The Corrosion Behavior Of 20# Steel In CO₂/H₂O Two Phase Bubble Flow

The effects of corrosion time,CO₂ partial pressure and CO₂ gas flow velocity on the corrosion rate,corrosion product morphology and corrosion product composition of 20# seamless steel tube under the condition of CO₂ /water vapor liquid bubble flow were comprehensively analyzed by means of weight loss method,SEM,EDS,XRD and XPS in the dynamic tube flow corrosion equipment originally designed by us.The results show that:(1)With the increase of time under different pressures,the corrosion rate firstly decreases and reaches the minimum value at 3h,and then reaches the peak value at 5h,and then slowly decreases.Corrosion specimen surface black gray coverage area of corrosion morphology by initial loose product acicular membrane and small amounts of flocculent composed of monolayer film gradually shift the relatively dense needle-like film+loose flocculent film product of dual structure characteristics,composed of membrane,the extension of time product membrane by perpendicular to the columnar film+compact film on the surface of the sample of the double structure of inner membrane and spherical particles in the outer membrane,eventually form a relatively dense columnar inner layer,the three layers of dense middle and spherical particle outer characteristics of corrosion product film layer.The transformation process of the product morphology in the light gray covering area with the corrosion rate lagging behind in the same time period is consistent with that in the dark gray area.The content of iron in the product film of the dense inner layer is high,while the content of carbon and oxygen in the loose outer layer is high.(2)With the increase of CO₂ partial pressure under different corrosion time,the corrosion rate firstly decreases and reaches the minimum value at 0.1MPa,then increases.Corrosion specimens under different CO₂ partial pressure tube wall surface can be observed that the characteristics of the two different morphology,and the corrosion product film is gradually with the increase of the CO₂ partial pressure are the densification,increases because of Fe²⁺and degree of supersaturation of CO₃^2-,at 0.1 MPa form grain fine and dense homogeneous Fe CO₃protective film,corresponding to minimize corrosion rate,as the CO₂ partial pressure increased to 0.15MPa the cavitation effect caused by the bubble breaking and impacting on the film layer,coupled with the alternating load caused by the bubble rolling motion and the high wall shear stress,resulted in the product film failure.its corrosion rate significantly increased.(3)The corrosion rate of 20# steel tube increases with the increase of gas phase flow rate.Under different gas phase velocity of the surface of the sample can be observed in two morphology characteristics of different area,low speed when the first area product feature film composed of loose tube wall surface film+flocculent membrane double structure of inner membrane,the outer layer of uniform spherical particles,augmenting the phase velocity of product after membrane by the direction perpendicular to the wall surface dense columnar membrane+the double structure of inner membrane of dense membrane,the outer layer is the product"fireworks"form the membrane;Similarly,the second characteristic region is from the needle film of lamellar products perpendicular to the surface of the matrix,and the double structure characteristic film of dense film and flocculent film is transformed after the gas phase flow rate increases.With the increase of gas phase flow rate,the product film is accompanied by cracking and failure phenomenon,which promotes the mass transfer process,increases the corrosion rate and densifies the product film.(4)According to the NACE RP-0775-91 standard,its corrosion rate is much greater than0.254mm/a,belongs to the very serious corrosion;According to EDS analytisches,the importish constituent elements of corrosion products are Fe,C and O,and the XRD outcome analysis indicate that the peimary constituent phases are Fe₃C,Fe CO₃,Fe OOH,Fe₃O₄ and Fe₂O₃.The XPS peaking spectrum shows that three potential fitting peaks appear in C1s,O1s and Fe2p,which is consistent with the XRD results.