Corrosion of copper in sulphide containing environment: the role and properties of sulphide films – Annual report 2020

In COCOS project, the role and properties of sulphide films on copper surface are studied. OFP-copper samples were exposed to sulfide-containing anoxic simulat-ed groundwater for different durations. In 2020, the second year of the COCOS project, the work continued with characterization of previously exposed OFP-copper samples by new techniques. Also, new 9-month long exposure tests were started with sulphide concentrations ranging from 0 to 320 mg/L. The electrochemical measurements were conducted across the duration of the exposure test. As a result, trends in corrosion behaviour of copper was seen throughout the test. EIS results indicated the formation of the different corrosion product films on sample surfaces exposed. The protective effect of surface layers changes with time of exposure, and the behaviour is influenced by the amount of sulphide in the environment. A distinct and sharp increases in the open circuit po-tential values of copper samples were observed in some cases during the test. These were more common in environments with low sulphide amount and may be related to time when sulphide has been consumed by chemical reactions. The observation will be further studied in future. Corrosion rate of samples exposed to 0 mg/L, 32 mg/L and 320 mg/L of sulphide was determined also by measuring mass loss during 4-month test. One interest-ing finding was that the highest corrosion rates, ca. 0.4 µm/a, were obtained for samples exposed to sulphide content of 32 mg/L. Sample characterisation showed that the highest mass loss was most likely due to local defects on the samples, not uniform corrosion. HEXRD studies revealed significant lattice changes as deformation extending several hundreds of micrometers into the bulk. This is attributed to H infusion. The results demonstrate the risk for H-induced stress corrosion cracking of copper as canister material during long-term storage of nuclear fuel when exposed to sulfide-containing groundwater. The last year of the project is going on at the moment. Longer term experiments have been performed. Further analysis of new samples and data from electro-chemical measurements from longer exposures continues. Also, the last new ex-perimental tests will be conducted in 2021.