Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 303

result is that there are wide differences in chemical activity throughout
the alloy, with multiple local galvanic cells (Figure 12.13).
Attack of this kind will remove metal with a regular periodic varia-
tion of effect at a microscopic level. It is peculiar to certain alloy systems
but can be more generally induced by two conditions:

1. The introduction into the solution around the metal of an agent
that attacks one component of the alloy selectively. For instance,
F − ion will selectively complex with and remove aluminum from
copper-aluminum alloys (alloys not suitable for implantation).

2. The presence of more than one phase in the alloy. Usually, all of
the grains of an alloy have the same general composition, within
a narrow range. The alloy is said to be single phase. However, it
is possible for there to be grains of two or more different discrete
compositions and thus chemical activities (see Chapter 7). Heat
and mechanical treatment may affect the grain size, but not the
composition disparity in such materials, called multiphase alloys .
For this reason, multiphase alloys are not generally used in corro-
sion applications. An exception is the 35% Ni-containing cobalt-
base alloy, F 582 (see Chapter 7), which is a multiphase alloy.
From an engineering standpoint, it performs well in vivo , being
quite corrosion resistant. However, biologic (local host response)
studies suggest that it may release Ni somewhat easily, and thus its
application is uncertain.

Leaching produces surface appearances that are similar to those that
result from pitting and intergranular attack. It can be verified only by
analysis of the corrosion products or the surface concentration of the
corroded metal part. Although corrosion products or corroded surfaces
of alloys will differ from the alloy composition, there are characteristic
ratios that may be established for uniform attack in each system, and
leaching produces deviations from these.

M +

O -

1/2 O 2 (d)

e -

M +

O -

1/2 O 2 (d)

e -

FIGUre 12.13 leaching.