Currentless Deposition of Niobium-Based Protective Coatings for Application in Molten Salts

Abstract

Molten salts, particularly alkali halides, can be used for developing new methods of rare and refractory metals production and pyrochemical reprocessing spent nuclear fuels. Molten salt nuclear reactor now again attracts attention as one of the prospective power generating systems. Wide scale application of fused salts in various areas of technology is hampered by the problem of finding suitable corrosion-resistant materials. Corrosion of metals in molten salts is electrochemical in nature, and therefore the most suitable constructive materials for these media are electropositive refractory metals, for example, niobium. High cost of these materials and the difficulties associated with the mechanical processing and welding limit their application. A way to overcome this problem is forming a protective layer of a refractory metal on the base material. In the present work a novel method of forming niobium-based coatings by currentless deposition in molten salts is proposed. Nicrofer 6020, Nicrofer 6616, Hasteloy N, and KhN65MVU nickel based alloys and metallic nickel were chosen as the base materials for the present investigation. Corrosion tests were carried out in NaCl–KCl–NbCln (n=3.5 or 5 wt. % niobium) and NaCl–KCl–UCl3 (1 wt. % uranium) melts at 750 oC. Duration of the corrosion tests was 30 h. Contacting nickel based alloys with NaCl–KCl–NbCln melt leads to increase of the sample mass due to the formation of an alloy between nickel (the base constituent of the studied alloys) and niobium on the sample surface, Figure. Nickel-niobium alloy is formed as a result of disproportionation of niobium (III) ions, 4Nb3+ + 3Nialloy → Ni3Nb + 3Nb4+. Nb(IV) ions produced in the same reaction can be reduced to Nb(III) by niobium metal placed in the melt. Formation of a Ni-Nb alloy layer on the sample surface decreases the rate of the base alloy corrosion. This Ni-Nb phase forms a continuous layer on the sample’s surface and, therefore, this phase can protect the studied alloy from further corrosion. Such protection would be very useful for construction materials used for niobium electrorefining in molten chlorides; especially since currently the problem of finding suitable materials is one of the limiting factors in producing high-purity niobium. The samples of Nicrofer 6616 and 6020 alloys with the formed niobium coatings were tested in NaCl–KCl–UCl3 (around 1 wt.% U) melts (30 h exposure at 750 ºС). Here the weight of the samples covered with Ni3Nb phase decreased and the corrosion rates were comparable with those obtained earlier for uncoated samples in uranium-containing melts. Figure. Microstructure of Hastelloy N (a), KhN65MVU (b), Nicrofer 6616 (с) and Nicrofer 6020 (d) alloy samples after 30 h contact with NaCl–KCl–NbCln melts (n=3.5, 5 wt. % Nb) at 750 ºС. Figure 1