Biomedical Engineering Reference
In-Depth Information
Fig. 3.8
Concentration cell
Δ
E
diffusion
barrier
M
M
oxygen poor
oxygen rich
Localized corrosion finds there its origin, but of course the overall process is the
combined result of the oxygen concentration gradient, the corroding medium and
the type of alloy. Figure
3.8
is a “macro-model” of a corrosion cell with electrodes
of the same metal but immersed in solutions of differing oxygen concentration.
11
The cathodic reaction (
3.13
) is pH dependent, but assuming in a first approxi-
mation a constant pH in both half-cells, the generated potential difference is easily
calculated by the Nernst equation. For the oxygen-rich side:
RT
4F
1
0
O
2
C
O
2
/
E
D E
ln
.p
(3.16)
and for the oxygen-pour side:
RT
4F
2
0
O
2
/
E
D E
O
2
C
ln
.p
(3.17)
E
1
from
E
2
Subtracting
log
!
O
2
p
0:059
4
2
1
E
E
D E D
(3.18)
p
O
2
shows
to be simply proportional to the logarithm of the oxygen concentration
ratio: oxygen concentration is directly related to the oxygen pressures
E
O
2
O
2
p
and
p
.
O
2
10p
O
2
For an increase of the log-term by 1 what means
p
, a potential difference
of
mV is generated. When the electrodes are short-circuited, corrosion may
occur for an increase of a few tens of mV above
C15
E
corr
for the given metal. A cell
as in Fig.
3.8
is also called a concentration cell, allowing to define the relationship
11
This set-up can be used to demonstrate the dependence of potential on concentration.
