3.3 - Boric-Sulfuric Anodizing - General Properties

Boric Sulfuric anodizing is an electro-chemical process of converting aluminum to a very thin coating of aluminum oxide. The thickness of the coatings range from .0002 to .0003 inches, and are used primarily on aircraft, electronics, computers, and OEM machine parts because it is an excellent paint base and does not leave a corrosive residue in riveted or welded joints. Rather, the yellow chromic acid that becomes trapped in any seams provides enhanced corrosion protection when moisture is formed by changing atmospheric temperatures, and can serve as an inspection device to verify properly welded joints and seams. This type of anodizing also produces a grey, opaque coating that is dielectric in function, and especially useful on aluminum chassis with metal to metal contact, or machine parts with recesses. Moreover, the coating can be dyed to result in an attractive black color if necessary. Chromic anodizing is a suitable alternative to sulfuric anodizing whenever a thinner coating is preferred. Boric-sulfuric anodizing is performed according to MIL-A-8625 Type IC.

Chromate on Aluminum MIL-C-5541
A conversion coating is applied to aluminum alloys by immersing them in a hexavalent chromium solution. A thin chromate coating, about .00005 inches, is formed on the surface. This coating ultimately provides good corrosion resistance and acts as an excellent paint base. Iridite and Alodine are the more commonly used brand name references to this process, however, conversion coating is the technical designation. properties are needed.

Most chemical conversion coatings are applied according to MIL-C-5541, divided into two process categories:

Type Ia Normally a deep yellow color, provides maximum corrosion protection for either painted or unpainted surfaces.

Type 3 A light yellow color for corrosion protection where low electrical resistance is required. Also, a clear color can be requested that helps ensure electrical conductivity.

To be certain the process is providing adequate corrosion protection, a monthly salt spray test is performed according to ASTM-B-117. Also, to verify good adhesion, a monthly wet tape test must be performed according to FED-STD-141.

For additional details on this process, call us and we can arrange to run samples or schedule a visit at your convenience.

BACK