Electrochemical Impedance Spectroscopy.
Electrochemical interfaces are not simply resistive. For a start the electrochemical double layer adds a capacitative term. Other electrode processes, such as diffusion, are also time (and frequency) dependant.
The measurement itself can be done using various instruments and techniques, ranging from a simple oscilloscope display to a Fast Fourier Transform (FFT) analyser. The usual instrument used is a frequency response analyser.
This generates a sine wave at a given frequency and measures the response of a circuit at that frequency. Normally two channels would be used, one to measure the voltage applied to the test cell, the other to record the resulting current (usually converted to a voltage first). The ratio of the two is the impedance of the test electrode. The measurement is then repeated at another frequency. Pseudorandom noise perturbation and FFT can be used to get data simultaneously at a number of frequencies. This does not always work :(
The response of real corroding electrodes is rarely simple. Additional contributions from processes such as diffusion or adsorption add extra circuit elements, giving further semicircles and lines on the plot.
Inhomogeneities (lumpy bits) of the electrode surface tend to distort the semicircles further. Powerful numerical analysis programs are available to deconvolute the measured response to its components. Even so, the analysis of impedance data can be something of a black art and the literature abounds with arcane equivalent circuits and explanations.
For more detailed notes have a look at Gamry's site