Anhydride curing of epoxy coating

In the previous post, we discussed the concept of epoxy coating and the reaction between an amine hardener and an epoxy resin. You can read this post here.

In this post, we will discuss the reactions between epoxy resins and other types of hardeners such as phenol, anhydride, and thiol.

Epoxy resin with anhydride hardener

Acid anhydride curing agents are diacid molecules from which water molecules have been eliminated. This removal of water is a reversible reaction, and the anhydride curing agent can change their molecular structure if they come in contact with moisture.

The following image shows the typical structure of an anhydride curing agent molecule.

Figure: Transformation of diacid molecule to anhydride molecule due to water elimination

This reaction differs from the one between amine and epoxy because here different parts of the epoxy resin react with the hardener. The amine reaction is only between the epoxide group and the amine reaction. Let us go through the main steps for the epoxy-anhydride reaction.

Reaction mechanism

The reaction proceeds as described in the following steps:

  • In this scheme of reactions, the O atom in the anhydride reacts with the epoxide group as well as the hydroxyl group in the resin but not in the same step.
  • In the beginning, the oxide ring in anhydride opens up. The O atom in anhydride takes the H atom from the OH group in the epoxy resin backbone chain. This O atom combines with one of the C atoms from the anhydride. Here, the epoxide ring is not affected.
  • Next, the combined anhydride-epoxy molecule reacts with another epoxy resin molecule. Here, the OH group in the anhydride loses the H atom. The remaining O atom breaks the epoxide ring and connects one of the C atoms in the epoxide ring. Please note that all the epoxide rings do not open up at the same time in the same reaction. Every epoxide ring requires an OH group to open up.
  • The tertiary molecules thus formed then react with each other or other unreacted epoxide groups. Here one epoxide group from one molecule reacts with the OH group from another molecule to form a crosslinking network.
  • This reaction continues until all the epoxy resins are crosslinked with ether (C-O-C-) groups in the backbone chain.
Figure: Reaction scheme of epoxy and anhydride reactions to form epoxy coating

Such a complex reaction mechanism results in a long pot life of the coating as the molecules take time to react with each other in multiple ways.

Features of the reaction mechanism

  • Requires heat for curing
  • Needs catalysts and a small amount of moisture for reaction initiation
  • There is no definite end for the cure cycle. The curing is deemed completed when the curing cycle is stopped. This does not mean that all the molecules have reacted completely.
  • Anhydrides can react with water before mixing and curing and lose their reactivity to epoxy resin.

Advantages of anhydride-cured epoxy

  • High chemical resistance – due to the low number of reactive sites and higher crosslinking
  • High Tg and thermal resistance – as the greater crosslinking and interlinking make it difficult for the polymer chains to become flexible
  • High dielectric strength – due to less moisture penetration and a greater amount of reacted molecules
  • Low toxicity – as no other chemical is used for reaction and the original chemicals react with each other till the end of the cure cycle
  • Low cure shrinkage and exotherm – as the high amount of crosslinking makes the coating stiffer and resistant to shrinking of polymer chains.

Wrap up

The properties of the coating are related to how the polymer chains are formed during the reaction. hence, to remember the properties, it is crucial to relate them to the reaction mechanism and the resulting morphology for the coating chains.

In this article, we dealt with the epoxy-anhydride reactions. In the next article, we will talk about the thiol and phenol curing agents and their reactions with epoxy.

What is epoxy coating?

The importance of epoxy coatings for corrosion protection cannot be overstated. The global market for epoxy coatings is predicted to grow at a CAGR of 5.32% to surpass USD 44.61 billion by 2028. It is no wonder that a quick search of the keywords ‘epoxy coating’ brings up more than 88 million results.

This article will go through the definition and main reactions that form the epoxy coating.

What is epoxy coating? Definition and working principle

Epoxy coating is a layer of protective organic polymer coated on a metallic or non-metallic substrate. It is the chemical product formed after a reaction between two components—resin and hardener. It consists of additional components that contribute towards specific properties.

The resin is a polymer that contains the functional group epoxide which is a triangular ring of one oxygen and two carbon atoms.

Figure: Epoxide ring

This epoxide ring may then be connected to a varying number of hydrogen atoms or other C-H groups in a long polymer chain. The backbone chain may be made up of esters, acrylics, ethyl or methyl groups, etc.

The key to recognising an epoxy coating is the epoxide ring in the resin chemical structure.

The hardener may vary depending on the curing properties required for the application. Here’s a quick list of the various hardeners:

  • Amines – aliphatic, aromatic, and cycloaliphatic
  • Anhydrides
  • Phenols
  • Thiols

The reactivity of the hardeners increases in the following order:

Phenols < Anhydrides < Aromatic amines < Cycloaliphatic amines < Aliphatic amines < Thiols

The following images show the typical chemical reaction for an epoxide resin with an amine hardener. The epoxy reaction occurs in three steps:

  1. The epoxide ring in the resin reacts with the amine group in the hardener to form a secondary amine. In this step, one H atom from amine goes and forms OH with the O in the epoxide group leading to the opening up of the ring. The N gets connected with the C in the open epoxide molecule.
  2. Next, the formed secondary amine once again reacts with the epoxide group. Here, the remaining H from the secondary amine breaks the epoxide ring. H goes to the O from the epoxide group. The C from the backbone molecule connects with the N from the amine group. This reaction forms the tertiary amine.
  3. The last step is the etherification of the tertiary amines. The tertiary amine reacts with one more epoxide ring. this step is different from the previous steps because the N in the amine group does not participate in this reaction. The Hydroxyl OH groups are the active sites. The H from the OH group of the tertiary amine breaks the epoxide ring and reacts with the O to form OH. Thus, it shifts from OH in tertiary amine to OH in the epoxide group. The remaining O in the tertiary amine now forms a crosslinking bond with the C of the backbone CH2 group of the epoxide ring. This is the final epoxy coating molecule.
Figure: Reaction scheme of the epoxy resin and amine hardener to form epoxy coating.

Wrap up

Epoxy coating is a popular coating. However, its reaction mechanism is often misunderstood or not understood at all. This series of articles will cover the principle, reactions, and failures of epoxy coatings.

This article dealt with the amine + epoxy reaction. In the next part of the article, we will deal with the reaction mechanisms of other hardeners used to form epoxy coatings.

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.

Electrochemical testing – part 3- What does an ammeter do?

 Electrochemical testing – 3 – Ammeter

Poll Question WITH Answers:


Tafel polarization is a potentiodynamic test, where the potential is changed at a certain mV/time rate.

The changes in the current flow occurring due to the potential change are measured using the ammeter.

The ammeter is connected between the working electrode and the counter electrode.

It is NOT connected to the reference electrode in any way.

Check out the video for a detailed explanation

Check out the previous parts here –

For concise and focusSed courses, check out the website below!

Important terms in API certifications – CRV & IOW – III


IOW can be understood by using a simple example of milk kept for boiling in a pot on a stove.

The definition as per API 584 is 
 “established limits for process variables (parameters) that can affect the integrity of the equipment
 if the process operation deviates from the established limits for a predetermined length of time”

What are the important words here? 

  • established limits 
  • process variables 
  • affect the integrity of the equipment
  • process operation deviates
  • predetermined length of time
In the above example, let us keep the time as 1 hour.
Now there are two critical variables here-

      • Temperature 

What is the IOW for temperature?

The melting point of milk is 92 degrees C. That is the optimum level.
For 1 hour, if temperature is below 40 degrees C, it may not boil at all.
If the temperature is  above 120 degrees C, it may condense.
So, the IOW = 40 – 120 degrees C

      • Vessel dimension = volume of milk is 1 ltr

What is the IOW for vessel?

Vessel volume = 1 ltr, milk will spill over.
Vessel volume = 1.5 ltr, milk will boil, and may not overflow.
Vessel volume = 3 ltr, milk will fit and not overflow.
So, the IOW = 1.5 – 3 ltr

Thus, the IOW tells us what the range is for the safest and cost-effective process.


For previous parts, click here –

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Important terms in API certifications – CRV & IOW – II

CRV can be understood by using a simple example of milk kept for boiling in a pot on a stove.

The motive, i.e. the reliability of this process has two main factors –
1) The milk should boil.
2) It should remain the vessel and not overflow.

There are three critical variables here –

      • Temperature 

Why is this a critical variable? 
The melting point of milk is 92 degrees C. That is the optimum level.
If temperature is too low (30 degrees C), the milk may not boil at all.
If the temperature is too high (200 degrees C), it may burn quickly.

      • Vessel dimension = volume of milk is 1 ltr

Why is this a critical variable? 
Vessel volume < 1 ltr, milk will not fit in it in the first at all.
Vessel volume = 1 ltr, milk will fit, but will overflow on boiling
Vessel volume >1 ltr, milk will fit and not overflow.

      • Time

Why is this variable critical?
If the other two variable are at the optimum level, then – 
Too less time (~ 1 min), insufficient for milk to reach boiling point
Too much time (~ 2 hours), milk will boil and condense, taste will change
Optimum time (~ 10 min), milk will boil, not overflow, and the heating will be stopped at the correct time.
Thus, maintaining the three CRVs will help to initiate and complete the process of boiling milk perfectly.

So, whenever one has to decide the CRV for larger processes, the important questions to ask are –



Reference: “Development of Corrosion Control Document Database System in Crude Distillation Unit”, Junghwan Kim, Wonsub Lim, Younghee Lee, Seungnam Kim, Sang-Rok Park, Sun-Kyu Suh, and Il Moon, Industrial & Engineering Chemistry Research 2011 50 (13), 8272-8277
DOI: 10.1021/ie101871a

Read the other parts here –

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Important terms in API certifications – CRV & IOW – I

CRV stands for critical reliability variables.

It notes the variables, i.e. parameters which are the most important in damaging the reliability of the equipment.

In simple terms, it is that parameter which if changed will damage your equipment.

This parameter is important in risk-based inspection.

It helps to note the exact parameters, may be temperature, pressure, pH, which will increase the risk of corrosion and damage.


Read the NExt parts here –

Reference: “Development of Corrosion Control Document Database System in Crude Distillation Unit”, Junghwan Kim, Wonsub Lim, Younghee Lee, Seungnam Kim, Sang-Rok Park, Sun-Kyu Suh, and Il Moon, Industrial & Engineering Chemistry Research 2011 50 (13), 8272-8277

DOI: 10.1021/ie101871a

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What is a Corrosion Loop – PART 2 – EXAMPLE

How to prepare a corrosion loop?

To prepare a corrosion loop, the person in charge will look at the overall picture and answer the following questions –

1. What is the process?

2. What are the components?

3. What are the materials for each component?

4. What are the common features among them?

5. Where does the commonality end? –  This is important as that will mark the beginning of a new corrosion loop.

6. Can I use the same inspection procedure for all of them?

7. Do they corrode/get damaged in the same manner due to the same reasons?



Read part 1 here!

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What is a Corrosion Loop – PART 1 – CONCEPT

Corrosion loop is used to simplify inspection procedures in refining processes.

Corrosion loop is defined as a group of components with common materials, processes, and operating parameters. 

Source: Rachman, A. and Ratnayake, R.M.C. (2020), “Corrosion loop development of oil and gas piping system based on machine learning and group technology method”, Journal of Quality in Maintenance Engineering, Vol. 26 No. 3, pp. 349-368.

It is needed when the scenario has –

1. Complex process

2. Several variables such a materials of construction, process parameters, functions

3. Multiple damage mechanisms


Click here for part 2!

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