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CorrosionissuesfornuclearreactormaterialsKeywordsforthispart•corrosion;gereralcorrosion;localcorrosion;pitting;crevicecorrosion;galvaniccorrosionSCC;•Whatarethemaincorrosionenvironmentfornuclearreactormaterials?Corrosion:theenvironmentaldegradationofmaterialsThedisintegrationofanmaterialintoitsconstituentatomsduetochemicalreactionswithitssurroundings.Erosionofthecarbonsteelreactorhead,causedbyapersistentleakofboratedwater.2002年3月6日，Davis-BesseUSA•Accordingtocorrosionmechanism:•Physicalcorrosion:physicalsolutioninduced•Chemicalcorrosion:chemicalreactioninduced.Oxidation•酸性气体腐蚀：H2S+FeFeS+H2•在800℃以上高温下进行迅速nMOOnM=+22•金属氧化的热力学判据–金属被氧化的化学腐蚀表现为氧化膜逐渐增厚，体积不断变化，但温度和压力不变。–根据热力学第二定律，从等温等压的吉布斯（Gibbs）自由能的△G值可判断金属是否自发氧化。即•△G＜0时，氧化反应能自发进行，•△G＞0时则不能，•△G＝0时为平衡状态。2222O//OOO11lnlnlnTPGRTRTRTPPPΔ=−+=R为普适气体常数，R=1.198717Cal/K.mol；PO2为标准氧分压P'O2为体系中的氧分压当环境体系氧分压P'O2大于氧化物分解压PO2时，氧化反应自发进行•Electrochemicalcorrosion:chemicalreactioninvolvingthetransferofelectronsatthemetalsurfaceComparationofchemicalcorrosionandelectrochemicalcorrosion•Chemicalcorrosion：金属表面与环境组分直接发生纯化学反应而引起的破坏（nocurrent）•Electrochemicalcorrosion：金属表面与环境组分(电解质溶液)发生电化学作用而产生的破坏。（Twoelectrodes，current）ZnH2SO4ZnPt•uniformcorrosion(generalCorrosion)•Uniformcorrosionischaracterizedbycorrosiveattackproceedingevenlyovertheentiresurfacearea,oralargefractionofthetotalarea.•Generalthinningtakesplaceuntilfailure.)/gtSWW2010hm⋅（－＝失重ν)/gtSWW2002hm⋅（－＝增重ν匀腐蚀速率的表示方法：均匀腐蚀速率－失重或增重zCreviceorcontactcorrosionzThecorrosionproducedattheregionofcontactofmetalswithmetalsornonmetals.Schematicillustrationofthemechanismofcrevicecorrosionbetweentworivetedsheets.Mostsensitivecontactwidthis0.025－0.15mm。Meansofpreventingorlimitingcrevicecorrosionredesignofequipmenttoeliminatecrevices•Useweldsratherthanboltedorrivetedjoints（不用铆接或螺栓连接）•designinstallationstoenablecompletedraining(nocornersorstagnantzones)不存在死角滞留区•Hydrofuge（防水）anyintersticesthatcannotbeeliminated•inparticular,greaseallsealsandsealplanes,useonlysolid,non-porousseals,etc.•theproperuseofsealants,andprotectivecoatingsareeffectivemeansofcontrollingthisproblem.Molybdenum-containinggradesofstainlesssteel(e.g.316and316L)haveincreasedcrevicecorrosionresistance.zIntergranularcorrosionzanlocalizedattackonoradjacenttotheGBofametaloralloy.•Thiscorrosionisusuallyassociatedwithchemicalsegregationeffects(impuritieshaveatendencytobeenrichedatgrainboundaries)orspecificphasesprecipitatedonthegrainboundaries.Suchprecipitationcanproducezonesofreducedcorrosionresistanceintheimmediatevicinity.•Aclassicexampleisthesensitizationofstainlesssteels.•Cr-richgrainboundaryprecipitatesleadtoalocaldepletionofCrimmediatelyadjacenttotheseprecipitates,leavingtheseareastobecorrodedattackincertainelectrolytes.1Cr18Ni9Ti—450～880℃—(Fe、Cr)23C6—Cr12%GBengineering•Inaseverecaseofgrain-boundarycorrosion,entiregrainsmaybedislodgedduetocompletedeteriorationoftheirboundaries.Inanycasethemechanicalpropertiesofthestructurewillbeseriouslyaffected.Pittingcorrosion,orpittingAformofextremelylocalizedcorrosionleadstothecreationofsmallholesinthemetal.表面孔很小，内部很大Pittingcanleadtounexpectedcatastrophicsystemfailure.Thesplittubing(aboveleft)wascausedbypittingofSS.Aboverightshowsatypicalpitonthistubing.•Thedrivingpowerforpittingisthedepassivationofasmallarea,whichbecomesanodicwhileanunknownbutpotentiallyvastareabecomescathodic,leadingtoverylocalizedgalvaniccorrosion.•10to100timesthatbygeneralcorrosion（均匀腐蚀的10~100倍）•Asmall,narrowpitwithminimaloverallmetallosscanleadtothefailureofanentireengineeringsystem.很窄小的孔造成重量损失很小，却可以导致整个工程系统的失效•verydifficulttodetect,predictanddesign难以发现、预测、设计•Pittingisconsideredtobemoredangerousthanuniformcorrosiondamagebecauseitismoredifficulttodetect,predictandprevent.比均匀腐蚀更危险PittingResistanceEquivalentNumberisanindexusedtomeasureandcompareresistancelevelofametal(alloy)topittingcorrosion.PREN=%Cr+m.(%Mo)+n.(%N)ForferriticgradesStainlessSteel,theformulaemployedis:PRE=%Cr+3.3(%Mo)ForausteniticgradesStainlessSteel,theformulaemployedis:PREN=%Cr+3.3(%Mo)+30(%N)ForInconel625,Hastelloy,etc,theformulaemployedis:PREN=%Cr+1.5(%Mo+%W+%Nb)•MinimumPRENrequiredformaterialexposetoseawateris40.–金属表面不均匀性使钝化膜某些部位较为薄弱，被击穿–晶界(特别是有硫化物析出时)、晶格缺陷(如位错露头处)–非金属夹杂物处，尤其是硫化物，如FeS、MnSGalvanicCorrosionzanelectrochemicalactionoftwodissimilarmetalsinthepresenceofanelectrolyteandanelectronconductivepath.•GalvanicCompatibility•Managedbyfinishesandplating.•Forharshenvironments(highhumidity,saltenvironments).Typicallythereshouldbenotmorethan0.15VdifferenceintheAnodicIndex.•Fornormalenvironments(storageinwarehousesornon-temperatureandhumiditycontrolledenvironments).Typicallythereshouldnotbemorethan0.25Vdifference.•Forcontrolledenvironments(temperatureandhumiditycontrolled)0.50Vcanbetolerated.electromotiveforceEachmetaloralloyhasauniquecorrosionpotential,Ecorr,whenimmersedinacorrosiveelectrolyte.全面腐蚀与局部腐蚀比较无保护作用可能有保护作用腐蚀产物损失量小，很难检测其腐蚀速率，往往导致突然的腐蚀事故造成大量损失，可以检测和预测，一般不会造成突然事故危害阳极电位小于阴极电位阴极电位＝阳极电位电位阴极面积》阳极面积阴极面积＝阳极面积电极面积阴阳极在微观上可分阴阳极变换不定，阴阳极不可辨腐蚀电池破坏主要集中在一定区域，其它区域不腐蚀腐蚀分布在整个表面形貌局部腐蚀全面腐蚀特征StressCorrosionCracking(SCC)•Stre