CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
2.10.6 Crevice corrosion
This type of corrosion occurs in crevices, gasket, and in lap joints of the fabricated material
and equipments. In general, systems that show pitting behaviour are susceptible to crevice
corrosion [36]. However, the reverse is not true always. Austenitic stainless steel does suffer from
crevice corrosion in nitric acid medium containing aggressive ions. Crevice corrosion indeed has
been observed in nuclear fuel reprocessing plant behind weld backing strip at the closing seam of
304L SS vessels handling hot concentrated nitric acid [15]. Corrosion by stagnant acid in the
crevices creates hexavalent chromium ions (Cr 6+ ) which causes accelerated attack while other
exposed surfaces remains unaffected.
2. 11 Need for present study
Based upon the literature survey it is found that even though the mechanism of different
forms of corrosion of austenitic stainless steel in nitric acid medium has been investigated, the
passive film initiation, formation and dissolution has not been addressed to date either in ex-situ or
in-situ condition combining electrochemical mechanism and surface morphology. Thus, in the
present thesis major emphasis has been given to understand the important aspects of passive film
such as nucleation, agglomeration, formation, dissolution, morphology and composition of the
passive film of AISI 304L SS in nitric acid medium in both ex-situ and in-situ condition using
electrochemical polarization and atomic force microscope. Similarly, the effect of surface
modification either by sub-surface or top-surface for understanding the corrosion resistance and
protection efficiency in nitric acid environment is not significantly investigated. Hence, an attempt
has been made to investigate the effects of sub-surface modification by nitrogen ion implantation
and top surface modification using protective coating (Ti, TiO 2 and duplex Ti-TiO 2 ) on the
corrosion resistance of AISI 304L SS in nitric acid medium.