In this chapter the Open Circuit Potential (OCP) and the Corrosion Potential are introduced. The principles of reference electrodes as well as the 0 V by convention potential of the standard hydrogen electrode (SHE) are explained.
The electrochemical potential can be measured. Technically it is only possible to measure the difference in potential between two points. If two metals are immersed in an electrolyte (conducting solution), a voltmeter connected to both of them will show their potential difference.
If the two metals are connected to each other with a cable, for example, electrons flow from the metal with the lower potential to the metal with a higher potential. As a result the potential difference between the two metals will get smaller. Another dynamic equilibrium will be created between the processes leading to the electrochemical potential and the electron flow, which leads to a new potential difference depending on the current flowing. The higher the current flow is the lower is the potential difference.
Open Circuit Potential
Due to this reason usually the open circuit potential (OCP) or open circuit voltage (OCV) is especially interesting. It is the potential where no current is flowing, because the circuit is open. Between two metals in the same solution the OCP is the highest potential difference possible without applying a potential from the outside. Measuring the OCP is also non-invasive, because no current is flowing.
For most applications the OCP and other potentials will be measured versus a reference electrode. Due to the fact that potentials are additive, one can easily calculate what the potentials would be versus other reference electrodes. There is no absolute potential, but electrochemists have agreed that the standard potential of the standard hydrogen electrode (SHE) is by convention 0 V. This makes it easy to identify noble metals.
All metals with a positive standard potential versus SHE are noble metals. They are resistant to acid corrosion, because to corrode the protons of the acid need to be reduced to hydrogen and for that to happens the potential of the metal would need to be lower (more cathodic) than the potential of the proton reduction.
OCP versus Corrosion Potential
Is the OCP the same as the corrosion potential? Yes, it is, despite the definition sounding 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 lose 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.