Chromium-Free Conversion Coating Technologies for Light Metals

Improving corrosion control is a significant technical challenge that must be addressed before lightweight metal alloys of aluminum and magnesium can be more widely used in manufacturing industries such as automobile, aerospace, and metal finishing. Use of lightweight metal alloys would provide many advantages for these industries. For example, a 40 percent reduction in the weight of an automobile would make 80 miles per gallon fuel efficiencies possible and also would make fuel cell technologies more attractive. Currently, chemical treatments based on hexavalent chromium (i.e., chromates) are used to develop corrosion-resistant films on aluminum and magnesium alloys. These films also promote the adhesion of subsequent coatings (e.g., paints) to give a highly effective corrosion-control coating. Chromates have been demonstrated to be potent mutagens and are considered to cause cancer in humans. As a result, there are mounting pressures to eliminate the use of chromates. A need exists for nonchromate conversion coating pretreatments that are effective on a range of aluminum and magnesium alloys. American Research Corporation of Virginia (ARCOVA) proposes the development of a nonchromium coating process based on interfacial precipitation reactions. These coating treatments will significantly improve the corrosion resistance of aluminum and magnesium alloys and also will act as excellent adhesion promoters. The objectives of this project include formulation of chromium-free coatings, evaluation of the coating corrosion and adhesion performance, and acquisition of families of test data to be used in the optimization of the corrosion-prevention coatings for lightweight alloys. ARCOVA will build on previous experience in the development of trivalent chromium conversion coatings for high-copper aluminum alloys to formulate and test new chromium-free pretreatment formulations for cast aluminum and magnesium alloys. Accelerated corrosion testing and electrochemical methods will establish the feasibility of corrosion control systems utilizing the new pretreatment. During Phase I, one or more pretreatment formulations will be developed and demonstrated in laboratory trials. During Phase II, the corrosion protection systems will be optimized and demonstrated using a pilot-scale metal finishing line.