镁合金,LPSO

ThisexplanationisconsistentwiththeOCPresultsthattheMDZ-Calloytakeslongertimetoreachingbalancepotentialanditspoten-tialfluctuatessignificantlyduetotheruptureofoxidationfilm.Ontheotherhand,theoxidationfilmremediationabilityalsoplaysanimportantroleincorrosionproperties.Itiswell-knownthattheoxidefilmonthesurfaceofMgalloysisnotcompact,andthenthesolutioncanpenetratethroughtheoxidationlayer.Takingintoaccountthereleaseofhydrogenduringcorrosionprocess,theoxidationfilmwillbebrokendownwithextendingimmersiontime.ThisisthereasonforthecontinuousdecompositionofMgalloys25,28.Therefore,inviewoftheformationfreshsurface,theoxidationfilmremediationabilityiscrucialtodeterminecorrosionrate.BasedonEISresults(Fig.9),thecapacitiveresistanceisreducedwithincreas-ingimmersiontestintheMDZ-Calloy,whileitisenhancedintheMDZ-545onemonotonously.Themainreasonliesinthedifferentoxidationfilmremediationability(Fig.10).AfterEIStesting,thepristineoxidationfilmisbrokendownandtheoxidationfilmreme-diationoccursimmediatelyintheMDZ-545alloy(Fig.10d-10f).Anewcompactoxidationfilmformsinthecracks,whicheffectivelyprohibitsthecontactbetweensolutionmediaandthematrix.Itisconsistentwiththeimmersiontest,whereMDZ-Ctakesmoretime(Fig.5d)toformacompleteoxidationfilm.Thisstrategyofimprov-ingcorrosionpropertiesbyformationofseparatedlayerwasemployedinMg-AlalloybySonget.al.28,29.

Theoretically,thesurfacefilmonMgalloysismainlycomposedofaqueoussolutionmagnesiumoxideormagnesiumhydroxide.Thecorrosionreactionsinneutralaqueousmediaarepresentedbythefollowingequations30–33.

Mg?Mg2zz2e{2H2Oz2e{?H2z2OH{

Figure11|XRDpatternsofalloysbeforeandafter12hofimmersionin0.9wt.%NaClsolution,(a)MDZ-C;(b)MDZ-545.

ð2Þð3Þð4Þð5Þ

Mg2zz2OH{<Mg(OH)2Dy3zz3OH{?Dy(OH)3

LPSOphasegenerateswhenitissolid-solutionheattreatedat545uC.Bothmorphologyanddistributionof14H-LPSOphasearedifferentfromthoseof18R-LPSOphase.Thewidthof14H-LPSOphaseisreducedanditismainlydistributedinsidegrains.TheseoccurrencesareconfirmedbyDSCresult,togetherwiththeSEMandTEMresults.Itisbelievedthatthe18R-typestructureisnotthermodyna-micallystableat/above500uC.Itisbecausethatitcontainsfoura-Mgatomicfaultlayerswhichprovideasimpletransitionalpathwaytoform14H-typeLPSO13,whichisconsistentwiththethermalcalcula-tionsofLPSOphasetransformations7.

AccordingtotheaboveSEMandEISresults,itcanbeconfirmedthatthecorrosionmechanismsaredifferentbetweentheMDZ-C(18R-LPSO)andMDZ-545(14H-LPSO)alloys.Specifically,thedetailedschematicdiagramsforcorrosionprocessexplanationwerepresentedinFig.12.IncontrasttotheMDZ-C(18R-LPSO)alloy,theMDZ-545(14H-LPSO)onepossesseslowercorrosionrate.Therearetwomainfactorsresponsibleforthedecreaseofcorrosionrate.Ontheonehand,differentfromtheMDZ-Csample,itisfaciletoformahomogeneousandcompactoxidationfilmattheinitialcorrosionstageintheMDZ-545one(Fig.5andFig.6),whichisascribedtodifferentformingsequencesofoxidationfilms.FortheMDZ-Calloy,theoxidefilmgeneratesonthesurfaceofMgmatrixfirstandthen

expandstowardthesecondaryphase.Thus,thethicknessoffilmisdifferent,whichresultsintensilestress27.Withincreasingtheimmer-siontime,thetensilestressisincreased,resultingintheruptureoftheoxidationfilmcontainingcorrosionproductsintheinterfacebetween18R-LPSOphaseandMgmatrix.Asaresult,theformedcracksprovidechannelsforthecorrosivesolutiontopenetratethroughandreachthefreshsurface,aswellasforhydrogentorelease.Consequently,thecorrosionprocessoccurssequentially.

However,thefollowingreactionpossiblyoccurswhentheenvir-onmentcontainschlorideion:

Mg2zz2H2Oz2Cl{?2Mg(OH)2Cl2?MgCl2z2H2Oð6ÞAccordingtocorrosionproducts,itcanbeconfirmedthatequation5ismorefaciletoproceedintheMDZ-CalloythanintheMDZ-545one.Chlorideionsareveryaggressivetomagnesium.ChlorideionsintheinterfacetransformMg(OH)2toeasilysolubleMgCl2.Asaresult,thethermodynamicalequilibrium(equation3)isreadilybro-kendownduetotheexistenceofchlorideions,whichdeterioratesthecorrosionpropertiesofthealloys5.ThiscorrosionexplanationagreeswellwiththecurrentopinionthatthecorrosionresistanceofMgalloysisdeterminedbyapartiallyprotectivesurfacefilmwherethecorrosionreactionoccursprincipallyatthebreaksofthefilm.Theseresultsarealsoconsistentwiththetendencyofchlorideioncausingfilmbreakdown,whichacceleratesthecorrosionofMgalloy34.However,thedirectevidencethatthechlorideioniseasiertobeabsorbedonthesurfaceofalloycontaining18R-LPSOphaseisnotdemonstratedinthisworkandfurtherresearchworksarenecessarytoclarifytheirdiscrepancies.

Insummary,paredwiththealloycontaining14H-LPSOphase,itispronetoformsomecracksintheinterfacesbetween18R-LPSOandMgmatrixatthebeginningofcorrosion,whichismostlyrelatedtotheinhomogeneous