Environmental Engineering Reference
In-Depth Information
Figure 5.19
Appearances of the surfaces for 0.16% C steel oxidized nonisothermally
up to 1273 K and held at this temperature for 2 h. (a) Coated with bentonite and CaSi
2
;
(b) uncoated.
der from a slurry bath, formation of CO
CO
2
gas bubbles could be prevented
by the achievement of very reduced oxygen activity at the alloy-scale interface.
Such coatings also minimized scale spallation even under thermal cycling, as
demonstrated in Fig. 5.19.
5.7.1 Consequences of Void Formation
Void formation in the scale and in the metal is expected to affect the rate and
mechanism of oxidation as well as the mechanical properties of metal/scale com-
bination. If vacancy injection in the metal predominates and if these are annihi-
lated at sites of vacancy sinks such as dislocations, interfaces, and grain bound-
aries, the metal surface is likely to recede. On the other hand, when the injected
metal vacancies partially condense as voids at the grain boundaries of the metal,
scale/metal contact may easily be maintained. However, accumulation of voids
in the region of grain boundaries can adversely affect the mechanical integrity
of the metal substrate.
When pores are formed at the metal-oxide interface or within the scale itself,
the path for outward solid state transport of the metal is partially blocked. This
may reduce oxidation rate but such effect may be temporary. It has been demon-
strated through a large number of investigations on nickel oxidation that after
