Environmental Engineering Reference
In-Depth Information
Figure 6.14
Concentration profiles of nickel and cobalt through alloy and scale for
Ni-10.9%Co alloy oxidized in 1 atm O
2
at 1273 K for 24 h [30].
could also be due to that fact that it diffuses more quickly than Ni in the oxide
solid solution. However, Co and Ni contents in the scale at the two reaction
interfaces had remained essentially constant with increasing oxidation time.
Partial Miscibility of Component Oxides and Formation of
Compound Oxides
Studies relating to oxidation behavior of Fe-, Ni-, and Co-based alloys have re-
ceived much attention in relation to the development of heat-resistant alloys,
where chromium is chosen as an important alloying element to provide high-
temperature corrosion resistance by the formation of chromia scales. Accord-
ingly, Fe-Cr, Ni-Cr, and Co-Cr alloy systems serve as the base alloys for many
commercial high-temperature alloys. It is pertinent that in such binary alloy sys-
tems there may be composition ranges in which noble metal oxide and reactive
metal oxide predominate but their oxides are partially miscible to form single-
phase solid solution scales, and the respective oxides also react to form a new,
distinct oxide phase [spinels of the type NiCr
2
O
4
, CoCr
2
O
4
, or FeFe
2
x
Cr
x
O
4
(0
2)]. Thus, the monoxides of Fe, Ni, and Co (MO) react with Cr
2
O
3
to
form spinels of composition MCr
2
O
4
. The main features of oxidation of these
three groups of alloys are similar, and as an illustration Co-Cr-O [31] is discussed.
At high temperatures, the thermodynamically stable phases that can exist in
a Co-Cr-O system are CoO, Co
3
O
4
(stable at
x
1243 K in 1 atm O
2
), Cr
2
O
3
, and
CoCr
2
O
4
, of which Cr
2
O
3
is the most stable one formed by selective oxidation.
However, if Cr
2
O
3
is to be formed externally as a protective layer, there has to
