Effect Of CO₂ In Natural Gas On Corrosion Behavior Of Pipeline Body And Weld

As the gas field development,the moisture content of output gas increased,CO₂ in produced gas dissolves in water to form carbonic acid,which will cause the pipeline corrosion and local corrosion,thickness thinning,perforation or break of tubing,leading to the tubing failure.Therefore,it is very necessary to clarify the corrosion behavior of the pipeline affected by natural gas containing CO₂,especially the weld part,for establishing the CO₂ corrosion failure mechanism of the pipeline,analyzing the failure reasons and formulating reasonable and feasible protective measures,which is of great significance to guarantee the gas treatment and pipeline transportation in the station.In this paper,based on the process parameters and transmission medium parameters of a natural gas station,through immersion experiment,electrochemical experiment and corrosion image analysis,the macroscopic corrosion behavior and corrosion dynamics process of the base material and weld of the pipeline transported by natural gas containing CO₂ were carried out,and the main conclusions were as follows:（1）Under the microscope,we can observe that the longitudinal and transverse microstructure of the finished sheet of pipeline steel has three kinds:acicular ferrite,quasi-multilateral ferrite and degenerated pearlite,and the acicular ferrite is the most,and the average grain size is 6μm.However,the grain size of the welded plate increases significantly,up to 150μm,and its internal microstructure is very similar to that of the coarse-grain zone,which all have granular bainite,lamellar martensite and coarse-strip bainite.The corrosion product composition analysis results show that the corrosion products at the base metal are mainly Fe CO₃,while the corrosion products at the weld are Fe₃C and Fe CO₃.（2）With the increase of soaking time and CO₂ partial pressure,the total corrosion rates of both the base metal and the weld showed an increasing trend,and the maximum pit depth increased rapidly at first and then remained basically unchanged.With the increase of CO₂partial pressure,the base metal and welding seam change from partial pitting corrosion state to total corrosion state.This is caused by the difference in solubility of CO₂ in solution and the structure of corrosion product layer between base metal and weld.（3）With the increase of immersion time and experimental temperature,the total corrosion rates of both the base metal and the weld increased,and the maximum pitting depth increased first and then tended to be stable.With the increase of temperature,the number of pitting pits on the base metal surface increased significantly,and the distribution of pitting pits showed obvious directionality,while the pitting pits on the weld metal surface gathered around cementite and gradually formed a typical and obvious pitting area.This is because under different temperature conditions,the structure of the corrosion product layer formed on the base metal and the weld surface changes,which leads to the different sensitivity of the base metal and the weld to temperature.（4）With the increase of flow rate and immersion time,the total corrosion rate of both base metal and weld increased,and the maximum pit depth of both increased,while the corrosion morphology showed obvious directional distribution along the direction of fluid flow.This is the result of the combined action of mechanical scour and electrochemical corrosion.（5）Under the steady-state action,the base metal surface is mainly covered by the Fe CO₃corrosion product layer at the initial soaking stage.In the long soaking process,the Fe CO₃coating dissolves and breaks,and the metal matrix is gradually re-exposed to the corrosive environment.The weld corrosion product Fe CO₃ forms a porous structure with Fe₃C as the skeleton,and the matrix corrosion process continues to occur.In the dynamic scouring process,it is difficult to form a stable corrosion product layer with high coverage rate on the metal surface,and the metal matrix is in contact with the corrosive medium all the time.（6）With the increase of CO₂ partial pressure,flow rate and temperature,the open circuit potential of the base metal and the weld moves negatively,and the negative shift velocity increases,indicating that the corrosion tendency of the base metal and the weld increases,and the weld is more prone to corrosion.The corrosion potential E_corr of the base metal and the weld moves negatively,while the corrosion current density I_corr increases.Under the action of CO₂ partial pressure and temperature,the corrosion reaction process between the base metal and the weld is controlled by the anode process and the anodic polarization degree increases,the capacitive reactance arc radius of the base metal and weld decreases gradually,the maximum phase Angle amplitudeθ_max decreases gradually,and the maximum phase Angle frequency fθ_-max changes to the low-frequency material diffusion process.However,under the action of flow rate,the corrosion reaction process of the base metal and the weld is controlled by the mixing process,and the slope r is basically unchanged,which indicates that under the condition of fluid flow,the mechanical action is enhanced,and it is difficult to form a stable corrosion product layer on the surface of the base metal and substrate.The maximum phase Angle amplitudeθ_max and the maximum phase Angle frequency fθ_-max change to the high frequency charge transfer process.The corrosion process is mainly mechanical scour and electrochemical corrosion.