Correlation between Electrical Properties of the Passive Film on Carbon Steel and Inhibition Efficiency by Phosphate and Nitrite Ions

The electronic properties of passive films grown at open circuit potential for 24 h and 7 days in pore simulating solutions are correlated to the inhibiting effect of nitrite and phosphate ions in chloride-rich alkaline solutions. Electrochemical impedance and Mott-Schottky plots were used to investigate the electronic properties of the films grown. Changes in composition were analyzed by X-ray photoelectron spectroscopy. Both anions are excellent inhibitors. However, the donor density (ND) decreases when nitrites are incorporated and remains constant when phosphates are added. ND increases with ageing time. These differences are interpreted in terms of the composition of each surface layer.