ASTM B921-08(2013)

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Designation:B921 −08 (Reapproved 2013)
Standard Specification for
Non-hexavalent Chromium Conversion Coatings on
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
Materials; Methods of Inspection
rinsed and non-rinsed non-hexavalent chromium conversion
coatings on aluminum and aluminum alloys intended to give 2.4 Military Specification:
protection against corrosion and as a base for other coatings. MIL-DTL-5541 Chemical Conversion Coatings on Alumi-
num and Aluminum Alloys
1.2 Aluminum and aluminum alloys are conversion coated
MIL-DTL-81706 Chemical Conversion Materials For Coat-
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
2. Referenced Documents
used on long coils of aluminum sheet stock that receive an
immediate subsequent paint or adhesive coating.
2.1 ASTM Standards:
B117 Practice for Operating Salt Spray (Fog) Apparatus
NOTE 1—Non-rinsed conversion coatings are finding increased usage
B602 Test Method for Attribute Sampling of Metallic and
on fabricated parts and castings.
Inorganic Coatings
3.1.2 rinsed—conversion coatings that are rinsed in water
D1730 Practices for Preparation of Aluminum and
prior to drying.
Aluminum-Alloy Surfaces for Painting
3.1.2.1 Discussion—Thistypeofcoatingistypicallyapplied
D3359 Test Methods for Measuring Adhesion by Tape Test
to extruded aluminum fabricated parts and castings.
2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test
4. Classification
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
Test)
4.1 A class one conversion coating provides for maximum
ISO 4519 Electrodeposited Metallic Coatings and Related
salt fog corrosion protection to bare metal. A class two
Finishes—Sampling Procedures for Inspection by Attri-
conversion coating is designed to give painted salt fog corro-
butes
sion resistance and some bare salt fog corrosion resistance. A
class three conversion coating provides little salt fog corrosion
resistance, but low electrical contact resistance (see MIL-DTL-
This specification is under the jurisdiction of ASTM Committee B08 on
5541). Class four conversion coatings only provide for painted
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
salt fog corrosion resistance.
B08.07 on Conversion Coatings.
Current edition approved May 1, 2013. Published May 2013. Originally
4.2 The finishes are divided into four classes; their most
approved in 2002. Last previous edition approved in 2008 as B921 – 08. DOI:
important characteristics are listed in Table 1.
10.1520/B0921-08R13.
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
Standards volume information, refer to the standard’s Document Summary page on
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B921−08 (2013)
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
Wrought Alloys Content >1 %
Content <1 %
2 Moderate corrosion resistance, used as a paint base and for
bonding to rubber
1 500 336 48
3 Decorative, slight corrosion resistance, low electrical contact
2 250 168 24
resistance
3 168 120 12
4 Moderate corrosion resistance, used as a paint base and for
4 500 336 48
bonding to rubber
A
The exposure times are indicative of the relative corrosion resistance of the
variou
...