Environmental Engineering Reference
In-Depth Information
Figure 6.36
Schematic representation of an idealized model for fluxing of a continu-
ous, protective oxide scale in a layer of molten salt deposit [61].
dissolved in the molten salt. It further implies that if the reaction rate is a linear
one, the protective scale is reformed at the same rate as it dissolves, i.e., the scale
has a stationary thickness.
The electrochemical model proposed by Rapp and Goto [62,63] states that
an accelerated attack during hot corrosion is sustained through a dissolution-
precipitation process whenever there is a negative gradient in the solubility of
the protective oxides across the salt film, i.e., at the oxide-salt interface, as illus-
trated in Fig. 6.36. Such gradients are developed by an electrochemical reduction
reaction that accomplishes oxidation of the metallic elements. Accordingly, the
dissolved protective oxide is transported away from the salt-oxide interface and
precipitated at some distance from it in a region of lower solubility as a noncontin-
uous and nonprotective phase. This model is based on the solubility data of Cr
2
O
3
and Al
2
O
3
in fused Na
2
SO
4
at 1200 K, and the slopes of the solubility lines
substantiate the following acidic and basic dissolution reactions:
Al
2
O
3
s 2Al
3
3O
2
acid dissolution
(6.41)
Al
2
O
3
O
2
s 2AlO
2
2
base dissolution
(6.42)
Cr
2
O
3
s 2Cr
3
3O
2
acid dissolution
(6.43)
Cr
2
O
3
2O
2
s 2CrO
4
2
base dissolution
(6.44)
The oxide stability gradient is established by the nature and site of the electro-
chemical reduction step, which will generate basic ions. The interrelation of the
gradient of the basic ions in the melt to the oxide solubility map determines the
extent and continuance of hot corrosion reaction. The major advantage of this
model over the foregoing fluxing model is that oxide ion production need not
occur as a result of removal of sulfur from Na
2
SO
4
. Therefore, the electrochemi-
