Influence of Alloying Elements and Chloride Concentration on the Galvanic Corrosion of Ferrous Alloys Coupled to Graphite

As the deployment of small modular nuclear reactors (SMRs) in Canada approaches, waste management considerations become increasingly critical. This study investigates the potential for galvanic degradation between the graphite outer casing of spent TRi-structural ISOtropic (TRISO) fuel and candidate container alloys during the interim dry storage phase. Four alloys, A516 Gr. 70 carbon steel, 405, 430, and 304L stainless steels, were evaluated to assess the influence of Cr and Ni content on galvanic corrosion interactions in chloride-containing environments. Electrochemical testing was conducted to investigate the galvanic corrosion properties of all four alloys coupled to graphite, in increasing salinity (0.001 M to 6.14 M NaCl). Potentiodynamic testing revealed that galvanic corrosion of A516 is governed by the concentration of dissolved oxygen, with corrosion rate peaking at 0.6 M NaCl and decreasing due to oxygen depletion with increasing Cl ions. Immersion testing highlighted microgalvanic coupling between ferrite and cementite phases in A516, although absent in oxygen-limited conditions. All stainless steel alloys demonstrated comparable corrosion rates up to 0.6 M NaCl, yet in 6.14 M NaCl, 405 and 430 exhibited a significant increase in corrosion rate due to a shift in pitting potential equating to the galvanic potential. 304L maintains a low corrosion rate at all concentrations of NaCl, highlighting the influence of increased Cr content and Ni additions on passive film stability.