1、Designation: B879 97 (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
2、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 th
3、e 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 p
4、rovide 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
5、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
6、 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 he
7、rein.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), Protec
8、tion 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
9、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-
10、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 (Trad
11、itional 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. Part
12、s 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,
13、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 in
14、formation, 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,Section D, 700 Robbins Ave., Phila
15、delphia, PA 19111-5098, http:/dodssp.daps.dla.mil.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15.1.2 The color, luster, and etch produced by the treatmentwill vary with the age and usage of the solution, alloycomposition, and heat
16、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 magnificati
17、on 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 on all
18、standard alloys except, EK41A, HM31A,HM21A, HK31A, WE54, WE43, and M1A on which thecoating does not form. The treatment causes no appreciabledimensional changes, is normally applied after machining, andis suitable for close clearance parts. Parts containing inserts ofbronze, brass, steel, or cadmium
19、 plated steel should not betreated unless the dissimilar metals are masked or it isdemonstrated that the treatment will not adversely affect them.For assemblies containing aluminum inserts or rivets, the acidfluoride treatment (see 7.2.3) should replace the hydrofluoricacid treatment in part prepara
20、tion.5.2.2 Coatings vary from light to dark brown dependingupon the alloy. On AZ91C-T6 and AZ92A-T6 castings thecoating is grey.5.3 Galvanic Chromate (Traditional Number 9) Treatment(see Practices D1732):5.3.1 This treatment can be used for all alloys and isspecifically used for those alloys which d
21、o not react or formsatisfactory conversion coatings in other baths. The treatmentrequires no external current but utilizes the relatively highpotential difference between suitably racked magnesium com-ponents and steel tank walls or other cathodes. As with thedichromate treatment, a prior immersion
22、in acid fluoridesolution is required to condition the magnesium surface. Thegalvanic chromate treatment causes no appreciable dimen-sional change and is normally applied after machining.5.3.2 Properly applied coatings vary from dark brown to adense black color depending on the alloy. The treatment i
23、sparticularly useful for application to optical equipment requir-ing a nonreflective black coating.5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-ment:5.4.1 This treatment can be applied to parts that requiretouch up. It is generally used in refinishing procedures orwhere parts or assemblie
24、s are too large to be immersed. It iseffective on most alloys and causes negligible dimensionalchanges.5.4.2 Coatings produced by this treatment can vary from abrassy iridescence to a dark brown depending upon treatmenttime. Prolonged treatment produces powdery coatings. For bestadhesion, dark brown
25、 coatings are preferred.5.5 Chromate Treatment (see DTD 911):5.5.1 This treatment is suitable for all magnesium alloys.The treatment causes no dimensional change and is normallyapplied after machining. The pickling procedures and thecomposition of the treating solution generally vary with thealloy b
26、eing processed.5.5.2 The coating will vary from dark brown to lightreddish-brown depending on the alloy.5.6 Chrome-Manganese Treatment:5.6.1 This treatment provides an improved paint base com-pared with the chrome pickle treatment and protection on allstandard alloys except EK41A, HM31A, HM21A, HK31
27、A,and M1A on which the coating does not form. The treatmentcauses no appreciable dimensional change, and normally isapplied after machining. It is suitable for close clearance parts.Parts containing inserts of bronze, brass, steel, or cadmiumplated steel should not be treated unless the dissimilar m
28、etalsare masked or it is demonstrated that the treatment will notadversely affect them.5.6.2 The bath generally gives dark brown to black films onboth cast and wrought magnesium alloys. Treatment of alumi-num containing alloys may require bath temperatures above50C.5.7 SemiBright Pickle (Traditional
29、 Number 21) TreatmentThis treatment provides a semibright silvery surface on mag-nesium parts that prevents tarnishing and corrosion for indoorstorage up to six months in non-air-conditioned environments.Extended storage times can be obtained by using air condition-ing. This process causes negligibl
30、e dimensional change. It is asimple, economical way to apply an attractive shelf-life finishand is a good base for clear lacquers. The treatment greatlyreduces or eliminates “filiform or worm-tracking” corrosionusually experienced when clear paints are used directly overpolished metal surfaces.5.8 P
31、hosphate Treatment:TABLE 1 Processing ChemicalsAcetic acid glacial, (CH3COOH)Aluminum sulfate (Al2SO3314H2O)Ammonium bifluoride (NH4HF2)Ammonium hydroxide (NH4OH), 30 %Ammonium phosphate monobasic (NH4H2PO4)Ammonium sulfate (NH42SO4)Ammonium sulfite (NH42SO3H2O)Calcium chromate (CaCrO4)Calcium fluor
32、ide (CaF2)Calcium sulfate (CaSO42H2O)Chromic acid (CrO3)Ferric nitrate (FeNO339H2O)Glycolic acid (HOCH2COOH), 70 %Hydrofluoric acid (HF), 60 %Magnesium fluoride (MgF2)Magnesium nitrate (MgNO326H2O)Magnesium sulfate (MgSO47H2O)Manganese sulfate (MnSO45H2O)Nitric acid (HNO3), sp gr 1.42Phosphoric acid
33、 (H3PO4), 85 %Potassium fluoride (KF)Potassium bifluoride (KHF2)Sodium bifluoride (NaHF2)Sodium bisulfate (NaHSO4)Sodium carbonate (Na2CO3)Sodium dichromate (Na2Cr2O72H2O)Sodium hydroxide (NaOH)Sodium metasilicate (Na2SiO3,orNa2SiO34H2O)Sodium nitrate (NaNO3)Sulfuric acid (H2SO4), sp gr 1.84B879 97
34、(2013)25.8.1 Phosphate treatments can provide a satisfactory paintbase on magnesium for many applications when it is necessaryto avoid the use of chromates. Commercial iron phosphatetreatments applied by spray or dipping have been successfullyused on magnesium die castings for automotive and otherco
35、nsumer product applications. The suitability of a particularphosphatizing process for magnesium should be verified bytesting. Iron phosphate treatments containing nickel or coppersalts as accelerators are detrimental to the corrosion resistanceof magnesium and should not be used.5.8.2 Phosphate trea
36、tments do not provide interim stand-alone protection against atmospheric oxidation and tarnishequal to that provided by some chromate conversion coatings.6. Part Preparation6.1 CleaningGeneral:6.1.1 Before considering the use of solvent degreasing,consult federal and state safety and environmental l
37、aws andregulations. Many of the commonly used solvents are nowbeing banned from use. Exposure to their vapor (VOC) is beingstrictly regulated for health, safety, and environmental reasons.Obtain current safe exposure levels for various solvents beforeuse. Follow all federal, state, and local regulat
38、ions for thedisposal of solvents.6.1.2 Solvent CleaningGrease or oil may be removed bymeans of vapor degreasing, ultrasonic cleaning, solventwashing, or an emulsion cleaning process that utilizes amineral oil distillate and an emulsifying agent. Chlorinatedsolvents, petroleum spirits, naphths, lacqu
39、er thinner, and simi-lar solvents that do not attack magnesium may be used. Methylalcohol (CH3OH) should not be used because it may react withthe magnesium surface.6.1.3 Mechanical CleaningMechanical cleaning may con-sist of sand, shot, pumice, grit or vapor blasting, sodiumcarbonate slurry, sanding
40、, hard bristle brushing, grinding andrough polishing. Sand, shot, or grit blasting leaves surfacecontamination that will greatly increase the corrosion rate ofthe magnesium on exposure to salt water or humid environ-ment. If these methods are used, specific pickling proceduresmust be employed after
41、blasting (see 6.4.2).6.1.4 Alkaline CleaningCleaning prior to application oftreatments other than the chrome pickle treatment (see 5.1),when used for protection during shipment or storage, should bedone in an alkaline cleaner recommended for steel or in acleaning solution as specified in 6.1.4.1. Ma
42、intain the solutionpH above 8.0. Alkaline cleaning prior to the application of thechrome pickle treatment (see 5.1), when used for protectionduring shipment and storage only, may be omitted provided theparts are free of grease, oil, and other deleterious deposits at thetime of application. Alkaline
43、cleaning solutions containingmore than 2 % sodium hydroxide will etch ZK60A, ZK60B,and some other magnesium alloys producing a change indimensions. If such a dimensional change is undesirable, usecleaners with lower alkali content.6.1.4.1 Alkaline cleaning may be carried out in solutions ofproprieta
44、ry cleaners. In this case the operating conditionsshould be as specified by the supplier. In no case should acleaner having pH lower than 8.0 be used. Most recommendedcleaners are used by simple immersion.After alkaline cleaning,rise parts thoroughly in cold running water. No water breaksshould be o
45、bserved in the rinse.6.1.5 Electrolytic CleaningUse of anodic current forcleaning is not generally recommended because of the possibleformation of oxide films, pitting of the magnesium surface, orboth. However, electrolytic cleaning using cathodic current at1 to 4 A/dm2may be carried out in properly
46、 formulatedcleaners.6.2 Graphite Lubricant Removal:6.2.1 Remove graphite-based lubricants from hot formedmagnesium sheet parts by soaking the parts for 10 to 20 min in100 g/L sodium hydroxide maintained at 88 to 100C. The pHshould be above 13.0.Add wetting agent (0.75 g/L), if needed,for the removal
47、 of heavy films of mineral oil. Then rinse partsthoroughly in cold water and immerse for 3 min in a chromic-nitrate pickle as specified in 6.5.2. Repeat the cycle until allparts are clean.6.2.2 Because of the difficulty of removing graphite fromchrome pickled sheet, such sheet should not be used for
48、forming unless the chrome pickle is removed as outlined in 6.3before forming.6.3 Previously Applied Chemical Finishes:6.3.1 Magnesium base alloys are often supplied with achrome pickle treatment to protect them during shipment,storage, and machining. The coating from this treatmentremaining on unmac
49、hined areas will impair the film producedby any subsequent chromate treatment and therefore must beremoved.6.3.2 Previously applied coatings may be removed with thealkaline cleaners recommended in 6.1.4.6.3.3 If the finish is difficult to remove, immerse the part inthe chromic acid pickle given in 6.5.1. Alternate immersion inthe alkaline cleaner and the chromic acid pickle may berequired to remove aged finishes. Rinse well in water betweenacid and alkaline pickling.6.3.4 The chromic acid brush-on treatment (see 5.4) may beapplied over the chrome pi
