CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
(E transpass = E coated - E uncoated ) in 1 M nitric acid was 150 mV (vs Ag/AgCl), and 50 mV (vs.
Ag/AgCl) in 8 M nitric acid, respectively. Similarly compared to Ti coated condition, the
improvement in transpassive potential was 104 mV (vs Ag/AgCl) in 1 M nitric acid, and 24 mV
(vs. Ag/AgCl) in 8 M nitric acid, respectively. Owing to decrease in passive current density and
increase in transpassive potential, corresponding decrease in corrosion current density was also
observed in both the test solutions. The decrease in corrosion current density for TiO 2 coated
specimen compared to that of Ti coated condition was significant in 1 M nitric acid whereas in 8
M nitric acid it was marginal. This is because the transpassive dissolution of oxide layer is faster
due to increase in auto-catalytic activity around 50 % concentration (8 M). Hence, breakdown of
passive film i.e. transpassivity was attained quickly leading to increase in corrosion current density
[22]. The protection efficiency in 1 M nitric acid was 95 % in 1 M nitric acid, and 43 % in 8 M
nitric acid, respectively.
The improvement in corrosion resistance of TiO 2 coated 304L SS specimens is because of
its inertness in electrochemical environment, largely due to its dielectric property, high lattice and
bond energy which inhibit the anodic dissolution process and provide the alloy a stable and
protective surface [179, 201]. Even though TiO 2 doesn’t behave as a perfect insulator [204], the
dielectric strength ensures sustain of high electric field during polarization, and thus resists field
assisted collapse of the passive film at the film-solution interface, and easy dissolution of passive
film. Furthermore, higher dielectric strength lowers electronic conduction through the passive
film, thus charge transfer process as well as activation energy for creation of physical “faults” in
passive film, which act as precursor sites for localized breakdown are also reduced. Apart from
this due to high lattice energy the cohesive force in the lattice is also high. Thus, large extent of
electrostatic potential barrier has to be overcome in order to polarize the surrounding environment,
and consequently to pull out an ion from oxide lattice leading to initiation of surface dissolution.