Passivity Breakdown on Copper: Influence of Chloride Ion

Passivity breakdown and pitting on pure copper, the assumed material for the outer canisters for the isolation of high-level nuclear waste in Sweden and Finland, has been studied in simulated, sulfide- and chloride-containing granitic rock groundwater 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 sodium sulfide solutions with different chloride concentrations and for variable pH (= 8-10) 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 × 1015 cm−2, is in excellent agreement with that calculated from the unit cell dimensions of the substrate Cu (ξ ≈ 1.53 × 1015 cm−2) and the barrier layer sulfide (Cu2S) (ξ ≈ 7.66 × 1014 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 on Cu sulfide- and chloride-containing granitic rock groundwater.