Evaluation of corrosion coupons exposed to molten-chloride electrorefiner salts long term

Corrosion mitigation has long been a challenge for the long-term service of both pyrochemical reprocessing and molten salt reactor applications. This work analyzed corrosion coupons made of 2.25Cr-1Mo, American Society of Mechanical Engineers Steel Alloy 387 (ASME SA387), Grade 22, Class 2 steel, that were installed in a uranium electrorefiner containing molten chloride salt at 500 °C. Seven of these coupons were removed in 2019 (i.e., after having been submerged in molten salt for 25 years) for analysis and characterization using neutron imaging, optical microscopy, scanning electron microscopy, and wave/energy dispersive x-ray spectroscopy. The results showed that the surface of all seven coupons degraded between 30 and 175 µm without evidence of brittle fracture. There was no evidence of accelerated corrosion or fractures, and the average steel surface loss was <5 µm per year. These results indicate that the environmental conditions inside the electrorefiner were favorable for the steel alloy, making these environmental conditions the primary driver for low corrosion.