Study On Identification Method Of Seriously Corroded Pipe Section Under High Voltage DC Interference

The line selection of HVDC transmission project is similar to that of oil and gas pipeline,and the two transmission destinations are highly coincident,so that HVDC transmission system will inevitably cause interference to the pipeline.Under the comprehensive action of interference factors and environmental factors,the problems of pipe corrosion and anticorrosion coating peeling are particularly prominent.In this paper,considering the environmental factors and interference factors,the influence of HVDC interference on the corrosion behavior of pipeline body and the peeling behavior of anti-corrosion coating is studied based on weight loss method,electrochemical method and cathodic stripping distance test method.Based on the improved FAHP(Fuzzy Analytic hierarchy process)and fuzzy comprehensive theory,the identification method of serious corrosion section under HVDC interference is established.The main results are as follows:The hierarchical structure of seriously corroded pipe section under high voltage DC interference is established,and the target layer,factor layer and numerical layer are divided.Combined with the grey correlation method to determine the construction principle of triangular fuzzy judgment matrix,the triangular fuzzy analytic hierarchy process is improved.According to the corrosion rate and cathode stripping distance,the grades are divided,and the membership function and membership matrix of the numerical layer are established.Combined with the fuzzy comprehensive evaluation method,the calculation model of the identification method of seriously corroded pipe section under high voltage DC interference is established: P = Ai × R× V.Among them,P is the evaluation result of corrosion severity grade,Ai is the matrix constructed by the relative weight value of each influence factor,R is the matrix constructed by each value relative to the membership degree of each grade,and V is the grade division evaluation set matrix of index factor.Interference current density,pH and chloride ion concentration are the key factors affecting pipeline body corrosion.The corrosion rate of X80 steel is positively correlated with DC current density and chloride ion concentration,the increase of current density will lead to the positive shift of corrosion potential and the increase of corrosion current density,the increase of chloride concentration will lead to the increase of corrosion current density,the corrosion rate of X80 steel is negatively related to pH,and the increase of pH leads to the decrease of corrosion current.The changes of interference time,interference time interval and interference frequency have little influence on the corrosion rate of X80 steel.Under DC interference,the whole corrosion process of X80 steel is controlled by cathode.Interference potential,pH,chloride concentration and interference time are the key factors affecting the stripping of 3PE coating.The negative shift of the interference potential leads to the intensification of the cathodic reaction and hydrogen evolution reaction,and the cathodic stripping distance increases gradually,and the interference time promotes the cathodic stripping process mainly by accelerating the cathodic reaction.On the one hand,the change of pH will accelerate the corrosion of the matrix metal,on the other hand,alkaline substances will be formed to destroy the bonding between the coating and the metal,thus promoting the cathodic stripping process.The increase of chloride ion concentration leads to more ions infiltrating between the coating and the metal substrate,which provides a prerequisite for cathodic stripping.The interference time interval and interference frequency have little influence on the cathodic stripping distance,and both of them may affect the cathodic stripping process of 3PE anticorrosion coating by affecting the formation process of corrosion products.