Influence of Nitrate on Pit Stability in Austenitic Stainless Steel

Sufficiently high concentrations of nitrate $(NO3−)$ additions to chloride (Cl⁻) solution resulted in the elimination of slow rise metastable pitting transients and, correspondingly, localized corrosion inhibition in Type 304L (UNS S30403) stainless steel (SS). In comparison, the nucleation frequency of sharp rise metastable pitting transients was independent of $NO3−$ concentration. Sharp rise transients in solutions containing only $NO3−$ were associated with the highest peak current and calculated pit current densities, 50 A/cm² to 200 A/cm². Thus, $NO3−$ does not appear to inhibit pitting corrosion by reducing the current density at the incipient pit surface. To evaluate the influence of $NO3−$ on repassivation, we developed a mathematical expression for fitting transients. In the case of sharp rise transients, the resulting parameters were used to generate plots of the anodic dissolution and the film growth components of the current as well as the fraction of passive film coverage with time. In comparison, the decay portion of slow rise transients could not be fit by our expression. It was concluded that, while the repassivation of sharp rise transients can be modeled as exposure of bare metal and subsequent oxide film formation, it appears that the repassivation of slow transients is governed by a different process.