Electrochemical processes Tafel region

The major error with Tafel extrapolation is the effect of concentration polarization introduced when the applied current is large. The theoretical basis of this method is only valid for the corrosion systems with an electrochemical control. If the applied current is significant, some corrosion systems may come under mass transport control, or at least under mixed electrochemical and mass transport control. This will make it very difficult to define the Tafel region from the polarization curve, introducing errors in the extrapolation process. Tafel extrapolation is not applicable in highly resistive environments for example, glacial acetic acid. 

Because the metal dissolution is an anodic process, for example, Fe(s) Fe +(aq) + 2e , the current of the process is assumed to be positive. When potential increases from Mez+zMe lo f (passivation or Flade potential), the current is increasing exponentially due to the electron transfer reaction, for example, Fe(s) -> Fe +(aq) + 2e", and can be described using Tafel s equation. At a E the formation of an oxide layer (passive film) starts. When the metal surface is covered by a metal oxide passive film (an insulator or a semiconductor), the resistivity is sharply increasing, and the current density drops down to the rest current density, 7r. This low current corresponds to a slow growth of the oxide layer, and possible dissolution of the metal oxide into solution. In the region of transpassivation, another electrochemical reaction can take place, for example, H20(l) (l/2)02(g) + 2H+(aq) + 2e, or the passive film can be broken down due to a chemical interaction with environment and mechanical instability. Clearly, a three-electrode cell and a potentiostat should be used to obtain the current density-potential curve shown in Figure 9.3. 

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