CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
OCP shifted in noble direction for titanium dioxide coated 304L SS specimens as
compared to uncoated, and Ti coated specimen. The shift of OCP in noble direction in both the
concentration of test solutions clearly indicates greater protecting power of TiO 2 coating in the
oxidizing environment of nitric acid as compared to uncoated, and Ti coated specimens. As
compared to Ti coated specimens, TiO 2 coated specimens showed mostly steady state with
increase in immersion time. This is largely due to the presence of perfect stoichiometeric oxide
layer on the surface as compared to Ti which in general forms a number of sub-stoichiometric
oxides in the oxidizing environment [197]. Nevertheless, marginal increase or decrease in OCP
prior to attaining of steady state in both the test solutions indicates stability of the coatingelectrolyte
interface by filling up of pinholes, pores, and void space at the inter-columnar
boundaries [22]. The increase in OCP is attributed to the greater passivation tendency of TiO 2 in
the auto-catalytic oxidizing environment of nitric acid leading to inhibition of anodic process as
compared to Cr 2 O 3 film which naturally occurs on stainless steel surface. In general, TiO 2 is
thermodynamically more stable over a wide potential region due to high lattice energy of
formation as compared to Cr 2 O 3 [201]. In the eventuality of film dissolution, subsequent
hydrolysis of the dissolved ions readily forms the TiO 2 film due to low free energy of formation in
solution [201,202].
Ti 4+ + 2 H 2 O TiO 2 + 4 H +
Thus, its healing power is also quite high, unlike Cr 2 O 3 [Cr(III)] where the film transfers to a
soluble Cr 2 O 2- 7 [Cr(VI)] form in the oxidizing environment. For uncoated specimen, OCP in 8 M
HNO 3 was higher as compared to 1 M HNO 3 because of large concentration of aqueous nitrous
acid (HNO 2 ) produced at higher concentration, which imposes higher redox potential to the
electrochemical environment. Thus, the auto-catalytic reduction rate of nitric acid increases, and