Environmental Engineering Reference
In-Depth Information
Class 1
Only one of two elements constituting the binary alloy oxidizes under the prevail-
ing conditions, producing BO.
(A) The reactive element, B (in smaller concentration), oxidizes under the
given conditions:
1.
Internally oxidized BO particles are produced in a matrix of A. For example,
oxidation of dilute Ag-Si alloys in an oxygen atmosphere less than the equi-
librium dissociation pressure of Ag
2
O, produces SiO
2
particles in a Ag matrix
(Fig 6.12a).
2.
Exclusively a single layer of BO is externally formed over the alloy matrix
depleted in B. For example, oxidation of Ag-Si alloys richer in Si at an
oxygen pressure less than the dissociation pressure of Ag
2
O produces an
external scale of SiO
2
(Fig 6.12b). However, when both the components of
the binary alloy have possibilities to oxidize but the kinetics and geometrical
conditions permit B to be selectively oxidized, as in the case of chromium-
rich Fe-Cr, Ni-Cr, and Co-Cr alloys, essentially an external scale of Cr
2
O
3
is developed, particularly at low oxygen pressure. In reality, a small amount
of Fe, Ni, or Co does enter the Cr
2
O
3
layer, producing solid solutions or
doping effects.
(B)
The reactive element, B (in major proportion), oxidizes exclusively:
1.
The nonoxidizable metal A gets dispersed in BO, as found in Cu-Au alloys
rich in Cu (Fig. 6.12c).
2.
An exclusive external scale of BO is formed leaving underneath the nonoxi-
dizable metal A in an enriched zone of the unreacted alloy as found in Ni-
Pt alloys (Fig. 6.12d).
Class 2
Both the elements (A and B) oxidize simultaneously at an oxygen pressure greater
than the dissociation pressures of both the oxides to produce AO and BO.
(A) AO and BO either are miscible (in the whole range or partially) to each
other or react chemically to produce a complex compound.
1.
AO and BO are completely miscible to produce a single solid solution of
(A,B)O in the external scale, as in the case of Ni-Co alloys (Fig. 6.12e).
However, in reality some internal oxide of (A,B)O richer in B than the sur-
face scale may also be present.
2.
The two individual oxides combine chemically, often to form a spinel
(ABO
2
). The particles are incorporated in a matrix of AO if the reaction is
incomplete, as in the case of some Ni-Cr alloys (Fig. 6.12f).
