High temperature oxidation of cold spray Cr-coated accident tolerant zirconium-alloy cladding with Nb diffusion barrier layer

Abstract

Cr-Nb bilayer coatings on Optimized ZIRLO™ light water reactor (LWR) fuel cladding tubes were developed using cold spray deposition technology to increase the limiting operational temperature (i.e., binary Cr-Zr eutectic temperature, 1332 °C) of Cr-coated Zr-alloy system. The properties of the dual-layered coatings were improved by varying the type of Cr powder and post-annealing of the cold sprayed Nb layer. High temperature oxidation behavior of Cr-Nb-coated cladding at 1200 °C and 1425 °C in flowing steam were compared with those of Cr-coated cladding and uncoated cladding. At 1200 °C, both Cr-coated cladding and Cr-Nb coated cladding exhibited outstanding oxidation resistance and a continuous intermetallic compound layer (1 – 3 μm thickness) of predominantly Zr-Cr and Cr2Nb formed at the Cr/Zr and Cr/Nb interface, respectively. At 1425 °C, the Cr-coated Zr-alloy cladding showed enhanced oxidation due to rapid consumption of Cr coating by interdiffusion and liquid phase formation, and consequent destabilization of the outer protective Cr-oxide layer. However, the Nb diffusion barrier layer between the Cr coating and Zr-alloy substrate prevented the melting and the outer Cr coating provided excellent oxidation resistance. For this bi-layered system, a diffusion layer devoid of intermetallic phases was observed at the Nb/Zr interface due to the miscibility of the two metals. Reaction pathways that include both surface oxidation and interdiffusion effects within the coating-substrate system have been proposed based on experimental results.