Investigation on the composition and corrosion resistance of cerium-based conversion treatment by alkaline methods on aluminum alloy 6063

Abstract

Cerium conversion coating (CeCC) and Ce–Mo conversion coating (CeMCC) were prepared on aluminum alloy 6063 (AA6063) by immersion in alkaline conversion baths. Surface morphology and composition of the conversion coatings were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). And electrochemical measurements were used to assess corrosion performance of the coatings. The SEM observations showed that CeMCC possessed a smoother and more uniform structure than CeCC, and the thickness of CeCC and CeMCC was about 0.8 and 1.2 μm respectively. The XPS depth analysis indicated that CeMCC contained a considerable amount of molybdenum and the cerium content was higher than that of CeCC at all coating depths. CeCC comprised of Al₂O₃, Ce₂O₃, CeO₂, and cerium hydroxides, and the composition of CeMCC also included MoO₂, MoO₃, Al₂(MoO₄)₃ and Na₂MoO₄ besides the above mentioned components. A potentiodynamic polarization (PDP) test revealed that the corrosion current density (i_corr) values for bare alloy and CeCC were 13.36 and 4.38 μA cm⁻² respectively in 3.5 wt% NaCl solution, while CeMCC exhibited the lowest i_corr value of 0.24 μA cm⁻², about two orders of magnitude lower than that of the substrate. Furthermore, the results obtained from both a cupric sulfate drop test and electrochemical impedance spectroscopy (EIS) characterization suggested that CeMCC possessed higher corrosion resistance in comparison with CeCC.