Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 299

during fabrication. Inclusions may be impurities or may be caused by
poor fabrication or heat treatment practices. In general, such inclusions
are noble with respect to the bulk metal and produce local corrosion in
the underlying metal. This occurs preferentially near the inclusion-metal
interface, since the electronic current density is highest there, resulting
in concentrated local attack that can physically release the inclusion.
Both acidic and basic cathode reactions are shown; in vivo conditions
(usually slightly alkaline) favor the basic one.

Crevice corrosion This is one of several forms of corrosion that are related to structural

details. The basic feature that initiates this process is a crevice or crack,
either between parts of an implanted device, such as between a screw
head and a fracture fixation plate (Figure 12.8), or a defect such as an
incomplete fatigue crack. The narrower and deeper the crack is, the more
likely crevice corrosion is to start. The details of initiation are unclear,
but once started, the process is characterized by oxygen depletion within
the crevice, anodic corrosion along the crevice faces, and cathodic pro-
tective conditions on the metal surface near the defect. Static, non-flow
conditions favor progression of crevice corrosion; this is probably asso-
ciated with the formation of a metallic ion concentration gradient away
from the mouth of the defect. The presence of chloride ion, such as in
vivo , accelerates the process. Since the area of attack is very concen-
trated, it can be easily seen in the mating areas between screws and
plates, and so on, such as shown in Figure 12.9. The design of this direct
compression type of plate, with a spherical screw underhead surface in
a semicircular “trough” in the plate, produces ideal conditions for crevice
corrosion: a line contact between two parts. This degree of attack, seen
after 15 months, is exceptional. However, studies of retrieved stainless
steel multipart internal fraction fixation devices show visible corrosion in
50%-75% of all devices. Both iron-base and cobalt-base alloys are rela-
tively more susceptible to crevice corrosion, the latter only very modestly,
but titanium-base alloys do not demonstrate it to any significant degree.

O -

Screw
head

e -

1/2 O 2 (d)

O -

M +

M +

e -

1/2 O 2 (d)

Plate

FIGUre 12.8 Crevice corrosion.