CHANGE OF OXIDATION-KINETICS IN THE ZR-AL SYSTEM

The oxidation of ZrAl2 in dry oxygen in the temperature range 883-1038 K obeys a logarithmic rate law, while the oxidation of Zr3Al, Zr2Al, ZrAl and Zr2Al3 follows a parabolic rate law. An activation energy of 115 kJ mol-1 is estimated for the oxidation of ZrAl2. The reaction starts by chemisorption of oxygen and rapid formation of a ZrO2 film. Subsequently, the reaction slows down and proceeds further at a low rate. Aluminium escapes oxidation and diffuses to the ZrAl2 bulk, forming the ZrAl3 phase at the oxide-alloy interface. The remaining zirconium is therefore selectively oxidized. Due to the higher content of aluminium in ZrAl2 than in the Zr-Al phases mentioned, aluminium accumulates very quickly at the oxide-alloy interface, releasing a correspondingly small amount of zirconium. Thus, the reaction is rapid as long as there is enough zirconium available for oxidation; subsequently, the reaction rate decreases abruptly. © 1990.