Environmental Engineering Reference
In-Depth Information
at elevated temperatures its reduction to the volatile SiO may occur or it may
react with other metallic ions to form complex molten oxides. It is also known
that the protective properties of Cr
2
O
3
are reduced at temperatures exceeding
1273 K and at high flow rates of oxidizing environment. Under such conditions,
Cr
2
O
3
may be converted to the volatile CrO
3
or, in the presence of water vapor,
to volatile Cr(OH)
3
.
At higher temperatures, an Al
2
O
3
scale appears to provide greater protection.
The diffusion rate of the cations through the Al
2
O
3
layer is also much smaller
than through the Cr
2
O
3
scale. Furthermore, evaporation of a volatile oxide does
not pose any serious problem in the case of an Al-O system in contradistinction
to a Cr-O system. It therefore emerges that the optimum protective simple oxide
scale should be alumina. However, thermodynamic considerations predict that
internal oxidation of Al will always occur unless the concentration of Al in the
alloy exceeds a critical value.
Mixed oxides of the general composition AB
2
O
4
(A and B represent two me-
tallic components) have often been identified as oxidation products of Fe-, Ni-,
and Co-based alloys. The most important properties of these compounds with
respect to oxidation are the diffusion coefficients of the cations and anions, which
are usually much smaller than in their parent oxides.
The diffusion coefficient very much depends on the stoichiometry of the par-
ticular spinel. However, the ability of a spinel to provide oxidation resistance
depends not only on diffusion coefficients but on the morphology of the spinel-
containing scale. Generally, two different modes of oxide layer growth are ex-
pected to occur. First, it can be the only oxidation product allowed to grow as
a continuous scale. Second, it can form as a byproduct of the solid-state reaction
of its parent oxides and eventually be interdispersed in these oxides. In the first
case, the diffusion coefficients of the transported species determine the oxidation
rate. However, in the second case, the effect of the spinel is only marginal because
the matrix of the parent oxides with their higher diffusion coefficients may serve
as easy transport channels, thus outflanking the beneficial effect of spinel phase.
A mechanism of this type has been identified for the oxidation of some alloys
in the Co-Cr system by Kofstad et al. [3], where the spinel CoCr
2
O
4
was found
to be dispersed in the CoO scale. Even in situations where the spinel forms the
bulk of the scale, the presence of very small CoO channels in the spinel has been
found to undermine the protective properties of the spinel. It is further empha-
sized that oxidation in such a situation proceeds at a much slower rate than
through bulk CoO scale, though at a faster rate than through Al
2
O
3
or Cr
2
O
3
scales. It should be kept in mind that only under very special conditions of tem-
perature, oxygen pressure, and alloy composition do compact spinel layers de-
velop, as has been reported for the case of Udimet 700 at 1311 K [4]. However,
the ability of a spinel phase to improve oxidation resistance of an alloy depends
largely on the type of spinel being formed.
