Environmental Engineering Reference
In-Depth Information
is to alter the diffusional transport process from predominantly outward cation
migration to predominantly inward diffusion of oxygen. It is also interesting to
note that the oxides of the reactive elements, e.g., Y
2
O
3
,La
2
O
3
, CeO
2
, ZrO
2
,
and HfO
2
, are all oxides in which oxygen defects predominate, making oxygen
diffusion much faster than cation diffusion. When inward oxygen diffusion pre-
dominates, oxide formation takes place at the scale-alloy interface, reducing void
or cavity formation, and thereby increased scale adhesion is achieved. The wrin-
kles and convolutions of the scale caused by countercurrent diffusion of oxygen
and chromium are also reduced. Such modifications in the scale growth process
in the presence of reactive elements are illustrated schematically in Fig. 6.26.
Modification of Scale Morphology and Microstructure.
For Al
2
O
3
-forming
alloys, in most cases the scales developed on alloys without reactive element
additions are non-uniform in thickness, convoluted, with protrusions at the scale-
gas interface and/or intrusions at the scale-alloy interface. Literature suggests
that Al
2
O
3
primarily grows by oxygen transport; however, the wrinkling and con-
volutions of alumina scales hint that oxide formation takes place within the scale
in a manner similar to that of chromia scales, and accordingly accompanied by
outward diffusion of aluminum also, probably along line defects in the oxide.
Hence, lateral growth of oxide produces rapid development of high compressive
stresses resulting in localized detachment of the scale from the underlying alloy.
Figure 6.26
Schematic representation of the chromia scales growth in (a) absence of
and (b) presence of reactive element [50].
