The corrosion and electrochemistry of copper in aqueous, anoxic sulphide solutions

A proposed method of disposal of Swedish/Finnish/Canadian high-level nuclear waste is to place it in corrosion resistant containers and bury it approximately 500--1000 m deep in granite environments. The chosen material for the fabrication of these containers is copper, selected primarily because of its thermodynamic stability in the aqueous anoxic environments anticipated in such repositories. The present design consists of an outer copper shell (∼5 cm thick) and an inner liner of nodular cast iron.;The presence of sulphide films shifts the corrosion potential for copper dissolution to more negative values, this could render copper susceptible to corrosion via the reduction of water. Possible components of the immediate vault environment, as well as the bentonite clay, include pyrite (FeS 2) and sulphate (SO42-) both of which are potential sources of sulphide, and it is well known that sulphate-reducing bacteria (SRB) exist which can convert sulphates to sulphides. A number of factors, such as the mechanical pressure from the swelling bentonite, the low water activity in the bentonite, gamma-radiation and the heat from radionuclide decay processes will ensure that there is no microbial activity in the vicinity of the container. Remotely produced sulphide could, however, be transported slowly through the compacted buffer to the container surface.;The mechanism and kinetics of Cu corrosion in anoxic aqueous chloride solutions containing sulphide have been investigated electrochemically and under natural corrosion conditions. Under these conditions, the anodic growth of a chalcocite (Cu2S)/digenite (Cu1.8S) films on Cu is supported by the cathodic reduction of water. Electrochemical experiments at rotating disc electrodes and impedance spectroscopy show that the film growth occurs under SH- transport control as stagnant conditions are approached. Film growth can follow two distinct pathways. The initially formed film grows rapidly via an ion (or associated defect) transport process. If this film remains coherent, subsequent film growth/corrosion is extremely slow. If the development of interfacial stresses leads to film fracture then growth continues and a much thicker nodular deposit is formed.;Scandinavian/Canadian high-level nuclear waste repository conditions are expected to evolve from initially warm and oxic to eventually cool and anoxic. During the warm, oxic period corrosion products will accumulate on the container surface. These deposits could impede the reaction of Cu with aqueous sulphide, the only reaction that could lead to the significant accumulation of additional corrosion damage under the long-term anoxic conditions. The evolution of the surface as oxides/hydroxides were converted to sulphides in the sulphide concentration range 10-5 to 10-3 mol/L were followed by corrosion potential measurements, in-situ Raman spectroscopy, and electrochemical impedance spectroscopy. The conversion of Cu2O to Cu2S was found to proceed via a chemical conversion reaction. Although this corrosion became slow at low sulphide concentrations, no evidence for a threshold sulphide concentration below which the oxide could protect Cu against corrosion by sulphide was identified.;Since proposed repository groundwaters are chloride-dominated, the corrosion of copper in sulphide solutions containing chloride has been investigated. Local pore chemistry within a copper surface film could allow for an exacerbated combination of low [SH-] and high [Cl-], especially in high groundwater with high chloride content when the Cu/SH- reaction has become transport controlled. The natural and electrochemical growth of sulphide films in solutions with constant sulphide concentration but various amounts of chloride have been followed with electrochemical (EIS, CSV) and surface analytical (SEM, EDX) techniques to examine the role of chloride in the copper corrosion process when exposed to anoxic, sulphide-containing brine. Chloride was found to influence the corrosion process primarily by destabilizing the CuxS films which slow down the corrosion process. Some evidence that CF could stimulate CuI transport by forming Cu(Cl)x(x-1)- complexes was obtained. This enhanced transport then leads to the accumulation of thicker outer sulphide deposits which are only marginally protective. (Abstract shortened by UMI.);Keywords: Copper, Sulphide, Chloride, Oxide, Corrosion, Electrochemistry, Film Growth, Mechanism, Current-Potential Modelling, Nuclear Waste Disposal...