Effect of Temperature on Passive Films of N80 Oil Tube Steel in Alkaline Solution

The electrochemical characteristics of passive films on N80 oil tube steel in aikaline solution (0.5 mol/L NaHCO₃ + 0.5 mol/L Na₂CO₃) at different temperatures were studied by potentiodynamic polarization and electrochemical impedance spectroscopy. The semiconductive behaviors of passive films were analyzed by Mott-Schottky plots. The results showed that with the increase of temperature, the passivation region decreased, the passive current density increased, the impedance of the system decreased, the stability of passive films of N80 steel deteriorated, and the protective effect on the substrate weakened. The N80 passive films had the features of n-type semiconductor. The donor density and flat-band potential increased, film resistance and transfer resistance decreased with increasing temperature, as results of increasing oxygen vacancies.