Corrosion of Corrosion-Resistant and High-Temperature Nickel-Based Alloys in Chloroaluminate Melts

Molten salt fast neutron nuclear reactors represent a new generation of safe energy producing systems that can burn both uranium and thorium fuel. However the industrial use of such type of nuclear reactors is limited by several problems. The most difficult and important task for the molten salt reactor concept development is development of new construction materials or selection/improvement of existing alloys and steels. The required materials must have corrosion resistance at relatively high temperatures during the contact with aggressive molten halide electrolytes under neutron irradiation.

Chloroalumninate melts are prospective media for the second loop of molten salt nuclear fast reactor due to their low melting temperatures and well-known physical properties. Application of chloroaluminates in such technologies is however hindered by the problem of finding corrosion resistant materials.

In the present work the corrosion behavior of Haynes 230, Hastelloy S and X high-temperature alloys and Hastelloy N, B-3, G-35, C-2000 corrosion-resistant alloys was studied in fused KCl-AlCl₃ mixtures. Corrosion behavior of studied materials was investigated in a wide temperature range (450–650 °C). The initial ratio of AlCl₃-to-KCl was equal to 1.1.

Gravimetric measurements (comparison of sample mass before and after the experiment) served as a basis for estimation of corrosion resistance of the studied materials. Quenched melt samples taken after each experiment were analyzed to determine the content of the elements of interest using ICP AES method (Optima 2100DV, Perkin Elmer). The surface of the alloy samples after corrosion tests was characterised using metallographic analysis (Olympus GX-71F), scanning electronic microscopy and Х-ray microanalysis (SEM Auriga with Oxford Inca attachment). To determine the composition of excessive phases formed at the grain boundaries of the studied alloys the foils of corroded materials were examined by transmission electronic microscopy (JEM 2100).

Metallographic analysis showed that the nature of corrosion varies for different types of alloys. The surface of the corrosion-resistant alloys (Hastelloy N, B-3, G-35, C-2000) after 100 h contact with KCl-AlCl₃melt was subjected to uniform corrosion, Fig. 1a. Intergranular corrosion was detected for all types of high-temperature alloys (Haynes 230, Hastelloy S and X) after 100 h exposure in KCl-AlCl₃ melt at 550 °C, Fig. 1b. Chemical analysis of the salt phase showed that chromium and iron species were the major corrosion products of all the alloys studied. This fact indicates the electrochemical nature of corrosion processes in molten chloroaluminates.

It is known that during the contact with high temperature melts the materials can be subjected to structural changes. For example, metallographic analysis revealed that secondary excess phases were formed along the grain boundaries of the studied high-temperature alloys. X-ray microanalysis of high-temperature alloy samples showed that chromium concentration near the grain boundaries decreased but within the grain boundaries Cr content greatly increased. This indicates the formation of chromium-containing carbide phases along the grain boundaries of such alloys. This process occurs mainly as a result of temperature-induced degeneration of initially segregated chromium carbides. Therefore heat resistant alloys with carbide hardening cannot be used as construction materials for molten salt nuclear reactors.

The corrosion rates of the corrosion resistant nickel-based alloys are determined by the red-ox processes resulting in dissolving the most electronegative alloy components (chromium, iron, manganese). The increase of temperature leads to noticeable increase of the corrosion rates. For example, the following results were obtained for Hastelloy B-3 corrosion resistance, g/(m²∙h): 0.05 at 450 °С; 0.09 at 550 °С; and 0.33 at 650 °С. Transmission electronic spectroscopy revealed that intermetallic phases (such as sigma-phase in case of Hastelloy G-35) along grain boundaries can be formed during long high-temperature exposure at 650 ºС. These phenomena can accelerate the processes of intergranular corrosion and stress corrosion cracking of studied materials in industrial conditions. The further investigation is required to determine the limits of corrosion resistance of nickel-based alloys at temperatures above 550 °С.