CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
of adsorption of oxygen on transition metals (Fe, Cr) mostly favours the formation chemisorbed
films as compared to low-energy films formed on non-transition elements due to physical
adsorption [19]. Thus, the chemisorbed film formed on the surface is thermodynamically more
stable then the oxide layer formed on the surface due to oxidation according to oxide layer theory.
However, the large recognition of adsorbtion theory does not preclude the existence of
oxide layer theory. It is argued that both the theories are supplementary to each other because the
adsorbed film in the process of nucleation and growth gradually changes into an oxide layer. The
inclusion of adsorbtion is necessary to explain the observed property of passive film, and by and
large is an overlap between the oxide layer and adsorbtion theory. The evidence for the change
over from adsorbed layer to oxide layer was first pointed out by Flade on the observation that
passive film on iron becomes more resistance to disruption when kept for longer duration in the
passive state [17]. The observation was subsequently confirmed by others and can be explained on
the basis of oxide film theory by formation of a thicker oxide film with time.
2. 8 Passivity of austenitic stainless steel
The passivation property of austenitic stainless steels primarily dependent upon chromium
(Cr) although iron (Fe) is the major alloying element. In addition, the contribution of austenite
stabilizers such as Ni, N and Mn also strengthen the passive film property. Apart from this,
molybdenum also significantly contributes to the passivity in the corrosive environments
containing aggressive ions. However, it is found that passive film formed on austenitic stainless
steel does not contain all the alloying elements those are added to stabilize the austenite phase
[18].
The thickness, composition, and structure of passive film in austenitic stainless steel are
highly dependent on the chromium content of the alloy as well as their formation potential. In
austenitic stainless steels passivation occurs by the rapid adsorbtion of hydrated complex largely