The electrochemical behavior and surface films of pure magnesium and Mg-xSn (x = 2, 5 wt%) alloys were characterized in detail in an NaCl solution for a better understanding of the role of Sn in the corrosion process and film formation. The cathodic hydrogen evolution rate decreased and the protectiveness of surface film increased with the increase of Sn content. All of the films were nanocrystalline and had a double-layered structure, which mainly consisted of Mg(OH)(2) and MgO. The porous outer layer of the film became more compact after the addition of Sn. The laminar inner layer in the Mg-Sn alloys was enriched in metallic Sn, because of the preferential dissolution of Mg atoms and the outward growth of the film. Owing to higher Sn content in the film of the Mg-5Sn alloy compared to the Mg-2Sn alloy, the Sn-rich layer could lead to a lower cathodic hydrogen evolution rate because of the higher hydrogen-evolution overvoltage of Sn than Mg. The decreased hydrogen evolution was beneficial to the deposition of Mg(OH)(2), which contributed to the increased compactness and corrosion resistance of the surface film. (C) 2016 The Electrochemical Society. All rights reserved.