Biomedical Engineering Reference
In-Depth Information
is scratched and the iron is exposed, the small size of the iron cathode limits the reaction,
and there is minimal corrosion. In contrast, tin is cathodic to iron. When a tin plate is
scratched, the small iron anode is coupled with a large cathode. Anodic corrosion and
removal of iron from the scratch result in an increased area of the exposed iron and thus
an increase in corrosion rate. This self-accelerating corrosion can be prevented by coating
the tin cathode with a nonconductive material such as paint or varnish.
Crevice corrosion can occur in a confined space that is exposed to a chloride solution.
The space can be in the form of a gasket-type connection between a metal and a nonmetal
or between two pieces of metal bolted or clamped together. Crevice corrosion involves a
number of steps that lead to the development of a concentration cell, and it may take six
months to two years to develop. Crevice corrosion has been observed in some implanted
devices where metals were in contact, such as in some total hip replacement devices, screws
and plates used in fracture fixation, and some orthodontic appliances.
The initial stage is uniform corrosion within the crevice and on the surfaces outside the
crevice. Anodic and cathodic reactions occur everywhere, with metal oxidation at the anode
and reduction of oxygen and OH- production at the cathode. After a time, the oxygen
within the crevice becomes depleted because of the restricted convection of the large
oxygen molecule. The cathodic oxygen reduction reaction ceases within the crevice, but
the oxidation of the metal within the crevice continues. Metal oxidation within the crevice
releases electrons that are conducted through the metal and consumed by the reduction
reaction on the free surfaces outside the crevice. This creates an excess positive charge
within the crevice that is balanced by an influx of negatively charged chloride ions. Metal
chlorides hydrolyze in water and dissociate into an insoluble metal hydroxide and a free
acid (Eq. (5.7)). This results in an ever-increasing acid concentration in the crevice and a
self-accelerating reaction.
M
þ
Cl
þ
H
þ
Cl
H
2
O
!
MOH
þ
ð
5
:
4
Þ
The surgical alloys in use today all owe their corrosion resistance to the formation of
stable, passive oxide films, a process called passivation. Titanium, which appears as an
active metal on the EMF series in Table 5.3, forms a tenacious oxide that prevents further
corrosion. Stainless steels and cobalt alloys form chromium oxide films. As indicated in
Table 5.3, they are active, but in the environment where this oxide film is formed, they
become passive or noble.
To be self-passivating, stainless steels must contain at least 12 percent chromium. How-
ever, carbon has a strong affinity for chromium, and chromium carbides form with the
average stoichiometry of Cr
23
C
5.
The formation of a carbide results from the migration of
chromium atoms from the bulk stainless steel alloy into the carbide. The result is that the
carbide has high chromium content, while the alloy surrounding the carbide is depleted
in chromium. If the chromium content is depleted and drops below 12 percent Cr, then
there is insufficient Cr for effective repassivation, and the stainless steel becomes suscepti-
ble to corrosion. As a safety factor, surgical stainless contains 17 to 19 percent chromium,
and the carbon content in surgical alloys is kept low at less than 0.08 percent or more than
0.03 percent.
The problem of carbide formation is especially important with welded stainless steel
parts. If steel is heated to the “sensitizing range” of 425
C to 870
C, the chromium can
