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.
In 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.
ASC/AASS is sponsoring a demonstration to evaluate the new coating's performance on operational aircraft. Test engineers chose the KC-135 and F-15 aircraft to represent the fleet's respective transport and tactical aircraft segments during operational testing of the AC-131BB surface treatment. They selected these aircraft because of the differences in their configurations; these differences make the coating application process unique to each aircraft. For example, while technicians have relatively easy access to treat, coat, or paint the F-15 from either the hangar floor or the top of the aircraft, their access to the KC-135 surfaces is much more difficult and requires automated lifts and elongated application tools. Whereas the F-15 has several sharp contours and access hatches on its exterior surface, the KC-135's more gradual contours (with few completely horizontal surfaces) facilitate better drainage of excess process solution.
After masking aircraft navigation lights, weapon attachment points, engine openings, and other applicable components, technicians used conventional painting equipment to apply the AC-131BB coating to one side of each aircraft. For comparison purposes, they then treated the other side of each aircraft with the typical chromatebased coating. Next, the crew applied the normal layers of primer and topcoat to both sides of both aircraft. The three-person crew took approximately 25 min to apply the AC-131BB coating to the right side of the F-15, which included a practice application on a test panel to refine their application techniques. Boeing Aerospace Support Center technicians applied the treatment, primer, and paint to the KC-135, and a Warner Robins Air Logistics Center crew treated, primed, and painted the F-15 (see Figures 1 and 2). Flight testing of both aircraft started in
The second phase of the demonstration includes plans for monitoring the AC-131BB coating performance and addressing the lessons learned during the application of the treatment. Every 4 months, maintainers are examining the aircraft surface for evidence of corrosion or related coating failure. Likewise, maintenance depots are providing feedback aimed at improving the efficiency of the AC-131BB treatment application process, optimizing the treatment for vehicleand depot-specific issues, and otherwise establishing critical insight as to the treatment's performance with respect to current and proposed aircraft performance requirements. The ultimate goals of the effort are to assess the AC-131BB coating performance, compare flight test and laboratory performance data, and optimize coating application procedures.
Mr. Steve L. Szaruga and Mr. Tim Anderl (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document ML-H-05-11.