NPFC - MIL-C-5541
CHEMICAL CONVERSION COATINGS ON ALUMINUM AND ALUMINUM ALLOYS
|Publication Date:||30 November 1990|
This specification covers the requirements for two classes of chemical conversion coatings formed by the reaction of chemical conversion materials and the surfaces of aluminum and aluminum alloys. This specification is intended specifically to provide components of military weapon systems with maximum corrosion resistance. The coating also provides a surface having better paint adhesion than uncoated aluminum. It is not intended as a general purpose coating for commercial and decorative applications, (see 6.1).
The chemical conversion coatings shall be of the following classes, as specified (see 6.2).
Class 1 A - For maximum protection against corrosion, for surfaces to be painted or left unpainted, (see 6.1.1). Class 3 - For protection against corrosion where lower electrical resistance is required, (see 6.1.2).
Class 1A chemical conversion coatings are intended to provide corrosion prevention when left unpainted as well as to improve adhesion of paint finish systems on aluminum and aluminum alloys.... View More
Class 1A chemical conversion coatings are intended to provide corrosion prevention when left unpainted as well as to 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 the interior surfaces but are required for the exterior surfaces.
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 184.108.40.206). The primary difference between a Class 1A and Class 3 coating is thickness, since current passes more readily through a thinner current resistant barrier(coating). 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 will improve adhesion of paint systems on aluminum and aluminum alloys.
When under a nominal electrode pressure of 200 psi, Class 3 coatings are qualified per MIL-C-81706 to have a resistance no 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-C-81706 or other similar methods.The following two variables may have a greater effect on electrical resistance values than the conversion coating thickness.
Panels 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.
If an electrode with a given surface area is not flat, the actual contact area will be lower than the theoretical value. Smaller contact area will result in a higher resistance value. The same reasoning can be applied to the specimen panel.View Less