Environmental Engineering Reference
In-Depth Information
Figure 5.22
Role of the Pilling-Bedworth ratio in producing compressive or tensile
stresses during nonuniform oxidation of a planar metal sample [Ref. 52].
exhibit a strong tendency toward linear nonprotective oxidation. The linear oxida-
tion of metals having particularly high PBRs (e.g., the ratio is 2.68 for Nb
2
O
5
and 2.5 for Ta
2
O
5
) follows an initial stage during which the scale remains fairly
coherent. The linear rate for such systems is always associated with the formation
of cracked or porous scales, probably due to development of stresses in the scale
as well as in the metal substrate. Cracking of scales is a common observation in
the cases where diffusion is primarily anionic. An ''extension'' to the PBR con-
cept has subsequently emerged for oxides having considerably large volume ra-
tios and for which oxide growth occurs primarily at the metal-oxide interface.
When oxygen ions diffuse from the surface to the metal-oxide interface where
they form new oxide which, owing to its high volume ratio, expands against the
resistance of the existing oxide layer, severe biaxial stresses develop that eventu-
ally lead to rupturing of the film [52].
The consideration of volume ratio turns out to be of minor importance when
