AUCSC Advanced Text 041211[1]

7/28/2019 AUCSC Advanced Text 041211[1]

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Advanced Course


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APPALACHIAN UNDERGROUND CORROSION SHORT COURSE

ADVANCED COURSE

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

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

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

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

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

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

TWO ELECTRODE METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

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

ANALYZING PIPE-TO-SOIL POTENTIAL DATA . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

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

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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

Remote Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

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

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


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

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

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

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

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

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

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

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

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

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

Electrolyte Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Total Circuit Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

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


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

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

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

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

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

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

April 12, 2011 Revision

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


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CHAPTER 1

PIPE-TO-SOIL POTENTIAL SURVEYS AND ANALYSIS

INTRODUCTION

The objective of this chapter is to present themost important corrosion control measurement,the pipe-to-soil potential measurement, and thevarious methods that can be used to make thismeasurement.

This chapter will also show how pipe-to-soilpotential data can be used to identify and

evaluate corrosion problems as well as assist indetermining the effectiveness of a cathodicprotection system.

Also included in this chapter is a discussion ofthe criteria for cathodic protection and theapplications for each.

All pipe-to-soil potential values given in thischapter will be with respect to a saturatedcopper/copper sulfate reference electrode

CORROSION MECHANISMS

The corrosion of an underground or submergedmetallic structure is electrochemical in nature.

There are basically two different mechanismswhich are responsible for this corrosion andthese are termed electrolytic corrosion andgalvanic corrosion.

Electrolytic corrosion, often called stray current

corrosion, results from currents which areintroduced into the ground from neighboringsources of direct current (DC) such as electricrailways, DC powered machinery, and foreigncathodic protection systems.

Galvanic corrosion is the result of the naturalelectrochemical process that takes place on a

buried metallic structure due to potentialdifferences which exist between points on thesame structure due to different surfaces,


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HIGH IMPEDANCEVOLTMETER

CURBBOX

TESTLEAD

COPPER/COPPER SULFATEREFERENCE ELECTRODE (CSE)

SOIL

PIPE

-0.85


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amount of time that the structure is effected, DCvoltage recording instruments can be

employed.

Non-fluctuating stray current or static straycurrent is not as readily apparent as thedynamic type. In this case, the interferenceeffect from a foreign cathodic protection systemwill usually remain constant and unless there isexisting historical potential data, or one has theopportunity to participate in cooperative

interference testing, this type of stray currentinterference may not be immediately detected.However, there are potential measurementtechniques that can detect this type of straycurrent interference such as lateral potentialsurveys, which will be discussed later in thischapter.

PIPE-TO-SOIL POTENTIAL MEASUREMENT

Before the various test methods and evaluationprocesses are discussed, a review of the pipe-to-soil potential measurement is required. Thismeasurement must be obtained using a highimpedance voltmeter, a calibrated referenceelectrode, an electrolyte present over the pipewhere the reference electrode can be placed,

and a method by which to contact the structureunder test. A high impedance voltmeter isrequired in order to obtain the most accurate

critical when recording these potential valueswhether it be as a negative value or a positive

value. The methods and instrumentationemployed for this testing are critical in order toprovide meaningful data that can be properlyevaluated.

TYPES OF POTENTIAL SURVEYS

A normal survey of a pipeline system in order toobtain either static or native potential values or

to ascertain the effectiveness of an existingcathodic protection system might consist ofmeasuring pipe-to-soil potentials at all availabletest locations such as those describedpreviously. However, there are occasions whenit is necessary to obtain additional potentialmeasurements between test points. This isaccomplished by placing the reference

electrode at regular intervals over the pipelineand measuring potentials at each referencelocation. The spacing of the intervals willdepend on the type of survey being performedand the type of detail required. This type ofsurvey will indicate the anodic locations alongan unprotected structure which will be thoseareas with the most negative potentials.However, these more negative potentials may

also correspond to a stray current pickuparea. In the case of a pipeline under cathodicprotection, this type of survey will reveal those


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It is also important to insure that the pipelineyou are testing is electrically continuous for the

length of pipe to be surveyed.

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

SINGLE ELECTRODE METHOD

The first method to be discussed is the singleelectrode survey, see Figure 1-2. This surveyutilizes one CSE, a high impedance voltmeter,and a reel of test wire. An additionalrequirement is a point of electrical contact tothe structure being tested.

There are two ways to perform this particularsurvey. In the first procedure, the pipeline iscontacted through a test lead or other suitableconnection point which is connected to a testreel. The test reel is connected to the positiveterminal of a voltmeter and the testingpersonnel measure potentials along thepipeline at prescribed intervals, carrying andmoving both the voltmeter and the reference

electrode together. Once again, the referenceelectrode is connected to the negative terminalofthevoltmeter.Thispolarityconventioncanbe

using the one electrode method may beaffected by voltage drop in the pipeline and

measuring circuit or by stray currentinterference.

The data can be recorded manually on fielddata sheets or electronically using a recordingvoltmeter or datalogger.

Figure 1-3 shows the basic components andhookups for recording the data electronically

using a computerized system. In this case, thevoltage measuring and datalogging equipmentcan be carried in a compact backpackarrangement by the operator. The operatorcarries one or two reference electrodes, whichmay be affixed to the bottom of extensionrod(s), which are then walked forward at somespecific interval, usually 2 to 3 feet. Potential

measurements are electronically recorded andstored by the instrument. The long test leadback to the connection to the pipeline at thesurvey starting point can be a one-time-usedisposable light gauge wire. During the courseof the potential survey the operator is able toelectronically note distances, terrain featuresand landmarks as well as other pertinentinformation.

The electronically collected data can then beprocessed by a personal computer in the field


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HIGH IMPEDANCE VOLTMETER

(-)(+)

SINGLE REFERENCE ELECTRODEMOVED ALONG LINE

PERMANENTTEST POINT

PIPELINE

ETC. GRADE

-0.85


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WIRE REEL

(+) ()EXPENDABLETEST WIRE

PERMANENTTEST POINT

PIPELINE

GRADE

BACKPACKCONTAINING

DATA ACQUISITION ANDLOGGING EQUIPMENT

WALKING REFERENCEELECTRODES


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VOLTMETER POSITIONS

(+) ()

PERMANENTTEST STATION

PIPELINE

ETC.

GRADE

1 2 3 4 5

A B A B A

NEXT PERMANENTTEST STATION

LEAP FROGGING REFERENCEELECTRODES A & B


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along and directly over the pipeline at somepreselected survey interval from reference

electrode A. The potential difference betweenthe two electrodes is then measured andnumerically added or subtracted from theprevious reading in accordance with themeasured polarity of the forward electrode (Bin this case). This leap frogging of the twoelectrodes is continued until the next permanenttest station is reached or any other locationwhere the pipeline can be electrically

contacted.

At this juncture, the cumulative potential up tothis point is compared to the actual potentialmeasured to a CSE at this second permanenttest station and adjusted as necessary. Thesurvey then continues following the abovedescribed procedure to the next contact point.

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

The two electrode method works well. However,it requires that a great deal of care be taken inrecording the data. An error made in thecalculating of potentials or the noting of polarity

at any given point will cause all subsequentcalculated potentials to be erroneous. In thecase of an error, the calculated pipe-to-

areas. Side-drain measurements are conductedutilizing two CSEs and a high impedance

voltmeter, as shown in Figure 1-5. The firstelectrode is placed directly above the pipe, incontact with the soil. The second electrode isplaced in contact with the soil at a 90 angle tothe pipe at a distance approximately equal to2 times the pipe depth. During testing, theelectrode placed directly above the pipe shouldbe connected to the positive terminal of thevoltmeter and the other electrode to the

negative terminal. Positive side-drain readingsindicate that current is being discharged fromthe pipe at this point, making it an anodic area.Negative side-drain readings normally indicatethat current is flowing towards or onto the pipeat this point, making it a cathodic area. Theside-drain measurements should be takentypically at no more than 5-foot intervals. Testsmust be made on both sides of the pipe, untilthe extent of the problem section has beendetermined.

This technique should be used with caution.Under certain conditions, a relatively stronglocalized anodic cell could exist on the bottomof the pipe with the top of the pipe serving as acathode and negative side-drain readings could

be measured while severe corrosion is actuallyoccurring on the bottom of the pipe at thislocation.


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(+)()

HIGH IMPEDANCEVOLTMETER

GRADE

PIPE BEING

TESTED

CUSO REFERENCEELECTRODE (TYP.)

4

C

-0.85


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Normally, after the survey data is plotted, the-850 mV value is red-lined on the plot. All areas

below this line are considered to beinadequately protected and in need of remedialmeasures. Figure 1-6 shows a typical pipe-to-soil potential profile of a cathodically protectedpipeline.

On pipelines which are not cathodicallyprotected, the pipe-to-soil potential profile andsurface potential surveys can be used to locate

corroding areas or hot spots along thepipeline. Experience has shown that when adifference in pipe-to-soil potential values existalong a pipeline, corrosion occurs at, and for agiven distance on either side of, the mostnegative or anodic points along the pipeline.

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

recorded.

Pipe-to-soilpotentialmeasurementstakenover

localized corrosion cells where the anode andthe cathode are located very close to each

other.

PIPE-TO-SOIL POTENTIAL SURVEYS AND

ANALYSIS

Figure 1-8 shows a typical potential profile on apipe on which the corrosion activity is one ofstraight forward galvanic action and is not beinginfluenced by interference currents or bimetallic

corrosion. Figure 1-9 shows a typical potentialprofile on a pipe which is exposed to damageas a result of a rectifier unit on a crossingpipeline. In comparing these two potentialprofiles, it should be noted that irrespective ofthe condition which exists, anodic areas alwaysexist at the locations where the over the pipepotentials are more negative than the off thepipe potentials. However, in a straightforwardgalvanic situation such as shown in Figure 1-8,the anodic areas occur at locations where theover the pipe potentials are of higher negativevalues than those measured in the cathodicareas. In an interference situation such asshown in Figure 1-9, the anodic areas are atthose locations where the potentials (bothover and off the pipe) are of lower negative

values than those at the cathodic areas.

It should be noted that although a potential


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-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

PROTECTED AREAS

(MORE NEGATIVE THAN -0.85VOLT)

-0.85 VOLT

UNPROTECTED AREAS

(LESS NEGATIVE THAN -0.85 VOLT)

NOTE: APPLICABLE CRITERION FOR THISLINE IS -0.85 VOLT TO Cu-CuSO

4

PIPE-TO-SOILPOTEN

TIAL(VOLTSTOC

U-C

USO

)4

0

-1.4


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CU-CUSO

REFERENCEELECTRODE

(TYP.)

DIRECTION OF SURVEY

GRADE

CATHODIC - ANODIC -CATHODIC

4

+18MV -10MV -2MV +3MV -2MV -4MV

PIPELINE

HIGHIMPEDANCEVOLTMETER(TYP.)

+8MV +12M

CATHODIC - ANODIC - CATHODIC


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CURRENTDISCHARGE-110

-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

CURRENTPICK-UP

CURRENTDISCHARGE

CURRENTPICK-UP

CURRENTDISCHARGE

ANODIC

AREA

CATHODIC

AREA

ANODIC

AREA

CATHODIC

AREA

ANODIC

AREA

25 FEET FROM PIPE

OVER PIPE

PIPETOSOILPOTENT

IAL(MILLIVOLTS)


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-110

-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

CURRENT

CURRENT

DISCHARGE

CURRENT PICK-UP

CATHODIC

AREAANODIC

AREA

CATHODIC

AREA

25 FEET FROM PIPE

OVER PIPE

-120

PIPETOSOILPOTENTIAL(MILLIVOLTS)


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-110

-100

-900

-800

-700

-600

-500

-400

-300

-200

-100

NORMAL GALVANIC

DISCHARGE TO

COPPER NORMAL

25 FEET FROM PIPE

OVER PIPE

-120

PIPETOSOILPOTENTIAL


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that type of situation, a complete investigationof the conditions which exist in a given area willusually yield the information necessary toestablish whether the potentials are attributableto interference or bimetallic activity.

Even without knowledge of the surroundingconditions, there are usually sufficientdifferences between two profiles to distinguishthem. Thus, in an interference situation, theportions of the pipe immediately adjacent to the

anodic area are almost always entirelycathodic. In a bimetallic situation, the profile ofthe pipe outside the area of bimetallic influenceis one which is similar to the normal galvanicprofile shown in Figure 1-8.

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

1. Anodic areas exist at locations where thelateral (off the pipe) measurements are lessnegative than the over the pipemeasurements. Conversely, cathodic areasexist at locations where the lateralmeasurements are more negative than the

over the pipe measurement. This conditionalways holds, irrespective of whether thecorrosion activity is a result of an electrolytic

INTERPRETATION OF POTENTIALS UNDER

NON-STRAY CURRENT CONDITIONS

After it has been determined that stray currentsare not present, potential measurements can beused in analyzing the galvanic corrosion patternwhich may exist. Surface condition of the pipe,chemical composition of the soil, and otherlocal conditions can greatly influence staticpotentials along the length of a pipeline.

Therefore, the more readings taken, the better

the evaluation. A more comprehensiveinterpretation of potential measurements can bederived from the following statements.

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

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

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

4. The normal potentials taken along a bare

pipe fall in the range of -500 to -600 mV. Anewly installed bare pipe will have highernegative potentials, and very old bare pipe


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

These statements are intended as ageneralized guide for interpretation of potential

measurements. They are not to be consideredas scientific principles and they do notnecessarily hold true under all circumstances.

They are merely a summation of fieldexperience. They are not derived fromcontrolled experiments or rigid reasoning.Despite the apparently severe qualificationsthat have been applied to these statements, ifthey are used with care and with completeappreciation of the theory of galvanic corrosion,most corrosion problems can be evaluatedsuccessfully.

COATED CROSS-COUNTRY PIPELINE

WITHOUT CATHODIC PROTECTION

The potential measurements shown in Table

1-2 were recorded during a pipe-to-soil potentialsurvey along a portion of a well-coated,cross-country pipeline. Examining the data in

piping at those locations.

Statements 1 and 2 indicate that older, barelines are of lower negative potentials thannewer, coated lines. The coupling of the coatedline to the bare line makes the potentials on thecoated line less negative at the shorted meterand regulator stations than along the balance ofthe pipeline. This situation shows theimportance of placing isolating flanges betweennew coated pipe and older bare pipe. This

isolation is needed whether or not cathodicprotection is provided for the coated line. Ifcathodic protection is not provided, the newcoated pipe will be anodic with respect to theolder, bare pipe. As a result, if there were noisolating flange installed between the twosections, or if the flange were shorted, thenewer coated section of pipe would besubjected to accelerated galvanic corrosionattack. When using isolating flanges or devices,safety precautions must be followed to preventa spark or arc across the isolator during a faultor power surge or static discharge. Devicesexist that will maintain DC isolation while actingas a shunt under the above conditions.

GAS SERVICE LINE

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


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closer inspection however, it was discoveredthat the water service lines in the developmentwere electrically continuous with the gasservice lines. Copper, being cathodic withrespect to steel, will when connected to a steelpipe, cause the potential of the steel pipe tobecome less negative than its natural potential.As a result of area relationships (the ratio ofareas of copper to steel is greater when thesteel is coated than when it is bare), the effectof copper on the potential of coated steel pipe

will be far greater than that on the potential ofbare pipes.

GROUNDING SYSTEM

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

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

nearly identical.

Potential profiles were also conducted along

corrosion activity exists. In fact, the condition atthis plant was so severe that leaks developedwithin the first year of operation despite the factthat the pipeline was extremely well-coated andhad been backfilled with sand. The very goodcoating no doubt contributed to the rapiddevelopment of leaks on the pipeline due to theconcentration of current discharge at holidaysin the coating. The corrosion current wasgenerated by the bimetallic coupling.

STEEL GAS AND WATER LINES

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

straightforward galvanic action resulting fromthe corrosiveness of the soil, these potentialswould be considered as very cathodic and,


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the soil to the radiant heat lines in the concrete(there were common structural connections inthe boiler room) made the potential on the gasand water lines less negative than normal.

These gas and water lines thus behaved as theanode in a mechanism which is similar to abimetallic couple. The conditions which madethis couple particularly severe were the lowresistivity soil, the proximity of the gas andwater lines to the radiant heat lines, and thefact that the area of the gas and water lines was

small compared to the area of the radiant heatlines. This area relationship was made stillworse by the fact that the gas and water lineswere coated.

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

CRITERIA FOR CATHODIC PROTECTION

Cathodic protection criteria are listed in NACEStandard Practice SP0169-2007 Control of

to-electrolyte boundary must be consideredfor valid interpretation of this voltagemeasurement.

2. A negative polarized potential of at least 850mV relative to a saturated copper/coppersulfate reference electrode.

3. A minimum of 100 mV of cathodicpolarization between the structure surfaceand a stable reference electrode contacting

the electrolyte. The formation or decay ofpolarization can be measured to satisfy thiscriterion.

SP0169-2007 also states It is not intended thatpersons responsible for external corrosioncontrol be limited to the criteria listed below.Criteria that have been successfully applied onexisting piping systems can continue to be usedon those piping systems. Any other criteriaused must achieve corrosion controlcomparable to that attained with the criteriaherein.

Some examples of other criteria that have beenused in the past are:

C

Net Current FlowC E-log I CurveC 300 mV Shift


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WHICH CRITERION?

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

POTENTIAL MEASUREMENT OF -850 mV TO

A Cu/CuSO4 REFERENCE ELECTRODE

WITH CATHODIC PROTECTION APPLIED

Of the three criteria listed above, the -850 mVcriterion with cathodic protection applied hashistorically been the one most widely used fordetermining if an acceptable degree of cathodicprotection has been achieved on a buried orsubmerged metallic structure. In the case of asteel structure, an acceptable degree ofprotection is said to have been achieved whenat least a -850 mV potential difference existsbetween the structure and a CSE contacting thesoil directly above and as close to the pipe aspossible.

Some sources indicate that this criterion wasdeveloped from the fact that the most negativenative potential found for coated steel was -800

mV. Therefore, the assumption was made thatif sufficient current is applied to raise thepotential of theentirestructuretoavaluemore

C Measuring or calculating the voltagedrop(s)

C Reviewing the historical performance of thecathodic protection system

C Evaluating the physical and electricalcharacteristics of the pipe and itsenvironment

C Determining whether or not there is physical

evidence of corrosion

Voltage drops can be reduced by placing thereference electrode as close to the pipe surfaceas possible. It should be noted that this meanselectrically close, not just physically close. Areference electrode placed physically close toa well coated pipe is not electrically close to itand voltage drops are not reduced.

The voltage drop can also be eliminated byinterrupting all sources of cathodic protectioncurrent and measuring the off potential. Theinstantaneous off potential should be free ofvoltage drop error. Comparison of the on andoff potentials and noting the differencebetween them will indicate the approximate

voltage drop included in the potentialmeasurement when the measurement is madewithprotectivecurrentapplied. This instantoff


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instruments are left in place overnight. Thesepotentials can provide a base-line from which toevaluate other measurements.

The -850 mV criterion with cathodic protectionapplied is the one which is almost always usedin areas with significant dynamic stray currentactivity. It is generally accepted that if thepotential of the structure to a CSE remainsmore negative than -850 mV at all times, evenif there are substantial fluctuations in potential

with time, then the pipe can be consideredprotected at that particular test point. It ispossible that the amount of test points surveyedand the frequency at which those surveys areperformed may have to increase in heavy straycurrent areas due to ever-changing conditions.

Limitations of the -850 mV Potential to a

Cu/CuSO4

Reference Electrode With

Cathodic Protection Applied Criterion

A limitation of this criterion for insuring totalprotection is that potentials can vary widelyfrom one area of the underground structure toanother as a result of coating damage,interference effects, etc. This suggests thepossibility of potentials being less negative than

-850 mV in sections of the pipe between twoconsecutive test locations. Also, the voltagedrop component in the potential measurement

that the polarized potential of the pipeline doesnot reach values at which coating damagecould occur due to hydrogen evolution at thesurface of the pipeline.

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

protective current than if one of the othercriterion were used.

Another limitation of this criterion is due to therequirement that potential readings be takenwith the reference electrode contacting theelectrolyte directly over or adjacent to thepipeline to minimize voltage drop. In cases ofriver crossings, road crossings, etc., where theelectrode cannot be properly placed, analternative criterion may have to be used.

POLARIZED POTENTIAL OF -850 mV

MEASURED TO A Cu/CuSO4 REFERENCE

ELECTRODE

As discussed in the previous criteria section,

one method of considering the voltage (IR) dropacross the electrolyte is to measure thestructure potential with all current sources off.


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Limi tations of the Polarized Potential of -850

mV Measured to a Cu/CuSO4 Reference

Electrode Criterion

Polarized potentials well in excess of -850 mVshould be avoided for coated structures in orderto minimize the possibility of cathodicdisbondment of the coating. SP0169-2007 alsopoints out that, Polarized potentials that resultin excessive generation of hydrogen should beavoided on all metals, particularly higher

strength steel, certain grades of stainless steel,titanium, aluminum alloys, and prestressedconcrete pipe.

100 mV POLARIZATION CRITERION

The 100 mV polarization criterion, like the -850mV polarized potential, is based on thedevelopment of polarization. This causes thestructure to exhibit a more negative potentialthan in its native state.

Measurement of the polarization shift can bedetermined by either measuring its formation ordecay. Determination of the amount ofpolarization is normally made during thepolarization decay (positive shift) period

subsequent to de-energizing the cathodicsystem, or in the case of galvanic anodes,when they are disconnected When cathodic

leave the structure unprotected for an extendedperiod of time. Normally however, the bulk ofthe depolarization will take place in the initialphase of the polarization decay; therefore itmay not be necessary to wait the full decayperiod except in those cases in which the totalactual polarization shift of the structure isrelatively close to 100 mV. If, during the earlyphase of polarization decay measurement, thepotential drops 100 mV or more, there is no realneed (unless the actual value is desired) to wait

for further de-polarization. If, on the other hand,the potential drop in the initial phase of thedecay period is only on the order of 50 to 60mV, it may be doubtful that the 100 mV shift willoccur. In this case, a determination should bemade as to whether a longer wait for totalde-polarization is required and justifiable.

In order to determine the formation ofpolarization on a pipeline/structure, it is firstnecessary to obtain static or native potentials(before the application of cathodic protectioncurrent) on the structure at a sufficient numberof test locations. Once the protection system isenergized and the structure has had time topolarize, these potential measurements arerepeated with the current source interrupted.

The amount of polarization formation can thenbe determined by comparing the staticpotentials with the instant off potentials.


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point of discharge. The results obtained underthese conditions could be misleading.Corrosion personnel must determine theeffectiveness of this criterion in areas of straycurrent activity.

The 100 mV polarization criterion is mostlyused on poorly coated or bare structures and insome instances could be useful in large pipenetworks as in compressor and regulatingstations where the cost of a cathodic protection

system to achieve either a -850 mV onpotential or a -850 mV polarized potential maybe prohibitive. However, in an economicanalysis, the additional cost of conductingfuture periodic surveys has to be taken intoaccount; as the use of the 100 mV criterion issomewhat more complicated and costly thanthe use of the -850 mV criteria with the cathodicprotection applied.

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

In piping networks, where new pipe is coupledto old pipe, it may be good practice to use the-850mVpolarizedpotentialcriterionforthenew

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

CRITERIA FOR SPECIAL CONDITIONS -

NET PROTECTIVE CURRENT CRITERION

This criterion is listed in Section 6 of SP0169-

2007 under a special conditions section tocover those situations for bare or ineffectivelycoated pipelines where it appears that long linecorrosion activity is the primary concern.

This protection criterion is based on thepremise that if the net current at any point in astructure is flowing from the electrolyte to thestructure, there cannot be any corrosion currentdischarging from the structure to the electrolyteat that point. The principle is based on firstlocating points of active corrosion and thenmeasuring current flow to or from the structure.

Some pipeline companies use the side drainmethod for application of this criterion on thebasis that if the polarity of the voltage readings

on each side of the structure indicates currentflow towards the structure, then the structure isreceiving protective current. If the electrode


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

In cases where there are sources of outsidegradients, it may be difficult to evaluate the

results of these tests properly.

Limitations of the Net Protective Current

Criterion

The use of this criterion is to be avoided inareas of stray current activity because potentialvariations could interfere with its use. Also,extreme care must be taken in commonpipeline corridors because other gradientsources may exist that could result in erroneousor misleading measurements.

Even though the results indicate a net currentflow towards the pipe/structure, that net currentflow is indicative only of what is happening atthe specific point of test and does not represent

what may be happening at other points on thepipeline/structure. Pipeline companies that usethe side drain method for application of this

Appl ications of the E-Log I Curve Cri ter ion

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

of the E-log I curve.

Once the current value and the potential to aremote electrode have been established, futuresurveys consist of checking the current outputof the cathodic protection system and thepotential of the structure to a remote electrode.It is important that the reference electrode belocated in the same place where it was locatedduring the E-Log I tests. Because the testmethod involved is rather elaborate, the use ofthis method is generally limited to structureswhere conventional means of determiningcurrent requirements would be difficult.Examples of such structures are pipeline rivercrossings, well casings, piping networks in aconcentrated area, and in industrial parks.

Limitations of the E-Log I Curve Criterion


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300 mV POTENTIAL SHIFT CRITERION

As opposed to working toward a certainminimum potential value to a referenceelectrode as discussed in some of the previoussections, the 300 mV potential shift criterion isbased on changing the potential of the structurein the negative direction by a specifiedminimum amount. The minimum potential shiftfor steel, as used in the past is 300 mV. Anystable reference electrode may be used with

this criterion because the method consists ofmeasuring a potential shift and is independentof the actual potential of the electrode.Determination of the voltage shift is made withthe protective current applied.

The development of this criterion appears tohave been largely empirical or experimental innature. Although 300 mV is the Figurecommonly used at this time, other values suchas 200 or 250 mV have also been used in thepast. There were, however, two considerationsthat supported this criterion, recognizing thatcorrosion prevention still may not be 100%complete.

The first consideration was that 300 mV may be

greater than the driving potential of most of thegalvanic cells on a structure to be protected. Byshiftingthestructureinthenegativedirectionby

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

Appl ications of the 300 mV Potent ial Shi ft

Criterion

This criterion has been used for entirestructures and also for hot-spot protection. It ismainly used for bare steel structures whichhave undergone a slow uniform corrosion ratedue to their age. These structures/pipesnormally have a natural potential range fromabout -200 mV to -500 mV as a result ofoxidation products developing on the externalsurfaces. This criterion has also been used onsome coated pipelines where soil conditionsalter the natural or static potential of the steel to-500 mV or lower. In this case, the 300 mV shiftmay also be easier to attain than achieving apotential reading of -850 mV, and could beexpected to stop the majority of corrosion.

The 300 mV potential shift criterion is almostalways more applicable to impressed current


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CSE even after the pipe potential is shifted inthe negative direction by 300 mV. If thepotential of the structure is fluctuating morethan the shift required, this criterion cannot bevalid.

If the steel structure is coupled to a more noblemetal, a 300 mV shift might indicate that muchof the adverse effect produced by the dissimilarmetal union has been overcome, but it wouldnot necessarily indicate that complete cathodic

protection is being provided to the steelstructure/pipeline itself.

USE OF CRITERIA

It must be noted that there will be situations orconditions where a single criterion cannot beused to evaluate the effectiveness of a cathodicprotection system and it is necessary to employa combination of criteria.

There will also be situations when theapplication of the criteria listed may beinsufficient to achieve protection. Someexamples pointed out in the SP0169-2007 aresituations where the presence of sulfides,bacteria, elevated temperatures, acid

environments, and/or dissimilar metals increasethe amount of current required for protection. Atother times values less negative than those


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CHAPTER 2

EVALUATION OF UNDERGROUND COATINGS USINGABOVEGROUND TECHNIQUES

INTRODUCTION

This chapter describes the indirect inspectionmethods intended for use as part of theExternal Corrosion Direct Assessment (ECDA)process for detecting coating flaws anddetermining cathodic protection levels on buriedpipelines. These methods are often morelaborious than surveys completed as part ofnormal daily operating practices as a result ofthe requirement for precise data set alignment.Standard pipeline surveys generally investigatedata trends over time or pipeline distance, whileECDA surveys look for small data variationsover short distances.

This chapter describes the methods of

conducting the following surveys for abovegrade indirect inspections. Other inspectionmethods can and should be used as required

stray current conditions, and large coatingholidays.

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

SURVEY SEQUENCE

The sequence in which the surveys areconducted is crucial to optimizing the surveytechniques and data analysis. Coating holidaysurveys (DCVG and ACVG) and AC attenuationsurveys should be completed prior to a CISsurvey. With surveys completed in this manner,

pipe to electrolyte potentials can be measureddirectly above the coating holiday indicationsfound using the DCVG or ACVG method


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NACE/CEA 54277, Specialized Surveys forBuried Pipelines 1988

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

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

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

DEFINITIONS

a) Anomaly: Any deviation from nominal

conditions in the external wall of a pipe, itscoating, or the electromagnetic conditionaround the pipe.

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

c) Close-Interval Survey (CIS): A method ofmeasuring the potential between the pipe

which the same indirect inspection tools areused.

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

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

i) External Corrosion Direct Assessment(ECDA): A four-step process that combines

pre-assessment, indirect inspections, directexaminations, and post assessment toevaluate the impact of external corrosion onthe integrity of a pipeline.

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

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


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

q) P ipe- to-S o i l P o tenti a l : SeeStructure-to-Electrolyte Potential.

r) Region: See ECDA Region.

s) Structure-to-Electrolyte Potential: Thepotential difference between the surface ofa buried or submerged metallic structure

and the electrolyte that is measured withreference to an electrode in contact with theelectrolyte.

SAFETY CONSIDERATIONS

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

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

Use properly insulated test lead clips and

terminals to avoid contact with anunanticipated high voltage (HV). Attach testclips one at a time using the single-hand

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

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

Observe appropriate Company safety

procedures, electrical codes, and applicablesafety regulations.

PIPELINE LOCATING

The pipeline must be located and marked toensure that subsequent measurements aremade directly above the pipeline. An inductiveor conductive pipe locating device can be used.

The pipe should be located within six (6) inchesperpendicular of the pipe centerline and surveyflags or paint marks placed directly above thepipeline every 100 feet using a slack chaindistance technique or a measuring wheel. Slackchain stationing error shall be no more than 2%+/-.

The locating flags/paint marks can benumbered using a permanent marker by writingthe flag number directly on the flag or painting


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DIRECT CURRENT VOLTAGE GRADIENT

SURVEYS (DCVG)

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

measured. Voltage gradients are located by achange in the interrupted signal strength atgrade.

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

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

disbonded coatings or cracking, produceselongated patterns.

resistance contact situations.

The current interrupter is installed in series with

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

DCVG surveys can be performed with

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

The IR drop is measured at the test stations in

the proximity of the DCVG survey. It isdesirable to have a minimum of 100 to 400 mVof IR drop in soil environments and more IRdrop when surveying on asphalt/concrete, in thesection of pipeline to be surveyed. If at theestimated daily survey section limits there is nota 100 to 400 mV IR drop, then the currentoutput of the CP current source should be

increased to achieve the desired result. If theoutput cannot be increased, then the section ofpipe with the 100 to 400 mV IR drop is the only


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parallel metallic structures by disconnectingelectrical bonds, negative drains to rectifiers,etc. In pipeline right of ways with multiple

electrically continuous pipelines or metallicconduits/structures, variations in the surveytechnique must be considered. For example, ifparallel pipelines (metallic structures) are lessthan ten (10) feet from the pipeline underinvestigation, then the perpendicular probeshould be placed at half the distance betweenthe two pipelines, however, difficulties may be

encountered with current flow to the parallelmetallic structure. The perpendicular probemust be placed on the side of the investigatedpipeline without a parallel pipeline (metallicstructure). If the pipeline under investigationhas pipelines (metallic structures) on eitherside, then the probe should be placedperpendicular, but not above or in closeproximity to the parallel pipeline (metallic

structures).

If the voltmeter indicates a coating holiday,additional measurements should be made toconfirm the coating holiday is on the pipelineunder investigation and not the parallel pipeline.

These tests include gradient measurements onboth sides of the pipeline and parallel with the

pipeline under investigation to confirm thecoating holiday location.

voltmeter at the same rate as the interrupterswitching cycle. The amplitude of the swingincreases as the coating holiday is approached

and decreases after it has been passed.Current flow from the interrupted current sourceto the pipeline indicates a possible coatingholiday while current flow away from thepipeline indicates current flow past the pipeline.

When a coating holiday is found, additionalgradient measurements can be beneficial to

confirm its location and that the indication is notcurrent traveling past the pipeline. Thesegradient measurements can be made on bothsides of the pipe and parallel with the pipe oneach side of the assumed coating holiday.

A straight-line attenuation effect is assumedbetween test station locations to calculate thesignal strength at intermediate coating holiday

locations. In order to calculate the coatingholiday size (%IR), the difference between theon and off potentials at each test station, valve,or other above grade appurtenance must bemeasured and recorded.

One reference electrode is placed at the baseof the test station or other electrical contact

point, in contact with the soil while the secondelectrode porous tip contacts the test stationwireorotherproperlycleanedelectricalcontact


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

Estimated signal strength at defect:

Precisely locating a coating holiday is achievedby marking the approximate location of theholiday at the area where the maximumamplitude is indicated. Near the approximatecoating holiday location and offset from the lineby approximately 10 ft, the probes are placed

along the voltage gradient to obtain a null (zero)on the meter. A right-angle line through thecenter of the probe locations passes over thecoating holiday epicenter, as shown in point Ain Figure 2-7. This geometrical procedurerepeated on opposite sides of the pipelinelocates the exact point above the holiday.

A survey flag, wooden stake, paint mark, orlathe is often placed at the indication epicenterand identified by a unique indication number

The percentage IR is used to develop a coating

condition classification system to prioritizecoating damage.

Once an indication is located, its size orseverity is estimated by measuring the potentiallost from the holiday epicenter to remote earth.

This potential difference is expressed as afraction of the total potential shift on the pipeline(the difference between the on and off

potential, also known as the IR drop) resultingin a value termed % IR. DCVG survey readingscan be broken into four groups based onapproximate size as follows:

Category 1: 1% to 15% IR - Indications in thiscategory are often considered of lowimportance. A properly maintained CP system

generally provides effective long-termprotection to these areas of exposed steel.

= 200 mV +1500

500 + 1500300 - 200 mV

= 200 mV + 75 mV

= 275 mV

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

= 55 mv

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

Signal Strength

=55*100

275

= 20%


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DEFECT

1500 yds 500 yds

200 mV 375 mV300 mV


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Category 4: 61% to 100% IR - The amount ofexposed steel indicates that this indication is amajor consumer of protective CP current and

that massive coating damage may be present.Category 4 indications typically indicate thepotential for serious problems with the coating.

These example categories are empirical innature and are based on the results of priorexploratory excavations at holiday locationsdetermined by DCVG surveys.

ALTERNATING CURRENT VOLTA GE

GRADIENT SURVEYS (ACVG)

Alternating current voltage gradient (ACVG)surveys are used to evaluate the coatingcondition on buried pipelines. Voltage gradientsarise as a result of current pickup or dischargeat coating holidays. In an ACVG survey, the AC

signal is created by a low frequency transmitterconnected to the pipeline and the voltagegradient in the soil above the pipeline ismeasured. Voltage gradients are located by achange in the signal strength at grade.

ACVG signal strength is not always proportionalto holiday size, as the orientation of the holiday

and other factors affect the measured signal.

ACVG surveys are capable of distinguishing

An AC current attenuation survey may beperformed with impressed current CP systemsenergized, however by turning off the rectifier

and using the positive and negative leads at therectifier station, the signal-generationcapabilities of the equipment can bemaximized. Sacrificial anodes and bonds thatare not disconnected show up as anomalies.Sacrificial anodes and bonds to other structuresare usually disconnected to prevent signal lossand enhance current flow down the pipeline.

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

anode bed or magnesium anode can be used toestablish an electrical ground.

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

The detector is used to measure the attenuationof the signal current that has been applied tothe pipe. An electrical current, when applied to


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magnitude will increase as the holiday isapproached.

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

When the holiday is centered between the two

probes, the magnitude will be zero and thedirection arrows will not indicate a consistentcurrent direction.

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

perpendicular to the pipe.

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

CLOSE-INTERVAL SURVEYS (CIS)

CIS is used to measure the potential difference

between the pipe and the electrolyte. Data fromclose interval surveys are used to assess theperformance and operation of the CP system.

evaluate CP system performance inaccordance with the NACE pipeline CP criteriaas found in SP0169. On and Off surveys

measure the potential difference between thepipe and the electrolyte as the CP currentsource(s) is switched on and off.

On and Off surveys rely on electronicallysynchronized current interrupters at each CPcurrent source, bond, and other current drainpoint that influences the pipeline potential in the

survey area. The ratio of the On-to-Offinterruption cycle should be long enough forreadings to be made but short enough to avoidsignificant depolarization. A three second On,one second Off cycle period or similar can beused to maintain pipeline polarization over timeand allow accurate Off potentials to berecorded.

The copper sulfate reference electrodes (CSE)are placed directly over the pipeline, typically at2.5 to 5 foot intervals such that both On and Offpotentials can be measured and recorded ateach reference cell location.

The accuracy of the on and off data can beverified by recording a continuous datalog

(waveprint) at test stations or points of electricalcontact such as valves or risers. This data logwill illustrateproper interruptersynchronization


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copper sulfate reference electrodes, smallgauge CIS wire (30 AWG), wire dispenser, andpipe/cable locating equipment (See above

section on pipe locating).

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

Prior to the CIS, a rectifier influence survey maybe completed to determine the CP currentsources which must be interrupted for theaccurate measuring of Off potentials. Theseinclude company rectifiers, galvanic anodesystems, foreign company rectifiers, andelectrical bonds to foreign company structures.Individually, each CP current source should betested. A current interrupter is used to

interrupt the CP current source suspected ofinfluencing the CIS pipeline segment. Typically,a slow interruption cycle is used such as a tensecond On, five second Off period.Pipe-to-electrolyte potentials are measured atthe furthest test points suspected of influencefrom the suspect CP current source.

Test points are monitored moving away fromthe suspect CP current source. Once a 10 mVor less difference between On and Off pipe to

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

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

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

Industry standard copper sulfate referenceelectrodes (CSE) should be used for potentialmeasurements. See NACE Standard TM0497.Reference electrodes should be calibrated with

an unused control reference electrode daily, theresults of which should be recorded in theproject field book. The control referenceelectrode should be a recently chargedelectrode not used to gather data in the field.

To calibrate the CSE, the ceramic porouselectrode tips are placed tip to tip to measurethe voltage difference between the two

electrodes or both tips are immersed in acontainer of potable water. A digital voltmeteron the millivolt scale is used to measure the


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the completion of CIS each day. The data canbe retrieved from the datalogger each day andanalyzed for the existence of dynamic stray

currents, de-energizing of a CP source, orimproper current interrupter operation.

On and Off pipe-to-electrolyte potentials arethen measured and recorded typically at 2.5 to5 foot intervals using a high-input impedancevoltmeter/datalogger. The datalogger shouldhave the ability to adjust the time during the Off

cycle at which the handheld datalogger storesthe Off potential value due to the possibility ofinductive/capacitive spiking. The dataloggershould be programmed such that the On andOff pipe to electrolyte potentials are measuredand stored from a time period beyond thespiking as determined by the waveprintsdiscussed above.

Pipe-to-electrolyte potential measurementsshould be measured and recorded at each teststation and foreign pipeline crossing from eachtest wire within accessible test stations. Nearground (NG), metallic IR Drop (IR), and farground (FG) On and Off pipe-to-electrolytepotential measurements should be made ateach point of pipeline connection.

If the Off metallic IR drop exceeds 5 mV, thesurvey should be halted and an investigation

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

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

Upon completion of the survey, all CIS wire

should be retrieved. Flags can be left in placeuntil the final ECDA process/surveys arecompleted and deemed appropriate for finalremoval.

AC CURRENT ATTENUATION SURVEYS

(ELECTROMAGNETIC)

AC current attenuation surveys are used to

provide an assessment of the overall quality ofthe pipe coating within a section or as acomparison of several sections. A current isapplied to the pipeline, and coating damage islocated and prioritized according to themagnitude and change of current attenuation.AC current attenuation surveys may beperformed with impressed current CP systems

energized, however by turning off the rectifierand using the positive and negative leads at therectifierstation,signal-generationcapabilitiesof


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along the pipe. The transmitter is energized andadjusted to an appropriate output. Typically, thelargest attainable current output is chosen to

maximize the length of pipe that can besurveyed. An impressed current anode bed ormagnesium anode can be used to establish anelectrical ground. If operating rectifiers areinterfering with the signal, then turn the unitsoff.

Signals are measured using the receiver unit.

The receiver consists of a handheld,symmetrical, multi-axis antenna array. Theelectromagnetic field radiating from the pipelineis measured by the detector. The detector isused to measure the attenuation of a signalcurrent that has been applied to the pipe. Anelectrical current, when applied to a well-coatedburied pipeline, gradually decreases asdistance increases from the point of current

application. The electrical resistivity of thecoating under test and the surface area incontact with the soil per unit length of pipe arethe primary factors affecting the rate of declineand the frequency of the signal.

The logarithmic rate of decline of the current(attenuation), which is effectively independent

of the applied current and marginally affectedby seasonal changes in soil resistivity, providesan indication of the average condition of the

accuracy of the readings may be affected bydistortions in the AC signal caused by otherunderground piping and conduits, traffic control

signaling, or vibrations due to passing vehicles.

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

SUMMARY

The indirect above ground inspectiontechniques discussed in this chapter are usedto identify and define coating faults and in turnthose areas where corrosion activity may haveoccurred or may be occurring. Two or more ofthese inspection techniques should be usedwhen conducting this indirect inspection testingso that different types of data can be comparedand analyzed to determine whether there is any

correlation. The effectiveness of the testingtechniques employed will depend on factorssuch as operator exper ience,pipeline/coating/CP circuit conditions, depth ofpipe, and type of cover at grade.

Should significant coating damage be indicatedfrom these tests, the pipeline should be

excavated and examined for possible corrosiondamage and the appropriate remedial actionsshould be taken.


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CHAPTER 3

MATERIALS FOR CATHODIC PROTECTION

INTRODUCTION

This chapter will discuss common materialsused for underground cathodic protectioninstallations. Some of the materials haveestablished track records and some are newwith very little known about the long term lifeeffects in a particular environment. For ourpurposes, we will define long term as any

material with a successful application record inunderground use of more than 20 years.

Knowledgeable persons in the corrosion controlindustry must have a grasp of the advantagesand limitations of a product and how theproduct relates to any given application.Furthermore, they must be able to convey inwriting, the specifics of how the product is to be

manufactured or supplied to assure that it willconform to predetermined design life criteria.

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

1. Specify materials completely.

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

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

Usage:

1. Follow the manufacturers recommendationsand instructions.

2. Use compatible components.

3. Use the proper tools.

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


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BURIED GALVANIC ANODE OF

MAGNESIUM SURROUNDED

WITH A PACKAGE OF

SPECIAL CHEMICAL

BACKFILL

STEEL UNDERGROUND

STRUCTURE (CATHODE)

DRAIN WIRE FROM

STRUCTURE TO ANODE

(OR ANODES)

CATHODIC PROTECTION

CURRENT FLOW

CONNECTING WIRESTO ADDITIONALANODES IF NEEDED


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schematic diagram of a typical galvanic anodeinstallation.

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

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

The important point to consider for maximumservice life is the cost per ampere hours ofcurrent capacity, once it has been establishedthat the driving potential is sufficient for thecathode metal and the resistivity of theelectrolyte in the circuit. When working in thehigher resistivity soils, long slender anodeshave a lower resistance to earth than theshorter anodes which are available. This meansthat the circuit resistance for the limited driving

Prepared Backfill and Packaging

Most magnesium and zinc anodes used in soils

require the use of a prepared backfill aroundthe anode for the following reasons:

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

C The bentonite clay absorbs and retains

moisture.

C The gypsum provides a uniform, lowresistance environment.

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

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

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

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


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TABLE 3-1

Capabilities and Consumption Rates

TypePotential*

(- volts)

Amp Hours

per lb.

Consumption

(lb/A-yr)

Magnesium

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

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

Zinc

ASTM B418-01

Type I (saltwater) 1.1 354 24.8

Type II (soil) 1.1 335 26.2

Aluminum

Mercury Alloys 1.10 1250 - 1290 6.8 - 7.0

Indium Alloys 1.15 1040 - 1180 7.4 - 8.4


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periods of inclement weather and handlingdamage. Prior to backfilling, the paper bag isremoved and discarded, permitting the cloth

bag containing the backfill to absorb moisture,allowing the anode to start putting out currentsoon after installation. During transportationand/or handling the anodes may shift in theprepared backfill. This may result in unevenconsumption of the anode, reduction of currentoutput and premature failure of the anode. Thiscondition should be avoided by careful

specification of transportation packaging andfield handling precautions. Anodes packaged incardboard boxes or bags with centralizingdevices may tend to reduce anode shifting.

Wire and Cable Attachment

Wire used to connect the anode to the structureis usually a No. 12 AWG solid copper, single

conductor, with TW (30 mils PVC), THW (45mils PVC), or THHN insulation.

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

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

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

Magnesium Alloys

Magnesium anodes are produced in a widevariety of sizes and shapes to fit designparameters. They may be cast in molds orextruded into ribbon or rod shapes, with steelspring, perforated strap or wire cores as shownin Figure 3-2. The core is important because itshould extend 85 percent or more through the

anode to reduce the internal circuit resistance.Table 3-2 shows some of the common weightsand corresponding dimensions for bare andpackaged anodes. Chemical composition isshown in Table 3-3.

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


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E

D

BAGGEDMAGNESIUM

ANODE

G

F

A

C

B

BOXED

MAGNESIUMANODEO

PEN

THIS

END

ONLY


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TABLE 3-2

Magnesium Anode Dimensions and Weights

Nominal Dimensions (Inches) - See Figure 3-2

Alloy A B C D E F GPackaged

Weight (lbs)

1 AZ63 3.2 Rd 2 6 6 - - 3.6

3 H.P./AZ63 3 3 6 8 6 - - 95 H.P./AZ63 3 3 10 12 5 - - 12

6 H.P./AZ63 3 3 10 - - 12.5 5 14

9 H.P./AZ63 3 3 13.5 17 6 - - 27

12 AZ63 4 4 12 18 7.5 - - 32

17 H.P. 3.5 3.5 25.5 30 6 - - 42

17 AZ63 3.5 3.5 28 - - 32 5.5 45

20 H.P. 2 2 60 - - 71 4.5 65

32 H.P./AZ63 5.5 5.5 21 25 8 - - 72

32 H.P./AZ63 5.5 5.5 21 - - 24 7.5 70

40 H.P. 3.5 3.5 60 64 6 - - 105

48 H.P. 5.5 5.5 32 36 8 - - 10650 AZ63 7 7 15 24 10 - - 110


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TABLE 3-3

Composition of Magnesium Alloy

ElementAZ63B

(H1A)

AZ63C

(H1B)

AZ63D

(H1C)

M1C

(High Potential)

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

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

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

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

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

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

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

Others (each) - - - < 0.05%

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

Magnesium (Mg) Balance Balance Balance Balance

Performance Characteristics*

AZ63B AZ63C AZ63D M1C


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manganese surrounds the ironparticles and decreases theirlocal cathodic effect.

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

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

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

Copper Decreases current capacity if>0.02%.

Others Not commonly found in amountsthat are detrimental.

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

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

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

The purchaser should request a verifiablespectrographic analysis of any anode purchase

system design. They are available in a varietyof sizes and shapes including bracelet anodesfor marine pipelines, docks and piers, hull

anodes for marine vessels, and in ribbon formfor use in utility ducts and for AC mitigation.Zinc is not recommended in environmentswhere carbonates or bicarbonates are found, orwhere the temperature of the electrolyte is over120 F. Under these situations zinc becomescathodic, rather than anodic, to steel and itsuse should be avoided. Tables 3-4 and 3-5show some common zinc anode sizes andcompositions respectively.

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

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

Zinc anodes are also used as stationaryreference electrodes.

Aluminum Alloys


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TABLE 3-4

Zinc Dimensions and Weights

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

Bare Zinc Anodes

5 1.4 1.4 9 0.250

12 1.4 1.4 24 0.250

18 1.4 1.4 36 0.250

30 1.4 1.4 60 0.250

30-A 2.0 2.0 30 0.250

45 2.0 2.0 45 0.250

60 2.0 2.0 60 0.250

Zinc Ribbons

2.4 1.0 1.250 -- 0.185

1.2 0.625 0.875 -- 0.135

0.6 0.500 0.563 -- 0.130

0.25 0.344 0.469 -- 0.115


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TABLE 3-5

Zinc Alloy Compositions

Element

ASTM B418-01

Type I

(sea water)

ASTM B418-01

Type II

(soil)

Aluminum 0.1 - 0.5% < 0.005%

Cadmium 0.025 - 0.07% < 0.003%

Iron < 0.005% < 0.0014%

Lead < 0.006% < 0.003%

Copper < 0.005% < 0.002%

Others 0.1%

Zinc Balance Balance

TABLE 3-6

Aluminum Alloy Composition and Performance

Element

Mercury Family

AI/Hg/Zn

Indium Family

Al/In/Zn

Zi (Z ) 0 35 0 60% 2 8 6 5%


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4

6

1

2

3


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1,000 ppm or more of chlorides.

The mercury alloy is used in free flowing sea

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

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