4.4SteelQuality—Thequalityofthesteelshouldbechar-acteristicoftherequirementoftheproductandtheelectroplat-ingoperation.Thesteelshouldbefreeofinjurioussurfacedefects,andofatleastaveragecleanliness.
5.PreparationofSteel,General
5.1PreparatoryTreatments—Awidevarietyofsurfaceconditionsareencounteredinhigh-carbonsteelarticlestobeelectroplated.Thesurfacemayrequiretheremovalofoneormoreofthefollowingcontaminants:grease,oilordrawingcompounds,burned-inoilscale,lighttoheavytreatmentscale,permeableoxidefilms,emeryandfinesteelparticlesresultingfromthegrindingoperation.Theremovalofsuchcontaminantsisaccomplishedbyoneormoreofthefollowingpretreatmentprocedureswhereapplicable:
5.1.1Substantialremovalofoil,grease,andcaked-ondirtbyprecleaningbeforethepartenterstheelectroplatingcycle(applicableinallcases).
5.1.2Mechanicaltreatmentofthesurfacebytumbling,sandorgritblasting,vaporblasting,orgrinding(optional).
5.1.3Finalandcompleteanodiccleaninginanelectrolyticalkalicleaner.
5.1.4AcidtreatmentinHCltoremovethelasttraceofoxideandscale.Thisshouldbeavoidedforspringtemperandcase-hardenedparts.Thistreatmentalsoremovesresidualtracesofleadthatmaybepresentfollowingproperlead-bathquenching.
5.1.5Smutremovalbycyanidedippingorbyanodictreat-mentincyanideoralkali.
5.1.6Finalpreparationforelectroplatingmaybeaccom-plishedbyananodicetchingtreatmentinHpossibleintheinterestofhighyieldand2SOadhesion).
4(usedwhen-ever5.1.7Conditioningofthesurfacetobeelectroplatedmaybeaccomplished,wherenecessaryfortheelectroplatingprocess,byashortdiporrinseinasolutionequivalenttotheelectroplatingsolutionwithoutitsmetalliccontent.
5.2Rinsing—Inadequaterinsingaftereachsolutiontreat-mentstepistherecognizedcauseofalargeportionofelectroplatingdifficulties.Notenoughrinsingischaracteristicofmostpretreatmentcycles.
5.3PretreatmentTime—Allprocessingstepsinvolvinghy-drogengenerationmustbedesignedtooperateforaminimumlengthoftime,toavoidhydrogenembrittlementofthehigh-carbonsteel.
5.4Control—Allpretreatmentstepsshouldbecarriedoutwithsolutionsthataremaintainedingoodworkingconditionbycontrolofcompositionandcontaminants,andusedunderconditionsoftime,temperatureandcurrentdensityspecifiedtomeettherequirementoftheworkbeingprocessed.
5.5PretreatmentCycleDesign—Dependinguponthere-quirementsfortheparticularhigh-carbonsteelpartstobeelectroplated,aminimumcycleshouldbeselectedfromthegeneralstepslistedin5.1.Differentclassesofmaterialsrequire
selectedprocessstepscombinedintopretreatmentcyclesofgreaterorlesscomplexityaccordingtotheconditionandpropertiesofthematerial.Theminimumnumberofstepsnecessarytoaccomplishtheelectroplatingsatisfactorilyisrecommended.
6.PreliminaryPretreatmentProcedures
6.1Application—Degreasingandmechanicalsurfacetreat-mentarenecessaryonlywherethehigh-carbonsteelpartsarecontaminatedtosuchanextentthatotherwisetheburdenimposedonthepretreatmentcyclewouldimpairitsefficiency,increaseitscomplexity,andtendtopreventtheattainmentoftherequiredqualityofthedeposit.Theoverallcostoftheelectroplatingprocessisusuallyreducedbyusingtheprelimi-narytreatmentswhereapplicable.Oil,grease,dirt,drawingcompounds,burnt-inoil,heavyscale,andemeryandsteelparticlesaretypicalofthegrosscontaminantsencountered.6.2Precleaning—Solvent-degreasingwithcleansolvent,spray-washing,oremulsion-cleaning,followedbyelectrolyticorsoak-alkalicleanersarerecommended.Theformertypesarepreferredtoreducetheburdenonthealkalitreatments.Soak-alkalicleaningisusualforpartsthataretobebarrelelectroplated.Electrolyticcleaningshouldalwaysbeanodicwherethecontrolofembrittlementisaproblem.
6.3StressReliefTreatment—Itisrecommendedthathard-enedhigh-carbonsteelpartsreceiveastress-reliefbakebeforethepartsaremechanicallypretreatedorenterthefinalpretreat-mentcycle,orboth.Foralistingoftypicalstress-reliefbakes,consultSpecificationB849.
6.4MechanicalTreatment—Thepurposeofmechanicaltreatmentistoreducesubsequentacidpicklingtoaminimum.Wheremechanicaltreatmenthasbeenaccomplishedwithprecision,itissometimespossibletoeliminateacidpicklingentirely,thusimprovingthecontrolofhydrogenembrittlement.Whenrequired,mechanicaltreatmentofsmallpartsisbesteffectedbytumbling.Allscaledandnearlyalloil-quenchedmaterialsrequiremechanicalcleaningsuchasbytumblingwithorwithoutabrasive,orbysand,grit,orvaporblasting.Theseoperationsshouldbecarriedoutsoastoavoidsevererougheningofthesurfacewithaccompanyingnotcheffect.Oneresortstogrindingincertaincaseswherethesurfacesmoothnessordimensionsofthepartsareofcriticalimpor-tance,forexample,inchromiumelectroplatingforengineeringuse.
7.FinalPretreatmentProcedures
7.1Application—Finalcleaning,oxideremoval,andanodicacidtreatmentarefundamentalstepsrequiredforpreparinghigh-carbonsteelforelectroplating.Thesepretreatmentstepsaredesignedtoassistinthecontrolofhydrogenembrittlementandinsecuringthemaximumadhesionoftheelectroplatedcoating.
7.2ElectrolyticAnodicCleaning:
7.2.1Allwork,exceptworktobebarrelelectroplated,shouldpreferablybecleanedinanelectrolyticanodicalkalinecleaner.Anodiccleaningisrecommendedtoavoidhydrogenembrittlementthatislikelytoresultfromcathodiccleaning.Anexceptionisbarrelworkwhich,becauseoftheworksize,ispreferablycleanedbysoakingortumblinginanalkaline
cleaningsolutionwithouttheuseofcurrent.
7.2.2Thepurposeofthiscleaningstepistoremovecompletelythelasttracesofcontaminants.Inallcasesitshouldbeprecededbyheavy-dutyprecleaningascoveredin6.2.7.2.3Theelectrolyticanodiccleanershouldbeusedatatemperature2of90°Corhigher,andatacurrentdensityof5A/dmorhigher,inorderthattherequireddegreeofcleanli-nessbeobtainedinatimeperiodnotexceeding2min.
7.2.4Onremovalfromthecleaner,theworkshouldbethoroughlyrinsed,firstwithwaterwarmedto50°C,andtheninacold-watersprayatroomtemperature,priortotheaciddip.7.3Rinsing:
7.3.1Themostthoroughfresh-waterrinsingoperationpos-sibleismandatoryaftereachprocessingstepifthebestresultsinelectroplatinghigh-carbonsteelaretobeobtained.Thepurposeofrinsingistoeliminatedrag-overbycompleteremovaloftheprecedingsolutionfromthesurfaceofthework.Manyexistingcommercialoperationsarecharacterizedbyinadequaterinsing.
7.3.2Warmtohotrinsesshouldbeusedfollowingalkalinesolutionsorwherethesubsequentprocessingsolutionishot.Therinsetemperatureshouldnotbesohighastoinducedryingofthesteelsurfacebetweenprocessingsteps.Roomtempera-turerinsesaresuitableforusefollowingacidsolutionswherethesolutioninthenextprocessingstepiscold.Innocaseshouldverycoldwaterbeusedforrinsing.
7.3.3Therecommendedrinsingpracticeincludestheuseofanimmersionrinse,alwaysfollowedbyasprayrinseoffreshwaterattherequiredtemperature.Notusingasprayrinsingisaninvitationtotroubleintheelectroplatingofhigh-carbonsteel.
7.4HydrochloricAcidTreatment—ThepurposeoftheHCltreatmentistoremovecompletelythelasttraceofoxidefromthesurfaceofthehigh-carbonsteel.TheintensityoftheHCltreatmentshouldbeheldtotheminimumrequiredbythenatureandamountofoxidepresent.TheuseofHofHClisnotrecommendedfordescalinghigh-carbon2SO4insteadsteelbecauseofitssmut-formingtendency,inspiteofthesomewhatloweredtendencytorustingofH2SOtotheHCl4-treatedsurfaces.Theadditionofwettingagentssolutionisnotrecom-mended.Careandcautionmustbeexercisedintheuseofinhibitorswheretheyarerequired,becausetheysometimesinterferewithadhesion.Inhibitorsareofbenefitonlyinspecialcaseswheresurfacefinishanddimensionsareofprimeimportance.
7.5TreatmentforSmutRemoval—WhentheHCltreatmentofthehigh-carbonsteelresultsinthepresenceofsmut,thesmutmustberemovedbeforethesurfaceiselectroplated.Lightoxidesformedonexposuretoairafteracidtreatmentmustlikewiseberemoved.Thiscanbedonebyananodiccyanideoralkalinetreatment.Air-formedoxide,ifnottooheavy,canberemovedbyacyanidedipaftertherinsefollowingtheacidtreatment.Aconcentrationof22g/LofNaCNissufficientforthecyanidedip.Whereaseveresmutconditionexists,itcanbeeliminated2bya1⁄2to1-minanodictreatmentat1.5to2A/dminasolutionofaNaCNofthenoncriticalconcentrationof45g/Lusedatroomtemperature.Analternativetreatmentforasomewhatlightersmutcondition
iselectrolyticanodictreatmentinthenoncyanidealkalinecleaningsolution(6.3)above70°C,for15to30sat2.5to5A/dm2.Thecurrentdensityisnotcritical.7.6AnodicAcidEtching:
7.6.1Theuseofananodicacidetchandsubsequentrinseasfinalstepsinthepreparationforelectroplatingofhigh-carbonsteelisofimportanceinsecuringadhesion.Withoutsuchananodictreatment,pooradhesionmayoccur.TheanodicacidtreatmentiscapableofremovingasmallamountofsmutformedbytheprecedingHCltreatment;moresubstantialamountsofsmutshouldberemovedaccordingtotheproce-duresdescribedin6.5.
7.6.2A150to600mL/LH2SO4solutionusedatatempera-tureofnotmorethan30°C,andpreferablybelow25°C,iseffectiveforanodicetchingofhigh-carbonsteel.SeeCautionin3.1.Theadditionof125g/LofNaanhydroussalt)isofbenefitformanysteel2SO4(basedonthegrades.Anodictreatmentinthissolutionforatimeusuallynotexceeding1minatacurrentdensityof16A/dm2(rangeof10to43A/dm2)issufficient.Ahighacidcontent,highcurrentdensity,andlowtemperature(withreferencetotherangesspecified)willminimizetheattackonthebasismetalandproduceasmoothersurface.ThisH2SO4solutionisverystableandnotaffectedappreciablybyironbuildup.Besidessecuringadhesionofthesubsequentelectrodeposit,itimprovestheuniformityofthedepositandreduceshydrogenembrittlement.
7.6.3Adipfor5to10sina55mL/LHNOfollowedbyrinsingandanodiccyanidetreatment3solution,forsmutremoval,hasincertaincasesbeenfoundeffective.Althoughnonelectrolytic,theHNO3treatmentrequiresanadditionalelectrolyticcyanidetreatmentbeforeelectroplating.7.7Electropolishing:
7.7.1Electropolishingisusedtoremovehighlystressedmetalandmetaldebrisfromthesurfaceofcold-workedsteel,therebyimprovingthebondstrengthandcorrosionresistanceofelectroplatedcoatings.Itaccomplishesthisfunctionwithoutthetendencytoformsmut,whichmayresultfromanodicetching.Becauseitdoesnotetchthesteel,itispreferredbysomeelectroplaterstoanodicetchingforpreparingsteelsurfacesfordecorativeelectroplating.Proprietarymixturesofphosphoricandsulfuricacids3forelectropolishingareofspecialinterestinviewoftheirabilitytoremovesmutfromcastironsurfaces.Theadditionofchromicacidtosulfuric-phosphoricacidmixtures4furtherprovidesasurfacepassiva-tionthatisbeneficialinpreventingrustingduringtransferorduringtemporarystorageofsteelpriortoelectroplating.
7.7.2Anactivatingtreatmentafterelectropolishingcanbebeneficialtosubsequentelectroplating.Anexampleofasuitableactivatingtreatmentisanodiccleaningfollowedbyaciddipping.Electropolishingcanbeusedinadditiontoorinsteadofanodicetching.
7.7.3Electropolishingoffine-grainedsteelprovidesauni-form,smoothfinish.Electropolishingofacoarse-grainedstructure(No.6orcoarser)resultsinlesssmoothness.Ineithercase,however,awordofcautionmustbeinterjected,namely
3U.S.Patent2,334,699.
4U.S.Patents2,366,712and2,338,321.
thatanyseams,voids,stringers,andothersurfacedefectscanbedamagingtotheappearanceoftheelectroplatedcoating.Yet,theremovalofsharpedgesorscratchesandtheremovalofnonmetallicinclusionsinseamsandstringerscanreducetheharmfuleffectofthesedefectsonthecorrosionresistanceoftheelectroplatedsteel.
8.ElectroplatingProcedures
8.1Standardelectroplatingprocedurescanbeusedonhigh-carbonsteelwhentheproperpreparatorystepsdescribedinSections6and7havebeenselectedandfollowed.Theconditionsofuseoftheelectroplatingbathshouldassureaminimumevolutionofhydrogenatthecathodesurface(highestcathodecurrentefficiency).Theuseoftheminimumlengthoftimeineachstepofpreparationandelectroplatingisrecom-mended.Thematerialshouldbehandledwiththeminimumnumberofstepsconsistentwithpropertreatment.
8.2Theelectrodepositionoftinandcadmiumonhigh-carbonsteeliseasiertoaccomplishthanthatofzinc.Elec-trodepositionofnickelandchromiumisnotdifficult,iftherecommendedpracticeisfollowed.Lowinternalstressofthedepositisdesirable.
8.3Springsandsimilarmaterialsshouldnotbeelectro-platedwhilesubjecttoexternallyappliedstress.
9.HeatTreatmentAfterElectroplating
9.1Application—Onepurposeofthespecialpreparatorytreatmentsofhigh-carbonsteel(Sections6and7)istominimizehydrogenembrittlement.Inmostcommercialpro-ductionitisnecessarytobaketheelectroplatedworkforfinal
embrittlementrelief.Withspringsandsimilarmaterials,pre-cautionsshouldbeobservedtoavoidflexingthearticlesbeforetheyarebaked.
9.2Procedure—Thehydrogenmaybelargelyremovedandthephysicalpropertiesofthesteelsubstantiallyrestoredbyheating,forexample,for1to5hattemperatureinanovenmaintainedatatemperatureof150to260°C,thetemperatureandlengthoftreatmentdependingontheseverityofembrittle-ment,thecrosssectionofthearticle,therequirementsofthesteel,andthekindandthicknessoftheelectrodepositedcoatings.Thebakingshouldbedoneassoonaspossibleafterelectroplating,andbeforeanysupplementarychemicaltreat-mentoftheelectroplatedsurfaces.Thebesttimeandtempera-tureinsomecasesmustbeestablishedexperimentally.Atemperaturenotexceeding205°Cisrecommendedforzinc-orcadmium-electroplatedarticles.Alowertemperaturemayberequiredifthecoatingistobegivenasubsequentchemicaltreatment.10.TestMethods
10.1Adhesion—Nouniversallysatisfactorynondestructivetestforadhesionisknown.Pooradhesionmayberevealedduringgrindingoperations.
10.2Embrittlement—Thetestfortheeffectivenessoftheproceduresusedtocontrolembrittlementliesinthesubsequentserviceuseofthematerial.Applicablecontroltestsforem-brittlementcanbeselectedbyanalogytotheparticularserviceinwhichthehigh-carbonsteelarticlebeingprocessedwillbeused.
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