Analysis Of Flow Accelerated Corrosion Kinetics And Electrochemical Mechanism

Flow-accelerated corrosion(FAC)is a kind of behavior in which the oxide film on the inner surface of carbon steel or low alloy steel pipe becomes thin under the action of high speed and high temperature water,and its protection effect on metal matrix is weakened.As a result,the corrosion rate of metal increases,resulting in the wall thinning and fracture failure of pipe.This phenomenon generally occurs in the secondary circuit of nuclear power plants and the heat recovery pipes of supercritical units.The FAC reaction kinetics model was established by further studying the mechanism and influencing factors of flow accelerated corrosion,and the FAC mathematical model,solubility model and mass transfer model were established based on THE M.I.T model.The solubility model is used to calculate the solubility of oxidation product film at different temperatures and p H values.Comsol Multiphysic software was used to simulate the flow field,calculate the mass transfer coefficient,and analyze the influence mechanism of fluid dynamics factors and electrochemistry on FAC.Through the research of spot weld,girth weld,75°elbow and 125°elbow.For the weld,by changing the weld residual height and observing the flow field around the weld and downstream of the weld,the influence of the weld residual height on the location and rate of flow accelerated corrosion when the inlet velocity is the same is explored.The results show that: When the inlet flow rate is 5m/s,the greater the weld residual height is from 1mm to 5mm,the greater the flow rate around the weld is,and the greater the corrosion rate is.For 75°bend and 125°bend,compared the first two different tube type,bend the distribution of internal flow field,to analyze the effect of tube type of flow accelerated corrosion second under the same tube type,by changing the radius of curvature,to study at the same time,when the inlet velocity facies elbow under different radius of curvature changes of the flow field,the results show that: When the inlet velocity and radius of curvature are the same,the outer side of 75°elbow is more easily corroded.When the tube type is constant,the more serious the corrosion is when the radius of curvature of inner bend is R=0.1,and the corrosion is more serious when the radius of curvature of outer bend is R=0.175.Aiming at the combination of chemistry and physics,Comsol Multiphysics software was used to simulate the coupling of multiple physical fields.The effects of p H and inlet flow velocity on the corrosion current density were studied respectively,so as to calculate the flow accelerated corrosion rate.The results show that when p H increases from 8.8 to9.7,the current density decreases.The corrosion current density is an important electrochemical factor affecting the flow accelerated corrosion.The higher the corrosion current density is,the higher the flow accelerated corrosion rate is,up to 0.7mm/a.The change of flow velocity will also cause the change of current density.The higher the flow velocity,the higher the limit current density,and the higher the corrosion current density,leading to the increase of corrosion rate.