AD/A-OOLI 083

CIVIL ENGINEERING CORROSION CONTROL

VOLUME 11,* CATHODIC PROTECTION TESTING METHODS AND

I NSTRUMENTS

I HINCHMAN CORPORATION

PREPARED FOR

AIR FORCE CIVIL ENGINEERING CENTER1

JANUARY 19751

lowi iomf s- -1. L 1U1 Uim a

.....

8I ?vCLA1111POCATHMU OF 73435 PAGE IMON Df MOMM_________________

REORT DOCUMENTATIO PAGE strong___________wow1. REnPORT SIURGR GoOVACCESS"ON ;& RECIPIENT'S CATALOG MISE

AlICFL-TR-74-b Volume 11

. TITLE(dawe TYP OFRPR PERIOD COCivil Engineering Corrosion Control, lnlRpr ay1 7 Volume II - Cathodic Protection Testing to N4ov 1974Methods and Ins truments S. PERFORMING ONG. REPORT NUMMER

AFCEC-TR-74-6 Volume 117. A~mO~4) S. CONTRACT OR GRANT IIUMEEnRe)

*Lewis 11. West, P.11., The Ilinchman Co.Thomas F. Lewicki, P.11.The Air Force Civil Engineering Center

9. PERFORMING ORGANIZATION NMEF AND ADDRESS 10. PROGRAM ELEMENT.% PROJECT. TASK 4AREA A WORK UNIT NUMMERS

ICONTROLLING OFFICE NAME AND ADDRESSa RPRTOT

Air Force Civil Engineering Center(AFCEC) Jan 197STyndall AFB, FL 32401 13. NUNIOEROF PAGES

14. MONITORING AGENCY NAME A ADOREM~II diffevet kern Caoceilla Office) it. SECUIT CLASS. (of Wrne io)

Unclassified

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W~ DISTRIBUTION STATEBMENT (of am*. Ripef)

Approved for public release: distribution unlimited,

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Reproduced by

NATIONAL TECHNICALINFORMATION SERVICF

Sprisgfld VA 22151IL PPIUTARY MOTUS_____________

This report is rublished in three volumes, Vol I Corrosionr Control-General, Vol 11I Cathodic Protection Testing Methods

and Instruments, Vol III - Cathodic Protection Design.

1S. NOY WORSE1 (Cbs noe &Afts cit amseeceap m 0#& by Week -A-nbe)

Civil Engineering, Corrosion, Corrosion Control, CathodicProtection, Impressed Current Systems, Galvanic Anode System

20. AUSTRACT (Cafhe en .ev. e N neoceandm hfeI by bloc no~bmThe report is specifically written for Air Force Civil Engineeringpersonnel but can be useful to all Agencies of the FederalGovernment. It covers mainly Real Property and Real PropertyInstalled Equipment. It deals with corrosion and corrosioncontrol of buried and submerged metal structures. Causes andtheory of corrosion,'material selection, protective coatings,and cathodic protection application are included. The informaticcontained herein will be useful for solving all corrosion probleml

U JANi 72 OTOOIOSUI.E UnclassIf ied1 SCURITY CLASIFICATIOM OF THIS PAGE- (RWeu Duo~

UnclassifiedSCCUMTy CLMSICATM~ OF T"IS PAG(hm Daft &Wfmd.

Block 20 - Abstract (Cont'd)

encountered on real property and real property installed equip-,ment.

UN:CLASSI FIED

AFCEC-TR-74-6, VOLLME 11

CIVIL ENGINEERING CORROSION CONTROL

VOLUME 11 - CAMhODIC PROTECTION TESTINGMETHODS AND INSTRUMENTS

BY

Lewis 1H. West, P.E.jThe Ilinchman Company

Thomas F. Lewicki, P.E.The Air force Civil Engineering Center

Approved for Public Release, Distribution Unlimited

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*~* isrc~r t SU.i U~K ~.. ~ v 1 ~2a~i. Nov !.74.111oinas F. L!'c; ic'k1 w~s I ro., c t Offi c r Tic ti~ijor portion of

LI* LL]r T! 0 1 : rnc L; 0 r'.; Lt TL; tu7

This report is published in three volumes, Volume I - CorrosionControl - General, Volume II - Cathodic Protection Te~tingMethods and Instruments, and Volmnne III -Cathodic ProtectionDesign.

Approved for public release: distribution unlimited.

This report has been reviewed by the Information Office (01)andl is releasable to the National Technical InformationService (INTIS) . At NTIS, it will be available to the gceneralpublic, including foreign nations.

This technical report has been reviewed and is approved forpublication.

THOMAS F. LEUWICKI, P.E s :SS:381Project officer

D "SGS~ LLJIA .RA- e C SAFTechnical Director Commander

PREFACE

The Air Force's major guide to corrosion control of realproperty and real property installed equipment has beencontained in Air Force Manual 88-9, Chapter 4 which waspublished August 1962. The corrosion control field has beenprogressing with new methods, materials and equipmentconstantly being introduced. Since the Air Force's corrosionmanual was old and outdated, a need existed for searching,investigating and documenting the new methods, materials andequipment for corrosion control. It was decided that the mostprompt and economical method of accomplishing this task wasby procurring the services of a prominent corrosion engineeringfirm. Because of the large volume of the documented findingsof this endeavor, the information has been published in threevolumes. The first volume is entitled Corrosion Control -General. The second and third volumes are entitled CathodicProtection Testing Methods and Instruments and CathodicProtection Design.

A

/1.

£I

ii - F IF

C ONT I NTS

Paragraph Patc

Test ing Methods -- - - - - - - - - - - - - - - - 1 1

Instruments -- - - - - - - - - - - - - - - - - - 2 37

References - - - - - - - - - - - - - - - - - -- 94

Bibl iography------------------------------ ------- 9

ApIpeadices --- - - - - - - - - - - - - - - - -

Ap-pendix A - Glossary of Corrosion Terms---------97

Appendix 13 - Electromotive Force Series-------------lul

Appendix C - Galvanic Series------------------1,

Appendix P-Underground Corrosion SurvcyChecklist------------------------------ 10

Indcx------------------------------------------------- 112

iv

ILLUSTRATIONS

Figure page

I Typical Galvanic Anode Circuit 4

2 Current Flow Around Insulating Joints b

3 Circuit Diagram: Parallel Resistance at Insulating Joint 7

4 Ammeter and Voltmeter in Circuit 8'

5 Convential Meter Hook - Ups 11

6 Structure-to-Electrolyte Potential Measurement 13

7 Structure-to-Soil Voltage Galvanic Conditions 18

8 Structure-to-Soil Voltage - External Current Conditions 19

9 Comparison of Off-to-Side and Over Structure Readings 21

10 Struct ire-to-Electrolyte Readings - Current PassingThrough Structure 22

11 Null Method of Current Measurement 25

12 Casing-to-Carrier Pipe Resistance Measurement 27

13 Soil Resistivity Measurement 30

14 Soil Resistivity: Four-Pin Alternating Current Method 33

i15 Soil Resistivity: Two-Probe Method 34lb Typical Single-Probe Resistivity Measuring Device 35

17 Voltage vs. p1l for Antimony Copper-Copper Sulfate Cell 36

18 Current Requirement leasurement 39

19 Couplings Determined in Current Requirement Tests 41

20 Stray Current Field 'Measurement 43

21 Record Sheet for Interference Tests 44

[222 Correlation Curves 45

Locating iligh-Resistance Joints in Underground Piping 48

24 Coating Conductance Measurement: Short Line s5

25 Coating Conductance Measurement: Long Line 52

Vode L L. I !

- ' , .. .

Figure Page

26 Electrical Connections for Duct Slug Test 53

27 Soil Box 56

23 Typical Field Survey Test Equipment 58

29 Galvanometer 60

30 Basic D'Arsonval 'Movement 00

31 Instrument Bearings 0l

32 Taut Band Suspension 02

33 Basic D.C. Ammeter Circuit U4

34 Measurement of Current Fot. by the Zero ResistanceAimeter Method

35 Basic D.C. Voltmeter Circuit o6

30 Basic D.C. Potentiometer - Voltmeter Circuit 63

37 Basic D.C. Vacuum - Tube Voltmeter Circuit 0)

33 Battery and Generator Energized Soil Resistance Tester 72

39 Typical Combination MUeter 7&

40 Wiring Diagram for iller Meter, Model M-3-M 73

41 Typical Uses of Low-Resistance and High-Resistance Voltmeter 79

42 Typical Uses of Ammeter and High-Resistance Voltmeter 3) ...

43 Wiring Diagram and Application of Current Interrupter i2

44 Typical Copper-Copper Sulfate Reference Electrode 4

45 Polarization of a Copper-Copper Sulfate Reference Electrode ub

46 Effect of Temperature on a Saturated Copper-Copper SulfateReference Electrode J7

47 Calomel Electrode for Seawater Testing 89

4S Typical Silver/Silver Chloride Reference Electrode

49 Typical Pipeline Probe and Contact Bar

vi

[A

TABLE

TABLE PAGE

1 Standard Potentials (E*) of Commronly -Used Reference lbE!lectrode (1Half-Cells)

2 Correction Factor for Coating Conductance Calculations 5o

viFLA

ACK.OIW LE )G EI.ENTS

Sections of this report have been prepared from material found inAFM 88-9, Chapter 4 "Corrosion Control" (1 August 1962) andDesigning Impressed Current Cathodic Protection Systems witil DurcoAnodes, edited by I'illiam T. Bryan, The Curiron Company, Inc.,Dayton, Ohio, 1970. In addition, the following sources haveprovided data founU in tables and figures, sometim~es revised orupdated:

F igures Source

is, 23, 26, 27, 34, 43, 44, 45, 46 AF.I 88-9, Cl-. 4 (1 August 19k2)

14 Associated Research, Inc.

32, 39, 40, 41, 42 1M.C. Miller Company, Inc.

48 The Corrosion ilandbook,11.11. Uhlig, Copyright 1948,John Wiley t" Sons, Inc.,New York.

Table Source

1 NACE Basic Course,"Introduction to Corrosion",F.L. LaQue, Copyright 1969,National Association ofCorrosion Engineers, Houston,Texas.

2 National Association ofCorrosion Engineers Procedure:NACE 2D157

viii

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"Tr I c Tij S .\ [., .... .:;I. .i.s

1 TESTING METHODS. Requirements for any corrosiontest program must be tailored to the particular structureor system on which it is to be used. Specific requirementswill, therefore, vary with different localities and typesof structures. The corrosion engineer must sele't methodsand procedures which best supply information needed to studythese problems.

Although other factors are often important, electricity isthe basis of the corrosion process. Electrical currentleaving a metal surface produces actual metal loss, and acorrosion cell is an electrical circuit. Therefore, quan-titles associated with this circuit are those most oftenmeasured in corrosion testing, although various chemicaland physical tests may also be required. Electrical testsusually involve direct current (D.C.) measurement, becausethis (rather than A.C.) is most often encountered in corros-ion cells. Some recent attention has been given to alterna-ting current, but direct current remains most important.

Tests may be made in field or laboratory, depending onspecific measurements and eqlipment employed. Generally,electrical measurements on the actual structure in itsactual environment provide most reliable information. Com-plicated chemical and mechanical analyses may require labor-atory measurements, but even these are sometimes included infield testing.

1.1 Background. To determine if corrosion is active,how much protection is required, how protective equipmentshould be installed, and finally whether or not corrosionis being brought under control, it is necessary to take in-struments into the field or laboratory, connect them, andmeasure electrical quantities. Analysis of thesc quantitiesgives a solution, or at least a key to the solution, of cor-

F/ rosion problems. Usually several types of measurements aretaken before the corrosion behavior can be understood.Success or failure of a corrosion control program dependsupon accuracy and proper use of these meter readings.Equaliy important is proper recording and subsequent analysisof this information.

1.1.1 Description of Electrical Quantities. Electri-cal current can be considered to flow through conductors(materials having the ability to carry it) in the same man-

* ner in which fluids flow through pipe lines. The rate ofthis flow depends upon voltage pushing it through, which is Ianalogous to pressure, and resistance to flow which affectsmoving electric current as inside pipe walls affect flowingfluid. (A rougher wall surface results in more friction

I

~t

which is overcome by increasing pressure to maintain con-stant flow.) Resistance tends to hold back electricity,and voltage pushes it through. As liquids are measured ingallons, electric current is measured in amperes. As fluidpressure is measured in pounds per square ,electricalvoltage is measured in volts. Electrical resistance is mea-sured in oh~ms. One amp-ere of current will flow through aconductoro-fone ohm resistance when pushed by a voltage ofone volt. Milliamperes and millivclts, quantities commonlyassociated with corrosion work, equal one one-thousandth ofan ampere and a volt, respectively.

The relationship between basic electrical quantities is

given by the all-important Ohm's Law:

V = IR

where

V = voltage in voltsI = current in amperes, andR = resistance in ohms.

(If any two of these quantities are known, the third can befound through algebraic solution.)

Some examples of conductors (materials which will pass anelectric current when voltage is applied) are pipes, cables,soils, and waters. Coatings and cable insulation are exam-ples of non-conductors (materials which will not pass anyappreciable electric current when voltage is applied). Thegreater the resistance per unit material, the poorer theconductor.

Electrical currents are classified as direct or alternating.Alternating current reverses its direction of flow severaltimes each second, while direct current flows continuallyin the same direction for longer periods of time. Unlessspecial attachments are provided, current and voltage meas-uring instruments (ammeters and voltmeters) constructed fortne study of alternating currents cannot be used for directcurrent work. Since direct current is most commonly en-count.red in corrosion work, it will be the only type re-ferred to in this discussion.

Voltage measurement between two points along the path offlowing current is called voltage drop or loss, as is a dif-ference of pressure between two poins in a-pe line. Arelectrical circuit is the path which flowing current followsand can consist of one or several conductors. Current canbe considered to leave its source, flow through the circuit,and return to its source again. The circuit which it fol-lows will always be the path of least resistance. In the

82

A

case of a galvanic anode, such as magnesium (Figure 1),current leaves the anodes, passes through backfill and soilrespectively, enters the structure, flows along this struc-ture to the anode wire connection, and finally flows throughthe wire back to the anode. This circuit is made up of fourconductors other than the anode itself. These are backfillmaterial, soil, structure, anL copper return wire.

1.1.2 General Methods. To study the behavior ofelectrical circuits, one must first decide what quantity tomeasure (current, voltage, or resistance), where it can bestbe done (location in the circuit), what instrument to use,and finally how to connect this instrument. It is oftennecessary to measure current and voltage across a portion ofa circuit in order to determine resistance through that por-tion. Ammeters, and voltmeters or potentiometers are com-monly used to determine current flow and voltage respective-ly. Instruments which impress a known current and give aresistance reading from the resulting voltage drop are alsowidely used.

When instruments are connected, they can affect flowing cur-rent. If test leads are employed to make contact betweenmeter and structure, considerable resistance might be addedat one or all of the following:

1. Where leads contact structure.2. Where leads contact meter.3. Through leads themselves.

4. Through the meter.

A- mentioned before, any increase in resistance will reduceor hold back flowing current, so the flow might differ afterthe meter is connected.

a. Current. When current flows through insulatedwire, one can usually break the wire at any point, and meas-ure total current flowing simply by connecting an ammeter be-tween the broken ends. However, when this instrument is con-nected, total circuit resistance is immediately changed byadded contact, lead wire, and ammeter resistances. FromOhm's law, less current flows because of the increased re-sistance, even though voltage remains constant. Therefore,the measurement is not totally accurate.

Similarly, a pipe line through which electrical current isflowing is cut or two sections of pipe are electrically sep-arated by an insulating Joint. An ammeter Xs connected be-tween the two sections. Current passes through this instru-ment ana is measured. Total circuit resistance is increasedwith this instrument connected, and less current flows thanbefore.

t ~ 4.._........

G rad e *, .

(Earth)Insulated Copper

Buried

Backfill

Note: Arrows Indicate Current f low

Fioure ITYPICAL GALVA4NIC ANODE CIRCLRT

II

IIf the pipeline is buried or submerged, additional inaccur-acies are introduced, because soil and water are conductors(Figure -2). Any soil or water contacting a metallic struc-ture might carry some percentage of current flowing on thestructure. The total circuit contains several possiblepaths for current flow, each of different resistance (Figure

3). The ammeter measures on,.y that part of the currentwhich flows through it.

Because of these inaccuracies, In corrosion field work cur-rent is generally not measured with Just an ammeter (para-graph l.2.l.b).

b. Voltage. As fluid needs energy to move in a pipe,electricity requires a "push" along its circuit. The ener-gy or push to electric current is voltage.To determine the voltage used up between two points, a

voltmeter is usod.' A tcst Ifad wireF is connectedfrom one terminal of the voltmeter to one point (A) in thecircuit and a wire, from the other terminal to the otherpoint (B). The voltage used up between points A and B isread in volts. Current and resistance are not known, butthe voltmeter does reveal direction. of current flow. Thatis, when the voltmeter is connected to leads from points Aand B, the needle moves either to the left or the right de-pending upon the direction voltage pushes flowing current.

Points A and B could be two points in the carthor wator, two on a structure or A couldbe a point on a structure while B is a point on anotherstructure or earth (water). Thus, these voltage measure-ments indicate how much "push" the electrical current isgetting whether it is flowing along a buried structure,through an electrolyte, between one structure or another,or between a structure and an electrolyte. Inaddition, al-though the amount of current flowing is unknown, one cantell the direction in which voltage pushes current at thelocation where voltage is measured.

With voltage measuring instruments, an observer can tracethe path of electrical current along a buried o' submergedstructure; determine when it enters or leaves, its direc-tion of flow through earth or water, and whether or not itis entering or leaving from another structure. Althoughfrequently amount of current is unimportant, it is oftenpossible, after gathering some other data, to use the volt-age reading to compute current flow in amperes by Ohm 's Law.

It is important to remember, here, as with all electricalmeasurements, that good electrical contact be made at allconnections. Rust and dirt on terminals, clips, and probebars mean trouble and erroneous readings. Contact to the

I e .

L.1 - .....-.....

Grade

Buried Pipe

a) CURRENT FLOWS ON PIPE

Insulating Joint

b) CURRENT FLOWS AROUND INSULATOR, THROUGH SOIL.TOTAL CURRENT IS LESS THAN FOR a)

c) AMMETER DOES NOT MEASURE TOTAL CURRENT,BECAUSE PART CONTINUES TO FLOW THROUGH SOIL.

Note: Arrows Indicate Current Flow

Figure 2CURRENT FLOW AROUND INSULATING JOINTS

$

.- ......

.. . .... , _ ; . .... -. , . 1

Ri -esisanceiAmtradTtLed

RI Resistance Thrug AmeeInlat L d

Figure 3CIRCUIT DIAGRAM: PARALLEL RESISTANCES AT INSULATING JOINT

buried or submaerged structure is a common source of error andthe most difficult to cope with.

To measure current with an ammeter, connect the instrument sothat all current to be measured flows into one terminal andout the other. Voltage measuremients require- only a smallportion of current to flow tilrougli the measuring instrument(voltymeter). IjD fact, many times it is advantag'eous to use avoltneter througa which only a very, very siaalle portion oftotal current passes. Figure -1 illustrates thc method ofconnecting bota anitcter and voltmeter in circuit.

c. Resistance. Often, it is necessary to determine hownuch resistance electrical current will encounter. If currentand' voltage are both known, Ohm's Law can be used to computeresistance. Resistance equals voltage divided by current(R - %/I). To determine resistance, one can use an ammeterr and a voltmeter connected in separately, or one of severalcommercially available instruments which read the answerdirectly in 'ohms.

Some current possesV - Voltmeter through voltmeter, amountA a Ammeter determined by rasistanceR a Resistor

i R

All current posses Athrough ammeter

- liiilNote: Arrows signify current flow

Figure 4AMMETER AND VOLTMETER IN CIRCLIT

Resistance plays an important role in measuring electricalquantities. When test leads are used, they of+ . add resis-tance in clip and terminal connections because of corrosionproducts forming on metal surfaces or weak clip springs.Probe bar-to-structure and/or test lead wire-to-structureconnections are equally important. When an ammeter is usedto measure current, these cannot be tolerated. They willintroduce serious errors in some types of voltage measure-ments. If not understood, they can also cause resistancemeasurements to give false results.

d. Electrolyte Resistance. Electrical current flowsthrough some soils or waters easier than others. If thesame voltage is applied between two contact points in onesoil as in another, more current may pass between them inone instance. Resistance is lower in this case. If resis-tance between outer surfaces of two points and earth is notpart of the measured resistance, only current flowingthrough soil is being considered. Therefore, soil resis-tance is being measured.

For convenience, when dealing with soil or water, electro-lyte resiltivity rather than resistance is determined.Resistivity is a property of the material itself, independent

S

,-. -.

of the amount or geomet;7 of material. Resistance, on theother ha-n, is a iunction of the amount and geometry ofmaterial. Resistance and resistivity of -a-sample of mater-lal of size L by W by H, are related thus:

R = L

4whereR = resistance in ohms3P = resistivity in ohm-centimetersL = length in centimetersW = width in centimetersH = height in centimeters

From this relationship, it can be seen that resistivity equalsthe resistance of a cube of material measuring one centimeteron each of its sides. When the length only of this cube isdoubled, resistance is doubled. When either the width or theFT Eis doubled, resistance is halved. This is analogousto an electrical circuit in which'reslstors are placed firstin series, then in parallel.

Noting units in the above equation, one can see that resis-tivity has units of ohm-centimeters, not "ohms per cubiccentimeter" or "ohms per entrimeter cube" as are often mis-takenly used. Units such as "ohm-inch" or "ohm-mile"are tech-nically correct also, but for corrosion control applicationsthe unit "ohm-centimeter" is preferred.

e. Chemical Analysis. In addition to electrical meas-urements, a partial chemical analysis can often help indicatecorrosivity of soil or water. Standard laboratory methodsare employed to determine pH, presence of sulfates, or otherapplicable factors.

1.2 Field Tests. It Is important that corrosion fieldwork be pFf_'or m experienced personnel. Meaninglessmeasurements or misinterpretations of results are likely

P when inexperienced people do field survey work. Corrosion.1 - testing is widely diversified involving many different tech-

niques, some, highly specialized. Tests may require dura-tions from a few minutes to a year or more, and measurementsmay vary over wide limits. For example, potential measure-ments can vary from a few millivolts to hundreds of volts,and currents may be a few milliamperes or hundreds of amp-eres. Structure size bears no relation to the type tests re-quired. A small complex structure may involve many intric-ate measurements. Often available data are fragmentary, andconditions that cannot be measured may be of greater signifi-cance than those obtainable. Consequently, Judgment and ex-perience in fiell-testing techniques are of great value. Dueto the many factors involved, a corrosion investigation mayinclude visual inspection, study of geographical areas, studyof records, chemical or metallurgical analyses, electrical

.1 ....'! -

measurements and, sometimes, biological studies. Theproper combination of tests depends largely upon availabledata and local conditions. Here again, the necessity forexperienced personnel is evident. Corrosion survey reports pshIould indicate not only the t3st results, but also reasonsparticular tests were used or excluded.

1.2.1 Basic Field Tests. Standard test methods aredescribed hereT;-eld surveys, often combining several basicmeasurements, are discussed in paragraph 1.2.2.

A general procedure, common to all field tests usingammeters or voltmeters, involves standard electrical con-nections. It is common practice to connect the negativemeter terminal to the structure being investigated and thepositive terminal to the reference electrode (electrolytecontact), foreign line, or other. This arrangement will befcllovied in this manual and is generally accepted in thecorrosion field.

Another convention accepted in this manual concerns designa-ting points in a circuit as "positive" and "negative". Posi-tive and negative terminals of standard instruments aremarked by their manufacturer, conventionally with negativeterminal on the left and positive terminal cn the right, asone faces the meter (Figure 5). When current flows intothe positive terminal and out the negative terminal, the in-strument pointer moves to the right (clockwise). Conversely,when current flows into the negative terminal and out thepositive terminal the instrument pointer moves to the left(counterclockwise). By knowing where the two terminals areconnected, the engineer can easily determine Mirection offlow and which point is "positive" and which is "negative".For example, to measure IR drop, the meter terminals are con-nected to two points on the structure as in Figure 5a.If the meter swings to the right (clockwise) as shown, thisindicates that current is entering the meter at the positiveterminal and leaving at the negative. Therefore, currentflows through the structure from right to left as shown -nthe drawing. (Here, as always, "current" refers to conven-tional current flow, not electron flow.) By convention,point A is positive to point B.

Figure 5b shows a similar set-up for measuring structure-to-soil potential. As always, the reference electrode(electrolyte contact) is connected to the positive terminaland the structure, to the negative terminal. The meterswings to the left as shown, when current flows through thesoil from point B to point A.

10

1

Note:

conventionl currntfnow

a) iR DROP

- + Soil Contact

tA

b) STRUCTURE-T0-SOIL POTENTIAL

t~ Figure5CONVENTIONAL METER HOOK-UPS

k 1.~;1.4

a. Structure-to-Electrolyte Potential. When metalsare in contact with an electrolyte such as soil or water,they exhibit a potential, referred to as a "half-cellIotcdtial ". 7 is Aol.nti,1 k.ee ds on caardct, i'-

istics of the metal and characteristics of the electrclyte.Various metals exhibit different potentials when subjectedto the same electrolyte. In like manner, a metal will ex-hibit different potentials when subjected to electrolytesof different compositions. Another variable which affectshalf-cell potential is metal surface, often altered by heattreatment or other processes. Dissolved gases in the elec-trolyte or gases absorbed or adsorbed on the metal surfacealso affect half-cell potential,.

In the absence of externally impressed currents (stray orcathodic protection), the more negative a metal, relativeto its environment, the more it tends to corrode. For thisreason, knowledge of structure-to-electrolyte potential isimportant.

Structure-to-electrolyte potentials are helpful in locatingpossible corrosive areas on structures. Varying structure-to-electrolyte potentials indicate electrical currents flow-ing in the electrolyte. Where current leaves the structure,deterioration of metal results. Potential readings are alsoimportant in cathodic protection and stray current analysis(paragraph 1.2.2).

(1) Measure-nent. Potential measurement for a struc-ture relative to its electrolyte requires a voltmeter orother instrument, a dependable contact to structure (fromnegative meter terminal), and a reference electrode (con-nected to the positive meter terminal) to contact the elec-trolyte. Reference electrodes are used becausepotential is not an absoLute quaiitity; it i[ :caiurcdrelative to something of khown or assigned potential. Agalvanic potential exists between any reference el-ctrodeand the metal structure. Therefore, even if no aduitionalcurrent flows, this potential difference registers on a volt-meter and is added (or subtracted) from any measured voltage.For example, a galvanic potential of -0.5 to -0.8 volts istypical between a buried steel structure and a copper-coppersulfate reference electrode. If current enters or leavesthe structure, its resulting voltage irop is added to orsubtracted from these. This set-up is shown on Figure 6a.Figure 6b indicates the length of pipe observed by thesoil contact (about 3.5 times the distance between pipeand that contact).

(a) Contact to Structure. Measurement of poten-tial requires a good contact to structure. This may beachieved through test wires, an accessible section of struc-tures, or a probe bar. Test wires generally provide thebest contact and have other advantages.

12

/.... ...

I -

voltw~t4

IV

Buriedd Pipe

b) STYIONL SEBAETT-TUEPBERE

3.D

b) SETO OFBR TUTUEOSRE

BYRFRNE LCRDFigure

If a section of the structure is easilyaccessible (e.g. above ground), contact can be made there;any paint, corrosion products, or other coverings mustfirst be removed to assure electrical contact. When aprobe bar is used, care must be exercised to obtain goodmetal-to-metal contact with the structure. If the structureis coated and offers high resistance to earth, an insulatedprobe bar might possibly be necessary to avoid erroneousreadings because of probe bar area relative to coated struc-ture area. A bare probe bar, in good con'act with a struc-ture, must be considered a part of that structure electri-cally. Therefore, any electrolyte contact placed close tothe bar will yield the same test results as one placed closeto the structure.

(b) Voltmeter. Structure-to-electrolyte poten-tial measurement always involves fairly high resistance inthe external circuit, because of soil contact resistance.Therefore, a high-resistance voltmeter (100,000 ohms pervolt or greater) or preferably a potentiometer or vacuum-tube voltmeter must be used. If a voltmeter is used wherecircuit resistance is an appreciable percentage of metalinternal resistance, the voltmeter reading will be less thanthe true voltage. In order to correct the reading, externalcircuit resistance must first be found. Either of the fol-lowing two methods can be used to determine external circuitresistance:1. Measure circuit resistance with a resistance-measuringinstrument.2. Read voltage on two different meter scales, then applythe formula:

RE = RM2 RMl (V2 - Vl)

RM2 Vl - N-1 V2

where

RE = External Resistance (Ohms)RM1 = Resistance of meter on lower scale (Ohms)

RM2 = Resistance of meter on higher scale (Ohms)

V1 = Lower scale meter reading (Volts)V2 = Higher scale meter reading (Volts)

After external circuit resistance has been determined, truevoltage can be found from thefollowing:

E V (RM + RE)

14

& ... . . ...... .. , ..... .. . ....k,...- o )

where

E = True Voltage (Volts)V = Voltmeter reading (Volts)RM = Meter resistance, using scale on which V was

read (Ohms)RE = External resistance (Ohms)

When electrical instruments are used in the field, severalcauses of excessive external circuit resistance should beconsidered. These may increase resistance in normally low-resistance circuits, producing unexpectedly inaccurate meterreadings.1. Test leads may be in poor condition (broken strands ofwire, terminals), or clips may be accidentally disconnected.2. Instruments and/or test lead terminals may be dirty orcorroded.3. Connection to the structure being tested may be poor(bar or probe contact, or a clip to some exposed portion ofthe structure).4. Connection to bar or probe may be poor.

For measuring potentials on long straight pipelines, it oftensaves time and requires fewer personnel to use a recordingvoltmeter (paragraph 2.2.3). The meter is located at theconnection to structure; the reference electrode is connectedto the meter positive terminal through a wire reel. With theequipment set-up, one person can move down the pipeline con-tacting the electrode to earth where measurements are desiredwhile the meter records readings. This method, then, usesthe meter to record potentials at various locations ratherthan at one location over a period of time. The recordingmeter should not be used in this manner for short lines orin congested areas.I (c) Contact to Electrolyte -

Nefo-eclc Eloctrode (Ialf-Ccll). A metal's puten-tial is not absolute and can best be measured relative to a

V reference electrode (half-cell). For convenience, a set ofStandard Potentials (EO values, see Appendix C.) has beendetermined relative to the standard hydrogen electrode whichis assigned the value of zero. The term "Standard Potential"L. ; refers only to potentials measured relative to the standardhydrogen electrode. To avoid confusion, potentials measuredrelative to other electrodes must be labeled "potential,relative to" that reference (e.g. potential relative tocopper-copper sulfate (saturated) electrode).

Potentials measured relative to one reference electrode canbe converted to potentials relative to another according tothe equation, using Table 1:

15-I-

.... .. ....... -,d .., b .

7

E2 = El + El ° - E2 ° f

where

El = measured potential, relative to referenceelectrode "1"

E' ° = Standard Potential of reference electrode "i"

E2 - potential relative to reference electrode "2"0

E2 = Standard Potential of reference electrode "2"

For example, the structure-to-electrolyte potential of a piperelative to a copper-copper sulfate electrode is found to be-0.7000 volts. All other data were measured earlier, rela-tive to a silver-silver chloride reference electrode, and itis desired to convert this value to be compatible with theothers. The Standard Potential of the copper-copper sulfateelectrode, from Table 1, is +0.3160 volts; that of thesilver-silver chloride electrode is +0.2222 volts. Substi-tuting into the above equation:

E2 = (-0.7000) + (+0.3160) - (+0.2222) = -0.6062

we find the converted potential is -0.6062 volts, relativeto silver-silver chloride.

Table 1

Standard Potentials (E°) of Commonly-UsedReference Electrode (Half-Cells)

Half Cell EIVolts,_at 250C)

Tenth Normal Calomel +0.3337Saturated Copper Sulfate +0.31'0Normal Calomel +0.2800Saturated Calomel +0.2415Silver-Silver Chloride +0.2222Hydrogen 0.0000

Various standard reference electrodes are discussed inparagraph 2.3. For field work, the copper-coppersulfate electrode is the most widely-used reference. Itis a simple instrument and very reliable when properly main-tained. It should be noted, however, when used in salt water,the cell should be kept completely filled to prevent dilu-tion. of the copper sulfate solution, which has a detrimentaleffect on the cell. The silver-silver chloride electrodeand saturated calomel electrode are also used for work in

16

i.

seawater. The lead-lead chloride electrode is sometimesused to measure potential of lead-sheathed cables. Inaddition, such things as steel bars, lead plates or ductcells, or zinc cells may be used to contact the electrolyte.Reference such as these are not standard, and resultingpotantial readings will vary greatly with conditions. Suchreferences are useful for voltage change readings, whenstandard references are not available or practical. Intheory, a steel bar used as a reference to a steel structureof identical composition will yield a zero voltage readingif soil makeup and all other conditions are identical andno corrosion is occurring. This is because no galvanic cellwould exist under these conditions. Any current read wouldtherefore be due to corrosion of the structure. In reality,these perfect conditions are never found in the field, and

ra non-zero voltage reading is encountered even with nocorrosion.

In any case, good contact between reference electrode-elec-trolyte is required; with soil, this may require pouringwater on the ground efore measurements, or varying refer-

ence electrode design to increase contact area.(2) Reference Electrode Location. Location ofI electrode relative to structure is an important considera-

tion in structure-to-electrolyte potential measurements.IMany locations are considered, both near the structure andfarther away. As was shown in Figure 6b, the electrodewill survey a distance along a bare structure equal toapproximately 3.5 times its distance from the structure.

(If the structure is well-coated, electrode position is notas critical.) Therefore, the greater the separation be-tween a bare structure and the electrode, the greater theportion of structure viewed. This is true only until acertain critical distance ("Remote Earth") is reached. Be-yond this, the reading is constant, and no more of the struc-ture can be included. This point is referred to as the"remote" location. Readings can be taken over the structure,remote, or anywhere between, depending on the informationdesired. It is important to locate other structures in thearea prior to reading structure-to-electrolyte potentialsto prevent "shielding". Pipes, tanks, anodes, or otherstructures located between test structures and electrolyte-contact distort readings.

Structure-to-electrolyte potential readings indicatewhether current is entering or leaving a structure. Analy-sis can be based on comparison of readings; both over-the-structure and farther away from it. Both readings, relativeto each other, indicate current flow to or from a structure.

As shown in Figures .7 and 8, a natural potential differ-ence exists between reference electrode and structure. Any

17

• .. .. ... ;L- ;C j " . . ' - 'I=

- 7

.e -0.5

- ~ Typical Data -0.3

12 3Test Locations

Natural Voltage Anodic Area - Cathodic Area -

between Structure Corrosion Current Returning Currentand Half-cell Reinforces Natural Opposes Natural

:1Voltage - High Voltage - Low ReadingReading

Figure 7STRUCTURE-TO-SOIL VOLTAGE - GALVANIC CONDITIONS

-0.8

-0.5

-0. Typical Data

Test Locations

Met r

Natural Voltage Discharging Pick-up ofBetween Structure Current Opposes Current Reinforcesand Half-cellI Natural Voltage - Natural Voltage -

Low Reading High Reading

Figure -8STRUCTURE-TO-SOIL VOLTAGE - EXTERNAL CUIRRENT CONDITIONS

19

currents flowing about the structure will either reinforceor oppose this potential difference (depending upon itsdirection of flow), producing a greater (more negative) orsmaller (less negative) voltage reading. Galvanic currentsaffect the voltage differently than external (stray or cath-odic protection) currents.

Under galvanic conditions (galvanic or electrolytic cells),corrosion current leaving the structure reinforces the nat-ural potential difference, producing a more negative reading(test location "2" in Figure 7. Where current returns tothe structure (test location "3"), the natural potential dif-ference is opposed; a less negative reading results.

Under conditions where external currents (stray or cathodicprotection) exist, the reference electrode acts as part ofthe structure (Figure 8). The electrode picks up currentwhere the structure picks up current (test point "3", inFigure 8); this reinforces the natural potential differ-ence producing a more negative reading. Where stray currentdischarges from a structure, current also discharges fromthe electrode, opposing the natural difference (test point"2"). A less negative reading results.

It is often advantageous to measure structure-to-soil volt-age with both over and "off-to-the-side" reference electrodeplacement (Figure 9). Each set of readings can be plottedon the same graph versus distance along structure. Result-ing peaks indicate areas of current pick-up or discharge;valleys indicate likewise. Comparison of relative positionof the two graphs determines which are which; current dis-charge is indicated when over-the-structure readings arehigher than remote ones (test points "2" and "5").

Sometimes, stray current may pick-up on one side of thestructure and discharge from the other (Figure 10).Since a side reading explores only one side of the struc-ture, remote readings on both sides, in addition to over-the-structure, may be required. As above, comparison ofreadings indicates current discharge wherever over-the-structure readings are higher than remote ones.

Frequently, structure-to-electrolyte potentials are re-quired on the internal surfaces of water storage tanks.Generally, the reference electrode is lowered into the tankthrough an opening on top of the tank. The electrode mustbe lowered well into the tank to obtain measurements of thelower surfaces. Often it is difficult to recognize whenthe electrode is touching the bottom surface, a conditionwhich must be avoided because errors in measurements canresult. One method of avoiding this is to fit a plasticcap with slots cut into it over the electrode's porous plug.

20

-_

-0.6Corrosion Indlicated-0.6\

-0.2I~ 5 Off-to-side

Test Locations Along Structure

Meter

Half-cellPlan Locations

View - ~ To Waf-cell

I At

I Elevaton

LGalvanic Conditions External Circuit

Figure 9COMPARISON OF OFF-TO-SIDE AND OVER-STRUCTURE

STRUCTUEE-TO-ELECTROLYTE READINGS

21

0

12 3Test Locations

M ter

v Reference Electrode

Figure 10STRUCTURE-TO-ELIECTROLYTE READINGS

SHOWING CURRENT PASSING THROUGH STRUCTURE

22

The plastic prevents contact between plug and tank, whileslots allow adequate electrode contact to the electrolyte.In addition, it may be necessary to weight the electrodewith a thin lead sheath around the body, covered withplastic to prevent contact between lead and tank.

b. Current. It is often desirable to learn quan-tity and direction of current flowing along a buriedstructure. Knowing this will help:

Locate active corrosion.Determine mitigation effectiveness.Determine mitigative current requirements.

As discussed in paragraph l.l.2.a, simple ammetermeasurements of current cannot be used, so other methodsare employed.

(1) Measurement. One of two methods is ganerallyused to determine current flow on a buried structure:1. Measure IR drop and use Ohm's Law.2. Use "Null Method".

In the first method, IR drop (voltage) in millivolts alonga structure is measured. The structurc iscontacted at two points, and IR drop is the voltage meas-ured between these points. This reading is then convertedto current.

Two mractical ways to convert current from millivolt (IRdrop) measurements between two points on a structure are:1. Multiply millivolt reading by the reciprocal of struc-ture resistance for length between the two contact points.Distance between contacts must be measured accurately, andstructure resistance per lineal foot is obtained fromtables. Temperature changes will affect resistance, andcompensation is difficult.2. Multiply millivolt reading by field measured "calibra-tion factor". This is the same as (1) except tables are

• not relied upon, and the section between contacts is testedunder field conditions so that temperature and other factors

are eliminated as sources of error. Permanent test leadscan be connected for this use. This factor is determinedby impressing a known current on the portion of structurebetween contacts and measuring the resultant IR drop. FromOhm's Law (V = IR), this IR drop (V) divided by the knowncurrent (1) yields resistance (R) or "calibration factor"for that section of structures.

When cable sheath, cable or pipe is being observed, arelatively large mass of metal per lineal foot is underconsideration. Therefore, small resistance and small volt-ages result when corrosion current flows. These voltagesor IR drops are most often read on a millivoltmeter andE recorded as millivolts. A millivoltmeter is generally a

23 j

,jJ j

low-resistance instrument and, therefore, reading may needcorrection before conversion to amperes (paragraph

1.2.1.a for method of correction).

The "Null Method" is another way to measure current flow.This uses a battery with adjustable resistor to buck struc-ture IR drop, a millivoltmeter to indicate balance, and anammeter (or milliammeter) to read current flow directly.This method of measurement is limited to nonfluctuatingcurrent. Equipment is connected as shown in Figure 11.With the switch open, the millivoltmeter indicates directionof current flow. The adjustable resistance "R" is set atmaximum resistance, the switch is closed, and the resistanceis decreased until the millivoltmeter reads zero. The am-meter then reads current formerly on the structure. Whenthe millivoltmeter reads zero, current between "A" and"B" is reduced to zero by a portion of battery voltage.The remaii.ing battery voltage overcomes IR drop in the bat-tery circuit from "A" to "B . As current increases or de-creases, the resistance must be decreased for increased cur-rent and increased for decreased current in order to main-tain zero IR drop between "A" and "B".

For ease of measuring current flow, IR drop test stationsare frequently installed on underground pipelinesThes.! provide oo. connections to structure, anr.1 :I cons :ltseparation jnU lucc't on for future tests.

The distance between two points, across which current ismeasured, must be great enough to produce an appreciablereading on the instrument used with significant currentflowing. In cable sheath work this is usually a minimum oftwenty feet; for larger diameter pipes, it will be in theneighborhood of fifty feet. It is important to rememberthat small currents may enter and leave the structure be-tween the two contacts. Also, it is possible that all thesignificant current is either being discharged or receivedbetween the two contacts so that total current flow is notbeing measured.

Current flow measurements are useful as coordination testswhen interference from cathodic protection is suspected.These tests show current flow at locations where cable-to-soil potentials give no indication. Also, these IR dropsgive valuable information on current flowing along down-guys and other above-grade structures.

In fluctuating stray current areas (current from machineryflowing on structure), current flow measurements are valu-able for tracing current source and locating the bestdrainage point. Correlation curves are plotted to obtain

--

. . .. . ..

Aa

I.

+ -R Batt. SWIAMO

44

AM -Ammeter -4 a Current Flowing in PipeV - VoltmeterRt a Variable Resistor 40 "o a. Impressed BatteryBatt. z Battery Current (SW ClosedSW aSwitch

Figure 1"NULL METHOD" OF CURRENT MEASUREMENT

25

--NMWMIt

the linear relationship between current flow and structure-to-soil potential at two different locations along the samestructure (paragraph 1.2.2.c). For valid correlation, atleast twenty to thirty readings per graph are necessary.They must be simultaneous. Therefore, radio communicationcan be used to advantage.

c. Internal Resistance. It is often necessary todetermine electrical resistance between two buried struc-tures, across an insulating joint (between two sections ofburied structure separated by an insulating joint), or be-tween a casing and pipe. These measurements, referred toas internal -esistance often give values less than one ohmor, when dealing wffhbare pipes and cables, sometimes lessthan one-tenth of an ohm. Therefore, measurements shouldbe taken so as not to include lead wire connection, or in-strument resistances.

Generally, two test leads are reccmmended at each locationfor ease of measuring internal resistance (two on eitherside of an insulating joint, two on the casing and two onthe pipe, or two on each buried structure). Measured cur-rent is applied through one pair and resulting voltagechange measured across the other pair. If test leads arenot available, other means of contact such as probe barsmust be used. Here, an understanding of test lead and con-tact resistance is particularly important.

For example: It is necessary to measure resistance betweena pipe line and casing. Test leads are not available at thelocation; bars are used. One bar is pushed through earth tocontact the casing, and another, to contact the pipe. Abattery and ammeter are connected between the two bars sothat when an ammeter terminal is touched with the loose testlead, current flows from the battery through the ammeter tothe probe bar. It then flows to the pipe, and from the pipeto the soil or point of contact between casing and carrierpipe. It next enters the casing, travels along the otherprobe bar, the lead wire, and back to the battery (Figure

12a). Flowing current encounters resistance all along itscircuit. Of interest is the current's behavior as it flowsfrom casing to pipeline. The pipe may or may not be inelectrical contact with the casing. If it is, there is verylittle resistance for the current to overcome, because metalsurfaces touching are a low-resistance path (low voltage re-quired for high current flow).

If the pipe is not touching the inside of the casing and theannular space is clean and dry, it will be necessary for thetest battery current to leave the casing outer surface andflow into earth. It passes through soil until reaching thepipe surface where it enters through "holidays" in the coat-ing. Less battery current flows along the path, and morevoltage is required to move that which does. Resistance ishigher here than in the previous case.

26

..

ThseCotatsInluedinReisane Ms-

a)tAry CNATRSSTNEICE

T7

- NtehAro sinie fonts cureRet ac Mesj

C) ASINGTCTIRP RESISTANCE MAUREMEN

AMIBatter

5v

A 0M\27

b) BR COTACTRESSTANE NO INCUDENoe roindct lwn etcr.4ur 1

If no current passes through the above-mentioned circuitwhen the ammeter terminal is touched, a bad connection some-where in the test set-up is probably the cause. It is mostlikely that probe bars are not making good electrical con-tact with casing and/or pipe. This is fairly simple to cor-rect. However, another inaccuracy still exists, becausethe voltmeter leads are connected to the wrong place in thecircuit. These wires contact the two probe bars. Test cur-rent from the battery also passes through these bars on itsway into the casing and out the carrier pipe. Because ofthis, probe bar-to-pipe contact resistances (of both thecasing and carrier pipes) are included in the circuit be-tween the two test leads connected to the voltmeter. Sub-stantial resistance is often encountered in probe bar-to-pipe contact and will be part of total computed pipe-to-casing resistance. This added resistance can greatly alterthe voltage reading and, therefore, change the resistance.For example, a reasonable pipe-to-probe bar point resist-ance is 0.25 ohms, so that the two bars together can add0.50 ohms. With well-coated lines and casing-to-pipe resis-tances of two or more ohms, this additional value does nothave as much effect as with poorly-coated or bare structuresand casing-to-pipe resistances of only a few tenths of anohm. Here, total resistance is often more than doubled.

Probe bar contact resistance can be excluded from resistancemeasurement by using four probe bars. The voltmeter testleads are removed from the two contact bars, two additionalprobes are installed (one contacting the pipe, the othercontacting casing, as shown in Figure 12b), and these areused only for voltmeter connections. When measured testcurrent is passed between casing and pipe, true change inpipe-to-casing voltage is found. Contact resistance throughwhich test current flows is no longer included between thetwo voltmeter leads. Therefore, it is no longer included intotal computed pipe-to-casing resistance. This calculatedresistance is now the correct pipe-to-casing value.

If one test lead were available on each of carrier pipe andcasing, contact resistance would probably be less, since abrazed or soldered connection has replaced the probe barpoint. However, the possibility of a high-resistant con-nection always exists and, when measured resistance is lessthan one ohm, any contact can easily add appreciable error.So it is always desirable to use two connections on eachstructure when determining resistance.

These same considerations are valid for measuring any inter-nal resistance as between two buried structures or acrossan insulating Joint. In these cases, too, the result willoften also be less than one ohm and, with bare pipes andcables, sometimes less than one-tenth of an ohm. Two

28-, j

..- .f

contacts on each structure, or two contacts on each side ofthe insulating joint would be used. Measured current wouldagain be applied through one pair, and resulting voltagechange measured across the other pair.

In these resistance determinations one is concerned withchange in voltage produced by current supplied from thetest battery. That is, voltage is read with and withouttest current flowing. The difference between these tworeadings is the voltage change between pipe and casing, be-tween two buried structures, or across an insulating jointproduced by mea3ured test current. This change, divided bytest current, gives resistance.

Resistance between pipe line and casing, between two buriedstructures, or across an insulating joint measures opposi-tion encountered by electrical current leaving the surfaceof one structure, passing through earth or water, and enter-ing the surface of another. The greatest percentage of thisresistance is found at structure-to-electrolyte interface.Even if structures are bare, deposits built up on these sur-faces are often poor'conductors and act as thin coatings,increasing resistance between structure and soil.

d. Resistivity of Environment. Electrolyte resistiv-ity plays a big part in corrosion rate. However, no single

* test is an absolute determination of corrosivity. Varia-tions in electrolyte resistivity are often critical. Fordesign of cathodic protection systems, a knowledge of resis-tivity values in a given area is valuable in locating ground-beds and providing information on current distribution. Re-

4 sistivity data are also valuable in design of electricalgrounding systems. Resistivity data combined with otherfactors can indicate a potentially corrosive situation. Ingeneral, low-resistivity areas are more conducive to corros-ion than high-resistivity areas. However, low-resistivityareas are advantageously used in design of groundbeds orgalvanic anode placement for cathodic protection systems.

Common methods of measuring resistivity are given below.The voltage drop principle is used to determine soil andwater resistivity.

(1) Wenner Four-Pin Method. In this method, soilresistivity is determined by passing a known amount of cur-rent through soil and determining the voltage change pro-duced. Four equally spaced rods in straight line configura-tion are pushed into the earth (Figure 13). Current ispassed between the two outer electrodes, and the resultingvoltage drop is measured between the two inner electrodes.Assuming that the depth to which pins penetrate soil issmall with respect to the spacing between electrodes (whichis usually the case), average soil resistivity measured toa depth of "A" is given in ohm-centimeters by the followingformula:

29

C .. . .

Ai

Soil Resistivity(in Ohm-centimeters)

Change in Voltage • Test Current x A (in Feet) x 191 .::Due to Test Current "

Voltmeter

Soil Being Tested is BetweenThese Two Rods to a Depth~~Equal to "A"

" Ammeter1 'Arrows Indicate FlowingTest Current

Battery (Source of Test Current)

Note: Soil resistivity measurement instrumenh usually include a power sourceand read directly in ohms.

Figure. 13

SOIL RESISTIVITY MEASLREMENT

30

7e

r

17 2-AR

From U.S. Bureau of Standards Bulletin No. 258 entitled, ."A Method of Measuring Earth Resistivity", by Dr. F. Wenner. p

where --

' = average resistivity of soil, ohm-centimetersA = distance between electrodes, centimetersR = resistance-between the two potential elec- I

trodes, ohms.

Substituting simplifying relations:

SR = E/I21YA = 191.5 D

where

E = measured voltage, voltsI = current, amperesD = distance between electrodes in feet,

so that

=191.5 AE/I

By increasing spacing (A), average soil resistivity can bemeasured to any depth. In practice, soil resistivity meas-urements usually exceed structure depth by several feet inorder to establish general soil conditions. The depth towhich pins penetrate soil need be only a few inches so long

as mistsoil is contacted. For accuracy of measurement,pin depth must be small compared to pin separation.

L@Soil included between the two inner rods to a depth equal totheir spacing is that which is actually measured. Anyburied structures in this region will affect results, andcare should be taken to keep rods clear of them. The twoinside (voltage) rods or electrodes should be kept away fromburied structures to a distance of at least one-half the rodspacing. If buried structures are located between theserods, they are included in the measured voltage drop andwill lower the computed value of soil resistance. It isgood practice to place the electrodes in a straight lineperpendicular to buried structures such as pipelines or~cables.

The 4-pin soil resistivity measurement is especially accur-ate because it eliminates soil-to-probe contact resistance.Two-probe methods of measurement, although simpler and oftenuseful, are less accurate because contact resistance is notconsidered.

31. .... I

(a) Specialized Instruments. In addition to thedirect current method using independent D.C. source andvoltmeter (illustrated in Figure 13), specialized instru-ments are available for resistivity measurements. Thesesupply either alternating current or pulsed direct current,and are self-contained 4-terminal instruments. The methodof measurement, illustrated in Figure 14, is basicallythe same as that described in paragraph 1.2.1.(d)(l).

Two terminals supply alternating or pulsed direct currentfor test purposes. The other two terminals are used tomeasure IR drop in soil. The current terminals are con-nected to the two outer pins, Cl and C2, and the potentialterminals are connected to the two inner pins, P1 and P2 .The instrument (paragraph 2.2.8) is actuated by pressinga switch button which causes current to flow through thesoil. The galvanometer is adjusted to read zero by meansof a potentiometer calibrated to read directly in ohms.Therefore, it is necessary only to multiply the instrumentreading by 191.5 times the pin spacing in feet to obtainresistivity in ohm-centimeters.

These instruments are particularly useful in stray currentareas. Alternating or pulsed direct current minimizeeffects of stray current on resistivity readings.

(2) Two-Probe Method. This method employs two soilcontacts spaced a few inches apart with an instrument thatindicates average soil resistivity directly in ohm-centi-meters, but only to a depth of approximately 8 inches belowthe steel probe points. This two-probe measuring device iscommonly referred to as "Shepard Canes" (named for theinventor). The probes can be used in inaccessible placesor where a large number of measurements are to be made in ashort time on the surface of the earth (as in existing ex-cavations). The measurements can be made only in soilswhich will allow penetration of probes (approximately 1/2inch in diameter). Where firm earth which will not allowpenetration of probes is encountered, small holes may bedrilled to the desired depth, and the probes then inserted.For meter arrangement and probes, see Figure 15.

Inaccuracies are encountered with this method because volt-age is measured across the same contact resistance throughwhich applied current passes. As a result, probe contactresistance is included as a portion of total measured resis-tance. Therefore, readings will be larger than actualresistance of the soil under test. Also, since the contactsare so close together, a soil sample truly representative ofthe particular area is difficult to obtain. However, thistest method is useful for field measurement of water, resis-tance and soil which may come in direct contact with aburied structure (in a ditch-bottom, etc.).

32

I J

C1 C2 Conveer

!. . ~ ~~P CP2 .'' ' '

SOIL RESISTIVITY: FOLR-PIN ALTERNATING CURRENT METHOD

(3) Soil-Rod Method. This method is similar tothe two-probe method, except both terminals are located inthe same rod (Figure 16). One terminal is the smallmetal tip, insulated from the rest of the rod; the other,the rod's main shank. This is driven into the ground tothe desired depth, and resistance of soil at the probe tipis measured with a resistance-measuring device. This mayread resistivity directly in ohm-centimeters or may read inohms and require multiplication by a conversion factor.The instrument uses an AC bridge or other AC measuring cir-cuit to eliminate polarization and stray current effects.

As with other two-terminal devices, contact resistance canintroduce inaccuracies. Also, only a small volume of soilis measured. Therefore, it is difficult to obtain goodvalues for average soil resistivity with this instrument.

e. pH of Environment. Acidity or alkalinity off. an electrolyte can affect its corrosivity.

pH is an indication of these quantities and is often meas-ured in field or laboratory to help determine corrosivity.

One method, which can be used at the site, employs an anti-mony half-cell. This metal's half-cell potential varieswith pH of the electrolyte it contacts. Therefore, measure-ments of potential between antimony half-cell and referenceelectrode (usually copper-copper sulfate) can be convertedto pH (Figure 17).

33

J

-Id

instrument

Recad Directly in

Ohm-Cm.

1 Aug, red Holes

Figure 15

SOIL RESISTIVITY: TWO-PROBE METHOD

Currents flowing in the earth (from cathodic protectionsystems or stray currents) interfere with true measurementof pH in this manner. Such areas should be avoided. Also,the two electrodes should be placed close together to ob-tain best results. Care should be taken with antimonyelectrodes, because this metal shatters easily. In addition,if copper sulfate contacts antimony, copper will plate outon the metal surface, interfering with accuracy.

Other methods of measuring pH commonly used in the labora-tory, such as sensitized paper or electrometric analysis,may be employed in the field. These methods are discussedunder "Laboratory Tests", paragraph 1.3.l.a.

f. Redox Potential. Redox potential (oxidation-reduction potential) measurements in an electrolyte may beused to give an inication of anaerobic bacterial corrosion.This test is still somewhat experimental and presently notin common use. Potential is measured to an inert metal sur-face such as platinum, using a calomel half-cell as refer-ence electrode. pH values are taken in conjunction withredox potential. This test does not observe or countbacteria; it merely indicates whether or not the electrolytetested is favorable to development of anaerobic bacteria(Sporovibris desulfuricans). These bacteria reduce sulfate

34

A hN, V

Cnect Terminos toAC Resiknce Measuring dle

Terrminal, connected by wire (inside shankc)~to metl Itip

Terminal, 1/2" Round Steel Shankconnected to,,.1 tip-

Metal tipinsuloted

1. .Figure 16

TYPICAL SINGLE-ROSE RESISTIVITY MEASIRING DEVICE

_.,I

.,,,.. 3--L '•

700-

i t

6400-

.--%00~~

1 - - -

0>300 .10

PHFigur 17

VOLTAGE VS. pH FOR ANTIMCNY COMP-COPPU SUFATE CELL

36

"- 2 A'

/1

compounds where little or no oxygen is present. They aremost often found in environments of near neutral pH (5.5 to8.5) and low redox potentials (below 100 to 200 millivolts).

For measuring these quantities, a soil probe has been devel-4 oped. It uses glass and calomel electrodes to determine pH

and platinum electrodes for measuring redox potentials.These instruments, however, are expensive and fragile. Gen-erally, simple laboratory measurements of pH and sulfide

k, content (paragraph 1.3.l.b) are used to detect bacteria.

g. Testing Rectifier Pffiriency. Rectifiers are themost commonly - used power source for impressed currentcatbo ic oroteCtion. One type rectifier, the most common onecontains selenium stacks. These stacks age, causing a lossof conversion efficiency from AC to DC power. Seleniumrectifier efficiency should be tested regularly, usuallyonce or twice a year. A decrease in efficiency of 20% orgreater generally indicates stack replacement is needed.

Rectifier efficiency is calculated by determining the ACinput power and the DC output power, and using the relation:

efficiency (%) = DC owerAC werxI0

AC input power is found with a watt-hour meter, substitutinginto the equation:

AC power = 3600 KNT

where

K = meter constantN = number of disc revoltuionsT - time (seconds)

The time interval for testing should be at least sixty sec-onds, for an accurate measurement.

DC power equals DC voltage in volts multiplied by DC currentin amperes. These values may be obtained directly from the jrectifier meter(s). If meter accuracy is doubtful, theymay be checked using the rectifier shunt.

1.2.2 Surveys. Prior to a corrosion survey, thecorrosion englneer mst decide which measurements will bestdescribe conditions on the buried or submerged structure.

He may determine this from his own past experience, from ex-perience of others operating in the same area, or fromavailable leak or failure history of the structure. A cur-sory check, prior to more detailed study, can often givesom Idea of conditions such as galvanic corrosion cells,

37

'II

stray current Lind cathodic interference, or bacterialactivity. The projected survey can then be tailored to in-clude proper measurements.

After survey data have been studied and protective or miti--ative measures designed and installed, further surveys arenecessary to determine effectiveness-i of work done. Meas-urements included in this 'ield work depend upon what typeinstallations have been emoloyed.

Recording data accurately and neatly is an important partof any survey. Data should be entered in a permanent recordarranged systematically for easy reference. A structurehistory, required by law, reveals changing conditions (soilmakeup, coating deterioration, etc.) that may arfect corros-ion behavior.

A checklist for underground corrosion surveys for both new(or under construction" structures and existing structuresis given in Appendix D

a. Current Requirements. Current requirement testsare run to determine amount of current required for cathod-ic protection, need for dielectric Insulation, spread ofprotection, feasibility of using magnesium anodes or impres-sed current groundbeds, and effects of protective current onother structures.

Basically, test current is impressed through a temporarygroundbed and the effect on structure-to-soil voltagesnoted. This test is useful in the design of both impressedcurrent and galvanic cathodic protection. Current must beimpressed at the point where the rectifier or galvanicanode would be permanently located. A portable rectifier,generator, battery, or other D.C. current source and ammeter,is used to supply known current through a temporary ground-bed (Figure 18). The temporary groundbed can be any handypiece of metal - a fence post, section of conduit, etc. -

driven into the ground (or water). The metal must not con-tact the structure directly, and it cannot be receiving anycathodic protection itself. Also, its configuration andsize should be roughly that of the anticipated permanentinstallation. Structure-to-electrolyte potentials are takenat various test points along the structure. It is not nec-Sar-' to impress sufficient current to bring the structure

to the desired level of protection, although test-currentshould approximate actual conditions to avoid distortedreadings. Results relate a unit of test current and theresultant structure-to-soil voltage. This is expressed interms of millivolts change per ampere and is called a"coupling" or "electrical constant". Required current canthen be calculated for the millivolt change desired. Re-o!ilts may indicate a change in point of current impression

38.4

IT

A/

t DC Current Source

v ReferenceGro Electrode

Structure

Groundbed

Figure 18CURRENT REQUIREMENT MEASUREMENT

(impressed current or galvanic anode location). Figure 19 k'is an example of typical couplings for two anode locations f;

(Ia and Ib). Test locations are for five structure-to-elec-trolyte potentials (Vl, V2 V3, V4, V6) and a foreign-line-crossing drainage bond (V5). Resulting couplings indicatevoltage changes at Locations 1 through 6 for a given currentimpressed at anode locations a and b. Each coupling is themillivolt per ampere change, based on one anode location andone test location. For example, AVI/AIa is the coupling(or constant) for potential readings at location Vl and anodelocation Ia . From these, a couplings and knowledge of therequired 6V at one point, the requireddI is determined byratio and proportion. This I is then used to calculate re-sulting voltage changes at other test points, using theouplings for these points.

Current rcquirement tests provide only an approximation ofveal current needs. Many factors affect true requirements:polarization effects, groundbed resista-ce, anode locations,and others.

b. Interference Tests. Cathodic protection sometimescauses undcesirable effects on structures not connected toth, protection system. This occurs because some current ispicked-up by these "foreign" structures at one point and, inattempting to return to the source, is discharged at anotherpoint. Corrosion occurs at the discharge point. Currentcan cause corrosion damage by passing between two structuresor across high-resistance joints in the same structure.Railway signal systems can become inoperative if sufficientcathodic protection current flows on rails. Tnese phenomenaare called cathodic interference, discussed in detail inAFM 85-5.

Any cathodic protection system can cause cathodic interfer-ence. Impressed current systems are studied and discussedmore often because they usually involve more current. Geo-metric location of anodes with respect to foreign structuresIs the most important single factor to consider. Bonds areoften used to eliminate areas of current discharge. Jointlyoperated systems are also employed; here two or more com-panies interconnect their underground structures and jointlyapply cathodic protection.

Testing requires cooperation by owners of structures involved.Such cooperation is best effected by a corrosion coordina-ting committee; all companies operating underground or under-water structures, and particularly those undo,. .athodioprotection, should be members of such a committee. Alist of most existing committees may be obtained from the

40

r~t

Sllb

VV

- 60 __ 1l

Couplings: Vl,1 ... AV6

AVi,AV2 ... AV6

Figure 19COUPLINGS DETERMINED IN CURRENT REQUIREMENT TESTS

/ 2

A%

/f - I-

National Association of Corrosion Engineers, Houston, Texas.Tests must be made on all structures adjacent to a cathodicprotection system to determine effects and design mitigation.Cathodic interference can be detected by measuring structure-

to-soil potentials and current flow (IR drop) with cathodicprotection on and off. (Changes are noted.) Structure-to-soil potentials will give indications only when measured inan area of current discharge or pick-up. Current flow (IRdrop) readings will indicate in between pick-up and dis-charge areas where structure-to-soil potential would shownothing. Figure 20 illustrates

Pipe-to-soil potential and current flow are measured atmany locations before the cathodic protection system isturned on. Then measurements are repeated at exactly thesame points with the system operating. The algebraic dif-ference between "on" and "off" readings gives the effect ofcathodic protection. The location showing greatest pipe-to-soil potential change in the positive direction is calledthe "critical" or "control" point. This is often at thepoint where protected and unprotected pipes cross. Mitiga-tion can usually be accomplished by installing a bond be-tween the unprotected structure (at location of natural dis-charge) and protected structure (as close as possible to thenegative connection). Method of calculating bond resistance(to obtain proper current drainage) is covered in AFM 85-5.

Another method of interference testing uses an experimentaldrainage bond between structures. Proper drainage can bedetermined by trial-and-error using a variable resistor toalter drainage current. Required drainage current can alsobe computed from test data. The form shown in Figure 21is useful for this.

When conducting current requirement tests and/or initiallyenergizing impressed current cathodic protection, all com-panies owning underground structures in the area should benotified and coordination tests made with those interested.Current drainage requirements for each structure, from testsat various anode locations, can be determined.

c. Fluctuating Stray Current Survey. While interfer-ence testing determines effects of steady stray currents,another type stray current survey analyzes fluctuatingstray currents. Such analysis is specialized and requiresstudy to master all tc.-.iques.

All the basic measurements can be used in studying fluctua-ting stray currents. Perhaps the most informative are meas-ur.ements similar to those used in interference testing:structure-to-electrolyte potentials and IR drops along

All

-.. . -- -"..-/--

Pipe-to-Soil Voltage (V) Indicates Pick-up andDischarge, IR Drop Indicates Current Flow

V V

Mv

r TIIII-ll

Current Pick-up Current DischargeFigure 20

STRAY CURRENT FIELD MEASUREMENTI

structure or electrolyte. Methods of analysis, however, aredifferent and specialized equipment is used. The major con-cern in performing a stray current survey is to ascertain* degree of damage and determine source of currents.

Fluctuating or periodic changes in structure-to-electrolytevoltage values and unusual or fluctuating currents are in-dicative of stray currents. Stray currents may affectstructures just as cathodic protection does. Structuresmay be protected or damaged by stray currents dependingupon whether current is flowing to or from the structure.

In order to determine pick-up and discharge points in fluct-uating stray current areas, correlation curves are plotted(Figure .22). Values of structure-to-electrolyte potentialare plotted against current flow along the structure (calcu-lated from IR drop), (graphs "A" through "E", Figure 22).In addition, current flow at different locations on the samestructures are calculated (graphs "A-C" and "C-E"). Whereforeign structures are nearby, structure-to-electrolyte

.- 3

INTERFERENCE TESTS

Structure

Date

Rectifier Location Rectifier Output Volts Amps

Tests for Determination of Exposure:

Location of Tests

A. Structure to Cu-CuSO4 - Rectifier On (volts)

B. Structure to Cu-CuS04 - Rectifier Off (voltsTC. Voltage between Structures - Rectifier On (olts)D. Voltage between Structures - Rectifier Off (voltsY

Tests for Determination of Required Drainage Current:(Using experimental drainage bond between structures)

Location of Experimental Test Drain _

E. Current drainage through test drain with rectifieroperating (amperes)

F. Structure to Cu-CuSO4 with test drain connected andrectifier operating (volts)

G. Voltage between structures with test drain connectedand rectifier operating (volts)

H. Voltage between structures with test drain connectedand rectifier not operating (volts)

I. Volts reduced (F-A)J. Volts Reduction per ampere (IT -

K. REQUIRED DRAINAGE CURRENT in Amperes _____

Figure 21

RECORD SHEET FOR INTERFERENCE TESTS

44

00

A V ByV C V

D V E V

A B C D E

rForeignLi ne

F V A-C ]A C-EIC

______E IC I

V = Structure-to-Soil PotentialI=Current Flow Along Structure

E =Voltage Between Structures- :Current Flow

Fi gure 22CORRELATION CURVES

45

potentials vs. voltage between structures are related(graph "F"). For each graph, at least 20 to 30 readingsare required to provide good correlation. Radio communica-tions may be required. because readings must be simultaneous.So-called "X-Y recorders" can also be used to advantage;these record and plot readings simultaneously. They arealso helpful when measurements are desired for long periodsof time (say 24 hours). The most effective way to use a re-cording voltmeter ("X-Y recorder") is to select a locationwhere corrosion has been experienced or an area where straycurrents are suspected. Meter range selected should be highenough to assure that voltages to be recorded will remain onthe 3hart and yet be sensitive enough to indicate small volt-age variations. The meter should be sheltered from weatherand located where it will be free from shock and excessivevibrations. Reliable connections are important and shouldnot be tampered with during recording. Before recording isbe,-un, the meter should be checked for accuracy by compar-ing readings with another voltmeter, the indicator armshould be adjusted to zero on the chart, and the date shouldbe recorded on the chart. The meter should be turned on tomake sure it is recording properly. No special attention isrequired other than an occasional check to assure the instru-merit is performing satisfactorily. Charts for recordingvoltmeters should show the voltage tap used, dates installedand removed, location of test, and any other pertinent data.

d. Continuity. Effective cathodic protection andcont-:'ol of stray or interference currents in an undergroundstructure requires that the structure be metallically con-tinuous to provide a low-resistance path for current to flow.It is often necessary, therefore, to locate and bond noncon-ducting or high-resistance joints before installing cathodicprotection. Such joints are located relatively simply, butthe procedure is time-consuming.

(1) Direct Current Method. Direct current voltageis applied to the structure or portion of structure in ques-tion causing current to flow in a given direction betweentwo fixed points. A series of voltage drop measurementsare taken along the structure to detect any abnormal voltagechanges. Where only one insulating joint exists betweenpoints of current application, a pronounced voltage changewill be observed. Where several high-resistance connectionsexist, the effect will be less pronounced but usually detect-able. Should there be no such joints, a nearly uniform po-tential drop between the two points of current applicationwill result (assuming the structure to be of uniform size).

A simple, but not positive method of detecting discontinu-ities in a pipe not uniform in size is by measuring struc-ture-to-electrolyte potential directly above the structureat equal intervals. If soil resistivity and depth of burialare uniform, the structure-to-electrolyte potential willvary uniformly from one end of the section under test to the

..... . .... .... . . ... ,

-'a

other end, except where there are variations in structureconductance. Lack of uniform readings indicates poor con-tinuity or insulating joints. A plot of readings can behelpful, because variations in slopc are readily detectable.

If soil resistivity and burial depth are not uniform, astudy of IR drop in the structure will give better results.One terminal is connected to a fixed point on the line andthe other, successively to equal distances along the linewith a contact bar (Figure 23). Again a plot of readingsshould be made so that irregularities in the curve can beobserved and studied. The IR drop method, while slow and

tedious, is almost completely independent of outside factorsand generally locates all high-resistance and insulatingjoints with reasonable accuracy. Sometimes a detailed studyis required to fully establish location and number of dis-continuities.

(2) Alternating Current Method. An alternatingcurrent method of testing insulating joints aboveground hasproven quite reli, ble. This method consists of Impressingan alternating current voltage across an insulating Jointand detecting current flow using a clamp-on device. If theinsulating joint is defective, current will flow, and areading will be noted on a detector meter. If the insula-ting joint is good, little reading will be noted on the de-tector meter. The test set consists of a battery which acti-vates a vibrator to convert direct current to alternatingcurrent for test purposes. Voltage is stepped up through atransformer and connected to the structure by means of twoclamps. A split core armature with wound coil on one endworks on the induction principle to detect any current flow-ing on the structure. The output of the detector coil is

V connected to a vacuum tube voltmeter or a similar indicatingdevice. This type instrument has high-inpuu impedance, onthe order of several million ohms, which requireb veiy littleenergy for operation. The instrument is capable of detect-ing leaky as well as defective insulating Joints.

e. Coating Conductance. Coating conductance (leakageconductance) of a pipe line is a measure of coating integ-rity; the greater the conductance, the poorer the coating.Conductance is inverse of resistance and is usually expres-sed in micromhos per foot. For comparisons on an area basis,the most common unit is micromhos per square foot. An ac-cepted procedure for making coating leakage conductance sur-veys has been established by the National Association ofCorrosion Engineers in their Procedure: NACE 2D157. Thismethod, summarized here, consists of impressing direct cur-rent through the coating, measuring voltage drop produced,and calculating conductivity from Ohm's Law and the relation-ship between conductivity and resistance.

-I.

Battery Voltmeter

G rode

FixedContact Bar

'%Pipe Insulating JointoMovable Contact Bar

IFDistance

Figure 23LOCATING HIGH-RESISTANCE JOINTS IN UNDERGROUND PIPING

A temporary groundbed and D.C. voltage supply are connectedto the pipe to be measured; an ammeter for measuring cur-rent is included in the circuit. The groundbed should belocated 400 feet from the beginning of the pipe sectionunder test. A test current of 2-10 amperes is required.This current is interrupted, and the change in current (AI,in amperes) is noted. The corresponding change in pipe-to-soil potential to remote earth is determined at variouspoints and averaged (AE, in volts).

Average leakage cor.ductance (k in micromhos per foot) isgiven by k = AI x 106 where L is the length in feet of theJELsection under test. The section between test stations shallhave no electrically connected laterals. Lines to be testedare classified as long or short and different techniques are

* required for each.

Careful interpretation of field data is very important.Coating conductance is affected by:

a) Physical properties and condition of coatingb Conductivity of surrounding soilc Contact resistance between pipe and soild) Casings.

() Short Line Method. A short line is defined forthis discussion as an electrically isolated section of line.Test procedure is shown schematically in Figure 24. Ifthe pipe is not of uniform nominal size and weight, it shouldbe treated as a long line and measured in sections. If thisis not possible, the various pipe sizes and locations shouldbe noted in the results.

To determine coatin leakage conductance, equipment is set-up as in Figure 2. The direct current source is turnedon and current measured with the ammeter in the anode cir-cuit; remote pipe-to-soil potentials are noted at the twotest locations. Test current is interrupted, and the changein current (Ai) and pipe-to-soil voltages (4E1 andAE2 ) aredetermined.

If the ratio AE 1/nE 2 is greater than 1.6, multiplyoE2 bythe appropriate factor from Table 2. The factor can beapplied to ratios less than 1.6, but the error without thefactor is less than 10%. If the ratioaEI/AE2 exceeds 3.0,measure the line in sections by the long line method. Aver-age coating leakage for the section tested is found from:

k = 4 x 106 (micromhos per foot)AE aveL-

where AE ave. = (AE1 I +E 2 )2

19BN-

A ~ -.

4g* Temporary Groundbed

Ammeter

A A

ReferenceDirect ElectrodeCurrent'--,h-Source ----

Test Point Test PointNo. 1 No. 2

H LBuried Pipe

Figure 24COATING CONDUCTANCE MEASUREMENT: SHORT LINE

Table 2

Correction Factor for Coating Conductance Calculations

E l/E2 Factor AE116E2 Factor

1.0 1.000 2.1 1.3451.1 1.032 2.2 1.3751.2 1.o66 2.3 1.4o41.3 1.099 2.4 1.4331.4 1.130 2.5 1.4611.5 1.163 2.6 1.4921.6 1.193 2.7 1.5201.7 1.224 2.8 1.5481.8 1.254 2.9 1.5761.9 1.286 3.0 1.6042.0 1.315

Am

50

(2) Long Line Method. When conditions for theshort line method are not met, the line must be measuredin sections as shown in Figure 25. Each section shouldbe of uniform nominal pipe size and weight. There is noset length for these sections, but the following is a guide:

(a) Maximum length is that at which the ratioAEI/AE 2 equals 3.

(b) Minimum length is that at which the differ-ence between AI 1 and W12 is 5 ma.

Test current is applied as for the Short Line method, withthe temporary groundbed 400 feet away from the test section.If the starting point is not at an insulating joint, thisseparation may be obtained by distance along the line. Theset-up is as shown in Figure 25. The direct currentsource is turned on and current is measured at the two testpoints by the IR drop method. Contact to pipe should bemade with leads attached to the pipe rather than with probebars. If the near end of the section terminates in an insu-lated joint or dead.end, current may be measured with anammeter in the anode circuit. Remote pipe-to-soil poten-tials are noted at the two test points also. The test cur-rent is then interrupted and the change in current at thenear and far points (AIl and A12) and the change in pipe-to-soil potentials at these points (AEl and AE 2 ) are determined.If the ratio AE1/4E2 is greater than 1.6, E2 should becorrected as described in paragraph 6.1.2.2.e.(l). If theratio AEl/AE2 exceeds 3.0, the test section should beshortened. Coating leakage conductance is calculated fromthe equation:

k =(/Il - A12) x 106

AE ave.L

where

AEave. = (AE1 .+AE 2 )

2

f. Duct Slug Survey. A duct survey can be made bypulling a lead slug (reference electrode) through a vacantduct adjacent to ducts carrying cable. Instruments are con-nected between this electrode and the lead sheath of theadjacent cables as shown in Figure 26. Measurements ofpotential, current, and resistance are made at 10-foot inter-vals along a cable run between two adjacent manholes. Theduct selected for the survey should be as near to the cables

as possible and preferably one of the lower ducts so as to

obtain the most severe conditions. The test electrode gen-erally consists of a piece of lead-sheath cable approxi-mately 12 inches long, fitted with two pulling eyes and testlead. The electrode must be as simple and rugged as possi-ble. A small galvanic potential may exist between cable j

51

..... b..o.A, -

.Temporary Groundbed

Ammeter

A ReferenceDi rect ElectrodeCurrent/Source

E E2 Test PointNo. 2

ITest Point '

No. 1 MillivoltmeterFigure 25

COATING CONDUCTANCE MEASUREMENT: LONG LINE

sheaths and test electrode, although the material in bothis the same. This difference is usually negligible (0.01volt or less) and does not seriously affect results. Poten-tial between cable sheath and lead electrode is measured bythe voltmeter connection shown in Figure 26. A potentio-meter-voltmeter should be used so as to minimize errors dueto electrode contact resistance.

The measurement is a close approximation of cable sheathpotential with respect to electrolyte from point to pointalong the duct run. Cabl, sheath corrosion Is proportionalto magnitude and density of current discharge from cablesheath tc electrolyte. This current can be estimated frompoint to point through the duct by connecting referenceelectrode to cable sheath through a resistance milliammeter.Milliammeter circuit resistance is relatively low, potentialwill be substantially that of the cable sheath. It may beassumed that current discharge per unit cable area at thetest point is approximately the same as that measuredthrough the electrode.

Current flowing from the cable sheath to slug is indicativeof corrosion. Duct resistance is an essential factor indetermining magnitude of current flow for any given poten-tial. Resistance between the test electrode as one terminaland the adjacent cable sheath as the other is measured at

52

j .. . ... ..

MilliammeterT

Voltmeter anolianole'er

Rubber Covered Conductor

Triing Rope et Pling RopeSott rib Electrode (Leau Slug)

Lead Sheath Cable

_j Ist Elctrode Manhole

Figure 26ELECTRICAL CONNECTIONS FOR DUCT SLUG TEST

each test point by connecting a low-voltage battery (1.5volts dry cell) in series with a sensitive millianmmeter(between cable sheath and test electrode). Resistance iscomputed from battery voltage and measured current. Themilliammeter used for this purpose should have a sensitiv-ity of approximately 0.05 milliampere per division, and itsresistance should not exceed 2 to 3 ohms. With suitableshunt resistance, the milliammeter range can be varied toany desired value. The same instrument can be used for bothcurrent and resistance measurements through switches, ar-ranged as in Figure 19. Duct resistance as measured ismostly contact resistance between the surface electrode con-tacting the duct wall. It represonts approximately the con-tact resistance per duct foot of cable when the duct isoccupied.

1.3 Laboratory Tests. In addition to field tests,laboratory tests can yield information helpful to the cor-rosion engineer. Laboratory tests are run on electrolyteor electrode samples, collected in the field and sent to

7-I53

the laboratory for investigation. Reasons for using labora-tory tests instead of field tests, which often supply thesame information, may be convenience or necessity. Manylaboratory tests, such as most metallur iical analyses, re-quire large and complicated equipment impossible or imprac-tical to bring to the site.

1.3.1 Field-Related Tests. Some laboratory tests,particularly those run on soils, provide information similarto field tests. Such things as soil resistivity and pH, al-ready discussed under field testing, can also be determinedin the laboratory. Many of these soil or water tests usesimple equipment which could easily be used at the site, aswell as in the laboratory. Some Important tests of thistype are described here.

a. pH of Environment. Common tests for pH determina-tion for small samples of soil or water are based on coloro-me ric or electrometric analysis. In either method, samplesof soil are analyzed by first mixing one tablespoon of soilin 15 milliliters of distilled water and filtering the re-sulting mixture. The pH of the resulting filtrate equalsthat of the soil sample.

In the colorometric method, the water sample or soil fil-trate reacts with sensitized paper or liquid indicators,producing a color characteristic of the pH. Various indi-cators are available for different pH ranges and accuracies.

The electrometric method uses a pH meter which measuresvoltage across two electrodes contacting the water sample orsoil filtrate. Typically a glass or antimony pH electrodeand a calomel or silver-silver chloride reference electrodeare used, often combined in a single assembly. The voltagemeasurement varies with solution pH, and the meter readsdirectly in units of pH. This is generally faster thancolor tests, but requires more expensive equipment.

b. Sulfide Content (Anaerobic Bacteria). A simpletest for detecting possible presence of anaerobic bacteriais to apply a dilute solution of hydrochloric acid (15%) tocorrosion product or to a soil sample near the structure.The familiar odor of hydrogen sulfide (smell of rotten eggs)can be detected if su3?ides are present. Sulfides are gen-erally found in soils only where anaerobic (sulfate-reducing)bacteria are present. Sulfides are a by-product of the lifeprocess of these bacteria.

c. Chloride Content. The Mohr method, based on dif-ferential precipitation of the chloride and chromate ionsof silver, is used to determine chloride content of water.Silver nitrate is titrated into the test sample to whichpotassium chromate has been added as indicator. When allthe chloride has precipated, titration produces a red colordue to precipltating silver chromate. This Indicates the, wI point. If" pr ,:tic of' -ulfites (whloh c:aii Inter'oLe

54. .. •

.- i

with test accuracy) are suspected, the test sample is firstpretreated with hydrogen peroxide. The amount of chloridepresent is then calculated from the known amount and concen-tration of silver nitrate added.

d. Soil (Box) - Resistivity of Environment. Resis-tivity of a sample of soil or water may be measured by afour-terminal soil box as shown in Figure 27. A directcurrent source is connected to the current terminals, and avoltmeter is connected to the potential terminals. Me, quredcurrent is passed through the sample, and the resulting volt-age drop is read across the potential pins. Box dimensionsare such that no multiplying factor is required to deter-mine resistivity. Potential change in millivolts, dividedby current in milliamperes, gives resistance in ohms, whichfor the soil box is numerically equal to resistivity in ohm-centimeters.

The measurement should be made as quickly as possible aftercurrent is turned on to minimize polarization of soil boxelectrodes. The'soil box can be used with a four-terminalalternating current soil resistivity meter to eliminatepolarization effects.

Resistivity readings taken at the site using methods dis-cussed in paragraph 1.2.l.d are generally preferred over

tsoil box measurements. Soil removed from the site andtamped into a soil box may take on different characteristicsof compaction, moisture and so on. However, where fieldmeasurements are impossible or impractical, the soil box canbe used.

For determining water resistivity, the soil box is mostpractical. Also, since water is relatively homogeneous,soil box readings for water samples are generally veryaccurate.

1.3.2 Sulfate Content. Sulfates in soil or water candestroy concrete, corroding this material and substitutingvoluminous corrosion products (gypsum - and calcium-sulphoal-uminate hydrate crystals) for the initial mix. The poresand voids of concrete become filled with these materialswhich grow and expand producing internal stresses anddestruction.

Sulfates in soil or water can be detected by several methods.Turbidimetric methods are generally used to determine concen-trations less than 100 ppm. These are based on precipitationof barium sulfate from the acidified, stabilized test sample.The stabilizer results in a stable suspension of uniform sizeparticles. Turbidity is then measured photometrically, re-sults indicating barium sulfate concentration.

55

A&I

NotestlCnecin

nL Cl, C2 = Current ConnectionsF igure 27SOIL BOX

The THQ (tetrahydroxyiquinone) method is used where highersulfate concentrations are suspected. Standard barium chlor-ide solution is titrated into the test sample, to which THQindicator, bromcresol green indicator, and alcohol have beenadded. Alcohol decreases solubility of barium sulfate.Endpoint is reached when the test sample color changes fromyellow to rose. Concentration of sulfate is calculatedfrom the amount of barium chloride added.

1.3.3 Standard Water Analysis. In many cases, a com-plete water analysis may be required for correct interpreta-tion of corrosion problems. Water treatment may be required

./ to mitigate problems from such sources as: hardness, alka-linity, dissolved oxygen, dissolved metals, nitrates, freecarbon dioxide.

2 INSTRUM"ENTS. As previously mentioned, corrosion ofburied or submerged metals is associated with electricalpotentials and current. Therefore, proper measurement andevaluation of these quantities are important to corrosioninvestigation work and cathodic protection. Instrumentcharacteristics and proper use of electrica.1 measuringdevices for acquiring field data should be understood. Thissection describes some basic instruments and shows how theymay be effectively applied in the field.

56

'II

AI

2.1 Why Special Instruments are Required. Because ofthe wide range of currents and voltages encountered in cor-rosion test work, test instruments must be designed formaximum usefulness and flexibility. They must be light-weight, sufficiently rugged for field use, and easily read-able. There are several manufacturers building combinationcorrosion meters capable of a variety of measurements witha single instrument. These are desirable, because they re-duce both number of instruments required and weight load.However, accuracy is usually not as good as when specializedsingle-purpose instruments are used. Figure 28 shows sometypical instruments and equipment used in corrosion work.Items are identified by numbers as follows:

1. Multipurpose corrosion meter.2. Storage bottle.3. Depth gauge.4. Copper sulfate crystals.5. Copper-copper sulfate half-cell.6. Copper-copper sulfate half-cell (with lead wire attached).7. Selector switch for duct slug test.8. Volt-ohm meter.9. Probe resistivity meter.10. Four-pin resistivity meter.11. pH meter.12. Measuring tapes.13. C clamp.14. 3oil probe pins.15. Map measure.16. 100-ampere shunt.17. Soil box.18. Equipment box.19. Probe for soil resistivity measurements.20. Pipe locator.21. Current interrupter.22. Calomel half-cell.23. Variable resistor.24. Beryllium scraper (nonsparking).25. Recording millivoltmeter.26. Lead wire.

* 27. Four-pin soil resistivity leads.28. Lead slug.29. Beryllium wire brush (nonsparking).30. Flashlight.

The questions of what instrument to use and where involvestudy of each individual situation and good working know-ledge of all instruments available. All corrosion work,from initial study through design and maintenance, is basedupon readings gathered in the field. Therefore, the effect-iveness of an entire program is dependent upon reliablereadings and proper use of instruments,

57

,-A

Figure 28TYPICAL FIELD SURVEY TEST EQUIPM'ENT

2.1.1 Design and Construction of Field Instruments.For many years, the basic meter movement found in corrosioninstruments was the galvanometric D'Arsonval type. Instru-ments are still available with this type movement, althoughthe new, more rugged "taut band" suspension is replacing itin some field instruments. These two types are discussedhere.

a. D'Arsonval Movement. The basic D'Arsonval metermovement is a galvanometer modified for practical usage.The basic galvanometer (Figure .29) acts on the principlethan an electromagnet placed between the poles of a horse-shoe magnet and enei.gized by current flow, will act againstthe tension of a spring which tends to force like poles to-gether. The force thus produced will move a pointer andindicate, on a calibrated scale, the amount of current pass-ing through the electromagnet. The greater the currentflow, the stronger the force, and the farther the pointermoves.

In the D'Arsonval meter movement (Figure 30), Jewel bear-ings, adjustable balance springs, and a modified horseshoemagnet are added to the basic galvanometer, and it is usu-ally calibrated to read some electrical quantity directly.

Instruments with D'Arsonval movement can easily be damagedby improper field handling. It is, therefore, as with allinstruments, important to have them calibrated at regularintervals. Damage to jewel bearings (Figure 31) or pivotpoints may cause a pointer to stick. When instruments aredropped or continually jarred (along the axis of pivots),these jewels may crack or break, or pivot points may becomej blunt or mushroom-shaped. This can happen easily becausepressures on the bearings of several tons per square inchare possible with the 300 milligram moving element restingon a.0002" diameter bearing. Dropping or rough handlingof these instruments should be avoided. They should be

Vtransported with the axes of their pivots parallel to theground so that the impact of any force will not be directlyon the pivot points. A sluggish, or sticking pointer mayalso result from its jewel bearings being adjusted tootightly, or from an accumulation of dirt. If the permanentmagnet of an instrument becomes weak with age, the instru-ment will no longer read correctly.

b. Taut Band Suspension. Taut-band suspension is a*fairly recent innovation. Tuistruments with this type meter

movement have no pivots, jewels, or hair-springs (Figure32). Instead, taut metal bands support the moving ele-

ment and carry current to the coil. They act like the hairspring of the D'Arsonval type movement to restrain theturning magnet. A more rugged, more sensitive instrumentwith frlctionless mounting that minimizes wear results.Taut band suspension is well-suited for portable field in-struments which are subjected to rough treatment. However,

59I

Figjure 29GAL*.VAN OMETER

Permatient Magn'et Zero Adjc.stment

Fi.,ureu30

BASIC DARSONBalMOVE e

Spin

Oi

Jewel Bearin6,

Ruc.-edized Type iJewel Feairing

Figure 31

INSTRUMENT BEARINGS

61.

ik.7

Tout BandSpring

Bond Guideand Anchor

mp rto

Bumper Stop Figure 32

bpFlfg btop TAUT DAND SUSPENSION

accuracy at extreme ends of meter scale ranges may be lessthan with D'Arsonval movements.

2.2 Types of Instruments. In general, high-sensitivityinstruments are those requiring a very small amount of cur-rent to deflect the pointer (500 microamperes or less).These high-sensitivity instruments are also high-resistanceinstruments because meter resistance (expressed in ohms pervolt) is inversely proportional to current (microamperes ormilliamperes) required for full scale deflection of themeter pointer. In use, the pointer of a high-sensitivitymeter swings slowly up-scale to the correct position, overtravels, swings back and forth (10% to 50%) and gradually

[. stops. There is no sharp division between "low-resistance"and "high-resistance" instruments. However, meters gener-ally considered low-resistance are those having internalresistance of 10,000 ohms per volt or less while high-resis-tance instruments arc in the range 100,000 ohms per volt or;,reater.

Dampin , descrtbes control of periodic swing of the movlngp,system, so that the pointer comes to rest more quickly.Aluminum damping vanes are sometimes attached to movingshafts of meter movements for this purpose. Five percentto ten percent overswing is usually considered satisfactorydamping.

62f(

Ai

The period of a meter movement is the length of time re-quired for its pointer to come to rest, after current hasbeen applied to the instrument terminals.

An instrument's accuracy, as given by the manufacturer, isusually stated as percentage of full-scale deflection andnot as percentage of actual meter reading. Therefore, a100-volt instrument which reads 2.0 volts and has an accur-acy of 0.5% can be in error by + 0.5 volts. (0.5% is acommon statement of instrument accuracy.)

2.2.1 Ammeters. Simple ammeters such as micro-ammeters, milliammeters, and regular ammeters use the basic

Y meter movement and employ three different circuit arrange-ments depending upon requirements.

1. All of 'he current flows through the moving coil.This is applicable to microammeters and milliammeters up toa range of about 200 milliamperes.

2. Part of the current flows through the moving coiland part, through a built-in shunt.

3. Part of the current flows through the meter andjpart, through an external shunt.Several shunts may be built into one instrument, so thatthe same meter movement can be used for various currentranges. Larger shunts, handling heavy currents, are oftenused externally. It is important to remember that all ofthe current to be measured must pass through an ammeter.Therefore, the shunt must be large enough to handle thiscurrent. A typical ammeter schematic is shown in Figure

33.

Another type ammeter, the zero-resistance ammeter, is actu-ally the combination of a millivolt-voltmeter and anammeter. These are connected in the circuit so as to meas-ure current flow between two points of different voltage ina given circuit without introducing additional resistance.A simple diagram of the electrical connection for measuring -

*current flow between a galvanic anode and pipe is shown inFigure 3P. The zero resistance ammeter operates on thenull principle. That is, the open circuit voltage betweenmagnesium (anode) and iron (pipe) registers on the volt-meter or galvanometer. A battery is connected in the cir-cuit with polarity opposite that of the open circuit volt-

* age indicated on the voltmeter. A variable resistor is ad-Justed to allow sufficient current to flow through resistor(R) until IR voltage drop matches open circuit voltage, andthe voltmeter reads zero. True current flowing betweenanode and pipe when bridged together through zero resistanceis then indicated on the ammeter.

63

Basic Meter Movement

Shunt wire connected across terminalsof Basic Meter Movement

Figure 33BASIC D.C. AMMETER CIRCUIT

This method eliminates any energy requirement from the volt-age source being measured, making this form of measurementdesirable where low energy sources are concerned.

2.2.2 Voltmeter - -High and Low Resistance. The volt-meter is the most versatile and widely used instrument incorrosion work. It consists of a basic meter movement Inseries with a multiplier resistor (Figure .35). Thisseries resistor limits current flow through the very finewire in the meter movement and provides a standard by whichvoltmeter range can be determined.

Internal resistance of the voltmeter should be high com-pared to resistance of test leads, reference electrode, andother components that make up the external circuit. Wherereadings of 1% accuracy are required, instrument resistancemust be ninety-nine times greater than total resistance ofthe external circuit. It is, therefore, important that high-resistance voltmeters be used in most corrosion investiga-tion work. For general use, instrument sensitivity shouldnot be less than 100,000 ohms per volt ("high-resistancevoltmeter") so as not to permit current flow great enoughto change the potential of the source being measured, or topermit polarization of the reference electrode which canaffect its stability as a reference.

&A

-An

Batt ry

R

Votee rGlaoee

IE

OpnCrci--

Grade

MEASUREMENT OF CURRENT FLOW BY'I THE ZERO RESISTANCE AMMETER METHOD

65

Busic Meter Movement

Vol tage tobe Measured

Multiplier Resistor Connected inSeries with Basic Meter Movement

Figure 35BASIC D.C. VOLTMETER CIRCUIT

For very low voltage readings such as those encountered inIR drops along a structure, however, "low-resistance"(1000 to 2000 ohms per volt) voltmeters are required. Suchlow voltages cannot force enough current through a high-resistance meter coil to register on the scale. Low-resis-tance meters require less current to register, and cantherefore be used.

Readings made with either type voltmeter are subject toerror (true voltage lower than that read) if external cir-cuit resistance is an appreciable percentage of meter inter-nal resistance. Methods of correcting such readings werediscussed in paragraph 1.2.l.a(l)(b).

2.2.3 Recorders. Recording instruments periodicallyor continuously monitor and record on a moving chart read-ings of direct current or voltage. These are especiallyuseful in areas of fluctuating stray current. The basicrecorder is a D.C. meter movement connected to an electri-cally-powered moving chart. The chart advances with time,providing a record of readings over the period of recording.

The typical chart is advanced by a sprinG-wound clock mech-anism or battery-powered motor. Generally, a bar-tap mech-anism periodically records readings: the bar depresses at

Iven intevvalo of time, marking the chart which is either

A6

A . . .

>-I

pressure-seisitivo or smoke-covered. The power source ill

this type recorder is generally too weak to record contin-uously, using pen and Ink. Continuous recorders are also foavailable which require an external A.C. power source; theseare, therefore, not always practical for field use. Battery-powered continuous recorders, using amplifier and self-bal-ancing potentiometer are powerful enough to operate pen and

~Ink.

Recorders are used for purposes other than stray currentanalysis. They may be employed as labor-saving devices, totake simultaneous readings at various locations. Build-upof polarization potentials can be monitored. Structure-to-electrolyte potentials on cross-country pipelines or cablescan be measured by coapling the recorder to a wheel-mountedor other moving reference electrode. The chart is thendriven by electrode movement, and records versus distance,not time.

Another recorder is the "X-Y" type, used in making correla-tion curves (paragraph 1.2.2.c) in stray current surveys.For example: this meter can simultaneously record struc-ture-to-electrolyte potential (y-axis) and IR drop (x-axis)along the structure. Data (automatically plotted by therecorder) are analyzed as in paragraph 1.2.2.c. Any othertwo voltages may, of course, be simultaneously measured andplotted.

2.2.4 Potentiometer-Voltmeter. The fundamental prin-ciple of the potentiometer-voltmeter is to measure potentialof a given voltage source without drawing external current.This is accomplished througi a special circuit arrangementas in Figure 36. The component parts include internalvoltage source, voltmeter, variable resistor, and galvano-meter. This combination uses a galvanometer of extremesensitivity, not suitable for direct measurement purposes,but highly desirable as a null indicator. Since the volt-meter element does not draw current from the external cir-cuit, it can be of the low-sensitivity type with strongsprings giving excellent accuracy and durability. The in-ternal voltage source is connected in the circuit with op-posite polarity from that of the voltage to be measured.

Operation of the potentiometer-voltmeter is simple. Inter-nal voltage is adjusted by means of the variable resistoruntil exactly equal and opposite to the voltage being meas-ured. This is indicated by zero deflection on the galvano-meter. At this point, no current is drawn from the externalcircuit, and the effect of infinite resistance is accom-plished. The voltmeter then gives the true open circuitpotential of the external source.

67

Z; _ 7

Internal Voltage Source

+

Variable Resistor VoltmeV er

IV

Galvanometer G

1 _4 Voltage to -

be MeasuredFigure 36

BASIC D.C. POTENTIOMETER-VOLTMETER CIRCUIT

2.2.5 Potentiometer. The potentiometer is generallya laboratory instrument, complex in circuitry and capable ofprecise potential measurement. The basic instrument issimilar to the potentiometer-voltmeter, with the indicatingvoltmeter replaced by calibrated resistance. Resistance isvaried by dials until the galvanometer reads zero; measuredvoltage is then read directly from the dials.

2.2.6 Electronic and Vacuum-Tube Voltmeter. Elec-tronic and vacuum-tube voltmeters have the high input resis-tance of a potentiometer without the disadvantage of manualbalancing each time the measured voltage changes, permittinotheir use in fluctuating stray current areas. However,static disturbances can produce errors, and zero drift issometimes a problem.

The principle of the vacuum-tube voltmeter is illustratedin Figure 37. A minute amount of current (around 10- 10amperes) is drained from the external circuit and amplifiedin the vacuum tube to produce deflection of a voltmeter.Vultaje applied to the vacuum-tubo tgrld a'fects changes invacuum-tube plate current. Therefore, current taken fromthe measuring circuit consists only of grid current andleakage current, which can be kept at a low value by properchoice of tube and circuit design. If ,rid potential is

--- . . _ - - ". ---- j

Vacuum TubeVariableResistor

Figure 37

BASIC D.C. VACUUM-TUBE

VOLTMETER CIRCUIT

negative with respect to cathode potential, essentially noelectrons flow. As grid potential approaches cathode poten-tial, electrons begin to flow from cathode to plate. Thehigher the grid potential, the greater the flow. The metermovement (calibrated in volts) measures the flow. A vacuum-tube voltmeter requires power to operate the vacuum-tubeand power for the D.C. bias circuit.

The newer solid state electronic voltmeter uses a field-effect transistor for amplification instead of a vacuum-tube. The resulting instrument can be used to measuresmaller voltages. Battery-powered electronic voltmetersare available. These may be used in the field without A.C.power.

The D.C. solid state amplifier used in electronic voltmetersis available separately, for use with high- or low-resis-tance instruments. The amplifier increases input resistancegreatly and improves response times on lower meter ranges.These battery-operated amplifiers have built-in D.C. biascircuits.

2.2.7 Earth-Current Meter.- The earth-current metercan be used to calculate current flowing in earth adjacentto an underground structure. A 2-inch hole must be boredfrom ground surface to the top of the structure into which Ia special probe, called a cantilever electrode, is inserted.This probe has four contact points; two are copper-coppersulfate reference electrodes, and two are copper, used todetermine relative soil resistivity. Voltage drop, meas-ured between reference electrodes, and relative soil

$69

-.- -

resistivity data are used to calculate current (density)nicked up or discharged from the structure.

The meter consists of battery, high-resistance voltmeter,milliammeter, and current commutator. Commutated batterycurrent flows into earth from the copper contacts, producingAC voltage drop at the reference electrodes. This voltage,commutated to DC voltage, is measured by the voltmeter. Aformula, associated with instrument constants, is used todetermine earth current:

I = QEeI c

'c

w,.here

I = current density in earth(milliamperes per square foot)

Q = instrument constant (inverse square feet)Ee = voltage between reference electrodes with

instrument in "potential" position (millivolts)Ec = voltage between reference electrodes with

instrument at "commutate" position andcommutator on (millivolts)

Ic = current from battery with instrument as forEc (milliamperes)

This instrument finds limited use because it is expensiveand relatively complicated to use, requiring a hole to beluug to structure.

2.2.8 Resistance Testers. Basic equipment for meas-uring soil resistivity consists of a direct current volt-meter, direct current ammeter, voltage source, soil contacts,and test leads. Where direct current is used for resistivitytesting, polarity must be reversed and average current valuesused to eliminace errors from polarization and galvaniceffects between electrodes and natural earth currents. Con-sequently, self-contained instruments are available whichchange direct current to alternating current for test pur-poses. Specialized soil resistivity instruments usually in-corporate this feature. Soil resistivities in any range canbe measured by proper choice of voltages, currents, and meas-uring devices. Specialized test sets are made to measure to2 to 3% accuracy resistivity values over wide ranges fromsea water, with resistivities of about 26 ohm-centimeters, tosoils with resistivities as high as 1,000,000 ohm-centimeters.

a. Instruments Using Four-Electrode Contact toElectrolyte. These instruments are used for measuring resis-t1V1y-by the Wenner Four-Pin Method (paragraph 1.2.1.d) orwith a four-contact soil box. These methods are most accur-ate because contact resistances are not included.

70

A •

Terminals of these instruments can b( conveniently "shortedout" to make them two or three terminal units, when neededfor measurements of circuit continuity or structure-to-soilresistance. Soil contacts connected to the four terminalscan be spaced over 100 feet apart without introducing appre-ciable errors in soil resistance measurements, and both in-struments are designed for minimum interference from straycurrents.

For special applications, such as large spacings, the Gish-Rooney method of measurement can be used. This method andthe instruments used are not commonly used and are there-fore not discussed here.

(1) Battery-Operated A.C. Type. The battery-operated, AC type meter is a specialized, self-containedinstrument used to measure soil resistivity by the four-electrode method. A schematic wiring diagram for one com-monly used design, the "Vibroground", is shown in Figure

38a. If the "push to read" button is pressed, the vibra-tor is excited by eight flashlight cells. A pair of vibra-tor contacts connects the dry cells alternately across eachhalf of the transformer primary. A voltage of approximately125 volts alternating current is inducted in the trans-former secondary winding which reaches terminals C1 and C2through the calibrated potentiometer circuit. A voltagedrop occurs across the potentiometer, matched against thevoltage drop through soil between electrodes P1 and P2.When voltage is adjusted so no current flows in the galvano-meter, the potentiometer reads directly in ohms.

(2) Hand Generator, Pulse D.C. Type. The hand-generator, pulse D.C. meter is a second type self-containedinstrument suitable for four-electrode soil resistivitymeasurements. A schematic circuit diagram for one frequent-ly used design, the "Megger", is shown in Figure 38b.Test current is supplied to this instrument from a hand-cranked generator; no batteries are required. Current is

i, supplied at approximately 50 cycles and 50 volts when gener-ator turns at recommended speed. (Current output is limitedto approximately 0.50 amperes.)

The "Megger" does not use a resistance-comparing circuitwith galvanometer but uses, instead, a D.C. ohmmeter withcurrent and potential coils in magnetic opposition.

A second pulse D.C. instrument uses a galvanometer resis-tance-comparing circuit similar to that described in para-graph 2.2.8.a(l), but with a mechanical rectifier andindicating dials replacing the potentiometer.

b. Instruments Using Two-Electrode Contact toElectrolyte. These instruments measure resistivity usingtwo contacts to the electrolyte. (The two contacts may beon two different rods, or both on one.) This method, lessaccurate than using four electrodes, includes rod contact

71€I

A/

RectifierSycronous

Vibrator

/1Push/Button

-~ I Blocking

Alterno'or---f( ICapaciftor

PotentiometerCalibrated in O.ims I

C1 PI P- C2a) BATTERY ENERGIZED TYPE

D.C. Generator 0Ohims 10

Potential Coil

Current ReverserCuarrent Coil

Curret Reerser f'5 7

b) GENERATOR ENERGIZED T PE

Fitlure 38BATTERY AND GENERATOR ENERGIZED SOIL RESIST;-NCE TESTERS

resistance in the measurement. Like soil boxes, they mean-ure only a small segment of the electrolyte which may notbe a true representation of the average soil conditionsalonrg the structure right-of-way.

(1) Two Probes. This two-probe earth resistivitymeter measures the earth or water resistivity between twoelectrodes on separate rods (often called "Shepar Canes")located approximately 18 inches apart. It is useful for ob-taining resistivity values of liquids or soils in excava-tions and trenches. Two separate rods can measure slightlylarger samples than a single probe (paragraph 2.2.8.b(2),which has fi:,:d electrode separation.

(2) Single-Probe. The single probe contains twoelectrodes in the same probe-rod: one is the tip; one, theprobe body. Elect -olyte resistivity, a function of resis-tance between the two electrodes, is either read directlyor calculated from resistance.. In either case, the meteremployed contains an A.C. bridge or similar circuit to elim-inate polarization of the electrodes. Generally, thesemeters are battery-powered.

2.2.9 Pipe Locator. Pipe -ocators are used to locateburied metal structures (pipes, cables, tanks, etc.) whenprecise locations are needed for corrosion testing or otherreasons. Most pipe locators impress an alternating currenton the structure which is then pinpointed by the electricalfiell produced. This field is traced with a receiver. Thealternating current signal may be impressed directly on thestructure through a wire connectc d to an exposed section(conductive), or it may be inducci on the structure (induc-tive). Some locators work both ways.

With the conductive type, batter:/-ssupplied direct current isconverted to A.C. by a vibrator cireuit and passes througha transformer to produce a maximum of several hundred volts.Voltage is induced in the receiver pick-up coil when nearthe structure, and this is amplified into headphones. Withthe receiver horizontal and near the structure, sound is ob-tained; none is obtained directly over the pipe, becausethere equal and opposite voltages are induced and cancel out.

The inductive type operate.- similarly, b'it no contact tostructure is used. The A.C. sinal is induced on the struc-ture by a coil in the transmitter and is detected as withthe conductive pipe locator. In some mod(,Is, the trans-mitter and receiver are mounted to, 2ther and used just tolocate structures. In others, they are separate and may beused to trace lon, structures such as pipelines.

Some locators include an interrupter to produce pulsationof the signal. This signal can then be distinguished fromilterfesence noise produced by other alternatin-, currentsin the vicinity.

73I - .

2.2.10 Holiday Detectors. Holiday detectors, some-times called "Jeeps", u1e an electrical si-nal. to detectcoating faults (holidays). These instruments impress avoltage across the coating. When an electrode is passr-dover tle '-oated surface, sparks occur between elc'trode andsuructure (pipe) at holidajs. These sparks set off an .lec-tri si<ni, .enerally an audible or visual alarm.

Most holida:.' W-tectcrs are battery-operated. They employpipe-encirclin;- (spring-coil) electrode for large pipes anda brush-type electrode for smaller pipes and structural sur-faces. The detector should be checked reg, ularly to assureproper operation. Also, proper voltage for the coatingbeing, tessud must be maintaine d.

The specific voltage required to locate holidas dcpends on

T:,pe and thickness of coating. The 7o]ta,,e must b" hig'henough to br,'ak down resistance of air of coating thlcknel.s,but low enough not to rupture the coating. Since actual

: coatings are not truly uniform in thickness, proper holidaydetector voltages are generally such that locations of verythin coating, as well as holidays, are detected. The ap-plied voltage actually ruptures thin places because of highstresses induced. This situation cannot be avoided, becauseLhere is no perfect coating. However, proper voltage levelslocate holidays and minimize damage.

Most coating manufacturers indicate a maximum value of voltsper mil coating thickness. Below this value, undue ruptureshould not occur. The National Association of CorrosionEngineers Committee TlOD-9A suggests a formula for determin-inf proper voltage for coatings. other than thin film, paint,or fusion types:

V = k V

where

V = output voltage, kilovoltsT = coating thickness, milsk = a constant (from 1.0 to 1.5 depending on

type coating)= 1.25 for coal tar or asphalt

The constant k is near 1.0 for rough coatings and near 1.5for smooth coatings. This formula is generally valid within1(% accuracy.

r. " i.; ., c .. , c vr J t], a! , t.

I, - .

I(

A/

methods, a voltage output of approximately 100 to 125 timescoating thickness in mils is sugge3ted (reference 17).When no accurate means of determining proper voltage outputis available, the coating manufacturer should be consulted.

2.2.11 Combination Meters. There are many "combina-tion" instruments useful to the corrosion engineer. Thesecombine features of individual instruments already mentionedand are valuable for measuring several different quantitiesduring a field survey. Two commonly-used types are des-cribed here.

a. Volt-Ohm-Milliammeter (VOM). Volt-ohm-milliam-meters consist ol an AC--DC voltmeter, DC ammeter and DCohmmeter. The AC-DC voltmeter is generally a basic D.C.meter movement with a rectifier connected to change A.C. toD.C. for measurement. The ohmmeter, powered by low-voltagebatteries, is mainly limited to checking circuit continuitythrough metallic circuits. These instruments are very use-ful for rectifier repair and maintenance work, and can alsobe used to "check out" other field test equipment. High-sensitivity type VOM can be used for some structure-to-soilpotential measurements.

b. Direct Current Combination Meter. Direct currentcombination meters are designed specifically for the corros-ion engineer. These instruments are available commerciallywith a variety of alternate features and covering a wideprice range.

Basically these instruments consist of two D.C. meter move-ments mounted in one case. One of these is a high-resis-tance meter; the other, relatively low-resistance. In-cluded also in the case are selector switches, rheostat con-trols, and batteries. These meters are used separately ortogether for various measurements. They can serve as high-resistance voltmeter and low-resistance voltmeter, volt-meter and ammeter, or sometimes zero resistance ammeter,potentiometer-voltmeter (with high-resistance meter as gal-vanometer and low-resistance meter as indicating meter),and in some models as an electronic voltmeter. Current fromthe internally-mounted batteries can be controlled by rheo-stats and routed throug' the ammeter in order to provide afacility for resistance measurement.

A typical combination meter is shown in Figure 39. Thisis of the type in common use by the military at this timeand is, therefore, described in detail. This instrument,the M.C. Miller Model "M-3-14", contains four specific meas-uring circuits:

ligh-Resistance Voltmeter 0.2 to 20 voltsPotentiometer-Voltmeter 0.2 to 3 voltsLow-Resistance Voltmeter 2 mv to 100 voltsMilliammeter/Ammeter 2 ma to 10 amps.

75

NO4 V-

-

Figure 39TYICAL COMB3INATION l'MER

76

LiJ

The wirin diagram for this instrument is )iven in FIigure40. Polarity switches and associated wiring are not shown

on this diagram, for simplicity.

This instrument may be used in a variety of ways. The low-resistance and high-resistance voltmeter may be used separ-ately or simultaneously for potential measurements (Figlre

41). The low-resistance meter may be used as an ammeterto measure currents, independent from the high-resistancemeter, or while the high-resistance me(er measures potential(Fijure 42). The potentiometer-voltmeter circuit employsboth meters. allowing only one measurement at a time; thiscircuit is useful for voltage measurement where external cir-cuit resistance is high, producing possible errors in read-ings on the high-resistance meter. Typically, this is en-countered with high values of: contact-to-electrolyte !'esis-tance (such as in frozen soil, concrete, or blacktop), soilresistivity, or coacing resistance on small structures.Readings above 3 volts are not noticeably affected by exter-nal circuit resistance and, foi, these readings, the high-resistance meter will suffice.

2.3 Accessories. In addition to standard instruments,accessories and auxiliary equipment are needed for corrosionsurvey measurements. A description of these accessories isriven here, including:

portable recti-ierbatteriescurrent interruptersoil boxreference electrodes (half-cells)shuntsva table resistorssoil probe pins

Vwire reeltest leadsclamps j

2.3.1 Portable Rectifier. Portable rectifiers can beused for convenience in determining current requirements forcathodic protection of buried or suomeriged structures (para-graph 1.2.2.a), testing newly-i'stallel groundbeds, testing;older rroundbeds with inoperative rectifiers, continuitytests, and rcsistance tests. This device acts as a permanentrectifier, impressing current on a groundbed (temporarygroundbeds are often used). A portable rectifier can consistof a transformer and rectifier for convertin,- A.C. to D.C.,and an ammeter and voltmeter for measuring output. An inter-rupter (paragraph 2.3.3) 1i allow "on-off" readings isoften included as well as a mounted shunt (paragraph2.3.10). This device is convenient, because it contains

77

II

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4431

---------------------------- -------

/E

+ U

AA

8E co-

Battery~- HafCl Half-Coil

Struturtructure

-.- G roundbed p Galvainic Anode

FA ~ ~~. LR - TEST CURREiNT IlI Cr UHR - STRUCTURE-TO-SOIL POTENTIAL L) LR - ANODE CURRENT

ChANGE riR - STRUCTURE-TO-SOIL POTENTIAL

CHANGEM-3- M-3-M

Butery, +-4Batter, -

c) SOIL RESISTIVIT" - 4-PIN METHOD d) -"OIL RESISTIVIT'( SOIL BOXMETHOD

1-3-

+

e) LR- CURENT LOW BTWEESTRUCUREl

e)L-CRRENTCFLO UEESTRULOWRESTNE(RAD

HIGH-RESISTANCE (HR) VOLTMETERS(MILLER METER M-3-M)

Structure StructL.

i.'aG roundbed Golvonic Anodea) LR -TEST CURRENT 6) LR - ANODE CURRENT

HR - STRUCTURE-TO-SOIL HR - STRUCTURE-TO-POTENT !AL CHANGE SOIL POTENTIAL

CHANGEM-3-M M-3-MReifr

'N tty or

c) LR - IR DROP A~LONG STRUCTURE d) LR OR HR - DC SOURCEHR - STRUCTURE-TO-SOIL POTENTIAL POTENTIAL

M-3-M

e) LR - STRUCTURE-TO-STRUICTURE POTENTIALHR - STRUCTURE-TO-SOIL POTENTIAL

Figure 42TYPICAL USES OF AMMETER (IR) AND HIGH-RESISTANCE VOLTMETER (HR)

(MILLER METER M-3-M)

to

all. ne'essary ,-quipment in one case. An A.C. power sourceis required to operate the rectifier; this may be a genera-tor power source or connection to service lines.

2.3.2 aitteries. Battevies, like a portable rect-i-fler, are useful in direct current sources or as supplement-ary D.C. sources in corrosion testing. Six or 12 volt ba-t-ries are frequently employed when available equipment islimited, and instruments at hand do not contain built-inbatteries. They are used to power such devices as currentinterrupters, and to supplement existing power when measur-ing, foj? exanpl, structure-to-soil potentials in high-resis-tance soils. Batteries may be used to supply test currentfor determining current requirements on well-coated struc-tures, or t- simulate galvanic anodes. Interrupting currentwill help conserve battery life.

Storage batteries may be used iL!stead of dry cells, whenhigher currents are reqaired.

2.3.3 Current Interrupter. A current interrupter isbasically a motor-driven cam or electronic circuit whichreqularly opens and closes a relay. This interrupts currentflow on a schedule (such as 20 seconds "on", 10 seconds"off"). This is useful for measuring, strum'tare-to-soil po-tentials and 3ther values with cathodic protection currenton and off. It is used in current requirement tests andfor checking the effectiveness of cathodic protection.

It is used advanta;reously on pipelines and complex struc-tures where several potential readings are taken for eachcurrent setting. When current is interrupted, readings aretaken at various points along the structure. "On" readingsare taken Just before current is interrupted, and "off"readings are taken as soon as possible thereafter. Switch-ing schedule should be set so there is a noticeable differ-ence between "on" and "off" periods. In this way, readingscan be differentiated at a distance.

A wiring diagram for a typical interrupter connected into acathodic protection circuit is shown in Figure 43. The6-volt battery which powers the interrupter may be built-inor external. Interrupters are also designed to operate fromA.C. power, or from either D.C. or A.C. at the engineer'sootion.

2.3.4 Soil Box. This measurement of electrolyte re-sistivity was discussed in paragraph 1.3.l.d. Most accur-ate is the 4-contact type soil box, made of high-dielectrie 'material (usually plastic) with current contacts at each endand potential contacts "long one side (Figure 27). Atypical soil box of this type is approximately 8-1/2 incheslong by 1-1/2 inches wide by 1-1/4 inches deep (inside

-" - j

.

2.-

2-Rpm, 6-Volt DC MotorApproximately 15 MA. ut 6 Volts

6-Volt Battery Special M-U Switch withBuilt-in Alinco Magnet to

Battery Snuf Ot DCArc WhenSwitch Opens

Structure epry rUndergoing TestAnd

Fi~,ure 43WIRING DIAGRAM AND APPLICATION OF CURRENT INTERRUPTER

b 2

dimensions). These measurements are such that resistance inohms of the sample equals resistivity in ohm-centimeters.Other size boxes are available, each characterized by a "boxconstant". Resistivity in ohm-centimeters for the electro-lyte sample Is obtained by multiplying rebIstance readingIn ohms, by the box constant (usually a convenient numbersuch as 1 or 10).

The soil box described above has a volume of 230 cubic cent-imeters. Its current electrodes are thin metal plates ap-proximately 1-3/8 inches by 1 inch in size. Potential elec-trodes arc metal pins about 1-1/2 inches long by 3/32 inchIn diameter, located 1-3/4 inches from the box ends.Effects of contact resistance are overcome with this type(four-connection) soil box, as with other 4 -pin methods of'measuring resistivity.

A simpler type soil box, less accurate because it does notconsider contact resistance, has only two electrodes. Theseare the metal ends of the box. Construction is similar tothat of the 4-electrode soil box, with the two-sIde(potential) contacts eliminated. This type box is alsoavailable in various sizes with characteristic "boxconstants".

2.3.5 Copper-Copper Sulfate Half-Cells. The copper-copper-sulfate half-cel is the reference electrode mostoften used. It consistz essentially of four parts: copperrod or tubing with necessary provision for connection tovoltmeter, saturated solution of copper sulfate crnfactingthe copper, porous plug through which contact to electrolyteis made, and nonconducting container. Figure 44 shows atypical rod-type reference electrode, consisting of plastictube with watertight cap at the top and porous wooden plugat the bottom. The plastic tube Is filled with saturatedcopper-sulfate solution and contains an excess of crystalsto insure saturation. In the half-cell of Figure 44, acopper rod in contact with the solution extends through thecap for connection to voltmeter. A rubber cap may be placedover the porous wooden plug to prevent evaporation when theelectrode is not in use.

Another design consists of saturated copper-sulfate solu-tion inside a section of copper tubing, surrounded by aplastic case. A porous plug for contact to electrolyte,and a provision for voltmeter connection are included.

The reference elecirode of Figure 44 is standard size,useful for contacting soils under most conditions. Twoother sizes (the "long" and the "stubby") are 1'requentlyused in corrosion work where the standard size would be im-f practical. They differ from the standard half-cell in

83

)J

Wino Nut

brass Washer

Hex Nut

Rubber WasherI Plastic Cap

Plastic Busning

I Copper Suif..te Solution

I Approx. g Plastic CartrideI 18"II I Copper RodI IL j

Surpl is Copp ,Sulfcte Cr/stols

Porous Plug

Approx. emovable Robioer Cop7/8"

Figure 44

TYPICAL COPPER-COPPER SULFATE REFERENCE ELECTRODE

U¢4

dimensions only. The "long" half-cell is identical to thestandard, but is 30- or 36-inches long. It is used to con-tact electrolyte in deep test holes or other places wherethe standard size would be inadequate or difficult to use.The "stubby" is made in various lengths, but the importantfeature is its broader base (typically 2-7/8 Inch diameter).This improves contact-to-el ctrolyte where poor contact isanticipated, or when readings are taken through concrete orin frozen ground. Almost any size copper-copper sulfateelectrode can be made to order, but the three discussed herefacilitate taking readings in soils under most conditionsand are therefore generally standard equipment for corrosionfield work.

a. Polarization of Copper-Copper Sulfate Half-Cell.With low-resistivity instruments, polarization of copper-copper sulfate half-cells can be a problem. The importanceof using high-resistance voltmeters for structure-to-electro-lyte potential readings is revealed in Figure 45. Pipe-to-soil voltages, relative to copper-copper sulfate, weretaken for various currents using a multirange voltmeter withinternal resistance of 1000 ohms per volt. At full scale,current through instrument and electrode is 1 milliampere.A plot of voltage versus current results in a straight line.When polarization occurs, the curve departs materially fromlinearity. In this case polarization began at 0.9 milli-ampere. Beyond this point the curve deviates considerablyfrom a straight line indicating that half-cell resistancehas changed. Since high-resistance voltmeters require onlya few microamperes for full-scale deflection, polarizationof the half-cell should not be a problem with these typemeters.

b. Temperature Effect on Copper-Copper Sulfate Half-Cell. The copper-copper sulfate half-cell contains crystalsof copper sulfate in equilibrium with solution. An increasein temperature causes some crystals to dissolve, increasingsolution concentration; a decrease in temperature causessome copper sulfate to precipitate, decreasing solution con-centration. A concentration change alters half-cell poten-tial, requiring a correction to potential readings at otherthan "room temperature" (77*F). The effect of temperatureon the copper-copper sulfate halI'-cell is shown in Figure

46. The temperature correction is about 0.5 millivolt perdegree Fahrenheit change. For example, a structure-to-electrolyte potential relative to copper-copper sulfate istaken when the outside temperature is 90*F. The reading is-0.70 volt. From Figure 46, the correction factor for90OF is +7 millivolts (+0.007 volts). The reading relativeto standard copper-co, ?er suifate half-cell (77°F) wouldthen be:

-0.70 + (+0.007) -0.693 volts

85

Al

-0.8

o-0.6

..

C

a. -0.4

-0.2.

Polarization

0. 4 0..6 0.8 1.0Current in Milliamperes

Figure 45POLARIZATION OF A COPPER- COPPER SULFATE REFERENCE ELECTRODE

The corrected value, -0.6Q3 volts, is still very-close tothe recorded value, -0.70 volts. Normally, no correctionis required for field work except where large temperaturechanges are encountered. The effect of temperature on acopper-copper sulfate half-cell is provided for informationpurposes and may be applied if precision of observationwarrants it.

2.3.6 Calomel Half-Cells. Although most often usedin laboratory, the calomel hal-cell also finds applica-tion as a reference for underwater measurements. Unlike thecopper-copper sulfate electrode which is easily contaminatedby saltwater and unable to withstand pressure, the calomelelectrode resists contamination and is not affected by lowpressure:;.

The saturated calomel electrode is basically a mercury elec-trode in contact with a solution of mercury ions; ion con-centration is controlled by a saturated solution of mercur-ous chloride. Other calomel electrodes (non-saturated) con-tain different concentrations of potassium chloride andtherefore have different half-cell potentials. The satur-ated electrode is most often encountered. Calomel electrodescontact the electrolyte by means of a porous plug, usuallyglass, and are enclosed in glass. They operate similarly to

86

5.25

20._ --

I15

5- -

0U

t5 ------- - -

-0)

-15-

S90 100 10 61 30

Temperature in Degrees FahrenheitFigure 46

EFFECT OF TEMPERATURE ON A SATURATEDCOPPER-COPPER SULFATE REFERENCE ELECTRODE

V7

OEM Ll

copper-copper sulfate electrodes, but are less rugged forfield work.

The seawater eiecl;rode (Figure 47) employs a simple, sat-urated calon.r half-cell as used for laboratory work; thisis enclosed in a protective case formed fro1 standardplastic fittings for underwater application. The resultinghalf-cell is lor: rugged and is satisactory for field ap-plications. I*,;ad ,hot is included for weight.

Half-cell poteitial of saturated calomel relative to stand-ard hydrogen electrode at 25'C is 0.2415 volts, compared to0.3160 volts for copper-copper sulfate. This differencenecessitates a value other than -0.85 volts structure-to-electrolyte peoential as a Priterion for adequate cathodicprotection of ferrous structares. It is -0.78 for saturatedcalomel. To confvert structure-to-electrolyte readings rela-tive to satarat, d calomel to equivalent readings relativeto copper-copper sulfate, add -0.07 volts. For example, ifthe potential of a steel pipe relative to saturated calomelis -0.61 volt, th same reading relative to copper-coppersulfate would b.*:

-0.61 + (-0.07) = -0.68 volts

2.3.7 Silver-Silver Chloride Half-Cells. Anotherreference electrode used for marine work is the silver-silver chloridc half-cell (Figure 48). It consists ofsilver gauze in contact with a mixture of silver chlorideand finely-dividend silver. No liquid electrolyte is used,eliminatinj possLble errors at liquid junctions. The silver-silver chloride half-cell is less likely to polarize thanthe copper-copper sulfate half-cell, and is more rugged thanthe saturated calomel half-cell. However, its half-cellpotential (normally 0.2222 volts, relative to standard hy-drogen elcctrode at 250 C) is dependent on salinity of thewater it contacts. Large errors in structure-to-electro-lyte potentials can result if variations in salinity exist.

The criterion for adequate cathodic protection of ferrousstructures relative to silver-silver chloride is -0.78volts. To convert structure-to-electrolyte potentials rela-tive to silver-silver chloride to those relative to copper-copper sulfate, add -0.07 volts.

A second type silver-silver chloride electrode, containing0.1 nitrot,,en potassium chloride solution, is sometimes used.The V.Llu,.;; orrenpond lng to those for the above-mentioned~lecixod' .c

-. 811 volts and -0.01 volts.

o d fo

DI

h.,

Insulated Copper Wire

Approx. 2-1/2"

______ - '*~ Rubber Stoppers

0 z~~ Approx. 2"

'~t 0 ~Lead Shot

St

PVC Calomel Electrode

-IolesRubber Stoppers

~ ~1 0

a0

tiur (7 KCALOMEL ~W ELECROD FO EWTRT ShG

0 0,04

0 C3

Fiur 47

CALME ELCRD FO SEWAE TESTI-NG

IL

Rubber-covered CableCementSoft Soldered Joint

Between 22 S.W.G.Silver Wire and Rubber-

-: covered Copper CableCement Seal

Silve Wire1/16" Dia. Holes

3 Tapered Pegs SecuringCork Disk Ring. Pegs are Spaced

GlassWoolSymmetrically Around

Silver/Silver Chlo;1:e -S ilver GauzeMixture, Silver F, 1.Divided Tubin'g

6-1/4" Dia. Holes, ....... Glass Wool Logging

Staggered Rows Equ 'III 2Spaced 6 Holes p~i Row 4

Porous Rod Pru o

3 Tapered Pecs

Disk End PlateFijure 48

TYPICAL SILVER/SILVER CHLORIDE REFERENCE ELECTRODE

Ile

h6- (

2.3.8 Lead-Lead Chloride Half-Cells. The lead-leadchloride half-cell is sometimes used as reference for meas-uring structure-to-electrolyte potentials of' lead-sheathedcables. This half-cell is constructed similar to thecopper-copper sulfate half-cell, with copper rod replacedby lead and copper sulfate solution replaced by a leadchloride-potassium chloride solution. The half-cell poten-tial of this electrode relative to standard hydrogen elec-trode at 25*C is 0.326 volts. This varies with concentra-tions of chlorides in solution. The potential is close tothat of lead in soil. Therefore, virtually no galvanicpotential exists between a buried lead structure and lead-lead chloride half-cell. This is sometimes consideredadvantageous.

2.3.9 Antimony Half-Cells. The antimony half-cellis used to measure electrolyte pH rather than structure-to-electrolyte potential (paragraph l.2.l.e). This half-cellconsists of a plastic tube with a tip ot' antimony metal,and terminal for connection to a voltmeter. (Voltage be-tween antimony and copper-copper sulfate half-cell is pro-portional to pH.)

Antimony is very brittle and the antimony half-cell must behandled with care. Copper sulfate solution must not contactthe metal tip for copper will plate out on antimony.

2.3.10 Shunts. Shunts of various s!zes are avail-able for measuring current flow. These are calibrated

Fmetal resistors in the form of wire or bar with two postsor appendages for connection to a voltmeter. They are madeof high-quality metals (such as maganin or constantan)which are virtually unaffected by environment and are there-fore considered "constant resistance". A typical size isthe 0.001 ohm shunt; voltage reading in millivolts acrossthe shunt equals current in amperes.

.. / 2.3.11 Variable Resistors. Variable resistors areuseful for estimating required resistance in situationswhere current splits between two or more paths. Resistorsmay be needed to obtain proper current divisiun in the var-ious circuits. Examples of situations where installation ofresistors may be necessary are current drainage bonds andmulti-circuit, impressed current groundbcds.

In the first case, a resistor may be needed to preventdrainage of excess current from a protected structure to aforeign structure. In the second case, one rectifier sup-plies current to several groundbeds; resistors may be need-ed to distribute current among the groundbeds so that nogroundbed supplies too much or too little current.

91j

.... .1i

I" Pipe, SteelJj~ 2"

T3Weld

1" Brazed Coating

DETAIL "A"

C.R. Steel Hex, or r -1Round Stock

2-'

Steliite G ;rind 3 F atInsertSurfaces to

4.50

-4 11/4"v

"Build up", withWelding Rod

3"1j See Detail "A"- F-igure 49

TYPICAL PIPELINE PROBE ND CONTACT BAR

92

When working with variable resistors, contact resistance issometimes an important consideration. Dirt and corrosionproducts can accumulate on the exposed portions of resistorwire turns and on the adjustable terminal, increasing con-tact resistance between the two. These may require clean-ing from time to time to assure good contact.

2.3.12 Probe Bars (For Structure Contact). Onemethod of contacting buried structure is by bar or probepushed through the soil. Figure 49 illustrates a probebar designed for pipeline corrosion test work. It can bedriven through thin blacktop or gravel fill with a minimumof damage to the handle. It is made of cold-rolled steel,a ductile and economical material, with a stellite point.Stellite holds a point well. The thick section of the barlocated a few inches above its point aids its removal frommoist soil. A brazed surface near the handle reduces pos-sible increased circuit resistance when battery clips arefastened to the probe rod surface which is always subjectto build-up of corrosion products. If the bar point isfirmly pressed against the buried structure, a simultaneousrotation will tend to penetrate scale or 3oating.

If a doubt exists as to whether electrical contact to struc-*ture has been made, it can be probed at two points. With

one test lead clipped to each bar, resistance is measuredwith a niegger, ohmmeter, or vibroground. As an estimate,if two bars are spaced 100 feet apart and number 14 orheavier test wires are used, a circuit resistance below1.0 ohms indicates a good connection. If it is only prac-tical to use one bar, as is often true with structure-to-soil voltage measurements, two or three repeated readingsgiving identical results are indicative of a good connec-tion. It is somewhat more difficult to get good probe barcontact in dry soil than moist, and every effort should bemade to keep bar points sharp.

2.3.13 Wire Reels. Long, insulated lead wires woundon reels are needed for electrical measurements in fieldcorrosion work. Structure-to-electrolyte voltage surveysfor long structures such as pipelines or cables, remote

A structure-to-electrolyte readings, IR drops where no testleads are available: these and other measurements requirea long test lead. A reel facilitates handling.

Metal reels of various types are available; typical reelsare hand-held or rest on a stand. The type with stand aregenerally more rugged, made of heavier material, and havea greater capacity. Wire lengths of 500 feet for the hand-held reel or 1000 feet for the stand-type reel are typical.Stranded copper wire of size 18 with heavy duty insulation

A&,is used.

93;f .J + ]

2.3.14 Test Leads. Shorter test leads, 3 to 6 feetor so are also needed. These may be straight or coiled.No. l AWG conductor is a typical size wire. Connectors ofvarious types are used; banana plugs, spring clips, orothers are common. Care must be taken when attaching theseconnectors to minimize contact resistance between cable andconnector base. Both parts must be free of dust and corros-ion products, and a good, tight connection must be made.A poor connection will introduce errors in measurement.

It is important to have good contact between connector andstructure or terminal, also. Clips and plugs must be keptfree of dirt, grease, and corrosion products, and paint orbuild-up on structure must be scraped away. Clean metal-to-metal contact is essential for accurate measurements.

REFERENCES

1. Romanoff, Melvin, "Underground Corrosion", NationalBureau of Standards Circular 579, April, 1957.

2. Rears, R. B., "Some Recent Developments in the Study ofCathodic Protection", Gas, December, 1947

3. Romanoff, Melvin, "Corrosion of Steel Pilings in Soils",Journal of Research of the National Bureau of Stanards.Vol. 66c, No. 3, July-September, 1962.

4 Sudrabin, R. C., "Foundation Piling Corrosion, Mechanismsand Cathodic Protection", Materials Protection, October,1963.

5. DeMarco, R. C., "Protection of Underground Steel in aHighly Corrosive Area", Materials Protection, February,1964.

6. Romanoff, Melvin, "Exterior Corrosion of Cast Iron Pipe",Journal AWWA, 56:1129, September, 1964.

7. Sherer, C. M. and K. J. Granbois, "Study of AC Currentsand Their Effect on Lead-Cable Sheath Corrosion",American Institute of Electrical Engineers Transactions,May, 1955.

! 8. Johnson, W. A., "Pipe Line Leaks Are Not Inevitable",Petroleum Engineer. August, 1953.

9. Wagner, John, "Cathodic Protection and Corrosion Controlfor Utilities in Urban Areas", Proceedings of the TwelfthAnnual Appalach an Underground Corrosion Short Course,

t 1967. (Technical Bulletin No. 86, Exp. Sta.,West Virginia University.)

10. )brecht, M. F. and L. L. Quill, "Hot Temperature, Velocityof Potable Water Affect Corrosion of Copper and ItsAlloys", Heating. Piping and Air Conditioning, 1960 andApril, 1961.

11. Kuhn, R. J., "Casings Promote Corrosion at CrossingsInstead of Providing Protection", Oil and Gas Journal,December, 1970.

12. Trouard, S. E., "A Corrosion Engineer Looks at the

Problem of Pipelines Crossing Railroads and Highways",A.S.C.E. Committee Meeting, February, 1960.

95

XA> N

REFERENCES

13. National Academy of Sciences, Feberal ConstructionCouncil Technical Report No. 47, Publication 114,1963, "Field Investigation of Underground HeatingSystems".

14. Hunter, A. D. and C. H. Horton, "Cathodic ProtectionChecks Corrosion", Underwater Engineering, November,1960.

15. Wagner, E. F., "Loose Film Wrap as Cast Iron Pipe

Protection", Journal AWA, Vol. 56, March, 1964.

16. Parker, Mar.iall, (the work of).

17. Eisele, C. W., "Effective Use of Holiday Detectors",Proceedings of the 16th Annual Appalachian UndergroundCorrosion Short Course, 1971, Technical Bulletin No. 103.

18. "Rectifier Service Manual", Good-All Electric, Inc.,1972.

19. Tudor, S., et al, "Electrochemical Deterioration ofGraphite and High-Silicon Iron Anodes in Sodium-ChlorideElectrolytes", Corrosion, Vol. 14, No. 2, (Feb. 1958),pp. 53-59.

20. LaQue, F. L. and H. R. Copson, Corrosion Resistance ofMetals and Alloys, Reinhold Publishing Company, New York,1965, p. 604.

21. Literature from Englehard Industries, Inc.

22. Tefankjian, D. A., "Application of Cathodic Protection",Materials Protection Vol. 11, No. 11, November, 1972.

962 .le

BIBLIOGRAPHY

LaQue, Wik. T., ed., Designing Imvressed Current CathodicTesting Materials. Proceedings, Vol. 51, 1951.

Mudd, 0. C., "Control of Pipe Line Corrosion", CorrosionVol. 1, No. 4, December, 1945, pp. 192-218, Vol. 2, No. 1,March, 1946, pp. 25-58.

Parker, Marshall E., "Fundamentals of Corrosion Surveys",The Petroleum Engineer, Vol. 27, No. 3, March, 1955, pp.

i) 022-2'7.

Parker, Marshall F., Pipe Line Currosion and CathodicProtection: a Field Manual, Gulf Publishing Company,Houston, Texas, 1954.

Peabody, A. W., Control of Pipeline Corrosion, NationalAssociation of Corrosion Engineers, Houston, 1970.

Uhlig, H. H., Corrosion Handbook, John Wiley and Sons, Inc.,New York, 1948.

J

97-

APPENDIX A.

GLOSSARY OF CORROSION TEVIS('Definitions from NACE Standard RP-01-69)

Aisorytion. The taking up of one substance at the surfaceor anotber. The tendency of all solids to condense upontheir surfaces a layer of any gas or solute which contactsuch solids.

Aeration cell (oxywen cell). An electrolytic cell in whicha difference in oxygen concentration at the electrodes ex-uits, producing corrosion.

A, m loteris. Materials subject to attack by both acid andalaline environments. Aluminum, zinc, and lead, commonlyused in construction, are examples.

SAerobic, Free of air or uncombined oxygen; an aerobicbacteria are tbhose which do not use oxygen in their lifecycle.

Anion, A negatively charged ion which migrates toward theanode under influence of a potential gradient.

Anode, An electrode at which oxidation of its surface ornse component of the solution is occurring. Antonym:cathode.

*Bell hole. An excavation to expose a buried structure.

Catbode. An electrode at which reduction of its surface orsome component of the solution is occurring. Antonym:anode....

sC odio corrosion. Corrosion resulting from a cathodic

condition or a structure, usually caused by the reaction ofalkaline products of electrolysis with an amphoteric metal.

C2tbodic Protection. A technique to prevent the corrosionof a metal surface by making that surface the cathode of aneleotroohemical cell.

fltoo A positively oharged ion of an electrolyte whichif as toward the cathode under the influence of a po-

tential gradient.

98

2

Concentration cell. An electrolytic cell in which a dif-ference in electrolyte concentration exists between anodeand cathode, producing corrosion.

*Continuity bond. A metallic connection that provideselectrical continuity.

*Corrosion. The deterioration of a material, usually ametal, because of a reaction with its environment.

*Current density. The current per unit area.

*Electrical isolation. The condition of being electricallyseparated from otber metallic structures or the environment.

*Electro-osmotic effect. Passage of a charged particletbrotgb..a membrane under the influence of a voltage. Soilmay act as the membrane.

*Electrode notential. TN potential of an electrode asmeasured against a reference electrode. The electrode po-tential does not include any loss of potential in the sol-ution due to current passing to or from the electrodes, i.e.it represents the reversible work required to move a unitcharge from the electrode surface through the solution tothe reference electrode.

Electrolyte. A chemical substance or mixture, usuallyliquid, cnaining ions that migrate in an electric field.Examples are soil and seawater.

Electromotive force series (E4F series) A list of ele-ments arranged according to their standard electrode po-tentials the sign being positive for elements havingpotentials that are cathodic to hydrogen and negative forthose elements having potentials that are anodic to hydro-gen.

*ForeWn structure. Any structure that is not intended asa part or bte system of interest.

*Galvanic anode. A metal which, because of its relativeposition in the galvanic series, provides sacrificial pro-tection to metal or metals that are more noble in the ser-ies, when coupled in an electrolyte. These anodes are thecurrent source in one type of cathodic protection.

99

4s

Galvanic cell. A corrosion cell in which anode and cath-ode are -ss Milar conductors, producing corrosion becauseof their innate difference in potential.

*Galvanic series. A list of metals and alloys arrangedaceording to their relative potentials in a given environ-meat.

eBolidav. A discontinuity of coating that exposes the met-alsrae to the environment.

Aydrosen overvolta.ae. Voltage characteristic for each met-al-environment comination above which hydrogen gas is lib-erated.

*Impressed current. Direct current supplied by a powersource external to the electrode system.

*Insulating coating system. All components comprising theprotective coating, tbe Wm of which provides effectiveelectrical insulation of tbe coated structure.

*Interference bond. A metallic connection designed to con-trol electrical current interchange between metallic sye-tems.

Ion. Electrically cbarged atom or molecule.

.IR drop. The voltage across a resistance in accordancewt M Law.*Line current. The direct current flowing on a pipeline.

Local action. Corrosion caused by local cells on a metalsurface.

ill scale. The heavy oxide layer formed during hot fabri-cation or'eat-treatment of metals. The term is appliedcbiefly to iron and steel.

f Concentration of a solution expressed as thenmbeo gram molecules of the dissolved substance per1000 grams of solvent.

O.A measure of hydrogen ion activity defined by pH -10 (1/aff) where aH+ - hydrogen ion activity - molal

concentration of bydrogen ions multiplied by the mean ionactivity coefficient (= 1 for simplified calculations).

100

Polarization. The deviation from the open circuit potent-ial of an electrode resulting from the passage of current.

*Reference electrode. A device whose open circuit potent-ial is constant under similar conditions of measurement.

*Reverse-current switch. A device that prevents the rever-sal of direct current thr6ugh a metallic conductor."Stray current. Current flowing throul' paths other thanthe intended circuit.

*Stray current corrosion. Corrosion resulting from directcurrent flow through pats other than the intended circuit.

*Structure-to-electrolvte voltsae. (also structure-to-soilpotential or pipe-zo-soil pozential). The voltage differ-ence between a buried metallic structure and the electro-lyte which is measured with a reference electrode in con-tact with the electrolyte.

*Structure-to-structure voltage. (also structure-to-struct-ure potential). Tbe difference in voltage between metallicstructures in a common electrolyte.

*Voltase. An electromotive force, or a difference in elec-trode potentials expressed in volts.

1.01

APPENqDIX

ELECTRO!OTIVE FORCE SERIES

Standard Electrode gotential

Electrode Reaction-. EU(volts)._25 C.

Potassium - K + + e- -2.922Calcium =Ca+ + 2e -2.87Sodium +a + + e -2.712Magnesium = Mg + + 2e- -2.34Beryllium Be + 2e- -1.70

Aluminum = Al++ + 3e -1.67Manganese Mn++ + 2e -1.05Zinc- , " Zn + 2e- -0.762Cbromium Cr:'++ 3e- -0.71Gallium Ga + + 3e -0.52

Iron Fe++ + 2e- -0.440Cadmium Cd++ + 2e -O.I02Indium = In++ + 3e -0.340Thallium =T1 + e -0.336Cobalt -o +0 + 2e- -0.277

Nickel - Ki+" + 2e -0.250Tin = Sn++ + 2e- -0.136SLead = P1+ + 2e- -0.126Hydrogen 2H+ + 2e 0.000Copper Cu+4 + 2e 0.345

Copper Cu++ e 0.522

Mercury Eg + 2e- 0.799Silver Ag+ 0.800Palladium - Pd ++ + 2e -0.83Mercury - Hg + 2e 0.854

Platinum - Pt' + 2e 1.2Gold u Au+ +-3e- 1.42Gold -Au4 + e 1.68

lot,or

sw

APPENDIX

TABLE D-1

GALVANIC SERIES WITH RESPECTTO SATURATED CALOMEL ELECTRODE1

Negative Potentialto Saturated Caloml1

Metal Electrode, volts

Zinc 1.03Aluminum (Alclad 3S) 0.94Aluminum (3S-H) 0.79Aluminum (61S-T) 0.76Aluminum (52S-H) 0.74Cast iron o.61Carbon §teel o.61Stainless steel type 430

17% Cr (active) 0.57Ni-resist cast iron, 20% Ni 0.54Stainless steel type 304

18% Cr, 8% ni (active) 0.53Stainless steel type 410,

13% Cr (active) 0.52

Ni-resist cast iron, 30% Ni 0.49Ni-resist cast iron, 30% Ni + Cu 0.46Naval rolled brass 0.40

• Yellow brass 0.36

Copper 0.36Red brass 0.33Composition G bronze 0.31Admiralty brass 0.2990-10 Cupro nickel, 0.8% iron 0.2870-30 Cupro nickel, 0.06% iron 0.2770-30 Cupro nickel, 0.47% iron 0.25Stainless steel type 430,

17% Cr (passive) 0.22

1 Based on potential measurements in sea water; velocityof flow, 13 ft. per sec.; temperature 25'C. (770EF).

10

: I03

II -ll I I- -=i

TABLE i (Continued)

GALVANIC SERIES 'AITH RESPECTTO SATURATED CALO1ML ELECTRODE1

Negative Potentialto Saturated Calomel

Metal Electrode, volts1

Nickel 0.20Stainless steel typ- -6, 18% Cr,

12% Ni, 35 -e (active) 0.18Inconel 0.17Stainless steel type 410,

13% Cr (passive) 0.15

Titaniujm (commercial) 0.15Silver 0.13Titanium (high purity frrr. iodide) 0.10Stainless steel type 304',

18% Cr, 8% Ni (passive) 0.08

Hastelloy C 0.08Monel 0.08Stainless steel type 316 18 Cr

12% Ni, 3% Mo (passive5 0.05

1 Based on potential measurements in sea water; velocity

of flow, 13.ft. per sec.; temperature 250 C. (770F.).

10/i

TABLE

GALVANIC SERIES W ITH RESPECT TOSATURATED COPPER-COPPER SULFATE ELECTRODE1

Negative Potential to SaturatedMetal Copper-Copoer Sulfate electrode

Magnesium (Galvormag alloy)a 1.75Magnesium (H-I alloy)a 1.55Zinc 1.10Aluminum (Alclad 3S) 1.01Cast iron 0.68Carbon steel 0.68Stainless steel type 430, 17% Crb 0.64Ni-resist cast iron, 20% 1i. 0.61Stainless steel type 304, 18% C. 8% jib 0.60Stainless steel type 410, 13% Cr6 0.59Ni-resist cast iron, 30% Nli 0.56Ni-resist cast iron, 20%, I + Cu 0.53Naval rolled brass 0.47Yellow brass 0.43Copper 0.43Red brass 0.40Bronze, composition G 0.38

* Admiralty brass 0.3690:10 Cu-Ni + 0.8% Fe 0.3570:30 Cu-Ni + 0.06% Fe 0.3470:30 Cu-Ni + 0.47% Fe 0.32

* Stainless steel type 430, 17% Crb 0.29Nickel 0.27Stainless steel type 316, 18% Cr,

12% Ni, 3% MoD 0.25Inconel 0.24Stainless steel type 410, 13% Crb 0.22Titanium (commercial) 0.22Silver 0.20Titanium (high purity from iodide) 0.17Stainless steel type 304,18% Cr,8% Nib 0.15Hastelloy C 0.15Monel 0.15Stainless steel btype 316, 18% Cr,

12% Ni, 3% MO 0.12

1 Based on potential measurements in sea water; velocityof flow, 13 ft. per sec.; temperature 25"C. (770F).

a Based on data by The Dow Chemical Company.b The stainless steels as a class exhibited erratic poten-

tials depending on the incidence of pitting and corrosion inthe crevices formed around the specimen supports. The valueslisted represent the extremes observed and, due to their er-ratic nature, should not be considered as establishing an in-variable potential reaction among the all3ys which are covered.

105

APNDIX

UNDERGROUND CORROSIOL: SURVEYCHECK LIST

New ConstructionI. Meeting with A/E or owner.A. Description of facilities to be constructed.1. What is included?a. Gasb. Waterc. Buried electrical and grounding systemd. Buried communications or signale. Tanksf. Pilingg. Bulkheadsh. Building structural membersi. Other

2. Materials to be aled and where?a. Steelb. Cast Ironc. Leadd. Concretee. Copperf. Aluminumg. Other

3. Construction methods specified.a. Coatings - Types?b. Insulation between structures?c. Are special fills being used?d. Road and railroad casings?

Are they insulated?e. Type pipe joints - weld, flange, dresser, other?f. Type grounding connecting cables?g. Layout of structures (distance between those of

varying materials, etc.)?h. Roadways - Will deicing salts leach down into

buried structures?i. Lawns - Their location. Is it objectionable to

install above grade test stations, etc, in ornear them?

a. Pavement - Its location and type. What buriedfacilities will be placed under it?

B. Get complete drawings of all facilities.1. The following are usually included:

a. Electricalb. Meebanicala. Communicationsd. Fire Protectiona. Pilingf. Fuel systems

106V

/, -'

L._

g. Storage tanks2. Be sure they are the latest.3. Ask to be kept advised of any changes.

C. Ownership of Facilities.1. Gas, water, power, telephone, e c. - Which are to

be included in project? 'dhich are "utility own-ed"?

2. Wbere does utility's ownership and plant's juris-diction begin?

3. Will the utility install insulation?D. What life does the owner expect from his facilities?

How many y~ccrs?E. What does a corrosion failure cost? (Each type

facility)F. Are any facilities extremely critical? (no failures

of any kind to be tolerated because of cost or has-ard.)

G.- Is direct current being used anywhere in this plantor nearby?

2. Get complete information on where and why.2. Wiring diagrams and schematics.3. Method of rrounding.

H. Are any abandoned facilities located in the vicinity?(Metal pipes, etc. migbt be used as groundbeds.)Are they connected or to be connected to anythingelse?

II. Field TestsA. Soil Resistivity

If site is uniform, take 5' and 10' (usual depth ofburied structures) readings at suitably spaced grid.(20' to 100' readings may be required.) Do not ex-ceed 100' spacing with Vibroground instrument. Ifroute of piping or structure known, follow route.Take readings of fill, if any.

B. Soil pHTake pH at same places resistivity, if soil is moist.

C. Soil Samples and/or Water (steam riser etc.)Take samples for sulfides and sulfate tand pH) at re-presentative grid locations. (Min. . 6)

D. Stray CurrentsUsing 2 copper sulfate cells, take soil potentialprofile reading in a rosette pattern as necessary.

III. ConsultingA. Contact Corrosion/Maintenance Engineers of operators

in area.1. Oil Transmission Pipelines2. Gas Transmission Pipelines3. Gas Distribution Company4I. Telephone Company

_of

5. Water Department6. Electrical Power Company7- Manufacturing Plants in area8. Corrosion Coordinaing Committee9. Railroad (do nearby railroads have signal systems?

Electrical Propulsion - AC or DC?)B. Data to get from those contacted in A.1. Failure and corrosion experience.2. Is cathodic protection being used?

Type?Rectifier locations?

3. Personnel to contact for coordination tests-names, addresses and telephone numbers.

4. Place and time of Coordinating Committee meeting.5. Is stray current a problem? Its source?

Wbat structures have beer. affected?6, Are deicing salts used in 'streets??* "Are underground structures coated?

Which ones?Type Coating?

8. Get drawings and/or other location information onall structures in the area.Mark those protected and locations of rectifiers.

9. Are other new facilities planned for this area?Utilities, pipelines, etc.

10. Will these new facilities be coated and/or cathod-ically protected?

11. Is it objectionable to use impressed current catb-odic protection?

Existing Structures1. Meeting with A/E or owner.A. Find out what facilities are to be covered by this

investigation. Also get data on all others in area.I. Look for the following:

a. Gasb. Watera. Buried electrical and grounding systemd. Buried communications or signale, Tanksf. Pilinggo Bulkbeadsb. Building structural membersi. Other

2. Wat materials have been used and where?a. Steelb. Cast ironco Leadd. Concrete*. Copper

[_o

.uPA

. .... ....... -, . . m . . . .. . ... E

-- --

f. Aluminum'g. Other

3. Construction methods used.a. Coatings - Types?b. Insulation between structures?e. Are special fills being used?d. Road and railroad casings?

Are they insulated?e. Type pipe joints - weld, flange, dresser, other?f. Type grounding connecting cables?g. Layout of structures (distance between those of

varying materials, etc.)?b. Roadways - Will deicing salts leach down into

buried structures?i. Lawns - Their location. Is it objectionable to

install above grade test stations, etc. in ornear them?

3. Pavement - Its location and type. What buriedfacilities will be placed under it?

k. Have test wire, been installed on buried struc-tures?

1. Where can connections to buried structures bemade? Exposed valves, sections of pipe, etc.

B. Get complete drawings of all facilities.1. The following are usually included:a. Electricalb. Mechanicalc. Communicationsd. Fire protectione. Pilingf. Fuel systemsg. Storage tanks

2. Be sure they are the latest.3. Ask to be kept advised of any changes.4. Test station locations.5. Test station wiring diagrams.6. Insulation joint locations.7. Insulation joint types.

C. Ownership of Facilities.1. Gas water, power, telephone, etc. -Which are to be

included in project?Which are "utility owned"?

2. Were does utility's ownership end and plant'sjurisdiction begin?

3. Will the utility install insulation?4. Are utility companies using cathodic protection?5- Have the utility company's made any tests or in-

vestigations on the systems covered by this survey?D. Wat life does the owner expect from his fact Lities?

How many years?

•-

E. What does a corrosion failure cost? (Each typefacility)

F. Are any facilities extremely critical? (No failuresof any kind to be tolerated because of cost or ha-zard. )

G. Have any corrosion failures been experienced?1. How many?2. When (dates)?3. Where? (Mark on drawings)4. WhLt was their appearance?

H. Have other failures occurred?(investigate to be sure they were not really cor-rosion.)

I. Is direct current being used anywhere in this plantor nearby?

1. Get complete information on where and why.2., Wiring diagrams and scbematics.. Method of grounding.

J. Are any abandoned Lacilities located in the vicinity?(Metal pipes, etc. bnigbt be used as groundbeds.) Arethey connected or to be connected to anything else?

K. Are additional facilities planned? (Immediate orlong range) If so, get information.

1. Type and methods of construction.2. Probable location.3. How will they be connected to existing facilities?4. Will direct current be used?

II. Field TestsA. Soil Resistivity

If site is uniform, take 5' and 10' (usual depthof buried structures) readings at suitably spacedgrid. (20' and 100' readings may be required.) Donot exceed 100' spacing witb Vibroground instrument.If route of piping or structure known, follow route.Take readings of fill, if any.

B. Soil pHTake pH at same places resistivity, if soil is moist.

C. Soil Samples and/or Water (steam riser etc.)Take samples for sulfides and sulfate (and pH) atrepresentative grid locations. (Min. = 6)

D. Structure-to-Soil Voltage (at descretion of engineer).1. Thorough test of bare structure requires one over

structure and one on each side every 25'.2. Coated Structure-less frequent.

E. I.R. Drop (get at least one on every structure.)1. Always test external circuit resistance.2. Correct readings if necessary.3. Be sure to indicate polarity of all readings.

F. Voltage between structures. Teat voltage between alluetallic structures. (Be sure to indicate polarity

10

of each reading.)G. Insulating Joint- Test resistance of all known and

look for others.1. Use four connections (two on each side of Joint)

with D. C. Method.H. Mecbanical pipe Joints.

1. Test each piping system to find if mechanicalJoints exist.

2. Test representative number of mechanical Joints todetermine quantitative resistance per joint.

3. Be sure to use four po.ant contact method with di-rect current.

I. Electrical and communications cables in duct.1. All electrical tests at each manhole.a. As in D, E, and F (above),b. Be sure to test voltage between all cables in

multiple run duct systems.2. Visually inspect all hardware in each manhole.a. Bracketsb. Bondsc. Condition of cablesd. Note material of each component and its conditione. Note fastening methods and insulation between

componentsJ. Stray Current Investigation

1. Stray currents will be indicated by abnormal struc-ture-to-soil voltages and/or IR drop. (Eithersteady or fluctuating.)

2. If stray current is suspected, investigate:a. Any possible source of direct current in area.b. Operating cathodic protection.

3. Have suspected source turned off and on to estab-lish its affect on any structure.

4. Get additional IR and voltage readings to estab-liash circuit.

!1 K. Current requirement tests (for cathodic protection).At least cursory current requirement tests shouldusually be conducted if there is any chance of usingcathodic protection at the site.

1. Test using artificial groundbed for both magnesiumanode and impressed current design.

2. Extent of testing will be determined by scope ofwork laid out by client. (Is all design data to beincluded with tbis survey?)

L. Existing cathodic protection.1. Visually inspect all equipment.2. Test to determine protection being afforded and

possible interference to other structures.3. Get operating record.4. Find out when installed and turned on.

M. Miscellaneous - Note any other corrosion problems(chemical, water, atmospheric, etc.) which could use

11 .1

4

further detailed study.

II. Consultingk. Contact all plant personnel who have knowledge of

structures being studied and get all possible in-fozmation from them.

B. Contact Corrosion/Maintenance Engineers of operatorsin area.

1. Oil Transmission Pipelines2. Gas Transmission Pipelines3. Gas Distribution Company4. Telephone Company5. Water Department6. Electrical Power Company7. Manufacturing Plants in area8. Corrosion Coordinatini Committee9.. Railroad (Do nearby railroads have signal systems?

Electrical Propulsion - AC or DC?)C. Data to get from those contacted in B.1. Failure and corrosion experience.2, Is cathodic protection being used? Type? Recti-

fier Locations?3*. Personnel to contact for coordination tests - names,

addresses and telephone numbers.4. Place and time of Coordinating Committee meeting.5- Is stray current a problem?

Its source?What structures have been affected?

6. Are deicing salts used in streets?7.. Are underground structures coated?

Which ones?Type coating?

8. Get drawings and/or other location information onall structures in the area. Mark those protectedand locations of rectifiers.

9. Are other new facilities planned for this area?Utilities, pipelines, etc.

10. Will these new facilities be coated an./or catbod-ically protected?

11. Is it objectionable to use impressed current cath-odic protection.

. .1

INDEX

Paragraph

*: A

Ammeter zero-resistance ............................. 2.2.1

Antimony reference electrode ........................ 1.2.1e

B

Bacteria, anaerobic ................................. 1.2.1f

Batteries ........................................... 2.3.2

C

Cable ducts, for lead sheath cable ................... 1.2.2f

Cathodic interference ............................... 12.2b

I Chloride content .................................. 1.3.1c

Circuit, electrical ................................. 1.1.1

Coatings, conductance .............................. 12.2e

Combination meter ................................. 2.2.11

Contact resistance .................................. 1.1.2

Continuity tests .................................... 1.2.2d

Correlation curves ......................... 1.2.1b, 1.2.2c

Current

alternating ..... .............................. 1.1.1

direct ........................................... 1.1.1interrupter ...................................... 2.3.3 A

measurement ............................ l..2a, 1.2.1b 4

"Null Method" of measurement ..................... 1.2.1b

D

D'Arsonval movement ............................... 2.1.la.

Drainage bonds.................................... 1.2.2b

Ducts, cable ........................................ 1.2.2f

? ~K .1* -.

Paragraph

E

Electrical connecitons ................................. .1.2.2

Electrical quantities...................................1.1.1

I

Instruments ........................................... 2

accessories ........................................ 2.3

accuracy ........................................... 2.2

typical field ...................................... 2.1

Interference .......................................... 1.2.2b

testing ............................................ 1.2.2b

M

Meter movement .......................................... 2.1.1

0

Ohm's Law ............................................. 1.1.1

P

pH measurement ........................................ 1.3.1a,e

Pipe locators ........................................ 2.2.9

Potential structure-to-electrolyte .................... 1.2.1a

Potentiometer-voltmeter ............................... 2.2.4

R

Recorders ............................................. 2.2.3

Rectifiers

efficiency ......................................... 1.2.lg

oortable ........................................... 2.3.1

Redox ................................................. 1.2.1f

A

Paragraph

Reference electrode................................ 1.2.la(l)(c)

antimony ....................................... 2.3.9

calomel ........................................ 2.3.6

copper-copper sulfate ............................ 2.3.5

lead-lead chloride.............................. 2.3.8

silver-solver chloride ........................... 2.3.7

Remote earth ...................................... 1.2.la(2)

Resistance

contact ........................................ 1.1.2

external circuit................................ I.2.la(l)(b)

internal ....................................... 1.2.1c

measurement.............................l1.1.2c, 1-2.1c

vs. resistivity.................................l1-1.2d

Resistivity ......................... l1.1.2d, 1-2-1d, 2.2.8

meters ......................................... 2-2.8b

soil box ....................................... l1.3. d

Resistor, variable................................. 2.3.11

S

Shepard canes ..................................... 2;2.8b

*shunt ................................. 2.3.10

Sol

box .................................... l1.3.1d, 2.3.4

rod..o..........................................l1.2.d03)

Stray current ................... o.................. 1.2.2f

Sulfate content ................................... 1.3.2

Sulfide content..................................1-3.1b

Paragraph

T

Tanks potential measurement inside ............... 1.2.la(2)

Taut band suspension ............................. 2.1.1b

Testing .......................................... 1

laboratory .................................... 1.3

V

Voltage

CL-op .......................................... 1.1.1

measurement ................................... 1.1.2b

Voltmeter ........................................ 2.2.2

recording ..................................... 2.2.3

vacuum-tube ................................... 2.2.6

Volt-ohm--millianmeter ............................ 2.2.11a

W

Water analysis ................................... 1.3.3

/

.... ... -- , . ... . .. .. . .. ... ..- - .., _ , 1