Appalachian Underground Corrosion Short Course … UNDERGROUND CORROSION SHORT COURSE BASIC COURSE ... Alternating current electricity is that which flows first in …

Basic Course

Appalachian Underground Corrosion Short CourseWest Virginia UniversityMorgantown, West Virginia

Copyright © 2011

APPALACHIAN UNDERGROUND CORROSION SHORT COURSEBASIC COURSE

CHAPTER 1 - BASIC ELECTRICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

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

ELECTRICAL FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Physical Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

The Two General Types of Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

BASIC TERMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

OHM'S LAW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

THE BASIC ELECTRICAL CIRCUIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

THE SERIES ELECTRICAL CIRCUIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8

THE PARALLEL ELECTRICAL CIRCUIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9

COMBINATION CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

CHAPTER 2 - CORROSION FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

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

FUNDAMENTAL CORROSION CELL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

TYPES OF NATURAL CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

Dissimilar Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

Dissimilar Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

Dissimilar Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

Differential Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

Stress Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5

Cast Iron Graphitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5

Microbiologically Influenced Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Combinations of Corrosion Cell Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Amphoteric Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

STRAY CURRENT CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Definition of Stray Current Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Differences Between Stray Current Corrosion and Natural Corrosion . . . . . . . 2-7

Severity of Stray Current Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Why Stray Current Occurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Alternating Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Types of Stray Current Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Sources of Stray Current Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Typical Static Stray Current Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

HVDC Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

Typical Dynamic Stray Current Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10

DC Transit Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10

DC Mining Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10

DC Welding Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

FACTORS AFFECTING THE RATE OF CORROSION . . . . . . . . . . . . . . . . . . . 2-11

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11

Electrolyte Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12

Voltage Difference Between Anode and Cathode . . . . . . . . . . . . . . . . . . . . . 2-12

Anode/Cathode Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12

Effect of the Metal Itself . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12

Passivation of the Metal Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-13

CHAPTER 3 - CORROSION CONTROL METHODS . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

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

SUMMARY OF CORROSION CONTROL METHODS . . . . . . . . . . . . . . . . . . . . . 3-1

COATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Theory of Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

How Cathodic Protection Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Types of Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

Galvanic (or Sacrificial) Anode Cathodic Protection . . . . . . . . . . . . . . . . . . . . . 3-5

Impressed Current Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

ISOLATING JOINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

DRAINAGE BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

REVERSE CURRENT SWITCHES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

FORCED DRAINAGE BONDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

CHAPTER 4 - INTRODUCTION TO PIPELINE COATINGS . . . . . . . . . . . . . . . . . . . 4-1

CHAPTER 5 - POTENTIAL MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

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

INSTRUMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Voltmeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Effect of Voltmeter Resistance on Potential Being Measured . . . . . . . . . . . . . 5-1

Care and Storage of Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

REFERENCE ELECTRODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

Electrode-to-earth Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

STRUCTURE-TO-EARTH POTENTIAL MEASUREMENTS . . . . . . . . . . . . . . . . . 5-6

CELL-TO-CELL POTENTIAL MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . 5-8

STRUCTURE-TO-STRUCTURE POTENTIAL MEASUREMENTS . . . . . . . . . . . . 5-9

POTENTIAL SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10

POLARIZATION EFFECTS ON POTENTIAL . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12

CRITERIA FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

MONITORING CATHODIC PROTECTION SYSTEMS . . . . . . . . . . . . . . . . . . . . 5-14

CHAPTER 6 - RESISTANCE MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

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

SIMPLE RESISTANCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

NON-ISOLATED RESISTANCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Resistance of Isolating Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Resistance-to-Earth of Anode Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Resistance-to-Earth of a Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

SOIL RESISTIVITY MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Soil Box Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

Single Rod Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

4-Pin (Wenner) Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

CHAPTER 7 - CURRENT FLOW MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . 7-1

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

DETERMINING CURRENT FLOW IN PIPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

Use of Pipe Resistance Calculations with 2-wire Pipe Span Test Points . . . . 7-1

Current Flow Measurements Using 4-wire Pipe Span Test Points . . . . . . . . . 7-3

CURRENT SURVEYS ON PIPELINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

CLAMP-ON AMMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

CHAPTER 8 - RECORD KEEPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

GENERAL CHARACTERISTICS OF RECORDS . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

Management Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

TYPES OF RECORDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3

Physical Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3

Corrosion Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4

Corrosion Control Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4

PHMSA REQUIREMENTS - FEDERAL AND STATE . . . . . . . . . . . . . . . . . . . . . . 8-5

COMPUTERIZED RECORDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8

FIELD DATA SHEETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9

2011 Revision

To submit comments, corrections, etc. for this text, please email: [email protected]

BASIC ELECTRICITY1-1

CHAPTER 1

BASIC ELECTRICITY

INTRODUCTION

In this chapter, we will be discussing:

S the kinds of electricity encountered incorrosion control work

S explanations of various applicableelectrical terms

S how the electrical units represented bythese terms interact

S how the electrical units apply to varioustypes of electrical circuits.

It is important that the principles set forth in thischapter be thoroughly understood beforeproceeding to the material included in theremaining chapters of the Basic Course.Similarly, a complete knowledge of this chapterwill be at least equally as important to anunderstanding of the subject matter included inthe Intermediate and Advanced Courses.

The treatment of the subject material in thischapter is intended to be such that a goodpractical understanding will result. This in turnpermits a better understanding of the logic andmechanics of interpreting, evaluating andsolving corrosion problems when encounteredin the field.

ELECTRICAL FUNDAMENTALS

Physical Matter

Electricity is directly involved with the makeupof physical matter. Although we will not spenda great deal of time on atomic structure ofmatter, we need to explore the subject

sufficiently to establish the stated relationshipbetween matter and electricity.

What is “Matter”

"Matter" is that which makes up the substanceof anything. It will occupy space and will havemass. It can be a solid, a liquid, or a gas.Whatever the form may be, however, it will bemade up of atoms or of atoms and/ormolecules. Atoms are the building blocks fromwhich elements are comprised - an elementbeing that form of matter which cannot bechanged by chemical means. Examples ofelements are copper (chemical symbol Cu) andoxygen (chemical symbol O). Molecules arecombinations of atoms that comprise thesmallest part of a substance that retains thephysical characteristics of that substance. Anexample of a molecule would be the smallestpart of copper sulfate (CuSO4) which is acombination of atoms of copper (Cu), sulphur(S), and oxygen (O).

When we take a closer look at atoms, we findthat they are made up of a positively chargednucleus surrounded by orbiting negativelycharged electrons. Each elemental atom has itsown characteristic combination of a nucleusand electrons. Electrons are the key wordinsofar as the relationship with electricity isconcerned. A flow of electric current involvesthe transfer of electrons through an electricalcircuit.

This relationship will be explored in greaterdetail in Chapter 2 - Corrosion Fundamentals.


The Two General Types of Electricity

There are two general types of electricity whichwill be involved with corrosion and corrosioncontrol work. These are alternating current (AC)and direct current (DC).

Alternating Current (AC)

Alternating current electricity is that which flowsfirst in one direction and then in the oppositedirection in accord with an established pattern.As an example, the usual alternating currentpower sources used in the United States havea frequency of 60 cycles per second. This isreferred to as 60 Hertz (or 60 Hz). A singlecycle can be illustrated as shown in Figure 1-1.

As can be seen from the figure, the current flowat the beginning of the cycle (left side ofillustration) is zero. The current builds up to apeak in the forward direction and then dropsback to zero at the end of the first half cycle. Itthen reverses its direction of flow and builds upto a maximum in the reverse direction.Following this, it again drops back to zero at theend of the second half cycle (which is the endof one full cycle). At this point, it again reversesdirection to start the next cycle.

From this, it can be seen that there are, ineffect, two net current reversals for one fullcycle. This means that for a normal 60 Hzalternating current power source, the currentflow changes direction 120 times per second.

The shape of the normal current flow plot fromthe usual alternating current commercial powersource is known as a sine wave.

Significance of Alternating Current

Alternating current electricity is a relativelyinsignificant factor as a cause of corrosionexcept in very special cases.

In the control of corrosion, however,commercial AC power sources are used as anenergy source to power corrosion controlequipment such as rectifiers (which convert AC

power to DC power) widely used in impressedcurrent cathodic protection systems. These arediscussed in Chapter 3 - Corrosion ControlMethods.

Direct Current (DC)

Direct current electricity is that which normallyflows in one direction only rather than changingdirection in accord with an established patternas was discussed for alternating current.

An example of direct current is that from abattery powering a common flashlight.

Significance of Direct Current

DC electricity is of prime importance in theconsideration of the corrosion process. It isdirectly involved in the various types ofcorrosion cells discussed in Chapter 2,"Corrosion Fundamentals.” It is also directlyinvolved in corrosion control by the use ofvarious types of cathodic protection asdiscussed in Chapter 3.

In this chapter, the prime emphasis will be onelectrical fundamentals as they apply to DCcircuits.

BASIC TERMS

The following discussion defines and explainsthe various electrical units and terms which areinvolved in DC electrical circuits. A thoroughunderstanding of these is necessary in orderthat various types of electrical circuitsencountered in corrosion work may be properlyanalyzed and evaluated. Once the meaning anduse of these units and terms are mastered, theywill be used in understanding the varioussample calculations used throughout thebalance of the Basic Course. Further, they willallow knowledgeable handling of the varioustypes of DC circuits and conditions whenencountered in the actual practice of detecting,evaluating and solving corrosion problems inthe field.

��

��

��

��

��

��

��


Volt

The volt is the basic unit of electrical pressurewhich forces an electrical current (electrons) toflow through an electrical circuit. Voltage can beused to indicate electrical pressure in general.The comparable term in a water system wouldbe water pressure expressed as pounds persquare inch (psi).

Another term used for electrical pressure iselectromotive force or EMF. Still, the volt is theunit used to express the quantitative amount ofan electromotive force.

The symbols representing electrical pressure informulas which you may use in corrosion workwill be either V (for volts) or E (for EMF -- whichis also voltage).

Although 1 volt is the basic unit, there areinstances where much smaller units are easierto use. One millivolt (mV or mv) is onethousandth of a volt, or:

1000 mV = 1 V

1 mV = 0.001V

Also, one microvolt (:V or :v) is one millionth ofa volt, or:

1,000,000 :V = 1 V

0.1 :V = 0.000001 V

Sources of DC voltages encountered incorrosion work include corrosion cell voltageswhich cause corrosion current to flow (seeChapter 2). Such voltages are typically in themillivolt or microvolt range. In some instances,stray current corrosion (caused by interferencefrom outside sources as discussed in Chapter2) can be caused by DC voltages measured involts.

In corrosion testing work, DC voltages used asa source of test current may be batteries, suchas common dry cell batteries or storagebatteries. A flashlight battery typically has a

voltage of nominally 1.5 volts while a storagebattery of the automobile type has a voltage ofnominally 12 volts. Where larger amounts ofcurrent are needed, DC generators may beused.

In corrosion prevention work, sources of DCvoltage used to provide cathodic protectioncurrent include:

S galvanic anodes of zinc, aluminum ormagnesium where the driving voltage maybe measured in tenths of a volt or inmillivolts

S higher capacity sources such as AC to DCrectifiers or DC generators of varioustypes. Such sources typically have outputvoltages measured in volts and areavailable in a wide range of voltages tomatch specific requirements.

All of the above are more fully discussed inChapter 3 - Corrosion Control Methods.

Ampere

The ampere (often abbreviated to amp) is thebasic unit of electrical current flow. This currentis caused to flow through an electrical circuit byelectrical pressure (or voltage) as discussedabove. The comparable term in a water systemto express the rate of water flow could be, forexample, gallons per hour.

The symbol commonly used to representcurrent in formulas is the letter I. However, acalculated or measured current can bedesignated by the letter A; such as 12 A torepresent 12 amperes. Typical applications willbe discussed later in this chapter.

Although 1 ampere is the basic current flowunit, there are instances where very smallfractions of an ampere may be involved incorrosion work. Smaller units may then be moreconvenient to work with. One milliamp (mA orma) is one thousandth of an ampere, or:

1000 mA = 1 A


1 mA = 0.001 A

For an even smaller unit, one microamp (:A or:a) is one millionth of an ampere, or:

1,000,000 :A = 1 A

1 :A = 0.000001 A

Knowledge of current flow, its evaluation, andits measurement is particularly important incorrosion work since, as will be discussed inChapter 2, there is a direct relationship betweenthe amount of current flow in a corrosion celland the amount of corrosion experienced. Thehigher the current flow in a corrosion cell, thegreater the amount of metal loss.

Coulomb

The term coulomb is seldom, if ever,encountered in practical corrosion control workon underground structures. It is, however,related to current. Earlier, in the section on"Physical Matter", it was indicated that electricalcurrent flow involves a transfer (or flow) ofelectrons through an electrical circuit. Thecoulomb is a representation of this electron flowand is the basis for the practical unit, theampere. A DC ampere is a flow of one coulombper second where the coulomb is 6.28 x 1018

electrons. Another way of saying it is that thecoulomb is 6.28 billion billion electrons -- a verylarge figure indeed.

The development of the coulomb is based onthe fact that when normal (or neutral) atoms ofa material either gain or lose electrons, they willdevelop positive or negative charges. When thiscondition exists, like charges repel each otherand unlike charges attract each other. Thisattraction or repulsion force is the source ofelectrical pressure, or voltage, as discussedearlier. Then when a suitable current path isestablished, there will be a transfer of electronsbetween dissimilarly charged materials tosatisfy a natural tendency to reestablishneutrality. This effect will be seen in Chapter 2which discusses corrosion fundamentals.

Although, as indicated earlier, you may not usethe term coulomb in your normal corrosionwork, the above information is included here asbackground material to give greater meaning tothe practical term "ampere.” You can see theclose relationship to the water systemcomparison mentioned in the section "ampere"where water flow could be indicated as gallonsper hour rather than electrons per second forDC electricity.

Ohm

The ohm is the basic unit for resistance to theflow of electrical current. With a fixed drivingvoltage applied to an electrical circuit, theamount of current flowing through the circuitdecreases as the circuit resistance increases.Conversely, the current flow increases as thecircuit resistance decreases.

The usual symbol for resistance in formulas isthe letter R. A calculated or measured figure forresistance may be represented by "ohms" or bythe Greek letter omega (S); for example, 10ohms or 10 S.

Resistivity

The term resistivity is used to indicate thecharacteristic ability of a material to conductelectricity. It can be applied to both metallic andnon-metallic materials.

Resistivity is commonly expressed asohm-centimeters (ohm-cm). The unit ohm-cm isthe resistance between opposite faces of acube of material which is 1 centimeter x1 centimeter x 1 centimeter in size. In practice,a cubic centimeter of a material is neverisolated for the purpose of measuring itsresistivity. Rather, the resistance across a bodyof the material of known dimensions ismeasured and, by calculation, determining itsunit resistivity. See Chapter 5 for furtherinformation.

In underground corrosion control work, thegreatest need for measuring resistivity is inconnection with soils and waters as needed for


the design of corrosion control systems.

Polarity

The term polarity is important in determining thedirection of conventional current flow inpractical usage. The direction of conventionalcurrent flow in DC circuits is from positive (+) tonegative (-). If for example, a 12-volt battery isconnected to an electrical circuit, electriccurrent will flow in the conventional sense fromthe positive (+) terminal of the battery throughthe circuit and back to the negative (-) terminalof the battery.

In Chapter 2, it will be demonstrated that theflow of electrons (which actually is the basis forelectric current as described earlier in thischapter under the heading "coulomb") is fromminus (-) to plus (+) which is in the oppositedirection from conventional current flow.Although it is important to know this,conventional current flow will be used in thegreat majority of practical corrosion controlapplications.

Conductor

The term conductor is used to designate amember of an electrical circuit that readilycarries an electrical current. This will commonlybe used to mean a wire or cable although inunderground corrosion control work, pipes orother metallic structures may serve asconductors.

Different metallic materials have differentcapability for carrying electric current. This isrelated to the characteristic resistivity of thematerial as has been discussed earlier.

Table 1 compares the relative current carryingcapability of some conductor materialscommonly encountered in undergroundcorrosion work based on copper.

TABLE 1

Relative Conductivity of Some Metals(Based on Smithsonian Physical Tables)

MaterialCurrent Conducting CapabilityBased on Copper as 100%*

Copper (annealed) 100.0%

Aluminum 60.0%

Magnesium 36.8%

Zinc 27.6%

Brass 24.6%

Steel 9.6%

Lead 8.0%

* Relative percentages will vary depending on alloyingcomponents.

Although copper is obviously the best conductormaterial, a steel pipe (even though a relativelypoor conductor material) can be a very goodpractical conductor because, particularly inlarger sizes, the amount of steel in the pipe isso much greater than the amount of copper inthe usual copper wire or cable. For example,the resistance of 1000 feet of 4/0 AmericanWire Gage (AWG) copper cable (which isapproximately 0.54 inches in diameter) willhave a resistance in the order of 0.051 ohmswhereas the resistance of 1000 feet of 12-inchsteel pipe with 0.375-inch wall thickness willhave a resistance of only about 0.0058 ohms -- or roughly one tenth that of the heavy coppercable.

Insulator

An insulator or insulating material will have avery high resistance to the flow of electricalcurrent and is used to confine or control theflow of current in electrical circuits. Examplesare wire or cable jackets of rubber, neoprene,plastic or similar insulating materials. Onunderground pipelines, insulating coatings ofplastics, coal tar enamel, asphalt, and similar


I

or

Volts

=ER

Amps =Ohms

I

or

VA

=ER

Current =3.512

3 42Ω

= .

R

or

Volts

=EI

Ohms =Amperes

materials are used to restrict the flow of currentto or from the earth.

Insulating materials are also used to electricallyisolate one metallic structure from another.When insulating materials are used for thispurpose they are called isolators or isolatingdevices.

How the Various Terms Go Together

In order to get a better idea of how the variousterms described above are used in a simpleelectrical circuit, Figure 1-2 illustrates first theelectrical usage and, for comparison, shows awater system analogy.

OHM'S LAW

The worker in the field of undergroundcorrosion control must have a thoroughunderstanding of Ohm's law as it applies to DCcircuits. Actually, the law is a very simplestatement of the relationship between volts,amperes and ohms in a DC circuit.

Basically, the law states that one volt ofelectrical pressure will force one ampere ofcurrent through a circuit having a resistance ofone ohm. This can be shown by the followingformula:

E = I × R

or

Volts = Amps × Ohms

Using the formula as shown above allows us tocalculate the voltage applied to a circuit if thecurrent flow and circuit resistance are known.For example, if we know that the current flowingthrough a circuit is 3.6 amps and the resistanceof the circuit is 1.7 ohms, the voltage needed tocause the 3.6 amps to flow will be:

E = I × R

Volts = 3.6A x 1.7S= 6.12 V

The Ohm's law formula is flexible in that if anytwo components are known, the other can becalculated.

We have just covered the case where thevoltage is the unknown component. Nowconsider the case where the current (I) isunknown while the voltage (E) and theresistance (R) are both known. To cover this,the formula is rearranged to:

As an example of the use of this form of theformula, assume that a voltage of 12.0 volts isapplied to a circuit having a resistance of 3.5ohms. Calculate the current flow by using theformula as just stated.

There is one more form of the statement of theOhm's Law formula to cover the case where thecircuit current and applied voltage are knownbut the circuit resistance is unknown. Theformula is rearranged to:

As an example, assume that 6.0 volts is appliedto a circuit and that this forces 1.5 amps to flowthrough the circuit. Calculate what the circuitresistance must be to restrict the current flow tothe 1.5 amps.

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

�� !��

��

��"��

��

��

��

��#


R

or

VoltsAmps

Ohms

=EI

Resistance =1.5

64=

Resistance =1

22

Voltsmilliamp

Ohms=

Resistance =0.001

22000

VoltsAmp

Ohms=

Resistance =1

20002000

millivoltsmillamp

Ohms=

Circuit Resistance =520

4VA

Ohms=

Voltage drop across (E) =

5A 4

resistance

ohms volts× = 20

Ohm's Law is simple and easy to use. Justremember the basic formula E = IR and theother two forms (I = E / R and R = E / I ) aresimple rearrangements of the basic formula.There is, however, one very important rule inthe use of any of the three forms of the formula.This is that values entered in the formula mustbe in the similar units. An example will showhow mixing units can cause a wrong answer.Assume that the applied voltage to a circuit is2.0 volts and the current measured in the circuitis 1.0 milliamp. To calculate the circuitresistance, we use the formula, R = E / I. Donot do it by setting up the calculation as:

THIS IS WRONG!!

What must be done is to convert the 1.0milliamp to amps (1.0 MA = 0.001 Amp). Thenthe calculation becomes:

The other approach would be to convert thevoltage to millivolts. In this case, 2.0 volts =2000 millivolts. The calculation then is:

Failure to observe the similar units rule canresult in very serious errors in the design ofcorrosion control circuits. In the use of the basicform of the formula, E = IR, it should be notedthat if I is expressed as amps and R isexpressed as ohms, the result will be in volts.

But if the current (I) is expressed as milliampsand the resistance (R) is expressed asmilliohms, the result will be in microvolts whichmay be misleading. It is much better to convertthe I to amps and the R to ohms and get theresult in volts.

THE BASIC ELECTRICAL CIRCUIT

Figure 1-3 represents a simple, or basic,electric circuit which will be used to illustratefurther applications of Ohm's Law discussed inthe preceding section. This will serve to solidifyan understanding of the principles of the usageof the law

The Figure 1-3 representation shows all of thecircuit resistance confined to a simple resistor.Now assume that a DC power source providingcurrent to a cathodic protection system forcorrosion control has instruments indicating thatthe supply voltage (E) is 20 volts and thecurrent flow (I) is 5 amps, the total circuitresistance (R) can be calculated by that form ofOhm's Law, R = E/I. Using the values given,

In such a simple circuit, the entire voltage dropfrom the voltage source appears across thecircuit resistance. To check this in the aboveexample, Ohm's Law can be used in its basicform, E = IR. The current is known to be 5amps. The resistance was calculated as 4ohms. Then:

In another application of Ohm's Law to thisbasic circuit, if a 20-volt DC power source wereconnected across a known resistance of 4ohms, the current flow (in the absence of anammeter) can be calculated by that form ofOhm's Law, I = E / R. Then:

��

��$

��

��

��

��

��

��

�

� ��

��

��

��

��%�� !��

��


Current flow (I) =20

45

Vohms

amps=

102

5V

ampsohms=

From the above, it becomes apparent that inthe basic electrical circuit, all of the currentflows without change throughout the circuit andthat all of the electrical voltage applied to thecircuit appears as a voltage drop across thecircuit resistance.

THE SERIES ELECTRICAL CIRCUIT

Not all electrical circuits conform to the basicelectrical circuit configuration described in thepreceding section.

Figure 1-4 represents an electrical circuit wherethe total circuit resistance comprises two ormore load resistances which are connected inseries. By "series", it is meant that the severalload resistances are connected end-to-end andthat the entire circuit current has to passthrough each of the resistances. This in turnmeans that the voltage of the DC potentialsource applied to the circuit will be distributedacross the several resistances in the circuit.

To illustrate the effect, assume that thefollowing values are known with respect to thecircuit of Figure 1-4:

Power supply voltage (ES) = 10 voltsCircuit current flow (I) = 2.0 ampsLoad resistance No. 1 (R1)= 3.0 ohmsLoad resistance No. 2 (R2)= 1.87 ohmsLoad resistance No. 3 (R3)(20 ft of No. 8 wire) = 0.13 ohms

Calculate the voltage drops across the threeresistances by the E = IR form of Ohm's law.

Drop across R1: V1 = 2A × 3 ohms = 6 volts

Drop across R2: V2 = 2A × 1.87 ohms = 3.74 volts

Drop across R3: V3 = 2A × 0.13 ohm = 0.26 volt

The sum of these voltages is:

6 + 3.74 + 0.26 = 10 volts

This matches the power supply voltage as itshould. Note that the voltage drops across theresistances are proportional to the ohmic valuesof the resistances - - the higher the value of theresistance, the greater the voltage drop acrossit for a given value of current flow.

The sum of the resistance in the example usedis:

3.0 + 1.87 + 0.13 = 5 ohms

This should match the value of total circuitresistance calculated from the known powersource voltage and current, ES = 10 volts and I= 2 amps. To check, use the R = E / I form ofOhm's Law.

Total Circuit resistance (R1 + R2 + R3) =

For another application of the Figure 1-4 circuit,assume that we do not know the ohmic valuesof the resistances R1, R2 and R3. All we know isthe power source voltage (ES = 10 volts) andpower current (I = 2 amps). The problem is nowto determine the amount of each resistance inohms. It soon becomes apparent that notenough information is known to determinethese values without taking an intermediatestep.

The necessary intermediate step is to measurethe voltage drop across resistances R1 and R2using a suitable DC voltmeter. The reason forneglecting the drop across R3 will becomeapparent shortly.

Assume that the intermediate step has beentaken with the following values recorded.

Drop across resistance No. 1 = 6 volts Drop across resistance No. 2 = 3.74 volts

��

��

��

��&

#

��

��

��

��

��

�

��

��

��

��

��

��

�

�

�

��

��

��

��

��

�

$

��#

��

��# ��

�

#

��$ �� $

��$


Resistance No. 1 =62

3VoltsAmps

ohms=

Resistance No. 2 =3742

187.

.Volts

Ampsohms=

Total Circuit Resistance =102

5Volts

Ampsohms=

I =1203

6 67Volts

Ohmsamps= .

I =2202

10Volts

Ohmsamps=

This gives all the needed information. Using theR = E / I form of Ohm's Law,

We know from the power source voltage (10V)and current (2A) that by Ohm's Law,

Therefore, the value of Resistance No. 3 has tobe 5 ohms minus the sum of Resistances R1and R2:

Resistance No. 3 = 5 - (3 + 1.87) = 0.13 ohms

Note that the values of resistance so obtainedagree with the values used earlier in theexample - as they should.

Remember the following things with respect toa series circuit:

1. The total current from the power sourceflows through each resistance element in thecircuit.

2. The sum of the voltage drops across theseveral resistance elements in the circuitmust equal the voltage of the power source.

3. The sum of the ohmic values of the severalresistance elements in the circuit must equalthe total circuit resistance:

RT = R1 + R2 + ... + RN

THE PARALLEL ELECTRICAL CIRCUIT

Equally as important as the series circuit is theparallel circuit. In such a circuit, two or moreload resistances are so connected that the plus(current input) ends of all resistances areconnected together and the minus (currentoutput) ends are connected together instead ofbeing connected end to end as is the case witha series circuit. This is illustrated by Figure 1-5.

As can be seen by examining Figure 1-5, thepower source voltage will be impressed oneach resistance element rather than beingdistributed as is the case with a series circuit.Further, the power source current will bedivided among the several resistance branches.

An example of how the parallel circuit workscan be developed by assuming that thefollowing values are known with respect toFigure 1-5:

Power supply voltage (ES) = 20 volts Power supply current (IS) = 16.67 amps Load resistance No. 1 (R1) = 3 ohms Load resistance No. 2 (R2) = 2 ohms

First calculate the current flow (I1 and I2)through each load resistance branch. Thevoltage drop across each branch equals thepower supply voltage (ES) or 20 volts. Then byOhm's Law, I = E / R:

Current flow through R1:

Current flow through R2:

The sum of these 2 currents should equal thepower source output current of 16.67 amps - -which it does.

Second, calculate the parallel resistance of R1

��'

��

��

�� (�

��

��

��

��

��

��

�� #

��

��

��

�(�� #�

��

��

��#

��# �#�

��

��

��

��

��

��


Parallel Resistance =R RR R

1 2

1 2

×+

Parallel Resistance =3 23 2

65

12

ohms ohmsohms ohms

ohms

×+

=

= .

2016 67

12VoltsAmps

ohms.

.=

R RR R

R1 2

1 21 2

×+

= −

R RR R

R1 2 3

1 2 31 2 3

−

−− −

×+

=

11 1 1 1 1

1 2 3 4R R R R RN+ + + + +...

1 1 1 1 1 11 2 3 4R R R R R RP N

= + + + + +...

and R2. This is done by using the formula:

In this case,

Note that the parallel resistance of any tworesistors is always less than the smallerresistance value.

The parallel resistance of load resistances R1and R2 must equal the effective total circuitresistance. This can be checked using thepower supply voltage (ES = 20 volts) and thetotal power supply current output (IS = 16.67amps). By Ohm's Law, R = E / I. In this case,

Effective total circuit resistance =

This checks the calculated parallel resistance ofthe two load resistances.

Should there be three or more parallelbranches, the parallel resistance can becalculated as follows:

1. Calculate the parallel resistance of R1 andR2 to obtain R1 - 2:

2. Calculate the parallel resistance of R1-2 andR3 to obtain R1-2-3:

3. Continue the same procedure for anyadditional branches until all branches have

been included in the calculations.

Note that the resulting resistance will always beless than the smallest branch resistance in thecircuit.

There is another way to calculate parallelresistances but the above procedure illustratesthe principal and becomes cumbersome only ifthere are a large number of parallel branches.For reference, the relationship to use by thealternate calculation method is:

Parallel resistance (RP) =

Or:

Remember the following important points withrespect to a parallel circuit:

1. The full power supply voltage is impressedacross each parallel branch.

2. The sum of the currents through theindividual parallel branches must equal thetotal current output of the power source.

3. The parallel resistance of two or morebranches will always be less than that of thesmallest branch resistance.

COMBINATION CIRCUITS

Electrical circuits can be a combination ofseries elements and parallel elements. A simplecase is illustrated by Figure 1-6.

Calculation procedures applicable to such acombination circuit are in general accord withthe principals covered in the sections on seriescircuits and parallel circuits. There are,however, certain important differences to be

��

��)

��

��

��

�(� ��

��

��

��

�� #

��

�(� �� #�

��

��

��#

��# �#�

��

��

��

��

��$ �$�


noted.

The power supply output current dividesbetween the two parallel branches R1 and R2and then combines again after passing throughthese resistances. The full power supply currentthen passes through the resistance element R3.

The voltage drop across parallel branches R1and R2 will be equal to each other. However,the amount of the voltage drop will be thepower supply voltages less the voltage dropacross series resistance R3.

The effective circuit resistance will be thecalculated parallel resistance of branches R1and R2 plus the resistance of series resistanceR3.

CHAPTER 2

CORROSION FUNDAMENTALS

INTRODUCTION

Corrosion is a major problem associated withunderground structures. Modern industrialplants, universities, hospitals, cities and utilitycompanies utilize many underground systemsand corrosion failures become expensive,cause shutdowns, hazardous conditions,occasional fires or other catastrophes, andcause inconvenience to both personnel and thepublic.

Corrosion is an electrochemical reactionbetween a metal and its environment.Essentially, it is the tendency of a refined metalto return to its natural state as an ore. Corrosionalways involves a flow of direct currentelectricity through an electrolyte such as soil orwater from one point to another on a metalsurface. This current is generated by a potentialor voltage difference between the two points.

There are two major types of undergroundcorrosion. These are natural corrosion andstray current corrosion. Both are variations ofthe fundamental corrosion cell.

This chapter describes the fundamentalcorrosion cell and its relation to undergroundstructures. Various types of natural cells arediscussed and examples are given. Some othertypes of corrosion, basically variations ofnatural cells, are shown also to give a generalpicture of this aspect of "what makes thestructure fail". The chapter then goes on todescribe stray current corrosion and givesexamples of both dynamic and static straycurrent situations. Finally, factors affecting therate of corrosion are explained.

FUNDAMENTAL CORROSION CELL

Corrosion is, in effect, similar to a dry cell. Inorder for corrosion to occur, we must have fourelements, namely: Electrolyte, anode, cathodeand a return circuit. Refer to Figure 2-1. Notethat it is basically a dry cell. The electrolyte is asubstance such as earth or water capable ofcarrying an electric current. For simplicity'ssake, we say it consists of hydrogen (H+) andhydroxyl (OH-) ions, positively and negativelycharged elements of water. What occurs isbasically this. At the anode, metal particles(positively charged ions) dissolve into theelectrolyte. This leaves electrons behind on thesurface of the anode. The electrons flow to thecathode through the return circuit. Now theelectrolyte must remain electrically neutral.Therefore, since positive ions have been addedto the electrolyte at the anode, some otherpositive ions must be displaced. One reactionthat may occur is the displacement of hydrogen.Hydrogen ions accept the electrons at thecathode and become hydrogen gas. Thisevolution of hydrogen is a type of polarization.When the film of hydrogen remains on thecathode surface, it acts as an insulator andreduces the corrosion current flow.

Another reaction also occurs. In the case ofsteel, ferric (iron) ions flow into the electrolyte atthe anode. These ions react with the hydroxylions to form rust. If the rust adheres to themetal surface, it may coat the surface and alsoslow down the flow of corrosion current. This isa type of anodic polarization. Polarization isdiscussed later in the chapter.

CORROSION FUNDAMENTALS2-1

ee RETURN CIRCUIT

ANODECATHODE

Fe

ELECTROLYTE

H

H OH

OH++

-

+ -

+H+H+

FUNDAMENTAL CORROSION CELL

FIGURE 2-1

ee

--

--

ee

--

ee

Here are reactions that can occur:

At the anode:

Aluminum Al ! Al3+ + 3e-

Zinc Zn ! Zn2+ + 2e-

Iron or steel Fe ! Fe2+ + 2e-

At the cathode:

2H+ + 2e- ! H2

3O2 + 6H2O +12e- ! 12OH-

The influence of oxygen on corrosion is easilyseen. The reaction produces additional hydroxyl(OH-) ions to react with the ferric (Fe3+) ions,thus increasing the propensity for ferric ionproduction (corrosion).

O2 + 4H+ +4e- ! 2H2O

This reaction and the proceeding one consumeelectrons at the cathode. The greater thepropensity to consume electrons at thecathode, the greater the tendency for theproduction of metallic ions at the anode(dissolution of metal - corrosion).

It is important to emphasize that the corrosionreactions shown above are not platingreactions. There is no migration of ions from theanode to the cathode, that is metal does notmove from the anode to the cathode.

We have shown so far that corrosion involvesthe flow of both ions and electrons. This is whyit is called an electrochemical process. Nowlet's simplify it a bit. We don't really care aboutthe electrons since they stay in the metal anddon't cause any trouble. What we care about,however, is the flow of ions (charged particles)into the electrolyte at the anode. This is whatcauses corrosion. As we have seen, the ionsdon't actually flow from the anode to thecathode in the electrolyte, but rather there is amigration of ions into the electrolyte at theanode and out of the electrolyte at the cathode.

We say, however, for simplicity's sake, thatthere is a flow of current from the anodethrough the electrolyte to the cathode. Since theanode and the cathode are connected by thereturn circuit, we say the current flows back tothe anode through this circuit. This is theso-called conventional flow - through a wirefrom the positive to the negative terminal of abattery.

Accepting the proposition of "conventional”current flow, the corrosion cell may be summedup as follows:

1. Current flows through the electrolyte fromthe anode to the cathode. It returns to theanode through the return circuit.

2. Corrosion occurs wherever current leavesthe metal and enters the soil (electrolyte).The point where current leaves is called theanode. Corrosion, therefore, occurs at theanode.

3. Current is picked up at the cathode. Nocorrosion occurs here. The cathode isprotected against corrosion. (This is thebasis of cathodic protection, by the way.)Polarization (hydrogen film build-up) occursat the cathode.

4. The flow of current is caused by a potential(voltage) difference between the anode andthe cathode.

Figure 2-1 showed the fundamental cell as abattery. Figure 2-2 shows how such a celldevelops on a pipeline, or similar structure. Theonly difference between natural and straycurrent corrosion is this: In stray currentcorrosion, some external source of current isinserted into the return circuit.

TYPES OF NATURAL CORROSION

Dissimilar Metals

Where dissimilar metals are in electrical contactwith One another underground, here will be apotential difference between them. This will


FUNDAMENTAL CORROSION CELL AS IT APPLIES TO A PIPELINE

A Electrochemical cellB Conventional current cell

FIGURE 2-2

Cathode(Protected Area)

Anode(Corroding Area)

Corrosion Current

Electrolyte (Soil)

Return Circuit (Structure)

e–e–

Structure

CathodeAnode

A

B

Fe Fe++

+2e–

Fe++

H2O H+

(OH)–

2H+

+ 2e– H2

H+H+

cause current to flow with the less noble of thetwo being the one to corrode.

There is a natural potential difference betweendifferent metals when immersed in a conductingelectrolyte. This is illustrated by Table 2-1 whichis a practical galvanic series indicating voltageswhich can be expected when measuring metalpotentials with respect to a standard copper-copper sulfate reference electrode when bothare in contact with neutral soils or waters. Thevalues shown in the table serve as a generalguide only since they are subject to somevariation as the environment changes.

If any two materials having different voltagesare connected together and placed in aconducting environment, there will be apotential between them equal to the algebraicdifference of the two voltages. The materialwhich is listed uppermost in the table will be theanode and the other (more noble) material willbe the cathode. As an example, if lead (-0.5volt) and copper (-0.2) are the pair of metalsselected, the potential difference will be 0.3 voltwith lead (uppermost in the table) being thecorroding anode and copper the cathode.

As another example, take zinc (-1.1 volt) andcarbon (+0.3 volt). The potential difference(since the carbon is plus) will be 1.4 volts andthe zinc will be the corroding anode. Thisexample is given because the 1.4 volt figuredoes not agree with the 1.6 volt value normallyfound with a new carbon-zinc dry cell battery.This simply illustrates that the materialpotentials are subject to variation withelectrolyte composition; the dry cell electrolytehas been selected to give maximum practicaloutput voltage.

If underground structures happen to be buriedin earth containing cinders where the cindersare allowed to be in direct contact with thestructure metal, there will be a very stronggalvanic cell causing corrosion of the structuremetal. This is because cinders are essentiallycarbon. Inspection of the figures in Table 2-1will show that the potential between, forexample, new steel and carbon can range

between 0.8 volt and 1.1 volt. These areseriously high galvanic cell potentials. Theeffect is illustrated by Figure 2-3. The corrosioncaused by cinders can be very difficult tocontrol on existing structures. It should beemphasized that contacts with cinders shouldalways be avoided on new construction.

Figure 2-4 illustrates steel and copper pipebeing electrically joined together. The galvanicseries of Table 1 indicates that the steel will beless noble and will be the corroding anode. Thecopper will be the cathode and not corroding. Itshould be noted that this is why copper oftenhas been considered to be a corrosion freematerial when used underground. This isbecause of widespread use of copper servicepipes on steel or cast iron systems such thatthe copper is made a cathode (free ofcorrosion) by virtue of being in contact with thesteel or cast iron. This relative corrosionfreedom is gained at the expense of the steel orcast iron.

Dissimilar Surfaces

Another not-so-obvious example of dissimilarmetal corrosion is the insertion of a piece ofnew steel pipe welded into an old undergroundrusty steel pipe. Again, Table 2-1 indicates thatthere will be a potential difference between thetwo and it will be the new steel pipe thatcorrodes. Typically, such a piece of new pipeused to replace a section that has failed fromcorrosion will not (in the absence of somemeans of corrosion control) last as long as theoriginal piece because of this new steel-oldsteel relationship. See Figure 2-5.

Another related example is dissimilarities insurface conditions on a pipe. In threadedunderground pipe assemblies, if a pipe wrenchis used on piping, the teeth of the pipe jaws canbite into the surface of the pipe exposing brightmetal. After rebackfilling, these pipe wrenchscars will be anodic to adjacent cathodic pipemetal and will corrode in the absence ofadequate corrosion control. Also, when threadsare cut into the pipe, the bright metal exposedby the thread cutting will likewise be anodic


Soil Contaminated with Cinders

Pipe Wall (Anode)

CathodePhysical ContactBetween Pipe andCinders

CORROSION DUE TO CINDERS

FIGURE 2-3

Gas

Wat

er

Steel GasService (Anode)

Gas Meter with no isolating fitting,shorted isolating fitting orby-passed isolating fitting

WaterHeater

Copper Water Service (Cathode)

DISSIMILAR METAL CORROSIONGAS AND WATER SERVICE LINES

FIGURE 2-4

TABLE 2-1

PRACTICAL GALVANIC SERIESP

rogr

essi

vely

mor

e an

odic

º(le

ss n

oble

) an

d m

ore

corr

osiv

e Metal Volts(1)

Commercially pure magnesium -1.75

Magnesium alloy (6% Al, 3% Zn, 0.15% Mn) -1.6

Zinc -1.1

Aluminum alloy (5% Zn) -1.05

Commercially pure aluminum -0.8

Mild steel (Clean and shiny) -0.5 to -0.8

Mild steel (rusted) -0.2 to -0.5

»P

rogr

essi

vely

mor

e ca

thod

ic

(nob

le)

and

less

cor

rosi

ve

Cast iron (not graphitized) -0.5

Lead -0.5

Mild steel in concrete -0.2

Copper, brass, bronze -0.2

High silicon cast iron -0.2

Mill scale on steel -0.2

Carbon, graphite, coke +0.3

(1) Typical potentials measured between metal (when immersed in neutral soils orwaters) and a copper-copper sulfate reference cell contacting the adjacent soil orwater.

Old Pipe(Cathode)

NEW-OLD PIPE CELL

FIGURE 2-5

New Pipe(Anode)

Old Pipe(Cathode)

where exposed to earth and will corrode in theabsence of suitable corrosion control. Thesereactions are shown in Figure 2-6.

Similarly, on existing underground pipesystems, if a sharp pointed probe rod is used tomake electrical contact with the pipe duringcorrosion control test work, the bright metalscars left by the probe rod will be anodic toadjacent pipe. Unless there is a suitablecorrosion control system in operation, dissimilarmetal corrosion cells can develop at thesescars.

Mill scale, although not a metal, is electricallyconductive and has a characteristic potentialwhich is more noble than that of the underlyingsteel. inspection of Table 2-1 indicates that thepotential between mill scale and new steel canbe in the range of 0.2 volt to 0.5 volt - with steelbeing the corroding material. In the absence ofeffective corrosion control, corrosion of thesteel can be rapid since, typically, the currentdischarge is concentrated in a small area.

Mill scale is a thin and usually tightly adherentsurface oxide film on hot rolled steel as itcomes from the steel mill. Where buriedstructures are built using steel from which suchscale has not been removed, corrosion cellscan occur at breaks in the mill scale such thatthe underlying steel is exposed to contact withthe environment. This is illustrated by Figure2-7.

Dissimilar Soils

Just as dissimilar metals are the source ofelectrical potential in corrosion cells, dissimilarsoils can also be a source of voltage difference.It is rare indeed to have a completely uniformsoil in which an underground structure isburied. In the more usual case, there will bechanges in characteristics of the soil from point-to-point along a buried structure. This isillustrated in principle by Figure 2-8. The figureshows two different soils, Soil "A" and Soil 'B".It also shows that if the potential of the steelpipe in soil "A" is measured with a voltmeterwith respect to a standard reference electrode

(such as copper-copper sulfate electrode), it willhave a different potential from that similarlymeasured in soil "B". In the case shown, thedifference between the two readings is 0.5 - 0.3= 0.2 volt with soil "A" being positive (+) to Soil“B”. With a current flow path being establishedbetween the two soils by the buried steel pipe,the current flow path will then be, (1), from themore positive soil "A" to soil "B", (2), from soil"B" onto the steel pipe in soil "B" - thus makingit cathodic and non-corroding, (3) along thepipe itself from the soil "B" area to the soil "A”area, and finally (4) discharging from the pipe insoil "A" to complete the circuit. This makes thepipe in soil "A" anodic and corroding.

Conditions are seldom as clearly defined asshown in Figure 2-8. By the time excavationsare made for a buried structure and thestructure is finally backfilled, there may be amixture of soil types in the backfill. This tends togive a condition illustrated by Figure 2-9 whichleads to numerous anode-cathode relationshipsalong the structure. The effects of this conditioncan be seen when an old steel undergroundstructure (such as a pipeline which has neverhad corrosion control measures applied) isuncovered for inspection. Typically, there will beheavily corroded areas (anodic) next to areas(cathodic) which can be in nearly as newcondition.

Differential Aeration

Another source of electrical potential causingcorrosion current to flow is differential aeration.This simply means that when oxygen from theair (aeration) is more readily available throughthe electrolyte to one part of a structure than toanother, there will be a difference in potentialthe between the two. When this conditionexists, that part of the structure havingrestricted oxygen availability will be anodic andwill corrode.

A typical example on a buried pipe is illustratedby Figure 2-10 which demonstrates the effect ofrestricted oxygen availability to the pipe undera paved road while there is good oxygenavailability through porous earth to the pipe on


Pipe(Cathode)

ThreadsBright Metal(Anode)

Scratches Caused byPipe Wrench(Anode)

CORROSION CAUSED BYDISSIMILARITY OF SURFACE CONDITIONS

FIGURE 2-6

Pipe WallPit Forming at Breakin Mill Scale (Anode)

Soil

Mill Scale(Cathode)

PITTING DUE TO MILL SCALE

FIGURE 2-7

SOIL “A”

SOIL “B”

(–)(+)

DISSIMILAR SOILS AS SOURCEOF CORROSION CELL POTENTIAL

FIGURE 2-8

-

-0.5 VOLT

+ -

-0.3 VOLT

+

VOLTMETER

REFERENCE ELECTRODE

DIRECTION OF CURRENT

STEEL PIPE

GRADE

ANODIC (CORRODING) CATHODIC

-0.50 -0.30

Direction of Current Flow

MIXTURE OF DISSIMILAR SOILS AS SOURCE OF CORROSION CELL POTENTIALS

FIGURE 2-9

STEEL PIPE

GRADE

A = ANODIC AREASC = CATHODIC AREAS

C A C C AA C A C A C

either side. This would be the case even thoughthe soil is otherwise uniform through the area.As was illustrated on Figure 2-8, it would befound that the pipe under the paving is morenegative with respect to earth than is the pipe inwell aerated soil on either side of the road. Thisis indicative of an anodic area under the roadand results in current flow in the directionsindicated by Figure 2-10.

Another example, again using pipe to illustratethe point, can occur if a buried pipeline is bed-ded in dense soil at the bottom of the pipelinetrench, while relatively loose backfill permitsgreater oxygen availability to the top of the pipethan to the bottom. In such a situation,corrosion cell current flow can be from thebottom of the pipe to the top. If this is the majorfactor causing corrosion, failures will be on theunderside of the pipe - which usually makespipe repair more difficult. See Figure 2-11.

Stress Corrosion

Where there is an area of differential stress onany part of an underground structure, that partwhich is more highly stressed will be anodic(and corroding) with respect to adjacent, lesshighly stressed parts of the structure. The effectis illustrated by Figure 2-12 which couldrepresent an underground bolt or tie rod havinga reduced cross-section at one point. In thisreduced cross-sectional area, the actual metalstress in terms of pounds per square inch ofcross-section is higher than in the remainder ofthe bolt or tie rod.

A reduced cross-section may not be present inthe member when installed. However, anothersource of corrosion such as dissimilar soils ordifferential aeration (discussed earlier) mayinitiate corrosion attack which will cause alocalized reduction in cross-section. This thenpermits the establishment of the differentialstress corrosion cell which can then act inaddition to the original corrosion cell - thusaggravating the attack.

Very often, the bolts of a coupling will be foundto be necked down or even broken in the

middle. The center of the bolt is more highlystressed than other parts because of stressconcentration at this location. The flow ofcurrent is shown on Figure 2-13.

Where high strength alloy steels are used inunderground structures which are highlystressed (such as high pressure pipelines), aphenomenon known as stress corrosioncracking may be encountered. In suchsituations, another source of corrosion caninitiate a corrosion pit which will act as a localstress raiser in the structure metal.

The resulting differential stress corrosion cellcan then act in conjunction with the originalcorrosion cell as above except that cracks maydevelop in the structure metal at the base of theinitiating pits with, in many cases, catastrophicfailures occurring. Sometimes the cracks areknown as “transgranular" where the crackpasses through metal grains or crystals. Inother cases, the cracks can be "intergranular"where they follow a zig-zag path along theboundaries of the metal grains or crystals.

The study of stress corrosion cracking hasbeen the subject of intense research as thephenomenon is subject to many variables suchas stress level, the metal alloy, nature of theenvironment, temperature, metal contaminants,etc., etc. An alloy which is resistant to stresscorrosion cracking under a given set ofconditions may behave quite differently underother conditions.

Cast Iron Graphitization

Where cast iron is used in undergroundconstruction and corrodes, the physicalappearance of the corrosion can be quitedifferent from the usual pitting on steel pipe.This is because cast iron contains in the orderof 3% to 4% carbon (in contrast with steel whichhas typically only a fraction of 1%). When graycast iron is poured in the molten condition, thecooling action converts much of the carbon tographite - another form of carbon. The graphiteis present in the form of particules or flakes inthe cast material. In a corrosive environment


CATHODIC

DIFFERENTIAL AERATION AS ASOURCE OF CORROSION CELL POTENTIAL

FIGURE 2-10

CURRENT FLOW PIPE

CATHODIC

SANDY SOIL(GOOD AERATION)

PAVED ROAD AERATION RESTRICTED BY PAVING COVER. PIPE ANODIC AND CORRODING.

StructureAerated Soil

OxygenAvailable(Cathode)

Poor or No Aeration(Anode)

CORROSION CAUSED BYDIFFERENTIAL AERATION OF SOIL

FIGURE 2-11

AREA OF SMALLEST CROSS-SECTIONAND HIGHEST STRESS IN POUNDSPER SQUARE INCH. ANODIC AND CORRODING.

CATHODIC

DIFFERENTIAL STRESS AS A SOURCEOF CORROSION CELL POTENTIAL

FIGURE 2-12

CONDUCTING ELECTROLYTESURROUNDING STRESSED MEMBER

APPLIED TENSION

STRESSED MEMBER

CURRENTFLOW

CATHODIC APPLIED TENSION

CouplingHigher StressedArea (Anode)

Lower Stressed Area(Cathode)

Lower Stressed Area(Cathode)

STRESS CORROSION

FIGURE 2-13

the iron corrodes, leaving behind the graphiteand products of corrosion. This leaves amechanically weak material which retains theoriginal shape of the structure without pitting asis the case with steel. This form of cast ironcorrosion is commonly known as"graphitization". The graphite material on thus-corroded cast iron can be easily cut or scrapedaway with a knife.

The condition can be accelerated by straycurrent attack.

Where cast iron is used in water mains, forexample, the pipe may become heavilygraphitized in severely corrosive areas and stillcontinue to carry water through the earth-supported graphitic structure. Eventually thepipe becomes so weak that a pressure surge orheavy traffic vibration will cause the pipe torupture causing a major leak. Such a rupturemay be the first indication that a severecorrosion problem exists the area.

Graphitization occurs to some extent on ductileiron, but not so aggressively as on cast iron.Ductile iron tends to pit like steel.

Microbiologically Influenced Corrosion

Colonies of certain types of bacteria canestablish conditions on the surface ofunderground structures which cause more rapidcorrosion by existing cells. One of the moresignificant types of bacteria which can involvea corrosion problem is the "anaerobic" type -which simply means that the bacteria thrive inthe absence of oxygen. The most commonproblem-causing bacterium of this type isknown as Desulfovibrio desulfuricans, a formwhich reduces any sulfates present at theunderground metal surface to producehydrogen sulphide and consumes hydrogen inthe process. At the metal surface, this actionconsumes hydrogen which forms at cathodicsurfaces of existing corrosion cells. Thisintensifies the action of the corrosion cell bydepolarization, as discussed under the heading“FACTORS AFFECTING THE RATE OFCORROSION.”

From the above it is apparent that the bacteriado not themselves attack the metal - but docause the intensification of existing corrosioncells. Anaerobic bacteria are apt to be found inheavy, dense, water logged soils where oxygenavailability is at a minimum. They can exist,however, under less favorable conditions orunder a covering material which locally restrictsoxygen availability. A disbonded pipelinecoating is an example.

Bacteria require organic material as a foodsupply. This is normally available to at leastsome degree in the earth. However, if a majorfood supply is combined with oxygen restriction,the action can be intensified. A simple exampleof this is a piece of wood lying against anunderground bare metal structure.

Combinations of Corrosion Cell Effects

A corrosion problem at a given point on anunderground structure is not necessarilyconfined to just one of the previously discussedtypes of corrosion cells. Two or more sources ofcorrosion cell current may in fact be acting at agiven location - such as, for example, adissimilar soil corrosion cell which is furtheraggravated by the presence of anaerobicbacteria and cinders in the backfill.

Amphoteric Metals

Certain metals when made cathodic cancorrode. These are referred to as "amphoteric",meaning that they are sensitive to stronglyalkaline conditions. Lead, tin, and aluminum areexamples of such metals although most metalstend to be susceptible to some degree ifsubjected to intense alkalinity. Of the threeexamples given above, aluminum tends to bethe most susceptible.

The term "amphoteric” means "partly one andpartly the other". In this particular application, itmeans that amphoterically-sensitive metals can(under certain conditions) be collecting currentfrom the environment, which is a cathodiccondition and yet be corroding which isnormally an anodic condition.


In a corrosion cell, the concentration of hydroxylions (OH-) at the cathode surface causes anincrease in alkalinity at the cathode surface.This action in connection with galvanic or othercorrosion cells will not normally create acorrosive condition on an amphoteric metalsuch as aluminum. However, where there isintense current collection on a cathodic surfaceunder certain stray current conditions or whereexcessive cathodic protection current (seeChapter 3) is collected on cathodic surfaces, aparticularly sensitive amphoteric metal such asaluminum can be subject to increasedcorrosion.

STRAY CURRENT CORROSION

Definition of Stray Current Corrosion

Stray current corrosion is that caused by earthpath direct current from some source externalto the underground metallic structure which canbe picked up by the structure at one point(creating a cathodic condition), flow along thestructure for a distance, and then discharge intothe environment in order to complete the circuitto the external source. Where the dischargeoccurs, an anodic condition exists with, in somecases, a very severe corrosive effect on theunderground structure.

Stray current problems tend to be most severeon long underground structures, such aspipelines, but can be a problem with anyunderground structure if stray currents arepresent in the earth.

Differences Between Stray CurrentCorrosion and Natural Corrosion

Whereas in natural corrosion, corrosion celldriving potentials are small (typically less thana volt) and corrosion currents tend to likewisebe small, the driving potentials from an externalsource causing stray current to flow can bemany volts, and in severe cases, the straycurrent on an underground structure can behundreds of amperes.

Severity of Stray Current Corrosion

If stray current is allowed to discharge from anunderground structure into the environmentwithout corrosion control measures, the rate ofcorrosion attack can be extremely rapid.Structure failure, under these conditions, canoccur within a short period of time. This isentirely logical (where, for example, hundred ofamperes are discharged) in accord with theelectrochemical equivalent of the metal involvedas will be discussed later.

Why Stray Current Occurs

Some stray current effects are result of parallelpaths where the affected structure serves as aparallel path for the earth current as shown inFigure 2-14.

As can be seen from the figure, some of thecurrent flowing through the earth to the DCsource is picked up by the metallic structure(typically a good conductor) which happens tobe going in the direction that the current wantsto go. There are now two parallel paths - thepath through the earth and the path through thestructure. As the current on the structureapproaches the source of stray current,however, it has to leave the structure to reenterthe earth in order to complete its circuit. This iswhere the corrosion damage occurs.

Alternating Current

Alternating current (AC) also causes corrosion.While no definitive conclusions have yet beendrawn, it is generally thought that AC causesonly about 1% of the corrosion of an equivalentamount of DC. Some tests have shown thatwhen AC is superimposed on a galvanic orother corrosion cell, the rate of corrosion mayincrease.

AC can also be a safety hazard. AC can causeboth sparks and shocks. You should beespecially careful on coated pipelines thatparallel high voltage overhead electrical lines.Underground tanks and piping may carry AC ifbuildings in the area do not have proper


PARALLEL EARTH PATHSCAUSING STRAY CURRENT CORROSION

FIGURE 2-14

EARTH PATH CURRENTSOURCE OF DIRECT CURRENT CAUSINGSTRAY CURRENT TO FLOW

(+(–)

UNDERGROUND STRUCTURE(PLAN VIEW)

CURRENT PICKUPAREA (CATHODIC)

CURRENT DISCHARGE AREA(ANODIC AND CORRODING)

electrical grounding.

Types of Stray Current Corrosion

There are two types of stray current, dynamicand static.

Dynamic stray currents are those which aresubject to variation with time - often to a verymarked degree. Sources of dynamic straycurrent can cause changes in current pickupand discharge areas on a structure and cancause reversal of current flow on a structure insome instances. This all results in changes inthe location of anodic and cathodic areas.

Static stray currents are those which are from asteady state external DC voltage source whichcauses fixed anodic and cathodic areas onaffected structure with a relatively constantamount of current flow on the structure.

Sources of Stray Current Corrosion

Sources of stray current can be either man-made (usually the more serious) or natural.

Man-made sources of stray direct currentinclude the following:

• DC powered transit systems• DC powered mining operations• DC welding operations• High voltage direct current (HVDC) electric

power transmission systems• Cathodic protection systems (see

Chapter 3) on underground structures otherthan the affected structures

A natural source of stray current is what isknown as "telluric" or earth current of magneticorigin. These are direct currents, of a variablenature, in the earth's crust which are a result ofvariations in the earth's magnetic field. Thesevariations in the magnetic field are in turncaused by variations in solar activity.

Telluric currents are dynamic in nature withareas of current pickup and discharge subjectto constant change and with reversals in

direction of current flow.

The effects of telluric current are generally moreapparent on long structures such as pipelinesand tend to be more significant in some areasof the world than in others. In extreme cases,short term currents may be seen which are inthe hundreds of amperes - although theseseldom last more than a few minutes.

Telluric currents are particularly apt to havesome of the characteristics of alternatingcurrent, with attendant reduction in corrosivity.This plus the usual movement of anodic andcathodic areas from point to point distributesthe damage that does occur. For thesereasons, specific correction measures fortelluric currents are seldom used except inextreme cases. Rather, corrective measuresapplied for corrosion control or for other formsof stray current are relied upon to absorb theimpact of telluric currents.

Typical Stray Current Sources

Cathodic protection systems will be describedin more detail in Chapter 3. For purposes ofstray current effects, however, it will besufficient to indicate that "impressed current"cathodic protection systems involve circulatingdirect current between a cathodically protectedstructure and an anode bed through whichcurrent is forced into the earth. A source of DCpower is used to force this current to flow. Thedriving voltage of this power source can rangefrom a few volts to (in unusual cases) over 100volts. Also the current can range from less than10 amps to (for large industrial installations)hundreds of amps.

In any event, this type of installation involvessteady state conditions once installed andadjusted. For this reason, any stray currenteffects on other underground structures in thearea are of the static type of effect rather thandynamic as is the case with other stray currentsources that have been described.

It should be noted that there may be sources ofstatic stray current other than cathodic


protection systems. This could be the case, forexample, with industrial processes. Cathodicprotection installations are used for thisdiscussion since stray current effects from themare encountered frequently in undergroundcorrosion control work - particularly onpipelines.

Stray current effects from static sources suchas cathodic protection systems are oftenreferred to as stray current interference.

Figure 2-15 illustrates a particularly seriouscase of stray current. In the figure, Pipeline "A"is equipped with a cathodic protection system.Pipeline "B", of other ownership, passes closeto the cathodic protection anode bed onPipeline "A" and then passes under Pipeline "A"without touching it. Under this condition, part ofthe current being discharged from the anodebed is picked up by Pipeline "B". This currentthen flows along Pipeline "B" toward the point ofcrossing where it discharges through the earthto Pipeline "A" in order to complete the circuit.At this point-of-crossing discharge area,Pipeline "B" is anodic and corrodes.

The type of situation illustrated by Figure 2-15is one that should never knowingly be allowedto exist. Some of the worst instances of thisnature have occurred when cathodic protectionwas installed not knowing that a pipeline ofother ownership existed in the vicinity. Therehave been cases of such instances where theinterfered-with pipeline has failed within weeksafter the cathodic protection system was placedin operation.

HVDC Systems

Stray currents associated with high voltagedirect current (HVDC) electric transmissionsystems may not demonstrate any particularpattern on recording instrument charts. Undercertain conditions, however, stray currenteffects can be intense over, usually, shortperiods of time. This is illustrated by Figure 2-16 showing the basic elements of an HVDCsystem under (A) normal operations and (B)emergency operation.

In normal operation (bipolar mode) shown inpart (A) of Figure 2-16 note that the twooverhead conductors will be carrying very closeto the same amount of current. Automaticequipment keeps the load balanced betweenthe two halves of the system to accomplish this.Any unbalance current (typically quite moderateas shown) will flow along the earth pathbetween grounding electrodes at the ends ofthe system. This small amount of unbalancecurrent will seldom have a serious stray currentimpact on other underground structures in thearea. This is because grounding electrodes arenormally built so as to be separated from otherstructures and are designed to have lowresistance to earth in order to accommodate thefull load current of the system. The unbalancecurrent can, as shown, be flowing in eitherdirection through the earth path. If this smallunbalance current should have an effect onunderground systems in the vicinity of theelectrodes, recording instrument charts of theaffected underground system voltage to earthwould not necessarily show any particularrepetitive pattern depending, however, on theoperating procedures of the particular HVDCsystem.

Conditions are quite different in the monopolarmode of HVDC system operation shown in part(B) of Figure 2-16. This illustrates whathappens if one half of the HVDC system is outof service because of an emergency or formaintenance.

When this occurs, the entire current flowsthrough one conductor in one direction andthrough the earth in the other direction. This iswhy the HVDC system grounding electrodesare designed to carry full system load current;to do this, they have to be massive affairs.

It is under monopolar HVDC system operatingconditions that the greatest chance of straycurrent effects is possible on other undergroundsystems especially in the vicinity of thegrounding electrodes. This is as shown on part(B) of Figure 2-16 which shows the direction ofstray current flow with a break in the upperconductor of the figure. But if the break were in


STRAY CURRENT CORROSIONFROM CATHODIC PROTECTION SYSTEM

FIGURE 2-15

PIPELINE“B” DISCHARGES CURRENT IN VICINITY OFCROSSING. CURRENT FLOWS THROUGH EARTH TO PIPELINE“A”. PIPELINE “B” IS ANODICAND CORRODING IN THIS AREA.

PIPELINE “A” NOT INELECTRICAL CONTACT WITHPIPELINE “B” AT POINT OF CROSSING

PIPE PICKS UP CURRENT(AND IS CATHODIC) IN VICINITYOF ANODE BED

ANODE BED

CATHODICPROTECTIONDC POWERSOURCE UNDERGROUND

PIPELINE “B”

UNDERGROUNDPIPELINE“A”

(+)(–)

HIGH VOLTAGE DC TRANSMISSION LINE

FIGURE 2-16

Bipolar Operation

No Current in the Earth

Source Load

Source Load

Stray Current

Monopolar Operation

Showing Earth

the lower conductor, the stray current flowwould be in the opposite direction. Thus twodiffering patterns of stray current effect arepossible.

Depending on geological earth conditions in thevicinity of a grounding electrode, the radius ofthe area around an electrode within which straycurrent pick-up or discharge may be expectedcan be measured in miles under monopolaroperating condition. This is because the drivingvoltage forcing the current to flow through thegrounding electrode resistance will be higherunder the monopolar than bipolar operation.

Operators of HVDC systems do their best tokeep the length of time that a monopolarcondition exists to as short a time as possible.

An HVDC system is considered a static sourcesince large fluctuations of current generally donot occur, even under monopolar operation.

Typical Dynamic Stray Current Sources

DC Transit Systems

Some types of transit systems which are DCpowered operate with an electrically isolatedpositive conductor to the car or train but use therails as a negative return conductor. Where therails are in contact with earth, part of the returncurrent leaves the rails and enters the earth.This earth current is the "stray" current. Wherean underground structure happens to befollowing the same path, it picks up some of thisstray current as described earlier. An exampleis illustrated by Figure 2-17.

A well maintained transit system will have therails on high resistivity ballast or concrete invert.This increases the resistance to earth andreduces the current discharge to the parallelearth path. All-welded rails or mechanical jointsin the rails that are equipped with bond cablesacross the joints keep the rail resistance as lowas practical. A poorly maintained transit system,on the other hand, will cause a greater amountof the return current to follow the earth and thusincrease the corrosion hazard on underground

structures in the area.

DC Mining Operations

Were DC power is used for underground miningoperations (mining cutters. ore transporttrackage, accessory equipment, etc) the basicstray current problem can then be generallysimilar to that described for DC transit systems.It is another example of dynamic stray currents.This is as illustrated by Figure 2-18.

Whereas the existence of a transit system isobvious, this is not necessarily the case withmining operation - particularly so with a crosscountry pipeline which could be some distancefrom surface facilities associated with the mine.Although DC substation equipment will usuallybe located at the mine entrance (and will beobvious) there may be additional DCsubstations located underground. These arenot obvious - - unless one finds an AC powercable going underground for no good apparentreasons and then deduces that it may be goingto an underground substation.

As shown in Figure 2-18, the current pick-upareas on a structure such as a pipeline move asthe DC power-using equipment moves,whereas the current discharge area tends to beconcentrated in the area(s) of the substation(s).This can be complicated, however, byoccasional moving of substations to newlocations as the pattern of mining operationchanges. But by keeping tabs on the locationsof all substations (once it is discovered thatthey are a problem), one will know where toexpect corrosion problems.

Just to make life more difficult for the corrosionworker, it is possible to encounter DC miningoperations where it is the positive (+) side of theDC power supply that is grounded to the railsystem while the insulated side of thedistribution system is negative (-). To visualizethe effect of this, just reverse the direction of allcurrent flow arrows on Figure 2-18 Now therewill be concentrated areas of current pick-up ona structure such as a line in the vicinity ofsubstations while the current discharge areas


SOURCE OF DC CURRENT (SUBSTATION)

DC TRANSIT SYSTEM AS A SOURCE OFSTRAY CURRENT CORROSION

FIGURE 2-17

DISCHARGE AREA(CORRODING)

OVERHEAD POSITIVE CONDUCTOR TYPICALLY600 VOLTS (+) TO GROUND

MOVING PICKUP

STEEL RAILSCAR OR TRAIN(LOAD)

UNDERGROUNDSTRUCTURE

(+)

(–)

CURRENT FLOW

ANODIC DISCHARGE AREAPIPE CORRODESCATHODIC PICKUP

AREA

(–)(+)

MINE HEADING

CORROSION BY STRAY CURRENTFROM DC MINING OPERATIONS

FIGURE 2-18

STEEL RAILS GROUNDED

ISOLATED POSITIVE FEEDER SYSTEM

CURRENT FLOW UNDERGROUNDDC SUBSTATION

ORE CARS CUTTER

PIPELINE

AC POWER TO UNDERGROUNDDC SUBSTATION VIA BORE HOLE

IE

R

(with attendant corrosion) move as theunderground DC power-using equipmentmoves. This makes it that much more difficult topredict where the worst stray current problemsmay be expected. DC Welding Operations

This is another example of dynamic straycurrent. Where DC welding is performed withone conductor connected directly to the piecebeing welded while the other conductor isconnected to the welding electrode holder,there is little chance for significant stray current.However, on major plant construction projects,shipyards, etc, the current may come from a DCwelding generator station some distance fromthe point where the welding is being performed.To reduce the amount of cable required, thestructure itself may serve as a current-carryingmember between the generator station andpoint of work just as do the rails in a DC transitsystem. Where part of the structure is buried orsubmerged, there can be parallel earth pathswith current pick-up and discharge areas; thelinear distance between welding generatorstation and point of work may be small enoughand the conductivity of the structure so great,however that this may not be a serious factorduring the construction period - - but shouldalways be a point of consideration.

What can result in serious stray currentdamage, is DC welding on a part of the buriedstructure which is not electrically connected tothe rest of the structure. This is illustrated byFigure 2-19.

With the structure on both sides of the gap incontact with earth, welding, current can passthrough the earth to complete the circuit. Witha small gap the current discharge may beconcentrated in a relatively small area. With thehigh currents typically associated with welding,corrosion damage can be rapid. There havebeen cases where failures have occurred oncritical elements of structures before initialconstruction was completed.

FACTORS AFFECTING THE RATE OFCORROSION

General

The rate of corrosion is directly proportional tothe amount of current leaving the anodesurface. This current is related to both potential(voltage) between the anode and cathode andthe circuit resistance.

Voltage, resistance, and current are governedby Ohm’s Law:

Where:

I = Current (Amperes or Milliamperes)

E = Voltage (Volts or Millivolts)

R = Resistance (Ohms)

Essentially, Ohm’s Law states that current isdirectly proportional to the voltage and inverselyproportional to the resistance.

Polarization

Polarization is a retardation in the rate ofcorrosion, or current flow, caused by areduction in the voltage difference or anincrease in resistance between the anode andcathode. There are two types of polarization,activation and concentration.

Activation polarization is inherent in the reactionitself. Essentially, it represents the speed atwhich electrons can be transferred from thecathode surface to a cation (ions attracted tothe cathode) such as hydrogen. Consequentlythe rate of hydrogen evolution, and hence therate of corrosion, differs for different metals. Astime progresses, hydrogen may adhere to thesurface of the cathode, thus reducing the rateof electron transfer to hydrogen ions in theelectrolyte.


GAP

RAPIDCORROSION

ELECTRICALLY ISOLATEDPORTION OF UNDERGROUNDSTRUCTURE

HYPOTHETICAL UNDERGROUNDPORTION OF STRUCTURE

(+)

DC WELDINGGENERATOR STATION

CORROSION BY STRAY CURRENTFROM DC WELDING OPERATIONS

FIGURE 2-19

WELDING ARC

(-)

Concentration polarization represents adepletion of cations adjacent to the cathodesurface. If hydrogen is being evolved, forexample, the availability of hydrogen ionsaround the cathode may be reduced, thusreducing the rate of corrosion.

The evolution of hydrogen may cause a film ofhydrogen to develop over the cathode surface.Likewise, corrosion products may adhere to theanode surface. Both of these reactions causean increase in the anode to cathode resistance,and thus a reduction in the rate of current flow.

Electrolyte Resistivity

As was mentioned above, corrosion obeysOhm’s Law.

Consequently, structures placed in highresistivity electrolytes tend to corrode moreslowly than those in low resistivity envi-ronments. For this reason, salt water isgenerally more corrosive than fresh water.

Voltage Difference Between Anode andCathode

This is particularly applicable to dissimilar metalcouples. The greater the voltage differencebetween two metals, the greater the rate ofcorrosion (again, after Ohm’s Law). Of course,the rate of corrosion may be reducedconsiderably from the initial rate due topolarization. Stainless steel polarizes muchfaster than copper, for example, when each areconnected to carbon steel. Consequently, thecorrosion rate between carbon steel andstainless may end up being much less than thatbetween carbon steel and copper, even thoughthe initial voltage differences may be about thesame.

Anode/Cathode Ratio

The relative area between the anode andcathode greatly affect the rate at which theanode corrodes. If the anodic area is small inrelation to that of the cathode (a galvanizedfitting in a brass pipe for example - Figure 2-

20A), the anode will corrode rapidly. This isbecause the corrosion current is concentratedin a small area. Also, the large cathode may notpolarize easily, thus maintaining a high rate ofcorrosion.

When a large anode is connected to a smallcathode (a brass fitting in a galvanized pipe forexample - Figure 2-20B), corrosion is spreadout over a large area. Thus, the concentrationof corrosion current is not so dense, and theanode corrodes more slowly. Polarization mayplay an important role here, too. The smallcathode may polarize rapidly, reducing the rateof corrosion current flow.

Effect of the Metal Itself

Different metals corrode at different ratesbecause of the difference in the amount ofenergy stored in them. The rate is expressed byFaraday's Law:

W = K x I x TWhere:

W = Weight Loss in One Year

K = Electrochemical Equivalent in PoundsPer Ampere Per Year

I = Corrosion Current in Amperes

T = Time in Years

Table 2-2 gives the electrochemical equivalentstated in grams per coulomb, and consumptionrates of some typical metals in pounds perampere-year. This is the weight of metal thatwill be corroded away by one ampere of directcurrent continuously discharging from a metalinto a surrounding electrolyte for a period of oneyear.

Table 2-2 also includes the volume of the metalwhich will be removed per ampere-year. This isthe critical figure as it illustrates the amount ofmetal that will be removed from, for example, ahigh pressure steel pipeline for a given amountof current.


ANODE-CATHODE SIZE RELATIONSHIP

FIGURE 2-20

BRASS COUPLING(CATHODIC)

DIRECTION OF CURRENT FLOW

GALVANIZED IRON COUPLING(ANODIC)

BRASS PIPE(CATHODIC)

A – SMALL ANODE – LARGE CATHODE (SERIOUS CORROSION)

GALVANIZEDIRON PIPE(ANODE)

B – LARGE ANODE – SMALL CATHODE (LESS INTENSE CORROSION)

TABLE 2-2

CONSUMPTION RATES OF TYPICAL METALS

MetalElectrochemical Equivalent

(Grams per coulomb)

Consumption Rate(Pounds per

Ampere-year)

Volume of MetalConsumed

(Cubic inches perAmpere-year)

Carbon* (C+++)** 0.4149 x 10-4 2.89 36.99

Aluminum (Al+++) 0.9316 x 10-4 6.48 69.99

Magnesium (Mg++) 1.2600 x 10-4 8.76 141.47

Iron (Fe++) 2.8938 x 10-4 20.12 70.81

Nickel (Ni++) 3.0409 x 10-4 21.15 67.06

Copper (Cu++) 6.5875 x 10-4 45.81 142.89

Zinc (Zn++) 3.3875 x 10-4 23.56 90.87

Tin (Sn++) 6.1502 x 10-4 42.77 162.43

Lead (Pb++) 10.736 x 10-4 74.65 181.68

* Carbon is not strictly classified as a metal but as a metalloid -- but subject to consumption as a metal.

** Each metal is followed by its chemical symbol. The number of (+) signs following the symbol indicatesthe valence (a chemical term) for a typical anode reaction. The electrochemical equivalents arecalculated on the valence shown. Other valences may apply under certain conditions for some metals.

As a example, if a steel pipeline weredischarging only 1 milliamp (0.001 ampere) ata given spot, the volume of iron removed peryear would be 70.81 cubic inches per ampereyear x 0.001 ampere or 0.07081 cubic inches.If concentrated in small areas (such as atdefects in an otherwise good coating), this issufficient to cause approximately 4 one quarterinch diameter holes in a three eights inch thickpipe wall in a years time. This example is givento show how important it is to control thedischarge of even very small amounts ofcurrent from an underground steel structure intosurrounding earth or water.

From the table it will be seen that the corrosionrates and volumes of metal removed varyconsiderably. Of the metals shown, lead hasthe greatest corrosion rate and the greatestvolume loss. This emphasizes the critical needfor effective corrosion control on, for example,lead sheaths on underground power andtelephone cables. Formerly, lead sheaths wereused widely but new construction favors plasticsheathing.

Passivation of the Metal Surface

The ability of a metal to form a protective filmgreatly at affects the rate at which it willcorrode. The film is often an oxide and isgenerally invisible. The development of suchfilms is the reason for the corrosion resistanceof such metals as aluminum, chromium,stainless steels, lead and others.

When a film breaks down, but is immediatelyrestored, passivity continues. Should the filmnot be restored, as in the case of a crevice instainless steel where there is little or no oxygen,then rapid corrosion may occur. Pittingcorrosion will frequently take place at a pointwhere film breakdown is sustained.

SUMMARY

The corrosion control worker needs tounderstand what causes corrosion. Thisrequires knowledge of the electrochemicalprinciples involved and the various mechanisms

which create corrosion cells. The factors thataffect the rate of corrosion need to beunderstood, too. Armed with this knowledge,corrosion control personnel are ready for thenext step - corrosion control. Methods ofcorrosion control are discussed in the nextchapter.


CORROSION CONTROL METHODS3-1

CHAPTER 3

CORROSION CONTROL METHODS

INTRODUCTION

In Chapter 2, Corrosion Fundamentals, some ofthe various sources of galvanic corrosion andstray current corrosion were discussed. In thischapter, we will outline various methods andprocedures that are in common usage forcontrolling the two general types of corrosion.

SUMMARY OF CORROSION CONTROLMETHODS

Following are the types of corrosion controlmethods applicable to galvanic corrosionproblems and to stray current problems. It willbe noted that some of these methods areapplicable to either category of corrosionproblems.

For Galvanic Corrosion Control:

S CoatingsS Cathodic protectionS Isolating joints

For Stray Current Corrosion Control:

S CoatingsS Cathodic protectionS Isolating jointsS Drainage bonds and forced drainage

bondsS Reverse current switches

Each of the several types of corrosion controlwill be described under following headings.

COATINGS

It has been learned in the preceding chapterthat wherever direct current flows from anunderground metallic structure into asurrounding electrolyte (earth or water), themetal will be consumed (corroded) at the pointsof current discharge. Following simple logic, ifan isolating barrier (a coating) were to beplaced between the metal and the surroundingelectrolyte, corrosion current could not flow andthere would be no loss of metal. This isabsolutely true.

Before we decide that coatings are the ultimateanswer to corrosion control on undergroundstructures and that we need look no further, itmust be understood that there is a veryimportant catch to the preceding statements onthe use of coatings. It is this. If a coating is toperform as stated, it must be perfect and it mustremain perfect. On extensive undergroundstructures such as pipelines, it is noteconomically or practically feasible to apply anisolating coating that will meet these criteria.

To illustrate the difference between a perfectcoating and one that has imperfections, Figure3-1 shows the difference in corrosionperformance under the two conditions. Thecorrosive environment shown is that for astructure in dissimilar soils as was first shownby Figure 2-8 of Chapter 2 under the heading,Dissimilar Soils.

Part "A" of Figure 3-1 shows that even thoughthere is a galvanic voltage difference of 0.2 voltbetween Soil "A" and Soil "B", the isolatingbarrier interposed by the perfect coating

��

��

�

��

��

��

��

��

��

��

��

�

��

��

��

��

��

��

��

�� !"

��

�#$%&'#() (*�+$$%)' �,(-

�(,% #) �'.%$-#/%�0&%,,%)' �(1'#)2

�%$*%&'�(1'#)2

�( �+$$%)' �,(-

�

�


prevents current flow to or from the pipe tocomplete an electrical circuit. With no currentflow to or from the pipe, there can be nocorrosion on the pipe exterior surface.

Part "B" of Figure 3-1 shows what happenswhen an otherwise excellent coating developspinholes, scrapes, or other imperfections(known as coating "holidays") which canexpose small areas of pipe surface to thesurrounding earth. With the exposed areas atcoating flaws, there is now a path for current toflow as a result of the existence of the dissimilarsoil galvanic corrosion cell driving potentialshown. Current will now discharge from thepipe at coating flaws in anodic areas (andcause corrosion at these locations) and willreturn to the pipe at coating flaws in cathodic(non-corroding) areas.

In Chapter 2 it was pointed out that one of thefactors controlling the amount of corrosioncurrent flowing (and amount of metal loss) in agiven galvanic corrosion cell is circuitresistance. In Part "B" of Figure 3-1, thepresence of the isolating coating, even thoughflawed, will substantially increase the totalcircuit resistance. This means that the totalgalvanic cell current flow will be substantiallyless. This means, further, that the amount ofmetal corroded away during a given period oftime will be much less than would occur if thestructure were uncoated under otherwisesimilar conditions.

Do not conclude from the preceding paragraphthat corrosion of a coated structure is notserious even with imperfections in the coating.What happens is this. The corrosion cell currentflowing, even though reduced, is concentratedat the metal exposed at holes in the coating.This means that at current discharge points inanodic areas the current discharge density interms of milliamps per unit area can be higherthan would be the case if the same structurewere entirely bare. Because of this, a structuresuch as a coated high pressure pipeline can (inthe absence of other corrosion controlmeasures) experience its first leak sooner thanwould have been the case if the pipeline had

been laid bare. This can be true even thoughthe total metal loss is less than on the bare pipebecause of the reduced total corrosion cellcurrent.

Some of the factors that operate to preventapplying and keeping a perfect coating on alarge underground structure are:

1. Undetected handling damage duringconstruction.

2. Stones or debris in backfill that force theirway through the coating.

3. Soil movement or structure movement withpressure changes (pipelines).

4. Tree roots working through coating.

5. Excavations by others which expose thestructure and damage the coating withexcavation equipment.

Even with these hazards, the use of suitablegood coatings and good application andmaintenance practices can result in a coatingwhich is initially 99.9% perfect or better -- andwhich will decrease only a few percentagepoints with time. This indicates that the majorityof the structure surface will be free of corrosiondamage. It is the remaining surface area thatneeds additional corrosion control measures.

In addition to reducing the total corrosionhazard from galvanic corrosion cells, coating ona structure will also reduce stray currentdamage by increasing the circuit resistance.This reduces the amount of current pickup.Even so, the total stray current can be muchgreater than that resulting from galvaniccorrosion cells. At points of stray currentdischarge on coated structures, theconcentration effect at coating imperfections asdescribed earlier, can be much more intensethan under galvanic corrosion cell exposurewith very rapid structure penetration. Again,additional corrosion control measures areneeded to supplement the coating.


Further detailed treatment of coatings onunderground structures is included in theChapter 4 of the Basic Course.

CATHODIC PROTECTION

This is a widely used and effective method ofcorrosion control. It can be used to controlcorrosion on bare underground structures. It iscommonly used as a supplement to coatings onunderground structures. For pipelines carryinghazardous materials, for example, the use ofcoatings and cathodic protection is required byFederal regulations on all new construction.And on existing facilities in this category,coated or bare, cathodic protection in corrodingareas is similarly required by regulations.

The following material under this heading willdiscuss (1) the theory of cathodic protectionand the way it works, (2) the types of cathodicprotection in common usage, and (3) theapplicability of cathodic protection to galvaniccorrosion cell conditions and to stray currentexposure conditions.

Theory of Cathodic Protection

The basic theory of cathodic protection issimple. In Chapter 2 it was demonstrated thatan unprotected underground structure will haveanodic areas where current discharges to earthand corrodes the metal and will have cathodicareas where current flows from the environmentonto the structure. Most importantly, there is nocorrosion in the cathodic areas.

Now if, by some means, the anodic areas couldbe converted to cathodic areas, the corrosionshould stop. This is exactly what cathodicprotection does. Direct current is forced to flowinto the earth through an anode connectionoutside the structure and then through the earthto the structure to be protected. The amount ofcurrent forced to flow onto the structure isadjusted to a level which will nullify currentdischarge in anodic areas and result in netcurrent collection in these areas. What hadbeen anodic areas are now cathodic and the

areas which were originally cathodic are stillcathodic.

In theory, once the entire exposed metalsurface of the structure is collecting currentfrom the conducting environment, the entirestructure becomes a cathode and corrosionstops. Hence the name "cathodic protection" forthis method of corrosion control.

How Cathodic Protection Works

An illustration of how corrosion works inprinciple is shown by Figure 3-2.

As can be seen from the figure, some source ofDC potential (discussed later) forces current toflow through a wire or cable connection fromthe protected structure to the anode bed, thenthrough the earth to all metal surfaces of thestructure which are exposed to surroundingearth or water, and finally along the structureitself to the drainage cable connection tocomplete the circuit. It should be noted thatcurrent which flows from an undergroundstructure through a metallic connection (suchas the cathodic protection drainage cable) doesnot cause corrosion of the structure.

In Chapter 2, it was established that any metalor metalloid that discharges direct current intoa conducting electrolytic environment (earth orwater) corrodes. This, then, means that theanode bed of Figure 3-2 should corrode - and itdoes. Accordingly, it can be said that in onesense, cathodic protection does not eliminatecorrosion; it simply transfers the corrosion fromthe structure being protected to the cathodicprotection anode bed. The anode bed can bereplaced periodically without endangering theworking structure. Further, anode beds aretypically designed to permit discharge of thedesired amount of current for a period of yearsbefore replacement is necessary.

The amount of current to be discharged fromany single cathodic protection installation willdepend largely on the amount of exposed metalon the structure (or portion of a structure) to beprotected by that installation.

��

��

��

��

�� 3 � ��

��3 ��

��

�� 4�

�� !�

��


At this point, it is important to stress thedifference between cathodically protecting anuncoated underground structure and protectingthe same structure with a good coating servingas an isolating barrier between structure andadjacent earth or water. If, for example, a barestructure were found to require 100 amperes ofcathodic protection current to attain completeprotection, the same structure with a goodcoating might require less than one ampere forthe same degree of protection.

To take an actual case history, a 70-mile lengthof 26-inch natural gas pipeline with an excellentcoating buried in a low resistivity corrosiveenvironment could, during its early life, becompletely cathodically protected throughout itslength with the current from a single flashlightbattery. Had this pipeline been bare, the currentrequired would have been in the order of 2,500amperes under the most optimistic conditionsbased on a current consumption of only onemilliampere per square foot of bare surface (atypical minimum amount for a new-steel barestructure).

Note that there is no fixed ratio that can beused between current required to protect a barestructure and current to protect the samestructure if coated. It depends very largely onthe quality of the coating used, the type ofstructure, and the design of the cathodicprotection installations used.

The preceding paragraphs do demonstrate,however, that cathodic protection and coatings,used together, comprise an effective team. Thecoating provides corrosion protection for thebulk of the structure surface while the cathodicprotection current seeks out and protects metalexposed to the environment at the inevitableflaws in the coating layer.

Granting that the theory of cathodic protectionis simple, the question always arises as to howit can be known that full cathodic protection hasactually been attained on an undergroundstructure. There are several criteria that can beused. These are discussed in detail in theintermediate course. At this point, however, one

criterion in common usage should bementioned. This is the measurement of thepotential between the structure and a standardreference electrode (such as a copper-coppersulfate reference electrode) contacting theadjacent environment. In the case of steelstructures, such potential measurements madebefore applying cathodic protection will typically(depending on the structure) be in the range of-0.2 volt to -0.7 volt with respect to acopper-copper sulfate reference electrode. Withthe application of cathodic protection current,these structure potentials will move in the morenegative direction. When values of at least-0.85 volt are reached at all points, it is a goodpractical indication that a sufficient amount ofcathodic protection current has been applied tostop corrosion. Note the words, "good practicalindication". It is always possible that there canbe local unusual conditions where less than fullprotection is reached in small areas -- thesemay not be revealed by the potentialmeasurements taken. With good structureconstruction practice and effective coatings (ifcoatings are used), these "unusual" situationsor anomalies should be at a minimum.

Types of Cathodic Protection

Figure 3-2 discussed under the precedingheading indicated that direct current was forcedto flow along the wire from the cathodicallyprotected structure to the anode connection oranode bed. Forcing current to flow along a wireimplies the presence of a driving potential (orvoltage). This will always be present as part ofa cathodic protection installation.

There are two general types of cathodicprotection installation which differ primarily inthe manner in which a voltage is obtained toforce cathodic protection current through itscircuit. These two types are commonly referredto as galvanic (or sacrificial) anode cathodicprotection and impressed current cathodicprotection. These are described below.


Galvanic (or Sacrificial) Anode CathodicProtection

When we use the word "galvanic", we mightconclude that a galvanic potential is what forcesthe current to flow. This is, in fact, the case.This type of cathodic protection depends on thevoltage difference between dissimilar metals tocause a current to flow.

Actually, practical use is made of a dissimilarmetal corrosion cell as was discussed inChapter 2. In this practical use, however, adissimilar metal cell is needed that has asubstantial driving potential (voltage) betweenthe two metals. For example, if a steel structureis to be protected, the practical galvanic series(Table 1, Chapter 2) indicates that the initialvoltage difference between magnesium alloyand steel could range from 0.8 volt to 1.4 volts(depending on the condition of the steel). Thisvoltage difference can be used to advantagesince, in the steel-magnesium couple, the steelis cathodic and the magnesium is anodic.

With the above in mind, the essentials of agalvanic anode cathodic protection installationare shown by Figure 3-3. The anode materialcan be magnesium (as shown) or may be zinc(lower cell voltage -- used in low resistivity soilsas a general rule) or may be aluminum (lowercell voltage -- particularly useful in a sea waterenvironment). The chemical backfill shown inthe figure is normally used with buriedmagnesium or zinc anodes to obtain uniformanode consumption and maximum anodeefficiency. The anode material is normallyavailable in cast shapes of various sizes to fitthe requirements of differing galvanic anodecathodic protection installation designs.

For very small current requirement at onelocation, a single galvanic anode may besufficient. If more current is needed, severalanodes may be connected in parallel as shownby the dashed lines in the figure. Whatever thecurrent output designed for a particularinstallation may be, the design typicallyprovides for sufficient anode material todischarge design current for a selected period

of years before replacement is necessary. Thisis based on the electrochemical equivalent ofthe anode metal used (as discussed in Chapter2) with allowances for anode efficiency and forreplacement before the anodes are completelygone.

Galvanic anode installations have certainadvantages such as:

1. They are self-powered. No dependence onoutside sources of power.

2. Low maintenance requirements.

3. Minimum probability of stray currentinterference on other undergroundstructures.

There are also limitations such as:

1. Low driving voltage.

2. Relatively low current output capacity.

Impressed Current Cathodic Protection

Whereas galvanic anode installations are self-powered, impressed current installations utilizea separate source of direct current. The currentfrom this source is impressed on the circuitbetween the structure to be protected and theanode bed. The essential components of sucha system are shown by Figure 3-4.

An AC to DC rectifier is shown as the DCsource in the figure. Such devices are widelyused in cathodic protection work. Basically, theyuse DC power from a commercial source (suchas 120, 240, or 440 volts, single phase or threephase), step down the voltage, and rectify it toDC. The DC output voltage can normally beadjusted so that the desired current can beforced to flow through a range of circuitresistances. Rectifiers are available with a widerange of current output capacity and DC outputvoltage.

Although the rectifiers more commonly used arethose having a fixed output, once adjusted to

��

�� !

��

��

�� 4��

��

��

��

�� !5

�� 3��

��

��

�� 6 �� 7��6��6 ��!�� 6 �� 4��

��

��

��


suit a specific cathodic protection installation,there are automatically controlled rectifiers withsolid state control circuitry. Units can beobtained which will maintain a desired currentoutput with changes in the circuit resistance.Others are available which will maintain a fixedstructure to earth (or water) protective potentialwith changes in the current needed to maintainthat potential.

Other sources of DC power can also be used.Among these are engine-generator sets whichuse (for example) propane or natural gas topower the engine; solar panels which convertsunlight to DC energy; thermoelectricgenerators which use a fuel source to heat aseries of metallic thermal junctions whichgenerate direct current; and (in some areas)windmill-generators which develop the neededDC energy.

Figure 3-4 illustrates one type of anode bedwith anodes installed vertically near the surfaceat a selected design spacing and connected inparallel. The anodes can be installedhorizontally in some installations. The line ofanodes can be perpendicular to the structurerather than parallel to it as shown. In still otherdesigns, the anodes may be placed in a singledeep hole (sometimes hundreds of feet deep)in order to reach a favorable soil formation or toavoid stray current effects on otherunderground structures in the area. These arecommonly referred to as deep anode beds.

The anodes, since they discharge current, willcorrode. In order to attain maximum life,low-consumption-rate materials such asgraphite, high-silicon cast iron, magnetite, orprecious metals are used. Additionally, suchanodes are normally backfilled with acarbonaceous material such as coke breeze orgraphite particles which tends to be consumedby the current discharged before the basicanode material is seriously attacked. The usualanode bed design is such that sufficient anodematerial is provided to last a period of years(such as 25) at the design current output beforereplacement is necessary.

Impressed current installations have certainadvantages such as:

1. A wide range of DC voltage and currentoutput capacities. This provides greatflexibility in system design.

2. Single installations which will protect muchlarger structures (or portions of structures)than is usually possible with single galvanicanode installations.

There are also disadvantages such as:

1. Greater maintenance requirements than forgalvanic anode installations.

2. Dependence on availability of adependable power supply or fuel supply.

3. Continuing cost of energy where AC poweror a fuel supply is required.

4. Greater possibility of stray currentinterference on other undergroundstructures than is the case with galvanicanode installations.

Cathodic protection is applicable to structuressubject to galvanic corrosion. It also can beused to offset the effects of lesser stray currenteffects such as those from relatively remoteman-made sources or from telluric currents.

In this introduction to cathodic protection, thedetails of cathodic protection system designswere not covered. Further information on theconstruction and design of such systems iscontained in the intermediate and advancedcourses.

ISOLATING JOINTS

This method of corrosion control utilizes variousisolating devices which are inserted in pipelines(or other structures where applicable) and willblock the flow of electrical current withoutdisturbing the functional requirements of thestructure itself.


Isolating joints are useful for the followingpurposes:

1. To separate dissimilar metals.

2. To confine cathodic protection current to astructure (or portion of a structure) to becathodically protected.

3. To confine and reduce the effect of straycurrent to manageable proportions (whereapplicable).

An example of the use of isolating joints isillustrated by Figure 3-5. The application shownis the electrical isolation of a new steel pipereplacement in an old steel pipeline system. InChapter 2 it was learned that if severe corrosionhas occurred on a section of old pipeline andthat section is replaced by a piece of new pipewelded into the old line, the piece of new pipewill be anodic and will corrode with relativerapidity because of being electrically joined toadjacent cathodic old pipe sections.

An effective replacement procedure, asillustrated, is to make the replacement withwell-coated pipe which is electrically isolatedfrom the adjacent system by use of isolatingjoints. This nullifies the effect of the new steel -old steel galvanic couple. Corrosion is furthercontrolled on the replacement by use ofgalvanic anode(s) for cathodic protection of thereplacement. The isolating joints serve toconfine the cathodic protection current to justthe replacement section.

Typically, isolating joints in common usageinclude but are not limited to, isolating flangesets, isolating compression couplings,proprietary weld-in prefabricated devices, andisolating unions or isolating bushings forthreaded pipe assemblies.

DRAINAGE BONDS

This type of corrosion control method isparticularly applicable to stray current corrosionproblems. Since direct current drained from an

underground structure by way of a wire or cabledoes not corrode the structure, the drainagebond concept is based on providing a metallicpath by which stray current discharging from ahighly anodic area on an affected structure canreturn to its source without damage to thestructure.

Figure 3-6 illustrates a simple case of the useof a drainage bond to control corrosion on anunderground pipeline caused by stray currentfrom a transit system. To be effective, thedrainage bond cable must be large enough, andas short as possible, to drain the maximumstray current involved with minimum voltagedrop in the resistance of the drainage bondcable itself.

It should be noted that once the drainage bondcable is installed as shown, the resistance tocurrent discharge will normally be less than wasthe case when current was being dischargedfrom pipeline directly to earth. This means thatthe stray current pickup in pickup areas tendsto be greater. Basically this is not harmful asthese areas are cathodic and non-corrosive.

There are situations where a simple bond, asdiscussed above, may not be fully effective.These conditions are described under thefollowing two headings.

REVERSE CURRENT SWITCHES

Where there are multiple sources of DC currentin a stray current source, such as multiple DCsubstations on a transit system, a problemarises if one or more of the substations is takenout of service during light load periods on thesystem. At an out-of-service substation, asimple drainage bond would serve as a paththrough which stray current could flow in thereverse direction through the bond onto theunderground structure. This could greatlyincrease the amount of stray current picked upby the structure and further complicate thedrainage problem at working substations.

To offset the above effect, reverse current

�� 3��

�� 8��

�� !9

�� 3 �� 3

�� 3��

�� ! ��

��8��

��8��

��3 ��

��

�� 3 ��

�� !:

��

��3 ��

��

��3 ��4��

�� !! ��

��


switches can be used in drainage bond cables.These permit current to flow in the desireddirection from the underground structure to thesubstation negative terminal; but when a straycurrent source system condition arises whichtends to cause current to flow in the wrongdirection through the drainage bond, the switchopens and prevents undesired stray currentpickup by the underground structure. Somedevices in this category utilize an actual switchoperated by reverse current sensing circuitry. Insome applications, one way solid state diodesmay be used.

The application of a reverse current switch to atypical instance is illustrated by Figure 3-7.Although, as shown on the figure, it would bepossible for stray current discharge from thepipeline to flow in the indicated direction to therails by way of the stray current drainage bondand the substation negative cable to rails, thiswould seldom occur. This would be becausethe rails would tend to be positive to thepipeline. With the substation out of service asshown, the more normal condition would be thatthe reverse current switch would be open andthat the current in the circuit would be zero.

FORCED DRAINAGE BONDS

In some instances where stray current drainageis required to correct a corrosive condition onan underground structure, the distance from thedrainage point on the underground structure tothe negative terminal of the stray current sourceis so great that a simple drainage bond willhave too much resistance to permit drainingenough current to fully correct the corrosiveexposure on the structure. Some means mustbe used to ensure that sufficient current isdrained by compensating for the voltage drop inthe drainage cable -- by Ohm's Law, this dropwill be the stray current drained in ampsmultiplied by the total cable resistance in ohms.

The desired compensation can be attained bymeans of forced drainage. This means insertinga voltage source into the drainage bond circuitto force current to flow through the bond in at

least the amount that would flow if the drainagebond had zero resistance. One means ofaccomplishing this is by inserting the output ofan automatically controlled AC to DC rectifierpower source into the drainage bond circuit.The automatic control circuitry in the unit willautomatically control the applied compensatingvoltage to match the voltage drop in thedrainage bond cable. The unit used will have tohave a current carrying capacity to match themaximum stray current to be drained.

An added advantage is that the forced drainageunit may be adjusted to have a selectedminimum current drainage. This means that ifthe DC substation to which the drainage bondis connected is out of service, there will be atleast some current being drained back to thestray current source system.

The concept of the system described above isas illustrated by Figure 3-8. The details of suchan installation are subject to forced drainageequipment used, method used for automaticcontrol, etc.

��

��!��!��

�� 3 ��

�� !;

��

��3 �� 3!! �� 3 � <��

��

��3 ��4��

��

�� 8�� 3 ��

��

�� 3 ��

�� !=

��

�� !�� 3�

��

��3 ��4��

��

��

��

��

�� 3 ��

CHAPTER 4

INTRODUCTION TO PIPELINE COATINGS

The pipeline coating is the most integral part ofa cathodic protection system. Without theproper use of pipeline coatings, a cost effectivecorrosion control program cannot bemaintained. Chosen and applied properly, apipeline coating can extend the design life of apiping system to well over 25 years. However,in order to be most effective, a pipeline coatingmust be used in conjunction with some form ofcathodic protection.

A pipeline coating is a barrier between the pipeand the electrolytic processes found in theearth. An effective pipeline coating system mustpossess excellent cohesive and adhesive bondstrength to the pipe, be impervious to waterpenetration and provide good electricalresistance.

A laboratory test program with a battery ofindividual tests can determine the strengths andweaknesses of a particular coating system. Acomparative laboratory analysis of cohesiveand adhesive bond strengths of coatings canbe determined through a standardized ASTMG-8 Salt-Crock Cathodic Disbondment Test.The test involves drilling an intentional defect or"holiday" into a coated pipe sample. The pipesample is then connected to an electricalsupply source consisting of a rectifier or anodeat typically 1.5 volts for 30, 60 or 90 days induration. The pipe sample is then submergedinto a chamber filled with a 1% salt concentrateat a temperature of 73EF. At the end of the testperiod, the pipe samples are then removedfrom the chamber. At the "holiday" area, coatingis then peeled back to where there is still goodadhesion and the area of disbondment ischecked and recorded.

The dielectric resistance is the amount ofvoltage necessary to breakdown a givencoating of specified coating thickness.

An ASTM G-9 Lab Test can determine thedegree of water penetration into a pipelinecoating given a dielectric constant. The test isoften comparative in nature. If the dielectricproperties of a coating are lost through waterabsorption, then coating effectiveness isseverely weakened.

When checking for defects or "holidays" on thepipe coating surface, an electrical inspectiontool or "holiday" detector should be utilized. Byusing a 100 volts per mil of coating thicknesscriteria, one can pinpoint "holidays" in thecoating without dielectrically breaking down thecoating. ASTM G-62 lists standard testprocedures for setting the "holiday" detectorwhen electrically inspecting for coating defects.If there is uncertainty in the field as to theproper setting, one may test simply by creatingan intentional "holiday" in the coating. Byrunning the "holiday detector" over the flawedcoating area several times, one can determinethrough trial and error the proper setting.Normally this is done by listening to a "beeping"sound and calibrating the length of spark givenoff by the "holiday" detector.

When using a "holiday" detector over periods oftime, it is imperative to check the batteries atleast once a day. It is also extremely importantto keep the tail piece grounded to the earth inorder to complete the circuit. When it is notpossible to achieve a proper ground to theearth, one must ground to the pipe.

INTRODUCTION TO PIPELINE COATINGS4-1

Pipeline coatings are applied either at a coatingfacility or in the field. In either case, propersurface preparation of the pipe is critical to thesuccessful performance of a coating. The SteelStructures Painting Council (SSPC) haspublished a standard listing various degrees ofsurface preparation. An SSPC #6, CommercialBlast Cleaning or an SSPC #10, Near-WhiteBlast Cleaning are generally the preferredcleaning processes required prior to theapplication of any plant coating.

The SSPC #1,2,3 (Solvent Cleaning, Hand ToolCleaning and Power Tool Cleaning) methods ofsurface preparation are normally utilized in thefield. Whenever possible, however, an SSPC#6, Commercial Blast Cleaning, SSPC #7,Brush-off Blast Cleaning or SSPC #10, Near-White Blast Cleaning should be used. Theminimal surface preparation of a field coating isoften determined by the type of coatingselected, field conditions and ultimately themanufacturer's recommendations.

Several types of pipeline coating materialsincluding fusion bonded epoxy, cross-head dieextruded polyethylene, calendar-typemulti-layered tape systems and a variety ofspecialty-type plant coatings can be applied atthe coating mill. The fusion bonded epoxyprocess of coating pipe has gained widespreadindustry acceptance over the past twenty years.Fusion bonded epoxy is normally appliedbetween 12-16 mil thickness. The processinvolves the baking of powder epoxy onto apipe surface heated between 450-500EF. AnSSPC #10, Near-White Blast Cleaning surfacepreparation is required prior to coatingapplication. It is also critical in the surfacepreparation stage that all chlorides (salts) beremoved with an acid bath application. Thecoating application is precise. Strict qualityassurance standards need to be monitoredthroughout the coating process.

Another plant coating that has been around for30 plus years is the extruded polyethylenecoating. Part of the surface preparation requiresremoval of moisture and contaminants from thepipe surface. An SSPC #6, Commercial Blast

Cleaning is the standard surface preparation.The coating is comprised of anasphaltic-rubberized 10 mil adhesive that isflowed onto the pipe surface. Through heatingand cooling processes, an extrudedpolyethylene layer is then applied over theadhesive. The extruded polyethylene layer ofnormally 40 mil thickness is basically amechanical barrier, which serves to lessen theamount of impact experienced during theshipping, handling, backfill, etc. Extrudedpolyethylene coating ranges from 40 to 60 milthickness.

Calendar-type multi-layered (50 mil and 80 milcomposite) polyethylene tape systems areanother example of a plant-applied coating. AnSSPC #6, Commercial Blast Cleaning is thestandard surface preparation for application ofcalendar-type multi-layered tape systems. The50 mil composite tape application consists of amechanical application of primer, a 20 mil blackinner-wrap layer and a 30 mil white outer-wraplayer. An 80 mil composite system, which isbetter suited for larger diameter pipeapplication, is composed of a primer, a 20 milblack inner-wrap, a 30 mil white outer-wrap andan additional 30 mil white outer-wrap. It is oftena good idea to use a conformable tape fillermaterial along the longitudinal seams prior toapplication of plant-applied tape.

Coal-Tar Enamel and Asphalt Enamel coatingsapplied at the coating plant were usedextensively in the past. However, due toenvironmental constraints, very few if anycoating plants still apply hot coal-tar or enamelcoatings. Normally, the Tar, Glass and Felt(TGF) or the Tar and Felt (TF) systems of thepast were very popular. Most coal-tar andenamel coatings were comprised of 120 milsplus in thickness. Since a lot of rehabilitationwork still involves coal-tar coated pipe, it isimperative to use field-applied repair coatingsthat are generically compatible with coal-tar.

Other plant coatings that can be consideredspecialty-type coatings include polyethylene,multi-layered polyethylene systems, epoxypolymer concrete, concrete-epoxy,


polyurethane epoxy and dual-layer fusionbonded epoxy. Many of the specialty-type plantcoatings listed above are becoming popular,because they fill an application need.Directional boring of pipeline has become acommon application practice in the pipelineindustry due to environmental concernsassociated with wetlands and land-use. Most ofthe specialty coatings listed above possessexcellent impact resistance qualities, whichmake them good candidates for that type ofapplication.

Once the pipe is shipped to the job-site from thecoating plant, the field coating of girth weldsand repair of damaged areas becomesimportant. Various types of commonfield-applied coatings used for new pipelineinstallation include hot-applied coal-tar tape,cold-applied polyethylene tape, heat-shrinksleeves, fusion bonded field-applied epoxy andliquid epoxy coatings.

Hot-applied tape is generally a 60 mil thickcoal-tar tape that consists of coal-tar pitchsaturated into a cotton fabric. Applied inconjunction with a primer, the tape is heatedand tightly wrapped around the pipe. A 50%overlap of tape can be used to attain 120 milthickness of coal-tar material.

Coal-tar hot applied tape possesses anexcellent hot-melt bond strength to steel,excellent impact resistance, good dielectricstrength and excellent chemical resistance.Hot-applied coal-tar tape provides goodconformability and performance when properlyheated with tension. Since hot-applied coal-tartape is very forgiving in its application, a clean,dry surface preparation of SSPC #1,2,3(Solvent Cleaning, Hand Tool Cleaning orPower Tool Cleaning) is all that is generallyrequired for its application.

Cold-applied polyethylene tape with anelastomeric synthetic-butyl adhesive isavailable in thickness ranging from 30 mils toupwards of 65 mils. Traditionally, cold-appliedtape requires liquid primer with its application.However, a new process whereby primer is

directly applied to the surface of the adhesive,thus creating a "dry" primer, has become verypopular. With the elimination of liquid primer inmost instances, the MSDS (Material SafetyData Sheets) hazards and disposal problemsassociated with primers are virtually eliminated.However, a clean and dry surface preparationincluding SSPC #1, 2 or 3 (Solvent, Hand ToolCleaning or Power Tool Cleaning) minimalbecomes more critical than ever to theapplication process.

The major advantage of cold-applied tape iswith its ease of application. The dielectricstrength of cold-applied tape is excellent as isthe pliability in cold weather. Cold-applied tapesshould be used where direct burial of pipe isinvolved. In the case of ploughing-in or boringapplications of pipeline, other field-appliedcoating materials should be considered.Rockshield or select backfill should always beused as an option when harsher than normalsoil conditions exist.

Most of the newer generation of cold-appliedtapes on the market possess special types offilm backings that are more abrasion resistantthan traditional polyethylene backings. Some ofthe newer cold-applied tape film backings alsopossess UV (ultra-violet) resistance in theirformulation. This now makes it possible to usecold-applied tape for both above andbelow-ground applications.

The heat-shrink sleeve is another method ofcoating girth welds. Heat-shrink sleeves areavailable in either tubular or split sleeve form.The heat-shrink sleeve is normally 70-90 mils inthickness and consists of cross-linked radiatedpolyethylene. On the surface of the sleeve,there are built-in design features that indicate tothe user when enough heat has been applied tothe shrink-sleeve.

The tubular shrink-sleeve application requiresthat the user remove a release paper and slidethe sleeve over the girth weld. The sleevesmust be in position near the place of applicationprior to welding. The application of heat shouldbe applied from the middle out towards the


edges in a horizontal and/or vertical fashion.

With the split shrink-sleeve application, a fillermaterial/sealant should be used over thelongitudinal seams and girth weld beads. Apre-cut shrink-sleeve of correct length with awidth of 12" or 18" is wrapped around the girthweld area. Heat then is applied with a torchproceeding from the weld seam outwardtowards the edge of the sleeve. Uponcompletion of the heating process, a closurestrip is often applied over the end of the lap toinsure that a 3" to 4" overlap of material ismaintained on top. Split shrink-sleeves areeconomical when used on larger diameter pipe.Tubular sleeves are more practical to apply onsmaller diameter pipe.

The heat-shrink sleeve possesses excellentimpact resistance and dielectric strength.Heat-shrink sleeves have generally had a goodhistory and remain very popular due to theirease of application in the field.

The fusion bonded epoxy (FBE) type of fieldapplication for girth welds can be used on largediameter pipe where substantial footage isinvolved. The process is similar to the plantcoating application, whereby the pipe must beabrasive blasted to an SSPC #10 Near-WhiteSurface Preparation. An induction heaterwarms the pipe to 450EF. Powder epoxy is thensprayed onto the weld surface using a wheelapplicator that transverses around thecircumference of the pipe. Typically, severalpasses of the wheel are required to achieve thespecified coating thickness.

By using the fusion bonded epoxy process inthe field, one can achieve a factory-type coatingthat is consistent in terms of application. Tightstandards regarding surface preparation (SSPC#10) and application temperature (450EF range)must constantly be maintained. A certifiedcoatings technician is usually hired to monitorthe coating operation. Liquid epoxies have beengaining in popularity and have a lower cost thanin the past.

Until now, the discussion has centered on

pipeline coatings used for new construction.However, several of the field coatingsdiscussed above i.e., hot-applied coal-tar tapes,cold-applied polymer tapes and liquid epoxiesare also used for maintenance applications. Inaddition, liquid mastics, sealants, hot-appliedwaxes, cold-applied waxes/petrolatum, liquidcoal-tar epoxies, high-temperature tapes andflange-fillers are used for numerousmaintenance coating applications.

As a result of in-line inspections (ILI), "hot-spot"reconditioning frequently requires the use ofheavy-duty maintenance coatings. Many ofthese coatings are selected based on pastexperience, cost and their compatibility withexisting coatings found on the pipe. Heavy-dutyreconditioning coating materials usually includehot-applied coal-tar tape, cold-appliedmesh-backed tape, hot-applied wax, liquidepoxy and liquid coal-tar epoxy.

Another reconditioning application involvesexposed piping strung along bridges. In theseinstances, atmospheric corrosion has corrodedthe pipe and/ or disturbed the original coatingthrough (UV) ultra-violet degradation. A UVresistant cold-applied tape coating or a UVresistant cold-applied wax (petrolatum) tapecoating works well in those instances. Ordinarypaint could be used as a short-term solution,but requires considerable ongoingmaintenance.

Risers are another application that requirereconditioning. Some of the most severecorrosion occurs at the pipe and soil interfacearea of a riser. Several maintenance coatingsthat are used for riser applications include UVresistant cold-applied tapes, UV resistantcold-applied wax/petrolatum tapes, liquidepoxies and hot-applied tapes (whitewashed).The coating of choice first must be durable andsecondly provide good resistance to theatmosphere.

Irregular bolted couplings, valve pits, fittingsassociated with the maintenance of pipingwould be another type of coating application. Inthese instances, ease of application with a "field


friendly" coating material are of paramountimportance. A liquid mastic or a wax/petrolatumtype of application have been used frequently inthese situations. In wet conditions or wherepipe constantly sweats, wax/petrolatum tapecoatings work very well.

Another area where existing coatings oftenbecome deteriorated and sometimes fail is onhigh-temperature discharge header typeapplications. These areas are found at naturalgas compressor sites where pipe runs out fromthe after-coolers. In the past, plant-appliedcoatings were used because little else wasavailable at that time. Coal-tar epoxy orhigh-temperature cold-applied tape combinedwith epoxy primer are currently being used torecondition those areas.

As part of an atmospheric inspection program,operators of gas meter, regulator, andcompressor sites often check flanges and boltswithin flanges. Because of the high cost of boltscoupled with the safety factor, there is awillingness on the part of station supervisors todeal with the problem from a maintenancemode. There are numerous ways to coatexisting flange bolts and arrest the corrosionprocess. One method involves pumpingflange-filler with a caulking gun into the flangeuntil the entire area is filled. This in essencestops any corrosion processes from continuing.The flange-filler, however, must not shrink,must provide dielectric resistance and must beeasily removable upon re-entry into the flange.

Another method involves flood coating theflange with hot-applied wax coating. Many othermethods have been tried over the years withlimited success. In conclusion, there are many excellent plantand field-applied pipeline coatings on themarket today. However, not every coating isgood for all applications. Good surfacepreparation and overall cleanliness of the pipewill create a better environment for coatingperformance.

REFERENCES 1. Gunther, Carl - Washington Gas Light

Company, "ASTM Series of Coating Tests,"pp. 21, 22 from the Proceedings of The1993 Appalachian Underground CorrosionShort Course.

2. Steel Structures Painting Council, Systemsand Specifications Volume No. 2, 1995.


POTENTIAL MEASUREMENTS5-1

CHAPTER 5

POTENTIAL MEASUREMENTS

INTRODUCTION

The making of accurate potentialmeasurements is critical to many areas ofcorrosion control work on undergroundstructures. If such measurements are not madecorrectly (or if unsuitable instrumentation isused) erroneous conclusions can be reachedas to the need for corrosion control. Also, whenpotential measurements are used for evaluatingthe effects of a cathodic protection system,inaccurate potential readings can lead toentirely incorrect conclusions regarding theperformance of that system.

Material to be covered in this chapter includesthe following:

– An overview of instrumentat ionrequirements

– Reference electrodes used in certainpotential measurements

– Types of potential measurements

– Potential measurement techniques

– Polarization effects

– Criteria for cathodic protection

– Monitoring cathodic protection systems

INSTRUMENTATION

Voltmeters

Voltmeters can be classified into analog anddigital types. Voltmeters used for measuringpotentials between a structure and a referenceelectrode contacting earth should be of the high

resistance type to avoid serious error.

An analog voltmeter uses a moving needle orpointer that takes a position on a calibratedscale from which the electrical quantity beingmeasured can be read. The movement of thepointer is known as a "D'arsonval movement."The operating principle is based upon electricalcurrent passing through a coil to form anelectromagnetic field. The intensity of themoving coil magnetic field is proportional to theamount of current passing through the coil.Even though a voltmeter measures electricalpotentials, it is still a current operated device.

A digital voltmeter is entirely electronic anduses a digital readout module to display thedigital characters as the applied electrical signalchanges. There is no actual physicalmovement. The input resistance of the digitalvoltmeters is typically very high (ten megohmsor higher). The high input resistance means thatthe current taken from the external circuit will bevery small. The advantage of this is that therewill be very little voltage drop through theexternal circuit.

Effect of Voltmeter Resistance on PotentialBeing Measured

In corrosion control work on undergroundstructures, the majority of potentialmeasurements are taken between the structureand a reference electrode contacting thesurface of the earth. The ohmic resistancebetween the structure and the connections tothe voltmeter can be very high - in some casesthousands of ohms. This resistance is calledthe external resistance, Re.


To obtain accurate potential measurements, theinternal impedance of the meter (Ri), referred toas the meter impedance, must be very high withrespect to Re. Failure to understand andobserve this fact will lead to seriouslyinaccurate field data.

The importance of taking accurate potentialdata cannon be overemphasized. These dataare not only required for compliance reports,but are also essential to determine theeffectiveness of cathodic protection.

Figure 5-1 illustrates a digital meter of the typecommonly used for structure-to-soil potentialmeasurements. The meter requires a very smallcurrent, usually only a few microamperes, tooperate the meter display. Placing even a lowvoltage across the meter terminals wouldinstantly blow out the display. Consequently, avery high impedance (or resistance [Ri]) isplaced in series with the display mechanism(Figure 5-1). The accuracy of the meterdepends on the ratio of the meter impedance tothe external resistance.

The external resistance (Re) consists of thestructure-to-earth resistance, the resistance ofthe earth path, the resistance of the electrodeto earth, the resistance within the electrode, theresistance of lead wire connections and theresistance of the lead wires between thevoltmeter and the structure and between thevoltmeter and the electrode (Figure 5-1). In highresistivity soils or under dry surface conditions,the resistance from the structure to theelectrode terminal may be several thousandohms. In many cases, a reel of perhaps athousand feet of small diameter wire is used toconnect the electrode to the voltmeter. Theresistance of this wire can amount to 100 ohmsor more.

The current required to operate the meterdisplay creates a small voltage (IR) drop in themeter impedance. The display is calibrated toallow for this IR drop, so the display shows theactual potential that appears across the meterterminals.

The operating current also creates an IR dropthrough the external resistance (Re). This IRdrop is directly proportional to both themagnitude of the current and to Re and affectsthe potential that appears across the meterterminals.

From Ohm's Law, I = E/R, we see that thehigher the input impedance of the meter (Ri),the lower the display operating current. Thelower the operating current, the lower the IRdrop through Re, and thus the more accuratethe potential that appears across the voltmeterterminals.

Modern digital voltmeters, such as those usedtoday for potential measurements, have a veryhigh meter impedance (Ri), typically 10megohms or more. This impedance is the samefor all the voltage ranges on the instrument.When these instruments are used, the externalresistance seldom causes a problem, as weshall discuss next.

The measured potential, that shown on themeter display (Pm), is directly proportional to theratio of Ri to the total circuit resistance (Rt).

Equation 1 shows the calculation of total circuitresistance (Rt):

Rt = Meter Impedance + External Resistance

= Ri + Re (1)

The ratio is shown by Equation 2:

Ratio = Ri / Rt (2)

The measured potential (Pm) then equals theratio times the actual potential (that at theelectrode location [Pa]), as shown in Equation 3:

Pm = Pa x Ratio (3)

The higher the ratio, the more accurate Pm is.

The actual potential (Pa) mentioned in thisdiscussion is the actual potential of thestructure that appears at the location of the

DIGITAL VOLTMETER OPERATION SHOWING COMPONENTS OF EXTERNAL RESISTANCE (Re)

FIGURE 5-1


electrode. Pa thus includes the IR drop causedby the cathodic protection current. Pa istherefore not the actual potential at the surfaceof the structure.

Modern digital voltmeters, such as those usedtoday for potential measurements, have a veryhigh meter impedance (Ri), typically 10megohms or more. Attempting to take structureto soil potentials with an incorrect instrumentcan yield seriously erroneous measurements asthe following examples show.

Let us suppose that Re in a certain situation is2500 ohms and that the actual potential (Ea) atthe electrode location is -0.950 V. Let us furthersuppose that a corrosion technician attempts totake potentials with an inexpensive little meterwith a meter impedance (Ri) =1000 ohms. Now,from Equation 1:

Rt = (Ri + Re) = 1000 + 2500 = 3500 ohms

From Equation 2, we see that

Ratio = Ri / Rt = 1000/3500 = 0.286

We find the measured potential (Pm) fromEquation 3 is:

Pm = Pa x Ratio = -0.950 V x 0.286 = -0.272 V

The measured potential is only 28.6% of theactual potential, a serious error.

Now let us see what happens when thetechnician uses a good digital meter with ameter impedance (Ri) of 10 Megohms(10,000,000 ohms).

From Equation 1:

Rt = (Ri + Re) = 10,000,000 + 2500

= 10,002,500

From Equation 2, we see that

Ratio = Ri / Rt

= 10,000,000 /10,002,500 = 0.999

We find the measured potential (Pm) fromEquation 3 is:

Pm = Pa x Ratio = -0.950 V x 0.999 = -0.949 V

The measured potential now is 99.9% of theactual potential, a negligible difference. The purpose of this discussion is to show howimportant it is to use the correct voltmeter whenmaking potential measurements. As mentionedearlier, accurate potential measurements areabsolutely essential to provide the data requiredfor compliance reports and to establish theeffectiveness of cathodic protection.

Care and Storage of Instruments

Corrosion test equipment has to be treated withcare. For reliability, it must be treated asdelicate equipment during transportation andfield use. Instruments must be kept clean andmaintained in good working order. Wheninstruments are returned to storage betweenfield usages, they should be completelychecked and calibrated (where necessary) forthe next time needed. Any batteries should beremoved to prevent the possibility of batteryleakage with internal corrosive damage to theinstrument.

REFERENCE ELECTRODES

In corrosion control testing of undergroundmetallic structures, we frequently need tomeasure the potential between the structureand its electrolyte (soil or water). Unfortunately,this is not a measurement that can be madedirectly. We can however measure the potentialdifference between the structure and someother metal that is also in contact with theelectrolyte. It is not as simple as inserting theend of the test wire into the ground orconnecting it to a steel or copper rod in theelectrolyte even though a reading is obtained.These readings are useless as they are notrepeatable. Further, they will not be applicableto some of the criteria for protection. This


problem was solved with the development ofreference electrodes which are used to contactthe electrolyte. A reference electrode is simplya device which is used to contact the electrolyteand to which one terminal of the voltmeter isconnected. To be suitable, a referenceelectrode must have a relatively constant“half-cell potential”. A reference electrode isone half of a corrosion cell consisting of a metalin a solution of its metal ions. The “half-cell”potential is constant if the concentration ofmetal ions in the solution around the metalremains constant.

The most common reference electrode used incathodic protection testing is the saturatedcopper/copper sulfate reference electrode. Thiselectrode is sometimes referred to as “ahalf-cell”, “copper sulfate”, “Cu/CuSO4" or“CSE”. The components of a typical saturatedcopper/copper sulfate reference electrode areshown in Figure 5-2.

As previously stated, in order to for a referenceelectrode to be suitable, it must have a constanthalf-cell potential. In order to have a constanthalf-cell potential, the concentration of metalions in the solution around the metal mustremain constant. With the saturatedcopper/copper sulfate reference electrode thisis achieved by having a constant concentrationof copper ions (saturated) in the solutionaround the copper rod.

The saturated copper/copper sulfate referenceelectrode is simple and reasonably rugged butthere are precautions that must be observed foraccurate results. These are:

1. The copper rod must be cleaned so that it isbright and shiny and free of contaminants.The copper rod should be cleaned by usinga new, unused, non-metallic scouring pador sandpaper. Green scouring pads arepreferred. When green scouring pads areunavailable, sandpaper can be used. Oxidetype sandpaper cannot be used since it willintroduce unwanted metals into the surfaceof the copper rod. Do not touch the copperrod with bare fingers after it has been

cleaned.

2. The copper sulfate solution must be free ofcontaminants. The solution should be madewith deionized or distilled water, or with agel made specifically for use incopper/copper sulfate reference electrodes.Only high purity copper sulfate should used.

3. The tube must contain a saturated solutionof copper sulfate. In order to ensure thesolution is saturated, there must be coppersulfate crystals visible in the solution.

4. The porous plug must be moist. The porousplug provides electrical contact between thereference electrode and earth. If the plug isdry, the measured potentials may beinaccurate. When new electrodes areprepared, the plugs should be moistenedfor at least 24 hours prior to use. When theelectrode is not in use, the porous plugshould be covered with a protective cap toprevent it from drying out.

5. The accuracy of reference electrodesshould be checked periodically. A newreference electrode can be prepared andused as a standard. This “standard”electrode should not be used in the field sothat it remains uncontaminated. The fieldelectrodes can be checked against thestandard electrode by placing the twoelectrodes in a glass or plastic containercontaining a dilute solution of copper sulfateand measuring the voltage differencebetween them. If the voltage differencebetween the standard electrode and fieldelectrode is more than 10 millivolts, the fieldelectrode should be refurbished. Somecompanies may require the voltagedifference to be some value less than 10millivolts.

There are two characteristics of saturatedcopper/copper sulfate reference electrodeswhen in use that should be known. The first ofthese is temperature effect. The half-cellpotential although basically constant in otherrespects is subject to some variation with

��

��

��

��

��

��

��

��

��

��


change in the temperature of the copper sulfatesolution above or below a median point of 77o

F. This potential change is in the order of -0.5millivolt per degree as per the following formula:

ECSE* = ECSE - 0.5 mv/oF (T - 77oF)

Where:

ECSE* = corrected potential

ECSE = potential read

T = temperature in oF of copper sulfatesolution

If a potential reading measured to a saturatedcopper/copper sulfate reference electrode is0.80 volt, it would, for example, be:

1. At an electrode temperature of 87o F:

0.80V - (0.5 mV X 10o temperature riseabove 77 o)

= 0.800V - 0.005V = 0.795 Volt

2. At an electrode temperature of 57o F:

0.80V + (0.5 MV x 20o temperature dropbelow 77o)

= 0.800V + 0.010V = 0.810 Volt

Knowledge of this effect could be useful whenmaking critical measurements.

The other characteristic is a photoelectric effectcaused by sun striking the copper sulfateelectrode. This appears to be a factor when thecopper rod of the electrode is not sufficientlyclean. This can be checked by shading theelectrode to see if there is any change in thevoltmeter reading. If there is a change, keep theelectrode shaded.

Another type of reference electrode that theunderground structure corrosion worker shouldbe aware of is the saturated silver/silverchloride electrode for use in sea water. This

would be used, for example, when working onoff-shore pipelines and drilling platforms.

The need arises for this because of thepossibility of contamination, by sea water, of thecopper sulfate solution in the saturatedcopper/copper sulfate reference electrode. Thesaturated silver/silver chloride referenceelectrode does not contain a liquid solution tobe contaminated. It comprises a core of silvergauze on which is deposited a film of silverchloride with the assembly enclosed in aprotective sleeve which allows free entry of seawater when the electrode is in use.

The half-cell potential of the saturatedsilver/silver chloride reference electrode in 25ohm-cm seawater is 0.05 volts less than that ofthe saturated copper/copper sulfate electrode.Accordingly, in order to convert saturatedsilver/silver chloride reference electrodereadings to saturated copper/copper sulfatereference electrode readings, add -0.05 volt.For example:

-0.84 volt to silver chloride in seawater =-0.89 volt to copper sulfate

Electrode-to-earth Resistance

It should be noted that the contact resistancebetween a saturated copper/copper sulfatereference electrode and earth is the usualcause of high external resistance in potentialmeasurements and is what necessitates theuse of high resistance analog voltmeters ordigital voltmeters. This is because the contactsurface between the electrode porous plug andearth is so small. The actual amount of thisresistance is a function of the resistivity of thesoil contacted by the electrode tip. Highresistivity soils give high electrode-to-earthresistances.

When placing the reference electrode to take areading, it helps to scrape away any dry surfacesoil to expose a moist surface. If soil conditionsare so dry that this is not effective, wetting thesurface with water can be helpful in reducingthe contact resistance.


Also, when placing the reference electrode,make sure that there is no contact betweenexposed metal (at the test lead connection endof the reference electrode) and grass, weeds,or anything else foreign to the measurementcircuit. This is necessary to avoid possibleeffect on the potential being measured.Likewise, it is essential that all test leads usedin setting up the potential measurement circuitbe well insulated and that the insulation be freeof cuts, breaks, or scrapes that could allowpossible contact to earth or vegetation. If thereshould be leakage paths to earth at any suchpoints, the potential being measured could beaffected making recorded values subject toerror of an unknown amount. This is particularlyimportant when working under wet conditions.

STRUCTURE-TO-EARTH POTENTIALMEASUREMENTS

With an understanding of instrumentationrequirements and the use of referenceelectrodes, consideration can be given to theiruse in making various types of potentialm e a s u r e m e n t s . S t r u c t u r e - t o - e a r t hmeasurements normally will be the mostcommon type made in underground structurecorrosion control work.

The potential measurement commonly calledthe pipe-to-soil potential should be made usinga high input resistance voltmeter (typically 10megohm or higher) connected between the pipeand a reference electrode located on the earthas close as is practicable to the pipeline asillustrated in Figure 5-3.

This potential difference measurement not onlyincludes the pipe polarized potential (Ep) withinthe influence of the reference electrode, butalso the voltage drop in the soil (IRsoil) betweenthe reference and the pipe and the voltage dropin the pipe (IRpipe) between the point of contactto the pipe and the point of measurement. Withthe cathodic protection current 'ON' (applied),the voltmeter measures the total potentialdifference without being able to distinguish thepipe polarized potential (Ep) and therefore:

Vm = Ep + IRsoil + IRpipe

In the example shown the IR drops are additivesince their polarity is same as Ep and thevoltmeter will indicate that the pipe is moreelectronegative than the actual polarizedpotential (Ep). If the cathodic protection currentin the pipe is in the opposite direction then thepipe IR drop would subtract from thepipe-to-soil potential measurement. It is theseIR drop voltages which must be taken intoaccount for valid comparison to the criteria.

Figure 5-4 illustrates common potentialmeasurements as made on an undergroundpipeline. The figure shows, with solid lines,potential measurement to a "close" referenceelectrode directly above the pipe. This is themore common measurement as the referenceelectrode is, normally, in the most neutral areawith minimum effect on the potential caused byvoltage drops in the earth resulting from directcurrent flowing to or from the pipe.

The dashed line showing a test lead extensionto a "remote" reference electrode location. Thismay be used on bare pipelines or complexunderground systems in checking cathodicprotection coverage to verify that "long line"corrosion cells have been neutralized. Long linecells being anodic and cathodic areasseparated by some distance as opposed tothose which may be a few feet or a few inchesapart.

As a rule of thumb, the area "seen" from areference electrode on a bare structure is thatsubtended by an angle of roughly 120o (seeFigure 5-5).

For this reason, the close reference electrode"sees" only a relatively small portion of the barestructure as opposed to the remote referenceelectrode location.

By "remote" reference electrode is meant areference electrode location which is electricallyremote from the structure. There is no fixeddistance figure for this. It can be determined inthe field (for a bare structure) by first taking and

��

��

��

��

��

��

��

��

!

�

�"#

��$%&'

�"#

�"#

�#

��#&#(

��

�� )��*��

�� )��*��

��

��

��

� ��

��

��+

�!� ��

��

��)�� *��

��

��

��

��

,�-�


recording a potential reading between thestructure and a reference electrode placed, forexample, 50 feet from the structure. Thereference electrode is then moved further awayfrom the structure by approximately 50-footincrements in a direction perpendicular to thestructure. The structure-to-reference electrodepotential is measured and recorded at eachlocation until there is no significant change inthe reading from one point to the next. This isan indication that electrically remote earth hasbeen reached. For large bare pipelines, forexample, this distance could be (but notnecessarily so) several hundred feet. Thereason for variation is variation in earthresistivity and its geological structure - bothaffecting the distance from the bare structureneeded to encompass essentially all of thestructure-to-earth resistance.

In contrast to bare structures, thestructure-to-earth resistance for well coatedstructures includes the resistance across thecoating as the major element of the totalstructure-to-earth resistance. For this reason, a"close" reference electrode location at a coatedstructure is, for practical purposes, in theequivalent of electrically remote earth.

On large bare structures, once the referenceelectrode has been placed in electrically remoteearth, the potential measured between thestructure and the reference electrode reflectsthe average of widely spaced anodic andcathodic areas from long-line corrosion cells.Experience has indicated that, on steelunderground structures for example, when apotential of -0.85 volts, structure-to-saturatedcopper/copper sulfate reference electrode, isobserved, the widely spaced (long-line)corrosion cells have been stifled. This does notnecessarily mean that small local cells withinthe larger cells are likewise stifled. They stillcan be active in local areas. For this reason,readings to remote reference electrode on barestructures, although useful for determining thestifling of large longline cells (which may well bethe major source of corrosion), cannot be usedas an indication of when complete protection isattained for all surfaces of the underground

bare structure.

On Figure 5-4, it will be noted that the (-)terminal of the voltmeter is connected to thereference electrode with the (+) terminalconnected to the pipe. This is the preferredmethod. With this polarity, pipe-to-soil potentialsare normally negative values (such as -0.750volts).

There can be situations in strong stray currentexposure areas where the structure can bepositive with respect to the reference electrode(reversed polarity from that shown on Figure5-4). Readings are then recorded accordingly(such as +0.425 volts). This means that thestructure is very strongly anodic and subject topossible heavy corrosion damage.

The age and condition of an undergroundstructure such as a pipeline can have anappreciable effect on the potentials measuredto a close saturated copper/copper sulfatereference electrode. An old, bare structure thatis not cathodically protected may havepotentials that are generally low in value (suchas in the -0.1 to -0.3 volt range). The valuesfrom point to point along the structure can showmarked variation. This is an indication of theexistence of corrosion cells. For any two pointson a non-cathodically protected structure, thepoint with the most negative reading is anodicwith respect to the other point. To helpunderstand this, Figure 5-6 will clarify thematter.

For the two points "A" and "B" which could, forexample, be five feet apart, we get theconventionally-recorded figures (the upper set)of -0.150 volts and -0.210 volts with respect tothe reference electrode. In the usual case,there would be no detectable difference in thepipe itself between points "A" and "B" causedby voltage drop in pipe resistance from galvaniccorrosion current flowing through the pipebetween the two points.

Accordingly, we can temporarily reverse ourthinking and treat the pipe as the referencerather than the saturated copper/copper sulfate

��

��.

��/��-0-.-� �!�

��-0,�-� ��-0�,-� �� 1�1��+

!-0,�-� !-0�,-� 1�1��+��

)�* )�*

��

��

�!� ��

��

��


reference electrode. Looking at it this way, thereadings (lower set of figures) are +0.150 volts(saturated copper/copper sulfate positive withrespect to pipe) at Point "A" and +0.210 volts atpoint "B". This means that the earth surface atpoint "B" is more positive than at point "A".Point "B" is then 0.060 volts positive withrespect to point "A" (difference between the twofigures). Corrosion current flow would then beas shown by the earth-path arrows confirmingthat point "B" is anodic with respect to "A".

Just two readings (as in the example above)are usually not enough to explore the completeextent of a corrosion cell. This is done bypotential surveys as discussed later.

Whereas old bare steel structures havegenerally low negative potentials with respect toa saturated copper/copper sulfate referenceelectrode, non-cathodically protected new steel(and especially well coated new steel) can havepotentials to a close reference electrode in theorder of -0.70 volt or more. -0.800 volt is aboutthe maximum for bright new steel.

Because the amount of corrosion currentflowing to or from a well-coated pipe isextremely small, there is little possibility ofsignificant voltage drops in the earth around thepipe. This means that a close referenceelectrode position directly above the pipe isessentially the same as one at a remotelocation. This would not be the case if therewere strong stray earth currents in the areafrom some external DC source.

Point-to-point changes along the surface abovea coated pipe tend to be much more gradualthan on bare pipe.

C E L L - T O - C E L L P O T E N T I A LMEASUREMENTS

These are measurements between tworeference electrodes. Two applications areillustrated by Figure 5-7.

Part A of Figure 5-7 shows one use of potentialreadings between a reference electrode over a

structure such as a pipe and another referenceelectrode at one side of the structure. Thisprocedure could be used to verify the flow ofcathodic protection onto a bare structure, forexample, or to trace the flow of stray earthcurrents around a structure.

If, in Part A of the figure, the voltage drop ismeasured between reference electrodes "A"and "B" and reference electrode "A" is positivewith respect to "B" as shown this means thatcurrent is flowing toward the pipe. This is fineas it indicates that the pipe is cathodic. This isnot necessarily the whole story. To confirm this,a potential reading between referenceelectrodes "B" and "C" will reveal what is goingon on the other side of the pipe. If the polarity isas shown on the figure ("C"+), current is alsoflowing toward the pipe confirming the cathodiccondition on the pipe.

If, however, reference electrode "B" is found tobe (+) with respect to "C", this would mean thatthe current flow is away from the pipe on itsright hand side - as could arise under certainstray current conditions. This results in the leftside of the pipe being cathodic while the rightside is anodic and corroding.

In Part B of Figure 5-7, the use of cell-to-cellmeasurements along a pipeline is illustrated.This usage, if there is a potential differencebetween the two electrodes, indicates atendency for current flow in the earth. Withelectrode "B" in the figure being (+) as shown,the indication is that there is earth path currentflowing from "B" to "A" as shown. It furtherindicates that there is an anodic areasomewhere in the vicinity of (but not necessarilyat) electrode "B". This assumes galvaniccorrosion conditions.

In making cell-to-cell measurements, it must beremembered that there are two high resistanceelectrode-to-earth contacts in the circuit. Thisdictates care in taking the potentialmeasurements to be sure that high circuitresistance is not affecting the readings - or ifso, to make required corrections.

��

��2

�

�/� �!�

��

��

��/��

��/��

��

��

�/� �!��/� �!�

)�* )*�

)�* )�*


STRUCTURE-TO-STRUCTURE POTENTIALMEASUREMENTS

Measurements between undergroundstructures (or between different parts of thesame structure) often are used in conjunctionwith structure-to-earth measurements. Theycan serve as a quick screening method toestablish continuity - or lack of it.

Figure 5-8 illustrate some typical uses.

Part A of Figure 5-8 shows a potentialmeasurement across an isolating joint used ina piping system to separate a cathodicallyprotected section of the system from anunprotected one. If everything is in goodworking order there will be a reading on thevoltmeter, which will depend on the potentials toearth of the cathodically protected andnon-protected piping. The difference betweenthe two potentials will be the numerical valueshown on the voltmeter. The amount couldrange from, for example, 0.25 volt or less to1.50 volts or more. The unprotected pipingsection would be positive in polarity to thecathodically protected pipe (which is morenegative to earth ).

If the voltmeter reads zero, it is an indicationthat something is wrong. The question is what?Here are some of the possibilities:

S The isolating joint is "shorted" (no longerisolating).

S The isolating joint is perfectly OK but aparallel metallic path has been establishedaround the isolating joint (such as gasinstrumentation piping for example).

S The cathodic protection system is "off" forwhatever reason and the potentials to earthon the two sides of the isolating joint areexactly the same. This is highly improbable- but possible.

It becomes obvious from the above that thereason for the zero voltmeter reading cannotnecessarily be deducted from the voltmeter

reading alone. Additional tests will benecessary which, depending on the conditionsof the situation, may involve structure-to-earthpotentials, current flow on the structure, use ofa pipe locator to trace the path of the pipelocator transmitter signal, etc.

Part B of Figure 5-8 represents a situationcommon to gas and water distribution pipingsystems in urban areas. Shown is acathodically protected gas main with services toa customer's property. Let's assume a coatedsteel main with a coated steel service line whichis electrically continuous with the main butprovided with an isolating fitting where it entersthe customer's building. Also shown is anunprotected water main with service line to thesame customer's property - and possibly buriedin the same ditch with the gas service line.Assume that the water main is bare cast ironand that it has a bare copper service line. Thetwo systems must be kept electrically separateif the cathodic protection installation on the gassystem is to be fully effective.

In Part B of Figure 5-8, several points areshown at which potentials between each suchpoint and the gas main can be measured. If allis as it should be, the potential between gasmain and Point A should be zero. This indicatesthat the service pipe is electrically continuouswith the gas main plus the probability that theisolating fitting in the service line is functioningas it should otherwise there would be currentflowing on the service line which would causesome voltage drop through the service piperesistance.

The potential between the gas main and PointB should be significant (0.25 volts or less to 1.5volts or more) with Point B being positive (+) tothe gas main. This is further indication that theisolating fitting is satisfactory.

Typically, the potential between the gas mainand Point C (on the water service line) will beequal to the value measured to Point B. This isbecause the various services entering acustomer's property (gas, water, electricity) areusually in electrical contact with each other

��

��3

��

�/� �!�

��

� ��

�� 4��

��

��

��

��

��

��

��

)�* )�*

)*

��

��5��

��


within the customer's building.

If the potential readings do not show anacceptable pattern, some things to look for withsupplemental tests include:

S Electrical discontinuity in the gas serviceline could result from the accidental use ofan isolating fitting at the main tap or couldbe from the use of a mechanicalcompression type coupling whichaccidentally did not provide electricalcontinuity.

S Failed isolating fitting in the service linewhere it entered customer's building.

S Electrical contact between gas service lineand water service line.

S Electrical contact between gas and water atsome property or at some point other thanthe one being tested.

When making contact with structures forpotential measurements, a permanent test wireis best - if there is one. In many cases,however, there will not be a test wire availableand it may be necessary to make theconnection by clipping or clamping theinstrument test lead to a convenient point onthe structure. Or if this is not possible, to use apointed device (such as a scratch awl with ahardened tip) held against the structure with theinstrument lead connected to the tool used.Obviously these are connections made to fit thesituation but are perfectly satisfactory ifcarefully done and provided that all suchconnections are to bright metal surfaces. If, forexample, there were a bit of mill scale betweenthe contacting surfaces, extraneous voltagescould be introduced into the circuit and thepotential observed on the voltmeter would notbe the true potential desired.

POTENTIAL SURVEYS

Potential surveys are useful in determining thelocation of anodic (corroding) areas onnon-cathodically protected structures or when

making a detailed evaluation of theperformance of cathodic protection systems.

In substance, the potential survey consists ofmaking close interval measurements to asaturated copper/copper sulfate referenceelectrode along extensive structures such aspipelines. This permits a reasonably completecoverage so that major anodic areas can beidentified on non-cathodically protected lines orso that below-standard protective potentials oncathodically protected systems will be revealed.

Potential surveys can be made in various ways.Some of these are illustrated by Figure 5-9.

In Part A of Figure 5-9, a two-electrode surveymethod is shown. It works in this fashion. At apermanent test point, the potential of the pipe ismeasured to saturated copper/copper sulfatereference electrode "A" and recorded. Thenreference electrode "B" is placed at thepreselected survey distance (5 ft. for example)along the pipe and directly above it. Thepotential between the two electrodes is thenmeasured and numerically added or subtractedfrom the prior reading in accord with the polarity[(+) or (-)] of the forward electrode ("B" in thiscase). Electrode "A" is then moved to thesurvey spacing distance beyond "B" and thepotential between the two electrodes measuredand numerically added or subtracted from theprior figure. This "leap-frogging" of the twoelectrodes is continued until the next permanenttest point (or other location) where the structurecan be directly contacted. At this point, thecumulative potential up to this point iscompared with the actual potential to asaturated copper/copper sulfate referenceelectrode and corrected as necessary. Thesurvey then continues in the same fashion tothe next access point.

�6�� 7��7��8��

��

��9

��/��

��/�� /��

�� 1�1��+�� )�*�� )�*

��

��

��

�6��

, � � + �

�!� ��

)�* )�* )�* )�* )�*

��

��

�!� ��

�� )��7��7*��8�� 8��

)��7� �*�1�1��

��/��

��

��

��

��

�!� ��


The manner in which the data would berecorded would be in general accord with thefollowing:

TestPipelineStation

No.

PotentialBetween

Electrodes

Polarity ofForward

Electrode

Pipe toCu-CuSO4Potential

Pipe toCu-CuSO4 atTest Point at

672+15 -0.573

Cu-CuSO4 atTest Point toCu-CuSO4 at

672+20 0.015 (+) -0.588

Cu-CuSO4 atSta 672+20 toCu-CuSO4 at

672+25 0.023 (+) -0.611

Cu-CuSO4 atSta 672+25 toCu-CuSO4 at

672+30 0.012 (-) -0.599

Although the two-electrode survey systemworks, it does take a great deal of care inrecording data both as to amount of potentialdifference between electrodes and the polarityof the forward electrode. An error at any onepoint is then present in all subsequentcalculated pipe to saturated copper/coppersulfate potential readings. Should this be thecase, the calculated pipe to reference electrodepotential will not check with the actual pipe toreference electrode potential when the next testpoint (or other pipe access point) is reached.This could possibly necessitate a repeat surveyof the section - but see the following paragraph.

It should be pointed out, however, that even ifthere were no data errors, there could still be adifference between the last calculated pipe tosaturated copper/copper sulfate potential andthe actual measured pipe to saturatedcopper/copper sulfate potential at the nextpipeline test point or other access point. Therecould be two possible reasons for this. First, ifthere is any long line direct current flowing onthe pipe, the voltage drop in the pipelineresistance between the two test points couldaccount for the difference. Second, if there isany stray current activity, natural or man-made,which is affecting the pipeline potential, it, too,could result in a difference.

It is important that the two electrodes used be

leap-frogged as described. This is tocompensate for any slight potential differencebetween the half-cell potentials of the twoelectrodes. The leap-frogging action makes thispotential difference additive on half thereadings taken and subtractive on the other half- thus nullifying the effect of any potentialdifference between electrodes.

In Part B of Figure 5-9, a single electrodesurvey method is shown. This accomplishes thesame purpose as describes for thetwo-electrode method. The difference is thatinstead of moving the voltmeter from point topoint along the line, it is stationed at apermanent test point (or other pipeline acrosspoint) and the reference electrode moved frompoint to point along the line using a longconnecting test lead.

This method avoids the problem of an error inreading at any one point affecting allsubsequent readings as is possible with thetwo-electrode method.

The disadvantage is that when the data hasbeen taken between two permanent test points(or other pipeline access points), it is necessaryto retrieve the long test leads before starting thenext section.

At the forward end of the section surveyed fromthe first test point, the pipe to referenceelectrode potential measured at the next testpoint via the long test lead may not agree withthe potential measured with the voltmeter at thesecond test point. Reasons for thedisagreement, if any, could be as set forth inthe prior discussion of the two-electrode surveymethod.

In Part C of Figure 5-9, a conceptualrepresentation is made of a high technologyover-the-line potential survey system. In thissystem, the voltage measuring equipment anddata logging equipment can all be carried in acompact back pack by the surveyor. Thesurveyor carrying the equipment carries twosaturated copper/copper sulfate referenceelectrodes which are "walked" forward at


approximately two and a half foot intervals.Potential measurements are recordedelectronically (avoiding human error) in theequipment storage module. The long test leadfrom the survey starting point can be aone-time-use disposable test lead. As it isreleased from the back pack reel as thesurveyor moves forward, the distance from thesurvey starting point is automatically measuredand recorded in the data storage module alongwith the potential measurements. The surveyoris also able to electronically enter notes onterrain features or other identifiable landmarks,as well as data necessary to identify thepipeline section being surveyed.

The electronically recorded data package isprocessed by the home office computerprogrammed to produce a plotted profile,complete with notes, for the pipeline sectionsurveyed. If desired, the last previous surveycan be computer-plotted at the same time sothat any changes since the last survey will bevisually displayed.

Common to all survey methods is the need tolocate a pipeline so that the referenceelectrodes are placed directly above the pipe asthe survey progresses. This is normally donewith a pipe locator operator (moving ahead ofthe data acquisition team) who locates andstakes the line.

Although a more detailed discussion ofpotential survey work and analysis thereof iscontained in Chapter 1 of the AdvancedCourse, Figure 5-10 is included to give an initialidea of what might be expected of plotted datafrom a pipeline potential survey. The figuregives an indication of variations typical of asurvey taken along a section of bare or verypoorly coated pipeline.

Within the section illustrated, the peaks in theplotted data (where the pipe is most negativewith respect to a saturated copper/coppersulfate reference electrode) represent anodicareas. This is in accord with the discussion onthis subject earlier in this chapter under theheading, Structure-to-Earth Potential

Measurements. The Figure 5-10 plot shows thatinsofar as potential differences are concerned,there is one major peak (designated as a majoranodic area) and three minor peaks within theslopes leading to the major peak. The majorpeak could be caused by a long line galvaniccorrosion cell while the lesser peaks could bethe result of more localized corrosion cells.

As has been discussed earlier in the course, acorrosion cell having a given driving potentialwill cause more current to flow through lowresistivity earth and hence cause morecorrosion than will be the case where the earthresistivity is high. This means, then, thatpotential is not the whole story. Earth resistivitymust be considered as well. If the plot of Figure5-10 represented an actual situation, it could bequite possible for one of the "lesser" anodicareas to be in a local section of very lowresistivity soil and causing more corrosion thanthe designated "major" anodic area which maybe in generally much higher resistivity earth.

The following chapter will include methods formeasuring earth resistivity.

A plot of over-the-line potentials made along awell coated pipe would not ordinarily (in theabsence of any stray current effects) have themarked variations as shown in the Figure 5-10plot. Rather, there would be gradual rises andfalls in a generally smooth curve. This isbecause the reference electrode directly abovethe pipes is, for all intents and purposes, inelectrically remote earth because of the highresistance coating barrier and will generally notreflect conditions right at small pinholes orsmall defects in the coating. Should howeverthere be a major coating holiday (such as a footor so of bare pipe), close-spaced readingscould be expected to reveal its presence by alocal variation in the otherwise smooth curve.There can be other effects as well which will bediscussed in the Advanced Course.

POLARIZATION EFFECTS ON POTENTIAL

The discussion earlier in this chapter under theheading, "Structure-to-Earth Potential

��

��,-

��4��

��

��1�1��+

� ��

��

��

��


Measurements,” covered the nature of apotential measurement between the structureand a saturated copper/copper sulfatereference electrode contacting adjacent earth.At this point, it is appropriate to explore theeffect of polarization (and depolarization) onsuch a reading.

Part A of Figure 5-11 shows the general shapeof a polarization curve. If a voltmeter isconnected to measure structure to saturatedcopper/copper sulfate potential at a test pointon a non-protected structure, a steady statepotential (in the absence of stray currenteffects) will be observed as represented by thehorizontal line of the Part A representation. If,while watching the voltmeter, cathodicprotection current is applied to the structure,there will be an immediate potential increasecaused by voltage drop in the earth resultingfrom cathodic protection current flow to thestructure. This will be followed by a moregradual increase as cathodic polarization buildsup.

If the structure is bare or poorly coated, the timerequired for full polarization may be days orweeks because of the large areas to bepolarized. On the other hand, a very well coatedstructure can polarize very quickly - seconds tominutes.

Part B of Figure 5-11 represents the reversesituation showing depolarization when cathodicprotection current is interrupted. Underdepolarization, as opposed to polarization,depolarization of a bare structure tends to berelatively rapid during early stages compared tothe rate of depolarization on a well-coatedstructure.

CRITERIA FOR CATHODIC PROTECTION

Although a more complete discussion of criteriafor cathodic protection is included in theIntermediate Course, there are twopotential-dependent criteria, which should bementioned here, which are in addition to the-0.85 volt potential to a saturated copper/coppersulfate reference electrode (for steel structures)

as discussed in Chapter 3 under the heading,"How Cathodic Protection Works.”

The first of these is the criterion which uses atleast 100 millivolts of structure polarization asan indication of cathodic protection. This can bedetermined on a structure where all sources ofcathodic protection current affecting thatstructure can be interrupted at the same instant.With a voltmeter set up to measure structure tosaturated copper/copper sulfate potential priorto interrupting the current, watch the voltmeteras the cathodic protection current is interrupted.Normally there will be an immediate dropfollowed by a slight hesitation (particularlydetectable on an analog voltmeter) which is thestart of the depolarization curve. Note thevoltmeter reading at that point. Then if there isat least 100 millivolts of depolarization beyondthat point, the criterion has been met. If thestructure is one that depolarizes very slowly, itcould take quite some time to reach the 100millivolts of depolarization. In this case, it maybe helpful to plot potential change with time andthen extend the curve to see if it appears thatthe 100 millivolts of depolarization will beattained. If the indications from this are positive,it is then worthwhile to continue the test untilthe objective is attained.

The second potential-dependent criterion usesa 300-millivolt shift in the negative directionfrom the structure to reference electrodepotential prior to applying cathodic protection tothe potential attained with cathodic protectionapplied. This may be used in instances wherea potential of -0.85 volt (on steel structures) isnot attained. This is not as sound a criterion asthe -0.85 volt criterion (for steel) or the100-millivolt polarization criterion. It has,however, had reasonably good results. Thiscriterion was dropped from the list of criteria inSection 6 of NACE SP0169. Although stillrecognized and listed as a criterion for cathodicprotection in Appendix D to Part 192 of theCFR, it is being interpreted by inspectors as300 mV of cathodic shift after correction for IRDrop. In effect, they are requiring a 300-mVpolarization shift.

��

��,,

��/��

��

��

��

��

��

��

��1��+��

��

��1��+��

��

��1�1��+

� ��

��

��

��1�1��+

� ��

��

��

��/��


MONITORING CATHODIC PROTECTIONSYSTEMS

Potential measurements are used toperiodically verify that cathodic protection on astructure, once attained, continues to beeffective. This monitoring can be performed onpipelines by periodic over-the-line potentialsurveys as discussed earlier in this chapter. Atintervals between complete surveys, pipe tosaturated copper/copper sulfate potentials canbe measured at test points along the line.

There is one caution that should be observedwhen taking such measurements at test points.This is to avoid placing the reference electrodeclose to a buried galvanic anode connected tothe line. If the reference electrode is placed onthe surface above such an anode, the potentialreading between pipe and reference electrodewill include the voltage drop in the earth causedby current discharge from the anode. For thisreason, the reading obtained will not reflect thecorrect potential between pipe and earth. Forexample, if the reference electrode were placedabove a magnesium anode, the readingbetween pipe and saturated copper/coppersulfate might be -1.15 volt which looks justgreat - whereas with the reference electrodeabove the pipe fifteen feet away from themagnesium anode, the reading might be only-0.65 volt. This is quite another story since thepipe is not fully protected.

As a rule of thumb, it would be well to place thereference electrode at least fifteen feet from thenearest working galvanic anode. Also, anyabnormally high readings could lead one tosuspect the presence of a galvanic anode evenif its existence is not known. Checkingpotentials either side of the high reading pointshould then clarify the matter.

R =1.210 Volts

0.0676 Amperes 17899. Ohms

CHAPTER 6

RESISTANCE MEASUREMENTS

INTRODUCTION

There are many instances in corrosion controlwork where it becomes necessary to measurevarious types of resistances. A related subjectis the measurement of soil resistivity which iscritical for the selection of suitable low resistivitysites for cathodic protection installations as wellas for the subsequent design of thoseinstallations.

Material to be covered in this chapter includesthe following:

C Measurement of simple resistances - resistors, shunts, bonds, test wires.

C Measurement of non-isolated resistances -resistance of isolating joints, resistancebetween galvanic anodes or impressedcurrent anode beds and earth, resistance ofpipeline sections to earth, resistance ofgrounding systems to earth.

C Measurement of soil resistivity

SIMPLE RESISTANCES

To start with the easier resistancemeasurements, consider the problem ofdetermining the ohmic resistance of itemswhich can be temporarily isolated, electrically,for the purpose of measurement. This includessuch items as resistors which may, for example,be used in bonds between pipelines atcrossings (for interference control), currentmeasurement shunts, stray current bondcables, instrument test leads, long reels of testwire, or resistors which occasionally are used to

reduce the current output of galvanic anodes.

These resistance measurements are simpleapplications of the basic electric circuit as wasdiscussed in Chapter 1. All that will be neededfor such tests are a battery, a multi-rangevoltmeter, a multi-range ammeter and testleads. Multicombination corrosion testinginstruments combine all these requirementsexcept for the connecting wires to the unknownresistance.

Figure 6-1 shows the resistance measurementcircuit based on the material contained inChapter 1.

To accomplish the measurement of theunknown resistance, connect it as shown in thesketch. Record the data. For example: E =1.210 volts and I = 67.6 milliamps. Now applyOhm's law where R = E/I. Remember to keepunits in the same order by either converting thevoltage reading to millivolts or converting themilliamps reading to amperes. Choosing thelatter course, the result is:

With the usual corrosion test instruments, it ispossible to easily and accurately measureresistors ranging from a thousandth of an ohm(0.001 ohm) or less, up to tens of thousands ofohms.

Referring to Figure 6-1, note that there is oneset of connections between the voltmeter andresistor and another set between the remainderof the circuit and the resistor (the connections

RESISTANCE MEASUREMENTS6-1

MEASUREMENT OF A SIMPLE RESISTANCE

FIGURE 6-1

BATTERY

UNKNOWN RESISTANCE

(–) (+)

(–) (+)

AMMETER

VOLTMETER

(–)

(+)

E = I R

=E

R

R =E

I

I

being represented by the dots). Keeping thevoltmeter connections independent from theconnections carrying the battery current isimportant, particularly when measuring lowresistance values. If the connections are madewith test leads having copper spring clipconnectors, the temptation is (after making thefirst set of connections) to clip the second set ofconnectors onto the first set. DO NOT DO IT! Ifthere is any resistance at all in the connectionsto the resistors, the test current flowing throughthat resistance will cause a voltage drop suchthat the voltmeter will read a potential greaterthan the actual voltage drop across theresistance being measured. The calculatedresistance of the unknown resistance beingmeasured will be higher than it should be.Keeping the connections independent avoidsthis. Having the voltmeter connections insidethe current connections, as shown on thesketch, is also good practice.

There are small general purpose testinstruments which are volt-amp-ohmmeters.These instruments are carried by electriciansand are a convenience to corrosion workers aswell. They may be analog or digital. Of interesthere is the fact that they include a resistancemeasuring circuit.

The analog type has a scale which iscompressed at the higher end making accuratereadings difficult and very small resistancevalues may not be accurately read either.

The digital type of volt-amp-ohmmeter will bemore accurate but may not be sensitive to verylow resistance values.

NON-ISOLATED RESISTANCES

In corrosion work on underground structures(and particularly pipelines), there are a numberof resistances to be measured which cannot beelectrically isolated to permit the simpleresistance measurement as discussed underthe preceding heading. How to handle some ofthese will be considered below.

Resistance of Isolating Joints

The first case will be that of measuring theresistance of an isolating joint in a pipeline. Thismay look easier than it actually is. Part A ofFigure 6-2 illustrates a set up using the simpleresistance measuring circuit with a battery,voltmeter and ammeter. Everything seems towork just fine. However (as an example) thevoltmeter reads 0.19 volt, right hand side ofisolating joint positive, before the battery circuitis connected. This, in itself, indicates that thereis some resistance in the isolating joint. It will,though, complicate the calculations a bit. Moreon this a little later.

After the battery circuit is closed, to continuethe example, the ammeter reads 0.0676 amp(or 67.6 ma) while the voltmeter reads 1.21volts, right hand side of isolating joint positive.But the voltmeter already read 0.19 volt beforethe test current was applied. Therefore theapplied current only changed the voltage dropacross the resistance from 0.19 volt to 1.21 voltor 1.02 volt .

Before proceeding further with the data taken,it is time to consider "delta" values and their usein Ohm's Law. The symbol ∆ means "changein" although normally read as "delta." In anyapplication of Ohm's Law we can use ∆E and∆I in the formula since if one changes, the othermust also change. The resistance (in theapplications which we will be considering) doesnot change so it is not shown as a delta value.So now the three arrangements of Ohm's Lawcan be shown as:


AMMETER

VOLTMETER

(+)

(–)

(+)(–) ISOLATING JOINT RESISTANCE

(+)(–)

RIGHT HAND PIPE SECTIONRESISTANCE TO EARTH

“EARTH”

LEFT HAND PIPE SECTIONRESISTANCE TO EARTH

PART B-- EQUIVALENT ELECTRICAL CIRCUIT OF PART A

RESISTANCE MEASUREMENTSAT ISOLATING JOINTS

FIGURE 6-2

VOLTMETER

AMMETER

(+)(–)(+)(–)(–) (+)

PIPELINE

ISOLATING JOINT

(+)(–)

MEASURE CURRENTIN PIPE SPAN--SEE CHAPTER 6

PART C-- HOW TO MORE ACCURATELY MEASURE THE RESISTANCE OF AN ISOLATING JOINT

VOLTMETER AMMETER

(+)(–)(+)(–)(–) (+)

BATTERY

PIPELINE

ISOLATING JOINT

GRADE

PART A-- HOW NOT TO MEASURE THE RESISTANCE OF AN ISOLATING JOINT

R =E

I 0.676A

102

15089.

.V

ohms

R =E

I

1.02 V

0.0006 A1700 ohms

Back to the example that was started earlier.The voltage and current readings were:

Voltage = 0.19V, OFF and 1.21V, ON!∆E = 1.02 V

Current = 0A, OFF and 0.0676 A, ON !∆I = 0.0676 A

Therefore:

But see following discussion.

In the data above, the "OFF" readings are thosetaken before applying the battery current whilethe "ON" readings are taken with the testcurrent flowing. Another note: the voltagereadings above were of the same polarity. Hadthey been of opposite polarity (one (+) and one(-)) , the figures would have been numericallyadded instead of subtracted as above.

After having gone through all this and obtaininga value of 15.089 ohms, THIS IS NOT THERESISTANCE THROUGH THE ISOLATINGJOINT!

Part B of Figure 6-2 shows why not. Actuallythe resistance that has been calculated is thatacross a parallel resistance circuit. One of theparallel resistors is the isolating joint. The otheris the resistance to earth of pipe on one side ofthe isolating joint plus the resistance to earth ofthe pipe on the other side of the joint. Now allthat is known is that the resistance that wasmeasured is less than the smaller of the twoparallel resistance paths. The chances are thatthe sum of the two pipe section resistances toearth is the smaller resistance element. Thereis nothing in the data taken which will provethis. But do not give up. If we can figure outhow much of the test current is going throughthe resistance of the isolating joint, we canagain apply the delta values to Ohm's Law andget closer to the actual resistance of theisolating joint.

Part C of Figure 6-2 illustrates one way to dothis. Measure the current flowing in the pipe onone side of the ammeter-voltmeter-batterysetup at the isolating joint. Chapter 7 on currentflow measurement will tell how to do this. Thecurrent flowing in the pipe is the current flowingthrough the resistance path which is in parallelto the resistance of the isolating joint. It follows,then, that if the current in the pipe is subtractedfrom the total battery current, the result shouldbe the current flowing through the isolatingjoint.

To expand the example being used earlier,assume that it has been determined that thecurrent in the pipe is 0.067 amp (which is mostof the test current from the battery). This meansthat the current flowing through the isolatingjoint is the 0.0676 amp battery current minusthe 0.067 amp pipe current or 0.0006 amp. Nowthe applicable data is:

Do not treat this figure as being all that preciseeither. It is simply a more accuratedetermination than was obtained with the firsttest set-up. Since a really good isolating jointcould have a resistance in the millions of ohms,the difference between the battery test currentand the pipeline current could be in the order ofa millionth of an amp (0.000001 amp) or less.At the present state of technology, the usualtest equipment available for field corrosiontesting simply cannot come close to measuringeither the battery current or pipeline current withthis level of precision. Nevertheless, thetechnique described does identify effectiveisolating joints.

Resistance-to-Earth of Anode Beds

Another case of measuring a non-isolatedresistance is that of determining the resistance-to-earth of a cathodic protection anode bed(either impressed current or galvanic).

Figure 6-3 illustrates a technique for doing this.Although the anode bed shown in the figure


R =E

I

in ohms

cannot be electrically isolated from earth it canbe temporarily isolated from the protectedstructure by opening the switch as shown in thefigure (or switching off the power supply). Thenit is simply a matter of determining the ∆voltageto the remote reference between current ONand OFF conditions and the ∆current (usuallyzero to the power source ON current) andcalculating the anode bed resistance as:

Figure 6-3 shows the remote reference locatedaway from the anode bed by five times themajor anode bed dimensions. The idea is to beoutside the area of earth which is affected bycurrent discharge from the anode bed. This issometimes referred to as the "area of influence"surrounding the anode bed. Normally, thefive-times rule of thumb will be safe although acloser distance may also be satisfactory.

An alternate to the five-times guide is to startwith the remote reference, say, 100 ft from theanode bed and measure the ∆V. Then movethe reference to a location 50 ft further from theanode bed and again measure the ∆V (with thepower source current the same for both sets oftests). If the two ∆V measurements are thesame, the reference is sufficiently distant fromthe anode bed. If the ∆V at 150 ft is lower thanthat at 100 ft, remote earth may not yet havebeen reached. Continue extending thereference from the anode bed by 50 ftincrements until the last two ∆V readings areessentially the same (at the same power sourcecurrent output). The reference will then be insatisfactorily remote earth. In general, remoteearth will be obtained at closer distances in lowresistivity soils than in high resistivity soils.

The remote reference may be a Cu/CuSO4

electrode as shown on the figure but does nothave to be. A steel pin pushed six inches to afoot into the earth will do as well and will havethe advantage of a lower contact resistancethan that of a Cu/CuSO4 electrode. The half-cellpo tential of whatever reference is used is

immaterial since it is only the change in themeasured potential that is important.

Note also in Figure 6-3 that the referenceelectrode is connected to the voltmeter negativeterminal rather than to the positive terminal asis normally the case when measuring structureto earth potentials on a cathodically protectedfacility. This is because the anode bedresistance should be measured under itsnormal current discharge mode (connected tothe positive terminal of the power supply). Thisis so that the effect of any positive polarizationof the anode bed will be included in theresistance measurement.

Figure 6-3 shows that the power supply for thetest may be either a cathodic protection rectifierat an established installation or a temporarypower source such as a 12-volt storage batteryor DC generator for newly installed anode beds.At established installations, periodicmeasurement of anode bed resistance canreveal deterioration of the anode bed with time.In the case of newly installed anode beds,some operators use the resistancemeasurement (made with a temporary powersupply) to size the permanent rectifier installa-tion. When making such a test on a new anodebed, the test current should be allowed to flowlong enough to permit making resistancemeasurements, say, every 15 minutes until itstabilizes (reflecting positive polarization) oruntil a curve of increasing resistance (if any)can be plotted and extended to estimate finalpolarized resistance.

Galvanic anode beds can be measured insimilar fashion using the anode dischargecurrent as the test current. Typically, the area ofinfluence around a galvanic anode bed is muchsmaller than for an impressed current anodebed. This means that remote earth will be foundto be closer than for tests on impressed currentinstallations.


MEASURING RESISTANCE OFANODE BED TO EARTH

FIGURE 6-3

CATHODIC PROTECTIONPOWER SOURCE OR OTHER TEMPORARY DC POWER SUPPLY

(+)(–)

(+)(–)

(+)(–)

IMPRESSED CURRENTANODE BED

5 TIMES MAJORANODE BEDDIMENSION

REMOTE REFERENCE(Cu-CuSOOR GROUND ROD)

PIPELINE (OR OTHERPROTECTED STRUCTURE)

4VOLTMETER

AMMETER

I = power supply current - I - I1 2

R =E

I

in ohms

I = power supply current - - I

= 3.45 -1.59 -1.60 = 0.26 amps

1 2I

VAVE =

(-1.35) + (-1.33) + (-1.34)

3= -1.34 V

R =V

I=

-1.34 V

0.26 A = 5.15 ohms

AVE

Resistance-to-Earth of a Pipeline

Applicable to coated pipelines is themeasurement of the non-isolated resistance toearth of a known length of the coated pipeline.This becomes useful in the determination of theeffective resistance of the pipeline coating.

Figure 6-4 illustrates one method of making thisdetermination. The essential requirements areto have a periodically interrupted test currentsupply to a temporary anode bed near thecenter of the section, pipeline currentmeasurement test stations at each end of thepipeline section under test, and facilities formeasuring the pipeline potential to remotereference electrode at least at the center and atthe two ends of the section under test. Onlonger sections, it is desirable to measure thepotential at intermediate locations as well.

With the test setup as shown, the DC powersupply is energized with the current interrupterin operation. This is allowed to stabilize untilsuccessive current ON ammeter readings areconsistent. Then pipe-to-soil voltage readingswith current ON and OFF are taken at the twoends, and at intermediate points, if any. ON,OFF and ∆V readings are recorded at eachlocation. At the pipeline current test stations ateach end of the section under test, the currentflowing on the pipe is measured and recordedwith the test current ON and OFF. From thesethe net pipeline current change attributable tothe test current is determined and recorded -indicated by ∆I1 and ∆I2 on the figure. Watchpolarities when making these determinations incase of current reversals in the pipe as thepower supply current switches from ON to OFF.

Now the data necessary to calculate thepipeline section resistance to earth is available.The delta test current flowing through theresistance between the pipeline section undertest and the earth is:

The ∆V between the pipeline and earth is theaverage of all the ∆V readings taken at the

pipeline-to-earth test stations along the section.Using these two values, the resistance betweenpipeline and earth is:

As an example, assume that the followingreadings have been taken:

1. Power source current, switch ON = 3.45amps

2. Pipe-to-earth potential at center of section:-2.22V, ON ; -0.87V, OFF; ∆V = -1.35V

3. Pipe-to-earth potential at left end of section:-2.19V, ON; -0.86V, OFF; ∆V = -1.33V

4. Pipe-to-earth potential at right end ofsection: -2.20V, ON; -0.86V, OFF; ∆V =-1.34V

5. Pipeline current at left end of section: 1.23A,ON. Flow to the right; 0.36A, OFF. Flow tothe left. ∆I1 = 1.23 + 0.36 = 1.59 Amp

6. Pipeline current at right end of section:1.95A, ON. Flow to the left- 0.35A, OFF.Flow to the left. ∆I2 = 1.95 - 0.35 =1.60 Amp

Applying the above data to determine thevalues needed to determine the sectionresistance to earth:

1. Current flow through the section-to-earthresistance:

2. Average ∆V across the section-to-earthresistance:

3. Resistance of the pipeline section to earth:


MEASURING RESISTANCE OFPIPELINE SECTION TO EARTH

FIGURE 6-4

VOLTMETER PIPELINE I

PIPELINE CURRENTMEASUREMENT TESTSTATION. SEE CHAPTER 7

(+)(–)

(-)(+)

(-)(+)

(+)(–)

(-)(+)

AUTOMATICCURRENTINTERRUPTER

PIPELINE SECTION

REMOTE TEMPORARYANODE BED

REMOTE REFERENCE

TEMPORARY DCPOWER SUPPLY

AMMETER

UNDERGROUND COATEDPIPELINE

REMOTE REFERENCE

REMOTE REFERENCE

VOLTMETER VOLTMETER

2PIPELINE I2

PIPELINE CURRENTMEASUREMENT TESTSTATION. SEE CHAPTER 7

R =E

I

in ohms

R of ft of pipe

V

I

100

= =0.0043V

10.2 A = 0.00042 ohm

R =(0.0006 0.5)

(0.00006 0.5)= 0.000599 ohms

The above demonstrates a technique formeasuring the resistance of a coated pipelinesection to earth. In Chapter 2, "PipelineCoatings", of the Advanced Course, there willbe a discussion of the use of such values in thedetermination and evaluation of coatingresistance.

The last non-isolated resistance to beconsidered involves the measurement of thelongitudinal resistance of an in-place pipeline.This is involved in pipeline current flowmeasurement to be discussed further inChapter 7.

Figure 6-5 illustrates the basic requirement formaking such a test. Basically, the test consistsof passing a known amount of direct currentthrough an accurately measured length of thepipeline and measuring the voltage drop acrossthat measured length caused by the currentflow. Delta values are used in order to eliminatethe effect of any existing current flowing throughthe pipe. Then the linear resistance of the pipein the 100-ft span is:

As an example, assume that the storage batteryin Figure 6-5 has been adjusted, by means ofthe controlling resistor, to provide a current inthe order of 10 amps when the circuit is closed.With this accomplished, the actual readingscould be:

1. Battery current = 10.2 Amps, ON; 0, OFF; ∆I= 10.2 amps

2. Voltage drop across 100-ft span = 0.0032V,ON, right end of span (+) = 0.0011V, OFF,left end of span (+), ∆V = 0.0043V, (4.3millivolts)

From these data, the following is obtained:

It may have been noted that no mention hasbeen made of the parallel earth path resistanceas was important when measuring theresistance of an isolating joint or of a pipe tocasing. This is because the pipe resistance isso low (whereas the others were high) that theparallel earth path resistance has little effect.

To illustrate this, a 100-ft length of 12-inch pipemight have a resistance in the order of 0.0006ohm. If, for example, the parallel earth pathresistance were 0.5 ohm (and could be muchhigher than this), the parallel resistance wouldbe (per Chapter 1):

The difference between this parallel resistancefigure and the pipe resistance is only 0.000001ohm which is a deviation of only approximately0.17%. This is acceptable being a betteraccuracy than probable for the various readingstaken to make the resistance determination.

SOIL RESISTIVITY MEASUREMENTS

Knowledge of the procedures for measuring soilresistivity is highly important in the evaluation ofsoils as to their corrosivity. It is equallyimportant for the selection of suitable locationsfor cathodic protection installations as well asfor the successful design of these installations.

As was discussed in Chapter 2, low soilresistivities result in low circuit resistances forgalvanic corrosion cells. This means highercurrent flow for a given cell potential and thehigher current means more corrosion.

Various ratings for soil corrosivity as a functionof soil resistivity are used by corrosion workerson underground systems. The following is anexample based on the standard unit of ohm-cmfor soil resistivity:


AMMETERRESISTOR TO LIMIT CURRENTOR REEL OF WIRE OF SUITABLE RESISTANCE

MEASURING LINEAR RESISTANCEOF PIPELINE

FIGURE 6-5

(+) (–)

(+)(–)

(+)(–)

BATTERY

VOLTMETER WITHMILLIVOLT RANGES

WIRE SPACING TO COMPAMY STANDARDS

PIPELINE

GRADE

TYPICALLY 100 FT OR MORE

RESISTIVITY CORROSIVITY

0 to 1000 ohm-cm Very corrosive

1000 to 2000 ohm-cm Corrosive

2000 to 10,000 ohm-cm Mildly corrosive

10,000 ohm-cm andabove

Progressively lesscorrosive

Such a rating system is only a general guide. If,for example, general soil resistivities in an areaare well above 10,000 ohm-cm, this does notmean that there will be no corrosion -- just thatthe tendency is less. And if there were local lowresistivity inclusions within the generally highresistivity soils, corrosion could be quite active.

Under the following headings will be describedtwo methods of measuring spot soil resistivityand a widely used method for measuring masssoil resistivity.

Soil Box Procedure

This procedure is used for measuring theresistivity of soil samples removed fromexcavations or auger holes. The resistivity maybe measured on site or samples may be placedin sealed plastic bags (to preserve moisturecontent) tagged as to location, date of removal,and other pertinent information and tested laterin the laboratory.

Figure 6-6 illustrates a typical type of soil boxtogether with a test setup for measuring a soilsample. Soil boxes are commercially available.The essential features are a non-conductingplastic container, metal end plates for passingtest current through the sample, and removablepotential pins between which the resistance ismeasured. It is essential that the distancebetween the potential pins in centimeters isequal to the cross sectional area of the interiorof the box in square centimeters. With thisrelationship, the resistance measured in ohmsbetween the potential pins is equal to the soilresistivity in ohm-cm.

In use, the potential pins are removed and the

box filled with the soil sample. The soil iscompacted to the same degree as it was in thelocation from which the sample was removed.The potential pins are then inserted and the soilrecompacted to ensure solid contact betweenthe soil and the pins. The soil in the box isstruck off flush with the top of the box so thatthe cross sectional area of the soil sample isequal to that of the box. The sample is nowready for the resistance measurement.

The illustration on the lower part of Figure 6-6shows the resistance being measured with anAC resistance measuring test set. This has notbeen discussed previously but is a type widelyused in soil resistivity test work. There arevarious makes of this type of equipment. Thetest set illustrated works in this fashion: currentfrom internal batteries passes through one halfof a synchronous vibrator which rapidlyalternates the direction of the current flowmaking it AC current. This AC current is thenpassed through a transformer which steps upthe battery voltage to a higher value. Thishigher AC voltage from the transformer isconnected to the end plates of the soil box topermit AC current to pass longitudinally throughthe sample in the soil box. This AC currentcauses an AC voltage drop between thepotential pins with the amount of the voltage be-ing proportional to the soil resistivity. The ACvoltage from the potential pins is then passedthrough the other half of the synchronousvibrator which changes it back to DC. A bridgecircuit then compares the resistance of the soilsample with that of an adjustable and calibratedresistor (identified as "Resistance scale inohms" on Figure 6-6). When making ameasurement, the calibrated resistor knob isrotated (with the energizing button depressed)until the galvanometer reads zero -- which isthe balance condition. The range selectorswitch may have to be adjusted in the process.Then the ohms reading indicated on thecalibrated resistor resistance scale is multipliedby the factor indicated by the range selectorswitch at the position where a balance wasobtained. The result is the soil resistivity inohm-cm.


DISTANCE BETWEENPOTENTIAL PINS INCENTIMETERS EQUAL TO CROSS SECTIONAL AREA OF BOX IN SQUARE CENTIMETERS

SOIL BOX MEASUREMENTOF SOIL RESISTIVITY

FIGURE 6-6

REMOVABLE POTENTIAL PINS

AC RESISTANCEMEASURING TEST SET

SOIL SAMPLE COMPACTEDIN BOX FLUSH WITH TOP

METAL END PLATEWITH OUTSIDE TERMINAL

BALANCINGGALVANOMETER

RESISTANCE SCALEIN OHMS

SIDE VIEW

PLASTIC CONTAINER

TOP VIEW

MEASURING RESISTIVITY OF SAMPLE

ENERGIZING BUTTON

P1

C1

P2

C2

RANGE SELECTORSWITCH

P1 P2

C1 C2

C1 C2

P1 P2

The use of equipment of the type described hasthe advantage of permitt ing rapidmeasurements with a minimum of calculationrequired.

The voltmeter-ammeter-battery method, as hasbeen described in other applications, may alsobe used for making the soil resistivitymeasurement with the soil box. The ∆I, ∆Vprocedure should be used to eliminatepolarization and galvanic effects. In the case ofvery high soil resistivities, resistance valuesmay be beyond the range of the AC test sets.The DC method may then be used.

The soil box may be used for measuring theresistivity of water samples in the same fashion.

The soil box should be cleaned thoroughlybetween tests to avoid contamination of thenext sample.

Where soil samples are taken for later resistivitymeasurement in the laboratory and are found tohave dried out for any reason, distilled wateronly should be used to remoisten the sample.Using distilled water avoids the possibility ofadding salts which could affect the resistancemeasurement.

It should be noted that the AC resistance testequipment described is not confined to soilresistivity measurements only. It can be usedas well for measuring other resistances withinits range. The range, depending on theequipment used, could be from about 0.1 ohmto 10,000 ohms.

Single Rod Test Procedure

For rapid accumulation of spot soil resistivitydata along an underground structure such as apipeline, the single rod resistivity equipment canbe used. There are various designscommercially available.

The essential features of such a device areillustrated by Figure 6-7. The two exposedmetal surfaces at the lower end of the testershaft, when pushed into the ground, will have a

resistance between them which is proportionalto the resistivity of the soil immediatelysurrounding these tips.

In use, the rod is pushed into the ground to thedesired depth (up to three or four feet), the testcircuit actuating button depressed, and the soilresistivity read directly from the calibratedread-out.

The single rod tester only measures the soil re-sistivity in a small local area -- which was alsotrue of the soil box technique. Because of thespeed with which it can be used, it isparticularly useful in screening prospectivesites, for example, for cathodic protectioninstallations and then using mass soil resistivitytechniques to explore the more promisinglocations indicated by the single rod equipment.

4-Pin (Wenner) Procedure

This procedure, as opposed to soil box tests orsingle rod tests, has the advantage ofpermitting the measurement of mass soilresistivities to various depths without having togo below the surface. This procedure is usedvery widely for final selection of cathodicprotection test sites and to accumulate the datanecessary for the installation design.

The "Wenner" designation is the name of theperson who developed the theory andapplication of the procedure.

The arrangement of a 4-pin test setup isillustrated by Figure 6-8.

The size of the soil pins shown is not critical butcan be ¼-inch or d-inch rod two to three feetlong and fitted with tee handles to facilitatehandling. At a test site, the pins are pushed ordriven into the ground (to a depth no more than5% of the pin spacing) along a straight line withspacing between pins equal to the depth towhich the average soil resistivity is to bemeasured. An AC resistance test set (as wasdescribed in connection with soil box tests) canbe used to measure the resistance betweenpins P1 and P2. The voltmeter-ammeter-battery


SINGLE RODSOIL RESISTIVITY TESTER

FIGURE 6-7

BATTERY OPERATED RESISTANCE MEASURING CIRCUITRY. MAY BESHAFT MOUNTED OR, IN SOME MODELS, MAY BE SEPARATE AND CONNECTED WITH TEST LEADS TO TERMINALS AT THE UPPER END OFTHE TESTER SHAFT. READOUT OFCIRCUITRY CALIBRATED TO INDICATE OHM-CM DIRECTLY.

READOUTFACES UP

TWO ELECTRICALLY SEPARATE METAL SURFACES WITH CONNECTING WIRESINSIDE SHAFT TO RESISTANCEMEASURING CIRCUITRY.

SHAFT COVEREDWITH INSULATINGMATERIAL

INSULATINGSEPARATOR

STEEL OR STAINLESSSTEEL SOIL PINS

WITH SOIL PINS PLACED IN STRAIGHTLINE CONFIGURATION AT EQUAL SPACING d.SOIL RESISTIVITY IS MEASURED IN A HEMISPHERICALLY SHAPED MASS OF EARTHHAVING A RADIUS EQUAL TO THE PIN SPACING.

AC RESISTANCE TEST SET

SOIL RESISTIVITY MEASUREMENTBY WENNER 4-PIN METHOD

FIGURE 6-8

GRADEC1 P1 P2 C2

P1 P2

C1 C2

d d d

d

1915. d R

1915 25 18 86175. . . ohm cm

method may also be used (using the ∆V, ∆Iprocedure) but it is slower.

The measured resistance value is not the soilresistivity. This is a function of both themeasured resistance and the pin spacing. Forany pin spacing, the soil resistivity isdetermined by the following formula:

Where:

ρ = soil resistivity in ohm-cm

d = pin spacing in feet

R = measured resistance between P1 and P2in ohms

The Greek letter "rho" (ρ) is commonly used todesignate resistivity.

As an example, assume that a test has beenmade with pins spaced 2.5 ft apart and that theresistance between P1 and P2 has beenmeasured and found to be 18 ohms. The soilresistivity will then be:

The soil resistivity so determined is a weightedaverage to the 2.5 ft depth. As indicated onFigure 6-8, the mass of earth "seen" by themeasurement is a hemispherically shapedmass with the flat cross sectional area of thehemisphere flush with the earth surface, andwith the radius equal to the pin spacing andcentered at a point midway between soil pinsP1 and P2. It will be seen, then, that theaverage will be affected more by the sub-stantially greater volume of earth in the top halfof the hemisphere than by the bottom half.

In order to determine whether the soil resistivityis constant with depth or is increasing ordecreasing with depth, it is advisable to take aseries of readings. Such a series, for example,might be as follows:

Pin Spacing(Feet)

MeasuredResistance

(ohms)Factor

Average SoilResistivity(ohm-cm)

2.5 18 191.5 8617.5

5 9.4 191.5 9000.5

7.5 5.9 191.5 8474.2

10.0 3.4 191.5 6511.0

12.5 3.1 191.5 7420.8

15 3.5 191.5 10,053.8

The preceding discussion illustrates the basictechniques for using the 4-pin method formeasuring mass earth resistivity. Information onthe use and interpretation of such data will becovered in Chapter 5, "Design of ImpressedCurrent Cathodic Protection", of the AdvancedCourse.


CHAPTER 7

CURRENT FLOW MEASUREMENTS

INTRODUCTION

The primary focus of this chapter is themethods used to measure current flow onmetallic structures such as pipelines, where anammeter cannot be inserted into the circuit inthe conventional manner.

There are two methods that can be used tomeasure current flow on a structure such as apipeline in this situation:

• Measure the voltage drop across a spanof the structure and apply Ohm’s Law tocalculate the current flow.

• Use a clamp-on ammeter.

Interpretation of data acquired using thesemethods for pipeline current surveys will alsobe discussed.

DETERMINING CURRENT FLOW IN PIPE

The most common method of measuringcurrent flow on a long structure such as apipeline involves determining the resistance ofa known span of the structure (also discussedin Chapter 6), measuring the voltage dropacross that span caused by the current flowthrough it, and then calculating the currentusing Ohm's Law. To do this successfullyrequires good instrumentation, very carefulmeasurement techniques, and observation ofthe various factors that could affect the results.

There are procedures which can be used with2-wire test points (1 test wire from each end of

a pipe span) and with 4-wire test points (2 testwires from each end of the span). Each of theseprocedures will be discussed in this chapter.

Use of Pipe Resistance Calculations with2-wire Pipe Span Test Points

A 2-wire test point may be either a permanentinstallation with test wires attached to the pipeor it may be a temporary set-up using proberods to contact the pipe. Of the two, thepermanent installation is preferred. The twotypes of 2-wire test points are illustrated byFigure 7-1. In each case, a millivoltmeter isshown connected to measure the voltage dropacross the span. An arrow on the pipeline ineach case shows the direction of the currentflow for the instrument polarity shown.

When making pipeline current measurements,the polarity of the instrument connections,which end of the span is (+) and which is (-), isjust as important as the accuracy of themeasurement itself. This is necessary so thatthe direction of current flow will be known. Theimportance of this will be discussed in a latersection of this chapter on current surveys. Forthe this reason, buried test wires for apermanent 2-wire test point must be colorcoded so that the tester knows where each wiregoes. Color codes vary with pipeline systemsbut should be uniform within that system.

The permanent 2-wire test point installation hasthe advantage of solid connections between thetest wires and the pipe. It also permits aminimum of setup time to make a test. Thetester, however, will be depending on the

CURRENT FLOW MEASUREMENTS7-1

TWO-WIRE CURRENTMEASURING TEST POINTS

FIGURE 7-1

ACCURATELY MEASUREDSPAN BETWEEN PROBERODS ( 1”)

TEMPORARY TWO-WIRE TEST POINT

PERMANENT TWO-WIRE TEST POINT

PIPELINE

MILLIVOLTMETER

LONG TEST LEADSPROBE ROD CONTACTING PIPE

+-

(+)(–)

GRADE

PIPELINE

MILLIVOLTMETERTEST WIRES ATTACHED TO THE PIPELINEWIRES MUST BE COLOR CODED

GRADE

COAT CONNECTION TO PIPE

ACCURATELYMEASURED PIPESPAN ( 1”)+-

(+)(–)

TEST WIRES TERMINATED ATTEST STATION

+-

Rweight ft

microhms ft 2891.

//

accuracy of pipe span lengths as entered onpipeline maps or other records. The tester willalso be depending on the accuracy of the colorcode. These possible sources of error shouldcause minimal problems on properly installedand inspected pipeline systems.

The 2-wire test point using probe rods is a slowprocedure and has certain undesirable featureseven when span length and polarity are known.To make good contact, probe rods should havesharp hardened tips. A pipe locator will beneeded to locate the pipe accurately so that theprobe rod can contact the top of the pipe. Oncea solid contact is made, the person making thetest has to depend on that contact staying solidfor the duration of the test. Some operators usesand bags or other weights to keep a steadypressure on the contact to ensure its continuity.

Using probe rods on a coated pipe is stronglydiscouraged, since at least one holiday in thecoating is made by each probe rod, and, if theperson probing has trouble hitting the top of thepipe, there may be a substantial amount ofscarring and cutting of the coating before asolid contact is made. Further, bright metalexposed where the probe rod makes contactwill be anodic to adjacent metal. This isparticularly true on bare pipe where thegalvanic potential between bright metal andadjacent surfaces exposed to soil will tend to bestronger than on coated pipe.

A pipeline current survey made using proberods on what was a very well coated pipeline(initially) can have a substantial detrimentaleffect on the effective coating resistance as aresult of additional coating holidays. There willbe also an increase in cathodic protectioncurrent requirements.

In order to determine pipeline current flow, aknown length of pipe is necessary. 100 footspans work well for pipeline sizes up to in theorder of 20". For large sizes such as 30", 36"and 48" pipe, spans of 200 to 400 feet may beneeded so that there will be enough resistancein the pipe to allow measurement of the pipelinecurrent with acceptable accuracy.

To make the actual measurement of pipelinecurrent with either the permanent or temporary2-wire test point setup, a millivoltmeter isconnected to measure the voltage drop acrossthe span (See Figure 7-1). Keep in mind,however, the resistance of the pipe in the spanmust be known before the pipeline current canbe calculated.

In Chapter 6 the method for measuring theresistance of a pipe span was described, butthat method required four connections to thepipe. The pipe resistance cannot be measuredwith sufficient accuracy using two pipeconnections because the resistance of the pipeconnections (test wires or probe bars) will beincluded in the measurement.

In order to determine the resistance of the pipespan in this situation, use is made of piperesistance tables. Table 1 shows resistancevalues for some standard steel pipe that areused to calculate the linear resistance per footof pipe. All that is needed from pipelineconstruction records is the weight per foot ofthe steel pipe used in the area where thecurrent measurement is to be made. With thisfigure, the linear resistance of the pipe per footwill be:

To see how the above is used, assume that avoltage of 2.35 mV has been measured acrossa 100-ft span of 12-inch, 0.375" WT (wallthickness) east-to-west pipeline and that thewest end of the span was (+).

Using Table 7-1, the resistance of 12-inch,0.375-inch WT pipe is 5.82 microhms per foot.For the 100-ft span in the example, theresistance of the span = 5.82 x 100 = 582microhms which is the same as 0.000582 ohm.

The measured voltage drop of 2.36 mV is thesame as 0.00236 V. By Ohm's Law, I=E/R.Therefore, in this example, the current flowingin the measurement span is:


I0.00236 V

0.000582 4.055 A flowing west to east

The current measured using the figures fromTable 1 or using the formula herein based onpipe weight per foot may not be strictly accurateas there are variations in steel resistivity. The18 microhm-cm figure on which Table 1 isbased is a general average.

Ideally, an instrument used to measure themillivolt drop across a pipe span should bereadable to the nearest one hundredth of amillivolt (0.01 mV). In many situations, aninstrument readable to the nearest one tenth ofa millivolt is suitable (0.1 mV).

In the example worked earlier, it was calculatedthat the pipeline current was 4.055 ampereswhich implies that this value is accurate downto the last milliamp. Aside from the possibleinaccuracy caused by variations in steelresistivity, there is also inaccuracy because oflimitations on instrument capability. In theexample, it was determined that the millivoltdrop across the span was 2.36 millivolts, whichwas as close as it could be read using theinstruments at hand. Although it could not beread from the instrument, assume that the truemillivolt drop was more like 2.3648. Then thepipeline current calculation would have been:

The difference from the 4.055 amps calculated(using a millivoltmeter sensitive to only ahundredth of a millivolt) is 0.008 amps (or 8milliamps). For a current of the magnitudemeasured, this is only an error of a fraction ofone percent. However, if the indicated pipelinecurrent had been 100 milliamps, the 8 milliampsinaccuracy would result in an 8 percent error.Another way of looking at it is that if there wereonly say 7 or 8 milliamps flowing on the pipe, amillivolt drop taken across the 100 feet of12-inch pipe might not detect any current at all.

The possible inaccuracies in the current

measurements do not detract from their value incorrosion test work on long structures such aspipelines. It is desirable, however, thatcorrosion personnel be aware of the limitationsdescribed when interpreting the results of thecurrent surveys.

Current Flow Measurements Using 4-wirePipe Span Test Points

The use of 4-wire permanent test points permitsthe most accurate measurement of currentflowing on a structure such as a pipeline. Thisprocedure does not require knowledge of thestructure weight per foot. It is not affected byvariations in the resistivity of the metal. It is notaffected by the temperature of the structure. Itis not affected by inaccuracies in the length ofthe structure in the measurement span. And, itcan be used on structures made of metals otherthan steel. For example, this procedure alsoworks on aluminum, brass, or copper pipe, orany alloys.

The reason for the accuracy of the 4-wiremethod is that it allows the resistance of thespan in which the current is flowing to beaccurately measured. This is true no matterwhat "surprises" may exist within the span. Forexample, in the case of a pipeline, if part of thespan was standard wall thickness pipe and partwas heavy wall (and possibly not shown ondrawings), the ability to accurately measure thespan resistance would allow the current to beaccurately calculated. If the currentmeasurement were made with the 2-wiretechnique, however, and if the presence oramount of heavy wall pipe was not knownaccurately, the current measurementcalculation could be seriously in error.

The procedure for measuring the resistance ofa span of pipe (or other structure) using fourconnections to the structure was described indetail in Chapter 6. Figure 7-2 illustrates atypical 4-wire permanent test point installation.The optimal length of the current measuringspan will vary with pipe size as was discussedfor 2-wire current measurement spans earlier inthis chapter. But, since we are measuring the

I0.0023648 V

0.000582 4.063 A flowing west to east


TABLE 7-1

RESISTANCE OF STEEL PIPE(1)

Nominal Pipe Size

(in)

OutsideDiameter

(in)

WallThickness

(in)

Weight perLinear Foot(2)

(lbs)

Resistance perLinear Foot(3)

(:S)

2 2.375 0.154 3.65 79.2

4 4.5 0.237 10.8 26.8

6 6.625 0.280 19 15.2

8 8.625 0.322 28.6 10.1

10 10.75 0.365 40.5 7.13

12 12.75 0.375 49.6 5.82

14 14 0.375 54.6 5.29

16 16 0.375 62.6 4.61

18 18 0.375 70.6 4.09

20 20 0.375 78.6 3.68

22 22 0.375 86.6 3.34

24 24 0.375 94.6 3.06

26 26 0.375 102.6 2.82

28 28 0.375 110.6 2.62

30 30 0.375 118.7 2.44

32 32 0.375 126.6 2.28

34 34 0.375 134.6 2.15

36 36 0.375 142.6 2.03

(1) From “Control of Pipeline Corrosion”, NACE, 1967, Peabody.

(2) Based on steel density of 489 lbs/ft3.

(3) Using 18 :S -cm resistivity for steel:

R = (16.061 x 18 :S -cm ) / weight per foot in pounds = 289.1 / weight per foot = resistance of one foot of pipe in :S

(+)

AMMETER

(–)

(+)(–)

(+)(–)

BATTERY RESISTOR TO LIMIT CURRENTOR REEL OF WIRE OF SUITABLE RESISTANCE

MILLIVOLTMETER

BLK WHT

FOUR-WIRE CURRENT MEASURINGTEST POINTS

FIGURE 7-2

RED GREEN

GREEN

WHITE

TEST STATIONGRADE

COAT ALL CONNECTIONS TO PIPE

DOWNSTREAM

PIPE SPAN IN WHICHCURRENT IS MEASURED

REDBLACK

PIPELINE

WIRE SPACING TO COMPANY STANDARDS

TEST WIRES ATTACHED TO THE PIPELINEWIRES MUST BE COLOR CODED

K factor =10.8 A

4.95 mv2.182 A / mV

resistance, the span can be any length.

Color coding of the buried conductors isessential. Different pipeline system operatorsmay use color codes adapted to their system. Apossible system is shown on the figure wherethe black and white wires measure millivoltsacross the current measurement span, whilethe red and green wires are the test currentwires (outside the current measurement span)used during span resistance determinations.The red and black wires shown on the figurecould always be at the downstream end of thespan (if pipeline product flow is always in thesame direction) or compass direction could beused (such as west for east-west pipelines ornorth for north-south pipelines). In any event, astandard code must be established andconsistently observed for a given pipelinesystem. Obviously, care must be taken duringthe installation of the test point so that the colorcoding is accurately done. Probe rods can beused to make the four pipeline connections ifabsolutely necessary. Such use is slow andinvolves the problems as discussed under theprocedures for 2-wire test points.

Figure 7-2 shows the connections formeasuring the span resistance per the Chapter6 procedure. The figure shows themillivoltmeter connections for measuring thevoltage drop across the span. The voltage dropacross the span is used to determine the spanresistance and then to calculate the current flowin the pipe.

Rather than actually calculating the spanresistance in ohms, it is simpler to determine acalibration factor (K factor) in units of amps permillivolt (A/mV). To illustrate this, assume thatin measuring the resistance of the pipe span, abattery test current of 10.8 amperes through thered and green wires results in a voltage dropacross the current measurement span of 4.95millivolts (or 0.00495 V). The calibration factorwill be:

After we have determined the K Factor, we canuse this value to calculate the amount of currentflowing on our pipeline. For example, if wemeasure 3.2 mV across our span, the amountof current flowing is:

For 4-wire test points on systems operating atconstant temperature, the span resistance needbe determined only once. The test station canthen be permanently tagged with this calibrationfigure.

If the pipeline on which the 4-wire test point isinstalled, however, is subject to substantialtemperature variation during its operation, it willbe desirable to measure the pipe spanresistance each time. For example, a 10º Ctemperature change on a steel pipe couldcause in the order of a 5% change in the spanresistance. It should also be noted thattemperature of a pipe could cause variationsfrom Table 1 resistance figures as used with the2-wire test points.

When taking voltage drop readings across thecurrent measurement span to determine currentflowing on the pipe, it is essential that thepolarity of the millivolt reading be properlyrecorded - such as "downstream (+)" whichwould indicate current flowing upstream alongthe pipe.

CURRENT SURVEYS ON PIPELINES

Now that the techniques for making pipelinecurrent measurements have been developed,the next step is to discuss some of the waysthat the procedures may be put to use on apipeline system.

Modern cross country pipelines are usuallyprovided with current-measuring test points atintervals of, for example, approximately fivemiles, with the permanent 4-wire test points

I = K factor EmV

I = 2.182 A / mV 3.2 mV = 5.382 A


being the preferred type for reasons that havebeen developed earlier in this chapter. The testpoints are normally located, where possible, atroad crossings or other points of easy access.It is also desirable to have current measuringtest points at each side of pump or compressorstations, at isolating flanges, and at otherlocations where the system configuration issuch that a possibility of current entering orleaving the pipeline may exist.

Before cathodic protection is applied to apipeline, there are advantages in taking acurrent survey along the line. A current surveycould detect:

• The presence of strong long linegalvanic corrosion cells (but not localcells).

• The effectiveness of isolating joints.

• The quality of the pipeline coating.

• The presence of stray currents.

Figure 7-3 illustrates two examples of such asurvey. Single lines to the pipeline are shown torepresent 4-wire test stations.

Part A of Figure 7-3 shows a possibledistribution of galvanic current along thepipeline. At Test Stations 1 and 2, there was nodetectable pipeline current (if any, it is less thanthe test procedure can pick up). This indicatesthat isolating joint A is effective. The currentthen gradually increases at Test Stations 3 to 6.At Test Station 7, the current has reverseddirection indicating that there is a long linegalvanic cell anodic area between Test Stations6 and 7 with current discharge to earth. TestStations 8, 9 and 10 show decreasing current.At Test Station 11, the current has againreversed showing a cathodic area of galvaniccurrent pickup between Test Stations 10 and11. From there on the current increases exceptfor a decrease at Test Station 13 whichindicates an area of current discharge in thatarea. The test station at isolating joint "B" stillshows substantial current flow which indicates

that the isolation either is not effective, or thereis a parallel path (sneak circuit) carrying thecurrent around the isolating joint.

Part B of Figure 7-3 gives an idea of what theeffect of stray current could be along a sectionof pipeline. As can be seen there is an area ofsevere stray current discharge between TestStations 5 and 6 with in excess of 8 amperesmax discharge. Early knowledge of such areasis critical so that corrective measures can betaken as soon as possible.

At the two ends of the pipeline sections shown,reversing currents are shown. These readingsare more than 15 miles from the area ofmaximum stray current discharge. Theindication is that galvanic current is read part ofthe time (during periods of low stray currentactivity) while at other times, the stray currenteffect is strong enough to overcome thegalvanic currents and reverse the direction offlow.

Figure 7-4 illustrates two examples of theapplication of current surveys on a cathodicallyprotected pipeline.

Part A of Figure 7-4 illustrates what may beexpected on a protected line where there are nocomplications. From the figures showingpipeline current, it is apparent that the 10.4amps being drained by the cathodic protectionrectifier is well divided between pipelinesections in each direction from the rectifier.There is a certain amount of attenuation(reduction in current pickup per 5 mile section)with distance from the rectifier. This is afunction of pipeline size (and its longitudinalresistance) and coating quality. The larger thepipe and the better the coating, the moreuniform the current distribution will be.

Part B of Figure 7-4 demonstrates the effect ofa contact between a foreign structure and theprotected pipeline. The first hint of trouble isthat the current at the first test point to the leftof the rectifier is much higher than at the firsttest point to the right of the rectifier. As thesurvey continues, it is found that current


CURRENT SURVEYS ALONG PIPELINEPRIOR TO CATHODIC PROTECTION

FIGURE 7-3

TEST STATION NUMBERS ALONG PIPELINEAT NOMINAL 5-MILE INTERVALS.

ISOLATINGJOINT “A”

PART A – GALVANIC CURRENT SURVEY ON PIPELINE PRIOR TO CATHODIC PROTECTION

ARROWS SHOW DIRECTION OF CURRENT FLOW.FIGURES ARE PIPELINE CURRENT IN AMPS.

1“B”

0 0

0.01

5

0.02

0

0.03

2

0.04

0

0.11

5

0.10

5

0.08

5

0.02

0

0.01

0

0.01

5

0.01

0

0.01

5

0.02

0

0.02

5

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

PART B – STRAY CURRENT SURVEY ON PIPELINE PRIOR TO CATHODIC PROTECTION

TEST STATION NUMBERS ALONG PIPELINEAT NOMINAL 5-MILE INTERVALS.

ARROWS SHOW DIRECTION OF CURRENT FLOW.DOUBLE HEADED ARROWS SHOW CURRENT REVERSALS.FIGURES ARE PIPELINE CURRENT IN AMPS.E AND W DESIGNATE FLOW EAST OR WEST.

EAST

1

0.20

5 W

to

0.39

0 E

2

0.18

0 W

to

0.41

0 E

3

0.11

0 E

to

0.55

0 E

4

0.65

0 E

to

2.75

0 E

5

2.15

0 E

to

4.65

0 E

6

3.75

0 W

to

1.95

0 W

7

2.16

0 W

to

1.45

0 W

8

1.75

0 W

to

0.95

5 W

9

0.07

5 W

to

0.21

0 W

10

0.15

0 W

to

0.09

5 E

11

0.07

5 W

to

0.15

0 E

WEST

4-WIRE TESTSTATION AT NOMINAL 5-MILEINTERVALS

CATHODIC PROTECTIONRECTIFIER AND ANODE BED

PART A – CURRENT DISTRIBUTION ALONG CATHODICALLY PROTECTED PIPELINE WITH NO COMPLICATIONS.


PIPELINE(–)(+)

1.70 2.30 3.10 4.30 4.25 3.00 2.15

10.4A

CURRENT SURVEYS ALONGCATHODICALLY PROTECTED PIPELINE

FIGURE 7- 4

CATHODIC PROTECTIONRECTIFIER ANDANODE BED

FOREIGNPIPELINECROSSING

PART B – CURRENT DISTRIBUTION ALONG CATHODICALLY PROTECTED PIPELINE WITH CONTACT TO FOREIGN STRUCTURE.


0.70 0.95 6.85 7.10 7.50 4.15 3.75

4-WIRE TESTSTATION AT NOMINAL 5-MILEINTERVALS

(–)(+)

12.6A

remains high on the pipeline to the left of therectifier until the foreign pipeline crossing ispassed, then the current at the first test stationpast the crossing is a relatively small value. Thefigures indicate that the foreign structure orpipeline contacting the protected line is takingroughly half of the rectifier output. Such asituation requires correction.

CLAMP-ON AMMETERS

Another way to measure current flow on aconductor such as a pipe or cable is with aclamp-on ammeter. A clamp-on ammeter has asensor in the form of a clip or clamp. When thesensor is placed around a conductor, theammeter will indicate the current flow on it.Current quantity and direction will be indicated.

Small sensors are in the form of clips or “jaws”.The clips are squeezed open and placedaround the conductor. The clips are thenreleased so that they close. The ammeter willthen indicate the current flow on the conductorwithin the sensor. The sensor may beintegrated into the ammeter or may be separatefrom the ammeter and connected to it with testleads. Sensors in the form of clips are availablefor use on conductors up to 6 inches indiameter.

Large sensors are in the form of 2 or 4 piececlamps that are assembled around theconductor. The assembled sensor is connectedto the ammeter with test leads. The ammeterwill then indicate the current flow on theconductor within the sensor. Large sensors areavailable for use on conductors up to 82 inchesin diameter.

The sensors may be permanently installedaround a buried or submerged pipeline andtheir test leads terminated in a test station sothat current flow can be monitored.

It is important to note that clamp-on ammetersmeasure the net current within the sensor. If 2conductors are placed within the sensor, theammeter will indicate the sum of the current

flow on the 2 conductors. For example, if thesensor is placed around both the positive andnegative leads from a rectifier, the ammeter willindicate 0, since the current on the 2 cables isin opposite directions.

Figure 7-5 shows a clamp-on ammetermeasuring current flow on a stainless steeltubing line. In this example, the tubing line wasinstalled across an isolating flange.


CLAMP-ON AMMETER MEASURING CURRENT ON STAINLESS STEEL TUBING

FIGURE 7-5

CHAPTER 8

RECORD KEEPING

INTRODUCTION

The subject of record keeping is just asimportant as everything that has been learnedfrom prior chapters on the technical aspects ofcorrosion and its control. No matter how skilledthe corrosion worker may become in fieldtesting, design of corrosion control measures,and maintenance of corrosion control systems,his work can be ineffectual if it is not suitablyrecorded for later reference. How effectively thecorrosion worker keeps his or her records canwell be a critical element when jobperformances are being evaluated.

There are three prime requisites for an efficientand useable system of records. These are:

1. They must be COMPLETE.

2. They must be ACCURATE.

3. They must be ACCESSIBLE for readyfuture reference.

The corrosion worker will learn quickly that if hisrecords meet these objectives, his ongoingwork will be made more efficient by eliminatingunnecessary repeat testing and by maintaininga consistent approach to corrosion controldesign and maintenance measures. Further,effective records, properly used, will assist himin the early recognition of changes in thecorrosion control system performance. Thesechanges may signal the development ofproblem areas requiring early correction.

With the ever-increasing efficiency and use ofcomputers in the business world, their use in

keeping records on corrosion and corrosioncontrol is continually increasing in importanceand efficiency. The larger the system beingcovered by a corrosion control program, themore important this becomes because of thesheer mass of data that are recorded. Probablythe greatest advantage of a suitablecomputerized system of records is the readyand rapid accessibility of data on any aspect ofthe corrosion control program when it isneeded.

In the remainder of this chapter, the specificaspects of corrosion control records will bediscussed in greater detail.

GENERAL CHARACTERISTICS OFRECORDS

As has been stated, records must bemaintained for subsequent use in any ongoingcorrosion control program. They must be insuch form that they can be analyzed todetermine the performance of the corrosioncontrol system so that any necessary changescan be initiated promptly to maintain theeffectiveness of the system.

In the introduction it was stated that the threeprime requisites of the records system arecompleteness, accuracy, and accessibility.These merit amplification.

Completeness refers to having all thenecessary information to make the record ofuse at a later date. If it is a field operation, therecord must include the details of the field testor other operation in such form that the results

RECORD KEEPING8-1

of the test or operation are clearly set forth.

It is most important that the location of the testor operation is stated as well as the date andthe name or names of those participating in thework. The completeness aspect can beexpedited by having forms (designed for theworker's specific system) that have provisionsfor entering all critical information.

Accuracy is of prime importance. No guessingor rough estimation of a test location, forexample, is permissible. If later analysis ofresults indicates the need for further work atthat specific location the original record (ifinaccurate) may be worthless or necessitaterepeat work in order to locate the point ofinterest.

Another example is the "as built" record of acathodic protection installation involving arectifier and anode bed. During the installationof such a system, the location of undergroundcable routing and the location of individualanodes may differ from the design drawing asmade necessary by unexpected field conditionsencountered once construction is started. Therevised locations must be accuratelydetermined and recorded on the "as built"drawing entered into the record system. If thisis not done, there can be time wasted in findingthese buried portions of the installation duringmaintenance operations requiring cable repairor replacement of anodes. This meansneedless expense which would have beenavoided had the record been accurate.

Accessibility, as the word suggests, means thatrecords of past work must be so handled thatwhen information is needed, it can be retrievedeasily.

If a manual filing system is used, it should beset up by categories of data according to anestablished procedure applicable to theparticular underground structure system. Nomatter what filing procedure is established,rapid accessibility of needed information is thekeyword and the prime consideration in thedesign of the filing system used.

If a computerized records system is used, itshould likewise be so set up that the requiredinformation, and that information only, can bequickly withdrawn from the computer records.

Whatever system is used, good recordsmanagement is essential to make the systemeffective. Paramount to this is seeing that allinformation that should be in the recordssystem actually reaches the system promptlyafter it is generated. This means having anorganized, routine procedure for getting thedata into the system with the least practicabledelay. Unentered data piled up on someone'sdesk is of no effective use as far as theaccessibility factor is concerned - and is subjectto loss or misplacement during this "loosestorage" period.

Management Use

The records system designed and used will bethe basis for summary reports for management.Such summary reports will be used for periodiccorrosion control system status reports to keepmanagement advised of how effectively thesystem is working. Such reports do not containall of the detail included in field test records.They do, however, need to include, (1) aconcise analysis of any corrosion leaks thathave occurred in the reporting period togetherwith comparative data from past reportingperiods, (2) the level of cathodic protection (ifused) that has been maintained during thereporting period (and again with comparativedata from past reporting periods), (3) summaryof any problem areas that may be developing,and (4) recommendations for any requiredaction. Effective management reports willgreatly expedite requests for funds necessaryfor corrosion control installations and othercorrosion control system funding requirements.

Obviously, a records system that incorporatesthe basic requirements of completeness,accuracy and accessibility (and which is keptup-to-date) is essential to the preparation ofeffective summary reports for management.

RECORD KEEPING8-2

TYPES OF RECORDS

There are basically three types of records whichwill be used by or which will be generated bycorrosion personnel (or other structureoperating and maintenance personnel). Theseare:

1. Records of basic structure physicalinformation.

2. Corrosion records.

3. Corrosion control records.

Physical Information

The basic structure physical records are the"working tools" essential to all else that is doneon the structure. Such records are notnecessarily developed by the corrosion controldepartment of an organization, but must bemade available to them.

These records include information such as:

1. When the structure was installed.

2. What the structure consists of.

3. Maps or drawings showing the location ofthe structure - with these maps or drawingsupdated on a systematic basis to showchanges, modifications, replacements, etc.The dates of all such changes should beknown.

4. Details of any protective coatings used:

a. Mill applied coatings.

b. Over-the-ditch hot applied coatings.

c. Cold applied tapes and waxes.

5. In the case of pipelines:

a. Pipeline size.

b. Wall thickness, grade of steel (or other

metallic material), weight per foot.

c. Location of cased crossings with dataon diameter and length of casing pipe,whether or not coated, and details ofprovisions for isolating the casing pipefrom the carrier pipe.

d. Type of construction (all welded,mechanical couplers, or combinationsof the two).

e. Location of branch taps, tees, andvalves.

f. Location of isolated joints used toseparate electrically the subject pipelinefrom other facilities.

g. Location of underground metallicstructures of other ownership (includingname of owner) that cross the subjectpipeline.

h. Pipeline operating temperature - mayvary from location to location alongpipeline.

i. Location of closely paralleling highvoltage electric transmission lines.

j. Location of possible sources ofman-made stray current that couldaffect the pipeline.

k. Pipeline detailed alignment sheetswhich may show much of the abovematerial and which will have a stationnumbering system so that the locationof any point on the pipeline may beaccurately located.

In connection with 5-k above, the corrosioncontrol department may work with thedepartment responsible for maintaining thealignment sheets to arrange for the inclusion ofkey corrosion control features such as thelocation and type of test points, location ofcathodic protection installations, location of anybonds to foreign pipelines, location of stray

RECORD KEEPING8-3

current control installations associated withman-made sources of variable stray current,location of electrical isolation and location ofcorrective measures to reduce induced ACvoltages or currents. Any additions or changesto the corrosion control data on the alignmentsheets should be reflected promptly in the latestissue of the alignment sheets.

Corrosion Records

These are the records that pertain to thedetection and repair of corrosion leaks orfailures. These comments will be based onpipeline practice. There are three categories ofthese corrosion records.

These are:

1. Inspection reports. 2. Leak reports. 3. Leak repair reports.

Pipeline inspection reports are made wheneverthe pipeline is exposed by excavation for anypurpose - whether or not corrosion was thereason for making the excavation. In any event,the report should include any evidence ofcorrosion on the external surface of the pipelineand on internal surfaces if the pipeline isopened as part of the job order necessitatingthe excavation. If the pipeline is coated, thecondition of the coating is observed andrecorded. Records of such inspections aremandatory for regulated pipelines.

Leak reports record the location of the reportedleak, when the leaks occurred, pertinent dataon the specific pipe that is leaking together withthe material that is leaking, and details ofneighboring facilities which could be affected bythe leaks - particularly if there is a hazardproblem. Answer the questions: WHERE,WHEN AND WHAT.

Leak repair reports record the details of whatwas done to correct previously reported leaks.Such reports include the nature of the leaks(which are not necessarily corrosion related)and how the leaks were repaired.

In a following section, Field Data Sheets,suggestions are given for the utilization ofpreprinted data sheets for recording many ofthe frequently made tests and inspectionstypically necessary on large corrosion controlsystems.

Corrosion Control Records

These are the records that apply to thecorrosion control program as opposed to therecords on corrosion itself as outlined above.Records which could apply to a pipeline system(and, as applicable, to other structures) mayinclude:

1. Soil resistivities along a pipeline route if asoil resistivity survey was made prior topipeline construction.

2. Measurements of effective pipelinecoating resistance.

3. Current requirement tests made todetermine the direct current needed forcathodic protection of the pipeline or otherstructure.

4. Site selection surveys for cathodicprotection installations.

5. Design calculations and drawings forcathodic protection installations togetherwith revisions (if necessary) to showaccurately the "as built" details of theinstallation.

6. Energization and adjustments of cathodicprotection installations to attain optimumcathodic protection levels along pipelines.

7. Interference tests at crossings withpipelines of other ownership and recordsof corrective bonds (or other measures)found necessary and installed.

8. Surveys made to evaluate the effects ofvariable stray direct current fromman-made or natural sources.

RECORD KEEPING8-4

9. Design calculations and drawings forcorrective measures (if found necessary)to offset the effects of man-made ornatural sources of variable stray directcurrent. Drawings to be revised, asnecessary, to show "as built" conditions.

10. Tests to determine induced AC voltagescaused by parallelism with high voltageelectric transmission lines.

11. Design calculations and drawings forcorrective measures (if found necessary)to mitigate induced AC voltages causedby parallelism with high voltage electrictransmission lines.

12. Periodic pipe-to-soil potential surveysalong cathodically protected pipelines toevaluate the level of protection beingmaintained.

13. Periodic tests of cathodic protectionrectifier installations or other sources ofimpressed current cathodic protection.

14. Periodic tests of bonds or other devices tocorrect interference at crossings withpipelines of other ownership.

15. Periodic tests of corrective measuresinstalled for mitigation of man-made ornatural sources of variable stray directcurrent.

16. Periodic tests of coupons or other meansused to evaluate the effect of internalcorrosion on pipelines carrying apotentially corrosive material.

Also see the following section on PHMSArequirements with additional records which maybe applicable to Federally regulated pipelinefacilities.

PHMSA REQUIREMENTS - FEDERAL ANDSTATE

For certain federally regulated gas and liquidpipelines, regulations pertaining to corrosion

control are initiated by and administered by ThePipeline and Hazardous Materials SafetyAdministration (PHMSA). States, in turn,through their own regulatory bodies, may haveregulations which supplement the applicableFederal requirements.

The Federal regulations include requirementsfor record keeping as well as other aspects ofpipeline corrosion control.

The key word to the Federal regulations is"minimum". The Federal requirements are theminimum needed for compliance by alloperators in all states.

Any state may, however, have regulations withrequirements that extend beyond the Federalminimums. For corrosion workers on regulatedpipelines, then, it is essential that they know theFederal requirements plus requirements by thestate or states within which they operate.

The point should also be made that there isnothing that says that the regulations confinecorrosion and corrosion control records to justthose set forth in regulations. Any additionalrecords which will improve the efficiency of thecorrosion worker's operations (and make theworker’s job easier) are certainly in order.

The degree of corrosion control compliancewith Federal and State requirements onregulated pipelines is subject to periodic reviewby authorized inspectors. A first class set ofcomplete, accurate and accessible records willgreatly expedite such inspections and will domuch to establish the credibility of the workbeing done.

As an illustrative example, the followingmaterial lists record-keeping requirementscurrently included in Federal regulationspertaining to gas transmission pipelines and,where applicable, to hazardous liquidspipelines. Requirements by individual statesmay or may not be more stringent.Requirements for other gas transportationfacilities (by pipeline), or for other liquidpipelines, may differ.

RECORD KEEPING8-5

1. Reports of Leaks

The reporting of gas pipeline leaks that are notintended by the operator and that requireimmediate or scheduled repair is covered byPart 191 - Transportation of Natural and OtherGas by Pipeline; Reports of Leaks, and SafetyRelated Condition Report 191.23 (a) (1), Title49 of the Code of Federal Regulations.

For hazardous liquids pipelines, Subpart B,Accident Reporting, of Part 195 - Transportationof Hazardous Liquids by Pipeline, Title 49 of theCode of Federal Regulations, covers pipelineaccidents and leaks. Those incidents involvingcorrosion are covered by Paragraph 195.55 (a)(1) and filed in accordance with paragraph195.56.

Also, Part 192 - Transportation of Natural andOther Gas by Pipeline; Minimum Federal SafetyStandards specifies, in Paragraph 192.7097that records covering each leak discovered andeach transmission line break shall be kept foras long as the segment of transmission lineinvolved remains in service. This coverscorrosion leaks or breaks caused by corrosionas well as leaks or breaks resulting from othercauses.

Part 195 - Transportation of Hazardous Liquidsby Pipeline - Specifies in Paragraph 195.404©,Maps and Records of Subpart F, Operation andMaintenance, that each operator shall maintainfor the useful life of that part of the pipelinesystem to which they relate, under © (1) (2), arecord of each inspection and each testrequired by this subpart.

2. Repairs of Leaks

Records of repairs of leaks or line breaks are tobe kept for as long as the pipeline segmentremains in service. This is as specified in Part192, Paragraph 192.709.

Records of repairs of pipeline parts, such asvalves, regulators, etc., only require a recordretention of five years.

3. Pipeline Inspections

In Part 192, Subpart I - Requirements forCorrosion Control, Paragraph 192.459 specifiesthat when any portion of a buried pipeline isexposed the exposed portion must beexamined for evidence of corrosion if bare or ifany coating has deteriorated. Similarly, Para-graph 192.475 provides for internal surfaceinspection for corrosion whenever any pipe isremoved from a pipeline for any reason.

Paragraph 192.491 (of Subpart I - CorrosionControl Records) specifies that external pipeinspection records be retained for at least fiveyears. Internal inspection records are to be keptfor the life of the pipeline.

Paragraph 195.416(e) of Part 195 -Transportation of Hazardous Liquids byPipeline, Subpart F, specifies that wheneverany buried pipeline is exposed for any reason,the operator shall examine the pipe forevidence of external corrosion and that if activecorrosion is found, it shall investigate further todetermine the extent of the corrosion.

Paragraph 195.418©, Subpart F, provides forperiodic examination of coupons or other typesof monitoring equipment to determine the effectof inhibitors or the extent of internal corrosion inliquid pipelines. Paragraph 195. 418(d)specifies that whenever any pipe is removedfrom the pipeline for any reason, the operatormust inspect the internal surfaces for evidenceof corrosion and, if general corrosion is found,that the extent of the corrosion shall bedetermined.

4. Locations of Cathodically Protected Pipingand Cathodic Protection Facilities

Paragraph 192.491 of Subpart I provides thatmaps or records shall be maintained to showthe location of:

a. Cathodically protected piping.

b. Cathodic protection facilities (other thanunrecorded galvanic anodes installed

RECORD KEEPING8-6

before August 1, 1971).

c. Neighboring structures bonded to thecathodic protection system.

This paragraph further stipulates that therequired maps or records must be retained foras long as the pipeline remains in service.

In Subpart D, Construction, of Part 195 -Transportation of Hazardous Liquids byPipeline - Paragraph 195.266(f) on recordsmaintained for the life of the pipeline facilityprovides that the location of each test stationshall be part of the permanent records.

5. Corrosion Tests, Surveys and Inspections

Paragraph 192.491 of Subpart I further providesthat records of all corrosion tests, surveys, andinspections required by Subpart I must beretained for the in-service life of the pipeline, orfive years, dependent on schedule. Thisparagraph further specifies that such recordsbe in sufficient detail to demonstrate theadequacy of corrosion control measures or thata corrosive condition does not exist. Some ofthe applicable tests, surveys and inspectionsare as listed below together with paragraphreferences.

Five Year Requirement

a. Soil resistivity measurements. Paragraph192.455(b).

b. Tests for bacterial activity. Paragraph192.455(b).

c. Pipe-to-soil potential measurements.Paragraph 192.455(b).

d. Soil acidity (pH) measurements. Paragraph192.455(e).

e. Cathodic protection current requirementtests. Paragraph 192.457(a).

f. Leak detection surveys. Paragraph192.457(b)(3).

g. Coating inspections. Paragraph 192.461©.

h. Periodic tests of impressed current cathodicprotection power sources. P a r a g r a p h192.465(b). For liquid pipelines, Paragraph195. 416©, Part 195.

i. Periodic tests of stray current controldevices. Paragraph 192.465©.

j. Periodic inspection of internal corrosionmonitoring devices. Paragraph 192.477.

k. Periodic reevaluation of effectiveness ofprotective coatings on pipelines exposed toatmospheric corrosion. Paragraph 192.481.

Lifetime Requirement

a. Periodic cathodic protection adequacytests. For natural gas pipelines Paragraph192.465(a). For liquid pipelines, Paragraph195. 416(a), Part 195.

b. Periodic reevaluation of noncathodicallyprotected pipelines. Leakage surveys whenused to monitor active corrosion onunprotected pipelines. Paragraph192.465(e).

The preceding review of minimum recordkeeping requirements for regulated gas andliquid transmission lines serves to demonstratethe variety of records that can be involved in asystem of records. With respect to the variousrecords listed above for the gas and liquidpipeline example, keep the following items inmind:

1. State regulations could have supplementaryrequirements.

2. The examples given were for one type ofpipeline only. Regulations for other types ofregulated pipeline systems are notnecessarily the same in all respects.

3. Regulations are subject to revision. Thepipeline corrosion worker should keep an

RECORD KEEPING8-7

updated file of the latest regulationsapplicable to the type of system with whichhe or she is associated.

It will be noted that the records requirements forhazardous liquids pipelines as set forth in Part195 are not as detailed as for gas pipelines asset forth in Part 192. However, a well organizedcorrosion control program for liquid pipelinesmay, nevertheless, incorporate the recordssystem as set forth for gas pipelines.

COMPUTERIZED RECORDS

Where feasible to do so, there are advantagesto the use of computerized records. Some ofthese are:

1. As stated earlier, ready and rapid access-ibility of data.

2. Plotting capabilities to permit showing datain graphic form. Pipe-to-soil potentialprofiles are an example. Latest profile maybe compared (on the same printout) withpreceding profiles to detect changes.

3. Cathodic protection installation drawings inelectronic storage form that may be recalledas needed. Updating possible to reflect"as-built" condit ions or periodicmodifications.

4. Rapid data sorting or data searching whenlooking for specific trends or changes withrespect to any part of the corrosion controlsystem. Or when separating applicable datato support a special report (or for otherreasons).

5. The capability of recalling data on aterminal screen for inspection or searchpurposes and of getting hard copy of itemsof specific need.

6. Freedom from loss as in manual filingsystems where data, maps, or other recordscan be pulled out "temporarily" for use onan assignment but are then lost, misplaced,or destroyed leaving a possibly

irreplaceable gap in the records system.

Whether all this (and more) can be utilizeddepends on the capabilities of the computersystem available for use and on the availabilityof (or development of) software for instructingthe machine for both storage and recallfunctions. Sophisticated computer systemshave these capabilities. With the ongoing rapiddevelopments in the computer field, more andmore can be accomplished with smaller andsmaller machines. Telephone links permittingdata transmittal from the field to computermemory are an interesting and usefuldevelopment. This permits earliest possibledata analysis at the home office and at thesame time provides a safeguard against loss offield data.

Economic justification is always a factor indetermining whether computer records shouldbe used rather than manual. Small systems orstructures may well be very nicely taken care ofwith a manual system. Very large systems,however, having a multitude of different types ofrecords plus having large amounts of dataunder each type should considercomputerization if at all possible.

Where computerization is under consideration,the corrosion worker (unless also a top notchcomputer specialist) should not attempt todesign his own system. This is a job where theaid of experts is needed. By working with suchexperts and making sure that they know yourobjectives for use of the system, you have thebest chance of attaining an efficient andeffective application.

FIELD DATA SHEETS

There are significant advantages in havingpreprinted field data sheet forms for use inrecording the results of different types of testsor inspections. These are useful for eithermanual or computerized records systems.

The principal advantages are uniformity andprotection against the human error of neglectingto include data or information applicable to the

RECORD KEEPING8-8

particular test or inspection being made.

In addition to provisions for entering technicaldata the forms for any specific test or inspectionshould include spaces for "standard"information such as location, date, names orpersonnel making the test or inspection, andspecific identification of any test equipmentused. Each form should be preprintedprominently with its intended use (such as"Periodic Rectifier Inspection" or "Pipe-to-SoilPotential Tests").

In addition to the "standard" information, formsfor a large system may include provisions forentering such things as identification of thedistrict or division within which the test orinspection is being performed and codedesignations for filing guidance. This latter itemcan be particularly helpful in computerizedsystems to permit automatic electronic data"tagging" for subsequent retrieval whenneeded.

Consistent with the capabilities of the computerequipment available, the data sheet formsshould be designed to require minimum manualtime requirements in transferring data to thecomputer memory. If capabilities exist, or willexist in the future, so that the complete datasheet can be "read" electronically, so much thebetter. The whole idea of prepared data sheetsfor specific purposes is to save work, not tomake work.

CONCLUSION

As the reader will have noted, no examples offorms for records systems have been includedin this chapter. This is because no two systemsare exactly alike - no one form for any phase ofa records system could be expected to meet allthe requirements of all systems. For thisreason, the emphasis has been on setting forththe principles involved so that records systemscan be planned to meet the needs of thecorrosion occurrence records and of thecorrosion control system applicable to theuser's own underground system.

The corrosion records system planner may befaced with designing a records system for anew underground system; or he or she may berevising or expanding an existing system; or theconsideration may be to change the type ofsystem (such as manual to computerized) foran existing underground structure. Whateverthe objective, the first order of business will beto take the time to thoroughly study everyaspect of the underground structure systemitself, probability for system expansion,management information requirements, Federaland State regulatory requirements (whereapplicable), relative real costs of variousrecords systems under consideration, systemmaintenance requirements, and the availabilityand capabilities of computer facilities.

Once the background study has beencompleted, the records system designer is thenin position to make decisions as to the recordsthat will be needed and the type of system -manual or computerized or, possibly, acombination of the two. In any event, thedesigner should keep the "flexibility" element inmind so that modifications are possible withminimum effort in case of changes in theunderground system itself or of changes inrecord keeping requirements.

RECORD KEEPING8-9

Documents

Appalachian Underground Corrosion Short Course … UNDERGROUND CORROSION SHORT COURSE BASIC COURSE ... Alternating current electricity is that which flows first in …