Kinetics and mechanism of hydrogen-induced disproportionation of ZrCo

The kinetics and mechnism of the hydrogen-induced disproportionation of ZrCo was studied experimentally. In general, the time dependence of the disproportionation has a sigmoidal shape. The disproportionation rate increased with increasing initial hydrogen pressure and showed a 3/2-power dependence on the pressure. As for the temperature dependence, the rate increased with temperature only up to 815 K and then decreased at higher temperatures. Namely, a volcano-type pattern was observed in the Arrhenius plot of the rate constant. In addition, a fairly large isotope effect was observed for the rate of disproportionation between hydrogen and deuterium. These results were explained well by a nucleation and nuclei growth model, in which hydrogen solubility in the ZrCo phase was assumed to play an important role.