Study On The Preparation And Corrosion Behavior Of Corrosion Resistant Coatings On Hastelloy N Alloy In Molten Salt Reactor

Molten salt reactor?MSR?is is one of the Generation?reactors,which has good incomparable advantages in terms of thermal conversion efficiency,inherent safety and sustainable utilization of nuclear fuel and non-proliferation.Currently,Hastelloy N alloy is chosen as the main structural material in MSR,however,the property requirement of the structural materials are very harsh for high-temperature and strong-corrosion MSR.Therefore,it is one of the most important problems to improve the corrosion resistance of Hastelloy N alloy in high temperature fluoride molten salt,in order to ensure the long-term stability of this new candidate of metal structure material in MSR.In this paper,the surface coating technology has been utilized to prepare the corrosion resistance coating on the surface of Hastelloy N alloy.On the one hand,NiCoCrAlY and pure metal?Ni,Co?coatings were prepared by the laser cladding technique,On the other hand,the pure Ni coating was prepared by the electrodeposition method,in order to obtain ideal coating with high temperature corrosion resistance to molten fluoride salt.The static corrosion experiment was carried out by using the self-made corrosion device.The phase composition,microstructure,corrosion behavior and corrosion mechanism of Hastelloy N substrate and coating were analyzed by XRD,SEM and weight loss corrosion method,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 coatings on Hastelloy N alloy for molten salt reactor.The main conclusions are as follows:?1?The surface of the laser cladding NiCoCrAlY coating is smooth,dense and uniform,and the coating shows good metallurgical bonding,without pores,cracks and other defects.The NiCoCrAl Y coating is mainly composed of?-Ni,Al_0.983Cr_0.017,AlNi₃ before corrosion,and only?-Ni phase exists after corrosion.The substrate is composed of?-Ni and Mo,and the Cr_1.12Ni_2.88 phase is precipitated after corrosion.Both NiCoCrAl Y coating and substrate are continuously corroded in molten salt,and the corrosion weight loss is 0.3257 mg/mm² and 0.1512 mg/mm²,respectively.?2?The surface of the laser cladding pure metal?Ni,Co?coating is flat,uniform and dense,without cracks,pores and other defects.The pure Ni coating is mainly composed of?-Ni,NiO,Cr₂O₃ phase before corrosion,and only?-Ni phase exists after corrosion.The pure Co coating is mainly composed of?-Co and Cr₂O₃ phase before corrosion,and only?-Co phase exists after corrosion.The corrosion weight loss of pure Ni and pure Co coating is 0.1263 mg/mm² and 0.1529 mg/mm²,respectively.?3?The coating quality of the electrodeposited pure Ni coating is of good and smooth surface,showing a bright silver metallic luster,and the microstructure is uniform and compact,without pores,cracks and peeling.The binding force of coating/substrate of electrodeposited pure Ni coating is enhanced after a heat treatment.The original pure Ni coating is composed of?-Ni and Cr₂Ni₃,and the Cr₂Ni₃ is transformed into Cr_1.12Ni_2.88 phase after corrosion.However,the pure Ni coatings after heat treatment were only containing?-Ni phase before and after corrosion.The corrosion weight loss of the original pure Ni coating and the heat treated samples is 0.337 mg/mm² and 0.169 mg/mm²,respectively.?4?The corrosion behavior of Hastelloy N substrate is mainly intergranular corrosion,in which the selective dissolution corrosion of Cr elements occur along grain boundaries.All the coated samples show a slight uniform corrosion,minor of elements diffuse from the coating to the molten salt.The Al element in the NiCoCrAlY coating acts as a"consumable"to uniform diffuse out from the coating,and the formed corrosion products can hinder the diffusion of other elements in the coating to protect the substrate.The Ni or Co elements in the pure metal coating and the electroplated pure Ni coating are corrosion resistance in molten salt,and block the diffusion of matrix elements?mainly Cr?into molten salt.