Electrochemical characterization of passive films formed on X80 and X70 pipeline steels in NaHCO₃

The passive behavior and electrochemical characterization of the passive films formed on X80 and X70 pipeline steels in 0.5 mol/L NaHCO₃ solution were discussed by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements. It is show that X80 and X70 pipeline steels present a wide passive anodic region (-0.16-0.90 V), the passive current density (i_p) of X80 and X70 steels are 6.76μA/cm² and 18.38μA/cm², respectively. EIS results indicate that the surface of the passive film formed on X80 steel is much denser and uniformer than that of X70, and the result is also show by AFM. Mott-Schottky analyses displayed that the passive films on X80 and X70 pipeline steels in 0.5 mol/L NaHCO₃ exhibit n-type semiconductive characters in the potential region of -0.20 V to 0.80 V. Compared with the passive film of X70 steel, the donor density is much lower and the width of space charge layer is thicker for the passive film of X80 steel, so the corrosion resistance of the latter is better.