CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
The improvement in magnitude of total impedance (|Z|) for duplex Ti-TiO 2 coating in 1 M
nitric acid was two orders as compared to Ti coating, and one order as compared to TiO 2 coating.
The plots for phase angle () vs. frequency in 8 M nitric acid revealed, near capacitive property
over wide frequency region from 10 0 Hz to 10 4 Hz (Fig. 6.15c). Moreover, unique single time
constant feature was observed in 8 M nitric acid what has not been observed for TiO 2 coating. It
shows that presence of interlayer in duplex coating is very effective in aggressive media by
containing low amount of retained charge, and hindering the charge transfer process at coatingsolution
as well as coating-substrate interface.
Fig. 6.15c: Log f vs. for uncoated and duplex Ti-TiO 2 coated 304L SS in 8 M HNO 3 .
The plots of total impedance (|Z|) vs. frequency (Fig. 6.15d) predominantly showed slope
of -1 up to high frequency indicating capacitive property, and is well in agreement with the plot of
phase angle vs. frequency (Fig. 6.15c). The improvement in magnitude of total impedance (|Z|) for
Ti-TiO 2 coated specimens in 8 M nitric acid was more than one order as compared to both Ti and
TiO 2 coating. The significant improvement in total impedance shows good passivation property