Study On The Corrosion Behaviors Of 316 Stainless Steel In Mixtures Of Molten Nitrate Salt

The heat storage medium is the core of solar thermal power conversion efficiency.Molten nitrate salt is a kind of mature heat transfer medium in solar thermal power generation system.Solar salt(60%KNO3-40%NaNO3)and HITECT(53%KNO3-7%NaNO3-40%NaNO2)has been widely used in solar thermal power generation system.Because molten nitrate salt is corrosive and easy to decompose at high temperature,the corrosion of salt storage pipe material in molten nitrate becomes an important problem in the development of solar thermal power generation technology.Material in a molten salt corrosion is mainly active metal is dissolved,and trace impurity and temperature are the main driving force for molten salt corrosion.The corrosion of the material in the molten nitrate salt is an intrinsically electrochemical process,and therefore can be investigated by some electrochemical techniques by which more information concerning the corrosion processes could be obtained.In this dissertation,the corrosion of 316 stainless steel in HITECT(53%KN03-7%NaN03-40%NaNO2)has been studied by electrochemical techniques.Chemical composition and morphology of the corrosion products were analyzed using SEM/EDS.The phase structure of the corrosion products and the molten salt were determined by XRD.The effects of adding rare earth elements,Cl-and temperature on the corrosion behavior were analyzed.The main results are as follows:1.The addition of Y2O3 can reduce the corrosion rate of 316 stainless steel.The corrosion current density decreased from 9.47mA·cm-2 to 7.13mA·cm-2 and 3.73mA·cm-2 respectively with the addition of 1wt‰ and 2wt‰ Y2O3 and correspondingly,Electrochemical impedance spectroscopy results show that the transfer resistance of 316 stainless steel in molten nitrate was enhanced.The addition of trace of rare earth element is an effective way to improve the corrosion resistance of 316 stainless steel in the molten nitrate.2.Presence of Cl-slowed down decomposition of ternary nitrate and increased corrosion rate of stainless steel markedly.The polarization curve obtained indicated that the corrosion current density increased from 3.02 mA.cm-2 to 8.76 mA.cm-2.Electrochemical impedance spectroscopy indicated a decrease in charge transfer resistance of 316 stainless steel in ternary nitrate containing NaCl,resulting in a decreased corrosion resistance.3.The corrosion current density decreased from 2.02mA·cm-2 to 8.09mA·cm-2 and 34.4mA·cm-2 respectively with the temperature rises from 450? to 600? and 680?.The transfer resistance of 316 stainless steel in molten nitrate was decreased.Indicating that the temperature rise will reduce the corrosion resistance of 316 stainless steel in molten nitrate salt.316 stainless steel corrosion from the morphology and composition can be seen,stainless steel in the ternary molten nitrate salt selective corrosion occurred,in the corrosion layer appears obvious element localized enrichment.The temperature rise promoted the decomposition of the ternary nitrate.So in the case of solar thermal storage to ensure the normal circumstances,a reasonable control of the temperature of molten nitrate salt work,help to reduce the corrosion of materials and molten salt decomposition.