CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 5
boundaries and kinks where passive film formation is highly unstable. However, the degree of
ammonium ion formation increases with increase in amount of nitrogen, and decreases with
applied potential [168]. To compensate the decreasing
amount of ammonium ions at higher
potential Kamachi Mudali et al proposed the subsequent formation of nitrates and nitrites ions
from ammonium ion, which are well known for their strong stabilizing effect on passive film
stability [169]. Similarly, it has also been observed the nitrogen implantation decreases the passive
current density as well as corrosion current density owing to enrichment of nitrogen beneath the
passive film, and segregation of nitrogen during polarization forming interstitial nitrides which
decreases the surface dissolution process. Abundant evidence for the existence of nitrogen beneath
the passive film subsist from surface analytical technique, and the decrease in passivation current
is because of charge associated with passive film formation is reduced by implanted nitrogen
[168]. Thus, less current is required for passive film growth with increase in nitrogen implantation
dose. The decrease in corrosion current density can also be attributed to surface segregation of
nitrogen to the oxide-metal interfaceduring anodic polarization forming stable interstitial nitride
phases of chromium [163, 166, 170, 171]. The formation of chromium nitride during polarization
has been confirmed by several studies, and has been argued on the basis of low solubility of
nitrogen in austenitic stainless steel, and thermodynamically favourable process of formation.
These phases are proposed [166] to act as a kinetic barrier to the dissolution of the alloy because
their dissolution reaction is slow due to multi-electron transfer process.
5.2. 8 Electrochemical impedance measurement
The electrochemical impedance measurements for both prior to, and after implantation
with different doses of nitrogen in open circuit potential condition are presented in Fig. 5.8. All the
Nyquist plots showed unfinished semi-circular arc and appropriate equivalent circuit is chosen as
shown in Fig. 3.9 to evaluate the obtained experimental results. The fitted parameters for double