CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
The surface morphologies in 0.5 M HNO 3 also shows the formation of the platelet like
structures, however the platelets formed were fewer, and the size was also smaller as compared to
0.1 M HNO 3 . The platelets, projected area (A S ), inclination (), disorientation angle () between
platelets for 0.1 M and 0.5 M nitric acid were calculated by using NOVA software assuming them
as rectangular platelets, and the root mean square roughness on the surface are shown in Table.
4.2. Inclination () was measured by taking three platelet i.e. two in the same layer and third in
adjacent layer and, disorientation angle () by taking three platelet in the same layer (top-bottom).
Conc of HNO 3
A S
(µm 2 )
(Deg)
(Deg)
R q
(nm)
0.1 M HNO 3 0.8-1 45-65 2 9
0.5 M HNO 3 0.4-0.7 10-20 1 6
0.6 M HNO 3 - - - 4
1 M HNO 3 - - - 20
Table. 2: Statistical EC-AFM data for platelet growth in 0.1 M , 0.5 M, 0.6 M and 1 M HNO 3
[20].
It was found that, the size of the platelet decreased from 0.8 µm 2 to 0.4 µm 2 , angle of
inclination () decreased from 45° to 10° with increase in concentration from 0.1 M to 0.5 M
HNO 3 . Similarly, disorientation angle decreased from 2° to 1° from 0.1 M HNO 3 to 0.5 M HNO 3.
The important feature in both 0.1 M and 0.5 M HNO 3 is that, no visible corrosion process was
observed on the surface and, the surface was fully covered by the platelets [20].
Figure. 4.8a-d represents the surface morphology of 304L SS under immersion condition in
0.6 M nitric acid in as polished, 500 mV (Ag/AgCl), 900 mV (Ag/AgCl), and 1100 mV
(Ag/AgCl), respectively. The interesting feature observed at this concentration was the formation
of platelet like structure as well as initiation of surface dissolution process around 1100 mV