ASTM B921-02
(Specification)Standard Specification for Non-hexavalent Chromium Conversion Coatings on Aluminum and Aluminum Alloys
Standard Specification for Non-hexavalent Chromium Conversion Coatings on Aluminum and Aluminum Alloys
SCOPE
1.1 This specification covers the requirements relating to rinsed and non-rinsed non-hexavalent chromium conversion coatings on aluminum and aluminum alloys intended to give protection against corrosion and as a base for other coatings.
1.2 Aluminum and aluminum alloys are conversion coated in order to retard corrosion; as a base for organic films including paints, plastics, and adhesives; and as a protective coating having a low electrical contact impedance.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory requirements prior to use.
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Designation: B 921 – 02
Standard Specification for
Non-hexavalent Chromium Conversion Coatings on
Aluminum and Aluminum Alloys
This standard is issued under the fixed designation B 921; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
Materials; Methods of Inspection
1.1 This specification covers the requirements relating to
2.4 Military Specification:
rinsed and non-rinsed non-hexavalent chromium conversion
MIL-C-5541 Chemical Films forAluminum andAluminum
coatings on aluminum and aluminum alloys intended to give
Alloys
protection against corrosion and as a base for other coatings.
1.2 Aluminum and aluminum alloys are conversion coated
3. Terminology
in order to retard corrosion; as a base for organic films
3.1 Definitions:
including paints, plastics, and adhesives; and as a protective
3.1.1 non-rinsed—conversion coatings that are dried imme-
coating having a low electrical contact impedance.
diately after the conversion coating step without receiving a
1.3 This standard does not purport to address all of the
water rinse.
safety concerns, if any, associated with its use. It is the
3.1.1.1 Discussion—Thisspecialtypeofcoatingistypically
responsibility of the user of this standard to establish appro-
used on long coils of aluminum sheet stock that receive an
priate safety and health practices and determine the applica-
immediate subsequent paint or adhesive coating.
bility of regulatory requirements prior to use.
NOTE 1—Non-rinsed conversion coatings are finding increased usage
2. Referenced Documents
on fabricated parts and castings.
2.1 ASTM Standards:
3.1.2 rinsed—conversion coatings that are rinsed in water
B 117 Practice for Operating Salt Spray (Fog) Apparatus
prior to drying.
B 602 Test Method for Attribute Sampling of Metallic and
3.1.2.1 Discussion—This type of coating is typically ap-
Inorganic Coatings
plied to extruded aluminum fabricated parts and castings.
D 1730 Practices for Preparation of Aluminum and
Aluminum-Alloy Surfaces for Painting
4. Classification
D 3359 Test Methods for Measuring Adhesion by Tape
4.1 A class one conversion coating provides for maximum
Test
salt fog corrosion protection to bare metal. A class two
2.2 ISO Standards:
conversion coating is designed to give painted salt fog corro-
ISO 2409 Paint and Varnishes—Cross-Cut Test
sion resistance and some bare salt fog corrosion resistance. A
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
class three conversion coating provides little salt fog corrosion
Test)
resistance, but low electrical contact resistance (see MIL-C-
ISO 4519 Electrodeposited Metallic Coatings and Related
5541). Class four and five conversion coatings only provide for
Finishes—Sampling Procedures for Inspection by At-
painted salt fog corrosion resistance.
tributes
4.2 The finishes are divided into four classes; their most
2.3 Federal Standard:
important characteristics are listed in Table 1.
5. Surfaces Preparation
5.1 The surfaces of the parts to be conversion coated shall
This specification is under the jurisdiction of ASTM Committee B08 on
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee be clean and free of any oxidation, scale, or soils such as metal
B08.07 on Chromate Conversion Coatings.
turnings, grinding dust, oil, grease, lubricants, hand-sweat, or
Current edition approved Sept. 10, 2002. Published November 2002.
any other contamination detrimental to the conversion coating
Annual Book of ASTM Standards, Vol 03.02.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 06.01.
5 6
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B921–02
TABLE 1 Classification of Non-hexavalent Chromium TABLE 2 Relative Corrosion Resistance
Conversation Coatings
A
Exposure Time, h
Class Corrosion Protection
Heat-Treatable Alloys
Cast Alloys with
Coating Non-Heat-Treatable and Cast Alloys with
1 Maximum corrosion resistance generally used as a final finish
a Nominal Silicon
Class Wrought Alloys a Nominal Silicon
2 Moderate corrosion resistance, used as a paint base and for
Content >1 %
Content <1 %
bonding to rubber
3 Decorative, slight corrosion resistance, low electrical cont
...
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