Environmental Engineering Reference
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different durations of exposure has established that the oxide grain size gradually
increases with time (scale densification). Accordingly, the parabolic oxidation
rate constant { k p
W )/ dt ]} decreases progressively, consistent with de-
creasing population of easy diffusion paths due to scale densification. In compari-
son, the oxide grain size formed on annealed Ni is larger in the beginning and
changes little with time; accordingly, k p remains fairly constant. The k p for CW
Ni is made up of two components, i.e., the normal lattice diffusion, plus a contri-
bution from leakage paths that is the greatest at initial stages of oxidation at lower
temperatures where the oxide grain size is small. By correcting oxidation rates
of CW Ni for lattice diffusion (using theoretical relation with contributions from
both processes of migration), and by taking into account the changes in leakage
path density during growth, it is possible to estimate the parabolic rate constants
and an activation energy for growth via leakage paths only. The estimated activa-
tion energy is reported to be 155
2[ d (
8.4 kJ/mol, which is in agreement with the
value for HB oxidation studies in the temperature range 573-973 K where growth
of oxide takes place exclusively by cation transport across the film via high-
diffusivity paths (grain boundaries and dislocation pipes).
5.10.3 Oxidation of Chromium
Similar to the oxidation behavior of Ni the electropolished Cr surface is also
reported to result a fine-grained oxide layer and, consequently, a very rapid rate
of oxidation as depicted in Fig. 5.39. Experiments conducted at 1363 K in 1 atm
O 2 on EP and ETCHED Cr show that EP Cr oxidizes at a much faster rate than
the ETCHED one. This is due to formation of fine-grained prior oxide film on
EP Cr during electropolishing. Such a situation leads to the development of a
fine-grained Cr 2 O 3 layer during a high-temperature oxidation test. In contrary, a
well-epitaxed coarse-grained oxide forms on ETCHED Cr. Some orientations of
ETCHED Cr are thought to develop monocrystalline scale for which the parabolic
growth constant and the associated activation energy are minimum. The evidence
of multilayered and highly blistered scale on EP Cr is a consequence of the com-
pressive stresses that develop in the growing oxide layers. In contrast to NiO,
Fe 3 O 4 , and single-crystal Cr 2 O 3 , which are considered to grow by outward cation
diffusion, polycrystalline Cr 2 O 3 , possibly forms by outward diffusion of cation
as well as ingress of oxygen along the oxide grain boundaries. As a result of
such two-way traffic of both Cr and oxygen, new oxide formation within the
growing scale leads to development of high compressive stresses. When the oxide
contains a large population of easy diffusion paths, the high flux of cation vacanc-
ies is expected to promote scale separation at the metal-oxide interface. However,
because of the high volatility of Cr, oxidation is sustained by Cr vapor transfer
across the gap; generation of compressive stress from the two-way diffusion be-
ing continued leads to buckling and wrinkling of scale. The progressive stress
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