Synthesis of plasma-nitrided Cr coatings on HT9 steel for advanced chemical barrier property in a nuclear cladding application

Chromium-chromium nitride multi-coating barriers were synthesized on HT9 steel disks using the electroplating and radiofrequency (RF) plasma nitriding techniques, respectively, to significantly mitigate fuel-cladding chemical interactions in sodium fast reactors. It was revealed that plasma nitriding not only synthesized an additional chromium nitride layer at the outer surface of the Cr coating but also had a significant crack-closure effect on hard Cr coatings. Further, Cr coatings nitrided for durations ranging from 0 to 72 h were investigated to observe crack-closure stages; notably, nitriding for 72 h at 600 degrees C and an RF power of 180 W resulted in the elimination of most visible interior and penetration cracks. Moreover, an additional 0.75-1 mu m thick nitride layer at the outer surface of the Cr coating included columnar grains of CrN and Cr2N phases aligned with the growth direction. Diffusion couple tests of Ce-Nd alloys at 650 degrees C for 25 h revealed that the 72-h-nitrided Cr coating possessed an excellent barrier property without any visible inter-diffusion, in contrast with the untreated Cr coating, for which a substantial inter-diffusion area of up to 17,500 mu m(2) was observed.

Automated summary

Chromium-chromium nitride multi-coating barriers were synthesized on HT9 steel disks using the electroplating and RF plasma nitriding techniques to mitigate fuel-cladding chemical interactions. It was found that plasma nitriding could not only create an additional chromium nitride layer on the outer surface of the Cr coating, but also significantly improve the crack-closure effect on hard Cr coatings. Additionally, the 72-hour nitrided Cr coating showed excellent barrier properties without visible inter-diffusion when subjected to diffusion couple tests of Ce-Nd alloys at 650 degrees C for 25 hours.