ASTM_B_879_-_97_2013.pdf

Designation:B87997(Reapproved 2013)Standard Practice forApplying Non-Electrolytic Conversion Coatings onMagnesium and Magnesium Alloys1This standard is issued under the fixed designation B879;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 practice covers a guide for metal finishers to cleanand then provide a paint base for the finishing of magnesiumand magnesium alloys using chemical conversion coatings.Where applicable(for example,aerospace)secondary supple-mentary coatings(for example,surface sealing)can be used(see Appendix X1).1.2 Although primarily intended as a base for paint,chemi-cal conversion coatings provide varying degrees of surfaceprotection for magnesium parts exposed to indoor atmosphereeither in storage or in service under mild exposure conditions.An example is the extensive use of the dichromate treatment(see 5.2)as a final coating for machined surfaces of die castmagnesium components in the computer industry.1.3 The traditional numbering of the coating is usedthroughout.1.4 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.5 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 The following documents form a part of this practice tothe extent referenced herein.2.2 ASTM Standards:2D1732 Practices for Preparation of Magnesium Alloy Sur-faces for Painting2.3 SAE Standard:3AMS 2475 Protective TreatmentsMagnesium Alloys2.4 Military Specifications:4MIL-M-3171 Magnesium Alloy,Processes for Pretreatmentand Prevention of Corrosion onDTD 911(British),Protection of Magnesium-Rich AlloysAgainst CorrosionDTD 5562(British),Clear Baking Resin for Surface SealingMagnesiumDTD 935(British),Surface Sealing of Magnesium RichAlloys3.Significance and Use3.1 The processes described in this practice clean andprovide a paint base for the finishing of magnesium andmagnesium alloys.Service conditions will determine,to somedegree,the specific process to be applied.4.Reagents4.1 The chemicals that are used to formulate and control theprocessing solutions are listed in Table 1.Commercial gradechemicals are satisfactory.The concentrations stated for chemi-cals that are normally supplied at less than a nominal 100%strength are those typically available.Other strengths may beused in the proportions that yield the specified processingconcentrations.Unless otherwise stated all solutions are madeup using water.5.Types of Coating5.1 Chrome Pickle(Traditional Number 1)Treatment(SeePractices D1732):5.1.1 With slight variations this treatment can be applied toall alloys and forms of magnesium.The treatment removes upto 15 m of metal per surface,30 m per diameter.Therefore,it may not be applicable to machined surfaces with closetolerances.Parts with steel inserts may be processed,but someslight etching of the steel surface may occur.1This practice is under the jurisdiction of ASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 onConversion Coatings.Current edition approved May 1,2013.Published May 2013.Originallyapproved in 1997.Last previous edition approved in 2008 as B879 97(2008)1.DOI:10.1520/B0879-97R13.2For 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 Society of Automotive Engineers(SAE),400 CommonwealthDr.,Warrendale,PA 15096-0001,http:/www.sae.org.4Available from Standardization Documents Order Desk,DODSSP,Bldg.4,SectionD,700RobbinsAve.,Philadelphia,PA19111-5098,http:/dodssp.daps.dla.mil.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 5.1.2 The color,luster,and etch produced by the treatmentwill vary with the age and usage of the solution,alloycomposition,and heat treatment of the alloy.The most desir-able paint base is a matte grey to yellow-red,iridescent coatingwhich exhibits a pebbled etch finish when viewed under lowmagnification(5 to 10).Bright brassy coatings,showing arelatively smooth surface with only occasional rounded pitsunder low magnification are unsatisfactory as a paint base butare acceptable for protection during shipping and storage.5.2 Dichromate(Traditional Number 7)Treatment(seePractices D1732):5.2.1 This treatment provides an improved paint base com-pared with the chrome pickle treatment,and for temporaryprotection