Environmental Engineering Reference
In-Depth Information
The high-temperature corrosion resistance of numerous practical alloys are
provided mainly by scales consisting of either Cr
2
O
3
,Al
2
O
3
, SiO
2
, or more com-
plex oxides of these compounds. Since the concentration of point defects in Cr
2
O
3
and Al
2
O
3
are of negligible order compared to those in base metal oxides like
NiO, Cu
2
O, FeO, CoO, etc., the growth of protective Cr
2
O
3
and Al
2
O
3
scales
are expected to take place by countercurrent grain boundary diffusion of metallic
and nonmetallic species whereby the new oxide formation takes place at the grain
boundaries of the growing scales. Therefore, at relatively low and intermediate
temperatures, for the interpretation of oxidation kinetics, one should consider the
contribution of grain boundary diffusion along with that of lattice diffusion. The
mathematical models suggested for such estimation have been discussed in detail
by Kofstad [4].
5.7 FORMATION OF VOIDS, POROSITIES, AND OTHER
MACRODEFECTS IN OXIDE SCALE AND
IN THE SUBSTRATE
Metals like Cu, Ni, Co, Fe, etc., which oxidize predominantly by outward cation
migration through the film/scale, are found to generate vacancies at the com-
pound layer-gas interface. These vacancies diffuse inward through the growing
oxide, e.g., Cu
2
O, NiO, CoO, FeO, etc., and accumulate at the metal-oxide inter-
face, thereby nucleating and subsequently coalescing in the forms of voids and
porosities near the inner interface and sometimes within the metal itself. It has
been demonstrated during oxidation of Fe-19%Cr alloy and many other metal/
alloy-oxide systems [4] that these interfacial voids lead to loss of adhesion and
subsequent spallation of the scale. Moreover, supersaturation of cationic vacan-
cies at the metal-oxide interface is accompanied by development of a vacancy
concentration gradient within the metallic substrate. In particular, vacancies have
been found to migrate into nickel by Hancock and Fletcher [3], into copper by
Jaenicke et al. [44] and Appleby and Tylecote [45], and into iron by Cagnet and
Moreau [46]. These vacancies have generally been found to precipitate preferen-
tially along grain boundaries of the metals.
When the growth of oxide on a metal solely involves outward migration of
metal ions (oxygen ions are considered to be essentially immobile in the oxygen
sublattice), fresh oxide is formed at the oxide-oxygen interface. If the situation
is such that the oxide formed is a completely compact envelope around the metal
core, voids and porosities should develop beneath the scales, and the total volume
of the void should be equal to the volume of the metal that is being consumed
and converted to oxide. A classical example of such circumstances has been
demonstrated by Mackenzie and Birchenall [47] who found that an iron wire rod
when fully oxidized formed a central cavity of dimensions almost identical to
