The Exchange Current

The exchange current, I0, is a fundamental characteristic of electrode behavior that can be defined as the rate of oxidation or reduction at an equilibrium electrode expressed in terms of current. Exchange current, in fact, is a misnomer since there is no net current flow. It is merely a convenient way of representing the rates of oxidation and reduction of a given single electrode at equilibrium, when no loss or gain is experience by the electrode material.

For the specific reaction describing the corrosion of iron this would imply that the exchange current is related to current of each direction of a reversible reaction, i.e. an anodic current (Ia)  and a cathodic branch (Ic).

anodic reaction ... Fe  Fe2+ + 2e-

reversed anodic reaction ...  Fe  Fe2+ + 2e-

Since the net current is zero at equilibrium, it implies that the resultant current of these two reactions is zero.

There is no theoretical way of accurately determining the exchange current for any given system. This must be determined experimentally. For the characterization of electrochemical processes it is always preferable to normalize the value of the current by the surface area of the electrode and use the current density often expressed as a small i, i.e. i = I/surface area. The magnitude of exchange current density is a function the following main variables:

Metal composition: Exchange current density depends upon the composition of the metal or alloy and the solution. For redox reactions, the exchange current density would depend on the composition of the metal supporting an equilibrium reaction.

Surface roughness: The exchange current density is usually expressed in terms of projected or geometric surface area and depends upon the surface roughness. The higher exchange current density for the H+/H2 system equilibrium on platinized platinum (10-2 A cm-2) compared to bright platinum (10-3 A cm-2) is a result of larger specific surface area of the former.

Soluble species concentration: The exchange current is also a complex function of the concentration of both the reactants and products involved in the specific reaction described by the exchange current. This function is particularly dependent on the shape of the charge transfer barrier (a) across the electrochemical interface.

Surface impurities: Impurities adsorbed on the metal surface usually affect its exchange current density. Exchange current density for the H+/H2 system is markedly reduced by the presence of trace impurities like arsenic, sulfur and antimony.