1、Designation: B879 97 (Reapproved 2013)B879 17Standard 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 th
2、e 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 clean and then provide a paint bas
3、e for the finishing of magnesium andmagnesium alloys using chemical conversion coatings. Where applicable (for example, aerospace) secondary supplementarycoatings (for example, surface sealing) can be used (see Appendix X1).1.2 Although primarily intended as a base for paint, chemical conversion coa
4、tings provide varying degrees of surface protectionfor magnesium parts exposed to indoor atmosphere either in storage or in service under mild exposure conditions. An example isthe extensive use of the dichromate treatment (see 5.2) as a final coating for machined surfaces of die cast magnesium comp
5、onentsin the computer industry.1.3 The traditional numbering of the coating is used throughout.1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.5 This standard does not purport to address all of the safety concerns, if an
6、y, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior to use.1.6 This international standard was developed in accordance with in
7、ternationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 The following docume
8、nts form a part of this practice to the extent referenced herein.2.2 ASTM Standards:2D1732 Practices for Preparation of Magnesium Alloy Surfaces for Painting2.3 SAE Standard:3AMS 2475 Protective TreatmentsMagnesium Alloys2.4 Military Specifications:4MIL-M-3171 Magnesium Alloy, Processes for Pretreat
9、ment and Prevention of Corrosion onDTD 911 (British), Protection of Magnesium-Rich Alloys Against CorrosionDTD 5562 (British), Clear Baking Resin for Surface Sealing MagnesiumDTD 935 (British), Surface Sealing of Magnesium Rich Alloys3. Significance and Use3.1 The processes described in this practic
10、e clean and provide a paint base for the finishing of magnesium and magnesiumalloys. Service conditions will determine, to some degree, the specific process to be applied.1 This practice is under the jurisdiction of ASTM Committee B08 on Metallic and Inorganic Coatings and is the direct responsibili
11、ty of Subcommittee B08.07 onConversion Coatings.Current edition approved May 1, 2013Nov. 1, 2017. Published May 2013November 2017. Originally approved in 1997. Last previous edition approved in 20082013 asB879 97(2008)97(2013).1. DOI: 10.1520/B0879-97R13.10.1520/B0879-17.2 For referencedASTM standar
12、ds, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 Available from Society of Automotive Engineers (SAE), 400 Commonwealth Dr., Warrendale,
13、 PA 15096-0001, http:/www.sae.org.4 Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http:/dodssp.daps.dla.mil.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indicati
14、on of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be consi
15、dered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14. Reagents4.1 The chemicals that are used to formulate and control the processing solutions are listed in Table 1. Commercial gradechemicals are satisfactory
16、.The concentrations stated for chemicals that are normally supplied at less than a nominal 100 % strengthare those typically available. Other strengths may be used in the proportions that yield the specified processing concentrations.Unless otherwise stated all solutions are made up using water.5. T
17、ypes of Coating5.1 Chrome Pickle (Traditional Number 1) Treatment (See Practices D1732):5.1.1 With slight variations this treatment can be applied to all alloys and forms of magnesium. The treatment removes up to15 m of metal per surface, 30 m per diameter. Therefore, it may not be applicable to mac
18、hined surfaces with close tolerances.Parts with steel inserts may be processed, but some slight etching of the steel surface may occur.5.1.2 The color, luster, and etch produced by the treatment will vary with the age and usage of the solution, alloy composition,and heat treatment of the alloy. The
19、most desirable paint base is a matte grey to yellow-red, iridescent coating which exhibits apebbled etch finish when viewed under low magnification (5 to 10). Bright brassy coatings, showing a relatively smooth surfacewith only occasional rounded pits under low magnification are unsatisfactory as a
20、paint base but are acceptable for protectionduring shipping and storage.5.2 Dichromate (Traditional Number 7) Treatment (see Practices D1732):5.2.1 This treatment provides an improved paint base compared with the chrome pickle treatment, and for temporary protectionon all standard alloys except, EK4
21、1A, HM31A, HM21A, HK31A, WE54, WE43, and M1A on which the coating does not form.The treatment causes no appreciable dimensional changes, is normally applied after machining, and is suitable for close clearanceparts. Parts containing inserts of bronze, brass, steel, or cadmium plated steel should not
22、 be treated unless the dissimilar metalsare masked or it is demonstrated that the treatment will not adversely affect them. For assemblies containing aluminum inserts orrivets, the acid fluoride treatment (see 7.2.3) should replace the hydrofluoric acid treatment in part preparation.5.2.2 Coatings v
23、ary from light to dark brown depending upon the alloy. On AZ91C-T6 and AZ92A-T6 castings the coating isgrey.5.3 Galvanic Chromate (Traditional Number 9) Treatment (see Practices D1732):5.3.1 This treatment can be used for all alloys and is specifically used for those alloys which do not react or for
24、m satisfactoryconversion coatings in other baths. The treatment requires no external current but utilizes the relatively high potential differencebetween suitably racked magnesium components and steel tank walls or other cathodes. As with the dichromate treatment, a priorimmersion in acid fluoride s
25、olution is required to condition the magnesium surface. The galvanic chromate treatment causes noappreciable dimensional change and is normally applied after machining.TABLE 1 Processing ChemicalsAcetic acid glacial, (CH3COOH)Aluminum sulfate (Al2 SO3314H2O)Ammonium bifluoride (NH4HF2)Ammonium hydro
26、xide (NH4OH), 30 %Ammonium phosphate monobasic (NH4H2PO4)Ammonium sulfate (NH42SO4)Ammonium sulfite (NH42SO3H2O)Calcium chromate (CaCrO4)Calcium fluoride (CaF2)Calcium sulfate (CaSO42H2O)Chromic acid (CrO3)Ferric nitrate (FeNO339H2O)Glycolic acid (HOCH2COOH), 70 %Hydrofluoric acid (HF), 60 %Magnesiu
27、m fluoride (MgF2)Magnesium nitrate (MgNO326H2O)Magnesium sulfate (MgSO47H2O)Manganese sulfate (MnSO45H2O)Nitric acid (HNO3), sp gr 1.42Phosphoric acid (H3PO4), 85 %Potassium fluoride (KF)Potassium bifluoride (KHF2)Sodium bifluoride (NaHF2)Sodium bisulfate (NaHSO4)Sodium carbonate (Na2CO3)Sodium dich
28、romate (Na2Cr2O72H2O)Sodium hydroxide (NaOH)Sodium metasilicate (Na2SiO3, or Na2SiO34H2O)Sodium nitrate (NaNO3)Sulfuric acid (H2SO4), sp gr 1.84B879 1725.3.2 Properly applied coatings vary from dark brown to a dense black color depending on the alloy. The treatment isparticularly useful for applicat
29、ion to optical equipment requiring a nonreflective black coating.5.4 Chromic Acid Brush-On (Traditional Number 19) Treatment:5.4.1 This treatment can be applied to parts that require touch up. It is generally used in refinishing procedures or where partsor assemblies are too large to be immersed. It
30、 is effective on most alloys and causes negligible dimensional changes.5.4.2 Coatings produced by this treatment can vary from a brassy iridescence to a dark brown depending upon treatment time.Prolonged treatment produces powdery coatings. For best adhesion, dark brown coatings are preferred.5.5 Ch
31、romate Treatment (see DTD 911):5.5.1 This treatment is suitable for all magnesium alloys. The treatment causes no dimensional change and is normally appliedafter machining. The pickling procedures and the composition of the treating solution generally vary with the alloy beingprocessed.5.5.2 The coa
32、ting will vary from dark brown to light reddish-brown depending on the alloy.5.6 Chrome-Manganese Treatment:5.6.1 This treatment provides an improved paint base compared with the chrome pickle treatment and protection on all standardalloys except EK41A, HM31A, HM21A, HK31A, and M1A on which the coat
33、ing does not form. The treatment causes noappreciable dimensional change, and normally is applied after machining. It is suitable for close clearance parts. Parts containinginserts of bronze, brass, steel, or cadmium plated steel should not be treated unless the dissimilar metals are masked or it is
34、demonstrated that the treatment will not adversely affect them.5.6.2 The bath generally gives dark brown to black films on both cast and wrought magnesium alloys. Treatment of aluminumcontaining alloys may require bath temperatures above 50C.5.7 SemiBright Pickle (Traditional Number 21) TreatmentThi
35、s treatment provides a semibright silvery surface on magnesiumparts that prevents tarnishing and corrosion for indoor storage up to six months in non-air-conditioned environments. Extendedstorage times can be obtained by using air conditioning. This process causes negligible dimensional change. It i
36、s a simple,economical way to apply an attractive shelf-life finish and is a good base for clear lacquers. The treatment greatly reduces oreliminates “filiform or worm-tracking” corrosion usually experienced when clear paints are used directly over polished metalsurfaces.5.8 Phosphate Treatment:5.8.1
37、 Phosphate treatments can provide a satisfactory paint base on magnesium for many applications when it is necessary toavoid the use of chromates. Commercial iron phosphate treatments applied by spray or dipping have been successfully used onmagnesium die castings for automotive and other consumer pr
38、oduct applications. The suitability of a particular phosphatizingprocess for magnesium should be verified by testing. Iron phosphate treatments containing nickel or copper salts as acceleratorsare detrimental to the corrosion resistance of magnesium and should not be used.5.8.2 Phosphate treatments
39、do not provide interim stand-alone protection against atmospheric oxidation and tarnish equal to thatprovided by some chromate conversion coatings.5.9 Plasma Electrolytic Oxidation (PEO)This process is a combination of the co-deposition on Magnesium and Magnesiumalloys from an electrolyte and the ox
40、idation of the surface of the metals in question with a plasma. Short pulses of plasmadischarges result in the metals being processed to oxidize and then melt the generated Magnesium oxides onto the surface of themetals being treated. The melting seals up any holes in the surface oxides to prevent c
41、orrosion, increase ware and provide for asurface that will accept secondary organic coatings (i.e. commercial paints).5.10 All Organic Hydrocarbon AcidThis process chemically reacts the Magnesium or Magnesium alloy, or both, in questionwith various organic acids that then attach themselves to the su
42、rface of the metal in question, seals the surface of the metal toprevent any oxidation and produces a surface that will easily accept applied secondary coatings (i.e. commercial paints).6. Part Preparation6.1 CleaningGeneral:6.1.1 Before considering the use of solvent degreasing, consult federal and
43、 state safety and environmental laws and regulations.Many of the commonly used solvents are now being banned from use. Exposure to their vapor (VOC) is being strictly regulatedfor health, safety, and environmental reasons. Obtain current safe exposure levels for various solvents before use. Follow a
44、llfederal, state, and local regulations for the disposal of solvents.6.1.2 Solvent CleaningGrease or oil may be removed by means of vapor degreasing, ultrasonic cleaning, solvent washing, oran emulsion cleaning process that utilizes a mineral oil distillate and an emulsifying agent. Chlorinated solv
45、ents, petroleum spirits,naphths, lacquer thinner, and similar solvents that do not attack magnesium may be used. Methyl alcohol (CH3OH) should not beused because it may react with the magnesium surface.B879 1736.1.3 Mechanical CleaningMechanical cleaning may consist of sand, shot, pumice, grit or va
46、por blasting, sodium carbonateslurry, sanding, hard bristle brushing, grinding and rough polishing. Sand, shot, or grit blasting leaves surface contamination thatwill greatly increase the corrosion rate of the magnesium on exposure to salt water or humid environment. If these methods areused, specif
47、ic pickling procedures must be employed after blasting (see 6.4.2).6.1.4 Alkaline CleaningCleaning prior to application of treatments other than the chrome pickle treatment (see 5.1), whenused for protection during shipment or storage, should be done in an alkaline cleaner recommended for steel or i
48、n a cleaningsolution as specified in 6.1.4.1. Maintain the solution pH above 8.0.Alkaline cleaning prior to the application of the chrome pickletreatment (see 5.1), when used for protection during shipment and storage only, may be omitted provided the parts are free ofgrease, oil, and other deleteri
49、ous deposits at the time of application.Alkaline cleaning solutions containing more than 2 % sodiumhydroxide will etch ZK60A, ZK60B, and some other magnesium alloys producing a change in dimensions. If such a dimensionalchange is undesirable, use cleaners with lower alkali content.6.1.4.1 Alkaline cleaning may be carried out in solutions of proprietary cleaners. In this case the operating conditions shouldbe as specified by the supplier. In no case should a cleaner having pH lower than 8.0 be used. Most recommended cleaners areused by simple imm
