Handbook of Corrosion Engineering Episode 2 Part 10 ppt

This potential V t is made up of two components: Needle deflection points toward defect Needle deflection points toward defect No needle deflection Buried pipeline Decreasing signal stre

Pearson survey. The Pearson survey, named after its inventor, is used

to locate coating defects in buried pipelines Once these defects havebeen identified, the protection levels afforded by the CP system can beinvestigated at these critical locations in more detail

Methodology. An ac signal of around 1000 Hz is imposed onto thepipeline by means of a transmitter, which is connected to the pipelineand an earth spike, as shown in Fig 11.25 Two survey operators makeearth contact either through metal studded boots or aluminum poles

A distance of several meters typically separates the operators.Essentially, the signal measured by the receiver is the potential gradi-ent over the distance between the two operators Defects are located by

a change in the potential gradient, which translates into a change insignal intensity

As in the CIPS technique, the measurements are usually recorded

by walking directly over the pipeline As the front operator approaches

a defect, increasing signal intensity is recorded As the front personmoves away from the defect, the signal intensity drops and later picks

up again as the rear operator approaches the defect The tion of signals can obviously become confusing when several defectsare located between the two operators In this case, only one personwalks directly over the pipeline, with the connecting leads at a rightangle to the pipeline

interpreta-In principle, a Pearson survey can be performed with an impressedcathodic protection system remaining energized Sacrificial anodes

Test station

Receiver

Aluminum pole

should be disconnected because the signal from these may otherwisemask actual coating defects A three-person team is usually required

to locate the pipeline, perform the survey measurements, place defectmarkers into the ground, and move the transmitters periodically Theoperator carrying the receiver should be highly experienced, especially

if the survey is based on audible signals and instrument sensitivitysettings Under these conditions, the results are completely dependent

on this operator’s judgment

Advantages and limitations. By walking the entire length of thepipeline, an overall inspection of the right-of-way can be made togetherwith the measurements In principle, all significant defects and metal-lic conductors causing a potential gradient will be detected There are

no trailing wires and the impressed CP current does not have to bepulsed

The disadvantages are similar to those of CIPS because the entirepipeline has to be walked and contact established with ground Thetechnique is therefore unsuitable to roads, paved areas, rivers, and soforth Fundamentally, no severity of corrosion damage is indicated and

no direct measure of the performance of the CP system is obtained.The survey results can be very operator dependent, if no automatedsignal recording is performed

Direct current voltage gradient (DCVG) surveys. DCVG surveys are amore recent methodology to locate defects on coated buried pipelinesand to make an assessment of their severity The technique againrelies on the fundamental effect of a potential gradient being estab-lished in the soil at coating defects under the application of CP cur-rent; in general, the greater the size of the defect, the greater thepotential gradient The DCVG data is intricately tied to the overallperformance of a CP system, because it gives an indication of currentflow and its direction in the soil

Methodology. The potential gradient is measured by an operatorbetween two reference electrodes (usually of the saturated Cu/CuSO4type), separated by a distance of say half a meter The appearance ofthese electrodes resembles a pair of cross-country ski poles (Fig.11.26) A pulsed dc signal is imposed on the pipeline for DCVG mea-surements The pulsed input signal minimizes interference from othercurrent sources (other CP systems, electrified rail transit lines, telluriceffects) This signal can be obtained with an interrupter on an existingrectifier or through a secondary current pulse superimposed on theexisting “steady” CP current

The operator walking the pipeline observes the needle of a voltmeter needle to identify defect locations (More recently devel-

milli-914 Chapter Eleven

oped DCVG systems are digital and do not have a needle as such.)

It is preferable for the operator to walk directly over the pipeline,but it is not strictly necessary The presence of a defect is indicated

by a increased needle deflection as the defect is approached, no dle deflection when the operator is immediately above the defect,and a decreasing needle deflection as the operator walks away fromthe defect (Fig 11.27) It is claimed that defects can be located with

nee-an accuracy of 0.1 to 0.2 m, which represents a major advnee-antage inminimizing the work of subsequent digs when corrective action has

to be taken

Cathodic Protection 915

Figure 11.26 DCVG measuring equipment (Courtesy of CSIR

North America Inc.)

An additional feature of the DCVG technique is that defects can beassigned an approximate size factor Sizing is most important for iden-tifying the most critical defects and prioritizing repairs Leeds andGrapiglia15have provided details on the sizing procedure An empiri-cally based rating based on the so-called %IR value has been adopted

in general terms as follows:

■ 0 to 15%IR (“small”): No repair required usually

■ 16 to 35%IR (“medium”): Repairs may be recommended

■ 36 to 60%IR (“large”): Early repair is recommended

■ 61 to 100%IR (“extra large”): Immediate repair is recommended

To establish a theoretical basis for the %IR value, the pipeline tial measured relative to remote earth at a test post must be consid-

poten-ered This potential (V t) is made up of two components:

Needle deflection points toward defect

Needle deflection points toward defect

No needle deflection

Buried pipeline

Decreasing signal strength (when leaving defect)

No signal (when directly above defect)

X

X Location of coating defect Equipotential lines

Figure 11.27 DCVG methodology (schematic).

potential difference across each of them Although V icannot be

mea-sured easily in practice, V scan be measured relatively easily with theDCVG instrumentation (one reference electrode is initially placed atthe defect epicenter, and the voltage change is then summed as theelectrodes are moved away from the epicenter to remote earth) In

practice, the V svalue measured at a test post has to be extrapolated to

a value at the defect location Two test post readings bracketing thedefect location and simple linear extrapolation are usually employed

For effective protection of the defect by the CP system, the V s /V tratioshould be small The overall shift in pipeline potential due to the appli-

cation of CP should be manifested by mainly shifting V i , not V s Higher

%IR values imply a lower level of cathodic protection

Because the DCVG technique can be used to determine the direction

of current flow in the soil, a further defect severity ranking has beenproposed As indicated in Fig 11.1, current will tend to flow to a defectunder the protective influence of the CP system Corrosion damage(anodic dissolution) at the defect has an opposite influence; it will tend

to make current flow away from the defect Using an adaptation of theDCVG technique, it has been claimed that it is possible to establishwhether current flows to or from a defect, with the CP system switched

ONand OFFin a pulsed cycle

Advantages and limitations. Fundamentally, the DCVG technique isparticularly suited to complex CP systems in areas with a relativelyhigh density of buried structures These are generally the most diffi-cult survey conditions The DCVG equipment is relatively simple andinvolves no trailing wires Although a severity level can be identifiedfor coating defects, the rating system is empirical and does not providequantitative kinetic corrosion information The survey team’s rate ofprogress is dependent on the number of coating defects present.Terrain restrictions are similar to the CIPS technique However, itmay be possible to place the electrode tips in asphalt or concrete sur-face cracks or in between the gaps of paving stones

V s

V t

Cathodic Protection 917

Corrosion coupons. Corrosion coupons connected to cathodically tected structures are finding increasing application for performancemonitoring of the CP system Essentially these coupons, installeduncoated, represent a defect simulation on the pipeline under con-trolled conditions These coupons can be connected to the pipeline via

pro-a test post outlet, fpro-acilitpro-ating pro-a number of mepro-asurements such pro-aspotential and current flow

A publication describing an extensive coupon development and itoring program on the Trans Alaska Pipeline System16 serves as anexcellent case study This coupon monitoring program was designed toassess the adequacy of the CP system under conditions where tech-niques involving CP current interruption on the pipeline were imprac-tical Although the coupon monitoring methodology is based onrelatively simple principles, significant development efforts and atten-tion to detail are typically required in practice, as this case studyamply illustrates

mon-Methodology. Perhaps the most important consideration in theinstallation of corrosion coupons is that a coupon must be representa-tive of the actual pipeline surface and defect The exact metallurgicaldetail and surface finish as found on the actual pipeline are thereforerequired on the coupon The influence of corrosion product buildupmay also be important Furthermore the environmental conditions ofthe coupon and the pipe should also be matched (temperature, soil con-ditions, soil compaction, oxygen concentration, etc.) Current shieldingeffects on the bonded coupon should be avoided

Several measurements can be made after a coupon-type corrosionsensor has been attached to a cathodically protected pipeline.17 ONpotentials measured on the coupon are in principle more accurate thanthose measured on a buried pipe, if a suitable reference electrode isinstalled in close proximity to the coupon The potentials recorded with

a coupon sensor may still contain a significant IR drop error, but thiserror is lower than that of surface ONpotential measurements Instant-OFF potentials can be measured conveniently by interrupting thecoupon bond wire at a test post Similarly, longer-term depolarizationmeasurements can be performed on the coupon without depolarizingthe entire buried structure Measurement of current flow to or from thecoupon and its direction can also be determined, for example, by using

a shunt resistor in the bond wire Importantly, it is also possible todetermine corrosion rates from the coupon Electrical resistance sen-sors provide an option for in situ corrosion rate measurements as analternative to weight loss coupons

The surface area of the coupon used for monitoring is an importantvariable Both the current density and the potential of the coupon are

918 Chapter Eleven

dependent on the area In turn, these two parameters have a directrelation to the kinetics of corrosion reactions.

Advantages and limitations. A number of important corrosion ters can be conveniently monitored under controlled conditions, with-out any adjustments to the energized CP system of the structure Themeasurement principles are relatively simple It is difficult (virtuallyimpossible) to guarantee that the coupon will be completely represen-tative of an actual defect on a buried structure The measurements arelimited to specific locations The coupon sensors have to be extremelyrobust and relatively simple devices to perform satisfactorily underfield conditions

parame-References

1 Ashworth, V., The Theory of Cathodic Protection and Its Relation to the Electrochemical Theory of Corrosion, in Ashworth, V., and Booker, C J L (eds.),

Cathodic Protection, Chichester, U.K., Ellis Horwood, 1986.

2 Peabody, A W., Control of Pipeline Corrosion, Houston, Tex., NACE International,

1967.

3 Eliassen, S., and Holstad-Pettersen, N., Fabrication and Installation of Anodes for

Deep Water Pipelines Cathodic Protection, Materials Performance, 36(6):20–23

(1997).

4 Sydberger, T., Edwards, J D., and Tiller, I B., Conservatism in Cathodic Protection

Designs, Materials Performance, 36(2):27–32 (1997).

5 Shreir, L L., and Hayfield, P C S., Impressed Current Anodes, in Ashworth, V., and

Booker, C J L (eds.) Cathodic Protection, Chichester, U.K., Ellis Horwood, 1986.

6 Shreir, L L., Jarman, R A., and Burstein, G T (eds.), Corrosion, vol 2, 3d ed., Oxford, Butterworth Heinemann, 1994.

7 Beavers, J A., and Thompson, N G., Corrosion Beneath Disbonded Pipeline

Coatings, Materials Performance, 36(4):13–19, (1997).

8 Jack, T R., Wilmott, M J., and Sutherby, R L., Indicator Minerals Formed During

External Corrosion of Line Pipe, Materials Performance, 34(11):19–22 (1995).

9 Kirkpatrick, E L., Basic Concepts of Induced AC Voltages on Pipelines, Materials

Performance, 34(7):14–18 (1995).

10 Allen, M D., and Ames, D W., Interaction and Stray Current Effects on Buried

Pipelines: Six Case Histories, in Ashworth, V., and Booker, C J L (eds.), Cathodic Protection Chicester, U.K., Ellis Horwood, 1986, pp 327–343.

11 NACE International and Institute of Corrosion, Cathode Protection Monitoring for Buried Pipelines, pub no CEA 54276, Houston, Tex, NACE International, 1988.

12 Goloby, M V., Cathodic Protection on the Information Superhighway, Materials

Performance, 34(7):19–21 (1995).

13 Pawson, R L., Close Interval Potential Surveys—Planning, Execution, Results,

Materials Performance, 37(2):16–21 (1998).

14 NACE International, Specialized Surveys for Buried Pipelines, pub no 54277,

Houston, Tex, NACE International, 1990.

15 Leeds, J M., and Grapiglia, J., The DC Voltage-Gradient Method for Accurate

Delineation of Coating Defects on Buried Pipelines, Corrosion Prevention and

Control,42(4):77–86 (1995).

16 Stears, C D., Moghissi, O C., and Bone, III, L., Use of Coupons to Monitor Cathodic

Protection of an Underground Pipeline, Materials Performance, 37(2):23–31 (1998).

17 Turnipseed, S P., and Nekoksa, G., Potential Measurement on Cathodically Protected Structures Using an Integrated Salt Bridge and Steel Ring Coupon,

Materials Performance, 35(6):21–25 (1996).

Cathodic Protection 919

Anodic Protection

12.1 Introduction 921 12.2 Passivity of Metals 923 12.3 Equipment Required for Anodic Protection 927 12.3.1 Cathode 929 12.3.2 Reference electrode 929 12.3.3 Potential control and power supply 930 12.4 Design Concerns 930 12.5 Applications 932 12.6 Practical Example: Anodic Protection in the Pulp and

Paper Industry 933 References 938

12.1 Introduction

In contrast to cathodic protection, anodic protection is relatively new.Edeleanu first demonstrated the feasibility of anodic protection in 1954and tested it on small-scale stainless steel boilers used for sulfuric acidsolutions This was probably the first industrial application, althoughother experimental work had been carried out elsewhere.1 This tech-nique was developed using electrode kinetics principles and is some-what difficult to describe without introducing advanced concepts ofelectrochemical theory Simply, anodic protection is based on the for-mation of a protective film on metals by externally applied anodic cur-rents Anodic protection possesses unique advantages For example,the applied current is usually equal to the corrosion rate of the pro-tected system Thus, anodic protection not only protects but also offers

a direct means for monitoring the corrosion rate of a system As an

Chapter

12

enthusiast and famous corrosion engineer claimed, “anodic protectioncan be classed as one of the most significant advances in the entire his-tory of corrosion science.”2

Anodic protection can decrease corrosion rate substantially Table 12.1lists the corrosion rates of austenitic stainless steel in sulfuric acid solu-tions containing chloride ions with and without anodic protection.Examination of the table shows that anodic protection causes a 100,000-fold decrease in corrosive attack in some systems The primary advan-tages of anodic protection are its applicability in extremely corrosiveenvironments and its low current requirements.2Table 12.2 lists severalsystems where anodic protection has been applied successfully

Anodic protection has been most extensively applied to protect ment used to store and handle sulfuric acid Sales of anodically pro-tected heat exchangers used to cool H2SO4manufacturing plants haverepresented one of the more successful ventures for this technology

equip-922 Chapter Twelve

TABLE 12.1 Anodic Protection of S30400 Stainless Steel Exposed to

an Aerated Sulfuric Acid Environment at 30°C with and without

Protection at 0.500 V vs SCE

Corrosion rate, my-1Acid concentration, M NaCl, M Unprotected Protected

Among the parameters that are particularly affected by sensitization

are i p and icc, as defined in Fig 12.1 In this example, the ability to

sus-tain passivity increases as the current density to mainsus-tain passivity

(i p) decreases and as the total film resistance increases, as indicatedfrom measurements obtained with different metals exposed to 67%sulfuric acid (Table 12.3) The lower or more reducing the potential atwhich a passive metal becomes active, the greater the stability of pas-sivity The depassivation potential corresponding to the passive-active

transition, called the Flade potential, can differ appreciably from Epp

measured by going through the active-passive process of the same tem This technical distinction is important for the control aspect of

sys-anodic protection where Eppis the potential to traverse to obtain sivation, and the Flade potential is the potential to avoid traversingback into active corrosion

pas-Passivity can also be readily produced in the absence of an externallyapplied passivating potential by using oxidants to control the redoxpotential of the environment Very few metals will passivate in nonoxi-dizing acids or environments, when the redox potential is more cathodicthan the potential at which hydrogen can be produced A good example

of that behavior is titanium and some of its alloys, which can be readilypassivated by most acids, whereas mild steel requires a strong oxidizing

924 Chapter Twelve

Log (Current density)

Ecorr(corrosion potential)

transpassive

passive

Figure 12.1 Hypothetical polarization diagram for a passivable system with active, sive, and transpassive regions.

pas-agent, such as fuming HNO3, for its passivation Alloying with a moreeasily passivated metal normally increases the ease of passivation andlowers the passivation potential, as in the alloying of iron and chromium

in 10% sulfuric acid (Table 12.4) Small additions of copper in carbon

steels have been found to reduce i pin sulfuric acid Each alloy system has

to be evaluated for its own passivating behavior, as illustrated by thecase Ni-Cr alloys where both the additions of nickel to chromium andchromium to nickel decrease the critical current density in a mixture

of sulfuric acid and 0.25 M K2SO4(Table 12.5).1

The parameters defining and controlling the passivation domain of

a system are thus directly related to the composition, concentration,purity, temperature, and agitation of the environment This is illus-

trated with the current densities required to obtain passivity (icc), and

to maintain passivity (i p), for a S30400 steel in different electrolytes,

as presented in Table 12.6 From the data in this table, it can be seenthat it is approximately 100,000 times easier to passivate large areas

of this steel in contact with 115% phosphoric acid than in 20% sodiumhydroxide The concentration of the electrolyte is also important, andfor a S31600 steel in sulfuric acid, although there is a maximum cor-rosion rate at about 55%, the critical current density decreases pro-gressively as the concentration of acid increases (Table 12.7).1

sen-The presence in the environment of impurities that retard the mation of a passive film or accelerate its degradation is often detri-mental In this context, chloride ions can be quite aggressive formany alloys and particularly for steels and stainless steels As anexample, the addition of 3% HCl hydrochloric acid to 67% sulfuricacid raises the critical current density for the passivation of a S31600stainless steel from 0.7 to 40 mAcm2 and the current density tomaintain passivity from 0.1 to 60 Acm2 Therefore, the use of thecalomel electrode in anodic-protection systems is not recommendedbecause of the possible leakage of chloride ions into the electrolyte,

for-926 Chapter Twelve

TABLE 12.3 Current Density to Maintain Passivity and Film

Resistance of Some Metals in 67% Sulfuric Acid

Metal or alloy i p ,Acm 2 Film resistance, Mcm

TABLE 12.4 Effect on Critical Current Density

and Passivation Potential of Chromium Content

for Iron-Chromium Alloys in 10% Sulfuric Acid

TABLE 12.5 Effect on Critical Current Density and

Passivation Potential on Alloying Nickel with Chromium

-500

0 500

Hastelloy cathode

Hg/HgSO4 reference electrode

Figure 12.4 Schematic of an anodic protection system for a sulfuric acid storage vessel.

equipment to be protected, considering any special operational tions As described earlier, the electrochemical parameters of concernare the potential at which the vessel must be maintained for corrosionprotection, the current required to establish passivity, and the currentrequired to maintain passivity The electrode potential can be deter-mined directly from polarization curves, and the required currents can

condi-be estimated from the polarization data However, condi-because the current

is so strongly time dependent, its variations with respect to time must

be carefully estimated Empirical data available from field installationsare the best source for this type of information.3

Special care and attention should also be focused on estimating thesolution resistivity of a system because it is important in determiningthe overall circuit resistance The power requirements for the dc powersupply should be as low as possible to reduce operating costs Thesolution resistivity should usually be sufficiently low so that the cir-cuit resistance is controlled by the cathode surface area It is essen-tial for a system to have good throwing power or good ability for theapplied current to reach the required value over complex geometryand variable distances In general, a uniform distribution of potentialover a regular-shaped passivated surface can be readily obtained byanodic protection It is much more difficult to protect surface irregu-larities, such as the recessions around sharp slots, grooves, or crevicesbecause the required current density will not be obtained in theseareas This incomplete passivation can have catastrophic conse-quences This difficulty can be overcome by designing the surface toavoid these irregularities or by using a metal or alloy that is easilypassivated with as low a critical current density as possible In therayon industry, crevice corrosion in titanium has been overcome byalloying it with 0.1% palladium.1

The actual passivation of a surface is very rapid if the applied rent density is greater than the critical value However, because of thehigh current requirements, it has been found to be neither technicallynor economically practical to passivate the whole surface of a largevessel in the same initial period For a storage vessel with an area of

cur-1000 m2, for example, a current of 5000 A could be necessary It istherefore essential to avoid these very high currents by using one of afew techniques It may be possible and practical, for example, to lowerthe temperature of the electrolyte, thereby reducing the critical cur-rent density before passivating the metal If a vessel has a very smallfloor area, it may be treated in a stepwise manner by passivating thebase, then the lower areas of the walls, and finally the upper areas ofthe walls, but this technique is not practical for very large storagetanks with a considerable floor area.1

Another method that has been successful is to passivate the metal byusing a solution with a low critical current density (such as phosphoric

Anodic Protection 931

sive) A potentiodynamic curve of each of these types of behavior isshown, respectively, in Figs 12.6 through 12.9 Astable behavior occursinfrequently because it requires a single anodic-cathodic intersection

on the negative resistance portion of the anodic curve This is an ble operating condition that results in continuous oscillations betweenactive and passive potentials Various alloys in elevated temperaturesulfuric acid are known to exhibit such behavior.6

unsta-The four types of mixed potential models presented in Figs 12.6 to12.9 are simplistic and do not necessarily reflect the complete behav-ior of carbon steel in Kraft liquors because the models all assume somesort of steady states Figure 12.10 depicts typical curves from an insitu test in a white liquor clarifier at different scan rates The passivestate does not exist until after the active-passive transition is tra-versed Therefore, unless sufficient anodic current density is dis-charged from carbon steel by a naturally occurring cathodic reaction or

an applied anodic protection current, the carbon steel liquor interfaceremains monostable (active) because the passive film and its low cur-rent density properties do not exist

Under normal operating chemistries in white and green Kraftliquors, carbon steel exhibits a monostable (active) behavior, and thebistable behavior occurs only after the passivation process has reachedsome degree of completion, as predicted by Tromans and verified by

Figure 12.6 Theoretical polarization curve illustrating the monostable (active) behavior

of mild steel exposed to Kraft liquors.