CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 7
Sub-surface modification using nitrogen ion implantation on 304L SS brings structural and
compositional changes on the surface which significantly increases the localized corrosion
resistance in the oxidizing environment of nitric acid. The factors responsible for the improvement
in corrosion resistance are homogenization of the surface, enrichment of nitrogen and formation
of selective chromium nitride (Cr 2 N) phase. Thus the beneficial aspect nitrogen on the corrosion
resistance is well reflected from the study on corrosion behaviour of nitrogen ion implanted 304L
SS in nitric acid medium.
Top surface modification using Titanium (Ti) layer gives good corrosion performance in 1
M nitric acid. The oxide film developed is stable enough to resist attack at structural
heterogeneous areas. At higher concentration of 8 M nitric acid, the oxidizing action in the pores
and intergranular boundaries increases which decreases the protection efficiency and
performance of the 304L SS. Titanium dioxide gives improved corrosion performance as
compared to titanium coating in both low (1 M) and high (8 M) concentration of nitric acid
despite presence of pores and other structural heterogeneities. This is mainly due to presence of
perfect stoichiometric oxide with good passivation property. Even though TiO 2 shows good
corrosion performance, uniform dissolution of the coating can be observed at 8 M nitric acid.
Duplex Ti-TiO 2 coating is extremely effective for corrosion protection in both low (1 M) and high
concentration (8 M) of nitric acid as compared to Ti and TiO 2 coatings due to combination of
good passivation property of titanium dioxide and minimization of structural heterogeneities by Ti
interlayer. The percentage of protection efficiency and performance is far superior to both the
single layer coatings. Top surface modification is more effective than sub-surface modification for
corrosion protection because of easy optimization of microstructure by tunable reduction of pores
and other structural heterogeneities on the surface.