Influence Of Microstructure Types On Evolution Of CO₂ Corrosion Product Film And Development Of Platform Corrosion In Oil And Gas Field Environment

In the process of oil and natural gas exploitation and gathering and transportation,the existence of CO2 will lead to serious corrosion behavior of oil casing and gathering and transportation pipeline(mostly platform corrosion)and lead to leakage,causing huge economic loss and environmental harm.An obvious characteristic of CO2 corrosion is the formation of corrosion product film on the metal surface after corrosion.The evolution of corrosion product film is closely related to CO2 corrosion behavior.At present,the research on the evolution of CO2 corrosion product membrane focuses on the composition and structure changes of corrosion product membrane,and fails to link the evolution process of corrosion product membrane with the development of platform corrosion.Organization of N80 steel in pearlite + ferrite and bainite ferrite + organization of X80 steel as the research object,through high temperature and high pressure reaction kettle corrosion and high temperature and high pressure flow loop corrosion simulation experiment,combined with modern analytical analysis testing technology,SEM,EDS and XRD)and electrochemical tests in situ,and focus on use of EBSD technology development of corrosion on the evolution of the corrosion product film and platform in the process of characterizing the corrosion product film crystal structure,explore the tissue types influence on the evolution of CO2 corrosion product film,and explore the development regularity of corrosion on the platform.The main research results are as follows:(1)Under static and dynamic conditions,the average corrosion rates of N80 steel and X80 steel decrease with the increase of corrosion time,and the corrosion rates of X80 steel in each cycle are higher than that of N80 steel,indicating that the steel with ferrite + pearlite structure has better corrosion resistance under this experimental condition.(2)Under static conditions,local corrosion of N80 steel and X80 steel did not occur.Two kinds of steel corrosion product film has a double-layer membrane structure,with the increase of corrosion time,the thickness of the inner and outer layer of the corrosion product film increases,the corrosion interface between the inner film and the substrate becomes flat,the outer film becomes uniform and continuous thickness and dense,the protection of the corrosion product film rapidly improves.(3)Under dynamic conditions,local corrosion occurred during the evolution of corrosion product film of X80 steel,and the thickness of corrosion product film and the volume of local corrosion pit increased with the prolongation of corrosion time.The corrosion product is a double-layer membrane structure,and the inner layer is thicker than the outer layer,which is different from the evolution law of the corrosion product membrane under static condition.Under the action of the flow rate,the ion mass transfer rate is accelerated at the early stage of corrosion,which accelerates the corrosion process of the matrix inward.The outer film is not easy to deposit and has little protection to the matrix.If the defects caused by Ca CO3 keep in contact with the corrosive medium,local corrosion will occur.The reason why there is no local corrosion in the evolution process of corrosion product film of N80 steel is that lamellar Fe3C has an anchoring effect,which can prevent the falling off of corrosion product film and provide nucleation sites to promote the deposition of FeCO3,so local corrosion is not easy to occur.(4)Through EBSD analysis and test,it can be found that under dynamic conditions,the corrosion product film grains of N80 steel and X80 steel have different evolution rules.With the increase of corrosion time,the grain morphology of the corrosion product film of N80 steel changes from columnar grain to equiaxed grain,and the grain size decreases,and the mechanical properties and corrosion resistance improve.The grain size of the corrosion product film of X80 steel is larger than that of N80 steel.(5)according to the different corrosion cycle test specimen macroscopic,microscopic morphology and EDS and corrosion product film EBSD analysis of X80 steel platform corrosion law of development is as follows: local corrosion was originally to FeCO3 grain at the beginning of the corrosion of amorphous transition happens,FeCO3 grain grew up in amorphous layer deposition,one regional excessive internal stress and fracture in amorphous layer falls off,Eva involves corrosion pit;Further development of pitting corrosion pit,because the effect of velocity of corrosion pits on both sides of the corrosion products of grain growth in a different way,the protection of matrix effect is different,so the corrosion pit direction parallel to the flow toward a protective corrosion product film on one side of the poor development,in the presence of passivation film protection,at the bottom of the corrosion morphology gradually developed into the platform.