The military specification referred to as “Military Specification, Chemical Conversion Coatings on Aluminum and Aluminum Alloys” covers metal surface coatings formed by the reaction of chemical conversion materials with the surfaces of aluminum and aluminum alloys. The current specification (2009) is MIL-DTL-5541F, which superseded MIL-DTL-5541f in July 2006.
These coatings are categorized by the following types and classes.
- Type I: Addresses compositions are containing hexavalent chromium. This chemical 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 chemical film compositions containing no hexavalent chromium. This film typically appears to have no color. (Having no color is described as “clear.”)”
- Class 1A: These chemical conversion coatings provide corrosion prevention on unpainted items, and they improve adhesion of paint finish systems on aluminum and aluminum alloys. Metal surface coatings of this type may be used, for example, on tanks, tubing, and component structures where paint finishes are not required for interior surfaces but are required for the exterior surfaces.
- Class 3: These chemical conversion coatings provide a corrosion preventive chemical film for electrical and electronic applications where lower resistant contacts, relative to class 1A coatings, and anodic coatings in accordance with
MIL-A-8625, are required. Because Class 3 coatings are thinner, they are more susceptible to corrosion than class 1A coatings. If it is required to paint areas surrounding electrical, Class 3 coatings improve adhesion of paint systems on aluminum and aluminum alloy material.
Chemical Film Process
Chemical film mask black anodize processes