This study explores the effect of cobalt ions on the formation behavior of corrosion oxide layers on carbon steel SA 106 GR.B, a material widely used in the primary coolant system of pressurized heavy water reactors (PHWRs). Oxide layers were generated under two high-temperature aqueous conditions. One condition contained hydrochloric acid, and the other contained hydrochloric acid with added cobalt sulfate. The specimen oxidized without cobalt exhibited a homogeneous magnetite (Fe3O4) layer with uniform morphology and composition. In contrast, the cobalt-containing condition produced a distinct bilayered oxide structure. The outer layer was primarily composed of iron oxides with minimal cobalt content, while the inner layer showed significant cobalt enrichment, with concentrations reaching up to 60 wt%. Cobalt was incorporated into the oxide scale, and a Co-enriched inner region was observed. These results demonstrate that cobalt alters the oxidation pathway and promotes the formation of compositionally heterogeneous layered oxide scales. These laboratorygrown oxides provide a non-radioactive surrogate for PHWR corrosion products, enabling controlled studies relevant to decontamination technology development.

Effect of cobalt on the oxide layer formation on carbon steel in hydrothermal acidic environment
    Abstract
    Highlights
    1 Introduction
    2 Materials and methods
        2.1 Materials
        2.2 Fabrication of oxide layer on carbon steel specimens
        2.3 Characterization of oxide layer
    3 Results and discussion
    4 Conclusions
    References

• Bum Kyoung Seo(Nuclear Facility Cleanup Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea)
• Sion Kim(Nuclear Facility Cleanup Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, South Korea)
• Kamal Asghar(Nuclear Facility Cleanup Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, South Korea)
• Miguta Faustine Ngulimi(Nuclear Facility Cleanup Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, South Korea)
• Changhyun Roh(Nuclear Facility Cleanup Technology Division, Korea Atomic Energy Research Institute, Daejeon 34057, South Korea, Nuclear Science and Technology, University of Science and Technology, Daejeon 34113, South Korea) Corresponding author
• Yahya Hoque Mozumder(Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK, Henry Royce Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK)
• Fabio Scenini(Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK, Henry Royce Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK)
• Dora Capone(Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK, Henry Royce Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK, Department Centre for Nuclear Engineering and Science, Paul Scherrer Institute, Forschungsstrasse 111, Villigen 5232, Switzerland)