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

摘要

High-level radioactive wastes, primarily consisting of concentrated sodium hydroxide (NaOH) and sodium nitrate (NaNO{sub}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 10M NaOH and various concentrations of NaNO{sub}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 VSCE, 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 ℃) controls the preference of the cathodic reaction, i.e. oxygen reduction versus nitrate reduction. When oxygen reduction is the preferred cathodic reaction, E{sub}(corr) is driven more noble than the active-passive transition peak.