1、Designation: B921 08 (Reapproved 2013)Standard Specification forNon-hexavalent Chromium Conversion Coatings onAluminum and Aluminum Alloys1This standard is issued under the fixed designation B921; 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 specification covers the requirements relating torinsed and non-rinsed non-hexavalent c
3、hromium conversioncoatings on aluminum and aluminum alloys intended to giveprotection against corrosion and as a base for other coatings.1.2 Aluminum and aluminum alloys are conversion coatedin order to retard corrosion; as a base for organic filmsincluding paints, plastics, and adhesives; and as a
4、protectivecoating having 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 theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-b
5、ility of regulatory requirements prior to use.2. Referenced Documents2.1 ASTM Standards:2B117 Practice for Operating Salt Spray (Fog) ApparatusB602 Test Method for Attribute Sampling of Metallic andInorganic CoatingsD1730 Practices for Preparation of Aluminum andAluminum-Alloy Surfaces for PaintingD
6、3359 Test Methods for Measuring Adhesion by Tape Test2.2 ISO Standards:ISO 2409 Paint and VarnishesCross-Cut Test3ISO 3768 Metallic CoatingsNeutral Salt Spray Test (NSSTest)3ISO 4519 Electrodeposited Metallic Coatings and RelatedFinishesSampling Procedures for Inspection by Attri-butes32.3 Federal S
7、tandard:Fed. Std. No. 141 Paints, Varnish, Lacquer, and RelatedMaterials; Methods of Inspection42.4 Military Specification:MIL-DTL-5541 Chemical Conversion Coatings on Alumi-num and Aluminum Alloys4MIL-DTL-81706 Chemical Conversion Materials For Coat-ing Aluminum and Aluminum Alloys43. Terminology3.
8、1 Definitions:3.1.1 non-rinsedconversion coatings that are dried imme-diately after the conversion coating step without receiving awater 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 c
9、oating.NOTE 1Non-rinsed conversion coatings are finding increased usageon fabricated parts and castings.3.1.2 rinsedconversion 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. Classificat
10、ion4.1 A class one conversion coating provides for maximumsalt fog corrosion protection to bare metal. A class twoconversion coating is designed to give painted salt fog corro-sion resistance and some bare salt fog corrosion resistance. Aclass three conversion coating provides little salt fog corros
11、ionresistance, but low electrical contact resistance (see MIL-DTL-5541). Class four conversion coatings only provide for paintedsalt fog corrosion resistance.4.2 The finishes are divided into four classes; their mostimportant characteristics are listed in Table 1.1This specification is under the jur
12、isdiction of ASTM Committee B08 onMetallic and Inorganic Coatings and is the direct responsibility of SubcommitteeB08.07 on Conversion Coatings.Current edition approved May 1, 2013. Published May 2013. Originallyapproved in 2002. Last previous edition approved in 2008 as B921 08. DOI:10.1520/B0921-0
13、8R13.2For 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 website.3Available from American National Standards Institute (A
14、NSI), 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, http:/dodssp.daps.dla.mil.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Cons
15、hohocken, PA 19428-2959. United States15. Surfaces Preparation5.1 The surfaces of the parts to be conversion coated shallbe clean and free of any oxidation, scale, or soils such as metalturnings, grinding dust, oil, grease, lubricants, hand-sweat, orany other contamination detrimental to the convers
16、ion coatingprocess. The parts shall therefore, as far as necessary, becleaned in a non-hexavalent chromium cleaner before conver-sion coating and, if necessary, be pickled.6. Methods of Application of Conversation Coatings6.1 Metallic material other than aluminum shall not betreated with the parts t
17、o be conversion coated.6.2 Conversion coatings are normally applied by dipping:the coating may also be applied by inundation, spraying, rollercoating, or by wipe-on techniques. The application methodused shall be taken from the operating instructions for theconversion coating process employed. Conve
18、rsion coatingsolutions are usually acidic and may contain various salts thatmay be varied to affect the appearance and hardness of the film.The type of conversion coating depends on the composition ofthe solution and may also be affected by the pH, temperature,duration of the treatment, and the natu
19、re and surface conditionof the alloy being treated.6.3 Those coatings receiving a final water rinse. If thecoating is meant to be a basis for additional coatings, the detailshall be subject to a rinse in deionized water with a conduc-tivity less than 100 S/cm. Hot water shall be used only ifallowed
20、by the manufacturer of the conversion coating inquestion. The drying of the coating shall be carried out inaccordance with the specifications of the manufacturer of theconversion or conversion coating in question.6.4 Any additional subsequent treatments will depend uponthe direction of the manufactu
21、re of the conversion or conver-sion coating in question.7. Coating Requirements7.1 Electrical ResistanceWhen specified, the purchaserand supplier shall agree to an electrical resistive maximum andfrequency of testing. The method shall be in accordance withMIL-DTL-81706.7.2 AdhesionThe coatings shall
22、 be adherent and non-powdery. A practical evaluation of the adhesion can be madeby measuring the adhesion of a secondary organic film appliedto the conversion coating in question. When specified, theconversion coating shall pass the organic coating adhesion testin Test Methods D3359 or the equivalen
23、t ISO 2409.7.2.1 Class 4 coatings intended for use under MIL-DTL-5541 shall have their adhesion evaluated by Method 6301 ofFed. Std. No. 141.7.2.2 Additional treatments applied under MIL-DTL-5541can be found in Practices D1730, Methods 5, 6, and 7.7.3 Corrosion ResistanceWhen subjected to the neutra
24、lsalt spray test specified in Test Method B117 or in theequivalent ISO 3768, three separate test specimens of thecoating shall withstand exposure for the hours shown in Table2 without giving evidence, to the unaided eye, of more than atotal of 8 isolated spots or pits. None shall be larger than 1 mm
25、in diameter. 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.7.4 Test Specimens and SamplingsUnless otherwisespecified, the sampling plans of Te
26、st Method B602 or theequivalent ISO 4519 shall be used to test the coatings.7.4.1 Test specimens shall be of the same alloy and surfacecondition as the articles represented. Test specimens shall be150 by 100 mm.8. Keywords8.1 conversion coating; non-hexavalent chromium conver-sion coating; non-rinse
27、d; rinsedTABLE 1 Classification of Non-hexavalent ChromiumConversation CoatingsClass Corrosion Protection1 Maximum corrosion resistance generally used as a finalfinish2 Moderate corrosion resistance, used as a paint base and forbonding to rubber3 Decorative, slight corrosion resistance, low electric
28、al contactresistance4 Moderate corrosion resistance, used as a paint base and forbonding to rubberTABLE 2 Relative Corrosion ResistanceExposure Time, hACoatingClassNon-Heat-TreatableWrought AlloysHeat-Treatable Alloysand Cast Alloys witha Nominal SiliconContent 1 %1 500 336 482 250 168 243 168 120 1
29、24 500 336 48AThe exposure times are indicative of the relative corrosion resistance of thevarious coating classes on the different alloys, but no direct relationship existsbetween performance in the neutral salt spray test and performance in service.B921 08 (2013)2ASTM International takes no positi
30、on respecting the validity of any patent rights asserted in connection with any 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
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