Passivity Breakdown on Copper: Influence of Temperature

Passivity breakdown and pitting on pure copper has been studied in simulated repository ground water (x M NaCl + 2 × 10−4 M Na2S, pH = 9.0 ± 0.2 at 40°C, as established by 0.2 M boric acid/sodium hydroxide buffer, with x = 0.05 to 5), at elevated temperatures (22°C to 82°C) by means of potentiodynamic polarization, cyclic voltammetry, and potentiostatic polarization, as well as by using SEM/ EDS to explore the elemental composition of the film. By assuming that the potential breakdown sites are normally distributed with respect to the cation vacancy diffusivity, a near normal distribution in breakdown potential is predicted by the Point Defect Model (PDM) and is found experimentally at different temperatures (42 - 82°C). From the potential sweep-rate dependence of the breakdown voltage, the critical, areal cation vacancy concentration, ξ, leading to passivity breakdown on copper is in the range of 9.15 × 1014 - 1.01 × 1015 cm−2. These values are in excellent agreement with that estimated structurally (1014 - 1015 cm−2), assuming vacancy condensation on either the copper substrate or upon the cation sublattice of the barrier layer (Cu2S).