CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
Results revealed that in both the test solutions, OCP shifted towards more noble potential for
Ti coated specimens as compared to uncoated condition. For uncoated specimens OCP in 8 M
nitric acid was higher as compared to that of 1 M nitric acid because of large concentration of
nitrous acid (HNO 2 ) produced at higher concentration due to auto-catalytic reduction which
imposes higher redox potential [3-7, 20-22, 41,42]. In 1 M nitric acid the uncoated 304L SS
specimens attained passive state quickly, and showed steady passivation behaviour with increase
in immersion time, however the OCP of titanium coated specimen started with noble potential
decreased to lower value before attaining steady state (Fig. 6.3a). In 8 M nitric acid the uncoated
specimens also showed steady OCP behavior. Similar to 1 M nitric acid, the Ti coated specimens
initially showed higher OCP in 8 M nitric acid, and decreased abruptly to lower value before
attaining steady state (Fig. 6.3b). In both the test solutions, Ti coated specimens showed one
transition point before attaining steady state justifying the observed OCP behaviour in
electrochemical solution [196].
The OCP of the Ti coated 304L SS specimens shows the stability of the developing oxide film
in both the concentration of test solution with increase in oxidizing nature. Titanium is known to
form thick oxide layer by oxidation of metallic Ti, but the composition and thickness of the oxide
layer largely depends upon the nature of the electrochemical environment. In acidic solutions, the
oxidation state changes from lower to higher value such as Ti 2 O 3 , Ti 3 O 5 and TiO 2 , respectively
[197]. The non-stoichiometric oxides are insoluble at lower concentration of oxidizing solution,
but rapidly dissolve in concentrated solution [198]. Moreover, the passive film does not stabilize
properly, if Ti fails to oxidize perfectly to TiO 2 , and OCP decreases to lower value justifying the
observed OCP behavior [197]. The structural heterogeneities such as pinholes, and inter-columnar
boundaries also effects the passive film stability because developing passive film is generally thin
in these regions, and provides local sites for breakdown of passive film.