CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
constant feature can be attributed to degradation of passive film at the pinholes due to permeability
of the solution resulting increase in charge transfer process with increase in aggressivity of the
medium [22]. Moreover, the high phase angle over wide frequency region as observed in 1 M
nitric acid was not observed in 8 M nitric acid. Significant improvement in magnitude of total
impedance (|Z|) was also observed in 8 M nitric acid for TiO 2 coated specimen as compared to
uncoated, and Ti coated specimens. The improvement with respect to uncoated 304L SS was one
order of magnitude, and two times as compared to titanium coating (Fig. 6.10d). Thus, although
there is a decrease in the stability of passive film from 1 M to 8 M nitric acid for TiO 2 coated
specimens, the magnitude of total impedance, and phase angle are quite superior to uncoated, and
Ti coated specimen in both the solutions signifying good passive film property. The impedance
parameters obtained using electrochemical circuit model as shown in Fig. 3.10a-b are presented in
Table. 6.3 [22].
Fig. 6.10c: Log f vs. for uncoated and TiO 2 coated 304L SS in 8 M HNO 3 [22].