Tafel Plot and Evans Diagram
To understand the foundation of corrosion current measurements the Tafel plot and the Evan’s diagram are explained. The connection between a polarization curve and the Evan’s diagram is explained and how to extract the corrosion current from a polarization curve.
As usual, it would be great if we can predict the corrosion current or corrosion potential. Julius Tafel studied the Hydrogen Evolution Reaction (HER) in the early 1900s. HER is a common reaction in corrosion because all water contains protons. He found that there is an exponential relationship between the applied current at a platinum surface and the potential.
This is also true the other way around (applied potential and measured current). A convenient way of plotting this relationship was to plot the potential versus the logarithm of the current, lg I, because using the logarithm leads to a linear plot.
In Figure 4.1 the slope of the line is called the Tafel slope. It is usually expressed in the units mV/decade. This approach is the ideal case. For many reasons real reactions often deviate from this behavior. Very common reasons are passivation and diffusion limitation. The influence of passivation will be discussed later (see chapter Features of Polarization Curves).
Oxygen reduction reaction
Diffusion limitation leads to a potential independent current. The amount of converted species, for example in the oxygen reduction reaction (ORR) the oxygen, is depleted within reach of the electrode. The reaction can only continue, and thus a current can only occur, if new oxygen diffuses towards the electrode. The current no longer depends on the potential, but the transport of oxygen in the solution. So the Tafel plot will no longer be linear (s. Figure 4.2).
Combining reduction and oxidation
Up to now we have only looked at the reduction or the oxidation, but we need to combine a reduction and an oxidation for corrosion to occur. This is also the situation in real environments.
If the Tafel plot of both side reactions is known, one can use the two Tafel plots to find the theoretical corrosion current and corrosion potential. This is possible due to two facts:
- An immersed conducting sample has one potential at any instant and thus all reactions must happen at that potential.
- The conversion of charge demands that all electrons donated need to be accepted, i.e. the reactions have to happen at the same rate which implies the same current.
From these two conditions it can be derived that the corrosion current and the corrosion potential are determined by the point where the two Tafel plots of the reduction reaction and oxidation reaction meet. Plotting the two Tafel plots (or more) into one plot is an Evans diagram (see Figure 4.3). It is helpful to estimate what influence a change in the oxidation or reduction rate has on the corrosion rate. Also the potential and corrosion current of a galvanic couple can be predicted.
Unfortunately, the Evan’s diagram is most of the times only used for qualitative estimations. The numbers of influences and missing quantitative data usually makes it necessary to evaluate the system with an experiment. Usually this is done with a polarization curve. To record such a curve a linear potential sweep is applied to the samples and the current is recorded.
The recorded current is the difference between the current of the oxidation and the reduction. This means that the measured current at the corrosion potential is 0. Since the plot is made in a logarithmic scale a 0 would correspond to a minus infinite (‑∞), which a potentiostat can’t measure. A scheme of a polarization curve is shown in Figure 4.4.
The goal of recording a polarization curve is usually to extract the corrosion potential as well as the corrosion current, but as in the previous paragraph discussed the point of interest, the intersection of the two Tafel plots, is not directly visible in the polarization curve.
Further away from the corrosion potential the polarization curve is mainly influenced by only one of the reactions. At very cathodic potentials the reduction dominates and at very anodic potentials the oxidation. Due to this the linear parts of the polarization curves can be used for extrapolation of the Tafel slopes and thus the corrosion potentials as well as corrosion current.
For a reliable extrapolation the linear behavior over a few decades is ideal and at least for one decade necessary. The more decades show the linear behavior the better the extrapolation. According to the theories we have looked at up to now, the curves should stay linear in the Tafel plot when the potential difference to Ecorr is increased.
Unfortunately, there are limitations that will lead to deviation from this behavior. We have already seen an example in Figure 4.2, where some reaction partners are limited by diffusion. Other examples can be the onset of another reaction or passivation of the surface. In the section about polarization curve processing alternatives to the extrapolation via Tafel slope fitting will be presented (see chapter Processing Polarization Curves).