Performance of oxygen-free phosphorous-doped and high-conductivity phosphorous-doped copper in ammonia-containing groundwater

High-purity copper is used for high-level waste (HLW) canister material due to its mechanical strength and corrosion resistance. While oxygen-free phosphorous-doped copper (OFE + P) is the current standard material in Finland and Sweden, high-conductivity phosphorous-doped (HCP) copper has emerged as a potential alternative. This study aimed to evaluate the corrosion performance, stress corrosion cracking (SCC) susceptibility, and hydrogen uptake of HCP relative to OFE + P under ammonia-disturbed groundwater conditions. Both copper grades were subjected to a 3-month exposure in an autoclave containing simulated groundwater and 100 mg/L of ammonia at room temperature. Constant potentials were applied to shift surface potentials into the Cu₂O/CuO stability region, mimicking thermodynamic conditions associated with SCC. Hydrogen content was assessed by hot-melt mass spectroscopy and thermal desorption spectroscopy supported by molecular dynamics simulations of hydrogen diffusion. U-bend specimens, constantly polarized at − 50 mV versus saturated calomel electrode, were analysed using plasma-focused ion beam (PFIB) and electron backscatter diffraction (EBSD) to characterize microstructural degradation. Both materials presented very-low corrosion rates (0.2–0.4 µm/year), and no hydrogen uptake was detected. However, PFIB and EBSD results revealed fully oxidized, intergranular cracks penetrating up to 10 µm, along random grain boundaries. These shallow features were attributed to ammonia-induced degradation facilitated by localized strain, even under anoxic conditions and room temperature. HCP copper exhibited comparable corrosion behaviour and susceptibility to ammonia-induced intergranular attack to that of OFE + P copper. These results support the potential use of HCP copper as a viable alternative for HLW canisters in ammonia-containing environments.