Common electrochemical tests for corrosion

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 (Ecorr), corrosion current density (icorr), and polarization resistance (Rp). The higher the icorr, the higher is the corrosion rate. In fact, it is also used to calculate the rate in mpy or mmpy. Further, the lower the Rp, 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!

Introduction to electrochemical techniques for corrosion testing

Need for electrochemical techniques

Corrosion is the electrochemical degradation of metals and alloys. A simultaneous occurrence of anodic and cathodic reactions is seen in aqueous corrosion. In the oil and gas industry, most of the corrosion is measured in terms of corrosion rates – mmpy or mpy. which are employed in terms of the ‘corrosion allowance’, namely the wall thickness. However, these terms assume that all the corrosion is uniform corrosion, with consistent thickness reduction everywhere in the component walls.

This leads to a neglect of the localized corrosion phenomenon with disastrous consequences. The localized corrosion, specifically pitting, starts at an atomic level and propagates in an autocatalytic manner. It remains undetectable till the pit perforates the wall and causes a leak or cracks to develop at the pits. Hence, the corrosion rate in mpy is not always the correct method to gauge or plan for the corrosion damage.

Categories and standards for electrochemical corrosion testing

Electrochemical testing methods work based on the changes in potential and current. As such, they can detect the conditions which lead to the formation of pits and other localized corrosion phenomena. They are categorized as follows —

  1. Cyclic and linear sweep voltammetry (CV)
  2. Chronopotentiometry (CP)
  3. Chronoamperometry (CA)
  4. Impedance spectroscopy (EIS)

ASTM standards

Some ASTM standards which specify the test used for corrosion testing  are given in the table 1—

Table 1: ASTM standards for electrochemical corrosion testing
No.ASTM StandardDescription
1.D8370-22Standard Test Method for Field Measurement of Electrochemical Impedance on Coatings and Linings
2.F1113-87(2017)Standard Test Method for Electrochemical Measurement of Diffusible Hydrogen in Steels (Barnacle Electrode)
3.G3-14(2019)Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
4.G5-14(2021)Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
5.G59-97(2020)Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements
6.G61-86(2018)Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys
7.G71-81(2019)Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
8.G96-90(2018)Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
9.G100-89(2021)Standard Test Method for Conducting Cyclic Galvanostaircase Polarization
10.G102-23Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements
11.G106-89(2015)Standard Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements
12.G108-94(2015)Standard Test Method for Electrochemical Reactivation (EPR) for Detecting Sensitization of AISI Type 304 and 304L Stainless Steels
13.G148-97(2018)Standard Practice for Evaluation of Hydrogen Uptake, Permeation, and Transport in Metals by an Electrochemical Technique
14.G150-18Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels and Related Alloys
15.G180-21Standard Test Method for Corrosion Inhibiting Admixtures for Steel in Concrete by Polarization Resistance in Cementitious Slurries
16.G199-09(2020)Standard Guide for Electrochemical Noise Measurement

Equipment and test setup

1.     Potentiostat

The most important equipment required for electrochemical testing is the potentiostat. The potentiostat is an arrangement of a voltmeter, an ammeter, a power source, and a rheostat (variable resistor). Figure 1 shows the typical arrangement. The function of the rheostat is to change the current flowing through the circuit when the power is switched on. The variations in the potential are measured by the voltmeter, and the corresponding current is measured by the ammeter. The potentiostat is connected to a power source.

Figure 1: Typical arrangement of a three-electrode setup with internal parts of potentiostat—A: ammeter, V: voltmeter, Rh: rheostat, WE: working electrode, RE: reference electrode, and CE: counter electrode.

2.     Electrodes

The test setup has three electrodes – working, counter, and reference. The working electrode (WE) is the material of the component which needs to be tested. The reference electrode (RE) measures the potential of the working electrode. The counter electrode (CE) is used to complete the circuit and support the chemical reactions.

The sequence of connections is specific to the role played by each component and each electrode. The working electrode is the one that is to be measured, namely the sample. Its potential is an indicator of its active or noble quality. The potential is called ‘open circuit potential’ or ‘OCP’. It is also the potential where the corrosion of the working electrode is freely occurring in the electrolyte without any externally applied voltage. It is measured against the reference electrode, which can be silver-silver chloride, saturated calomel, etc, with the help of the voltmeter. Hence, one wire will go from the working electrode to the reference electrode via the voltmeter.

In the second step, the rheostat must be in series with the power source, so that the current flow can be changed. To measure the current flow, the ammeter is required, which is also in series. This external voltage must be imposed on the working electrode. So, one wire will connect the working electrode to the series circuit of ammeter and rheostat. The voltage will effect chemical reactions (either oxidation/reduction) at the working electrode. The corresponding reactions will occur at the counter electrode. Hence, the counter electrode is now connected to the ammeter. This will ensure that there is a complete circuit for the current flow that is measured by the ammeter.

Part Two coming soon. Subscribe to the blog for regular updates!

** Also, check out this quiz for cathodic protection certification.