The metastable pitting behaviors of mild steel in bicarbonate and nitrite solutions containing Cl−

The metastable pitting behaviors of mild steel in NaHCO3 + NaCl and NaNO2 + NaCl solutions were studied with electrochemical methods. In NaHCO3 + NaCl solution the potential range between metastable pitting potential Em and pitting potential Eb is relatively large, and most of the current fluctuations are individual. While in NaNO2 + NaCl solution the Em and Eb values are much higher and the potential range between them is very narrow, leading to overlapped current peaks. The changes of Em and Eb with chloride concentration follow similar tendency. In both testing solutions, the repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, leading to accumulated corrosion damages on the surface. Mild steel shows stronger passivity in NaNO2 + NaCl solution than in NaHCO3 + NaCl solution, which explains the higher pitting resistance of the steel in the former. In addition, the competitive adsorption of NO2- anions also plays an important role in the increase of Em and Eb, especially in inhibiting nucleation of metastable pits.