Environmental Engineering Reference
In-Depth Information
the metal oxide reacts with SO
3
and gets dissolved in the molten salt as a cationic
species. This is known as
acid fluxing
. If the oxygen ion activity of the sulfate
melt is high compared to that required for complex anion formation according
to the equilibrium
MO
O
2
s MO
2
2
(6.37)
then complex ion formation can take place and the metal oxide gets dissolved
in the melt as a complex anion. This is referred to as
basic fluxing
. Figure 6.30
further demonstrates that from a minimum solubility of NiO corresponding to
activity of Na
2
Oas10
10.3
, the solubility goes on increasing with the increase of
a
Na
2
O
(i.e., decreasing
p
SO
3
) according to the reaction:
1
2
O
2
s 2NiO
2
2NiO
O
2
(6.38)
and this corresponds to basic fluxing. It has been proposed [57] that NiO can
dissolve in Na
2
SO
4
as nickelate (NiO
2
), possibly due to instability of Na
2
NiO
2
.
Similarly, for the right-hand side of the curve, solubility of NiO increases with
decreasing
a
Na
2
O
according to the reaction:
NiO s Ni
2
O
2
(6.39)
which corresponds to acid fluxing. Such solubility behavior in Na
2
SO
4
is also
reported for other oxides, such as Co
3
O
4
,Fe
2
O
3
,Al
2
O
3
, and Cr
2
O
3
, which are of
principal interest in high-temperature alloys and coatings. The reported solubility
curves for the different oxides maintain the same nature but with different minima
values and are displaced to the left or the right of each other. The differences in
magnitude of -log
a
Na
2
O
values as observed for the solubility minima between the
different types of oxides further emphasize that the local chemistry within a fused
salt film is important. The terms
acidic
and
basic
are relative and refer to the
reaction that occurs rather than the condition of the melt.
To simplify the fluxing processes with reference to hot corrosion of alloys in
the presence of a molten Na
2
SO
4
deposit, it is useful to consider the appropriate
stability diagrams as a means for predicting the feasible fluxing reactions. It is
to be recognized that the initially formed oxides on the metal/alloy surface will
virtually determine the nature of the salt to be either basic or acidic. This in turn
will be governed by the oxide ion concentration at the salt-gas interface of the
deposited salt and the affinity of the oxides as well as their metal ions for oxide
ions. The affinity of different relevant metals for the oxide ions can be best de-
scribed by using their stability diagrams superimposed on the Na-S-O diagram
(for Na
2
SO
4
) at the temperature under consideration. As an illustration, such a
superimposed stability diagram is presented in Fig. 6.31 for the phases of Ni,
Al, and Cr that can exist in contact with a molten Na
2
SO
4
layer on Ni-Cr-Al
