Corrosion Behavior of Austenitic Stainless Steel in Supercritical CO2 Containing O2 and H2o

Abstract

The corrosion behavior of 347H stainless steel was investigated in supercritical CO2 (sCO2) containing H2O and O2 to simulate heat exchanger conditions that would exist in direct sCO2 power cycles. To understand the thermodynamic properties of oxygenated sCO2 aqueous systems related to the corrosion phenomena, thermodynamic modeling was performed using REFPROP software. Samples were exposed to oxygenated water-containing CO2, and oxygenated CO2-containing H2O. The exposure tests were carried out at a pressure of 80 bar and two temperatures, 50 °C and 248 °C. The environmental conditions were simulated by filling each 1100 ml autoclave with 400 ml of deionized water, bringing each autoclave to its respective test temperature, and then pressurizing it with a CO2:O2mixture having a molar ratio of 95:1. The corrosion rate of each sample was determined by mass loss measurements. The surface morphology and the composition of the corrosion product layers were analyzed using surface analysis techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS) for oxygen and chromium, and X-ray photoelectron spectroscopy (XPS). Weight gain results reveal that the samples exposed to higher temperatures experience a significant increase in oxide growth compared to the lower temperature counterparts. XRD results confirmed this result in that corrosion products present on the samples exposed to sCO2 containing H2O and O2 and H2O containing sCO2 and O2 at 50 °C were too thin for conventional Bragg-Brentano XRD to detect anything aside from the base material. However, the samples exposed to the environments at 248 °C had sufficiently thick corrosion layers for this method. The XRD results for the corrosion products on the sample surfaces formed in the oxygenated CO2-containing H2O and water-containing CO2 environments at 248 °C revealed the presence of primarily hematite (Fe2O3) and magnetite (Fe3O4), respectively. These results indicate that the corrosion degradation mechanism for 347H in oxygenated H2O containing sCO2 is different from the mechanism in oxygenated sCO2 containing H2O at 248 °C.