Corrosion of a Nickel-Based Alumina-Forming Alloy in Molten NaCl–MgCl2 at 600 °C For the Development of a Molten Salt Nuclear Reactor

Abstract

Molten chloride salts represent a very corrosive medium due to the amount of impurities they contain and that essentially comes from moisture. In this work, an industrial nickel-based alumina-forming alloy was preoxidized and corroded for 500 h in the NaCl–MgCl₂ eutectic. Electrochemistry and SEM analyses were used to prepare and analyse the corrosion test. Both the nickel-rich matrix and the alumina scale formed during preoxidation seemed to remain stable during the corrosion test contrary to some of the chromium carbides initially present in the columnar microstructure of the alloy. The use of X-ray tomography coupled with SEM observation revealed a preferential dissolution of the chromium carbides connected to the alloy/salt interface. X-ray tomography reveals a chromium carbides network enabling a deep molten salt infiltration within the alloy due to their preferential dissolution. Molten salt infiltration in the dissolved carbides network then leads to the oxidation of aluminium present in the alloy into a mixed MgAl₂O₄ spinel. An oxoacido-basic reaction between the alumina scale formed at the alloy surface during preoxidation and MgO dissolved in the salt is also discussed. This work shows that nickel-based alumina-forming alloy present a realistic interest and that the microstructure of the alloy should be optimized in further work to enhance corrosion resistance.