OXIDATION OF ZIRCALOY-4 WITH HIGH-TEMPERA TURE AIR

The oxidation of zircaloy-4 in air was studied in the temperature rang e 350-750-degrees-C. The oxidation kinetics cannot be described by a s ingle rate law. At the beginning of the reaction, the rate decreases r egularly. Then, after a few hours, an acceleration is observed and fin ally a linear law is obeyed. The morphological observations show that the acceleration is related to the formation of cracks in the oxide sc ale. Moreover, two different zirconia phases (monoclinic and tetragona l) are identified. When the kinetic transition occurs, the tetragonal phase partially disappears. All these observations show that a mechani sm, similar to that proposed to describe the oxidation of zircaloy-4 i n steam, may be used to describe the reaction. In parallel, the oxidat ion reaction was studied by using acoustic emission. The frequency, th e amplitude and the energy were used to characterize the signal. Typic al groups were identified and their appearance was strongly correlated to the different parts of the reaction.