<p>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 laboratory-grown oxides provide a non-radioactive surrogate for PHWR corrosion products, enabling controlled studies relevant to decontamination technology development.</p> Graphical abstract <p></p>