Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 296

medium will provide a reasonably reliable estimate of the corrosion rate
to be expected for a particular metal. In fact, a little reflection will show
that, allowing for possible errors introduced by the quasi-static condi-
tions (slowly changing potential), an anodic polarization diagram rep-
resents a vertical cross section of a Pourbaix diagram. This approach to
corrosion prediction has proven particularly useful in examining relative
changes in corrosion behavior produced by changes in alloy processing
and finishing.

Forms of corrosion

Electrochemical series, both ideal and practical, Pourbaix diagrams for
selected elements, and anodic polarization diagrams for specific alloys
help predict the occurrence of corrosion. However, the study of corrosion
in engineering applications has proceeded from recognition of incidents
of attack, grouping of these incidents by similarities in their appearance
and performance (exposure) conditions, and then by reasoning back to
the fundamental aspects underlying the observed phenomena. Such an
engineering approach has resulted in classifying corrosive attack phe-
nomena into eight forms.* These forms of corrosion will be discussed
in a general way, taking into account aspects specific to orthopaedic
applications.

Uniform attack This is the overall corrosion process that takes place in the corrosion

region (by ionization) and the passivation region (by passive film dis-
solution or reaction) of the Pourbaix diagram (Figure 12.6). It is the
most common form of corrosion. In the absence of equilibrium con-
centrations of constituent ions in the bathing solution or of significant
cathodic protection, uniform attack will occur for all metals. Even in the
immunity region, uniform attack will result in slow removal of metal
from implants, especially if the bathing solution has a very low level of
constituent ions. Thus, it is fair to say that, owing to uniform attack, all
metals have a finite corrosion rate in vivo but may not have any clinical
consequences.
Figure 12.6 shows the general mechanism. Anodic and cathodic areas
exist on the surface of the implant, resulting from small local variations
in surface structure and environment. An electron current flows within
the metal from anodic to cathodic regions. A characteristic potential,
intermediate between the half-cell potentials for each reaction, can be
measured with reference to another electrode; this is called the mixed or
corrosion potential.
Uniform attack may not be noticed until or unless a visible amount
of metal is lost. Typical corrosion rates for chromium-containing cobalt-
base alloys, under normal general tissue conditions, are estimated

* This classification was developed by M.G. Fontana and N.D. Greene and is discussed at
greater length in Fontana MG (ed): Corrosion Engineering , 3rd. Ed. McGraw-Hill, New
Yo r k , 19 8 6 .