Cheng, Chin-Hua (2021-05). Corrosion Behavior on Mild Carbon Steel C1018 in Supercritical CO2 Environment. Doctoral Dissertation.
The first part of this dissertation was designed to evaluate three different types of tungsten-based electrodes, which were made in house, and find out the best electrode to measure stable signals in a severe environment (high temperature and high pressure). Electrochemical performance and crystal microstructures of three different types of tungsten-based electrodes were evaluated in pH range of 3 to 10 at room temperature and elevated temperatures. Tungsten/tungsten oxide electrodes were prepared by thermal-oxidized and electrochemical-polarized methods and compared with pure metallic tungsten electrode. The electrochemical properties of the electrochemical-polarized tungsten/tungsten oxide electrode were examined by Cyclic Voltammetry and Mott Schottky testing. Crystal microstructures of the thermal-oxidized and electrochemical-polarized tungsten/tungsten oxide electrodes were confirmed as W/WO3/WO2 and W/H2WO4. The pH measurements of all the prepared tungsten-based electrodes responding to temperature over a wide range of pH values were examined by Open Circuit Potential at room temperature and elevated temperatures. The second part of this study was aimed to compare the corrosion rate predictions obtained from a state-of-the-art CO2 corrosion modeling with experimental results on mild carbon steel (C1018) exposed in brine solutions in both sub-critical CO2 and supercritical CO2 (SC-CO2) environments. Corrosion behavior of mild carbon steel were investigated by immersing in a still 1 wt.% NaCl solution at temperatures from 60 - 120oC under both 400 psi (sub-critical CO2) and under 1600 psi (supercritical CO2), respectively. The kinetics of the corrosion process on C1018 were studied by Linear Polarization Resistance (LPR) and Electrochemical Impendence Spectrum (EIS). Surface morphology, element distributions on the surface, and crystal structures of C1018 were evaluated by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and X-ray Photoelectron Microscopy (XPS). The LPR results indicated that the corrosion rates of C1018 in the sub-critical CO2 environment at steady state were higher than the predicted theoretical values at high temperature, whereas the corrosion rates of C1018 in the supercritical environment at steady state decreased with the increase of temperature and were found consistently lower than its predicted values. The last part of this study focused mainly on the kinetics and corrosion behavior of C1018 in supercritical CO2 environment with the presence of H2S gas. Considering the solubility limit of H2S gas in water, low concentrations of H2S gas (5 ppm, 50 ppm, 100 ppm, and 200 ppm) were created via a wet chemical method before temperature and pressure were increased to the same level of our previous research interests (same as in the second part of this study). The kinetics and corrosion behavior were studied by using Weight Loss method (WL), LPR, and EIS technologies. A corrosion mechanism was tentatively proposed to explain the complicated corrosion phenomenon caused by CO2 corrosion with H2S corrosion in the supercritical CO2 environment. Several crucial factors that could largely influence the corrosion rate in the supercritical environment such as temperature, pressure, H2S concentration, and exposure time, were interpreted and ranked.