Corrosion Mechanism Of Low-carbon Steel Materials In CO₂ Flooding Storage And Transportation Facilities In Sulfur-containing Environment

CO₂-enhanced oil recovery（CO₂-EOR）is an important part of carbon capture,utilization and storage（CCUS）technology,which is widely recognized as one of the technologies that can effectively reuse CO₂,thereby mitigating global temperature rise.CO₂ in CO₂ flooding is generally captured from emission sources such as coal-fired power plants and smelters.Although it has undergone dehydration and desulfurization processes,a certain amount of carbon source impurities（such as O₂,NO_x,and H₂O）will remain,and the CO₂ transmission pipelines pose a corrosion threat.In fact,the corrosion factor in the carbon capture process not only comes from the carbon source impurities,but also from the substances generated by the cross-reaction between them.Secondary sulfur（S）is one of them.These often overlook secondary products also bring corrosion hazards and threaten the overall safety of CO₂ transmission pipelines/oil and gas transportation and storage carriers along the process flow,and have a wider impact on the safety of pipelines.However,the corrosion mechanism of low carbon steel in the main storage and transportation facilities of CO₂ flooding in the sulfur-containing environment is not completely clear.In this paper,the existence form of element S,corrosion inhibitors,sediments and low carbon steel materials are used as research variables to study the corrosion mechanism of its main surface storage and transportation facilities in CO₂ flooding.Including the corrosion effect of element S on X80 steel in the form of solution and deposition in CO₂ supercritical transport system and the corrosion of element S in the form of sediment to L245NS steel in oil and gas gathering and transportation systems and Q235B steel in oil and gas storage tank systems.Digital camera,SEM,3D topography instrument and other instruments are used to observe the corrosion products and substrate surface morphology of the samples before and after corrosion,and using EDS,XRD,XPS and other analytical methods to analyze the element information and material composition of the corrosion products.The weight loss method is also used to judge whether the sample is pitted or not.The corrosion mechanism of the sample under high S-containing sediments during initial stage was analyzed by electrochemical methods such as potentiodynamic scanning,wire bundle electrode real-time potential/current scanning and AC impedance spectroscopy.The experiment results show that element S promotes pitting corrosion of X80 steel in the supercritical CO₂-NO_x-O₂ saturated water environment and reduces the uniform corrosion rate;element S in the water-saturated supercritical CO₂-NO_x-O₂ phase generates sulfur-containing corrosion products The products are Fe SO₄ and Fe S₂,while in the water-rich phase,the products are Fe SO₄·7H₂O and Fe S;element S,under the conditions of supercritical CO₂ saturated water and water saturated supercritical CO₂,generates the same isomers of Fe₂O₃ and Fe OOH.In this paper,the effect of secondary element S on the corrosion of X80 steel in the dynamic environment of supercritical CO₂saturated water was also studied,and it was found that the corrosion was more serious in the dynamic environment.Through the corrosion experiments of L245NS steel under high S-containing deposits,it is found that under the condition of deposits in the early stage of corrosion,the reaction process is controlled by diffusion,and the corrosion rate increases.Under these conditions,the cathodic polarization process is controlled by complete concentration diffusion,which reduces the concentration of substances involved in the cathodic reaction,resulting in a"back hook"in the cathodic polarization curve.Through the research on the initial corrosion of Q235B steel in the S-containing deposit system,it is found that when the left and right parts of the sample surface are covered with deposits of different thicknesses,the potential nephogram of the wire bundle electrode under the thinner deposit is always more negative,and the corrosion is more serious.When there is no deposit on the left half of the sample and there is deposit on the right half（with a thickness of 5 cm）,the diffusion impedance under the deposit is the barrier diffusion impedance of the spherical electrode,which combines the spherical electrode and the planar electrode.