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 Specifications
  • Military and commercial aircraft Al components
  • Paint adhesion on Al components
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. 

Common Cr6+Alternatives Examples of Authorizations and Implementations Specifications
Sulfuric acid anodizing Widely used. Authorized for most NAVAIR applications MIL-A-8625 Type II
Thin film sulfuric acid anodizing (TFSAA) Fatigue-sensitive components 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 BAC 5632
Phosphoric acid anodizing Adhesive bonding of Al SAE-ARP1575
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  
EC2 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.