Steel or cast iron containers with an outer copper shell have been adopted for the long-term containment of used nuclear fuel in a deep geological repository by several waste management organizations worldwide. While copper is generally stable in anoxic groundwater, corrosion can occur due to the presence of sulphide produced via microbial activity under anoxic conditions. Once oxygen trapped in the repository is consumed by reaction with minerals, microbial respiration and minor container corrosion, the dominant threat to the long-term durability of the container is corrosion by sulphide. This presentation will outline a detailed mechanism based on the results of our studies to elucidate the corrosion mechanisms of copper exposed to solutions containing anions anticipated in groundwaters such as sulphide, chloride and sulphate.
When the sulphide concentration is low (i.e., ≤ 1 ´ 10-4 mol/L), the copper sulphide (Cu2S) film formed has a cellular structure and its growth kinetics appear to be linear. The film growth process is controlled primarily by SH- diffusion in the aqueous solution, implying that the sulphide film is not protective under these conditions up to exposure times of ~ 4000 h. However, when the sulphide concentration is ≥ 5 ´ 10-4 mol/L, the film appears compact but grows non-uniformly with a logarithmic rate. In this latter case, micro-galvanic corrosion can occur between “thick film-covered” Cu surface locations, acting as net cathodes, and “thin film-covered” Cu surface locations acting as net anodes. Cu transport via copper sulphide complexes (e.g., Cu(SH)2-) and possibly clusters or nano-particles of Cu3S3 stoichiometry, from the net anode to the net cathode sustains the growth of the sulphide film at the net cathode.
At these high sulphide concentrations, the chloride concentration has a significant influence on the properties of the deposited film. When the chloride concentration is low (e.g., 0.1 mol/L), the film appears compact and its growth obeys a logarithmic law. The film growth process is controlled mainly by Cu+ diffusion in the sulphide film, leading to the formation of a protective deposit. When the chloride concentration is > 1.0 mol/L, the film becomes more porous, and at an extremely high concentration (e.g., 5 mol/L), the film grows with a two-dimensional structure within which particulates accumulate and deposit. Under these conditions, the film growth process is controlled by SH- diffusion in the bulk of the aqueous solution and Cu(SH)2-/Cu3S3 transport in the pores of the cellular sulphide film with the accumulation of the particulate deposit partially suppressing the corrosion process. No obvious pits are found based on surface and FIB-cut cross-sectional analyses.
The involvement of sulphate in the growth of the sulphide film is dependent on its concentration. When the sulphate concentration is low (e.g., 0.1 mol/L), the anion has only a minor effect on the structure and properties of the sulphide film and its growth kinetics. With an increase in sulphate concentration, the sulphide film changes from compact to porous at an intermediate sulphide level (5 ´ 10-4 mol/L) due to an increase in ionic strength within the pores in the film. Electrochemical studies suggest film growth is impeded by the co-adsorption of SO42- and H2O, which displaces the adsorbed SH- essential for the first step in the film formation reaction.
In anoxic groundwaters, the precursor to the deposition of the sulphide film on the Cu surface is the chemisorption of SH-, which competes with the adsorption of other groundwater anions (i.e., chloride and sulphate). This competition changes the structure and properties of the sulphide film, and the film growth kinetics. Other possible groundwater anions, such as carbonate and bicarbonate, also influence the porosity of the sulphide film, and this influence on film growth kinetics is still under investigation.
Copper; Sulphide; Corrosion; Nuclear waste disposal