ASTM_D_1732_-_03_2013.pdf

Designation:D173203(Reapproved 2013)Standard Practices forPreparation of Magnesium Alloy Surfaces for Painting1This standard is issued under the fixed designation D1732;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.This standard has been approved for use by agencies of the U.S.Department of Defense.1.Scope1.1 These practices cover two classes of treatment forpreparation of magnesium alloy surfaces for painting,asfollows:Class IChemical Treatments.Class IIAnodic Treatments.In general,the latter treatments are the more protective of thetwo classes.Mechanical(abrasive)treatments,solventcleaning,alkaline solution treatments,and acid pickles notresulting in protective conversion coatings are suitable prelimi-nary treatments only for metal to be exposed under mildlycorrosive(indoor)exposures.When a high degree of corrosionprotection and paint adhesion are desired,as in many outdoorenvironments,surface preparation by one of the aboveconversion-coat classes is necessary.The hexavalent chromiumbased methods given are not recommended as hexavalentchromium is a known carcinogen.NOTE1Testing of CoatingsQuality control tests of coatings arefrequently desirable,and these generally consist of exposures,with orwithout paint,to salt spray,humidity,or natural environments,withsuitable procedures for assessing the degree of breakdown suffered afterfixed time intervals.It is recommended that quality control tests ofcoatings shall be made as far as possible with high-purity material(forexample AZ31A alloy),2the inherent corrosion rate of which is relativelyconsistent from batch to batch)and that precautions shall be taken toremove surface contamination before coatings are applied.Such contami-nation shall be removed by acid pickling to a depth of at least 0.001 in.(25m)per side.1.2 This standard may involve hazardous materials,operations,and equipment.This standard does not purport toaddress all of the safety concerns,if any,associated with itsuse.It is the responsibility of the user of this standard toestablish appropriate safety and health practices and deter-mine the applicability of regulatory limitations prior to use.(See Note 11.)PRELIMINARY TREATMENT OF SURFACES2.Procedure2.1 Certain anodic treatments simultaneously produce con-version coatings on,and remove contamination from,magne-sium alloy surfaces.In general,however,apply conversioncoatingsonlytosurfacespreviouslyfreedfromallcontamination,including oxide,rolling-scale,corrosionproduct,burned-on drawing and forming lubricant,and thecontamination introduced by blast cleaning and fabricationoperations.Contamination in or under surface conversioncoatings seriously reduces their protective values(Note 2).Forthe removal of tenacious surface contamination,such asrolling-scale or casting skin,an acid pickle to dissolve some ofthe actual surface is essential.When organic contamination,such as grease or oil,is also present,an initial degreasingoperation in solvent or in an alkaline degreasing solution isusually necessary to allow the subsequent acid to wet thesurface.These matters are discussed in more detail under theheadings of the specific cleaners or treatments(Note 3),asfollows:2.2 Alkaline CleanersOil,grease,and old(but not baked)chrome-pickle coatings are readily removed by most commer-cially available heavy-duty alkaline cleaners;but such cleanersare not suitable for removing oxide and the like,for whichpurpose use acid pickles,preceded by alkaline cleaners.Remove graphite lubricant and also baked chrome-picklecoatings by a solution conforming to the following composi-tion:Caustic soda(NaOH)12 oz(90 g)Wetting agent0.1 oz(0.74 g)Water(Note 4)1 gal(1 litre)Soak the parts in the above cleaner for 10 to 20 min atboiling-point,and a treatment shall follow either in the chromicacid-nitrate pickle described under 2.3.3 or,for parts machinedto fine tolerances,in the chromium trioxide solution describedunder 2.3.1.After alkaline cleaning,rinse in water verythoroughly.Alkaline cleaners may be held in plain steel tanks.1These practices are under the jurisdiction ofASTM 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 1960.Last previous edition approved in 2008 as D1732 03(2008).DOI:10.1520/D1732-03R13.2For information concerning magnesium and aluminum alloys,see ASTMSpecification B80,B90,B91,B93,B107,and B209 covering these alloys,in thesection on Aluminum and Magnesium and Their Alloys,Annual Book of ASTMStandards,Vol 02.02.See also ASTM Practice B275,for Codification of CertainNonferrous Metals and Alloys,Cast and Wrought.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700