Study On Thermal Properties And Corrosion Behavior Under Thermal Cycling Conditions Of Ternary Eutectic Chloride Salt In Solar Thermal Power Station

As a kind of heat transfer and heat storage material with superior performance,molten salt has been widely used in concentrated solar thermal power stations.The traditional nitrate has the disadvantage of instability at high temperature.It has important engineering application significance to study the chloride salt with superior performance at high temperatore.In this paper,the ternary eutectie NaCl-KCl-CaCl2 was used as the research object,and its thermal properties and thermal stability were analyzed by TG-DSC test method.In the actual solar thermal power station,there is a phenomenon that the corrosion of the pipeline container in the molten salt is aggravated by thermal cycle.This paper uses the intelligent temperature-controlled box furnace to simulate the thermal condition of the solar thermal power station.Corrosion weight loss test,SEM and EDS analysis were adopted to study the corrosion behavior of high temperature chloride salt in stainless steel 304.The main findings obtained are as follows:(1)15 different ratios of temary eutectic NaCl-KCl-CaCl2 were prepared.It was difficult to form a eutectic molten salt or the eutectic effect was not good,when the content of two components was too low.The melting point of the molten salt was fluctuations in the range of 531.86℃ to 562.83℃.When the ratio of sodium chloride,potassium chloride and calcium chloride is 2:5:3,the ternary eutectic chloride salt has good thermal properties.The latent heat value of the molten salt is 154.3J/g,the melting point is 531.86℃,and the average specific heat capacity at 600-900℃ is 2.1 J/K·g.(2)The mixed chloride salt has good thermal stability at 700℃,and the mass loss is small at high temperature for a long time.After several times of heat absorption and exothermic,the melting point and freezing point of the molten salt are almost unchanged.After experiencing a long period of cold and heat cycle,the heat flow curve of the molten salt tends to be stable,and has the ability of long-term heat storage and heat release.(3)Thermal cycling(500-700℃)accelerates the corrosion rate of stainless steel alloys compared to alloys immersed in eutectic chloride salts at isothermal(700℃).By plotting the relationship between metal mass loss and square root of time,after 100 hours of corrosion,the corrosion rate of the alloy under thermal cycling is about 1.8mg/cm2,which parabolic kinetics were observed;The corrosion rate of the alloy at isothermal conditions is 0.8mg/cm2,which linear kinetics were observed.(4)The surface of the alloy subjected to thermal cycle chloride salt corrosion has a larger area of exposed matrix structure than the isothermal case,and the corrosion layer has obvious delamination,with maximum corrosion layer thickness reaches 60μm.Thermal cycling accelerates the shedding of the outer oxide layer,causing the chloride salt to penetrate more easily into the inner matrix.(5)Thermal cycling results in a decrease in the intergranular corrosion resistance of the alloy,and precipitation of chromium carbides at the grain boundaries.The combination of thermal cycling and chloride salt causes the alloy to appear along the crystal crack,and chromium oxide is formed at the grain boundary.The corrosion of the stainless steel 304 by the chloride salt is mainly grain boundary corrosion,and the preferred de-solubility of Cr compared with the Fe element is the reason why the alloy is not resistant to chloride salt corrosion.The basic theory provided by this study has wide application value for the application of chloride salt in solar thermal power stations and the failure of actual pipelines under thermal conditions.