Tech Briefs

AFRL scientists develop advanced corrosion protection for aircraft.

AFRL scientists and engineers recently completed development of a nonchromated treatment for aluminum aircraft surfaces and structures. The new treatment method is the result of a collaborative effort between AFRL, Boeing Phantom Works, and the Aeronautical Systems Center's Aging Aircraft Systems Squadron (ASC/AASS). The development of a non-chromatebased aluminum conversion coating fulfills one of several Air Force (AF) initiatives intended to provide aircraft manufacturers and maintainers with an environmentally safe corrosion protection method. Conversion coating is a metal finishing process that involves the application of a coating to a base metal to increase corrosion resistance and prepare the surface for additional coatings.

Due to the excellent corrosioninhibiting properties of chromates, aircraft manufacturers and maintainers employ chromate-based surface treatments, primers, and inhibitors to control and mitigate corrosion in AF aircraft. One of the constituent elements of these chromate-based coatings, however, is hexavalent chromium—a known carcinogen. Federal and state regulatory agencies designate material containing this substance as hazardous and thus strictly control its use and disposal. Environmental scientists estimate that by replacing all of its existing chromiumcontaining treatments, the AF could eliminate 90% of its hazardous waste stream and significantly reduce associated handling and disposal costs. Consequently, researchers have evaluated a variety of other surface treatments but have found none offering corrosion protection equal to that of chromate-based treatments.

ImageIn 1998, the Defense Advanced Research Projects Agency initiated and funded an AFRL applied research program with the goal of developing a non-chromate-based alternative for treating aluminum aircraft surfaces. As a result of this program, researchers created a coating known as AC-131BB using a dilute, sol-gel-based, aqueous mixture of zirconium alkoxide and a silane coupling agent. Aluminum test panels treated using the new coating have successfully completed more than 1,000 hours of salt spray testing to determine the performance limit of the new treatment; this testing will continue until coating failure occurs. Concurrent to the ongoing test activity, AFRL researchers have also performed adhesion experiments and filiform corrosion tests, and their results indicate that all of the treated panels meet the appropriate specifications.