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EnvironmentallyAssistedCracking
(CorrosionEngineering)
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EnvironmentallyAssistedCrackingLectureOutline Stresscorrosioncracking
CrackInitiation CrackGrowth MetallurgicalEffects ElectrochemicalEffects TypicalSCCMaterials
HydrogenInducedCracking SourcesofHydrogen
CorrosionFatigue TestMethodstoexamineEAC FractureMechanics MechanismsforEAC
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TypesofCorrosion
Uniform Galvaniccorrosion Crevicecorrosion Pittingcorrosion Intergranularcorrosion Selectiveleaching Erosioncorrosionandfretting Environmentallyinducedcracking Hydrogendamage
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EnvironmentallyInducedCrackingDefinition
Brittlemechanicalfailuresthatresultsfromthesynergismbetweentensilestressandacorrosiveenvironment Stresscorrosioncracking(SCC) CorrosionFatigue Hydrogeninducedcracking MetalInducedEmbrittlement
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StressCorrosionCracking
Stresscorrosioncrackingistypicallydefinedasafailurethatoccurswhenyouhaveasusceptiblematerialthatissimultaneouslyexposedtoarelativelylowconstanttensilestressinanaggressiveenvironment
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ExampleofSCC
Surfacecracksonstainlesssteelexposedtoseawater
Intergranularandtransgranularcrackson
stainlesssteelexposedtoseawater
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SCCTextbook
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BasicSCCphenomenology(CrackInitiation)
Crackinitiationdoesnotnecessaryoccurallthetimefromsmoothsurface Machiningmarks Fatiguecracks
Crackscaninitiatefromcorrosionpitsorcrevicecorrosion,orintergranularcorrosion
Thetransitiondependsupontheoriginaldefectshape
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BasicSCCphenomenology(CrackGrowth)
Typicallythereisalackofmacroscopicdeformationandabsenceofgeneralcorrosion
Crackbranchingandsecondarycracksaretypical
IntergranularfracturesurfacesproducedbySCCaresmooth
Transgranularexhibitnumerousfinescalefeaturedsuchasserratedsteps
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SCC MetallurgicalEffects Puremetalstendtobeless
susceptibletoSCCthanalloys,butcanstillundergocracking Copperinammonia
AlloycompositionandmicrostructurehavealargeeffectonSCCresistance Especiallyaroundgrain
boundaries Generaltrendisthat
increasingSCCsusceptibilitywithincreasingstrength
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SCC MetallurgicalEffects
Grainboundarymorphology(andsize)canhaveamajoreffectonSCCsusceptibility ElongatedgrainboundarycanleadtoreducedSCCresistance
Coldworkcanalsoaffectthesusceptibilityofmaterials
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SCCElectrochemicalEffects
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SCCElectrochemicalEffectsExamplesP.MScott.2000
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TypicalSCCMaterialEnvironmentCombinations(StainlessSteel)
Proseketal.2009
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TypicalSCCMaterialEnvironmentCombinations(StainlessSteel)
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FerricstainlesssteelsaregenerallymoreresistanttoSCCthanausteniticSS,howeverwhensmallamountsofnickelisaddedtheycanbecomesusceptible
DuplexstainlesssteelaregenerallyalsomoreresistanttoSCCcomparedtotheausteniticstainlesssteels.However,theyarenottotallyimmune
TypicalSCCMaterialEnvironmentCombinations(StainlessSteel)
AlShamarietal.2009
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TypicalSCCMaterialEnvironmentCombinations(CarbonSteel)
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TypicalSCCMaterialEnvironmentCombinations(NickelAlloys)
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TypicalSCCMaterialEnvironmentCombinations(CopperAlloys)
Wetaeratedenvironmentscontainingsmallquantitiesofammonium,nitrites,andnitratesledtoSCCincopper SCCinitiatedatshallow
pitswherethepassivelayerofCuOhadbeendestroyed
Brasscartridgecasesin1920crackedfromhighresidualstressesandtraceamountsofammonia
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TypicalSCCMaterialEnvironmentCombinations(Aluminum)
Extensivelyusedinaircraft Mostcommonalloystocrackare2xxx,7xxx,and5xxxseries
Susceptibletohumidair,seawater,andpotablewater
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TypicalSCCMaterialEnvironmentCombinations(Titanium)
Titanium(Ti)isverycorrosionresistant
Tiusuallysuffersfromenvironmentallyinducedcrackingfromhydrogenembrittlement
Formationofhydridesembrittlethematerial
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HydrogenInducedCracking
Damagingeffectsfromhydrogenisaphenomenaknownashydrogenembrittlementorhydrogeninducedcracking
HICisrelevanttoSCCashydrogenisgeneratedatcracktipsinaqueousenvironments
Numeroustypesofhydrogenembrittlementprocesses Concentrationofhydrogeninregionsofthemetal Hydrideformation
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SourcesofHydrogen Solutionsusedtocleanandapplyprotectivecoatings
Weldinginmoistatmospheres Heattreatmentinhydrogenbearingatmospheres
Cathodicprotection Aqueoushydrogensulfide(OilandGasIndustry)
Corrosionprocess Highpressuregaseoushydrogen(fuel)
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HydrogenInducedCracking
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CorrosionFatigueCracking
CorrosionfatigueissimilartoSCCinthattheenvironmentenhancesthecrackgrowth
Crackscangrowbycyclingofcrackfront
ASMEcodeisbasedonfatiguecrackgrowthinair needtoapplysomeenvironmentalfactor
Argonne.2011
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CorrosionFatigueCracking
Crackgrowthratepercycletypicallyincreaseswithlowerfrequency
Increasingtheratio,R,ofminimumtothemaximumstresstendstodecreasethecrackgrowthpercycle
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TestingMethods
ConstantDeformationTests SustainedLoadTests SlowStrainRateTesting CorrosionFatigueTesting
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ConstantDeformationTests Ubendtestsareusedto
measurecrackinitiationsusceptibility Largestressandstrainmuch
greaterthanyieldstress CRingandbentbeamareused
tomeasurecrackinitiationsusceptibility Canapplymuchlessstressand
measuredappliedstressstrain(straingauges)
Techniqueusedependsuponformofmaterial(i.e.plateorpipe)
Tensilespecimensallowgreatestcontrolofappliedstressstrainbecauseoftheknowngeometry
Mainproblemisthatwhencracksgrow,appliedloaddrops
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ConstantLoadTests SustainedLoadTestsalso
examinecrackinitiation CanusespringloadedCring ProvingRingsaretypically
usedwithtensilesamples Deadweightloadshavebeen
usedbutmuchmorebulksodifficulttouseexceptforsimplesolutiontests.
DifferencebetweenConstantLoadvs.ConstantDeformationisthatduringConstantLoadtests,stressincreaseswithcrackgrowth
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SlowStrainRateTests SlowStrainRate(SSR)testingor
constantelongationratetensile(CERT)testingisusedtoexamineSCCsusceptibilitybetweenalloysorenvironments
Strainrateusedwillprovidedifferentresultssoneedtochoosestrainratecarefully Minimizetimefortestandprovide
accuratesusceptibilitycharacterization
Typicallyexamineparametersinenvironmentdividedbyparametersinair Closerthevalueisto1,less
susceptibletoSCC TimetoFailure Ductility Reductioninarea Yieldstrength %SCConfracturesurface
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TestingMethods
FatiguePrecracksampleinairandthenexposetoenvironmentofinterest
Applycyclicload R=Smin/Smax Rcangofrom1to1
Comparecrackgrowthrateinenvironmenttoinair
Examinefracturesurfaceforenvironmentaleffects
WhatwouldanRof1mean?
WhatwouldanRof1mean?
WhatwouldanRof0mean?
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FractureMechanicsTesting
TypicaltestingisconductedusingmodeI KIc isthevaluetypicallyconsideredbecauseitislowerthanothermodes
CrackswillgrowbelowKIC whenitissusceptibletoSCC
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MechanismsofSCC
HydrogenEmbrittlement AdsorptionInducedCleavage FilmRupture FilmInducedCleavage
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HydrogenEmbrittlement
Hydrogenweakensinteratomicbondsandfacilitatesdecohesion
Hydrogenfacilitatesdislocationactivityintheplasticzoneaheadofcrackpromotingcrackgrowthbyslipmicrovoidcoalescence
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AdsorptionInducedCleavage
Similartohydrogeninductedcrackingthisisspeculatedforaggressiveions
Aggressiveionsweakenbondingbetweenadjacentatomsatcracktip.
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FilmruptureorSlipDissolution Mostwidelycited
mechanism Basedonruptureof
protectivefilmbyslipbandsthatintersectcracktip
Dissolutionalonggrainboundariesorlowindexcrystalplanes
Filmmayreformandprocessstartsagainorsurfacemaycontinuetobefilmfree
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FilminducedCleavage Filminducedcleavage
includesrepeatedsequencesof: formationofan
environmentallyinducedbrittlefilmatcracktip
Rapidbrittlefractureofthefilm
Continuationofthefractureintotheunderlyingsubstrate/metal
Crackarrestandblunting
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LectureReview Stresscorrosioncrackingisthefailuremodethatoccurswhen
youhaveasusceptiblematerial,inanaggressiveenvironment,withlowconstanttensileload Crackinitiationcanoccuratmachinemarks,fatiguecracks,pits,or
intergranularcorrosion Crackgrowthcanoccurinvariousformsincludingintergranularfracture
andtransgranularfracture Metallurgicaleffectscanincreasesusceptibility(weldedregionstendto
beweakpoints) Materialstendtobesusceptibleinvariouselectrochemicalpotential
ranges(outsiderangenocrackingmayoccur) HydrogenInducedCrackingoccurswhenhydrogenembrittle
materialthatleadstofailureatlowerstresses Environmentleadstoanenhancedeffectoncorrosionfatigue
crackgrowthrate
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LectureReview
VarioustestmethodstoexamineEACincludingconstantdeformation,sustainedloads,slowstrainrate,andcorrosionfatigue
Environmenttypicallydecreasesthestressintensityvaluewherecrackswillbegintopropagate
MechanismsforEACincludehydrogenembrittlement,adsorptioninducedcleavage,filmrupture,andfilminducedcleavage
