Anodic polarization behavior of low-carbon steel in concentrated sodium hydroxide and sodium nitrate solutions

Abstract

High-level radioactive wastes, primarily consisting of concentrated sodium hydroxide (NaOH) and sodium nitrate (NaNO 3) solutions, are stored in large underground storage tanks made of low-carbon steel. The anodic polarization behavior of low-carbon steel in concentrated solutions of 10 M NaOH and various concentrations of NaNO 3 (0.01–2.0 M) was determined in order to predict the caustic stress corrosion cracking (CSCC) susceptibility of the tanks. The active–passive transition peak exhibited during anodic polarization of low-carbon steel in 10 M NaOH, typically associated with CSCC, at −0.25 and −0.75 V SCE, is still present at the lower and higher concentrations of nitrate. However, there is a mid-range of nitrate concentrations (0.5–1 M) within which the peak is suppressed by the strongly oxidizing nitrate in the presence of oxygen, a cathodic depolarizer. Temperature also affects the magnitude of this mid-range of nitrate concentrations where CSCC is seen to be electrochemically prevented. The data suggest that the oxygen solubility at the relatively low temperatures tested (<95 °C) controls the preference of the cathodic reaction, i.e. oxygen reduction versus nitrate reduction. When oxygen reduction is the preferred cathodic reaction, E corr is driven more noble than the active–passive transition peak.