Three types of AZ91 magnesium alloy with various weight percentages of calcium oxide ranging from 0 to 0.6 wt.% are examined using electrochemical techniques and surface analyses. The results show that AZ91 magnesium alloy has improved corrosion resistance with increasing calcium oxide content due to higher pitting potential and charge transfer resistance. In addition, the benefits of calcium oxide addition are the refinement of the precipitates and increased the formation of a more protective surface film in mixed Mg–Al–Zn–Ca oxides, resulting of the extra resistance between the a-Mg matrix and the second phase.