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The iron rust phase has been analyzed by using EPMA, TEM and alternating current (AC) impedance methods after wet/dry corrosion tests using 0.5 mass%NaCl solution. The steel containing 0.8 mass%Al showed higher corrosion resistance than carbon steel in the test. Aluminum was identified in the spinel oxide of inner rust of Al-bearing steel by TEM and EPMA, which is confirmed by thermodynamic calculation (Pourbaix analysis). By AC impedance it is demonstrated that the resistance of the rust (Rrust) corresponds to the structural factor of the rust of steels. The Rrust value of Al-bearing steel increases after the continuous formation of inner rust, which implies that Al takes part in the conversion of spinels into fine structure that prevents the penetration of Cl-ions.
Nishimura et al. (Mon,) studied this question.