The passive behavior and electrochemical characterization of the passive films formed on X80 and X70 pipeline steels in 0.5 mol/L NaHCO ₃ 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 (ip) of X80 and X70 steels are 6.76 μA/cm ² and 18.38 μA/cm ², 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 NaHCO ₃ 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.

The corrosion behavior of X70 steel in Yingtan acidic soil was studied by using polarization curve technique and electrochemical impedance spectroscopy (EIS) in 13%~40% water content. It is found that X70 steel corrosion rate is rised with the increasing of water content and reached a maximum value when the water content of soil is saturated, and samples is corroded selectively expanding roundly. Cathode reaction is oxygen reduction when water content is 13%, hydrogen ion then participated in with the water content increasing. Water content and pH both affect X70 steel electrochemical behaviors. Corrosion resisting property of X70 steel is bad in Yingtan acidic soil.