Environmental Engineering Reference
In-Depth Information
probably behave partly like Fe-Cr and partly like Ni-Cr alloys in situations
where doped Cr
2
O
3
can be developed on them.
6.5.4 Oxidation Behavior of Fe-Cr-Al, Ni-Cr-Al, and
Co-Cr-Al Alloys
The main principles already described for oxidation of binary alloys also apply
to the ternary alloy systems, at least in simple atmospheres. It is to be envisaged
that the reduction in oxidation rate and improvement in scale adhesion, with
suitable alloying element additions, and within the restrictions of desired mechan-
ical properties and cost, can provide one of the most promising ways for the
development of high-temperature materials without resorting to the use of coat-
ings. Most alloys/alloy coatings for high-temperature applications are based on
iron, nickel, and/or cobalt and rely on the establishment of chromia (Cr
2
O
3
),
alumina (Al
2
O
3
), or silica (SiO
2
) healing layers for protection against oxidation.
These oxides are thermodynamically very stable, have high melting points, and
the transport processes through such scales are generally slow. For many high-
temperatures applications, Cr
2
O
3
scales can be effective on binary M-Cr alloys
(where M is Fe, Ni, or Co). However, at temperatures above 1173 K, CrO
3
may
further react with oxygen to form its higher oxide, such as CrO
3
(a volatile spe-
cies), limiting the long-term application of such alloys to temperatures below
about 1173 K.
Accordingly, for applications in highly oxidizing environments at high temper-
atures up to 1473 or 1573 K, alloy/alloy coatings often are designed to develop
surface layers of Al
2
O
3
or SiO
2
that do not form volatile oxides (formation of
volatile SiO limits applications of the latter in environments of low oxygen partial
pressure). Unfortunately, the concentrations of Al or Si needed to establish such
scales on the respective binary alloys often result in unacceptable mechanical
properties. Fortunately, in practice, addition of Cr to alumina or silica-forming
binary alloys reduces the concentration of aluminum or silicon required to estab-
lish the respective Al
2
O
3
or SiO
2
scale to tolerable levels, e.g., 3-4% Al in an
Fe-14% Cr or an Fe-27% Cr alloy or 1-2% Si in an Fe-14% Cr alloy [34]. Si
is also expected to compete with Cr to form a surface layer of SiO
2
rather than
that of Cr
2
O
3
, but it is reported [30] to be unsuccessful with Fe-26% Cr-1% Si
and Fe-29% Cr-5% Si alloys at 1273-1473 K, even under preferential oxidation
conditions. This is because SiO
2
grows so slowly that the initial nuclei are over-
grown by Cr
2
O
3
, which continues to thicken, and SiO
2
fails in coalescing to
provide a protective layer, slowing down the oxidation rate below that of Fe-Cr-
Al alloys.
α
-Al
2
O
3
can be more readily formed and maintained on Fe-Cr-Al alloys than
on binary Fe-Al alloys. This is partly because Cr tends to stabilize
α
-Al
2
O
3
rather
than the less protective
γ
-Al
2
O
3
(stable below 1173-1223 K). The more general
