Environmental Engineering Reference
In-Depth Information
uted in the alloy matrix. Even though several mechanistic models have been
postulated to explain the improvement in scale adherence, these are not mutually
exclusive and a consensus view is yet to emerge. The various suggested models
are discussed below.
Enhanced Scale Plasticity.
Many researchers believe that the scales formed on
metal/alloy-containing reactive element/oxide dispersoid have higher plasticity
contributing to better adhesion than those formed on metal/alloy without RE. It
is well known that improvement in scale plasticity can be obtained by a finer
grain structure of the oxide scale. There is no direct evidence for improvement
in scale plasticity of alumina scales by active elements, though there is extensive
literature that shows that the reactive elements considerably improve the scale
adhesion. In most cases, presence of the reactive element produces a columnar
fine-grained oxide microstructure segregated at the grain boundaries. This will
definitely affect the plastic deformation behavior. Diffusional creep is the most
important mechanism for plastic deformation in fine-grained polycrystalline alu-
mina. Besides, the creep strain rate is extremely sensitive to grain size and in-
creases with decreasing grain size. Plastic deformation will occur more easily in
doped scales than in undoped ones. The growth stresses will be relieved by plastic
flow during oxidation.
Vacancy Sink Model.
There is much clear evidence in the literature to justify
that the presence of a reactive metal or its oxide dispersion in the alloy minimizes
the development of voids at the alloy-scale interface. In the usual alloys, the
voids arise from condensation of vacancies at the alloy-scale interface. It has
been proposed that the internal oxide particles of the reactive element, the reactive
element atoms themselves, or the stable oxide dispersions provide alternative
sites for vacancy condensation, thus eliminating interfacial porosity. This, in turn,
helps to maintain better scale-alloy contact and minimize the possibility of scale
spallation. Moreover, for Cr
2
O
3
-forming alloys with reactive element additions,
the scale growth mechanism is reversed, with the oxide-forming reaction shifting
from the scale-gas interface to the scale-alloy interface, whereas Al
2
O
3
scales
grow primarily by oxygen transport. So the possible source of vacancies is con-
fined to the early transient stages of oxidation when faster-growing base metal
oxides, such as NiO and CoO, are being formed. Once such vacancies are formed
on reactive element-free alloys, voids continue to persist. But the presence of
reactive element or its stable oxide dispersion in the alloy curtails this transition
period, leaving only a few vacancies. It has been reported [40] that 0.05% addi-
tion of Y or Hf to Co-10% Cr-11% Al alloy could successfully eliminate all the
voids.
Graded Seal Mechanism.
The graded seal mechanism is based on the assump-
tion that the compound oxide layer developed between the surface scale and the
alloy possesses a thermal expansion coefficient intermediate between the scale
