CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
However, the morphological features in 8 M nitric acid were quite different from 1 M and 4
M nitric acid, where homogenization as well as local breakdown of passive film over the ridged
lines on polished surface can be observed with increase in immersion time. Figure. 4.3a-d,
represents the surface morphology of 304L SS in 8 M nitric acid during the immersion up to 27 h.
a
b
c
d
Fig. 4.3: Ex-situ surface morphology of 304L SS in 8 M nitric acid with increase in immersion
time, (a) As polished, (b) 9 h, (c) 18 h, and (d) 27 h.
The morphology after 9 h of immersion (Fig. 4.3b) was mostly similar to that of as polished
condition (Fig. 4.3a) showing polishing lines, and ridged structure. However, the surface appeared
to be smoother by filling up of the gaps in between the ridged structures after 18 h of immersion
indicating uniform passive film covering the surface. The breakdown of passive film on the
surface was also observed as thin layer distorted topography over ridged structure after immersion
for 27 h (Fig. 4.3d) [124]. The interesting point in 8 M nitric acid as compared to 1 M, and 4 M