Ruslan R. Alimgulov

A protective coating deposited by high-velocity oxygen fuel (HVOF) spraying is used to improve the corrosion resistance of structural materials in molten salts. Austenitic 12Kh18N10T stainless steel and a KhN62M-VI nickel alloy are studied. A self-fluxing nickel-based PG-12N-01 powder is a base material for an HVOF coating, which was subjected to additional remelting with an acetylene torch to decrease porosity. Corrosion studies are conducted under static conditions in an FLiNaK melt at 650°C for 100 h. The corrosion rates were calculated using gravimetric and chemical methods. The results of corrosion tests demonstrate that the coating significantly decreases the corrosion rate of 12Kh18N10T steel, from 288–294 to 85–93 μm/year, and the effect for the KhN62M-VI alloy is less pronounced, from 85–91 to 76–79 μm/year. Microstructural analysis demonstrates that the coatings are dense and homogeneous, through porosity is absent, and defects are rare; however, local signs of selective dissolution of individual phases are detected. The obtained results confirm the feasibility of using nickel-based HVOF coatings for protecting materials in the FLiNaK melt, and further investigations should be aimed at optimizing the phase composition of the resulting composite material and at reducing the susceptibility of the coating to selective corrosion.

Corrosion of nickel alloys in molten salts is a complex process dependent on many factors. The paper describes the influence of microstructural changes in several nickel-based alloys (Hastelloy^® G-35^®, VDM^® Alloy 59, KhN62M-VI, Hastelloy^® B-3^®) on the mechanism of their corrosion in molten fluoride salts. The corrosion experiments were performed in LiF–NaF–KF and (LiF–NaF–KF) + UF₄ melts at 550–750 °C. Formation of excess secondary phases along the grain boundaries of the alloys led to heterogeneity of the alloy microstructure. As a result of these changes, microgalvanic couples formed on the surface of the alloys, leading to the development of intergranular corrosion upon contact with molten electrolytes. Secondary phases acted as microanodes or microcathodes depending on a number of factors. Formation and composition of the secondary phases affected the depth and extent of the corrosion damage to nickel alloys.

The corrosion resistance of several types of steel (AISI 410, 321, 316L, 904L) was determined in a liquid Bi-Li (5 mol.%) alloy (BLA) medium at 650 °C combining gravimetric analysis of steel samples and chemical analysis of corrosion products’ content accumulating in the BLA phase. Energy dispersive X-ray spectrometry, scanning electron microscopy, X-ray fluorescence and inductively coupled plasma–atomic emission spectrometry analysis were employed for characterizing steel structure and alloy composition. AISI 321, 316L and 904L nickel-containing corrosion-resistant steels underwent severe corrosion in BLA, and their corrosion rates depended on the nickel content in the material. AISI 410 steel exhibited the lowest corrosion rate of all the materials investigated, and this type of steel can be considered as a reasonable structural material for work in BLA environments. The corrosion rates of AISI 410, 321, 316L and 904L steels in BLA at 650 °C were 77, 244, 252 and 280 µm/year, respectively. It was also found that chromium was etched more intensively than iron from the surface of steel samples.

Bismuth–lithium alloys are considered primary candidates for the reductive extraction step in the on-line reprocessing of molten salt reactor fuel. The corrosion behavior of molybdenum-based alloys and Hastelloy^® B-3 alloy (taken for comparison) was examined here in a liquid Bi–Li (5 mol.%) alloy at 650 °C. MoW10, MoW30, and TZM corrosion-resistant alloys were studied as prospective construction materials for holding liquid bismuth–lithium alloy. Rates of corrosion were determined by the gravimetric method as well as by chemical analysis of corrosion products formed in liquid-phase Bi–Li alloy. The microstructure and chemical composition of samples of the materials and Bi–Li alloys containing the corrosion products after the tests were determined using inductively coupled plasma–atomic emission spectroscopy, X-ray fluorescence analysis, scanning electron microscopy, and energy dispersive spectroscopy. TZM molybdenum-based alloy corrodes in the bismuth-lithium alloy due to the formation of a zirconium–bismuth intermetallic compound, which passes into the liquid phase. The corrosion rates of MoW10, MoW30, and TZM alloys at 650 °C were 16, 16, and 23 µm/year, respectively. Hastelloy^® B-3 alloy, despite its high molybdenum content, was subjected to severe corrosion in liquid Bi–Li alloys due to dissolution of nickel in liquid bismuth. The corrosion rate of this alloy was 14 mm/year.

Bismuth–lithium alloys are considered as primary candidates for the reductive extraction step of on-line reprocessing of a molten salt reactor fuel. The corrosion behavior of pure metals and nickel-based alloys was studied in a liquid Bi–Li (5 mol.%) alloy at 650 °C. The tantalum, molybdenum, and corrosion-resistant alloys VDM^® Alloy C-4, Hastelloy^® G-35^®, KhN62M, VDM^® Alloy 59 were studied as prospective materials for this liquid metal media. The corrosion rates were determined by gravimetric method as well as chemical analysis of corrosion products in Bi–Li alloy. Microstructure and chemical composition of samples of the materials and Bi–Li alloys containing the corrosion products after the tests were evaluated using inductively coupled plasma–atomic emission spectroscopy, X-ray fluorescence analysis, scanning electron microscopy, and energy dispersive spectroscopy. Metallic tantalum and molybdenum do not chemically interact with liquid Bi–Li alloy; the corrosion rate of these metals is determined only by the solubility in this medium. The corrosion rates of Ta and Mo at 650 °C were 0.09 and 0.07 mm/year, respectively. Nickel alloys are subjected to severe corrosion in liquid Bi–Li alloys due to dissolution of nickel in liquid bismuth. Alloys of this type cannot be used in such an environment.

Corrosion resistance of molybdenum, tantalum, and nickel-based alloys was studied in molten salt systems based on LiF–NaF–KF mixture in the temperature range 550–750°C. The rates of corrosion of these materials were determined, the nature of the degradation of the surface of materials established, and the effect of the salt medium and temperature on the corrosive properties of metals and nickel-based alloys considered.

Corrosion resistance of ceramic and carbon materials was studied in molten salt systems based on LiF–NaF–KF mixture in the temperature range 550–750°C. The rates of corrosion of silicon nitride, boron nitride, carbon-carbon composites, and high density graphite in the fluoride electrolyte were determined. The mechanisms of materials degradation after the contact with molten FLiNaK are proposed, and the effect of the salt media and the temperature on the mechanical properties of ceramic and carbon materials is considered.

The corrosion of various metallic materials was investigated in 3LiCl–2KCl and 3LiCl–2KCl–UCl₃ (5 wt. % U) melts under inert atmosphere. Metallic tantalum and molybdenum, nickel-chromium-molybdenum alloys (VDM® Alloy C-4, Hastelloy® G-35®, and specially designed KhN62M alloy) were investigated. The corrosion tests run for 100 h, and the temperature was varied from 450 to 750°C. The corrosion rates were determined by gravimetric technique and chemical analysis of quenched melts.

Corrosion resistance of VDM Alloy С-4, Hastelloy G-35 and ХН62М (KhN62M) nickel based alloys, and AISI316L type steel was studied in alkali chloride melts containing uranium (III) chloride and metallic zinc. These alloys are considered as prospective construction materials in pyrochemical reprocessing of spent nitride nuclear fuels including decladding fuel rods by liquid zinc dissolution process. Rates of corrosion of these materials were determined at 550 °C.