Environmental Engineering Reference
In-Depth Information
associated with low diffusivity of oxygen into the alloy. Under such conditions,
the outward flux of the solute element exceeds the inward flux of the oxygen
atoms. The decrease in the inward flux of oxygen can be achieved by lowering
N
(s
O
, i.e., by maintaining lower
p
O
2
in the atmosphere; the increased outward flux
of solute B can be achieved by cold-working the alloy (increased contribution
of short circuit diffusion paths), and by doing so, one can reach transition to
external scale formation at lower solute concentrations in the alloy systems.
According to Wagner, at the critical alloy composition, the volume fraction
of precipitated oxide is just sufficient in developing a physical barrier to the
ingress of oxygen and the existing oxide particles grow laterally to form a contin-
uous protective oxide film. Therefore, the conditions under which transition from
internal to external oxidation will take place are represented by:
1/2
π
g
D
O
V
D
B
V
N
(O)
B
N
(s
O
(6.29)
2
ν
′
where
g
volume fraction of internal oxide zone occupied by BO
ν
phase
D
O
diffusion coefficient of oxygen in A
D
B
diffusion coefficient of B in alloy
′
V
,
V
molar volumes of alloy and BO
ν
oxide, respectively.
With larger amount of precipitation due to the presence of increased reactive
element concentration, the oxygen partial pressure at the advancing precipitating
front is reduced to so low a level such that oxygen ingress to the nobler metal
matrix decreases. Since reactive solute supply from the interior to the precipita-
tion front is maintained, it favors more oxidation of the reactive metal, their
lateral growth and subsequent coalescence to provide a continuous protective
scale, thereby completely stopping internal oxidation. The critical solute concen-
tration for an alloy will, however, be decided by the partial pressure of oxygen
at any given temperature. The lower the oxygen partial pressure in the environ-
ment, the lower will be the critical amount of reactive solute requirement for
exclusive external scale formation as demonstrated by Rapp and illustrated in
Fig. 6.5 [15] for Ag-In alloy system at 823 K. At
p
O
2
1 atm (10
5
N/m
2
), the
transition was observed at
N
(O)
In
0.15, while by decreasing the oxygen partial
pressure to 10
7
atm (10
2
N/m
2
), the transition occurred at
N
(O)
In
0.04. Such a
decrease in the concentration of solute (In) is due to reduction in the flux of
oxygen into the alloy. In the same investigation [15], the transition from internal
to external oxidation at a particular oxygen pressure was demonstrated to depend
on the initial surface preparation of the alloy. For mechanically polished speci-
mens exposed to air at 823 K, the transition took place at
N
(O)
In
0.10, while for
specimens etched in hot concentrated HNO
3
and electroetched in 1-N HNO
3
, the
