Biomedical Engineering Reference
In-Depth Information
and (1), respectively. After etching at constant voltage 10 V and for
different time, the initial starting current is almost constant (curve
2), but the end current decreases with the progress of etching
time (curve 1). The observed current decay is attributed to a high
electrical ield oxide formation. For the samples etched in electrolytes
containing different HF concentration, the current density increases
with increasing HF concentration (Fig. 9.13b). The gap between
the current measured at start and end time signiicantly increases
with increasing HF concentration. The HF plays an important role
in titanium etching. Tsuchiya
et al
. [88] observed that for higher
applied voltages (>80 V), the current signiicantly increases after
initial decreasing stage, which was related with breakdown of the
metal oxide. In acidic electrolytes containing HF, the main and minor
part of the current is consumed in dissolution reaction and TiO
2
formation, respectively [5, 50]. On the other hand, Macak
et al
. [50]
found that using NaF instead of HF leads to the growth of thicker
porous layer, because etching of TiO
2
in neutral electrolytes is slower
than in acidic electrolytes. Increasing of the HF concentration in
the electrolyte, results in high dissolution rate of the oxides. This
phenomenon is possible even without anodization conditions
(chemical etching).
Figure 9.13
Variation of the current density for different etching time
(1M H
3
PO
4
+ 2% HF at 10 V) (a) and for different HF content
(1M H
3
PO
4
, 10 V for 30 min) (b); lines number: 1 and 2
present current density after end and start etching time,
respectively [28].
The polarization curves clearly indicate the etching behavior.
The polarization curves for Ti immersed in 1M H
3
PO
4
with different
HF content is shown in Fig. 9.14 [4]. The current densities of the
polarization curves, recorded by Bauer
et al
. [4], increase two
decades with HF concentrations and increasing from 0.0 to 1.0 wt%.
