Corrosion Characteristics Of N80 And The Effect Of Adding A Small Amount Of Cr On The Corrosion Behavior Of N80

N80 oil casing is seriously corroded in CO₂-H₂S-H₂O corrosion environment in an oil field.Aiming at this problem,this paper studies and analyzes the corrosion behavior and mechanism of N80 and 3Cr-80 under simulated field conditions through high temperature and high pressure immersion corrosion simulation test and in-situ high temperature and high pressure electrochemical test,and discusses the influence of adding 3%Cr element on the corrosion behavior and law of N80 in this environment,It provides experimental support and theoretical basis for preventing CO₂/H₂S corrosion of oil casing and oil casing material selection.The test results show that:In 1MPa CO₂ corrosion environment,a small amount of Fe CO₃corrosion products are generated on the surface of N80 steel,and pitting corrosion occurs on the surface.After adding 3%Cr,the quantity and particles of Fe CO₃ products on the surface are improved,but there is still no continuous corrosion product film.The products are rich in Cr,which reduces the uniform corrosion rate and pitting rate of N80.In 0.3MPa H₂S corrosion environment,the corrosion product film formed on the surface of N80 steel is Fe S film,and there is a gap between the films.After adding 3%Cr element,The voids of Fe S corrosion product film on the surface are reduced and becomes more dense.The enrichment of Cr element in the corrosion product film not only reduces its conductivity,but also reduces the anion diffusion channel and improves the corrosion resistance of N80.In the corrosive environment of1MPa CO₂+0.3MPa H₂S,H₂S is preferentially adsorbed on the steel surface for corrosion,and the corrosion product film on the surface of N80 steel is Fe S,with local bulge and falling off.After adding 3%Cr,the surface bulging and falling off disappeared,and the film was enriched with Cr,which enhanced the stability of the product film.Through the comprehensive analysis of the immersion corrosion test results,it is found that Cr mainly affects the corrosion product film of N80 and then reduces the corrosion rate,which does not change the corrosion law of N80 in CO₂/H₂S coexistence environment,that is,the corrosion of N80 and 3Cr-80 in CO₂/H₂S coexistence environment is dominated by H₂S corrosion,in which CO₂ plays a relatively small role.The results of high temperature and high pressure electrochemical test show that the electrochemical corrosion laws of 3Cr-80 and N80 are similar under the three test conditions.In CO₂ environment,the EIS diagrams of N80 and 3Cr-80 show the characteristics of single capacitive reactance arc,and the cathodic reaction is dominated by H₂CO₃ reduction;In H₂S and CO₂/H₂S environments,the EIS diagrams of N80 and 3Cr-80 show the characteristics of double capacitive reactance arc,and the cathodic reaction is dominated by H₂S reduction.In the three environments,the anodic reaction of N80 is mainly the dissolution of Fe,and the anodic reaction of 3Cr-80 is mainly the dissolution of Fe and Cr.According to the analysis of polarization curve,after adding 3%Cr element,the self corrosion potential of N80 steel moves forward and the corrosion tendency decreases;The self corrosion current and corrosion rate decreased significantly.The main reason is that Cr element is enriched in the product film,which not only improves the coverage and density of the product film,but also reduces its conductivity,increases the resistance of charge passing through the matrix and corrosive medium,and improves the corrosion resistance of N80.According to the analysis of AC impedance spectrum,after adding Cr element,the radius of capacitive reactance arc increases,the polarization resistance and the film resistance of corrosion product film are increased,which increases the difficulty of current passing and slows down the progress of corrosion.