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.
MIL-A-8625F, Type 1, 1B
ESOH IssuesCr6+ (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 ExposurePersonnel 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, StatusPrimary alternatives are other acids, such as sulfuric and boric acids. Alternatives are commercially available and some have been authorized and implemented.
Examples of Authorizations and Implementations
Sulfuric acid anodizing
Widely used. Authorized for most NAVAIR applications
MIL-A-8625 Type II
Thin film sulfuric acid anodizing (TFSAA)
MIL-A-8625 Type IIB
Other acids, such as boric, oxalic, tartaric
Tartaric-sulfuric acid anodize, used by Airbus (Europe)
MIL-A-8625 Type III
Boric-sulfuric acid anodizing (BSAA)
Spars, longerons, etc. used by Boeing commercial aircraft
Phosphoric acid anodizing
Adhesive bonding of Al
Keronite (plasma electrolytic oxidation, PEC)
Most applications and authorizations in Europe. Approved by NAVAIR
Tagnite (PEO), brush Tagnite
Used for many commercial helicopter gearboxes, all Apache gearboxes. NAVAIR approval. Brush Tagnite for repair.
Sikorsky spec SS8518s
Ceratronic PEO, France
Used by Liebherr on Al aerospace valves and by Messier-Bugatti for corrosion and wear on Airbus components
Used in vehicles. In evaluation for aircraft
For detailed technical and performance information, validations, authorizations and implementations see Surface Engineering Database. Search for Chromic Acid Anodize Alternatives.