Unveiling the mechanism of selective dissolution-repassivation for effective oxide film removal from alloy 690 in alkaline H₂O₂ solutions

This study investigates the corrosion behavior and oxide film removal of Alloy 690 in a boron-free Small Modular Reactor (SMR) environment, which focuses on the electrochemical and structural mechanisms of oxide film formation and selective removal under different conditions. Using hydrogen peroxide (H₂O₂) as a green oxidant, this study evaluates the effectiveness of various oxide removal schemes, including acidic, lithium-removal, and alkaline H₂O₂-based treatments. The results show that the without lithium-removal scheme under alkaline conditions (pH = 9.5) achieves the most efficient oxide removal, selectively dissolving Cr oxides while preserving the Ni-based substrate. This process leads to the formation of a new, non-radioactive, protective oxide film, ensuring substrate integrity. Electrochemical analyses reveal that the selective dissolution of Cr and the rapid repassivation of the alloy surface contribute to enhanced corrosion resistance after treatment. These findings introduce a novel electrochemical dissolution-induced repassivation mechanism for oxide removal, providing a material-friendly alternative to traditional decontamination methods in SMRs. The study offers valuable insights for optimizing decontamination strategies in boron-free reactor environments, contributing to the safety and sustainability of next-generation nuclear reactors.