Environmental Engineering Reference
In-Depth Information
Figure 5.31
Notional spallation map for alumina on FeCr alloy steel cooled from
1373 K. Note expansion of region of no damage [Ref. 51].
Fig. 5.31. This is thought to be representative of an alumina layer on FeCr alloy
steel being cooled from an oxidation temperature of 1373 K. The movement of
the wedging line to the right (a weak substrate) and the buckling line to the left
(a small defect size) has widened the central area of the map so that no serious
spallation damage will be expected for most experimental conditions. It seems
likely that this combined influence of low creep strength of the alloy and absence
of large interfacial defects is the principal reason for the excellent spallation resis-
tance of this type of alloy.
In conclusion, it can be stated that there exists a direct influence of oxide
growth stresses on defect diffusion through the layer and hence on the high-
temperature oxidation behavior of metals. In particular, the stress factor becomes
significant when constrained volume changes accompany growth of the layer.
5.9 DISSOLUTION AND DIFFUSION OF OXIDANT
IN METALS
All metals exhibit a tendency to absorb oxygen and other oxidants (like nitrogen,
sulfur, carbon, etc.) to a lesser or greater extent, especially at elevated tempera-
