CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 6
exhibits wide passive range as compared to both the single layer coating. It is also evident from,
Table. 6.2, Table. 6.4, and Table. 6.6 that duplex Ti-TiO 2 coated specimen exhibited lowest I corr
value showing highest corrosion resistance out of the three different coatings investigated in the
present investigation. The protection efficiency for the duplex Ti-TiO 2 coating calculated using
Eq. 13 are 97 % in 1 M nitric acid, and 84 % in 8 M nitric acid, respectively.
The low passive current density as well as higher transpassive potential is due to the
combination of good passivation property of outer TiO 2 layer as well as reduction in structural
heterogeneity by Ti interlayer. For duplex coating the extent of structural heterogeneities has
greater role in predicting the corrosion rate. If the porosity is less, the specimen behaves totally as
a coating, and pores are passivated if the coating passivates showing good corrosion resistance.
Similarly, if significant pores are there, the coating behaves like base material, and it fails to
passivate at the pores and other structural inhomogenities regardless of how noble the coating may
be. As can be observed from the Fig. 6.2d, Fig. 6.8d, and Fig. 6.13d, significant reduction of pore
density in the duplex coating was observed for duplex Ti-TiO 2 coating compared to Ti and TiO 2
single layer coatings which increases the susceptibility to corrosion by increasing the dissolution
rate of the base material. Moreover, from morphology and pore size analysis it is evident that the
structure of duplex Ti-TiO 2 layer coating is less columnar, more compact and has fewer pinhole
with small surface area as compared to Ti and TiO 2 coating which are essential to restrict the
solution path to the base material. Hence, the effective isolation of the base material from
oxidizing environment leads to low passive current density as well as corrosion current density.
Nevertheless, pores and pinholes also can be observed from Fig. 6.13c in the duplex coating.
However “repassivation” ability of the passive film in these region is higher because of small
surface area and Ti intermediate layer which can oxidize itself to form protective oxide layer [195,
197, 209].