Environmental Engineering Reference
In-Depth Information
The nickel sulfide particles in the scale become oxidized and the released sulfur
moves deeper into the alloy where chromium sulfide is formed.
Acid Fluxing
Acid fluxing differs from base fluxing by the fact that the acid-induced attack is
usually self-sustaining. Hence, a small amount of salt deposit produces much
more attack in acid fluxing processes than in basic fluxing. Salt deposits can
be made acidic by two different factors: alloy-induced acidity and gas-induced
acidity.
In alloy-induced acid fluxing, the acidic conditions in the salt melt are estab-
lished by the dissolution of such oxides as MoO
3
,WO
3
, and V
2
O
5
formed on
the alloy surface, which have a greater affinity for Na
2
O. As the oxides are dis-
solved in the melt, oxide ion concentration of the melt decreases, thus making
the salt more acidic. The dissolution of oxides in the salt can occur by dissociation
reaction according to Eq. (6.36). Giggins and Pettit [58] discussed in detail the
sulfate-induced hot corrosion behavior of two important alloys, Co-25% Al-12%
W and Ni-8% Cr-6% Al-8% Mo, which are widely known to suffer from alloy-
induced acidic fluxing. During the hot corrosion process, oxides of such metals
as tungsten and molybdenum become dissolved in Na
2
SO
4
, forming tungtates
and molybdates, with some SO
3
being displaced from Na
2
SO
4
. The time required
for such dissolution of the refractory metal oxides in Na
2
SO
4
depends on the
oxidation characteristic of the alloy. In certain alloys, the refractory elements are
oxidized at the initial stage of oxidation, whereas in others selective oxidation
of other elements causes delay before these are available to react with Na
2
SO
4
.
The solubility of these oxides in Na
2
SO
4
is known to be substantially high (e.g.,
WO
3
dissolves to the extent of more than 50 mol % at 1023 K). As dissolution
continues, the Na
2
SO
4
solution gradually becomes enriched with the oxides of
refractory metals. Al
2
O
3
,Cr
2
O
3
, and CoO can also be dissolved in such refractory
metal oxide-enriched melts by donating oxide ions to the melt according to
Al
2
O
3
3WO
3
2Al
3
2Al
3W
6O
2
→
3WO
4
2
(6.47)
2Cr
3W
6O
2
→
Cr
2
O
3
3WO
3
2Cr
3
3WO
4
2
(6.48)
Co
W
2O
2
→
CoO
WO
3
Co
2
WO
4
2
(6.49)
Subsequently, these ions diffuse through the solution to the outer surface of the
melt where the above-mentioned reactions proceed in reverse direction due to
the prevalence of lower activity of the refractory metal oxides in that region.
Such low activity is caused by evaporation loss of refractory metal oxides to the
gas phase. Hence, Al
2
O
3
,Cr
2
O
3
, and CoO are dissolved at one side of the molten
salt and reprecipitated as a nonprotective scale at the melt-porous oxide interface.
The precipitation process results in some of the melt being incorporated into the
outer, porous part of the scale.
