CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 4
encountered by all materials used for engineering applications. Although, many metals and alloys
can naturally form air formed passive layer, it breaks down depending upon the process condition
of the environment they are used. Thus, better understanding on the morphology, composition and
“role of passivity” on the corrosion resistance of engineering materials is important for application
in critical circumstances.
Passivity results from the formation of natural, ultra thin, tenacious, and self-healing oxide
layer on the surface which acts as a barrier layer from the corrosive environment [3]. The
protectiveness of the passive film is mostly due to insoluble nature of oxide layer, and is
dependent on the corrosive nature of the electrochemical environment. If damaged, the passive
film normally reforms rapidly. However, a change in the character of the environment may cause
the passive material to revert to an active state. Subsequent damage to the pre-existing passive film
can result in substantial increase in corrosion rate due to accelerated degradation of the passive
film. As passive film constitutes a protective surface layer against localized corrosion, its chemical
and structural properties play major role on the rate and extent of transient removal from the
surface. The characterization of structure and composition of the passive film is thus vital for
understanding their nature of interaction with corrosive environments. A large number of
techniques are available to elucidate the properties of passive film, such as (a) polarization study
to find out the potentials and current densities during passive film growth and dissolution, (b) AC
impedance technique for analyzing capacitance, resistance and space charge within the passive
film (c) electrochemical quartz crystal micro balance for studying the extent of oxide layer
dissolution, (d) spectroscopic measurement to elucidate the structural information, (e) chemical
study to locally resolve the chemical composition, and (f) topographical study to understand the
morphological features etc.