Objective:
This project will evaluate the performance of a complete nonchrome coating system for corrosion protection of aerospace aluminum alloys, which relies on a novel magnesium-rich primer to provide cathodic protection of aluminum substrates. The evaluation process will include comprehensive laboratory testing, demonstration of the coating system via application onto selected Air Force and Navy aircraft, field evaluation after exposure to various service environments, and critical review of performance data for potential acceptance as an approved nonchrome coating system for select Department of Defense (DoD) aerospace applications. Technology Description: The concept of a magnesium-rich primer for cathodic corrosion protection of aluminum alloys was developed by researchers at North Dakota State University and is analogous to a zinc-rich primer for protection of steel. Using this concept, Akzo-Nobel Aerospace Coatings has successfully developed a viable formulation that retains all the desired properties of a conventional aircraft primer. The volatile organic compound (VOC)-compliant solvent-borne formulation utilizes magnesium pigment particles as a sacrificial anode to protect aluminum. Magnesium is more active in the galvanic series than aluminum. Therefore, when contact between the pigment particles and the aluminum substrate is obtained, the pigment must corrode preferentially to the substrate. Fortunately, the magnesium oxide corrosion product is denser than the magnesium pigment, minimizing any negative aesthetic effects due to corrosion protection. Since magnesium is a very active metal, concerns over the flammability of the pigment powder are alleviated through the presence of a very thin oxide layer present on the surface of the particles. No abnormal flammability issues have been detected to date in either the wet formulation or dry films. Laboratory corrosion evaluation using ASTM B-117 Neutral Salt Spray has resulted in performance that rivals that of conventional hexavalent chromium-based coating systems. The magnesium-rich primer will be evaluated as part of a complete nonchrome coating system. As such, it will utilize established nonchrome surface cleaning/treatments that permit electrical contact between the primer and the substrate (i.e., noninsulating treatments). It also will utilize an advanced performance coating (APC) grade polyurethane topcoat. The resultant complete coating system is not only chrome-free, but also does not contain significant heavy metal constituents, unlike many other nonchrome inhibitor technologies. Expected Benefits: Hexavalent chromium (Cr[VI]) compounds are toxic and known carcinogens. The primary benefit to the DoD, as well as potential benefits to the commercial sector, from implementation of the magnesium-rich primer coating technology would be a significant reduction in environmental impact through the elimination of toxic Cr(VI) species in painting and depainting operations used today. Current coating systems rely on Cr(VI) in either the surface treatment, primer, or both. The waste stream generated through these materials represents the highest annual hazardous materials cost to the Air Force. Furthermore, recent reductions in the Cr(VI) personnel exposure limit by the Occupational Safety and Health Administration will result in increased compliance costs unless a viable alternative is identified and implemented. A nonchrome coating system would extend the life of existing aluminum-based assets and facilitate expanded use of economical aluminum alloys in both DoD and commercial applications. (Anticipated Project Completion - 2009) Principal Investigator: Dr. Joel Johnson Air Force Research Laboratory 2941 P Street, Building 654, Room 136 Wright-Patterson AFB, OH 45433-7750 Telephone: (937) 255-4651 Fax: (937) 255-2176 E-mail: joel.johnson@wpafb.af.mil
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