Introduction to corrosion science-E.McCafferty, P127
Overpotential
$\eta = E-E_o$
where E is the electrode potential for some condition of current flow and $E_o$ is the electrode potential for zero current flow (also called the open-circuit potential, corrosion potential). Note that the electrode potential of zero current flow Eo should not be confused with the standard electrode potential $E^o$, which plays a prominent role in corrosion thermodynamics.