Direct and pulse electroplating effects on the diffusion barrier property of plasma-nitrided Cr coatings on HT9 steel

The introduction of a Cr and chromium nitride multi-coating barrier between the nuclear metallic fuel and the cladding is a potential candidate to mitigate fuel cladding chemical interaction (FCCI). Cr coatings were electroplated on HT9 disks using direct and pulse currents followed by plasma nitriding for synthesizing additional nitride coating. The pulse current induced Cr coatings revealed severe microstructural changes from crack-free to the abundance of cracks during the plasma nitriding due to phase transformation during the plasma nitriding. The phase transformation of hexagonal CrH to cubic Cr occurred at a temperature as low as 110 °C resulting in the liberation of hydrogen pores agglomerated at Cr grain boundaries and internal stress. Finally, the diffusion barrier property of the pulse current induced Cr coatings was significantly reduced, while the plasma-nitrided Cr coating produced by a direct current exhibited a crack-closure effect, and an additional nitride layer successfully hindered the elemental diffusion of Ce and Nd at 650 °C for 25 h. This current study points to the critical effect of current waveforms on the final barrier property of Cr-chromium nitride coatings for nuclear cladding applications.