Study On High-temperature Corrosion Behavior Of Ultrafine Grained Nickel-based Alloy In Polychloride Molten Salt

In this paper,based on nickel base alloy（In625）,surface plastic deformed nickel base alloy（SM In625）and ultrafine grained nickel base alloy（SC In625）,the microstructure characterization and mechanical properties were tested by backscattered electron diffractometer（EBSD）and microtensile test.The electrochemical corrosion behavior of different nickel-based alloys in binary and ternary chloride molten salts was studied by open-circuit potential and potentiodynamic polarization,and the static corrosion rate of different nickel-based alloys in chloride molten salts was analyzed by weight loss method.Field emission electron microscopy/energy spectrum analysis（SEM/EDS）,laser confocal micro-Raman spectrometer（Raman）and field emission electron probe analyzer（EPMA）were used to characterize the surface and cross section morphology,and analyze the composition of corrosion products,and finally explore the high-temperature corrosion mechanism.The surface plastic rheological nickel base alloy（SM In625）was obtained by high speed impact and grinding of In625.The grain size decreased obviously,there was a lot of residual stress at the grain boundary,the yield strength and tensile strength increased obviously,but the elongation decreased obviously.After isothermal annealing at 900℃,the grain size was further reduced and the residual stress was eliminated,and the superfine crystal nickel-base alloy（SC In625）was obtained with high yield strength and tensile strength,as well as good tensile plasticity.According to the results of open-circuit potential and electrokinetic potential polarization,the corrosion resistance of SC In625 is the best and In625 is the worst in binary and ternary chloride molten salts,and the corrosion rate of the three alloys is lower in binary molten salts.According to the static corrosion results,the corrosion rate of SC In625 is the lowest and In625 is the highest before 168h corrosion of the two molten salts.This is because the surface plastic flow treatment results in finer grain size and more twins,and the nano twin layer has lower corrosion sensitivity and higher corrosion resistance.After 900℃isothermal annealing,the alloy has finer grain and stronger corrosion resistance.After 168h corrosion,the EPMA results of the sections of the three samples all showed the absence of Cr and Nielements.According to the depth and weight loss of the Cr-poor layer,the corrosion resistance of SC In625was the strongest and In625 was the worst,which was due to the long-term corrosion.SM In625 surface plastic flow layer and SC In625 surface ultra-fine crystal layer have been exhausted basically,SC In625 matrix after isothermal annealing more grain boundaries,intergranular corrosion is more serious.At the early corrosion stage,the corrosion products of the three alloys are Cr₂O₃and NiCr₂O₄in molten salt.In the later stage of corrosion,the corrosion products of binary molten salt before 168h are mainly NiO,and at 336h,Cr₂O₃and NiCr₂O₄are reformed.In terpolymer molten salt,due to the strong water absorption of Ca Cl₂,new Cr₂O₃and NiCr₂O₄oxide layers have been formed at 168h.