The tool used to measure a polarization current (1 amp = 1 coulomb per second) is called a potentiostat. A potentiostat is an essential device used in corrosion studies to investigate electrochemical parameters such as reduction, oxidation and other chemical characteristics. This tool allows an operator to alter the potential of a metal specimen in a controlled manner and measure the current that flows through the electrochemical cell as a function of the applied potential. (To learn about other tools, read Corrosion Assessment: 8 Corrosion Tests That Help Engineers Mitigate Corrosion.)
The potentiostat is so named because it polarizes the metal using a potentiostatic method. That is, polarization occurs under potential-controlled conditions while the current is measured. This device works by controlling three electrodes during electroanalytical experiments: a working electrode, a counter electrode and a reference electrode. A potential is maintained at the working electrode with respect to the reference electrode by adjusting the current at the counter electrode.
These three electrodes are described in detail below:
- The working electrode is a sample of the corroding metal being studied. Typically, it is not the actual metallic structure; rather, it is a small specimen with similar properties. At the working electrode, the potential is controlled and the current is measured during polarization. This electrode can be either bare or coated metal.
- The reference electrode’s purpose is to act as a reference for measuring and controlling the working electrode potential with no current flowing through it. Ideally, the reference electrode should also have a constant potential at low current densities.
- The counter electrode (also called an auxiliary electrode) is the conductor that completes the electrochemical cell. This is generally an inert conductor such as graphite or platinum and is immersed in the same electrolyte as the working electrode. In field probes, these electrodes can also be another piece of the working electrode material.
Although the electrodes are the most critical elements of the potentiostat, the device itself also consists of an electrometer, an I/E converter, a control amplifier and a signal circuit. All of these elements are responsible for operations such as current and voltage measurements.