Amethod and apparatus have been conceived for using electrochemical impedance spectroscopy (EIS) for determining rates of corrosion of coupled metals. EIS has been used heretofore for determining rates of corrosion of single metals. However, many structures used in corrosive environments include different metals in electrical contact. Moreover, the concept of using a sacrificial metal to suppress or reduce the corrosion of another metal is applied in some structures. The coupling of two different metals can affect the rates of corrosion of both. Hence, there is a need to extend EIS to coupled metals.

This EIS Apparatus resembles a conventional EIS apparatus in most respects. In this case, the working electrode is a specimen of two coupled metals.
In most respects, the present method and apparatus for EIS testing of coupled metals are essentially the same as the conventional method and apparatus EIS testing of a single metal. The apparatus (see figure) includes a three-electrode electrochemical cell filled with an electrolyte that simulates the corrosive environment to which the metal specimen is exposed in use (e.g. an aqueous solution of NaCl to simulate seawater). One of the electrodes, denoted the working electrode, is the metal specimen to be tested. The portion of the working electrode exposed to the electrolyte can be coated or uncoated, depending on whether or not one intends to examine the anticorrosion effect of a coating material. The working electrode may be either a conventional single-metal specimen or a coupled-metal specimen. A typical coupled-metal specimen consists of a circular cylindrical plug of a first metal embedded tightly, with its end face flush, in a matching cylindrical hole in a plate of a second metal. Preferably, the second-metal plate area exposed to the electrolyte is about three times the area of the exposed end face of the first-metal plug.

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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Measuring Corrosion-Related Properties of Coupled Metals

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This article first appeared in the August, 2007 issue of Defense Tech Briefs Magazine.

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