Formation mechanisms of multilayered cobalt deposits on 304 stainless steel in primary cooling systems of pressurized water reactor nuclear power plants

Radioactive corrosion products, specifically ⁵⁸Co and ⁶⁰Co, accumulate on the surface of components in pressurized water reactors (PWRs), posing significant challenges to nuclear plant safety. In this study, the microstructure and chemical composition of the surface layer of 304 stainless steel (304SS) exposed to cobalt-containing boron/lithium water at a high temperature of 573 K for 5, 10, 15, 20, and 25 days were investigated. The cobalt deposition behaviour and mechanisms were analysed via material characterization techniques, E–pH diagrams, Gibbs free energy calculations, and analysis of the preference energies of metal cations in crystallographic structures. The results revealed that after 15 days of soaking, three distinct cobalt deposition layers formed on the 304SS surface. The outer layer (∼65 nm) consisted of Co₃O₄, the middle layer (∼16 nm) consisted of CoFe₂O₄, and the inner layer (∼4 nm) consisted of CoCr₂O₄. The composition of these layers was relatively independent of the soaking time. CoCr₂O₄ primarily formed through migration of Co²⁺ from solution, coprecipitation with dissolved Cr³⁺ from the specimen surface or ion exchange with FeCr₂O₄ and NiCr₂O₄. CoFe₂O₄ formed through coprecipitation with dissolved Fe³⁺ or ion exchange with NiFe₂O₄ and Fe₃O₄. Co₃O₄ was derived from oxidative decomposition of Co(OH)₂ at high temperatures. This study provides key insights into the formation mechanisms of cobalt deposition layers on 304SS in PWRs and provides a theoretical reference for optimization of primary water chemical environmental parameters, improvement of structural materials, and selection of decontamination methods during operation or decommissioning.