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1、3.051/BE.3401Lecture4BiomaterialsSurfaces:Chemistry(Continued)Inthecorrosionofmetallicimplants,furtherconsiderationsarisewiththecomplexityofbiologicalfluids.Invivoenvironment�pH7.4�T=37°C---�anions:Cl,PO3,HCO3(bicarbonate)++2+2+�cations:Na,K,Ca,Mgz+PittingandCreviceCorrosion:locali
2、zedattacksresultinginrapidMreleaseatdiscretesites�initiatedatlocalfailureofpassivefilmpitting:surfacedefect;crevice:regionofrestrictedtransport�remainingsurfaceactsascathode®rapidpenetrationatdefect(oxidation=reduction)-�catalyzedbypresenceofCl,whichreplacesboundwater�autocatalyzed
3、byacidityfromrxninconfinedvolumeImageremovedduetocopyrightconsiderations.PhotoofCoCrhipprosthesis,fromJ.J.Jacobsetal,“CorrosionofMetalOrthopaedicImplants”,J.BoneandJointSurgery80-A,268(1998).3.051/BE.3402Ex.StainlesssteelsinAqueousSoln:ForCrcontentsbelow12.5at%:FeOOHresidesatsurfac
4、einhydratedform:FeOx(OH)y·nH2O(ironoxyhydroxidehydrate)-InabsenceofCl,slowdissolution:3+FeOOH+H2O®Fe+3OH--WithClpresent:FeOOH+Cl®FeOCl+OH3+-FeOCl+2H2O®Fe+Cl+2OH3+LowsolubilityFeprecipitatesasFe(OH)3(rust)restrictedtransportoxideFe2O3S.S.(Fe,Cr)2O3Fe-9CrBreakdownofpassivationlayerun
5、tilreachingmetal:2+Fe®Fe+2eacceleratedattractsClcorrosion!2+-Fe+2Cl+2H2O®Fe(OH)2+2HCllocalacidityinpitorcrevice3.051/BE.3403Outsidepitorcrevice:2+3+-Fe®Fe+e3+-Fe+3(OH)®Fe(OH)3-O2+2H2O+4e®4OHprecipitates,furtherrestrictingtransportSteelcompositioneffects:Cr:ForCr>12.5at%:surfaceishy
6、dratedCrOOH(chromiumoxyhydroxide),i.e.,CrOx(OH)3-2x·nH2O�rendersS.S.surfacepassive�increasessusceptibilitytopitting/crevicecorrosion(CrCl3solutionshavelow/negativepH!)Mo:¯dissolutionrateinpits/crevices(mechanism?)Influenceofotherionsfoundinvivo:2+2+CaandMg:decreasecorrosionrate�for
7、mprecipitatesonmetalsurfaces+-(CO2+H2O®H+HCO3)2+--Ca+HCO3+OH®CaCO3+H2O2+-precipitatesMg+2OH®Mg(OH)2�O2influxrestricted®inhibitscathodicreduction3.051/BE.3404BiologicalFactorsInfluencingCorrosivity:·O2levels((O2Þcorrosion;anodicºcathodic)·pHvariations(inflammation/infectionÞ¯pH)·Cel
8、lularactivity(e.g.,phagocy
