CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 3
corrosion science in recent years due to development of sophisticated electronics which employs
latest digital electronics and computer control overcoming the earlier limitations.
EIS study is carried out in properly designed standard electrochemical cell consisting of
specimen as working electrode, counter electrode, and a reference electrode as shown in Fig. 3.8
[93]. The basic design feature of the electrochemical cell is to measure the impedance at the
working electrode-electrolyte interface only, and to eliminate the impedance contribution from the
counter electrode. This is done by making the resistance of the counter electrode negligible
compared to that of working electrode, and capacitance of the counter electrode very large
compared to that of working electrode [98]. Since, resistance in series add and capacitance in
series add reciprocally, the influence of the counter electrode can be eliminated by making its area
large compared to that of working electrode. Often platinised platinum with large area is used as
counter electrode.
In EIS measurements a perturbing sinusoidal voltage E = E 0 Sin (t) with frequency is
applied to the electrode system under test [95,102,103]. The response is analysed in terms of the
resultant current I = I 0 Sin (t + ), where represents a characteristics phase angle shift. The
frequency response is analyzed by instruments known as Frequency Response Analyzers (FRA)
interfaced with potentiostat. Frequency response analyzer provides scope for studying wide range
of frequencies, faster analysis, and removal of harmonic distortions. The corresponding complex
impedance spectrum Z() obtained by varying the signal frequency is expressed in terms of
displacement vector Z(). In the plane of cartesian coordinates impedance is expressed by its real
and imaginary parts as presented in Eq. 8.
Z() = Z Re + jZ Im. (8)