Environmental Engineering Reference
In-Depth Information
may promote an irregular alloy-oxide interface, which is desirable for better
scale adherence to the substrate alloy. Such corrogated interface provides better
keying action of the scale to the underlying alloy substrate.
Similarly, cold working of an alloy can promote the establishment and mainte-
nance of a protective oxide layer, not always produced immediately on annealed
alloy, by increasing the alloy interdiffusion coefficient. But it may also cause
spalling of this layer due to the remaining residual stresses in the alloy. Reduction
of oxygen potential is conducive to early development of a protective scale and
minimization of internal oxidation. This is because of the reduced oxygen flux
into the alloy without affecting the flux of the less noble constituent of the alloy
to reach the alloy surface. Thus, there is a tendency to precipitate a compact,
more prohibitive layer on the alloy surface rather than form an array of internal
oxide particles.
Addition of a second alloying element to binary alloys that can form an oxide
intermediate stability between those of the eventual protective oxide and the no-
ble metal sometimes plays an overriding role in gettering oxygen and helping
the main reactive alloying element to develop a protective layer. The oxidation
behavior of Cu-Zn-Al is one such classical example [23]. A Cu-30wt%Zn solid
solution alloy on oxidation forms a relatively slow-growing ZnO scale. A binary
Cu-5wt%Al alloy suffers from internal oxidation of Al and rapid scaling. How-
ever, addition of 2-4wt%Al to Cu-Zn alloy can substantially cut down its oxida-
tion rate as depicted in Fig. 6.11, where protection is provided by the healing
layer of Al
2
O
3
formed immediately beneath the thin initially nucleated scale.
When the surface of the bare ternary alloy is exposed to an oxidizing environ-
ment, Cu
2
O, ZnO, and Al
2
O
3
are nucleated at the beginning of oxidation. In view
of depletion of Al in the alloy next to the surface, Al atoms migrate to the surface
without being converted to Al
2
O
3
on their way to the surface. Hence, there is
sufficient supply of Al atoms with which oxygen can react preferentially in com-
parision to Cu and Zn in view of higher negative standard free energy of forma-
tion of Al
2
O
3
. Moreover, Al atoms diffusing toward the surface convert the ini-
tially formed nuclei of Cu
2
O and ZnO to Al
2
O
3
by virtue of displacement
reactions, such as
2Al
3Cu
2
O
6Cu
Al
2
O
3
(6.30a)
and
2Al
3ZnO
3Zn
Al
2
O
3
(6.30b)
Thus, the scale is supposed to consist exclusively of Al
2
O
3
as depicted in Fig.
6.11c.
Although Zn atoms do not enter the steady-state scale but here Zn acts as a
gettering element, reducing oxygen flux into the alloy interior and allowing Al
to diffuse up to the surface in forming the protective layer. Basically the lowering
