Environmental Engineering Reference
In-Depth Information
Figure 5.35
Proposed mechanism for film growth and failure in zirconium and zirco-
nium alloys [Ref. 62].
expected to occur at sites of maximum stress in the oxide, preferentially at edges
and corners. However, it is hoped that further studies on Zr-O
2
system will present
a better picture of the different stages and mechanism of its oxidation.
Hafnium
The oxidation behavior of hafnium is very similar to at of Ti and Zr. It is reported
that the oxidation of Hf in the temperature range 623-1473 K could be well
described by a logarithmic followed by parabolic and ultimately by a linear rate
equation at higher temperatures. The logarithmic law is valid at low temperatures,
and the duration of this stage decreases with a rise in temperature. Similar to Ti
and Zr, the oxidation of hafnium also leads to simultaneous oxygen dissolution
in the metal and formation of monoclinic HfO
2
film. Marker studies suggest that
oxygen migrates inward through oxygen vacancies in the scale. The parabolic
rate constant is reported to be independent of oxygen pressure variation at 1073
K, hinting at the n-type nature of HfO
2
. The temperature dependency of the para-
bolic rate constant has resulted in an estimated activation energy of 150 kJ/mol,
similar to that for ZrO
2
scale growth. It is also proposed that the transition from
protective to nonprotective behavior may be associated with cracking of the em-
brittled oxygen-enriched zone of the metal. However, any conclusive evidence
for such a mechanism is still awaited.
