by Steve Szaruga
AFRL, Materials and Manufacturing Directorate

Accomplishment: Scientists and engineers at the Air Force Research Laboratory’s (AFRL) Materials and Manufacturing Directorate (ML) recently completed development of a non-chromated surface treatment for aluminum aircraft surfaces and structures. The treatment is the result of a collaborative effort between the directorate, Boeing Phantom Works, and the Aging Aircraft Systems Squadron. Operational flight tests on the KC-135 and F-15 aircraft commenced in late 2004 at Hickam and Eglin Air Force Bases.

Payoff: Development of an environmentally safe, non-chromated surface treatment for aluminum aircraft structures is one of several Air Force initiatives concerned with providing aircraft with advanced corrosion protection that is increasingly environmentally friendly. Replacing existing chromate-containing treatments is expected to eliminate 90 percent of the Air Force’s hazardous wastestream and to reduce costs associated with handling and disposal of the current chrome-based treatments, which are carcinogenic. Feedback provided by maintenance depots during the second phase of the program is expected to improve the efficiency of the treatment application process, optimize the treatment for vehicle- and depot-specific issues, and provide new concepts in the way new non-chromate materials perform against the current and proposed performance requirements of aircraft systems.

Background: Due to the excellent corrosion-inhibiting properties of chromates, chromate-based surface treatments, primers and inhibitors have been used to control and mitigate corrosion in Air Force aircraft. However, hexavalent chrome is a known carcinogen, and environmental and health regulations have designated the materials as hazardous, which requires careful handling and additional disposal expense. Though a variety of non-chromium based surface treatments have been developed and evaluated, never before has a treatment offered corrosion protection equal to chromium-based treatments.

In 1998, the Defense Advanced Research Projects Agency ( DARPA) initiated and funded an applied research program with ML’s Nonstructural Materials Branch to develop a non-chromated alternative for aluminum aircraft surface treatment. The surface treatment the Directorate developed, AC-131BB, uses a non-chrome, sol-gel-based coating process to coat aircraft structures prior to the application of a primer and top coating. This coating system was then charted as an Advanced Technology Demonstration (ATD) and the Aging Aircraft Systems Squadron (ASC/AASS) initiated funding to demonstrate this technology on operational aircraft.

To date, aluminum test panels prepared using the new treatment have completed 1,000 hours of salt spray testing, and will continue being tested until coating failure to determine whether the treatment performs well. In addition, the Directorate has performed filiform and adhesion testing, with all panels meeting the appropriate requirements.

KC-135 and F-15 aircraft were chosen to represent transport and tactical aircraft, respectively, during operational flight testing of the non-chromated surface treatment. These systems were selected because their production environment is very different. Access to treat, coat or paint the F-15 can be made from the hanger floor or the top of the aircraft; the KC-135 has more difficult access that requires special lifts. The F-15 has several sharp contours and offers access to hatches on the exterior surface. Contours on the KC-135 aircraft are more gradual with few completely horizontal surfaces, which makes draining the process solution from the aircraft easier than with the F-15.

AC-131BB was applied to one side of each aircraft using conventional painting equipment after masking the aircraft navigation lights, some weapon attachment points, engine surfaces, etc. The other half of the aircraft was treated in the normal fashion with a chromated surface treatment for comparison during the test. The normal primer and topcoat were then applied to both sides. A crew of three workers applied the coating to the right side of the F-15 in 25 minutes, which included practice on a test panel and discussion of application techniques. Following application of the treatment to both aircraft, the solution was allowed to drain, and drips and drainage were removed. The treatment was applied and the surfaces were painted at Boeing Aerospace Support Center and Robins AFB in November 2004.

Plans for the second phase of the program include monitoring the performance of the coating systems and addressing lessons learned during the application of the treatment. The aircraft will be examined every four months for evidence of corrosion or coating failure. The ultimate goals of monitoring the operational test aircraft are to assess the performance of the coating, allow comparison with laboratory data, evolve the technology, and optimize application procedures and overall performance.