Corrosion Protection of Magnesium Alloys by Ce, Zr and Nb oxide layers

Abstract

This chapter describes the recent advances in the corrosion protection of AZ91 alloys, achieved with new conversion and anodizing Cr6+ free treatments. The new green conversion coating was produced on AZ91 magnesium alloys, by a treatment in a solution containing cerium, zirconium, and niobium compounds. The new anodizing treatment was designed, based on a zirconium, niobium, phosphate, and borate containing solution. The electrochemical measurements showed a decrease of the corrosion and anodic currents, and a higher charge transfer resistance for both new conversion and anodizing treatments compared to the untreated alloy and to other existing treatments. The chemical characterization of the new conversion coating by XPS revealed the presence of stable oxides such as CeO2, ZrO2, Nb2O5, as the main components. The positive effects of the new coatings on the corrosion resistance can be attributed to the high stability of the compounds that constitute the surface oxide layers. The coatings could be employed either as final treatment or as supports for painting. They particularly offer interesting alternatives to the Cr6+-based coatings for magnesium alloys.