The effects of dissolved oxygen concentration and mass-transport conditions on the dissolution behavior of copper nuclear waste containers

The effects of solution composition, dissolved oxygen concentration and rate of mass transport on the corrosion behavior of copper in Cl--containing solutions have been determined. The potentials at which copper dissolves directly as Cu(I) and Cu(II) have been measured in Cl-, SO42- and mixed Cl-/SO42- solutions using rotating ring-disc and rotating splitting disc electrodes. By comparison to corrosion potentials measured in O2-containing solution, it is shown that copper dissolves as Cu(I) in Cl--containing solutions, in the form of the CuCl2- species. Subsequent precipitation of Cu(II) solids is a consequence of homogeneous oxidation of Cu(I) by O2. Corrosion potentials measured with a clay-covered copper electrode are ∼0.1 V more positive than those measured in bulk solution of the same chloride and O2 concentration. The ennoblement of ECORR under these restrictive mass-transport conditions is a result of the change from anodic transport/cathodic kinetic control in bulk solution to transport control of both anodic and cathodic reactions in the presence of compacted clay. The potential of the clay-covered electrode is not sufficiently positive, however, for copper to dissolve directly as Cu(II). Copper(II) is formed, therefore, by homogeneous oxidation of Cu(I) by O2, as in bulk solution. A simple mathematical model can account for the corrosion behavior of copper and the oxidation states of copper corrosion products in O2-containing Cl solutions over a wide range of mass-transport conditions. The extent of formation of Cu(2) depends on the relative rates of diffusion and oxidation of Cu(I). The rate of Cu(I) oxidation by O2 in compacted clay may be lower than that in bulk solution because of spatial restrictions in the compacted medium