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