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

Thursday, October 15, 2015: 11:20
102-A (Phoenix Convention Center)
P. Lu (University of California, Berkeley), S. Sharifi-Asl (University of California, Berkeley), B. Kursten (SCK.CEN), and D. D. Macdonald (University of California at Berkeley)
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 (Vc) 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 Vc 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.