Hydrothermal corrosion of inconel 625 in high-chlorine, high-alkalinity supercritical water oxidation environments: A new mechanism

Abstract

This study investigates the hydrothermal corrosion behavior of Inconel 625 in high-chlorine, high-alkalinity supercritical water oxidation (SCWO) environments. A significant reduction in corrosion rate was observed between 450°C and 500°C due to a transition from electrochemical to chemical corrosion mechanisms. The transition temperature increased from approximately 420°C to 450–500°C at 25 MPa, attributed to molten NaOH stabilizing ion transport. At 600°C, interactions between molten NaOH and oxide films cause melting and sintering, leading to oxide film resolidification but raising concerns about mechanical instability. Under oxygen-rich conditions, chromium from the protective inner oxide layer oxidizes to soluble Cr⁶⁺, depleting the inner layer and accelerating outer layer growth, with a significant increase in the outer-to-inner layer thickness ratio compared to near-pure supercritical water (SCW). The study highlights the synergistic effects of alkalinity, oxygen, and chlorine, provides new insights into corrosion mechanisms, and offers guidance for mitigation strategies in SCWO systems.