Environmental Engineering Reference
In-Depth Information
In contrast, Ni-Cr-Al alloys containing 15-30% Cr and 1% Al exhibited a
total protective behavior (type 2) with no sign of break-away during its exposure
period. On further increase of the Al content (4-4.5% Al) with the same Cr level,
the alloy showed superior protective performance at both temperatures (type 1).
The different morphologies of the scales developed on the three alloy systems
are presented schematically in Fig. 6.17.
Comparison of the scaling behavior of the three alloy systems [21,25,30,35]
highlights the following interesting trends:
1.
The ease of formation of a protective
-Al
2
O
3
layer on the alloys and the
lowest mass gain necessary to achieve this protective layer are in the order
Fe-Cr-Al
α
Ni-Cr-Al
Co-Cr-Al.
2.
Under rapid scaling conditions, as a result of break-away of protective
α
-
Al
2
O
3
,Cr
2
O
3
,orCr
2
O
3
-
-Al
2
O
3
oxide layers, the most reliable scale is pro-
duced on Ni-Cr-Al. Fe-Cr-Al gives the most disastrous local break-away by
locally formed nodules that grow at a considerably faster rate than on Co-
Cr-Al alloys, even though its surface area undergoing break-away at a cata-
strophic rate is less. The Co-Cr-Al system yields the most generally distrib-
uted break-away over the alloy surface for which the overall mass gain is
found to be greater than the corresponding Fe-Cr-Al alloys.
α
3.
α
-Al
2
O
3
is the most desirable surface scale and doped Cr
2
O
3
with an inner
layer of doped
-Al
2
O
3
is the next most protective scale. Doped Cr
2
O
3
sur-
face scale located above
α
-Al
2
O
3
-rich internal oxide is a less desirable mode
of scaling but is definitely more protective than the various nodular and fully
stratified scales containing the respective oxides of Fe, Ni, or Co, which may
develop during the early stages of oxidation or after mechanical breakdown
of the
α
α
-Al
2
O
3
or Cr
2
O
3
scales.
So it is clearly revealed that at certain critical concentrations of the alloy con-
stituents the reaction during steady-state changes from one mechanism to the
other. For a particular alloy system, with the help of available data, one can
construct an ''oxide map'' in which the composition ranges of the alloys are
delineated for the formation of different types of oxide scale and the reaction
behavior. An example of such oxide map for Ni-Co-Al alloys at 1273 K [32] is
illustrated in Fig. 6.18. In region 1, the scale consists of NiO, NiCr
2
O
4
, and
NiAl
2
O
4
spinels with internally oxidized particles in the alloy; region 2 represents
Cr
2
O
3
as the external scale, and in addition Al is internally oxidized; region 3
represents selective oxidation of Al producing
-Al
2
O
3
scale. There exists a great
challenge for future investigators to derive such maps for different alloy systems
with the help of more basic data involving the interplay of thermodynamics,
diffusion data, and interfacial reactions in predicting the expected scale morphol-
ogy utilizing the diffusion path approach of Dalvi et al. [27].
α
