Module Five of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Activation Polarization

The term activation polarization makes reference to retarding factors that are an inherent part of the kinetics of all electrochemical reactions. For example, consider the evolution of hydrogen gas illustrated previously for zinc in air-free hydrochloric acid: (reference)

While this reaction seems to be relatively simple, the rate at which hydrogen ions are transformed into hydrogen gas is in reality a function of several factors, including the rate of electron transfer from a metal to hydrogen ions. In fact, there is a wide variability in this transfer rate of electrons by various metals and, as a result, the rate of hydrogen evolution from different metal surfaces can vary greatly.

The exchange current density (i₀) is surely the single most important variable that explains the large differences in the rate of hydrogen on metallic surfaces. Table 5.1 contains the approximate exchange current density for the reduction of hydrogen ions on a range of materials.

Approximate exchange current density (i₀) for the hydrogen oxidation reaction on different metals at 25^oC

Note that the value for the exchange current density of hydrogen evolution on platinum is approximately 10^-2 A/cm² whereas on mercury and lead it is 10^-13 A/cm², eleven orders of magnitude for the rate of this particular reaction, or one hundred billion times easier on platinum than on mercury or lead!

This is the reason why mercury is often added to power cells such as the popular alkaline primary cells to stifle the thermodynamically favored production of gaseous hydrogen and prevent unpleasant incidents. This is also why lead acid batteries (car batteries) can provide power in a highly acidic environment in a relatively safe manner.

However, there is no simple method to estimate the exchange current density for a specific system. The exchange current density must be determined experimentally by scanning the potential with a laboratory set-up such as shown in Figure 5.1. In this experimental arrangement a potenstiostat/galvanostat power controller is used to pass current through the sample, or working electrode (W), and an auxiliary electrode (AUX) immersed in solution while monitoring the potential of the working electrode with a reference electrode and a Luggin capillary.

Electrochemical instrumentation to carry out potentiodynamic measurements in which a potenstiostat/galvanostat power controller is used to pass current through the sample, or working electrode (W), and an auxiliary electrode (AUX) while monitoring the potential of the working electrode with a reference electrode

The following theory explains the basic mathematics that may then be used to extract exchange current density from the results obtained. A general representation of the polarization of an electrode supporting one specific reaction is given in the Butler-Volmer equation:

where:

i_reaction is the anodic or cathodic current;

b is the charge transfer barrier (symmetry coefficient) for the anodic or cathodic reaction, usually close to 0.5

n is the number of participating electrons

R is the gas constant, i.e. 8.314 J mol^-1 K^-1

T is the absolute temperature (K)

F corresponds to 96,485 C/(mole of electrons)

The exchange current density is a fundamental characteristic that can be defined as the rate of oxidation or reduction of the electrode at equilibrium expressed in terms of current. The presence of two polarization branches in a single reaction expressed in equation is illustrated in the following Figure for the polarization of a palladium electrode immersed in a solution containing similar concentrations of ferric (Fe³⁺) and ferrous (Fe²⁺) ions with a completely reversible reaction :

Current vs. overpotential polarization plot of the ferric/ferrous ion reaction on palladium showing both the anodic and cathodic branches of the resultant current behavior

When h_reaction is cathodic, i.e. negative, the second term in the Butler-Volmer equation becomes negligible and the cathodic current density (i_c) can be expressed by a simpler equation and its logarithm:

where b_c is the cathodic Tafel coefficient described in equation that can be obtained from the slope of a plot of h against log |i|, with the intercept yielding a value for i₀ as shown in the following Figure.

E of a plot of h against log |i| or Tafel plot showing the exchange current density can be obtained with the intercept

Similarly, when h_reaction is anodic, i.e. positive, the first term in the Butler-Volmer equation becomes negligible and the anodic current density (i_a) can be expressed by equation and its logarithm, with b_a obtained by plotting h_reaction vs. log |i|: