Low melting temperatures and well-known physical properties make chloroalumninate melts prospective heat transfer media for the second loop of MSNFR. However, application of chloroaluminates in such technologies is limited by the problem of finding suitable corrosion resistant materials. In the present work the corrosion behavior several alloys from VDM Metals was studied at 450–650 oC in fused KCl-AlCl3 mixture with the initial AlCl3-to-KCl ratio of 1.1. One high-temperature alloy, VDM® Alloy 600 (Nicrofer 7216), and two corrosion-resistant alloys, VDM® Alloy C-4 (Nicrofer 6616) and VDM® Alloy 825 (Nicrofer 6020), were selected for the investigation. The corrosion rates of the studied materials were determined after exposure in the melt for different periods of time under high-purity argon atmosphere. Up to 550 oC all studied alloys were subjected to the slow frontal corrosion. Surface of the corroded alloy samples was depleted in chromium and iron. Chemical analysis of the quenched melts showed that chromium and iron species were the major corrosion products present in the salt phase. These facts indicate the electrochemical nature of the corrosion processes in molten chloroaluminates. At 650 oC the surface of the alloys after 100 h contact with KCl-AlCl3 melt was subjected to local and sometimes intergranual corrosion. Transmission electronic microscopy showed that secondary sigma-phases were formed in the alloys after prolonged contact with KCl-AlCl3 melt at 650 oC. The relationship between the corrosion rate and the amount of the excessive phases was determined. The extent if the secondary phase formation decreases in the following order: 600 &gt; 825 &gt; C-4. This result agrees well with thermodynamic analysis, thermochemical properties of the alloys, and constructed "time-temperature-precipitation" (TTP) diagram. However the time of precipitation of secondary phases according to the TTP-diagram should be longer than observed experimentally during the corrosion tests at the surface layer. A possible reason for this is a selective leaching of chromium from the samples' surface resulting in changes of the chemical composition of the alloys’ surface layer. The results obtained here allowed determining maximum working temperature and exposure time of the alloys thus providing the information on the limitation of their application in MSNFR technologies. The work was supported by the Government of the Russian Federation (Act 211, contract 02.A03.21.0006).

Nickel-based alloys from VDM Metals© for molten-salt, nuclear fast-reactor (MSNFR) applications

251st National Meeting (2016)

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