Environmental Engineering Reference
In-Depth Information
3.
Careful measurements of the front penetration as a function of time for an
alloy of known solute concentration can yield a value of the solubility-diffu-
sivity product,
N
(s
O
D
O
(permeability) for oxygen in the metal matrix.
The results of similar derivation for a cylindrical specimen [10] yield
(
r
1
)
2
2
r
1
r
2
1
2
2
N
(s
O
D
O
ν
(
r
2
)
2
ln
t
(6.27)
N
(O)
B
and that for a spherical specimen [10] is
(
r
1
)
2
3
2
3
(
r
2
)
3
(
r
1
)
2
N
(s
O
D
O
ν
(
r
2
)
2
t
(6.28)
N
(O)
B
where
r
1
is the specimen radius and
r
2
the radius of the unoxidized alloy core.
Many of the effects of internal oxidation, both on the overall corrosion process
for an alloy and on the mechanical, electrical, and magnetic properties of the
alloy, are intimately related to the morphology of the oxide precipitates. Forma-
tion of oxide particles by internal oxidation is a nucleation and growth process.
Accordingly, for precipitation of very stable oxides, the driving force for the
process is large, which facilitates nucleation of finer particles. On the other hand,
for less stable oxides, the driving force being less, nucleation occurs with much
difficulty rather favoring growth of existing particles.
Transition from Internal Oxidation to External-Scale Formation
Upon increasing the concentration of reactive solute in an internally oxidizable
alloy system (e.g., Cu-Be, Cu-Si, Cu-Al, Ni-Cr, Ni-Al, etc.), a critical solute
concentration will be reached above which a protective external scale is formed
and the alloy is no longer oxidized internally. Assuming the formation of a com-
pact, pore-free oxide scale, Wagner [14] has given a theoretical analysis of the
transition from internal oxidation to external scale formation. This model is based
on the fact that the cross-sectional area available for oxygen diffusion into the
alloy is reduced due to the presence of internally formed oxide particles. So above
a critical volume percent of oxide particles formed in the metal matrix, further
oxygen diffusion into the metal gets retarded, creating a situation for formation
of surface oxide layer only. This transition to external scale formation is the basis
for the design of Fe-, Ni-, and Co-based high-temperature alloys, which contain
sufficiently high concentration of a solute, such as Cr, Al or Si, to produce an
external layer of a slow-growing stable oxide (i.e., Cr
2
O
3
,Al
2
O
3
or SiO
2
), which
prevents further degradation of the alloy.
The condition that favors external scale formation is
N
B
D
B
N
O
D
O
, i.e., (1)
high concentration of solute B associated with its rapid diffusion to form a contin-
uous blocking layer of BO
ν
and stop internal oxidation and (2) low solubility
