Environmental Engineering Reference
In-Depth Information
Figure 6.27
Phase stability diagram for the Na-S-O system at 1173 K [50].
reasons the corrosion process may lead to the formation of oxides, sulfides, and
sulfates (if these are thermodynamically stable under reaction conditions).
6.7.1 Thermodynamic Considerations
In order to have an understanding of Na
2
SO
4
-induced hot corrosion, one requires
knowledge not only of the properties and stability of Na
2
SO
4
but also of the
reactions of the metal under consideration with the two oxidants such as sulfur
and oxygen. The stability of Na
2
SO
4
is illustrated in terms of phase stability at
a constant temperature, as depicted in Fig. 6.27, where the activities of O
2
and
SO
3
are used as coordinates. In this figure, the activity of SO
3
can alternatively
be expressed in terms of activity of Na
2
O, as the equilibrium of the reaction:
Na
2
SO
4
s Na
2
O
SO
3
(6.33)
can be expressed as:
a
Na
2
O
a
SO
3
Constant
⋅
a
Na
2
SO
4
For pure Na
2
SO
4
,
a
Na
2
SO
4
is unity. Thus,
a
Na
2
O
Constant
⋅
a
SO
1
3
