Biomedical Engineering Reference
In-Depth Information
Chapter 3
Corrosion
Three peculiarities discriminate metals from most other materials:
Metals are used in a vulnerable oxidation state. It makes them apt to be oxidized
or to be corroded.
Corrosion observed on one spot can simply be the result of a wrong environment
at the distant end of a metal object. Metals are highways for electric currents.
No electrochemical process will occur unless at least two reaction partners are
involved.
These statements do not apply to ceramics or polymers. Some authors want to
extend the term corrosion to degradation processes of all classes of materials but
we prefer to keep it confined to degradation of metals. Atoms of a ceramic are in a
higher if not the highest oxidation state. These atoms are held together by a mix of
ionic and covalent bonds or by the weaker hydrogen bonds or Van der Waals forces.
Except for the special class of highly conducting polymers such as poly-acetylenes,
discovered in 1977, and some ceramics such as substoichiometric titanium dioxide,
all polymers and ceramics are all electric insulators. In metallic bonds, however, all
the atoms share their valence electrons: it is a more or less ordered array of positively
charged nuclei bathing in a sea of delocalized electrons. Atoms tied up by a prepon-
derately pure ionic bond, for example
LiCl
, can hardly be called “molecules”. In
metals no molecules either: long-range collective interactions constitute the bond-
ing forces between the atoms. It is not only for their specific mechanical properties
that metals constitute a generic class of materials, but also the statements formulated
above drive them together into that specific class.
3.1
It Should not Have Happened
A patient had a primary total hip replacement with the insertion of a prosthe-
sis. Seventy months postoperatively radiographs revealed a fractured prosthetic
neck (Fig.
3.1
a). A similar case was reported for the same type of prosthesis but
85 months postoperatively. Both were modular PCA prostheses implanted without
