Environmental Engineering Reference
In-Depth Information
Figure 5.37
Effect of surface pretreatment on the early stages of oxidation of Ni at
600°C. H.B. (''hot-bare'') oxidation of polycrystalline Ni in 0.5 torr O
2
, F.R. (''furnace-
raised'') oxidation of ETCHED and E.P. polycrystalline Ni in 0.1 torr O
2
, F.R. oxidation
of E.P (112) Ni in 5
10
3
torr O
2
producing a singly orientated overgrowth [Ref. 64].
formation of oxide via lattice diffusion only. Experiments on other EP surfaces of
Ni single crystals have exhibited decreasing oxidation rate in the following order:
(100), (111), and (112). This difference in rate with orientation as well as changes
in oxidation rate with surface pretreatments can be explained in terms of modifi-
cation in the structure of the growing oxide. When a single crystal oxide, as on
(112) Ni, persists during growth the oxidation rate is very low; when the oxide
contains twin boundaries [as on (111) Ni] or becomes polycrystalline [as on (100)
Ni], the oxidation rate is enhanced because of the availability of additional easy
diffusion paths. The role of easy diffusion paths in governing oxidation anisot-
ropy has been addressed by many research groups.
The effect of cold work (CW) on oxide grain size and oxidation rate of Ni is
depicted in Fig. 5.38. This figure clearly exhibits that at each temperature CW Ni
oxidizes at a faster rate than the corresponding annealed ones. Such observations
suggest that CW Ni oxidizes faster because of more oxide nucleation sites, lead-
ing to growth of finer grained oxide. Observation of surface scale morphology at
