Another electrode, denoted the counter electrode, is used to apply a known voltage and current, via the electrolyte, to the working electrode. The counter electrode typically is made of platinum. The third electrode, denoted the reference electrode, is used to measure the opencircuit potential between the working electrode and the counter electrode. This typically is a standard saturated calomel electrode. The three electrodes are connected via an interface circuit and a frequency- response analyzer to a three-lead potentiostat, which is a commercially available item. The potentiostat communicates with a personal computer or similar apparatus, which controls the potentiostat and analyzes the EIS current and voltage measurement data.

In operation, the potentiostat generates a small known AC potential (typically having an amplitude of about 5 mV) that is gradually swept over a wide frequency range — typically from about 10-3 to 105 Hz. This potential is applied via the counter electrode, giving rise to potential and current waveforms that propagate through the electrolyte to the working and reference electrodes. The phase angle (relative to the phase angle of the potential on the counter electrode) of the current through the counter electrode, the amplitude of this current, and the phase and amplitude of the potential sensed by the reference electrode are measured and recorded for each increment of frequency. The data are used to determine the magnitudes and phase angles of the impedances of the electrolyte and the specimen as functions of frequency.

In subsequent analysis, one seeks correlations between the impedance-versusfrequency data from the measurements and the corresponding data computed for theoretical model resistor-and-capacitor circuits. Once a close match is found between the measurement and the impedance-versus-frequency data of one of the models, the model resistance and capacitance values are taken to be indicative of corrosion mechanisms and corrosion rates. For example, under some circumstances, a capacitance could be attributed to a protective coat; a decrease in that capacitance over time could be attributed to permeation of the coat by electrolyte and, thus, could be regarded as indicative of the onset of corrosion.

This work was done by Michelle Skoorka Burgess of the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. NRL-0013

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