Characterization of surface films on constructional steel in carbonated media containing chloride and nitrite ions

Abstract

The inhibition effect of NaNO2 on the electronic properties of the passive film was investigated for carbon steel exposed to solutions of pH = 13 and pH = 9 keeping the ratio [inhibitor]/[chloride]=1. Cyclic voltammograms, polarization curves, electrochemical impedance spectroscopy, Mott-Schottky plots and X-ray photoelectron spectra were analyzed. Steel is passive at pH=13 and active at pH=9 (carbonated solutions). Passivity can be regained in carbonated solutions when nitrite ions are also present. The thickness and composition of the surface films depend on alkalinity. Both films behave as n-type semiconductors. Donor density values are similar for films grown in solutions containing nitrite ions, regardless of the presence of carbonate ions and the difference in alkalinity. Nitrite ions inhibit pitting in both solutions, although the difference between pitting and open circuit potentials is 2.7 times higher at pH=13. Donor density values cannot be used as the single parameter to evaluate pitting risk.