Corrosion Behavior Of X65 Pipeline Steel In Gaseous CO₂ Environment Containing Impurities

High-pressure CO₂ pipeline transportation is a very important part in Carbon Capture and Storage（CCS）process.High-pressure gaseous CO₂ is one of the common CO₂ pipeline transportation phases and gaseous CO₂ pipelines have important industrial applications at home and abroad.Limited by the processing technology and economic cost,the gaseous CO₂ fluid transported by pipeline inevitably contains a certain amount of impurities such as H₂O,O₂,N₂,etc.,which could corrode the pipeline and greatly increase the operational risks of high-pressure CO₂ transportation pipeline.At present,domestic and foreign researches on corrosion in CO₂transportation pipelines mainly focus on supercritical CO₂.There are few studies on the internal corrosion of gaseous CO₂ transportation pipeline containing impurities.Considering the application status and development plan of China’s CCS technology,the corrosion behavior and mechanism of CO₂ transportation pipeline,proposing a reasonable corrosion rate prediction model,and setting the critical water content of high-pressure gaseous CO₂ transportation pipeline containing impurities are of very important practical significance.Using gaseous CO₂ transportation pipeline steel X65 carbon steel as the experimental material,referring to the actual operating conditions of the gaseous CO₂ transportation pipeline in Shengli Oilfield,the influence of impurities on the corrosion behavior of X65 carbon steel is studied through corrosion simulation experiments combined with modern surface analysis techniques.The study found that（1）X65 carbon steel undergoes severe uniform corrosion in gaseous CO₂-saturated H₂O system.X65 carbon steel undergoes typical"CO₂ corrosion"with a uniform Fe CO₃ corrosion protection film formed on the surface without impurities.O₂ provides an additional cathodic reaction and oxidizes Fe²⁺to become Fe³⁺at the same time.A double layer corrosion product film formed on the surface with a Fe CO₃ inner layer and a Fe₂O₃ outer layer,which deteriorates corrosion.Although N₂ won’t directly participate in corrosion reaction,the addition of N₂ reduces the solubility of gaseous CO₂ in H₂O,resulting in a decrease of uniform corrosion rate.（2）In the H₂O-saturated gaseous CO₂ system,X65 carbon steel undergoes severe localized corrosion,and localized corrosion occures on the surface covered by condensate water droplets.When O₂ is present,the pit depth is deeper;N₂ reduces the solubility of H₂O in gaseous CO₂,and water is more likely to condense and precipitate from H₂O-saturated gaseous CO₂fluids,resulting in the number and volume of condensate droplets are larger,which deteriorates localized corrosion.The effect of flow on the corrosion behavior of X65 pipeline steel in high-pressure gaseous CO₂ environment containing impurities is studied.（1）Flow aggravates corrosion,and the erosion of fluid makes the forming of a dense and complete Fe CO₃ corrosion product film for a longer time.（2）When O₂ is present,localized corrosion pits occur on the surface.The vortex continuously scours the corrosion product film in the pit,in addition,the corrosion pits cause galvanic corrosion of"large cathodes and small anodes"on the surface.It is more difficult to form a double-layer corrosion product film with good protection performance on the inner surface of the pit,which continuously deteriorates localized corrosion.（3）In the dynamic H₂O-saturated gaseous CO₂ system,the influence of flow on localized corrosion is not significant.The corrosion rate prediction model of X65 pipeline steel in high-pressure gaseous CO₂environment is established based on the water chemistry and electrochemical model of the CO₂-H₂O system.The applicable temperature range is 25～75℃,and pressure range is 2～6 MPa.In general,the model has good accuracy under the conditions of gaseous CO₂ pipeline transportation.The experimental study on the critical water content for corrosion of X65 pipeline steel in high-pressure gaseous CO₂ environment containing impurities has been carried out.It is pointed out that in the dynamic gaseous CO₂ environment containing 4%O₂ and 4%N₂ at 50℃and 4MPa,the critical water content for corrosion of gaseous CO₂ transportation pipeline could be set at 80%RH（Relative Humidity）.