Environmental Engineering Reference
In-Depth Information
Figure 5.29
Variation of critical temperature drop to initiate chromia spallation from
austenitic steel as function of gross weight gain or oxide thickness [Ref. 51].
and spallation occurs by the formation of cracks in tensile regions around the
buckle. This buckle configuration is stable, i.e., it will not extend laterally by
tensile crack growth along the oxide-metal interface. Such lateral growth will
occur in a small region as shown at the top of the map bounded by the buckling
and wedging lines after their interaction. The dominant failure mode for the ex-
ample cited is the wedging as indicated by a large area to the right-hand side of
the map. Of much interest is the region to the bottom of the map within which
the oxide layer suffers no significant mechanical damage. Alloys having high
spallation resistance will have this area in an enlarged form.
A significant feature of Fig. 5.30 is the rapid increase in the value of
T
c
for
buckling with only modest increases in oxide thickness, e.g., over the range 0.5-
1.0
∆
m. A consequence is that buckling failure is likely to be important only for
thin layers or for those that already exhibit large-scale decohesion. This appears
unlikely in alloys that have been designed for high-temperature oxidation resis-
tance where extensive interfacial void formation is rare. A recent study for the
interfacial voids during high-temperature oxidation of alumina forming FeCr
alloy (Fe-22Cr-5A1) and Haynes 214 (Ni-16Cr-4.5A1) alloys exhibited nothing
µ
