1、Designation: D 1732 03Standard Practices forPreparation of Magnesium Alloy Surfaces for Painting1This standard is issued under the fixed designation D 1732; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、 A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 These practices cover two classes of treatment for
3、preparation 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, solvent clean-ing, alkaline solution treatments, and acid pickles not re
4、sultingin protective conversion coatings are suitable preliminarytreatments only for metal to be exposed under mildly corrosive(indoor) exposures. When a high degree of corrosion protec-tion and paint adhesion are desired, as in many outdoorenvironments, surface preparation by one of the aboveconver
5、sion-coat classes is necessary. The hexavalent chromiumbased methods given are not recommended as hexavalentchromium is a known carcinogen.NOTE 1Testing of CoatingsQuality control tests of coatings arefrequently desirable, and these generally consist of exposures, with orwithout paint, to salt spray
6、, 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 r
7、ate 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 mate
8、rials, opera-tions, 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
9、 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 conversioncoatings only to surfaces previously freed from all co
10、ntamina-tion, including oxide, rolling-scale, corrosion product,burned-on drawing and forming lubricant, and the contamina-tion introduced by blast cleaning and fabrication operations.Contamination in or under surface conversion coatings seri-ously reduces their protective values (Note 2). For the r
11、emovalof tenacious surface contamination, such as rolling-scale orcasting skin, an acid pickle to dissolve some of the actualsurface is essential. When organic contamination, such asgrease or oil, is also present, an initial degreasing operation insolvent or in an alkaline degreasing solution is usu
12、ally neces-sary to allow the subsequent acid to wet the surface. Thesematters are discussed in more detail under the headings of thespecific cleaners or treatments (Note 3), as follows:2.2 Alkaline CleanersOil, grease, and old (but not baked)chrome-pickle coatings are readily removed by most commer-
13、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:Cau
14、stic soda (NaOH) 12 oz (90 g)Wetting agent 0.1 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 c
15、hromium 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 of ASTM Committee B08 on Metallicand Inorganic Coatings and is the direct responsibility of Subcommit
16、tee B08.07 onChromate Conversion Coatings.Current edition approved Sept. 10, 2003. Published Sept. 2003. Originallyapproved in 1960. Last previous edition approved in 1998 as D 1732-67 (1998).2For information concerning magnesium and aluminum alloys, see ASTMSpecification B 80, B 90, B 91, B 93, B 1
17、07, and B 209 covering these alloys, in thesection on Aluminum and Magnesium and Their Alloys, Annual Book of ASTMStandards, Vol 02.02. See also ASTM Practice B 275, for Codification of CertainNonferrous Metals and Alloys, Cast and Wrought.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box
18、 C700, West Conshohocken, PA 19428-2959, United States.2.3 Acid Cleaners (Note 1)Acid picking removes mill-scale, oxide, corrosion product, and the like. Use as a prelimi-nary treatment for surface conversion coatings when thehighest degrees of surface uniformity and protective values arerequired. A
19、cid cleaners are as follows:2.3.1 For Sand and Permanent Mold Castings:2.3.1.1 Nitric-Sulfuric Acid Solution Use a solution of 8volume % of concentrated nitric acid (HNO3) plus 2 volume %of concentrated sulfuric acid (H2SO4) in water (see Note 1(a)at 70 to 90F (21 to 32C) as a preliminary treatment
20、for newsand castings and to remove the surface-contaminating effectsof blast cleaning. Immerse for about 10 to 15 s, or until 0.002in. (51 m) per surface is removed. The solution may be heldin ceramic, rubber, synthetic rubber, or vinyl-lined tanks.2.3.1.2 Chromic Acid SolutionUse a boiling 20 weigh
21、t %solution of chromic anhydride (CrO3) in water to remove oldchemical and anodic treatments, corrosion product, and oxidelayers, without significant dissolution of metal and hencewithout changing the dimensions of machined parts. Immer-sion time varies from 1 to 5 min, depending upon the conditiono
22、f the surface. The solution may be held in lead-lined steel orASTM alloy No. 990A or its Aluminum Association equiva-lent, alloy No. 1100 aluminum tanks.2.3.2 For Die Castings:2.3.2.1 Chromium Trioxide-Nitric-Hydrofluoric AcidSolutionThis solution is used to produce a smut-free surfaceon die casting
23、s, without violent attack of the metal. Thesolution shall conform to the following composition:Chromium trioxide (CrO3) 37.5 oz (280 g)Hydrofluoric acid (60% HF) 1 fl oz (8 ml)Nitric acid (70% HNO3) 3.25 fl oz (25 ml)Water (Note 4) to 1 gal (1 litre)Immerse the parts in the above solution at 70 to 9
24、0F (21 to32C) for 30 s to 2 min, or until a bright, clean surface isobtained. The solution may be held in tanks lined withsynthetic rubber or vinyl-base materials.2.3.3 For Wrought Products:2.3.3.1 Acetic Acid-Nitrate SolutionThis solution rapidlyremoves surface contamination to 0.001 in. (25.4 m).
25、Use forwrought parts subsequently to be finished for the maximumprotective value. The solution shall conform to the followingcomposition:Glacial acetic acid 25.5 fl oz (199 ml)Sodium (NaNO3) 6.6 oz (49.5 g)Water (Note 4) to 1 gal (1 litre)Immerse the parts in the above solution at 70 to 90F (21 to32
26、C) for 30 s upwards, or until a bright, clean surface isobtained. When heavy surface contamination, such as hot-rolled mill-scale is to be removed, immersion times shall besufficient to remove at least 0.001 in. (25 m) per surface. Thesolution may be held in No. 990A aluminum, ceramic, orrubber-line
27、d tanks.2.3.3.2 Chromium Trioxide-Nitrate Solution Use this so-lution following the use of the method described in 2.2 for theremoval of burned-on graphite lubricants from hot-formedparts. The solution shall conform to the following composition:Chromium trioxide (CrO3) 1.5 lb (180 g)Sodium nitrate (
28、NaNO3) 2 oz (15 g)Water (Note 4) to 1 gal (1 litre)Immerse the parts in the above solution at 70 to 90F (21 to32C) for approximately 5 min, for the removal of graphite.2.4 Abnormally slow reaction in the above solution indi-cates that it is depleted and that the pH has risen to 1.7 orhigher. Revivif
29、y the solution by the addition of chromiumtrioxide to bring the pH back to 0.5 to 0.7. Attempt no morethan four revivifications. The solution may be held in ceramic,No. 990A aluminum, 18-8 stainless steel, or synthetic rubber-lined tanks.NOTE 2Removal of Contamination by Welding FluxesWhen a partto
30、be painted has been welded by an operation involving the use of flux,such flux shall be removed before the subjection of the part to any surfacepreparation process. Such removal shall be made immediately by immers-ing the part in hot water with scrubbing, and finally by immersing it for 1h in a boil
31、ing 5 % solution of sodium dichromate, after which the partshall be well rinsed.NOTE 3Suspension of Articles for TreatmentThe use of magnesiumalloy suspension wires is preferred for use in acid pickles in order to avoidobjectionable contamination of the solutions through dissolution of thewire mater
32、ials. Heavy metal contamination, particularly of copper, maydeposit on the magnesium surface and lead to seriously reduced corrosionresistance. Copper suspension wires in the hot dichromate solutions arenot objectionable.NOTE 4Quality of WaterIn the preparation and makeup of acidpickles, dichromate
33、solutions, and hot-water rinses, precautions shall betaken against the use of water contaminated with heavy-metal impurities,or excessive chlorides or sulfates. No upper limits can be specified at thisstage for soluble impurities in the water, but powdering of coatings andpoor resistance to corrosio
34、n are known to result from the use ofcontaminated water. Thus, when a choice exists, water from steamcondensate or ion-exchange-treated water shall be employed in preferenceto well water or hard tap water.SURFACE PREPARATION PROCEDURESCHEMICAL3. Class I, Type I (Chrome Pickle)33.1 ScopeClass I, Type
35、 I treatment is applicable to allforms and alloys of magnesium except certain special alloyscontaining silver, but since it may remove as much as 0.0006in. (15 m) of metal per surface, it shall not be used on partsmachined to fine tolerances. When properly applied, theprocess constitutes a good pain
36、t base, but rigid control isrequired at each step. The treatment is applicable to magne-sium alloy containing inserts of, or attached to, other metals.3.2 ProcedureFor wrought parts the bath shall conform tothe following composition:Sodium dichromate (Na2Cr2O72H2O) 1.5 lb (180 g)Nitric acid (HNO3) (
37、sp gr 1.42) 1.5 pt (187 ml)Water (Note 4) to 1 gal (1 litre)3.2.1 For die-, sand- and permanent-mold castings thesolution shall conform to the following composition:Sodium dichromate (Na2Cr2O72H2O) 1.5 lb (180 g)Nitric acid (HNO3) (sp gr 1.42) 1.5 pt (187 ml)Sodium potassium, or ammonium acid fluori
38、de 2 oz (15 g)(NaHF2, KHF3,orNH4HF2)Water (Note 4) to 1 gal (1.0 litres)For wrought products, sand, and permanent-mold castingsthe above solutions operate at 70 to 90F (21 to 32C). The3Conforming to Class I, Type I treatments are the Dow No. 1 process, the AMC“A” process, and the Type I process of M
39、ilitary Specification MIL-M-3171A.D1732032immersion times shall be from 1 to 2 min, the necessary timeincreasing with use of the solution. For die-castings give theparts a 15 to 30 s dip in water at 160 to 180F (71 to 82C),followed immediately by a 10-s dip in the second of the abovebaths, operated
40、at 120 to 140F (49 to 60C). Failure to preheatthe castings results in no coating in 10 s.3.2.2 Following immersion remove the parts, allow to drainfor not less than 5 s nor more than 30 s, then wash thoroughlyin cold running water, followed by a dip in hot water at 160 to180F (71 to 82C) to facilita
41、te drying. Do not allow the partsto drain following the chromate treatment for more than thespecified 30 s; excessive drainage times result in powderycoatings of poor value as paint bases. Such coatings also resultfrom the use of hot-water rinses, the temperatures of which arein excess of 180F (82C)
42、. Paint the parts preferably immedi-ately after they are dry. The solution may be held in Type 316stainless steel, or ceramic tanks or in steel tanks lined withsynthetic rubber or vinyl-base materials. Tanks of ASTM alloyNo. 990 A, or its Aluminum Association equivalent, Alloy No.1100 aluminum are s
43、atisfactory for the nonfluoride-containingpickle.3.3 Revivification of SolutionsSluggish reaction with themetal, associated with pale yellow, lustrous coatings, indicatesthat the solution is depleted. Revivification is accomplished bythe addition of dichromate and nitric acid, to raise the dichro-ma
44、te again to 1.5 lb (180 g)/gal (1 litre) and the free nitric acidto levels indicated in the table below. Revivify when the freenitric acid content of the solution is depleted to 0.5 pt/gal (62.3ml/1 litre) (see Section 11 for analytical procedure) and shalltake place once only for M1 and ZK60A alloy
45、s,3and not morethan six times for other alloys if good paint-base properties aredesired. Excessive use of the solution or too many revivifica-tions result in smooth, lustrous coatings not possessing thedegree of etch necessary for the best paint adhesion withconventional primers. Revivify in accorda
46、nce with the follow-ing table:RevivificationNumberAdjust Concentrationof HNO3to:1 1.3 pt/gal (162 ml/1 litre)2 1.1 pt/gal (137 ml/1 litre)3 to 6 0.9 pt/gal (113 ml/1 litre)4. Class I, Type II (Sealed Chrome Pickle)44.1 ScopeClass I, Type II treatment is applicable to alltypes and forms of magnesium-
47、base alloys, subject only to thelimitations of the Class I, Type I treatment, since it isessentially the same process as the latter, followed by sealing.In protective qualities and as a paint base, this treatment issomewhat superior to that of Class I, Type I treatment undersevere exposure condition
48、s (Note 5).4.2 ProcedureFollowing chrome pickling as specifiedunder Section 3 and rinsing in cold water, transfer the partsimmediately to a boiling solution conforming to the followingcomposition:Sodium dichromate (Na2Cr2O72H2O) 1.5 lb (180 g)Calcium or magnesium fluoride (CaF2or MgF2)13 oz (2.5 g)W
49、ater (Note 4) to 1 gal (1.0 litre)Boil the parts in the above solution for 30 min, after whichrinse them in cold running water, followed by a rinse in hotwater at a temperature of not less than 160F (71C) nor morethan 180F (82C) to facilitate drying. Preferably apply thepaint coating immediately after the parts are dry. The solutionmay be held in a steel tank.NOTE 5Causes of Defective CoatingsThe following information isintended to provide guidance on the causes of the most usual defectsarising in the application of either Class I,
