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Evans Diagram

Last updated: March 9, 2017

What Does Evans Diagram Mean?

An Evans diagram is a pictorial that depict the graphical representation between an electrode’s kinetic data (or current densities) and thermodynamics (or potential) in a corrosive process. An Evans diagram is also known as a mixed potential plot, where the mixed potential is the effective potential of a metallic surface in contact with an electrolyte that is driving electrochemical corrosion. Mixed potential theory explains metal corrosion as a reaction of two or more electrodes working simultaneously at the interface of the metal’s surface and an electrolyte.

An Evans diagram is useful for understanding and determining the corrosion process.

An Evans diagrams may also be known as a polarization diagram, mixed potential diagram or mixed potential plot.


Corrosionpedia Explains Evans Diagram

Two partial reactions occur when a metal object corrodes: oxidation and reduction. A basic premise in any corrosion reaction is that it will only occur when the total rate of oxidation becomes equal to the total rate of reduction. This means that the sum of anodic oxidation currents must equal the sum of cathodic reduction currents.

Evans diagrams and mixed potential plots are based on the mixed potential theory, where the x-axis of the plot is the current density and the y-axis is the potential. They consider a metal surface dipped in an electrolyte as being made up of two or more electrodes. These electrodes cause electrochemical corrosion with oxidation directly coupled to the reduction reaction of the depolarizer. The resulting potential of the electrodes is the mixed potential that drives the corrosion process. Cathodically polarizing the cathode reduces the corrosion current density, ultimately causing the corrosion rate to decrease.



Polarization Diagram

Mixed Potential Diagram

Mixed Potential Plot

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