Corrosion Resistance Studies of Cerium Conversion Coating with a Fluoride-Free Pretreatment on AZ91D Magnesium Alloy

An alternative of toxic hydrofluoric acid (HF) commonly used in the pretreatment process of magnesium (Mg) alloys prior to the conversion coating treatment in a cerium (Ce) nitrate and hydrogen peroxide bath was systematically investigated in this study. Comparing with the HF- and HCl-pretreated samples, the corrosion film formed in the vanadate solution at pH 8 effectively retarded the reaction rate of the Ce conversion coating. As a result, the average size and population density of blisters on the Ce conversion coating were markedly reduced. Both the electrochemical impedance spectroscopy and salt spray test showed that the Ce conversion coating with improved corrosion resistance can be made on the AZ91D that was pretreated in the vanadate solution for 120 s. Consequently, the vanadate solution is a feasible alternative to the HF solution for the pretreatment of the AZ91D prior to the Ce conversion coating treatment.