Once the connections are made, the next step is to decide which electrochemical test to perform. The following are the major tests used for corrosion testing –

1. Potentiodynamic polarization—Tafel and Linear
2. Potentiostatic polarization
3. Electrochemical impedance spectroscopy

Out of these tests, the first two are destructive tests, while the last one is the non-destructive test. The tests are briefly described subsequently.

1.     Potentiodynamic polarization

The term ‘Potentiodynamic’ means a varying potential. In this test, the potential of the WE is imposed from a large negative value w.r.t the OCP to a large positive value w.r.t OCP. The rationale behind this variation is as follows—

When the WE is at a highly negative potential, it works as a cathode and supports cathodic reactions such as oxygen reduction, hydrogen evolution, and others. Here, the CE acts as the anode. The cathodic current density is higher than the anodic current density.

As the potential approaches the OCP, the rates of anodic and cathodic reactions become equal, and the potential is the corrosion potential. At this point, the WE is now anodic, but it is corroding at an equilibrium potential. The CE now supports cathodic reactions, which have the same rate as the WE.

As the potential is increased above OCP, the anodic current density of WE increases, and it starts corroding faster. The CE continues to support cathodic reactions.

The current changes with each step change in the potential. This value is measured and converted into current density (current per unit of area exposed to electrolyte). A plot of potential vs. current density is plotted. This is called the Tafel plot.

If it is a linear polarization, the steps in the test remain exactly as given above. The only difference is that the plot is now potential vs. current density. The typical plots look as shown in Figure 2.

Figure 2: (a) Tafel plot; and (b) Linear polarization

The major quantities derived from these tests are corrosion potential (E_corr), corrosion current density (i_corr), and polarization resistance (R_p). The higher the i_corr, the higher is the corrosion rate. In fact, it is also used to calculate the rate in mpy or mmpy. Further, the lower the R_p, the higher is the corrosion.

Most of the tests given in Table 1 are variations of this polarization.

2.     Potentiostatic polarization

The term ‘potentiostatic’ means the potential is kept constant for a pre-determined length of time. The potential may be OCP, or a value positive or negative w.r.t OCP. The current will vary depending on the reactions occurring at the WE.

If the WE and CE are identical, then this technique measures the ‘electrochemical noise (EN)’. Electrochemical noise indicates the pit formation-destruction cycles in terms of current variation. Figure 3 shows the typical plots for potential and current measurements with time.

The data is usually detrended, i.e., any variation due to external factors, is mathematically removed, to arrive at normalized values. These are further analyzed to obtain the noise resistance, which is the resistance to pitting.

The ASTM standard G199-09(2020) specifies the requirements of EN.

Figure 3: Electrochemical noise: (a) Potential vs. time, with detrended line, and (b) Current vs. time with detrended line

3.     Electrochemical Impedance spectroscopy

Electrochemical impedance spectroscopy uses an alternating current (AC) method of application of potential. The level of potential applied is low (around 10 mV), and it is cycled between +10 to -10 at different frequencies. The resulting current is mathematically resolved into impedance. It is plotted into Bode and Nyquist plots to arrive at the extent of damage of the WE.

The Bode plot shows the variation of the total electrochemical impedance vs. frequency in Hz. This impedance is resolved into real and imaginary components using Euler’s equation, which are plotted in the Nyquist plot.

Figure 4: (a) Bode plot presenting impedance s. frequency, and (b) Nyquist plot presenting imaginary component of impedance vs. real component of impedance

The Bode impedance represents the barrier to corrosion. For coatings, it represents the barrier to the ingress of the electrolyte. The shape may include a slope, a plateau or a combination of both, depending on the condition of the system.

Typically, a slope represents a capacitive behaviour, aka more barrier protection/corrosion resistance. The evolution of a resistive plateau signals a degradation of the barrier properties.

Final thoughts

Electrochemical corrosion testing is crucial in the oil and gas industry to arrive at a more accurate corrosion rate. It considers localized corrosion of the material during testing. Hence, it can be applied to especially those materials which do not show uniform corrosion before failure.

It indicates those materials which are more likely to pit and crack. Thus, electrochemical corrosion testing should be made a more integral part of the testing portfolio of the oil and gas industry, than it is now.

While most of the testing is done by third-party laboratories, field personnel should know its nitty-gritty to be able to recommend the tests and use their results to prevent corrosion in their facilities.

If you would like to revisit the basics, here is the first part of this article!