CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 5
correlation between surface analytical studies and corrosion resistance of 304L SS in 1 M nitric
acid is well observed. With increase in nitrogen ion dose, decrease in passivation current density,
corrosion current density and marginal increase transpassive potential revealing improvement in
corrosion resistance was observed. Similarly, in both SIMS and XPS analysis increase in nitrogen
intensity revealing enrichment of nitrogen as well as formation of chromium nitride was observed.
Thus, the beneficial role of nitrogen from corrosion results with surface analysis using SIMS and
XPS can be well correlated.
5.2. 9 Surface morphology study after potentiodynamic polarization
Surface morphology analysis of the unimplanted and nitrogen ion implanted samples with
dose of 1×10 16 and 2.5×10 17 N + /cm 2 after potentiodynamic polarization experiment investigated
using AFM are as shown in Fig. 5.9a-c [21]. Distinctive surface morphologies were observed
depending on the dose of nitrogen implantation, as clear from the topographic images indicating
effect of nitrogen in slowing down the surface dissolution process leading to improvement in
corrosion resistance. The unimplnated samples showed grain boundary dissolution in nitric acid
medium with visibility of well demarked grain boundaries (Fig. 5.9a). The large damage, and
highly corrugated surface morphology is due to intergranular corrosion of austenitic stainless steel
occurring in nitric acid medium [20,21].
The extent of grain boundary dissolution lessened at a dose of 1×10 16 N + /cm 2 (Fig. 5.9b) as
less clear grain boundaries were visible. At highest dose of implantation of 2.5×10 17 N + /cm 2 the
grain boundary dissolution disappeared and at some places selective attack was observed (Fig.
5.10c). The selective attack is attributed to the presence of surface inhomogenities on the surface.
The root mean square roughness (R q ) calculated for the aforementioned polarized test samples in
1 M nitric acid are, 238 nm for unimplanted, 110 nm for dose of 1×10 16 N + /cm 2 and 46 nm for