New Alloy for Application in Molten Chlorides: Design and Properties

Fused chlorides can be used in technologies of nuclear fuel reprocessing and as a working media for molten salt nuclear fast reactors (MSNFR). However, application of such technologies is limited by the problem of finding suitable corrosion resistant materials stable in contact with molten salts. Existing corrosion-resistant alloys are typically manufactured for application in aggressive aqueous-based media, whereas high-temperature alloys are mostly designed for application in aerospace industry where the mechanical strength under extreme conditions is required. On the basis of the comprehensive studies of corrosion resistance of different kinds of stainless steels and various nickel-based superalloys we concluded that a new alloy with «nickel – chromium - molybdenum» matrix is needed for molten chlorides application. In the present work the results of the new alloy fabrication process and detailed examination of the alloy’s properties are presented. The required levels of the basic component contents were determined on the basis of thermodynamic calculations (Thermocalc Software AB was used to simulate the phase content of new alloy) and previously obtained experimental data on metals and alloys corrosion in molten chlorides and sensibility of these materials to intergranular corrosion. Carbon content in the material should be maintained at a minimal level. The influence of other elements presence in the γ-phase austenite matrix was analyzed. The alloy of the required composition was cast in a vacuum induction furnace. A special 4-stage refining casting procedure was developed to provide the required carbon content. The obtained ingot was hot rolled into a strip. Mechanical and thermophysical properties, susceptibility to intergranular corrosion of the new nickel alloy with a low carbon content in «as received» conditions were investigated. The obtained results correlate well with the data for 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 maximal temperature and time of alloy’s operation in contact with molten salts. Corrosion resistance of the material was studied in molten KCl-AlCl 3 electrolytes at 450–650 °C. Chloroaluminates are prospective media for the second loop of MSNFR and acidic KCl-AlCl 3 electrolytes have extremely high aggressive properties among other chloride mixtures. To create 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. The samples were tested in «as received» conditions 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. 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 400 h in the bulk of the material. Formation of the secondary phases in the surface layer is caused by selective chromium leaching and degradation of nickel-based solid solution. Prolonged contact of the material with the high-temperature melt leads 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 o C and above) becomes undesirable due to possible development of integranular corrosion. Tests performed on welded and bent samples showed that their corrosion rates were higher that could be explained by changes in the structure of the alloy caused by high temperatures (in case of welds) and increased amounts of defects. Figure. Microstructure of the new alloy after 100 h exposure in KCl–AlCl 3 melt at 450 (a, b); 550 (c, d) and 650 °С (e, f). Figure 1