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ExternalPolmericPioeline oatingailureModes

SANKARAPAPAVINASAM,MICHAEL ATIARD, AND R. WINSTON REvIE,CANMET Materials TechnologyLaboratory

Pipelinecoatingsamajordefenseagainstcorrosion.Cathodicprotectionprovidesprotectionatcoatingholidays.Anychangenthechemical.physical.orelectrochemicalropertiesofanexternalpipelinecoatingcanbeconsideredsafailure.Somefailuresarecatastrophic.whereasothershavelittleornoeffectonthecoatingperformance.heeightmostcommonailuremodesofexternalpolymericpipelinecoatingsareassessednthisarticle.

s long as pipeline coatingsare intact and completelyisolate the pipeline from theenvironment, corrosionshould not occur. But overthe years, coatings undergohanges that affect their

ability to isolate the pipeline from theenvironment. Generally, any changes inthe properties of a coating are consideredas a coating failure. The degree of influ-ence of the changes on pipeline integrityvaries, depending on the extent and thenature of changes. The predominant fail-ure modes are discussed in this article.ModeofFailure

AIR PERMEATIONPolymeric pipeline coatings are perme-

able because of the presence of pores at themolecular level. Gas can permeate through

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the pores. When gases permeate a coatingthat is well bonded to steel, the pressurewithin the coating increases. At high levelsof permeation, the build-up of pressure maybe reduced by the liberation of gases, caus-ing disbondment of the coating. At lowlevels of permeation, the state of equilib-rium is reached without any chemical orphysicalchangesto the coating.I

WATER PERMEATIONIn addition to gases, water and salts can

also penetrate the coating. Permeation isfurther facilitated by osmosis and electro-osmosis. When a semipermeable mem-brane (e.g., pipeline coating) separates asolution of different concentrations, thewater permeates from the concentratedsolution side to the dilute solUtion side sothat the concentrations at both sides of themembrane become the same. This processis called osmosis. The presence of salts onthe contaminated steel surface results inthedevelopment of the osmosis process. Ifosmosis is facilitated by the electrical cur-rent flow caused by the application of ca-thodic protection (CP), it is called electro-osmosis.2-4

LOSS OF ADHESIONAdhesion is a measure of the degree of

attachment between the coating and thepipeline steel with which it is in contact.The adhesion is a force that keeps the coat-ing on the steel surface.5 Adhesion may becaused by chemical, physical, and me-chanical interactions. When these interac-tions are diminished, the coating loses itsadhesion.

LOSS OF COHESIONThe cohesive strength is the bonding

within the coating itself that holds the coat-ing together as an entity. A coating withgreat cohesion will break the adhesive bondwith the surface and then peel from thesurface to form a free-standing coating (Fig-ure 1). On the other hand, if the cohesivestrength is less than the adhesive strength,the coating will break within itself, leavingpart of the coating on the surface and parrof it off the surface (Figure 2).6 Becausethepipe surface is protected by the remainder

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the coating, the damage is not as severethat caused hy adhesion failure.

BLISTERINGThe swelling of coatings from watersorption causesa lateral distortion ofe filmwith respect to the steel.Stressesthe coating/steel interfacearisingfrome distortion produce a lossof adhesion,using a blister. If CP completely pene-ates through the blistered coating, thene pH of the solution will be in the al-line range (>7). Under blistered coat-gs, a pH as high as 12 has been ob-

DISBONDMENT ANDPASSAGE OF CATHODICPROTECTION CURRENT

Degradation of adhesion by CP isown as cathodic disbondment.9 Factorscluding pH, cathodic potential, theability of the interfacial oJ{ide,the sub-rate surface roughness, defect geometry,ating formulations, cyclic wetting andying, and water uptake contribute to/lOdic disbondment. As in the case ofblistered coating, if CP completely pen-rates the disbonded coating, the pH ofe solution below the disbonded coatingill be in the alkaline range.IO The in-ease in pH comes from the consump-on of hydrogen ions and the generationhydroxyl ions. Maintaining a high pHvironment helps to protect the steelneath a disbonded coating by passivat-g the pipeline steel. The interruptionremoval of the CP system may cause acrease in pH and cause the potentialsshift to more positive values. Atore positive potentials, corrosion may

IICoating disbondment leads to the for-tion of crevices. Sizesof disbondments

epend on the coating, the species in thevironment, the morphology of the dis-ondment, and the level of CP. Whereccess to the inside of a crevice is re-icted, a significantly different chemistryy be present compared to the chemistrythe groundwater.Where the solution is trapped andere is little or no exchange with fresh

FIGURE1

/' Coating partly separated from steelCohesion failure ~~ ""''.., .-~ , ,.- PipelinePart of coatingadhering onto steel

Sketchshowingadhesionfailureof a coating.

FIGURE2

Sketchshowingcohesivecoatingfailure.

solution, the corrosion rate of steel willdecrease rapidly and remain at low values.Where an exchange of solution (the flowof water between the coating and thepipe) can occur, a frequent or continualgeneration of alkalinity by external polar-ization is required to control corro-sion.12In high-conductivity solutions, CP can

be effective within a crevice. Clean crev-ices with large openings are easier topolarize than those containing mill scaleor corrosion products. The pH of thesolution inside the crevice is alkaline,s-12whereas the bulk solution just outside thecrevice remains neutral.

DISBONDMENT ANDPREVENTION OF THE PASSAGEOF CATHODIC PROTECTIONGenerally, if the solution resistance is

high, there is a large ohmic drop and es-sentially no flow of current into the dis-bonded region. 13If CP does not penetrate,then corrosion occurs at the disbonded

region even when the pipe-to-soil poten-tial at the ground surface meets the -0.85V or other criterion.

INCREASE IN CATHODICPROTECTION CURRENT

Even on a microscopic level, poly-meric coatings exhibit pores or holidays.The origin of holidays can be traced tothe mode of crystal formation/growth.14During construction, crews are carefulnot to damage the protective coating. Inspite of these precautions, sometimescoatings are removed and/or damaged.These areas act as potential locationswhere disbondments may initiate. Exter-nal coatings and CP work synergisticallyto mitigate the corrosion of pipelines.It is difficult to cathodicaUy protect abare pipeline because the magnitude ofthe current required for protection ishigh. A good-quality coating can de-crease the current required by a factor of1,000 or more. As the coating deterio-rates and/or more and more holidays are

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lo_usaLiningsformed, the CP current demand in-creases, until it is economically not fea-sible to protect the pipeline with a dete-riorating coating.Six coating systems were evaluated overa period of25 years: fusion-bonded epoxy

(FBE), coal-tar enamel, asphalt enamel,polyethylene tape, asphalt mastic, and ure-thane. 15Several locations exhibited pittingcorrosion. These locations were distributedthrough all four coating types and weregenerally located in areaswhere the coatingconditions were poor. Many of the defectswere in areas where the CP was initiallyconsidered to be adequate, indicating that,where coatings were in poor condition, CPwas not completely effective.RankingfCoatingailureModesAlthough any chemical, physical, or

electrochemical changes may be consideredas a coating failure, not all changes affectthe ability of coatings to protect the pipe-line. In an ideal situation, polymeric coat-ing protects the pipeline and, when it fails,the CP acts as the backup. Only after bothdefense mechanisms failwould the pipelinebecome susceptible to corrosion.The worst-case scenario of coating fail-

ure is the one in which the coating nolonger protects the pipeline, and, in addi-tion, the coating prevents the CP fromprotecting the pipeline. This type of failuremode is primary in terms of the impact ofthe failure on the protection of the pipe.The presence of holidays is the secondmost common cause offailure because the

CP current increases as the holiday sizeand number increase. The alkaline pHcreated by the CP can easily become dif-fused, and hence, the CP should be ap-plied continuously.The formation of disbondment behinda coating that passes CP presents the thirdranking. In this case the coating has failed,but CP can act as the backup. Because ofthe diffusion limitation of the hydroxide(OH-) ions, the amount of CP requiredis smaller.The formation of blisters is the fourth-

ranked cause of coating failure. This fail-ure is associated with the penetration ofwater, so that CP can prevent corrosionat this location. This case is better than30 MATERIALSPERFORMANCEOctober 2006

that with disbonded coating protectedwith CP because CP reaches the steelsurface readily and uniformly.Loss of adhesion makes the coating

unable to perform its primary function(Le., to cover the steel surface). This is thefifth-ranked mode of failure. Loss of cohe-sion is the sixth because at least part of thecoating still covers (and hence protects)the steel surface.Water permeation through the coatingis the seventh-ranked failure mode. This

mode of failure establishes electrochemi-cal cells, facilitating conditions for corro-sion to occur.The permeation of gases may break

certain chemical bonds, but may havelimited effect on the overall performanceof the coating. This is the eighth-rankedfailure mode.

SummaryAny change in the chemical, physi-cal, or electrochemical properties ofthe external polymeric pipeline coat-ings can be considered as a failure.

Some failures are catastrophic,whereas others have little or no effecton the coating performance. The eight most common failuremodes of external polymeric pipelinecoatings are ranked in the order of theimpact of the failure on the protectionof the pipe.

AcknowledgmentsThe authors would like to acknowledge

the helpful discussionsand financialsuppottfrom the Canadian Federal Government'sProgram of Energy R&D (PERD) and itsmembers (Alberta Energy Corporation[AEC], Enbridge, KOGAS, Reilly Indus-tries, Shaw Pipe Coating, SpecialtyPolymerCoatings [SPC], TransCanada Pipeline[TCPL] and 3M) of the CANMET/NRC/Industry Consortium on External PipelineCoatings for Prevention of Corrosion andStress-Corrosion Cracking.

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13. J.J. Perdomo. I. Song, "Chemical and EIecrrochemicalConditions on Steel under Disbonded Coatings: The EffectofApplied Potential. Solution Resistivity. Crevice Thickness. andHol iday Size; Corros. ScL42 (2000) : p. 1.389.

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SANKARAPAPAVINASAMs a research sci-entist and project leader at CANMETMaterialsTechnology laboratory (MTU, 568Booth St.,Ottawa, ON, Canada. He has been involved inpipeline corrosion control issues since joiningCANMET MTL in 1994. He has developed threesoftware packages for prediction and controlof internal and external corrosion of oil andgas pipelines. He has an M.Sc. ,M.Phil,and Ph.D.