Environmental Engineering Reference
In-Depth Information
Figure 6.15
(a) Schematic illustration of processes taking place during transient oxi-
dation stage of Co-Cr alloy at 1273-1473 K in 10 torr to 1 atm O
2
. (b) Steady-state scaling
of Co-Cr alloys with insufficient Cr (
30%) showing a two-layered external scale and
internal oxidation of Cr. (c) Development of a continuous layer of Cr
2
O
3
on Co-Cr alloys
with more than 30% Cr [32].
due to formation of voids and porosities in the inner part of the external scale,
allowing rapid gaseous transport across the pores that overrides the effect of
diffusional blockage by CoCr
2
O
4
-spinel particles in the inner layer.
On increasing the concentration of Cr in the Co-Cr alloys (Cr greater than
30%), an increasingly larger fraction of the inner layer consists of CoCr
2
O
4
spinel.
Diffusion through the spinel being slow in comparison with CoO phase, the avail-
able area in the scale for easy diffusion continues to decrease. As a consequence,
the rate of oxidation gradually decreases with increasing Cr concentration in the
alloys until a critical concentration is reached where a continuous blocking layer
of Cr
2
O
3
is formed (Fig. 6.15c). The dependence of parabolic rate constants on
Cr concentration for oxidation of Co-Cr alloys at 1373 K is illustrated in Fig.
6.16.
6.5.3 Comparison of the Behavior of Fe-Cr, Ni-Cr, and
Co-Cr Alloys
The possible oxide phases that may be formed during oxidation of binary Fe-Cr,
Ni-Cr, and Co-Cr alloys are:
1.
Fe-Cr alloys—FeO, FeFe
2-
x
Cr
x
O
4
(0
2), Fe
2
O
3
, and Fe
3
O
4
, of which
the last two form a complete range of solid solutions
x
2.
Ni-Cr alloys—NiO, NiCr
2
O
4
, and Cr
2
O
3
