Effect of pre-oxide and hydrogen on creep of Zircaloy-4 at 1123 K

The creep behaviour of pre-oxidized and pre-hydrided Zircaloy-4 was investigated at 1123 K. Outer only and outer + inner pre-oxidation were considered in the study. The cladding creep rate decreases when the thickness of the pre-oxide layer increases and stabilizes for thicknesses higher than about 20 μm. The effect of pre-oxide is stronger in double-sided than in single-sided pre-oxidized samples and depends on the applied stress. Hydrogen increases Zircaloy-4 creep rate on both bare and pre-oxidized samples. The presence of high amounts of hydrogen, which induces the formation of significant quantities of β phase at high temperature, modifies the creep mechanism. During pre-oxidation, a network of regularly spaced radial cracks is formed on thick pre-oxide layers. At the beginning of the creep test, these cracks spread out throughout the oxide layer up to the α(O) layer, where they stop. Finally, strain develops in the oxide-free zones.