ABSTRACT
Ordinary metals have been found in the form of oxides and/or salts in nature. us, in nature where oxygen and water exist, the oxidized state is chemically more stable than the metallic state, and the metallic state is formed by reduction from the oxidized state. However, metallic materials are generally separated from oxygen and water in nature due to coverage by an air-formed oxide lm, and their oxidation is signicantly slower because oxidation is controlled by outward diusion of cations and inward diusion of oxygen through the lm. is is called spontaneous passivity. If the air-formed lm is broken by the attack of given environments, corrosive dissolution and/or rusting of materials occur. us, metallic materials other than precious metals should be and can be practically used only in the spontaneously passive state. However, the uniform passive lm with uniform protectiveness cannot be formed on heterogeneous metallic materials.
