Environmental Engineering Reference
In-Depth Information
rate of vaporization due to dissociation alone decreases steadily with increasing
environmental pressure. However, vaporization due to volatile oxide formation
increases initially with increasing oxygen pressure because the rate of oxide for-
mation and subsequent vaporization depends on the rate of oxygen availability.
Ultimately, the vaporization rate will decrease with increasing pressure due to
formation of a blanket of the volatile oxide.
If metals from the platinum group are used as coatings in oxidizing environ-
ments at high temperatures, they are eroded through volatilization of the oxides
like PtO 2 [2].
Protective coatings are classified into three main groups depending on the type
of protective scales they form during service: (1) chromia formers, (2) alumina
formers, and (3) silica formers. Among these three, the chromia- and silica-form-
ing coatings suffer from the following limitations contributed by reactions with
the environment producing volatile oxides.
(i).
At high temperatures and high oxygen pressures, Cr 2 O 3 is oxidatively evap-
orated to CrO 3 according to 1 / 2 Cr 2 O 3
CrO 3(gas) . Therefore, chromia
scale-forming coatings should not be used at temperatures above 1273 K
in oxygen at or near atmospheric pressures, but may be used at higher tem-
peratures in lower oxygen partial pressures.
3 / 4 O 2
(ii).
Silica scales on silicide coatings are not stable at reduced oxygen pressures
and at high temperatures due to the formation of volatile SiO according to
Si
2SiO (gas) . High-temperature oxidation studies of elemental
silicon have established that SiO (g) can form at the silica film-element inter-
face by active oxidation process [50] under specific partial pressures of
oxygen. The SiO (g) so formed tries to move upwardly and finds its escape
route through pinholes and fissures (microchannels) in the SiO 2 film, thus
rupturing the film locally. The SiO (g) formation at a particular temperature
continues so long as the pressure of SiO (g) is greater than the ambient oxy-
gen pressure, i.e., p SiO
SiO 2
p O 2 , thus destroying protective property of the
silica film. In general, SiO 2 -forming coatings should not be used at temper-
atures above 1873-1973 K because low viscosity of silica is maintained
only below such a temperature range. It is the viscosity of silica, rather
than its melting point, that deserves due consideration in applications, even
though in practice, silicide coatings are sometimes used at temperatures
above silica's melting point. Silica is relatively fluid in the temperature
range 2273-2773 K [80]. Moreover, the glassy films of silica are not prone
to break-away, possibly because of their excellent ductility and thinness.
Vapor phase loss of silica is possibly responsible for the oxide film thinness
formed on silicides. Such losses can occur by two processes: simple evapo-
ration and decomposition to volatile SiO, which bubbles out through the
outer protective layer of silica, thus causing early coating failure [80].
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