Overvoltage passage

Other contributions to the polarization are activation polarizations (nact) caused by inhibition of the passage of ions through the phase boundary which may arise in the discharge mechanism. Films on the electrode may also contribute (e.g. oxide, metal already deposited, impurity) by offering a resistance to current flow differing from the bath resistance (ohmic polarization, Tlohm)- Hence the observed overvoltage is given by... 

If the potential of an electrode deviates from the reversible or equilibrium value, a current flows in either the anodic or cathodic direction. The deviation of the potential from its equilibrium value is the anodic or cathodic overpotential of the electrode. The terms emf and voltage are used here to refer to a cell, whereas the term potential refers to a single electrode (Section 12-1). Overvoltage represents the additional voltage above the reversible cell emf required to permit the passage of a finite current, and overpotential refers to the deviation of the potential of a single electrode from its reversible value. In both cases the ohmic voltage drop iR is first subtracted, as seen below. 

Polarization is the departure of the electrode potential from its theoretical Nernst equation value on the passage of current. Overvoltage is the potential difference between the theoretical cell potential from Equation 22-2 and the actual cell potential at a given level of current. 

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