Study On The Mechanism Of Micro-turbulence On Local Corrosion Induced By Local Defect Of X80 Pipeline Steel Under Dynamic Liquid Film

CO₂ corrosion is one of the key issues that make it difficult for long-term safe and stable operation of natural gas long-distance pipelines.The main cause of CO₂ corrosion is the liquid film formed in the pipeline due to wet gas transportation,which provides conditions for the formation of the corrosion environment of the pipeline steel by the dissolution of CO₂gas.The liquid film in the pipeline is subjected to the shearing force of the high-speed airflow,and the liquid film is usually in a flowing state except for maintaining a static state at some corners.When there are mechanical damage or local corrosion pits on the pipe surface,the flow field will produce local turbulence induced by local defects,which makes the local corrosion process under dynamic liquid film more complex.The interaction between corrosion behavior under dynamic liquid film and flow field involves many parameters such as velocity,pressure,shear force and turbulent kinetic energy in local flow field,and also interacts with mass transfer process in corrosion process,composition and morphology of corrosion product film.Therefore,it is necessary to deeply study the interaction mechanism between flow field and corrosion under the influence of local turbulence.In this paper,the CO₂ corrosion of X80 pipeline steel in static and non-uniform dynamic flow field is studied.Combined with the theories of fluid dynamics,corrosion dynamics and mass transfer dynamics,the effects of mass transfer process and local turbulence on CO₂ corrosion of pipeline steel with surface defects in dynamic flow field are studied by means of experimental research,numerical simulation and theoretical analysis.（1）Based on the electrochemical test and analysis of the whole and micro area corrosion under static liquid film,the local corrosion process under different thickness of liquid film and the influence of surface defects on the local corrosion process were studied,and the change rule of corrosion electrochemical information under static liquid film with time was obtained.Combined with the mass transfer process,the influence mechanism of ion concentration and mass transfer process on local corrosion was analyzed.The results show that the corrosion mechanism of X80 pipeline steel under static liquid film is basically the same,the thickness of liquid film only affects the reaction time of each process in the corrosion reaction,and the overall reaction process is basically the same.The CO₂ corrosion in static solution can be divided into two stages:in the early stage,the ion concentration inside and outside the defect pit is the same,and the corrosion is uniform everywhere;In the later stage,due to the influence of CO₂ dissolution and mass transfer distance,the cathodic reaction is the main reaction outside the defect pit,while the anodic reaction is the main reaction inside the defect pit and accelerates the local corrosion process in the defect pit.Under the condition of no interference in the laboratory,although the corrosion in the defect pit is accelerated,the corrosion acceleration in the whole defect pit is uniform,and there is no smaller scale point corrosion or crevice corrosion.The corrosion enhancement process in the defect pit is considered as a local corrosion process of overall enhancement.Under the condition of no interference in the laboratory,although the corrosion in the defect pit is accelerated,the corrosion acceleration in the whole defect pit is uniform,and there is no smaller scale point corrosion or crevice corrosion.The corrosion enhancement process in the defect pit is considered as a local corrosion process of overall enhancement.In the micro-area electrochemical test,when the CO₂ in the initial solution is consumed,affected by the CO₂dissolution rate and diffusion mass transfer rate,the defect pit is mainly in the anode area and the outer plane is mainly in the cathode area.It is further confirmed that the defect pits with surface morphology will show a stronger local corrosion tendency in the long-term corrosion reaction.（2）Based on the self-built dynamic liquid film corrosion on-line test platform under high shear force,the experiment was conducted to obtain the CO₂ corrosion process induced by surface morphology defects in a heterogeneous field,and the influence mechanism of local turbulence on local corrosion was clarified.The results show that,compared with the corrosion process under static conditions,the CO₂ dissolution rate and diffusion mass transfer rate in the dynamic flow field have less influence on the corrosion process,and the influence of convective mass transfer and corrosion product film composition and integrity in dynamic flow field on the corrosion process is greater.The single wall shear force in the plane flow field can not effectively strip off the loose corrosion product film,resulting in the formation of a complete loose corrosion product film on the surface of the sample,which is mainly composed of Fe₃C framework attached with Fe CO₃.However,the shear stress in the plane flow field will lead to the fatigue cracking of the loose corrosion product film at some locations,and the cracking location is random,which leads to random local corrosion.（3）Based on the micro-area electrochemical test of the wire beam electrode with surface defect characteristics reconstructed in the dynamic flow field,it is found that under the influence of the local turbulence induced by the surface defect pit,the existence of the surface defect pit will significantly affect the flow field distribution and induce the occurrence of local turbulence,which affects the formation of corrosion product film in different areas,and then affects the corrosion process.The corrosion process can be divided into a variety of situations:at the initial stage of the corrosion,X80 pipeline steel is exposed to corrosive solution,and the corrosion is mainly concentrated in the areas of flow field enhancement,and these areas are connected with a piece,which can be considered as local enhanced overall corrosion,and the corrosion rate at high flow rate is faster than that at low flow rate;under low flow rate,the corrosion residue Fe₃C not stripped by flow field forms the loose corrosion product film,which strengthens the diffusion mass transfer in some areas,and the enhanced diffusion mass transfer is dominant in the later stage of corrosion,which controls the distribution of the anode area;under medium flow rate,the loose corrosion product film in the local turbulence influence area is completely stripped,and a dense and complete corrosion product film is formed to hinders the corrosion mass transfer,and the appearance of the anode area is mainly due to the enhancement of convective mass transfer caused by local turbulence;under the high intensity flow field,the loose corrosion product film is stripped to form a dense corrosion product film,but the dense corrosion product film at downstream of the defect pit is locally destroyed under the influence of local turbulence to form scattered anode points,and the local corrosion occurs in the downstream area.（4）Numerical simulation was carried out to study the correlation between local turbulence and corrosion induced by surface defects.The distribution of flow field parameters near the wall induced by surface defects was obtained,and the correlation between local flow field and corrosion mass transfer process was studied.The results show that for the dense corrosion product film Fe CO₃,the wall shear force generated by local turbulence is not enough to achieve the strength required for its destruction,and the energy generated by the fluid is not enough to make it dissolve and desorb.The destruction of the dense corrosion product film downstream of the surface defect pit is due to the release of CO₂ in the saturated CO₂-NACE solution,CO₂ bubbles formed under pressure fluctuation,and then broke the dense corrosion product film by collapsing in the pressure recovery zone.The results show that the mechanical strength range of the loose corrosion products is 137.2-148.3 Pa.