| Flow-accelerated corrosion (FAC) is in feed water and drain systems is one of the typical problems,which means the dissolved phenomenon of carbon steel or low alloy steel in fast flowing water or two phase flow.FAC will lead to wall thinning even leakage of the tube which can influence immensely the safety of the worker and equipment in power plant. Currently,plenty of theoretical models are constructed by most researchers to analyse the mechanism of FAC,while rarely using the experimental method.A FAC experiment system is established in this research,and the method of numerical calculation is also applied to the study of influence of flow distribution on FAC in power plant.The purpose of this study is to obtain FAC thinning rate of carbon steel electrode in different places of 90° elbow using electrochemical methods.The constructed FAC experimental system including water tank,temperture control system,flow velocity control system,water-chemistry control unit and electrochemical measurement unit.20# carbon steel is being used as working electrode which owns a surface area of 2.7mm*3.7mm, and thirty-six specimens are mounted into the elbow test section with the same distance in flow direction.FAC test is running within 25? of temperture,atmospheric pressure,3m/s of velocity and 9.3 of pH,while electrochemical impedance spectroscopy measurements (EIS) are performed on electrode system with OCP and a frequency range from 10000Hz to 0.1 Hz in order to obtain the corrosion rate of working electrodes.According to measurement data,the distribution pattern of the working electrode's corrosion current density along the symmetric position on both sides of the 90° bend through the center section.In the area of bend's inlet pipe,the corrosion situation of outer wall is more serious than the innermost one.In the area of 90° bend,it is also obvious to see that the maximum value appears on the center position of outer wall,while the minimum value appears on the inlet nearby position of innermost wall.In the area of bend's outlet pipe,the corrosion current density of outer wall is bigger than the innermost one.At last.the experimental results are compared and analysed with the actual statistics of 90° bend's failure position from power plants,which indicates that the measurement data are basically consistent with the real situation,and provides a convinced foundation for the following numerical research.Meanwhile,CFD software is being applied on the numerical simulation of flow-field distribution and shear-stress distribution in 90° elbow,which aiming at the study of influence of fluid flow on FAC.The results show that,Corrosion current density at the bend's innerwall is affected mainly by the tangential velocity of fluid.along the flow direction,the velocity and wall-shear stress in the inner wall of elbow presents a pattern of increasing at first and then decreasing,while the corrosion current density shows same trend.while in the outer wall,corrosion is mostly affected by the fluid's radial force as a result of direct flush to the outer wall.there is a rising trend of radial force on the outer wall owing to the influence of Dean Vortices,and fluid's velocity is also increasing,which is basically consistent with the corrosion rate.In addition,the corrosion difference in 90° bend where flow velocities are 3m/s and 1.5m/s is compared.Results show that the influence of Dean Vortices when flow velocity is 1.5m/s is lower than the other one,which leads to the discending of corrosion current density in outermost wall of 90° bend.The results of this study provide a fundamental direction on further improvement of 90° bend's flow-fieid to reduce the corrosion rate.Meanwhile,the positions which FAC is easier to occur in the area of 90° elbow is assured on the basis of analysis result,in order to provide a guideline of the feed water and drain systems in power plants on the task of corrosion monitoring and controlling. |
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