Development of a rapid screening test for SCC susceptibility of copper in disposal vault conditions
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Development of a rapid screening test for SCC susceptibility of copper in disposal vault conditions Iva G. Betova1, Martin S. Bojinov 2, Jussi Heinonen 3, Petri Kinnunen 4, Christina Lilja5, Timo Saario 4 1 Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Department of Physical Chemistry, University of Chemical Technology and Metallurgy, Sofia, Bulgaria 3 Radiation and Nuclear Safety Authority, Helsinki, Finland 4 VTT Industrial Systems, VTT Technical Research Centre of Finland, Espoo, Finland 5 Swedish Nuclear Power Inspectorate, Stockholm, Sweden ABSTRACT Stress corrosion cracking is a potential failure mechanism of the copper canister proposed to be the main corrosion barrier in the Scandinavian design of the final nuclear waste disposal vault. The main target in SCC-oriented research is to determine the limiting values for the critical concentration of harmful species, the potential range in which these species cause SCC and the threshold stress level required for SCC. The present paper attempts to redefine the criteria for SCC susceptibility of copper in acetate solutions through analysis of potentiodynamic curves using both rotating disc and thin plate electrodes insulated on one side. A potential range in which the current densities at a slow sweep rate are lower than those at a fast sweep rate exists for Cu in 0.001 M - 0.1 M acetate. However, the values of the current densities within this range are significantly smaller than those postulated for active anodic dissolution, implying dissolution mediated by a surface film. New criteria for the SCC susceptibility of pure copper in acetate solutions have been formulated through Stern-Geary analysis of the polarisation curves. Several electrochemical characteristics, such as the current density at the positive crossover potential, the ratio between the corrosion current densities determined from the slow and fast sweeps, as well as the anodic current density in a fast sweep show a clear jump for concentrations between 0.01 M and 0.05 M. This, however, cannot yet be considered as an indication of SCC susceptibility at higher acetate concentrations, as evidenced by complementary SSRT tests. INTRODUCTION Stress corrosion cracking is a potential failure mechanism of the copper canister. This possible problem can be approached in several ways. One possibility is to perform crack growth tests with large fracture mechanical specimens. Such tests are useful in the sense that by varying experimentally controllable parameters, i.e. stress intensity, concentration of chemicals or electrode potential one may be able to determine minimum/maximum levels in these parameters that give rise to SCC. Another possibility is to use slow strain rate tests (SSRT) which give a more on-off type of understanding of the susceptibility of copper to SCC in a given environment. The main target in SCC research with any technique would be to determine the limiting values for the above mentioned critical parameters and to reveal the mechanism of
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