Passivation Behavior of Mild Steel Used for Nuclear:Waste Disposal Package

Mild steel has been investigated as one of the overpack materials for geological disposal of high-level nuclear waste. The present paper discussed the passivation behavior of mild steel using an electrochemical approach to evaluate the critical condition for passivation. The critical current density for passivation, icris, of mild steel in 0.1 mol /L [HCO3 + CO32−] solution at room temperature was found to decrease with increasing pH of the solution. The depassivation pH, pHd, at room temperature was determined to be about 7.5 by comparisons between icris and the diffusion-limiting current density, iL02, for dissolved oxygen of 8.1 ppm in equilibrium with atmospheric oxygen. The anodic current density of mild steel in the solution of pH = 8 at 90°C was also measured and was found to decrease rapidly with time, leading to the absence of a critical current density for passivation. Thus, mild steel can be self-passivated in the system. An empirically determined corrosion phase diagram is provided for mild steel in 0.1 mol/L [HCO3− + CO32−] solution at 90°C, based on these findings and some published data.