The conversion coatings covered by this specification are intended for use, throughout the Department of Defense, 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 commercially available products due to the nature of their use on aircraft.

Class 1A.

Class 1A chemical conversion coatings are intended to provide corrosion prevention on unpainted items as well as 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 are intended for use as 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 (see 6.1.2.1). Coating thickness is varied by immersion time, and as a result, the same conversion material can be listed on QPL-81706 for both classes. 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 contacts, class 3 coatings improve adhesion of paint systems on aluminum and aluminum alloys.

Electrical resistance testing.

When under a nominal electrode pressure of 200 psi, class 3 coatings are qualified under MIL-DTL-81706 to have a resistance not greater than 5,000 microhms per square inch as supplied and 10,000 microhms per square inch after 168 hours of salt spray exposure. In addition to the coating or coating thickness (see 6.1.2), other variables heavily influence resistance values when using the test method specified in MIL-DTL-81706 or other similar methods. The following two variables (see 6.1.2.1.1 and 6.1.2.1.2) may have a greater effect on electrical resistance values than the conversion coating thickness.

Surface roughness of the specimen panel.

Test specimens having rough surfaces will yield lower resistance values when subjected to a contact electrode pressure due to coating fracture. This reasoning can also be applied to the contact electrode.

Flatness of the contact electrode.

If an electrode with a given surface area is not flat, the actual contact area will be lower than the theoretical value. A smaller contact area results in a higher resistance value. The same reasoning can be applied to the test specimen.