Passivity of Dual-Phase Carbon Steel with Ferrite and Martensite Phases in pH 8.4 Boric Acid-Borate Buffer Solution

Abstract

Passivity of a dual-phase carbon steel with ferrite and martensite phases was investigated in pH 8.4 boric acid-borate buffer solution. The corrosion potential of the dual-phase steel was higher than that of pure martensitic steel and lower than that of pure ferritic steel. In dynamic polarization measurements, the passivity-maintaining current of the dual-phase steel was an intermediate value of those of pure ferritic and martensitic steels. EIS revealed that both the charge transfer resistance and capacitance of the oxide film formed on the dual-phase steel were intermediate between those of pure ferritic and martensitic steels. Although the donor density in the oxide film of the dual-phase steel was similar to that of pure ferritic steel, it was smaller than that of pure martensitic steel. SECM showed that the passive film formed on the martensite phase of the dual-phase steel had better electronic conductivity than that of the passive film formed on the ferrite phase. These differences in local passivity of the dual-phase steel suggested that the passive film formed on the ferrite phase has better passivity but larger scattering of the property than that of the passive film on the martensite phase. The local passivity of the dual-phase steel was strongly dependent on the texture of the substrate material.