Chem-Film, sometimes called Alodine or Iridite, is a chemical conversion coating that is used to essentially passivate aluminum. It protects aluminum from corrosion and serves as a base for organic (paint) coatings while leaving the part dimensions unchanged.
Chem-Film is applied by dipping, spraying, or brushing. It is sometimes applied alongside Type II Anodizing and can be used to repair minor damage to anodized surfaces. The resultant coating is dependent on the aluminum condition. Chem-Film is generally not intended for decorative use and has very little abrasion resistance.
Aerospace/Aviation: Brush-on chem-film is used in the repair of anodized surfaces and is used for general corrosion protection on a multitude of aluminum aircraft components.
Military & Defense: Chem-Film is used as a base coat for primer and CARC paint on ordnance, vehicles, and tactical equipment.
Maritime/Naval: Chem-Film provides corrosion protection in saltwater environments.
Electrical Industry: Chem-Film is used for corrosion protection on housings and other parts where there will not be opportunities for abrasion.
The MIL-DTL-5541 specification covers chemical conversion coatings that form protective coatings through chemical reactions with aluminum and aluminum alloy; these coatings are categorized by the following types and classes.
Type I: Addresses compositions containing hexavalent chromium. This film typically appears to be gold or brown in color, but in some cases may be optionally specified as having no color (having no color is described as “clear”).
Type II: Addresses compositions containing no hexavalent chromium. This film typically appears to have no color (having no color is described as “clear”).
Class 1A: Provides maximum protection against corrosion, whether painted or unpainted. This is a relatively thick coating used as a final finish or pre-treatment to paint or powder coating.
Class 3: Provides protection against corrosion where low electrical resistance is required. This is a thin coating providing low contact resistance, and the coating weight is lower as is the corrosion resistance. There may be some advantages to using this thinner film in bonding applications.
Dow 7 is a good choice for both new and overhauled parts that have dissimilar materials such as inserts, or where conditions will tolerate a less robust and less costly alternative to Dow 17 anodizing. It makes an excellent paint base and provides the best corrosion protection of the available chemical coatings for magnesium.
Dow 7 is a chemical conversion coating that results in no appreciable dimensional change to the component. The surface of components coated with Dow 7 is changed to a brassy or dark brown finish depending on the alloy. The SAE AMS-M-3171 Type III (formally Mil-M-3171 Type III) covers the Dow 7 process.