Nitrate ion effect on the passive film breakdown and current oscillations at iron surfaces polarized in chloride-containing sulfuric acid solutions

Current oscillatory phenomena have been used to study the effect of nitrates on pitting corrosion of passive iron surfaces in chloride-containing sulfuric acid solutions. From the quasi steady-state current-potential and potentiostatic current-time curves of the Fe | 0.75 M H₂SO₄+10 mM Cl⁻ system it is deduced that at lower potentials nitrates stimulate pitting acting as activators of the oxide dissolution. At higher potentials nitrates act as passivators causing a sudden passivation of Fe during a mass-transport controlled process across a salt film. Current oscillations appear over a wide potential region. The oscillation period as well as the induction period of time occurring before the onset of oscillation, both decrease by increasing the nitrate concentration. The effect of nitrates at lower and higher potentials is discussed in terms of the electrochemical and redox reactions of nitrate ions.