Study On Diffusion Barriers Against Interdiffusion Of Ni/GH3535 System In Molten Fluoride Environment

Molten salt reactor(MSR)is the only liquid fuel reactor of the Generation-?nuclear reactors,which uses molten fluoride salts as the carrier of the fuel and the coolant of the reactor.It has many advantages,such as sustainable utilization of nuclear fuel,high inherent safety,high thermal conversion efficiency,and less nuclear waste.However,the molten salts have strong corrosiveness to the structure materials used in the loop pipe and the core container of the MSR at high temperature,which restricts the development of MSR.Ni-based alloys,as one of the candidate structural materials,have good corrosion resistance to molten salts,but it still can not meet the use requirements of molten salt reactor.Therefore,the high temperature molten salt corrosion resistance of Ni-based alloys needs to be improved.In this dissertation,the Ni coating,as the molten salts corrosion resistant layer,was prepared on the surface of GH3535 alloy by electroplating and the corrosion behaviors of GH3535 alloy and Ni/GH3535 system were studied in a eutectic(Li,Na,K)F melt at 700?.Furthermore,CrN,TiN,AlN,Ta,Ni8Ta and Ni-Re interlayers were prepared and their effects on the interdiffusion behavior of Ni/GH3535 system had also been investigated.The results were summarized as following:1.Corrosion behavior of GH3535 alloy is characterized by the fast selective dissolution of Cr and Mo in molten(Li,Na,K)F salt at 700?.Applying a Ni coating on the alloy surface can significantly improve the corrosion resistance of it.However,Cr and Fe in the underlying alloy can diffuse to the surface of the Ni coating and dissolve in the molten salt during long-term service.In addition,the interdiffusion between the coating and substrate causes the formation of Kirkendall voids at the interface between them,which degrades the protective performance of the coating.2.The CrN,AIN and TiN ceramic diffusion barriers(DB)have excellent thermal stability in the Ni/GH3535 system,which can effectively inhibit the diffusion of Cr and Fe from substrate alloy into the coating.Besides,Ni/DB composite coating has better corrosion resistance than that of Ni coating in molten(Li,Na,K)F salt at 700?.Moreover,the adhesion strength between AlN layer and nickel coating can be significantly enhanced through mechanical interlocking effect.The penetration of nickel metal into AlN coating improves the ductility of the AlN coating and effectively prevents the coating from cracking due to thermal expansion mismatch.In conclusion,AlN coating,as the barrier layer in Ni/GH3535 system,has the best comprehensive performance among the three nitride ceramic coatings.3.The Ta diffusion barrier can effectively inhibit the outward diffusion of Cr and Fe in the alloy,because a diffusion blocking Ni-Ta intermetallic compound is formed by the reacting of Ta with Ni in the nickel coating or base alloy.However,the diffusion of Ni from the base alloy into Ta diffusion barrier caused the formation of voids at the coating/substrate interface,which will reduce the spallation resistance of the coating.On the other hand,Ni8Ta diffusion barrier can restrain the outward diffusion of the elements in the base alloy,and can prevent the formation of the pock-marked area at the coating/substrate interface.4.The thermal stability of Ni-Re diffusion barrier prepared by electroplating is poor,thus it cannot effectively abate the outward diffusion of Cr and Fe from GH3535 alloy.