Environmental Engineering Reference
In-Depth Information
be a continuous supply of Cr from the inner part of the alloy to the alloy-oxide
phase boundary, which means that both the concentration and diffusivity of Cr
in the alloy are sufficiently high that it is in ample supply at the alloy-oxide
reaction interface. If this condition is not fulfilled, the other component (more
noble) of the alloy will be oxidized and the reaction mechanism becomes a func-
tion of several factors, including composition of the alloy and rates of transport
in the alloy phase as well as in the oxide scale.
As a clean Co-Cr alloy surface is exposed to an oxidizing atmosphere at high
temperatures, both Co and Cr oxidize to form CoO (and Co
3
O
4
at less than 1243
K in 1 atm O
2
) and Cr
2
O
3
as a thin film, which thickens by solid-state diffusion.
Thereafter several competing processes take place at or near the interface between
CoO and the alloy during the transient oxidation stage (Fig 6.15a): (1) Co contin-
ues to get dissolved in CoO and diffuses outwardly through CoO to react with
oxygen at the oxide-oxygen interface; (2) Cr in the alloy also diffuses to the
alloy-oxide interface and reduces CoO to metallic Co while it itself gets oxidized
to Cr
2
O
3
; (3) the released O
2
from CoO dissolves and diffuses into the alloy
where it reacts with Cr to form internal oxide particles of Cr
2
O
3
beneath the
scale-alloy interface.
The establishment of a protective continuous Cr
2
O
3
layer depends on factors
like (1) the concentration and diffusivity of Cr in the alloy, (2) the diffusivity of
oxygen in the alloy and (3) the growth rate of CoO layer. This suggests that only
under certain specific conditions a protective continuous Cr
2
O
3
scale can be
grown, at and above a critical concentration of Cr in the alloy. This concentration
in turn is a function of exposure temperature and partial pressure of oxygen.
Under steady-state conditions, at lower concentration of Cr (less than 30%),
a continuous layer of Cr
2
O
3
fails to develop. The external scale consists of an
outer CoO layer (growth rate of CoO is faster than that of Cr
2
O
3
) with an inner
layer of CoO
CoCr
2
O
4
(formed by solid-state reaction of CoO and Cr
2
O
3
),
and internally oxidized Cr
2
O
3
particles are dispersed in the alloy beneath the
external scale (Fig. 6.15b). Since the overall oxidation process is governed by
solid-state outward diffusion of Co
2
through CoO of inner and outer layers, the
inner layer in particular develops voids and closed porosity. Inward dissociative
transport of oxygen taking place across the voids allows the inner external layer
to grow into the alloy, thereby oxidizing metallic Co to CoO in the internally
oxidized zone. Since CoO grows inwardly, the internally oxidized Cr
2
O
3
particles
get embedded in CoO and finally react to form CoCr
2
O
4
spinel. So one has to
realize that for dilute Co-Cr alloys (Cr less than 10%), the increase in oxidation
rate with increasing Cr concentration is partly due to the doping effect contributed
by dissolution of Cr in CoO, which increases the vacancy concentration of the
oxide and in turn enhances the diffusion of Co
2
in CoO (the native defect concen-
tration in CoO is 1% in 1 atm O
2
at 1273-1473 K, and the solubility of Cr in
CoO is 2.6% at 1473 K [30]). However, the effect is primarily believed to be
