金属在某些环境中的腐蚀课件

第三章金属在某些环境中的腐蚀第一节在高温气体中的腐蚀HighTemperatureCorrosion1第三章金属在某些环境中的腐蚀第一节在高温气体中的腐蚀1Hightemperaturecorrosionisaformofcorrosionthatdoesnotrequirethepresenceofaliquidelectrolyte.Sometimes,thistypeofdamageiscalled“drycorrosion”or“scaling”.高温腐蚀是在无液相电解质存在下的腐蚀，也称干腐蚀或脱皮。1.Introduction2HightemperaturecorrosionisThetermoxidationisambivalentsinceitcaneitherrefertotheformationofoxidesortothemechanismofoxidationofametal,i.e.itschangetoahighervalencethanthemetallicstate.“氧化”有双重含义，狭义为生成氧化物，广义为化合价的升高。3ThetermoxidationisambivaleStrictlyspeaking,hightemperatureoxidationisonlyonetypeofhightemperaturecorrosion.Infact,oxidationisthemostimportanthightemperaturecorrosionreaction.严格说来，高温氧化只是高温腐蚀的一种。4Strictlyspeaking,hightemperInmostcorrosivehightemperatureenvironments,oxidationoftenparticipatesinthehightemperaturecorrosionreactions,regardlessofthepredominantmodeofcorrosion.绝大多数高温环境下的腐蚀中都有氧的参与。5InmostcorrosivehightemperaAlloysoftenrelyupontheoxidationreactiontodevelopaprotectivescaletoresistcorrosionattacksuchassulfidation,carburizationandotherformsofhightemperatureattack.合金通常通过高温腐蚀如硫化和渗碳等在表面生成保护性膜。6AlloysoftenrelyupontheoxiIngeneral,thenamesofthecorrosionmechanismsaredeterminedbythemostabundantdominantcorrosionproducts.通常按最主要的腐蚀机理命名高温腐蚀。7Ingeneral,thenamesofthecForexample:Oxidationimpliesoxides,Sulfidationimpliessulfides,Sulfidation/oxidationimpliessulfidesplusoxides,andCarburizationimpliescarbides.8Forexample:Oxidationimplies2.ThermodynamicPrinciplesItispossibletouseplotsofthefreeenergyofformationofmetaloxidesvs.temperaturetopredictthetemperaturesatwhichametalisstableandthetemperaturesatwhichitwillspontaneouslyoxidize.Fortemperaturesatwhichthefreeenergyofformationoftheoxideispositive,thereversereactionisfavoredandtheoxidewillspontaneouslydecomposetothemetal.92.ThermodynamicPrinciplesIt1010pO2>pMeO，G<0，oxidization。pO2=pMeO，G=0，equilibrium。pO2<pMeO，G>0，decomposition。Foragivenreaction(Me+½O2=MeO),thefollowingequationcanbeusedtoexpresstheoxygenpartialpressureatwhichthemetalandoxidecoexist,i.e.thedissociationpressureoftheoxide:11pO2>pMeO，G<0，oxidization。F3.THEPROCESSOFMETALOXIDATION123.THEPROCESSOFMETALOXIDATI1313ProtectiveAbilityofOxideFilms

Thedegreetowhichanoxidefilmisprotectivetoametaldependsuponmanyfactors,ofwhichthefollowingareimportant:Thevolumeratio（体积比）ofoxidetometalafteroxidationshouldbecloseto1:1.Thefilmshouldhavegoodadherence.Themeltingpointoftheoxideshouldbehigh.14ProtectiveAbilityofOxideFiTheoxidefilmshouldhavealowvaporpressure.Theoxidefilmshouldhaveacoefficientofexpansion（膨胀系数）nearlyequaltothatofthemetal.Thefilmshouldhavehigh-temperatureplasticitytopreventfracture.Thefilmshouldhavelowconductivityandlowdiffusioncoefficientsformetalionsandoxygen.15TheoxidefilmshouldhavealThecalculationofthevolumeratioofoxidetometalafteroxidationisafirststepthatcanbetakentofindoutifanoxideofametalmightbeprotective.ThisratioiscalledthePilling-Bedworth(P.B.)ratioandcanbeexpressedinequationformas16ThecalculationofthevolumeIfametalhasaP.B.ratiooflessthan1,asisthecaseforthealkalimetals(forexample,NahasaP.B.ratioof0.576),themetaloxidewillbeporousandunprotective.IftheP.B.ratioismorethan1,asisthecaseforFe(Fe2O3hasP.B.ratioof2.15),compressivestresseswillbepresentandtheoxidewilltendtocrackandspalloff.17IfametalhasaP.B.ratioofIftheP.B.ratioiscloseto1,theoxidemaybeprotective,butsomeoftheotherfactorspreviouslylistedhavetobesatisfied.Thus,theP.B.ratioalonedoesnotdetermineifanoxideisprotective.18IftheP.B.ratioisclosetoProtectiveoxidesNonprotectiveoxidesBe

1.59K

0.45Cu

1.68Ag

1.59Al

1.28Cd

1.21Cr

1.99Ti

1.95Mn

1.79Mo

3.40Fe

1.77Hf

2.61Co

1.99Sb

2.35Ni

1.52W

3.40Pd

1.60Ta

2.33Pb

1.40U

3.05Ce

1.16V

3.18P.B.RatioofMetals19ProtectiveoxidesNonprotectiv4.MechanismsofOxidation

Whenanoxidefilmformsonametalbytheoxidationofametalbygaseousoxygen,itisformedbyanelectrochemicalprocess.Theoxidationandreductionpartialreactionsfortheformationofdivalentionsare:204.MechanismsofOxidationWhenIntheveryearlystagesofoxidation,theoxidelayerisdiscontinuousandbeginsbythelateralextensionofdiscreteoxidenuclei.Afterthenucleiinterlace,themasstransportoftheionsoccursinadirectionnormal(法线)tothesurface.21IntheveryearlystagesofoxMarkersburiedM+OxideOxidedriftOxidee-MetalMetalZoneofMetalConsumptionGasZoneofOxideFormationcationdiffuse——Themetaldiffusesascationsand

electronsacrosstheoxidefilm.Inthismechanismtheoxygenisreducedtooxygenionsattheoxide-gasinterface,andthezoneofoxideformationisatthissurface.(a)Whencationsdiffuse,theinitiallyformedoxidedriftstowardthemetal.22MarkersburiedM+OxideOxidedriMetalConsumptionGasO2-OxideOxidedriftOxidee-MetalMetalMarkersremainonsurfaceOxideFormationaniondiffusion——forsomeheavymetaloxides,theoxygenmaydiffuseasO2-ionstothemetal-oxideinterfaceandelectronsdiffusetotheoxide-gasinterface.Inthiscasetheoxideformsatthemetal-oxideinterface.(b)Whenanionsdiffuse,theoxidedriftsintheoppositedirection.23MetalConsumptionGasO2-OxideOxThesignificantdissolutionofoxygenatomsinsomemetals,thehighvolatilityofsomeoxidesandmetals,thelowmeltingpointsofsomeoxides,andgrainboundariesinthemetaloftencomplicatetheoxidationmechanismsofpuremetals.24Thesignificantdissolutionof5.DamageoftheoxidefilmThebreakawayofthefilmisinconnectionwiththecompressivestressesdevelopedintheoxidescales.255.DamageoftheoxidefilmDamageoftheoxidefilmMeMeMeMeMeMe（a）未破裂的空泡（b）破裂的空泡（c）气体不能透过的微泡（d）剥落（e）切口裂开（f）在角和棱边上裂开26DamageoftheoxidefilmMeMeMe6.Kinetics

Threebasickineticlawshavebeenusedtocharacterizetheoxidationratesofpuremetals.Itisimportanttobearinmindthattheselawsarebasedonrelativelysimpleoxidationmodels.Practicaloxidationproblemsusuallyinvolvealloysandconsiderablymorecomplicatedoxidationmechanismsandscalepropertiesthanconsideredinthesesimpleanalyses.276.Kinetics

ThreebasickinetTimeLogarithmicLinearParabolicWeightgainedperunitarea28TimeLogarithmicLinearParabolicTheparabolic(抛物线)ratelawassumesthatthediffusionofmetalcationsoroxygenanionsistheratecontrollingstepandisderivedfromFick'sfirstlawofdiffusion.Theconcentrationsofdiffusingspeciesattheoxide-metalandoxide-gasinterfacesareassumedtobeconstant.Thediffusivityoftheoxidelayerisalsoassumedtobeinvariant.Thisassumptionimpliesthattheoxidelayerhastobeuniform,continuousandofthesinglephasetype.29Theparabolic(抛物线)ratelawasStrictlyspeaking,evenforpuremetals,thisassumptionisrarelyvalid.Therateconstant,kp,changeswithtemperatureaccordingtoanArrheniustyperelationship:x——theoxidefilmthickness(orthemassgainduetooxidation,whichisproportionaltotheoxidefilmthickness)t——timekp——therateconstant,directlyproportionaltothediffusivityoftheionicspeciesthatisratecontrollingx0——aconstant30Strictlyspeaking,evenforpuManymetalsoxidizeaccordingtotheparabolicratelaw,andtheseareusuallyassociatedwiththickcoherentoxides.Iron,copper,andcobaltareexamplesofmetalsthatshowparabolicoxidationbehavior.31ManymetalsoxidizeaccordingThelogarithmicratelawisanempiricalrelationshipwithnofundamentalunderlyingmechanism.Thislawismainlyapplicabletothinoxidelayersformedatrelativelylowtemperatures,andthereforerarelyapplicabletohightemperatureengineeringproblems:ke——therateconstantcandb——constants32ThelogarithmicratelawisanThelinearratelawisalsoanempiricalrelationshipthatisapplicabletotheformationandbuild-upofanon-protectiveoxidelayer.kL——therateconstant33ThelinearratelawisalsoanItisusuallytobeexpectedthattheoxidationratewilldecreasewithtime(parabolicbehavior),duetoanincreasingoxidethicknessactingasastrongerdiffusionbarrierwithtime.Inthelinearratelaw,thiseffectisnotapplicable,duetotheformationofahighlyporous,poorlyadherentorcrackednon-protectiveoxidelayers.Clearly,thelinearratelawishighlyundesirable.34ItisusuallytobeexpectedtPracticalHighTemperatureCorrosionProblems

Theoxidationratelawsdescribedabovearesimplemodelsderivedonthebehaviorofpuremetals.Incontrast,practicalhightemperaturecorrosionproblemsaremuchmorecomplexandinvolvetheuseofalloys.Forpracticalproblems,boththecorrosiveenvironmentandthehightemperaturecorrosionmechanism(s)havetobeunderstood.35PracticalHighTemperatureCorHigh-TemperatureMaterials

Themechanicalandphysicalpropertiesnecessaryordesiredforagoodhigh-temperaturemetaloralloyare:Goodroom-temperaturemechanicalpropertiesAdequatehigh-temperaturemechanicalproperties

Structuralstabilityathightemperatures

36High-TemperatureMaterialsTheOxidationResistanceForrelativelylow-performancehigh-temperatureoxidationservice——iron-nickel-chromiumalloys.Iron-,cobalt-,andnickel-basealloyspossesssatisfactoryoxidationresistanceattemperaturesabovethosewheremechanicalpropertiesbecomeinadequate.Forvery-high-temperatureservice,molybdenumandotherhigh-meltingmetalssuchastungsten,columbium铌,andtantalum.

37OxidationResistance37HydrogenDamageHydrogencancauseanumberofcorrosionproblems.Hydrogenembrittlementisaproblemwithhigh-strengthsteels,titanium,andsomeothermetals.Controlisbyeliminatinghydrogenfromtheenvironmentorbytheuseofresistantalloys.38HydrogenDamageHydrogencancHydrogendamagemaybeclassifiedintofourdistincttypes:Hydrogenblistering氢鼓泡Hydrogenembrittlement氢脆Decarburization脱碳Hydrogenattack氢侵蚀39HydrogendamagemaybeclassifHydrogenblisteringHydrogenblisteringcanoccurwhenhydrogenenterssteelasaresultofthereductionreactiononametalcathode.Single-atomnascent初生的hydrogenatomsthendiffusethroughthemetaluntiltheymeetwithanotheratom,usuallyatinclusions夹杂物ordefects缺陷inthemetal.40HydrogenblisteringHydrogenblTheresultantdiatomichydrogenmoleculesarethentoobigtomigrateandbecometrapped.Eventuallyagasblisterbuildsupandmaysplitthemetalasshowninthepicturebelow.41TheresultantdiatomichydrogeHydrogenblisteringiscontrolledbyminimizingcorrosioninacidicenvironments.Itisnotaprobleminneutralorcausticenvironmentsorwithhigh-qualitysteelsthathavelowimpurityandinclusionlevels.42HydrogenblisteringiscontrolHydrogenEmbrittlementThisisatypeofdeteriorationwhichcanbelinkedtocorrosionandcorrosion-controlprocesses.Itinvolvestheingressofhydrogenintoacomponent,aneventthatcanseriouslyreducetheductilityandload-bearingcapacity,causecrackingandcatastrophicbrittlefailuresatstressesbelowtheyieldstressofsusceptiblematerials.43HydrogenEmbrittlementThisisSourcesofHydrogenmakingofsteelprocessingpartsweldingstorageorcontainmentofhydrogengashydrogenasacontaminantintheenvironmentthatisoftenaby-productofgeneralcorrosion(eg.nuclearindustry).44SourcesofHydrogenmakingofPreventionofhydrogenembrittlementcontroltheamountofresidualhydrogenControltheamountofhydrogenpickupinprocessingdevelopalloyswithimprovedresistancetohydrogenembrittlementdeveloplowornoembrittlementplatingorcoatingprocessesrestrictingtheamountofin-situ(inposition)hydrogenintroducedduringtheservicelifeofapart45PreventionofhydrogenembrittDecarburizationandHydrogenAttack

AtelevatedtemperaturesTheprimaryeffectofhydrogen—decarburization,removalofcarbonfromanalloy.C(Fe)+2H2=CH4

resultsin—areductionoftensilestrengthandanincreaseinductilityandcreeprate.

long-termexposuretohydrogenatelevatedtemperatures—steelstendtolosetheirstrength.

46DecarburizationandHydrogenAHydrogenSulfideAndSulfur-ContainingGases

Hydrogensulfidemayactasanoxidizingagentintheformationofsulfidescalesonmetalsubstancesathightemperatures.nickelandnickel-richalloysareusuallyrapidlyattackedinthepresentofhydrogensulfideandothersulfur-bearinggases.Iron-baseareoftenused;chromiumadditionsareingeneralbeneficial.47HydrogenSulfideAndSulfur-Co第三章金属在某些环境中的腐蚀第一节在高温气体中的腐蚀HighTemperatureCorrosion48第三章金属在某些环境中的腐蚀第一节在高温气体中的腐蚀1Hightemperaturecorrosionisaformofcorrosionthatdoesnotrequirethepresenceofaliquidelectrolyte.Sometimes,thistypeofdamageiscalled“drycorrosion”or“scaling”.高温腐蚀是在无液相电解质存在下的腐蚀，也称干腐蚀或脱皮。1.Introduction49HightemperaturecorrosionisThetermoxidationisambivalentsinceitcaneitherrefertotheformationofoxidesortothemechanismofoxidationofametal,i.e.itschangetoahighervalencethanthemetallicstate.“氧化”有双重含义，狭义为生成氧化物，广义为化合价的升高。50ThetermoxidationisambivaleStrictlyspeaking,hightemperatureoxidationisonlyonetypeofhightemperaturecorrosion.Infact,oxidationisthemostimportanthightemperaturecorrosionreaction.严格说来，高温氧化只是高温腐蚀的一种。51Strictlyspeaking,hightemperInmostcorrosivehightemperatureenvironments,oxidationoftenparticipatesinthehightemperaturecorrosionreactions,regardlessofthepredominantmodeofcorrosion.绝大多数高温环境下的腐蚀中都有氧的参与。52InmostcorrosivehightemperaAlloysoftenrelyupontheoxidationreactiontodevelopaprotectivescaletoresistcorrosionattacksuchassulfidation,carburizationandotherformsofhightemperatureattack.合金通常通过高温腐蚀如硫化和渗碳等在表面生成保护性膜。53AlloysoftenrelyupontheoxiIngeneral,thenamesofthecorrosionmechanismsaredeterminedbythemostabundantdominantcorrosionproducts.通常按最主要的腐蚀机理命名高温腐蚀。54Ingeneral,thenamesofthecForexample:Oxidationimpliesoxides,Sulfidationimpliessulfides,Sulfidation/oxidationimpliessulfidesplusoxides,andCarburizationimpliescarbides.55Forexample:Oxidationimplies2.ThermodynamicPrinciplesItispossibletouseplotsofthefreeenergyofformationofmetaloxidesvs.temperaturetopredictthetemperaturesatwhichametalisstableandthetemperaturesatwhichitwillspontaneouslyoxidize.Fortemperaturesatwhichthefreeenergyofformationoftheoxideispositive,thereversereactionisfavoredandtheoxidewillspontaneouslydecomposetothemetal.562.ThermodynamicPrinciplesIt5710pO2>pMeO，G<0，oxidization。pO2=pMeO，G=0，equilibrium。pO2<pMeO，G>0，decomposition。Foragivenreaction(Me+½O2=MeO),thefollowingequationcanbeusedtoexpresstheoxygenpartialpressureatwhichthemetalandoxidecoexist,i.e.thedissociationpressureoftheoxide:58pO2>pMeO，G<0，oxidization。F3.THEPROCESSOFMETALOXIDATION593.THEPROCESSOFMETALOXIDATI6013ProtectiveAbilityofOxideFilms

Thedegreetowhichanoxidefilmisprotectivetoametaldependsuponmanyfactors,ofwhichthefollowingareimportant:Thevolumeratio（体积比）ofoxidetometalafteroxidationshouldbecloseto1:1.Thefilmshouldhavegoodadherence.Themeltingpointoftheoxideshouldbehigh.61ProtectiveAbilityofOxideFiTheoxidefilmshouldhavealowvaporpressure.Theoxidefilmshouldhaveacoefficientofexpansion（膨胀系数）nearlyequaltothatofthemetal.Thefilmshouldhavehigh-temperatureplasticitytopreventfracture.Thefilmshouldhavelowconductivityandlowdiffusioncoefficientsformetalionsandoxygen.62TheoxidefilmshouldhavealThecalculationofthevolumeratioofoxidetometalafteroxidationisafirststepthatcanbetakentofindoutifanoxideofametalmightbeprotective.ThisratioiscalledthePilling-Bedworth(P.B.)ratioandcanbeexpressedinequationformas63ThecalculationofthevolumeIfametalhasaP.B.ratiooflessthan1,asisthecaseforthealkalimetals(forexample,NahasaP.B.ratioof0.576),themetaloxidewillbeporousandunprotective.IftheP.B.ratioismorethan1,asisthecaseforFe(Fe2O3hasP.B.ratioof2.15),compressivestresseswillbepresentandtheoxidewilltendtocrackandspalloff.64IfametalhasaP.B.ratioofIftheP.B.ratioiscloseto1,theoxidemaybeprotective,butsomeoftheotherfactorspreviouslylistedhavetobesatisfied.Thus,theP.B.ratioalonedoesnotdetermineifanoxideisprotective.65IftheP.B.ratioisclosetoProtectiveoxidesNonprotectiveoxidesBe

1.59K

0.45Cu

1.68Ag

1.59Al

1.28Cd

1.21Cr

1.99Ti

1.95Mn

1.79Mo

3.40Fe

1.77Hf

2.61Co

1.99Sb

2.35Ni

1.52W

3.40Pd

1.60Ta

2.33Pb

1.40U

3.05Ce

1.16V

3.18P.B.RatioofMetals66ProtectiveoxidesNonprotectiv4.MechanismsofOxidation

Whenanoxidefilmformsonametalbytheoxidationofametalbygaseousoxygen,itisformedbyanelectrochemicalprocess.Theoxidationandreductionpartialreactionsfortheformationofdivalentionsare:674.MechanismsofOxidationWhenIntheveryearlystagesofoxidation,theoxidelayerisdiscontinuousandbeginsbythelateralextensionofdiscreteoxidenuclei.Afterthenucleiinterlace,themasstransportoftheionsoccursinadirectionnormal(法线)tothesurface.68IntheveryearlystagesofoxMarkersburiedM+OxideOxidedriftOxidee-MetalMetalZoneofMetalConsumptionGasZoneofOxideFormationcationdiffuse——Themetaldiffusesascationsand

electronsacrosstheoxidefilm.Inthismechanismtheoxygenisreducedtooxygenionsattheoxide-gasinterface,andthezoneofoxideformationisatthissurface.(a)Whencationsdiffuse,theinitiallyformedoxidedriftstowardthemetal.69MarkersburiedM+OxideOxidedriMetalConsumptionGasO2-OxideOxidedriftOxidee-MetalMetalMarkersremainonsurfaceOxideFormationaniondiffusion——forsomeheavymetaloxides,theoxygenmaydiffuseasO2-ionstothemetal-oxideinterfaceandelectronsdiffusetotheoxide-gasinterface.Inthiscasetheoxideformsatthemetal-oxideinterface.(b)Whenanionsdiffuse,theoxidedriftsintheoppositedirection.70MetalConsumptionGasO2-OxideOxThesignificantdissolutionofoxygenatomsinsomemetals,thehighvolatilityofsomeoxidesandmetals,thelowmeltingpointsofsomeoxides,andgrainboundariesinthemetaloftencomplicatetheoxidationmechanismsofpuremetals.71Thesignificantdissolutionof5.DamageoftheoxidefilmThebreakawayofthefilmisinconnectionwiththecompressivestressesdevelopedintheoxidescales.725.DamageoftheoxidefilmDamageoftheoxidefilmMeMeMeMeMeMe（a）未破裂的空泡（b）破裂的空泡（c）气体不能透过的微泡（d）剥落（e）切口裂开（f）在角和棱边上裂开73DamageoftheoxidefilmMeMeMe6.Kinetics

Threebasickineticlawshavebeenusedtocharacterizetheoxidationratesofpuremetals.Itisimportanttobearinmindthattheselawsarebasedonrelativelysimpleoxidationmodels.Practicaloxidationproblemsusuallyinvolvealloysandconsiderablymorecomplicatedoxidationmechanismsandscalepropertiesthanconsideredinthesesimpleanalyses.746.Kinetics

ThreebasickinetTimeLogarithmicLinearParabolicWeightgainedperunitarea75TimeLogarithmicLinearParabolicTheparabolic(抛物线)ratelawassumesthatthediffusionofmetalcationsoroxygenanionsistheratecontrollingstepandisderivedfromFick'sfirstlawofdiffusion.Theconcentrationsofdiffusingspeciesattheoxide-metalandoxide-gasinterfacesareassumedtobeconstant.Thediffusivityoftheoxidelayerisalsoassumedtobeinvariant.Thisassumptionimpliesthattheoxidelayerhastobeuniform,continuousandofthesinglephasetype.76Theparabolic(抛物线)ratelawasStrictlyspeaking,evenforpuremetals,thisassumptionisrarelyvalid.Therateconstant,kp,changeswithtemperatureaccordingtoanArrheniustyperelationship:x——theoxidefilmthickness(orthemassgainduetooxidation,whichisproportionaltotheoxidefilmthickness)t——timekp——therateconstant,directlyproportionaltothediffusivityoftheionicspeciesthatisratecontrollingx0——aconstant77Strictlyspeaking,evenforpuManymetalsoxidizeaccordingtotheparabolicratelaw,andtheseareusuallyassociatedwiththickcoherentoxides.Iron,copper,andcobaltareexamplesofmetalsthatshowparabolicoxidationbehavior.78ManymetalsoxidizeaccordingThelogarithmicratelawisanempiricalrelationshipwithnofundamentalunderlyingmechanism.Thislawismainlyapplicabletothinoxidelayersformedatrelativelylowtemperatures,andthereforerarelyapplicabletohightemperatureengineeringproblems:ke——therateconstantcandb——constants79ThelogarithmicratelawisanThelinearratelawisalsoanempiricalrelationshipthatisapplicabletotheformationandbuild-upofanon-protectiveoxidelayer.kL——therateconstant80ThelinearratelawisalsoanItisusuallytobeexpectedthattheoxidationratewilldecreasewithtime(parabolicbehavior),duetoanincreasingoxidethicknessactingasastrongerdiffusionbarrierwithtime.Inthelinearratelaw,thiseffectisnotapplicable,duetotheformationofahighlyporous,poorlyadherentorcrackednon-protectiveoxidelayers.Clearly,thelinearratelawishighlyundesirable.81ItisusuallytobeexpectedtPracticalHighTemperatureCorrosionProblems

Theoxidationratelawsdescribedabovearesimplemodelsderivedonthebehaviorofpuremetals.Incontrast,practicalhightemperaturecorrosionproblemsaremuchmorecomplexandinvolvetheuseofalloys.Forpracticalproblems,boththecorrosiveenvironmentandthehightemperaturecorrosionmechanism(s)havetobeunderstood.82PracticalHighTemperatureCorHigh-TemperatureMaterials

Themechanicalandphysicalpropertiesnecessaryordesiredforagoodhigh-temperaturemetaloralloyare:Goodroom-temperaturemechanicalpropertiesAdequatehigh-temperaturemechanicalproperties

Structuralstabilityathightemperatures

83High-TemperatureMaterialsTheOxidationResistanceForrelativelylow-performancehigh-temperatureoxidationservice——iron-nickel-chromiumalloys.Iron-,cobalt-,andnickel-basealloyspossesssatisfactoryoxidationresistanceattemperaturesabovethosewheremechanicalpropertiesbecomeinadequate.Forvery-high-temperatureservice,molybdenumandotherhigh-meltingmetalssuchastungsten,columbium铌,andtantalum.

84OxidationResistance37HydrogenDamageHydrogencancauseanumberofcorrosionproblems.Hydrogenembrittlementisaproblemwithhigh-strengthsteels,titanium,andsomeothermetals.Controlisbyeliminatinghydrogenfromtheenvironmentorbytheuseofresistantalloys.85HydrogenDamageHydrogencancHydrogendamagemaybeclassifiedintofourdistincttypes:Hydrogenblistering氢鼓泡Hydrogenembrittlement氢脆Decarburizatio