A Comparative Study of Cerium and Aluminum Conversion Coatings on AZ31 Magnesium Alloy in Aqueous and Ethanol Solutions

Abstract

Regarded as a potential replacement for chromate conversion coating, cerium-based conversion coating system for magnesium alloys has been widely discussed for many years. As revealed in previous research, trivalent cerium conversion coating formed in aqueous solution is found to have a loose structure with a fibrous layer on the top, which leads to a limited barrier protection. To enhance the corrosion resistance of trivalent cerium conversion coating, some approaches that include non-aqueous system and quadrivalent cerium conversion coating have been investigated. In contrast, an aluminum conversion coating based on aluminum nitrate, which is expected to have a protection effect similar to that of cerium nitrate, has also been proposed. However, the corrosion behavior of aluminum conversion coating and its feasibility of being conducted in non-aqueous solution have not been well studied so far. Hence, this study is focused on a comparative understanding through a combination of electrochemical analysis and microstructural characterization.Prior to conversion coating process, the AZ31 plates were abraded using emery paper up to 2500 grit and rinsed with deionized water. The conversion coating process was conducted in aqueous or ethanol solution for 60 s under room temperature, after which the coated plates were heated by hot air of approximately 75 °C for drying.Gray coatings can be observed after both cerium and aluminum coating processes conducted in aqueous solution. As observed in electrochemical impedance spectroscopy (EIS), cerium conversion coating slightly increases the total impedance from around 5,000 to 7,000 Ωcm2 and exhibits a partially blocked electrode behavior, which is coherent with its loose microstructure. However, aluminum conversion coating shows a different behavior with an apparent barrier protection, which strongly elevates the total impedance to over 50,000 Ωcm2.As conducted in ethanol solution, the formation of cerium and aluminum conversion coatings are both retarded. Without the existence of citric acid, aluminum conversion coating can still provide certain corrosion protection, while cerium conversion coating can barely formed. With citric acid added, however, cerium conversion coating formed in ethanol solution exhibits an impedance even higher than that of the coating formed in aqueous solution in EIS, while the total impedance of aluminum conversion coating is further decreased.