影响腐蚀的结构因素课件

第二章影响腐蚀的结构因素第三节异种金属组合因素1第二章影响腐蚀的结构因素第三节异种金属组合因素1Dissimilarmetalcorrosionresultsfromthecouplingoftwodifferentmetalsinacorrosiveelectrolyte.Itisalsocalledbimetalliccorrosionorgalvaniccorrosion.Whenagalvaniccoupleforms,oneofthemetalsinthecouplebecomestheanodeandcorrodesfasterthanitwouldallbyitself,whiletheotherbecomesthecathodeandcorrodesslowerthanitwouldalone.异种金属彼此接触或通过其他导体连通，处于同一个介质中，会造成接触部位的局部腐蚀。其中电位较低的金属，溶解速度增大，电位较高的金属，溶解速度反而减小，这种腐蚀称为电偶腐蚀，或称接触腐蚀、双金属腐蚀。

2DissimilarmetalcorrosionresThisisoneofthemostcommonformsofcorrosionaswellasoneofthemostdestructive.Here’saclassicexampleofgalvaniccorrosion;astainlessscrewincontactwithacadmiumplatedsteelwasher.电偶腐蚀是最常见的腐蚀形式之一，也是最具破坏性的腐蚀之一。图为不锈钢螺丝与镀镉垫片接触，发生电偶腐蚀。3Thisisoneofthemostcommon一、电偶腐蚀原理从混合电位的角度分析电偶腐蚀电池形成原理，可以清楚地理解电偶腐蚀过程。4一、电偶腐蚀原理从混合电位的角度分析电偶腐蚀电池形成原理，可lgi+Eicorr1icEcicorr1’icorr2’icorr2总氧化反应总还原反应偶接后：M2为阳极，M1为阴极，电偶电流从M1流向M2。M2发生阳极极化，腐蚀电流从icorr2增加到icorr2’；M1发生阴极极化。腐蚀电流从icorr1降到icorr1’。阳极体金属腐蚀速度增加的效应，称为接触腐蚀效应；阴极体金属腐蚀速度减小的效应，称为阴极保护效应。电偶腐蚀电池形成原理5lgi+Eicorr1icEcicorr1’icorr2’一、电偶腐蚀原理Thefigureshowstheratedeterminationwhenathirdelectrodeprocessisaddedatapotentialbetweenthefirsttwoelectrodereactions.TherulethatmustbeappliedisthattheTOTALOXIDATIONRATEMUSTEQUALTHETOTALREDUCTIONRATE.Thedashedlinesrepresentthetotalrates.下图表示当电极电位处于原两个电极的电极电位之间的第三个电极加入系统中时的腐蚀速率的确定。所应用的规则是总氧化反应速率必然等于总还原反应速率。6一、电偶腐蚀原理ThefigureshowstheCorrosionratedeterminationforathreeelectrodesystem7CorrosionratedeterminationfThecorrosionrateforelectrode2hasincreasedfromicorrtoicorr1+2asitistheonlyanodicreaction.Thisisthecasewhenthecorrosionpotentialforthreeelectrodesisabovethetwoelectrodepotential.Theresultingcorrosionpotentialismorenegativethanthethirdelectrodereversepotential.电极2的腐蚀速率从icorr增加到icorr1+2，因为它是唯一的阳极。这是三电极的混合电位正于双电极的腐蚀电位的情况。最终系统的腐蚀电位负于外加的第三个电极的腐蚀电位。8ThecorrosionrateforelectroAssuchitcanonlycontributetothecathodicreactionrate.Thethirdelectrodeisthereforeprotectedfromcorrosion.Thesecondelectrodedissolutionrateincreasedsignificantlybytheintroductionofthethirdelectrodeprocesses.这种情况只能加速第三个电极的阴极反应的速率。第三个电极因而得到保护。而第二个电极的溶解速率由于第三个电极的加入而大大增加。9AssuchitcanonlycontributeThefigureshowsthatintroducingamoreanodicmetalwilldecreasethecorrosionrateinamorenoblemetal.Thisistheprocessbehindgalvaniccorrosion.Itcanalsobeusedforprotectionbygalvanizing.该图表明，阳极性金属的加入将减缓贵金属的腐蚀速率。这就是电偶腐蚀的原理。它也可以用来解释电镀保护。10Thefigureshowsthatintroduc二、电偶腐蚀影响因素

FactorsAffectingGalvanicCorrosion1.AreaEffectWhencurrentflowsbetweentheanodeandcathode,theCURRENTwillbethesameintheanodeandcathodeindependentofthesurfaceareaofeachelectrode.ItistheCURRENTratherthantheCURRENTDENSITYwhichisequalfortheanodicandcathodicreactions.当电流流过阴阳极时，阴阳极电流应是相等的，而与面积无关。要注意是电流而不是电流密度与阴、阳极的反应速率相等。11二、电偶腐蚀影响因素

FactorsAffectingGTherefore,ifthecurrentflowingbetweentheanodeandthecathodeisoneampandthesurfaceareasareonecm2,thenthecurrentdensityineachelectrodeisoneA/cm2.However,iftheareaoftheanodeisonly0.1cm2,thenthecurrentdensityintheanodewiththesameoneampflowingis10A/cm2.ThecorrosionratedependsontheCURRENTDENSITYintheanode.Inthiscasedecreasingthesurfaceareaoftheanodeincreasesthecorrosionratebyafactorof10.12Therefore,ifthecurrentflowTheanodetocathodeareaeffectisanimportantcharacteristic.Itisimportantinseveralotherformsofcorrosionincludingpittingcorrosion,crevicecorrosion,stresscorrosioncrackingandcorrosionfatigue.Remembertheimportanteffectsoftheanodetocathodearearatios.阴、阳极的面积比是一个非常重要的影响因素。在其他类型的腐蚀中，如点蚀、缝隙腐蚀、应力腐蚀破裂和腐蚀疲劳中，阴、阳极的面积比也十分重要。13Theanodetocathodeareaeffe2.DistanceEffectAnotherimportantfactorforgalvaniccorrosionisthedistanceeffect.Galvaniccorrosionratesarethelargestattheinterface.Asthedistancebetweentheanodicreactionsiteandthecathodereactionsiteincreasesthetransportoftheionsbecomesmoredifficultandthecorrosionratedecreases.电偶腐蚀的另一个重要因素是距离。电偶腐蚀速率在阴、阳极界面处最大，离阴、阳极反应界面越远，离子的迁移越困难，腐蚀速率越小。142.DistanceEffect14三、防止电偶腐蚀的途径

PreventionofGalvanicCorrosion

1.ElectricallyInsulatetheMetalsElectricalinsulationofthedissimilarmetalswillavoidgalvaniccorrosion.2.MaterialsSelectionTrytoavoidselectingdissimilarmetals.3.Designa)Designforthecheapestreplacement.b)Insealedsystemsusetheareaeffect.15三、防止电偶腐蚀的途径

PreventionofGal法兰连接的正确绝缘加中间金属的结构16法兰连接的正确绝缘加中间金属的结构163.DesignSeveralfeaturescanbedesignedintominimizetheeffectsofgalvaniccorrosion:a)Designforthecheapestreplacement.Allowcorrosiontooccurinacomponentthatcanbereplacedwithoutlossofproductioncapability.Thismaymeansomeredundant（附加）systemssuchasmultiplevalvesandpipesinonearea.However,thiswouldallowonepipesystemtobeusedasabypass（旁路）whilethecorrodedpipefromthefirstsystemisreplacedwithoutlossofproduction.

173.Design17Afterthesecondpipeexhibitscorrosionthenusethefirstpipesystemandreplacethesecondpipe.Thisiscorrosioncontrolratherthanprevention.However,thereisnoplantdowntime（停工检修）andthecorrodedcomponentisapieceofpiperatherthananexpensiveanddifficulttoreplacevalve.18Afterthesecondpipeexhibitsb)Insealedsystemsusetheareaeffect.Inasealedsystemwherethesupplyofdissolvedoxygenforthecathodicreactionislimitedandeasilyexpended,ifalargeanodetosmallcathodeareaisdesignedthenonlyaverysmalldegreeofcorrosionoverthelargeanodewilloccurbeforetheoxygenconsumed.Thennomorecorrosionwilloccur.在封闭系统中，当阴极氧的供应不足，氧气很快耗尽的情况下，设计成大阳极小阴极的结构，可以使氧耗尽时阳极的腐蚀仍很轻微。当氧气耗尽后，腐蚀将不再发生。19b)InsealedsystemsusetheaIfabrassfittingisplacedonawatertankfroaclosedsystemwherethesteelsurfaceareaofthetankismuchlargerthebrassarea,thenthesteelwillcorrodeslightlyuntilalltheoxygenisusedupandthenitwillcease.如将黄铜构件置于通往密闭系统的水槽上，水槽的钢制表面面积比黄铜的面积大得多，则钢将遭受轻微腐蚀，直至氧气耗尽后，腐蚀将不再继续。20Ifabrassfittingisplacedo4Coatings涂层Paintthecathodetopreventthecathodicreaction.

5.SacrificialProtection牺牲阳极保护Theadditionofasacrificialelementwillavoidcorrosion.

6.ImpressedCurrentProtection外加电流保护Ifelectronsaresuppliedtotheanodethenitwillnotcorrodedespitethegalvaniccontactwithamorenoblermetal.7.Inhibitors缓蚀剂214Coatings涂层21GalvanicCorrosion-QuestionsWhichisbetter,asteelbolt（螺栓）

inabrassstructure,orabrassboltinasteelstructure?Thesteelwillbetheanode,soasmallsteelboltinalargebrassstructurewillsufferseveregalvaniccorrosion.Abrassboltwillbeprotectedbythesteelstructure.22GalvanicCorrosion-QuestionsWhichisbetter?BrassboltinasteelstructureSteelboltinabrassstructureSmallbrasscathodewillcausesmallincreaseincorrosionofsteelstructure.BoltwillbeprotectedfromcorrosionbycouplingtosteelSmallsteelanodewillsufferlargeincreaseincorrosionduetocouplingwithbrassstructure.

23Whichisbetter?BrassboltinWhatshouldwepaint?Amilkcoolerhasaninnerlinerofstainlesssteel,andanoutercasingofmildsteel.Chilledbrine（冷盐水）

isusedbetweenthesteelandstainlesssteeltocoolthemilk.Corrosionofthemildsteelisoccurring-woulditbeagoodideatopainttheinsideofthemildsteelcasing?Thiswouldbeaverybadideaasanypin-holesinthepaintwouldactastheanode,withthefullareaofthestainlesssteelasthecathode.24Whatshouldwepaint?AmilkcoBeneficialApplicationsMosthouseholdsilveris