Biomedical Engineering Reference
In-Depth Information
Figure 5.5
Anodic and cathodic part of the polarization curve with Tafel
extrapolations, presenting corrosion current and potential
[56].
Among the few metals used for implants preparation, Co, Ni,
Cu, and V are knows as toxic, while Zr, Ti, Nb, Ta, and Pt exhibited
excellent biocompatibility and belong to the vital group in tissue
reaction [36]. Al has causal relations with neurotoxicity and senile
dementia of the Alzheimer type [23, 27, 36, 54]. The corrosion
behavior and biocompatibility of these materials plays a key role for
theirs successful implant applications.
Titanium and its alloys are the most corrosion-resistant of the
alloys. This is assured by the very high stability of the TiO
2
passive ilm
that forms on the alloy surface in the oxygen-containing atmosphere.
Typical thickness of the native oxide is only a few nanometers, but
is suficient to protect against corrosive dissolution. The TiO
2
is
thermodynamically stable in the pH range between 2 and 12, and
only species, such as HF or H
2
O
2
, lead to its substantial dissolution
[41, 53]. In acidic solutions, the Al-containing alloys such as Ti-6Al-
4V, Ti-6Al-7Nb are signiicantly more susceptible for dissolution
than commercially pure Ti (cp-Ti) [39, 53].
The Ti-6Al-4V alloy is one of the most commonly used in the
implant applications. The titanium, aluminum, and vanadium are
released into the cell tissue by the dissolution of the TiO
2
passive ilm
and wear corrosion process. Hence, the more stable the TiO
2
passive
layer on the Ti-based implant alloys is required for the better the
corrosion resistance. Implanted alloy corrode in the biological body
