Biomedical Engineering Reference
In-Depth Information
component analysis, was used to process the map into the individual components (1-4)
and a combined overlay (C) based on
Figure 9.2
reference spectra. Each individual compo-
nent was present in the RT TiO
2
sample (C) and associated spectra (i-iv, solid line) were pro-
cessed according to
Figure 9.2
standard spectra (i-iv, dashed line). The RT component (C, i)
along with the actual spectrum (i) at the arrow point compared to the standard RT spectrum (i, black
dashed line). The 400°C component (C, ii) along with the actual spectrum (ii) at the arrow point com-
pared with the 400°C standard spectrum (ii, black dashed line). Next, the 700°C component (C, iii)
along with actual spectrum (iii) compared with the 700°C standard spectrum (iii, black dashed line).
Finally, the 1000°C component (C, iv) along with actual spectrum (iv) compared with the 1000°C stan-
dard spectrum (iv, black dashed line). In the actual spectra for the RT (i), 400°C (ii), and 700°C (iii)
components the peaks for anatase (395, 515, and 637 cm
1
) are more dominate than rutile (445 and
609 cm
1
). On the other hand, in the actual spectra for the 1000°C (iv), it is apparent that the peaks for
rutile are more prominent than anatase. The combination of Raman mapping (C) along with the 3D sur-
face plot for individual components (1-4) illustrate the overall TiO
2
crystalline distribution; whereas,
the separate crystalline forms in the bright field image are indistinguishable (
Figure 9.3A and B
).
The temperature effect of anatase to rutile transformation was verified spectroscopically. Raman
microspectroscopy is a powerful technique to distinguish different crystalline phases by using Raman
mapping at selected characteristic peaks related to individual crystalline form.
9.4
CORROSION TESTS WITH ELECTROCHEMICAL TECHNIQUES
Various electrochemical techniques have been applied to assess the behavior of corrosion, e.g., open-
circuit voltage (OCV), potentiodynamic polarization (Tafel analysis), and electrochemical imped-
ance spectroscopy (EIS). Tafel analysis is a well-established electrochemical technique, the current is
recorded when the open-circuit potential is imposed on a metal sample. Equation (9.1) shows that the
current and voltage have a linear relationship, and the slope is the polarization resistance (R
p
). R
p
is
the resistance of the metal to oxidation during the application of an external potential. Combined Eqs
(9.1) and (9.2), it shows that the corrosion rate is inversely related to R
p
and can be calculated if the
anodic and cathodic Tafel slope were known, respectively (Eq. (9.1)). The anodic and cathodic Tafel
slope can be obtained from linear polarization measurements. The equation used for calculating the
corrosion rate (mmpy) is as indicated below in Eq. (9.2).
β
β
a
c
I
corr
2 .
R
(
β
β
)
(9.1)
p
a
c
where,
I
corr
corrosion current (in A),
R
p
polarization resistance (in Ω),
β
a
anodic Tafel slope,
β
c
cathodic Tafel slope.
I
K
EW
corr
Corrosion Rate (CR)
d
A
(9.2)
K
constant that defines the units of the corrosion rate
3,272 mm/(A cm year), EW
equivalent
weight (in g/equivalent)
11.768 g/eq,
d
density (in g/cm
3
)
4.420 g/cm
3
,
A
sample area
(in cm
2
)
0.151 cm
2
, for all current Ti alloy samples.
