Preparation and corrosion resistance of cerium conversion coatings on AZ91D magnesium alloy by a cathodic electrochemical treatment

Abstract

Cerium conversion coatings were prepared on AZ91D magnesium alloy by a cathodic electrochemical treatment. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electrochemistry workstation and X-ray photoelectron spectroscopy (XPS) were used to characterize the compositions, micro-morphology, corrosion resistance and the oxidation state of cerium and oxygen elements of the coatings. The results showed that the conversion coatings presented an intact and uniform morphology when the cathodic current density was 0.20Adm−2, the temperature was 35°C and the treatment time was 2min. When the applied current densities were higher than 0.25Adm−2, the coatings showed a micro-cracked morphology. The better corrosion resistance of the cerium-based conversion coatings could be obtained when the current density was 0.20Adm−2 when tested in 3.5% NaCl (wt.) solution. There are Ce(IV) and Ce(III) oxidation states in the coating.