Study On Localized Corrosion Mechanism And Life Eveluation Of Non-metallic Lined Tanks

With the development of China's crude oil exploitation into the middle and late stage,the wastewater content in the produced liquid increases gradually.However,the composition of the medium in the wastewater is complex and the environment of the medium is harsh,so the corrosion of the wastewater storage tank is very serious.The most common anti-corrosion measure is to coat the lining inside the tank,but once the lining material is eroded,it suffers from partial damage,and the medium will enter into the lining,which will lead to the peeling between the lining and the inner wall of the tank,causes serious local corrosion of the inner wall of the tank.In this paper,the influence of wastewater on the corrosion of Q235 steel under different temperature and ion conditions was analyzed by electrochemical test with the method of combining macroscopic and microscopic.Through immersion test,the correlation between different shapes and sizes of coating defects and the local corrosion of lining tank was explored.Combined with cathodic stripping test,the influence of different cathodic protection potential on the corrosion resistance of the coating was studied.With the help of COMSOL Multiphysics software,the finite element analysis of residual life of Q235 steel in wastewater medium was carried out,and the influence of defect size,depth and other factors on the effective stress was analyzed.The main corrosion type of Q235 steel in wastewater is pitting corrosion.When different concentrations of Cl^-,HNO₃and Cu²⁺are added to the wastewater,the corrosion resistance of the material decreases and the corrosion resistance weakens.When Na OH and S^2-are added,the resistance and corrosion resistance of the material increase,which indicates that the two ions can inhibit the corrosion.Analysing the corrosion condition of Q235 steel in wastewater medium at different temperatures,it can be concluded that when the temperature is 25?,the corrosion resistance is the least and the corrosion resistance of the material is the best.The results of immersion test show that when the shape of the coating defect is the same and the area is different,the larger the area is,the more severe the reaction is.With different shapes and the same area,the larger the perimeter,the more intense the reaction,and the corrosion products are easy to gather at the tip corner of the defect.The weld is more prone to corrosion than the base metal,and the corrosion products generated at the weld are mostly concentrated at the junction of the weld and the base metal.Applying different cathodic protection potential to the damaged coating,it can be analyzed that when the potential is-850m V,both the passivated film and the substrate can produce the maximum corrosion resistance,which can provide the most stable protection for the specimen.According to the finite element analysis,the length and depth of the defects under uniform corrosion are positively correlated with the effective stress of the storage tank,while the width is negatively correlated.The pore size of the defects under local corrosion is positively correlated with the effective stress of the storage tank,while the depth is negatively correlated.The calculation of the residual life of the storage tank shows that the larger the aperture is,the shorter the residual life of the storage tank is,and the residual life of the weld is the shortest,followed by the lining defect,the residual life of the storage tank matrix is the longest.