Non-Chromate Talc Conversion Coatings for Aluminum

Abstract A method has been developed for applying an inorganic conversion coating on aluminum that is procedurally similar to chromate conversion coating methods. This new method, however, does not use or produce hazardous or toxic chemicals. The coating forms by a precipitation mechanism involving Al3+, Li+, OH-, CO32-, and possibly other anions present in the bath as impurities. This polycrystalline coating is continuous, conformal and persistent in aggressive environments. Coating thicknesses range from several tenths to ten micrometers depending on the substrate alloy composition and the coating process conditions. The outer portions of the coating are porous, but pores do not penetrate to the coating-substrate interface. These coatings, as currently fabricated, do not match the levels of performance offered by commercially available chromate conversion coatings, but are capable of meeting many of the corrosion resistance, electrical resistivity, and paint adhesion requirements established in MIL-C-5541E "Chemical Conversion Coatings on Aluminum and Aluminum Alloys". In this paper, methods for producing the talc coating on aluminum alloys 1100 and 6061-T6 are described and compared to traditional chromate conversion coating methods. The resulting coating structure and composition are described. Additionally, performance data for the talc coatings in MIL-C-5541E required tests are presented along with comparative data from samples prepared using commercial chromate-based coating processes.