Rate controlling processes during the pre-transition oxidation of zirconium alloys

Abstract

A study of the potentials developed across growing oxide films on zirconium alloys, and the current-voltage characteristics of these oxide films has shown that a simple treatment of the oxidation potential to give transport numbers for ions and electrons is not possible for these alloys. Many of the conditions for such a treatment to apply are not met. The shape of the current-voltage curves shows that the ionic current obeys the Tafel relationship, whereas the electronic current is best fitted by a Schottky emission process. The predicted effect of polarisation during oxidation (predicted from the current-voltage curve) was observed for zirconium specimens. An anomaly in the current-voltage curves for Zircaloy-2 resulted in similar predictions being obeyed only for very thin films. The cause of the anomaly was not established but its occurrence was correlated with a sharp change in the oxidation potential.