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