Corrosion of stainless steel 316 and Nitronic 50 exposed to supercritical and ultrasupercritical water was studied as a function of temperature and exposure time. Post-exposure surface analysis was performed using Raman and X-ray photoelectron spectroscopies to determine the chemistry of the oxides formed as a result of the exposure. When exposed to supercritical water, Nitronic 50 and stainless steel 316 were observed to have similar weight gains; however, stainless steel 316 was found to gain less weight than Nitronic 50 in exposure tests performed in ultrasupercritical water. Stainless steel 316 developed surface films primarily composed of iron oxides, while the surface of Nitronic 50 contained a mixture of iron, chromium and manganese oxides. Based on these analyses, the differences in weight gain and oxidation characteristics of the two materials are attributed to the higher concentration of Cr and Mn in Nitronic 50 compared to stainless steel 316.
Corrosion behavior of Inconel 625 and 718 in subcritical, supercritical and ultrasupercritical water was studied as a function of temperature and time. The change in the chemistry of the as-received surface film on Inconel 625 and 718 after exposure to subcritical water at 325°C and supercritical water at 425°C and 527.5°C for 2 hours was studied. After exposure to 325°C subcritical water, the CrO4 2based film formed; however minor quantities of NiFexCr2-xO4 spinel compounds were observed. The oxide film formed on both alloys when exposed to supercritical water at 425°C consisted of NiFexCr2-xO4 spinel. The surface films on both alloys were identified as NiFe2O4 when exposed to supercritical water at 527.5°C. To characterize the fully developed oxide layer, studies were conducted at test solution temperatures of 527.5 and 600°C. Samples were exposed to these temperatures for 24, 96, and 200 hours. Surface chemistry was analyzed using Xray diffraction, as well as Raman and X-ray photoelectron spectroscopies. Inconel 718 exhibited greater mass gain than Inconel 625 for all temperatures and exposure times. The differences in corrosion behavior of the two alloys are attributed to the lower content of chromium and increased iron content of Inconel 718 as compared to Inconel 625.
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 Xray photoelectron spectroscopies. It is found that the native CrO 4 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) 2 O 4 , structure and a thin outer layer of NiO. The concentration of Fe in the spinel was found to increase at 525°C.
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