Preparation And Properties Of Cr₂O₃/Ni Composite Coating On 316L Stainless Steel

Six types of nuclear reactors were put forward at the Fourth Generation Nuclear Reactor International Forum,among which molten salt reactor is the only liquid fuel reactor.Molten salt reactors have attracted worldwide attention due to their advantages of high safety,high heat conversion efficiency,and low nuclear waste.In molten salt reactor systems,molten fluoride salts are nuclear fuel carriers and coolants.Due to the strong corrosiveness of molten fluoride at high temperature,the corrosion of structural materials in molten fluoride has become one of the unavoidable problems in the application and development of molten salt reactor.Ni is a metal with excellent corrosion resistance to molten fluoride,Ni coating can significantly improve the corrosion resistance of Cr-containing alloys to molten fluoride,but it still cannot suppress the external diffusion of Cr and the formation of pores in the stainless-steel matrix in long-term operation.The ceramic diffusion barrier has certain advantages in inhibiting the interdiffusion of elements.Thereinto,the oxide ceramic materials have been successfully applied as a diffusion barrier in the field of high temperature coatings.Therefore,in this paper,the transition layer of Ni?NiO?was electrodeposited on316L stainless steel,then the pure Ni layer was also prepared by electrodeposition,and finally the Cr₂O₃/Ni composite coating was formed by high temperature oxidation.The inter-diffusion properties of Cr₂O₃ coating as a diffusion barrier to suppress the element diffusion between the matrix alloy and Ni coating were studied,the corrosion behavior of Cr₂O₃/Ni composite coating in molten salt at700?was also studied.The main results achieved in this article are as follows:?1?Ni?NiO?transition layer and outer Ni layer were prepared on the surface of 316L stainless steel substrate by pulse electrodeposition method.By adjusting the parameters of pulse power supply,the most suitable process conditions of Ni?NiO?transition layer were obtained,average current density 10 A·dm^-2,duty cycle 80%,positive duty cycle 60%,frequency 100 Hz,temperature 55°C,content of nano-NiO in the plating solution 50 g·L^-1,plating time 200 s.?2?Cr₂O₃ diffusion barrier was in-situ generated on the surface of 316L stainless steel substrate by vacuum high temperature oxidation of Ni?NiO?transition layer,forming the Cr₂O₃/Ni composite coating.The effect of heat treatment?such as time and temperature?on the growth of Cr₂O₃ diffusion barrier was studied,establishing a kinetic model of Cr₂O₃ diffusion growth,Under the heat treatment conditions of 900?and 8 h,continuous Cr₂O₃ diffusion barriers with an average thickness of 1?m that meet the experimental requirements can be obtained;and through the diffusion kinetics of Cr and other elements in a single Ni coating and Cr₂O₃/Ni composite coating,it is concluded that compared with a single Ni coating,Cr₂O₃/Ni composite coating can effectively prevent the external diffusion of Cr element in stainless steel.?3?The corrosion behavior of 316L stainless steel,Ni coating and Cr₂O₃/Ni composite coating in molten fluoride was studied by potentiodynamic polarization and electrochemical impedance spectroscopy.The results show that:compared with the substrate,the corrosion potential of the Cr₂O₃/Ni composite coating is more positive,the corrosion current density is reduced by more than five times,and the corrosion rate is reduced;electrochemical impedance spectroscopy shows that the Cr₂O₃/Ni composite coating has a better resistance to molten salt.