2. Understanding Corrosion

2.7 Activation Polarization [1/2]

When a certain step in a half cell reaction controls the rate of electron flow, the reaction is said to be under activation charge transfer control and activation polarization results. For example, consider the reduction of Hydrogen ions

2H+ + e H2

Hydrogen evolution occurs in 4 major steps:

  1. H+ is absorbed in the surface at the electrode

  2. H+ + e Had  the species is reduced on the surface

  3. The two reduced species combine to form a Hydrogen molecule

  4. Hydrogen bubbles are formed by combination of hydrogen molecule

The rate of hydrogen reduction is determined by the slowest of the steps. The rate controlling step varies with the metals, current density and environment. There is a critical activation energy needed to surmount the energy barrier associated with the slowest step. The rate of transformation is controlled by the magnitude of energy barrier that an atom or ion must surmount to transform from metal to ion. The energy that must be acquired is the activation energy, DG*.

Pronounced activation polarization also occurs with the discharge of OH- at an anode accompanied by oxygen for hydrogen evolution. The corresponding polarization term is hydrogen over-voltage. Similarly, activation polarization also occurs with discharge of OH- ions at an anode accompanied by oxygen evolution. The corresponding polarization term is oxygen over-voltage.

The relationship between activation polarization and the rate of reaction is given by

ha = ba Log (ia/io)

hc = bc Log (ic/io)

where i = current density

io = exchange current density

A plot of over-polarization (hact) versus Log(i) is linear for both anodic and cathodic polarization. ba and bc are called the Tafel slopes. The activation polarization drops for hydrogen reaction is shown in the figure below.