Environmental Engineering Reference
In-Depth Information
Basic Processes
A. Dissolution of reaction product (i.e., AO) due to removal of sulfur and oxygen from
Na
2
SO
4
by the metal or alloy:
1
2
3
2
SO
4
2
(sulfate deposit)
→
S
2
(for reaction with alloy)
O
2
(for reaction with alloy)
O
2
(for reaction with AO)
Reaction between AO and oxide ions can follow two courses:
(i) Continuous dissolution of AO
1
2
A(alloy)
O
2
O
2
→
AO
2
2
Na
2
SO
4
is converted to Na
2
AO
2
and the attack is dependent on the amount of Na
2
SO
4
initially present.
(ii) Solution and reprecipitation
1
2
A(alloy)
O
2
O
2
→
AO
2
(solution)
→
AO(precipitate)
O
2
A supply of SO
3
is required in order the attack can proceed indefinitely; otherwise the
attack will stop when melt becomes sufficiently basic at the precipitation site.
B. Solution and precipitation of AO as a result of a negative gradient in the solubility of
AO in Na
2
SO
4
Acidic Processes
Gas phase-induced
:
C. Formation of ASO
4
in Na
2
SO
4
:
1
2
A(alloy)
SO
3
O
2
→
A
2
SO
4
2
Continuous solution of ASO
4
in Na
2
SO
4
requires continuous supply of SO
3
and O
2
from the gas phase.
D. Solution and precipitation of AO in Na
2
SO
4
due to reduction of SO
3
:
A(alloy)
SO
3
(from gas)
→
A
2
SO
3
2
(in melt)
1
2
A
2
SO
3
2
O
2
(from gas)
→
AO(precipitate)
SO
3
E. Solution and precipitation of AO as a result of a negative gradient in the solubility of
AO in Na
2
SO
4
.
Alloy phase-induced
:
F. Solution of AO in Na
2
SO
4
modified by a second oxide from the alloy (i.e., BO
3
)
Modification of Na
2
SO
4
by BO
3
:
3
2
B(alloy)
O
2
SO
4
2
→
BO
4
2
SO
3
Solution reaction for AO; Na
2
SO
4
becomes enriched in ABO
4
:
A(alloy)
B(alloy)
2O
2
→ A
2
BO
4
2
Solution and precipitation:
A(alloy)
B(alloy)
2O
2
→ A
2
BO
4
2
→ AO
BO
3
Precipitation of AO in Na
2
SO
4
as a result of BO
3
loss from Na
2
SO
4
permits substantial
attack with small amounts of Na
2
SO
4
.
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