Environmental Engineering Reference
In-Depth Information
change
), and then solve the
relevant equations of elasticity subject to the constraints in question. Therefore,
it is natural that an anisotropic strain can be produced by the locally isotropic
stress distribution.
Other related problems arise when alloys of differing oxidation resistance are
forced into close contact by means of external constraint. A particularly subtle
example was the problem of bolt failures experienced in UK gas-cooled nuclear
reactors [51] where rimming steel washers (low silicon) were used in conjunction
with fully killed steel (high silicon) bolts. The low-silicon steel oxidized consider-
ably faster than the bolt material in CO
2
environment at and around 673 K and
the differential increase in volume produced was accompanied by straining and
fracture of the oxidation-resistant bolts.
Early evidence for the deformation produced by oxidation has been provided
by Noden et al. [54] on austenitic stainless steel oxidized at 1173 K in CO
2
and/
or air. Some of their results clearly exhibit that creep strain of about 2% were
developed in the thinnest samples (0.1 mm) in relatively short time. Even for the
thickest samples (0.38 mm), creep strain of about 0.5% was recorded. Subse-
quently, similar effects have been observed in other metals and alloys. It has
been concluded that the ''origins'' of such growth stresses are volume changes
induced by the oxidation of process, occurring in a confined space.
Attention was subsequently focused on the oxide-metal interface and the role
it may have on stress development when the outward cation transport occurs by
vacancy interchange. In such cases, there is a flux of vacancies counter to the
direction of cation flux. It becomes important to recognize that for the metal to
be consumed, such vacant sites, within the oxide must be annihilated at suitable
locations (metal-oxide interface) generating stresses.
δ
V
(
ξ
) associated with the point defect density
C
(
ξ
5.8.2 Epitaxial Relationships Between Structures of Metal
and Its Corresponding Oxide
Atoms in the planar surface of a parent metal crystal are arranged in a periodic
fashion. The periodicity is determined by the crystal structure of the metal and
the relative crystal orientation of the surface. The surface atoms exert short-range
molecular forces on any newly deposited or chemically formed atomic and mo-
lecular layer at the surface. A newly formed molecular layer of oxide is thus
subjected to influences other than those associated with bulk thermodynamic
properties of the oxide. Accordingly, the crystal structure and lattice constant in
the monolayer may be quite different from those found in bulk quantities of the
oxide. Such influence of the substrate in determining the structure, orientation,
and lattice constant of deposited or chemically formed layers is referred to as
epitaxy
. When formation of the first oxide monolayer is complete, the periodic
arrangement of atoms and molecules in this first layer exerts similar short-range
