The passive films formed on X80 and X70 pipeline steel in 0.5 mol·L-1NaHCO3solution are investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), and the corrosion resistance of both steel is compared. The results show that X80 steel has a much lower passive current density than X70 steel. EIS measurements indicate that the film formed on X80 steel is much more compact and uniform than that of X70 steel. Mott-Schottky analysis demonstrates that the passive film of X80 steel has a lower donor density and a thicker space charge layer than X70 steel. These factors lead to a better corrosion resistance for X80 steel than X70 steel.

To improve the corrosion resistance of Ni-P electroless coating, the KI was added in the electroless solution. The surface micrograph was observed using scanning electron microscopy (SEM), and the corrosion resistance of Ni-P coatings in 3.5%NaCl solution was examined by polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that KI reduced the amount of defects and the size of crystal grain of Ni-P coating, and the surface of Ni-P coating became more homogenous, smoother and compact by KI. In addition, the phosphorus content of Ni-P coating was increased by KI. These factors increased the ability to corrosion protection of Ni-P coating.