Is Copper Immune to Corrosion When in Contact With Water and Aqueous Solutions

Objectives The aim of this project has been to increase knowledge and to contribute to the research community in the area of copper corrosion in a repository environment. For SSM, the most important subject is to provide better conditions for a science based evaluation of a repository for spent nuclear fuel. In this respect, this project aimed at conducting a comprehensive theoretical study on corrosion of copper in repository environment based on an expected composition of dissolved species in the groundwater in the Forsmark area. In addition the thermodynamic immunity of copper in pure anoxic water has been especially addressed as this was one of the initial conditions made by SKB for selecting copper as canister material. Results The authors have shown, in so-called corrosion Domain Diagrams, that copper in a thermodynamic sense can be considered as immune in pure anoxic water (without dissolved oxygen) only under certain conditions. It is shown that copper will corrode in pure anoxic water with very low concentrations of [Cu+] and very low partial pressures of hydrogen gas. At higher concentrations of [Cu+] and partial pressures of hydrogen, copper is found to be thermodynamically immune and will not corrode. The rate of copper corrosion in the repository water environment will thus depend on the transport of corrosion products away from the copper surface or the transport of corroding species to the copper surface. The degree to which this affects the corrosion of copper canisters in the repository environment has not been further studied. Still, the result shows that copper cannot be considered as thermodynamically immune in the presence of pure anoxic water, this implicate that one of SKB:s initial conditions for selecting copper as a canister material can be questioned. To what degree this may influence the corrosion of copper canisters in the repository environment still needs to be investigated. Of other species present in the water at repository depth in Forsmark, different sulphur species was found to be most deleterious causing copper to corrode in an anoxic environment under hydrogen gas evolution. In order to find out what species that can be present in a repository environment a Gibbs Energy Minimisation algorithm was employed. By this method it was concluded that (S2-, HS- and H{sub 2}S) are predicted to be present in the entire anoxic period at sufficient concentrations to cause corrosion of copper. It is finally concluded that the corrosion rate of copper canisters will be determined by the very complex interaction between copper, buffer material and bedrock in order to reduce corrosion of copper to an acceptable level