The high temperature oxidation kinetics of a Zr-3 Nb-1 Sn alloy

The high temperature oxidation kinetics of a Zr-3 wt% Nb-1 wt% Sn alloy has been measured in the range 600 deg - 900 deg C in flowing oxygen and carbon dioxide. The oxidation behavior is characterized by a transition to more rapid linear kinetics after the initial parabolic oxidation. This rate transition has been observed at each temperature. The oxidation behavior in CO2 is similar to that in O2 except that the weight gain is smaller and the time-to-transition longer in CO2. The oxide scale in the pre-transition region comprises an essentially nonstoichiometric black oxide which transforms to the stoichiometric white oxide after the transition. The rate transition is associated with a breakdown of the protective oxide scale. X-ray powder patterns have shown the presence of the 6 ZrO2.Nb2O5 phase in the scale. In the pre-transition reqion, the alloy oxidizes by one mechanism above 700 deg C and by a different mechanism at lower temperatures. This is attributed to a phase change in the alloy. (author)