ASTM B921-08

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Designation:B921 −08
StandardSpecification for
Non-hexavalent Chromium Conversion Coatings on
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Aluminum and Aluminum Alloys
This standard is issued under the fixed designation B921; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.3 Federal Standard:
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
1.1 This specification covers the requirements relating to
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Materials; Methods of Inspection
rinsed and non-rinsed non-hexavalent chromium conversion
2.4 Military Specification:
coatings on aluminum and aluminum alloys intended to give
MIL-DTL-5541 Chemical Conversion Coatings on Alumi-
protection against corrosion and as a base for other coatings.
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num and Aluminum Alloys
1.2 Aluminum and aluminum alloys are conversion coated
MIL-DTL-81706 Chemical Conversion Materials For Coat-
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in order to retard corrosion; as a base for organic films
ing Aluminum and Aluminum Alloys
including paints, plastics, and adhesives; and as a protective
coating having a low electrical contact impedance. 3. Terminology
1.3 This standard does not purport to address all of the
3.1 Definitions:
safety concerns, if any, associated with its use. It is the
3.1.1 non-rinsed—conversion coatings that are dried imme-
responsibility of the user of this standard to establish appro-
diately after the conversion coating step without receiving a
priate safety and health practices and determine the applica- water rinse.
bility of regulatory requirements prior to use.
3.1.1.1 Discussion—This special type of coating is typically
used on long coils of aluminum sheet stock that receive an
2. Referenced Documents
immediate subsequent paint or adhesive coating.
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2.1 ASTM Standards:
NOTE 1—Non-rinsed conversion coatings are finding increased usage
B117 Practice for Operating Salt Spray (Fog) Apparatus
on fabricated parts and castings.
B602 Test Method for Attribute Sampling of Metallic and
3.1.2 rinsed—conversion coatings that are rinsed in water
Inorganic Coatings
prior to drying.
D1730 Practices for Preparation of Aluminum and
3.1.2.1 Discussion—Thistypeofcoatingistypicallyapplied
Aluminum-Alloy Surfaces for Painting
to extruded aluminum fabricated parts and castings.
D3359 Test Methods for Measuring Adhesion by Tape Test
4. Classification
2.2 ISO Standards:
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ISO 2409 Paint and Varnishes—Cross-Cut Test
4.1 A class one conversion coating provides for maximum
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
salt fog corrosion protection to bare metal. A class two
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Test)
conversion coating is designed to give painted salt fog corro-
ISO 4519 Electrodeposited Metallic Coatings and Related
sion resistance and some bare salt fog corrosion resistance. A
Finishes—Sampling Procedures for Inspection by Attri-
class three conversion coating provides little salt fog corrosion
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butes
resistance, but low electrical contact resistance (see MIL-DTL-
5541). Class four conversion coatings only provide for painted
salt fog corrosion resistance.
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This specification is under the jurisdiction of ASTM Committee B08 on
4.2 The finishes are divided into four classes; their most
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
B08.07 on Conversion Coatings. important characteristics are listed in Table 1.
Current edition approved April 1, 2008. Published April 2008. Originally
approved in 2002. Last previous edition approved in 2002 as B921–02. DOI:
5. Surfaces Preparation
10.1520/B0921-08.
2 5.1 The surfaces of the parts to be conversion coated shall
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
be clean and free of any oxidation, scale, or soils such as metal
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
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B921−08
TABLE 1 Classification of Non-hexavalent Chromium TABLE 2 Relative Corrosion Resistance
Conversation Coatings
A
Exposure Time, h
Class Corrosion Protection
Heat-Treatable Alloys
Non-Heat- Cast Alloys with
Coating and Cast Alloys with
1 Maximum corrosion resistance generally used as a final
Treatable a Nominal Silicon
Class a Nominal Silicon
finish
Wrough
...