The working electrode in an EIS apparatus is made of coupled metals.

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.