The effect of β (Mg17Al12) phase distribution on the corrosion behavior of AM50 alloy in NaCl solution was studied using scanning vibrating electrode technique (SVET) in conjunction with potentiodynamic polarization scan. The β phase distribution was modifi ed by casting AM50 alloy at different cooling rates. For all cooling rates, the following phases were present: primary magnesium (α), eutectic mixture of α and β phase, β phase and Al8Mn5 phase. In 0.17 wt%and 1.6 wt% NaCl solutions, the free corrosion potential (FCP) of the moderate cooled alloy was more noble than that for the fast and slow cooled alloy. The corrosion current densities calculated from SVET analysis at zero current potential (ZCP) were in agreement with that determined from potentiodynamic polarization method. This study clearly indicated that for a given magnesium alloy composition, the corrosion resistance of the alloy can be greatly affected by size and distribution of secondary phases.