CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
[17]. They concluded from their study that the air formed film on 18-8 stainless steel gets
strengthened when passivated in nitric acid media. According to this theory, the surface of the
metal/alloy is oxidized or the atoms on the surface are in such a relation with the oxygen of the
electrolyte, that it is equivalent to an oxidation process. The properties characterizing a typical
oxide film are of low ionic conductivity, and low solubility. Moreover, it is assumed that such
oxide layers are formed as a diffusion barrier separating metal/alloy from electrochemical
environment thereby decreasing the reaction rate. Due to these properties, the oxide layer prevents
a large extent transport of metal ions from sites in crystal lattice to the electrochemical
environment i.e., it prevents the anodic dissolution. Nevertheless, there is slow dissolution
corresponding to passive state of the materials since oxide layers directly formed in close
connection with crystal structure often contains defects which allow electron conduction, and
electrochemical reaction can occur at the top surface of the oxide layer.
2.7.2 Adsorption theory
The oxide film theory suffered set beck, when it was observed that, the lack of coincidence
of the potential marking the active to passive state transition with the potential for the reverse
transition [17]. It was thought that the lack of coincidence in passivation potential could not be
explained by oxide layer theory rather by a variable layer of adsorbed oxygen on the surface either
by physical or chemical adsorbtion. Later on, Uhlig [19] proposed that adsorbed oxygen film is
commonly the source of passivity, and such film forms on transition metals (Fe, Cr) as well as 18-
8 stainless steel in accord with their uncoupled d-electrons interacting with oxygen atoms to form
stable bond. The high sublimation energy of these metal-oxygen bond favours retention of the
metal atoms in their lattice in preference to their removal to form the oxide lattice as per oxide
layer theory. Even less than a monolayer of adsorbed film has passivating effect, increases the
anodic overvoltage and decreases the exchange current density. The characteristics of high energy