Corrosion Behavior Of 20# Steel At Initial Stage Under The CO₂/Aqueous Solution Gas-liquid Two-phase Plug Flow Condition

In this paper,the actual corrosion environment of petroleum transportation pipeline is simulated by dynamic tube flow corrosion test device.The corrosion time is greatly shortened by modifying experimental equipment.The corrosion behavior of20#steel in the initial stage of CO₂/aqueous solution gas-liquid two-phase plug flow environment is studied.And the effects of environmental factors such as corrosion time,CO₂ partial pressure,liquid phase flow rate on corrosion at the initial corrosion behavior were systematically explored.The corrosion rate,the macroscopic morphology and microstructure of the corrosion products,the content of corrosion products and the composition of the phases were studied by weight loss method,SEM,EDS,XRD and XPS.The result shows a high initial corrosion rate under different pressure conditions.There is a significant corrosion rate deceleration transition phase in the early stage of corrosion process.The difference of corrosion rate under different CO₂ partial pressures decreases with the increase of corrosion time,the precipitation of corrosion products starts from the defect or grain boundary on the surface of the pipe wall,The corrosion product extends outside until the corrosion product substantially covers the substrate The corrosion products grew into the flocculated corrosion products on the surface of acicular corrosion products layer deposited preliminary with the extending corrosion time.the growth of corrosion product on the upper wall lags behind the lower wall surface,and the main constituent phases in the corrosion product layer are Fe₃C,FeCO₃,FeOOH and Fe₃O₄,and there is no polygonal FeCO₃ crystal observed.the partial pressure of CO₂ has different effects on the initial corrosion rate at different corrosion time.the corrosion rate increases first and then decreases with the increase of partial pressure under the shorter corrosion time,The minimum corrosion rate?4.85mm/a?and the maximum value?7.38mm/a?were observed at 0.05MPa and0.18MPa respectively.The change of corrosion rate is relatively insignificant with partial pressure increase under longer corrosion time.With the increase of partial pressure,the plaque feature of corrosion products low magnification is gradually obvious,and the acicular corrosion products are gradually dense.The corrosion rate increases with the increase of flow rate under different corrosion time conditions.The minimum corrosion rate?4.85mm/a?and the maximum value?9.05mm/a?were obtained at 0.11m/s and 0.18m/s respectively after 5 minutes of corrosion.As the liquid phase flow rate increases,the corrosion product gradually decreases,and the surface of the substrate gradually changes from smooth to rough.According to the NACE RP-0775-91 standard,the above corrosion rates are far greater than 0.254mm/a,which is extremely severe corrosion.The EDS elemental analysis shows that the corrosion products of the lower walls are composed of elements such as Fe,C,O,The XPS peak spectrum shows that there are three fitted potential peaks in C 1s,O 1s and Fe 2p.The three peaks in C 1s correspond to the binding energy of carbon in cementite?Fe₃C?and iron carbonate?FeCO₃?.The three spectral peaks of O 1s correspond to the[OH^-]ions in Fe₂O₃,Fe₃O₄ and ferric oxyhydroxide?FeOOH?respectively,and the three peaks in Fe 2p correspond to Fe₂O₃,Fe₃O₄ and FeOOH.Combined with XRD analysis,the main constituents of the corrosion products are Fe₃C,FeCO₃,Fe₂O₃,Fe₃O₄ and FeOOH.