OXIDATION OF ZIRCALOY-4 WITH HIGH-TEMPERATURE AIR

The oxidation of zircaloy-4 in air was studied in the temperature range 350-750-degrees-C. The oxidation kinetics cannot be described by a single rate law. At the beginning of the reaction, the rate decreases regularly. Then, after a few hours, an acceleration is observed and finally a linear law is obeyed. The morphological observations show that the acceleration is related to the formation of cracks in the oxide scale. Moreover, two different zirconia phases (monoclinic and tetragonal) are identified. When the kinetic transition occurs, the tetragonal phase partially disappears. All these observations show that a mechanism, similar to that proposed to describe the oxidation of zircaloy-4 in steam, may be used to describe the reaction. In parallel, the oxidation reaction was studied by using acoustic emission. The frequency, the amplitude and the energy were used to characterize the signal. Typical groups were identified and their appearance was strongly correlated to the different parts of the reaction.