CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...
CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...
CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...
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Chapter 6<br />
The impedance parameters for uncoated, and Ti coated 304L SS in 1 M and 8 M nitric acid<br />
obtained by fitting with circuit models as mentioned in Fig. 3.10a-b are shown in Table. 6.1. From<br />
Table. 6.1, it is clear that Ti coated 304L SS specimens showed improvement in polarization<br />
resistance (R ct2 ) as compared to uncoated condition, however the polarization resistance of Ti<br />
coated specimen in 8 M nitric acid was much lower as compared to that of 1 M nitric acid.<br />
Moreover, the pore resistance (R ct1 ) in 8 M nitric acid was also significantly lower compared to 1<br />
M nitric acid, which shows increasing charge transfer process, and anodic activities at structural<br />
heterogeneities in the coating-substrate interface. The stability of the coating in corrosive solution<br />
is determined by the coating capacitance, and in general lower the coating capacitance value<br />
higher is the stability signifying insulating nature [199].<br />
In 1 M nitric acid, the coating<br />
capacitance (C dl1 ) showed lower capacitance value which indicates low amount of retained charge<br />
on the coated surface. However with increasing concentration (8 M), the retained charge capacity<br />
increased which showed higher capacitance value. The double layer capacitance (C dl2 ) for Ti<br />
coated 304L SS specimens also showed lower capacitance value in 1 M nitric acid, which<br />
indicates good passive film stability at the coating-substrate interface. Increase in C dl2 value was<br />
observed at higher concentration (8 M) indicating decreasing passive film stability at the coatingsubstrate<br />
interface with increasing oxidizing nature.<br />
Conc.<br />
Substrate<br />
condition<br />
R s<br />
cm 2 CPE 2<br />
R ct2<br />
cm 2 CPE 1<br />
R ct1<br />
cm 2<br />
C dl2<br />
-1 cm -2 S n<br />
n<br />
(0-1)<br />
C dl1<br />
-1 cm -2 S n<br />
n<br />
(0-1)<br />
1 M Uncoated 2 6.5×10 -5 0.76 9.35×10 2 - - -<br />
1 M Ti coated 5 4.6×10 -5 0.83 2.13×10 4 6.5×10 -6 0.84 4×10 2<br />
8 M Uncoated 1 5.8×10 -4 0.65 1.8×10 2 - - -<br />
8 M Ti coated 2 8.2×10 -5 0.71 4×10 2 2.7×10 -5 0.76 1×10 1<br />
Table. 6.1: EIS fitted values for the uncoated and Ti coated 304L SS in 1 M and 8 M HNO 3 .
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