Steel In Concentrated Solutions Research Articles

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

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.

Anodic Behaviour of Mild Steel Strongly Alkaline Solutions

In connexion with the well-known intercrystalline failure of mild steel in concentrated solutions of sodium hydroxide, it would appear that a study of the anodic behaviour of iron in such solutions might lead to results of considerable interest. A search of the literature soon reveals, however, that but little attention has been devoted to this matter. The work of Grube and Gmelin1, who carried out a careful and extensive investigation in this field, is therefore of special importance. These workers obtained evidence that anodic dissolution takes place in two distinct potential stages: at low current densities electrolysis proceeds at a potential of about — 0·85. V. (hydrogen), whereas at higher values it changes suddenly to + 0·65 V., when oxygen evolution commences. The critical current density was found to increase rapidly with temperature. Evidence is adduced by Grube and Gmelin that the lower (more negative) potential stage is associated with the formation of ferrous ion, and the higher with oxidation to the hexavalent ion Fe6.; anionic complexes being formed having the probable compositions (FeO2)″ and (FeO4)″ respectively.