Investigation On An In-situ Corrosion Monitoring Method For Inner Pipe Surfaces

The existing online corrosion monitoring methods mainly include weight-loss method,electricalresistance probe method,electrochemical linear polarization probe method,field signature method,etc.Except the field signature method,magnetic flux leakage method and hydrogen probe method,corrosion information of the monitored system cannot be directly acquired by means of the above corrosion monitoring methods.Only the corrosion rate of the probe can be measured,which is used to indirectly characterize the corrosion rate of the monitored system.While,in fact,the differences of the state of materials and the flow state of the surface medium environment between the probe and the pipe surface may make their corrosion behavior react in different ways.To this end,this paper has investigated a new monitoring method and a matched device,which helps fulfill to obtain the corrosion information such as surface polarization resistance and corrosion rate from the pipe itself.In addition,this paper has tried to establish a model of potential distribution on the pipe inner surfaces through handling the mathematical model by COMSOL Multiphysics,through which the results can be proved and the influence of various factors can be studied.This paper has established a monitoring system which simulated corrosion of inner pipe surfaces and in this system we hassuccessively used galvanostatic polarization method,potential dynamic scanning method and AC impedance method in different conductive medium to study the corrosive state of carbon steel pipe and carbon steel electrode.The experimental results have indicated that good uniformity has been proved by comparing the test results of taking the whole pipeas working electrode with the small electrode of homogeneous material.Base on the results fromthree different diameter pipes and a combination with the COMSOL Multiphysics simulation results,this paper has analyzed the influence of several factors such as the medium conductivity?G?,pipe inner diameter?D?on the potential distribution of the inner pipe surfaces.When compared with pipe diameter,the pipe length?L?is infinitely long and the pipe surface polarization was controlled in the linear area,we can put the potential distribution on the inner surface of the pipe into a formula:???,V is the difference between the polarization potential and the self-corrosion potential of a certain place,V₀ stands for the difference between the polarization potential and the self-corrosion potential where x equals zero,?stands for the resistivity of the medium and R_p stands for polarization resistance.By monitoring the potential information of two or more locations,the surface polarization resistance and corrosion rate could be obtained gradually.Under the condition of these experiments in the paper,when the whole pipe was applied withgalvanostaticpolarization,the pipe surface polarization was controlled in the linear area,the range of the current distribution in the pipelines was 0 to 5D.The bigger the pipe diameter was,the morehomogeneous the polarization value distribution was.The smaller the size of the tube,the greater the shielding effect of the space,which resulted in the decrease the current and and the range of the current.The smaller tube medium conductivity was,the greater the errorsexisted between the experimental results and simulation results due to reasons such as solution resistance.Thus solution resistance is needed to be eliminated.