In view of the electrochemical nature of corrosion, it is not surprising that measurements of the electrical properties of the metal solution interface are so extensively used across the whole spectrum of corrosion science and engineering, from fundamental studies to monitoring and control in service. Electrochemical testing methods involve the determination of specific interface properties that can be divided into three broad categories:
Potential difference across the interface: the potential at a corroding interface arises from the mutual polarization of the anodic and cathodic half-reactions constituting the overall corrosion reaction. Potential is intrinsically the most readily observable parameter and, with proper modeling of its value in relation to the thermodynamics of a system, can provide the most useful information on the state of a system.
Reaction rate as current density: partial anodic and cathodic current densities cannot be measured directly unless they are purposefully separated into a bimetallic couple. By polarizing a metal immersed in a solution, it is possible to estimate a net current for the anodic polarization and for the cathodic polarization from which a corrosion current density can be deduced.
Surface impedance: a corroding interface can also be modeled for all its impedance characteristics, therefore revealing subtle mechanisms not visible by other means. Electrochemical impedance spectroscopy (EIS) is now well established as a powerful technique for investigating corrosion processes and other electrochemical systems.
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