Comparison of pitting susceptibility of passive films formed on iron with different inorganic inhibitors

Abstract

The pitting susceptibility of passive films on iron in chloride containing solution, formed using three different inorganic inhibitors, chromate, nitrite and bicarbonate, is determined using linear sweep voltammogram. It is found that the pitting susceptibility becomes stronger in the order of the passive films formed in 0 · 01M chromate, 0 · 1M nitrite and 0 · 5M bicarbonate. The difference in pitting susceptibility is explained by the difference in electronic property of the passive films, which is determined using Mott–Schottky measurements. The donor density of the passive films formed in bicarbonate is highest, followed by nitrite and then by chromate, although the concentration of bicarbonate is five times that of nitrite and 50 times that of chromate. This suggests that the passive film formed in bicarbonate is most susceptible to attack by chloride ions.