EIS study on aerobic corrosion of copper in ground water: influence of micro-organisms

The nuclear waste disposal concept in Finland and Sweden is based on a multi-barrier system, where the solid spent fuel is placed in a cast iron container that is further enclosed in a copper canister. The copper canisters will be placed vertically in holes drilled in a deep bedrock which are then filled with a bentonite clay. The near-field environment of bentonite clay and groundwater is then expected to maintain integrity of the copper canisters for at least 100 000 years. The ground water at the final disposal depth contains micro-organisms with a vast metabolic potential, and it is known that micro-organisms may induce or accelerate the corrosion of metals. Therefore, it is of vital importance to understand the surface interactions of copper and the expected near-field environment. In this study, the results from two types of experiments that investigate the corrosion of copper in oxic ground water environment with and without micro-organisms and electrochemical impedance spectroscopy (EIS) as one of the key research methods are reported. First, long-term immersion experiments with a range of in-situ electrochemical measurements were designed to simulate the initial oxic stage of the deep geological nuclear waste repository in the presence of bentonite. Second, electrochemical laboratory tests were planned to improve understanding on the interactions between copper and the micro-organisms in ground water environment: copper specimens were incubated with micro-organisms for different time periods and then subjected to electrochemical measurements to provide data. The results collected from these two test series are reported and discussed here by focusing on the EIS data and its role in interpreting the surface processes. It is evident that EIS helps in understanding the influence of micro-organisms on the copper degradation process.