Effects Of Dissimilar Material And Ion Valence On Corrosion Mechanism Of Structural Alloy In Molten Salts

Due to its excellent thermal conductivity,high volumetric heat capacity,and fairly good stability and almost no corrosion will happen in pure molten salt,however,impurities such as chemical stability,molten salts have been widely used in various energy fields,such as the nuclear reactor,high thermal storage,hydrogen production,fuel cells and nuclear fuel reprocessing.However,structural alloys are required to have good anti-corrosion properties when used in molten salts environment.Different materials are commonly used in molten salt system,for instance the alloy-graphite system.Because of the electronegativity differences,the graphite and alloy can be formed a galvanic couple,which would enhance the corrosion of alloy.The salts with high purity are chemical moisture,oxides,metallic constituents and oxyacid ions are always contained in molten salts and cannot be removed by H₂/HF purification.These impurities with oxidation may accelerate materials corrosion.Therefore,it is required to investigate the effect of dissimilar materials and molten impurities on corrosion behavior and corrosion mechanism of structural alloys in the molten salt environment.Based on this background,the static corrosion studies,high electrochemical tests and comprehensive analysis methods were undertaken to systematically study corrosion behavior and corrosion mechanism of alloy in molten salts,and the main contents are summarized and listed as follows:?1?The corrosion process of structural materials in molten salts is mainly controlled by the selective dissolution of Cr and Fe from metal matrix into salts,and the Cr-depletion layer is evidently formed on the alloy surface.In the alloy-graphite system,it was found that electric contact between alloy and graphite would enhance the corrosion of alloy.When the alloy and graphite was nonelectric and isolated,the average weight loss of GH 3535 alloy is about 0.41mg/cm² and the evident depth of Cr-depletion layer is around 20?m in FLiNaK salts at 700?.The main corrosion mechanism is attributed to the nonelectric transfer happened.Since the Cr is firstly oxidized on the alloy surface and then dissolved into salts in the form of Cr²⁺ion.Then,the Cr²⁺was subsequently disproportionated to Cr⁰ and Cr³⁺ion by the reaction of 3Cr²⁺?2Cr³⁺+Cr⁰,and the formed Cr⁰ can react with carbon?graphite?to form Cr₇C₃.Thus,the existence of graphite accelerates the preferred dissolution of Cr by forming carbides.However,when the alloy and graphite was electric contact,the average weight loss of GH 3535 alloy is 2.73 mg/cm²and the evident depth of Cr-depletion layer is around 30?m,which obviously enhance the corrosion rate of alloy.This can be explained by galvanic corrosion,which formed between alloy anode and graphite cathode due to their electronegativity differences.Two variation tendency can be detected with the exposure time increase.The corrosion of alloy increases rapidly in the first variation tendency,and then slows down and gradually reaches a plateau.The first rapid corrosion sage is mainly drived by the impurity.After the impurities depleted,the corrosion rate decreases and eventually reaches the stability between 200h and 400h.When the alloy and graphite was nonelectric and isolated in molten salts,the annual corrosion rate is 8?m/year,whereas the annual corrosion rate is 75?m/year in the system of alloy and graphite electric contact.And the corrosion kinetics curves follow a power function relationship between the mass changes and the exposure time.?2?Impurities such as total oxygen,oxygenated anions and metallic ions in salts can result in attack to alloy,among which the SO₄^2-would cause the intergranular corrosion of alloy and its concentration must be controlled at a low level.Oxide impurities can come from many sources in fluoride salt applications.An initial source of O^2-is often inevitable by production of salt fabrication due to the concentration of impurities in raw fluoride salt materials.The pyrohydrolysis process also produces O^2-in molten fluorides.Although O^2-is not oxidative towards metals and has no direct influence on the structural material corrosion,it can have an indirect effect through reactions with corrosion products.In this part,the corrosion performance of 316SS in FLiNaK with and without the additive O^2-was investigated.EIS measurements showed that the R_ct?charge transfer resistance?has raised about 10 times and the the calculated capacitance C_B decreased 8times after 1000 ppm of O^2-was added into FLiNaK salt,indicating that the dissolution of 316SS can be alleviated by the additive O^2-.Through the analysis of ICP-OES,SEM/EDS,EMPA,it was observed that 316SS suffered corrosion attack in original FLiNaK salt at 650 ^oC.The most prevalent corrosion products on the surface of 316SS corresponded to Cr₂O₃ and Fe₃O₄,which were detected by both Raman spectroscopy and XRD.With the addition of 1000 ppm of O^2-to the FLiNaK salt,the corrosion product changed to LiCrO₂,which could protect the alloy constituents from diffusion into the salt.?3?The impurity iron is one of the common oxidizing metallic ions in molten salts.The electrochemical methods including cyclic voltammetry?CV?and square wave voltammetry?SWV?are applied to ascertain the stable valence states of Fe in molten salts,and the results show that both Fe???and Fe???can stably existed in molten FLiNaK salts.The Fe???species is reduced to Fe through one step with two electrons transferred.Whereas the reduction of Fe???proceeds through two steps:the Fe???is initially reduced to Fe???at-0.15V,followed by subsequent reduction to Fe at-0.58V.But the dissolution of Fe metal in FLiNaK salt is in the form of Fe???analyzed by electrochemical analysis.When the Fe???was contained in molten salts,it was found that the Cr composition of alloy will react with Fe ion through the reaction of Fe²⁺+Cr?Cr²⁺+Fe.Thus the Fe metal will accumulate on the surface of the alloy and then diffuse into the alloy bulk.While the Cr can be simultaneously converted into ionic state and gradually diffused from alloy matrix to alloy surface.The diffusion of Cr/Fe in alloy is provided by the grain boundary analyzed by SEM-EDS.