Dear Dr. Sunil Sinhmar ,

in simple terms, corrosion potential is the characteristic or property of metal and nonmetal surfaces to lose electrons in the presence of an electrolyte. During the process of corrosion, two electrodes are formed spontaneously, a cathode and an anode.

For more details, please see the source:

Corrosion potential by Corrosionpedia (2019)

Available at: https://www.corrosionpedia.com/definition/335/corrosion-potential

The Open Circuit Potential (OCP) or Open Circuit Voltage (OCV) is the potential between two points with no current flowing between the two points. Without an external potential source,this is usually the highest possible potential difference between these two points. If a current flows between these two points, the potential difference is reduced.The OCP is often used to check if a system behaves as expected. In corrosion research the OCP of a sample is used to monitor the development ofcorrosionon the sample surface.In general OCP measurements require high impedance measurement circuitry to avoid errors in the OCP due to leakage currents.

For more details, please see the source:

Open Circuit Potential by PalmSens

Available at: https://www.palmsenscorrosion.com/knowledgebase/corrosion-potential/

Best regards, Pierluigi Traverso

Dear Pierluigi Traverso

Thank you for nice explanation. But I have one more doubt. It is given in literature that 'the corrosion potential is the potential of a corroding surface in an electrolyte relative to a reference electrode measured under open-circuit conditions' (as given in ASTM G15). Does the corrosion potential also measured under open circuit condition as the OCP?

With Regards, Sunil

Dear Dr. Sunil Sinhmar,

is the OCP the same as the corrosion potential? Yes, it is, despite the definition sounds slightly different: The corrosion potential is the potential a sample has, when no external current is applied. A current can still flow to enable two reactions, one oxidation and one reduction, at the sample. For example a steel sample in contact with an acid solution will oxidize (depending on the steel composition) and protons will be reduced at the surface. If you connect a voltmeter and a reference electrode, the measured OCP is the corrosion potential of the steel-proton reaction.

Observing the OCP or corrosion potential allows you to monitor corrosion processes as well. High corrosion potentials are good. The system will rather take up electrons than loose electrons, so a reduction is more likely. If the corrosion potential is dropping, it means that the sample is oxidizing and negative charges are accumulating in it.

For more details, please see the source:

OCP versus Corrosion Potential by PalmSens

Available at: https://www.palmsenscorrosion.com/knowledgebase/corrosion-potential/

Best regards, Pierluigi Traverso

Dear Sunil Sinhmar,

Simplest and easiest definition for non chemistry background people is that the corrosion potential is a voltage pertaining to the electrode reactions occurring in the two boundary layers. While one electrode reaction is dominant in the boundary layer of the reference electrode, more electrode reactions may be proceeding in the boundary layer of the corroding metal.

Another way of definition for Corrosion potential is that this is a mixed potential (also an open-circuit potential or rest potential- The open circuit (OCP) potential is the potential of the working electrode (metal specimen) relative to the reference electrode when no potential or current is being applied to the corrosion system. The metallic materials with nobler OCP are more thermodynamically stable than materials with less noble OCP) at which the rate of anodic dissolution of the electrode equals the rate of cathodic reactions and there is no net current flowing in or out of the electrode.

Meaning of corrosion potential

This is the characteristic or property of metal and nonmetal surfaces to lose electrons in the presence of an electrolyte. During the process of corrosion, two electrodes are formed spontaneously, a cathode and an anode. High corrosion potentials are good. The system will rather take up electrons than loose electrons, so a reduction is more likely. If the corrosion potential is dropping, it means that the sample is oxidizing and negative charges are accumulating in it. Free corrosion potential is the absence of a net electrical current that flows to and from a metal's surface. The free corrosion potential is measured through the voltage difference between the immersed metal and the appropriate reference electrode in a given environment. The anodic or cathodic Tafel plots are described by the Tafel equation: η = overvoltage, the difference between the potential of the specimen and the corrosion potential. This equation has the form y = mx + b, so a plot of η vs. log i is a straight line with slope β. The Tafel slope simply tells you how much you have to increase the overpotential(Overpotential is typically calculated as applied potential minus standard potential. In a reaction if involving only one step with one electron transfer, the Tafel slope will be determined by the symmetry factor, ploting the log value as x axis and potential as y axis.

Ashish