Study Of The Corrosion Mechanism And Corrosion Product Characterization For 13Cr Martensite Stainless Steel In CO₂ Environments

During the exploitation and transportation of oil and gas,CO2 corrosion often results in the failures of pipelines and types of equipment,which makes the oil and gas fields suffer heavy economic losses.the potential material is known as super 13Cr with a martensitic microstructure have been widely used in oil and gas containing CO2 environments However,the corrosion behaviour in terms of general and localized corrosion of super 13Cr and the formation mechanism of the corrosion product scales studies are rare at high temperature ranges between 150-200? and CO2 partial pressure(2.7-28.5 bar).It is obvious that the formation of passive film or corrosion product film on the surface at high temperature is complex.The evolution and film growth is essential to understanding the nature of the film in terms of corrosion product kinetics and protective capability in HTHP environmentswithin this work,high temperature and high pressure(HTHP)mass loss and electrochemistry tests were performed to simulate the HTHP CO2 corrosion environment in oil and gas field.The chemical composition of the corrosion product,the morphology and their evolution formed on the surface are analysed via a combination of SEM/EDX,XRD,XPS,Raman spectroscopy and FIB/TEM and related to the level of protective capability at various temperatures,CO2 partial pressures,flow rates,and stresses.The results indicate that the passive film formed at 90? is an amorphous-like structure with high Cr content.The corrosion product scales with multilayer structures formed on super 13Cr stainless steel as the temperature raised from 90?to 200° at 1 bar CO2(25?),comprising an inner FeCr2O4 layer and outer scattered FeCO3 crystal layer.The formation of the corrosion scales on 13Cr at 200? varied with CO2 partial pressure:when the CO2 partial pressure is below 6.5 bar,the primary composition of the corrosion scales was nano-polycrystalline FeCr2O4,which is controlled by electro-migration process.As the CO2 partial pressure is beyond 15.4 bar,the primary composition of the corrosion scales was amorphous Cr(OH)3,which is controlled by dissolution-precipitation process instead.Although Cr(OH)3 layer is thicker and more uniform than nano-polycrystalline FeCr2O4 layer,the formation of later provides better corrosion resistance compared to that of forming highly defective Cr(OH)3 layer.The effect of Ni depends on the aggressive conditions.For low CO2 partial pressure(1 bar at 25?)at 90?/200?,it could be noticed that significant nickel exists covering the steel matrix,which improve the resistance of Cr2O3/FeCr2O4 inner layer.As the CO2 partial pressure increased to 28.5 bar at 200?,a complicated corrosion product scale with a mix of Cr(OH)3 and austenite based which is enrich in nickel formed.The local enrichment of nickel makes it difficult to form a continuous interface against corrosion.The formation and evolution of the corrosion product films has significant influence on localised corrosion for super 13Cr stainless steel.Dynamic fluid suppresses the localised corrosion through the formation of a uniform inner FeCr2O4 layer at CO2 partial pressure of 2.7 bar.The delay of the precipitation of the crystalline FeCO3 at 28.5 bar of CO2 decreases the risk of the localised corrosion by avoiding the generation of the small anode.The chemical composition and structure of the corrosion product scales influence the SCC sensitivity as well.The formation of FeCO3 increases with the elevated temperature at 15.4 bar CO2,A thicker corrosion product scale that composes a higher ratio of FeCO3/Cr(OH)3 contributes to the increase of phase interface,accelerating the diffusion of Cl" ions.The thick and porous film is sensitive to crack under stress.Localised corrosion occurred where no corrosion products covered,then crack would nucleate and develop under stress.