Environmental Engineering Reference
In-Depth Information
systems can be explained in terms of the interfacial segregation of reactive metal
ions to metal/alloy-scale interface and the scale boundaries. They diffuse out-
wardly from the metal substrate to the gas interface, and the driving force for
such diffusion is the oxygen potential gradient in the scale. By diffusing outward
along the scale grain boundaries, the reactive element ions inhibit outward cation
transport.
Similar types of mechanisms have also been suggested for Al
2
O
3
-forming
alloys [40-42]. Even though in the literature there is a general agreement that
reactive elements influence the transport processes in Al
2
O
3
scales, the situation
is not as clear as for Cr
2
O
3
scales. There exists considerable disagreement regard-
ing the detailed mechanism. This is mainly due to the fact that the transport
mechanism in undoped alumina has always remained a subject of controversy
[50]. Many researchers have suggested that the preponderant diffusing species
is the oxygen, while others hold the view that Al
2
O
3
scales grow either by outward
diffusion of aluminum or by diffusion of both oxygen and aluminum. So the
beneficial effect of active elements is assumed to be due to a decrease of oxygen
or aluminum diffusion. However, there is general agreement that, in contrast to
the situation of chromia scales, active elements do not reverse the growth mecha-
nism of alumina scales, although they influence the transport processes. There
also exists controversy [52] about the beneficial or detrimental effects of certain
active elements like Ti, Zr, Hf, etc., and this appears to be dependent on their
amount, chemical state in the scale, alloy nature, oxidation conditions (tempera-
ture, gas composition), and so forth. Sulfur is always recognized as a detrimental
element, whereas the effect of yttrium on the growth kinetics of Al
2
O
3
scales is
marginal compared to the growth kinetics of Cr
2
O
3
scales. In some cases, Y
decreases (marginally) the oxidation rate of alumina-forming alloys, but an in-
crease has also been observed (for FeCrAl alloys at
T
1373 K) [52]. Therefore,
at this stage it may be inferred that the predominant mechanism by which active
elements improve the oxidation resistance of Al
2
O
3
-forming alloys probably is
not directly related to their influence on transport properties but rather associated
with some effects in the improvement of scale adhesion.
Mechanisms for Improved Scale Adherence
The most beneficial effect of active element additions on the oxidation behavior
of Cr
2
O
3
- and Al
2
O
3
-forming alloys is the considerable improvement in scale
adherence to the alloy substrate. This aspect has been studied in detail, and a
number of hypotheses [40,42,48,49,50,52] have been advanced to account for
this. It has to be realized that this effect is irrespective of whether the alloy con-
tains RE in its elemental form or as a dispersion of its stable oxide. Only distribu-
tion of the oxide particles may be different in the two cases. The RE oxides
formed during the high-temperature exposure often exist in the vicinity of the
alloy grain boundaries at intersections with the scale-alloy interface. However,
in the case of dispersion containing alloys, the oxides are more randomly distrib-
