Investigation On Corrosion Characteristics And Mechanism Of 316L Stainless Steel In Gas And Liquid With Chloride Ions Slug Flows

316L stainless steel is widely used in industrial fields due to its excellent processing properties and corrosion resistance.So far,researches on corrosion behaviors of 316 L stainless steel are mainly focused on the single-phase fluid medium.In the actual production process,the media in pipelines usually contain both the gas and the liquid.The configurations of the two-phase flows are more complicated than those of the single-phase flow,resulting in different corrosion characteristics of 316 L stainless steel.In this paper,experimental investigations,numerical simulations and theoretical analysis were conducted to study the corrosion behaviors and characteristics of 316 L stainless steel in the gas-liquid two-phase slug flows.The corrosion characteristics of 316 L stainless steel in slug flows were established and the corrosion mechanisms were revealed.This research is based on the common problem in industrial fields and may supply engineering significance for application and risk prediction of 316 L stainless steel.The main contents and results of this research are as follows:(1)The gas-liquid two-phase flow experiments were carried out to investigate slug flow configurations at different flow rates in vertical pipes.The polarization curves,electrochemical impedance spectrum(EIS)measurements and morphology analysis were used to investigate corrosion characteristics of 316 L stainless steel under variable working conditions.The computational fluid dynamics(CFD)method was used to calculate the fluid shear stress and mass transfer coefficient in the fluid field to reveal the corrosion mechanism of 316 L stainless steel.The results show that flow rates have extremely significant effects on structures of Taylor bubble and liquid slug.As the gaseous flow rate is increased,the two-phase flow pattern changes from bubbly flow to slug flow.,leading to the increase of the lengths of Taylor bubble and liquid slug.In this process,the solution resistance gets larger,while the polarization resistance gets smaller,leading to greater corrosion rates of 316 L stainless steel.As the liquid flow rate is increased,the solution resistance and polarization resistance get smaller,leading to greater corrosion rates of 316 L stainless steel.(2)Effects of inclination angle on configurations of slug flow were experimentally studied.The change laws of slug flow at different inclination angles were developed.The electrochemical technology was used to investigate corrosion characteristics of 316 L stainless steel at different inclination angles,and the relationship between slug flow configuration and corrosion characteristics of 316 L stainless steel was established.The results show that a new flow pattern,namely columnar bubble flow is formed in the horizontal straight pipe.As the inclination angle is increased,the columnar bubble length is gradually decreased.The resistances of polarization and solution are large at small inclination angles,resulting in low corrosion rates of 316 L stainless steel.The flow pattern in the pipe transforms from columnar bubble flow to slug flow,leading to the decrease in resistance and increase in corrosion rate of 316 L stainless steel.(3)The evolution regularities of slug flow when passing through elbows were developed by experiments and theoretical analysis methods.A time-space evolution model of slug flow in the elbow was established.For a vertical to horizontal elbow system,when the liquid slug passes through the elbow,the inner wall surface is wetted by liquid water.The outside wall is contacted with the Taylor bubble,without being wetted by liquid water.When the Taylor bubble leaves the elbow and enters the horizontal straight pipe,it still moves along the upper wall due to the buoyancy effect,exhibiting the columnar bubble flow.For a horizontal to vertical elbow system,the columnar bubble flow in the horizontal pipe changes to the slug flow in the vertical pipe.The outside wall of the elbow is always wetted by the liquid water,while the inner wall is intermittently wetted by the liquid water.The region contacted with the Taylor bubble is not wetted by liquid water,leading to relatively lower corrosion rates of 316 L stainless steel.In this study,a new gas-liquid two-phase flow pattern was discovered at small inclination angles,and the time-space evolution model of slug flow in inclined pipes and elbows was established.Corrosion characteristics of 316 L stainless steel were studied by electrochemical technology.The corrosion mechanism of 316 L stainless steel in the slug flow based on the wetting state of the wall was revealed by numerical simulation and theoretical analysis methods,and the key characteristic parameters of pitting corrosion in slug flows were established.The prediction method of 316 L stainless steel in gas-liquid two-phase slug flow was formed.