Environmental Engineering Reference
In-Depth Information
Figure 5.39
Oxidation curves for E.P. and ETCHED Cr at 1090°C in 1 atm O
2
using
the ''cold insertion'' (C.I.) oxidation procedure. Metallographic cross-sections show the
oxide on E.P. Cr after 21 h to be a six-layered polycrystalline, wrinkled scale, whereas
thin single-crystal oxide forms on some metal grains of ETCHED Cr [Ref. 64].
development and wrinkling leads to eventual failure of the scale by cracking
and subsequently another fine-grained oxide layer is formed on the metal. This
sequential process of scale wrinkling, failure, and new oxide formation may prog-
ress in segmental or repeated manner producing multilayered separated scales.
In situ oxidation studies on high-purity Cr under scanning electron microscopy
with different pretreatments have further demonstrated [65] that there exists a
close correlation between the morphology of the thin films formed and the oxida-
tion procedures.
5.10.4 Oxidation of Iron
The oxidation rate of pure Fe is also reported to increase with a decrease in the
initial oxide grain size. The behavior of CW Fe as depicted in Fig. 5.40 clearly
shows a similar trend of decreasing
k
p
with exposure time as discussed above
for CW Ni. This obviously signifies subsequent oxide grain growth with exposure
time. The very high value of
k
p
is primarily due to rapid grain boundary diffusion
