**AC Impedance**

The Gill AC; Gill 8; Gill 12 and the Field Machine are
all used for this test. A
typical experiment sweeps from 10 kHz to 0.01 Hz with a 10 mV perturbation around the rest
potential. The usual result is an nyquist impedance plot of half a semi-circle: the high
frequency part giving the solution resistance and the width of the semi-circle giving the
corrosion rate
in the same manner as LPR. The analysis of this data is performed by circle
fitting in the analysis software. One useful benefit of AC is the ability to measure the
solution resistance at high frequency. This allows any instrument that incorporates AC to
perform automatic IR compensation during DC tests.

At each frequency a sine wave is generated and fed into the potentiostat. This wave is
then imposed on the cell and its potential and current flow measured. The measured
values of current and voltage are compared for amplitude and phase and an impedance
calculated. This is repeated for the rest of the frequencies and a plot generated. The
standard starting point with AC impedance is the basic Randles circuit below.

An alternate name for AC impedance is Electrochemical Impedance
Spectroscopy (EIS).