CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
but the kind of film formed, and its enthalpy of formation decides the stability of the oxide layer.
The mechanism of passivation initiates with the formation of Ti 3+ ions, which consequently forms
certain metastable oxides which further oxidizes to TiO 2 [197]. However, the composition of
oxide layer depends upon the oxidizing power of the solution, and generally consists a mixture of
anatase with certain quantity of rutile phase [197]. Various oxide layers that develops in acidic
solution when titanium undergoes passivation are Ti 3 O 5 (H f = - 2459.15 kJ/mole), Ti 2 O 3 (H f = -
1520.84 KJ/mole), TiO 2 (H f = - 944.75 KJ/mole), respectively. The non-stoichiometric oxides
are having low enthalpy of formation, and are generally porous in nature [200]. The perfect
stoichiometric oxides are compact and have high enthalpy of formation. The non-stoichiometric
oxides are stable in low concentration, and forms good passive film but dissolve in strong acid
solution due to low oxide density as well as increase in aggressivity in the exposed area of the
pores [196-198]. Apart from this, the electrochemical environment at structural heterogeneities
such as pinholes, flaws, and inter-columnar boundaries is more aggressive as compared to that of
matrix due to accumulation of solution, and passive film formed in these regions is thinner. Thus,
the protective oxide film formed electrochemically offers less resistance to corrosive medium and
fails usually in the depth of the pores at higher concentration. Moreover, galvanic potentials do
exist at pinholes, and inter-columnar boundaries and these regions prevent passive oxide layer
formation and lead to enhanced corrosion of base material. Once initiated, corrosion proceeds at
the boundaries between the coating and substrate, and the protection of base material is lost. Apart
from this, accelerated corrosion reduces the coating bond strength, and uniform dissolution of the
coated surface occur leading to peeling up of the coating.
6.2.1.5 Morphological investigation after potentiodynamic polarization study
The results for surface morphological study for uncoated and Ti coated 304L SS specimens
after potentiodynamic polarization in both the concentration of test solution are shown in Fig.