Environmental Engineering Reference
In-Depth Information
results in formation of a thin amorphous layer of silica between the chromia and
the steel. It has been speculated that this amorphous interlayer might have im-
proved adherence by its ability to accommodate epitaxial misfit strains. However,
for a condition of good oxide-metal bonding, it is still possible to demonstrate
prevalence of both wedging and buckling modes while characterizing spallation
at different oxide thicknesses even within the same oxide-alloy system.
Researchers normally conduct oxidation experiments for a number of thermal
cycles in order to have a better assessment on the performance of high-tempera-
ture alloys. It will be of interest to determine the value of
T
c
(critical temperature
drop to initiate spallation) for spallation in the first cycle when the protective
surface oxide remains undamaged and its properties can be well characterized.
A number of methods such as use of thermobalances, acoustic emission, or
a combination of both have been developed to determine the
∆
T
c
values. How-
ever, the most direct approach is the use of thermobalances where spallation is
readily detected as a loss of mass. Use of this simple technique on the oxidation
behavior of 20Cr-25Ni-Nb-stabilized austentic stainless steel has been exten-
sively employed and reviewed. The detailed attention paid to this material derives
from its use as thin-walled (0.38-mm) cladding for UO
2
fuel in advanced gas
cooled (nuclear) reactors. On oxidation at temperatures around 1123 K the alloy
forms a protective oxide layer of chromia with an underlying thin interlayer of
silica (amorphous).
The variation of the critical temperature drop,
∆
T
c
, to initiate spallation with
the estimated thickness of the chromia layer is shown in Fig. 5.29. It depicts a
distinct trend of decreasing
∆
T
c
values with increasing oxide thickness. This is
the dependence expected from the wedging process, whether this is described in
the phenomenological manner of a critical strain energy or by the mechanistic
finite element calculations.
It was already stated that the initiation of oxide spallation in a given thermal
cycle may occur either by a wedging or buckling mode as illustrated in Fig. 5.28.
Both of these mechanisms have been theoretically examined and expressions
derived [51] that adequately predict the critical temperature drop,
∆
T
c
to effect
spallation. These expressions differ in their parametric dependence on oxide
thickness such that the preferred mode of mechanical failure for any given oxide-
metal system will change as the oxide layer thickens. These different relations
can be demonstrated graphically by using ''spallation maps.'' An example of
such maps is presented in Fig. 5.30 for chromia scale formed on 20Cr-25Ni aus-
tenitic stainless steel and being cooled from 1173 K. The line curving downward
and the line curving upward with increasing oxide thickness correspond to the
wedging and the buckling criteria, respectively.
Basically, this diagram (Fig. 5.30) consists of four regions within which four
distinctly different modes of mechanical response to the cooling cycle occur. To
the left-hand side of the map there is a region in which buckling failure dominates
∆
