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 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.

Accelerated estimation of corrosion rate in supercritical and ultra-supercritical water

“…Therefore, it is reasonable to assume that the exposure of these materials to SCW and USCW will result in uniform corrosion. 4,21,22,29 The values of atomic mass and equivalents exchanged for austenitic alloys were 55.85 and 2, respectively. These values correspond to Fe, which has been reported in literature to be the main corroding species in SS316.…”

“…4,7,8,[12][13][14][15][16][17][18][19][20] Calculations of the corrosion rates of Inconel alloys were performed using the values of 58.69 for a and 2 for n since Ni has been identified as the primary corroding element. 4,6,15,[29][30][31][32] The intermodulation spectra obtained from SS316 exposed to SCW at 530°C and USCW at 600°C are shown in Fig. 1a and 1b, respectively.…”

“…On the surfaces of Inconel alloys, Cr compounds, specifically NiCr 2 O 4 and Cr 2 O 3 , form porous layers that allow Ni and Fe to diffuse to the surface and form NiFe 2 O 4 . 4,6,15,[29][30][31][32] Based on this surface analysis, an equivalent circuit was constructed to represent all alloys tested within this study, and it is shown in Fig. 2a.…”

Oxides Film Formed on Fe- and Ni-Based Alloys: An Ellipsometry Insight

“…The electronic structure of the oxide films is thus similar to a p-n heterostructure, where the space charge regions formed by the two oxides limit the electronic and ionic transfers and thus increase the effective corrosion protection. In the case of nickel-based alloys, the outer layer is mainly based on NiO while the inner layer is rich in Cr [3]. The various semiconductors (Fe 2 O 3 , Cr 2 O 3 , NiO) contained in the passivation layer of these alloys are transparent conductive oxides that grow spontaneously or when the alloy is subjected to thermal and/or chemical stress.…”