AbstractThedepositsalloWmoreNiPK1.1.74thetheirofpropertiesThelurandfectiallomagnesiummagnesiumordinarycanmagnesiumshould0925-8388/$doi:10.1016/j.jallcom.2004.07.083JournalofAlloysandCompounds391(2005)104109ElectrolessNiPplatingonAZ91DmagnesiumalloyfromasulfatesolutionChangdongGu,JiansheLian,GuangyuLi,LiyuanNiu,ZhonghaoJiangTheKeyLabofAutomobileMaterials,MinistryofEducation,CollegeofMaterialsScienceandEngineering,JilinUniversity,NanlingCampus,Changchun130025,ChinaReceived14May2004；accepted9July2004Availableonline8December2004DirectelectrolessNiPplatingontheAZ91Dmagnesiumalloyfromaplatingbathcontainingsulfatenickelwasinvestigatedinthepaper.nucleationmechanismofNiPdepositsontheAZ91DmagnesiumalloywasstudiedbyusingXRDandSEM.TheelectrolessNiPwerepreferentiallynucleatedontheH9252-Mg17Al12phaseandextendedtotheprimaryandeutecticH9251phasesoftheAZ91Dmagnesiumy.TheeffectoftheacidpickletreatmenttimeoftheAZ91DmagnesiumalloysubstrateontheelectrolessNiPplatingwasalsoinvestigated.ithprolongingtheacidpickletreatmenttime,theclusterofelectrolessNiPdepositionbecamesmallerandthedepositswerefoundtobecompact.ThedepositionrateofelectrolessNiPplatingwasproportionaltotheacidpickletreatmenttime.Thehardnessvaluesofthecoatingswereabout660VHNandwerenotinfluencedbythepretreatments.2004ElsevierB.V.Allrightsreserved.eywords:Surfacesandinterfaces；Electrochemicalreaction；Metals；X-raydiffractionIntroductionMagnesiumisoneofthelightestmetalswithadensityofg/cm3andmagnesiumalloyshavebeenusedwildlyinaerospace,electronicsandautomobilefieldsbecauseofhighstrength-to-weightratio.However,theapplicationmagnesiumalloyshasbeenlimitedduetotheirundesirableincludingpoorcorrosionandlowwearresistance.corrosionofmagnesiumalloysdependsontheirmetal-gyandenvironmentalfactors.Metallurgicalmanipulationsomeothercreativesynthesistechniquesprovideanef-vewaytoimprovethecorrosionresistanceofmagnesiumys1,2.Theothereffectivewaytopreventcorrosionofalloyistocoatthesubstratematerial3.Sinceisoneofthemostelectrochemicallyactivemetal,coatings,suchasnickel,copperandzinccoatings,onlyprovideaphysicalbarriertocorrosionattackofsubstrate.So,anycoatingsonmagnesiumalloysbeasuniform,adheredandpore-freeaspossible.Correspondingauthor.Fax:+864315095876.E-mailaddress:(Z.Jiang).hasnumerouspropertieswearductilitytionwithouteasofplatingthatmagnesiumnearlypapersiumsulfandalloinseefrontmatter2004ElsevierB.V.Allrightsreserved.ElectrolessdepositiontechniqueofNiPalloycoatingsbeenawell-knowncommercialprocessthathasfoundapplicationsinmanyfieldsduetoexcellentofcoatings,suchashighcorrosion-resistant,high-resistant,goodlubricity,highhardnessandacceptable47.Anotheradvantageoftheelectrolessdeposi-techniqueisthatveryuniformedcoatingscanbeobtainedspecialrequirementsforsubstrategeometries.How-ver,electrolessnickelplatingondifficultsubstrates,suchmagnesiumalloys,hasmanychallengesintheprocessingplatingandthereisverylimitedreportsonelectrolessonmagnesiumalloys.Furthermore,itcanbenotedinmanypreviousreportsontheelectrolessplatingonalloys811,thenickelionswereprovidedbybasicnickelcarbonateintheplatingbath.Inthis,directelectrolessNiPplatingontheAZ91Dmagne-alloywasundertakenbyusingaplatingbathcontainingatenickel.ThenucleationmechanismofNiPdepositstheeffectoftheacidpickletreatmenttimeofmagnesiumysubstratesontheelectrolessNiPplatingwerealsovestigated.C.Guetal./JournalofAlloysandCompounds2.ExperimentalThesubstratematerialusedwasAZ91Ddiecastmag-nesiumalloywithasizeof30mm40mm5mm.ThechemicalcompositionofthealloyisgiveninTable1.ThesampleswereabradedwithNo.1500SiCpaperbeforethepretreatmentprocesses.Thetechnicalflowchartoftheelec-trolessdepositionontheAZ91DmagnesiumalloyisshowninFig.1.Thesampleswerecleanedthoroughlywithde-ionizedwtreatments.allocroscopethephologywanalysiscontentstructuretometerTChemicalAlZnMnFeMgFig.magnesiumaratenessesdepositiontestertionallothe24010as-depositednotheasRwhered(cm3.3.1.andideaterasquicklyaspossiblebetweenanytwostepsofthepre-Theacidpickletreatmenttimeofthemagnesiumywaschangeableinthisstudy.Ascanningelectronmi-(SEM,JEOLJSM-5310,Japan)wasemployedforobservationsofthesurfaceandthecross-sectionmor-oftheNiPalloycoatingsandanEDXattachmentasusedforqualitativeelementalchemicalanalysis.EDXwasalsousedforthedeterminationofphosphorusoftheelectrolessNiPdeposition.CrystallographicofthesamplewasstudiedbytheX-raydiffrac-(XRD,RigakuDymax,Japan)withaCutargetandable1compositionoftheAZ91Ddiecastmagnesiumalloy(wt.%)9.1(wt.%)0.64(ppm)0.17(ppm)0.01Balance1.ThetechnicalflowchartofelectrolessNiPplatingontheAZ91Dalloy.haThus,gettinglotrolessXiangorersafterledlayercleaning.signed(b)40coarsermicrostructureprimaryandthethetoallotheBypretreatmenton391(2005)104109105monochromatorat50kVand300mAwiththescanningandstepbeing4/minand0.02,respectively.Thehard-ofthemagnesiumalloybeforeandafterelectrolesswereevaluatedusingaHXD-1000microhardnesswithVickersindenter,ataloadof100gandadura-of15s.ThedepositionrateRoftheelectrolessNiPycanbeexpressedintermsoftheweightgainduringdepositionprocess.AnanalyticalbalanceMETTLERAE(METTLER-TOLEDO,Switzerland)withaprecisiontoH9262gwasemployedtoweightheas-depositedsamples.ThesamplewasdriedandweighedrepeatedlyuntilfurtherchangeinthereadingtoensurecompletedryingofNiPalloy.Accordingly,thedepositionrateisexpressedthefollows12,13:=104dAt(1)Risthedepositionrate(H9262m/h),theweightgain(g),thedensityofthedeposit,Athesurfaceareaofdeposition2)andtthedepositiontime(h).ResultsanddiscussionFunctionsofindividualtreatmentsMagnesiumanditsalloyshavehighlychemicalactivitywhentheycontactwithairorwater,oxideandhydrox-filmswouldbeformedonthesurface1,14,whichwouldveadetrimentaleffectoncoatingadhesionanduniformity.thepre-treatingprocessesplayaveryimportantroleinagoodprotectivecoatingonmagnesiumanditsal-ys.Astudyonsurfacestateduringthepretreatmentofelec-nickelplatingonmagnesiumalloyswasreportedbyetal.15.Alkalinecleaningwasusedtoremovesoilsgreasesonthesurfaceofmagnesiumalloy.Butsuchclean-arenotsuitableforremovingoxideandthelike16.So,alkalinecleaning,thesamplesshouldbeetchedorpick-inachromicacid/nitricacidsolutiontoremoveanyoxideorotherchemicalcoatingsnotfullyremovedbyalkalineInthispaper,theacidpickletreatmenttimewasde-as0,5,10,20,40and60s,respectively.Fig.2(a)andgivethemorphologyofacidpickletreatmentafter10ands.ItcanbeseenfromFig.2thatthetreatedsurfacebecamewithprolongingtheacidpickletreatmenttime.TheoftheAZ91DmagnesiumalloyconsistedofH9251-MggrainssurroundedbyaeutecticmixtureofH9251-H9252-Mg17Al1217.Mg17Al12iscathodicwithrespecttomatrix,resultinginanon-uniformsurfacepotentialacrosssubstrate1.So,thenextfluorideactivationtreatmentiscreateanequipotentializedfilm(MgF2)onthemagnesiumysurface.Itisanindispensablestepofpretreatmentinprocessingofelectrolessplatingonmagnesiumalloys.XPSanalysis,Xiangetal.11indicatedthataftertheoffluorideactivation,thefluoridefilm(MgF2)surfaceofmagnesiumalloywasdiscontinuous.According106andCompoundsFig.pickletothansurfogywithoutsurfterFig.dif(seethanmaytionandmetal-reducingcompletiontobath3.2.thetreatmentdepositstreatmenttheaciddepositsC.Guetal./JournalofAlloys2.ThemorphologiesoftheAZ91Dmagnesiumalloyafterpretreatments:(a)40sfollowedbyfluorideactivation.Ref.9,thetimeoffluorideactivationshouldnotbeless10minsoastoformthecompletelyMgF2filmsontheaceofthemagnesiumalloy.Fig.2(c)showsthemorphol-oftheAZ91Dmagnesiumalloyafterfluorideactivationacidpickletreatment.Itcanbeseenthatthesubstrateacewasslightlycorrodedafterthefluorideactivation.Af-acidpickle40sfollowingbythefluorideactivation(see2(d),thesubstratesurfacedoesnotshowsignificantlyferencefromthatwiththeacidpickletreatmentof40sFig.2(b).Iftheacidpickletreatmenttimewaslarger60s,thesubstratewouldbeseverelycorroded,whichinfluencethepracticalusageofthemagnesiumalloy.DuringtheprocessofelectrolessNiPplating,thefunc-ofsulfatenickel(NiSO46H2O)istoprovidenickelions,thesodiumhypophosphite(Na2H2PO2H2O)actsasaagent.Sodiumacetate(NaC2H3O2)isthexingagentofthenickelions.Thioureaactsasasolu-stabilizerandtheammoniasolution(NH3H2O)isusedadjustPHoftheplatingbath.Thefluorideintheplatingistoinhibitcorrosionofthesubstrateduringplating.EffectofacidpickletimeThemorphologiesofelectrolessNiPdepositsafter1honAZ91DmagnesiumalloyasafunctionoftheacidpickletimeareshowninFig.3.ItcanbeseenthattheNiPontheAZ91Dmagnesiumalloywithoutacidpickle(seeFig.3(a)showedmorebumpsandloosethansampleswithacidpickletreatment.Withprolongingthepickletreatmenttime,theclusteroftheelectrolessNiPbecamesmallerandtheNiPdepositswerefoundtoposit12takratesEq.depositionItacidtreatmentwfwNiPmorphologyingbethepicklemechanicalaandtointhewillFig.NiP1theFig.391(2005)104109acidpickle10s,(b)acidpickle40s,(c)fluorideactivation,and(d)acidbemorecompact.ThephosphoruscontentintheNiPde-wasabout3.72wt.%byEDXanalysis.Accordingto,thecorrespondingdensityoftheNiPdepositcanbeenas8.5g/cm3.Bysungthisdensityvalue,thedepositionoftheelectrolessNiPplatingweredeterminedfrom(1).TheeffectoftheacidpickletreatmenttimeontherateofelectrolessNiPplatingisshowninFig.4.canbeseenthatthedepositionratewasproportionaltothepickletreatmenttime.Withprolongingoftheacidpickletime,thesubstratewasmuchmorecorroded,whichouldprovidemorechemicallyactiveonthesubstratesur-acefortheelectrolessplating.Furthermore,theacidpicklingouldresultinbettermechanicaladhesionoftheelectrolessplatingonthemagnesiumalloy3,9.Fig.5showstheofthecross-sectionofelectrolessNiPcoat-ontheAZ91Dmagnesiumalloy.FromFig.5(a),itcanseenthattheelectrolessNiPcoatingwasembeddedinmagnesiumalloysubstrateattheinterface.So,theacidpretreatmentprovidessurfacepitstoactassitesforinterlockingtoimproveadhesion3.Incontrast,distinctinterfacewithoutinterlockingbetweenthecoatingthesubstratewasobservedforthesample(Fig.5(b)duelackoftheacidpickletreatment.However,someporesthecoatingsinFig.5mayresultfromtheevaluationofhydrogenduringtheelectrolessdeposition.Furtherworkbedonebythepresentauthorstoeliminatethesepores.6givesthequalitativechemicalanalysisofelectrolessplatingontheAZ91DmagnesiumalloyafterplatinghbyEDXanalysis.Fromtheelementsdistributingfromcoatingsurfacetothesubstratealongthelinelabeledin6,itcanbeseenthatthecoatingwasconnectedcloselytoFig.(d)theofandAZ91DFig.theC.Guetal./JournalofAlloysandCompounds3.ThemorphologiesofelectrolessNiPdepositionontheAZ91Dmagnesium20s,(e)40sand(f)60s.substrate.Thehardnesstestsindicatedthatthehardness