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Polarization and Its Effect on Corrosion

Polarization is the change of potential from a stabilized state, e.g. from the open-circuit electrode potential as the result of the passage of current. It also refers to the change in the potential of an electrode during electrolysis, such that the potential of an anode becomes more noble, and that of a cathode more active, than their respective reversible potentials. Often accomplished by formation of a film on the electrode surface.

Polarization is sometimes also referred to as "overvoltage" or "overpotential". In some electrochemistry books, there is a differentiation between overvoltage and overpotential. The former refers to the difference between the actual electrode potential at which appreciable electrolysis begins and the reversible electrode potential (the standard redox potential); the latter refers to the potential deviation from the equilibrium potential of a system.

In the context of corrosion, polarization refers to the potential shift away from the open circuit potential (free corroding potential) of a corroding system. If the potential shifts in the "positive" direction (above E_corr ), it is called "anodic polarization". If the potential shifts in the "negative" direction (below E_corr ), it is called "cathodic polarization".

Fo all metals and alloys in any aqueous environment, cathodic polarization always reduce the corrosion rate. Cathodic protection is essentially the application of a cathodic polarization to a corroding system.

For a non-passive system (e.g. steel in seawater), anodic polarization always increases the corrosion rate. For systems showing active-to-passive transition, anodic polarization will increase the corrosion rate initially and then cause a drastic reduction in the corrosion rate. Anodic protection is essentially the application of anodic polarization to a corroding system.

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