Mesoscopic Transformation And Behavior Mechanism Of 20# Steel In The Initial Stage Of CO₂/Water Two-phase Stratified Corrosion

CO₂ has always been a by-product in the oil and gas industry.The low carbon steel is widely used in Oil and gas transmission pipeline.Thus,CO₂ corrosion of low carbon steel has become an inevitable problem in oil and gas production.It is crucial to master the CO₂ corrosion mechanism and take corresponding measures to reduce accidents and operating costs for the oil and gas industry.In this project,a self-design dynamic pipeline platform is used to systematically study the corrosion behavior of 20#pipeline steel under the CO2/aqueous solution gas-liquid stratified flow condition.The corrosion rate,corrosion morphology and component of corrosion products of 20#steel were calculated,observed and analyzed by means of weight loss method,SEM,EDS,XRD and XPS.The corrosion behavior mechanism of 20#steel under simulated service condition was discussed based on the comprehensive analysis.?1?The results show that the corrosion rate of 20#pipeline steel under different CO₂ pressure conditions in aqueous solution under the gas-liquid stratified flow condition increased with the increasing CO₂ pressure.The corrosion rate increased significantly when the pressure increased from 0.03MPa to 0.11MPa,then tended to be flat when the CO₂ pressure increased from 0.11MPa to 0.26MPa,but it was still increasing slowly.At the beginning of corrosion process,the pipe wall surface presents a network-like morphology that was composed of Fe and Fe3C.The corrosion preferentially started from the defects and spread to its surrounding area,and the corrosion products were preferentially deposited at the defects area.With the increasing CO₂ pressure,the deposition products had a tendency to change from filamentous shape,flocculent shape,granular shape to scaly shape.The content of C and O elements in the products with all kinds of morphological characteristics increased with the increasing CO₂ pressure.The main phases of the corrosion product film were Fe,Fe₃C,FeCO₃ and Fe₂O₃.A double-layered product film with a dense outer layer and a loose-porous inner layer was formed on the pipe wall surface.There were forming and densifing stages for the corrosion products film during the corrosion process.?2?The corrosion under different CO₂ pressure and gas flow velocity conditions showed approximately static corrosion characteristics.When the gas flow velocity was unchanged,the greater the CO₂ pressure was,the greater the corrosion rate was.The changing trend of corrosion rate was almost similar under the same CO₂ pressure and different gas flow velocity condition.When the CO₂ pressure was 0.06MPa and the gas flow rate was 0.234 m/s,the minimum and maximum corrosion rate were 1.19356 mm/a and 1.2136 mm/a,respectively.When the CO₂ pressure was 0.21MPa,the corrosion rate increased from 1.26071 mm/a to 1.283343 mm/a.The change of gas flow velocity had little effect on the corrosion rate.The corrosion products were easy to adhere to the lamellar cementite Fe₃C to form a loose and porous corrosion products layer on the pipe wall surface.The EDS results indicated that the elements Fe,C and O contents in the corrosion product layer were similar at different gas phase flow velocity.The main phases of the corrosion products in gas under gas-liquid stratified flow condition were Fe,Fe3C,FeCO₃,FeO,Fe₂O₃,Fe₃O₄.?3?The corrosion rate in aqueous solution increased with the increasing liquid flow velocity.The corrosion rate was affected by the mass transferring process that was intensified through the increasing velocity of aqueous solution when the other experimental conditions were not changed.When the CO₂ pressure was 0.05 MPa,the liquid flow velocity increased from 0.036 m/s to 0.126 m/s,the corrosion rate increased from 1.3131 mm/a to 1.5452 mm/a.When the CO₂ pressure was 0.16 MPa and the liquid flow velocity was 0.126m/s and the maximum corrosion rate was 1.8548mm/a.When the liquid flow velocity was 0.036 m/s and 0.126 m/s,the phase and binding energy corresponding to the peak of the three elements Fe,C and O were approximately the same as shown by XPS result.The binding energy corresponding to the peak of C1s was slightly shifted.The fitted peaks of C1s at 284.6eV and 288.35eV corresponded to Fe₃C and FeCO₃,respectively.The fitted peaks of Fe 2P_3/2 at 710.4eV,712.19eV,and724.58eV respectively showed FeO,Fe₃O₄,Fe₂O₃.The fitting peaks of O1s at 529.77eV,530.94eV,and 531.94eV corresponded to Fe₂O₃,Fe₃O₄,and FeO,respectively.It was known that the corrosion products were composed of FeO,Fe₂O₃,Fe₃O₄,FeCO₃according to XPS result.and Fe and Fe₃C were matrix residual phases.