The electrochemical behaviour of a commercial magnesium alloy containing rare-earth elements, the WE43, was investigated by electrochemical techniques in both sulphate and chloride solutions and compared to that of pure magnesium (99.95 wt %). A particular attention was paid to the oxides film that formed during the corrosion process. Electrochemical impedance data analysis allowed the oxides films thickness to be determined. The film was thinner and more protective for the WE43 Mg alloy than for the pure Mg. ToF-SIMS analysis showed the incorporation of alloying elements, mainly yttrium and zirconium in the corrosion products layer. A higher compactness due to a higher Pilling-Bedworth ratio can explain the electrochemical results. (C) 2018 Elsevier Ltd. All rights reserved.