Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 295

+ 1.0

Trans-passive
corrosion

+ 0.5

Pitting
passivation

E p
E (V)

0

- 0.5

NoCI - 0.1 M Cl -

corrosion

- 1.0

Immunity

I 0 I m

Current (Log I )

FIGUre 12.5 anodic polarization curves.

then to reduce the potential (move toward anodic conditions) at a con-
stant rate of 10-50 mV/mm. A continuous plot of the logarithm of cur-
rent in the external circuit (horizontal scale) versus the external potential
of the polaristat (vertical scale) produces the curve seen in Figure 12.5,
which represents one that might be obtained for a pure chromium elec-
trode (nonpassivated) at pH 7.2.
The curve has the following (typical) regions:

At large cathodic potentials, a minimum current, I 0 , called the
exchange current flows. This represents the minimum corrosion
rate possible and reflects the immune region, if one exists for the
metal being examined. In a practical electrochemical series, met-
als are ordered in increasing I 0 as one moves down (from noble to
base).
As positive potential decreases, the current will increase, especially
if a complexing ion, such as Cl − , is present, reaching a maximum
current, I m . If the metal is passivated or if passivation is very rapid
and no complexing ion is present, there will be no current increase.
At a lower potential, the surface can be seen to grow a passive film,
current is reduced, often to I 0 , and the metal passes into the pas-
sive region.
At a still lower potential (more anodic), current starts to rise as active
corrosion, called trans-passive corrosion, takes place. The poten-
tial at which this occurs is often called the breakdown potential. If
pitting is possible, owing to the presence of Cl − or another suitable
ion, the current may begin to rise at a lower anodic potential called
the pitting potential ( E p ).
It should be clear that reference to a generic Pourbaix diagram to
determine the pH and pO 2 expected in a particular application combined
with performance of an anodic polarization experiment in a suitable