Ce Research On Corrosion Mechanism And Application Of 10Cr5MoV Corrosion Resistant Oil Casing Steel In CO₂/H₂S Environment

Oil casing is an important component of oil and gas extraction equipment,which is played a role in supporting the wellbore and oil and gas extraction channels during the oil and gas extraction process.CO₂ and H₂S,as the accompanying gases of oil and gas extraction,dissolve in the produced liquid,which causing extremely serious corrosion to the oil casing.It not only brings incalculable economic losses,but also constantly threatens life and environmental safety.To deal with corrosion,corrosion resistant material selection is the first choice for corrosion.However,most of China’s oil and gas fields have the characteristics of poor oil and low permeability.Chromium based and nickel based corrosion resistant materials for oil casings are expensive and sensitive to stress corrosion.In addition,the positive effect of rare earths on the corrosion resistance of seawater corrosion resistant steel and atmospheric corrosion resistant steel is evident,while there are few reports on research and application in high-temperature and high-pressure CO₂/H₂S oil and gas field corrosion steel.In this paper,the CO₂/H₂S resistance special oil well pipe steel 10Cr5MoV independently developed by Baotou Steel was taken as the research object.Fact Sage thermodynamic calculation software was used to predict the type,precipitation temperature and percentage of non-metallic inclusions and carbide in test steel with different rare earth Ce content（0-120ppm）.Combined with OM,SEM,TEM,and XRD,the effects of rare earth Ce content on the modification of the inclusions,carbide precipitation behavior and microstructure transformation of the test steel were studied.The influence mechanism of rare earth Ce on electrochemical corrosion behavior and sulfide stress corrosion cracking（SSC）behavior of test steel under high temperature and high pressure CO₂/H₂S service environment were systematically studied by means of high temperature autoclave simulation corrosion test,electrochemical corrosion test and SSC test.The main conclusions are as follows.With the addition of rare earth Ce,the chain shaped Al₂O₃ non-metallic inclusions in the test steel gradually moved to the ellipsoidal or spheroidal CeAlO₃,Ce₂O₃,Ce₂O₂S,CeS rare earth inclusions and composite rare earth inclusions,and the content of rare earth Ce reached 63ppm.The modification of Al₂O₃ non-metallic inclusions in the test steel was completed.With the increase of rare earth Ce content,the inclusions evolution sequence is Al₂O₃,CeAlO₃,Ce₂O₃,Ce₂O₂S and CeS.Within the range of 25ppm to120ppm of rare earth Ce content,as the rare earth Ce content increases,the number of rare earth non-metallic inclusions in the test steel increases,but the size of rare earth inclusions is obviously refined,and the distribution of non-metallic inclusions in steel is further dispersed.The carbides precipitated from the test steel with different contents of rare earth Ce are Cr₂₃C₇ and Cr₇C₃.The large carbide particles are distributed in the grain boundary and subgrain boundary,and the small carbide particles are distributed in the dislocation defects.The addition of rare earth Ce and the increase of Ce content have no obvious effect on the type of carbide precipitation.The percentage of carbide decreases gradually,the size of carbide decreases gradually,and the rare earth Ce prevents carbide precipitation in the experimental steel.The interaction between rare earth Ce and Cr was analyzed based on the first principles,and the mechanism of influence of rare earth Ce on carbide precipitation behavior was explored from the microscopic perspective.The addition of rare earth Ce causes the CCT curve of the test steel to move downwards,expanding the austenite phase zone of the test steel,and improving the stability of undercooled austenite;Austenitizing at 880-1180℃,test steel without rare earth Ce,with austenite growth activation energy of 202656.76J/mol and austenite grain growth diffusion frequency factor M₀ of 5.16×10⁹.Test steel with a rare earth Ce content of 120ppm,austenite growth activation energy of 236966.17J/mol,and grain growth diffusion frequency factor M₀ of 1.61×10¹¹.With the increase of rare earth Ce content,theα-Fe grain size is refined and the dislocation density is increased.Under high temperature and high pressure CO₂/H₂S corrosion environment,the simulation corrosion test results show that the test steels with different rare earth Ce content all have comprehensive and uniform corrosion,and the uniform corrosion rate decreases with the increase of rare earth Ce content.The corrosion product films of the test steels are all double-layer film structure,and the surface corrosion product films are mainly Fe S and a small amount of Fe CO₃.The inner corrosion product films are mainly Fe CO₃,Fe S and Cr₂O₃.With the increase of rare earth Ce content,Cr rich integration in the second layer corrosion product films increases.The electrochemical test results show that the addition of rare earth Ce and the increase of the content of rare earth Ce increase the tendency of the test steel surface activation to form a dense corrosion product film.The key to the corrosion resistance of the test steel is the protection of the corrosion product film.NACE 0177-A method and NACE 0177-D method were used to test the SSC resistance.The results showed that the SSC critical stress intensity factor K_ISSC increased and the SSC tendency weakened under the precondition that the rare earth Ce content was not greater than 120ppm,and the SSC resistance of the test steel was improved.The hydrogen penetration test results show that the order of precipitated phases and defects affecting the SSC performance of the test steel is non-metallic inclusions,carbides,grain boundaries,dislocations.The mechanical properties of BT80S-5Cr,which is made of 10Cr5MoVRE steel,meet the requirements of technical indexes.The performance of SSC is good when evaluated by NACE TM-0177-2016 standard A method,and the uniform corrosion rate of BT80S-5Cr is reduced by 30%-50%when compared with 3Cr steel of the same grade（planned replacement steel）,and it has been tested in a domestic oil field and served well.