Kinetics of Coupled Half-Cell Reactions

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

The conventional approach to corrosion is to start directly with the concept of a mixed electrode of indistinguishable distribution of sites for the anodic and cathodic reactions. The approach taken in this chapter is to first examine the behavior of distinguishable anodic and cathodic sites. This is the classical case of galvanic couples of joined dissimilar metals in contact with a common solution. In this case, local movement of a reference electrode through the solution can map the 

Before developing the concept of the mixed electrode in greater detail, it is important to establish an understanding of the relationship between the potential difference across the metal/solution interface and the potential difference within the solution. 

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In Chapter 4, analysis of the kinetics of coupled half-cell reactions shows how the corrosion potential and corrosion current density depend on the positions of the anodic and cathodic polarization curves. The anodic polarization curves are generally represented as showing linear or Tafel behavior, and the cathodic curves are shown with both Tafel and... 

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