Microstructural characteristics of multilayers and interfaces of Cr-coated Zircaloy-4 cladding based on elemental diffusion under high-temperature steam oxidation

Abstract

A dense and well-adhered Cr coating was prepared on the surface of Zircaloy-4 cladding by arc ion plating. Oxidation tests on Cr-coated Zircaloy-4 claddings were conducted in steam at 700-1300 °C. The surface morphology and cross-sectional microstructure were investigated using SEM, EBSD, FIB and TEM techniques. The results showed that no significant cracking or spalling was observed after oxidation. The oxidized cross-sections showed characteristics of multilayers and interfaces. From the outer to the inner layer, these are, in order, a Cr oxide film and Sn precipitates on its grain boundaries, an amorphous structural layer, a residual Cr coating and (Zr,Cr)xOy oxides on its grain boundaries, a C15-type Zr(Cr,Fe)2 intermediate layer, and a (Cr,O)-rich substrate Zr. The multilayers and interfaces alter the oxygen diffusion path and reduce the oxygen diffusion rate, thus slowing down the oxidation process of the substrate. In addition, the location and cause of amorphous structures and Kirkendall cavities and their effect on microstructure and adhesion properties were discussed.