Study On The Corrosion Mechanism Of Ternary Eutectic Nitrate Molten Salt On Three Kinds Of Stainless Steel

Solar thermal power generation technology is one of the key concerns in new energy and energy-producing technologies.This dissertation focues on the heat storage system with molten salts as heat storage materials.Invesitagation on the compatibility between the molten salts and heat storage tank is also carried out,such as the failure mechanism caused by corrosion of molten salt or molten salt composite on stainless steel tanks at high temperature.In this paper,a ternary salt of KNO₃-LiNO₃-Ca?NO₃?₂ and KNO₃-LiNO₃-Ca?NO₃?₂/10%EG composite were used as heat transfer and heat storage media.The static corrosion immersion test and electrochemical method were used to study the compatibility of 304SS,316SS and 316LSS with selected molten salt at 300°C and 500°C and the molten salt/EG composite at 300°C,to evalue their corrosion behavior and corrosion mechanism.The microstructure,cross-section and distribution of metal elements on the surface of the samples were observed by metallographic microscope and scanning electron microscope?SEM?combined with X-ray energy spectrometer?EDS?.The exact chemical composition of the corrosion products of the metal samples was determined by X-ray diffractometry?XRD?.The main results are summerized as following:1.The results showed that the corrosion of three stainless steels in the ternary molten salt of67.2wt%KNO₃-13.5wt%LiNO₃-19.3wt%Ca?NO₃?₂ at 500°C were severe.The surface topography was seriously damaged with an obvious high corrosion weight loss.The corrosion current density were large and the impedance value were small.More corrosion products were produced.The out layer of the sample showed porous structure,and some parts were fallen off.2.The corrosion damage of the three samples was weaker in the ternary salt of 67.2wt%KNO3-13.5wt%LiNO3-19.3wt%Ca?NO3?2 and 67.2wt%KNO3-13.5wt%LiNO3-19.3wt%Ca?NO3?2-10wt%EG composite at 300°C.The corrosion rate was low.Only a small amount of corrosion products were detected.The microscopic morphology showed that the corrosion layer had a slightly loose structure,and corrosion damage on the surface of the sample was not observed by the eyes.At the same temperature,the effects of the molten salt with and without EG on the corrosion damage of the same sample were similar,indicating that the addition of EG on the corrosion behavior of the molten salt during static corrosion was not obvious.3.Electrochemical tests showed that the Nyquist plot characteristic of the molten salt with EG were consistent with that of the molten salt without EG,indicating that the electrochemical principles of the two mixtures were the same.The corrosion process was controlled by charge transfer.However,the molten salt/EG composite showed totally different electrochemical properties with pure molten salt due to the electrical conductive characteristic of EG.4.The soluble chromate K2CrO4 was detected in 304 stainless steel in all three tested environments.The content of Cr on the surface of the sample was obviously reduced,therefore the corrosion was the worst among the three stainless steel samples,the followed by 316SS and316LSS.The corrosion resistance test of different stainless steel samples by the pure molten salt and molten salt/EG composite showed the application potential and safe operation of them in molten salt tank made from 316LSS.