Environmental Engineering Reference
In-Depth Information
to form additional sulfide. As a result, both the earlier formed pervious Cr
2
O
3
layer and the inner sulfide layer grow; consequently, the inner scale-salt interface
and the scale-alloy interface migrate inward (Fig. 6.40d). At a longer time of
exposure when the salt at the gas-salt interface attains the maximum activity of
CoSO
4
(as dictated by the gas composition), the outwardly migrating cobalt forms
CO
3
O
4
(Fig. 6.40e). So, during LTHC of cobalt alloys, high degradation rates
result from the continuous dissolution of cobalt and/or cobalt oxide at the scale-
salt interface and precipitation of cobalt oxide occurs near the salt-gas interface.
Such a process makes the initially formed Cr
2
O
3
film discontinuous and pervious
at the alloy-scale interface. Since the solubility of Cr
2
O
3
or Al
2
O
3
in the melt
at the prevailing atmosphere is low, chromium or aluminum remains below the
original alloy-scale interface as a pervious oxide layer containing Cr
2
O
3
and/or
Al
2
O
3
and/or CoCr
2
O
4
.
Thermodynamic considerations suggest that Ni-based alloys containing no co-
balt are not supposed to undergo LTHC as the Co-based alloys do at intermediate
concentrations of SO
3
. This is justified by the fact that at temperatures above the
melting point of Na
2
SO
4
-NiSO
4
eutectic (944 K), NiSO
4
(s) is not expected to
form under normal gas turbine operating conditions (
p
SO
3
10
4
atm) even
though Na
2
SO
4
-NiSO
4
liquid may form. However, at high SO
3
levels, where
NiSO
4
(s) is stable at the scale-gas interface, nickel dissolution and sulfate precip-
itation may occur, resulting an accelerated attack.
In the above discussion, acid and basic fluxing mechanisms have been consid-
ered independent of each other. But in reality the situation may not be so simple,
and interactive effects of both the processes leading to catastrophic attack are
often encounted with multicomponent alloy systems under a vaguely defined at-
mosphere. The possible salt fluxing reactions with Na
2
SO
4
deposit for basic and
acidic processes are illustrated in Table 6.6 [58]. In this table, category A includes
the basic processes that occur due to production of oxide ions in Na
2
SO
4
as a
result of removal of oxygen and sulfur from the melt by the alloy. As a result,
the attack may occur either due to solution of the oxide in Na
2
SO
4
or solution
and reprecipitation. In both cases, the attack is not self-sustaining but rather is
controlled by the amount of Na
2
SO
4
deposit unless the gas phase contains SO
3
.
Category B illustrates the Rapp-Goto electrochemical model [62], whereby the
attack is explained not in terms of sulfur removal but in terms of the existence
of a negative solubility gradient of the corrosion product at the oxide-salt inter-
face in the Na
2
SO
4
film.
Categories C, D, E, and F include the different acidic processes wherein C
and D refer to gas-induced acidic fluxing processes involving dissolution and
reprecipitation, with the acidic component supplied by the gas phase. Category
E illustrates the Rapp-Goto concept for acidic melts. Category F demonstrates
the alloy-induced acid fluxing processes whereby the acidic component comes
from the alloy undergoing degradation.
