The effect of stress on the polarization curve of high pressure die cast aluminum alloy in sodium chloride solution was evaluated by mechano-electrochemical experiments and boundary element electrostatics simulations. The polarization curve was measured under a constant stress condition, and a decrease in natural potential due to stress was observed. A periodic arrangement of intact and damage surfaces was assumed in the mechanically damaged oxide film, and the polarization curve of the damage surface was identified. The simulation results showed that the decrease in natural potential is explained by the nucleation of the damage surface by plastic strain.