High potential inhibition of the 17-4 PH stainless steel in the presence of Nitrate and evolution of metastable pitting

In the present investigation, the potential effect of Nitrate ions' inhibition on pitting corrosion of 17-4 precipitation hardening stainless steel in different Chloride and Nitrate concentrations was investigated using potentiostatic and potentiodynamic examinations. Furthermore, metastable pitting was examined in depth to clarify Nitrate inhibition. The results demonstrate that adding Nitrate as an inhibitor to the Chloride containing solution decreases the metastable pit peak current, stability products, and pit radius. In contrast, no significant difference between metastable pit lifetime was observed in the presence and absence of Nitrate in the Chloride solution. It was also shown that introducing 17-4PH stainless steel to the high potential resulted in the repassivation of the surface, even if the Nitrate/Chloride concentration ratio was less than the critical value needed for Nitrate inhibition. Overall, Nitrate ions have an inhibition effect in the Chloride solution. Especially at very high potential, whether the Nitrate concentration was less than the critical value or not, Nitrate forces the surface to get repassivated.