CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 3
rate of 10Å/min using 5 keV Ar + ion beam with current of 50 µA/cm 2 at an incidence angle of 45°,
and at a pressure of 1×10 -5 Pa. The pressure in the analyzer chamber was of the order 1×10 -7 Pa
throughout the measurement.
3.1.5 Electrochemical corrosion studies
3.1.5.1 Open circuit potential vs. time measurement
Out of several electrochemical techniques used for the investigation of corrosion, open
circuit potential (OCP) measurement over time of immersion is successfully employed over the
years for understanding the corrosion process by analyzing the natural fluctuation of the corrosion
potential. When a specimen is immersed in corrosive solution both oxidation and reduction
reaction takes place, and in general specimen oxidizes and the solution reduces. Thus, open circuit
potential or free corrosion potential is defined as the potential of a working electrode in steady
state measured with respect to a reference electrode in absence of any perturbation to the working
electrode surface [93]. The magnitude of open circuit potential is dependent on the metal/alloy
itself, composition of the solution, temperature, and hydrodynamics of the electrolyte. OCP-time
measurement provides insight into the instantaneous electrochemical process occurring in the
electrode surface, and is important for determining tendency of materials to free corrosion [94].
Moreover, variation in OCP is an indication of the protective power of the passive film building
on the surface, and its stability from surface electro-chemical phenomenon occurring in a
corrosive environment as a function of time. The change in potential in the anodic direction is
termed as noble potential, and indicates the presence of stable passive film whereas the shift of
potential in the cathodic direction i.e. active direction indicates the onset of corrosion process.
Apart from this, it provides the idea regarding the transient steps in the passive film formation
from the signals spontaneously generated from the corroding surface. However, the OCP
measurement gives only qualitative information of the corrosion behaviour, and is limited