ASTM B921-08(2023)

Standard Specification for Non-hexavalent Chromium Conversion Coatings on Aluminum and Aluminum Alloys

ABSTRACT
This specification covers the requirements relating to rinsed and non-rinsed non-hexavalent chromium conversion coatings on aluminum and aluminum alloys intended 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. Coatings are categorized into four classes according to corrosion protection and finish. The type of conversion coating depends on the composition of the solution and may also be affected by pH, temperature, duration of the treatment, and the nature and surface condition. Films are normally applied by dipping, but may also be applied by inundation, spraying, roller coating, or by wipe-on techniques. Coatings shall adhere to specified electrical resistance, adhesion, and corrosion resistance requirements.
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, health, and environmental practices and determine the applicability of regulatory requirements prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2023
• ICS: 77.120.10 - Aluminium and aluminium alloys
• Technical Committee: B08 - Metallic and Inorganic Coatings
• Drafting Committee: B08.07 - Conversion Coatings

Relations

• Replaces: ASTM B921-08(2018) - Standard Specification for Non-hexavalent Chromium Conversion Coatings on Aluminum and Aluminum Alloys
• Effective Date: 01-Nov-2023
• Refers: ASTM B117-19 - Standard Practice for Operating Salt Spray (Fog) Apparatus
• Effective Date: 01-Nov-2019

Overview

ASTM B921-08(2023) is the international standard specification for non-hexavalent chromium conversion coatings on aluminum and aluminum alloys. Developed by ASTM International, this standard defines the requirements for both rinsed and non-rinsed non-hexavalent chromium conversion coatings designed to provide corrosion protection, serve as a base for organic coatings (such as paints, plastics, and adhesives), and offer protective coatings with low electrical contact impedance. The document classifies coatings into four categories depending on corrosion protection and finish, and it outlines criteria regarding electrical resistance, adhesion, and corrosion resistance.

Key Topics

• Scope of Coatings: Addresses both rinsed and non-rinsed conversion coatings on aluminum and its alloys, providing versatility for a range of industrial applications.
• Purpose and Benefits:
  • Retards corrosion on aluminum surfaces
  • Acts as a reliable base for organic coatings such as paints and adhesives
  • Provides protective coatings with low electrical contact resistance
• Classifications:
  • Class 1: Maximum corrosion resistance, suitable as a final finish
  • Class 2: Moderate corrosion resistance, primer for painting or bonding
  • Class 3: Decorative, slight corrosion resistance, low electrical contact resistance
  • Class 4: Moderate painted corrosion resistance, used as a coating under paint or bonding
• Methods of Application:
  • Dipping (most common)
  • Inundation
  • Spraying
  • Roller coating
  • Wipe-on techniques
• Testing and Quality Assurance:
  • Electrical resistance verified per agreed upon parameters
  • Adhesion tested typically using ASTM D3359 or ISO 2409
  • Corrosion resistance evaluated by exposure in salt spray tests (ASTM B117, ISO 3768)

Applications

Non-hexavalent chromium conversion coatings offer a safer, RoHS-compliant alternative to traditional hexavalent chromium treatments for aluminum and aluminum alloy components. Common industries and applications include:

• Aerospace: Protects aircraft aluminum structures against corrosion while enabling strong adhesion for primers and topcoats.
• Automotive and Transportation: Used on body panels, heat exchangers, and structural parts for both corrosion protection and as an underlayer for paints.
• Electronics: Applied where low electrical contact resistance is essential, such as in connectors or enclosures.
• Construction and Architecture: Enhances longevity and appearance of aluminum frames, facades, and decorative elements.
• General Manufacturing: Facilitates reliable bonding for adhesives, rubber, and other polymers to aluminum substrates.

The standard supports sustainable manufacturing by eliminating hexavalent chromium, mitigating environmental and worker safety risks while maintaining performance in challenging environments.

Related Standards

ASTM B921-08(2023) references and aligns with a number of other national and international standards to ensure consistency and interoperability, including:

• ASTM B117 - Practice for Operating Salt Spray (Fog) Apparatus
• ASTM B602 - Guide for Attribute Sampling of Metallic and Inorganic Coatings
• ASTM D1730 - Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting
• ASTM D3359 - Test Methods for Rating Adhesion by Tape Test
• ISO 2409 - Paint and Varnishes-Cross-Cut Test
• ISO 3768 - Metallic Coatings-Neutral Salt Spray Test
• ISO 4519 - Electrodeposited Metallic Coatings and Related Finishes-Sampling Procedures
• Fed. Std. No. 141 - Paints, Varnish, Lacquer, and Related Materials; Methods of Inspection
• MIL-DTL-5541 and MIL-DTL-81706 - Chemical Conversion Coatings on Aluminum and Materials for Coating Aluminum

For companies seeking to meet international requirements for aluminum coating and corrosion protection, compliance with ASTM B921-08(2023) and these related standards is essential. This supports quality assurance, product durability, and regulatory goals across global supply chains.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B921 − 08 (Reapproved 2023)
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 2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test
1.1 This specification covers the requirements relating to
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
rinsed and non-rinsed non-hexavalent chromium conversion
Test)
coatings on aluminum and aluminum alloys intended to give
ISO 4519 Electrodeposited Metallic Coatings and Related
protection against corrosion and as a base for other coatings.
Finishes—Sampling Procedures for Inspection by Attri-
1.2 Aluminum and aluminum alloys are conversion coated
butes
in order to retard corrosion; as a base for organic films
2.3 Federal Standard:
including paints, plastics, and adhesives; and as a protective
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
coating having a low electrical contact impedance.
Materials; Methods of Inspection
1.3 This standard does not purport to address all of the 2.4 Military Specification:
MIL-DTL-5541 Chemical Conversion Coatings on Alumi-
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- num and Aluminum Alloys
MIL-DTL-81706 Chemical Conversion Materials For Coat-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory requirements prior to use. ing Aluminum and Aluminum Alloys
1.4 This international standard was developed in accor-
3. Terminology
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the 3.1 Definitions:
Development of International Standards, Guides and Recom-
3.1.1 non-rinsed—conversion coatings that are dried imme-
mendations issued by the World Trade Organization Technical diately after the conversion coating step without receiving a
Barriers to Trade (TBT) Committee. water rinse.
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.
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 Guide for Attribute Sampling of Metallic and Inor-
3.1.2 rinsed—conversion coatings that are rinsed in water
ganic Coatings
prior to drying.
D1730 Practices for Preparation of Aluminum and
3.1.2.1 Discussion—This type of coating is typically applied
Aluminum-Alloy Surfaces for Painting
D3359 Test Methods for Rating Adhesion by Tape Test to extruded aluminum fabricated parts and castings.
4. Classification
4.1 A class one conversion coating provides for maximum
salt fog corrosion protection to bare metal. A class two
This specification is under the jurisdiction of ASTM Committee B08 on
conversion coating is designed to give painted salt fog corro-
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
sion resistance and some bare salt fog corrosion resistance. A
B08.07 on Conversion Coatings.
Current edition approved Nov. 1, 2023. Published November 2023. Originally
approved in 2002. Last previous edition approved in 2018 as B921 – 08 (2018).
DOI: 10.1520/B0921-08R23. Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 4th Floor, New York, NY 10036, http://www.ansi.org.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
Standards volume information, refer to the standard’s Document Summary page on Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
the ASTM website. dodssp.daps.dla.mil.
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B921 − 08 (2023)
TABLE 2 Relative Corrosion Resistance
class three conversion coating provides little salt fog corrosion
A
resistance, but low electrical contact resistance (see MIL-DTL- Exposure Time, h
5541). Class four conversion coatings only provide for painted Heat-Treatable Alloys
Non-Heat- Cast Alloys with
Coating and Cast Alloys with
salt fog corrosion resistance.
Treatable a Nominal Silicon
Class a Nominal Silicon
Wrought Alloys Content >1 %
Content <1 %
4.2 The finishes are divided into four classes; their most
1 500 336 48
important characteristics are listed in Table 1.
2 250 168 24
3 168 120 12
5. Surfaces Preparation
4 500 336 48
5.1 The surfaces of the parts to be conversion coated shall A
The exposure times are indicative of the relative corrosion resistance of the
be clean and free of any oxidation, scale, or soils such as metal various coating classes on the different alloys, b
...