Environmental Engineering Reference
In-Depth Information
The most important feature of alloy-induced acidic fluxing is that a zone of
refractory metal oxide-rich liquid is formed in the immediate vicinity of the alloy
surface and the oxides that are commonly relied on for protection against attack
(e.g., Al
2
O
3
,Cr
2
O
3
) become nonprotective due to a solution-precipitation process.
Therefore, it is the result of a local increase in salt acidity by removal of oxide
ions in the salt through the formation of basic complexes (e.g., WO
4
2
,VO
3
,
or MoO
4
2
) with other components in the alloy or salt.
Gas-induced acid attack occurs when the atmosphere contains relatively high
partial pressures of SO
3
or V
2
O
5
, which are introduced to the gas phase during
combustion of fuel containing sulfur and vanadium. Under such situations, reac-
tion (6.37) is forced to the left, resulting in a salt of low oxide ion or Na
2
O
activity. This form of attack is prevalent at low temperatures between 873 and
1073 K, where the salt deposit (Na
2
SO
4
) is expected to be solid. Although the
applicability of the proposed fluxing theory is justified for the high-temperature
hot corrosion (HTHC) processes of pure metals and alloys, its validity has been
contested for low-temperature hot corrosion (LTHC), particularly in Co-based
binary and ternary alloys. Turbines operating in marine environments suffer from
severe degradation of Co-based CoCrAlY coatings at relatively low temperatures
(873-1073 K), where Na
2
SO
4
salt depositing on the surface of blades is expected
to be solid. Damage is manifested irregularly in the form of pits rather than as
a broad frontal attack. Luthra et al. [56,64] demonstrated that a minimum
p
SO
3
of the order of 10
5
atm is required in the gas phase to stabilize an Na
2
SO
4
-
CoSO
4
liquid salt at temperatures below the melting point of Na
2
SO
4
(less than
1157 K). Under such conditions, a liquid melt of Na
2
SO
4
-CoSO
4
can form on
cobalt-containing alloys by the interaction of cobalt oxide and Na
2
SO
4
with SO
3
in the gas phase according to
CoO(s)
SO
3
(g)
CoSO
4
(s,l)
(6.50)
1
3
Co
3
O
4
(s)
1
6
O
2
(g)
SO
3
(g)
CoSO
4
(s,l)
(6.51)
where the underline implies that sulfate is present in solution at an activity less
than unity. Similar to HTHC of alloys, the degradation of Co-based alloys by
LTHC is also a two-stage process: an initial stage during which an Na
2
SO
4
-
CoSO
4
liquid forms on the alloy surface, followed by a propagation stage during
which the reaction occurs by transport of various reactants through the liquid
melt. Degradation results from rapid dissolution of Co and/or CoO/Co
3
O
4
on the
alloy surface by Na
2
SO
4
. This prevents the formation of a continuous, impervi-
ous, protective film of Cr
2
O
3
and/or Al
2
O
3
on the alloy surface. Based on detailed
considerations of the mechanisms of transport of various oxidants through the
liquid melt, a model for LTHC has been proposed [56]. This model has been
developed on the basis of dissolution of the more noble metal or metal oxide in
