Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 291

forms a very stable passive layer that, unlike that which forms on aluminum
and magnesium, is highly resistant to attack by Cl − and thus resists dissolu-
tion in seawater. As a result, passivated titanium appears relatively noble in
seawater and is highly corrosion resistant in vivo . For stainless steel alloys,
which are less reactive with oxygen, deliberate passivation during manufac-
ture greatly increases their practical nobility in the presence of Cl − , render-
ing many of them useful as implant materials.
However, such practical series should be used with caution, as the
actual implant conditions may affect the relative activity of the metals in
question. For instance, wear, as in a bearing interface, or fretting, as in a
modular junction, may disrupt the passive layer on the metallic surface.
effects of environmental variation
Beyond the general role of the environment, it is desirable to be able to pre-
dict the effects of changes in pH, PO 2 , and so on, within a specific envi-
ronment on the corrosion process that is favored to take place. There are
many possible chemical reactions in any system consisting of a metal and its
environment; it is necessary to make some order out of them so that we can
approach the problem of corrosion prediction in a systematic way.
All the possible chemical reactions in a system can be classified by
answering two questions:
1. Does the reaction depend on pH?
2. Does the reaction depend on local electrical potential?
This classification makes it feasible to sort out the possible reactions
in a given system and to begin to understand the effects of pH and PO 2
on them.
The Pourbaix diagram
It is possible to plot the outcome of reactions in a given metal-electro-
lyte (liquid containing [dissolved] ions) system by connecting points of
equal ionic product concentrations, on a diagram whose axes are pH
and electrical potential (with reference to the H/H + half-cell). Such dia-
grams are called pH-potential or Pourbaix diagrams (named for Marcel
Pourbaix [1904-1998], a Belgian electrochemist who has been respon-
sible for popularizing their use). They further simplify consideration of
competing chemical reactions since it turns out that in any area of such
a diagram, a single reaction, among the 20-30 possible in any metal-
electrolyte system, dominates and is the major contributor of dissolved
ions. Preparation of such diagrams is difficult and time consuming, requir-
ing the assembly of data from many different types of experiment. Thus,
despite their utility, they are available only for most elemental metals in
water and are not available for alloys and metals in most other solutions.
Figure 12.4a is the Pourbaix diagram for chromium in water. The solid
boundaries connect points at which the concentration of all chromium