Biomedical Engineering Reference
In-Depth Information
Fig. 3.3
Simplified Pourbaix
diagram for chromium,
normally given with respect
to an equilibrium aqueous
concentration of
10
6
gdm
3
.Forthe
vertical dotted line, see the
end of the next section
Fig. 3.4
Pourbaix diagram
for chromium in a chloride
solution of 0.85 M l
1
.
A classic, already displayed
by Black [51
, p. 43]
is in the zone of passivity, is greatly reduced by the presence of chloride and situ-
ated above pH
8. However, conservative the body is, pHs below that value occur:
during the onset of infection, for example in the surrounding of an osteosynthesis
plate, pH may go down to 5.5 [
89
, p. 240] or [
90
]. Pure chromium is mechanically
speaking not suitable, for say a dental implant and indeed, it is always used as alloy-
ing element in implants. There too, however, the protective layer is Cr
2
O
3
but the
practice is, fortunately, somewhat more favorable than the prediction by Fig.
3.4
.
The merit of Pourbaixs diagrams is that they are offering the corrosion scientist
a perspective on what occasionally can happen, however, not what will. They are
purely thermodynamic, composed of pure metals, without accounting for effects of
other ions, surface texture, etc. and above all, with which speed it all happens. Let
us say that a Pourbaix diagram is like “a room with a view” but, if the room is on
the backside, one does not see who is ringing at the front door. The front door is, in
modern electrodics, the study of kinetics of corrosion reactions.
D
3.2.3
Corrosion Rate
In Fig.
3.5
are displayed the basic components of a corrosion cell: two short-
circuited dissimilar metals in an aqueous solution. Electrons are flowing from anode
