Evan's Diagram

The Evan’s diagram shows the relationship of current and potential for the oxidation and the reduction reaction. These are usually plotted as potential versus the logarithm of the current (E-lg I) curves.

Figure 1 | Evan’s Diagram

If these two reactions are responsible for the corrosion and no currents flow from or into the systems, all the electrons released by the oxidation must be accepted by the reduction. So the two reactions can only corrode at the potential where both reactions currents are the same. This means the curves of the two reactions should intersect in the Evan’s Diagram.

This means, if the intersection in the Evan’s Diagram of the two reactions is know, the corrosion potential and corrosion current are known.

The popular Tafel Analysis is based on these theories.

Figure 2 | Polarization curve (green)
with Evan’s diagram (blue)

Related articles

Polarization Curves: setup, recording, processing and features

In this extensive section Polarization Curves are discussed. How to setup your equipment, the choice of parameters as well as the data processing is discussed. This will enable you to record a polarization curve and extract the corrosion rate from it by using PSTrace 5. Furthermore, the polarization curves and Evan’s diagrams for passivation films (thick and thin) are discussed. This section closes with a brief description of crevice and pitting corrosion.