Environmental Engineering Reference
In-Depth Information
Figure 6.40
Schematic representation of the reaction sequence during low-tempera-
ture hot corrosion of a Co-30% Cr alloy exposed to O
2
-SO
2
-SO
3
environment where both
Na
2
SO
4
-CoSO
4
liquid and Co
3
O
4
are stable. At higher concentration of SO
3
where Co
3
O
4
is unstable at the gas-salt interface, the outward migrating cobalt will form CoSO
4
(s) or
Co
3
O
4
and CoSO
4
(s) [56].
porous Cr
2
O
3
film (Fig. 6.40b). Further reactions occur by inward migration of
SO
3
and outward transport of cobalt. Inwardly migrating SO
3
oxidizes Cr to form
Cr
2
O
3
and SO
2
. The released SO
2
further reacts to form both sulfides and oxides
of cobalt and chromium (Fig. 6.40c). With increasing exposure time, the earlier
formed sulfide is oxidized, generating S
2
or SO
2
which can migrate further inward
