Environmental Engineering Reference
In-Depth Information
of oxygen in the void gradually builds up and eventually becomes adequately
high to sustain reaction for new oxide formation at the interior surface. With
continued oxide dissociation and new oxide formation, the void is expected to
move gradually outward in the scale. This will ultimately lead to opening of the
void at the oxide-gas interface.
Development of Microchannels
A model for the development of microchannels in the scale has been proposed
by Mrowec [50]. In this scheme an initial void is assumed to be formed at the
inner interface just below an oxide grain boundary as shown in Fig. 5.21. The
Figure 5.21
Model for the development of microchannels above cavities at the metal-
oxide interface or in the oxide scale. Solid state diffusion in the oxide takes place by
outward metal diffusion in the lattice and along grain boundaries. When grain boundary
diffusion is much faster than lattice diffusion, the grain boundary opens into a microchan-
nel. This, in turn, permits inward transport of molecular oxygen [Ref. 4].
