The anoxic corrosion behaviour of copper in the presence of chloride and sulphide

Abstract

The Canadian used fuel container for the long‐term containment of spent nuclear fuel in a deep geological repository comprises a 3‐mm copper corrosion barrier applied directly to a strong carbon steel container. Although a final site for the Canadian deep geological repository has not yet been chosen, the site selection process has narrowed down to two candidate locations with unique groundwater chemistry, particularly with respect to salinity. Therefore, to ensure the long‐term integrity of the used fuel container, the effect of a range of groundwater chemistries on copper corrosion must be understood. The primary variables of interest are the influence of chloride concentration and the consequences of the interaction of hydrogen sulphide, naturally present in the groundwaters in very low concentrations, with the copper cladding. An additional aspect that has been investigated is the behaviour of copper in pure water. The anoxic corrosion of copper can be inferred by the very slow release of hydrogen gas. In this paper, new data are presented, which demonstrate an extremely sensitive approach to quantify the very limited hydrogen release at 75°C. Initially, corrosion rates under conditions approximating an anticipated Canadian deep geological repository were significantly less than 0.5 nm/year and they invariably declined over the course of several months/years. The presence of chloride or hydrogen sulphide was found, under specific conditions, to stimulate short‐term corrosion behaviour, which was consistent with anion‐assisted surface rearrangement, not bulk corrosion.

Investigating the effect of bentonite compaction density and environmental conditions on the corrosion of copper materials

“…Such results are in line with other studies of Cu embedded in bentonite and Cu immersed in anoxic brines. [24,25] Coupons embedded in bentonite compacted to lower densities showed initial average corrosion rates higher than those measured in higher-density experiments, with ranges of 7.5-8.5, 6.0-6.5, and 0.4-1.9 µm/ year for experiments conducted in bentonite with dry densities of 1100, 1400, and 1600 kg/m 3 , respectively. This discrepancy in initial corrosion rates could have been due to a few factors, including: increased pore space in lower-density pressure vessels, which would have initially contained more oxygen leading to corrosion; faster time to saturation, leading to a greater proportion of trapped oxygen, causing corrosion as opposed to other consumption routes, such as microbial respiration or mineral reactions; or increased MIC, due to the lower swelling pressure of less-dense clay.…”

The use of hydrogen in monitoring the anoxic corrosion of copper

“…In both simulated ground waters, and making the assumption that all recorded hydrogen is due to corrosion, the initial corrosion rate of the various copper types was of the order of several nm year −1 , which is consistent with previous work. [4,5] The corrosion rates declined similarly across all cells over time. The uniform corrosion rate for electrodeposited copper was marginally lower than either the cold spray or junction material, and by approximately 1 year of exposure, all copper cells were demonstrating uniform corrosion of less than 0.1 nm year −1 .…”

“…[2] In Sweden (and Finland), the approach is to use wrought copper to protect a cast iron insert. [3] Previously, the early period of corrosion behaviour of copper under simulated anoxic repository conditions was indirectly monitored via the evolution of very small amounts of hydrogen gas, [4,5] typically at corrosion rates below 10 nm year −1 . The interpretation was that, while oxidation did occur during hydrogen evolution, it was likely a copper surface phenomenon only and not a bulk corrosion process that could affect the canister cladding.…”

Stress corrosion of copper in sulfide solutions: Variations in pH‐buffer, strain rate, and temperature

“…The walls of the cavity may attain a smooth appearance without any adhering films of Cu 2 S, at least not at a thickness visible in SEM. The low corrosion rate at the walls of the cavity can be explained in part because corrosion at the grain boundary has left a near‐perfect crystal surface, [ 23 ] in part also because of near electrochemical equilibrium between the perfect crystals and the copper‐containing solution. The absence of a visible copper sulfide film may be due to an unfavorable interaction between metal and Cu 2 S. [ 24 ] However, a smooth appearance without any adhering films may also indicate a fresh intergranular attack corresponding to the left image in Figure 18.…”