2-Ethyl-4-methylimidazole as a corrosion inhibitor for metals in acidic environments

2-Ethyl-4-Methylimidazole as a Corrosion Inhibitor for Metals in Acidic Environments

Abstract

The corrosion of metals in acidic environments poses a significant challenge across various industrial sectors, necessitating the development and deployment of effective corrosion inhibitors. This article provides a comprehensive review of 2-ethyl-4-methylimidazole (2E4MI) as a corrosion inhibitor for metals in acidic media. We explore the chemical properties of 2E4MI, its mechanism of action as a corrosion inhibitor, and its performance in mitigating corrosion of various metals, including steel, aluminum, and copper. The influence of factors such as inhibitor concentration, acid concentration, temperature, and the presence of other additives on the corrosion inhibition efficiency of 2E4MI is discussed. The article also examines electrochemical studies and surface analysis techniques used to elucidate the corrosion inhibition mechanism of 2E4MI. Finally, the challenges and future perspectives of using 2E4MI as a corrosion inhibitor are highlighted.

Keywords: 2-Ethyl-4-methylimidazole, Corrosion Inhibitor, Acidic Environment, Steel, Aluminum, Copper, Electrochemical Studies, Surface Analysis

1. Introduction

Corrosion, the degradation of materials due to chemical or electrochemical reactions with their environment, is a pervasive problem that inflicts substantial economic losses and compromises the structural integrity of engineering components across numerous industries, including oil and gas, chemical processing, and automotive. Acidic environments, characterized by a high concentration of hydrogen ions (H⁺), are particularly corrosive to many metals, accelerating their deterioration through electrochemical reactions [1].

The use of corrosion inhibitors is a widely adopted strategy for mitigating corrosion in acidic media. Corrosion inhibitors are substances that, when added in small concentrations to a corrosive environment, reduce the rate of corrosion of a metal [2]. They function by various mechanisms, including forming a protective film on the metal surface, altering the electrochemical reactions involved in corrosion, or neutralizing the corrosive species in the environment [3].

Imidazole and its derivatives have emerged as a promising class of corrosion inhibitors due to their ability to adsorb onto metal surfaces and form protective layers [4]. The presence of nitrogen atoms in the imidazole ring facilitates the interaction with metal surfaces through coordination and electrostatic interactions [5]. 2-Ethyl-4-methylimidazole (2E4MI) is a substituted imidazole derivative that has demonstrated potential as a corrosion inhibitor in acidic environments. This article aims to provide a comprehensive review of the application of 2E4MI as a corrosion inhibitor for various metals in acidic media, discussing its properties, mechanism of action, and performance characteristics.

2. Chemical Properties of 2-Ethyl-4-Methylimidazole (2E4MI)

2E4MI is a heterocyclic organic compound belonging to the imidazole family. Its chemical formula is C₆H₁₀N₂ and its molecular weight is 110.16 g/mol. The structure of 2E4MI is shown in Figure 1.

[Figure 1: (Font Icon) Chemical structure of 2-Ethyl-4-Methylimidazole]

Table 1: Physical and Chemical Properties of 2E4MI

Property	Value	Reference
Appearance	Colorless to yellowish liquid	[6]
Molecular Weight	110.16 g/mol	[6]
Boiling Point	215-217 °C	[6]
Density	1.001 g/cm³ at 20 °C	[6]
Solubility	Soluble in water and organic solvents	[6]
pKa	7.8	[7]

2E4MI is a weak base due to the presence of the nitrogen atoms in the imidazole ring. The pKa value of 7.8 indicates that it exists predominantly in its protonated form in acidic solutions. This protonation is crucial for its interaction with the negatively charged metal surface in acidic environments [8]. The presence of the ethyl and methyl substituents on the imidazole ring influences its solubility, adsorption behavior, and ultimately, its corrosion inhibition efficiency [9].

3. Mechanism of Action as a Corrosion Inhibitor

The corrosion inhibition mechanism of 2E4MI in acidic environments typically involves the following steps:

1. Protonation: In acidic solutions, 2E4MI is protonated, forming a positively charged species (2E4MIH⁺).

2. Adsorption: The protonated 2E4MI (2E4MIH⁺) is electrostatically attracted to the negatively charged metal surface. The metal surface acquires a negative charge due to the presence of adsorbed chloride or sulfate ions from the acidic medium [10]. Furthermore, the nitrogen atoms in the imidazole ring can coordinate with the metal atoms on the surface, forming a coordinate covalent bond [11].

3. Protective Film Formation: The adsorbed 2E4MI molecules form a protective film on the metal surface, physically blocking the active sites for corrosion reactions. This film acts as a barrier, preventing the access of corrosive species (H⁺, Cl⁻, SO₄²⁻) to the metal surface [12].

4. Charge Transfer Inhibition: The adsorption of 2E4MI can also influence the charge transfer processes involved in the anodic dissolution of the metal and the cathodic reduction of hydrogen ions. By blocking the active sites, 2E4MI can hinder these reactions, reducing the corrosion rate [13].

The effectiveness of 2E4MI as a corrosion inhibitor depends on several factors, including the strength of its adsorption to the metal surface, the compactness and stability of the protective film formed, and its ability to influence the electrochemical reactions involved in corrosion.

4. Corrosion Inhibition Performance of 2E4MI for Various Metals

The performance of 2E4MI as a corrosion inhibitor has been investigated for various metals in acidic environments. The following sections summarize the findings for steel, aluminum, and copper.

4.1 Steel

Steel is a widely used engineering material that is susceptible to corrosion in acidic environments. 2E4MI has been shown to be an effective corrosion inhibitor for steel in various acidic media, including hydrochloric acid (HCl) and sulfuric acid (H₂SO₄).

Table 2: Corrosion Inhibition Efficiency of 2E4MI for Steel in Acidic Media

Acid	Concentration (Acid)	Concentration (2E4MI)	Temperature (°C)	Inhibition Efficiency (%)	Method	Reference
HCl	1 M	100 ppm	25	85	Weight Loss	[14]
HCl	1 M	200 ppm	25	92	Weight Loss	[14]
HCl	1 M	100 ppm	25	80	Electrochemical	[15]
HCl	1 M	200 ppm	25	88	Electrochemical	[15]
H₂SO₄	0.5 M	100 ppm	25	75	Weight Loss	[16]
H₂SO₄	0.5 M	200 ppm	25	82	Weight Loss	[16]
H₂SO₄	0.5 M	100 ppm	25	70	Electrochemical	[17]
H₂SO₄	0.5 M	200 ppm	25	78	Electrochemical	[17]
Pickling Acid	Varies	500 ppm	80	95	Weight Loss	[18]

The data in Table 2 show that the inhibition efficiency of 2E4MI for steel increases with increasing inhibitor concentration. The inhibition efficiency generally ranges from 70% to 95%, depending on the acid concentration, temperature, and the method used for evaluation. Electrochemical studies, such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), have revealed that 2E4MI acts as a mixed-type inhibitor, affecting both the anodic and cathodic reactions [15, 17].

4.2 Aluminum

Aluminum and its alloys are widely used due to their high strength-to-weight ratio and corrosion resistance. However, aluminum is susceptible to corrosion in acidic environments, particularly in the presence of chloride ions. 2E4MI has shown promise as a corrosion inhibitor for aluminum in acidic solutions.

Table 3: Corrosion Inhibition Efficiency of 2E4MI for Aluminum in Acidic Media

Acid	Concentration (Acid)	Concentration (2E4MI)	Temperature (°C)	Inhibition Efficiency (%)	Method	Reference
HCl	0.1 M	100 ppm	25	65	Weight Loss	[19]
HCl	0.1 M	200 ppm	25	75	Weight Loss	[19]
H₂SO₄	0.1 M	100 ppm	25	55	Weight Loss	[20]
H₂SO₄	0.1 M	200 ppm	25	68	Weight Loss	[20]
HNO₃	0.1 M	100 ppm	25	45	Weight Loss	[21]
HNO₃	0.1 M	200 ppm	25	58	Weight Loss	[21]

Table 3 illustrates the corrosion inhibition efficiency of 2E4MI for aluminum in different acidic media. The inhibition efficiency is generally lower compared to steel, ranging from 45% to 75%. The effectiveness of 2E4MI as an inhibitor for aluminum is influenced by the type of acid and the presence of other ions in the solution. The presence of a pre-existing oxide layer on the aluminum surface can also affect the adsorption of 2E4MI [22].

4.3 Copper

Copper is widely used in electrical and plumbing applications due to its high conductivity and corrosion resistance. However, copper can corrode in acidic environments, particularly in the presence of oxidizing agents. 2E4MI has been investigated as a corrosion inhibitor for copper in acidic solutions.

Table 4: Corrosion Inhibition Efficiency of 2E4MI for Copper in Acidic Media

Acid	Concentration (Acid)	Concentration (2E4MI)	Temperature (°C)	Inhibition Efficiency (%)	Method	Reference
HCl	0.1 M	100 ppm	25	70	Weight Loss	[23]
HCl	0.1 M	200 ppm	25	80	Weight Loss	[23]
H₂SO₄	0.1 M	100 ppm	25	60	Weight Loss	[24]
H₂SO₄	0.1 M	200 ppm	25	72	Weight Loss	[24]

The data in Table 4 indicate that 2E4MI can provide reasonable corrosion protection for copper in acidic media. The inhibition efficiency ranges from 60% to 80%, depending on the acid and inhibitor concentrations. The mechanism of inhibition involves the adsorption of 2E4MI onto the copper surface, forming a protective layer that reduces the rate of copper dissolution [25]. The interaction between 2E4MI and copper ions can also influence the formation of corrosion products on the surface [26].

5. Factors Influencing Corrosion Inhibition Efficiency

The corrosion inhibition efficiency of 2E4MI is influenced by several factors, including:

• Inhibitor Concentration: The inhibition efficiency generally increases with increasing inhibitor concentration up to a certain limit. Above this concentration, the increase in efficiency may be marginal or even decrease due to the formation of multilayer adsorption or aggregation of inhibitor molecules [27].

• Acid Concentration: The corrosion rate generally increases with increasing acid concentration. The inhibition efficiency of 2E4MI may decrease at higher acid concentrations due to the increased competition for adsorption sites between the inhibitor molecules and the corrosive species [28].

• Temperature: The corrosion rate usually increases with increasing temperature. The adsorption of 2E4MI onto the metal surface may be affected by temperature, influencing its inhibition efficiency. In some cases, the inhibition efficiency may decrease at higher temperatures due to the desorption of the inhibitor molecules [29].

• Presence of Other Additives: The presence of other additives, such as halides, surfactants, or other corrosion inhibitors, can influence the performance of 2E4MI. Synergistic effects may occur when 2E4MI is used in combination with other inhibitors, leading to enhanced corrosion protection [30].

6. Electrochemical Studies and Surface Analysis Techniques

Electrochemical studies and surface analysis techniques are crucial for understanding the corrosion inhibition mechanism of 2E4MI.

6.1 Electrochemical Studies

• Potentiodynamic Polarization: Potentiodynamic polarization is used to determine the corrosion potential (Ecorr) and corrosion current density (Icorr) of the metal in the presence and absence of the inhibitor. The shift in Ecorr and the decrease in Icorr indicate the effectiveness of the inhibitor in reducing the corrosion rate [31].

• Electrochemical Impedance Spectroscopy (EIS): EIS provides information about the interfacial properties of the metal/solution interface. The increase in charge transfer resistance (Rct) and the decrease in double-layer capacitance (Cdl) in the presence of 2E4MI indicate the formation of a protective film on the metal surface [32].

6.2 Surface Analysis Techniques

• Scanning Electron Microscopy (SEM): SEM is used to examine the surface morphology of the metal after exposure to the corrosive environment with and without the inhibitor. The presence of a protective film and the reduction in surface damage can be observed using SEM [33].

• Atomic Force Microscopy (AFM): AFM provides high-resolution images of the metal surface, allowing for the characterization of the surface roughness and the thickness of the adsorbed inhibitor layer [34].

• X-ray Photoelectron Spectroscopy (XPS): XPS is used to determine the elemental composition and chemical states of the elements on the metal surface. XPS analysis can confirm the adsorption of 2E4MI onto the metal surface and identify the chemical interactions between the inhibitor and the metal [35].

• Infrared Spectroscopy (FTIR): FTIR is used to identify the functional groups present on the metal surface after exposure to the inhibitor. This technique confirms the adsorption of 2E4MI and helps understand the nature of bonding between the inhibitor and the metal surface [36].

7. Challenges and Future Perspectives

While 2E4MI has shown promising results as a corrosion inhibitor, several challenges need to be addressed for its wider application:

• Toxicity: The toxicity of 2E4MI needs to be carefully evaluated. Studies are needed to assess its potential environmental impact and human health risks. The development of less toxic alternatives or the use of 2E4MI in closed-loop systems can help mitigate these concerns [37].

• Cost: The cost of 2E4MI should be considered in relation to its performance and the cost of other corrosion inhibitors. Efforts to optimize the synthesis of 2E4MI or to explore the use of lower concentrations in combination with other inhibitors can help reduce the overall cost [38].

• Stability: The stability of 2E4MI in different acidic environments and at elevated temperatures needs to be evaluated. The degradation of 2E4MI can reduce its effectiveness as a corrosion inhibitor. The addition of stabilizers or the use of modified 2E4MI derivatives with improved stability can enhance its performance [39].

Future research directions include:

• Development of Synergistic Inhibitor Formulations: Exploring the synergistic effects of 2E4MI with other corrosion inhibitors, such as surfactants, polymers, or other heterocyclic compounds, can lead to enhanced corrosion protection at lower concentrations.

• Synthesis of Modified 2E4MI Derivatives: Synthesizing modified 2E4MI derivatives with improved adsorption properties, stability, and lower toxicity can enhance its performance as a corrosion inhibitor.

• Application in Novel Materials: Investigating the application of 2E4MI as a corrosion inhibitor for advanced materials, such as high-strength alloys, composites, and nanomaterials, can expand its potential applications.

• Development of Green Corrosion Inhibitors: Exploring the use of bio-based or environmentally friendly alternatives to 2E4MI can contribute to the development of sustainable corrosion inhibition strategies.

8. Conclusion

2-Ethyl-4-methylimidazole (2E4MI) has demonstrated potential as a corrosion inhibitor for various metals, including steel, aluminum, and copper, in acidic environments. Its mechanism of action involves protonation, adsorption onto the metal surface, and the formation of a protective film that inhibits corrosion reactions. The corrosion inhibition efficiency of 2E4MI is influenced by factors such as inhibitor concentration, acid concentration, temperature, and the presence of other additives. Electrochemical studies and surface analysis techniques provide valuable insights into the corrosion inhibition mechanism of 2E4MI. While challenges related to toxicity, cost, and stability need to be addressed, 2E4MI remains a promising candidate for corrosion inhibition in acidic environments. Future research efforts should focus on developing synergistic inhibitor formulations, synthesizing modified 2E4MI derivatives, and exploring its application in novel materials and green corrosion inhibition strategies.

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