The Modelling of Pitting Corrosion of Carbon Steel in High Level Nuclear Waste Supercontainer

Passivity breakdown and pitting on carbon steel, the material for the overpack holding the waste canister inside high-level nuclear waste supercontainer in Belgium, has been studied in simulated concrete pore solution (SCPS) with a pH of 13.5 and the data are interpreted in terms of the Point Defect Model (PDM). The near normal distribution in the critical breakdown potential (V_c) measured in deaerated SCPS with different chloride concentrations is in satisfactory agreement with the quantitative prediction of the PDM. The critical areal concentration of condensed cation vacancies at the metal/film interface that leads to passivity breakdown, as predicted by the PDM from the experimental dependence of the V_c on the square root of the voltage sweep rate, ξ < 1.09 × 10¹⁶ cm⁻², is in excellent agreement with that calculated from the unit cell dimensions of the substrate Fe (ξ ≈ 1.53 × 10¹⁶ cm⁻²) and the barrier layer oxide (Fe₂O₃) (ξ ≈ 6.89 × 10¹⁵ cm⁻²) for vacancy condensation upon the metal lattice or upon the cation sublattice of the film, respectively. This observation provides convincing evidence for the validity of the PDM for modeling passivity breakdown.