Study On Corrosion Mechanism Of Localized Corrosion In Dense CO₂ Tranportation Pipelines

Carbon Capture and Storage technology and injtecting CO₂ to reservoir for enhanced oil recovery are promising methods to inhibit global warming caused by CO₂ from fossil fuels combustion.Due to high corrosivity of wet CO₂,it is important to carry out specific researches on the corrosion behaviors of different materials in supercritical CO₂ systems to ensure the safety relability of CO₂ tansportation systems.Due to China’s actual state,the CO₂ for EOR purpose is mainly captured from the flue gas of coal-fired power plants.In the captured CO₂,there will be various impurities,such as SO₂,NO₂,H₂O and O₂.There might be liquid water in EOR pipelines and the water will lead to a severer corrosion attack after the dissolution of flue gas impruities.Most researchers focused on the corrosion of steels in supercritical CO₂ with single impurity and paid less attention to the corrosion behaviors of steels in water phase with multiple impurities.In this paper,autoclave was used to simulate supercritical CO₂environment with multiple impurities.Scanning electron microscopy（SEM）,energy dispersive scopy（EDS）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and 3D profilometer were used to analyze the morphologies and compositions of corrosion product scales.The differences of corrosion behaviors of steels in supercritical CO₂ phase and water phase were sutided and the localized corrosion mechanism under different conditions was analyzed.The main research contents in the paper are as follows:（1）The differences of 13Cr steel corrosion behaviors in supercritical CO₂ phase and water phase were studied in the paper.The 13Cr showed a great resistance to general corrosion in both supercritical CO₂ phase and water phase and the general corrosion rates in both phases were lower than 0.1 mm/y.Pitting corrosion is the predominant corrosion occurred in supercritical CO₂ environmet.The pitting factor in supercritical CO₂ phase was 28.33 and it was 17.33 in water phase and the pitting rate in supercritical CO₂ phase was larger than that in water phase,suggesting that the pitting corrosion in supercritical CO₂ phase was a greater threat to pipelines safety.The formation mechanism of corrosion scales in supercritical CO₂ phase and water phase was different.Flow would inhibit the further developmet of pitting corrosion.Cl^-promoted the general corrosion in both phases and was able to break down the corrosion products and create crevices which caused the formation of peaks at the boundary of pit holes,corresponding to“secondary localized corrosion”.（2）The effect of CO₂ phase change in transportation pipelines on the corrosion behaviors of X80 steel was investigated.The results showed that the general corrosion rate and pitting rate in liquid CO₂ were larger than those in supercritical CO₂.X80 steel was more susceptible to corrosion attack in liquid CO₂.When pressure was stable,CO₂phase change caused by change of temperature would promote the general corrosion rate and pitting rate,and the boundary of pit holes became irregular.The general corrosion rate of X80 steel increased with the increasing of CO₂ density.The greatest effect of pressure change or temperature change on corrosion rate of X80 steels was obtained at CO₂ critical point and the phase change of CO₂ would have greatest influence on the change of corrosion rate of X80 steels.