# What is an Overpotential?

When two or more complementary processes such as those illustrated for zinc in air-free hydrochloric acid, for magnesium in neutral water, or for zinc in aerated hydrochloric acid occur over a single metallic surface, the corrosion potential that results from this situation is a compromise between the various equilibrium potentials of all the anodic and cathodic reactions involved. The difference between the resultant potential (E) and each individual reaction equilibrium potential (Eeq) is called polarization and quantified in terms of an overpotential (h) as in equation:

The polarization is said to be either anodic, when the anodic processes on the electrode are accelerated by changing the specimen potential in the positive (noble) direction or cathodic when the cathodic processes are accelerated by moving the potential in the negative (active) direction. There are three distinct types of polarization and these are additive, as expressed in equation:

where:

hact is the activation overpotential, a complex function describing the charge transfer kinetics of an electrochemical reaction. hact is always present and mostly dominant at small polarization currents or voltages;

hconc is the concentration overpotential, a function describing the mass transport limitations associated with electrochemical processes. hconc is predominant at larger polarization currents or voltages;

iR is the ohmic drop. This function takes into account the electrolytic resistivity of an environment when the anodic and cathodic elements of a corrosion reaction are separated by this environment while still electrically coupled.

Example problem 5.1

What is the relation between the overpotential and standard potential of an electrochemical reaction?

Example problem 5.2

What is the relation between polarization and overpotential?

Activation polarization is usually the controlling factor during corrosion in strong acids. Concentration polarization usually predominates when the concentration of the active species is low; for example, in dilute acids or in aerated waters where the active component, dissolved oxygen, is only present at very low levels. The ohmic drop will become an extremely important factor when studying corrosion phenomena for which there is a clear separation of the anodic and cathodic corrosion sites, e.g. crevice corrosion. The ohmic drop is also an important variable in the application of protective methods such as anodic and cathodic protection that force a potential shift of the protected structure by passing a current in the environment.

Knowing the kind of polarization which is occurring can be very helpful, since it allows an assessment of the determining characteristics of a corroding system. For example, if corrosion is controlled by concentration polarization, then any change that increases the diffusion rate of the active species (e.g., oxygen) will also increase the corrosion rate. In such a system, it would therefore be expected that agitating the liquid or stirring it would tend to increase the corrosion rate of the metal. However, if a corrosion reaction is activation controlled then stirring or agitation will have no effect on the corrosion rate.

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