Flow Accelerated Corrosion Behavior Of The Gradual Contraction Pipe Under High Pressure CO₂ Environment

In oil and gas production,gradual contraction pipes cause severe localized corrosion due to high pressure corrosive fluids,which threatens the safe operation of pipelines.Up to now,there is still no quantitative study on the correlation between the flow accelerated corrosion mechanism,internal corrosion rate and hydrodynamics of gradual contraction pipes under high pressure conditions.This project adopts a high pressure circulation pipeline device designed and built by ourselves to realize high pressure dynamic in-situ electrochemical testing.The flow-accelerated corrosion mechanism of X65 carbon steel gradual contraction tubes under high pressure is investigated by using electrochemical testing techniques and microscopic characterization methods（SEM,XRD,XPS）.CFD software simulations were used to obtain the flow velocity,wall shear stress and turbulent kinetic energy distribution in the gradual contraction tube under different flow conditions.Electrochemical test results show that increasing the partial pressure of CO₂increases the concentration of cathodic reaction species and thus increases the corrosion rate of CO₂.The corrosion rate under dynamic high-pressure conditions is higher than that under dynamic atmospheric pressure and static high-pressure conditions.The corrosion rate increases in the direction of fluid flow in the gradual contraction tube under dynamic high-pressure conditions,while the corrosion rate decreases in the direction of fluid flow under dynamic atmospheric conditions.High-pressure conditions of the flow accelerated corrosion mechanism of the gradual contraction tube and atmospheric conditions have significant differences,the mechanism and the composition of corrosion products film and morphology related.Fluid flow rate and CO₂partial pressure on the corrosion product film morphology has a significant impact,increasing the partial pressure of CO₂can improve the product film denseness.During the process of product film denseness,the internal stress increases leading to product film cracking,which weakens the protection performance of the product film on carbon steel.The corrosion rate distribution in the gradual contraction tube is related to the fluid dynamics.CFD simulation results show that the fluid flow rate remains stable in the straight section before the gradual contraction,gradually increases along the gradual contraction section,and reaches the maximum at the junction of the gradual contraction area and the straight section after the gradual contraction,and the flow rate basically remains stable in the straight section after the gradual contraction.The wall shear stress and turbulent kinetic energy distribution in the gradual contraction tube are similar to the flow velocity.Turbulence effect to accelerate the reaction medium transfer and accelerate the cathodic reaction rate,while hindering the formation and development of protective corrosion products film,increasing the corrosion rate.The flow accelerated corrosion rate distribution in the gradual contraction tube is consistent with the distribution of fluid dynamics characteristics.