6. Evans Diagrams

This chapter deals with the potential-current density diagrams, also called Evans diagrams, which relate the variation of potential of a reaction—either anodic or cathodic—with the current density exchanged in the process, starting from its equilibrium potential in the corresponding environmental conditions. These diagrams allow to identify the corrosion working conditions, E_corr and i_corr, where the anodic and cathodic processes proceed with the same rate. The cases of active and active–passive metals are described. Ohmic drop can also be represented on the diagram, modifying corrosion rate. Both corrosion conditions and the imposition of a polarization are discussed with reference to the modification of the electrode working condition that they cause. Finally, experimental polarisation curves are introduced.