1、Designation: B 449 93 (Reapproved 2004)Standard Specification forChromates on Aluminum1This standard is issued under the fixed designation B 449; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number i
2、n 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 This specification covers the requirements relating torinsed
3、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 10546 and istechnically equivalent.1.2 Aluminum and aluminum alloys are chromate c
4、oated 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 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibil
5、ity 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 ASTM Standards:2B 117 Practice for Operating Salt Spray (Fog) ApparatusB 602 Test Method for Attribute Sampling of
6、 Metallic andInorganic CoatingsB 767 Guide for Determining Mass Per Unit Area of Elec-trodeposited and Related Coatings by Gravimetric andOther Chemical Analysis ProceduresD 1730 Practices for Preparation of Aluminum andAluminum-Alloy Surfaces for PaintingD 3359 Test Methods for Measuring Adhesion b
7、y Tape Test2.2 ISO Standards:ISO 2409 Paint and VarnishesCross-Cut Test3ISO 3768 Metallic CoatingsNeutral Salt Spray Test(NSS Test)3ISO 3892 Conversion Coatings on Metallic MaterialsDetermination of Mass Per Unit AreaGravimetricMethod3ISO 4519 Electrodeposited Metallic Coatings and RelatedFinishesSa
8、mpling Procedures for Inspection by At-tributes3ISO/ DIS 10546 Chemical Conversion CoatingsRinsedand Nonrinsed Chromate Conversion CoatingsOn Alu-minum and Aluminum Alloys32.3 Federal Standard:Fed. Std. No. 141 Paints, Varnish, Lacquer, and RelatedMaterials; Methods of Inspection42.4 Military Specif
9、ication:MIL-C-5541 Chemical Films for Aluminum and AluminumAlloys43. Terminology3.1 Definitions:3.1.1 nonrinsedchromate coatings that are dried immedi-ately after the chromating step without receiving a water rinse.3.1.1.1 DiscussionThis special type of coating is typicallyused on long coils of alum
10、inum sheet stock that receive animmediate subsequent paint or adhesive coating.NOTE 1Nonrinsed chromate coatings are finding increased usage onfabricated parts and castings.3.1.2 rinsedchromate coatings that are rinsed in waterprior to drying.3.1.2.1 DiscussionThis type of coating is typically ap-pl
11、ied to extruded aluminum fabricated parts and castings.4. Classification4.1 Chromate finishes can be applied ranging in color frombrown, thick coatings (Class 1) providing maximum corrosionprotection to yellow, intermediate thickness coatings (Class 2)suitable as an organic film base or to colorless
12、, thin coatings(Class 3) suitable for low electrical contact resistance. Theyellow coatings vary from golden yellow to iridescent lightyellow. Chromate-phosphate finishes (Class 4) can be applied1This specification is under the jurisdiction of ASTM Committee B08 onMetallic and Inorganic Coatings and
13、 is the direct responsibility of SubcommitteeB08.07 on Chemical Conversion Coatings.Current edition approved April 1, 2004. Published April 2004. Originallyapproved in 1967. Last previous edition approved in 1998 as B 449 93 (1998).2For referenced ASTM standards, visit the ASTM website, www.astm.org
14、, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.4Available from Standa
15、rdization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-50981Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.ranging in color from green to iridescent light green. The Class4 coatings comply
16、 with the requirements of MIL-C-5541.4.2 Finishers can seldom guarantee to supply exact shadesof color with chromate conversion coatings. If 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
17、, but they can only be expected to givean order of added corrosion resistance similar to that providedby the undyed coatings. It should be noted that color and coloruniformity will vary somewhat between one alloy and anotherand from a polished surface to an etched surface. Iridescenceand variations
18、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 divided 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
19、cleanand free of any oxidation, scale, or soils such as metal turnings,grinding dust, oil, grease, lubricants, hand-sweat, or any othercontamination detrimental to the chromating process. The partsmust therefore, as far as necessary, be cleaned before chromat-ing and if necessary be pickled. Fig. X2
20、.1 shows the variousprocessing step options.6. Methods of Application of Chromate Coatings6.1 Metallic material other than aluminum should not betreated with the parts to be chromated.6.2 Chromate conversion coatings are normally applied bydipping: the coating may also be applied by inundation,spray
21、ing, roller coating, or by wipe-on techniques. The appli-cation method used should be taken from the operatinginstructions for the chromating process employed. Chromatingsolutions are usually acidic and may contain hexavalentchromium salts together with other salts that may be varied toaffect the ap
22、pearance and hardness of the film. The color of thefilm, and, therefore, the type of conversion coating, depends onthe composition of the chromating solution, but it is alsoaffected by the pH and temperature, the duration of thetreatment, and the nature and surface condition of the alloybeing treate
23、d.6.3 These coatings receive a final water rinse. If the coatingis meant to be a basis for additional coatings, the detail shall besubject to 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 t
24、he time of rinsingshould be kept as short as possible in order to prevent thedissolution of the hexavalent chromium. The drying of thecoating shall be carried out at a temperature not exceeding60C to prevent cracking due to dehydration, which causes lossof adhesion and performance of the chromate co
25、ating.6.4 Any additional subsequent treatments depend upon thepurpose for which the chromated parts are intended.7. Coating Requirements7.1 GeneralChromate conversion coatings harden withage by gradual dehydration. They should, therefore, be handledcarefully for the first 24 h after treatment, and a
26、ny tests(including corrosion tests) shall be deferred until the expirationof that period. The green chromate-phosphate coatings usuallycontinue to improve in corrosion resistance after initial forma-tion. They achieve their maximum corrosion resistance after 1to 2 months at room temperature. It is n
27、ot required to storeparts for this purpose.7.2 Electrical ResistanceColorless, light yellow, or lightgreen iridescent chromate layers of low 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 curren
28、t the resistance should be less than 0.1 V.Highly colored brown, yellow, or green coatings show amarked increase in electrical contact resistance with increasingmass per unit area of the chromate layer and may reachresistances of 10 000 V or more.7.3 AdhesionThe coatings shall be adherent and non-po
29、wdery. There are no practical tests for measuring theadhesion of a chromate conversion coating on aluminum.However, a practical evaluation of the adhesion can be madeby measuring the adhesion of a secondary organic film appliedto the chromated aluminum. When specified, the chromateconversion coating
30、 shall pass the organic coating adhesion testin Test Methods D 3359 or the equivalent ISO 2409.7.3.1 Class 4 coatings intended for use under MIL-C-5541shall have their adhesion evaluated by Method 6301 of Fed.Std. No. 141.7.3.2 Additional treatments applied under MIL-C-5541 canbe found in Practices
31、D 1730, Methods 5, 6, and 7.7.4 Corrosion ResistanceWhen subjected to the neutralsalt spray test specified in Practice B 117 or in the equivalentISO 3768, three separate test specimens of the coating shallwithstand exposure for the hours shown in Table 2 withoutgiving evidence, to the unaided eye, o
32、f more than a total of 8isolated spots or pits. None shall be larger than 1 mm indiameter. Each individual test specimen shall not have morethan 5 isolated spots or pits, none larger than 1 mm in diameteron their respective surfaces. Spots within 10 mm of the edgesof the panels are not counted.7.5 C
33、oating Mass Per Unit AreaThe mass of the coatingper unit area of coated surface shall conform to the valuesgiven in Table 1. Use the procedure in ISO 3892 as referencedin Guide B 767.7.5.1 Coating Mass SignificanceIt should be recognizedthat heavier coating masses do not always provide betterTABLE 1
34、 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 rubber3 co
35、lorless 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 notreflect a ranking of corrosion resistance.BThe exposure times are indicative of the relative corrosion resistance of thevarious coating classes on the different alloys,
36、but no direct relationship existsbetween performance in the neutral salt spray test and performance in service.B 449 93 (2004)3quantity 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 th
37、e layer, a sampleis taken whose surface to be tested is about 100 cm2; this istreated with 50 mL of approximately 65 % nitric acid at roomtemperature until the layer is either completely dissolved or atleast until the surface has clearly been attacked. The resultingsolution is filtered through glass
38、 wool, and 25 mL of the filtrateare neutralized with 20 % caustic soda until phenolphthaleinindicator gives a red coloration. The 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 precip
39、itate shows the presence of zinc. Theleast quantity of zinc detectable by this process corresponds toabout 20 mg zinc per square 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 par
40、ts to be treated, the aluminum alloy used, and thedesired appearance of the parts. See Fig. X2.1.FIG. X2.1 Treatment Stages of the Chromating of Aluminum andAluminum AlloysB 449 93 (2004)4ASTM International takes no position respecting the validity of any patent rights asserted in connection with an
41、y item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical
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43、t a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor
44、Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).B 449 93 (2004)5
