Study Of Corrosion And Erosion Mechanism For Cr-Bearing Alloy Steel In CO₂-Containing Wet Gas Pipeline

With the development of deep water offshore gas fields in recent years,more and more long distance subsea gas pipelines have been built up.The need to maintain gas production at reducing reservoir pressure and to reduce the accumulation liquid had already resulted in much higher gas velocities in the field.Carbon steel is the most widely used pipeline material in the natural gas industry.However,a high gas velocity can be detrimental to the corrosion inhibitor performance on the internal surface of a pipeline,which will increase the erosion risk especially for wet gas transportation.Low Cr-bearing alloy steel which shows an excellent performance-price ratio with a better CO2 corrosion resistance is expected to replace the carbon steel used for pipelines.In this paper,the phenomenon and mechanism of "pseudopassivation" of low Cr-bearing alloy steel and 6.5Cr steel were investigated through immersion tests and electrochemical measurements.In addition,the high pressure and high speed wet gas flow loop and rotating cylinder electrode(RCE)were used to investigate the erosion behavior and mechanism of carbon steel,erosion behavior and mechanism of low Cr-bearing alloy steel and 6.5Cr steel,performance and mechanism of corrosion inhibitor in CO2-containing wet gas pipeline with high gas velocity.The main conclusions were listed as follows:(1)The corrosion resistance of 1Cr?6.5Cr steels was closely related to the“pseudopassivation”property.As the Cr content of Cr-bearing alloy steel increased,the“pseudopassivation" property enhanced and the corrosion rate decreased.Cr-bearing alloy steel could be pseudopassivated due to the formation of corrosion film during the immersion tests or anodic polarization tests.The corrosion film was mainly composed of Cr(OH)3.When the Cr content in the substrate was sufficient(3%),the corrosion film could isolate the matrix from the corrosion medium effectively and provide enough protection to the substrate,and the steel could be pseudopassivated.Meanwhile,the Ca2+ in the solution could strengthen the pseudopassivation,resulting in decreasing of corrosion rate.(2)The erosion risk of carbon steel increased rapidly as the wall shear stress and water content increased in CO2-containing wet gas pipeline with high gas velocity.The main corrosion product was FeCO3,which could be easily partial damaged by the high wall shear stress.It was difficult for FeCO3 to deposit on the surface of carbon steel when the wall shear stress reached 212Pa,and only some Fe3C and little FeCO3 existed on the surface.(3)The salt deposition in the natural gas well was simulated by the flow loop for the first time.With the high temperature and low water content,the salt could deposit on the steel surface even if the salinity was low in CO2-containing wet gas pipeline with high gas velocity.The main components of the salt deposit were NaCl and CaCO3?(4)The 3Cr steel showed much better erosion resistance than the carbon steel under in CO2-containing wet gas pipeline with high gas velocity.The main corrosion product of 3Cr steel was Cr(OH)3 and FeCO3.The shear-resistance property of this corrosion scale was much better then FeCO3 film.It could form and keep intact even if the wall shear stress reached to 212Pa.It is the corrosion scale that provides the excellent erosion resistance for the 3Cr steel.(5)Increasing the flow velocity could accelerate the rich of Cr(OH)3 on the 6.5Cr steel surface,which could enhance the corrosion resistance of 6.5Cr steel.When the rotating speed of RCE reached to 2000rpm,6.5Cr steel was passivated.(6)The performance of corrosion inhibitor in CO2-containing wet gas pipeline with high gas velocity was worse than that in the water phase.On the one hand,increasing wall shear stress was beneficial to the dissolution and mass-transfer process of corrosion inhibitor in the corrosion medium,accelerating the adsorption of corrosion inhibitor molecules.Besides,the high wall shear stress could promote the corrosion inhibitor molecules desorption from the steel surface.Hence,increasing the wall shear stress did not necessarily decrease the performance of corrosion inhibitor.(7)Choosing the right corrosion inhibitor for 3Cr steel pipelines under high gas velocity conditions should be a special concern.Some corrosion inhibitors(e.g.OAI)which have high inhibitor efficiency on the carbon steel surface may not play its role well on the 3Cr steel surface.However,some corrosion inhibitors may have higher inhibitor efficiency on the low Cr steel surface due to the synergistic effect between the corrosion inhibitor and corrosion scale.