Study On Corrosion Resistance Of 5Cr Oil Tubing And Casing Steel

Grasp the corrosion status of CO₂ corrosion on oil casing in oil and gas well production,select and evaluate oil casing materials that take into account corrosion resistance and economy,which can effectively prevent and reduce oil casing corrosion failure accidents.The purpose of this paper is to study the film-forming mechanism and corrosion characteristics of 5Cr oil tubing and casing steel corrosion product film in the CO₂ environment of oil and gas field.The combination of physical and chemical performance test,atmospheric pressure,high temperature and high pressure corrosion loss test,electrochemical test are used to study.The research content and conclusions of each chapter are as follows:（1）The morphology and composition changes of the product film in the long-term corrosion of 5Cr oil tubing and casing steel under atmospheric pressure were studied.The product film evolution mechanism model in different corrosion times was characterized.The overall and local information of 5Cr oil tubing and casing steel corrosion was integrated and confirmed by combining the polarization curve,electrochemical impedance spectroscopy（EIS）and wire beam electrode（WBE）.Under atmospheric pressure conditions,the inner layer of the 5Cr oil tubing and casing steel surface corrosion product film is composed of Fe CO₃,Cr（OH）₃ and Cr₂O₃intercalation,and granular Ca CO₃ is attached to it.Electrochemical corrosion shows semi-passivation characteristics,with the progress of corrosion,the resistance caused by charge transfer resistance and product film cover gradually increases.The anode reaction is inhibited,the charge transfer process becomes more and more difficult,and the resistance of electrochemical reaction gradually increases.So that its resistance to passivation breakdown is better,showing excellent resistance to local corrosion.With the corrosion of 1 d～28 d,the corrosion potential of the electrode changes from-0.58 V to-0.61 V,and the negative shift phenomenon occurs,and the anode current region on the electrode surface is greatly reduced.The tendency of pitting corrosion in the early stage increases,and when the corrosion progresses to 14 d～28 d,the Cr enrichment in the corrosion product film is greater than 30%,and the Cr/Fe ratio reaches 0.8,which is about 15times that of the matrix,and the thickening accumulation of the film effectively inhibits the development of the pit.（2）The total pressure conditions are increased to realize the high temperature and high pressure CO₂ environment.The corrosion behavior of 5Cr oil casing steel is studied in 50°C,70°C and 90°C environment.The local corrosion research is studied by EIS and WBE technology.Under high temperature and high pressure conditions,with the increase of temperature,the electrode corrosion potential of 5Cr oil tubing and casing steel has different degrees of negative shift,and the corrosion tendency increases.In the electrochemical impedance spectrum,a capacitive arc with a large radius is formed,the film coverage and compactness of the corrosion products increase,the charge transfer resistance in the equivalent circuit becomes larger,and the resistance of the electrochemical reaction increases significantly.The polar transformation phenomenon of the matrix area current under the corrosion product film occurs from the cathode to the anode,and the pores in the product film cause the base metal to be corroded,resulting in the appearance of anode current.The formation and expansion of the local potential anode region on the surface of the electrode has a tendency to produce pitting corrosion,which is formed in the void of the film of the product in the early stage of corrosion.The corrosion products were gradually deposited on the inner wall of the pit,and a protective layer with obvious Cr enrichment was formed on the inner wall of the pit,and the original pitting corrosion area was changed from the original anode active point to the cathode area,which inhibited the development of pitting corrosion,which echoed the local electrochemical test results.（3）The effects of 0.01 MPa,0.1 MPa,0.5 MPa and 1 MPa CO₂ partial pressure on corrosion under high temperature and high pressure environment of 5Cr oil tubing and casing steel were studied,supplemented by polarization curve and impedance test technology.Under high temperature and high pressure conditions,as the partial pressure of CO₂ increases from 0.01 MPa to 1 MPa,the depth and diameter of the pitting pit on the surface of 5Cr oil tubing and casing steel do not change much,and the difference in maximum pitting corrosion rate is small,and even a trend of decrease.The layered corrosion products and the bottom of the pitting pit close to the surface of the matrix were enriched by Cr elements,and the degree of enrichment increased with the increase of CO₂ partial pressure,and the proportion of Cr₂O₃ in the products increased from30.07%to 45.95%,and the protective properties of the surface corrosion product film were enhanced.Electrochemical corrosion shows semi-passivation characteristics,with the progress of corrosion,the resistance caused by charge transfer resistance and product film cover gradually increases,and the resistance of steel to local corrosion in corrosive environment is better.