Study On The Corrosion Mechanism Of Metal Materials In Molten Nitrates

As a high temperature heat storage material,molten nitrate salt has been successfully applied to the tower solar thermal power generation system with heat storage and industrial waste heat recovery.It is important to study the corrosion resistance and mechanism of the metal materials in the molten nitrates of the heat storage system.From metal contact molten salts,the change of ions in molten salts and the solid products attached to metal surfaces point of view,to reveal the mechanism of metal corrosion in molten salt.According to the monitoring method of NOx in the air of the national standard,monitoring NOx emission from binary molten nitrate salts(Solar Salt)and ternary molten nitrate salts(Hitec)contacting with metal meterials at high temperature.Experimental results show that,with the increase of air humidity,the NOx emission of molten salt is increased.The amount of NOx released by Hitec in the process of heat storage is greater than solar salt;the order of the NOx emission of the molten salt contact with different materials is: quartz boat > corundum boat > platinum boat.The NOx emissions of solar salt and Hitec contact Ni base alloy,carbon steel,stainless steel increased with the increase of temperature;the amount of NOx emission reduced after molten salts contact the nickel base alloy treated at 500 ? for a long time.Thermodynamic analysis shows that,NOx derived from the reaction of molten nitrate salts with metal and container materials,K/NaNO3 can react with Al2O3 and SiO2 to generate NO and NO2 in the thermodynamics,react with metals(Fe,Cr,Ni,Mo,W)to generate NO;Mo,W react with K/NaNO3 to generate NO2 and react with the NaNO2 to form NO can occur in thermodynamics.The results of elemental analysis by atomic absorption spectrometry show that,after a certain period of storage,the metal elements Ni,Fe,Cr,Mo trace into the molten salt,the main reaction product between metal and molten salt is attached to metal surface in solid form;The content of NO2-in Solar salt samples increases with the increase of temperature,while the content of NO3-and NO2-in the Hitec samples have irregular changes with the temperature.Thermodynamic analysis shows that,in addition to the thermal decomposition of K/Na NO3 to formNO2-,it can also produce K/NaNO2 through reacting with metals,which makes the increase of NO2-in the sample of Solar salt contact metals;In addition to these two kinds ofreaction,the NaNO2 of the molten salt in Hitec can be oxidized by oxygen in air or involved in the reactions with metals.The results of several kinds of reactions will cause the irregular change of NO3-and NO2-content in molten salt.Short term corrosion test showed that the Ni based In625 and Has C276 formed a denser oxide film than the stainless steel 310/321 after contact with Hitec at high temperature,it shows that the metal ions are mainly attached to the surface of the metal in the form of solid oxide.The corrosion behavior of solar salt was studied by electrochemical method and weight loss method.The corrosion resistance of Has X was better than that of In600,Has X showed good corrosion resistance because it contains Mo elements.SEM(scanning electron microscopy),EDX(energy spectrum analysis)and XRD(X-ray diffraction)analysis showed that,with the corrosion time extended from 400 h to 2000 h,the metal surface gradually formed a dense oxide film,the corrosion products of NiO and Fe3O4,which are more compact and have good adhesion with metal matrix,have been produced,which can effectively slow down the corrosion reaction,reflected in the NOx emissions decreased by 12% ~ 34%.The research contents of this paper can provide the reference data for the safe operation of solar thermal power generation system and the environmental effects.