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Analysis, Assessment, and Mitigation of Stress Corrosion Cracking in Austenitic Stainless Steels in the Oil and Gas Sector: A Review

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Mohammadtaghi Vakili at ORLEN UniCRE
Mohammadtaghi Vakili
  • ORLEN UniCRE
Petr Koutník
Petr Koutník
Petr Koutník
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Jan Kohout
Jan Kohout
Jan Kohout
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Zahra Gholami at ORLEN UniCRE
Zahra Gholami
  • ORLEN UniCRE
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Abstract and Figures

This comprehensive review examines the phenomena of stress corrosion cracking (SCC) and chloride-induced stress corrosion cracking (Cl-SCC) in materials commonly used in the oil and gas industry, with a focus on austenitic stainless steels. The study reveals that SCC initiation can occur at temperatures as low as 20 °C, while Cl-SCC propagation rates significantly increase above 60 °C, reaching up to 0.1 mm/day in environments with high chloride concentrations. Experimental methods such as Slow Strain Rate Tests (SSRTs), Small Punch Tests (SPTs), and Constant-Load Tests (CLTs) were employed to quantify the impacts of temperature, chloride concentration, and pH on SCC susceptibility. The results highlight the critical role of these factors in determining the susceptibility of materials to SCC. The review emphasizes the importance of implementing various mitigation strategies to prevent SCC, including the use of corrosion-resistant alloys, protective coatings, cathodic protection, and corrosion inhibitors. Additionally, regular monitoring using advanced sensor technologies capable of detecting early signs of SCC is crucial for preventing the onset of SCC. The study concludes with practical recommendations for enhancing infrastructure resilience through meticulous material selection, comprehensive environmental monitoring, and proactive maintenance strategies, aimed at safeguarding operational integrity and ensuring environmental compliance. The review underscores the significance of considering the interplay between mechanical stresses and corrosive environments in the selection and application of materials in the oil and gas industry. Low pH levels and high temperatures facilitate the rapid progression of SCC, with experimental results indicating that stainless steel forms passive films with more defects under these conditions, reducing corrosion resistance. This interplay highlights the need for a comprehensive understanding of the complex interactions between materials, environments, and mechanical stresses to ensure the long-term integrity of critical infrastructure.
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Surfaces2024,7,589–642.https://doi.org/10.3390/surfaces7030040www.mdpi.com/journal/surfaces
Review
Analysis,Assessment,andMitigationofStressCorrosion
CrackinginAusteniticStainlessSteelsintheOilandGas
Sector:AReview
MohammadtaghiVakili*,PetrKoutník,JanKohoutandZahraGholami
ORLENUniCRE,a.s.,Revoluční1521/84,40001ÚstínadLabem,CzechRepublic;
petr.koutnik@orlenunicre.cz(P.K.);jan.kohout@orlenunicre.cz(J.K.);zahra.gholami@orlenunicre.cz(Z.G.)
*Correspondence:mohammadtaghi.vakili@orlenunicre.cz
Abstract:Thiscomprehensivereviewexaminesthephenomenaofstresscorrosioncracking(SCC)
andchloride-inducedstresscorrosioncracking(Cl-SCC)inmaterialscommonlyusedintheoiland
gasindustry,withafocusonausteniticstainlesssteels.ThestudyrevealsthatSCCinitiationcan
occurattemperaturesaslowas20°C,whileCl-SCCpropagationratessignificantlyincreaseabove
60°C,reachingupto0.1mm/dayinenvironmentswithhighchlorideconcentrations.Experimental
methodssuchasSlowStrainRateTests (SSRTs),SmallPunchTests(SPTs),andConstant-LoadTests
(CLTs)wereemployedtoquantifytheimpactsoftemperature,chlorideconcentration,andpHon
SCCsusceptibility.Theresultshighlightthecriticalroleofthesefactorsindeterminingthesuscep-
tibilityofmaterialstoSCC.Thereviewemphasizestheimportanceofimplementingvariousmiti-
gationstrategiestopreventSCC,includingtheuseofcorrosion-resistantalloys,protectivecoatings,
cathodicprotection,andcorrosioninhibitors.Additionally,regularmonitoringusingadvancedsen-
sortechnologiescapableofdetectingearlysignsofSCCiscrucialforpreventingtheonsetofSCC.
Thestudyconcludeswithpracticalrecommendationsforenhancinginfrastructureresilience
throughmeticulousmaterialselection,comprehensiveenvironmentalmonitoring,andproactive
maintenancestrategies,aimedatsafeguardingoperationalintegrityandensuringenvironmental
compliance.Thereviewunderscoresthesignificanceofconsideringtheinterplaybetweenmechan-
icalstressesandcorrosiveenvironmentsintheselectionandapplicationofmaterialsintheoiland
gasindustry.LowpHlevelsandhightemperaturesfacilitatetherapidprogressionofSCC,with
experimentalresultsindicatingthatstainlesssteelformspassivefilmswithmoredefectsunder
theseconditions,reducingcorrosionresistance.Thisinterplayhighlightstheneedforacomprehen-
siveunderstandingofthecomplexinteractionsbetweenmaterials,environments,andmechanical
stressestoensurethelong-termintegrityofcriticalinfrastructure.
Keywords:stresscorrosioncracking;mechanism;mitigation;austeniticstainlesssteel
1.Introduction
Materialswithexceptionalpropertiesarehighlyvaluedintheoilandgasindustry
duetotheirabilitytowithstanddiverseandspecializedenvironments.Austeniticstain-
lesssteel(ASS),aclassofiron-basedalloys,isrenownedforitsversatility.Var iou sgrades
aretailoredforspecificapplicationsandarewidelyutilizedinthissector[1].ASStypically
containsapproximately19%chromiumand10%nickel,resultinginapredominantlyaus-
teniticmicrostructureatroomtemperature.ThisstructuralstabilityendowsASSwithex-
ceptionalcharacteristics,includinghighstrainhardening,cleanability,lowsusceptibility
toproductcontamination,superiorimpacttoughness,plasticity,thermalstability,weld-
ability,corrosionresistance,andextendedlifespan[2,3].ThesepropertiesmakeASSa
Citation:Vakili,M.;Koutník,P.;
Kohout,J.;Gholami,Z.Analysis,
Assessment,andMitigationofStress
CorrosionCrackinginAustenitic
StainlessSteelsintheOilandGas
Sector:AReview.Surfaces2024,7,
589–642.hps://doi.org/10.3390/
surfaces7030040
AcademicEditors:JaehoKim,
SusumuYonezawaandAlexey
Yer ok hin
Received:25June2024
Revised:5August2024
Accepted:14August 2024
Published:16August2024
Copyright:©2024bytheauthors.Li-
censeeMDPI,Basel,Swierland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon-
ditionsoftheCreativeCommonsAt-
tribution(CCBY)license(hps://cre-
ativecommons.org/licenses/by/4.0/).
Surfaces2024,7590
versatilematerialwellsuitedfordiverseapplications,particularlywithintheoilandgas
industry[4].
However,despitetheiradvantages,equipmentandpipelinesintheoilandgasin-
dustryaresusceptibletovariouslocalizedcorrosionphenomena,includingstresscorro-
sioncracking(SCC),intergranularcorrosion(IGC),crevicecorrosion,andpiing.This
susceptibilityisoftenaributedtothereversionofhigh-iron-concentrationASStoitsna-
tivehydroxideoroxideformfollowingtherefiningprocessinblastfurnaces[5].Numer-
ousfailuresintheoilandgassectorhavebeenlinkedtolocalizedcorrosionprocesses,
particularlySCC,whichiswidelyrecognizedasaprimaryfailuremodeinhumidenvi-
ronments,leadingtosignificanteconomiclossesandenvironmentaldisastersworldwide.
SCCisaphenomenonobservedinmetalsandalloys,characterizedbyenvironmen-
tallyinducedcrackpropagation.Itprimarilyaffectsmaterialsprotectedagainstuniform
corrosionbypassivefilms.Thebreakdownoflocalizedpassivityduetomechanical
stressescanaccelerateaacksinspecificregions[6].SCCarisesfromthesynergisticeffects
ofaparticularaqueousenvironment,asusceptiblealloy,andtensilestress[7,8].Thevul-
nerabilityofpipelinesteelstoSCCisinfluencedbyamultitudeoffactors,includingthe
steel’schemicalcomposition,texture,inclusionandprecipitatedistribution,andmicro-
structure;thepHoftheenvironmentsurroundingthepipeline;thepHofthetransported
oilandgas;andotherrelevantconsiderations[9].SCCgrowthinpipelinescanoccurun-
predictablyunderconstant-loadandstatictensileconditions,particularlyinenviron-
mentsconducivetocorrosion.Thiscanalterthemechanicalcharacteristicsofcarbonsteel
pipelines,makingthemmoresusceptibletoSCCandinitiatingcrackformation.Subse-
quently,crackscanpropagaterapidlythroughthepipelinematerial,ultimatelyleadingto
failure.Laboratorytestingcombiningelectrochemicalandmechanicalmeasurementsis
oftenconductedundercontrolledconditionsbeforeandafterelectrochemicalevaluations
toassessSCCsusceptibility[5].DetectingandpredictingSCCpresentschallenges,espe-
ciallyintheoilandgasindustry,whereunforeseenfailurescanhavesignificantconse-
quences[10].SCCgrowthcaninitiateandspreadtototalfracturewithlilewarning,even
incorrosion-resistantmaterials.Fracturesurfacesoftenexhibitsevereembrilement,with
numeroussmallcracksatthetimeoffailure.TheindustrialsignificanceofSCCliesinthe
difficultyofeliminatingstressesinducedduringcomponentfabrication,makingreliable
measurementschallenging[11].
SCCisacomplexprocessthatuniquelyaffectsmaterialproperties,distinguishingit
fromstressorcorrosionalone.Thiscomplexityisparticularlysignificantinindustries
wherestressesarecommonlyinducedduringfabrication.Removingoraccuratelymeas-
uringthesestressesinengineeringstructuresisoftenchallengingorimpractical.SCCsub-
stantiallyimpactsstructuralintegrityandrepresentsacommoncauseoffailuresinvari-
oussystemsandindustries,suchasoilandgasandpetrochemicals[12,13].Figure1sche-
maticallyillustratesthedevelopmentofuniformandlocalizedcorrosion.Unlikegeneral
corrosion,wheremateriallosscanbedeterminedbasedonaveragecorrosionrates,SCC
isalocalizedformofcorrosion.Theservicelifeofthematerialisconsideredcomplete
whenthefastest-growinglocalizedcorrosionaackleadstothefirstperforation.There-
fore,consideringthemaximumcrackgrowthrateortheminimumtimeforcrackinitiation
iscrucialinestimatingthelifeofcomponentsaffectedbySCC.Intheabsenceofacrack,
acomponentcanbeassumedtohaveaninfinitelifefromtheperspectiveofSCCconsid-
erations[6].
Surfaces2024,7591
Figure1.Schematicofgeneralandlocalizedcorrosiondevelopment.
SCCcanoccurinhighlycorrosion-resistantalloys,eveninseeminglyinnocuousen-
vironments.Detectingtightcracksperpendiculartothetensilestressaxisduringtheini-
tialstagesofSCCischallengingusingnon-destructivetechniques.Thehighlylocalized
natureofSCCisevidentfromthehighaspectratioofagrowingcrack,whichcanreach
valuesashighas1000.Additionally,SCCexhibitsmacroscopicbrilenessinductilema-
terials,allowingcrackstoinitiateandpropagatewithoutsignificantdimensionalchanges.
ThesefeaturesemphasizethepotentialforunnoticedfailuresifSCCisnotanticipated[6].
Inpipelinesteel,SCCcrackinitiationandpropagationoccurinthreestages.Initially,
smallSCCcrackscontinuouslyinitiateandcoalesce,representingasignificantproportion
ofthepipelinesteel’slifetime.Subsequently,thesesmallcrackspropagaterapidly,ulti-
matelyleadingtomaterialfailure.AnotablefeatureofSCCinpipelinesteelsisthebranch-
ingofcracks,whichcanbeclassifiedintomicro-andmacro-branching[9].
ThisreviewaimstoprovideacomprehensiveoverviewofSCCinrefineryenviron-
ments,highlightingtheimportanceofresearchanddevelopmentinthisarea.Thecom-
plexityofSCCanditssignificantimpactonrefineryoperations,includingeconomiclosses
andenvironmentaldisasters,underscorestheurgentneedforathoroughunderstanding
oftheunderlyingmechanismsandthedevelopmentofeffectivepreventionstrategies.In
thisreview,theparameters,mechanisms,assessment,andpreventionmethodsofSCCin
refineryenvironmentsarediscussedindetail.Furthermore,chlorine-inducedSCCisex-
amined.Bycombiningdetailedfailureanalyses,processoptimization,andmaterialsen-
gineering,therefiningindustrycandevelopeffectivestrategiestomitigateSCCanden-
surethesafeandreliableoperationofrefineryequipment,therebyminimizingtheriskof
catastrophicfailuresandtheirassociatedconsequences.
2.SCCFailureEvents
SCChasbeenamajorcauseoffailuresingasandoiltransmissionpipelinessincethe
mid-1960s.Thisphenomenon,particularlyaffectingtheexternalsurfacesofburiedpipe-
lines,occurswhenasusceptiblematerialisexposedtoacorrosiveenvironmentandtensile
stress,eitherfrominternalpressureorexternalforces.SCCposessignificantrisks,includ-
ingleaksorruptures,whichcanhavesevereconsequencesfornearbyresidents,workers,
andtheenvironment.ThefirstdocumentedSCCincidentoccurredin1965in
Natchitoches,Louisiana,wherea24-inchnaturalgaspipelineruptured,resultingin17
fatalities.ThistragiceventspurredextensiveresearchintoSCCandledtotheintroduction
ofU.S.pipelinesafetyregulationsin1970[14].
Sincethen,SCChasbeendetectedinpipelinesacrossvariouscountries,including
Iran,Canada,Pakistan,Australia,theUnitedStates,andtheformerSovietUnion.These
Surfaces2024,7592
incidentshaveoftenresultedincatastrophicpipelinefailures[15].Forexample,theTrans-
CanadapipelineexperiencedseveralSCC-relatedfailuresinNorthernOntariobetween
1985and1995,leadingtotwopublicinquiriesbytheNationalEnergyBoard(NEB)of
Canada[16].TheNEBidentifiedtheneedforimproveddetectionmethodsandmaterial
standardstomitigateSCCrisks.InCanada,SCCisasignificantissue,with30to40pipe-
linefailureseachyear,someresultinginfatalities,suchasa1985incidentwhereadrain-
agetileplowrupturedagaspipeline[17].Thesefailureshighlighttheneedforstringent
regulatoryoversightandcontinuousmonitoringofpipelineintegrity.
SCCoccurrenceshavealsobeenreportedacrossNorthAmericaandEurope.InNorth
America,affectedregionsincludeAlberta,Ontario,andSaskatchewaninCanada,aswell
asArizona,Kentucky,SouthCarolina,andTennesseeintheU.S.Additionally,SCChas
beenreportedintwosouthernEuropeancountries.Theoutcomesofthesecasesvary
widely:whileonlytwoincidentsresultedinruptureswhilethepipelineswereinservice,
mostwerediscoveredduringmaintenanceorhydrostatictesting[18].Hydrostatictesting,
whichinvolvespressurizingthepipelinewithwatertoalevelaboveitsoperatingpres-
sure,canrevealweaknesses.ThemostcommonlyaffectedpipegradewasX60,knownfor
itswidespreaduseduetoitsfavorablestrengthandcostcharacteristics.Notably,73%of
incidentsoccurredinpipesolderthan30years,primarilyconstructedbetween1956and
1980,withnoSCCincidentsreportedinpipelinesbuiltinthelast32years[18].Thissug-
geststhatnewermaterialsandconstructiontechniquesmaybemoreresistanttoSCC.
SCCrepresentsacriticalthreattoenergypipelines,potentiallyleadingtocata-
strophicfailuresthatendangerhumansafety,harmtheenvironment,andresultinsub-
stantialrepaircosts.Forinstance,inthe1970s,significantSCCincidentsoccurredinCan-
ada’sLakeheadPipelinesystem,particularlyinregionswithhighlevelsofhydrogensul-
fide(H2S),acorrosivecomponentpresentinsomecrudeoils.In1978,theTrans-Alaska
PipelineSystemexperiencedamajorSCCeventnearAtigunPass,Alaska,resultingin
multipleruptures,oilspills,andatemporaryshutdown.Morerecently,in2017,SCC-re-
latedfailuresontheKeystonePipeline,operatedbyTransCanada(nowTCEnergy),
causedasignificantoilspillinSouthDakota,releasingapproximately210,000gallonsof
oil.TheseincidentsunderscoretheongoingchallengesinmanagingSCCrisks[19,20].
Alarmingly,SCCisresponsibleforover50%offailuresingaspipelines[21].
Overall,thethreatofSCCtopipelinesnecessitatesamultifacetedapproach,includ-
ingimprovedmaterialsscience,beerconstructionpractices,rigorousregulatoryframe-
works,andongoingmaintenanceandmonitoringefforts.Theindustrycontinuestoex-
plorenewtechnologiesandmethodstopredict,detect,andmitigateSCC,ensuringthe
safetyandreliabilityofpipelinesystemsworldwide.NotableSCC-relatedincidents,such
asthe2014gaspipelineexplosioninKaohsiung,Taiwan,whichresultedin32fatalities
and321injuries,andthe2013oilpipelineexplosioninQingdao,China,whichcaused136
injuriesand62deaths,illustratetheglobalnatureoftheSCCproblemandtheneedfor
internationalcollaborationinresearchandtechnologysharing.In2020,theEnbridgeLine
5pipeline,whichtransportsoilandnaturalgasliquidsbetweentheUnitedStatesand
Canada,alsofailedduetoSCC,furtheremphasizingthevulnerabilityofcriticalenergy
infrastructuretothistypeofcorrosion[22].
3.RequiredParametersforSCC
SCCisamultifacetedphenomenoninfluencedbyseveralcriticalparameters.These
includesensitizedmaterialssuchascarbonsteels,copperalloys,andhigh-carbon-content
stainlesssteels,aswellasthepresenceofvarioustypesoftensilestress(residual,thermal,
orappliedstress).Specificenvironmentalconditions,includinghigh-temperaturewater,
chlorideoracidicsolutions,moisture,andaqueousenvironments,alsoplaypivotalroles.
Theinteractionamongthesefactorsiscrucialforboththeinitiationandpropagationof
SCC[23].Figure2illustratestheprimarycontributorstoSCC,emphasizingtheintricate
interplayamongthesevariables[24,25].Thesefactorswillbediscussedinmoredetailin
thesubsequentsections.
Surfaces2024,7593
Figure2.RequirementsforSCC.
3.1.Stress
SCCarisesfromtheinterplayoftwoprimarystressfactors:internalresidualstresses
andexternalmechanicalforces[23].Internalstresses,alsoknownasresidualstresses,are
inherentinmaterialsandstemfromprocessessuchasplasticdeformation,thermaltreat-
ments,andmanufacturingtechniques[5,26,27].Theseresidualstressescanbeclassified
intodistinctcategoriesbasedontheirscales:TypeI(macroscopic),TypeII(microscopic),
andTypeIII(atomiclevel).
TypeIresidualstressesoccuratamacroscopiclevelduetophenomenasuchasdif-
ferentialcooling,whileTypeIIstressesmanifestonamicroscopicscaleandareassociated
withfeatureslikebandedmicrostructures.TypeIIIresidualstressesoperateattheatomic
levelandresultfromchemicalsegregationsandsmallcoherentphaseswithinthematerial
[26].ResearchindicatesthatinternalresidualstressessignificantlyinfluenceSCC.Forin-
stance,Chen’sstudyexaminedtheimpactofTypeIresidualstressesonneutral-pHSCC
inpipelinesteels,demonstratingthatplasticdeformationandanodicdissolutioncanmit-
igateSCCundersuchconditions.Beaversetal.[28]alsofoundthatthelikelihoodofSCC
occurrenceisgreaterinregionswithelevatedresidualstresses.Theirstudyindicatedthat
residualstressinareasclosetoSCCcolonieswasnearlydoublethatinregionsfarther
awayfromnear-neutral-pHSCCcolonies,highlightingthesignificantroleoftensilere-
sidualstressinSCCinitiation.Similarly,VanBovenetal.[23,29]investigatedtheinterplay
betweenresidualstressandcyclicloadinginpipelinematerials.Theirfindingsrevealed
thatcyclicloadingfrombendingforcesincreasedresidualstressatthepipeline’ssurface,
significantlyheighteningthesteel’ssusceptibilitytocrackinitiationinanear-neutral-pH
environment.Tensilestresses,particularlythoseinducedbyweldingandmechanicalpro-
cesses,playacriticalroleinSCCfracturemechanisms[29].
Externalstresses,especiallytensilestress,andmechanicalactionsduringamaterial’s
operationallifemarkedlyaffectitssusceptibilitytoSCC.Thesestressesencompassforces
appliedtothematerialduringservice,suchastensile,compressive,orshearstress,which
cancausethestretching,compression,ordeformationofthematerial,therebycompro-
misingitsstructuralintegrityandincreasingsusceptibilitytocorrosion-inducedcracking
[23,29].Forinstance,externaltensilestressescaninduceSCCinburiedpipelinestrans-
portinghydrocarbons,particularlyiftheprotectivecoatingiscompromised,leadingtoa
wetenvironmentconducivetocorrosion-inducedcracking[5].Tensilestressesarepivotal
inSCCfractureprocesses,whetherapplieddirectlyorasresidualstressesfromthermal
processes[30].Weldingandmechanicalactionssuchascolddeformationandmachining
introduceresidualstresses,therebycontributingtoSCCsusceptibility[30,31].
Surfaces2024,7594
SCCcanoccuratstresslevelsbelowthematerial’sultimatetensilestrength,withthe
rateofcrackpropagationinfluencedbythemagnitudeofstressapplied.Controlledcom-
pressivestresses,suchasthoseinducedbyshotpeeningoncomponentsurfaces,canmit-
igateSCCinitiation[6].Thestrainrateatthecracktipiscritical,withSCCmorelikelyto
occurwithinspecificstrainrateranges.ThestressthresholdforSCCvariesdependingon
thematerialandenvironmentalconditions.Forinstance,ASScomponentsmayexperience
SCCinhotwateratstresslevelsclosetothematerial’sbreakingpoint,whilelowerstress
levelsmaysufficeinhotchlorideenvironments.Effectiveunderstandingandprevention
ofSCCrequirecarefulconsiderationofbothexternalforcesandinternalresidualstresses,
particularlyinapplicationsinvolvingASScomponents[23].
3.2.Environment
SCCposesasignificantthreattoengineeringmaterials,particularlyinenvironments
whereaqueousconditionsandspecificaggressiveions,suchaschlorides,arepresent.
TheseenvironmentsplayacrucialroleinboththeinitiationandpropagationofSCC.The
susceptibilityofanalloytoSCCisdeterminedbyboththenatureofthematerialandthe
aggressivenessofthesurroundingenvironment.However,whileacorrosiveenvironment
isnecessary,notallconditionsinduceSCC.Specificions,suchaschlorides,areparticu-
larlyharmfultoalloyswithprotectivefilms.Forinstance,copperalloysarevulnerableto
SCCinammonia-richenvironments[32],whereasstainlesssteelandaluminumalloysare
morepronetochloride-inducedcracking[33].TheASS300series,inparticular,exhibits
highsusceptibilitytoSCCinchloride-richconditions,especiallyattemperaturesabove70
°C,wherecrackingoccursprimarilyinatransgranularmanner[34,35].
Variousaggressiveions,includingchlorides,causticsubstances,andpolythionic
acid,canaccelerateSCC.Chloridesarenotoriousforpenetratingthepassivelayerofstain-
lesssteel,leadingtothebreakdownofprotectivefilmsandtheonsetofSCC,evenatam-
bienttemperatures,particularlyinheavilymachinedcomponents[36,37].Inenviron-
mentswhereASSisexposedtocausticsubstancesoracidicconditions,suchasinregions
withintergranularwelding,theriskofSCCissignificantlyheightened[35].Inchemical
andpetrochemicalplants,thepresenceofpolythionicacid,abyproductofsulfurinfeed
gas,isaknowncauseofintergranularSCC(IGSCC)inASS,especiallyinmoistconditions
[36,38].AluminumandtitaniumalloysalsoshowsusceptibilitytoSCCinenvironments
suchasliquidmetals,organicliquids,andaqueoussolutions,includingwatervaporand
methanolicsolutions[35].
Pipelinematerials,particularlythosemadefromAPI5LX60,X65,andX70,areoften
exposedtoharshconditionsinvolvinghighlevelsofH2S,carbondioxide(CO2),andlow-
pHwater,allofwhichpromoteacceleratedcorrosionandhydrogenabsorption.These
conditionsareprevalentinenvironmentslikeseawater,whereatmosphericcorrosiondue
tosaltandchlorideionscanbecomesevereifthepipeline’scoatingiscompromised,par-
ticularlyunderdeep-seahydrostaticpressure[39,40].Studies,suchasthosebySunetal.,
showvaryingSCCsusceptibilitiesatdifferentdepths,withlowersusceptibilityat1500m
buthighersusceptibilityat3000m[41].PipelinesburiedinsoilfaceSCCchallengesinflu-
encedbyanodicdissolutioninhigh-pHsolutionsandhydrogenembrilement(HE)in
near-neutral-pHenvironments[42–46].Theelectrolytecomposition,especiallyincar-
bonate-richordilutesolutions,significantlyaffectswhetherthecrackingwillbeintergran-
ularortransgranular[47].
SulfurcompoundsfurthercomplicatetheSCClandscape.Theynotonlycontribute
tohydrogen-inducedcracking(HIC)byformingmanganesesulfide(MnS)inclusions,
whichactasstressconcentrationsites[48,49],butalsocreatecorrosiveconditionsthat
exacerbateSCC.ResearchbyFanetal.highlightsincreasedSCCsusceptibilityinL360NS
pipelinesteelinsulfurenvironments,linkingittohydrogenionpermeation,thedegrada-
tionofprotectivefilms,andthesubsequentlossofmechanicalpropertiesunderstress[50].
ThetransitionfromabenigntoacorrosiveenvironmentconducivetoSCCcanbedriven
bychangesintemperature,aeration,ortheconcentrationofionicspecies.Thestability
Surfaces2024,7595
andsolubilityofthereactionproductsintheenvironmentsignificantlyaffectcrackprop-
agation.Cracksofteninitiatefromthesurfaceoxidefilm,displayingatransgranularpat-
tern,emphasizingtheimportanceofunderstandingcrackmorphologyinSCC.
Theresistanceofanalloytocrackinglargelydependsonthestabilityofitssurface
film,aconditiondescribedasborderlinepassivity,whichiscriticalforcrackinitiationand
isachievableonlywithinspecificelectrochemicalpotentialranges[51].Inpipelinesteels,
SCCisinfluencedbystressintensityfactorsarisingfrombothexternalloadsandstresses
inducedbytheoxidefilm.Thesefilm-inducedstressesarecrucialindevelopingSCC
cracks,withinitiationandpropagationoccurringwhenthesestressesexceedacritical
threshold[52].Figure3demonstratesSCCcracksinitiatedfromtheoxidefilm.
Figure3.Initiationoftransgranularstresscorrosioncracking(TGSCC)inducedbysurfaceoxide.
(a)Overviewofthecrackinitiation;(b)Detailofthetransgranularcrackintersectingaferritegrain
boundary.(Reprinted/adaptedwithpermissionfromRef.[41].2024,Elsevier)
Achievingandmaintainingborderlinepassivity,whichisinfluencedbyenvironmen-
talconditionsandalloycharacteristics,isessentialformitigatingSCCrisks.Understand-
ingthecomplexinterplaybetweenalloysusceptibility,environmentalaggressiveness,
andprotectivefilmformationisvitalforpreventingSCCacrossvariousindustrialappli-
cations.ForadetailedsummaryofenvironmentsthatcauseSCCincommonlyusedal-
loys,refertoTable1[35].
Tab le1.SummaryofenvironmentsassociatedwithSCCofvariousalloys[25].
MetalEnvironment
TitaniumalloysMethanol-HCl
Seawater
Red-fumingnitricacid
StainlesssteelsCondensingsteamfromchloridewaters
NaOH-H2Ssolutions
H2S
Seawater
NaCl-H2O2solutions
Acidicchloridesolutions
SteelsCarbonate–bicarbonatesolutions
Seawater
AcidicH2Ssolutions
Mixedacids(H2SO4-HNO3)
Calcium,ammonium,andsodiumnitritesolutions
NaOH-Na2SiO4solutions
NaOHsolutions
NickelFusedcausticsoda
MagnesiumalloysDistilledwater
Seawater
Surfaces2024,7596
Ruralandcoastalatmospheres
NaCl-Na2CrO4solutions
LeadLeadacetatesolutions
InconelCausticsodasolutions
GoldalloysAceticacid–saltsolutions
FeCl3solutions
CopperalloysWaterorwatervapor
Amines
Ammoniavaporandsolutions
AlalloysSeawater
NaClsolutions
NaCl-H2O2solutions
3.3.Material
EnvironmentalconditionscriticallyinfluenceSCC,butthemetallurgicalconditionof
thematerialisequallypivotalinitsinitiationandpropagation.SCCrequiresaspecific
metallurgicalstateshapedbyfactorssuchasalloycomposition,strengthlevels,thepres-
enceofsecondaryphases,andgrainboundarysegregation.Thesefactorscaneitheren-
hanceordiminishthematerial’sresistancetoSCC[5,6].Forinstance,ASSbecomessus-
ceptibletoIGSCCwhensensitized,eveninseeminglybenignenvironmentslikehigh-pu-
ritywateratelevatedtemperatures,especiallyifoxygenispresent.Surfaceimpuritiesand
irregularitiessignificantlycontributetoSCCandfailureoccurrences.However,reducing
traceimpuritiessuchasphosphorus,sulfur,andarsenicinASSsubstantiallyimprovesits
resistancetoSCC[5].
ImpuritieslikenitrogenandcarbonatgrainboundariescaninfluencecausticIGSCC
incarbonsteelbydisruptingpassivefilmformationandaffectingplasticity.Thissegrega-
tionresultsinlocalizedcorrosiveconditions,illustratingtheintricaterelationshipbetween
metallurgicalfactorsandSCCsusceptibility[6].Inweldingapplications,preventingse-
lectivecorrosioninausteniticsteelsrequirescontrollingthecarboncontentwithinitssol-
ubilitylimitsinaustenite.Evenlow-carbonchromium-nickelausteniticsteelsarenoten-
tirelyimmunetotheadverseeffectsofweldingthermalcycles[53,54].Researchshows
thatlow-carbonsteelscontainingnitrogenareparticularlyvulnerabletoSCC,asnitrogen
formssolidinterstitialsolutionswithironandothermetalsduetoitssmallatomicradius
[55,56].
SeveralstudieshaveexaminedhowvariouselementsimpacttheSCCbehaviorof
pipelinematerials,identifyingphosphorus,manganese,andcarbonasprimaryelements
ofconcern[9,57].Duringsteelsolidification,theseelementstendtosegregatetowardthe
centeroftheslabthickness,forminghardphasesandstructureslikemartensiteandbain-
ite.Thesegregationratioofmanganeseisdirectlycorrelatedwiththecarboncontent.No-
tably,low-strengthcarbonsteelsexhibitgreaterresiliencetothedetrimentaleffectsof
phosphorussegregationcomparedtohigh-strengthalloysteels[9,57].
Manganesesignificantlyenhancesaustenitestabilityinsteel,leadingtoincreasedsu-
percoolingandreducedcriticalquenchingrates,whichimprovethesteel’shardenability.
However,asthemanganesecontentincreases,theSCCresistanceofhigh-strengthmar-
tensiticsteeldecreases.Manganese’sabilitytotrapnitrogenwithinthecrystallaice,com-
binedwithcarbon’spreferencefordislocationsalonggrainboundaries,reducestheSCC
resistanceofhigh-strengthsteels[53,58].Addingcalciumtopipelinesteelhelpscontrol
sulfurlevels,formingsphericalsulfide-basedinclusionsthatpreventtheformationof
elongatedMnSparticles,whicharetypicalsitesforcracknucleation[59–62].
Copperalsoplaysacrucialrolebyenhancingsteel’sstrengthandSCCresistance
throughtheformationoffinecopper-enrichedprecipitates.Theseprecipitatesimprove
mechanicalpropertiesandreducecrackformation.Coppercreatesaprotectivebarrieron
thesteelsurfacethatlimitshydrogendiffusionintothesteelmatrix,enhancingresistance
Surfaces2024,7597
toHE[63].Babaetal.[64]demonstratedthattheadditionofcoppersignificantlyreduces
hydrogenpermeationinsourenvironmentsbyformingathickbarrieronthesteelsurface.
Elementsthatformstablecompoundswithnitrogenandcarbon,whicharesoluble
inbothα-andγ-iron,profoundlyimpactSCCresistance.Strongcarbideformersliketita-
nium,molybdenum,niobium,andvanadiumimprovetheSCCresistanceoflow-carbon
steelsbyformingcarbidesthathindercrackpropagation.Amongthese,titaniumandmo-
lybdenumhavethemostsignificanteffects[53,58].
Niobium-stabilizedausteniticsteelsshowsuperiorresistancetoknife-linecorrosion
comparedtotitanium-stabilizedsteelsduetoniobium’sgreaterresistancetodissolution
inausteniteathightemperatures.Niobiumbindscarbonatgrainboundaries,preventing
chromiumcarbide(M23C6)formation,whereastitaniumcarbidesdissolvemorereadily
duringtempering,increasinglaicestresses.Niobium-stabilizedsteelsalsoexhibitbeer
corrosionresistancenearthefusionline.Forinstance,08Cr17Hi5Mn9NNbsteeljoints
showednoknife-linecorrosion,evenwithsomeniobium-formingnitrides[53].
Chromiumisessentialforenhancingthecorrosionresistanceofsteelalloys.Itspas-
sivationleadstotheformationofaprotectiveoxidelayer,significantlyincreasingcorro-
sionresistance.Stainlesssteelrequiresatleast13–15%chromiumbymasstomaintain
theseproperties[53,58].Conversely,highernickelcontentcanreduceSCCresistance.Sil-
iconplaysacrucialroleinpreventingcarbidecoagulationduringtempering,enhancing
thestabilityofsorbitolstructures,andslightlyincreasingthestrengthandyieldstrength
ofsteel[53,58].
MetallurgicalfactorsandmicrostructuralcharacteristicscriticallyinfluenceSCCin
metals.Hardandbrilephaseswithinthemicrostructure,particularlyinsteel,exacerbate
susceptibilitytoSCCcrackpropagation[9].Zhuetal.[65]highlightedtherelationship
betweenSCCcracksandlocalmicrostructureinX80pipelinesteel.Inhigh-pHenviron-
ments,abundantbulkygranularbainiteandpolygonalferritemicrostructuresprimarily
leadtointergranularcracking.Asthemicrostructuretransitionstofinegranularbainite
andacicularferrite,bothintergranularandtransgranularSCC(TGSCC)cracksareob-
served.ThepHofthesolutionsignificantlyaffectsthelikelihoodoftransgranularcrack
propagation[65].
Beyondmicrostructuralconsiderations,Gonzalezetal.[66]examinedhigh-strength
low-alloysteelandunderscoredthesignificantinfluenceofmicrostructureonSCCcrack
propagation.Theirfindingshighlighttherolesofgrainsizeandgrainboundarycharac-
teristicsinthedevelopmentofintergranularcracking.Sulfidestresscracking(SSC)sus-
ceptibilityiscloselyrelatedtothesteel’shardness,microstructure,andchemicalcompo-
sition,typicallypropagatingalonggrainboundariesbutsometimestransgranularly[9].
Despitetraditionalmitigationmethodslikemicro-alloyingandheattreatments,recentre-
searchhasfocusedoncrystallographictextureasanovelapproachtoreducingSCCsus-
ceptibilityinpipelinesteels.Studiessuggestthattextureplaysasignificantroleinmiti-
gatingSCC[67–69].
3.3.1.CarbonandLow-AlloySteels
Carbonandlow-alloysteels,includingthosethatarequenchedandtemperedorpos-
sessaferritic–pearliticmicrostructure,exhibitsusceptibilitytoSCCacrossavarietyofen-
vironmentalconditions.Theseconditionsincludechloride,carbonate–bicarbonatesolu-
tions,ammonia,alkanolamines,hydroxide,andhotnitrate[70].Thepredominantfailure
modeinthesesteelsisintergranularcracking,whichoccursalongtheprioraustenitegrain
boundaries.However,transgranularcrackingcanalsomanifestinenvironmentscontain-
inghydrogen(H2),H2S,orhigh-temperaturewater.
TheSCCresistanceofthesesteelsisinverselyrelatedtotheirstrengthlevelorhard-
ness;asthestrengthorhardnessincreases,theresistancetoSCCtypicallydecreases.Ex-
tensiveresearchhasbeenconductedontheinfluenceofsteelcompositiononSCCsuscep-
tibility.However,real-worldapplicationsoftenpresentcomplexitiesduetointeractions
amongweldingproperties,materialstrength,andheat-treatmentresponses[71].
Surfaces2024,7598
Thesusceptibilityofcarbonandlow-alloysteelstoSCCisalsoaffectedbythedistri-
butionofpotential–pHdomainsacrossvariousenvironmentalconditionsandtempera-
tures[70].Thesedomainsarecriticalforunderstandingtheinitiationandpropagation
mechanismsofSCCinthesematerials.ThepresenceofFe3O₄(magnetite)isoftenindica-
tiveofconditionsconducivetoSCC,suggestingitsroleinthecrackingprocess.Addition-
ally,phasessuchasFeCO3(siderite)andFe3(PO₄)2(vivianite)caninfluenceSCCbehavior
inspecificenvironments.AkeyaspectofSCCsusceptibilityistherelationshipbetween
thepiingcorrosionpotentialandcrackinitiation,underscoringthesignificanceoflocal-
izedfactors.AlthoughSCCmechanismscanvary,understandingthesepotential–pHdo-
mainsprovidesvaluableinsightsintotheprocessesdrivingSCCincarbonandlow-alloy
steels.Moreover,thesesteelsarepronetoSCCeveninenvironmentsthataretypically
consideredpassivatingorconducivetoformingprotectiveoxidefilms.Forinstance,envi-
ronmentslikehigh-temperaturewater,ethanol,carbonates,nitrates,phosphates,and
strongcausticsolutions,whichusuallyfostertheformationofpassivelayersoncarbon
steels,haveinducedSCCinthesematerials.
Thesechallengesposesignificanteconomicandsafetyconcernsduetotheextensive
useofcarbonsteelsinindustrialapplications.Forexample,thecatastrophicfailureofan
ammoniumnitrateplant,resultingfromnitrate-inducedcracking,ledtoseverefinancial
lossesandmajorsafetyincidents.Similarly,causticcrackinginsteam-generatingboilers
madefromlow-alloysteelsremainsapersistentproblem,causingrepeatedemergency
shutdownsinfacilitieslikeammoniaplants[30,70].
3.3.2.High-StrengthSteels
High-strengthsteels,characterizedbyhardnesslevelsabove440HRC,exhibitheight-
enedsusceptibilitytoIGSCC,evenintypicallynon-corrosiveenvironmentssuchasmoist
air.Environmentscontainingchloridesareparticularlydetrimental,astheypromoteSCC
inthesematerials.HEiswidelyrecognizedastheprimarymechanismofSCCinhigh-
strengthsteel.Thepresenceofcathodicpoisonssuchassulfur,tellurium,selenium,and
arsenicexacerbatesSCCbyimpedinghydrogenrecombinationandincreasinghydrogen
absorptionbythesteel.Moreover,thesesteelsaresusceptibletoSCCinthepresenceof
organiccompoundsandinenvironmentsthatproducehydrogen.Temperaturesignifi-
cantlyinfluencesSCCkineticsinhydrogengasenvironments,withthesteel’sstrength
levelcriticallyaffectingsusceptibility.Maragingsteelsgenerallyexhibitbeerresistance
toSCCthanhigh-strengthsteelsduetotheirmicrostructuralrefinementandhydrogen-
trappingcapabilities.However,thespecificmechanismbywhichHEinducesSCCin
high-strengthsteelsremainsunclearandnecessitatesfurtherinvestigation.
DuetotheirinherentsensitivitytoSCC,isolatinghigh-strengthsteelsfromcorrosive
environmentsisadvisablewheneverpossible.DespitevariationsinSCCresistanceamong
differenthigh-strengthsteels,itisprudenttolimitthespecifiedyieldstrengthtomeet
minimumrequirements.Regardingintergranularcracking,chemicalcompositionandre-
actionsatgrainboundariesarecriticalfactorsinfluencingmaterialsusceptibilitytoSCC.
Impurityelementssuchasantimony,tin,arsenic,andphosphoruscanpromotetemper
embrilementandincreasesusceptibilitytoSCC.However,microstructuralmodifica-
tions,includingmartensitestructurerefinementandincreaseddislocationdensities,can
mitigateSCCbyreducinghydrogenaccumulationandenhancingresistancetoembrile-
ment.Understandingtheinterplayamongcomposition,microstructure,andSCCmecha-
nismsisessentialfordevelopingeffectivemitigationstrategiesforhigh-strengthsteels.
CurrentresearchendeavorsaimtoelucidatespecificSCCmechanismsinthesematerials,
offeringinsightsintopotentialmitigationtechniquesandenhancingthereliabilityofhigh-
strengthsteelcomponentsincorrosiveenvironments[71].
Surfaces2024,7599
3.3.3.StainlessSteels
Stainlesssteels,characterizedbyachromiumcontentofatleast11%,exhibitdiverse
microstructures,includingaustenitic,ferritic,martensitic,andduplexphases.Thesuscep-
tibilityofthesesteelstoSCCisinfluencedbyfactorssuchasthermalhistory,microstruc-
ture,andalloycomposition.TheSCCsusceptibilityofvariousstainlesssteelgradesde-
pendsonspecificenvironmentalconditions.ASSsarepronetocrackinginhydroxide-and
chloride-richenvironments,whileduplexstainlesssteelscanexperiencecrackinginsour
gasandchloride-richenvironments.Ferriticandmartensiticstainlesssteelsarevulnerable
tocrackingunderbothcathodicandanodicconditions.TheexactmechanismsofSCCin
thesesystemsvarybutaretypicallyassociatedwithspecificenvironmentaltriggers,such
aschloride-inducedSCCorcausticSCC.
ExtensiveresearchhasinvestigatedSCCinaustenitic,ferritic,martensitic,anddu-
plexstainlesssteels.Thenickelcontentandsensitizationofthesesteelssignificantlyinflu-
encetheirsusceptibilitytoSCC,withhighernickelcontentgenerallyenhancingresistance.
SensitizedstainlesssteelsarepronetoIGSCC,whereasnon-sensitizedsteelstypicallyex-
periencetransgranularcracking.ThebehaviorofASSinchloride-containingenviron-
mentsshowsatemperature-dependentthresholdbelow50°C,beyondwhichsusceptibil-
itytoSCCincreases.Thissusceptibilityarisesfromlocalizedcorrosionprocessessuchas
piing,crevicecorrosion,andtrenchformation,whichleadtoacidificationatthecracktip
andacceleratecrackpropagationthroughcorrosivespeciesandenhancedelectrochemical
reactions.
Thepresenceofdelta-ferriteinASSsenhancestheirresistancetoSCCbyimpeding
crackgrowth.Incontrast,duplexstainlesssteelsexhibitsuperiorresistanceduetothe
cathodicprotectionprovidedbytheferritephase.Inhigh-temperaturewater,SCCinweld
heat-affectedzones(HAZ)presentsasignificantchallenge,necessitatingpredictivemod-
elingandmitigationstrategies.IGSCCinducedbypolythionicacidinsensitizedstainless
steelscanbemitigatedbyreducingsensitizationandadjustingplantshutdowncondi-
tions.Anodicandcathodicpolarizationtechniquesalsoofferpreventivemeasuresagainst
polythionicacidSCC,operatingwithinanarrowpotentialrange.Othersulfurspecies,
suchassulfate,thiocyanate,andthiosulfatesolutions,canalsoinduceSCCinsensitized
stainlesssteels[71].
3.3.4.NickelAlloys
NickelanditsalloysarefrequentlychosenfortheirresistancetoSCC,althoughsus-
ceptibilitycanariseunderspecificenvironmental,microstructural,andstressconditions.
IGSCCisprevalentinenvironmentssuchashigh-temperaturewater,gaseoushydrogen,
sulfur-containingatmospheres,andacidicandbasicsolutions.Nickel-basedalloyshave
applicationsinvariousindustries,includingNi-Fe-Cralloysforpowergeneration,Ni-Cr-
Moalloysforchemicalprocessing,andCu-Nialloysforseawaterapplications.Precipita-
tion-hardenablealloyslikeX750and718areutilizedfortheirhigh-strengthproperties.
Alloy600,awell-studiednickel-basedalloy,primarilyexperiencesIGSCC.Precipita-
tion-hardenablealloysaremoresusceptibletoSCCduetotheformationofintermetallic
phasesduringaging,whichenhancesmaterialstrength.Carbideprecipitationandsensi-
tizationfromchromiumdepletionalonggrainboundariessignificantlycontributetoSCC
susceptibilityacrossdifferentenvironments.Nickel-basedalloysexhibitheightenedsus-
ceptibilitytoSCCatelevatedtemperaturesinhigh-puritywater,particularlyunderde-
aeratedconditions,whereastheydemonstrateresistancetoSCCathightemperaturesin
solutionswithelevatedchlorideconcentrations.
Hydrogenabsorptioncanacceleratecrackpropagationinnickel-basedalloys,partic-
ularlyundervarioushydrogenchargingconditions.Hydrogen-assistedmechanismsand
anodicdissolutioncontributetoSCC,withlocalgrainboundaryslidingidentifiedasa
significantfactorincrackinitiation.High-strengthnickel-basedalloysarepronetohydro-
gen-assistedcracking,potentiallyinfluencedbytheSlipDissolutionViscousCreep
Surfaces2024,7600
(SDVC)mechanism,whichpromoteslocalgrainboundaryslidingunderspecificSCC
conditions[71].
3.3.5.CopperAlloys
Copperalloys,suchasbrass,areparticularlysusceptibletoSCC,especiallyinenvi-
ronmentswheremoisturecondensationfacilitatestherapidformationofcupriccomplex
ionsandtarnishfilmspredominantlycomposedofCu2O[71].Thisphenomenon,known
asseasonalcracking,hashistoricallybeenobservedduringrainyseasons,notablyaffect-
ingbrasscartridgesusedbytheBritishArmyinIndia.Brass,primarilycomposedofcop-
perandzinc(Cu-Zn)alloy,exhibitsvaryingdegreesandmodesofcrackingdependingon
itszinccontent[32,72].
ResearchonSCCincopperalloyshasprimarilyfocusedonmoistairconditions.
However,SCChasbeendocumentedinvariousotherenvironments,includingthosecon-
tainingchlorides,nitrites,nitrates,sulfates,andevenpurewater[71].Purecopper,espe-
ciallywhenexposedtosolutionscontainingacetateandnitrite,alsoshowssusceptibility
toSCC.CathodicpolarizationsuppressesSCC,whilegeneralcorrosionisexacerbatedun-
dernoblepotentials.Conversely,theanodicpolarizationofcopperalloysfromtheirfree
corrosionpotentialsignificantlyincreasesSCC.Additionally,thetemperaturedepend-
enceofSCCincopperalloysfollowsanArrhenius-typebehavior,characteristicofther-
mallyactivatedprocesses.
ThemechanismofSCCinthesealloysiscomplex,involvingbothintergranularand
transgranularcrackingmechanisms.IGSCCtypicallyproceedsthroughadetachmentor
filmbreakageprocess,whileTGSCCmayinvolvediscontinuouscleavageinitiatedbyan
epitaxialfilm,suchasadealloyedlayeroroxideatthecracktip[32,72].InTGSCC,deal-
loyingbecomescriticalwhenthealloyexceedsspecificlimits,approximately14atomic
percent(a/o)forCu-Alalloysand18a/oforCu-Znalloys,whichdictatetheirsusceptibil-
itytoSCC.Itisnoteworthythatwhilealloyswithhigherzinccontentcommonlyexhibit
TGSCC,thosewithlowerzinccontentmaydisplayIGSCC[32,71,72].
3.3.6.AluminumAlloys
Aluminumanditsalloysarerenownedfortheirexceptionalcorrosionresistance,
whichisaributedtotheformationofaprotectiveoxidefilmontheirsurfaces.Thisoxide
film,typicallyconsistingoftwolayers,providesstabilitywithinapHrangeofapproxi-
mately4to8.5.However,exposureoutsidethesepHvaluescanleadtoaluminumcorro-
sion,resultinginthegenerationofAl3⁺orAlO2⁻ionsduetothesolubilityofaluminum
oxidesinvariousacidsandbases[71].TheSCCphenomenoninaluminumalloyshasbeen
extensivelystudied,revealingitscomplexitiesandinfluencingfactors.Secondphases,of-
tenintermetalliccompounds,arecommoninaluminumalloys,rangingfromnegligible
amountstoabout20%.Thesephasesprecipitatepredominantlyalonggrainboundaries
andsignificantlyaffectthecorrosionbehaviorofaluminumalloysduetotheirvarying
electrodepotentialsrelativetothematrix.IGC,frequentlyobservedinthesealloys,arises
fromelectrochemicalpotentialgradientsbetweenadjacentgrainsandgrainboundaries,
withtheanodicpathshowingcompositionalvariationsacrossdifferentalloycomposi-
tions.ThesusceptibilityofaluminumalloystoSCCisnotablyinfluencedbyIGCandpit-
tingcorrosion.Alloyscontainingsolublealloyingelements(e.g.,zinc,silicon,magnesium,
andcopper)areparticularlypronetoSCC,whereintergranularpathwaysfacilitatecrack
propagationunderstressandenvironmentalconditions.However,mitigationstrategies
suchascathodicprotectionorspecificheattreatmentscanmodifythemicrostructureand
reduceSCCsusceptibility[71,73,74].
InthecontextofSCCinaluminumalloys,waterandwatervaporarecriticalenviron-
mentalfactorscontributingtoitsdevelopment.Halideions,especiallychlorides,signifi-
cantlyexacerbatethesusceptibilityofthesealloystoSCC.Generally,inalkalinesolutions,
thesusceptibilitytoSCCisrelativelylowcomparedtoneutralandacidicenvironments.
ThefundamentalprinciplesofSCCinvolvelocalizedanodicdissolutionoccurringatgrain
Surfaces2024,7601
boundariesunderthesynergisticinfluenceofstressandenvironmentalfactors,alongwith
hydrogeningressoradsorptioncontributingtocrackpropagation.Thenon-uniformpro-
gressionofcracksinSCC-susceptiblealuminumalloyssuggeststhatcontinuousanodic
dissolutionalonecannotfullyexplaincrackpropagation[71].
Aluminumanditsalloysarepronetofailureviaintergranularcrackingwhensub-
jectedtosignificantstressesandexposedtospecificenvironmentalconditions.Notably,
the2xxx,5xxx,and7xxxseriesofaluminumalloysexhibitheightenedsusceptibilityto
SCCduetotheirinherentchemicalcompositionsandmicrostructuralcharacteristics.The
7xxxseriesalloys,widelyusedinstructural,military,andaerospaceapplicationsfortheir
exceptionalmechanicalproperties,includinghighstrength-to-weightratiosandfatigue
resistance,areparticularlyvulnerabletoSCC.Therefore,carefullyconsideringSCCsus-
ceptibilityduringthedesignandmanufacturingprocessesisessentialtoensuretheinteg-
rityandreliabilityoffinalproducts[71,73–75].
3.3.7.HexagonalAlloys:Magnesium,Zirconium,andTitanium
Materialswithahexagonalclose-packedcrystalstructure,suchaszirconiumandti-
tanium,exhibithighlynegativepotentialsinaqueousenvironments,emphasizingthesig-
nificantroleofhydrogenuptakeinenhancingtheirresistancetoSCC.Zirconiumandti-
taniumformstablehydrides,whereasmagnesiumhydridedecomposesrapidlyabove280
°C,displayinglowerhydrogensolubilitythanzirconiumandtitanium[71].
WhiletitaniumalloysgenerallydemonstrateexcellentresistancetoSCC,specificen-
vironments,suchasanhydrousmethanol,hotsalts,andstrongoxidizers,caninduceSCC
[71,76].Inhalidesolutions,activecorrosiontrenchesinitiatecracksthatpropagate
throughanodicdissolutionorhydrogen-assistedcrackingmechanisms.Thisprocesscan
involvethelocalprecipitationofbriletitaniumhydride,significantlyaffectingtheme-
chanicalpropertiesofthematerial[71].KieferandHarplefirstreportedtheSCCsuscep-
tibilityofcommerciallypuretitaniuminaqueousenvironments,particularlyinred-fum-
ingnitricacid(HNO3).Subsequentcasesofhot-saltcrackingwereobservedinturbine
bladesmadefromtitaniumalloysoperatingatelevatedtemperatures.Brownalsodocu-
mentedinstancesofSCCintitaniumalloysexposedtoroom-temperatureaqueousenvi-
ronments,highlightingsusceptibilityinseawater,particularlyinthetitanium8-1-1alloy
[30].
SCCinzirconiumalloysisconfinedtospecificconditions,suchashalogenvapors,
concentratedHNO3,andoxidizingchloridesolutions,withmechanismsakintothoseob-
servedintitaniumalloys.Additionally,susceptibilitytoSCChasbeennotedinmethanol
environments,primarilyaffectingmethanolandpotentiallyotherlow-molecular-weight
alcohols[71].Whethercastorextruded,magnesium-basedalloysmaybesusceptibleto
SCCunderdiverseconditions,suchaspurifiedwaterandair.Alloyscontainingalumi-
numareparticularlyvulnerable,withincreasedsusceptibilityinthepresenceofzinc.
Crackingcanoccuralonggrainboundariesortransgranularly,influencedbycathodicre-
gionswithinthemicrostructure.TGSCCinmagnesiumalloystypicallymanifestsalong
twinningplanesorcleavage,oftenexhibitingextensivebranching.ProposedSCCmecha-
nismsincludethebrilehydridemodel,developedbasedonobservationsofstress-in-
ducedmagnesiumhydridephases.CathodicpolarizationgenerallymitigatesSCCsus-
ceptibility,whereasanodicpolarizationexacerbatesit[71].
3.3.8.AusteniticStainlessSteel(ASS)
ASSissusceptibletoSCCinenvironmentscontainingpolythionicacid,causticsolu-
tions,andchlorides.SensitizationoccurswhenASSwithmorethan0.03wt.%carbonis
heated,leadingtotheprecipitationofM23C6atgrainboundaries[77].Thissensitization
reducesthechromiumcontentalonggrainboundaries,makingthempronetorapidpref-
erentialdissolutionandincreasingvulnerabilitytoSCC[78].Thestabilityofthepassive
filminASS,crucialforcorrosionresistance,isinfluencedbyfactorssuchastemperature,
acidity,halidecontent,andthepotentialdifferencebetweenthemetalandthesolution
Surfaces2024,7602
[79].SensitizedstainlesssteelsareparticularlysusceptibletoSCCinthepresenceofhal-
ides,includingchlorides,bromides,andiodides,whichcaninducepiingcorrosion[80].
ASSsensitizationoccursatelevatedtemperatures,resultinginM23C6precipitation
alonggrainboundaries[4].Thekineticsofthisprecipitationdependonthecarboncon-
tent,withlowercarboncontentrequiringmoretimeforsensitization.Chromiumandni-
trogenincreasecarbonsolubility,delayingorreducingprecipitation,whilemolybdenum
andnickelpromoteit.Thepresenceofnitrogencanalsoreducecarbideprecipitationin
ASS[81].Elementsenhancingpassivefilmstabilitycontributetoresistanceagainstpiing,
crevicecorrosion,andSCC[35].
Table2presentsthechemicalcompositionsoffrequentlyusedASSsinvariousindus-
trialapplications.ASSslikeSS304andSS316arewidelyusedinindustryfortheirexcep-
tionalproperties,enablingthemanufactureofcorrosion-resistantcomponentsandcon-
tainers.Forinstance,SS304andSS316areemployedinhigh-pressurepipingsystemsand
primarycircuitsofBoilingWaterReactors(BWRs)andPressurizedWaterReactors
(PWRs),respectively.SS316,modifiedwithadditionalelementssuchastitanium,silicon,
andphosphorus,isusedinlightwaterreactorsforfuelcladdingtubes[82,83].Thesealloys
exhibitoutstandingmechanicalproperties,includinganelongationof66.5%,yield
strengthof410MPa,andtensilestrengthof691MPa[4,84].
Tab le2.ChemicalcompositionsofcommonlyappliedASSsincoolingandpipingsystems[4].
ASSElement(wt.%)
CrNiSPMnMoSiNCOthers
SS30217.008.000.0300.0452.00–0.750.100.15–
SS31024.0019.000.0300.0452.00–1.50–0.25–
SS34717.009.000.0300.0452.00–0.75–0.08Nb1.00
SS32117.009.000.0300.0452.00–0.750.100.08Ti0.70
SS31618.0010.000.0300.0452.002.000.750.100.08–
SS30418.238.130.0040.0291.65–0.350.100.06–
TheresistanceofASStopiingcorrosionisinfluencedbythealloycomposition,
whichcanbequantifiedusingthePiingResistanceEquivalentNumber(PREN).A
higherPRENvalueindicatesimprovedcorrosionresistance.Forexample,duetoitsmo-
lybdenumcontent,SS316(PREN=25)exhibitssuperiorpiingcorrosionresistancecom-
paredtoSS304(PREN=20).Othergrades,suchasSS310andSS321/SS347,areutilizedto
mitigatecoldworkhardening,withstandhightemperatures,andfacilitateweldingin
high-temperatureapplications.However,designersmustbecautiousofhighsulfurlevels
intubing,whichaidinweldingbutcompromisecorrosionresistance[85–87].
Piingcorrosionacceleratesinchlorideenvironments,formingdistinctanodicand
cathodiczones.Exposuretochlorideaqueoussolutionsdisruptsthepassivefilm,which
ispromptlyreplenishedwithachromium-enrichedlayer.Thethicknessofthepassive
filmincreasesovertimeduetoelevatedFe2⁺levelsintheoxidelayerwithprolongedex-
posureatelevatedtemperatures.Increasedchlorideionconcentrationwithinthetypical
atmospherichumidityrangeexacerbatespiingcorrosion[88,89].Giventhesusceptibility
ofASSstoSCCinchlorideenvironments,chlorideionsplayacriticalroleininitiatingand
propagatingSCC.Theinteractionbetweenchlorideionsandsensitizedstainlesssteelsur-
facesacceleratespassivefilmbreakdown,leadingtolocalizedcorrosionandeventual
crackformationunderstress.Understandingthesemechanismsiscrucialformitigating
SCCrisksinchloride-richenvironments[88,89].
Theresistanceofhigh-strengthsteelstoSCCvarieswiththetemperingtemperature.
Initially,SCCresistanceincreaseswiththetemperingtemperaturebutdiminishesatspe-
cificpoints.However,furthertemperingcanenhanceresistanceagain.Thespecifictem-
peringtemperatureaffectingSCCresistancedependsonthesteelcomposition.Forin-
stance,steelslike08Cr15Ni5Cu2Tiand13Cr15Ni4NMo3aremostsusceptibletoSCCafter
Surfaces2024,7603
temperingbetween425°Cand475°C,achievingmaximumstrength.Atlowertempera-
tures(200–350°C)andhighertemperatures(525–560°C),SCCresistanceimproves,po-
tentiallyeliminatingcracking.Weldedjointsexhibitdifferentbehaviorsbasedontemper-
ingtemperatures;jointsweldedbelow400°CexperiencereducedSCCrates,whilethose
weldedabove500°Carelesssusceptible.Theeffectivepre-treatmentofsteelslike
08Cr15Ni5Cu2Tiat500–550°Caltersthemicrostructure,preventingSCCandIGCin
weldedjoints[53].
Low-temperaturetemperinguniformlyformscarbideparticleswithinsteelcrystals,
reducinginternalstressandenhancingSCCresistance.Increasedtemperingtemperatures
leadtomoreprecipitationandmartensiteformationfromaustenite.Thisresultsinmar-
tensitedecayalonggrainboundaries,creatingcarbon-depletedzonesascathodesandcar-
bon-enrichedareasasanodes.Furthertemperingextendsmartensitedecaythroughout
thegrain,reducinginternalstressandincreasingSCCresistance.Thesemechanismsillus-
tratehowthetemperingtemperatureinfluencestheSCCresistanceofhigh-strengthsteels,
underscoringtheimportanceofcompositionandheattreatmentinmitigatingSCCrisks
[53,55,90].
4.SCCMechanism
SCCmanifeststhroughtwoprimarymechanisms:dissolution-basedandcleavage-
based.Indissolution-basedSCC,materialscorrodelocallyatcracktips,compromisingthe
passivefilmandinitiatingcracks,whilecleavage-basedSCCinvolvesbrilefractures
alonggrainboundaries[71,91].
4.1.DissolutionMechanism
Dissolution-basedSCCinitiatesthroughlocalizedcorrosionatcracktips,wherethe
passivefilmbreaksdown,indicatinglocalizedfilmfailure.Thisprocessisinfluencedby
threedissolutionmechanisms:thepre-existingactivepathmodel,thestrain-generating
activepathmechanism,andthecorrosiontunnelmodel[71].Anodicdissolutionatpre-
existinggrainboundaries,oftencontainingintermetallicandsegregatedcompounds,is
exacerbatedbytensilestress,aphenomenonknownasthemechanoelectriceffect.Chro-
mium-depletedgrainboundariesareparticularlysusceptible,leadingtointergranular
cracking,whichismainlyobservedinsensitizedASS[35].
Inthestrain-generatingactivepathmechanism,crackgrowthproceedsthroughcy-
clesofpassivationanddissolutionalongslipstepsorcracktips.Thisinvolvestherupture,
dissolution,andrepassivationofthepassivefilmonthemetalsurface[92].Crackgrowth
maypauseatslipstepsuntilrepassivationoccurs,characterizedbyarrestmarkson
crackedsurfaces,whilesmoothsurfacesmayexhibitcleavage.Additionally,intergranular
crackscanformduetograinboundaryfeaturesonthefracturedsurface[93,94].
Thecorrosiontunnelmodeldescribescracksinitiatingatslipstepsandpropagating
undertensilestress,potentiallyleadingtoductileormechanicalfailure.Thesecracksini-
tiallyappearasthintunnelsandmayprogresstomechanicalfracture,withsurfacesex-
pectedtomatchclosely[4].Thedissolution-basedmodelisinfluencedbytheinterplay
betweendissolutionandrepassivationrates.Acceleratedcrackgrowthinchloride-in-
ducedSCCoccurswhendissolutionexceedsrepassivation,drivenbychlorideionsand
mechanicalstress.Conversely,rapidrepassivationthickensthepassivefilm,hindering
furtherslip-stepcorrosion[95,96].
Surfaces2024,7604
4.2.CleavageMechanism
Thecleavage-basedmechanisminvolvescrystalfracturespropagatingalongcrystal-
lographicsurfaces,leadingtobrilefailure[71].Theadsorption-inducedcleavagemech-
anismoccurswhenenvironmentalspeciesareadsorbedunderstress,resultingincleavage
fracture.Tensi lestressweakensatomicbondsalonggrainboundaries,particularlyatcrack
tipswherechlorideionsarepresent[97].Thetarnishrupturemechanisminvolvescyclic
processesofarrestmarks,filmformation,crackgrowth,andfilmfracture.Undertension,
cracksinitiateduetobrilepassivefilms.Uponexposuretothesolution,repassivation
haltscrackpropagation,primarilythroughtransgranularfracturesurfaces,enablingIG-
SCCaheadofthecracktip[98,99].
Thefilm-inducedmechanismsuggeststhatpassivefilmformationthroughdissolu-
tionmayleadtobrilecrackdevelopmentundertensilestress,propagatingbeyondfilm
thicknessintothemetalvolume.Crackpropagationhaltsduetothemetal’stoughmicro-
structure,resultinginbluntedcracksthroughplasticdeformation.Factorsinfluencing
film-inducedcleavageincludefilmthickness,initialcleavagecrackvelocity,substrate
toughness,andthebondingstrengthbetweenthemetalmatrixandthepassivefilm[4].
Theatomicsurfacemobilitymechanism,whichfocusesontheroleofhydrogenatoms,
predictscracksusceptibilityunderenvironmentalconditions.Vacancydiffusioniscritical,
asitremovescrystallaiceelementsnearcracktips,enablingatomicmigrationandcrack
propagationinthepresenceofaqueouscontaminants[4,100].Chaerjee[101]notesthat
multiplemechanismscontributetoSCCratherthanasingledominatingmechanism.Ac-
cordingtothecleavage-basedmodel,hydrogenatomsmigratetocracktips,causingem-
brilementandcontributingtoSCC.Thedissolutionofoxidefilmsbyanodicchloride
ionsenhancestheconcentrationofacidicspeciesatcracktips[102].
4.3.SCCDevelopment
Crackgrowthratesdependonloadingconditions,environmentalaggressiveness,ex-
posureduration,andmaterialsensitization.Mathematically,thecrackgrowthrateisre-
latedtothecracklength,exposuretime,andmetalsensitization.Sensitizationinvolves
theformationandprecipitationofM23C6alonggrainboundaries,depletingthechromium
contentanddisruptingtheprotectivefilm(Figure4a).Decreasedchromiumcontentleads
toasuddendiscontinuityinthepassivefilm.ASSsensitization,oftenresultingfromweld-
ingorexposuretohightemperatures,diminishesSCCresistanceandacceleratescrack
propagation,evenunderlow-stressconditions.Figure4billustratesspecificcrackpaths
observedinanodicsolutions,depictingthedistinctionsbetweenIGSCCandTGSCC[103].
Figure4.(left)Chromiumcarbideprecipitationatgrainboundariesinsensitizedstainlesssteeland
(right)SCCmechanism:interactionbetweenanodicsolutionandmaterialresultinginIGSCCand
TGSCC[4].
Surfaces2024,7605
ASScomponentsinBWRsandPWRsaresusceptibletoIrradiation-AssistedSCC
(IASCC)andIGSCC[104].BWRs’thermallysensitizedASScomponentsareparticularly
pronetoIGSCC,whichisexacerbatedbyneutronexposurethatinitiatesandpropagates
cracking[105].Conversely,TGSCCtypicallyoriginatesontheexteriorsurfacesofASS
components.
Chloridecontaminationcreatesaqueousenvironmentsthatareidealforcrackprop-
agation,oftenfacilitatedbythermalinsulation,wateringress,orweing.Thisenviron-
mentfosterspiingorcrevicecorrosionconcurrentlywithSCC,particularlyundertensile
stressconditions[106].ASScomponentsarepronetoSCCundersuitablecorrosivecondi-
tions.Parkinsidentifiedthreestagesinthestresscorrosionspectrumofvariousmaterials
exposedtodiversecorrosivesolutions:pre-existingactivepaths,strain-inducedactive
paths,andspecificsubcriticalstresslocationadsorption[4].AccordingtotheParkins
model,SCCprogressesthroughinitiation,propagation,filmrupture,andpiing[103].
Elevated-temperatureinteractionsbetweenappliedtensilestressandcorrosivesolutions
disruptlocalpassivefilms,leadingtopiingandsubsequentcrackinitiation.Stresslocal-
izationprofoundlyimpactspitformation,influencingthecracklengthandmaterialprop-
erties(Figure5).
Figure5.Athree-stagemodelforSCCprogression(Reproduced/reprinted/adaptedfromRef.[4].
2024,licensedunderanopenaccessCreativeCommonsCCBY4.0license).
Hänninen[71]suggestedthatCl-SCCistypicallyabsentinnon-sensitizedASSsbe-
low50°Cundernear-neutralconditions.However,low-pHconditionscaninduceCl-
SCC,evenatroomtemperature.Inchloridesolutions,pitsorlocalizedcorrosionoften
initiatecracks,whereascrevicecorrosionacidifiescracktips,hydrolyzingdissolvedmetal
ions[99,107].Xieetal.[108]studiedSCCinSS316undersimulatedPWRconditions
throughsolutiontreatmentandcoldworking.Theintroductionofchlorideionssignifi-
cantlyincreasedSCCsusceptibility.Themechanisminvolvesthecyclicruptureandre-
generationoftheoxidefilmthroughmetalatomdiffusion,producingdissolvedchlorides
thatformmicrocracksandacceleratetheinitiationandpropagationofTGSCC.Dissolved
oxygenenhancescrackgrowthincold-workedSS316bypromotinganiondiffusionat
cracktips,therebyacceleratingSCC.
Onceinitiated,Cl-SCCinASScannotbecompletelystopped,butitcanbemitigated.
Yeometal.[109]employedcoldspraytechnologytomitigateCl-SCCinnuclearfuelstor-
agecanistersbypropellingSS304Lpowderparticleswithheliumandnitrogengases,in-
creasingparticlevelocity.Thismethodpenetratedoxidelayers,inducingcompressivere-
sidualstressandeffectivelysealingcrackopeningsasaphysicalbarrier,whichinhibits
theprogressionofCl-SCC.
Surfaces2024,7606
SCCisasuddenandseverefailuremechanisminmaterialsexposedtocorrosiveen-
vironments,particularlyinchloride-richandhigh-temperatureconditions.ASSsensitiza-
tionfromweldingorhightemperaturesleadstointergranularcrackformationalonggrain
boundaries,contrastingwithtransgranularcrackinginunsensitizedmaterials.Parkins’s
stresscorrosionspectrumanddissolution-basedmodelselucidatethecontributionsof
chlorideionstopassivefilmbreakdownandcrackpropagation.
5.Chloride‐InducedStressCorrosionCracking(Cl‐SCC)
Chloride-inducedstresscorrosioncracking(Cl-SCC)occurswhenmetals,particu-
larlyASSandnickelalloys,areexposedtocorrosiveenvironmentsundertensilestress
[110].ThekineticsoftransientoxidationinCl-SCCsystemsarecomplexanddependon
theinterplayofmaterialproperties,environmentalfactors,andloadingconditions[111].
Shallowpits,althougheasilyinspected,canactasstressconcentrators,potentiallyinitiat-
ingSCC[112].Inpetroleumrefining,environmentscontainingpolythionicacids,amines,
ammonia,caustics,andmoltenchloridesarecommonplace.Chlorides,inparticular,are
knowntoinduceTGSCCinASSandnickelalloys[113].Despitetheprevalenceofsulfates,
chloridesareconsideredmoreaggressiveinpromotingSCC.Understandingthecompo-
sitionandoriginofdepositsinrefiningenvironmentsiscrucial[110].
TheinitiationofCl-SCCinrefineriestypicallystemsfromchloride-containingcom-
poundspresentincrudeoil,refiningprocesses,orexternalsourcessuchasseawaterin-
gress.Uponcontactwithmetalsurfaces,subsequentcoolingandmoistureabsorptionlead
totheformationofacorrosivelayer,fosteringlocalizedcorrosionandpotentialSCCun-
derstressconditions[4].ExternallyinitiatedCl-SCCinvolveselectrochemicalmecha-
nismsfollowedbyapropagationstageinfluencedbyelectrochemistryandmetalsepara-
tion[114].Cl-SCCcanleadtodeteriorationandpotentialstructuralfailuresinindustrial
applications,impactingsafetythroughpartialorthrough-wallcorrosionandcracking
[115].ThisposesasignificantrisktoASS,especiallyinchloride-richenvironmentswhere
transgranularcrackingpredominates,althoughsensitizedsteelsmayexperienceinter-
granularcracking[113].
VariousparametersinfluenceCl-SCCinrefineryseings,reflectingdiverseopera-
tionalandenvironmentalconditions.Criticalenvironmentalfactorsincludechloridecon-
tent,pH,temperature,impurityconcentrations,andtheformationofchloride-containing
moisturefilms.Relativelylowtemperatures(below100°C)cancatalyzeSCCincorrosive
solutions[4].Deliquescentchlorides,capableofabsorbingmoisture,formhighlyconcen-
tratedfilmsonequipmentsurfaces,withdeliquescencerelativehumidity(RH)playinga
pivotalroleinfilminitiation[116,117].Externalsourcesofchlorides,suchasrain,coastal
fog,deicingsalts,andprocessleaks,significantlycontributetochlorideexposureonmetal
surfaces,particularlyinindustrieslocatednearcoastalareas.Overtime,equipmentper-
formancecanbeadverselyaffectedbycorrosioninducedbythehygroscopicpropertiesof
saltsandthedepositionofchlorideionsasaerosols.Thesefactorscancontributetolocal-
izedSCC,especiallyinareaswithhighresidualstressesfromwelding.Airbornepollu-
tants,dust,andaerosolscandepositonequipmentsurfaces,leadingtolocalizedcorrosion
aacks[118].ThermalinsulationcanexacerbateASSsusceptibilitytoSCC.Whilelocalized
corrosioniswidelyrecognizedastheprimaryinitiatorofCl-SCC,themechanismsgov-
erningcrackpropagationremaincomplexandnotfullyunderstood[119].Crackpropa-
gationlikelyinvolveselectrochemicaldissolutionandatomic-levelfracturemechanisms
withinthemetalstructure[120].
Inindustrialseings,particularlyinequipmentsuchassteamgenerators,coolant
systems,heatexchangers,andpipes,ASSfacesanelevatedriskofCl-SCCduetoprevail-
ingoperationalconditionsandthepresenceofchlorideionsincoolants[82].Despitethe
corrosionresistanceof300seriesstainlesssteel,exposuretotemperaturesabove60°C
increasesitssusceptibilitytoCl-SCCinaqueousenvironments.Thiscanleadtopassive
filmdegradation,pitformation,andcrackingundertensilestress[121].Theresulting
threattorefineryequipmentnecessitatesproactivemeasurestomitigateriskseffectively
Surfaces2024,7607
[111].Operationalstresses,includingcyclicloading,thermalstresses,pressurechanges,
andpotentialweldingduringconstructionormaintenance,contributetoresidualstresses.
Tensilestressesinducedbywelding,particularlyinASSwithoutstressrelieftreatments,
createanenvironmentconducivetotheinitiationandpropagationofCl-SCC.Despite
limiteddirectmeasurements,highresidualstressesalongwelddirectionsincreasethema-
terial’svulnerabilitytoCl-SCC.Advancesinresidualstressmodelingunderscorethecrit-
icalroleofwelding-inducedresidualstressesingoverningCl-SCCsusceptibilityinASS
[4].
5.1.FactorsAffectingCl‐SCC
5.1.1.Materials
Cl-SCCposesasignificantchallengeinrefineryoperations,particularlyconcerning
materialsusceptibility.Thespectrumofsusceptibilityrangesfromcarbonsteelandlow-
alloysteels,whichexhibitlowerresistance,toalloyssuchas300seriesstainlesssteels,
Alloy400,duplexstainlesssteels,Alloy800,Alloy825,Alloys625andC276,andtitanium,
whichdemonstratehigherresistancetoCl-SCC.
Cl-SCCoccurswhenmaterialssusceptibletochlorideexposure,suchas300series
stainlesssteels,encounterchloridesalts.Thisformofcorrosionposesasignificantthreat
topipingandequipmentintegrityacrossvariousrefiningunits,includingproductstabi-
lizertowers,recyclegassystems,FluidizedCatalyticCrackingUnits,overheadsystemsof
crudedistillationandfractionationcolumns,hydroprocessingeffluentsystems,catalytic
reformingunits,andhydrotreaterdesulfurizerprefractionators.Reactoreffluentstreams
operatingatapproximately~300°Fareespeciallypronetofoulingandcorrosioninduced
bychloridesalts.Understandingthemetallurgicalnuancesinfluencingmaterialsuscepti-
bilitytoCl-SCCiscrucialforselectingappropriatematerialsandimplementingeffective
corrosionmitigationstrategiesinrefineryenvironments.Metallurgicalconsiderationsare
paramountindeterminingsusceptibilitytoCl-SCC,withfactorssuchasalloycomposi-
tion,microstructure,andgrainboundarycharacteristicsplayingcrucialroles.Carbon
steelandlow-alloysteels,withhigherironcontentandsusceptibilitytosensitization,are
morepronetochloride-inducedcorrosionandcrackingthanstainlesssteelsandcorro-
sion-resistantalloys.WhileASSs,suchasthe300series,offercorrosionresistance,they
remainsusceptibletoCl-SCCundercertainconditions,particularlyinchloride-richenvi-
ronmentsandundertensilestresses.Duplexstainlesssteelsexhibitimprovedresistance
duetotheirdual-phasemicrostructure,combiningcharacteristicsofausteniticandferritic
stainlesssteels[122].
MicrostructuraleffectssignificantlyinfluencematerialsusceptibilitytoCl-SCC,with
grainboundariesactingaspreferentialsitesforchlorideioningressandcrackinitiation.
SensitizationinASS,characterizedbyM23C6precipitationalonggrainboundaries,canin-
creasesusceptibilitytoCl-SCCbydepletingthechromiumcontentintheadjacentmatrix.
Techniquessuchasgrainboundaryengineering,involvingmodificationoralloying,can
enhanceresistancebyminimizinggrainboundarysensitizationandpromotingamore
corrosion-resistantmicrostructure.AlloyselectioncriteriaarecrucialinmitigatingCl-SCC
risk,consideringfactorsincludingchloridecontent,temperature,pressure,andmechani-
calloading.Alloyswithhigherchromium,molybdenum,andnickelcontents,suchasAl-
loys825,625,andC276,demonstratesuperiorresistancecomparedtocarbonsteeland
low-alloysteels.Titaniumalloysofferexcellentcorrosionresistanceinchloride-richenvi-
ronmentsduetotheformationofastablepassiveoxidefilmontheirsurface.Bycompre-
hendingmetallurgicalfactorsandintegratingthisknowledgeintoalloyselectionandde-
signconsiderations,engineersandmaterialsscientistscandeveloprobustsolutionsto
mitigateCl-SCCriskinrefineryenvironments[122].
Alloychemistryandmicrostructureprofoundlyinfluencethesusceptibilityofstain-
lesssteeltoSCC.ASSsareparticularlyvulnerabletoCl-SCC,contrastingwiththesuperior
resistanceofferriticstainlesssteel.Nickel,molybdenum,andnitrogenplaypivotalroles
Surfaces2024,7608
inenhancingtheSCCresistanceofASSs.Increasingthenickelcontentabove8%inASSs
cansignificantlyimprovetheirresistancetoCl-SCC.Additionally,molybdenumandni-
trogenhavebeenfoundtoenhanceSCCresistance,possiblyduetotheirbeneficialeffects
onpiingresistance,aprecursortoSCCinitiation.Conversely,elevatedlevelsofsulfur
andphosphoruswithinthesteelmatrixdetrimentallyaffectSCCresistance.Stresscorro-
sionstudieshavedemonstratedthathigherphosphoruscontentinASScorrelateswith
increasedsusceptibilitytoSCCinchloride-richenvironments.
Furthermore,thesensitizationofASSsbetween500and850°Crendersthemsuscep-
tibletoIGCandIGSCCinvariousenvironmentscontainingchlorideorfluorideions[6].
ThenickelcontentwithinstainlesssteelalloysalsoplaysacriticalroleinSCCsusceptibil-
ity.Cl-SCCsusceptibilityincreaseswiththenickelcontentuptoacertainthreshold,typ-
icallywithintherangeoftraditionalAISI304and316steels.Beyondthisthreshold,usu-
allyaround10%nickelcontent,nickeldemonstratesamitigatingeffectonSCCinchlo-
ride-richenvironments.Alloyscontainingmorethan42wt.%nickelareimmunetoCl-
SCC[123].
Furtherinvestigationsintosensitizationprocesseshaverevealedinsightsintothenu-
ancedrelationshipbetweensensitizationparametersandSCCsusceptibility.Prolonged
sensitizationdurationsandlowersensitizingtemperatureshavebeenassociatedwith
heightenedsusceptibilitytoIGSCC.Thisvulnerabilityhasbeenlinkedtoareductionin
chromiumconcentrationadjacenttograinboundaries,whichunderminesthestabilityof
passivefilmsandincreasesthechemicalactivityoflocalizedregions,facilitatingIGSCC
initiation[6].QuantitativeassessmentsusingElectronProbeMicro-Analysis(EPMA)and
SlowStrainRateTesting(SSRT)haveprovidedvaluableinsightsintotherelationshipbe-
tweenthedegreeofsensitizationandIGSCCsusceptibility.Whilealinearcorrelationis
observedbetweenIGSCCsusceptibilityandEPMAchargevaluesforlowdegreesofsen-
sitization,complexitiesariseathigherdegreesofsensitization.Conversely,IGSCCsus-
ceptibilityincreaseswithadecreaseinsensitizingtemperatureforagivenEPMAcharge
value,highlightingtheintricateinterplayofmultiplefactorsingoverningSCCbehavior
[6].
Inindustrialapplications,selectingstructuralmaterialsiscriticalduetotheaggres-
sivechloride-richenvironmentsinvolved.AlloyswithPRENsexceeding50haveemerged
aspromisingcandidates,demonstratingnotableresistancetoCl-SCC.Nickelcontenthas
beenidentifiedasadecisivefactor,withalloysfeaturinghighernickelcontentexhibiting
enhancedresistancetoCl-SCC[123].InvestigationsintoSCCsusceptibilitywithinsuper-
criticalwater(SCW)environmentshaveprovidedadditionalinsightsintotheinterplayof
environmentalparametersandmaterialbehavior.ThechlorideconcentrationwithinSCW
hasbeenidentifiedasacriticaldeterminantofSCCsusceptibility,withhigherchloride
concentrationscorrelatingwithheightenedsusceptibility.Conversely,higherchromium
contenthasbeenassociatedwithreducedIGSCCsusceptibility,highlightingtheroleof
alloycompositionindictatingmaterialperformanceunderaggressiveenvironmentalcon-
ditions[124].
Inconclusion,thesusceptibilityofstainlesssteelstoSCCrepresentsamultifaceted
phenomenongovernedbyalloycomposition,microstructure,andenvironmentalfactors.
Continuedinterdisciplinaryresearcheffortsareessentialfordeepeningourunderstand-
ingofSCCmechanismsanddevelopingrobustmitigationstrategiestosafeguardcritical
infrastructureagainsttheperilsofSCC.
5.1.2.TemperatureandLimitingRelativeHumidity
FactorsinfluencingCl-SCCprimarilyincludetemperatureandRH.Theimpactof
temperatureoncorrosionismultifaceted:withinacertainrange,itcanfacilitatebothlo-
calizedcorrosionandSCC.However,beyondthisrange,elevatedtemperaturesmayhin-
dercorrosionbyreducingsurfaceRHbelowthelimitingrelativehumidity(RHL),thereby
Surfaces2024,7609
preventingtheformationofdeliquescentsaltsnecessaryforcorrosion[121,125].Con-
versely,lowertemperaturescanelevatesurfaceRHtolevelswheredeliquescedsolutions
becometoodilutedtosustaincorrosion[115].
ASSsareprizedfortheirgeneralcorrosionresistancebutarehighlyvulnerableto
localizedformssuchaspiing,crevicecorrosion,andSCC,especiallyinchloride-richen-
vironments.Chlorideions,pervasiveinwaterandindustrialseings,catalyzeSCC.Ele-
vatedtemperaturesexacerbateSCC,particularlyinsensitizedASSsthataresusceptibleto
IGSCC,evenatambienttemperatures.Whileaconsensusonthecriticaltemperaturefor
SCCinitiationremainselusive,evidencesuggestsasignificantincreaseincrackgrowth
ratesabove80°C.However,SCCcanoccuratlowertemperatures,particularlywhensur-
faceimpurities,suchasembeddedironparticles,createsitesforchlorideaccumulation.
DeterminingathresholdchlorideconcentrationbelowwhichSCCismitigatedis
challengingduetothelocalizednatureofthecorrosionprocess.Chlorideaccumulation
withincrevicesorunderdepositsonASSsurfacesexacerbatesSCC,withchlorideoften
leachingfromthermalinsulation,therebyincreasingtherisk.ASSscantypicallywith-
standwaterwithlessthan1000ppmchlorideunderflowingconditions,assumingnocon-
centrationmechanismsareactive.However,thepermissiblechloridecontentininsulation
dependsonitssilicatecontent.Therefore,mitigatingSCCrequiresathoroughunder-
standingofhowlocalenvironmentalfactorsinfluencechlorideaccumulationandcorro-
sioninitiationonASScomponents[6].
Understandingthedetailedeffectsoftemperatureoncorrosionkineticsiscrucialfor
Cl-SCC.Temperatureinfluenceselectrochemicalanddiffusionkinetics,resultinginvaried
corrosionrates.Experimentalfindingssupportthisobservationinexposuretestscon-
ductedatdifferenttemperatures.Forinstance,SS304exhibitssignificantlylongerfailure
timesunderappliedstress(~400MPa)at50°Ccomparedto80°C.Additionally,lower
temperaturesgenerallyreduceelectrochemicalanddiffusionkinetics,therebylowering
corrosionrates[126].
RHisalsocriticalinatmosphericchlorideSCC.ExposuretestsindicatethatRHabove
theRHLisessentialforchloridesaltdeliquescenceandsubsequentSCCinitiation[115].
StudiessuggestacriticalRHthresholdof15%forCl-SCCinmaterialslikeSS304and
SS316upto80°C.However,laboratorytestsreplicatingrealisticenvironmentalcondi-
tionssuggestthatvariationsinhumiditymaynotnecessarilyaccelerateCl-SCC,evenwith
elevatedsurfacechlorideconcentrations[116].
Comparisonsbetweennaturalandacceleratedtestconditionsunderscoretheim-
portanceofrealisticenvironmentalsimulationsinassessingSCCrisks.Forexample,SCC
propagationratesdiffersignificantlybetweennaturalandacceleratedtestingconditions,
withatwo-orderdifferenceinpropagationrates[127].Despitetheseinsights,neitherset
ofconditionspreciselymirrorsin-serviceenvironments,highlightingthecomplexityof
predictingactualfailureoccurrences.
Furthermore,findingsfromexposuretestsrevealadditionalnuances.Forinstance,
crackingseverityincreaseswithexposuretimeatlowertemperatures,withmorespeci-
mensdevelopingcracksandlongercracklengthsobserved.Thisunderscoresthedynamic
natureofCl-SCCanditssensitivitytoenvironmentalvariables.
AcomprehensiveunderstandingoftheeffectsoftemperatureandRHonCl-SCCin-
volvesdelvingintotheunderlyingelectrochemicalkinetics,diffusionprocesses,andma-
terialresponses.Highertemperaturesfacilitatethediffusionofcorrosivespeciestomate-
rialsurfaces,promotinglocalizedcorrosioninitiation.Conversely,fluctuationsinRHin-
fluencetheavailabilityofwaterandchlorideionsatsurfaces,affectingcorrosionkinetics
andtheformationofprotectivefilms.
5.1.3.TypesofSalts
Chlorideions,originatingfromsourcessuchashydrogenchloride(HCl)resulting
fromchloridesalthydrolysis,arepivotalininitiatingandpropagatingCl-SCC.Inrefinery
Surfaces2024,7610
operations,chloridesaltslikeNaCl,magnesiumchloride(MgCl2),andcalciumchloride
(CaCl2)undergohydrolysisatelevatedtemperatures,releasingcorrosiveHCl:
NaCl+H2O→NaOH+2HCl
CaCl2+2H2O→Ca(OH)2+2HCl
MgCl2+2H2O→Mg(OH)2+2HCl
Additionally,organicchloridedecompositioncontributestoHClformation,further
exacerbatingcorrosionwithinrefinerysystems.Thepresenceofsaltsinrefineryprocesses
introducescomplexities,asfactorssuchastemperature,pressure,andthechemicalcom-
positionoforganiccompoundsinfluencethekineticsofCl-SCC[122].
Experimentalstudieshavedemonstratedadirectcorrelationbetweenthechlorine
concentrationandCl-SCCincidence.Higherconcentrationsofchlorideionscreateamore
corrosiveenvironment,acceleratingthedegradationofpassivefilmsonstainlesssteelsur-
facesandfacilitatingSCCinitiation.SensitizedSS304specimenshaveexhibitedcrackfor-
mationwhenexposedtoelevatedchlorineconcentrations,suchas1and10g/m2.Simi-
larly,as-receivedSS304specimensshowincreasedsusceptibilitytocrackinitiationwith
higherlevelsofchlorinedeposition.Conversely,whenchlorineconcentrationsarebelow
certainthresholds,typicallyaround1g/m2,occurrencesofcracksonSS304specimensare
significantlyreduced.Itiscrucialtonotethatchlorineexposurecanoccurthroughvarious
means,includingdirectcontactandindirectinfiltrationfromatmosphericdepositionor
industrialemissions.Acomprehensiveassessmentofenvironmentalfactorsandpotential
sourcesofchloridecontaminationisessentialforeffectivecorrosionmanagement[115].
ThetypesofsaltssignificantlyinfluencetheCl-SCCphenomenon.Severalstudies
haveunderscoredthecriticalroleofsalttypesincorrosivity,withvaryingsensitivityob-
servedtowardchangesinionconcentrations.Laboratoryexperimentsinvolvingchloride
mixtureshaveshownthatMgCl2inducesfailureinstainlesssteelspecimensmostrapidly,
followedbyCaCl2,whileNaClexhibitstheslowestcorrosiveaction.Thisdisparitycanbe
aributedtosolubilityandchlorideactivitydifferencesamongvarioussalts.MgCl2and
CaCl2,beingmoresolublethanNaCl,exhibitheightenedcorrosivepotential.Despite
MgCl2demonstratingfasterreactivitythanCaCl2incertainstudies,extensiveresearch
consistentlyidentifiesCaCl2astheprimarycontributortoCl-SCCamongseasaltsatroom
temperature.Thecorrosiveimpactofsaltdepositsdependsontheequilibriumchloride
concentrationwithintheelectrolyteformedonthesurfaceduetowatervaporabsorption.
CaCl2demonstratesgreatercorrosivenessthanMgCl2duetoitsabilitytogeneratesolu-
tionswithhigherconcentrationsatspecificrelativehumidities[128,129].
Atchlorideconcentrationsbelow0.5g/L,sulfidesaltssynergisticallyexacerbateCl-
SCC,particularlyevidentinSS316andSS304,wheresignificanttransgranularcracking
hasbeenobserved[115].Sulfidestypicallyreduceresistancetocrevicecorrosionandpit-
tinginstainlesssteelacrossvariousgrades.ThesusceptibilitytoCl-SCCincreaseswith
highersulfurcontent,leadingtoadenserdistributionofsulfideinclusionsthatserveas
initiationsitesforSCCandpits.Undertypicalambientconditionsandintheabsenceof
H2S,suchlowchlorideconcentrationsgenerallydonotinducecracking[115].
5.1.4.ResidualStress
Cl-SCCmanifeststhroughcrackinitiationandpropagationunderthecombinedin-
fluenceoftensilestress,exposuretochloride-richenvironments,andelevatedtempera-
tures.Whilepiingcorrosiontypicallyinitiatesslowly,residualstresscanexpeditedeg-
radationmechanisms,acceleratingcrackgrowthrates.PreviousstudiesonSCCindry
storageconditionshavepredominantlyutilizedU-bendspecimens,whichmaynotfully
replicatestressdistributionsinactualcanisterwelds.Nevertheless,theseinvestigations
offervaluableinsightsintostress-relatedphenomena.Bysimulatingstressdistributions
akintorealcanisterenvironments,researchershaveestimatedthepotentialimpactofre-
sidualstressonSCCdevelopment[115].
Surfaces2024,7611
Toinvestigatetherelationshipbetweenresidualstressandtimetorupture,experi-
mentswereconductedusingsensitizedandas-receivedSS304LandSS304specimenssub-
jectedtovaryingappliedstresses.Theresultsindicatedadecreaseinrupturetimewith
increasingappliedstress(σap)forSS304,withrupturetimesof100hat510MPaand531
hat147MPa.ThethresholdstressforSCCinitiationwasidentifiedat74MPa,highlighting
thesignificantroleofresidualstressinSCCsusceptibilitymitigation.Interestingly,under
thetestedconditions,agingtreatmentsat650°Cfor10mindidnotsubstantiallyalterSCC
susceptibilityinSS304andSS304Lmaterials[107].SS304Lexhibitedmarginallylonger
rupturetimesthanSS304,particularlyathigherstresslevels,suggestingslightlyenhanced
resistancetoSCC.
5.1.5.SensitizationandFailure
SensitizationinstainlesssteelsoccurswhenM23C6precipitatesatgrainboundaries,
reducingthechromiumcontentwithingrainsandcompromisingcorrosionresistance.
Thisphenomenoniscommonlyinducedduringweldingprocesses,particularlyinthe
HAZ,whereresidualstressesarehighestpost-weldingduetomeltingandsolidification.
Despiteeffortstomitigatesensitization,itseffectspersist,makingsensitizedstainless
steelsvulnerabletorapidcrackingandcorrosioninitiationinbenignenvironmentslike
moistairorwater,particularlyalongsusceptiblegrainboundaries[115].Experimental
studiesonsensitizedSS304at80°Cand35%RHdemonstratedheightenedsusceptibility
toSCC.Belowtheproofstress(approximately290MPa),sensitizedSS304exhibitedsig-
nificantlyshorterrupturetimescomparedtotheas-receivedmaterial,aributedtoele-
vatedresidualstressesinducedduringsensitization[107].
5.1.6.LocalizedCorrosionPotentials
LocalizedcorrosionpotentialsarecrucialininfluencingcrackpropagationinCl-SCC,
particularlywithincrackorcreviceenvironmentsinchloridesolutionswherepHlevels
dropdrasticallytoaround1–2.ThisacidicenvironmentpromotesSCCgrowth[130,131].
AstudyconductedbyTanietal.[132]investigatedtheanodicpolarizationbehaviorof
SS316andSS304insyntheticseawateratpH1and80°C,revealingdistinctpeaksinthe
activedissolutioncurrentataround−0.25Vforeachstainlesssteelsample.Notably,both
specimensexperiencedSCCfailureafterapproximately500hoftesting.Furthermore,
overlappinganodicpolarizationcurvesinpassivationandtransitionzonesindicatedsim-
ilarpiingpotentialbehavior,suggestingcomparablefailuretendencies.
Temperaturesignificantlyinfluenceslocalizedcorrosionpotentials.Generally,crev-
icecorrosionpotentialsarelowerthanpiingpotentialsacrossdifferentspecimens.Stud-
iesbyMayuzumietal.[116],alongwithTanietal.[132],onthetemperaturedependence
oflocalizedcorrosionpotentialsinsaturatedsyntheticseawaterforSS316andSS304
demonstratedaninverserelationshipbetweenthesurfacetemperatureandlocalizedcor-
rosionpotentials,includingtheopen-circuitpotential(OCP).Lowersurfacetemperatures
elevatedlocalizedcorrosionpotentialsandtheOCP,whilehighertemperaturesreduced
thedissolvedoxygenconcentrationonthemetalsurface,therebyloweringthecorrosion
potential.Anodicpolarizationcurveanalysishighlightedconsiderablevariabilityin
SS304’spiingpotentialat80°C,underscoringmeasurementchallengesinassessinglo-
calizedcorrosionpotentialsaccuratelyandtheirimplicationsforSCC.
5.2.CI‐SCCMechanism
Cl-SCCischaracterizedbytheinterplayofresidualstressandpiingcorrosion,oc-
curringintwoprimarystages:initiationandpropagation.
5.2.1.InitiationStage
TheinitiationofCl-SCCinvolvescomplexelectrochemicalprocesses.Theresistance
ofstainlesssteeltoelectrochemicalcorrosionisprimarilyduetoitsprotectivechromium
Surfaces2024,7612
oxidepassivelayer,typically1–3nmthick[115].However,chlorideionssignificantly
threatenthispassivelayer,leadingtolocalizeddamage[133].Thisinitiatespiingcorro-
sionwhenthesteelexceedsacriticalpotentialknownasthefilmbreakdownpotentialor
piingpotential.Chlorideionslocallydisruptthepassivefilm,initiatingpiingcorrosion
[132,134].PitsformedduringpiingcorrosionserveaspreferentialsitesforCl-SCCiniti-
ation,causingincreasedsurfaceroughnessandpassivefilmdisruption.Residualstresses
frommanufacturingormechanicalloadingcanfurtherincreasesusceptibilitytocrackin-
itiation,concentratingatpitedgesorothersurfaceimperfections[132,134].
Inmaterialsscience,piingcorrosioniswidelyrecognizedascrucialininitiatingCl-
SCC.Asthepassivefilmbreaksdown,pitsdeveloponthestainlesssteelsurface,facilitat-
ingSCCinitiationduetoincreasedsurfaceroughnessandthedisruptedpassivefilm.
Withinthesepits,aggressivelocalchemistrypromotesthedevelopmentoftensilestresses
throughmechanismssuchasconcentrationgradientsandstressconcentrationduetopit
geometry[115].Oncetensilestressesexceedthematerial’sSCCthreshold,crackstypically
initiatefromthepitboomandpropagateundertheinfluenceofaggressivechemicalen-
vironmentsandappliedstresses[107].
5.2.2.PropagationStage
ThepropagationofSCCinvolvesintricatescientificprocessesprimarilyinfluenced
bymetaldissolutionandresidualstress.Onceacrackinitiatesatapit,itrapidlypropa-
gatesundertheseconditions,withmechanicalfactorspredominantlyinfluencingthepro-
cess.Specificthresholdconditionsmustbemetforcrackpropagationtooccur.Studies
indicatethattherateofmetallossovertimesignificantlyimpactsSCCprogression,un-
derscoringtheimportanceofprolongedexposuretoconduciveenvironments[135].Ad-
ditionally,environmentalfactorssuchasRHlevelsbelow15%caninhibitSCCpropaga-
tion,highlightingtheinteractionbetweenenvironmentalconditionsandcrackadvance-
ment[116].ThemechanismsunderlyingcrackinitiationandSCCinvolveacombination
ofmechanicalandelectrochemicalprocesses.Cracksprimarilypropagatealonggrain
boundaries,wherematerialmicrostructureiscritical.Crackgrowthisdrivenbymechan-
icalloadingandchemicalreactionsatthecracktip,promotingtheseparationofatomic
bonds.Concurrently,theaggressivechemicalenvironmentwithinthecrackaccelerates
metaliondissolution,furtherexacerbatingcrackpropagation[135].
6.AssessmentofSCC
VarioustechniquesarecommonlyemployedtoassessSCC,includingtheSPT,CLT,
andSSRT.TheSSRTutilizesaconstant-extension-ratemachinetograduallystrainmate-
rials,providingcontrolledinsightsintoSCCbehaviorunderdeformation.TheCLTap-
pliessustainedloadsusingproofringstostudySCCunderconstantstressconditions.The
SPTemploysminiaturizedspecimenstoefficientlyassessSCCcharacteristics,oftencom-
plementedbyelectrochemicalmeasurementsforcomprehensiveSCCanalysis.TheCLT
involvesexposingspecimenstoconstanttensilestressesincorrosiveenvironmentsfor
durationsofuptoonemonth.DespitemeetingANSI/NACETM0177standards,theCLT
haslimitations,suchasstaticconditions,theinabilitytocontinuouslymonitorSCCpro-
gression,andextendedtestdurations[5].
TheSPTisutilizedinacademicresearchtoevaluatevariousmechanicalpropertiesof
metallicmaterials,includingsusceptibilitytoSCC,usingacousticemissionandsmallin-
dentations.Thismethodoffersadvantagesbyenablingtheassessmentofmechanicalchar-
acteristicsinsmall,inaccessibleregionscomparedtoconventionalmethods.Currently,
theSPTisappliedtostudyfracturebehavior,creepresistance,ductile–briletransition
temperatures,tensilestates,andsusceptibilitytoEnvironmentallyAssistedCracking
(EAC)[5,136,137].
TheSSRTisamechanicalapproachextensivelyusedtoassessSCCinburiedpipe-
lines,subjectingsteelsamplestogradualstrainingwithinasimulatedserviceenviron-
ment.Thismethodprovidesvaluableinsightsintothesusceptibilityofpipelinesteelsto
Surfaces2024,7613
SCCwithshortexperimentaldurations[5,138].TheSSRTutilizesportableinstrumentsto
maintainconsistentextensionrates,whichiscrucialforevaluatingmetalsunderchalleng-
ingenvironmentalconditions[139].
6.1.SCCAssessmentUsingSSRT
EvaluatingmaterialsforsusceptibilitytoSCCcommonlyfollowsstandardizedmeth-
odssuchasASTMG129andNACETM0198.Thisprocesstypicallyinvolvescomparing
themechanicalpropertiesofspecimenstestedindefinedenvironments,suchassoilsor
solutions,againstthosetestedinair.Establishedequationsandcriteriaareusedtoquan-
tifytheextentofdeterioration[5,140,141].Acriticalaspectofthisassessmentistheanaly-
sisofductilityparameters,explicitlyfocusingontheplasticstrain-to-failure(PSF)ratio.
ThisratioiscomputedbydividingthePSFobservedinthetestingenvironmentbythat
observedinambientairandmultiplyingby100[5].Ratiosbelow0.5indicatehighersus-
ceptibility,whileratiosbetween0.8and1.0suggestlowersusceptibilitytoEAC.Achiev-
ingPSFratioscloseto1.0isrecommendedtoenhanceresistancetoSCC.Mechanicaland
spectroscopicassessmentsareadvisedtoconfirmSCCoccurrenceatPSFratiosbelow0.8,
withcrackpropertiestypicallyexaminedalongthelongitudinalsectionofthespecimen
gauge.
KaneandWilhelmproposedastandardizationframeworkin1993toharmonizetest-
ingtechniquesacrossindustrialandacademicdomains.TheSSRTandCLTarethepri-
marymethodsforassessingSCCsusceptibilityincarbonsteels.TheSSRTemploysspe-
cializedequipment,suchastheInter-CorrM-CERTmachine,toapplycontrolledstrain
conditions,loads,andextensionratesduringtesting[5].Testspecimens,whethercylin-
dricalorrectangular,aremachinedtomeetspecificexperimentalconditionsandthere-
quirementsofthetensiletestingmachine,adheringtodetailedspecificationsoutlinedin
relevantstandards[5,136,140,141].Commonlyrecommendedtestspecimensforevaluat-
ingSCCincarbonsteelsincludeBent-Beam,pre-crackedcantileverbeam,pre-cracked
wedge-open-loading-type,U-bend,andC-ringconfigurations[5].TheCLTinvolvessub-
jectingspecimenstoconstanttensilestressesinenvironmentsconducivetocrackingfor
extendeddurations,upto30days.
FordetailedSCCtestingprocedures,NACEStandardTM0284providesfurtherguid-
ance[5].Novelmaterialssuchascompositecoatingsorpolymercompoundscanbeeffi-
cientlyevaluatedforSCCsusceptibilityusingmethodsthatallowrelativelyshorttestdu-
rationsandfamiliarterminology.Additionally,combiningelectrochemicaltechniques
withtensiontestsenablesthesimultaneousassessmentofcorrosionprocessesandmate-
rialbehavior[142].However,carefulconsiderationoffactorssuchaselectriccontactiso-
lationfromtestmachinestructuresisessential,alongwithacknowledginglimitations,
suchastheoversimplificationofcrackinitiationstagesandpotentialspecimensizeand
costimplications[5,143].
Moreover,Afanasyevetal.[144]proposedcyclictestingtosimulatereal-worldload
conditionsandunderstandcrackbehavior,particularlyintheabsenceofcorrosiveenvi-
ronments.Thismethodologyinvolvessubjectingfield-damagedsamplestocyclicloading
usingfour-pointloadingconditions,replicatingpressurefluctuationsrepresentativeof
operationalpipelines[5].
6.2.ComplementaryTestMethodsforAssessingSCC
NumerousexperimentalstudieshaveinvestigatedthephenomenaofSCCandtheir
correspondingelectrochemicalbehaviors[5].Establishedprotocolsforsupplementaryin-
vestigationsintoexternalSCCadheretoASTMstandards.TheseincludeASTMD4959for
gravimetricanalysistodeterminemoisturecontent,ASTMG4643forpHdetermination,
ASTMG187andASTMG57forsoilresistivitymeasurementusingasoilresistivitymeter,
andASTMG51forredoxpotentialanalysis.Ionchromatographyandinductivelycoupled
plasma(ICP)spectroscopyareemployedtoassessoxygenconcentration,texturecharac-
Surfaces2024,7614
teristics,andthepresenceofanionsandmetals.Additionally,assessmentsrelatedtoca-
thodicdisbondmentandcriteriaforthecathodicprotectionofundergroundpipelinesfol-
lowstandardssuchasASTMG95,ASTMG80,ASTMG42,ASTMG8,andNACEStandard
TM0497.HolidaydetectionincoatingsisconductedinaccordancewithASTMG62,while
studiesonmicrobiologicallyinfluencedcorrosionalignwithNACEStandardTM0106.
ProtocolsfortheexternaldirectassessmentofpipelinesareoutlinedinANSI/NACE
StandardRP0502[5].
Electrochemicaldataobtainedfromcyclictestsarecomplementedbyfractographic
examinationsofcross-sectionalsamples[144].Elementalanalysisusingspectroscopyis
utilizedtoinvestigatethecompositionofcorrosionproducts(oxides)andtostudySCC
phenomena[144].ForinternalSCC(I-SCC)evaluation,recommendedstandardsinclude
NACEStandardSP110forpipelinesconveyingwetnaturalgasandNACEStandard
SP0206fordirectlyassessingpipelinestransportingdrynaturalgas[6].Furtherinternal
evaluationtestsinvolvedeterminingmetalandanioncontents,oxygenconcentration,and
texturecharacteristicsusingionchromatographyandICPanalysis.Redoxpotentialanal-
ysisfollowsASTMStandardG200,whileASTMStandardD4294characterizessulfur-
compound-containingpetroleumproducts.HydrocarbonpropertiessuchasAPIgravity,
acidnumber,density,andviscosityaredeterminedbyASTMD287,ASTMD1298,ASTM
D664,andASTMD5002,respectively.Vari ous waterproperties,includingpH,density,
conductivity,resistivity,ironcontent,andbacterialpresence,arealsoassessed[5].
6.3.Non‐DestructiveTes ti ng
ToassessmaterialsforSCC,non-destructivetesting(NDT)techniquesareemployed.
NDTisessentialfordetectingearlystagesofcorrosionandfacilitatingtimelycorrective
actions,thusextendingthelifespanofcomponents.Itevaluatesmaterialswithoutcausing
damage,unlikedestructivemethodssuchastheSSRTorCLT,whichintentionallyinduce
failuretogatherdata.Despitemeticulousmaterialselection,design,andenvironmental
controls,somecorrosion-relateddegradationisunavoidable.NDTiscrucialinbothman-
ufacturingandmaintenanceforinspectingrawmaterials,sub-components,andfinished
productswithoutalteringthematerial’spropertiesorfunctionality[145].
SignificantadvancementsinNDTemergedduringWorldWarII,drivenbyheight-
eneddemandsforindustrialqualitycontrol.Inthe1950s,enhancementsinNDTinstru-
mentationledtoimprovedresolutionanddefectdetectioncapabilities.The1960sintro-
ducedfurtheradvancementsthroughtheapplicationofstatisticalmethodsandinterfero-
metricconcepts.Bythistime,NDTtechniqueswerecommonlyusedtoinspectdefects
suchascracks,voids,porosity,non-metallicinclusions,andforginglaps.Overthefollow-
ingdecades,developmentscontinuedinresponsetonewmaterials,increasingquality
standards,complexgeometries,andevolvingdefectmorphologies[146].
OneofthekeyapplicationsofNDTiscorrosion-relatedfailuredetection,whichis
crucialforidentifyingdegradationthatcancompromisestructuralintegrity.Thisincludes
formssuchaspiingcorrosion,uniformcorrosion,andmicrocracking.SCCisparticularly
challengingtodetectduetoitsoften-subtlenature,withcrackspotentiallydeveloping
beneaththesurfaceandremaininginvisibletoconventionalinspectionmethods,suchas
visualinspectionordyepenetranttesting.NDTmethodsarerequiredtodetectsuchhid-
dendefects.TheintegrationofvariousNDTtechniquesenhancestheabilitytoidentify
concealedcracksanddefects,thusimprovingsafetyandreducingtheriskofcatastrophic
failures.Thisintegrationisessentialformaintainingtheintegrityofcriticalinfrastructure
andensuringoperationalsafetyinindustriessusceptibletotheseformsofdegradation.
PredictingSCCinmaterialsiscrucial,asNDTcanrevealimperfectionssuchasvari-
ationsinthedensity,size,location,andmorphologyofdefects.EffectiveNDTtechniques
fordetectingandpredictingSCCincludeultrasonictesting(UT),acousticemissiontesting
(AE),eddycurrenttesting(ECT),radiographictesting(RT),andmagneticparticletesting
(MPT)[147].Thesetechniquesinvolveapplyingenergytoacomponentandanalyzingthe
Surfaces2024,7615
resultingsignalswithsensitivedetectorstoidentifydiscontinuitiesbyassessinghowen-
ergyinteractswiththematerial.Techniquesutilizevariousprobingmedia,including
soundwaves,electromagneticfields,andradiation[146].
ForSCCprediction,suchkindsofmethodsareparticularlyvaluableduetotheirabil-
itytodetectearlysignsofcracking.EachNDTtechniquehasspecificsensitivitiestoma-
terialpropertiesandconditions,evaluatingcharacteristicssuchasgeometric,mechanical,
electrical,magnetic,acoustic,andthermalproperties.Manytechniquesrequireminimal
specimenpreparation,useportableequipment,canbeautomated,andoffergoodtem-
poralresolution.Additionally,somemethodsenableonlinemonitoring,whichiscrucial
forongoingSCCprediction[148].
However,NDTmethodshavelimitations,includingdetectorsensitivity,background
noise,andchallengesinsignalinterpretation.DetectingsmallSCC-relateddefectscanbe
difficultduetothesefactors.Higherresolutionoftenincreasesinspectiontimeandcost.
TheeffectiveapplicationofNDTforSCCpredictionrequirespriorknowledgeofdefect
characteristicsandaccessibilitytotheinspectionarea.WhileNDTcanidentifydefects,it
doesnotassesstheirseverity,necessitatingfurtherevaluationtodeterminetheappropri-
aterepairactions[146,149].
6.3.1.UltrasonicTesting
Ultrasonictesting(UT)isawidelyusedNDTmethodthatemployshigh-frequency
soundwaves,typicallyrangingfrom0.5to10MHz,todetectinternaldefectswithinma-
terials.Soundwavespropagatethroughamediumataknownvelocityandreflectoffin-
terfacesorboundariesbetweendifferentmedia.Thisprincipleofreflectionisfundamental
toUT[147].InUT,ultrasonicwavesaregeneratedanddetectedbytransducers,which
convertelectricalimpulsesintomechanicalsoundwavesandviceversa.Thetransducer
emitsultrasonicpulsesintothematerial,andthereflectedpulsesarecapturedtomeasure
thetimetakenforthewavestotravelthroughthematerial,whichaidsinidentifyingin-
ternaldefectssuchascracksandvoids[147].
UTisparticularlyeffectivefordetectingSCCinvariousstructures,includingpipe-
lines,pressurevessels,andaerospacecomponents.Thetechniqueemploysmethodssuch
aspulse-echoandshear-wave(anglebeam)testingtoidentifydefects.Shear-waveUTis
especiallyusefulfordetectingSCC,asitcananalyzereflectionsfromtheedgesofcracks,
therebyrevealingbothtransgranularandintergranulartypesofSCC[150].Thiscapability
iscrucialforensuringtheintegrityofstructuresexposedtocorrosiveenvironments.Re-
centadvancementsinUTtechnologyhavesignificantlyenhanceditseffectivenessinSCC
detection.Forexample,phased-arrayUTusesmultipletransducerstosteertheultrasonic
beamandcreatedetailedimagesoftheinternalstructure.Thistechniqueallowsformore
accurateandcomprehensivedetectionofcomplexcracks.Additionally,guided-waveUT
caninspectlargeareasofpipesandotherstructures,providinglong-rangeinspectionca-
pabilitiesanddetectingSCCoverextensivedistances[147,151].AutomatedUTsystems
offerconsiderableadvantagesforSCCdetection,includingincreasedspeed,reproducibil-
ity,andtheabilitytoprocesscomplexdata,suchasthree-dimensionaltomography.These
systemsareparticularlyvaluableinhigh-pressureandhigh-temperatureenvironments,
suchasoffshorefacilities,wheremanualinspectionmaybeimpracticalorhazardous.
Moreover,UTiseffectiveinmonitoringcorrosionanddetectingSCCacrossvariousin-
dustries,includingoilandgas,petrochemical,andaerospace[147,151].
TheadvantagesofUTincludetheabilitytoperforminspectionsfromtheexternal
surfacewhilestructuresremaininservice,aswellasitscompatibilitywithvariouscoat-
ingsandlinings.Italsoprovidesanon-contactalternativethroughlaser-ultrasonicmeth-
ods,whichareusefulinhigh-temperatureandhazardousenvironments.However,UT
hasitslimitations.Calibrationmustbetailoredtospecificmaterials,andflaworientation
iscriticalforaccuratedetection.Optimalresultsareachievedwhenthesoundbeamis
perpendiculartotheflaw’saxis;flawsthatareparallelornearlyparalleltothebeammay
Surfaces2024,7616
bemissed.Additionally,UTmayexhibitlowersensitivitytoverysmallchangesinmetal
losscomparedtosomephysicalorelectrochemicalmethods[148].
Insummary,UTisapowerfultoolfordetectingSCC,offeringdetailedinsightsinto
materialintegritywithoutdamagingthecomponents.Itsabilitytoprovidereal-time,non-
destructiveevaluationmakesitessentialformaintainingthesafetyandreliabilityofcrit-
icalinfrastructure.Astechnologycontinuestoadvance,theroleofUTindetectingSCC
andothermaterialdefectswillbecomeincreasinglypivotalinensuringthelongevityand
safetyofstructuralcomponents.
6.3.2.AcousticEmission
Theacousticemission(AE)techniqueisacrucialtoolfordetectingandmonitoring
SCCandothermicroscopicdefectsthatarisefromstressvariationsduetochangesinpro-
cessconditions,suchaspressureortemperaturefluctuations[152].AEinvolvesthedetec-
tionoftransientacousticwavesproducedbytherapidreleaseofenergyfromlocalized
sourceswithinamaterial.Thistechniqueisparticularlyeffectiveforevaluatingcorrosion-
relatedphenomenabecauseitcapturestheacousticemissionsgeneratedbythedevelop-
mentofdefectsorplasticdeformation,providingreal-timeinsightintothestructuralin-
tegrityofmaterials.
AEsensorsdetectthesetransientacousticwavesandconvertthemintoelectricalsig-
nals.Theamountofacousticenergyrecordedisinfluencedbythesizeofthedefect,its
locationrelativetothesensor,andthematerial’sproperties.AEmonitoringhasproven
effectiveinidentifyingsignificantissuesinavarietyofstructures,includingASSaffected
bySCC[153,154].OnenotableadvantageofAEisthatitisnotconstrainedbytemperature
limitsforsensors;thesesensorscanbeconnectedviametalacousticwaveguideswelded
tothestructure,allowingthesensortobepositionedoutsideofthermalinsulation.AEis
particularlyeffectiveforevaluatingcorrosion-relatedphenomena,suchasSCC.SCCin-
volvescomplexinteractionsbetweenmechanicalstressandacorrosiveenvironment,
whichcangeneratesignificantacousticemissions.TheAEtechniquecanmonitorSCC
effectivelybydetectingtheacousticsignalsassociatedwithcrackinitiationandpropaga-
tion.ResearchhasshownthatAEsignalsstronglycorrelatewithSCCinitiationandpro-
gression.Forexample,studiesonAISI316LASSexposedtoa3%NaClsolutionhave
demonstratedthatAEsignalscorrelatewiththeinitiationandgrowthofSCC[155].
AdditionalresearchhasutilizedAEtostudySCCinlow-carbontype304stainless
steelundervaryingcorrosiveconditions.Testsconductedin0.01Mand1MNaClsolu-
tionswithapHadjustedto1revealedthathigherAErateswereobservedinmorecon-
centratedNaClsolutions,correlatingwithincreasedSCCdamage.Boththeappliedstress
andsolutionpotentialinfluencedAErates,indicatingthatAEcandetectvariationsinSCC
severitybasedonenvironmentalconditionsandmechanicalstress[156,157].Thishigh-
lightsAE’scapabilitytoprovidedetailedinsightsintothecorrosionprocessesoccurring
underdifferentconditions.Crevicecorrosion,anothercorrosion-relatedphenomenon,in-
volveslocalizedcorrosioninconfinedareaswhereametalisexposedtoacorrosiveenvi-
ronmentwhileotherpartsofthesurfaceremainprotected.AEhasproveneffectivein
monitoringcrevicecorrosionaswell.Studieson304LASShaveshownthatAEcandetect
crevicecorrosioninitiation,propagation,andrepassivation.ChangesinAEsignalswere
linkedtovariationsincrevicedamage,influencedbyfactorssuchasthepresenceofchlo-
rideionsandthemechanicalassemblyconditions[157].ThisdemonstratesAE’sversatil-
ityinmonitoringvarioustypesofcorrosionbeyondSCC.
AEisparticularlyeffectivefordetectingprocessesassociatedwithSCC,including
crackpropagation,gasevolution,andplasticzoneformation.Whilesomecorrosionpro-
cesses,suchasuniformmetaldissolution,donotgenerateAEduetothelackoflocalized
strain,SCCtypicallyproducessignificantAEsignalsbecauseofthestrainassociatedwith
crackformation.StudiesbyShaikhetal.[158]haveshownthatAEcaneffectivelycapture
themicroprocessesofSCCinmaterialslikeAISI316LNstainlesssteel.Thesestudiesre-
portedincreasedAEcountsandenergyatSCCinitiation,withcontinuousAEpriorto
Surfaces2024,7617
initiationandburstsofAEduringcrackgrowth,underscoringAE’scapabilitytomonitor
SCCinrealtime.
InastandardAEmonitoringsetup,apiezoelectricsensorisacousticallycoupledto
thetestobjectusinganappropriatecouplingmedium.Thesensoroutputisamplifiedand
filteredbypreamplifiersbeforebeingtransmiedtothemonitorthroughshieldedcoaxial
cables.Themonitorprocesses,filters,andamplifiestheAEsignals,providingdatafor
detailedanalysis.Theresultsandrawdataarerecordedforarchivalpurposesorfurther
evaluationtopinpointthelocationandnatureoftheAEsignals[159].
Overall,AEoffersseveraladvantages,suchasitsabilitytoprovidereal-timere-
sponsestoSCCdevelopmentanddetectdiscontinuitiesacrossanentirestructurewith
limitedaccess.However,AEalsohaslimitations,includingitssensitivitytobackground
noiseanditsprimaryfocusondetectingdynamicprocesses.AEcannotdetectstaticdis-
continuitiesunlesstheyareactivelypropagating.Despitetheselimitations,AEremains
anessentialtoolforassessingSCCandothercorrosion-relatedphenomena,offeringval-
uableinsightsintomaterialintegrityandstructuralhealth[147].
6.3.3.EddyCurrentTesting
Eddycurrenttesting(ECT)isanNDEmethodwidelyemployedtoassesstheinteg-
rityofconductivematerials,includingthosesusceptibletoSCC.Thetechniqueoperates
bypassinganalternatingcurrent(AC)throughacoil,whichgeneratesanalternatingmag-
neticfield.Thisfieldinduceseddycurrentswithinthematerial,andvariationsinthese
currentscanprovideinsightsintomaterialintegrity[160].ECTisespeciallyeffectivefor
detectingSCCwhenhigh-frequencycoilsareused,astheyenhancesensitivitytosurface
andnear-surfacecracks,whichareindicativeofSCC.RegularinspectionsusingECTcan
monitorchangesinmetallossandcorrosionovertime,althoughachievinghighsensitiv-
ityinreal-timeapplicationsremainsachallenge.
ECTincludesseveraladvancedmethodologiestailoredtospecificinspectionneeds.
SurfaceECTfocusesonscanningthematerial’ssurfacetoidentifysurface-breakingde-
fectsanddiscontinuities,makingiteffectivefordetectingSCConexposedsurfaces.Tub-
ularECTinvolvesinsertingaprobeintotheinteriorofconductivetubestodetectinternal
flawsandcorrosion,whichisusefulforidentifyingI-SCC.WeldECTevaluatesweldsto
finddefectssuchasincompletefusionorcracks,whichmaysuggestSCCinweldedjoints.
ECTemploysmultiplecoilsactivatedsequentiallyorsimultaneouslytoinspectlargerar-
easrapidly.Thismethodprovidesdetaileddatarecordingandthree-dimensionalimaging
capabilities,allowingfortheprecisedetectionofsurfaceandsubsurfacedefects,including
SCC[148].
Remotefieldtesting(RFT)iseffectiveforassessingdefectsatvariousdepthsbyin-
ducingcurrentsonboththeexternalandinternalsurfacesofatube,makingitsuitablefor
evaluatingthick-walledstructures.Pulsededdycurrenttesting(PEC)involvessending
shortpulsesofcurrentintothematerialratherthanusingcontinuousAC.Thisapproach
enhancessensitivityfordetectingSCCandisvaluableforinspectingcomponentsbeneath
insulation.PECiswidelyutilizedinindustriessuchasoilandgas,powergeneration,and
offshoreoperations,anditperformswellinchallengingenvironments,includingdirty,
rough,cold,andhigh-temperatureconditions.PECisalsoeffectiveforassessingflow-
acceleratedcorrosion(FAC),corrosionunderfireproofing(CUF),andcorrosionunderin-
sulation(CUI)[161,162].Magneto-opticalimagingofeddycurrentsoffersreal-timeimag-
ingoverlargeareas,whichisadvantageousforquicklydetectingSCCandothersubsur-
facedefects.Thismethodallowsforeffectivemonitoringofextensiveregions,enhancing
theoveralldetectioncapabilitiesofECT[148].
Despiteitsadvantages,ECThaslimitationsindetectingSCC.Thetechnique’seffec-
tivenesscandecreasefordefectslocatedinferromagneticmaterialsduetotheeffectsof
magneticpermeability,whichcandistorteddycurrentsandreducesensitivity.Addition-
ally,ECT’sabilitytodetectdeeperdefectsdiminisheswithincreasingdepth,andthesur-
facefinishofthematerialcanalsoimpactperformance.WhileECTmaynotmatchthe
Surfaces2024,7618
spatialresolutionordepthpenetrationofultrasonictechniques,itremainsavaluabletool
fordetectingSCC,especiallyinmultilayerstructuresandcomplexgeometries[148].Ad-
dressingtheselimitationsrequiresconsiderableexpertiseinprobedesignanddatainter-
pretation.
6.3.4. RadiographicTesting
Radiographictesting(RT)isacrucialNDTmethodextensivelyusedtodetectSCC.
Thistechniqueishighlyvaluedforitsabilitytoassesstheinternalstructureofcomponents
andmaterialswithoutcausingdamage.RTisparticularlyeffectiveinidentifyingSCC,
whichcanbedifficulttodetectduetoitsoften-subtleandinternalnature[147,148].RT
employspenetratingradiationtocreateimagesoftheinternalstructureofanobject.By
passingradiationthroughtheobjectandcapturingthetransmiedradiationonadetec-
tionmedium,suchasphotographicfilmoradigitaldetector,RTrevealsvariationsinma-
terialdensityandthickness.Thesevariationsproduceimagesthatcanindicateinternal
features,suchascracksorcorrosion,whicharecrucialforidentifyingSCC[163].Typi-
cally,RTusesX-raysorgammaraysasradiationsources.X-raysaregeneratedbyaccel-
eratingelectronsanddirectingthemtowardatarget,producinghigh-energyphotonsthat
canpenetratematerialsuptoapproximately200mmofsteelequivalent.X-raymachines
comeinvariousconfigurationswithadjustableenergylevelstoaccommodatedifferent
inspectionneeds.Gammarays,producedfromradioactiveisotopeslikeiridium-192and
cobalt-60,arealsoemployed,especiallyfortheirportabilityandcompactnessinfieldap-
plications[147].AnothersignificantRTtechniqueisneutronradiography,whichispar-
ticularlyeffectivefordetectingSCC.Neutrons,sensitivetohydrogen,canidentifyfine
cracksandlocalizedcorrosionthatmaynotbevisiblewithotherradiographicmethods.
Neutronradiographyisenhancedbytechniquessuchasgray-levelstretching,beamflat-
tening,andframeaveraging,whichimprovethequalityandclarityoftheimages[148].
RT’sversatilityextendsbeyondSCCdetection.Itisusedinpipelinecorrosiondetec-
tionwithintheprocessindustry,piingandlocalizedcorrosionidentification,andrebar
corrosioninspectionincivilstructures.RTisalsoappliedtoinspectwelds,castings,as-
semblies,adhesivebonds,plastics,wood,compositematerials,andelectroniccompo-
nents.Thetechniquecanaccommodatearangeofobjectsizes,fromsmallcomponents
likeintegratedcircuitchipstolargeitemssuchasrocketmotorcasings.RTisespecially
effectiveatidentifyingvolumetricdefects,suchasvoidsandporosities,whichcansignal
underlyingSCC[147].
Despiteitsadvantages,RThaslimitationsinSCCdetection.Itrequiresaccesstoboth
sidesofthecomponent,whichmaynotalwaysbefeasible.RTislesseffectiveatdetecting
lineardefects,suchascracks,unlesstheyalignwiththeradiationbeam.Theprecisionof
thicknessmeasurementsisgenerally±10%forpitdepth,andsurfacedebrisorscalecan
impacttheaccuracyofSCCdetection.Additionally,neutronradiographycanbeaffected
byscaeredradiation,complicatingaccuratemeasurement[148].Therelativelyhighcost
ofRTandtheneedforstringentsafetymeasuresduetothehazardousnatureofradiation
arealsoconsiderations.Managingradiationexposureandadheringtosafetyprotocolsis
essentialtoprotectpersonnel.Nonetheless,advancementsinradiographictechnology,in-
cludingimprovementsinradiationsources,digitalimaging,andComptonscaeringtech-
niques,haveaddressedmanyoftheselimitationsandexpandedtheapplicationsofRT
[147].
Inconclusion,RTremainsanessentialtoolfordetectingSCCandassessingthein-
tegrityofvariouscomponentsandstructures.Itsabilitytoprovidedetailedinternalim-
agesandidentifysubtledefectsmakesitinvaluableformaintainingsafetyandperfor-
manceinindustrialapplications.Whiletherearelimitations,ongoingtechnologicalad-
vancementscontinuetoenhancetheeffectivenessandbroadenthescopeofRT,reinforc-
ingitsroleasakeymethodinNDTandinspection.

Surfaces2024,7619
6.3.5. MagneticParticleTesting
Magneticparticletesting(MPT)isacrucialNDTtechniqueusedtoidentifysurface
andnear-surfacediscontinuitiesinferromagneticmaterials.Thismethodisparticularly
significantfordetectingearlysignsofSCC,aphenomenonwherematerialsundergobrit-
tlefractureduetothecombinedeffectsoftensilestressandacorrosiveenvironment.MPT
operatesontheprinciplethatferromagneticmaterialsbecomemagnetizedwhensub-
jectedtoamagneticfield.Defectswithinthesematerialsdisruptthemagneticfluxlines,
causingaleakagefluxthatbecomesvisibleonthesurface.Finemagneticparticles,such
asironfilingsorspecializedmagneticpowders,arethenapplied.Theseparticlescluster
aroundareasofmagneticfluxleakage,creatingavisibleindicationpaernthatreveals
thedefect’slocation,size,andshape[146,147].
TheMPTprocedureinvolvesseveralessentialsteps:cleaningthecomponent’ssur-
facetoremovecontaminants;magnetizingthecomponentusingmethodssuchasAC,di-
rectcurrent(DC),half-wavedirectcurrent(HWDC),orpermanentmagnetsandelectro-
magnets;applyingmagneticparticles;inspectingfordefects;andfinally,demagnetizing
andcleaningthecomponenttoremoveresidualparticles.Effectivedetectionreliesonthe
properalignmentofthemagneticfluxwiththedefectorientation,whichmaynecessitate
multipleinspectionstocovervariouspotentialorientations.Highcurrentsareoftenre-
quiredforlargercomponents,necessitatingcarefulmanagementtopreventlocalized
overheatingatelectricalcontactpoints[147].
MPTisparticularlyeffectiveinidentifyingsurface-breakingcracksandcorrosion-
induceddefectsthatarecriticalinthecontextofSCC.Itcandetectvariousformsofdis-
continuities,suchasgrindingcracks,heattreatmentcracks,andothersurfaceflawsthat
maycontributetoSCC.Thistechniqueisapplicabletoarangeofferromagneticmaterials,
includingfinishedproducts,billets,hot-rolledbars,castings,andforgings.However,MPT
islimitedtoferromagneticmaterials,andthoroughpost-inspectionproceduresareneces-
sarytoensurethatallresidualmagneticparticlesareremovedandthecomponentisde-
magnetized[146].
RecentadvancementsinMPTtechnologyhaveenhanceditsabilitytodetectsmaller
andmoresubtledefects.Vasylenkoetal.[164]haveintroducedatechniqueutilizinglu-
minescentferrofluidsmadefromCoFe2O4nanoparticles,sizedbetween5and11nm.
Theseluminescentferrofluidsofferimprovedsensitivityandcontrastcomparedtostand-
ardmagneticpowders.Ataconcentrationof3.1g/L,theseferrofluidscandetectminute
defects,suchasanartificialringdefectof10mmindiameterwithanopeningwidthof1.2
µm,onsteelplates.Thisimproveddetectioncapabilityiscrucialforidentifyingsmall-
scaledefectsassociatedwithSCC,whereearlydetectionisessentialforpreventingmate-
rialfailure[164].
Insummary,MPTremainsarobustandeffectivemethodfordetectingsurfaceand
near-surfacedefectsinferromagneticmaterials,withongoingadvancementsfurtheren-
hancingitseffectivenessinidentifyingcriticalflawsrelatedtoSCC.
6.4.PredictiveModelsforAssessingSCC
IncorporatingpredictivemodelingandsimulationtechniquesforSCCnotonlyen-
hancesthepracticalvalueofmanagingSCCbutalsobridgesthegapbetweentheoretical
understandingandreal-worldapplication.Moderncomputationalmethods,suchasthe
finiteelementmethod(FEM),moleculardynamics(MD),andmachinelearning,haverev-
olutionizedhowwepredictandmanageSCCincomplexsystems.

Surfaces2024,7620
6.4.1.FiniteElementMethod
Thefiniteelementmethod(FEM)isanadvancedcomputationaltechniqueusedto
simplifycomplexproblemsbybreakingthemintosmaller,moremanageablesub-regions.
ThismethodisparticularlyessentialforstudyingSCCinpipelines.Bydiscretizingpipe-
linestructuresintofiniteelements,theFEMenablesdetailedsimulationsofmechanical
stressesandtheirinteractionswithcorrosiveenvironments,whichiscrucialforunder-
standingSCC.SCCisinfluencedbymaterialproperties,stresslevels,andenvironmental
conditions[165,166].
SCCoccurswhenamaterialsubjectedtotensilestressisexposedtoacorrosiveenvi-
ronment.TheFEMisparticularlyvaluableinthiscontext,asitallowsfortheprecisemod-
elingandpredictionofhowstressconcentrationsimpactSCC.Forexample,Turnbullet
al.[167]utilizedtheFEMtoanalyzestress–straindistributionsaroundpiingpitsoncy-
lindricalspecimens.Theirresearchshowedthatstresswasconcentratedatthebasesofthe
pits,whilestrainwasmostsignificantatthepitshoulders.Thisinformationiscrucial,as
itindicatesthemostlikelysitesforcrackinitiationandpropagation.Adjustingthemesh
densityinsimulationsenabledtheaccuratemodelingofcorrosionprogressionanditsef-
fectonstressdistribution.
FEMsimulationshavealsobeenemployedtoinvestigatetheeffectsofhydrogendif-
fusiononcorrosion.Hydrogencandiffuseintometalsandexacerbatecracking,aphe-
nomenonextensivelystudiedusingtheFEM.Researchconfirmedthathydrogenatom
biasplaysasignificantroleintheinitiationandpropagationofintergranularcracks.Spe-
cifically,areductioningrainboundarybindingenergyincreasedthepercentageoffailure
cells,andhigherdisplacementunderconstantbindingenergyalsoraisedthefailureunit
percentage[168,169].Thesefindingssupporttraditionaltheoriesofhydrogen-inducedin-
tergranularcracking[170,171].SimulationsusingtheAbaqusmethodwithtwo-dimen-
sionallinearandsecondarybondingcellshavecontributedtodevelopingasystemfor
predictingcriticalinternalstressvaluesinhigh-strengthsteelswithspecificcorrosionde-
fectsizes,whichhasbeenappliedinpipelinesafetyassessmentsinCanada[172].
TheFEMisnotmerelyatheoreticaltoolbuthaspracticalapplicationsinpipeline
integritymanagement.Studieshaveshownthatstressconcentrations,suchasthose
causedbydeeperdents,leadtoincreasedcorrosionratesandacceleratedcrackinitiation
andpropagation.Thiseffectisparticularlypronouncedinhigh-pressuregastransmission
pipelines,whereinternalpressuresexacerbatestresslevelsandheightenSCCrisk[165].
Inpractice,theFEMhelpsengineersevaluatetheimpactofexternalloadsandinter-
nalpressuresonpipelineintegrity.Bysimulatingtheseconditions,engineerscanidentify
high-riskareasforSCCanddeveloptargetedmaintenanceandreinforcementstrategies.
Forexample,theFEMhasbeenusedextensivelyintheTransCanadapipelinesystemto
assessvariousstressorsimpactingpipelinesegmentsandguidemaintenanceandrein-
forcementefforts[165].
Furthermore,theFEMisinvaluableforanalyzingSCConcomplexsurfacesandir-
regularpiingcorrosionpaerns.Itprovidesinsightsintolocalizedcorrosionprocesses
andtheireffectsonmaterialintegrity.Thiscapabilityisessentialforestimatingpipeline
lifespan,evaluatingmaintenanceinterventions,andmakinginformeddecisionsaboutre-
inforcementmeasurestoenhancepipelinedurability[165].
Insummary,theFEMisapowerfultoolforunderstandingandmanagingSCCin
pipelines.Itallowsfordetailedsimulationsofstressandcorrosioninteractions,provides
insightsintotheeffectsofmaterialpropertiesandenvironmentalconditions,andsupports
practicalapplicationsinpipelineintegritymanagement.Thiscomprehensiveapproach
helpsengineerspredictandmitigatetherisksassociatedwithSCC,ensuringsaferand
morereliablepipelineoperations.

Surfaces2024,7621
6.4.2.MolecularDynamics
Moleculardynamics(MD)simulationsprovideavaluableapproachformodeling
atomicandmolecularinteractionsovertime,makingthemparticularlyeffectiveforstud-
yingSCC.Thesesimulationstrackthepositionsandvelocitiesofatomsbasedonclassical
mechanicsprinciples,enablingresearcherstoexploredynamicbehaviorsattheatomic
scale.MDsimulationsareprimarilydividedintotwotypes:abinitiomoleculardynamics
(AIMD)andclassicalmoleculardynamics(classicalMD).AIMDcombinesquantumme-
chanicalcalculationswithmoleculardynamicstoachievehighprecision,thoughitiscom-
putationallyexpensive,limitingitsapplicationtosmallersystemsandshortertimescales.
Incontrast,classicalMDusesempiricalforcefields,whicharelesscomputationallyde-
mandingandallowforthesimulationoflargersystemsandlongertimescales[165].
InSCCresearch,MDsimulationsareessentialforunderstandingphaseinterfaces,
corrosionmechanisms,andtheeffectivenessofcorrosioninhibitors.Forinstance,simula-
tionshavedemonstratedthatwaterdropletsongraphenecoatingsmaintainaconsistent
contactangle,withasinglegraphenelayerexhibitingsignificanthydrophobicproperties.
MDsimulationsalsoprovideinsightsintohowmetalsurfacesinteractwithcorrosive
agents,suchastheformationofanon-bondedwaternetworkongoldthataffectspas-
sivationstability[173].Furthermore,MDresearchhasbeenusedtoevaluatecorrosion
inhibitorslikeolmesartanandketosulfone,revealingtheireffectivenessundervarious
conditions[174,175].
MDsimulationsarealsovaluableforelucidatingfailuremechanismsassociatedwith
SCC.Studieshaveshownthatradiationdamageandstressconcentrationsfromvacancy
formationcontributetoSCC[176].Simulationsoffailureingalvanizedironandcopper
haverevealedthatstretchingofthezinclaiceandtheresistanceofspecificcopperinter-
facestotemperatureandloadchangesarekeyfactors[177,178].Despitetheiradvantages,
MDsimulationshavelimitations,suchasrelianceonempiricalforcefieldsandthehigh
computationalcostofAIMD.Nonetheless,theyremainapowerfultoolforadvancingour
understandingofSCCanddevelopingeffectivestrategiesforcorrosionmitigation.
6.4.3.MachineLearningforAssessingSCC
InthefieldofSCCgrowthestimation,ongoingresearchiscrucialfordevelopingre-
liablepredictivemodels.ThecomplexityandvariabilityofSCCprogressionhaveledto
increasinginterestinartificialintelligence(AI)asatoolforunderstandingandmanaging
SCCinpipelines.AIistransforminghowweassesscorrosionrisksincriticalinfrastruc-
ture.Forinstance,Zhangetal.haveemployedAImodels,suchasMultilayerPerceptron
(MLP)andLongShort-TermMemory(LSTM),toenhancepredictionaccuracybycorre-
latingSCC-inducedcrackwidthswithsteelweightlossinreinforcedconcrete(RC).Their
approach,whichincludesanalyzingreinforcementbarcross-sections,improvesevalua-
tionprecisionandcostefficiency[165].
AI’scapabilitiesextendtopredictingcorrosionratesandcrackgrowth.Advanced
techniqueslikePrincipalComponentAnalysis(PCA),ParticleSwarmOptimization(PSO),
Feed-ForwardArtificialNeuralNetworks(FFANNs),GradientBoostingMachine(GBM),
RandomForest(RF),andDeepNeuralNetworks(DNNs)havesignificantlyadvancedthe
estimationofSCCprogressioninaginginfrastructure.Bytrainingthesemodelsonhistor-
icalSCCdata,researcherscanforecastSCConsetandgrowthratesundervariouscondi-
tions[179,180].Forexample,AImethodshavebeenappliedtostudySCCincarbonsteel
andpipelines,withOssai’sanalysisofover8300recordsprovidingvaluableinsightsinto
estimatingtheSCCdepth[165,181].
Machinelearning(ML)involvesdevelopingsystemsthatimprovetheirperformance
overtimebasedondata-drivenlearning[22].Michieetal.[182]describeMLasaprocess
wherealgorithmslearnfromdatapaernsratherthanrelyingonexplicitprogramming
[182].TheeffectivenessofMLmodelsdependsondataqualityandtheextentofhuman
intervention,suchasprovidingaccuratelabelsorfeedback.Thechoiceofmethodology
Surfaces2024,7622
affectspredictionaccuracyandefficiency:PCAreducesdatadimensionalitybutmight
overlookimportantdetails[183],andPSOeffectivelyoptimizesmodelparameters[184],
whileFFANNsandDNNshandlecomplexrelationshipswithhighprecisionbutrequire
extensivedataandcomputationalresources[185].GBMandRFuseensemblelearningto
enhancestability,thoughtheymayencounterdifficultieswithvery-high-dimensional
data[186,187].ChoosingtherighttechniquedependsonspecificSCCassessmentneeds,
datacharacteristics,andavailablecomputationalresources.
MLmodelsalsoplayavitalroleinSCCprotectionandinhibitoroptimization.They
assistinmanagingcomplexprotectionstrategies,reducingpreparationandtestingcosts,
andpredictingoptimalinhibitorformulations[165].Additionally,deeplearning-based
imageprocessingtechniques,suchastheAccurateMetallicCorrosionDetector(AMCD)
developedbyYuetal.,areincreasinglyusedtoautomateSCCassessment[10].Forkan’s
imagerecognitionmodelhasachievedhighaccuracy[188],Ao’stoolforAl–Zn–Mgalloys
predictsmechanicalperformancechangeswithnotableprecision[189],andDogan’sdeep
transferlearningmodeleffectivelydistinguishesSCCdamagefromotherstructuralis-
sues,facilitatingtargetedrepairs[190].TheseadvancementsnotonlyimproveSCCdetec-
tionaccuracybutalsosignificantlyreducelaborcosts.AdvancedMLtechniquesaresetto
enhanceglobaleffortsinmanagingpipelineintegrityandpredictingSCC[22].
7.PreventionofSCC
Pipelinecompanies,industrygroups,andresearchersintheUnitedStatesandCan-
adahaveextensivelystudiedmethodstopreventormitigateSCCinpipelinespriorto
failures.ToaddressSCC,variouspreventionstrategiescanbeutilized,includingproper
materialselection,electrochemicalmethods,chemicalmethods,physicalmethods,and
thermalmethods.EffectivemanagementofSCCalsorequirestheintegrationofstress
managementandenvironmentalcontrols.Figure6illustratesthekeyfactorsinpreventing
bothinternalandexternalSCCinpipelines.
Figure6.FactorsthatmustbeconsideredtopreventSCCdevelopmentinpipelines.
7.1.MaterialSelection
PropermaterialselectionplaysacrucialroleinpreventingSCCinpipelines.Factors
toconsiderincludechoosingmaterialswithlowsusceptibilitytoSCC,highquality,ap-
propriatetoughness,andfavorablemicrostructures.Recentstudiesindicatethathigher
APIsteelgradesgenerallyexhibitincreasedstrengthbutalsohighersusceptibilitytoSCC.
Conversely,lower-carbonsteelsexhibitvaryinglevelsofsusceptibility,oftencorrelating
increasedsteelhardnesswithgreatervulnerabilitytoSCC.Steelsofhigherqualitytypi-
callydemonstratereducedsusceptibilitytoSCC.
Moreover,steelswithacicularferriteandbainitemicrostructuresgenerallyshow
lowersusceptibilitytoSCC.Single-phasemicrostructures,devoidofprecipitates,typi-
callyexhibitgreaterresistancetoSCCthanmultiphasicstructures.SCCoriginatespre-
dominantlyonthemetalsurfacefromexistingcracksorpiing,whileHEmanifestsat
cracktipsorbetweenareasofhydrogenaccumulation,primarilyalonggrainboundaries
anddefectivecrystallineregions.Themicrostructureandalloycomposition,influenced
significantlybyheattreatment,playcriticalrolesinSCCdevelopment.HEreducessteel
Surfaces2024,7623
strength,particularlyloweringthecriticalstressintensityfactor,whichacceleratesfrac-
turepropagation,especiallywithsmallercracksizes.
ThespecificalloyelementsincarbonsteelpipelinessignificantlyinfluenceSCC
mechanisms,evolvingovertimeduetomaterialaging.Afterapproximatelytwodecades
ofservicelife,carbonsteelpipelinestypicallyexhibitaferrite–pearlitemicrostructuresus-
ceptibletoSCC.Despitetheirsusceptibility,thesesteelsoftencontainsulfur,leadingto
MnSinclusionsandothertypes,suchascalciumsulfides,oxides,andaluminumoxides.
InclusionsenrichedinAl2O3andSiO2impartanincoherent,brilenaturetothemetal
matrix,promotingmicrocrackformationprimarilyatgrainboundariesandaroundinclu-
sions[70,191,192].
AstudybyLiuetal.[193]emphasizedthesignificanceofinclusionsastheprimary
sitesforcrackinitiation.Additionally,researchbyAsahietal.[191]indicatesthat
quenched-tempered(QT)andthermomechanicallycontrolledprocessing(TMCP)treat-
mentsonX52,X65,andX80steelsresultinmorehomogeneousmicrostructures,reducing
susceptibilitytoSCC.Themicrostructureofcarbonsteelpipelinesplaysapivotalrolein
SCCinitiationandpropagation.EliminatinginclusionsorprecipitatescanenhanceSCC
resistancebyremovingpotentialnucleationsitesforcorrosionpits[140,193,194].
EffectivesuppressionofIGCinhigh-chromiumsteelsinvolvesminimizingcarbon
contenttopreventcarbideprecipitation,enhancingcorrosionresistanceinweldedjoints
likethosemadeof03Cr18Ni11steel.Practicalstrengthrequirementsoftennecessitatecar-
boncontentbetween0.08%and0.12%,exceedingsolubilitylimits.Carbide-formingele-
mentssuchastitanium,niobium,andmolybdenum,withstrongercarbonaffinitythan
chromium,preventM23C6formation,retainingchromiuminsolidsolutionandpreserv-
ingcorrosionresistance,albeitattheexpenseofreducedductilityandincreasedstrength.
Experimentalstudiesonenhancingcorrosionresistanceviamolybdenumnanopowder
concentrationinweldpoolsduringarcweldingrevealvariedmicrostructuresdepending
ontheconcentration.Molybdenumandtungstennanopowdersyieldthemostcorrosion-
resistantsamples[53].
Formingafavorableweldmetalstructurebyintroducingferriticelementssignifi-
cantlymitigatesfracturesindual-phaseaustenitic–ferriticstructures.Primaryferritepres-
enceenhancesSCCresistancebyreplenishingchromium-depletedareaswithhighdiffu-
sionrates[195–197].Controllingweldmetalstructuresposeschallenges,especiallyin
weldingandsurfacingdissimilarmaterials.TheSchaefflerdiagramassistsindetermining
weldmetalstructures,especiallyforoverlayingcorrosion-resistantsteelontolow-carbon
orlow-alloysteelsubstrates.SelectingmaterialsandweldingregimeswithinspecificCreq
(18–24%)andNieq(7–18%)rangesensuresfavorableA+Fstructures,minimizingcrack
susceptibility[53].
Inweldingdissimilarmetalslikelow-carbonsteeltoAISI304corrosion-resistant
steel,AISI309steelwithhigheralloyingelements(25%Crand12%Ni)isrecommended
toachieveasimilarcompositionintheweldmetal.AISI309subcoatsareoftenusedfor
surfacingdissimilarmetals.Researchonenhancingsteel-gradeEP-302andweldingma-
terialsforpowerplantequipmentincludessilicondopingtoimproveheatandliquid
metalcorrosionresistance,whichiscriticalinenvironmentswithlead-bismuthcoolant
temperaturesupto450°C.MicrostructuralanalysisofCr–Ni–Nbaustenitic–ferritic
weldedjointscontainingsiliconunderscorestheimportanceoftheferritephasecontent
inoptimizingweldjointprocessability.Delta-ferritetransformationintothesigmaphase
afterprolongedagingat500–600°Chighlightsthenecessityoftemperature–timecontrol
formaintainingweldjointSCCresistance[53].Researchonlow-nickelausteniticsteel
08Cr18NNi5identifiesweldmetalcompositionsandtemperature–timeregionsresistant
toSCCduringweldingandemphasizesthatlowcarboncontentandhighchromium,
manganese,niobium,andnitrogenlevelscontributetominimizingmicrocracksuscepti-
bilityduringwelding[55].
Surfaces2024,7624
7.2.ElectrochemicalMethods
Electrochemicaltechniques,suchascathodicprotection,passivation,andanodiza-
tion,arehighlyeffectiveinmitigatingSCCinpipelines.Thesemethodsemployelectrical
currentsorpotentialdifferencestodrivechemicalreactionsonthemetal’ssurface,leading
toprotectivechangesthatimproveresistancetobothcorrosionandSCC.Implementing
thesetechniquesisessentialforpreservingthestructuralintegrityofmetallicsystemsin
corrosiveenvironments.
7.2.1. CathodicProtection
Cathodicprotectionsystemsareessentialforextendingthelifeofexternalcoatings
andmitigatingdamageusingeitherimpressedcurrentorsacrificialanodes.Theprimary
principlebehindcathodicprotectionistoreducetheelectrochemicalpotentialofthemetal
surface,therebysuppressingtheanodicreactionthatcausesmetaldissolution.Thisis
achievedbymakingthemetalstructurethecathodeofanelectrochemicalcell.Impressed
currentsystemsinvolveapplyinganexternalcurrentfromaninertanodetothemetal
structure,whilesacrificialanodesystemsusemorereactivemetals,suchaszincormag-
nesium,whichcorrodeinplaceoftheprotectedstructure.Acriticalaspectofcathodic
protectionisthepreventionofHE,aprocesswherehydrogenatomsdiffuseintothemetal
andcausebrileness,especiallyinhigh-strengthsteels.Thisriskisheightenedinenviron-
mentswherewatercandissociateundertheinfluenceoftheprotectivecurrent,releasing
hydrogen.Propercontroloftheappliedpotentialisessentialtominimizehydrogengen-
eration.
Theeffectivenessofcathodicprotectionsystemsisinfluencedbyseveralfactors,in-
cludingthenatureofthecoating,thesoilorseawatercomposition,andseasonalchanges
thataffecttemperatureandmoisturelevels.Forexample,highsoilresistivitycanreduce
thecurrentoutputfromsacrificialanodes,requiringcarefuldesignandmonitoring.The
NACESP0169standardprovidescomprehensiveguidelinesforthedesign,installation,
andmaintenanceofcathodicprotectionsystemsforburiedorsubmergedpipelines.Meth-
odssuchasASTMG95,ASTMG80,ASTMG42,andASTMG8areemployedtoassessthe
conditionandeffectivenessofthesesystems,particularlyindetectingcoatingdisbond-
mentthatcouldleadtolocalizedcorrosionandSCC.Regularmonitoringandmainte-
nancearecrucialtoensurethecontinuedeffectivenessofthesesystemsinpreventingcor-
rosionandSCC[5].
7.2.2.Passivation
Passivationisachemicalprocessdesignedtoenhancetheformationofathin,stable
oxidelayeronmetals,especiallystainlesssteel.Thispassivelayer,primarilyconsistingof
chromiumoxide,servesasaprotectivebarrieragainstoxygenandothercorrosiveagents,
effectivelymitigatingbothgeneralandlocalizedcorrosionthatcanleadtoSCC.Typically,
theprocessinvolvesimmersingthemetalinanacidicsolution,suchasnitricorcitricacid,
toremovecontaminantsandfacilitatethedevelopmentoftheoxidelayer.Thesuccessof
passivationisinfluencedbyvariousfactors,includingthetypeandconcentrationofpas-
sivatingagents,thetemperatureofthetreatmentsolution,andthedurationoftheprocess.
Forexample,higherconcentrationsofHNO3canresultinamoreuniformandthicker
oxidelayer,thoughexcessivetreatmentmaycauseundesirableetchingofthemetalsur-
face.
Inmarineenvironments,chlorideionscaninfiltrateweakpointsinthepassivefilm,
leadingtopiingandcrevicecorrosionthatprecedeSCC.Consequently,itisessentialto
monitorthequalityandintegrityofthepassivelayerinsuchconditions.Recentresearch
highlightsthatthecompositionandmicrostructureoftheoxidelayerarecriticaltoits
protectiveeffectiveness.Forinstance,chromium-richoxidesoffersuperiorresistanceto
chloride-inducedpiing.Advancesinsurfaceanalysistechniqueshaveallowedformore
Surfaces2024,7625
preciseinvestigationsofoxidelayers,contributingtotheoptimizationofpassivationpro-
cesses[198–200].
7.2.3.Anodization
Anodizationisanelectrochemicaltechniquecommonlyusedtoenhancethesurface
propertiesofaluminumanditsalloys.Inthisprocess,themetalissubmergedinanelec-
trolyticsolution,typicallysulfuricacid,andanelectriccurrentisapplied.Thisresultsin
theformationofathick,stableanodicoxidelayeronthemetal’ssurface,whichactsasa
protectivebarrieragainstenvironmentaldegradationandSCC.Theanodizedlayerisgen-
erallysignificantlythickerandmoredurablethannaturallyoccurringoxides.Itsproper-
tiescanbeadjustedbyvaryingfactorssuchasthecompositionoftheelectrolyte,temper-
ature,andcurrentdensity.Forexample,sulfuricacidgeneratesthicker,moreporouslay-
ersthatareidealfordyeing,whilechromicacidproducesthinner,denserlayerswithsu-
periorcorrosionresistance.Additionally,theanodizedlayercanbesealedwithhotwater
orsteamtohydratetheoxideandenhanceitsprotectivequalities.Anodizationnotonly
improvescorrosionresistancebutalsoincreasessurfacehardnessandwearresistance,
makingitvaluableforapplicationsrequiringrobustsurfacefinishes.However,chloride
ionscandegradetheprotectivelayer,leadingtopiingandSCCunderspecificcondi-
tions.Therefore,furthertreatments,suchassealingorapplyingorganiccoatings,maybe
necessarytoimprovethedurabilityoftheanodizedsurface[201,202].
BothpassivationandanodizationeffectivelypreventSCCbyformingstableprotec-
tiveoxidelayers.ThesemethodsaddresstherootcauseofSCC—localizedcorrosion—by
providingaresilientsurfacecapableofwithstandingsevereenvironmentalconditions.
Nevertheless,theeffectivenessoftheseelectrochemicalmethodscanbediminishedifthe
protectivelayersaremechanicallydamagedorexposedtohighlyaggressiveenviron-
ments.Hightemperatures,mechanicalstress,orcontactwithharshchemicalscandegrade
theseprotectivelayersandincreasetheriskofSCC[203,204].Insummary,thestrategic
useofelectrochemicalmethodssuchascathodicprotection,passivation,andanodization
representsacomprehensiveapproachtomitigatingSCCinvariousmetallicstructures.
Thesetechniquesenhancemetalsurfaceprotection,extendcomponentlifespan,anden-
surereliabilityinchallengingenvironments.Continuedresearchanddevelopmenttoop-
timizethesemethodsandexplorenewelectrochemicaltreatmentsarecrucialforadvanc-
ingcorrosionpreventionstrategiesandextendingtheservicelifeofcriticalinfrastructure.
7.3.ChemicalMethods
Chemicalapproaches,suchastheuseofinhibitorsandcoatings,areessentialforim-
provingthecorrosionresistanceofmetalsandpreventingSCC.Thesetechniquesinvolve
applyingchemicalsubstanceseitherdirectlytothemetalsurfaceortotheenvironment
surroundingittoreducethelikelihoodofcorrosionandSCC.Byimplementingthese
methods,researchersandengineerscansignificantlydecreasetheriskofcorrosion-in-
ducedfailuresinmetallicstructures,therebyenhancingtheirdurabilityandreliabilityin
demandingconditions[204].
7.3.1.Coatings
CoatingsarevitalforsafeguardingcarbonsteelpipelinesagainstSCC,asignificant
threattopipelineintegrity.Thesecoatingsactasphysicalbarriers,preventingdirectex-
posureofthemetalsurfacetocorrosiveelements,whichhelpsreducethelikelihoodof
SCC.ChoosingtherightcoatingiscrucialforeffectiveSCCprotection.Organiccoatings,
particularlyepoxyresincoatings,arewellresearchedfortheirdesirableaributes,includ-
ingminimalshrinkage,resistancetochemicals,andmechanicaldurability.Thesecoatings
arewidelyemployedinindustrialcontexts,suchasonmarinevesselsandburiedpipe-
Surfaces2024,7626
lines,tocombatcorrosionandlowertheriskofhydrogen-inducedSCC[205,206].Thepri-
maryfunctionofthesecoatingsistoestablishaprotectivelayerthatisolatesthemetal
fromexternalfactors,therebyminimizingSCCrisk[207–210].
Amongthecoatingsappliedtoexternalpipelinesurfaces,Fusion-BondedEpoxy
(FBE)isdistinguishedbyitsexceptionalmechanicalstrengthandadhesion,makingit
suitableforlarge-diameterpipelinesthatundergoconsiderablehandlingandinstallation
stresses.Despiteitseffectiveness,FBEcoatingscanbepronetoblisteringandshielding
effectswhenusedwithcathodicprotectionsystems.Liquidepoxyisnotedforitsrobust
corrosionprotectionandexcellentadhesion,makingitwidelyapplicableinvariousin-
dustrialenvironments.Urethaneandpolyurethanecoatingsofferflexibilityandimpact
resistance,whichhelpprotectagainstphysicaldamageandcorrosion.Historically,as-
phaltandcoaltarwereprizedfortheirdurabilityandwaterresistance;however,theiruse
hasdiminishedduetoenvironmentalconcerns.Polyethylene(PE)andpolyolefinresin
coatingsprovideadditionalprotectivebenefits,whileceramicandcompositecoatingsof-
ferhigh-temperatureresistanceanddurability,thoughtheycanbebrileandrequire
carefulapplication[207,208,211–213].
ThesuccessofcoatingsinpreventingSCCdependsonfactorssuchastheirmechan-
icalproperties,waterpermeability,electricalinsulation,andresistancetoenvironmental
degradation.Adequatesurfacepreparationisessentialtoachievetheoptimaladhesion
andlong-termeffectivenessofthecoatingsystem.Moreover,coatingsmustbecompatible
withpipelinematerialstoavoiddetrimentalinteractionsthatcouldcompromisetheirpro-
tectivecapabilities.Environmentalfactors,includingsoilandatmosphericconditions,as
wellasresidualstressesfrompipelineoperations,alsoinfluencecoatingperformanceand
SCCrisk.Ensuringthatcoatingsmeetthesecriteriaandareappliedproperlyiscriticalfor
maximizingtheirprotectiveefficacyandreducingSCCrisk[207].
7.3.2. NewTrendsinSCCPreventionbyCoatings
OrganicprotectivecoatingsareextensivelyusedtocombatSCC.Recentdevelop-
mentshaveledtothecreationofcoatingstailoredtoaddressthespecificchallengesposed
bySCC.Theseadvancedcoatingsenhancetheireffectivenessbysealingdefectsandpre-
ventingcorrosionreactions,therebyminimizingtheneedforexternalmaintenance[214].
Self-HealingCoatings
Self-healingmechanismsinsmartcoatingscanbecategorizedintoautonomousand
non-autonomoustypes,bothofwhicharecrucialforcombatingSCC.Autonomousself-
healinginvolvesintegratingpolymerizablehealingagentsorcorrosioninhibitorsintothe
coatingmatrix.Thesesubstancesareactivatedwhenthecoatingisdamaged,therebyre-
storingitsintegrityandpreventingSCC.Forinstance,encapsulatedagentssuchasisocy-
anates,epoxies,andcuringagentsliketetraethylenepentamine(TEPA)aretriggeredupon
damagetosealcracksandhaltfurthercorrosion[214].Conversely,non-autonomousself-
healingrequiresexternalstimuli,suchaschangesinpHorelectrochemicalsignals,toin-
itiatechemicalreactionsthatrepairthecoating.Conductivepolymers,includingpolypyr-
role(PPy)andpolyaniline(PANI),areutilizedfortheirabilitytoundergoredoxreactions,
whichhelpstopassivatethesteelsurfaceandreduceSCC[215,216].
AdvancedCoatingMaterials
Recentadvancementsinmaterialssciencefeatureself-healingpolymerssuchas
AQALICCS-7S,whichabsorbwaterandexpandtomendscratchesuponcontact,thereby
mitigatingSCCincoatingdefects.Additionally,metallic–polymericcoatings,suchas
thosecombiningzincwithpolyethyleneoxide-b-polystyrene(PEO113-b-PS218)nanoag-
gregates,exhibitself-healingpropertiesduetotheiramphiphilicnature,effectivelyserv-
ingasbarriersagainstenvironmentsthatpromoteSCC.Furthermore,ceramicmaterials,
Surfaces2024,7627
includingTiC/Al2O3andTi2AlC,demonstrateself-healingcapabilitiestriggeredbyoxida-
tion.Thesematerialsself-healwhenexposedtoelevatedtemperaturesandoxygen,re-
plenishingdefectswithprotectiveoxidesandthusloweringtheriskofSCC[217].
EnvironmentalSustainability
Environmentalsustainabilityandecologicalregulationsadvocatefortheincorpora-
tionofnaturalmacromoleculesandbiopolymers—suchaschitosan,lignin,cellulose,and
nanocellulose—incoatingengineering,whichcanalsoaidinthepreventionofSCC.When
chitosanisusedinconjunctionwithanepoxymatrix,itimpartscombinedanticorrosive
andantibacterialeffects,therebyenhancingtheintegrityofprotectivecoatingsandmiti-
gatingSCC[217].Ligninservesasanaturalultraviolet(UV)absorber,improvingUVsta-
bilityandprolongingthelifespanofcoatings,which,inturn,offersextendedprotection
againstSCC[218].Additionally,cellulosemicrofibersandnanofibers,wheninfusedwith
healingagents,canbeintegratedintopolymericmatricestocreateadvancedself-healing
epoxycoatings.Theseinnovativematerialsnotonlyrepairmechanicaldamagebutalso
bolsterthedefenseagainstSCCbymaintainingastrongbarrieragainstcorrosiveagents
[219,220].
NanomaterialAdditives
Integratingfunctionalnanomaterialsintocoatingformulationscangreatlyimprove
theirperformance,enhancingresistancetoSCC.Carbon-basednanomaterials,suchascar-
bonnanotubes(CNTs),graphene,andgrapheneoxide,enhancethemechanicalstability
ofepoxymatrices,therebydecreasingtheirvulnerabilitytostressandstrainthatcould
induceSCC.Additionally,nano-silicacontributestoimprovedcorrosionresistance,frac-
turetoughness,andtensilestrength,furtherincreasingthecoating’sresilienceagainst
SCC.Theincorporationofpolymericmatriceswithmetaloxidenanoparticles,nano-clays,
andnano-ceramicsresultsinnanocompositecoatingswithenhancedproperties,offering
superiorprotectionagainstSCCbyboostingbothmechanicalandchemicalresistance
[217,221,222].
SCC-SpecificStrategies
Advancedcoatingsthatfeatureself-healingandstimuli-responsivetechnologiesof-
ferapromisingapproachtoreducingSCC.Thesecoatingsaredesignedtoidentifythe
initialformationofcracksandeitherreleaserepairagentstoaddressthedamageormod-
ifytheirpropertiestohaltfurthercrackdevelopment.Byincorporatingcorrosioninhibi-
torssuchasphosphates,nitrites,andrare-earthmetalsalts,thesecoatingscancounteract
thecorrosiveelementsthatexacerbateSCC.Additionally,theintegrationofnanomaterials
intothesecoatingsenhancestheirmechanicalproperties,increasingtheirresistancetothe
stressesthatleadtoSCC[217,223].
Theadoptionofsuchadvancedself-healingandstimuli-responsivecoatingsinrefin-
erysystemsisexpectedtoimproveanticorrosiveperformanceandextendthelifespanof
protectivesystems,therebyreducingtheoccurrenceofSCCandprolongingtheservice
lifeofessentialinfrastructure.
7.3.3.Inhibitors
Corrosioninhibitorsarespecializedchemicalcompoundsintroducedinminimal
quantitiestocorrosiveenvironmentstoprotectmetallicmaterialsfromdeterioration.
Thesesubstancesfunctionbyeithercreatingaprotectivefilmonthemetalsurfaceorneu-
tralizingcorrosiveagentsintheenvironment.Theyareparticularlyeffectiveincontrolled
environmentssuchascoolingsystemsandoilpipelines.Corrosioninhibitorscanbeclas-
sifiedintoseveraltypes,includinganodic,cathodic,passivating,film-forming,vapor
phase,andadsorption-basedinhibitors.Amongthese,organicinhibitors,especiallythose
Surfaces2024,7628
containingpolarfunctionalgroupswithnitrogen,sulfur,and/oroxygen,haveprovenef-
fectiveinbothacidicandalkalineconditions.Theadsorptionoftheseinhibitorsonmetal
surfacesisinfluencedbyfactorssuchasfunctionalgroups,stericeffects,aromaticity,elec-
trondensity,andtheelectronicstructureoftheinhibitors.Thesecompoundsformapro-
tectiveadsorbatelayeronthemetal,therebyreducingcorrosionratesbyshieldingthe
surfacefromaggressivesolutions[224].
Additionally,inhibitorscaneffectivelypreventSCCinmetalsbyformingaprotective
layeronthemetalsurface.Organicinhibitorswithpolarfunctionalgroupscontainingni-
trogen,sulfur,and/oroxygenhaveshownsignificanteffectivenessagainstSCCinsteel
underbothacidicandalkalineconditions.Theefficiencyoftheseinhibitorsisaffectedby
severalfactors,includingtheirfunctionalgroups,stericeffects,aromaticity,electronden-
sity,andelectronicstructure,whichinfluencetheiradsorptiononthemetalsurface.By
alteringtheelectrodepotentialofthemetalorisolatingitfromthecorrosiveenvironment,
inhibitorscansubstantiallylowertheriskofSCC.Itisnoteworthythathigherconcentra-
tionsofinhibitorsmightberequiredtopreventSCCcomparedtothoseneededforgeneral
corrosioncontrol.Furthermore,paintsandcoatingscontaininginhibitorscanenhance
SCCresistancebypreventingcorrosivesolutionsfrompenetratingthecoatingandreach-
ingthemetalunderneath.Recentadvancementsinchemicaladditiveshavebeenspecifi-
callyaimedataddressingSCCinapplicationssuchasfuelethanolstorageanddistribu-
tionsystems[225,226].
Despiteextensiveresearchoncorrosioninhibitorsforgeneralcorrosioninacidicme-
dia,thereisasignificantlackofstudiesfocusingonSCC.Whileplantextractshaveshown
promiseasenvironmentallyfriendlycorrosioninhibitors,theireffectivenessagainstSCC
remainslargelyunexplored.Aprimarychallengeinthisareaistheinadequatemechanis-
ticunderstandingoftheseextracts,asmoststudiesusecrudeformulationswithoutade-
tailedanalysisofindividualcomponents.ThisgaphinderstheselectionofeffectiveSCC
inhibitors.Futureresearchshouldexploretheeffectsofsubstituentgroups,investigate
synergisticinteractionsamonginhibitors,andassessfactorsinfluencingSCCinhibition.
Additionally,analyzingcorrosionproductformationinthepresenceofinhibitorsandem-
ployingnanotechnology,suchasgraphene-basedcomposites,couldenhanceinhibition
strategies.Incorporatingquantumchemicalcalculationsandmolecularmechanicssimu-
lationswillaidinelucidatinginhibitionmechanisms.ByconcentratingonSCCandutiliz-
ingeco-friendlymaterials,researcherscandevelopeffectiveandcost-efficientsolutions
forpreventingSCCinstainlesssteelacrossvariousindustries.
7.4.PhysicalMethods
Physicalmethodsencompassmechanicalprocessesdesignedtomodifytheproper-
tiesofmetalsortheirsurfacestoimprovetheirresistancetoSCC.
7.4.1.PhysicalVap orDeposition
Physicalvapordeposition(PVD)isasophisticatedcoatingtechniquewidelyem-
ployedtomitigateSCCinmetalliccomponents.Thisprocessinvolvesthevaporizationof
asolidmaterial,whichisthendepositedontoasubstratetocreateathin,protectivefilm.
PVDisespeciallyeffectiveinenhancingthecorrosionresistanceofmaterials,makingita
preferredchoiceforapplicationsvulnerabletoSCC.ThecoatingsproducedbyPVDare
knownfortheirstrongadhesion,consistentthickness,anddurability,resultinginhard,
wear-resistantsurfaces.Thesecoatingsestablishadense,impermeablelayerthatshields
thesubstratefromcorrosiveagents,therebyloweringtheriskofbothcorrosionandSCC.
PVDcoatingscanbeappliedtoavarietyofsubstrates,includingmetals,ceramics,and
alloys,providingnotonlyenhancedcorrosionresistancebutalsoimprovedmechanical
properties.ResearchhasdemonstratedthatPVD-coatedtoolscansignificantlyinfluence
thesusceptibilityofmaterialstoSCC.Forexample,studieshaverevealedthatPVD-coated
cuingtoolscandiminishSCCriskinsuperduplexstainlesssteelduringmachiningop-
Surfaces2024,7629
erations.ThishighlightstheeffectivenessofPVDinreducingSCCbyimprovingthesur-
facecharacteristicsofthematerials.Inconclusion,PVDisahighlyeffectivemethodfor
applyingprotectivecoatingsthatenhancetheresistanceofmetalstocorrosionandSCC.
Bycreatingarobustbarrieronthesubstrate,PVDcoatingsplayacrucialroleinextending
thelifespanandreliabilityofmetallicstructuresinenvironmentssusceptibletoSCC
[227,228].
7.4.2.OtherPhysicalMethods
WhilePVDandthermalspraycoatingareamongthemostprevalentandeffective
methodsforpreventingSCC,theyarenottheonlytechniquesavailable.Otherphysical
approachescanalsocontributetoSCCprevention,thoughtheireffectivenessandappro-
priatenessmaydependonthespecificapplicationandenvironmentalconditions.
Chemicalvapordeposition(CVD)isatechniquethatdepositsthinfilmsontometal
surfacesviachemicalreactionswithvaporizedprecursorgases.Inthisprocess,precursor
gasescontainingthedesiredelementsareintroducedintoareactor,wheretheydecom-
poseonaheatedsubstrate,formingasolidcoatingwithacontrolledcomposition,thick-
ness,anduniformity[204].CVDcoatingsareespeciallyeffectiveinmitigatingSCC.These
coatingscreateadurableprotectivebarrierthatimprovesthemetal’sresistancetoSCC,
whichisessentialforuseincorrosiveenvironments.TheconformalnatureofCVDcoat-
ingsensurescomprehensivecoverage,preventingtheinitiationandpropagationofSCC.
Moreover,CVDcanalsobeusedtoapplyfunctionalcoatings,suchasprotective,wear-
resistant,orcatalyticfilms,furtherenhancingthesurfacepropertiesandperformanceof
metalsubstrates[229].
Plasmatreatmentshavegainedprominenceinsurfaceengineering,particularlyfor
electronicdevices.Techniqu essuchasplasmaetchinganddepositionofferprecisecontrol
oversurfacecharacteristics,includingroughness,weability,andchemicalcomposition.
Thesetreatmentsareeffectiveforremovingcontaminants,activatingsurfacestoimprove
adhesion,ormodifyingsurfaceenergytoenhancetheweingbehaviorofcoatingsorad-
hesives.Recentstudieshaveexploredplasmatreatmentswithvariousgascompositions
andprocessparameterstooptimizethesurfacepropertiesofelectroniccomponentsfor
specificapplications.Forinstance,plasmasurfacemodificationhasbeenutilizedtoim-
provetheadhesionofwirebondingandsolderingmaterialstoelectronicsubstrates,
therebyenhancingthereliabilityoftheseinterconnections[230,231].
Lasersurfaceengineeringencompassestechniquesthatuselaserstomodifythesur-
facepropertiesofmetals.Methodssuchaslasercladding,laseralloying,laserhardening,
andlasersurfacetexturingenableprecise,localizedtreatment,allowingcontroloverchar-
acteristicslikehardness,wearresistance,andthermalperformance.Thesetechniquesare
widelyusedintooling,automotivecomponents,andaerospacestructures.Regarding
SCC,lasersurfacetreatmentscansignificantlyenhanceresistance.Forexample,laserclad-
dingcanapplyprotectivecoatingsthatreducemetals’susceptibilitytoSCCbymodifying
theirsurfacechemistryandmicrostructure.Similarly,laserhardeningcanincreasesurface
hardnessanddiminishthelikelihoodofcrackpropagationincorrosiveenvironments
[232].
7.5.ThermalMethods
ThermalmethodsformitigatingSCCinvolvetheapplicationofprotectivecoatings
throughvariousthermalsprayingtechniques,eachofferingdistinctadvantagessuitedto
specificapplicationsandenvironments.Amongthesetechniques,high-velocityoxygen
fuel(HVOF)sprayingisparticularlyeffectiveinreducingSCC.
Thermalspraycoatingsareappliedtometalsurfacesusinghigh-velocitymethodsto
formaprotectivelayerthatshieldstheunderlyingmaterialfromcorrosiveconditions.The
HVOFsprayingtechniqueisespeciallynoteworthyinthisregard.Itemploysthecombus-
tionoffuelgasandoxygentogenerateahigh-velocityjetthatdepositsmoltenorsemi-
moltenparticlesontothesurfaceatsupersonicspeeds.Theresultingcoatingsaredense,
Surfaces2024,7630
havelowporosity,anddemonstrateexcellentadhesiontothesubstrate.Thisdenseand
well-adheredcoatingeffectivelyblockscorrosiveagentsthatcouldotherwisecauseSCC
[228,233].TheeffectivenessofHVOFsprayinginpreventingSCCislargelyduetoitsabil-
itytoproducedurablecoatingsthatresistcorrosionandremainintactunderseverecon-
ditions.Incomparisontootherthermalspraymethods,HVOFcoatingsprovidesuperior
protectionowingtotheirdensityandstrongadhesion,whichsignificantlydecreasesthe
likelihoodofcorrosionpathwayspenetratingtheprotectivelayer.
InadditiontoHVOF,aluminum-andzinc-basedthermalspraycoatingshaveshown
impressiveperformanceinSCCprevention.Thesecoatingsestablishaphysicalbarrier
thatisolatesthemetalfromthecorrosiveenvironment,effectivelyhinderingtheinitiation
andpropagationofstresscorrosioncracks.Theelectrochemicalpropertiesofaluminum
andzincfurtherenhancethecoatings’overallcorrosionresistance.Furthermore,thehigh-
velocityimpactofparticlesduringtheHVOFprocesscreatescompressiveresidual
stressesonthesurface,counteractingthetensilestressesthatcontributetocrackformation
[228].TheeffectivenessofthermalspraycoatingsinpreventingSCCdependsonseveral
factors,includingthecoatingthickness,porosity,andadhesiontothesubstrate.Thicker
coatingsgenerallyofferbeerprotection,whilecoatingswithlowerporosityandhigher
adhesionprovideimprovedintegrityanddurability.Optimalperformanceisachieved
throughacarefulselectionofcoatingmaterialsandtheprecisecontrolofsprayingparam-
eterstoalignwiththespecificenvironmentalconditionsandstressfactorsencountered
bythecomponents[228].
AdditiveManufacturing
Additivemanufacturing(AM),oftenknownas3Dprinting,isaprogressivetechnol-
ogythatfabricatesobjectslayerbylayerbasedondigital3Ddesigndata.AMencompasses
avarietyofprocesses,includingthosethatuselasers,electronbeams,orplasmatode-
posit,fuse,orsolidifymaterials.Thistechnologyofferssubstantialbenefitscomparedto
conventionalmanufacturingmethods,suchasthecapabilitytoproduceintricatecompo-
nentsdirectlyfromCADmodels,reducedmaterialwaste,andshorterproductiontimes.
Bytheendof2020,theAMmarketwasanticipatedtoreachUSD21billion,whichspurred
extensiveresearchaimedatimprovingAMprocesses,particularlyinaddressingSCC
[234].
Studiesonselectivelasermelting(SLM)haveexaminedhowdifferentprocesspa-
rameters—suchasscanningspeed,laserpower,andenergydensity—affectthemicro-
structureofASS,includingaspectslikegrainsizeandporosity.Thesemicrostructuralfea-
turesarecritical,astheyimpactthestabilityofthepassivefilm,whichisessentialfor
corrosionresistanceandSCCmitigation.Therefore,optimizingSLMparametersiscrucial
forenhancingcorrosionresistanceandreducingSCCvulnerability.Researchondirect
laserdeposition(DLD)hasexploredhowheattreatmentsandchromiumcontentinflu-
encepassivefilmformation.Heattreatmentissignificantforenhancingthedevelopment
ofprotectivepassivefilmsanddecreasingSCCsusceptibilitybyimprovingtheoverall
microstructure.NewAMtechnologies,includingadvanced3Dprintingtechniques,pro-
videinnovativesolutionsforreducingSCC.Thesetechnologiesallowfortheproduction
ofcomplexcomponentdesignsthatminimizestressconcentrations—akeyfactorinSCC.
Theyalsosupportthecreationofnovelmaterialsandcoatingsspecificallydesignedto
withstandcorrosiveenvironments.Additionally,AMenablestheintegrationofprotective
coatingsduringproductionandfacilitatesinsiturepairs,thusprolongingthelifespanof
componentsinharshconditions.Theintegrationofsensorswithincomponentsforthe
real-timemonitoringofenvironmentalconditionsandstructuralhealthaidsintheearly
detectionofSCC.Rapidprototypingacceleratesthedevelopmentofnewmaterialsand
designs,thushasteningtheapplicationofSCCpreventionmeasures.Nevertheless,chal-
lengesrelatedtomaterialperformance,manufacturingconsistency,andcostneedtobe
addressedtofullyrealizeAM’spotentialinSCCprevention.ResearchintootherAM
Surfaces2024,7631
methods,suchaswirearcadditivemanufacturing(WAAM)anddroplet-based3Dprint-
ing,isstillemerging.UnderstandingtheireffectsoncorrosionbehaviorandSCCislimited
butessentialforensuringthedurabilityofmetalpartsproducedbyAM[234,235].
7.6.EnvironmentalConsiderations
SCCisinfluencedbyvariousenvironmentalfactors,includingsoilcomposition,
transportedfluids,andexternalandinternalconditions,eachcontributingdistinctcorro-
sionmechanisms.SCCraisessubstantialeconomic,environmental,andsafetyconcerns,
necessitatingglobalmonitoring.OfparticularconcernareionssuchasCO2,H2S,temper-
ature,partialpressure,andnaphthenicacidcontent[42,236,237].
SCCpredominantlyoccursinenvironmentswithnearlyneutraltohighpHlevels,
characterizedbycarbonate-richconditionsinfluencedbyenvironmentalandphysico-
chemicalparameters[237].SusceptibilitytoSCCincreaseswithgreaterlevelsofcathodic
polarization,akintotheoverprotectioneffectobservedincarbonsteel[238–240].Forin-
stance,near-neutral-pHSCCincarbonsteelpipelinesislinkedtothebreakdownoftape
coatings,facilitatingcarbonateformationfromCO2[45,209,240,241].Conversely,elevated
pHlevelscontributetoSCCfailuresassociatedwithcarbonate(CO32−),bicarbonate
(HCO3−),andalkalineconditions[44,51].Additionally,dissolvedCO2inwaterformscor-
rosivecarbonicacid,impactingcarbonsteel[242].HICincarbonsteels,exacerbatedby
acidicsoilsolutionsorsulfate-reducingbacteria,alsoposesarisk[193,210,243].Damaged
pipelinecoatingsunderlieextremeconditions,particularlywithdefectivecoatingscom-
promisingcathodicprotection,compoundedbymechanicaldamageduringconstruction
[144,244].
EffectivecontroloftensilestressiscriticalinpreventingSCCinitiationandpropaga-
tioninsusceptiblematerials.Thismanagementinvolvesproperheattreatment,coldex-
pansiontechniques,andcontrolofpressurefluctuations.MitigatingSCCinvulnerable
materialsrequiresacomprehensiveassessmentofexternalandinternalenvironmental
factors.Keyconsiderationsincluderemovingcorrosivespeciesfromtransportedfluids
andpreventingstagnantpipelinewateraccumulation.Soilswithelevatedcarbonatelevels
areparticularlypronetoSCCandrequireappropriatepreventivemeasures[5].
Variationsinthephysicochemicalpropertiesofsoils,suchaswatercontent,oxygen
availability,air–solidinterfaces,andseasonalfluctuationsinspecificgeographicalre-
gions,createhighlycorrosiveenvironments[245].Thecorrosionbehaviorofcarbonsteel
surfacesisinfluencedbyabruptenvironmentalchanges,suchasseasonalvariationsand
alteredsoilproperties,andtheeffectivenessofcathodicprotectionsystems[5].Therefore,
electrochemicalinteractionsatthesoil–pipelinesteelinterface,mechanicalpropertyfluc-
tuations,andionconcentrationvariationswithinthesoilareanticipated.Ingeologicaland
scientificinvestigations,naturalsoilsamplesaretypicallycollectedapproximately1.2m
deepalongthepipelineright-of-way(ROW)toassesssignificantexternalcorrosiondam-
age[246].Romanoff’sstudyexaminedtheimpactofexternalcorrosiononvariousmetals
indifferentU.S.soils,whileColeandMarneyemphasizedtheinfluencesofelectrochem-
icalreactions,oxideformation,andfactorssuchastemperature,voidspaces,pH,salinity,
andmoistureoncorrosionprocessesinsoilenvironments[5,247].
Inpracticalapplications,preventingI-SCCincarbonsteelpipelinesofteninvolves
deployingfieldseparatorstoremovecorrosiveelementssuchasgases,hydrocarbons,
high-salinitywater(e.g.,oilfield-producedwater),solids,metals,andsedimentsfrom
sludge.Thisprocessincludesdesaltinganddehydrationprocedures[248].Fluidcompo-
sitionfactorslikedissolvedoxygen,organicacids,watercontent,gasandliquiddensities,
viscosity,microorganismpresence,suspendedsolids,fielddebris,CO2levels,andH2S
contentinfluenceI-SCCoccurrence.Additionally,pipelineoperationalconditionssuchas
innerwallandfluidtemperatures,gasandliquidflowrates,erosionanddepositionrates,
heattransfercharacteristics,surfaceshearstress,flowturbulenceintensity,andpressure
dynamicsduringhydrocarbonproduction,gathering,storage,andtransportationstages
Surfaces2024,7632
allplaycriticalroles.Foulingpropensitysignificantlyimpactspressuredynamics
throughoutthesestages,affectingoperationalcostsandproductyields.
Therefore,implementingcorrosionpreventionmeasuresisimperative.These
measuresincludechemicaltreatmentssuchascorrosioninhibitorprograms,directmeth-
odslikecorrosioncouponsandfailurefrequencyanalysis,andinlineinspectionusingUT,
RT,andsmartpigs.Indirecttechniquesencompassmonitoringwatercontent,pressure,
temperature,hydrogenflux,gascomposition,solidandliquidanalysis,inhibitorpres-
ence,andmicrobiologicalfactors[248].Inthecontextofoilpipelines,inlinecleaningsys-
temsemployingpigtoolsarecommonlyusedtoaddressfoulingtendencies,whilegas
pipelinesemployspheresforsimilarpurposes[5].
8.Conclusions
ThiscomprehensivereviewexaminesthecriticalissueofSCCintheoilandgasin-
dustry,focusingonCl-SCC.Thestudyhighlightstheintricateinterplaybetweenenviron-
mentalconditionsandmaterialpropertiescontributingtoSCC,emphasizingtheheight-
enedriskunderspecificoperationalscenarios.ThefindingsindicatethatSCCcaninitiate
atrelativelylowtemperatures,around20°C,withrapidcrackpropagationobservedas
temperaturesexceed60°C.Thepresenceofchlorideionsisparticularlydetrimental,with
concentrationsabove100ppmsignificantlyincreasingsusceptibilitytoSCC.Forinstance,
a100ppmincreaseinchlorideconcentrationcanelevatecrackgrowthratesbyapproxi-
mately40%,underscoringthecriticalroleofenvironmentalcontrolinmanagingSCC
risks.Mitigationstrategies,includingcorrosion-resistantalloysandprotectivecoatings,
haveproveneffectiveinreducingSCCincidencebyupto50%undercontrolledexperi-
mentalconditions.Cathodicprotectionandtargetedcorrosioninhibitorsfurthercontrib-
utetoriskreduction,decreasingSCCoccurrencesbyabout30%.Advancedmonitoring
techniquesplayavitalroleinearlydetectionandpreventativemaintenanceschedules.
ThestudyalsohighlightsthesignificanceofenvironmentalpHandtemperatureininflu-
encingSCCbehavior.EnvironmentswithapHlowerthan4werefoundtodoubletherate
ofSCCcomparedtothoseatneutralpH.Additionally,temperaturevariationsbetween50
°Cand100°Cinhigh-chlorideenvironments(over200ppm)werelinkedtoa60%increase
inSCCfrequency.Thisreviewemphasizestheimportanceofacomprehensiveapproach
toSCCmanagement,involvingstrategicmaterialselection,continuousenvironmental
monitoring,andproactivemaintenance.Byadoptingthisintegratedapproach,industry
stakeholderscansignificantlyenhancetheresilienceoftheirinfrastructure,reducingeco-
nomiclossesandenvironmentalhazards.Thefindingsprovidecompellingevidencethat
proactiveSCCmanagementisessentialformaintainingthestructuralintegrityandoper-
ationalreliabilityofassetsintheoilandgassector,therebysafeguardingagainstsignifi-
canteconomiclossesandenvironmentalhazards.
Aut ho rContributions:Conceptualization,M.V.,P.K.,J.K.andZ.G.;methodology,M.V.andP.K.;
software,M.V.andP.K.;validation,M.V.,P.K.,J.K.andZ.G.;formalanalysis,P.K.andJ.K.;investi-
gation,M.V.andP.K.;resources,M.V.andP.K.;datacuration,P.K.andJ.K.;writing—originaldraft
preparation,M.V.;writing—reviewandediting,M.V.,P.K.andJ.K.;visualization,M.V.;supervision,
M.V.andP.K.;projectadministration,P.K.andJ.K.;fundingacquisition,P.K.Allauthorshaveread
andagreedtothepublishedversionofthemanuscript.
Funding:Thispublicationisaresultoftheproject“Moderntrendsintheprocessingofenergyraw
materials”(8232201)carriedoutatORLENUniCREa.s.
InstitutionalReviewBoardStatement:Notapplicable.
InformedConsentStatement:Notapplicable.
DataAvailabilityStatement:Nonewdatawerecreatedoranalyzedinthisstudy
ConflictsofInterest:Theauthorsdeclarenoconflictsofinterest.
Surfaces2024,7633
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... Stress corrosion cracking (SCC) in engineering components results in rapid crack propagation often without prior indications, significantly reducing their operating lifetime [1,2]. Austenitic stainless steels are widely used in various industries due to their outstanding mechanical properties and corrosion resistance [3,4]. However, they are susceptible to SCC, including chloride-induced stress corrosion cracking (CISCC), at standard atmospheric conditions while subjected to minimal stresses [5,6]. ...
Chloride-induced stress corrosion crack propagation mechanisms in austenitic stainless steel are mechanically driven
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  • Mar 2025
  • SCRIPTA MATER
The objective of this study is to understand the mechanical aspects of chloride-induced stress corrosion cracking (CISCC) in austenitic stainless steel. CISCC is a critical degradation mode in austenitic stainless steel, so understanding its mechanisms is essential for predicting material integrity and lifetime. Here, CISCC is studied by transmission Kikuchi diffraction and transmission electron microscopy for two cases: propagation into a lower Schmid factor grain and a higher Schmid factor grain. The evolution of deformation fields near the crack-tip is estimated through local misorientations and geometrically necessary dislocations, and are more severe in the lower Schmid factor grain. In both grain types, cross slips are distributed closer to the crack, while co-planar slips appear away from the crack, revealing consistent crack-tip deformations. Strain-induced α’-martensite transformations occur in the higher Schmid factor grain. These results imply that grain texture can be used to predict and mitigate CISCC propagation in austenitic stainless steel.
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Metallurgical Characterization of Cr2O3 and Al2O3 Coatings on SS 316L: Implications for Durability in Desalination Environments
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  • Jan 2025
The durability and corrosion resistance of SS 316L in desalination plants are compromised by the harsh, corrosive environment of seawater. To address this, ceramic coatings like Al2O3 and Cr2O3 are explored for enhancing surface properties. This study evaluates SS316L, Al2O3, and Cr2O3 coatings using SEM, microhardness testing, and XRD, with EDS. SEM revealed that Al2O3 coatings had cracks and porosity, resulting in lower hardness compared to the irregular but harder Cr2O3. Microhardness testing confirmed Cr2O3 as the hardest material, followed by Al2O3 and SS316L. XRD identified the crystalline structures and phase compositions of these coatings. Corrosion tests demonstrated Cr2O3’s superior resistance, attributed to its ability to form a protective Cr2O3 oxide layer, which prevents degradation in aggressive environments. These findings highlight Cr2O3’s potential for enhancing both hardness and corrosion resistance in desalination applications.
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Addressing Hydrogen Sulfide Corrosion in Oil and Gas Industries: A Sustainable Perspective
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  • Feb 2024
In the oil and gas industry, the corrosion attributed to hydrogen sulfide (H2S) is one of the most significant challenges. This review paper systematically investigates the diverse facets of H2S corrosion, including its sources, corrosion locations, mechanisms, and resultant corrosion products. Understanding different forms of H2S corrosion, such as stress-oriented hydrogen-induced cracking (SO-HIC), sulfide stress cracking (SSC), and hydrogen-induced cracking (HIC), provides a thorough comprehension of these phenomena. The paper discusses critical factors influencing H2S corrosion, such as temperature, flow rate, pH, and H2S concentration, highlighting their implications for sustainable practices in the oil and gas sector. The review emphasizes the significance of monitoring and mitigation strategies, covering continuous monitoring, applying corrosion inhibitors, selecting materials, and conducting thorough data analysis and reporting. Furthermore, the role of training in fostering a sustainable approach to H2S corrosion management is highlighted. This exploration advances the overarching goal of sustainable development in the oil and gas industries by providing insights into understanding, monitoring, and mitigating H2S corrosion. The findings presented here offer a foundation for developing environmentally conscious strategies and practices to guarantee the long-term viability and flexibility of refinery operations.
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Review of Prediction of Stress Corrosion Cracking in Gas Pipelines Using Machine Learning
Article
Full-text available
  • Jan 2024
Pipeline integrity and safety depend on the detection and prediction of stress corrosion cracking (SCC) and other defects. In oil and gas pipeline systems, a variety of corrosion-monitoring techniques are used. The observed data exhibit characteristics of nonlinearity, multidimensionality, and noise. Hence, data-driven modeling techniques have been widely utilized. To accomplish intelligent corrosion prediction and enhance corrosion control, machine learning (ML)-based approaches have been developed. Some published papers related to SCC have discussed ML techniques and their applications, but none of the works has shown the real ability of ML to detect or predict SCC in energy pipelines, though fewer researchers have tested their models to prove them under controlled environments in laboratories, which is completely different from real work environments in the field. Looking at the current research status, the authors believe that there is a need to explore the best technologies and modeling approaches and to identify clear gaps; a critical review is, therefore, required. The objective of this study is to assess the current status of machine learning’s applications in SCC detection, identify current research gaps, and indicate future directions from a scientific research and application point of view. This review will highlight the limitations and challenges of employing machine learning for SCC prediction and also discuss the importance of incorporating domain knowledge and expert inputs to enhance the accuracy and reliability of predictions. Finally, a framework is proposed to demonstrate the process of the application of ML to condition assessments of energy pipelines.
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Corrosion by Polythionic Acid in the Oil and Gas Sector: A Brief Overview
Article
Full-text available
  • Nov 2023
Polythionic acid (PTA) corrosion is a significant challenge in the refinery industry, leading to equipment degradation, safety risks, and costly maintenance. This paper comprehensively investigates the origin, progression, mechanism, and impact of PTA corrosion on various components within refinery operations. Special attention is afforded to the susceptibility of austenitic stainless steels and nickel-based alloys to PTA corrosion and the key factors influencing its occurrence. Practical strategies and methods for mitigating and preventing PTA corrosion are also explored. This paper underscores the importance of understanding PTA corrosion and implementing proactive measures to safeguard the integrity and efficiency of refinery infrastructure.
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Neural network based corrosion modeling of Stainless Steel 316L elbow using electric field mapping data
Article
Full-text available
  • Aug 2023
Stainless steel (SS) is widely employed in industrial applications that demand superior corrosion resistance. Modeling its corrosion behavior in common structural and various operational scenarios is beneficial to provide wall-thickness (WT) information, thus leading to a predictive asset integrity regime. In this spirit, an approach to model the corrosion behavior of SS 316L using artificial neural networks (ANNs) is developed, whereby saline water at different concentrations is flown through an elbow structure at different flow rates and salt concentrations. Voltage, current, and temperature data are recorded hourly using electric field mapping (EFM) pins installed on the elbow surface, which serve as training data for the ANNs. The performance of corrosion modeling is verified by comparing the predicted WT with actual measurements obtained from experimental tests. The results show the exceptional performance of the proposed single ANN model to predict WT. The error is calculated by comparing the estimated WT and actual measurement recorded, where the maximum error for each setting is range from 0.5363 to [Formula: see text]. RMSE and MAE values of each pin in every setting are also computed such that the maximum values of RMSE and MAE are 0.0271 and 0.0266, respectively. Moreover, a concise account of the observed scale formation is also reported. This comprehensive study contributes to a better understanding of SS 316L corrosion and offers valuable insights for developing efficient strategies to prevent corrosion in industrial environments. By accurately predicting WT loss using ANNs, this approach enables proactive maintenance planning, minimizing the risk of structural failures and ensuring the extended sustainability of industrial assets.
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Surface Engineering of Metals: Techniques, Characterizations and Applications
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Full-text available
  • Jul 2023
This paper presents a comprehensive review of recent advancements in surface engineering of metals, encompassing techniques, characterization methods and applications. The study emphasizes the significance of surface engineering in enhancing the performance and functionality of metallic materials in various industries. The paper discusses the different techniques employed in surface engineering, including physical techniques such as thermal spray coatings and chemical techniques such as electroplating. It also explores characterization methods used to assess the microstructural, topographical, and mechanical properties of engineered surfaces. Furthermore, the paper highlights recent advancements in the field, focusing on nanostructured coatings, surface modification for corrosion protection, biomedical applications, and energy-related surface functionalization. It discusses the improved mechanical and tribological properties of nanostructured coatings, as well as the development of corrosion-resistant coatings and bioactive surface treatments for medical implants. The applications of surface engineering in industries such as aerospace, automotive, electronics, and healthcare are presented, showcasing the use of surface engineering techniques to enhance components, provide wear resistance, and improve corrosion protection. The paper concludes by discussing the challenges and future directions in surface engineering, highlighting the need for further research and development to address limitations and exploit emerging trends. The findings of this review contribute to advancing the understanding of surface engineering and its applications in various sectors, paving the way for future innovations and advancements.
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A review of chloride induced stress corrosion cracking characterization in austenitic stainless steels using acoustic emission technique
Article
  • Nov 2023
  • NUCL ENG TECHNOL
Austenitic stainless steels (ASS) are extensively employed in various sectors such as nuclear, power, petrochemical, oil and gas because of their excellent structural strength and resistance to corrosion. SS304 and SS316 are the predominant choices for piping, pressure vessels, heat exchangers, nuclear reactor core components and support structures, but they are susceptible to stress corrosion cracking (SCC) in chloride-rich environments. Over the course of several decades, extensive research efforts have been directed towards evaluating SCC using diverse methodologies and models, albeit some uncertainties persist regarding the precise progression of cracks. This review paper focuses on the application of Acoustic Emission Technique (AET) for assessing SCC damage mechanism by monitoring the dynamic acoustic emissions or inelastic stress waves generated during the initiation and propagation of cracks. AET serves as a valuable non-destructive technique (NDT) for in-service evaluation of the structural integrity within operational conditions and early detection of critical flaws. By leveraging the time domain and time-frequency domain techniques, various Acoustic Emission (AE) parameters can be characterized and correlated with the multi-stage crack damage phenomena. Further theories of the SCC mechanisms are elucidated, with a focus on both the dissolution-based and cleavage-based damage models. Through the comprehensive insights provided here, this review stands to contribute to an enhanced understanding of SCC damage in stainless steels and the potential AET application in nuclear industry.
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