镁合金的表面处理Surface treatment of magnesium alloy.docx

镁合金的表面处理Surface treatment of magnesium alloy.docx

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镁合金的表面处理Surfacetreatmentofmagnesiumalloy

镁合金的表面处理（Surfacetreatmentofmagnesiumalloy）

Surfacetreatmentofmagnesiumalloy

LiuZumingLebanon

Abstract:

thecorrosionmechanismandcorrosionprotectionofmagnesiumalloysarereviewed.Thestatus,progressandproblemsarereviewed.Thesurfacecharacteristics,surfacetreatmentprinciple,process,applicationexamplesandfuturedevelopmentdirectionofdiecastingmagnesiumalloyproductsaremainlyintroduced.

Keywords:

magnesiumalloycorrosionandprotectionsurfacetreatment

0Preface

WithlightandRecyclableformagnesiumalloyapplicationcharacteristics,hasbecometheidealmaterialformodernindustrialproducts,especiallytheapplicationofalargenumberofautoparts,telecommunicationsproductstothedevelopmentoflightandthinandshortandsmalldirectionofdemand,makesthedevelopmentofthisnewmaterialisextremelyoptimisticabouttheprospects.

Withthedevelopmentofmoderntechnology,hasbeenplaguedbymagnesiumalloyindustryrelatedtechnicalproblemssuchasdie-casting,recyclingproblemhasbeenresolved,afterthemajorprocessofmagnesiumalloyproducts-surfacetreatmenttechnologyhasmadeprogress,whichmakestheapplicationofmagnesiumalloyisconstantlyexpanding,consumptionhasincreasedrapidly,themagnesiumindustryisthegeometricspeedgrowth.

Propersurfacetreatmentcanmaketheproductprotectiveanddecorative,andcangivesomespecialfunctions.Foractivemagnesium,itisimportanttoobtainhighdecorativeandfunctionalpropertiesbysurfacetechnology,andimprovingitscorrosionresistanceisthemostimportantaspectatthisstage.

Corrosionandprotectionof1magnesiumalloy

Propertiesof1.1magnesium

MagnesiumelementsymbolMg,atomicnumber12,theelectronicstructureof2-8-2,anegativestandardelectrodepotential（-2.36V）,avolatileelectronicandoxidized,resultinginmagnesiumandmagnesiumalloycorrosionresistanceisverypoor,inthecorrosivemediumispronetoseverecorrosion.Theoxidefilmformedbymagnesiumalloyislooseandporous.ThedensitycoefficientofMgOandMg2+filmisabout0.8（<1）,whichhaspoorprotectionabilitytothematrix,andisnotsuitableformostcorrosiveenvironment.

Corrosionmodeof1.2magnesiumalloy

Thecorrosionofmagnesiumalloyusuallyhastwoways:

oneisthecorrosioninthegeneralenvironment,"saidgeneralcorrosion"or"corrosion",alsoknownasthe"corrosion",thetwoistheelectrochemicalcorrosionintheoriginalcellenvironment"".

Whenexposedtodryair,athinfilmisformedonthesurfaceofmagnesiumalloy,whichisstableintheabsenceofwatercontact,andtherewillbenocorrosionatthistime.Butintheactualoperatingenvironmentmaybeincontactwiththewaterorwatervapor,thenitwillleadtothefilm（surface）darkcolors（fromlightgraytodarkgray）.Iffurtherexposedtoliquidwaterandair,itwillcontinuetoreactwithCO2andH2Ointheairtoformcarbonates,whichmakethefilmthicker.Ingeneral,thefilmisstableenoughandhasaprotectiveeffect.Itdoesnotrequireanytreatmentandcanbeused.Forexample,theautobrakehousingdoesn'tneedextraprotectivemeasures.

WhenhighpurityAZ91Disused,noprotectivemeasuresarerequired.Evenifthecarcontactswiththesplashwater,itdoesnotneedtobeprotected,becausethefilmisstableinneutral,alkaline（orgeneralconditions）,andhasaprotectiveeffect.

However,ifthereiswaterintotheexistinginthedesign（orresidual）problem,orwhenthesaltintheprocessofusingtheenvironment（suchaswater）,itwillformamicrocellenvironment,causingelectrochemicalcorrosionismoreserious,includingwillhavesimilarcorrosioninneutralandalkalineenvironment.Atthispoint,agoodsurfacetreatmentprotectionisneeded.

Asfarasweknow,electrochemicalcorrosionisthemainmodeofcorrosionofmagnesiumalloy.Inneutralandalkalineenvironment,theprincipleofelectrochemicalcorrosionisasfollows（Figure1）:

Schematicdiagramofelectrochemicalcorrosion

Thechemicalreactionprincipleisasfollows:

Mg-2e=Mg2+（anodicreaction）

2H2O+2e=H2+OH-（cathodereaction）

Mg2++2OH-=Mg（OH）2（corrosionproduct）

ThetotalreactionwasMg+2H2O=Mg（OH）2+H2

Intheabovereaction,magnesiumiscorrodedasananodebylosingelectrons,andelectronsarereleasedfromthecathodetoproducehydrogen.Ingeneral,thecathodemaybeothermetalpartsincontactwiththemagnesiumalloy,

Theymayalsobealloyelementsorimpurityelements,whichformgalvaniccellwithmagnesiumalloymatrix,induceelectrodereactionandproduceelectrochemicalcorrosion.

Corrosionmorphologyof1.3magnesiumalloy

Thecorrosionpatternsofmagnesiumalloysaremainlyasfollows:

（1）galvaniccorrosion:

Thisisthemosteasilyoccurringcorrosionpatternofmagnesiumalloys.Severecorrosionisusuallyobservedinthelocalareasadjacenttothecathodeandthemagnesiummatrix.

（two）pittingcorrosion:

inthenonoxidizingmediumcontainingCl,thepittingcorrosionofmagnesiumalloyoccursattheselfcorrosionpotential;inneutraloralkalinemedium,thecorrosionmorphologyofmagnesiumalloyisusuallypittingcorrosion.

（three）,stresscorrosioninneutralsolutioncontainingClordistilledwater,magnesiumalloyshaveatendencytocorrosioncracking;inalkalinemedium,whenPH>10.2magnesiumalloyshowsgoodabilitytoresiststresscorrosion;influoridesolution,magnesiumalloyhasgoodantistresstheperformanceofstresscorrosion.

（four）intergranularcorrosion:

magnesiumalloysaregenerallyinsensitivetointergranularcorrosion,andcorrosioncannotdevelopalonggrainboundaries.

（five）filiformcorrosion:

undertheprotectiveorganiccoatingoranodicoxidationfilm,thecorrosionofthebatteryonthesurfaceofmagnesiumalloyleadstofiliformcorrosion.

CorrosionComparisonbetween1.4magnesiumalloyandothermaterials

In1980s,thedevelopmentofmagnesiumalloymaterialshasmadegreatprogress.ItisfoundthattheimpurityelementssuchasFe,Ni,CuandtheratioofFetoMnhaveagreatinfluenceonthecorrosionresistanceofmagnesiumalloys.Therefore,anewgenerationofhighpuritydiecastingmagnesiumalloyAZ91Dhasbeendeveloped.Thecorrosionresistanceofhighpuritymagnesiumalloywasincreasedby100times,whichisbeyondtheothercompetingmaterials,withouttheprotectionofthestatecanmeetmostoftherequirementsofautomotiveapplications,andevenexposedinthewinterroadcondition.Thetechnicalbreakthroughisveryimportantforthedevelopmentofdiecastingmagnesiumalloy,anditisalsoanimportantreasonthatmagnesiumalloycanbewidelyapplied.Intermsofsaltspraycorrosionresistance,mostofthediecastingmagnesiumalloybrandshaveexceededthealuminumalloyA380（asshowninFiguretwo）.

Corrosionprotectiontechnologyof1.5magnesiumalloy

Inviewofthecorrosionmechanismofmagnesiumalloy,theresearchworkofcorrosionprotectiontechnologyofmagnesiumalloyismainlyfocusedonthefollowingaspects

1.5.1partdesign（asshowninFigurethree）

Agoodprotectivemeasureshouldstartwithagooddesign.Thisisasystematicproject.Fromthepointofviewofcorrosionprotection,thefollowingfactorsshouldbeconsideredinthedesignofagoodmagnesiumalloypart:

（1）waterorotherliquidscannotbeaccumulatedtogethertoformagalvaniccellenvironment.

（two）thecornershouldbedesignedasroundarc,andsharpedgeanglewillreducetheprotectiveeffect.

1.5.2materialselection（asshowninFigurethree）:

Intheproductdesign,thechoiceoftherawmaterialsusedintheadjacentpartsisalsoveryimportant.Itisnecessarytoensurethatthestructuralfunctionisnotaffected,andthepotentialdifferencebetweenthetwomaterialsissmall,soastoensurethatnoseriouscorrosionoccurs.

Anotherproblemtonoteistheconnectionbetweenthecomponentsandthematerialoftheconnectingmediumandtheanticorrosiontreatmentprocess

It'salsoamatterofgreatconcern.

Figurethreecorrosionprotectionmethodsformagnesiumalloys

Theelectrochemicalactivityorderofsomecommonlyusedmaterials（atroomtemperature,seawater）islistedbelow.TheclosertoMg,thebetterthecompatibilitywithmagnesium:

magnesium

magnesiumalloy

Aluminumalloys5000,30000,1100,6000

cadmium

Aluminumalloy2000

steel

13%chromiumplatedstainlesssteel（reactive）

18-8notplatedsteel（active）

aluminum

zinc

Admiraltybrass

Nickel（active）

AlCualloy

Copper,copper

Nickel（inert）

Nickelcopperalloy

titanium

18-8stainlesssteel（inert）

gold

1.5.3metallurgicalcontrol

Metallurgicalfactorsincludealloyelements,impurityelements,phasecompositionandmicrostructure.

Differentelementshavedifferenteffectsonthecorrosionresistanceofmagnesiumalloys.Accordingtothealloyelementscommonlyusedinmagnesiumalloys,theyaregenerallydividedintothreecategories,asshownintable1.Effectofalloyelementsoncorrosionrateofmagnesiumalloy,

Usually,thecontentofthethirdkindsofelementsisreducedtobelowthecriticalvalue,andthemethodofincreasingpurityisadoptedtoincreasethecorrosionresistance.

1Al,Mn,Na,Si,Zrandotherelementswhichhavelittleinfluenceoncorrosionrate

2,thecorrosionrateofZn,Cd,Ca,Agandotherelementsslightlylarger

3,thecorrosionrateofNi,Fe,Cu,Coandotherelementsgreatlyincreased

Table1Effectofdifferentelementsoncorrosionresistanceofmagnesiumalloy

Phasecompositionhasagreatinfluenceonthecorrosionresistanceofmagnesiumalloy.Theprocessingmechanismofalloyisdifferent,thephasecompositionandcontentaredifferent,andtheinfluencemechanism