Phosphate-induced corrosion of nickel alloys in supercritical water: Mechanistic insights into passivation and accelerated corrosion at elevated temperatures

Abstract

This study examines the corrosion behavior of Inconel 600 and Inconel 625 alloys in supercritical water environments containing phosphates, with a focus on the temperature-dependent effects of phosphate in both dissolved and molten states. At 400 °C, dissolved phosphate acts as a passivating agent, forming protective metal phosphate films that reduce corrosion. At 600 °C, the low solubility of phosphate causes it to precipitate into a molten state. This significantly accelerates corrosion by dissolving the protective oxide film and enhancing ion diffusion, increasing the film thickness from 0.16 μm to 0.675 μm. The higher Cr content in Inconel 625 resulted in an 65.9 % reduction in oxide thickness relative to Inconel 600, indicating enhanced oxidation resistance. The core theoretical innovation of this study lies in identifying the dual role of phosphate in alloy corrosion: acting as a passivating agent at lower temperatures and as a corrosion accelerator at higher temperatures. This study further clarifies the competitive relationship between phosphate and oxygen, the mechanism by which molten salts accelerate corrosion, and the synergistic molten corrosion effect of Na⁺. The findings provide valuable insights into corrosion protection strategies for supercritical water oxidation systems, contributing to the optimization of material design in extreme environments.