Coatings for alloys used in molten salt nuclear reactor

Abstract

Rising CO2 levels due to the production of energy from fossil fuels are major contributors to environmental pollution. Nuclear energy, on the other hand, is clean energy and does not contribute to environmental pollution. Conventional water-cooled reactors are safe; however, they still suffer the consequences of the Fukushima accident. A molten salt-cooled reactor, on the other hand, is intrinsically safe and more efficient. But the problem of corrosion of structural alloys in a molten salt environment is an obstacle to the success of Molten Salt Reactors (MSR). The purpose of this research is to analyze the corrosion behaviour of different alloys in a molten salts environment and develop coatings for inhibiting this corrosion. The alloys Hastelloy® N, Haynes® X-750, AISI 304, and AISI 316 were tested for corrosion in molten FLiNaK for 100 h at 700 °C under an argon gas cover. The Cr depletion was found to be the major cause of corrosion in the FLiNaK environment. The highest corrosion observed in the Haynes® X-750, is related to the presence of Al and Ti. However, Hastelloy® N is well protected against corrosion in the FLiNaK environment. Furthermore, AISI 316 was coated with Ni of varying thicknesses to see its corrosion resistance behaviour in FLiNaK at 700 °C. The results indicate thicker Ni coatings of 75 µm are sufficient to provide corrosion resistance to the stainless-steel samples under the testing conditions. But the chromium was still observed to be diffusing toward the Ni coating from the steel substrate and could eventually lead to corrosion during longer exposures to FLiNaK salt. The Ni coating was also modified with the addition of Mo to evaluate the corrosion resistance in the FLiNaK environment. The AISI 304 samples coated with Ni-Mo did not protect well against corrosion. The presence of Mo in the Ni-Mo coating was associated with the formation of carbides at the surface of the Ni-Mo coated sample, which led to accelerated corrosion. Finally, a SiC diffusion barrier was deposited between the Ni plating and the steel surface. The results obtained showed that this coating improved the corrosion resistance of the AISI 304 samples by five times.