Environmental Engineering Reference
In-Depth Information
prolonged exposure the nature of the NiO scale becomes double-layered with an
outer, fairly compact, columnar crystal layer followed by an inner porous layer
of more fine-grained oxide. The following two mechanisms have been proposed
to explain such continued oxidation:
1.
Dissociative transport across the voids
2.
Development of microchannels in the outer layer that allow the oxidant to
penetrate into the inner layer.
Dissociative Gas Phase Transport Across Voids
When a void coalesces at the inner metal-oxide interface, metal ions will con-
tinue to move outward from the void surface to the outer oxide-gas interface.
Due to depletion of metal ions at the region of dissociation, there will be a corre-
sponding increase in the partial pressure by the liberated oxygen in the void. The
oxidant is thus transported in the gas phase across the void and new oxide is
formed on the interior surface of the void. Such a model is presented in Fig.
5.20. At the initial stage of void formation, partial pressure of the oxidant is
equal to the equilibrium dissociation pressure at the metal-oxide interface at the
temperature under consideration. For example, the partial pressure of oxygen
at the Ni-NiO interface at 1273 K is approximately 1
10
10
atm. Therefore,
instantaneous pressure of oxidant is very small. But with continued outward
nickel transport from the void surface to the NiO/O
2
(g) interface, the pressure
Figure 5.20
Model for the growth of scales by dissociative transport across voids in
the scale. Metal ions migrate outward through the scale by solid state diffusion, while
oxygen is transported inward across the voids as gaseous species [Ref. 4].
