Study On Corrosion Behavior Of Wear Resistant Alloy In High Temperature Fluoride Molten Salt

Fluoride molten salt has a series of superior physical and chemical properties such as high thermal conductivity,large specific heat capacity,small viscosity,low saturated vapor pressure,small neutron absorption cross section and high thermal stability.Therefore,as a functional material,it has a wide application prospect in the fields of nuclear energy,metallurgy and chemical industry.In the field of nuclear energy,fluoride molten salt is used as the fuel carrier and coolant for molten salt reactors.Since fluoride molten salt is highly corrosive at high temperatures,in addition to excellent high temperature mechanical properties,neutron radiation resistance and good processability,structural materials for molten salt reactors also need to have excellent molten salt corrosion resistance.GH3535 alloy has excellent resistance to molten salt corrosion,so it is widely used as the main structural material in the manufacture of the pipe,core vessel,and heat exchanger in molten salt reactors.However,special components such as the molten salt pump bearing in the reactor and the valve seat of the molten salt mechanical valve not only need to have excellent molten salt corrosion resistance.Therefore,these components often need to use wear-resistant alloy materials.At present,little research has been done on the compatibility of such alloys with fluoride molten salts.To screen wear-resistant alloys suitable for molten salt reactor,the corrosion behavior of different wear-resistant alloys in fluoride molten salt environment was studied and compared in this paper,the related corrosion mechanism was systematically studied and the possible improvement measures were explored,which provided a foundation for the follow-up research and the application of wear-resistant alloys in molten salt reactors.The main research contents and conclusions of this paper are summarized as follows:The corrosion properties of three Stellite alloys（Stellite4,Stellite6,and Stellite12）in FLiNaK molten salt were evaluated in graphite crucibles using a static corrosion method.The study found that after corrosion in FLiNaK molten salt at 700°C for 100h,the Cr in the three Stellite alloys is selectively lost along the dendrite boundaries,and the loss depths are more than 50μm,which leads to the local decomposition of the interdendritic carbides.Since the molten salt corrosion resistance of the three Stellite alloys selected in this study is unsatisfactory,it is far weaker than the nickel-based alloys commonly used in molten salt reactors.Corrosion properties of T-400 alloy and its compatibility with GH3535 alloy in FLiNaK molten salt were investigated.The study shows that the T-400 alloy suffers from non-uniform corrosion and the Cr-enriched alloy matrix is more vulnerable to the corrosion than the Mo-enriched Laves phase.The major elements dissolved from T-400 into salts are Cr and Co,whereas Mo is conversely enriched in the near surface region of the alloy after corrosion.When T-400 alloy and GH3535 alloy coexist in the FLiNaK molten salt,the presence of GH3535promotes the corrosion of T-400 by a galvanic corrosion mechanism and induces a mass transfer of Co from T-400 to GH3535 through the molten salt.The carbon from the graphite crucible transports to the surface of T-400 and forms Mo enriched carbides.The Cr-enriched matrix in the T-400 specimen is more susceptible to corrosion than the Mo-enriched Laves phase,so the Cr content in T-400 alloy was reduced next,and the corrosion mechanism of T-400 alloy with low Cr content in FLiNaK molten salt was studied.It is found that the non-uniform H₂O distribution and alloy microstructure cause the non-uniform corrosion of the alloy,showing pitted and roughened surface morphologies in different regions.The pitted morphology is a result of the preferential corrosion of Mo-enriched Laves phase particles induced by H₂O,whereas the roughened morphology is a result of Co₃Mo particles and Co layers formation induced by secondary corrosion.In addition,carbon from the graphite crucible transfers to the alloy surface and diffuses inward,causing carburization to a depth of 15μm.Therefore,T-400 alloy is not recommended for a typical MSR environment where GH3535 alloy and graphite exist extensively.The corrosion behavior of WC-Ni cemented carbide in FLiNaK molten salt and corresponding salt vapor at 700℃was studied.The results show that the microstructure and hardness of WC-Ni cemented carbide are almost unaffected by the corrosion,and the mass and alloy composition change little,indicating that it has good compatibility with fluoride salt.The WC-Ni cemented carbide immersed in FLiNaK molten salt still lost Cr and formed Ni and Ni₅P₂ particles on the surface due to direct corrosion caused by initial impurities H₂O and P and secondary corrosion caused by corrosion products such as Ni F₂ and WF₆.The oxide impurities in molten salt and steam lead to the formation of flake Cr₂O₃ oxidation products on the surface of WC-Ni cemented carbide in salt steam,and the K₂Na Cr F₆ complex is formed on the surface of the sample by the interaction of the salt steam.As the reaction continues,the initial impurity content in molten salt is gradually exhausted,and the loss rate of Cr in the molten salt soaked sample and the amount of Cr₂O₃ in the sample placed in steam decrease accordingly.WC-Ni cemented carbide is expected to be used in molten salt reactor environment,but the impact of initial impurities in molten salt on materials is still worth paying attention to,so it is necessary to strictly control the amount of trace impurities introduced in the production,transportation and preservation of molten salt.