ASTM_B_252_-_92_2014.pdf

Designation:B25292(Reapproved 2014)Endorsed by AmericanElectroplaters SocietyEndorsed by NationalAssociation of Metal FinishersStandard Guide forPreparation of Zinc Alloy Die Castings for Electroplatingand Conversion Coatings1This standard is issued under the fixed designation B252;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.1.Scope1.1 This guide is intended as an aid in establishing andmaintaining a procedure for preparing zinc alloy die castingsfor electroplating and conversion coatings.It is primarilyintended for the preparation of Alloys UNS Z33521(AG-40A)and UNS Z35530(AC-41A)(Specification B86)for electro-plating with copper,nickel,and chromium(SpecificationB456).1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2B6 Specification for ZincB86 Specification for Zinc and Zinc-Aluminum(ZA)AlloyFoundry and Die CastingsB456 Specification for Electrodeposited Coatings of CopperPlus Nickel Plus Chromium and Nickel Plus Chromium2.2 Military Standard:MIL-S-13165C Shot Peening of Metal Parts33.Summary of Practice3.1 The normal sequence of preparation steps is as follows:(1)smoothing of parting lines;(2)smoothing of rough ordefective surfaces,if necessary;(3)buffing,if necessary;(4)precleaning and rinsing;(5)alkaline electrocleaning and rins-ing;(6)acid dipping and rinsing;and(7)copper striking.4.Significance and Use4.1 The performance and quality of electroplated orconversion-coated zinc alloy die casting depends upon thesurface cleanliness and condition.Various metals are electro-plated or conversion coatings are established on zinc alloys fordecorative or engineering finish.The common electroplatesapplied are usually copper,nickel,and chromium for decora-tive and functional uses.The common conversion coatingsapplied are phosphates,chromates,and anodized coatings.Electroplated zinc die castings and conversion coatings on zincdie castings are used in many industries such as the marine,automotive,plumbing fixtures,and appliance industries.5.Composition and Characteristics of Zinc Alloy DieCastings5.1 The alloys used in the manufacture of zinc alloy diecastings are made with special high-grade zinc conforming toSpecification B6,alloyed with about 4%of aluminum,0.04%of magnesium,and either 0.25(max)or 1.0%copper(AlloysUNS Z33521 and UNS Z35530).Impurities such as lead,cadmium,tin,and iron are held at or below the specified lowlevels in Specification B86.5.2 Die castings made of Alloys UNS 233521 and UNS235530 are usually dense and fine grained but do not alwayshave smooth surfaces.Defects sometimes encountered in thesurface layers include cracks,crevices(cold shut),skin blisters,and hemispherical pores.Burrs are usually left at parting lineswhere fins and gates are removed by die trimming.5.3 Cast surfaces are frequently contaminated with partingcompounds applied at frequent intervals to die surfaces tofacilitate the ejection of the castings and with water-solubleoils added to quenching tanks for corrosion inhibition.1This guide is under the jurisdiction of ASTM Committee B08 on Metallic andInorganic Coatings and is the direct responsibility of Subcommittee B08.02 on PreTreatment.Current edition approved Nov.1,2014.Published November 2014.Originallyapproved in 1951.Last previous edition approved in 2009 as B252 92(2009).DOI:10.1520/B0252-92R14.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.3Available from Standardization Documents Order Desk,Bldg.4 Section D,700Robbins Ave.,Philadelphia,PA 19111-5094,Attn:NPODS.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 5.4 Zinc alloy die castings are chemically active and aredissolved or etched during prolonged contact with concentratedsolutions of many mineral or organic acids or strongly alkalinesolutions with a pH greater than 10.Immersion periods in suchsolutions should be of short duration to avoid roughening.6.Smoothing of Parting Lines6.1 Parting lines are smoothed by(1)mechanical polishingwith abrasive-coated wheels or belts,(2)tumbling with abra-sive media,or(3)vibration with abrasives.6.2 Abrasives with a size range of 220 to 300 mesh glued oncloth wheels or continuous cloth belts that run