CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 5
N + Dose
E corr
(mV vs. Ag/AgCl )
I corr
(µA/cm 2 )
I pass
(µA/cm 2 )
E transpass
(mV vs. Ag/AgCl)
AISI 304L SS 120 1×10 0 6×10 1 975
1×10 15 N + /cm 2 195 5×10 -1 2×10 1 973
1×10 16 N + /cm 2 300 3×10 -1 1.6×10 1 971
1×10 17 N + /cm 2 354 8×10 -2 1.2×10 1 978
2.5 ×10 17 N + /cm 2 375 5×10 -2 7×10 0 985
Table. 5.1: Polarization parameters for unimplanted and nitrogen implanted
304L SS in 1 M HNO 3 [21]
Nitric acid is an oxidizing acid, and the E corr , I corr , I pass and E transpass are dependent on the
extent of reduction of nitric acid, which itself is concentration dependent [3-7, 20-22, 41,42]. With
higher auto-catalytic activity of nitric acid, corrosion current density, passivation current density,
corrosion potential increases, and transpassive potential decreases because of faster transpassive
dissolution of oxide film on the surface. In the present investigation the improvement in corrosion
potential, reduction in passivation current density, and corrosion current density and marginal
improvement in transpassive potential reflects the beneficial effects of nitrogen on corrosion
resistance of 304L SS in nitric acid medium [21, 147, 167].
The overall improvement in corrosion potential, passivation current density and corrosion
current density is attributed to the combined effect of implanted nitrogen as well as formation of
chromium nitride. The implanted nitrogen plays major role in modifying electrochemical
properties mainly by forming ammonium ions which can further form nitrite or nitrate ions,
thereby increase the local pH, and shift the corrosion potential in the noble direction. The
formation of ammonium ion was first proposed by Osozawa et al [165] and suggested that,
ammonium ions formed restricts the decrease of pH at active sites on the surface such as grain