Application Research Of 718 Alloy For Blowout Preventer At High Temperature And High Sulfur Content Condition

Well control equipment is a reliable guarantee for safe drilling,which implements pressure control on oil and gas wells,and prevents,monitors also controls accidents,especially the blowout preventer plays an important role in controlling blowout.To some degree,the application of nickel-base alloy for wellhead device with excellent overall performance can meet the needs of high acid oil and gas field development,however,as the corrosive environment becomes more and more severe,the corrosion resistance of materials in high temperature and high sulfur environment is still worth studying.Based on the research and application of nickel-base alloys at home and abroad,this paper firstly studied the corrosion law and corrosion behavior of nickel-base alloy 718 in the simulated waterfield formation water environment with high temperature and high H₂S/CO₂content.The results show that under four different test conditions,the corrosion of nickel-base alloy 718 is slight and shows typically uniform corrosion characteristics,with the increase of temperature or the partial pressure ratio of H₂S/CO₂,the corrosion rate of nickel-base alloy718 is slightly increased,but the change was not obvious.After 720h of corrosion under the most severe corrosion conditions,the four-point bending SCC specimen did not break,the local characteristics of the crevice corrosion specimen were not obvious and the weight loss was small,showing good resistance to stress corrosion and crevice corrosion.Secondly,the XPS deep sputtering of the the passivation film for both uncorroded sample and corroded sample shows that the passivation film before etching is a bipolar film composed of two layers,the surface layer is mainly hydroxide and oxide of Ni and Cr,and the inner layer is mainly oxide of Ni and Cr.After the corrosion,a large amount of metal sulfide exists in the outer layer of the passivation film,the transition layer is mainly composed of hydroxides of Ni and Cr,and the inner layer is a metal oxide and its corresponding metal element.The transition of the film structure to the sulfide increases the corrosion of the nickel-base alloy 718.Finally,the polarization curves,dynamic potential cyclic polarization curves and AC impedance spectra of the nickel-base alloy 718 under simulated high H₂S/CO₂ corrosion conditions were tested.The electrochemical test results of the seamless nickel-base alloy 718sample show that the addition of H₂S causes the stability of the passivation film to decrease and the activation curve has multiple activation-passivation transitions compared with the CO₂corrosion system alone.And the fitting results of the AC impedance curve also show that the presence of H₂S reduces the polarization resistance,the polarization resistance decreases and the resistance of the active ions through the passivation film and the protective properities of the passive film are reduced.Electrochemical test results of crevice corrosion samples indicate that in the H₂S/CO₂ coexisting corrosion system,the polarization curves of different temperatures are intersected in the anodic polarization zone,showing good passivation and repassivation ability.With the increase of temperature,the radius of the arc resistance of the EIS spectrum is gradually reduced,and the crevice corrosion sensitivity increases.It is found that the existence of high content of CO₂ in the solution increases the crevice corrosion tendency of the alloy by comparing the cyclic polarization curves containing only CO₂ and H₂S only,while in the H₂S corrosion system,the closed hysteresis loop area is small,showing good corrosion resistance,and the corrosion polarization resistance in the H₂S environment is greater than the polarization resistance under CO₂ corrosion conditions.