Evan's diagram

In a corroding system an oxidation and a reduction must take place.

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 4.3 | 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 4.4 | Polarization curve (green)
with Evan’s diagram (blue)