Electrochemical characterization of passive film formed under different potential condition on X70 pipeline steel in NaHCO₃ solution

The passive behavior and electrochemical characterization of passive film formed under different potential on X70 pipeline steel in NaHCO₃ solution were discussed by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and capacitance measurements. It is shown that the X70 pipeline steel has a wide passive anodic region between -50 mV to 900 mV. EIS result indicates that the passive film is much denser and uniform with the increasing potential. Mott-Schottky analysis presents that the passive films on X70 pipeline steel in 0.5 mol·L^-1 NaHCO₃ solution exhibits n-type semiconductive character in potential region of -0.2 V to 0.8 V. The donor density of the passive film decreases and the thickness of the space-charge layers increases with the increasing potential. The corrosion resistance of the passive film increases with the forming potential.