LASER PROCESSING OF ORTHOPEDIC BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 291

materials tested showed a clear active to passive transition at E Corr values close to
0V. There is not much signifi cant difference in the free potential E corr values for
these materials. The active-to-passive transition corresponds to the formation of
a passive protective oxide fi lm on the surface of the material. In the case of the
conventional alloys Ti - 6Al - 4V and CP - Ti, the fi lm that forms breaks down at a
potential close to 1.5V, typically referred to as the breakdown potential, while in
the case of LENS™ deposited TNZT alloys the passive fi lm does not show any
breakdown up to a voltage of 5V and above. The critical current density values
for CP Ti and Ti - 6Al - 4V are

∼ 8.4 uA/m 2 and ∼ 5.3 uA/m 2 , respectively, whereas

that of TNZT is marginally higher, 10.4 uA/m 2 . Though the critical current

density of TNZT alloys are higher than CP-Ti and Ti-6Al-4V, the E tp or the trans-
passive potential for this alloy is higher than 5V, while E tp for Ti - 6Al - 4V and CP
Ti are

∼

1.5V respectively. Thus it can be concluded that LENS™ depos-
ited TNZT forms a more adherent surface oxide layer, exhibiting better corrosion
resistance in 0.1 N HCl as compared to CP - Ti and Ti - 6Al - 4V.
Anodic polarization tests were also performed in simulated body fl uid
(Ringer ' s solution). The materials compared were Ti - 6Al - 4V ELI, TNZT and
boride reinforced TNZT composite. The processing and tribological properties of
the boride reinforced TNZT composite have been discussed in the succeeding
section. The testing Ringer's solution used comprised of NaCl 9 g/l, KCl 0.43 g/l,
CaCl 2 0.24 g/l, and, NaHCO 3 0.2 g/l. Similar to the case of the testing in 0.1 N Hcl,
described previously, the potential was measured with reference to a Saturated
Calomel electrode and was varied from

∼ 1.3 and ∼

2V to 5V with a scan rate as low as
0.166 mV/s. Current density was plotted simultaneously and the resulting anodic
polarization curves for the three materials are shown in Figure 9.12. All three

−

Polarization Curve-Ringers

6

Ti-6A1-4V
TNZT
TNZT+B

5

4

3

2

1

0

-1

-2

0.00001 0.0001 0.001 0.01 0.1 1 10 100

Current Density A/m^2

Figure 9.12. Anodic Polarization curve for Ti-6Al-4V ELI, TNZT and TNZT + 2B in Ringers Solu-
tion. [Sonia Samuel, (M.S. Thesis, Dept of Mater. Sci. and Eng., University of North Texas, 2007)]