Corrosion Resistance and Formation Analysis of a Molybdate Conversion Coating Prepared by Alkaline Treatment on Aluminum Alloy 6063

Abstract

A molybdate-based conversion coating (MBCC) was prepared on aluminum alloy 6063 (AA6063) by treatment in an alkaline conversion bath. The obtained MBCC is a gray and uniform conversion coating. Scanning electron microscope and electrochemical measurements were used to analyze the basic properties of the coating. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition and formation process of the coating. The surface morphology and cross-sectional observation show that the coating grows rapidly under given conditions, its thickness can reach more than 30 μm in 10 min, and the MBCC prepared with the alkaline method is a non-cracked coating. The potentiodynamic polarization test indicates that the corrosion current of MBCC is more than one order of magnitude lower than the corrosion current of an untreated surface. The XPS measurement illustrates that the coating is made up of Al, Mo, O and F. According to the high-resolution XPS spectral analysis, the elements detected in the coating are in the form of Al2O3, AlF3, MoO3 and Al2(MoO4)3. Based on the XPS results, the formation process of the coating under alkaline conditions was discussed.