The effect of the metal cations, Na + , K + , Ca 2+ , Mg 2+ , Zn 2+ , and Ni 2+ , on the oxide film structure and morphology changes during long term immersion corrosion tests of aluminum alloy (A3003) in model tap waters was investigated by X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The effect of the metal cations on the corrosion behavior was also investigated with mass change and electrochemical tests. The hardness of the metal cations, X, based on the Hard and Soft Acids and Bases (HSAB) concept was applied to explain the effect of metal cations on the passive oxide film structure and corrosion resistance. The mass change rate and corrosion current density decreased with increasing metal cation hardness. The XPS results showed that hard metal cations like Zn 2+ and Ni 2+ were incorporated in the oxide films, while the four soft metal cations were not incorporated in the oxide films.The results are in good agreement with those which could be expected from the HSAB hardness of the metal cations.