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David C. Silverman |
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Electrochemical impedance spectroscopy (EIS) (sometimes also called AC impedance) is an electrochemical technique in which a low amplitude alternating potential (or current) wave is imposed on top of a DC potential (often the corrosion potential and zero imposed current). As such, this technique is in essence built on DC techniques as shown in this figure EIS adds another level to the analysis. A low amplitude repeating wave of varying frequency is imposed on top of the DC input signal. Usually the wave is a sinusoidal voltage wave but other waveforms have been used. Assuming a sinusoidal input, the resulting output signal is of the same frequency but is shifted in time is shown in this figure  (1)
 (2)
 (3)
 (4)
where V and I are the total voltages and the subscripts 'inphase' and 'outofphase' refer to the real or in phase and imaginary or out of phase contributions to the total signal. The relationship can be seen in the following figure The voltage and current can be pictured as similar rotating vectors in the following figure The voltage vector is divided by the current vector to yield the impedance voltage divided by current) as shown in this figure  (5)
 (6)
 (7)
 (8)
 (9)
According to ASTM G-15, the definition of "Electrochemical Impedance" is the frequency dependent, complex valued proportionality factor, ΔE/ΔI, between the applied potential (or current) and the response current (or) potential in an electrochemical cell. This factor becomes the impedance when the perturbation and response are related linearly (the factor value is independent of the perturbation magnitude) and the response is caused only by the perturbation. The value may be related to the corrosion rate when the measurement is made at the corrosion potential. The goal of the EIS technique is to measure the impedance as a function of frequency by proper procedures and then analyze the resulting spectrum to estimate corrosion rates and mechanisms the might give rise to the spectra. Further reading is available in D.C.Silverman, "Primer on the AC Impedance Technique", Electrochemical Techniques for Corrosion Engineering (R. Baboian, ed.), p. 73, NACE, 1987 1  (370k))
,J. R. Macdonald, 'Impedance Spectroscopy', John Wiley & Sons, NY, 1987
and D. C. Silverman and J. E. Carrico, "Electrochemical Impedance Technique - A Practical Tool For Corrosion Prediction", Corrosion, Vol. 44, No. 5, p. 280 (1988) 1 (517k).
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David C. Silverman, Ph.D. - Primary Consultant |
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