Pitting of 18-8 Stainless Steel In Ferric Chloride Inhibited by Nitrates^*

Pitting of 18-8 stainless steel in 1-15 percent FeCl₃ solutions is completely inhibited by NaNO₃ additions. The critical activity of NO₃⁻ required for inhibition is related to Cl⁻ activity by the relation: log(Cl⁻) = 1.92 log(NO₃⁻) + 1.34. This equation can be derived on the view that competitive adsorption occurs between NO₃⁻ and Cl⁻ for sites on the passive surface. Adsorbed NO₃⁻ increases anodic overvoltage for metal ion dissolution, but adsorbed Cl⁻ reduces anodic overvoltage by displacing adsorbed oxygen of which the passive film is composed. The mechanism can also be viewed as one of NO₃⁻ shifting the critical potential for breakdown of passivity in the noble direction. When the critical potential reaches the oxidation-reduction potential of FeCl₃ solutions, the operating potential is no longer sufficient to break down passivity, and FeCl₃ + NaNO₃ solutions are, therefore, unable to initiate pitting. Increasing chloride concentrations, on the other hand, shift the critical potential in the opposite or active direction, hence increased nitrate is required as FeCl₃ concentration increases in order to return the critical potential to the value of the oxidation-reduction potential.