Oxidation of Superalloys in Supercritical Water

Abstract

The formation and characterization of oxide films on nickel based superalloys as a result of exposure to supercritical water is of critical importance to thermal power industry. In this work, the oxide films formed on Inconel 625 and 718 after exposure to liquid water at 325°C and supercritical water at 425°C and 525°C are characterized using electron microscopy as well as Raman and X-ray photoelectron spectroscopies. It is found that the native CrO4 2- based films that provide these alloys their passive nature in liquid water are not present after exposure to supercritical water. When subjected to exposure to supercritical water, both alloys formed a duplex oxide film consisting of a base spinel, Ni(Fe,Cr)2O4, structure and a thin outer layer of NiO. The concentration of Fe in the spinel was found to increase at 525°C.