Environmental Engineering Reference
In-Depth Information
Figure 5.34
The parabolic rate constant for oxidation of zirconium as a function of
1/T. Data of Cubiciotti [66], Gulbransen and Andrew [67], Mackay [68], Wallwork et al.
[69] and Hussey and Smeltzer [61]. The broken line shows the calculated maximum possi-
ble reaction rate due to volume diffusion of oxygen in α-Zr [Ref. 57].
then the contribution due to oxygen dissolution at 673-773 K is appreciably
higher than would be expected from the maximum estimated mass gain due to
volume diffusion of oxygen in zirconium (Fig. 5.34). The results as such indicate
a contribution from grain boundary or short-circuit diffusion.
Since the oxygen dissolution makes a parabolic contribution to the total oxida-
tion process, the deviation from parabolic behavior must be due to oxide scale
formation. At relatively lower temperatures (
873 K), the nonparabolic oxidation
is attributed to enhanced diffusion along dislocation pipes and grain boundaries
in the initially formed oxide. Similar to Ti and Hf, a simple model has been
proposed assuming the density of oxygen sites in the short-circuit diffusion paths
to decrease by a first-order rate mechanism. This implies that the effective diffu-
