Effect Of Rare Earth And Re Content On The Corrosion Resistance Of Environment With CO₂/H₂S In BT80S-5Cr Oil Well Pipe

There are some oil and gas fields with low permeability,low pressure and low abundance in China,with low oil and gas reserves and short development life,which belong to typical"three low oil fields",mainly distributed in Sichuan Basin,Changqing,North China,etc.For these"three low"oilfields,the development and use of economical well pipes has become a hot spot for development.At present,the oil well pipes commonly used in China in high-yield oilfields are mainly 9Cr,13Cr,etc.,which have strong resistance to CO₂corrosion,but the cost is too high.Although the existing economic well pipes 1Cr and 3Cr are used in ordinary poor oilfields,due to the"three low"oil exploitation process often accompanied by CO₂,H₂S gas and high temperature and high pressure environment,the exploitation environment is increasingly harsh,and its corrosion problems in high CO₂partial pressure area and CO₂/H₂S coexistence and high corrosive environment are serious,and the use performance is limited.Therefore,it is necessary to further improve the corrosion resistance of existing economical well pipes on the basis of reducing costs.Previous studies have shown that the addition of rare earths to steel can improve the corrosion resistance of steel.The region is rich in rare earth resources,which provides a resource guarantee for the research and development of rare earth oil well pipes.Based on 3Cr oil well pipe,this thesis studies the corrosion resistance of rare earth 5Cr economic well pipe steel in the harsh environment of CO₂/H₂S coexistence by appropriately increasing the Cr content and adding trace rare earth element cerium,which has certain theoretical and practical guiding significance for making full use of local resource advantages and developing new economic oil well pipe.In this thesis,BT80S-5Cr steel with different rare earth contents smelted in a steel mill was used as the research object,and the micromorphology,inclusions and second phase after heat treatment were observed and analyzed by metallographic microscopy（OM）,field emission scanning electron microscopy（SEM）and transmission scanning electron microscopy（TEM）.The corrosion resistance of experimental steel in CO₂/H₂S coexistence simulation environment was studied by high temperature autoclave experiment.The morphology of corrosion products and the composition of corrosion product film elements of experimental steel were observed and analyzed by combining SEM and energy spectrum analysis.X-ray diffraction（XRD）was used to qualitatively and quantitatively analyze the corrosion products of experimental steel.The weight loss on corrosion and corrosion rate were calculated by weight loss method,and the corrosion kinetics law was analyzed.The electrochemical impedance and dynamic polarization curve of the experimental steel were tested and analyzed by electrochemical method.The corrosion behavior and electrochemical law of rare earth BT80S-5Cr oil well pipe in CO₂/H₂S coexistence environment were studied,and the influence law and mechanism of rare earth and addition amount on CO₂/H₂S corrosion resistance of BT80S-5Cr oil well pipe were explored.The results show that after 50 minutes of water quenching at 900°C and 90 minutes of tempering at 690°C,the microstructure of BT80S-5Cr steel with different rare earth contents is tempered tostenite+tempered cordite,but the uniformity of tempered structure increases with the increase of rare earth addition.The second phase precipitated particles were aggregated at the grain boundary,the size and number were reduced,and the appearance was mostly irregular polygonal blocks,and the types were Cr₇C₃and Cr₂₃C₆.Rare earth aluminate and rare earth oxygen sulfide gradually increased,some large-size inclusions became smaller,the addition of rare earth played a role in modifying inclusions,and the work function of rare earth inclusions was higher than that of the same type of inclusions,so the early corrosion dissolution point was reduced.The corrosion products of BT80S-5Cr steel in CO₂/H₂S simulation environment with different rare earth dosages are composed of loose Fe S,and the inner layer is mainly Fe₃O₄,Cr（OH）₃and Ca CO₃.With the increase of rare earth addition,the average thickness of the inner corrosion product film decreases,the content of Fe₃O4 and stable amorphous substance Cr（OH）₃increases,the content of Ca CO₃in the sediment gradually decreases,the compactness of the product increases,and the protective effect on the matrix is enhanced.The corrosion rate decreased from 0.4055mm/a to 0.3499mm/a after 5 days of corrosion.After 15 days of corrosion,the corrosion rate decreased from 0.2644mm/a to0.2247mm/a,and the corrosion rate decreased with the increase of rare earth addition.With the increase of rare earth and the amount added,the corrosion potential of the experimental steel moves in the positive direction,the self-corrosion current density gradually decreases,the impedance increases,and the corrosion resistance increases.The inclusion of rare earth modification and the improvement of Fe₃O₄and Cr（OH）₃content in the inner layer of corrosion products are the main reasons for the improvement of CO₂/H₂S environmental corrosion resistance of BT80S-5Cr steel.