A Study On Crevice Corrosion Of Heat Exchanger Influenced By Mass Transfer Process

Heat exchanger is a kind of heat exchange equipment which has been widely using in several industries,such as refining,chemical,food and pharmaceutical.In the petrochemical industry,heat exchanger are exposed in corrosion environments inevitably during its life and the crevice corrosion is the one do the most serious damage to heat exchanger among other corrosions.Usually,when the heat exchanger is corroded,the most significant crevice corrosion could be found in the area between heat exchange tubes and tube sheets and the area among welds,heat exchange tubes and baffles.This corrosion could mix the heat exchanger tubes and fluids,which result in low product quality or products cannot be used,or more serious,fire and explosion may threaten human life.Therefore,it is valuable to do research in the crevice corrosion which exists on the heat exchange tube,tube plate and welds.Using the Comsol finite element software to conduct the analog simulating research into the crevice corrosion of OCr18NilOTi austenitic stainless steels in the 3%NaCl solution.In the material transfer module,controlling the system quality conservation and the material migration by making use of the Ner-Plank formula and the Nernst-Einstein formula respectively and with the consideration of the neutral condition,6 electrochemical reactions and 6 hydrolysis reactions.Under the secondary corrosion module,the electrode deformation of the crevice coupling with the deformation geometry,choosing suitable boundary conditions.Simulating and calculating electrode deformations,interface potentials and local current densities,chloride ion concentrations and pH values(at three different positions of the crevice)under time-variation.The result obtained from this model corresponds with the value shown in the literature experiment and it can be the reference of pitting corrosion of austenitic stainless steel and issues caused by crevices corrode seam accumulations,such as deformation.Artificial rectangular gap experimental device has been built for simulating crevice corrosion environment in the experiment.Soaking experiment has been used in order to study the crevice corrosion performance of gap leakage points and several areas inside the gap within 98 hours.According to the results,when the experiment time length expand,the electrode potential and the pH value gradually decrease and the Cl concentration increases inside gaps.In addition,a significant Cl concentration change could be found at the bottom of gaps at which we also found the highest concentration value and the strongest acidity.Furthermore,when the gap size is relatively small,the electrode potential inside gaps is more negative,the change magnitude of Cl concentration is higher and Bulk solution is more acid.By using numerical simulation and experiment,this report has studied the crevice corrosion performance of heat exchanger under Mass transfer process.This could provide theoretical supports for relieving the crevice corrosion on the structure which made by different materials and used in the circulating cooling water system in refineries.In addition,this report could be used as the reference when tackling problems such as the austenitic stainless steel pitting and the deformation problem of seam deposits caused by the crevice corrosion.