CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
a
b
c
d
Opening up of grain
boundaries
Opening up of grain
boundaries
Fig. 4.9: In-situ surface morphology of 304L SS in 1 M nitric acid (a) as polished, (b) 500 mV,
(c) 900 mV and (d) 1100 mV [20].
In general, the in-situ results demonstrate the formation and breakdown of passive film with
increasing oxidizing power of nitric acid. According to the passivation model for austenitic
stainless steel in acidic media [12, 16-19], passive film is a gel-like structure, which breaks down
at local inhomogenities, thereby initiating surface dissolution. The decrease in size as well as
formation of the platelets up to 0.6 M HNO 3 indicates homogenization and decay of passive film.
Supportive evidence for the breakdown of passive film, and initiation of surface dissolution is
apparent from morphology at 0.6 M HNO 3, where surface dissolution was observed in the
presence of platelet like structures marked by arrows in Fig. 4.8d [20]. In 1 M nitric acid the
platelet like structures were not observed, thus aggressive media has direct access to high energy