Biomedical Engineering Reference
In-Depth Information
diffuse in the solid and form carbides. At temperatures above 870
C, the carbon is soluble
in the atomic lattice. Below 425
C, the mobility is too low for carbide formation. If the peak
temperature in the metal away from the weld is in the sensitizing range, carbides can form.
This is known as weld decay or corrosion of the sensitized metal on each side of the weld. By
heat treating after welding, the carbides can be redissolved, and the metal quickly quenched
to avoid reformation.
With the oxide film intact, surgical alloys are passive and noble. If the film is damaged, as
with scratching or fretting, the exposed metal is actively corroding. Reformation or repassiva-
tion results in restoration of the passive condition. Fretting corrosion involves continuous dis-
ruption of the film and the associated oxidation of exposedmetal. Devices that undergo crevice
corrosion are also examples in which fretting corrosion has accelerated crevice corrosion.
How often does corrosion of metallic implants happen in the patient? The most detailed
information on
corrosion of biomaterials comes from retrieval studies in which the
implant is removed from the patient. In one study in which 148 total modular hip implants
made with mixed -(Ti-6Al-4V/Co-Cr) and similar-(Co-Cr/Co-Cr) alloy femoral stem and
head combinations were examined, moderate to severe corrosion was observed in 16 percent
of necks and 35 percent of heads in the mixed-metal cases and in 14 percent of necks and
23 percent of heads for similar-alloy cases. This corrosion process is an example of mechani-
cally assisted crevice corrosion attack.
In another study, Type 316 stainless steel multicomponent devices were removed from
the human body, and 91 percent were found to have undergone visible corrosion, with
crevice corrosion being the dominant form. So to conclude, corrosion does occur to metallic
implants in the body, and this damages the metallic biomedical devices. Prior to implanta-
tion, these devices had been screened in simple laboratory tests under ideal conditions at
neutral pH that did not replicate the complex environments that arise as a result of inflam-
mation and wound healing cellular activity, so the
in vivo
corrosion was not predicted.
Since metal ion release and other corrosion products formed by pitting and crevice corro-
sion cause adverse reactions in the patient, this is an area that requires further research.
in vivo
EXAMPLE PROBLEM 5.6
Old cars in the northern United States are often rusted on the bottom of their doors and on
their trunk lids, and the tailgates of old pickup trucks are also often rusted. Name and discuss five
reasons for this. Would bolting on a zinc block help?
Solution
(1) The edges and bottoms of doors and lids are formed by bending the metal back on itself.
This causes cold working at the bend, which makes it anodic to the rest of the metal. (2) The
crimps are then spot-welded closed. This creates an area of different microstructure, which leads
to a galvanic situation. (3) The roads are salted in the winter, and the saltwater spray gets caught
in the crimp. That is the electrolyte. Sand may also get in the crimped space and help maintain a
moist environment. (4) Car manufacturers put a decorative strip of chromium-plated steel along
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