CommonDefects

Defect AnodizingBurn

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Defectvisibleafter Anodizing

Description • Brightareasonanodiccoatingsurroundedbyblackordarkerarea.• Usuallynoanodicfilmorverysoftfilmpresentinthedefectivearea.

Cause • Excessivecurrent• Poormovementofelectrolyteacrosspart• Poorelectricalcontactbetweenrack/jigandthepart.

Remedy • Reducecurrent• Checkelectrolyteconcentrationandensuretemperatureiscontrolledproperly• Improveelectricalcontactareabetweenrack/jigandthepart

Defect ChlorideCorrosion

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Defectvisibleafter Anodizing

Description Pitsformedonallsurfacesofpart.Relativelydeepandusuallyblackinappearance.Thereisnoanodicfilmoverthepits.

Cause Exceededcriticallevelofchlorideof80ppmmeasuredastheCl-ioninthesulfuricacidelectrolyte,corrodesthehighlyactivealuminumduringtheanodizingprocess.

Remedy Useofdeionizedwaterinanodizingelectrolytemake-upandtop-upandensurethoroughrinsetoeliminatingthisproblem.

Defect ColorDifferences

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Defectvisibleafter Coloring

Description Poorvisualcolormatch,outsidetheagreedupperandlowercolorlimits.

Cause • Alloyinconsistency• Differencesintheanodizingparameter• Incorrectdyeingconditions• Lackofcontrolofelectrolyticcoloringprocess

Remedy • Makesurethebasemetalsarefromthesamebatch.• Checkanodizingelectrolyteconcentration,PH,anodizingtimeandtemperature• Checkdyeconcentration,dyetime,temperature,PHandforcontaminationwithsulphatesand

chlorides.• Inthecaseofelectrolyticcoloring,thiscanbeduetothewrongcoloringconditionorcontaminationof

thecoloringelectrolytewithsodium.Fixrackingconfigurationtoensurecoloruniformity

Defect Crazing

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Defectvisibleafter Sealing

Description Hairlinecracksformintheanodiccoating.

Cause • Stressproducedintheanodicoxidecoatingwhenpartsaretransferredfromrinsetanktosealingtankduetodifferencesinthermalexpansioncoefficientbetweentheanodicoxidecoatingandthebasemetal.

• Causedbyamechanicaldeformationofananodizedpartbybendingorimpact.Canbeenhancedbyhighcurrent,lowelectrolytetemp,thickanodicfilms(hardanodizingconditions).

• Anodicoxidecoatingsexposedtohighambienttemperatures(e.g.lighting)andstrongsunlightaremorelikelytodevelopcrazingthanthoseexposedinthemoretemperateclimates

Remedy • Reducethermalshock(70°Cwaterbeforeboilingsealing)• Anybendingofthepartsshouldbecarriedoutbeforeanodizing.Preheatpartstoevenoutstressin

material(130°Cfor40min).Reducecurrent(1.4-1.6A/dm2).Electrolytetemperatureshouldbearound20°C.Reduceanodicfilmthickness(<30µm)Don’theatthickfilmabove120°C

Defect EtchStaining

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Defectvisibleafter Etching/Coloring

Description Stainingdevelopsonthepartafteretching.Thismaynotbeimmediatelyvisibleafteretching.

Cause • Etchingsolutiondriesonpartduringtransfertorinse• Poorcleaning/rinsingafteretching.• Poorjiggingincombinationwithpoorcleaning• Wrongetchsolutioncomposition• Etchsolutiontoohot

Remedy • Improvetimefrometchingtorinse• Ensuregoodandfastcleaning/rinsingafteretching.• Properjiggingofdifficultorthickwalledprofiledparts• Checkthelevelofwettingagentintheetchingsolutionisadequate

Ensureetchsolutiontemperatureisn’ttoohigh(<60°C)

Defect GasTrapping

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Defectvisibleafter Anodizing/Coloring

Description Randomspotsorareasthatareuncolored,differenthueorlighterontheundersideoftherackedworkpiece.

Cause Gasorairbubblestrappedonundersideofworkpiece.Theseairorgaspocketsactasinsulatorsforanodizingprocessormaskingagentsforthecoloringprocess

Remedy Rackpartssotherearenohorizontalsurfacesorinvertedchannelsontheundersidesogascanescape.

Defect InsufficientDegreasing

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Defectvisibleafter Etching/Brightening

Description NonuniformDegreasing

Cause Poordegreasesolutionorpractice(e.g.temperaturetoolow),oilorgreaseisstillleftonsomeareas.Thisleadstodifferentialetching/brighteningofthesectionsurface

Remedy Renewthedegreasingsolutionorextentthetreatmenttimeorincreasethetemperature.

Defect JigMarks

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Defectvisibleafter Anodizing/coloring

Description Exposedareas/marksontheanodizedpart

Cause Iftheworkpiecedonothavethreadedholes,jigmarksareinevitableasthepartsmustbeclampedtobeanodized/colored

Remedy • Usethreadedrodsinexistingthreadedholes• Selectlocationofcontactareastobeinanunimportanthiddenplace

Defect Blistering/Delaminating

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Defectvisibleafter Coloring

Description Blistersformontheanodiccoatingofthework-piece.Itoccursonhighzincalloys(Aluminum7000series)

Cause Duringanodizingprocess,thealloyingelement,inthiscasezinc,whenoxidizes,istoobigtojumpthroughthealuminumoxidesporouslayerandremainsattheinterfacebetweenthesubstrateandtheanodicfilmlayer.Thisnaturallyservesasstresspointandsubsequentthermalcyclingduringcoloringprocesscausestheseareastoblister.Note:Identifydelaminationbydeterminingifacoatingispresentatthespot.Aquicktestistoapplyadropofacidifiedcoppersolution(about1Nhydrochloricacidwithabout10g/lcoppersulfate)tothespot.Ifnocoatingispresent,thespotwillturnredwithinafewminutes.

Remedy Whenanodizing,extendingtherampupperiodtoallowzinctomoveawayfromtheinterfacebetweensubstrateandtheanodicaluminumoxidelayer.Thisreducestheblisteringeffect.

Defect WhiteSpots

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Defectvisibleafter Afterelectrolyticcoloringordyecoloring

Description Littlewhitespotsdevelopintheanodizedlayerafterelectrolyticordyecoloration.

Cause • Whitespotthatcan’tbewipedoff,andinroundshape- Likelyduetothesurfacecontaminationthatisresultofresiduethatwasnotadequatelyremovedby

anodizerthatinhibitdyefrompickingup.Anothermaybethepresenceofalkalineparticlesfloatingaroundintheairandadheringtotheanodizesurfacewhilebeingtransferredtothedyetank.

• Whitespotthatcan’tbewipedoff,andnotinroundshape–Usuallytheresultofentrappedanodizeacidintinypits,cavitiesorgroves.Pitsmaybeduetounwantediron-containinginclusionsinthemetalthatareetchedoutduringtheanodizestep.Cavitiesmaybeduetotheporosityofcastaluminum.Tinygrovesmayoccuronmachinedsurfaces.Anodizingcanopenupsurfaceornear-surfacedefects,leavingvoidsthatcanbeverydifficulttorinseclean.Asaresult,dyepickupwillbeinhibitedintheareawhereanyacidremains.Usuallytheacidoozesoutofthepitorcavitywhileinthedyetank,formingirregularshapedspots,includingcomettails.

Remedy

• Thoroughlycleantherinsetanksorimprovefume/gasextractioninalkalineetchingstage.Topreventdisturbingaircurrentsfrometchingareatocolorationarea.

• A30-to60-secondrinsein5-10percentnitricacidfollowedbyafreshwaterrinse.