The current fluctuations and accumulated pitting damage of mild steel in NaNO2–NaCl solution

Metastable pitting behavior of mild steel in 0.1 M NaNO2 + 0.05 M NaCl solution at constant potentials was studied. The current fluctuations showed the feature of quick rise and relatively slow decay, indicating quick breakdown and slow repassivation of the passive film on the surface. The current fluctuations resulted in observable pits on sample surface. A bigger pit may be composed of several small pits, which suggests that a repassivated metastable pit may still be active site for following metastable pits to nucleate. Repeated local dissolution events around a certain active site caused accumulated corrosion damage. The pit size distribution calculated from the current fluctuations was very close to that observed with SEM, indicating that there is good correlation between current fluctuations and the growth process of metastable pits. Corrosion products may pile up at pit mouth and lead the pit to an occluded state, thus metastable pitting process was replaced by stable pitting. Once the pit mouth was completely blocked up by corrosion products, the pit would stop growing.