CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
C H A P T E R 4
Passive film properties of 304L SS in nitric acid medium were investigated in both ex-situ and
in-situ conditions using AFM and EC-AFM techniques. Ex-situ study in 1 M, 4 M, 8 M, and 11.5
M nitric acid revealed that variation in passive film morphology occurs depending upon the
concentration, and time of immersion. In-situ passive film surface morphology investigation
showed formation of chromium hydroxide layer in the form of platelet like structures at lower
concentrations (0.1 M, 0.5 M), and towards higher concentration (0.6 M) the platelets formed
homogenous oxide layer on the surface which started depleting at still higher concentration (1 M)
leading to opening up of oxide boundaries. Compositional analysis of the passive film revealed
duplex nature consisting of hydroxide and oxide layers at lower concentration, and with
increasing concentration oxide layer predominates over the surface.
4.1 Introduction
Electrochemical corrosion initiation at sub-microscopic region involves several
phenomenon starting from breakdown of passive film in stochastic and sporadic way to localized
dissolution of oxide covered material, and mass transport of atoms across the surface to support
the continuing dissolution process [114,115]. The crucial factor controlling all the above
phenomenon is the passive film whose structure, property, and stability is of immense important in
understanding the many details of electrochemical corrosion process as well as central to
controlling corrosion, and use of metallic materials in many technologies. The desirable, and
beneficial property of passivity is directly related with sustainability of material, since corrosion is