CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
nitric acid is that the breakdown of passive film can be well observed due to increase in
aggressivity of the medium (Fig. 4.3d). The root mean square roughness value increased from 15
nm in as polished condition to 28 nm after immersion for 27 h which indicates degradation of
passive film with increase in aggressivity of the medium. The situation in 11.5 M nitric acid was
totally different from 1 M, 4 M and 8 M nitric acid, where a well formed passive film was not at
all observed (Fig. 4.4a-b), instead stained surface with degraded passive film and corrosion
product on the polished surface was observed with increase in immersion time (Fig. 4.4c-d) [125].
a
b
c
d
Fig. 4.4: Ex-situ surface morphology of 304L SS in 11.5 M HNO 3 with increase in immersion
time, (a) As polished (b) 9 h, (c) 18 h, and (d) 27 h.
The root mean square surface roughness value increased significantly from 15 nm in as
polished condition to 35 nm after 18 h of immersion to 52 nm after 27 h of immersion. The data