Investigation On The Corrosion Behavior And Mechanism Of Metallic Ni,Cr And Fe In Molten Chloride Salts

Molten chloride salts have been widely applied in electrochemical metallurgy,concentrated solar power,biomass pyrolysis and many other fields because of their beneficial characteristics,such as splendid reserve,low cost,wide operating temperature ranges and good heat transfer property,etc.In these widely used molten chloride salts,molten ZnCl₂,NaCl and KCl play important roles.Especially in the field of molten salt concentrated solar power generation,ZnCl₂-NaCl-KCl ternary molten salt is one of the most important candidate salts for heat transfer and thermal energy storage.However,these three kinds of molten chloride salts also have obvious disadvantage,that is,strong corrosiveness.The corrosion of metallic Ni,Cr and Fe,and of metal structural materials containing these metals by these three kinds of molten chloride salts seriously hindered their applications.Therefore,investigation on the corrosion behavior and mechanism of metallic Ni,Cr and Fe in molten ZnCl₂,NaCl and KCl is significant to understanding the behavior of product diffusion into molten salt in the electrochemical metallurgy process and enhancing the service performance of metal structural materials in molten salts,and then improving the applications of molten chloride salts.In addition,since the strong covalent ZnCl₂ and weak covalent NaCl-KCl have quite different properties,the corrosion behavior and mechanism of metallic Ni,Cr and Fe in molten ZnCl₂ and molten NaCl-KCl have been investigated in this dissertation,respectively.The results show:1.The corrosion resistance of metallic Ni,Cr and Fe blocks in molten ZnCl₂reduces in this order:Ni>Cr>Fe.The molten ZnCl₂ corroded metals became suspensions or solutions.In Ar gas,nanostructured ZnO coatings were obtained on the surfaces of corroded metallic Ni and Fe blocks.The ZnO coatings on the surfaces of metallic Ni block contained some Zn₅?OH?₈Cl₂ and the ZnO coatings on the surfaces of metallic Fe block contained some Zn₂OCl₂ and ZnFe₂O₄.In air,the molten ZnCl₂corroded metallic Cr and Fe blocks became suspensions,but the molten ZnCl₂corroded metallic Ni net became solutions containing Ni²⁺ions.In the temperature range of 370-470�C and time range of 0-3 h,for Cr,the suspended particles diffused into the molten salt evenly,and the particle sizes as well as element Cr concentrations grew over time and temperature;for Fe,the suspended particles kept floating up and aggregating on the salt-air interface,and the particle sizes as well as element Fe concentrations trended to decrease with temperature,while first increased?the increase time reduced with temperature?then decreased slowly over time.The underlying mechanisms of the corrosion of metals in molten ZnCl₂ can be attributed to chemical or electrochemical reactions among metal,ZnCl₂ and impurities.2.In Ar gas,the corrosion rates of metallic Ni,Cr and Fe blocks in molten NaCl-KCl at 750�C decrease in this order:Cr>Fe>Ni.Elements Cr and Fe in the molten salt only had a zero valence state.According to first-principles molecular dynamics simulation calculations,Cr atoms in the molten NaCl-KCl would aggregate to form small clusters.The zero-valence Cr in the molten salt could further react with graphite,and formed a 5?m thick conversion coating on the graphite surface.The main compositions of the coating are graphite,metallic Cr and some chromium carbides.The underlying mechanisms of metallic Cr and Fe blocks dissolving into the molten NaCl-KCl only had a zero valence state were ascribed to two simultaneous processes:1)Surface metal atoms diffused into the molten salts by physical diffusion;2)Metallic Cr and graphite make up a galvanic cell,and chromium ions on the surface of Cr anode diffused into the molten salts,and then reacted with F-center electrons released from graphite cathode in molten salt to form Cr atoms very quickly.In this dissertation,the corrosion behavior and mechanism of metallic Ni,Cr and Fe in strong covalent molten ZnCl₂ and weak covalent molten NaCl-KCl have been deeply respectively investigated.These works are not only helpful to understand the behavior of metal diffusion into molten salt in the electrochemical metallurgy process,but also the basis for studying the corrosion behavior and mechanism of metal structral materials with Ni,Cr and Fe as the main components in molten salts.These results are significant to the applications of molten salts in electrochemical metallurgy,concentrated solar power,biomass pyrolysis and many other fields.