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

Abstract

The metastable pitting behaviors of mild steel in NaHCO 3 + NaCl and NaNO 2 + NaCl solutions were studied with electrochemical methods. In NaHCO 3 + NaCl solution the potential range between metastable pitting potential E m and pitting potential E b is relatively large, and most of the current fluctuations are individual. While in NaNO 2 + NaCl solution the E m and E b values are much higher and the potential range between them is very narrow, leading to overlapped current peaks. The changes of E m and E b 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 NaNO 2 + NaCl solution than in NaHCO 3 + NaCl solution, which explains the higher pitting resistance of the steel in the former. In addition, the competitive adsorption of NO 2 - anions also plays an important role in the increase of E m and E b, especially in inhibiting nucleation of metastable pits.