Characterization of the Anoxic Sulfide Corrosion of Copper Materials in Gaseous and Aqueous Environments

Abstract

International consensus on the best practice for the disposal of used nuclear fuel is to bury it in a deep geological repository (DGR), which employs a multi-barrier system. In Canada, this DGR is being developed by the Nuclear Waste Management Organization (NWMO). A key component in this system is the used fuel container (UFC), which isolates the fuel from the environment. The Canadian UFC design uses a copper-coated carbon steel container, where the copper acts as a corrosion barrier, and the steel provides structural strength. Copper is applied via electrodeposition and cold-spray deposition techniques to a final thickness of 3 mm [1]. Once the UFC is emplaced, the potential corrosion processes governing container degradation evolve with the change in environmental conditions in the DGR. From a corrosion perspective, the relevant environmental conditions are temperature, radiation field levels, humidity, redox potential, groundwater chemistry, and water saturation timelines, all of which will evolve over the DGR’s lifetime. Sulfides, which are potentially produced during the anoxic period from the anaerobic metabolic processes of sulfate reducing bacteria, are of particular interest. This study aims to examine the corrosive effects of a gaseous hydrogen sulfide environment (100 ppm H₂S in N₂ carrier gas) on the electrodeposited copper material. Tests are conducted on both polished and pre-oxidized materials to determine what effect, if any, oxide layers formed during initial oxidizing periods may have on the corrosion processes of later anoxic corrosion. Samples are characterized with electron microscopy techniques to determine the nature of the film. Results are compared with aqueous-formed films to evaluate differences and facilitate a more complete understanding of the copper corrosion processes in the DGR.