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 Standard	Description
1.	D8370-22	Standard 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.	G102-23	Standard 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.	G150-18	Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels and Related Alloys
15.	G180-21	Standard 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.

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