Investigating the Potential for Stress Corrosion Cracking of Copper in Sulfide and Nitrite Environments

Canada’s proposed nuclear waste disposal method includes sealing used fuel bundles inside copper-coated used fuel containers (UFCs) which are then stored in deep geological repositories (DGRs). The UFCs must maintain integrity until the fuel’s radioactivity decreases to background levels. In this project, copper was tested in sulfide and nitrite (expected in a DGR) environments to further understand copper’s susceptibility to stress corrosion cracking (SCC), a corrosion mechanism characterized by a susceptible material cracking when exposed to a corrosive environment and tensile stress. Electrochemical testing methods, specifically potentiodynamic scans, were used to evaluate the copper and identify active/passive transitions. By varying the applied potential, the resulting current quantitatively described the different corrosion regimes of the material, including the target regions where the copper might be susceptible to SCC. Stressed copper samples were submerged in nitrite solutions and held in these target regions to determine if SCC was achievable. When exposed to aerated sulfide solutions, the copper’s behaviour depended on the sulfide concentration, with a passive film (a pre-requisite for many SCC mechanisms) only forming at the highest concentration. In deaerated conditions, the film formed in all concentrations. This led to the hypothesis that oxygen reduces the concentration of sulfide, and that low sulfide concentrations may form a passive film in low-oxygen environments (expected in a DGR). Future work should focus on determining the conditions required to form this film. In the nitrite solutions, it was observed that various passive films could be formed, depending on the applied potential. A stressed copper sample was immersed in a solution for 24 hours with a high nitrite concentration (beyond any real-world application) and held at a potential suspected to allow SCC. Scanning electron microscopy (SEM) analysis revealed that SCC had occurred. Future work should focus on determining the boundary conditions for SCC in nitrite.