In the present work, the effect of Sn addition on the corrosion behavior of Mg–5Al–1Zn alloys was investigated. Microstructure, potentiodynamic polarization and immersion tests were carried out in 3.5% NaCl solution of pH 7.2 to estimate the corrosion behavior of AZ51 alloys with and without Sn addition. Mg17Al12 and Mg2Sn phases were mainly precipitated in inter-dendrite structures. With increasing the Sn content, the volume fraction of the Mg2Sn phase was increased and coarsening tendency was observed. The corrosion resistance was increased by Sn addition. Especially, the AZ51-5wt.%Sn alloy characterized the superior corrosion resistance among the four alloys. The Sn is known for a high hydrogen overvoltage and the secondary phases effectively formed the network structure, resulting in a drastically decreasing corrosion rate of AZ51 alloy.