The effects of solute Zn on the corrosion resistance and surface film of Mg-5Sn single-phase alloy were investigated. The corrosion resistance of alloy decreased with increasing Zn content. The surface film formed on the Mg-Sn-Zn ternary alloy in NaCl solution had three layers and mainly contained Mg(OH)(2) and MgO. The pore defect density of film outer layer increased with the addition of Zn, which could enable more chloride ions to penetrate into the surface layer and form a film. MgCl2(H2O)(6), which can readily dissolve in aqueous solutions and accelerate film dissolution, was present in the middle and inner layers. The film/substrate interface and inner film layers of the Mg-Sn-Zn were both enriched with elements Sn and Zn owing to the selective corrosion of active Mg atoms. The enriched Zn could be responsible for the decreased protectiveness of film and for the decreased corrosion resistance of alloy. (C) 2016 The Electrochemical Society. All rights reserved.