Microstructural characteristics and different effects of 800–1200 °C preformed oxides on high-temperature steam oxidation of a zirconium alloy cladding

Highlights: • Preformed oxides enhanced oxidation resistance during subsequent oxidation. • The preformed oxide formed at higher temperature was more effective. • Phases of precipitates were changed depending on the pre-oxidation temperature. Zr-1Nb-1Sn-0.1Fe nuclear fuel cladding tube was pre-oxidised at five different temperatures (803, 908, 1012, 1112, and 1200 °C), and subsequent oxidation tests were performed at 1012 °C. All the pre-oxidised specimens showed a little or no breakaway oxidation after the subsequent oxidation at 1012 °C for 5130 s, but the non-pre-oxidised specimen showed a severe breakaway oxidation after the subsequent oxidation at 1012 °C for 5130 s. In addition, comparing the weight gains and oxide thicknesses of the pre-oxidised specimens with those of the non-pre-oxidised specimens, all the preformed oxides were effective in enhancing the oxidation resistance, and the preformed oxide formed at higher temperature was more effective. The preformed oxides were examined using electron probe microanalyser and transmission electron microscopy, which showed that the fraction of Zr-Sn type precipitates increased with increasing the pre-oxidation temperature. This phase change of the precipitates may cause the preformed oxide formed at higher temperature to exhibit better oxidation resistance enhancement.