pH-dependent corrosion mechanisms of alloy 690 in pressurized water reactor environments: Atomic-scale insights and modeling

This study investigates the pH-dependent corrosion mechanisms of Alloy 690, a critical material in pressurized water reactor (PWR) coolant systems, using in situ electrochemical measurements and mixed potential modeling (MPM). The findings reveal that increasing pH in deaerated subcritical aqueous environments results in a reduction in corrosion potential, and an increase in electrochemical impedance modulus, but a rise in corrosion current density. MPM analysis of electrochemical impedance spectra indicates that pH notably impacts the direct corrosion driving force, quantified by the substrate/oxide interfacial potential drop, which increases with rising pH. At the atomic scale, the study reveals that the corrosion driving force at the alloy substrate/oxide film interface is enhanced as pH increases, accelerating the transformation of metal atoms into ions and the subsequent electron loss reaction. This phenomenon provides a fundamental theoretical explanation for the increased corrosion rate of Alloy 690 in alkaline environments. This work innovatively links pH to atomic-scale corrosion processes, offering critical insights into the mechanisms governing corrosion in PWR environments.