| Molten salt reactor(MSR)is the only liquid fuel reactor of the candidate reactors in the advanced nuclear reactor of the Generation ?reactors,which has good advantages in terms of thermal conversion efficiency,inherent safety and sustainable utilization of nuclear fuel and non-proliferation.Hastelloy-N alloy with excellent comprehensive properties is generally recognized as the most suitable metal structural material for loops of MSR,however,the environment of high-temperature and strong-corrosion are very harsh for the structural material.Therefore,by applying protective coating on the surface of Hastelloy-N alloy to improve its corrosion resistance,it is one of the effective solutions to ensure the long-term stability of the structure material in MSR.In this paper,the pure Ni coating was prepared on the surface of Hastelloy-N alloy by DC electroplating technology and dipulse electroplating technology,and using the self-made corrosion device to carry out the static corrosion experiment.The phase composition,microstructure,corrosion behavior and surface microhardness of Hastelloy-N alloy substrate and nickel coating were characterized and analyzed by XRD,SEM,weight loss corrosion method and micro vickers hardness meter,etc.This work is aimed to provide experimental evidence and theoretical reference for the engineering application of the preparation of high temperature molten salt corrosion resistant coating on the surface of Hastelloy-N alloy for MSR.The main conclusions are as follows:(1)The the grains of nickel coating by DC electroplated are inhomogeneity,the thickness of the coating is only 30?m,the coating/substrate is well bonded,and there is a transition layer of 0.3 ?m at the interface between the coating and the substrate.After heat treatment,the grain size grew from 1~4?m to 5~7?m,the transition layer between coating and substrate disappeared,and the thickness of the coating increased to 33.5?m.In dipulse electroplating,with the increase of forward current density and forward period,the thickness of coating increased,and the microhardness increased first and then decreased.When the forward current density is 30A/dm2 and the forward period is 70 ms,the coating had the maximum hardness(483.06HV).The growth mode of Ni coating is the process of nucleation and growth.(2)The Ni coating by dipulse electroplating is nanocrystalline,bright and dense,a large number of fine grains are polymerized together to form a larger cellular structure,and uniform distribution of coating thickness(135?m).After corrosion by molten salt,cracks appeared on the surface of the coating,but it did not fall off,and the substrate was not obviously corroded.The Ni coating after heat treatment,the size of nanocrystalline increased,the larger cellular structure of grain disappeared,the thickness of coating increased(140?m),and the bonding between coating and substrate became denser.After corrosion by molten salt,the coating shows a slight uniform corrosion characteristic,which effectively prevents the diffusion and dissolution of Cr elements into fluorine salt in the substrate.(3)The substrate of Hastelloy alloy was corroded by molten salt with typical intergranular corrosion characteristics.Before corrosion,the substrate was composed of ?-Ni and Mo phases.After corrosion,the Cr1.12Ni2.88 phase was newly precipitated.The corrosion weight loss was 0.0837mg/mm2 after corrosion by molten salt at 900 for 100 h.? The Ni coating of dipulse electroplated was mainly composed of ?-Ni and Cr2Ni3 phases before corrosion.After corrosion,Cr2Ni3 transformed into Cr1.12Ni2.88 phase,however,the Ni coating after heat treatment were only including ?-Ni phase before or after corrosion.The corrosion weight loss of the original and heat treated samples were 0.0532mg/mm2 and 0.0324 mg/mm2,respectively.The corrosion rate of the original sample is 3/5 of that of the substrate,while the corrosion rate of the heat treated sample is 2/5 of the substrate.The heat-treated Ni coating of dipulse electroplated is more effective to improve the corrosion resistance of the Hastelloy-N alloy to molten salt. |
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