Mixed Potential Diagrams

Electrode kinetic data are typically presented in a graphical form called Evans diagrams, polarization diagrams or mixed potential diagrams. These diagrams are useful in describing and explaining many corrosion phenomena. According to the mixed-potential theory underlying these diagrams, any electrochemical reaction can be algebraically divided into separate oxidation and reduction reactions with no net accumulation of electrical charge. In the absence of an externally applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface. Under these circumstances the net measurable current is zero and the corroding metal is charge neutral, with no net accumulation of charge.

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The net measurable current is zero and the corroding metal is charge neutral, with no net accumulation of charge... anodic current = cathodic current

It is also important to realize that most textbooks present corrosion current data as current densities, since this normalized variable is more descriptive of the interfacial properties. However, one must use absolute current values to construct mixed potential diagrams, to properly balance the charges in question.

In order to construct mixed potential diagrams to model a corrosion situation, one must first gather the information concerning the 1) activation overpotential for each process that is potentially involved and 2) any additional information for processes that could be affected by concentration overpotential.