ASTM_B_482_-_85_2013.pdf

Designation:B48285(Reapproved 2013)Standard Practice forPreparation of Tungsten and Tungsten Alloys forElectroplating1This standard is issued under the fixed designation B482;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.INTRODUCTIONBecause of the high melting point of tungsten,(3410C)this metal and its alloys are used inultra-high-temperature applications.However,tungsten oxidizes readily and the oxides offer little orno protection to the metal because they melt or sublime below 2200C.Tungsten has a body-centered-cubic structure as does chromium.Its coefficient of thermal expansion is 4.3 m/m C;the coefficientof chromium is 6.1 m/m C,therefore,chromium-coated tungsten offers a reasonable match basedon crystal structure and coefficient of thermal expansion.The effect of hydrogen must be consideredin the design of coating systems for tungsten.Molecular hydrogen when trapped under a coating canresult in failure of the part.Although failure can occur if the adhesion of the coating to the tungstenis inadequate,failure within the basis metal as rupture at laminations of stringers has been observedwith off-grade tungsten.1.Scope1.1 This practice makes recommendations of several re-ported practices for electroplating on tungsten and its alloysalong with comments as a guide in the selection of a processingmethod for a given application.Because data on methods andresults of evaluation with electroplated coatings on tungstenare limited,a recommendation of one method over anothercannot be made.1.2 This standard does not purport to address the safetyconcerns,if any,associated with its use.It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2.Processes2.1 The several processes reported for electroplating ontungsten can be classified as either(1)providing as-depositedadhesion of the coatings or(2)depending on diffusion alloybonding to provide such adhesion.Appraisal of the relativemerits of these processes depends on the application for thecoating.2.2 Precleaning:2.2.1 The need for descaling,solvent cleaning,and alkaline-soak cleaning of tungsten is dictated by the degree of cleanli-ness of the surface to be coated.Remove gross scale andsurface imperfections by mechanical means.Remove greaseand oil by organic solvents.Remove saponifiable and dispers-ible surface dirt by soak cleaning in a hot alkaline cleaner.2.2.2 Anodic treatment of tungsten surfaces in a 5 to 25mass%sodium hydroxide solution at 71 6 6C and 16 to 25A/dm2can be used to remove undesired surface metal.Anodicetching in a 10 mass%hydrofluoric acid electrolyte at24 6 3C and 5.4A/dm2can be used to remove light scale andundesired surface metal before activation and electroplating oftungsten.2.3 Activation and ElectroplatingTwo alternative methodsare presented below.The selection of one process over theother should be based on preliminary experimentation.Bothprocedures produce as-deposited adhesion of electroplatedmetals on tungsten.2.3.1 Methods 12Treat the precleaned tungsten in a hy-drofluoric acid electrolyte(30 6 10 mass%HF)at 24 6 3C,using 2 to 5 V,60-Hz ac,and 2 to 5A/dm2for 1 to 2 min.Eitherdivide the work between two work rods or make one electrodeof platinum.After rinsing,the work is electroplated in acidelectrolytes.1This practice is under the jurisdiction of ASTM Committee B08 on Metallicand Inorganic Coatingsand is the direct responsibility of Subcommittee B08.02 onPre Treatment.Current edition approved Dec.1 2013.Published December 2013.Originallyapproved in 1968.Last previous edition approved in 2008 as B482 85(2008).DOI:10.1520/B0482-85R13.2Cannizaro,J.J.,U.S.Patent No.2,433,651,June,1948.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 2.3.2 Method 2Electroplate the precleaned tungsten witha chromium strike prior to subsequent electroplating with othermetals3,4using the following processing steps:2.3.2.1 Anodic EtchEtch anodically at 10 to 30 A/dm2inthe following solution for 2 to 5 min.KOH30 mass%Temperature48 to 60C2.3.2.2 Rinse.2.3.2.3 Acid NeutralizeImmerse for 1 min in 10 mass%sulfuric acid at 27 6 6C.2.3.2.4 Rinse.2.3.2.5 Chromium StrikeStrike at 15 to 25 A/dm2in thefollowing chromium solution for 1 to 3 min.Enter the solutionwith the parts cathodic.CrO3250 g/LH2SO42.5 g/LTemperature60 to 72C2.3.2.6 Rinse.2.3.2.7 Acid ActivationImmerse in 20 mass%hydrochlo-ric acid at 21 to 33C for 2 to 5 s.2.3.2.8 Nickel StrikeStrike at 5 to 10 A/dm2in thefollowing nickel solution for 2 to 5 min.NiSO4 6H2O240 g/LH2SO440 g/LTemperature21 to 33C2.3.2.9 Rinse.2.3.2.10 ElectroplateElectroplate with desired metal.2.4 Alloying and Heatin