Corrosion And Electrochemical Behavior Of N80 Steel In High Temperature And High Pressure CO₂/O₂ Environment

During the exploitation of oil and gas fields,serious corrosion would occur due to the presence of corrosive gas such as CO₂ and O₂,leading to the equipment failure,potential safety hazard and huge economic losses.Numerous researches have been devoted to CO₂corrosion at home and abroad,and reported detailed reaction mechanisms.However,there are relative few studies related to the corrosion under the coexistence of CO₂ and O₂conditions,respectively in the high temperature and high pressure environment.In this paper,CO₂/O₂ corrosion under high temperature and high pressure was studied and discussed.The CO₂/O₂ corrosion behavior of N80 steel under high temperature and high pressure was studies in detail,and for comparison,the corrosion behavior in the presence of only CO₂or O₂ was also investigated.Electrochemical measurements combined with weight loss analysis were utilized to study corrosion behavior of N80 steel.And the composition and morphology of the corrosion scale formed in different time and temperature were characterized through XRD,SEM and EDX detection.The effects of the temperature?40°C²40°C?on corrosion behavior of N80 steel were studied.Besides,the growth and evolution process of the corrosion product film at 120?were studied.Furthermore,the mechanism was proposed for the generation of corrosion scale.Collectively,we proposed or verified the following conclusions:?1?the corrosion rate under the coexistence of CO₂ and O₂ condition was remarkably higher than that under the presence of only CO₂ or O₂ and also greater than of the sum of both,indicating the presence of O₂ can significantly increase the corrosion rate of N80 steel.The mass loss of steel under CO₂ corrosion condition mainly occurred in the early stage of corrosion process,and the corrosion rate remarkably lowered after scale formation.The corrosion rate is maintained at a high level under the coexistence of CO₂ and O₂.?2?CO₂ corrosion scale exhibited dense and complete structure accumulated by the FeCO₃ grain,which is favorable for protecting the substrate.CO₂/O₂ corrosion scale was composed of the mixture of FeCO₃ and oxides of iron.The oxides of iron could not protect the substrate and also cause the FeCO₃ crystallization defects,as well as the weak adhesion between crystals.CO₂/O₂ corrosion scale exhibited loose porous structure,which can provide fast channel and accelerated corrosion rate.?3?The supersaturation of FeCO₃ is the main driving force for the formation of corrosion product film.During the CO₂/O₂corrosion process,the oxygen not only directly participated in the corrosion process but also destroyed the integrity of the scale,resulting in the expedited corrosion rate.The latter was proposed as the main factor to increase the corrosion rate in our experiment.?4?CO₂ and CO₂/O₂ corrosion products have the obvious inhibiting effect on anodic reaction.And the corrosion products FeCO₃ is preferentially deposited in the anode region.The corrosive film formed in the two gas conditions has the same effect on the mechanism of corrosion electrochemical process.The main difference is that the integrity of the film is different.