Influence of microstructure on the passivation and corrosion behaviour of a high carbon steel

The effect of bainite and tempered martensite on the passivation and corrosion behaviour of a steel with composition 0.70C, 0.22Si, 0.60Mn, 0.017P, 0.012S (wt.%) is investigated in aerated borate buffer solution, pH = 9.2, and with addition of 100 mM KCl. In the absence of chloride ions, free corrosion potential measurements and impedance spectroscopy tests have shown lower barrier properties of passive film grown on tempered martensitic microstructure as compared to the one formed on bainitic microstructure. Moreover, the electrochemical measurements highlight superior corrosion resistance displayed by the bainitic steel in the presence of chloride ions with respect to the tempered martensitic steel. The observed behaviour is ascribed to the shape, size, and distribution of the ferrite and cementite.