Biomedical Engineering Reference
In-Depth Information
Raja
et al
.
[37] in Ti anodization experiments found that
addition of luoride ions results in the corrosion potential to be
more negative and the current density increases in the passive
range. The critical passivation current density increases orders of
magnitude with comparison to NaCl solution. Figure 5.19 shows
sequence of oxide ilm formation during anodic polarization [37].
Dissolution of Ti metal occurs at free corrosion potential and
a reprecipitated salt ilm is seen (a) till the critical passivation
current density reaches (peak position). At this peak position, the
Ti is maximally attacked and no external layer is formed (b). When
the current decreases, a thin oxide layer covers the attacked grain
facets (c) and further increase of potential results in nucleation of
another layer of oxide over previous one is observed (d).
Figure 5.19
Stages of oxide ilm formation during polarization of Ti in
acid luoride solution. (a) A thick defective re-precipitated
layer on the surface, (b) at the critical peak current density no
visible ilm could be observed, (c) a thin ilm formed (inner-
barrier layer) immediately after reaching critical current
density for passivation, increases in potential resulting in
nucleation of secondary (outer) oxide layer and (d) growth of
outer oxide layer [49].
The surface treatment (not only anodic oxidation) is mainly
responsible for the good corrosion resistance. For example, the
effect of the laser surface remelting on corrosion resistance and
pitting potential of pure titanium was investigated by Sun
et al
.
