Study of oxidation behavior and associated microstructure evolution of Zircaloy-4 exposed to high temperature pure steam

Abstract

Oxidation reaction is inevitably consuming the thickness of load bearing matrix of Zircaloy-4 and releases hydrogen that can be ingress into the matrix, thereby resulting in the degradation of its structural integrity. The present work aims to simultaneously investigate the oxidation behavior, hydrogen pickup, and the associated microstructure evolution of Zircaloy-4 exposed to 800 °C–1000 °C pure steam. Gain weight, growth of the oxide layer and α-Zr(O) layer in Zircaloy-4 follow the parabolic law. The intensification of oxide corrosion versus temperature and duration time is attributed to the severe Sn segregation that leads to the oxide layer delamination. α-Zr(O) layer just appeared when the oxidation temperature does not exceed the phase transition temperature. Radial cracks are prone to initiating at the concave position of the oxide/metal undulation interface. The hydrogen pickup measured by vacuum hot extraction was no more than 40 ppm, and no precipitated hydrides were identified. A mild Sn segregation in the outer oxide layer has a small influence in deteriorating the outer oxide layer protectiveness, the hinder effect of an unbroken outer oxide layer to the diffusion of hydrogen into the α-Zr(O) layer and matrix metal, and then resulting in a smaller influence of hydrogen embrittlement in this study.