New Industrial Alloy for Molten Salt Application: Design and Properties

Abstract

Molten salts (MS) can be used for nuclear fuel reprocessing (NFR) and as a working media in molten salt nuclear reactors (MSNR). However, application of MS in NFR and MSNR technologies is limited by the problem of finding suitable corrosion resistance materials. In the present work the results of examining structure and properties of a new prospective alloy are presented. Existing corrosion-resistant alloys are normally manufactured for an intended application in aggressive aqueous-based media, whereas high-temperature alloys are mostly designed for aerospace industry where high mechanical strength under extreme conditions is required. On the basis of the comprehensive studies of corrosion resistance of different types of stainless steels and nickel-based superalloys we concluded that a new alloy with a “nickel–chromium–molybdenum” matrix is required for molten salts applications. In the present work the results of the new alloy fabrication process and detailed examination of the alloy’s properties are presented. The requirements for the alloy composition were determined on the basis of thermodynamic calculations (Thermocalc Software AB was used to simulate the phase content) and previously obtained experimental data on metals and alloys corrosion in molten chlorides and sensibility of these materials to the intergranular corrosion. Carbon content in the material should be maintained at a minimal level. The influence of other elements presence on stability of the γ-phase austenite matrix was analyzed. The alloy of the required composition was produced by induction vacuum melting in several stages. First a low carbon Ni–Mo master-alloy was manufactured. Then this master alloy was further melted and chromium and microelements added. Sheet, tube, rod and forged samples were fabricated. Mechanical and thermophysical properties, susceptibility to the intergranular corrosion of this new nickel alloy in the “as received” state were investigated. The results obtained correlated well with the properties of typical nickel-based superalloys with austenite structure. In a special series of experiments thermodynamic stability of the fabricated alloy was studied. No formation of carbide phases was observed. Phase structure of the excessive intermetallic phases was characterized. The constructed “time – temperature – precipitation” diagram allowed to determine maximum temperature and time of alloy’s operation in contact with molten salts. The phase stability of the alloy designed exceeded that of a variety of other nickel-based superalloys. The corrosion resistance of the material was studied in molten KCl-AlCl3 and 3LiCl-2KCl electrolytes at 450–750 °C. Chloroaluminates are prospective media for the second loop of MSNFR and acidic KCl-AlCl3 electrolytes are very corrosive compare to other chloride mixtures. From the other hand, 3LiCl-2KCl system is a prospective medium for NFR technologies. To model the worst possible conditions for the alloy, the electrolytes containing excess aluminum chloride were selected for the present investigation. Time of exposure varied from 6 to 1000 h, and the samples were tested in the “as received” state as well as after typical technological manipulations (bending, welding, heat treatment, etc.). Metallographic analysis showed that the alloy was subjected to slow frontal corrosion up to 550 ºC (Figure). Experimentally determined corrosion rates were below 50μm/year depending on the melt composition. The surface of the corroded samples was slightly depleted in chromium. Formation of secondary phases was not observed. Excessive sigma phases along the grain boundaries were formed after 100 h exposure at 650 ºC in the surface layer and after 512 h in the bulk of the material. Formation of the secondary phases in the surface layer was caused by selective chromium leaching and degradation of the nickel-based solid solution. Prolonged contact of the material with the high-temperature melt led to unavoidable decomposition of thermodynamically metastable nickel-based austenite. Such behavior correlates very well with the constructed “time – temperature – precipitation” diagram. Using the alloy at such temperatures (650 oC and above) becomes undesirable due to possible development of integranular corrosion. Figure. Microstructure of the new alloy after 100 h exposure in KCl–AlCl3 melt at 450 (a, b); 550 (c, d) and 650 °С (e, f) Figure 1