The Tafel Plot
Using the Tafel equation, useful plots can be drawn to help find corrosion rates.
In a plot of i vs. E, for a single electrode, the following is seen:
Considering the sum of the currents and then ignoring the signs and then taking log of current gives a plot known as a Tafel plot, which is described in the animation below for a single electrode:
As can be seen, at Ee, the net current flow is 0, as must be the case for equilibrium (the anodic and cathodic currents are equal and opposite). The straight-line sections have gradients related to the Tafel slopes – anodic, ba and cathodic bc. (If we had plotted E on the vertical axis and log i horizontally the gradients would be equal to ba and bc. )
There are several important points to note:
- ia and ic never reach zero individually. However, the resultant net current flow will be zero if anodic and cathodic currents are equal in magnitude.
- This derivation applies both to dissolution (corrosion) of metals and deposition (electroplating) of metals.
- This derivation also applies to hydrogen evolution and oxygen reduction, even though they don’t involve metal ions – they can still activation controlled.
- The signs may be dropped since they serve only to define direction of current flow. Had current been defined the opposite way around, all signs would be reversed, so dropping signs (to allow logs to be taken) is not unreasonable.
- α is usually 0.5 for a single step reaction.
- Multiple step reactions can have different steps with different stoichiometric numbers of electrons (different z). In this case, the value of z for the rate determining step should be used, not the overall stoichiometric number.
- However, the overall stoichiometric value of z is used to relate current density to rate constant and in the Nernst Equation. If these are different, it is standard to rename the value for the RDS (the value inside the exponential) as n.
N.B. There is no universally adopted standard to plot log (i) on the y-axis and E on the x-axis. In fact, it is more common to see polarisation curves plotted as E vs. log (i). In this TLP graphs will be plotted as log (i) vs. E.
Tafel plots can be linked to Pourbaix diagrams:
Corrosion occurs when two electrodes with different equilibrium potentials are in both electronic and electrolytic contact. We can use Tafel plots to predict corrosion rates as explained in the animation below.