Corrosion of copper in sulphide containing environment: the role and properties of sulphide films – Annual report 2021

In NKS-R COCOS project, the role and properties of sulphide films on copper surface were studied. This is the third annual and final report of the project. OFP-copper samples were exposed to sulfide-containing (0-640 mg/L) anoxic simulated groundwater for different durations between 2 to 9 months. A pre-oxidation procedure was used to simulate the effect of oxic period of repository to copper surface. The corrosion behaviour was studied using mass loss determination, electrochemical measurements and several modern material characterisation methods after the exposure. In addition to sulphide concentration the chemistry of test solution was varied by additions of chloride and hydrogen carbonate. Specimens and surface deposits were studied with electron microscopy (SEM+EDS, EBSD). In-depth characterisation was done using GD-OES analysis, HE-XRD techniques and SIMS analysis. During the project, also several other analysis methods were used. The measurable corrosion was not proportional to sulphide concentration. The corrosion was not always uniform. The corrosion rates were 0-12.3 µm/a, depending on the sulphide concentration, temperature and other variables of the environment. EBSD results suggested that both the pre-oxidation treatment and the exposure to sulphide environment introduced the strain localization in the near-surface areas of samples. HE-XRD results showed significant heterogeneous structure deformation in the copper lattice after the exposure to test environment, both in the near surface region and deep inside the microstructure. The sulphide-copper interactions and formation of sulphide film on copper surface were found complex processes. In this project, the world-class analytical techniques were used to investigate the penetration of chemical species into the copper microstructure. More work is needed to define the role of these chemical species as well as the nature and chemistry of the surface films that develop on copper surfaces in repository conditions.