Environmental Engineering Reference
In-Depth Information
liquid salts. In the proposed mechanism, it is considered that the predominant
oxidant at the scale-salt interface is SO
3
, which rapidly diffuses inward by an
S
2
O
7
2
/SO
4
2
exchange reaction [S
2
O
7
2
SO
4
2
SO
4
2
→
SO
4
2
S
2
O
7
2
SO
4
2
]. At the scale-salt interface, Cr and/or Al are preferentially oxidized,
and Co and/or cobalt oxide dissolves in the melt, leaving behind a porous film
of Cr
2
O
3
and/or Al
2
O
3
penetrated by an Na
2
SO
4
-CoSO
4
liquid. At longer times,
when the liquid salt gets saturated with Co, a steady state is reached wherein Co
dissolving at the scale-salt interface migrates outwardly through the liquid as
Co
2
ions by the exchange reaction 3Co
2
2
/2Co
3
[3Co
2
S
2
O
7
2
1
/
2
O
2
(dis-
solved) s 2Co
3
2SO
4
2
]. The location of the interface, where it forms oxide
and/or sulfate of cobalt, and the resultant product morphology depend on the
relative transport rates of reactants. The major reactions involved in the dissolu-
tion of the oxide and precipitation process are identified as follows:
A. Dissolution only (at the scale-salt interface until the salt is saturated with
Co)
CoO(s)
S
2
O
7
2
CoSO
4
(l)
SO
4
2
(6.52)
B. Dissolution at the scale/salt interface and precipitation in liquid salt under
steady-state condition:
(a)
Dissolution
Co
2
2e
2Co
3
3Co
2
(6.53)
(from the (inward
inner scale) diffusion
through the
liquid)
(b)
Precipitation (at intermediate levels of SO
3
, i.e., of the order of 10
4
to 10
3
atm at 1023 K):
2
3
1
3
3Co
2
O
2
(g)
Co
3
O
4
(s)
2Co
3
(6.54)
The reaction sequence during LTHC of a Co-30% Cr alloy covered with an
Na
2
SO
4
deposit exposed to an O
2
-SO
2
-SO
3
environment at 1023 K is shown
schematically in Fig. 6.40 [56]. Even though CoCr
2
O
4
formation is also likely,
in the figure only Cr
2
O
3
is shown for simplicity's sake. Initially as soon as the
alloy is exposed to the environment (Fig. 6.40a) both cobalt oxide and Cr
2
O
3
form below the surface of the salt layer. Subsequently, cobalt oxide interacts
with SO
3
from the environment forming CoSO
4
which gets dissolved in Na
2
SO
4
.
Consequently, formation of a liquid phase with cobalt oxide leaves behind a
