MIL-DTL-5541 Chemical Conversion Coating
Military specification MIL-DTL-5541 (Chemical Conversion Coatings on Aluminum and Aluminum Alloys) applies to chemical conversion coatings formed by the reaction of chemical conversion materials within the surfaces of aluminum and aluminum alloys. This specification is approved for use by all departments and agencies of the United States Department of Defense (DoD). Although prepared specifically for DoD applications, the standard is used for many commercial applications as well. It has gained worldwide use across many fields versus the more verbose ISO equivalent (ISO 8081:1985).
Chemical Conversion Coatings
Chromate Conversion coatings have been used for over 50 years as treatment to coated or as-fabricated surfaces to boost corrosion resistance and provide a good basis for the subsequent application of paint. Chromate aluminum coatings are an inexpensive solution to prevent aluminum from oxidizing. They make excellent primers for painting and can stand alone as highly corrosion-resistant metal surface coatings. Though primarily used for military/defense and aerospace applications, customers in all industries can benefit from chemical film coatings on their parts and components.
Types & Classes of Chemical Conversion Coatings
Current specifications denote two types and two classes of chromate conversion coatings. The MIL-DTL-5541 specification covers chemical conversion coatings that form protective coatings by chemical reaction with aluminum and aluminum alloy; these coatings are categorized by the following types and classes.
Type (Hexavalent Chromium)
The type within this specification refers to chemical conversion coatings with or without hexavalent chromium as an included ingredient of the chemical.
- Type 1: 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 2: Addresses compositions containing no hexavalent chromium. This film typically appears to have no color (Having no color is described as "clear").
Class (Corrosion Protection)
The class within this specification refers to the amount of corrosion protection within the given chemical conversion coating.
- 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 using this thinner film in bonding applications.
Chromate Conversion Coating Applicable Uses
Chemical conversion coatings are RoHS compliant and are intended for use on aluminum and aluminum alloy substrates that are not anodized. They are used to repair anodized coatings on aluminum. They are designated as a post-treatment to ion-vapor deposition (IVD) aluminum used on many military platforms as a cadmium alternative or galvanic corrosion inhibitor.
Type I and II conversion coatings provide corrosion protection on unpainted items, as well as improve adhesion of paint finish systems on aluminum and aluminum alloys. The conversion coatings covered by this specification exceed most commercially available products. This is a requirement due to their use on military aircraft. Some of the usages by class chemical conversion coatings, include:
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. 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
Class 3 chemical conversion coatings provide a corrosion preventive 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.
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