Environmental Engineering Reference
In-Depth Information
the development of dispersion-strengthened alloys gained momentum from the
understanding of this phenomenon.
The earliest comprehensive literature on internal oxidation, published by
Rhines et al. in 1942 [12], provides not only experimental data on a number of
copper alloys but also a formal treatment of the kinetics of the process. Subse-
quently, Darken [13] presented a more generalized treatment of the process com-
bining occurrences of diffusion and precipitation. However, the most lucid treat-
ment on kinetics of internal oxidation was provided by Wagner [14] in 1959,
and the same approach is presented below in estimating the depth of internal
oxidation zone (IOZ) as a function of exposure time. Internal oxidation requires
that the rate of diffusion of oxygen in the alloy is appreciably faster than that of
the solute element. In such a situation, an oxygen gradient is established in the
alloy, and the dissolved oxygen reacts to form an oxide of the solute element in
a zone beneath the alloy surface.
For simplicity, one may consider the planar specimen geometry and the quasi-
steady-state approximation. A binary alloy, A-B is chosen in which B is a dilute
solute that forms a very stable oxide. It is further assumed that the ambient partial
pressure of oxygen is too low to oxidize A but high enough to oxidize B. In such a
situation, the concentration profiles of dissolved oxygen and the alloying element
during internal oxidation of A-B alloy free from any initially formed surface
scale is schematically presented in Fig. 6.4. The quasi-steady-state approximation
implies that the dissolved oxygen concentration varies linearly across the IOZ.
Therefore, the oxygen flux through the IOZ will be given by Fick's first law as
Figure 6.4
Schematic concentration profiles of dissolved oxygen and solute element
for the internal oxidation of A-B alloy.
