A clean low cost anti-corrosion molybdate based nano-particles coating for aluminum alloys

Abstract

A molybdenum based treatment prepared by sol–gel method has been investigated as a replacement for toxic chromate. A treatment process in which the surface was etched followed by oxide thickening prior to molybdate treatment is being proposed. The process involves surface cleaning, etching followed by oxide thickening in distilled water and then, treatment in molybdate sol. The effect of surface preparation prior to molybdate treatment on the corrosion resistance of AA6061-T6 in 3.5% NaCl solution was measured using ac impedance spectroscopy and dc polarization techniques. Optical microscope and SEM showed a marked decrease in the number and depth of pits for the samples pre-etched and oxide thickened as compared to the non-etched treatments. XPS analysis revealed presence of relatively higher amount of Mo 3p1 and Mo 3p3 for the samples etched followed by oxide thickening compared with the non-etched samples. Conversely, non-etched samples revealed high amount of Mo 3d3 than the etched samples. Accordingly, it was suggested that the protection is due to conversion of molybdate from less corrosion resistant species such as Mo 3d3 to more corrosion resistant oxides such as Mo 3p1 and Mo 3p3. AFM showed that the film formed in the etched samples is more compact and has smoother appearance than the non-etched ones. It has been shown that the etching process prior to oxide thickening enhances formation of a uniformly distributed highly protective compact molybdate oxide film.