Environmental Engineering Reference
In-Depth Information
Figure 6.39
Superimposed stability diagrams for Ni-S-O and Na-S-O systems show-
ing schematic reaction path during hot corrosion of pure Ni [16].
Once suitable conditions have been established, the basic fluxing of Cr
2
O
3
and Al
2
O
3
can occur by the following reactions:
Cr
2
O
3
O
2
s 2CrO
2
2
(6.45)
Al
2
O
3
O
2
s 2AlO
2
2
(6.46)
forming chromate and aluminate ions in solution in the melt. These ions subse-
quently migrate through the melt to sites of higher oxygen potential close to the
salt-gas interface where they are subsequently reprecipitated as Cr
2
O
3
and Al
2
O
3
by reverse reactions according to Eqs. (6.45) and (6.46), releasing oxide ions.
The prevailing high oxygen potential at the salt-gas interface shifts Eq. (6.35)
in the reverse direction such that the oxide ion or Na
2
O activity becomes too low
to support the existence of complex anions. Automatically they decompose by
reverse reactions as per Eqs. (6.45) and (6.46). It is worth noting that sulfate-
induced hot corrosion attacks of pure Ni as well as those of Ni alloys containing
chromium and aluminum are not self-sustaining processes. As long as there is a
steady supply of sulfate ions from Na
2
SO
4
, the hot corrosion process proceeds
at approximately a linear rate. But as the source of sulfate ions get depleted, the
attack also diminishes, and oxygen becomes plentiful in the corrosion product.
