Environmental Engineering Reference
In-Depth Information
of the substrate. Besides, precipitation within metallic matrix will create local
differential stress distribution and strain rate. With increased point imperfections,
the scale is expected to have a better tendency to relaxation through elastic and
plastic deformation. Moreover, at elevated temperatures, individual oxide grains
may undergo some sort of sintering process facilitating a better barrier to mass
transport rendering protectiveness to the underlying substrate. However, it is to
be realized that stress and relaxation would be decided by a complex interplay
of mechanical and chemical properties of any metal-scale combination.
5.8.4 Effect of Vacancies Generated During Oxidation
For p-type oxide growth like NiO on Ni, Cu
2
O on Cu, CoO on Co, etc., due to
oxygen pickup at the scale-gas interface, cation vacancies are generated that
migrate through the oxide layer toward the metal-oxide interface, where they
are annihilated or consumed at suitable sinks like dislocation defects, grain
boundaries, etc. These metal vacancies may accumulate at the metal-oxide inter-
face followed sometimes by their coalescence, producing voids that result in
scale-metal decohesion at some locations. Alternatively, they may diffuse
through grain boundaries into the substrate leading to formation of discontinuous
or continuous pores, or be annihilated at dislocation sites. Sometimes voids may
also form within the scale. Compactness of the scale and its integrity to substrate
depend on mechanical properties of both the growing scale and the substrate
metal. In such cases, stresses are of course generated but of lower magnitude
because new oxide formation takes place at the free oxide-gas interface.
However, in cases of n-type product layer formation such as ZrO
2
,Ta
2
O
5
,
Nb
2
O
5
, TiO
2
, etc., on the corresponding metal substrates, oxygen vacancies are
created at the metal-oxide interface and are annihilated at the outer surface of
the scale. In such cases, oxidant migrate inward and new oxide formation mainly
takes place at the metal-oxide interface, generating more stresses and strains in
the layer.
Evans [51] has pointed out that at the oxide-metal and oxide-gas interfaces,
the free energy of formation of point defects (vacancies) will be changed by an
amount
σ
H
∆Ω
, where
σ
H
is the hydrostatic component of the local stress and
∆Ω
is the local change in volume. This factor will change the vacancy concentra-
tions at the boundaries of the oxide layer. In addition, the presence of a stress
gradient across the oxide layer will impose a bias to the random migration of
defects.
A treatment of this problem has been advanced [51] for the growth of a protec-
tive zirconia film on zircalloy-2 (Zr-1.5Sn) where the stress at the oxide-gas
interface is considered to be zero, so that the anion vacancy concentration (
C
II
)
there remained in equilibrium with the environment. Furthermore, a biaxial com-
pressive stress is assumed to exist at the oxide-metal phase boundary. Since the
