1、Designation: B449 93 (Reapproved 2015)Standard Specification forChromates on Aluminum1This standard is issued under the fixed designation B449; 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
2、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 This specification covers the requirements relating torinse
3、d and nonrinsed chromate conversion coatings on alumi-num and aluminum alloys intended to give protection againstcorrosion and as a base for other coatings. This edition of thespecification has been coordinated with ISO/DIS 10546 and istechnically equivalent.1.2 Aluminum and aluminum alloys are chro
4、mate coated inorder to retard corrosion; as a base for organic films includingpaints, plastics, and adhesives; and as a protective coatinghaving a low electrical contact impedance.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstand
5、ard.1.4 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. Referenc
6、ed Documents2.1 ASTM Standards:2B117 Practice for Operating Salt Spray (Fog) ApparatusB602 Test Method for Attribute Sampling of Metallic andInorganic CoatingsB767 Guide for Determining Mass Per Unit Area of Elec-trodeposited and Related Coatings by Gravimetric andOther Chemical Analysis ProceduresD
7、1730 Practices for Preparation of Aluminum andAluminum-Alloy Surfaces for PaintingD3359 Test Methods for Measuring Adhesion by Tape Test2.2 ISO Standards:3ISO 2409 Paint and VarnishesCross-Cut TestISO 3768 Metallic CoatingsNeutral Salt Spray Test (NSSTest)ISO 3892 Conversion Coatings on Metallic Mat
8、erialsDetermination of Mass Per Unit AreaGravimetricMethodISO 4519 Electrodeposited Metallic Coatings and RelatedFinishesSampling Procedures for Inspection by Attri-butesISO/DIS 10546 Chemical Conversion CoatingsRinsedand Nonrinsed Chromate Conversion CoatingsOn Alu-minum and Aluminum Alloys2.3 Fede
9、ral Standard:4Fed. Std. No. 141 Paints, Varnish, Lacquer, and RelatedMaterials; Methods of Inspection2.4 Military Specification:4MIL-DTL-5541 Chemical Films for Aluminum and Alumi-num Alloys3. Terminology3.1 Definitions:3.1.1 nonrinsedchromate coatings that are dried immedi-ately after the chromatin
10、g step without receiving a water rinse.3.1.1.1 DiscussionThis special type of coating is typicallyused on long coils of aluminum sheet stock that receive animmediate subsequent paint or adhesive coating.NOTE 1Nonrinsed chromate coatings are finding increased usage onfabricated parts and castings.3.1
11、.2 rinsedchromate coatings that are rinsed in waterprior to drying.3.1.2.1 DiscussionThis type of coating is typically appliedto extruded aluminum fabricated parts and castings.4. Classification4.1 Chromate finishes can be applied ranging in color frombrown, thick coatings (Class 1) providing maximu
12、m corrosion1This specification is under the jurisdiction of ASTM Committee B08 onMetallic and Inorganic Coatings and is the direct responsibility of SubcommitteeB08.07 on Conversion Coatings.Current edition approved March 1, 2015. Published April 2015. Originallyapproved in 1967. Last previous editi
13、on approved in 2010 as B449 93 (2010)1.DOI: 10.1520/B0449-93R15.2For 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 websi
14、te.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.4Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098.Copyright ASTM International, 100 Barr Harb
15、or Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1protection to yellow, intermediate thickness coatings (Class 2)suitable as an organic film base or to colorless, thin coatings(Class 3) suitable for low electrical contact resistance. Theyellow coatings vary from golden yellow to
16、 iridescent lightyellow. Chromate-phosphate finishes (Class 4) can be appliedranging in color from green to iridescent light green. The Class4 coatings comply with the requirements of MIL-DTL-5541.4.2 Finishers can seldom guarantee to supply exact shadesof color with chromate conversion coatings. If
17、 it is necessary tohave exact shades of color, it is possible to dye chromatecoatings having a coating mass greater than 0.4 g/m2to obtaina wide range of colors, but they can only be expected to givean order of added corrosion resistance similar to that providedby the undyed coatings. It should be n
18、oted that color and coloruniformity will vary somewhat between one alloy and anotherand from a polished surface to an etched surface. Iridescenceand variations in color density from one area of the surface toanother are normal and shall not be considered a sign of poorquality.4.3 The finishes are di
19、vided into four classes; their mostimportant characteristics are listed in Table 1.5. Surfaces Preparation5.1 The surfaces of the parts to be chromated must be cleanand free of any oxidation, scale, or soils such as metal turnings,grinding dust, oil, grease, lubricants, hand-sweat, or any otherconta
20、mination detrimental to the chromating process. The partsmust therefore, as far as necessary, be cleaned before chromat-ing and if necessary be pickled. Fig. X2.1 shows the variousprocessing step options.6. Methods of Application of Chromate Coatings6.1 Metallic material other than aluminum should n
21、ot betreated with the parts to be chromated.6.2 Chromate conversion coatings are normally applied bydipping: the coating may also be applied by inundation,spraying, roller coating, or by wipe-on techniques. The appli-cation method used should be taken from the operatinginstructions for the chromatin
22、g process employed. Chromatingsolutions are usually acidic and may contain hexavalentchromium salts together with other salts that may be varied toaffect the appearance and hardness of the film. The color of thefilm, and, therefore, the type of conversion coating, depends onthe composition of the ch
23、romating solution, but it is alsoaffected by the pH and temperature, the duration of thetreatment, and the nature and surface condition of the alloybeing treated.6.3 These coatings receive a final water rinse. If the coatingis meant to be a basis for additional coatings, the detail shall besubject t
24、o a rinse in deionized water with a conductivity lessthan 100t S/cm. If hot water is used as the final rinse after thechromating process, it is essential that the time of rinsingshould be kept as short as possible in order to prevent thedissolution of the hexavalent chromium. The drying of thecoatin
25、g shall be carried out at a temperature not exceeding60C to prevent cracking due to dehydration, which causes lossof adhesion and performance of the chromate coating.6.4 Any additional subsequent treatments depend upon thepurpose for which the chromated parts are intended.7. Coating Requirements7.1
26、GeneralChromate conversion coatings harden withage by gradual dehydration. They should, therefore, be handledcarefully for the first 24 h after treatment, and any tests(including corrosion tests) shall be deferred until the expirationof that period. The green chromate-phosphate coatings usuallyconti
27、nue to improve in corrosion resistance after initial forma-tion. They achieve their maximum corrosion resistance after 1to 2 months at room temperature. It is not required to storeparts for this purpose.7.2 Electrical ResistanceColorless, light yellow, or lightgreen iridescent chromate layers of low
28、 mass per unit areaincrease the electrical resistance between an electrical contactand the aluminum to a very small extent. When measured at9-V and a 2-A current the resistance should be less than 0.1 .Highly colored brown, yellow, or green coatings show amarked increase in electrical contact resist
29、ance with increasingmass per unit area of the chromate layer and may reachresistances of 10 000 or more.7.3 AdhesionThe coatings shall be adherent and non-powdery. There are no practical tests for measuring theadhesion of a chromate conversion coating on aluminum.However, a practical evaluation of t
30、he adhesion can be madeby measuring the adhesion of a secondary organic film appliedto the chromated aluminum. When specified, the chromateconversion coating shall pass the organic coating adhesion testin Test Methods D3359 or the equivalent ISO 2409.7.3.1 Class 4 coatings intended for use under MIL
31、-DTL-5541 shall have their adhesion evaluated by Method 6301 ofFed. Std. No. 141.7.3.2 Additional treatments applied under MIL-DTL-5541can be found in Practices D1730, Methods 5, 6, and 7.7.4 Corrosion ResistanceWhen subjected to the neutralsalt spray test specified in Practice B117 or in the equiva
32、lentISO 3768, three separate test specimens of the coating shallwithstand exposure for the hours shown in Table 2 withoutgiving evidence, to the unaided eye, of more than a total of 8isolated spots or pits. None shall be larger than 1 mm indiameter. Each individual test specimen shall not have moreT
33、ABLE 1 Classification of Chromate CoatingsClassAAppearanceCoating Mass perUnit Area, g/m2Corrosion Protection1 yellow to brown 0.4 to 2 maximum corrosion resistancegenerally used as final finish2 colorless to yellow 0.1 to 0.4 moderate corrosion resistance,used as a paint base and forbonding to rubb
34、er3 colorless 1 %1 500 336 482 250 168 243 168 120 124 500 336 48AClass numbers are assigned on the basis of traditional usage and do not reflecta ranking of corrosion resistance.BThe exposure times are indicative of the relative corrosion resistance of thevarious coating classes on the different al
35、loys, but no direct relationship existsbetween performance in the neutral salt spray test and performance in service.B449 93 (2015)3hexavalent chromium. The least quantity of total chromiumdetectable by this method corresponds to about 5 mg chromiumper square metre of surface.X1.3 Testing for Phosph
36、ateX1.3.1 To test for the presence of phosphate in the chro-mated layer, a sample is taken whose surface to be tested isabout 100 cm2; this is treated with 100 mL of 5 % sodiumhydroxide solution at 80 to 90C until the layer is completelydissolved or at least until the surface has been clearly attack
37、ed.The resulting solution is filtered, and 25 mL of the filtrate areacidified with approximately 38 % nitric acid then 10 mLammonium molybdate reagent and 5 g ammonium nitrate areadded. The sample is allowed to stand for at least 15 min. Ayellow precipitate shows the presence of phosphate. The least
38、quantity of phosphate detectable by this process corresponds toabout 40 mg P2O5per square metre of surface.X1.4 Testing for ZincX1.4.1 To test for the presence of zinc in the layer, a sampleis taken whose surface to be tested is about 100 cm2; this istreated with 50 mL of approximately 65 % nitric a
39、cid at roomtemperature until the layer is either completely dissolved or atleast until the surface has clearly been attacked. The resultingsolution is filtered through glass wool, and 25 mL of the filtrateare neutralized with 20 % caustic soda until phenolphthaleinindicator gives a red coloration. T
40、he solution is then weaklyacidified by addition of about 10 drops of 25 % sulfuric acid.After addition of 5 mL of 5 % potassium ferrocyanide solution,a greenish-white precipitate shows the presence of zinc. Theleast quantity of zinc detectable by this process corresponds toabout 20 mg zinc per squar
41、e metre of surface.X2. TREATMENT SEQUENCE OPTIONSX2.1 The chromate process necessitates a specified se-quence of treatments that depends upon the surface conditionof the parts to be treated, the aluminum alloy used, and thedesired appearance of the parts. See Fig. X2.1.FIG. X2.1 Treatment Stages of
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