Effect Of Impurity On The Corrosion Behavior Of CO₂ Transport Pipeline Steel In Water-saturated Supercritical CO₂ System

The transmission of CO₂ is a vital part in Carbon Capture and Storage?CCS?process.However,it is inevitable that some impurities such as H₂O,O₂,H₂S,SO₂ and NO₂ will be found in the supercritical CO₂ fluid.The presence of theses impurities may increase the corrosion rate of carbon steel,and pose great risk to the security of pipelines.Therefore,it is necessary to explore the effect of impurities on the corrosion regulation and mechanism,which would promote the development and application of CCS.To achieve this objective,the simulation experiments under different conditions were conducted in a high temperature autoclave.The corrosion rate of X70 steel used in supercritical CO₂ transport pipeline was determined through weight loss tests.The morphologies and composition of corrosion scales were characterized by scanning electron microscope?SEM?,energy dispersive spectroscopy?EDS?and X-ray diffraction?XRD?.What's more,the electrochemical behaviors of X70 steel were investigated by a self-designed experimental device according to the thin electrolyte layer characteristics of corrosion in supercritical CO₂ system.The primary conclusions are as follows:?1?There were a lot of protuberances on the corrosion scales of X70 steel,under which corrosion pits often emerged in the supercritical CO₂-H₂O system containing various impurities.At the initial stage corrosion pits were distributed on the metal surface and got deeply,then they would connect together with time.At the later stage of the immersion period,general corrosion tended to occur on steel surface.At the same time,the corrosion rate was extremely high but sharply decreased at the initial stage.Then the decrease relatively slowed down at the interim stage.The corrosion rate tended to be steady at the latter stage.The main constituents of corrosion products were FeCO₃,Fe₂O₃·H₂O,FeSO₃·x H₂O and FeSO₄·x H₂O.?2?The addition of O₂,H₂S,SO₂,NO₂ all increased the corrosion rate of X70 steel in water-saturated supercritical CO₂ system.NO₂ or SO₂ had highest corroding influence on X70 steel,followed by H₂S and O₂.Especially,in the system containing NO₂,localized corrosion would occur in X70 steel.The corrosion scale was single layer of FeCO₃ in water-saturated supercritical CO₂ system.Oxygen corrosion forming Fe₂O₃ occurred when O₂ was contained.While with the addition of H₂S,corrosion scales were double layers.FeS existed in the outer scale and the inner sacle was FeCO₃.The corrosion process was governed by CO₂ and H₂S.The corrosion products were mostly FeSO₃·x H₂O when SO₂ was contained.The corrosion process was mainly governed by SO₂.However,corrosion product was Fe₂O₃ without FeCO₃ in the supercritical CO₂-H₂O-NO₂ system,NO₂ completely controlled the corrosion process.?3?The polarization resistances of X70 steels covered with electrolyte layers containing NO₂,SO₂ or O₂ all tended to increase with time in the process of immersion.At the latter stage,the polarization resistances were highest in the CO₂-H₂O system than other systems containning impurities.Electrochemical impedance spectroscopy testing showed that,the forming process of corrosion scales changed when impurities was contained.The thin electrolyte layer resistance under different impurities system was different,and all rapidly decreased with time.