Corrosion behaviour of copper under biotic and abiotic conditions in anoxic ground water: electrochemical study

Finnish nuclear waste disposal program proposes to dispose spent nuclear fuel in tightly sealed copper canisters in a geological repository; here, copper acts as a corrosion barrier. The key argument for such materials selection is that copper is resistant to corrosion in oxygen-free water. However, the presence and colonization of microbial species in ground water in the geological repository may initiate, facilitate and accelerate corrosion of copper under such conditions. In this study, the results from experiments that were designed to simulate the final stage of the deep geological nuclear waste repository, when the temperature has already stabilized to the level of the surrounding bedrock and all oxygen included at the construction stage has been consumed, are reported, both in the presence and absence of micro-organisms retrieved from the repository site. The experiments were performed for 10 months, during which the copper specimens were subjected to electrochemical measurements. After the tests, the specimens were investigated in terms of microstructure and weight changes, while the test media was characterized with respect to selected chemical species. Analysis concentrated on EIS data in the light of results obtained by other methods. In the presence of micro-organisms, e.g., sulphate-reducing bacteria, the development of Cu2S is the primary surface process on copper specimens, with its corrosion protection properties being regulated by the growth of the biofilm. As compared to two-layered Cu2O films that grow on the specimen surfaces in the absence of bacteria, Cu2S films were essentially less susceptible for the attack by Cl-. Under abiotic conditions, passivity breakdown by Cl- resulting in the localized type of attack detected on the surfaces is the probable reason for an essentially higher corrosion rate than in the biotic system.