Environmental Engineering Reference
In-Depth Information
enhanced selective oxidation of chromium is interpreted as being due to forma-
tion and stabilization of a fine-grained substructure with high dislocation density
by the thoria particles. This substructure favors rapid short-circuit diffusion of
chromium, resulting in rapid development of a protective Cr
2
O
3
layer on the alloy
surface. The oxide particles of active element act as nucleation sites and promote
selective oxidation of the protective scale-forming element, thereby curtailing
the transient stage of oxidation as schematically illustrated in Fig 6.23 for Ni-
Cr alloy.
Essentially three main mechanisms have been proposed [48] to explain the
reduction in scale growth rate and possible modification to the transport processes
brought about by the reactive metals or their oxide additions. These are (1) doping
effects, (2) formation of a partial or complete blocking layer in the scale, and
(3) short-circuit diffusion model.
Figure 6.23
Schematic illustration of reactive element on scale nucleation in Ni-Cr
alloy with and without reactive element oxide dispersion.
