Environmental Engineering Reference
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utmost importance in prolonging the service life of many engineering components
because the failure or rupture of the surface film creates easy access of the oxidant
to the fresh metal surface, facilitating further degradation. The net result is a
significant loss of metal each time the overlying film/scale failure occurs. There-
fore, properties of the growing film/scale must be modified in such a fashion so
that it can withstand both internal and superimposed stresses (e.g., thermal and
growth stresses) yet remain protective to the underlying metallic substrates.
The most exhaustive and systematic recent review of stress effects on high-
temperature oxidation behavior has been put forward by Evans [51] who dis-
cussed different theoretical models of stress development and its measurement.
The main factors that have been identified to influence the development of
stresses in metal/scale combinations during isothermal oxidation are as follows:
1.
Molar volume ratio of the oxide to the metal transforming into oxide (PBR),
2.
Epitaxial relationships between the structures of metal and its oxide,
3.
Compositional changes that occur in both the metal and the surface oxide,
4.
Vacancies generated during oxidation, and
5.
Component geometry.
5.8.1 Stresses and Strains Due to Volume Changes
The concept of the Pilling-Bedworth ratio (PBR) [5] concerning protective or
nonprotective scale growth on metals has already been discussed in an earlier
section of this chapter. According to this, if the PBR (
) is greater than unity,
one can expect protective, compact, and adherent scale formation on a metal
surface, whereas if
φ
φ
is less than unity, porous nonprotective scale growth will
take place.
It is important to note that these authors had confined their studies entirely to
cylindrical geometries because their aim was to determine the qualitative oxida-
tion rates of wire specimens of various diameter, and the effect outlined a geomet-
rical one that would strongly depend on the diameter of the wire in question. It
is readily realizable that the geometrical constraint indeed approaches zero as the
radius of curvature of the metal surface approaches infinity as that for planar
metal specimens oxidizing uniformly. Even in the case of planar metal surface,
if nonuniform oxidation takes place, it could give rise to some effects from the
variation of mechanisms because the geometry would then deviate from the pla-
nar metal-oxide interface. Such situations are illustrated schematically in Fig.
5.22 [52].
Though the consideration of PBR nearly approaching unity acted as a handy
guideline for prediction of protective scale formation, it was felt necessary to
modify the concept in order to explain nonprotective oxide formation for those
metals having PBRs greatly exceeding unity. Kubaschewski and Hopkins [2] cite
evidence where metals having PBRs either much less or much more than unity
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