Environmental Engineering Reference
In-Depth Information
sion coefficient is time-dependent and its value as a limit becomes equal to that
of lattice diffusion. Under such circumstances, the rate of reaction would become
faster than parabolic during initial stages, whereas after an extended period it
will tend to approximate parabolic behavior. During oxidation of Zr at tempera-
tures above 1248 K, voids have been found within the compact oxide layer. Such
voids should act as barrier to the diffusion process resulting slower kinetics than
parabolic. It has been proposed that void formation takes place through condensa-
tion of oxygen vacancies.
But if oxygen diffusion takes place solely through a vacancy mechanism, it
becomes difficult to account for condensation of vacancies in the middle of the
oxide layer. Therefore, it appears that another migration mechanism would also
have to operate in the oxide scale for condensation to take place. If one assumes a
defect structure of Frenkel type for near-stoichiometric ZrO
2
, where both oxygen
vacancies and interstitial oxygen are involved, it may be assumed that vacancy
condensation within the scale might be a possibility. It has further been suggested
through emf measurement studies across growing ZrO
2
film at 973 K that electron
transport is the rate-limiting factor in the film growth process.
After the initial protective oxidation, Zr and Zr-base alloys start oxidizing at
an accelerated rate (breakaway oxidation). During protective oxidation, a com-
pact oxide film is supposed to be formed that adheres tightly to the metal substrate
even after cooling down to room temperature. The onset of breakaway oxidation
is found to be accompanied by formation of white surface oxide, particularly at
edges and corners of the specimens, and this color suggests that the oxide at the
surface is close to being stoichiometric. On continued oxidation in the breakaway
region, the surface is completely covered with white oxide, which is probably
porous and poorly protective. It has been suggested that such change in kinetics
may be due to a change in the modification of ZrO
2
from cubic and tetragonal
to monoclinic, the latter having poor protective properties.
It is significant to note that such breakaway oxidation is delayed during oxida-
tion of high-purity zirconium and the presence of impurities or some specific
alloy additions is considered to be an important cause of breakaway oxidation.
Such observations have lended support to the fact that inhomogeneously distrib-
uted impurities, alloy additions, or intermetallic particles might play an important
role. Analogous to the proposed mechanism by Smeltzer et al. [61], diffusion
through the oxide is assumed to take place along some preferred paths. These
may be provided by the misorientation in the oxide at grain boundaries and inho-
mogeneous region due to foreign elements originally present in the metal. A
scheme of oxide film growth as suggested by Cox [62] is depicted in Fig. 5.35.
This suggests that transport mechanism may lead to small-scale local variations
in the growth rate, which, in turn, result in stresses nucleating failure in the oxide.
Such nucleation sites are expected to increase with time. The cracks are also
