Chromic Acid Anodizing Alternatives
Current Usage
Anodizing is primarily used for providing a wear- and corrosion-resistant layer on the surface of Al alloys. The surface is created by electrochemical processing in a chromic acid bath. Chromic acid anodizing is used for corrosion prevention, increased hardness, and improved paint adhesion. In addition, many non-chromate anodized layers are sealed with a chromate conversion coating.
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Typical Applications
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Specifications
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- Military and commercial aircraft Al components
· Paint adhesion on Al components
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MIL-A-8625F, Type 1, 1B
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ESOH Issues
Cr6+ (CrVI, hexavalent chromium) is a known carcinogen that is strongly regulated under:
· EPA Clean Air Act regulations
· OSHA Occupational Exposure to Hexavalent Chromium (Cr6+ Permissible Exposure Limit is currently 5 μgm-3)
· European regulations (RoHS and REACH)
· DoD Policy Memorandum on Minimizing the Use of Hexavalent Chromium
Exposure
Personnel may be exposed at the OEM level and during depot level overhaul and repair.
· OEM and depot personnel may be exposed during anodizing (electrolytic tank) and sealing (dip and spray) processes for components and for repaint operations. Emission controls and PPE are required during these processes.
· Maintainers are exposed to chromate sealer in scuff sand and paint operations. PPE is required during these operations.
Note that chromic acid anodize layers, like all anodize layers, are oxides that are not hazardous. Exposures come only from the chromic acid anodizing process and the chromate sealers.
Alternatives, Applications, Status
Primary alternatives are other acids, such as sulfuric and boric acids. Alternatives are commercially available and some have been authorized and implemented.
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Common Cr6+ Alternatives
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Examples of Authorizations and Implementations
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Specifications
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Sulfuric acid anodizing
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MIL-A-8625 Type II
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Thin film sulfuric acid anodizing (TFSAA)
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Fatigue-sensitive components
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MIL-A-8625 Type IIB
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Other acids, such as boric, oxalic, tartaric
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Tartaric-sulfuric acid anodize, used by Airbus (Europe)
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MIL-A-8625 Type III
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Boric-sulfuric acid anodizing (BSAA)
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Spars, longerons, etc. used by Boeing commercial aircraft
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BAC 5632
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Phosphoric acid anodizing
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Adhesive bonding of Al
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SAE-ARP1575
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Keronite (plasma electrolytic oxidation)
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Most applications and authorizations in Europe
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For detailed technical and performance information, validations, authorizations and implementations see Surface Engineering Database. Search for Chromic Acid Anodize Alternatives.
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