Environmental Engineering Reference
In-Depth Information
and the substrate alloy. However, very few alloy systems develop such a continu-
ous intermediate scale.
Mechanical Keying/Oxide Pegging.
This is perhaps the most important factor
for improved scale adhesion during long-term exposures and under thermal cy-
clings. Many authors suggest that mechanical keying/oxide pegging is a result
of internal oxidation of the reactive element additions, or of oxide dispersoid
particles growing in size to form oxide stringers in the form of thin elongated
intrusions extending into the alloy substrate and thereby anchoring the scale to
the alloy as illustrated in Fig. 6.25.
Improved Chemical Bonding.
The adhesion of the scale to alloy substrate is
primarily governed by the atomic bonds that develop across the oxide-alloy inter-
face. It is reported that there are certain harmful impurities, such as S and P,
which are often present as minor impurities in the alloy, segregate to the scale-
metal interface, and embrittle the interface affecting scale-metal adhesion. This
is better known as the ''sulfur effect'' [53]. It has been proposed that reactive
element (like Y) additions reduce this segregation by interacting with the sulfur
and thereby improved scale adhesion is achieved.
Modification of the Scale Growth Process.
The addition RE or oxide dispersion
to chromia forming alloys in general reduces the oxide growth rate by a factor
of 10 or so at high temperatures. Such a reduction in rate is not significant (if it
happens at all) with alumina-forming alloys. Any reduction in rate is associated
with a decrease in the growth stresses. It has already been mentioned that in the
growth of chromia scales the important major effect of the reactive elements
Figure 6.25
Schematic diagram showing the mechanical keying of oxide scale devel-
oped on Fe-26wt%Cr-4wt%Al-0.82wt%Y alloy at 1200
°
CinO
2
at 1 atm [39].
