Environmental Engineering Reference
In-Depth Information
of specimen surface preparation often include either chemical polishing fol-
lowed by electropolishing (EP) or electropolishing followed by light etching
(ETCHED). Abrasion, the most widely used technique, produces a cold-worked
(CW) distorted surface. Etching of such a surface to get rid of the worked layer
may leave a rough surface that may be enriched with a more noble metal of the
alloy constituents by preferential dissolution or redeposition. In a similar way,
the procedure for oxidation tests could also vary. The test runs are generally
carried out either in an ultrahigh-vacuum manometric apparatus, which is a closed
system, or with the help of a continuously recorded thermobalance in a gas flow-
ing system. ''Hot-bare'' (HB) run may be conducted on metal surface free from
prior oxide after electropolishing followed by cathodic reduction with hydrogen.
In such experiments, oxygen is allowed to come in contact with oxide-free speci-
men at the predetermined experimental temperature. In the ''furnace-raised''
(FR) procedure, EP or ETCHED specimens are heated in the presence of oxygen
to the oxidation temperature. Cold insertion (CI) experiments are conducted by
quickly lowering the test samples into the hot zone of a vertical tube reactor in
flowing oxygen at a particular pressure.
5.10.2 Oxidation of Nickel
Results reported by Graham et al. [64] on oxidation of Ni at 973 K in 0.5 torr O
2
pressure are presented in Fig. 5.36. This figure clearly depicts that HB specimens
oxidize at a higher rate than the FR ones of either EP or ETCHED surfaces.
Moreover, the oxide formed on the HB run is reported to be fine-grained and
uniformly thick over the entire surface, whereas the oxides formed on the
ETCHED specimens are of large variation in oxide thickness with differences
in substrate grain orientation.
NiO is a p-type semiconductor with a predominance of point defects such as
vacancies on Ni sites. Thus, NiO on Ni grows by outward transport of Ni ions
through cation vacancies (lattice diffusion) and preferentially via oxide grain
boundaries. With a finer oxide grain size there are more oxide grain boundaries
to act as easy diffusion paths and therefore the oxidation rate is faster. Under
HB oxidation conditions a large number of randomly oriented oxide nuclei ini-
tially form on all metal orientations, yielding the maximum oxidation rate. At
temperatures of 973 K and below, the kinetic data obey a parabolic rate law with
an associated activation energy of 155 kJ/mol, an approximate value for the
growth of NiO via easy diffusion paths. On the other hand, under FR conditions
the epitaxial relationships between NiO and the different Ni orientations produce
overgrowths with a relatively restricted range of leakage path populations. In Fig.
5.36, the FR oxidation rate of ETCHED Ni is found to be slowler than the EP
Ni, probably due to formation of a few NiO grains which contribute to high-
leakage paths during oxidation.
