Corrosion Mechanism of Inconel 600 in Oxidizing Supercritical Aqueous Systems Containing Multiple Salts

Abstract

The corrosion behavior and mechanism of Inconel 600 exposed to the oxidizing high-salinity supercritical water (SCW) containing chlorides, sulfates, and phosphates at 450, 500, and 580 °C were investigated and proposed. The corrosion film had an outer layer dominated by NiO, Fe₂O₃, and eutectic phosphate deposit and an inner layer consisting of Cr-rich oxides. Some produced oxides may generate ionic reactions with the low-melting phosphates precipitated out from SCW, triggering the generation of the eutectic FePO₄–Ni₃(PO₄)₂–Na₃PO₄–Na₂HPO₄ salts in the outer layer. The formation and thickening of the outer layer results from the eutectic phosphate disposition and the common solid-state growth mechanism. Nonmolten chlorides and sulfates in SCW predominantly exist on the outmost surface and are easily washed off, being supported by the absence of S and Cl on sample surfaces. Based on this investigation, a comprehensive corrosion mechanism, involving the solid-state growth of oxide scales, salt precipitation, and phosphate melting processes, was proposed and discussed.