影響腐蝕的結(jié)構(gòu)因素課件

第二章影響腐蝕的結(jié)構(gòu)因素第三節(jié)異種金屬組合因素1第二章影響腐蝕的結(jié)構(gòu)因素第三節(jié)異種金屬組合因素1Dissimilarmetalcorrosionresultsfromthecouplingoftwodifferentmetalsinacorrosiveelectrolyte.Itisalsocalledbimetalliccorrosionorgalvaniccorrosion.Whenagalvaniccoupleforms,oneofthemetalsinthecouplebecomestheanodeandcorrodesfasterthanitwouldallbyitself,whiletheotherbecomesthecathodeandcorrodesslowerthanitwouldalone.異種金屬彼此接觸或通過其他導(dǎo)體連通，處于同一個介質(zhì)中，會造成接觸部位的局部腐蝕。其中電位較低的金屬，溶解速度增大，電位較高的金屬，溶解速度反而減小，這種腐蝕稱為電偶腐蝕，或稱接觸腐蝕、雙金屬腐蝕。

2DissimilarmetalcorrosionresThisisoneofthemostcommonformsofcorrosionaswellasoneofthemostdestructive.Here’saclassicexampleofgalvaniccorrosion;astainlessscrewincontactwithacadmiumplatedsteelwasher.電偶腐蝕是最常見的腐蝕形式之一，也是最具破壞性的腐蝕之一。圖為不銹鋼螺絲與鍍鎘墊片接觸，發(fā)生電偶腐蝕。3Thisisoneofthemostcommon一、電偶腐蝕原理從混合電位的角度分析電偶腐蝕電池形成原理，可以清楚地理解電偶腐蝕過程。4一、電偶腐蝕原理從混合電位的角度分析電偶腐蝕電池形成原理，可lgi+Eicorr1icEcicorr1’icorr2’icorr2總氧化反應(yīng)總還原反應(yīng)偶接后：M2為陽極，M1為陰極，電偶電流從M1流向M2。M2發(fā)生陽極極化，腐蝕電流從icorr2增加到icorr2’；M1發(fā)生陰極極化。腐蝕電流從icorr1降到icorr1’。陽極體金屬腐蝕速度增加的效應(yīng)，稱為接觸腐蝕效應(yīng)；陰極體金屬腐蝕速度減小的效應(yīng)，稱為陰極保護(hù)效應(yīng)。電偶腐蝕電池形成原理5lgi+Eicorr1icEcicorr1’icorr2’一、電偶腐蝕原理Thefigureshowstheratedeterminationwhenathirdelectrodeprocessisaddedatapotentialbetweenthefirsttwoelectrodereactions.TherulethatmustbeappliedisthattheTOTALOXIDATIONRATEMUSTEQUALTHETOTALREDUCTIONRATE.Thedashedlinesrepresentthetotalrates.下圖表示當(dāng)電極電位處于原兩個電極的電極電位之間的第三個電極加入系統(tǒng)中時的腐蝕速率的確定。所應(yīng)用的規(guī)則是總氧化反應(yīng)速率必然等于總還原反應(yīng)速率。6一、電偶腐蝕原理ThefigureshowstheCorrosionratedeterminationforathreeelectrodesystem7CorrosionratedeterminationfThecorrosionrateforelectrode2hasincreasedfromicorrtoicorr1+2asitistheonlyanodicreaction.Thisisthecasewhenthecorrosionpotentialforthreeelectrodesisabovethetwoelectrodepotential.Theresultingcorrosionpotentialismorenegativethanthethirdelectrodereversepotential.電極2的腐蝕速率從icorr增加到icorr1+2，因?yàn)樗俏ㄒ坏年枠O。這是三電極的混合電位正于雙電極的腐蝕電位的情況。最終系統(tǒng)的腐蝕電位負(fù)于外加的第三個電極的腐蝕電位。8ThecorrosionrateforelectroAssuchitcanonlycontributetothecathodicreactionrate.Thethirdelectrodeisthereforeprotectedfromcorrosion.Thesecondelectrodedissolutionrateincreasedsignificantlybytheintroductionofthethirdelectrodeprocesses.這種情況只能加速第三個電極的陰極反應(yīng)的速率。第三個電極因而得到保護(hù)。而第二個電極的溶解速率由于第三個電極的加入而大大增加。9AssuchitcanonlycontributeThefigureshowsthatintroducingamoreanodicmetalwilldecreasethecorrosionrateinamorenoblemetal.Thisistheprocessbehindgalvaniccorrosion.Itcanalsobeusedforprotectionbygalvanizing.該圖表明，陽極性金屬的加入將減緩貴金屬的腐蝕速率。這就是電偶腐蝕的原理。它也可以用來解釋電鍍保護(hù)。10Thefigureshowsthatintroduc二、電偶腐蝕影響因素

FactorsAffectingGalvanicCorrosion1.AreaEffectWhencurrentflowsbetweentheanodeandcathode,theCURRENTwillbethesameintheanodeandcathodeindependentofthesurfaceareaofeachelectrode.ItistheCURRENTratherthantheCURRENTDENSITYwhichisequalfortheanodicandcathodicreactions.當(dāng)電流流過陰陽極時，陰陽極電流應(yīng)是相等的，而與面積無關(guān)。要注意是電流而不是電流密度與陰、陽極的反應(yīng)速率相等。11二、電偶腐蝕影響因素

FactorsAffectingGTherefore,ifthecurrentflowingbetweentheanodeandthecathodeisoneampandthesurfaceareasareonecm2,thenthecurrentdensityineachelectrodeisoneA/cm2.However,iftheareaoftheanodeisonly0.1cm2,thenthecurrentdensityintheanodewiththesameoneampflowingis10A/cm2.ThecorrosionratedependsontheCURRENTDENSITYintheanode.Inthiscasedecreasingthesurfaceareaoftheanodeincreasesthecorrosionratebyafactorof10.12Therefore,ifthecurrentflowTheanodetocathodeareaeffectisanimportantcharacteristic.Itisimportantinseveralotherformsofcorrosionincludingpittingcorrosion,crevicecorrosion,stresscorrosioncrackingandcorrosionfatigue.Remembertheimportanteffectsoftheanodetocathodearearatios.陰、陽極的面積比是一個非常重要的影響因素。在其他類型的腐蝕中，如點(diǎn)蝕、縫隙腐蝕、應(yīng)力腐蝕破裂和腐蝕疲勞中，陰、陽極的面積比也十分重要。13Theanodetocathodeareaeffe2.DistanceEffectAnotherimportantfactorforgalvaniccorrosionisthedistanceeffect.Galvaniccorrosionratesarethelargestattheinterface.Asthedistancebetweentheanodicreactionsiteandthecathodereactionsiteincreasesthetransportoftheionsbecomesmoredifficultandthecorrosionratedecreases.電偶腐蝕的另一個重要因素是距離。電偶腐蝕速率在陰、陽極界面處最大，離陰、陽極反應(yīng)界面越遠(yuǎn)，離子的遷移越困難，腐蝕速率越小。142.DistanceEffect14三、防止電偶腐蝕的途徑

PreventionofGalvanicCorrosion

1.ElectricallyInsulatetheMetalsElectricalinsulationofthedissimilarmetalswillavoidgalvaniccorrosion.2.MaterialsSelectionTrytoavoidselectingdissimilarmetals.3.Designa)Designforthecheapestreplacement.b)Insealedsystemsusetheareaeffect.15三、防止電偶腐蝕的途徑

PreventionofGal法蘭連接的正確絕緣加中間金屬的結(jié)構(gòu)16法蘭連接的正確絕緣加中間金屬的結(jié)構(gòu)163.DesignSeveralfeaturescanbedesignedintominimizetheeffectsofgalvaniccorrosion:a)Designforthecheapestreplacement.Allowcorrosiontooccurinacomponentthatcanbereplacedwithoutlossofproductioncapability.Thismaymeansomeredundant（附加）systemssuchasmultiplevalvesandpipesinonearea.However,thiswouldallowonepipesystemtobeusedasabypass（旁路）whilethecorrodedpipefromthefirstsystemisreplacedwithoutlossofproduction.

173.Design17Afterthesecondpipeexhibitscorrosionthenusethefirstpipesystemandreplacethesecondpipe.Thisiscorrosioncontrolratherthanprevention.However,thereisnoplantdowntime（停工檢修）andthecorrodedcomponentisapieceofpiperatherthananexpensiveanddifficulttoreplacevalve.18Afterthesecondpipeexhibitsb)Insealedsystemsusetheareaeffect.Inasealedsystemwherethesupplyofdissolvedoxygenforthecathodicreactionislimitedandeasilyexpended,ifalargeanodetosmallcathodeareaisdesignedthenonlyaverysmalldegreeofcorrosionoverthelargeanodewilloccurbeforetheoxygenconsumed.Thennomorecorrosionwilloccur.在封閉系統(tǒng)中，當(dāng)陰極氧的供應(yīng)不足，氧氣很快耗盡的情況下，設(shè)計(jì)成大陽極小陰極的結(jié)構(gòu)，可以使氧耗盡時陽極的腐蝕仍很輕微。當(dāng)氧氣耗盡后，腐蝕將不再發(fā)生。19b)InsealedsystemsusetheaIfabrassfittingisplacedonawatertankfroaclosedsystemwherethesteelsurfaceareaofthetankismuchlargerthebrassarea,thenthesteelwillcorrodeslightlyuntilalltheoxygenisusedupandthenitwillcease.如將黃銅構(gòu)件置于通往密閉系統(tǒng)的水槽上，水槽的鋼制表面面積比黃銅的面積大得多，則鋼將遭受輕微腐蝕，直至氧氣耗盡后，腐蝕將不再繼續(xù)。20Ifabrassfittingisplacedo4Coatings涂層Paintthecathodetopreventthecathodicreaction.

5.SacrificialProtection犧牲陽極保護(hù)Theadditionofasacrificialelementwillavoidcorrosion.

6.ImpressedCurrentProtection外加電流保護(hù)Ifelectronsaresuppliedtotheanodethenitwillnotcorrodedespitethegalvaniccontactwithamorenoblermetal.7.Inhibitors緩蝕劑214Coatings涂層21GalvanicCorrosion-QuestionsWhichisbetter,asteelbolt（螺栓）

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

23Whichisbetter?BrassboltinWhatshouldwepaint?Amilkcoolerhasaninnerlinerofstainlesssteel,andanoutercasingofmildsteel.Chilledbrine（冷鹽水）

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