Biomedical Engineering Reference
In-Depth Information
TABLE 3.6. Passivation Parameters Obtained from Potentiodynamic Polarization Curves
in Hank's Solution at 37 °C. The Passivation Parameters are Zero Current Potential: ZCP,
Breakdown Potential: E b , Passive Current Density: i pass and the Passivation Range ( E b - ZCP )
Sample
ZCP (mV vs. SCE)
E b (mV vs. SCE)
A/cm 2 )
E b - ZCP (mV)
i pass (
μ
Ti - 6Al - 4V
− 276
1277
3.0
1553
Ti - 6Al - 4Nb
− 170
1182
1.4
1352
Ti - 6Al - 4Fe
− 257
1152
1.3
1409
Ti - 5Al - 2.5Fe
− 461
1244
1.2
1705
[105] .
TABLE 3.7. Corrosion Rates of the T i Alloys Determined by Tafel Extrapolation Method in
Hank's Solution at 37 °C and pH of 7.4. The Parameters are, Zero Current Potential: ZCP, Zero
Current Potential: The Cathodic (
β
c ) and Anodic (
β
a ), The Corrosion Current Densities ( i corr )
ZCP (mV
vs. SCE)
β
c (mV/
decade)
a (mV/
decade)
β
i corr (
A/
cm 2 )
μ
Corrosion rate
(
Sample
μ
m/year)
Ti - 6Al - 4V
− 231
− 176
168
0.16
1.39
Ti - 6Al - 4Nb
− 596
− 158
185
0.10
0.86
Ti - 6Al - 4Fe
− 390
− 122
181
0.04
0.35
Ti - 5Al - 2.5Fe
− 588
− 102
175
0.13
1.13
[105] .
tigated were of the same order of magnitude. The highest breakdown potential
(w) was exhibited by Ti-6Al-4V. The passive range in the case of materials exhibit-
ing stable passive behavior is provided by the difference between breakdown and
zero current potential. The addition of aluminum decreased the passive range
signifi cantly, as can be noted from the data for Ti-15Al.
On the other hand, Nb addition increased the passive range but not as much
as that for Ti-6Al-4V (Table 3.6). The zero current potential, the cathodic (
β
c ) and
anodic (
a ) Tafel slopes, the estimated corrosion current densities (i corr ) and corro-
sion rates are tabulated in Table 3.7. Considering the maximum corrosion rate
determined for each alloy (a conservative estimate), it is noted that the corrosion
rates of Ti - 5Al - 2.5Fe, Ti - 6Al - 4V, Ti - 6Al - 4Fe and Ti - 6Al - 4Nb are comparable.
Importantly, the corrosion rate is not drastically affected by the amount of Fe
substitution. The results of Choubey et al. clearly revealed that all the alloys
exhibited stable passive polarization behavior.
β
3.6.2 Ti-Based Alloys
Titanium and its alloys are being developed largely for different implant applica-
tions. They encompass mechanical properties that are suitable for the applica-
tions. Due to their excellent corrosion resistance in body fl uid, they are the most
accepted metallic materials for clinical applications. The interactions between
titanium and the body tissues, which allow osseointegration in contact with bone,
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