Electrochemical Techniques for Corrosion Quantification

Corrosion is the deterioration of a solid body upon interaction with atoms or molecules that originate outside the solid body. Corrosion is so widespread because most metallic materials of practical significance are chemically metastable. Therefore, nature provides the inherent tendency and driving force for the material to revert to its most stable thermodynamic state, which could be, e.g., an oxide, a hydroxide, a sulfide, a carbonate, or a sulfate. Thus the corrosion scientist and engineer must constantly battle with nature to preserve the metastability of the metallic material for long duration. Herein lies their major challenge. The direct economic impact of corrosion has been variously estimated. While one may take issue with specific figures, it is clear that the costs are substantial. In addition to these direct costs, indirect costs are also associated with corrosion, resulting from factors such as loss of production or the need for alternative procedures during the replacement of a corroded component or structure, gradual decrease in process efficiency resulting from corrosion product buildup and its impact on process flow and heat transfer, the need to overdesign certain components because of uncertainty due to corrosion, and loss of property and lives. Accurate measurement of corrosion is essential to predict the life of critical metallic components in service. Corrosion measurement also becomes very important in evaluating the effectiveness of corrosion control strategies. The techniques used to measure corrosion vary depending upon whether one is dealing with gas phase corrosion or liquid phase corrosion. Electrochemical techniques, which are the subject of the article, are applicable specifically to corrosion occurring in liquid media. Corrosion can be either general is localized. Much of the discussion in this article will focus on the phenomenon of general atom loss from metallic surfaces. When a metal corrodes in a liquid, the corrosion can involve a direct dissolution of the metal into the liquid, the conversion of the metal into an inorganic corrosion product layer on the surface, or a combination of both reaction layer formation and dissolution. The most widely spread corrosion processes in liquids occur in aqueous environments. To a more limited extent, corrosion is an issue in molten salt environments, e.g., in molten carbonate fuel cells or in the molten salt corrosion of turbine blades. Corrosion in liquid hydrocarbons is of interest to the petrochemical industry; however, in view of the nonconducting nature of these liquids, the applicability of electrochemical techniques to such systems is questionable. Three electrochemical techniques are described for the quantification of corrosion processes in liquid systems. The first two are direct current (dc) techniques, the first of which requires the use of relatively high applied potentials, while the second one uses applied voltages in the vicinity of the corrosion potential. The third technique, which uses alternating current (ac) voltages and currents, has the advantage of independently measuring different frequency-dependent processes in corrosion. Keywords: corrosion; quantification; electrochemical techniques; Tafel technique; principles; practical aspects; problems; linear polariation; electrochemical impedance spectroscopy