Study Of Corrosion Mechanism Of Steels In Supercritical CO₂ Environments

With the development and application of Carbon Capture and Storage（CCS）and enhanced oil/gas recovery（EOR/EGR）by CO₂ injection,supercritical CO₂corrosion was found widely existed on the corresponding engineering structure materials and devices,which caused serious influence on their service life and production safety.Deeply studying the corrosion behavior and failure mechanism of the metal materials in the supercritical CO₂ environments is an important way to ensure material service performance and improve the relevant technology.Supercritical CO₂ corrosion environments mainly conclude supercritical CO₂-containing aqueous phase（Water-rich phase）and H₂O-containing supercritical CO₂ phase（H₂O-rich phase）.At present,the previous studies mainly focused on evaluating the corrosion behavior of carbon steel in the static supercritical CO₂-saturated aqueous phase.In this study,the corrosion experiments were carried out in a high temperature and high pressure autoclave to investigate the effect of CO₂ partial pressure,flow rate,small amount of gas impurities（especially H2S）on the corrosion mechanisms of carbon steel,low Cr alloy steel and stainless steel in different supercritical CO₂ environments.The Scanning electron microscopy（SEM）.X-ray diffraction（XRD）.X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy,nanoindentation,Laser scanning confocal microscope and Surface profiler were employed to analyze the sanple surfece as well as the corrosion product.The results will be expect to provide theoretical support to predict the service safety of the materials in supercritical CO₂ environments,guide the materials selection for CCS technology in our country and promote the development of CCS and the application of EOR/EGR.The main research works are as the follows:（1）By comparing the corrosion behaviors of carbon steel in the aqueous phase under supercritical and low CO₂ partial pressure conditions.the corrosion mechanism of carbon steel in supercritical environments was investigated.The formation model of corrosion product scale on the carbon steel surface in the supercritical CO₂-saturated aqueous phase was proposed,which established theoretical foundation for the subsequent research of the effect of environmental factors（flow rate,H2S impurity,exposure angle）and the compositions of materials（Cr element）on the supercritical CO₂ corrosion of the steel.（2）The effect of flow rate on the corrosion behavior of carbon steel in the supercritical CO₂-saturated aqueous phase was investigated,and two localized corrosion mechanisms were proposed.The relationship between the minimum critical size of the localized corrosion pits that could exist and the flow rate was obtained by experimental statistics and data fitting,and the formation model of corrosion product scale inside the localized corrosion pit was established.（3）The effect of Cr content on the corrosion behavior of the steel in supercritical CO₂-saturated aqueous phase was investigated,and the corrosion mechanism of low Cr alloy steel was determined.In-situ formation of the Cr-rich layer and the transition from Cr-rich layer to mixed layer were two important prerequisites for the good corrosion resistance,especially local corrosion resistance of low Crally steel.（4）The corrosion mechanisms of carbon steel,low Cr alloy steel and stainless steel in the aqueous phase containing small amount of H2S and supercritical CO₂ were investigated,and found that the presence of H2S caused the corrosion type of carbon steel changing from localized corrosion to general corrosion,while low Cr alloy steel and even stainless steel suffered serious localized corrosion.In the supercritical CO₂ phase containing H2S,localized corrosion was dominated for carbon steel and low Cr alloy steels,while stainless steel was highly resistant to corrosion.The model of corrosion mechanisms of carbon steel and low Cr alloy steel in the supercritical CO₂ environments containing small amount of H2S were established.（5）The effect of pressure and exposure angle on the corrosion behavior of carbon steel in water-saturated supercritical CO₂ phase was investigated.The solubility（g/L）of H₂O in CO₂ and the solubility（mol/L）of CO₂ in H₂O with temperature and pressure were calculated.More H₂O condensed and higher concentration of corrosive reactants were the reasons the corrosion ofcarbon steel in the supercritical CO₂ phase was more serious than that in the gaseous CO₂phase,and the former was the main reason.In the supercritical CO₂ phase,the effect of surface downward（180°）on the corrosion of carbon steel was the most significant.followed by the surface upward（0°）and vertical（90°）.（6）In the supercritical CO₂ phase,the condensation of H₂O on the steel surface dominated by droplet condensation.The calculation model of maximum critical size of the condensed water droplet on the steel surface with different exposure angles was established,the accuracy of which was verified by experiments.The central zone and surrounding zone of carbon steel in water droplet were proved to constitute a galvanic cell during the corrosion process.which resulted in the uniform anodic dissolution of the central zone and the significant localized corrosion of the surrounding zone.