CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
As compared to uncoated specimens, morphology of TiO 2 coated specimens after
polarization did not reveal any grain boundary opening up, thereby showing greater resistance to
surface dissolution as well as intergranular corrosion (Fig. 6.12b and d) [22]. The improved
resistance to intergranular corrosion is due to high passive film stability as well as low anodic
dissolution rate of titanium dioxide in nitric acid medium. The morphology of TiO 2 coated
specimens after polarization in 1 M nitric acid did not show much change in surface morphology
(Fig. 6.12b). However, in 8 M nitric acid uniform dissolution of the coated surface was observed
(Fig. 6.12d), leading to change in shape, and decrease in particle size of the oxide layer [22].
Moreover, the pinholes were widely open, and apparent separation between the particles were well
observed. This is due to shrinkage in size of the particles as a result of dissolution along the
periphery at the inter-columnar boundaries in the coated surface [22]. Nevertheless, in both
concentrations, the extent of dissolution was less as compared to uncoated specimens, and no
intergranular corrosion was observed [22].
6.2.3 Corrosion behaviour of duplex Ti-TiO 2 coated 304L SS in nitric acid medium
6.2.3.1 Phase and morphology characterization
The phase analysis of the duplex Ti-TiO 2 coated 304L SS revealed anatase phase as shown
for TiO 2 coated 304L SS (Fig. 6.7). The morphological analysis of the duplex Ti-TiO 2 coated
specimen is shown in Fig. 6.13a-d. The morphology of the coated surface showed spherical
particles uniformly covering the surface (Fig. 6.13a). The histogram showing the distribution of
the particles as a function of frequency of formation is shown in Fig. 6.13b. The sizes of the
particles generally range from 12×10 -3 µm 2 to 1.25 µm 2 . However, certain agglomerated particles
were having sizes up to 4.5 µm 2 . The exact location, and morphology of the pores present on the
surface is shown in Fig. 6.13c. The histogram showing the distribution of the size of the pores as a
function of frequency of formation is shown in Fig. 6.13d. The size of the pores in the duplex Ti-