Environmental Engineering Reference
In-Depth Information
(B)
AO and BO are virtually insoluble in each other.
1.
Internal oxide particles of the reactive element B (solute) lie beneath a two-
phase outer layer of AO and BO (Fig. 6.12g), as found in the case of Cu-
Ni, Cu-Zn, Cu-Al, Cu-Be, etc., alloys. In such alloy systems, conditions may
develop such that the internal oxide particles may link up to form a complete
healing layer of BO at the scale base.
2.
When the reactive element is the solvent in the alloy system, no internal
oxidation can take place (Fig. 6.12h). In practice, the second phase, as de-
picted in Fig. 6.12g, may not be present in the outer regions of the scale if
AO grows rapidly to produce an external scale exclusively of it. However,
the outer regions of such a scale may be oxidized to higher oxides. For exam-
ple, a thin outer layer of CuO is observed over Cu
2
O during oxidation of
Cu-Si alloys.
It is therefore apparent that there cannot be a single comprehensive theory
of alloy oxidation. It is to be recognized that in certain alloy systems several
classifications of behavior are possible, depending on the alloy composition, oxi-
dizing atmosphere, temperature, time of exposure, and so on. Changes from one
type of behavior to the other may occur even for a single alloy.
6.5.2 Simultaneous Formation of Oxides of Both
Components in the External Scale
At concentrations lower than that at which B becomes selectively oxidized, the
external scale consists of oxides of both A and B, e.g., AO and BO. The various
limiting cases of scale growth under such situations are considered below.
Figure 6.12
Schematic presentation of the different modes of oxidation of alloy AB
with variable composition, where B is the less noble metal. (a) Solute B only oxidizes,
producing internal oxide BO in the matrix of A. (b) Solute B only oxidizes, producing
external scale of BO above the alloy depleted in B. (c) Solvent B only oxidizes, producing
a matrix of BO in which A is distributed. (d) Solvent B only oxidizes, producing an
external scale of BO above the alloy depleted in B. (e) A and B oxidize simultaneously,
producing an external scale of a solid solution or compound of variable composition. (f )
A and B (solute) oxidize simultaneously, producing a compound ABO
2
dispersed in a
matrix of AO as the external scale with internal oxide particles of BO at the alloy-oxide
interface. (g) A and B (solute) oxidize simultaneously to give insoluble oxides with BO
in a matrix of AO as the external scale and internal oxide particles of BO at the alloy-
oxide interface. (h) A and B (solvent) oxidize simultaneously to give insoluble oxides,
with AO in a matrix of BO as the external scale [25].
