAl/In/Zn

Zinc (Zn) 0.35 - 0.60% 2.8 - 6.5%

Silicon (Si) 0.14 - 0.21% 0.08 - 0.2%

Mercury (Hg) 0.035 - 0.060% ---

Indium (In) --- 0.01 - 0.02%

Copper (Cu) 0.004% max 0.006% max

Iron (Fe) 0.10% max 0.12% max

Aluminum (Al) Balance Balance

Consumption (lb/Ay) 6.8 - 7.0 7.4 - 8.4

Ampere hours/lb 1250 - 1290 1040 - 1180

Potential - Ag/AgCl 1.05 1.10

- Cu/CuSO4 1.10 1.15

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1,000 ppm or more of chlorides.

The mercury alloy is used in free flowing seawater and its principle advantage is the highnumber of ampere hours of capacity. Refer toTable 3-6. Its primary disadvantages are that itcannot be used in brackish water, in silt/mudzones, or at elevated temperatures. Under highhumidity atmospheric storage conditions somegrades have been known to degrade andbecome totally unusable prior to installation.There is some controversy also concerning thelong term environmental effects of this alloy insea water.

The indium alloy is used in free flowing seawater, brackish water, silt/mud zones, and atelevated temperatures. Its basic limitation is areduction in ampere hours of capacity.

Aluminum anodes have a variety of standardcores such as pipe, strap, bar, end type, sidetype, etc. The type of core must be specified.Cores should never be galvanized prior topouring the anode.

On major projects it is advisable to monitor thequality control process and inspect the finishedmaterials prior to shipment.

Just as with the magnesium, it is important thata certified analysis accompany each anodeheat.

The purchaser should request a verifiablespectrographic analysis of any anode purchaseto assure that the alloy falls within thepredetermined range of acceptable impurities.The anodes should be identified by heatnumber and date of manufacture. The chemicalanalysis should detail the heat number, the dateof manufacture, a listing of the impurities withthe percentages of each, and be signed by atechnically qualif ied person. Somemanufacturers identify each anode with a stampor tag, others may label each pallet of bareanodes with an identity number painted on theside.

IMPRESSED CURRENT CATHODICPROTECTION SYSTEMS

Unlike galvanic systems, impressed currentsystems utilize an external power source suchas a rectifier, electrically connected to lesssoluble anodes. The anodes are connected tothe positive terminal of the rectifier and thecircuit is completed by attaching the negativeterminal to the structure to be protected. Referto Figure 3-4 for the components in animpressed current system.

An impressed current system is used to protectlarge bare and coated structures and structuresin high resistivity electrolytes. Care must betaken when employing this type of system sothat cathodic coating damage and the potentialfor the development of stray currents adverselyaffecting other foreign structures, areminimized.

Prior to the 1970's there were only three typesof anodes primarily used for impressed currentanode beds: high silicon cast iron, graphite andscrap steel. As technology progressed, so didthe materials used for impressed currentanodes. When installation and operating costsare assessed, very few anodes can be useduniversally for any type of application and stillachieve a desirable design life.

In most soils, anodes evolve oxygen and theanode oxidizes as the current is discharged. Inchloride containing soils or water, anodesevolve chlorine gas which forms hydrochloricacid, and the anodes can breakdownchemically. Some anodes perform well in thepresence of oxygen and others in the presenceof acids.

Specifications

It goes without saying that if corrosionpersonnel do not effectively detail the type ofanode construction they expect, they willprobably get less than they expected. Thefollowing are minimum details to address in ananode specification.

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1. Anode Size - composition, dimensions andweight, within 2%.

2. Cable - size, type, insulation/thickness,length.

3. Type of Connection - end, center, <0.02ohms resistance.

4. Termination - epoxy cap, full encapsulation,shrink cap.

5. Packaging - bare, canister size/gauge,backfill type, weight.

6. Palletizing - size, padding, number perpallet.

7. Options - lifting rings, centering devices.

The sections of the specification covering thevarious types of impressed current anodesshould include additional information onconstruction methods.

High Silicon Cast Iron

The typical alloy for cast iron anodes is ASTMA518 Grade 3. Table 3-7 shows themetallurgical composition of this alloy.

The principal reason for the good performanceof cast iron anodes is the formation of a siliconoxide (SiO2) film on the anode surface, reducingthe rate of oxidation, and retarding theconsumption rate. Chromium is added toproduce a chlorine resistant alloy and improveits life in chloride containing soils and water.

Cast iron anodes have good electricalproperties and the resistivity of the alloy is 72micro ohm-cm at 20º C. In soils, the anodes areusually backfilled with metallurgical or calcinedpetroleum coke breeze to increase the effectiveanode surface area and provide uniformconsumption. By increasing the diameter orlength of a cylindrical anode, the anode toelectrolyte resistance is lowered. Increasing thelength has a greater effect than increasing thediameter.

The high tensile strength of the anode is anasset in some circumstances, but it is brittleand subject to fracture from severe mechanicaland thermal shock.

Cast iron anodes are manufactured in a widevariety of dimensions, shapes and weights.Refer to Table 3-8 for a description of thevarious sizes.

Solid cast iron anodes have the lead wiresattached at the end as shown in Figure 3-5. Theconnection is encapsulated with epoxy or aheat shrinkable cap to protect it from electricaldischarge.

Current typically discharges from conventionalend connected anodes at a greater rate (3:1)from the area of the cable connection than fromthe body of the anode. This causes anexaggerated attack, called “end effect”. Aconnection in the center of a tubular anode,suitably sealed at each end, has beendemonstrated to be effective against this rapidloss of the anode at the cable connection. SeeFigure 3-6.

Testing of chromium bearing tubular anodes inartificial sea water has shown the consumptionrate to be about 0.7 pounds per ampere year ata discharge rate of 3.5 amps per square foot ofanode surface area. No data has been providedfor soil or mud conditions but consumption inthese environments could double or triple theconsumption rate.

Tubular anodes range in size from 2.187" to4.750" in diameter, and from 46 pounds to 175pounds in weight, with surface area rangingfrom 50 percent - 170 percent greater thanconventional rod type anodes. The anode cableconnection strength should be tested to beequal to the breaking strength of the cablewithout any change in the 0.004 ohms or lessconnection resistance.

Thus, for the following cable sizes, connectionpull test values should be:

TABLE 3-7

Cast Iron CompositionASTM A518 Grade 3

ELEMENT COMPOSITION

Silicon 14.2 - 14.75%

Carbon 0.70 - 1.10%

Manganese 1.50% max

Molybdenum 0.20%

Chromium 3.25 - 5.00%

Copper 0.50% max

Iron Balance

TABLE 3-8

Typical Cast Iron Rod Anode Dimensions

NOMINALWEIGHT lbs (kgs)

NOMINALDIAMETER

in (mm)

NOMINALLENGTHin (mm)

NOMINALAREAft² (m²)

1.0 (.5) 1.1 (28) 9 (230) .22 (.02)

5.0 (2.3) 2.0 (51) 9 (230) .39 (.04)

9.0 (4.1) 2.5 (64) 9 (230) .50 (.05)

26 (12) 1.5 (38) 60 (1520) 2.0 (.19)

43 (20) 2.0 (51) 60 (1520) 2.6 (.24)

44 (20) 2.0 (51) 60 (1520) 2.6 (.24)

60 (27) 2.0 (51) 60 (1520) 2.7 (.25)

110 (50) 4.0 (102) 60 (1520) 4.0 (.37)

220 (100) 4.5 (114) 60 (1520) 5.5 (.51)

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CENTER CONNECTION

NOMINALWEIGHTlbs (kgs)

NOMINALDIAMETER

in (mm)

NOMINALLENGTHin (mm)

NOMINALAREAft² (m²)

31 (14) 2.6 (66) 41 (1067) 2.4 (.22)

50 (23) 2.6 (66) 60 (1520) 3.5 (.33)

46-50 (21-23) 2.2 (56) 84 (2130) 4.2 (.39)

63-70 (29-32) 2.6 (66) 84 (2130) 4.9 (.46)

85-95 (39-43) 3.8 (97) 84 (2130) 7.0 (.65)

110-122 (50-55) 4.8 (122) 84 (2130) 8.8 (.82)

175-177 (79-80) 4.8 (122) 84 (2130) 8.8 (.82)

230 (104) 4.8 (122) 84 (2130) 8.8 (.82)

260 (118) 6.7 (170) 76 (1981) 11.4 (1.06)

270 (122) 6.7 (170) 84 (2130) 12.3 (1.14)

TYPICAL TUBULAR ANODE DIMENSIONS

FIGURE 3-6


Connection Strength Cable Break Strength

No. 8 - 799 pounds 525 pounds



Graphite Anodes

Graphite rods have been used as an impressedcurrent material for many years. The basicconfigurations consist of round or square rods,manufactured from a slurry of powderedpetroleum, coke and coal tar resin. The coal taris used as a bonding agent to hold the graphiteparticles together and then baked at hightemperatures to fuse the mixture. This processincreases the resistance to oxidation andsubsequent breakdown.

There are many types of graphite compositionsand the type used for cathodic protection anodebeds is one of the most porous. Graphitecathodic protection anodes should have aspecific gravity of 1-6 grams/cc minimum. Theporosity allows moisture penetration to migrateto the connection, causing failure at the cableconnection. The porosity is reduced byimpregnating the rods with filler of linseed oil,micro-crystalline wax, or phenolic based resin.

There is controversy concerning the best typeof filler and even whether a filler really reducesmoisture penetration over long periods of time.Whatever filler is used, it should be applied bya method that assures complete penetration ofthe cross-section of the graphite anode. Insoils, graphite anodes must be backfilled withmetallurgical or calcined petroleum coke breezeto increase the effective anode surface areaand provide a uniform low consumption rate.

Anode to lead connection is just as critical aswith cast iron. See Figure 3-7. Both end andcenter connections are available. Additionalspecification details should include:

1. Type of connector - lead, brass, molten,compression.

2. Connection depth/diameter - 3", 4", 5",centered.

3. Connection sealant - thermoplastic,thermosetting (epoxy).

4. Cable sealant - TFE tubing, shrink cap,encapsulation.

5. Impregnation - wax, linseed oil, resin.

6. Sizes - 3" x 30" - 2 sq. ft., 3" x 60" - 4 sq. ft.,4" x 40" - 3.5 sq. ft., 4" x 80" - 7 sq. ft.

Graphite should not be operated at currentdensities exceeding one ampere per squarefoot in soil with coke breeze backfill. Foroptimum life in soils, most engineers designgraphite anodes for a maximum density of 0.20amperes per square foot, or one ampere per 3"x 60" rod. If current densities are within theseranges, the consumption rate will beapproximately 2 pounds per ampere year (Ay).Exceeding these limits causes the material tobecome mushy and less conductive, due tochemical breakdown of the crystal boundary.

As with cast iron, graphite is brittle and may beeasily damaged during transportation, eitherbare or packaged. Special handling andpadding is necessary to prevent cracking andbreaking.

Aluminum Anodes

Occasionally aluminum is used as animpressed current anode for protecting theinterior of water tanks, particularly where icedestroys the anodes annually. The consumptionrate of 9 pounds per ampere year may limit thecost effectiveness on the basis of cost per yearof service compared to other anode systems.There are also several proprietary systemsusing other anode materials that will withstandicing.

Lead Silver Anodes

Lead alloy anodes are only used in free flowingsea water applications and may employ variousmetals such as antimony lead, tin, and 1 or 2%silver. Commonly supplied in rod or strip form of

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1.5" diameter by 10" long, they have been usedextensively in Europe with a 2% silver alloy,which doubles its life.

Upon initial start up, the consumption rate isabout three pounds per ampere year andeventually a black, passive film of lead peroxideforms to extend the life of the anode, resultingin consumption of about 0.2 pounds per ampereyear. Normal current density ranges from 3 to25 amps per square foot. In silting or lowchloride conditions, this oxide film does notform and the anode is consumed rapidly.

Cable connections are made by drilling a holeand silver soldering the lead wire at the base ofthe hole. The connection cavity is then filledwith epoxy to prevent moisture penetration.

Installation is accomplished by hanging theanodes from a structure, dock or pier in aperforated fiberglass pipe or by a supportdevice to maintain its position. This support isimportant to prevent ice damage and keep theanodes from coming in contact with mud or silt.

Magnetite Anodes

Magnetite (Fe3O4), a European inspired anode,is itself an iron based corrosion product.Therefore, there is little or no corrosion involvedduring current discharge, but the relatively smallsurface area is better suited to brackish or seawater applications than soil.

The anodes are manufactured and shippeddirect from the factory with the lead wireattached and distributors splice extra wire tomeet user requirements. The material is brittleand a factory installed lead wire attachment tothe internal wall of the anode is critical. Figure3-8 shows the anode construction.

Some applications have involved offshoreplatforms, power plants, and limitedunderground use. As of this writing, there islimited background information on the use andsuccess of this anode.

Mixed Metal Oxide Anodes

Mixed metal oxide anodes were developed inEurope during the early 1960's for use in theindustrial production of chlorine and causticsoda. The first known use of the technology forcathodic protection use occurred in Italy toprotect a sea water jetty in 1971. These anodesexhibit favorable design life characteristicswhile providing current at very high densitylevels. The oxide film is not susceptible to rapiddeterioration due to anode acid generation,rippled direct current, or half wave rectification,as is common with other precious metalanodes.

The composition of the anode consists of atitanium rod, wire, tube or expanded mesh(refer to Figure 3-9) with the oxide film bakedon the base metal. Sometimes these anodesare referred to as dimensionally stable, ceramicor linear distributed anodes.

For oxygen evolution environments such assoils, the anode oxide consists of rutheniumcrystals and titanium halide salts in an aqueoussolution that is applied like a paint on the basemetal and baked at 400 to 800º C forming arutile metal oxide. After baking, the rutiledevelops a matte black appearance and ishighly resistant to abrasion.

For chlorine evolving environments such as seawater, the oxide consists of an aqueoussolution of iridium and platinum powder that isalso baked at high temperatures to achieve adesirable film.

Some manufacturers produce variations of theoxide films specifically for chloride or non-chloride electrolytes and they are notinterchangeable.

Normally, titanium will experience physicalbreakdown around 10 volts, but the oxide film isso highly conductive (0.00001 ohm-cmresistivity) that the current is discharged fromthe oxide rather than the base metal even witha rectifier voltage of 90 volts in soils. This is incontrast to the insulating titanium dioxide film

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that naturally forms on the surface of baretitanium. When the mixed metal oxide film hasbeen consumed, the insulating titanium dioxidefilm will cover the anode and not allow currentto discharge unless the applied voltage isgreater than 10 volts in sea water or 50 to 70volts in fresh water.

The maximum recommended current densitiesfor various electrolytes are:

Soil, Mud, Fresh Water 9.0 amps/ft2 (20 years)

Sea water 55.7 amps/ft2 (15 years)

Anodes in soil or mud must be backfilled withfine, low resistance, calcined petroleum cokebreeze for maximum life and performance.Even when the anode is pre-packaged withpetroleum coke, conservative engineeringjudgment would dictate that the anode packagebe surrounded with metallurgical coke, prior tofinishing the backfilling with native soil.Consumption rates at these densities rangefrom 0.5 mg/Ay in sea water to 5 mg/Ay in cokebreeze, fresh water and sea mud.

As with any anode, the connection must beconstructed so as to be moisture proof, watertight and have no more than 0.001 ohmresistance.

Advantages of mixed metal oxide anodes:

C Lightweight and Unbreakable

C Dimensionally Stable

C Negligible Consumption Rate

C High Current Density Output

C Inert to Acid Generation

C Cost Effective

Mixed metal oxide anodes are available in avariety of shapes including wire, rods, tubesand mesh.

Platinum Anodes

Platinum can be used as an anode coating formany types of cathodic protection installations.

Structures in a vast array of environments suchas underground, offshore, concrete, powerplants, and the internal surfaces of piping, tanksand machinery have utilized platinum forcathodic protection systems. In sea water,platinum has a low consumption rate, 0.00018pounds per ampere year, so only a smallamount is needed for a twenty-year anode life.In soil and fresh water, the consumption mayrange up to 0.005 pounds per ampere yeardepending on the resistivity of the electrolyteand the amount of coke breeze surrounding theanode in soil applications. Pure platinum, byitself, would be too expensive. Therefore, it isnormally coated over noble base metals suchas titanium and niobium. When anodes are inthe form of wire and rods, there may be acopper core to increase the conductivity forlengths in excess of 25 feet since titanium andniobium are relatively poor electrical conductorscompared to copper. Wire anodes for use indeep anode beds, and packaged anodes forsurface anode beds are manufactured. Figure3-10 shows different types and sizes ofplatinum anodes.

The passive film on titanium starts to breakdown at 10 volts, anode to cathode potential,therefore, these anodes are limited to lowresistance environments such as sea water.Niobium has a break down voltage of 120 volts,anode to cathode potential, and is used inhigher resistivity electrolytes.

Current densities range from 50 amps persquare foot in soils to 500 amps per square footin sea water, with a platinum thickness of 300micro inches, depending on the anode surfacearea and the method of coating application.Platinum has been coated on base metalsusing many techniques including sputteredelectrode position, cladding, and metallurgicallybonded.

In metallurgically bonded anodes, the metalsare compressed together in an oxygen freevacuum. This provides an oxide free, lowresistance bond between the metals.

Cladding involves wrapping a thin sheet of

20% NiobiumDiameterInches

Nb ThicknessInches

Resistancemicrohm/ft

Pt Thicknessµ-in (2X)*

.750 .038 22 300 (600)

.500 .025 50 200 (400)

.375 .019 89 150 (300)

.250 .013 201 100 (200)

.188 .009 356 75 (150)

.125 .006 806 50 (100)

40% NiobiumDiameterInches

Nb ThicknessInches


Pt Thicknessµ-in (2X)*

.375 .038 113 150 (300)

.250 .025 256 100 (200)

.188 .019 453 75 (150)

.125 .013 1025 50 (100)

.093 .010 1822 38 (75)

.063 .007 4102 25 (50)

.031 .0035 16,408 12.5 (25)

100% TitaniumDiameterInches

Ti ThicknessInches


Pt Thicknessµ-in

.750 Solid 468 300

.500 Solid 1054 200

.375 Solid 1874 150

.250 Solid 4215 100

.188 Solid 7454 75

.125 Solid 16,862 50

* Double Platinum Thickness

PLATINUM ANODES

FIGURE 3-10


platinum around a rod and spot welding theplatinum to the base metal at the overlap area.

Electrodeposition techniques plate a film ofplatinum on the base metal.

Thermal decomposition and welded techniquesexhibit the same problems as cladding, and asof the late 1980's are rarely used.

Each process has its advantages anddisadvantages and the corrosion worker shouldevaluate them for each application.

The anode to cable connection is critical andimproper connections will result in very earlypremature failure. Users should assure that theanodes are manufactured in compliance withtheir specifications by skilled personnel underthe guidance of established quality controlmethods.

The major disadvantage to platinum is poorresistance to anode acid evolution in staticelectrolytes, rippled direct current, and halfwave rectifiers. Use of a 3-phase transformerrectifier in sea water systems has been knownto double the life of platinum anodes byreducing the ripple in the DC output.

Very rapid deterioration will occur if the anodeis driven at current discharges exceeding themanufacturer’s limitations. In undergroundapplications, platinum anodes have had onlylimited success as of this writing.

Polymer Conductive Anodes

In 1982, a new anode material was testmarketed to provide a small amount of currentin restricted spaces such as internal pipesurfaces, heat exchangers, utility ducts, andareas shielded from conventional anode bedcurrent.

The material resembles electrical cable butactually consists of a stranded copperconductor with an extruded, conductivepolyethylene (see Figure 3-11). This concept isused in underground concentric power cables

as a conductive shield around the ground wires.The polymer contains carbon granules whichdischarge the current. The anode should bebackfilled in carbonaceous coke breeze formaximum life.

An optional plastic mesh is available, toseparate the anode from the cathode inrestricted spaces, preventing electrical shortingbetween the anode and cathode. Currently thematerial is available in four different diametersand the current output ranges from 3 to 9milliamperes per linear foot.

Scrap Steel Anodes

For operating companies who have access toscrap pipe or old rails, this type of anode maybe economical if properly installed. If pipe isused, multiple lead wire connections are madeat close intervals and carefully coated at eachlocation to prevent premature connectionfailure. The major cost of the system is in labor,welders and heavy construction equipmentnecessary to install the components. If anexisting abandoned line is used, the costs maybe less, depending on depth and number oflead wire connections required.

A major disadvantage in the use of scrap steelanodes is the high consumption rate of 20lb/A-yr.

Backfill for Impressed Current Anodes

Special carbonaceous backfill is used tosurround most impressed current anodes in soilto reduce the anode bed resistance and extendthe anode life. The only exception applies toanodes used in photovoltaic power supplysystems, where magnesium anode backfill isused due to the high back EMF voltageencountered with carbonaceous backfill.

Coke breeze is conductive and transfers currentfrom the anode to the interface of native soil.Some types pass the current more efficiently bymore electronic conductance and others areless efficient and current passes electrolytically.


The first type is metallurgical coke breeze,derived as a waste by-product of coking(heating) coal, associated with steel production.The composition and particle size of coke maygreatly enhance the life of an anode and shouldbe considered at the design stage. Most oftenthe particle size is about 0.375" (d") andsmaller for surface anode bed backfill. Thesmaller the particle size, the greater thecompaction and conductivity. Metallurgical cokeshould not exceed 50 ohm-cm when measuredusing ASTM G-58 Soil Box Test Method,temperature corrected. The carbon, iron,copper, sulfur content and weight per cubic footare important values to analyze prior toinstallation. Refer to Table 3-9. Metallurgicalcoke breeze is available bagged or bulk and isoften used in prepackaged impressed currentanodes.

Calcined petroleum coke consists of finelyscreened particles of coke that are typicallyderived as a waste by-product of crude oilrefining. It consists predominately of roundcarbon granules and is much more conductivethan metallurgical grades of coke. Theresistance should be less than 2 ohm-cm usingASTM G-58 Soil Box Test Method, temperaturecorrected. Since resistance values are low, highcurrent density is less prone to gas generationand the incidence of gas blocking in deepanode beds using a 92 percent carbon contentcoke is rare. Some grades incorporate asurfactant, similar to detergent, to reduce watertension and promote pumping and compactionin deep anode beds.

WIRE AND CABLE

Conductors typically used for undergroundservice are made of solid or stranded copper,rated at 600 volts, with a variety of insulationmaterials designed for the type of electrical andchemical exposure to be encountered (seeTable 3-10).

Conductors are rated on their ampacity undercertain temperature and service conditions (seeTable 3-11). Insulation values also varydepending on wet or dry conditions. The

following data are general in nature, coveringthe most common types, and specification datashould be obtained directly from manufacturerspertaining to their product.

CABLE CONNECTIONS AND SPLICES

Most connections to structures are made withan exothermic welding process which thermallybonds the conductor to the structure. Othermethods may change the temper of theconductors causing them to become brittle anddamage the insulating properties of the cableinsulation.

Exothermic welding uses a graphite mold tocontain the mixture of copper alloy andmagnesium starter powder. After igniting thepowder with a flint gun, the powder becomesmolten and drops on the cable and structure.The slag on the connection is removed bylightly striking the weld with a chipping hammerafter cooling. Figure 3-12 shows a typical set-upfor an exothermic weld.

Structures that contain certain types offlammable substances may require the use ofgrounding clamps to make the cableattachment.

Special attention should be given to coating anyattachment or cable splice to prevent bi-metalliccorrosion attack. The coating should be solventfree to prevent deterioration of the insulation.

Splicing cables together may involveexothermic welding, high compression crimpits,or split bolts as shown in Figure 3-13. Split boltsmay loosen with age and are generally used formagnesium anode header cable splices only.

SPLICE ENCAPSULATION

There are many methods available for moistureproofing cable splices:

1. Hand wrapping

2. Epoxy mold encapsulation

3. Shrink tubing and sleeves

TABLE 3-9

Coke Breeze Composition

Type Bulk Density(lb/cu ft)

Porosity(%)

Carbon(lb/cu ft)

Metallurgical 45 48.0 32.51

Petroleum, calcined

Delayed 48 59.5 47.76

Fluid 54 56.7 49.93

70 44.0 64.73

74 40.8 68.53

TABLE 3-10

Wire And Cable Insulation Designations

Designation Insulation Thick (in) Cable Size Specification

HMWPE High Molecular Weight Polyethylene .110.125

No 8 - No 2No 1 - 4/0

D-1248, Type 1Class C, Cat. 5

TW Polyvinyl Chloride (PVC) .030.034.060

No 14 - No 10No 8

No 6-No 2

U.L. Standard 83(60º C wet/dry)

THW Polyvinyl Chloride (PVC) .045.060

No 14 - No 10No 8 - No 2

U.L. Standard 83(75º C wet/dry)

THHN PVC/Nylon Jacket(.0004” Nylon)

.015

.020No 14 - No 12

No 10U.L. Standard 83

(90º C dry)

THWN PVC/Nylon Jacket (.004” Nylon)

.015

.020No 14 - No 12

No 10U.L. Standard 83

(75º C wet)

PVF/HMWPE Polyvinylidene (.020”)HMWPE jacket (.065”)

.085 No 8 - No 2 Kynar™ASTM D-257

ECTFE/HMWPE Ethylene Chlorotriflora-ethylene (.020”)HMWPE jacket (.065”)

.085 No 8 - No 2 Haylor™

TABLE 3-11

Conductor Ampacities And ResistancesNACE Corrosion Engineer’s Reference Book

SizeAWG

Ampacity*(Copper)

Resistance(Ohms/1000 ft @ 25º C)

No. 16 6 4.18

No. 14 15 2.62

No. 12 20 1.65

No. 10 30 1.04

No. 8 50 0.652

No. 6 65 0.411

No. 4 85 0.258

No.2 115 0.162

No. 1 130 0.129

No. 1/0 150 0.102

No. 2/0 175 0.0811

No. 3/0 200 0.0642

No. 4/0 230 0.0509

250 MCM 255 0.0423

300 MCM 285 0.0353

350 MCM 310 0.0302

400 MCM 335 0.0264

500 MCM 380 0.0212

* Ampacity based on THW and HMWPE insulation

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4. Elastomer/urethane impregnated wrap

Hand wrapping has historically been the mostreliable method if done properly, although it isalso the most expensive since more labor timeis required. The exposed splice area is cleanedwith a clean rag, dampened with solvent toremove release oil and human oil on thestrands. The area is wrapped with 3 laps ofelectrical rubber insulating tape extending 2inches on adjacent insulation, paying specialattention to the crease area if wrapping a “Y”connection, as in an anode header cable splice.The entire area is then wrapped with 3 laps ofPVC electrical tape and coated with electricalseals to complete the splice.

Epoxy molds (Figure 3-14) have been usedextensively over the years with some mixedresults. Epoxy does not bond to the release oilsused on wire, permitting moisture to traveleventually into the connection. Since there is nochemical bond to polyethylene, the mold mustextend far enough over the adjacent insulationto prevent hygroscopic moisture migration.

Epoxy molds should not be disturbed orbackfilled for 30 minutes at 70º F so that themixture begins to cross link and bonds to thesurfaces. Any movement will distort the materialand reduce the adhesion. During cold weather,this time will be extended to approximately 3hours at 32º F.

Heat shrinkage tubing of irradiated polyethylenewith an effective electrical sealant is used toinsulate in-line cable splices. This method hasproven to be very effective and inexpensive.Care must be taken when using a propanetorch so as not to melt or distort the cableinsulation while shrinking the sleeve.

Elastomer sealant kits are a recentdevelopment that involves wrapping a pliablestrip of sealant around the splice area tomoisture-proof the connection. An outer wrap ofelastic fabric impregnated with a quick setting,moisture cured urethane is wrapped around thesealant and sprayed with water to harden thefabric. This forms a hard shell to eliminate cold

flow of the sealant and prevent rocks or soilstress from damaging the encapsulation.

POWER SUPPLIES

Many sources of direct current (DC) power areavailable, as discussed in detail in Chapter 7 ofthe Intermediate Course, for use withimpressed current systems as follows:

1. Transformer-Rectifier

2. Solar Photovoltaic Cells

3. Thermoelectric Generators

4. Turbine Generator Units

5. Engine Generator Units

6. Wind Powered Generators

Transformer-rectifiers are the most frequentlyused power source for impressed current anodesystems. The unit consists of a step downtransformer to reduce alternating current to anacceptable level, and a rectifying element toconvert the alternating current (AC) to directcurrent (DC). Output terminals and a host ofoptions and accessories complete the finalassembly of the unit. See Figure 3-15.

Specifying the various options is easilyaccomplished using the following options menu.Selecting them properly requires knowledge.

Options1. Enclosure Type Utility

Swing OutCabinetsSlide Out RacksCustomSmall Arms ProofPole MountWall MountPad MountPhotovoltaic (solar panel)

2. Cooling Type Air CooledOil CooledExplosion ProofSubmersible

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Fan CooledForced Oil Cooling

3. Control Type Standard ControlCurrent RegulatedVoltage RegulatedAutomatic Potential ControlSolid State Control (no taps)IR Drop Free

4. Rectifying Selenium BridgeElement Selenium Center Tap

Silicon BridgeSilicon Center Tap

5. Circuit Type Center Tap, Single PhaseBridge, Single PhaseThree Phase WyeThree Phase BridgeMultiple Output Circuit

6. AC Input 115 volts230 volts, single or

3 phase460 volts, single or 3 phase115/230 volts230/460 volts, 3 phase115/460 volts

7. DC Volts Specify maximum DC output in volts

8. DC Amperes Specify maximum DCoutput in amperes

9. Options AC and DC LightingArresters

AC Arresters onlyDC Arresters onlyCommunications FilterEfficiency Filter (choke)Meter SwitchesPilot LightDC Failure LightDC FusePainted Cabinet

Hot Dipped GalvanizedCabinet

Anodized Aluminum CabinetStainless Steel CabinetCabinet Gauge -

16,14,12,11Cabinet LegsCabinet Side DoorSlide Out RacksHigh Ambient OperationSpecial TapsCoolant - Oil cooled unitsNon-Standard Access KnockoutsAC Frequency -

other than 60 hzElapsed Time MeterConvenience Outlet -

115 volts

JUNCTION AND RESISTOR BOXES

Junction boxes are an extra enclosure forterminating large numbers of anode cables to acommon electrical bus bar. The rectifier positivecable is fed through a conduit to the mainanode bus and a perpendicular bus terminatesthe lead wires at individual terminals. Leadsmay be attached to fixed value current limitingresistors and the current output can bemeasured by connecting a DC voltmeter acrosseach respective shunt. The negative cable fromthe rectifier is directly attached to the structureand is not normally brought into the box. SeeFigure 3-16.

Where several anode circuits are supplied fromone power supply, it may be desirable to controlthe current to each anode using variableresistors connected in series with the outputcable. Many of these control options can nowbe built into the rectifier circuitry.

PERMANENTLY INSTALLED REFERENCEELECTRODES

There are 3 types of reference electrodes thatare used for monitoring the potential of astructure or as a sensor in conjunction withpotentially controlled rectifiers. They are:

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High Purity Zinc

Copper/Copper Sulfate Electrode

Silver/Silver Chloride Electrode

If the electrode is to be buried, it must besurrounded with special backfill to increase theion trap area. Zinc is particularly subject to theeffects of polarization and should besurrounded with a 75/20/5 mixture of preparedbackfill to prevent this passivation effect.

Copper bearing electrodes with a saturatedsulfate solution will become contaminated in thepresence of chlorides and are limited to freshwater and chloride free environments formaximum life.

Silver bearing electrodes with a saturatedchloride solution are used in sea water andbrackish waters where chlorides are found.Other types utilize a perforated plastic tubecontaining a chloridized silver rod and dependon the natural chlorides in sea water to providemeaningful readings. This second type will giveerroneous readings in brackish waters due tothe constant changes in chloride levels.

TEST STATIONS

As simple as a test station is, it seems to causea great deal of confusion, particularly to thosepeople who are unfamiliar with the concept ofcathodic protection. The variables are manyand here is a partial listing of the variations.

Choices1. Box Type Post Mount

Flush MountFlange MountRoundSquareExplosion Proof -

Specify Type, ClassCondulet

2. Construction PolycarbonateFlake Filled PolyesterCast IronCast aluminum

ABS PolymerThermosettingComposite

3. Terminals Specify NumberPlated BrassCopperBrassBanana PlugsStainless SteelLock Washers

4. Hub Type Slip Fit, sizeThreaded, sizeDouble HubSingle Hub

5. Hub Size Specify size and quantity

6. Post Type Treated wood, sizePVC Conduit, sizeSteel Conduit, sizeWith or without anchors

7. Post Length Specify Length

8. Flush Type Cast Aluminum TopCast Iron TopPlastic Top, magnetic

9. Wiring NonePre-wired, specify size,

length, insulationShunts - specify quantity,

size, type

10. Covers Name cast on top“O" Ring Seal, round onlyWeather Proof SealWaterproof SealSlip FitExposed TerminalsLocking Type

11. Terminal Polyester Laminate - 0.125"Blocks Polycarbonate

12. Options Shorting Bars - specify quantity, materialLightning Arresters -


specify typeFixed Current Limiting

Resistors -specify amperage

Variable Slide WireResistor - specifyamperage

Rotary Rheostat - specify amperage

Meters - specify type, rangeLocking DevicesShunts - specify amperage

Figure 3-17 shows some typical test stations.

FLANGE ISOLATORS AND DIELECTRICUNIONS

To isolate cathodically protected structuresrequires the mechanical insertion ofnonconductive materials between the metalcomponents. The isolation material should notdeform or deteriorate due to the operationalconditions of the external or internalenvironment or the structure itself. Once again,there are a wide variety of materials to choosefrom depending on the particular application.

Flange gaskets (Figure 3-18) are selected onthe basis of pipe size, ANSI pressure rating, fullface or raised face, and material composition tosuit the pipeline product. The components of aflange isolation kit consist of a gasket, sleeves,isolating washers, and steel washers.

Components

1. SizeS Specify

2. Gasket TypeS Full Face “E” (with bolt holes)S Ring “F” (without bolt holes)S “D” for RTJ flanges

3. MaterialsS Plain Phenolic - 225º F, max.S Neoprene Faced Phenolic - 175º F, max.S Glass/Phenolic - 350º F, max.S Aramid Fiber - 450º F, max.

S Epoxy/Glass - 350º F

4. Sealing ElementS Nitrile Sealing Element - 250º F, max.S Viton - 350º F, max.S Teflon - 450º F, max.

5. Pressure RatingS ANSI Class - 150 lbs to 2500 lbs

6. Isolating SleevesS Polyethylene - 180º F, max.S Phenolic - 225º F, max.S Mylar - 300º F, max.S Acetal - 180º F, max.S Nomex - 450º F

7. Isolating WashersS Glass/Phenolic - 300º FS High Temp. Phenolic - 350º FS Silicon/Glass - 350º FS Epoxy/Glass - 280º F to 350º F

8. Steel WashersS Cadmium Plated, c" thick

Check with the manufacturer for more specificand absolute data. Flange isolation kits are bestsuited for above ground electrical isolation onnew installations.

Dielectric isolating unions (Figure 3-19) performthe same basic function as flange gaskets byinserting a high resistance plastic between theunion faces. They are normally available as “O”Ring Type or Ground Joint Type in pressureratings of 150 to 3,000 psi working pressure.Common installations include hot waterheaters, service station pumps, natural gasdistribution service lines, and hydraulic lines.

MONOLITHIC WELD IN ISOLATORS

Monolithic weld in isolators are factoryassembled, one piece, isolating devicesdesigned to eliminate underground isolatingflanges. They eliminate bolts and washerstherefore providing a uniform surface to easilyapply an exterior dielectric coating.

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They are available in pipe sizes up to 120inches diameter in all ANSI pressure ratings.Factory pretested, they assure high dielectricqualities and are available with an internalcoating to reduce bridging of the insulating gapwhen the product carried in a conductive liquid.

CASING ISOLATORS AND END SEALS

In the mid-1980's, the effects of electricallyshorted casings received wider attention fromregulatory agencies, due to some unfortunateincidents. The cause and effect relationship ofelectrically shorted casings should be wellknown to the reader by now, and the purpose ofthis section is to acquaint the reader with thetypes of materials that are currently available.Careful selection and installation of thecomponents during construction, either new orrehabilitative, will reduce the possibility of ashorted casing.

Casing isolators (Figure 3-20) generally consistof molded polyethylene segments, joinedtogether to form rings at frequent intervals onthe pipe, to support the weight of the pipe withproduct and keep the carrier (product line)centered in the casing. On occasion thesesegments become damaged during installationor during operation due to soil shifting and pipemovement, resulting in a shorted condition.

Other types are made of steel, shaped like asaddle, and are lined with a thick rubber or PVCsheeting to isolate the carrier pipe from thesaddles. These too may become damaged andineffective as mentioned previously if notproperly specified. To order, specify a highcompressive strength runner suitable for thepipe and product weight and the length of thecasing or crossing. The isolator should have aquality coating and be properly designed for theparticular project.

Another form of casing isolation is anonconductive wax filler, used in conjunctionwith isolating rings, to fill the annular spacebetween the carrier and the casing. If doneproperly, the filler will prevent ground waterpenetration and condensation from filling the

space with electrolyte, thus preventing ion flow.It will also prevent atmospheric corrosion attackfrom occurring within the casing.

If the casing has been in service, it should beflushed with water to remove debris. The endseals are usually replaced with new seals thatcan accommodate the weight and installationpressures encountered during the fillingprocedure. For new and rehabilitated casings,the vents are temporarily sealed off and thecasing is pressurized to determine if the fillerwill be contained.

Complications that may arise are, stitch weldedcasing, perforated casing, out of round casing,improper seals and constricted vent openings.

Casing end seals (Figure 3-20) havetraditionally consisted of rubber boots held inplace with hose clamps, shrink sleeves withsupport skirting, and a proprietary rubber linksystem. Only careful selection and supervisionduring installation can prevent the end sealsfrom being the weakest link in the system.

CONCLUSIONS

It is important for corrosion control personnel tounderstand the characteristics of the variousmaterials and equipment associated withcathodic protection systems.

Before designing a system, the corrosionworker should acquire catalogs from variouscathodic protection equipment suppliers todetermine what materials and equipment areavailable and to determine their characteristics.

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DYNAMIC STRAY CURRENT ANALYSIS4-1

CHAPTER 4

DYNAMIC STRAY CURRENT ANALYSIS

INTRODUCTION

This chapter will review the causes andcommon means of detecting and mitigatingdynamic stray current effects. Steady state, orstatic, stray current has been covered inChapter 5 in the Intermediate Course text.

This chapter presents detailed information ondynamic stray currents emanating from directcurrent (DC) sources. While alternating current(AC) may create a potential safety hazard, itseffect on corrosion of ferrous structures is notfully understood. A brief discussion on stray ACappears at the end of this chapter.

STRAY CURRENTS

Stray currents are defined as electrical currentsflowing through electrical paths other than theintended paths. Stray, or interference currentscan be classified as being either static ordynamic.

Static interference currents (Chapter 5,Intermediate Course text) are defined as those,which maintain constant amplitude andgeographical path. Examples of typical sourcesare railroad signal batteries, HVDC groundelectrodes and cathodic protection systemrectifiers.

Dynamic interference currents, on the otherhand, are defined as those which arecontinually varying in amplitude, direction orelectrolytic path. These currents can bemanmade or caused by natural phenomena.Typical examples of man-made sources are DCwelding equipment, electrical railway systems,

chlorine and smelter plants. Telluric, or naturalsources of dynamic stray currents, are causedby disturbances in the earth’s magnetic fielddue to sun spot activity. Telluric effects havenot been shown to contribute to corrosion, butthey do, however, create measurementdifficulties and can interfere with one’s ability toassess cathodic protection systemperformance.

THE EARTH AS A CONDUCTOR

In order to understand the control of straycurrent, one must first understand what thesestray currents are. In underground corrosion,we are dealing with the earth as a hugeelectrolytic medium in which various metallicstructures are buried and often interconnected.The earth is an electrolyte because of the watercontained in the soils. This water causesionization of various soluble salts and makesthe earth an electrolytic conductor.Conductance is measured in electrical unitsknown as Siemens, or mho centimeters. Morecommonly, the reciprocal of conductivity,resistivity, is measured and expressed as ohmcentimeters. The more conductive the earth, thelower the resistivity. Seawater, for example, hasa resistivity of 25 to 30 ohm centimeters.Natural soils, depending on the amount ofionizable material present and the moisturecontent, range in resistivity from near that ofsea water up to the hundreds of thousands ofohm centimeters. When one considers straycurrents, it should be borne in mind that thelower the resistivity of the soil, the more severethe effects of stray currents may be.

From the values of resistivity cited, it is quite


evident that a metallic structure in the earthmay be a far better conductor of electricity thanthe earth itself, even seawater. Resistivities forcopper and iron are less than 10 x 10-6 ohmcentimeters, while that for lead is approximately22 x 10-6 ohm centimeters. It is readilyunderstandable, therefore, that should currentflowing in the earth produce a potentialdifference (voltage gradient) crossed by such ametallic conductor, the conductor will readilyacquire a part of the current that is flowing. SeeFigure 4-1. Thus, pipelines and cables canbecome major conductors of stray currents inthe earth environment.

POTENTIAL GRADIENTS IN THE EARTH

How do the stray currents that accumulate onmetallic structures underground get into theearth? As in any electrical circuit, it isnecessary to have potential differencesbetween two points in order to have a currentflow. One also must have an electrical circuit,that is, a source of the current and a return forthat current. The magnitudes of current will bedependent upon the electrical potential andelectrical resistance. The amount of currentwhich will flow on any one structure will beinversely proportional to the resistance of thevarious parallel electrical paths that join the twopoints of different electrical potential in thecircuit. This is true of any electrical circuit whereparallel resistances are involved. A metallicelectrode in earth that is positive with respect tothe earth around it will discharge current intothe earth and onto any other metallic structuresthat may be in the vicinity. Any element that isnegative to earth will have the opposite effect ofcollecting current from the earth or from metallicstructures in its vicinity.

Many companies feel that any positive pipe-to-soil voltage swing of less than 10 millivolts on astructure is tolerable and does not requiremitigation, since interference is negligible. TheBritish “Joint Committee for Coordination ofCathodic Protection of Buried Structures”, hassuggested that the allowable limit of positivechange be 20 millivolts. It is possible thatpositive voltage changes within these ranges

may be indications of strong adverseelectrolytic currents, but in the majority of caseswhere voltage swings are less than 10 millivoltsnegligible interference exists.

In practice, if the positive swing does not causea potential less negative than -0.850 volts (IRdrop free), then negligible corrosion can beexpected.

DETECTION OF DYNAMIC STRAYCURRENTS

The easiest way to keep abreast of possibleinterference problems in an area is throughcontact with Corrosion or ElectrolysisCoordinating Committees. These committeesact as clearinghouses for information onunderground plant locations and knowninterference sources.

Dynamic stray currents are present if thestructure-to-soil potential is continuallyfluctuating while the reference electrode is keptin a stationary position in contact with the soil.See Figure 4-2. These potential changes are adirect result of current changes at the source ofthe interference due to loading variations. Thiswould also be detected by constantly changingline current measurements (Figure 4-3).

In efforts to locate the source of man-madedynamic stray currents, the following stepsshould be taken:

1. Determine if any DC electrical railwaysystems, mines or industrial plants(aluminum, chlorine, etc) exist in the areawhere the interference was detected.

2. Inquire of other pipeline operators, cableoperators or corrosion committees in thearea if any dynamic earth current sourcesin the area, such as rapid transit systemsor electrical DC substations exist.

3. If there are mines or DC railways in thearea, contact them to determine thelocation of their operations and currentsources.

-2.00

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

12:00 PM 2:24 PM 4:48 PM 7:12 PM 9:36 PM 12:00 AM 2:24 AM 4:48 AM 7:12 AM 9:36 AM 12:00 PM

Vg

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s to

Cu

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Time

Pipe-to-Soil Potential vs Time

Vg-Min Vg-Ave Vg-Max -0.85 V

TYPICAL PIPE-TO-SOIL POTENTIAL (Vg) PROFILEINDICATING DYNAMIC STRAY CURRENT

FIGURE 4-2


β VE

gsc

sc=

∆∆

4. Trace the current flow along the structureback to its source. This can be done byobserving the current flow at intervalsalong the interfered structure by utilizingmillivolt drop test station lead wires, todetermine the direction of the current flow.This technique is illustrated in Figure 4-3.

Let us assume that a situation exists where asingle generating unit is causing interferenceproblems. A voltmeter connection, to be usedfor pipe-to-soil potential (Vgsc) measurements, ismade between the pipeline and a referenceelectrode, somewhere within the earth currentpattern of the generator and its load. Observingthe fluctuating potential readings at this pointalone would not enable one to determine if thereadings are being taken at a point where thepipeline is picking up or discharging current. Ifmeasurements are being taken at a point ofcurrent pickup, a negative potential swing wouldoccur. A positive swing would indicate adecrease of current flow and a condition of thepipe returning to its steady state condition.Readings observed at a discharge point,swinging in the positive direction would indicatean increase in current leaving the line and anegative swing would indicate a decrease incurrent discharge.

The determination of whether test locations areeither pick-up or discharge points is done byobtaining correlations of the pipe-to-soilpotential to the open circuit voltage between theinterfered pipeline and the stray current source.This is known as a Beta Plot. Figure 4-4illustrates a typical Beta plot measurementconnections. The locations for the pipe-to-soilmeasurements may be remote from the opencircuit measurements. Communication must beestablished between the locations, as bothmeasurements must be made simultaneously.Sufficient data points must be obtained toindicate trends in the data. Figure 4-5 is anexample of a Beta Plot of one locationindicating a current pick up area and Figure 4-6shows a current discharge area at anotherlocation.

As the connection shown in Figure 4-4

indicates, a positive value for Esc occurs whencurrent flows from the stray current source tothe pipeline. Conversely, a negative value of Escindicates a flow from the pipeline to the straycurrent source. The slope of a straight linedrawn through the data points is the value ofBeta ($):

As indicated, this type of testing requires thatvoltage measurements be taken at two differentlocations. Many readings should be taken at thetwo locations simultaneously. The meters usedat the two locations must be identical or elsecomparison of the two sets of readings will bedifficult.

In most cases, a dual channel recorder such asan X-Y plotter is used or a multi-channel datalogger is used.

Interpreting Beta Curves

Beta curves, which are sloped from the bottomright hand side of the graph to the top left side,would indicate an area of current pick up. Whatthis means is that as the current output of thesource increases, the current pick up by theinterfered structure also increases. An exampleof such a curve is shown in Figure 4-5.

An area of current discharge is indicated by acurve which slopes from the bottom left to thetop right of the graph, indicating that the rate ofcurrent discharge increases as the currentoutput of the source increases. An example ofsuch a curve is shown in Figure 4-6.

If the plotted points describe a vertical line, thiswould indicate a neutral curve. This means thatthere is no influence on Vg by the outputfluctuations of the current source. Such a curveis rare in practice and if one is plotted, thepossibility of equipment malfunctions or errorsduring testing should be investigated.

If the plotted points do not describe a straightline, this may indicate that the interfered

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TYPICAL BETA CURVE - PICKUP AREA

FIGURE 4-5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-1.60 -1.40 -1.20 -1.00 -0.80 -0.60 -0.40

ES

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Vgsc1 (VOLTS)

∆Vgsc

∆Esc

∆Vgscβ =

∆Esc

TYPICAL BETA CURVE - DISCHARGE AREA

FIGURE 4-6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20

ES

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OL

TS

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Vgsc2 (VOLTS)

∆Vgscβ =

∆Esc


structure may be picking up stray currents froman additional source or that the suspectedsource is not causing the interference. Plotsdescribing two or more distinct lines wouldindicate the presence of more than oneinterfering current sources. The slopes of theselines will indicate whether currents aredischarging or being picked up according totheir slopes and the polarity of the meterconnections.

Determining the Point of MaximumExposure

The point of maximum exposure can bedetermined based on the slopes of the plotteddischarge beta curves. The beta curve whichhas the most horizontal (largest) slope indicatesthe location of maximum exposure. Figure 4-7shows this graphically.

In practice, the calculated slopes can be plottedversus distance to indicate clearly the locationof the largest slope (indicative of the mosthorizontal line).

METHODS OF MITIGATING THE EFFECTSOF DYNAMIC STRAY CURRENT

There are several general methods used toreduce or mitigate the effects of dynamic straycurrent. These methods include: control at thesource, installation of mitigation bonds andreverse current switches, use of sacrificialanodes, and use of impressed current cathodicprotection to counter the stray current. Each ofthese methods is discussed in the followingsections.

Controlling Stray Currents at the Source

The most effective way to minimize dynamicstray current is to minimize the electrical currententering the earth in the first place. In the caseof transit and any other systems involving railreturns, the best procedure is to ensure that therails are installed with insulated fasteners onwell-ballasted roadbeds or concrete inverts.The DC negative bus of the traction powersubstation should be ungrounded, and the

substations should be spaced at a maximum ofabout one-mile apart. Modern rail transitsystems use welded rail; this reduces thevoltage drop in the rail, which in turn reducesthe stray current induced in the earth.

Similarly, when dealing with equipment, ifisolated electrical positive and negative circuitscan be used, stray current problems will beavoided because the currents never reach theearth. When welding is done, care should betaken to assure that the welding electrode andthe ground are relatively close together and theelectrical path between them is of negligibleresistance. Thus, no voltage drop of anysignificance will exist between them.

Mitigation (Drainage) Bonds and ReverseCurrent Switches

Mitigation bonds and reverse current switchesare used to mitigate the effects of stray currentcorrosion on a structure.

The purpose of a mitigation, or drainage bondis to eliminate current flow from a metallicstructure into the electrolyte by providing ametallic return path for such current. Thedrainage bond will allow the stray current toflow through the structure and back to thesource, through the bond. Figure 4-8 shows thetypical current flow when a drainage bond isinstalled. As a result, corrosion will not occur ifthe current is not allowed to flow from the metalsurface into the electrolyte.

It is not good practice to install drainage bondson modern rail transit systems that have beendesigned to minimize stray current at thesource. The high resistance to ground of thesesystems minimizes stray current. Installation ofa bond lowers this resistance and in turnincreases stray current flow.

When designing a bond, the goal is to find theproper size resistance to be inserted betweenthe affected structure and the source so thatstray current discharge into the electrolyte iseliminated at all points. The proper bond willcause the affected structure to return the pickup

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current to the interfering structure through thebond.

Before the mitigation bond can be sized, thepoint of maximum exposure must be located.This is done as described earlier under“Determining the Point of Maximum Exposure”.

As previously indicated, dynamic stray currenttesting requires potential measurements takensimultaneously at two different locations. Thesereadings can be taken by two technicians whoare in communication with each other by radioso that one can tell the other when to take areading. Many readings should be taken at thetwo locations simultaneously. Meters used atthe two locations must have identicalcharacteristics or comparison of the two sets ofreadings will be difficult.

To facilitate the taking of these potentialreadings, strip chart recorders can be set up atthe two locations. Recordings obtained over a24-hour period will normally provide all the datarequired. The corresponding readings taken atthe two locations can then be plotted on lineargraph paper. This plot of points is known as a“beta curve”. Several locations should be testedlongitudinally along the pipeline within theexposure area. The location of the recorder atthe source remains constant. A separate set ofsimultaneous readings should be made at eachtest location. This type of survey is known as abeta profile.

State-of-the-art computerized multi-channel testequipment is available to facilitate obtaining abeta profile. These computerized recorders canbe set to record the potentials at as many as 28different locations simultaneously for anydesired length of time. The recorder will storeall readings in a permanent memory until it isdown-loaded into a computer terminal.Computer programs are available which willthen plot beta curves. Similarly X-Y recorders,which simultaneously record pipe-to-soilpotential and pipe to source voltage arefrequently used.

The points when plotted should describe a

straight-line plot referred to as the beta curve.The slope of the plotted line will indicatewhether the interfered structure in the area ofthe test is picking up current, dischargingcurrent or neither.

Once the location of maximum exposure isdetermined and its negative slope or beta curveplotted, the size of the resistance bond can bedetermined. The required size of the resistancebond is such that its installation will cause thebeta curve at the point of maximum exposure toassume a neutral or pick-up slope. Figure 4-9shows a beta curve at a point of maximumexposure as well as the required mitigationcurve.

Sizing the Mitigation (Drain) Bond

Two methods are available to size the bond,trial and error and mathematical. Both areexplained below.

Trial and Error Method

In relatively simple cases, such as where asingle source of stray current is involved, a trialand error solution may be possible. The size ofthe resistance bond can be determined byinstalling temporary cables and variableresistors and determining when stray currentcorrosion has been mitigated. Since draincurrents of up to 200 to 500 amperes can beinvolved, test cables must be sized accordingly.A mitigation curve such as that shown in Figure4-9 must be obtained from the test bond.

When using the trial and error method on straycurrent problems involving transit systems, careshould be exercised to ensure that thetemporary bond does not affect signaling.

Mathematical Method

Where a more complex interference problemexists that precludes the use of the trial anderror method, the following mathematicalmethod can be used to size the resistancebond. As noted earlier, dynamic stray currentsmay involve large values of current flow, so the

TYPICAL BETA CURVE - DISCHARGE AREA(MITIGATION CURVE)

FIGURE 4-9

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40

ES

C(V

OL

TS

)

Vgsc (VOLTS)

∆Vgscβ =

∆Esc

MITIGATION CURVE


β x E I x VI

sc 1g

1= ⎛

⎝⎜⎞⎠⎟

∆

anticipated currents must be known to sizecable and associated electrical equipmentproperly. A mathematical solution can providethe designer with these anticipated values.

This method is based upon the electricalrelationships between the source and thepipeline. The measured potential-to-earth of apipeline is a combination of its natural potential-to-earth plus the sum of all potential changescaused by the DC sources influencing it.

The test set ups for the mathematical methodare shown in Figures 4-10 and 4-11. Figure4-10 explains the test procedure for determiningthe internal resistance (Rint) between thepipeline and the stray current source. Figure 4-11 shows the procedure to determine thechange in pipe to soil potential per ampere ofdrain current.

The beta curve at the point of maximumexposure, such as the one shown in Figure4-12, is plotted. The equations used in reachingthe solution are as follows:

Equation 1

Vg = )Vgo + )Vgcp + )Vgsc + )Vgb

Where:

Vg = Pipe-to-soil potential with all currentsources operating

)Vgo = Natural pipe-to-soil potential of thepipeline

)Vgcp = Change in pipe-to-soil potentialcaused by cathodic protection ofpipeline

)Vgsc = Change in pipe-to-soil potentialcaused by stray current sources

)Vgb = Change in pipe-to-soil potentialcaused by mitigation bond

In the case of a dynamic stray current:

Equation 2

)Vgsc = $ x )Esc (See Figure 4-4)

Where:

$ = )Vgsc /)Esc

If no cathodic protection is present, Equation 1simplifies to Equation 3:

Equation 3

Vg = Vgo + $ + )Esc + )Vgb

For the dynamic stray currents to be mitigated:

Equation 4

$ x )Esc = )Vgb (Vg - Vgo must = 0)

From Figure 4-12, the value for Vgo = 0.570volts and $ (the slope of the plotted points) =0.017.

)Vgb is the change in potential-to-soil of theinterfered structure caused by a bond current.The value of )Vgb can be calculated for anygiven bond current from Equation 5:

Equation 5

(See Figure 4-11)∆∆

V I x VI

gb 1g

1= ⎛

⎝⎜⎞⎠⎟

Combining Equation 4 with Equation 5 yieldsEquation 6:

Equation 6

The value of the bond current, Ib, is calculatedfrom Equation 7:

# ./0

&

121345!10!6784044310491/:;./0133<=0./ 0>1"<331/0".3"<.0%<331/04/826504 14310491/!.?<504/16<!5;@.0>0>[email protected]">"56!186/4/86=1/677*&%>131!.!04/"1:10@11/0>10@6!03<"0<31!.!"45"<54018736?>?A!4@

***&./0*B

#

121345!10!6784044310491/:;./0133<=0./ 0>1"<331/0".3"<.0%<331/04/8=.=106!6.5=601/0.453148./ !4310491/!.?<504/16<!5;@.0>0>[email protected]">"56!186/4/86=1/677*&%>1">4/ 1./=.=106!6.5=601/0.45=134?=13167"<331/0756@ B.!"45"<54018736?>?A!4@4/8.!1C=31!!184!2650!=134?=131B

* **&B* B


R E x V I

x E - Rb sc

g 1

scint=

∆β

R V I

- Rbg 1

int=∆

β

R 0.00169

0.017 0.070 = 0.0294b = − Ω

E I x (R R )

I E

Ri R

I 12.0

0.070 0.0294

12.00.0994

I 120.7 amperes

sc b int b

bsc

nt b

b

b

= +

=+

=+

=

=

Equation 7

I x EVI

b scg

= ⎛⎝⎜

⎞⎠⎟1

β∆

The resistance of the bond can be calculatedfrom Ohm’s Law:

Equation 8

Esc = Ib x (Rint + Rb)

Or, rearranging:

Equation 9

R EI

Rbsc

bint= −

Substituting:

Equation 10

Or:

Equation 11

Tables 4-1 and 4-2 present sample data for asingle bond stray current problem. From Table4-1, the average resistance between structuresis found to be 0.070 ohms. Table 4-2 shows thevalue of )Vg/I1 to be 0.00169 V/A. Using the $value from Figure 4-12 of 0.017 and substitutingthe values into Equation 11, the bondresistance, Rb, is calculated:

The bond resistance value, 0.0294 ohm for thisexample, can be a simple cable or acombination of cable and variable or fixedresistors. The bond must also be sized topermit the maximum current flow and stillremain in the ampacity range for the bond. In adynamic stray current situation, the maximumcurrent can be calculated once the maximumopen circuit voltage between the structures isknown. This value is usually obtained by usinga recording voltage instrument over severaloperating cycles of the stray current source tomeasure the peak value. For most stray currentsources the typical cycles is 24 hours.

For example, if the maximum value of opencircuit voltage, Esc, is 12.0 volts, substitution ofthis value into Equation 8 and rearranging theequation gives the value of the maximum straycurrent through the bond 120.7 amperes asshown below:

Table 4-1 also shows some typical measuredvalues for E1 and I1, using the test setup shownin Figure 4-10, as well as the calculated internalresistance.

Table 4-2 also shows some typical measuredvalues for Vg and l1, using the test setup shownin Figure 4-11, and the correspondingcalculated )Vg/I1.

In many instances of stray current work wheretraction systems are involved, there may belocations where drainage bonds are required inareas where electrical reversals of potentialcould occur. Therefore, it is often necessary to

TABLE 4-1

Rint Data - See Figure 4-10

E1 (Volts) I1 (Amperes) Rint (ohms)

On +1.30 36.0

Off -1.15 0

Delta +2.45 36.0 0.068

On +0.80 39.0

Off -1.80 0

Delta +2.60 39.0 0.067

On +0.50 41.0

Off -2.30 0

Delta +2.80 41.0 0 .068

On +2.10 29.0

Off 0.00 0

Delta +2.10 29.0 0.072

On +2.00 29.5

Off -0.15 0

Delta +2.15 29.5 0.073

On +1.30 35.0

Off -1.25 0

Delta +2.55 35.0 0.073

On +0.35 43.0

Off -2.65 0

Delta +3.00 43.0 0.070

Average Rint = 0.070

TABLE 4-2

)Vg/I1 Data - See Figure 4-11

Vg (Volts) I1 (Amperes) )Vg/I1 (V/A)

On 0.645 34.0

Off 0.590 0

Delta 0.055 34.0 0.00162

On 0.635 23.0

Off 0.600 0

Delta 0.035 23.0 0.00150

On 0.770 82.0

Off 0.620 0

Delta 0.150 82.0 0.00183

On 0.770 86.0

Off 0.625 0

Delta 0.145 86.0 0.00168

Average )Vg/I1 = 0.00169


install an electrolysis switch or silicon diode intothe circuit to prevent current flow from therailroad back onto the pipeline through thebond. The resistance to the forward flow ofcurrent created by these devices must beincluded in the sizing of the bond.

Use of Sacrificial Anodes

Sacrificial or galvanic anodes may be used tomitigate stray current effects in situations wheresmall current flows or small voltage gradientsexist. A misconception surrounding the use ofgalvanic anodes for stray current mitigationstates that the anode draws the stray currentsand prevents currents from entering into theelectrolyte from the interfered structure. Thisover-simplification does not consider the entireelectrical network involved. In effect, a potentialgradient field produced by the galvanicanode(s) counteracts the interference current.The effect is a net current flow to the interferedwith structure. A version of the electrical circuitis shown in Figure 4-13.

The potential gradients of the stray currentsource and of the anodes counteract eachother. Galvanic anodes produce limitedvoltages, however, and as such are limited tothe stray current voltages, which they canovercome.

Another consideration when using a galvanicanode system to overcome stray currents is theexpected life of the anodes. As the anodesdissipate, their resistance to ground increases.The increased resistance reduces the currentflow from the anode and decreases theresulting voltage gradients. Sacrificial anodesshould be sized to provide a sufficientanticipated life span. As with any other straycurrent mitigation procedure, the anodes shouldbe placed on an active monitoring schedule.

Galvanic anode drains are commonly used inlieu of bonds where small drain currents areinvolved. In areas of large current drains, theuse of galvanic anode drains would beimpractical due to the high consumption rate ofthe anode material; frequent anode

replacement would be required. Galvanicanodes would also be impracticable wherevoltage gradients are encountered that aregreater than those galvanic anodes canproduce.

Use of Impressed Current Systems

When the magnitude of stray currents arebeyond the ability of galvanic anodes tocounteract, impressed current systems cansometimes be utilized. Impressed currentsystems have much higher voltage capacitiesthan galvanic anodes and a greater life perpound of anode material. Built-in control andmonitoring circuits in the impressed currentrectifier can be used to adjust the protectivecurrent output based upon the voltage-to-earthfluctuations of the interfered pipeline.

The design of galvanic or impressed currentmitigation systems is done by the trial and errormethod. Simulated systems are placed in thefield and the results measured. Based upon theresults, a full-scale system can be designed.

STRAY CURRENT FROM ALTERNATINGCURRENT (AC) SOURCES

Stray current from AC sources is more of asafety hazard than a source of corrosion.Recent work however, is beginning to show thatAC discharge from a pipeline may indeed causesignificant corrosion1. Both steady state anddynamic stray AC current may be found.

Steady state stray AC is most commonlyassociated with pipelines laid in close proximityand paralleling high voltage AC electricaltransmission lines. AC induction on suchpipelines can create dangerous voltages.Mitigation of the voltage on the line as well assafety grounding of test stations and aboveground appurtenances to buried equi-potentialmats are recommended if the AC pipe to soilpotential exceed 15 volts.

Dynamic AC stray currents may be generatedby AC welding operations, bad grounds inbuildings or AC electrified railroads.

BETA CURVE PLOTTED AT POINT OF MAXIMUM EXPOSURE

FIGURE 4-12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20

ES

C(V

OL

TS

)

Vg (VOLTS)

D D #D #

ED

FD

G


For information on AC mitigation, the reader isreferred to NACE International RecommendedPractice RP01-77 (latest revision), Mitigation ofAlternating Current and Lightning Effects onMetallic Structures and Corrosion ControlSystems2.

CONCLUSIONS

Stray current interference on a pipeline cancause severe local corrosion that could lead toa hazardous situation, product leakage andcostly repairs. Therefore, every effort should bemade to avoid interference problems and, ifthey exist, to mitigate their effects.

Stray currents can be classified as being eitherstatic or dynamic. Static interference currentsare defined as those, which maintain a constantamplitude, direction and electrolytic path.Dynamic interference currents are defined asthose, which continually vary in amplitude,direction and electrolytic path. Staticinterference is always caused by man-madesources; dynamic interference, however, canalso be caused by telluric activity.

Stray current control at the source is the bestmethod of stray current mitigation.

The use of bonds or cathodic protectionsystems to mitigate stray current corrosion canbe very effective when properly designed andinstalled. It is very important that all field testingand remedial actions be conducted or taken inthe presence of, or with the permission of,representatives of all companies/ownersinvolved.

Sacrificial anode or impressed current cathodicprotection can also be used to mitigate straycurrent.

Stray AC is more of a safety hazard than asource of corrosion. Its effects are not fullyunderstood, however, so the corrosiveness ofstray AC should not be discounted.

REFERENCES

1. R. A. Gummow, “AC Corrosion – AChallenge to Pipeline Integrity,” MaterialsPerformance, 38,2 (Feb. 1999)

2. Recommended Practice RP01-77 (latestrevision), Mitigation of Alternating Currentand Lightning Effects on Metallic Structuresand Corrosion Control Systems, NACEInternational, Houston, Texas

DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION5-1

CHAPTER 5

DESIGN OF IMPRESSED CURRENT CATHODIC PROTECTION

INTRODUCTION

The objective of this chapter is to providecorrosion control personnel with informationnecessary to design an impressed currentcathodic protection system for undergroundstructures.

This chapter will discuss the variousparameters which must be considered in thedesign of an impressed current cathodicprotection system. These parameters includethe following:

1. Protective current requirements

2. Anode bed resistance

3. Anode bed layout

4. Anode life expectancy

5. Rectifier sizing

REVIEW OF IMPRESSED CURRENTSYSTEM FUNDAMENTALS

Definition

Cathodic protection can be defined as thereduction or elimination of corrosion by makinga metallic structure cathodic with respect to theelectrolyte in which it is installed. This isaccomplished by applying current to thestructure by means of an impressed directcurrent or attachment of a galvanic anode to thestructure.

Cathodic protection makes a metallic structurethe cathode of a purposely designedelectrochemical cell thereby protecting it from

corrosion. The protective current is eitherdeveloped from within the cell or introduced tothe cell from an external source which is largeenough to overcome the effect of corrosioncurrents being discharged from the anodicareas of the structure. When the surface of thestructure is fully polarized, corrosion of thestructure is controlled. In a sense, the corrosionprocess is not eliminated but merely transferredto an expendable, or sacrificial, material(anode).

Theory of Operation

The impressed current cathodic protectionsystem utilizes an external power source toprovide a potential difference between theanode(s) and the protected structure. Theanodes are connected to the positive terminaland the structure to the negative terminal of thepower source. Current flows from the anodesthrough the electrolyte and onto the surface ofthe structure. It then flows along the structureand back to the power supply through anelectrical conductor. Since the structure ispicking up current from the electrolyte, thestructure is protected. The protective currentoutput of the power supply is adjusted to deliversufficient current to overcome the corrosioncurrents trying to leave the anodic points on thestructure. As the anodes discharge current theyare consumed. Therefore it is desirable to useanode materials that are consumed at lowerrates than magnesium, zinc, and aluminumwhich are used in sacrificial or galvanicsystems. Special alloys and combinationmaterials are used in impressed currentsystems to obtain a long, useful life expectancyfrom the system.


Application

The impressed current cathodic protectionsystem, in most cases, utilizes a variable powersource. Impressed current systems are capableof protecting large structures or structureswhich require greater magnitudes of currentthan can be provided economically by agalvanic anode system. These systems can bedesigned to protect bare (uncoated) structures,poorly coated, or well coated structures. As theamount of surface area to be protectedincreases, the total current requirementincreases and impressed current systemsbecome the system of choice.

Advantages and Disadvantages

Impressed current systems can be designed fora variety of applications with a reasonabledegree of flexibility. This is a distinct advantageof this type of system, along with the ability toincrease or decrease the current output, eitherautomatically or manually, by changing theoutput voltage. The disadvantages of thesystem are the increased maintenancerequirements, tendency for higher operatingcosts, and possibility of contributing to straycurrent interference on neighboring structures.

INFORMATION USEFUL FOR DESIGN OF ANIMPRESSED CURRENT CATHODICPROTECTION SYSTEM

Before embarking on the design of a system,certain information should be gathered,analyzed, and factored into the design:

1. What is the pipe material? Is it steel(including grade of steel), cast iron,wrought iron or other material? What is theknown electrical resistance?

2. Is the line bare or coated? If coated, whatis the coating material and what coatingspecifications were used?

3. If it is an existing line, is there a leakrecord? If so, information on the locationand date of occurrence of each leak will

positively indicate the more seriousproblem areas.

4. What is the pipe diameter, wall thicknessand weight per foot? Is there any data onany changes in these items along the routeof the line?

5. What are the location and constructiondetails of all corrosion test points whichhave been installed along the line? If notest points have been installed forcorrosion test purposes, determinelocations where contact can be made withthe pipeline for test purposes (other thanby driving contact bars down to the pipe).

6. Is the line of all-welded construction or aremechanical couplers used?

7. What are the locations of branch taps?

8. What are the locations of purposelyisolating flanges or couplers, if any, used tosectionalize the line or to isolate itelectrically from other portions of thesystem or from piping of other ownership?

9. Obtain route maps and detail maps givingas much data as is available.

10. What are the locations of undergroundstructures of other ownership that cross thepipeline to be surveyed? If any of thesestructures are cathodically protected,determine the location of cathodicprotection current sources (particularlyrectifiers) that may be in close proximity tothe structure to be protected.

11. Location of possible sources of man-madestray current (such as DC electric transitsystems or mining operations) that couldaffect the line under study.

12. Do any sections of the pipeline closelyparallel (within 200 feet or so) high-voltageelectric transmission lines? If so, what isthe length of such exposure, how close isthe pipeline to the towers, at what voltage


does the electric line operate, and whatmethod is used for grounding the towers?(This information is significant becauserectifier installations and isolating joints inwell coated pipelines, closely parallel tohigh-voltage electric lines, may bedamaged by induced AC voltage surgesunder electric system fault conditions ifpreventive measures are not taken.)

13. Is the line now operated at elevatedtemperatures, or will it be so operated inthe foreseeable future? (High temperaturecould cause deterioration of coatings usedand increase the current requirement toachieve corrosion control.)

14. Is AC power available to provide power tothe rectifier?

DESIGN OF AN IMPRESSED CURRENTANODE BED

General

There are several steps to be taken indesigning an impressed current anode bed. Theinitial steps include:

C Selecting an anode bed site.

C Selecting an anode bed type.

Once the anode bed site and type have beenselected, the designer can proceed to evaluatethe different parameters required to completethe design of the selected impressed currentanode bed.

Selecting an Anode Bed Site

The following factors should be taken intoaccount when considering an anode bed site:

a. Soil Resistivity - Soil resistivity is one of themost important factors to consider whenselecting an anode bed site. Soil resistivity isone major component in evaluating theanode bed resistance. The anode bedresistance is a critical component in the

determination of protective current outputand rectifier size. Usually, the lower theresistivity, the fewer number of anodesrequired to deliver a given current. In manycases, the area of lowest resistivity isconsidered a prime location for an anodebed site.

b. Soil Moisture - Another factor to consider inthe selection of an anode bed site is the soilmoisture at the proposed anode depth. Inmost cases, soil resistivity decreases as themoisture increases until the soil becomessaturated. When possible, anodes should beinstalled below the water table (assumingthe water resistivity is not too high), or inareas of high moisture content such asswamps, ditches, creek beds, etc.

c. Interference With Foreign Structures -Structures such as pipelines, undergroundmetallic sheathed electrical cables, or wellcasings, may be subjected to stray currentinterference problems. Therefore, anodebeds should be located away from foreignstructures whenever possible. Local utilitiesshould be contacted to determine ifunderground structures exist in the area ofthe proposed anode bed location.

d. Power Supply Availability - The selectionof an anode bed site may be influenced bythe availability of an economical source ofpower to be used to provide the requiredcathodic protection DC current.

e. Accessibility for Maintenance andTesting - The anode bed site selected mustbe accessible to construction vehicles foranode bed installation, testing and repairs.

f. Vandalism - Anode bed locations should beselected so as to minimize the potential forvandalism.

g. Purpose of Anode Bed and SiteAvailability - Two additional factors thathave to be considered in the process ofselecting an anode bed site are the intendedpurpose of the anode bed and the possibility


of acquiring the required right-of-way for theinstallation. Is the intent of the anode bed toprotect a long section of the pipeline(assuming the pipeline is well coated), or isit the intent to afford localized protection to ashort section of a pipeline (poorly coated) ora structure? In the first case, the approachmay be the installation of one remote typeanode bed near the midpoint of the pipelinesection, or perhaps the installation of a deepanode bed near the midpoint area. In thesecond case, the approach could be theinstallation of a distributed anode bed.

Once a determination has been made as tothe intent and purpose of the installation andthe most suitable type of installation, theland availability should be investigated.

Selecting Anode Bed Type Based on SiteSelection

Prior to selecting the anode bed type, it is wiseto consider the major types of anode beds usedand application conditions associated witheach.

Distributed Anode Bed

Distributed anode beds consist of anodes usedto protect a limited area of the pipeline. Theanodes are generally installed close to thestructure and, hence, are also called “closelycoupled distributed systems”. These systemsare used to reduce potential interference effectson neighboring structures, to protect sections ofbare or ineffectively coated pipelines, andpipelines in congested areas where electricalshielding might occur with other anode bedtypes.

Remote anode beds, also called “conventionalanode beds”, consist of anodes installed in theearth at a distance away from the pipeline. Thistype of anode bed is normally used to distributeprotective current over a broad area of thepipeline to be protected.

The anode bed used to protect a pipeline isusually installed perpendicular to the pipeline,

and the first anode may be more than 300 feetaway. The anodes are normally installed on 15to 30 foot spacings.

Remote anode beds can also be installedparallel to or at an angle to the pipeline,depending on right-of-way availability. The maindifference between a perpendicular and aparallel installation is that the gradient effect onthe pipeline from a perpendicular anode bedwould be less than from a parallel anode bed(assuming the line of parallel anodes is at adistance from the pipeline equal to that of thefirst anode in a perpendicular anode bed).Figure 5-1 is an example of a remote anodebed design with the anodes parallel to the linesto emphasize the anodic gradients around theanode bed and the cathodic gradients aroundthe pipeline.

The term “remote”, as associated with this typeof anode bed design, means that the pipeline isoutside of the anodic gradient of the anode bedcaused by the discharge of current from theanodes to the surrounding soil. This wouldallow a better spread of the protective current tobe achieved, thus protecting a larger section ofa pipeline from one installation. In real lifehowever, remote anode beds are installed atdistances from a pipeline where the remaininggradient around the pipeline is normally small(not zero).

Deep Anode Bed

A deep anode bed utilizes anodes installedelectrically remote from the structure in avertical hole drilled to a minimum depth of 50feet and, in some cases, as deep as 1,000 feet.This approach achieves similar results as aremote surface anode bed.

Deep anode beds are usually effective whenhigh surface soil resistivities exist over deeperlow-resistivity areas where the anodes areinstalled.

Hybrid Anode Bed

In some cases, on complex structures, different


anode bed types are used in combination toafford complete protection to the pipeline. Anexample may be the case where a remoteanode bed has been used to protect severalmiles of a well coated pipeline, but shieldingfrom foreign structures occurs in a localizedarea. In this case, a distributed anode bedmight be considered to supplement theeffectiveness of the remote anode bed. Thiscombination of anode beds is sometimesreferred to as a hybrid approach.

The designer may find himself involved in caseswhere site availability is scarce and where thetype of anode bed to be selected is dependenton the sites available. A similar case woulddevelop if the size of the site available isrestricted. These conditions may develop inurban areas or in instances in which the landowner may not wish to sell or lease any land forthe installation.

Land limitations may dictate that the installationhas to be restricted to the existing right-of-way,in which case a distributed type anode bed maybe selected for a localized condition or a deepanode installation may be selected forprotection of extended areas.

In cases where the site availability may dictatethe type of anode bed, it would be advisable toanalyze further the soil conditions to be able todetermine which is the most economical type ofdesign that would do a proper job.

As previously mentioned, soil with a highresistivity layer near the surface and a low orlower resistivity layer underneath would be wellsuited for a deep anode type of installation.This type of soil condition can be encounteredalong a pipeline at areas where a relativelyshallow (sometimes slightly more than pipelinedepth) surface soil overlies a high resistivitylayer of rock which, in turn, overlies a layer ofsoil of relatively low resistivity. The currentdischarge from a deep anode bed in this type ofsoil arrangement could extend laterally forconsiderable distances and work its way upthrough the higher resistivity rock layer to thesurface soil where the structure to be protected

is installed.

In contrast with the above example, the casemay be where the surface resistivity is low andthe underlying soil is of high resistivity. In thiscase, a surface remote or distribution type ofanode bed can be effectively utilized and adeep anode bed should not be considered.

In the cases where there are no restrictions tothe type of anode bed to be utilized from any ofthe parameters listed in the preceding sections,economics would then be the deciding factor.

Tests Required for Anode Bed Design

The design of an impressed current anode bedis normally based on test data acquired in thefield. The basic tests that should be conductedare soil resistivity and current requirement tests.

Soil resistivity tests are conducted along theroute of the pipeline and at the locationsselected for the anode bed installation.

Referring to the soil resistivity data shown inTable 5-1, let us assume that we are going toinstall a vertical anode bed to a depth of ten(10) feet. The weighted average soil resistivityto that depth (pin spacing) is shown as 19,533ohm-cm. For design calculation purposes, anaverage resistivity of 20,000 ohm-cm should beused.

It should be noted again that the resistivityvalue used for design purposes should beobtained in the specific area where the anodebed will be installed, as there can beconsiderable variations in soil resistivity withinshort distances. An actual case example of thisproblem was an anode bed designed on thebasis of soil resistivity data taken about 50 feetaway from the foot of a hill. Because of right-of-way considerations, the anode bed was moved30 feet towards the foot of the hill without newsoil resistivity measurements. When the systemwas energized, the resistance of the anode bedwas found to be about ten (10) times higherthan it would have been at the locationoriginally selected.

TABLE 5-1

Typical Soil Resistivity Data

GalvanometerDial Reading Multiplier Setting Resistance

(Ohms) Spacing (ft) Factor Resistivity(Ohm-cm)

2.4 10 24 2.5 191.5 11,490

1.6 10 16 5 191.5 15,320

10.2 1 10.2 10 191.5 19,533

6.2 1 6.2 15 191.5 17,801

2.6 1 2.6 20 191.5 9,958

8.2 0.1 0.82 25 191.5 3,926


I I x VV

T AREQ

g=

ΔΔ

I 10 x 400260

15.385 AmperesT = =

The second set of tests that should beconducted are current requirement tests todetermine the amount of current required toachieve protection of a pipeline/structure or asection thereof. In the event that the structureor pipeline is proposed and not yet installed, thecurrent requirement must be estimated byassuming a current density.

A temporary anode bed must first be set up,preferably at the location of the proposed anodebed installation. The temporary anode bed mayconsist of several ground rods driven into theground and connected to a portable rectifier orother DC source. Soil resistivity test pins mayalso be used. Temporary power sources, suchas batteries and DC welding machines, mayalso be used.

A creek or small lake, in close proximity to thestructure requiring protection, can be used toset-up a temporary anode bed. Householdgrade aluminum foil unrolled into the water andconnected to a DC power source using clipleads can provide an excellent temporaryanode bed for use when conducting currentrequirement tests.

Current requirement tests are conducted byenergizing the temporary anode bed and taking“ON” and “OFF” potential readings along thepipeline/structure using a high input resistancevoltmeter and a portable copper-copper sulfate(Cu-CuSO4) reference electrode. While takingpotential readings, the current output of thetemporary anode bed is increased untilprotection is achieved in the entirepipeline/structure, based on establishedcathodic protection criteria, as discussed inChapter 1 of this course. Figure 5-2 shows atypical setup for current requirement tests.

At times, it may be difficult to obtain an outputfrom the temporary anode bed that is largeenough to achieve full protection of the entirepipeline/structure. Should this be the case, theactual current requirements can be calculatedby extrapolating the data obtained to thedesired protective levels.

As an example, let us consider a test setup inwhich the output of the temporary anode bed is10 amperes, and the potential shifts obtainedare:

Location Vg-ON Vg-OFF )V

East End -0.75 V -0.45 V 0.30 V

Midpoint -0.95 V -0.55 V 0.40 V

West End -0.71 V -0.45 V 0.26 V

From the above data it can be seen that thelargest shift required to meet the -0.85 voltprotective criterion is 400 millivolts, [0.45 (Eastor West End) - (-0.85)] and the lowest shiftobtained with 10 amperes is 260 millivolts. Thetotal current required can be calculated asfollows:

Where:

IA = Applied Test Current

)VREQ = Potential Shift Required to Meet-0.85 Protective Criterion

)Vg = Lowest Voltage Shift ObtainedDuring Test

IT = Total Current Required

A total of approximately 16 amperes would berequired as a minimum to meet the -0.85 voltcriterion.

It is important to note that the amount of currentrequired to cathodically protect a pipeline orstructure is greatly affected by the condition ofthe coating on the structure. A bare structuremay require many times the amount of currentto protect the same pipeline/structure, if it wereproperly coated. Table 5-2 shows typicalcurrent requirements for a 5 mile section of18-inch diameter steel pipe with variousdegrees of coating effectiveness.

!

"

#

$

$

%

TABLE 5-2

Typical Current Requirements Based On Coating System Effective Resistance

Effective Coating Resistance (1)

(ohm-ft²) Current Requirement

(Ampere)

Bare Pipe (2) 187.50

10,000 3.73

25,000 1.49

50,000 0.75

100,000 0.37

500,000 0.075

1,000,000 0.0373

5,000,000 0.007

“Perfect Coating” 0.000015

(1) Effective coating resistance, as defined in the above table, of 10,000 to 25,000 ohm-ft² indicates poorapplication or handling during installation. Resistance of 100,000 to 5,000,000 ohm-ft² indicates goodto excellent application. Installation in 1,000 ohm-cm soil.

(2) Bare pipe in this table is assumed to require a minimum of 1.5 milliamperes per sq. ft. of pipe surface.In practice, most design engineers use 2 milliamperes per sq. ft. for pipe-in-soil, unless theenvironment is acidic, contains high concentrations of chlorides, bacteria, or the pipe is operating atelevated temperatures. In these cases, as much as 3.5 to 5.0 milliamperes per sq. ft. may berequired.


ΔΔ

VV

II

or I I x Ag

REQ

A

TT d S= =

−−

= ∴ =0.0310.289

0.40

I I 3.73 amperes

TT

DESIGN CALCULATIONS

The following examples show the various stepsnecessary to design a cathodic protectionsystem for underground pipelines.

Existing Structures

For existing structures, the design can bebased upon data obtained in the field throughthe previously mentioned current requirementtesting to determine the actual currentnecessary to achieve cathodic protection.

Current Requirement

A temporary anode bed is set up using 5ground rods and a portable rectifier. The groundrods are driven approximately 1½ feet into theground at 10-foot spacing. The measuredoutput of the rectifier is 34 volts and 0.40amperes. While interrupting the rectifier output,the following pipe-to-soil potential readingswere taken at various locations with respect toa portable copper-copper sulfate (CuCuSO4)reference electrode, placed directly over thepipeline in the area under the influence of thetemporary anode bed, and the change inpotential ()V) was calculated.

PIPE-TO-SOIL POTENTIAL (Volts)

Location On Off )V

1 -0.592 -0.561 -0.031

2 -0.570 -0.523 -0.047

3 -0.603 -0.545 -0.058

4 -0.598 -0.527 -0.071

5 -0.693 -0.575 -0.118

6 -0.833 -0.635 -0.198

7 -0.865 -0.650 -0.215

8 -0.814 -0.611 -0.203

9 -0.731 -0.605 -0.126

10 -0.655 -0.590 -0.065

11 -0.630 -0.575 -0.055

12 -0.640 -0.580 -0.060

Step No. 1

Using the lowest change in potential shift ()V)measured during the tests, calculate the voltageshift required to satisfy the -0.85 volt cathodicprotection criterion.

Lowest Potential Shift = -0.031 VoltsObtained at Location No.1 (based on data intable)

Off Potential = -0.561

Required Voltage Shift =-0.850 - (-0.561) = -0.289 Volts

Step No. 2

Using the previously calculated values,calculate the required current using thefollowing relation:

Substituting the Values:

Step No. 3

Verify that, with an output of 3.73 amperes, apotential of -0.85 volts can be achieved at thepoint of lowest “OFF” potential obtained duringthe test.

Lowest “OFF” Potential = -0.523 Volts atLocation No.2

Potential Shift ()V) at That Location During

Test = -0.047 Volts

Potential Shift Required = -0.850 - (-0.523) =0.327 Volts

Total Current Required can be calculated asfollows:


ΔΔ

VV

II

or g

REQ

A

T1=

Lowest Voltage ShiftRequired Voltage Shift

Applied Current UsedCurrent Required (I )T1

=

−−

= ∴ =0.0470.327

0.40

I I 2.783 amperes

TT

Substituting the Values:

Step No. 4

The current required for achieving -0.850 voltsthroughout the tested section of the pipe is 3.73amperes.

There are several points that the designershould bear in mind when interpreting theresults of current requirement tests:

1. The results indicate current requirement onlyin the area where the temporary anode bedwas installed, and where the pipe-to-soildata indicated an influence from the anodebed.

2. If two or more anode beds are going to beinstalled for protection of an electricallycontinuous pipeline/structure, the effects ofthe anode beds are additive. Therefore, if ata point of the pipeline, 1 anode bed with 1ampere output achieves half of the requiredshift, and a second anode bed with 1ampere output at a different location alsoachieves half of the required shift at thesame location, the total current required is 2amperes and not 2 amperes per anode bed.

3. The tests do not take into account futurechanges which might necessitate morecurrent, such as additional deterioration ofthe coating.

In cases where the pipeline/structure has notbeen installed, or when current requirementtests are not feasible, current requirements canbe estimated by selecting a current density and

applying that current density to the pipelinesurface area.

There are many different tables that have beenpublished outl ining current densityrequirements. One such table is shown asTable 5-3, and was taken from the NACECORROSION ENGINEER’S REFERENCEBOOK.

As an example, if a proposed pipeline were tobe coated and buried in relatively neutral (notalkaline nor acidic) but corrosive soil (due tonon-homogeneous conditions), one mightselect 2 ma/ft2 as the appropriate currentdensity to establish the total currentrequirement. If the pipeline was anticipated tohave a 95% coating efficiency, then 2 mA wouldbe applied to the 5% of the pipeline that wasbare.

EXAMPLE:

A 24-inch diameter pipeline two miles in lengthis being installed with a 95% efficient coating.Determine the total current required to achievecathodic protection.

Step No. 1

Calculate the area to be protected using thefollowing formula:

AS = p A d A L A 0.05

Where:

AS = Total Surface Area (ft2)

d = Pipeline Diameter (ft)

L = Length of Pipeline (ft)

.05 = 5% of Pipe Requiring Protection(Bare Area)

p = 3.1415

AS = p A 2 A 5280 A 2 A 0.05

AS = 3317.5 ft2

TABLE 5-3

Approximate Current Requirements for Cathodic Protection of Steel

Environment mA/ft²

Sea Water - Cook Inlet 35 - 40

- North Sea 8 - 15

- Persian Gulf 7 - 10

- US - West Coast 7 - 8

- Gulf of Mexico 5 - 6

- Indonesia 5 - 6

Bare Steel in Soil 1 - 3

Poorly Coated Steel in Soil or Water 0.1

Well Coated Steel in Soil or Water 0.003

Very Well Coated Steel in Soil or Water 0.003 or less


Step No. 2

Calculate the current required based on a 2ma/ft2 current density as follows:

IT = Id x AS

Where:

IT = Total Current Required (Amperes)

Id = Current Density (mA/ft2)

AS = Total Surface Area (ft2)

IT = (.002)(3317.5)

IT = 6.64 Amperes

Regardless of the method that is used todetermine the current required to achieveprotection, it is wise to consider increasing thatvalue in the design stage for futurerequirements over the life of the system. Thisincrease may be necessitated by coatingdegradation or additions to the structure. Mostdesigners add 20% to cover theserequirements.

Determining the Number of Anodes

After obtaining all the field data required for thedesign of the anode bed, the next step is theselection of the type of anode to be used andthe calculation of the number of anodesrequired. The factors to be considered whencalculating the number of anodes are:

1. Anode Material

2. Rate of Consumption of Anode

3. Current Required for Protection

4. Shape and Size of Anode

5. Maximum Anode Material Density

6. Life Expectancy

Of the above items, Nos. 1, 2, 4, and 5 areinterdependent. The consumption rate andmaximum allowable density depend on theanode material as do the shape and size of theanodes.

The first step in this phase of the design should,in most instances, be the selection of the anodematerial. Items to be considered for thisselection are environmental conditions,consumption rate, and sizes available.

Although not a general problem with mostunderground structures, there could beinstances where the anode material could besubjected to acidic or alkaline soil environmentswhich could be detrimental to the anodematerial. Under these conditions, it would bewise to check with the anode manufacturers toinsure that the actual environment will notadversely affect the anode material. Anotherenvironmental factor that could affect thematerial selection is whether the anodes will beinstalled in soil or water, or a combination ofboth.

As mentioned in Chapter 3 of this course, thereare many different anode materials that couldbe used for impressed current systems. Evenscrap steel has been used for anode material.Scrap steel, however, has a high consumptionrate (approximately 20 lbs/amp-year). The mostcommonly used anode materials forconventional anode bed installations aregraphite and high-silicon chromium-bearingcast iron anodes. Normal operating densitiesrange between 0.2 and 1.0 amp/ft2 for graphitewith an average rate of consumption of 2.0lb/amp-year, and between 0.5 and 2.5 amp/ft2

for the high-silicon chromium-bearing cast ironanode with an average consumption rate of0.75 lb/amp-year. These consumption rates aremarkedly reduced if the anode is installed incoke breeze backfill. Other anode materials thathave been used for soil installations in recentyears are the platinized, mixed metal oxide andmagnetite anodes.

The next step in the selection of the anode is todetermine the sizes and shapes available. Forsoil installations, the 5-foot long anodes arenormally used, and they are available indiameters ranging from 1½" to 4" (graphite andhigh-silicon iron only). The selection of theanode size will depend on the desired anodeoutput and the required life expectancy.


Desired Life Weight x Utilization Factor

Req'd Current Output x Consumption Rate

or

Weight Consumption Rate x Desired Life x Req'd Current

Utilization Factor

0.75 x 20 x 15

0.60 375 lbs.

=

=

= =

Number of Anodes 37560

6.25 7 anodes= = =

Number of Anodes Required Current

Max. Output per Anode

152.5

6 anodes

=

= =

Another variable to consider is the use ofprepackaged canister anodes. These anodesconsist of graphite, high-silicon chromium-bearing cast iron or other suitable anodesencased in a steel stove pipe filled with a selectbackfill. As discussed in Chapter 3, “AnodeMaterials”, there are advantages anddisadvantages to the use of canister anodes.

Now that we have selected the anode materialand the type and size of the anode, we canproceed with calculating the number of anodesrequired.

EXAMPLE:

The current required to protect a pipeline,based on current requirement tests, is 15 ampsincluding spare capacity for future demand.High-silicon chromium-bearing cast iron anodes(2" x 60") are to be used in the anode beddesign. Required anode bed life expectancy is20 years.

Step No. 1

Calculate the number of anodes required basedon a consumption rate of 0.75 pounds per amp-year and a utilization factor (amount of anodematerial that can be lost to consumption beforeanode replacement is required) of 60%, and ananode weight of 60 lbs., using the following lifeexpectancy formula:

Step No. 2

Calculate the number of anodes required basedon a nominal current discharge per anode of2.5 amps per anode (recommended output bymanufacturer). Since the surface area of theseanodes is 2.8 ft2, the anodes would beoperating at a current discharge density ofapproximately 0.9 amp/ft2. This value fallswithin the range of 0.5 to 2.5 amp/ft2 mentionedearlier in this chapter as the output range forthis type of anode.

Based on the above calculations in Step 1, thenumber of anodes required is 7.

An alternative approach to the method outlinedabove for the determination of the number ofanodes required for a conventional anode bedis the “cut and try” method. With this method,an actual anode installation is made and testsare conducted to ascertain the degree ofprotection achieved. The number of anodes isincreased until the desired level of protection isreached. Calculations should then be made todetermine if the life expectancy would be metwith this method. The size of the rectifier can bedetermined from the actual field tests.

Calculating the Anode Resistance-to-Earth

The next step in the design process is thecalculation of the resistance-to-earth of theanodes. There are exceptions to this approach,such as in those cases in which the utility or thecompany has established a preferred range ofanode bed resistance. If this is the case, thenthe preferred anode bed resistance becomesthe primary factor in calculating the number ofanodes required for the installation.

There are two basic approaches to calculatingthe resistance of an anode bed:

1. By using existing formulas, and


R 0.00521

L ln

8 L

d1V =

⋅⋅

⋅−⎛

⎝⎜⎞⎠⎟

ρ

R 0.00521

N L ln

8 L

d1

2 L

Sln 0.656 NV =

⋅⋅

⋅⋅

− +⋅

⋅ ⋅⎛⎝⎜

⎞⎠⎟

ρ

R 0.00521

L ln

4L + 4L S L

dS

S

L

S L

L1H

2 2 2 2 2

=⋅

⋅+

+ −+

−⎛

⎝⎜⎜

⎞

⎠⎟⎟

ρ

R 0.00521

N L ln

8 L

d1

2 L

Sln 0.656 NV =

⋅⋅

⋅⋅

− +⋅

⋅ ⋅⎛⎝⎜

⎞⎠⎟

ρ

R 0.00521 10000

25 7 ln

8 7

0.6671

2 7

15ln 0.656 25V=

⋅⋅

⋅⋅

− +⋅

⋅ ⋅⎛⎝⎜

⎞⎠⎟

2. By using existing charts.

The anode bed resistance-to-earth can becalculated using one of the following formulas,as applicable, for the anode bed design:

Single Vertical Anode (H. B. Dwight formula)

Where:

RV = Resistance-to-earth of single verticalanode in ohms

ρ = Effective soil resistivity in ohm-cm

L = Anode length in feet

d = Anode diameter in feet

Multiple Vertical Anodes in Parallel (Erling D.Sunde Formula)

Where:

RV = Resistance-to-earth, in ohms, of thevertical anodes connected in parallel


L = Anode length in feet

N = Number of vertical anodes in parallel


S = Spacing between anodes in feet

Single Horizontal Anode (H. B. Dwight formula)

Where:

RH = Resistance-to-earth, in ohms, of thehorizontal anode


L = Horizontal anode length in feet

N = Number of vertical anodes in parallel


S = Twice anode depth in feet

NOTE: When using prepackaged anodes oranode backfills (such as coke breeze), thedimensions of the canister or the backfillcolumn must be substituted in the aboveformulas for the anode dimensions.

Using the anode bed/anode resistance-to-earthformulas listed above can be somewhatcomplicated and time consuming. Therefore,graphs have been developed such as the onesshown in Figures 5-3 and 5-4 to expedite theprocess of determining anode bed/anoderesistance-to-earth.

Figure 5-5 shows a typical vertical anodedesign chart for an impressed current anodebed. The chart was developed on 2" x 60"anodes in an 8" x 7' column of 50 ohm-cm cokebreeze backfill. The chart is based on anodesinstalled vertically in a straight line.

EXAMPLE:

The following example shows how both theformulas and the graphs can be used todetermine the anode bed resistance-to-earth.

A conventional anode bed consisting of twenty-five (25) 2" x 60" anodes is to be installed in an8" x 7' column of 50 ohm-cm coke breeze.Anodes are connected in parallel at 15'spacings. Soil resistivity is 10,000 ohm-cm.

Method No. 1 -- Using Sunde’s formula tocalculate the anode bed resistance-to-earth:

Substituting the values in the above formula:

5.0

6.0

7.0

8.0

9.0

10.0

ST

AN

CE

-TO

-EA

RT

H I

N O

HM

S R with ρ = 10000 ohm-cm

R with ρ = 8000 ohm-cm



RESISTANCE-TO-EARTH OF 3" X 60" ANODES IN A VERTICAL INSTALLATIONIN 8" X 7' COLUMN OF COKE BREEZE.

SPACING = 10 FEET.

ρ = 10000 ohm-cm

ρ = 8000 ohm-cm

ANODE BED RESISTANCE-TO-EARTHOF 8" x 7' ANODES AT 10 FT SPACING

AT VARIOUS SOIL RESISTIVITY VALUES

FIGURE 5-3

0.0

1.0

2.0

3.0

4.0

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

NUMBER OF ANODES

ρ = 6000 ohm-cm

ρ = 4000 ohm-cm

0.5

0.6

0.7

0.8

0.9

1.0

ST

AN

CE

-TO

-EA

RT

H I

N O

HM

S R at 10' Spacing

R at 15' Spacing

R at 20' Spacing

R at 25' Spacing


ρ = 1000 OHM-CM.

10' Spacing

15' Spacing

20' Spacing

ANODE BED RESISTANCE-TO-EARTHOF 8" x 7' ANODES IN 1000 OHM-CM SOIL

AT VARIOUS SPACINGS

FIGURE 5-4

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

NUMBER OF ANODES

25' Spacing

0.5

0.6

0.7

0.8

0.9

1.0

TA

NC

E-T

O-E

AR

TH

IN

OH

MS R at 10' Spacing

R at 15' Spacing

R at 20' Spacing

R at 25' Spacing

10' Spacing

15' Spacing

20' Spacing


ρ = 1000 OHM-CM.

USING A GRAPH TO DETERMINE ANODE BED RESISTANCE-TO-EARTH

FIGURE 5-5

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30

AN

OD

E B

ED

RE

SIS

T

NUMBER OF ANODES

25' Spacing

25 Anodes at 15' SpacingR = 0.18 ohms


R 100001000

0.18 1.8 ohmsV = ⋅ =

After making all the calculations, the result is:

RV = 1.8 ohms

Method No. 2 -- Using the graph in Figure 5-5to determine the anode bed resistance-to-earth:

Step No. 1:

Locate the number of anodes on the horizontalaxis of the graph (25) and draw a vertical lineup to the curve representing anodes at 15'spacing.

Step No. 2:

Draw a horizontal line from that point on thecurve to the vertical axis of the graph and readthe resistance which, in this case, is 0.18 ohmin 1,000 ohm-cm soil.

Step No. 3:

The resistance is directly proportional to theresistivity of the soil. Therefore, the resistancein 10,000 ohm-cm soil would be:

Calculating Total Circuit Resistance

The total circuit resistance includes thefollowing:

C Resistance-to-earth of the pipeline orstructure

C Resistance of the CP system cables

C Resistance-to-earth of the anodes (anodebed resistance)

The resistance-to-earth of the pipeline/structuremay be significant or negligible, depending onlength of pipe and soil resistivity. Using thevalues on Table 5-2 and assuming, forexample, that the current requirements listedare necessary to shift the pipe-to-soil potential

approximately 0.40 volt, then, applying Ohm’sLaw, the resistance-to-earth values for thoseconditions range from 0.0021 ohm (0.40/187.5 -negligible) to 57.14 ohms (0.40/0.007 - verysignificant). The mid-range current requirement(0.37 ampere) represents a resistance-to-earthvalue of 1.081 ohms (0.40/0.37) which might bea significant percentage of the total circuitresistance.

The last factor to calculate for the circuitresistance is the resistance of the positive andnegative cables. The first step would be toselect the proper size of the cable. The cablesize should be appropriate to carry the outputcurrent and to minimize the IR drop in the cabledue to the current flow. Once the size of thecable is determined, the total length of cable tobe used is multiplied by the per-foot resistanceof the cable selected.

The next step is to calculate the total circuitresistance using the following formula:

RT = RGbed + RC + RS

Where:

RT = Total circuit resistance

RGbed = Anode bed resistance (usingdimensions of carbonaceous backfillcolumn for calculations)

RC = Cable resistance

RS = Pipeline/structure-to-earth resistance

EXAMPLE:

Calculate the total circuit resistance of theconventional anode bed shown in Figure 5-6,using ten (10) 2" x 60" graphite anodes in 8" x7' columns of 50 ohm-cm coke breeze backfill.Soil resistivity is 8,000 ohm-cm. Currentrequired for protection is 15 amperes. The pipeto be protected is 3-mile long, 16-diametercarbon steel pipe. Assume a coating resistanceof 500,000 ohm-ft2.


R 0.00521

N L ln

8 Ld

12 LS

ln 0.656 N

R 3.08 ohms

Gbed

Gbed

=⋅

⋅⋅

⋅− +

⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

=

ρ

Step No. 1

Calculate RGbed:

Step No. 2

Calculate RC, based on the effective systemcable length and the resistance of the cable perlinear foot. The effective cable length of thesystem (referring to Figure 5-6) can bedetermined using the following formula:

Lcable = Lneg + Lpos1+ ½ Lpos2

Where:

Lcable = Effective cable length (ft)

Lneg = Negative return cable length (ft)

Lpos1 = Length of positive cable to first anode(ft)

Lpos2 = Length of positive cable between firstand last anodes (ft)

Lcable = 300 +16 + 135/2 =383.5 ft

The cable resistance (RC) can be calculatedusing the following formula:

RC = Rcable x Lcable

Where:

Rcable = Resistance per linear foot of cablebased on a standard cable resistancetable such as the one shown in Table5-4. In this example, No.4 AWG cableis being used with a resistance of0.2540 ohm per 1,000 ft.

RC = 383.5 x 0.2540/1000

RC = 0.0974 ohm

Step No. 3

Calculate RS for the 3 miles of coated 16-inchdiameter carbon steel pipe, assuming a coatingresistance of 500,000 ohm-ft2.

RS = R/AWhere:

R = Coating resistance (ohm-ft²) = 500,000ohm-ft²

A = Surface area of pipe (ft²) = 66,350 ft²

RS = 500,000/66,350 = 7.53 ohms

Step No. 4Calculate RT:

RT = RGbed + RC + RS

RT = 3.08 + 0.0974 + 7.53

RT = 10.71 ohms

Sizing the Rectifier or DC Power Supply

The final step in designing a conventionalanode bed system is sizing the DC output of thepower supply to be used. The following factorsmust have been determined in order to size theDC output of the power supply.

1. Total Circuit Resistance. As indicated inthe previous section, the total circuitresistance consists of the anode bed-to-soilresistance, the cable resistance and thepipeline (structure)-to-earth resistance.

2. Total Current Required. The total currentrequired to protect the pipeline/structurebased on current requirement tests. Asmentioned earlier, the test results arenormally increased by 20% or more toprovide for future requirement increases.

3. Back Voltage. The back voltage is thevoltage that exists in opposition to theapplied voltage between the anodes and theprotected structure. Anode bed anodes withcarbonaceous backfill will usually have aback voltage of about 2 volts. In areas of

*

!()*&(+(),- . /&

*

!'

'&&

#

'

*

TABLE 5-4

Concentric Stranded Single Conductor Copper Cable Parameters

SizeAWG

Overall Diameter NotIncluding Insulation

(Inches)

Maximum DCResistance @

20º C (Ohms/1000 ft)

Maximum AllowableDC Current Capacity

(Amperes)

14 0.0726 2.5800 15

12 0.0915 1.6200 20

10 0.1160 1.0200 30

8 0.1460 0.6400 45

6 0.1840 0.4030 65

4 0.2320 0.2540 85

3 0.2600 0.2010 100

2 0.2920 0.1590 115

1 0.3320 0.1260 130

1/0 0.3730 0.1000 150

2/0 0.4190 0.0795 175

3/0 0.4700 0.0631 200

4/0 0.5280 0.0500 230

250 MCM 0.5750 0.0423 255


unusual soil composition, the back voltagemay be slightly higher, but, for designpurposes, 2 volts should be used unlessexperience in a specific area dictatesotherwise.

Using these three factors, the minimum DCoutput of the power supply can bedetermined/calculated. The required currentoutput of the power supply is equal to the totalcurrent required plus a percentage for laterdeterioration. The DC output voltage can becalculated using the following formula:

V0 = IR x RT + Vb

Where:

V0 = Required Power Supply Voltage

IR = Total Current Required

RT = Total Circuit Resistance

Vb = Back Voltage

EXAMPLE:

Assume the pipeline in the previous examplehas a current requirement of 0.003 mA/ft² andcalculate the required power supply voltage

Step No. 1

Calculate total current required, plus 20% fordeterioration.

IR = Surface Area of Pipe x Current Density

IR = 66,350 ft2 x 0.003 mA/ft2

IR = 0.199 Amp

With 20% spare capacity IR = 0.239 Amp.

Step No. 2

Calculate required power supply voltage usingRT from previous example.

V0 = IR x RT + Vb

V0 = (0.239x10.71) + 2

V0 = 4.56 Volts

The required output rating as calculated is notwhat would be considered a standard off-the-shelf rating. The actual rating of the rectifier unitpurchased should be based, wheneverpossible, on standard available ratings. In thiscase, the closest rating might be 4 amps and 8volts.

DESIGN OF DISTRIBUTED ANODE BED

The distributed anode bed consists of anodesinstalled in close proximity to thepipeline/structure to be protected. Distributedanode beds are used primarily when it isdesired to protect a limited area of a pipeline, orto protect sections of a bare or coated pipelinein areas where electrical shielding precludeseffective protection with remote anode bedinstallations. Figure 5-7 shows a typicaldistributed anode cathodic protection system.

Many companies designed this type of anodebed using a combination of the current pickupby the structure and the gradient established inthe soil by the anodes. The current pickupmakes the potential of the pipeline/structureswing in the negative direction, while the anodegradient makes the soil surrounding thepipeline/structure swing in the positive direction.The result is an additive effect.

Figure 5-8 shows how one anode swings thepotential of the earth in the vicinity of astructure. The earth potential swing conceptinvolves having the protected structure interceptthe anodic gradient field surrounding thedistributed anodes, as shown in the figure. Thepart of the structure that is within the gradientfield of the anode is in soil which is electricallypositive with respect to remote earth. Thismeans that, within the anodic gradient field, thestructure is negative with respect to adjacentearth, even though it may not be made morenegative with respect to remote earth. Withinthe anodic gradient field, that part of thestructure closest to the anode is normally mostnegative with respect to adjacent earth. Figure5-9 shows how the gradient from the anodeinfluences the potential of the pipeline-to-earth.

,

!(

!#(

+

8

/

&+

&

&


R 0.00521

L ln

8 Ld

1V =⋅

⋅⋅

−⎛⎝⎜

⎞⎠⎟

ρ

In designing a distributed anode bed, thedesigner considered that each anode created agradient around itself and that the effect on thepipeline/structure is additive. Figure 5-10 showsthe additive effect of two adjacent anodes.

The design of a distributed anode bed is similarto that of the conventional anode bed as far asdetermining current requirements, calculatinganode bed resistance-to-earth and sizing theDC Output of the power supply.

DESIGN OF DEEP ANODE BED

When it is desirable to install an anode bedsystem which has many of the operatingcharacteristics of a conventional anode bed, butfor which there is no space available at thedesired site location, the only way to install theanodes at a distance from the pipeline/structureis to install them deep below thepipeline/structure.

Deep anode beds are also used when the soilstrata are so arranged that the upper layers ofsoil are of high resistivity with underlying layersof lower resistivity.

A deep anode bed is usually more expensive tobuild than a conventional anode bed ofcomparable capacity. In addition, the anodes ofa deep anode bed are well below grade leveland are inaccessible for maintenance shouldfailure occur in the anode material or in theconnecting cables. Although there arereplaceable deep anode bed systems on themarket, it may be more economical to abandona malfunctioning system in place and install anew anode bed close to the original one, ratherthan to attempt system repairs.

The design procedure for a deep anode bed is,for all practical purposes, the same as that fora conventional anode bed. The deep anode bedoperates on the same concept as theconventional anode bed, making the protectedpipeline/structure more negative with respect tothe surrounding soil, as illustrated in Figure5-11.

Graphite and high silicon-content cast ironanodes in coke breeze backfill are often usedfor deep anode beds (see Figure 5-12). Inrecent years, platinized anodes andmixed-metal oxide anodes in fluidized cokehave become available for this use.

Other designs include the use of a steel casingas the anode material itself, or in cases wherethe drilled hole stays open and the water tableis high enough, plain anodes without specialbackfill are installed inside the hole. In allcases, the upper portion of the installation(normally 50 to 100 feet) does not dischargecurrent in order to avoid the gradients in thearea of the pipeline/structure. This can beaccomplished by installing a plastic casing inthat section of the hole or by covering thatsection of the steel casing with a high dielectrictape/coating.

However, it should be noted that, whendesigning a deep anode bed, the anode bedresistance-to-earth should be calculated usingthe total length of the backfill column install asillustrated by the following example:

EXAMPLE:

A deep anode bed consists of four 2" x 60"high-silicon chromium-bearing cast iron anodeswith a column 40' long of coke breeze backfill inan 8" diameter hole. The effective soil resistivityat anode depth is 10,000 ohm-cm.

The deep anode bed resistance-to-earth can becalculated using H. B. Dwight’s formula for asingle vertical anode as follows:

Where:

RV = Resistance-to-earth of single anode

D = Effective soil resistivity = 10,000ohm-cm

L = Anode length = 40 ft

d = Anode diameter = 0.667 ft

%

&

&+

&

&

!

. /


R 0.00521 10000

40 ln

8 400.667

1

R 6.73 ohms

V

V

=⋅

⋅⋅

−⎛⎝⎜

⎞⎠⎟

=

Many deep anode beds use conventionalanodes with coke breeze backfill. One mainproblem that can develop with this type ofinstallation is gas blocking. Chlorine gasevolves from the anode as a result of thecurrent discharge. Gas blocking can render adeep anode bed useless. In order to eliminate,or at least reduce this problem, it is standardprocedure to install a vent pipe which allows forthe dissipation of the gas. Plastic materials notsubject to deterioration by chlorine gas shouldbe used for this vent pipe. Figure 5-12 showsthe details of a typical deep anode bed.

Other problems associated with deep anodebeds are:

1. Cable Insulation - Some types of cableinsulation, such as the HMWPE, areattacked by chlorine gas and ozone.Special insulation, such as Kynar® orHalar®, should be used.

2. The anode-lead connection is a verycritical point of the system, and stepsshould be taken to preclude later problems.

3. As the steel casing consumes, theresistance of the anode bed may increasebecause of the layer of iron oxide thatdevelops.

CONCLUSIONS

Many parameters must be considered whendesigning an impressed current cathodicprotection system. As previously discussed,these parameters include the following:

1. Current density required for protection,

2. Life expectancy of installation,

3. Anode bed layout,

4. Anode bed resistance, and

5. Rectifier output.

The corrosion control designer must be awareof these parameters to determine the mosteffective and economical system design.

There are basically three (3) different types ofanode bed designs:

1. Remote or conventional,

2. Distributed, and

3. Deep anode.

Each type of anode bed has its ownapplications, and it is up to corrosion controlpersonnel to ascertain which type would bemore suited for each installation beingconsidered.

DESIGN OF GALVANIC ANODE CATHODIC PROTECTION6-1

CHAPTER 6

DESIGN OF GALVANIC ANODE CATHODIC PROTECTION

INTRODUCTION

Galvanic anodes remain an important anduseful means for cathodic protection ofpipelines and other buried structures. Theapplication of cathodic protection utilizinggalvanic anodes is nothing more than theintentional creation of a galvanicelectrochemical cell in which two dissimilarmetals are electrically connected while buried ina common electrolyte. In the “dissimilar metal”cell, the metal higher in the electromotive series(or more “active”) becomes anodic to the lessactive metal and is consumed during theelectrochemical reaction. The less active metalreceives cathodic protection at its surface dueto the current flowing through the electrolytefrom the anodic metal. The design of a galvaniccathodic protection system involvesconsideration of all factors affecting the properselection of a suitable anode material and itsphysical dimension, placement, and method ofinstallation to achieve a sufficient level ofprotection on the structure.

GALVANIC ANODE APPLICATION

General Uses

Galvanic anode installations are normally usedfor cathodic protection applications whererelatively small amounts of protective currentsare required and the resistivity of the electrolyteis low enough to permit their use. If currentrequirements are such that a large number ofgalvanic anodes would be required, it may bemore economical to use an impressed currentsystem, if possible.

Specific Uses

The following are some specific uses forgalvanic anode cathodic protection systems:

1. For protecting structures where a source ofDC or AC power is not available.

2. For clearing interferences caused byimpressed current cathodic protectionsystems or other DC sources, where theinterferences are not of great magnitudes.

3. For protecting sections of well-coatedpipelines.

4. For providing hot-spot protection on barepipelines where complete cathodicprotection is impractical.

5. For supplementing impressed currentsystems where low spots or unprotectedareas remain on the structure due toshielding effects or other reasons.

6. For temporary protection during constructionof a pipeline until the impressed currentsystem is installed.

7. For AC mitigation.

G A L V A N I C A N O D E C A T H O D I CPROTECTION DESIGN PARAMETERS

Galvanic Anode Selection

The first step in designing a galvanic anodesystem is the selection of the anode material.Corrosion control personnel must consider the


following to determine the most economicalanode material to use:

1. Required amount of protective current

2. Total weight of each type of anode

3. Anode life calculations

4. Desired life of installation

5. Efficiency of anode types

6. Theoretical consumption rate

7. Driving potential

8. Soil or water resistivity

9. Cost of anode material

10. Shipping costs

11. Degree of skill required for installing thegalvanic anode system

Table 6-1 shows some of these parameters forseveral anode materials. The order of priority ofthe above factors will probably change for eachinstallation.

Anode Current Efficiency

Anode current efficiency is a measure of thepercentage of the total anode current outputwhich is available in a cathodic protectioncircuit. The remaining current is dissipated inthe self-corrosion of the anode material.

From Table 6-1 it should be noted that zinc hashigh current efficiency. Magnesium on the otherhand, is shown to have an efficiency ofapproximately 50 percent. Although thisefficiency figure is used quite commonly, it isnot a constant and can be even less at lowcurrent outputs.

Current Requirements

The determination of the amount of currentrequired to protect a structure is one of themost important design parameters. Currentrequirements for a structure can be determinedby conducting current requirement tests on thestructure. This method was discussed in detail

in the previous chapter for the design of theimpressed current system. These approachesare identical for the galvanic anode system.

Electrolyte Resistivity

The selection of the galvanic anode material isprimarily dependent upon the resistivity of thesurrounding electrolyte (soil). As discussed inChapter 3, the low driving potentials of thevarious galvanic anode material alloys limit theireconomical use to the lower resistivity soils.Zinc anodes are seldom used in soils withresistivity values greater than 1,500 ohm-cm.Magnesium anodes are generally used in soilswith resistivity values between 1,000 and10,000 ohm-cm. Ideally, the galvanic anodebed should be installed in the area of lowestresistivity. Soil resistivity surveys should beconducted as described in Chapter 5 of theBasic Course.

Total Circuit Resistance

Total circuit resistance depends upon thefollowing:

C Anode bed resistance - Rabed

C Resistance of interconnecting cables - Rckt

C Resistance to earth of the cathode(structure) - Rcathode

The electrical circuit equivalent of the currentoutput of a galvanic anode is depicted in Figure6-1.

It should be noted that as corrosion personneldevelop experience, they will be able todetermine the conditions under which some ofthe above factors can be considered negligible.

Anode Bed Resistance - (Rabed)

The resistance of the galvanic anode bed ismore critical than that of the impressed currentsystem due to the fact that the driving voltage ofthe anodes Is limited. The current output islimited by the potential difference between thestructure and the anode material.

TABLE 6-1

Typical Operating Characteristics of Galvanic Anodes

GalvanicAnode Material

Theoretical OutputCharacteristics

(amp-hr/lb)

Actual Output*Characteristics

(amp-hr/lb)

ConsumptionRates

(lb/amp-yr)

CurrentEfficiency

NegativePotential (Volts)

to CuCuSO4

Zinc(MiI-A-18001 U) 370 370 23.7 90% 1.10

Magnesium(H-1 Alloy) 1000 250 - 580 15 - 35 25 - 58% 1.40 - 1.60

Magnesium (High Potential) 1000 450 - 540 16 - 19 45 - 54% 1.70 - 1.80

* Based on shown current efficiencies.

!

"

#


R 0.00521

N L ln

8 Ld

12 LS

ln 0.656 Nabed =⋅

⋅⋅

⋅− +

⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

ρ

R 0.00521 1500

5.5 6 ln

8 5.50.417

12 5.5

15ln 0.656 6

R 1.10 ohms

abed

abed

=⋅

⋅⋅

⋅− +

⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

=

Anode bed resistance of a galvanic anodesystem can be calculated using the sameformulas used to determine the resistance ofthe impressed current anode bed.

Example:

Calculate the resistance of a galvanic anodebed consisting of six prepackaged 60 lb zincanodes (5" x 66" long) spaced 15' apart,center-to-center, average soil resistivity in thearea is 1,500 ohm-cm.

Where:

Rabed = resistance of the total number ofvertical anodes in parallel (ohms)

D = electrolyte resistivity (ohm-cm)

L = length of anode (feet)

d = anode diameter in feet

N = number of anodes in parallel

S = spacing between anodes in feet

Resistance of Interconnecting Cables (Rckt)

The resistance of the interconnecting cables(Rckt) is normally negligible in the design ofgalvanic anode cathodic protection systems.The IR drops in the cables are usuallyinsignificant because of the low current outputof this type of installation. When the resistanceof the interconnecting cables is to be includedin the overall system resistance, Rckt can becalculated for all practical purposes, using thefollowing formula:

Rckt = Resistance per ft of cable (Table 6-2) xLength of cable

Cathode-to-Earth Resistance (Rcathode)

The cathode-to-earth resistance is resistancebetween the structure and electrolyte.

On a coated pipeline this resistance can becalculated by first determining the effectivecoating resistance (Rc) (as discussed inChapter 2) and then using the followingformula:

Rcathode = Rc/n

Where:

Rcathode = resistance of pipeline to earth (ohms)

Rc = effective coating resistance - ohmsper average square foot of surfacearea (ohm-ft² )

n = the number of square feet of pipelinesurface area

Example:

Assuming a 10-mile section of 36" diameterpipe with a coating resistance of 50,000 ohms(Rc) for one average square foot. The pipe-to-earth resistance (Rcathode) can be calculated asfollows:

Step No. 1Calculate surface area of pipeline (n):

n = p A pipe diameter (ft) A length of pipe (ft)

n = 3.14 x (36/12) x (10 x 5280)

n = 497,376 sq. ft.

Step No. 2Calculate pipe-to-earth resistance:

Rcathode = Rc/n = 50,000/497,376

Rcathode = Rc/n = 0.10 ohm

Anode Life

Having arrived at an anode configuration thatwill produce the required current output is not

TABLE 6-2

Resistance of Concentric Stranded Copper Single Conductors

Size AWG Max. DC Resistance@ 20º C (ohms/1000 ft)

14 2.5800

12 1.6200

10 1.0100

8 0.6400

6 0.4030

4 0.2540

3 0.2010

2 0.1590

1 0.1260

1/0 0.1000

2/0 0.0795

3/0 0.0631

4/0 0.0500

250 MCM 0.0423


Years Life Th A E UF

Hr I=

× × ××

Years Life 0.114 * A E UF

I=

× × ×

Years Life 0.0424 * A E UF

I=

× × ×

Years Life 0.114 10 0.50 0.85

0.10 4.8 years=

× × ×=

Years Life 0.0424 10 0.90 0.85

0.10 3.2 years=

× × ×=

sufficient in itself. An examination of theestimated life of the anodes must beundertaken in order to determine whether thedesign will provide protection for a reasonableperiod of time. The following expression may beused to calculate the estimated life of theanode:

Where:

Th = Theoretical amp-hours/lb

A = Anode weight (lbs)

E = Current Efficiency expressed as adecimal

UF = Utilization Factor - typically 85 percentexpressed as a decimal (0.85) -- thepoint at which the anode should bereplaced even though the anode metalmay not be entirely consumed.

Hr = Hours per year

I = Current (amps)

The utilization factor accounts for a reduction inoutput as the surface area of the anodedecreases with time, limiting the anode output.This factor is usually assumed to be 0.85.

1. For magnesium:

2. For zinc:

* These constants are derived by dividing thetheoretical ampere-hours per pound by thenumber of hours in a year.

As an illustration, compare the lives of ten

pound anodes of magnesium and zinc, eachdischarging 0.1 amp. Assume 90% currentefficiency for zinc and 50% for magnesium.Each anode is assumed to require replacementwhen it is 85% consumed.

Using the above formulas, the life expectancieswork out as follows:

1. For magnesium:

2. For zinc:

The calculated life figures reflect the effect ofdifferent rates of consumption of the two metalsas well as the effect of current efficiency.

G A L V A N I C A N O D E D E S I G NCALCULATIONS

A well-coated pipe is 36 inches in diameter and2,500 feet long. The average soil resistivity inthe area is 1,000 ohm-cm. The desired systemlife expectancy is 20 years.

Step No. 1

Choose the anode material. Based on the factthat magnesium has a higher amp-hr/lbcharacteristic and lower consumption rate thanzinc (see Table 6-1) magnesium is selected.

Step No. 2

Calculate the surface area (n) to be protected:


n = 3.14 x (36/12) x (2500)

n = 23,550 ft2


Years Life 0.114 W E UF

Iprotect=

× × ×

20 0.114 W 0.50 0.85

0.283=

× × ×

R 0.00521

N L ln

8 Ld

12 LS

ln 0.656 Nabed =⋅

⋅⋅

⋅− +

⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

ρ

R 0.00521 1000

7 2 ln

8 20.625

12 215

ln 0.656 7abed =⋅

⋅⋅

⋅− +

⋅⋅ ⋅⎛

⎝⎜⎞⎠⎟

Step No. 3

Calculate current requirements. The anode bedshould be designed to achieve a satisfactoryamount of polarization. After polarizationgalvanic anodes tend to self-regulate and thecurrent output at the anode bed will declinewhile protection is maintained. As a rule ofthumb, the amount of current required topolarize the pipe is four times the amount ofcurrent required to maintain protection.

a. Current required to maintain protection(lprotect) on the pipe can be calculated asfollows, assuming a current requirement of0.003 mA/ft2, typical for a well coated,electrically isolated pipe. This is based on1.5 mA/ft2 of exposed metal and on 0.2%coating damage.

lprotect = 0.003 mA/ft2 x surface area

lprotect = 0.003 mA/ft2 x 23,550 ft2

lprotect = 71 mA = 0.071 Amperes

b. The current required to polarize (Ipolarize) thepipe, by rule of thumb is based on a currentrequirement of 0.012 mA/ft2 (four timescurrent required for protection). This alsoapproximated the current required shouldcoating deteriorate to a total of 0.8%damage.

lprotect = 0.012 mA/ft2 x surface area

lprotect = 0.012 mA/ft2 x 23,550 ft2

lprotect = 283 mA = 0.283 Amperes

Step No. 4

Determine the minimum amount of anodematerial (W) required to provide an anode bedlife of 20 years, based on the current requiredto polarize the pipe.

W = 117 lbs of magnesium

Based upon the above calculated anodematerial weight requirement, it appears that 7 -17 lb anodes (4" x 4" x 17"), with backfill outerdimensions of 7.5" x 24", will provide sufficientanode material for the desired life.

Step No. 5

Calculate the anode bed resistance (Rabed) foran anode bank assuming center-to-centeranode spacing of 15 feet.

Rabed = 0.99 ohm

Step No. 6

Calculate the cathode to earth resistance(Rcathode) assuming an effective coatingresistance (Rc) of 15,000 ohm-ft2.

Rcathode = Rc/surface area

Rcathode = 15,000 ohm-ft2/23,550 ft2

Rcathode = 0.64 ohm

Step No. 7

Calculate the connecting cable resistance (Rckt)assuming the use of approximately 105 feet ofNo. 6 AWG wire.

Rckt = Resistance of wire x length

Rckt = 0.403 ohm/1000 feet x 105 ft

Rckt = 0.042 ohm

Step No. 8

Calculate anode bed current output (Iabed).Assume the anode potential to be -1.55 voltsand you want to polarize the pipe to -1.0 volt.


I Anode Potential Cathode Potential

R R Rabed

abed cathode ckt=

−+ +

I 1.55 1.0

0.99 0.64 0.042 0.329 Ampereabed =

−+ +

=

i 120000 f y

m =⋅ ⋅

ρi 150000 f y

m =⋅ ⋅

ρ

i 50000 f y

z =⋅ ⋅

ρ i 40000 f y

z =⋅ ⋅

ρ

The anode bed current output (Iabed) calculatedabove exceeds the previously calculatedcurrent values required to polarize and protectthe structure, therefore the design will beeffective in providing cathodic protection. Oncethe pipe polarizes (usually within a few months),the anode current will drop to the protectivelevel of about 0.071 amperes, thus furtherextending anode life. Also, using the polarizingcurrent in these calculations allows for coatingdeterioration over time.

Figure 6-2 shows a detail of a typical systemlayout for this design.

Simplified Calculations

From the previous example, we can see that itcan be rather time consuming to calculate thevarious resistive factors required to design agalvanic anode bed, and often certainassumptions must be made that result in anapproximate current output calculation. Theoutput of magnesium and zinc anodes hasbeen fairly well documented under varyingconditions, and many graphs, tables, and chartshave been developed. These references can beused by corrosion control personnel to providea simplified and reasonably accurate means ofdetermining anode current outputs undernormal operating conditions. One suchreference, widely used, was developed by D. A.Tefankjian(1). The following equations can beused to determine the current output formagnesium or zinc anodes, utilizing correctionfactors provided in Tables 6-3 and 6-4.

Bare or Poor Coating Good coating

Where:

im = current output for magnesium anode inmilliamperes

iz = current output for zinc anode inmilliamperes

D = soil resistivity in ohm-cm

f = anode shape factor from Table 6-3

y = driving voltage factor from Table 6-4

The equations assume a minimum soilresistivity of 500 ohm-cm and a distancebetween anode and structure of 10 feet. Theequations and factors were developed basedon bare or poorly coated structures. Because ofthe increased circuit resistance occurring withwell coated structures, the calculated currentvalue should be reduced by 20 percent. Thiscan be done by multiplying i as shown above,by 0.80.

Example

Calculate the number of anodes required toprotect 10,000 feet of well coated, electricallyisolated 16" pipe at a pipe-to-soil potential of0.85 volts in a soil of 2,500 ohm-cm. Assume acurrent requirement of 1.5 mA/exposed squarefoot of metal and coating damage of 1%. Use32 lb pre-packaged standard alloy magnesiumanodes and design for a 20 year anode life.

Step 1

Calculate the surface area (n) to be protected.


n = p A (16/12) A 10000 = 41,888 ft2

Surface area to be protected = 41,888 x 1%

= 419 ft2

Step 2

Calculate the current required:

419 ft2 x 1.5 mA/ft2 = 629 mA

$

%

## &$'(

)

$$

)

$

$&$'(

$$

%*+*

,*

TABLE 6-3

Anode Shape Factors (f)

Anode Weight (lbs) Anode Factor (f)

Standard Anodes

3 (Packaged) 0.53

5 (Packaged) 0.60

9 (Packaged) 0.71

17 (Packaged) 1.00

32 (Packaged) 1.06

50 (Packaged - anode dimension 8" dia x 16") 1.09

50 (Packaged - anode dimension 5" x 5" x 31") 1.29

Long Anodes

9 2.75" x 2.75" x 26" backfill 6" x 31" 1.01

10 1.50" x 4.50" x 72" backfill 4" x 78" 1.71

18 2.00" x 2.00" x 72" backfill 5" x 78" 1.81

20 2.50" x 2.50" x 60" backfill 5" x 66" 1.60

40 3.75" x 3.75" x 60" backfill 6.5" x 66" 1.72

42 3.00" x 3.00" x 72" backfill 6" x 78" 1.90

Extra-Long Anodes

15 1.6" dia x 10' backfilled to 6" dia 2.61



TABLE 6-4

Driving Voltage Correction Factors (Y)

P/S Standard Magnesium High-Potential Magnesium Zinc

-0.70 1.21 2.14 1.60

-0.80 1.07 1.36 1.20

-0.85 1.00 1.29 1.00

-0.90 0.93 1.21 0.80

-1.00 0.79 1.07 0.40

-1.10 0.64 0.93 --

-1.20 0.50 0.79 --

TABLE 6-5

Multiple Anode Adjustment Factors

No of Anodesin parallel

Adjustment Factors(anode spacing in feet)

5' 10' 15' 20'

2 1.839 1.920 1.946 1.965

3 2.455 2.705 2.795 2.848

4 3.036 3.455 3.625 3.714

5 3.589 4.188 4.429 4.563

6 4.125 4.902 5.223 5.411

7 4.652 5.598 6.000 6.232

8 5.152 6.277 6.768 7.035

9 5.670 6.964 7.536 7.876

10 6.161 7.643 8.304 8.679


i 120000 f y

i 120000 1.06 1.00

2500m m=

⋅ ⋅⇒ =

⋅ ⋅ρ

Number current requirement

anode output

Number 629 mA

50.9 mA / Anode

Number 12.4 or 13 anodes

=

=

=

Life 0.114 W E UF

Current (amps)

Life 0.114 32 0.50 0.85

0.0509

=× × ×

=× × ×

Step 3

Calculate the anode current output: (im)

im = 50.9 mA (0.0509 amps)

Step 4

Calculate the number of anodes required:

Anodes could be spaced evenly along the lineat intervals of 10,000/13 or about every 769feet.

Step 5

Calculate anode life:

Life = 30.5 years

The twenty year design life is satisfied.

Other Factors

The calculated result assumes an evendistribution of current over the calculated lengthof pipe, which is not realistic. The greatest is inclose proximity to the anode and decreaseswith distance. Therefore, assuming uniform

coating and soil, a greater amount of current isavailable to the pipe closest to the anode. Theattenuation of current is more pronounced ascoating quality decreases, so an individualgalvanic anode protecting a “hot spot” on a barepipe will protect a much smaller area of pipethan that indicated above. For a series ofindividually connected anodes, the potentialincrease on the pipe that results betweenadjacent anodes, is additive.

Anodes are commonly connected in parallel inorder to achieve higher current output at agiven location. The approximate current outputof a group of anodes can be calculated bymultiplying the calculated current output of asingle anode by the appropriate adjusting factorindicated in Table 6-5.

Example

The current output (It) of six 32 lb pre-packagedstandard alloy magnesium anodes connected inparallel can be calculated as follows, based ona current output of 50.88 mA for an individualanode. Assume an anode spacing of 10 feet.

It = 50.88 mA x Adjusting Factor (Table 6-5)

It = 50.88 mA x 4.902

It = 249.41 mA

SPECIFICATION AND MAINTENANCE OFGALVANIC ANODE INSTALLATIONS

Specifications must be written to insure that thematerials and methods, upon which the designhas been predicated, are utilized. There shouldbe no deviation from these specificationsunless approved by the person in charge ofdesign. Inspection should be performed duringsystem installation to guarantee that thespecification of material and procedures areadhered to.

The maintenance of galvanic anode systemsconsists of routine surveys and, whennecessary, repairs/replacement. A survey todetermine the operating levels of the systemshould be conducted annually and the data


recorded in a permanent log.

Current measurements will enable the life of theanode to be predicted, and their timelyreplacement scheduled accordingly. Additionalanodes should be installed where low potentialswarrant their installation. Damaged facilitiesshould be repaired to ensure that adequatelevels of protection are maintained.

CONCLUSIONS

As was the case with the design of theimpressed current cathodic protection system,the design of the galvanic anode systemwarrants the consideration of various factors.These factors include the following:

1. Soil resistivity

2. Current Requirements

3. Anode Material Selection

4. Life Expectancy

The resistivity of the soil and the resistance toearth of the structure being protected have agreat impact on design and systemperformance when using galvanic anodes. Thisis due to the fact that the driving potential of thesystem is limited to the natural potentialdifference between the anode material and thestructure being protected.

Reference

1. Tefankjian, D.A., 1974. “Application ofCathodic Protection”. Proceedings of the 19th

Annual Appalachian Underground CorrosionShort Course, pp 122-138.

MIC INSPECTION AND TESTING7-1

CHAPTER 7

MIC INSPECTION AND TESTING

INTRODUCTION

Microbiologically induced corrosion (MIC) hasbeen actively studied in recent years becauseof its potentially deleterious effect on importantunderground structures, such as pipelines. Themost well known bacteria associated withpipeline corrosion are the sulfate reducingbacteria (SRB). These bacterial are active indeaerated environments often associated withwet, poorly drained soils (that promote oxygenexclusion), require a nutrient that is near thepipe and often the carbonaceous material of thecoating, and have a sulfurous (rotten eggs)odor associated with the sulfate reductionprocess. If conditions change, they maybecome dormant and another type of bacteriamay become active. Because bacterial live incommunities, there is also a synergistic effectamong them whereby the reaction products ofone may act as a nutrient for another. Thus, itis important to test for more than one type ofbacteria. However, it is important to recognizealso that the presence of bacteria does notnecessarily mean that they are active or thatthey pose a threat to the integrity of thestructure.

Research studies have shown that MIC can becharacterized by the chemical, biological, andmetallurgical features associated with thesuspected site. The type and form of pittingcorrosion is often distinctively associated withMIC. Chemical and biological data aresupportive, and are useful for estimatingwhether a potential for MIC exists.

The following sections give a procedure forinspecting the dig site, assessing the coating

and steel at coating damage and suspectedMIC areas; sampling the soil, groundwater, orpipe-surface products for MIC testing; carryingout the MIC testing using commerciallyavailable field test kits; and visually inspectingthe corroded areas for MIC characteristics.

SITE INSPECTION

Before excavation of the pipeline, thetopography of the surrounding area of the digand the soil type should be noted. Thisinformation is useful in assessing the likelyconditions that can support various types ofbacteria. For example, low, wet areas with claysoils collect and hold water, and tend toexclude oxygen to promote deaeratedconditions. Soils with more sand or rock allowwater and/or air to migrate more readily to thepipe and promote more aerated conditions.Higher elevations with rocky soils tend to bebetter drained and drier, and tend to promotemore aerated conditions.

Exposure of the pipe should be slow andcareful to avoid damage to the pipe and areasof sampling. Remove soil from the pipe surfacecarefully to expose the coating surface. Do notremove any products adhering to the coatingsurface. Note and record the following in anappropriate form recommended by KMEP:

• Coating type (e.g., coal tar, asphalt,bitumen, tape), type of damage (e.g.,disbonding, holidays, blistering, seamtenting, cracking, and wrinkling), extent ofdamage (% of exposed area), and location(circumferential and longitudinal position onpipe in relation to weld seams and coating


seam, if present)

• Corrosion and/or cathodic protection surfaceproducts present

N Location (see above)

N Type (e.g., deposit, nodule, or films)

N Color (e.g., brown, black, white, or gray)

N Smell (e.g., none, earth, rotten eggs)

• Soil type (e.g., sand, gravel, silt, clay, rock)

• Soil moisture (e.g., wet, dry).

Do not remove any coating or surface productsat this time. Only the person performing thefield test should remove coating or productsfrom the pipe surface at his discretion untiltesting is complete.

COATING INSPECTION

Coating inspection for MIC testing purposesshould precede any other type of coatingevaluation planned.

Identify likely area of MIC or other forms ofcorrosion from the visual inspection above.Carefully remove the damaged coating using aclean knife to expose the steel beneath. If aliquid is present, take a sample using a syringeor cotton swab following procedures to bedescribed below for testing purposes. If thearea is not to be tested for MIC, the steelsurface condition and liquid pH should beevaluated.

Visually inspect the steel surface for corrosion.Where possible, use a gauge to measure thedepth of corrosion. Also, determine the lengthof the corroded area in relation to thecircumferential and longitudinal position.

The pH of the liquid may be tested usinghydrion paper or its equivalent. Carefully slicethe coating to a length to allow the test paper tobe slipped behind the coating. Press the

coating against the pH paper for a few secondsand remove, then remove the pH paper. Noteand record the color of the paper in relation tothe chart provided with the paper. Determinethe pH of groundwater away from the pipe inthe ditch if possible for reference. Compare thetwo pH values to determine if the pH near thepipe is elevated. An elevated pH indicates thepresence of cathodic protection currentreaching the pipe. A pH above about 9 wouldbe considered elevated for most soils. It is notuncommon to determine a pH of 12 to 14 forwell protected steel.

MIC AND DEPOSIT SAMPLING

Once the pipe is exposed, the soil and anysuspected deposits must be sampled andtested immediately. The coating around thesuspected area of corrosion should be carefullyremoved using a knife or similar instrument.Sample contamination must be kept to aminimum. Therefore, avoid touching the soil,corrosion product, or film with hands or toolsother than those to be used in sample collectionand provided with the test kits.

MIC SAMPLING AND TESTING

A minimum of three samples should becollected and tested. Two samples should betaken from one or more of the followinglocations:

• Undisturbed soil immediately next to theexposed pipe steel surface or at an area ofcoating damage

• A deposit associated with visual evidence ofpipe corrosion

• A scale or biofilm on the steel surface or thebackside of the coating

• Liquid trapped behind the coating.

The third sample should be taken from fresh,undisturbed soil at pipe depth, at least 1 mtransverse to the pipe. This location, such asthe ditch wall, will act as a reference from which


to determine if the bacteria count near the pipeis elevated.

Additional samples may be taken and testedfrom other locations where bacterial activity issuspected.

Collect the soil, film, or liquid sample followingthe detailed procedures outlined in the MlCkit IIIor V. Inoculate the vials at the dig site. Mark theside of each vial with the dilution number (1 to5 for MlCkit III and 1 to 4 for MlCkit V), as wellas the tray next to it using the permanentmarker supplied. Each tray should be marked toidentify the following:

• Date and time sample collected• Location where the sample was taken• Location of the dig site.

This same information must be put on theoriginal box in which the sample tray is to bestored. The box of samples should be stored atambient temperature with the box top closed; itis not necessary to incubate the samples atelevated temperature. All unused test kitsshould be stored in a refrigerator to prolongshelf life.

After one (1), two (2), and five (5) days, viewthe sample bottles and record the result. Usethe rating number (1 through 5) correspondingto the highest bottle number to give a positiveresult. Also, record the number of the type ofresponse, such as 2-Cloudy, shown on thePositive Reaction Sheet supplied with theMlCkit III. At the end of 15 days, record the finalresults. Typically, the results are unchangedafter about two to five days of incubation.

Each MlCkit III (or MlCkit V) tests for thepresence and count (bacterium/ml) of four (five)types of viable bacteria. The bacteria tested inthe MlCkit III are

• Sulfate reducing bacterial (BlO-SRB)• Acid producing bacteria (RIO-APB)• Aerobic bacteria (BIO-AERO)• Anaerobic facultative bacteria (BlO-THIO)

and the bacteria tested in the MlCkit V are

• Sulfate reducing bacterial (BlO-SRB)• Acid producing bacteria (BlO-APB)• Aerobic bacteria (BlO-AERO)• Iron-related (depositing) bacteria (BlO-IRB)• Low-nutrient bacteria (BlO-LNB).

The presence and viability of these four (or five)types of bacteria are tested in each of the four(or five) colored strings of bottles in each tray.The bottles in each string contain a fast-actingnutrient specific to the bacterium tested. ForMlCkit III, the tests give a quantitative indicationof the viability of each bacterium from 10 to 105

bacterium/ml by the serial dilution of the sampleand subsequent growth in each string; theMlCkit V gives a quantitative indication from 10to 104 bacterium/ml.

SUPPORTING ANALYSES

Corrosion and other types of deposits on thepipe may be analyzed in the field using theMlCkit IV to assist in interpretation of MIC andother corrosion, or cathodic protection products.This kit can be used to qualitatively analyze forthe presence of carbonate (CO3

+2), sulfide (S-2),ferrous iron (Fe+2), ferric iron (Fe+3), calcium(Ca+2), and hydrogen (H+1, pH) ions.

Collect a sample of soil, deposit, film, or liquidfrom the area of interest. Use only a clean knifeor spatula provided with the test kit. The films ordeposits may be from the steel surface, coatingsurface, interior of a corrosion pit, or thebackside of the coating. In all cases, note thecolor and type of sample. Carefully transfer thesample to the test kit vial for testing. Follow thedetailed procedure given in the MlCkit IVinstruction sheets. For comparison purposes,obtain a reference sample taken at least 1 mfrom the previous collection site.

Record the results of the analysis on theResults Sheet provided with the kit or in acompany designated format. Determine thelikely type of deposit according to thecharacteristics shown below. A corrosion filmother than a carbonate, an oxide for example,


may only show the presence of ferric or ferrousion, but with a pH that is similar to that of thereference sample or lower. In addition, cathodicprotection films may not necessarily show thepresence of calcium or carbonate, but will havean elevated pH compared with the referencesample. For most types of soils, the pH isgreater than about nine (9). An elevated pH isnot usually associated with the presence ofSRB.

PositiveResult

SRB CorrosionFilm

CorrosionOxide

CPFilm

CO3 -2 Yes Yes

S-2 Yes

Fe+2 Yes Yes Yes

Fe+3 Yes Yes

Ca+2 Yes

pH Elevated

METALLURGICAL INSPECTION

The form of the corrosion pits associated withMIC is reasonably distinctive. These featurescan be observed in the field with the unaidedeye or a low power microscope.

After any films or products sampled above havebeen obtained from a corroded area, theremaining product should be removed using aclean spatula or knife, being careful not toscratch the metal. Clean any remaining materialwith a clean, dry, stiff brush, such asnylon-bristle brush. Do not use a metal brush ifpossible, because the metal bristles will mar thepit features. If not all of the product can beremoved with this method, use a brass bristlebrush in the longitudinal direction only. Dry thearea with an air blast or an alcohol swab. Ashiny metallic surface of the pit suggests thepossibility of active corrosion. However,judgment must be used to differentiate thiscondition from one created by scraping thesteel surface with a metallic object, such as theknife or spatula use to clean the surface or

obtain the sample product.

Examine the corroded area newly cleaned firstvisually with the unaided eye. Then use a lowpower magnifying lens at 5X to 50X power toexamine the detail of the corrosion pits. MICoften has the following features:

• Large craters up to 2 or 3 inches (5 to 8 cm)in diameter or more

• Cup-type hemispherical pits on the pipesurface or in the craters

• Some times the craters or pits aresurrounded by uncorroded metal

• Striations or contour lines in the pits orcraters running parallel to longitudinal pipeaxis (rolling direction)

• Sometime tunnels can be seen at the endsof the craters also running parallel to thelongitudinal axis of the pipe.

Some examples of these features are shown inother materials. The cupping action of the pitsis a result of the local action of the bacteria.The striations follow the rolling direction of thesteel made into the pipe and are believed to bea result of preferential attack of the steelmicrostructural components.

ACKNOWLEDGMENT

This chapter was written by K. C. Garrity andwas edited by the AUCSC CurriculumCommittee.

MIC OF 304 SS WELD Under Tubercle Illustrated

From Little, Wagner,and Mansfeld

TUBERCLE BUILD-UP WITH IRON-RELATED BACTERIA

Fe-Related Bacteria

Aerated

ChlorideDeaerated

Low pH

IRB Create a Differential Oxygen Corrosion Cell with Low pH Environment

METALLURGICALPope and GRI

MICROBIOLOGICALAerobic and Anaerobic

• Aerobic Require O2and Nutrient

• Consume O2 and Produce Organic Acids Corrosive to Steel

• Protection Requirements Less Well Defined

• Occur with Anaerobic Bacteria

• Form Complex Communities Dominated by Acid Producing Bacteria

• MIC a Result of Microbe Community, Not SRB Alone

MICROBIOLOGICAL COMMUNITYExample APB and SRB

• Organic Nutrients Feed APB

• APB Products Nutrients for SRB

• SRB Fed by Organic Acids and Sulfate

• Produce more APB and SRB

• Chlorides with Acids (H+) Lower pH and Corrode Steel

APB

Organic Acids(Lactic, formic, acetic, etc)

SRB Sulfate

APB APB

SRB

H2S CO2Acids

Organic Nutrients

MICROBIOLOGICALSRB Theory (Anaerobic)

• 8H2O 8OH-+8H+

• 4Fe 4Fe+2+8e- (A)• 8H++8e- 8H (C)• SO4

-2+8H +Bacteria S-2+4H2O

• Fe+2+S-2 FeS (A)• 3Fe+2+6OH- 3Fe(OH)2

(A)• FeS Depolarizes

• Need Nutrient– Soil biomass– Coating adhesive

• Need No or Low O2– Wet soil– Low areas– Crevices

• More Protection (Current) Needed to Overcome Effects

MIC KIT III OR V TESTING

• Field Test Kits for Viable Bacteria– SRB, APB, anaerobic, aerobic, iron related– Bacteria count—10 to 105 (104) colonies/ml

• Test Soil or Surface Product• Create Slurry• Inoculate Culture Media Vials• Compare After 2 - 5 Days, 15 Days

MIC KIT INTERPRETATION

• Presence of Bacteria Not Conclusive of MIC

• SRB Often Do Not Dominate• Not Uncommon to Find All Tested Bacteria

Present• Interpret Indications with Caution

– 1 to 3 bottles – may have problem– 1 to 5 bottles – possible problem

MIC KIT III CULTURE TESTResults

MIC KIT III OR VAerobic Bacteria Test

MIC KIT III OR VAPB Bacteria Test

Documents

C:AUCSCAUCSC BIA Edits 2009Advanced Chapter 3 Tables 2009