Investigation of the deposition behaviour of cobalt on 304 stainless steel in a simulated spent nuclear fuel pool

• When 304SS samples are soaked in a 333 K boric acid solution containing 500 mg/L Co 2+ and 2000 mg/L B 3+ , Co was deposited on their surface in the form of stable compounds. • The main stable Co compounds were Co(OH) 2 , CoFe 2 O 4 , and CoCr 2 O 4 , and their formation depended on the solution pH. • Stable CoCr 2 O 4 and CoFe 2 O 4 were formed via coprecipitation between Cr 3+ /Fe 3+ and Co 2+ ions in solution, whereas Co(OH) 2 was formed through the precipitation of Co 2+ with OH − ions. The accumulation of radioactive corrosion products, specifically 58 Co and 60 Co, on metallic material (304 stainless steel) surface in spent nuclear fuel (SNF) pools is among the main factors of radioactive contamination. In this study, the microstructural characteristics and chemical composition of the surface layer of 304 stainless steel (304SS) exposed to 333 K Co-containing boric acid solution for 10, 30, 50, 70, 90, and 125 days were investigated. The cobalt deposition behaviour was analysed via material characterization techniques, E–pH diagrams, and Gibbs free energy calculations. The results revealed that CoFe 2 O 4 and CoCr 2 O 4 were deposited on the 304SS surface when the solution pH value was less than 6.08, and Co(OH) 2 and Co(Fe, Cr) 2 O 4 were deposited on the 304SS surface when the solution pH was greater than 6.08. After 125 days of soaking, 166 nm thick Co(OH) 2 layer was deposited on the surface of 304SS, and 6 nm thick Co(Fe, Cr) 2 O 4 layer beneath it. It was further analyzed that Co(OH) 2 was primarily produced by the precipitation of Co 2+ with OH – in solution, whereas CoFe 2 O 4 and CoCr 2 O 4 were primarily produced by the coprecipitation of Co 2+ in the solution with Fe 3+ and Cr 3+ dissolved from the substrate. This study provides key insights into the formation mechanisms of cobalt deposition layers on 304SS in SNF pool and provides a theoretical reference for optimizing primary water chemistry, improving structural materials, and selecting decontamination strategies during operation or decommissioning.