Electrochemical and Corrosion Examination of Copper under Deep Geologic Conditions for the Application of Nuclear Waste Containers

Copper is the selected material for the fabrication of nuclear fuel waste containers in Sweden, Finland and Canada due to its corrosion resistance in the anoxic aqueous environments, anticipated in deep geological repositories. A potential oxidant for copper is bisulphide produced either by the dissolution of minerals or by sulphate-reducing bacteria. The primary goals of this thesis were to determine the mechanism of corrosion of copper in bisulphide environments and to investigate the chemistry of the bentonite porewater that will ultimately be in contact with the copper container. The mechanism of sulphide film growth and the details of copper corrosion in anoxic aqueous bisulphide solutions in the presence of various groundwater anions have been investigated electrochemically. The chalcocite films were analyzed via voltammetry, electrochemical impedance spectroscopy and surface analysis. Under these conditions, growth of a coherent film occurred initially but the development of interfacial stresses led rapidly to film fracture. Upon fracture, a resistive outer chalcocite deposit formed with continued growth governed by the mass transport of bisulphide. Specific anion effects were observed with chloride, a common groundwater ion, found to maintain porosity in the chalcocite film while the oxyanions, sulphate and bicarbonate were found to suppress film growth. At high bisulphide concentrations and positive potentials growth of the outer deposit caused an apparent partial passivation of the surface, which theoretically could lead to localized film breakdown and pitting. However, the required high bisulphide concentrations are not attainable in a deep geologic repository and pitting should not occur. Bentonite porewater chemistry was successfully monitored using a novel experimental set-up. Electrochemical sensors monitoring pH, chloride concentration and the copper corrosion potential were placed both on the surface and inside of a bentonite clay disk. The results showed that the bentonite acted like a pH buffer and chloride from bulk solution adsorbed onto the montmorillonite structure. Corrosion potential measurements suggested that corrosion likely occurred via formation of a copper chloride complex or growth of a cuprite film. Data collected from these studies will be used directly by SKB via incorporation into models