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1、MaterialsTransactions,Vol.49,No.5(2008)pp.1057to1064SpecialIssueonPlatformScienceandTechnologyforAdvancedMagnesiumAlloys,IV#2008TheJapanInstituteofMetalsMechanismofCorrosionProtectionofAnodizedMagnesiumAlloysKojiMurakami1,MakotoHino1,KiyomichiNakai2,SengoKobayashi2,AtsushiSaijo3andTerutoKanadani41In
2、dustrialTechnologyResearchInstituteofOkayamaPrefecturalGovernment,Okayama701-1296,Japan2GraduateSchoolofScienceandEngineering,EhimeUniversity,Matsuyama790-8577,Japan3HoriMetalFinishingIndustry,Co.Ltd.,Takahashi716-0061,Japan4FacultyofEngineering,OkayamaUniversityofScience,Okayama700-0005,JapanCorros
3、ionresistanceofanodizedsurfacesonhigh-puritymagnesium(99.95mass%),rolledsheetsofASTMAZ31B(Mg-2.9Al-0.85Zn)magnesiumalloyanddie-castplatesofASTMAZ91D(Mg-9.1Al-0.75Zn)magnesiumalloyhasbeenstudied.Anodizationwasconductedbyenvironment-friendlyelectrolysiswhoseelectrolyteconsistsofphosphateandammoniumsal
4、t.Theanodizedsurfacewascoveredwithamorphousfilm,andshowedonlydiscolorationduringsaltspraytestwhereformationofcorrosionproduct(magnesiumhydroxide)waswellsuppressedwithin605ks.Evenwhentheanodizedsurfacesweretrenchedwithceramicknifetoformlocallyexposedsubstrate,corrosionwaswellsuppressedbyformationofnew
5、typeofdenseprotectivefilmsforeachsubstratewhichconsistofoxygen,magnesium,aluminumandphosphorus.Anodicpolarizationcurvesindicatethattheanodizedsurfacesshowsacrificialfunctionduetothethermodynamicallyunstablestateofphosphorusintheanodizedlayersanditsresultingreleaseofelectrons.Fromtheviewpointofkinetics
6、incorrosionontheanodzedsurfaces,thecurvesshowthattheanodizedlayersdissolvequiteslowlyintotheelectrolytecomparedwiththecaseoftheuntreatedsubstrates.Theexcellentcorrosionprotectivityobtainedbytheanodizationisconsideredtobebasedontheformationofadenseprotectivefilmontheexposedarea,aswellassacrificialfunct
7、ionoftheamorphousanodizedlayer.[doi:10.2320/matertrans.MC200718](ReceivedOctober5,2007;AcceptedMarch3,2008;PublishedApril16,2008)Keywords:magnesiumalloy,surfacetreatment,anodization,corrosionprotectio
