Environmental Engineering Reference
In-Depth Information
those of the original metal core. Similar observation was also made by Bose and
Sircar [48] who found that iodination of a copper wire ultimately transformed it
into a pipe of CuI with a central hole throughout the length of the wire. Such
observation hints to outward migration of Cu ions and ingress of cation vacancies
through the CuI layer, leading to vacancy coalescence and formation of central
cavity. However, depending on the exposure conditions (temperature and oxidant
pressure in the gas phase), only a fraction of this volume might develop as voids
and pores. Kofstad [4] has illustrated the following two major reasons for decohe-
sion of scale from the substrate:
1.
Extent of plastic deformation of the oxide scale
2.
Vacancy injection and void formation within the metal
Due to vacancy injection, voids in the metal may also be formed by hot defor-
mation or creep of the metal substrate induced by the oxidation process. The
voids can then be considered to nucleate at the grain boundaries due to grain
boundary sliding accompanying the hot deformation process. When the oxide
scale grows by outward metal ion migration and is less nonstoichiometric in
nature, being too thick to deform or to maintain contact with the receding metal
substrate, vacancies condense out and form porosities near the metal-oxide phase
boundary. For this reason, void formation has often been termed ''vacancy con-
densation.'' However, voids may also form at the inner metal-oxide interface
when metal ions diffuse outward through the scale as interstitials.
It is established that the tendency of void formation in the retreating metal as
well as in the scale is a function of geometry of the specimen and its surface-
to-volume ratio (radius of curvature). For a metal having an infinite plane surface
and oxidizing uniformly over the entire surface, the oxide being formed will
continuously collapse on the retreating metal surface without any deformation
of the scale. If, instead of uniform attack, nonuniform oxidation takes place for
a specimen with finite dimensions, constraints are imposed on the system, particu-
larly at the edges and corners which become highly prone to formation of voids
and cavities. Study on oxidation of high-purity nickel rods of various diameters
in 1 atm O 2 at 1373 K has demonstrated that the density of voids formed increased
with increasing oxidation time and with decreasing diameter of the rods. The
effect of surface-to-volume ratio has been attributed to the fact that the oxide
envelopes (with equal thickness) have less ability to deform, the smaller the diam-
eter of the rod.
The presence of carbon in metal has a dramatic effect on the extent of cavity
formation and detachment of scale from the metal. Kuiry et al. [49] have shown
excessive cavitation, wrinkling, and bulging of the scale in mild steel due to the
formation of CO and CO 2 beneath the scale. It is further demonstrated that during
oxidation of plain carbon steel with superficially applied coatings containing mix-
tures of ferromanganese, ferrosilicon, and bentonite or CaSi 2 and bentonite pow-
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