Study On The Corrosion Mechanism Of L360 Steel In CO₂/H₂O Gas-Liquid Two Phase Stratified Flow

Acidic media are generally common in petroleum and natural pipelines,and carbon steel is widely used due to its remarkable cost-effectiveness.CO₂corrosion control in oil and gas pipeline steel is a stubborn issue for engineers and designers because the processes are complex.Therefore,the corrosion mechanism of the L360 steel in CO₂/H₂O gas-liquid two-phase stratified flow was studied through self-designed dynamic corrosion device.The corrosion rates of L360 steel,the micro-appearance of corrosive surface and the composition of corrosion products were studied systematically under different corrosion conditions.The corrosion mechanism of L360 steel was summarized on the basis of the above research.The study showed that:（1）Under different corrosion times conditions,the corrosion rates of L360 steel in gas phase and liquid phase gradually decreased with the extension of time.The SEM micrograph showed that the corroded surface presented dark and light feature region.The corrosion process of the dark color region was faster.The corrosion products were preferentially deposited and the product membrane had better compactness.At 4h,the substrate was covered by loose and porous corrosion products,the ionic mass transfer process was inhibited,and the diffusion mass transfer had become the main method of corrosion.At6h,the porosity decreased.The surface showed a double-layer membrane structure,and the corrosion rate reached the minimum value.With the extension of time,the compactness,homogeneity,and thickness of the product membrance were improved,and the corrosion was in stable stage.（2）Under different CO₂ pressure conditions,the corrosion rate of L360steel showed a trend of rising first and then declining with the increasing pressure.The critical partial pressures were 0.10MPa and 0.13MPa.When the CO₂ partial pressure was 0.10MPa,the corrosion rate reached the maximum value.A large amount of H₂CO₃ would generate with the increase of CO₂ partial pressure,which would accelerate the cathodic hydrogen evolution reaction and promote carbon steel corrosion process.When the pressure was 0.13MPa,the corrosion rate reached the minimum value.At this time,the concentration of HCO3-and CO32-ions in the solution increased and the supersaturation increased,which promoted the formation of product membrane and inhibited the corrosion reaction process.When the CO₂pressure continued to rise,the product membrane in the light feature region was severely damaged.The corrosion products were quickly deposited at defects area to ensure the compactness of the product membrane.The localized corrosion became the main method of corrosion,and the corrosion rate changed slightly.（3）Under different liquid phase flow rates conditions,the corrosion rate of L360 steel continued to increase with the increase of liquid flow velocity.When the liquid flow rate was below 50L/h,the corrosion rate slowly increased and the liquid phase rate was slightly higher.When liquid flow rate was higher than 50L/h,the gas phase corrosion rate increased slowly and the liquid phase rate increased significantly.On the one hand,the high-liquid flow rate corresponds to the high shear stress and the product membrane is prone to mechanical fatigue and fall off.On the other hand,the increase of fluid flow velocity effects the convective mass transfer process and hinders the formation of product membrane.In the liquid phase environment,it is easier to form groove corrosion products that cause localized corrosion.And the corrosion rate increases rapidly.（4）EDS results showed that the corrosion products were composed of three elements:Fe,C,and O.XRD and XPS results showed that the main phase compositions were Fe CO₃,Fe₂O₃,Fe₃C,Fe₃O₄,and Fe O.