The Semiconducting Properties and Impedance Analysis of Passive Films on Copper in Anoxic Sulfide-Containing Solutions from the Viewpoint of the Point Defect Model

The corrosion of pure copper in sulfide-containing aqueous solutions that are typical of crystalline rock repositories in Sweden and Finland for the isolation of High Level Nuclear Waste has been studied using potentiostatic and potentiodynamic polarization, Mott-Schottky analysis, and electrochemical impedance spectroscopy. The results, which are interpreted in terms of the Point Defect Model, indicate that a bi-layer sulfide film forms, comprising of a p-type barrier layer of Cu2S and probably an outer layer of CuS, which is n-type in electronic character. The outer layer is not observed to form at 25 degrees C and at 50 degrees C, but is observed to form intermittently at 75 degrees C. Thus, the outer layer is unstable and frequently disappears from the surface by sloughing, resulting in large excursions of the corrosion potential. This phenomenon is found to induce considerable instability in the electrochemical response of the system, such as that under potentiodynamic polarization.