Environmental Engineering Reference
In-Depth Information
Figure 6.47
Comparative plot of diffusion coefficients in some spinels [2].
simple oxides. These spinels are contemplated to serve as effective diffusion
barriers since they exhibit low diffusion rates [2] in comparison with simple ox-
ides, as illustrated in Fig. 6.47.
The above discussion clearly brings out the following characteristic properties
of the three most commonly preferred alloying elements, Cr, Al, and Si, which
can form stable, self-healing, oxide layers on alloy surfaces in providing protec-
tion under oxidizing environments:
1. Transport rates of aluminum cations are the slowest among the three simple
oxides in the temperature range below 1673 K.
2. In the cases of Cr
2
O
3
and Al
2
O
3
layers, the diffusing species are the respec-
tive cations, whereas in the case of pure SiO
2
, it is mainly nonionized oxygen.
3. At temperatures above 1573 K, the mobility of the diffusing species, i.e.,
oxygen in SiO
2
, is the lowest among the three simple pure oxides.
