Study On The Corrosion Mechanism Of 20 Steel And Low Cr Alloy Steel In The CO₂-H₂S Environment

In the process of exploitation,gathering and transportation of oil and gas,the service life of oil and gas well pipeline is greatly shortened by serious CO₂-H₂S corrosion.Carbon steel and low Cr alloy steel are the main pipeline steel materials.However,there are few studies on the corrosion mechanism of carbon steel and low Cr alloy steel in the CO₂-H₂S environment.Therefore,it is particularly important to study the corrosion mechanism of carbon steel and low alloy steel containing Cr in the CO₂-H₂S environment.Corrosion is a complex process influencing by a dozen factors of environment and material properties,such as,temperature,p H,partial pressure of aggressive gas,concentrations,kinds of ion in solution,stress,fluid,alloying elements and microstructure of material,etc.Mutual competition and mutual dependence relations coexist among influence factors,which creates a great barrier to found the quantitative relationship between thermodynamic and kinetic of electrochemical corrosion.Considering the“dissolution–ionization-deposition”process in the metal and solution interface,the aim of this work is to present a‘dissolution-ionization–deposition'model in order to found a bridge between the thermodynamic and kinetic of electrochemical corrosion.Then,this model was applied for better understanding corrosion of the carbon and low alloying steels in the CO₂,H₂S and CO₂-H₂S environments.In the CO₂ environment,no corrosion product film appeared on the surface of 20 steel at25?,and the product film composition was Fe CO₃ at 80?and 160?.With the increase of temperature,the corrosion rate of 20 steel first increased and then decreased,80?>160?>25?.The corrosion rate of 20 steel increased slightly from 4MPa to 5MPa.The corrosion rate of 20 steel with 3m/s flow rate was higher than that at the static state at 4MPa CO₂ environment.The addition of Cr element in steel will promote the dissolution of Fe and reduce the film forming temperature of Fe CO₃.At 25?,No product film appears on the surface of 20Cr and 20steel and the corrosion rate of 20Cr is higher than that of 20 steel.At 80?and 160?,the denser corrosion product film formed on the 20Cr and the corrosion rate of 20Cr is lower than that of 20 steel.In the H₂S environment,at 25?and 80?,the composition of product film on the surface of 20 steel is mainly Fe S(M),and when the temperature increasing to 160?,the product film is mainly Fe S(P).With the increase of temperature form 25?to 160?,the corrosion rate of20 steel increases.At 25?,the corrosion rate of 20 Cr is higher than that of 20 steel.At 80?,Cr element has little influence on corrosion rate.At 160?,the corrosion rate of 20Cr is lower than that of 20 steel and with the H₂S pressure decreasing from 0.1mpa to 0.02mpa,and the corrosion rate of 20 steel decreases.The denser product film containing Cr will form on the surface of 20Cr,resulting in the reduction of the corrosion rate of 20Cr.In the CO₂-H₂S environment,the partial pressure ratio of CO₂/H₂S controls the concentration of ions in the solution,which determines the composition of the product film,which is also affected by the temperature.According to the calculation of corrosion thermodynamics and kinetics,at 25?,no product film appears on the surface of 20 steel.At80?,When the partial pressure ratio of CO₂/H₂S is 0?140,H2S dominants the corrosion process,and when the partial pressure ratio of CO₂/H₂S is 140?300,H₂S and CO₂ jointly control the corrosion process.At 160?,When the partial pressure ratio of CO₂/H₂S is 0?10,H₂S corrosion plays a leading role,and when the partial pressure ratio of CO₂/H₂S is 10?300,H₂S and CO₂ jointly control the corrosion process.The results obtained by immersion experiment are consistent with the calculated results.In the CO₂-H₂S environment,the addition of Cr element can accelerate the deposition of product film,but the mixed product film of Cr and Fe has poor adhesion force and is susceptible to the pitting corrosion.