Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor

*Guocai Tian and Weizhong Zhou*

### **Abstract**

Three sets of ionic liquids such as 1-alkyl-3-methylimidazole chloride [Cnmim] Cl, 1-alkyl-3-methylimidazolium acetate [Cnmim]Ac and 1-octyl-3 methylimidazole salt [Omim]Y (n = 2, 4, 6, 8, and Y = Cl, BF4, HSO4, Ac and TFO) were used as corrosion inhibitor medium for corrosion protection of carbon steel. Electronic structures and reactivity of these ionic liquids, surface energy and electronic structures of the iron surface were systematically analyzed by density functional theory. By increasing the alkyl chain length of the [Cnmim]Cl and [Cnmim] Ac systems, the lowest unoccupied molecular orbital energy (*E*LUMO), the highest occupied molecular orbital energy (*E*HOMO), the softness (*S*) and polarizability (*α*) increased gradually, whereas electronegativity (*χ*), energy gap (Δ*E*), hardness (*η*), dipole moment (*μ*)and electrophilic index (*ω*) gradually decreased. For the [Omim] Y system, the structure parameters of ionic liquids are quite different, and only the polarizability (*α*) decreases gradually by increasing the length of the alkyl chain. The results show that inhibition is mainly [Cnmim]<sup>+</sup> cations of the [Cnmim]Cl system, and the order of inhibition efficiency follows as [C2mim]Cl < [C4mim]Cl < [C6mim]Cl < [C8mim]Cl. Both [Cnmim]+ cations and the Ac anion have inhibition effect for the [Xmim]Ac system, and the order of inhibition efficiency is [C8mim]Ac > [C6mim]Ac > [C4mim]Ac > [C2mim]Ac. For the [Omim]Y system, [Xmim]<sup>+</sup> cations and anions (BF4 , HSO4 , Ac, TfO) have inhibition effect, and the order of inhibition efficiency is [Omim]TfO > [Omim]Ac > [Omim] HSO4 > [Omim]BF4 > [Omim]Cl.

**Keywords:** inhibitor, carbon steel, density functional theory, active site, global and local activity parameters, geometry structure

#### **1. Introduction**

Metal corrosion is a ubiquitous phenomenon in the national economy, national defense construction, science and technology and other relative fields [1–3]. It has tremendous negative effects and need to be treated. Corrosion of metal and its alloys in various fields in each country has caused serious problems in different fields and caused a series of major losses. Therefore, we must adopt effective corrosion protection measures. As latest reported from the National Association of Anticorrosive Engineers, the world's annual corrosion cost can decrease by 15–35% (US \$ 375–75 billion) by adopting appropriate anticorrosion technology [1–3].

Due to its low cost and excellent physical, chemical and mechanical properties, iron and its alloys are widely used in many fields, and most of them are in contact with corrosive environments, such as tanks, pipelines, boilers, oil and gas production units and refineries [1]. Therefore, it is necessary to take certain protective measures, and the addition of an inhibitor can significantly reduce the corrosion rate of iron and ferroalloy, and the amount of inhibitor is small, which has become an economic and effective method [2]. Traditional inhibitors, such as chromate, nitrite, silicate and molybdate, can react with metals to constitute a passive film or a dense metal salt protective film on the metal surface, preventing the corrosion of metals and alloys [3]. However, this kind of inhibitor is usually used in large amount and high cost. In addition, when the amount of inhibitor is insufficient, it will lead to serious local corrosion of metals and alloys. In addition, these inhibitors have certain toxicity and poor biodegradation ability. When they are accustomed, there are serious environmental and ecological problems and they are increasingly restricted. Therefore, it is very important to develop environment-friendly inhibitors. As an environment-friendly inhibitor, imidazolines, amides and other inhibitors have the characteristics of high efficiency, low toxicity and easy biodegradation, which are commonly used in the corrosion inhibition of iron and alloy [1–3].

(trifluoromethyl-sulfonyl) imide ([Hmim]NTF2), 1-propyl-3-methylimidazoliumbis(trifluoromethyl-sulfonyl) imide ([Pmim]NTF2), 1-propyl-2,3-dimethylimidazolium bis(trifluoromethyl-sulfonyl) imide [PDmim]NTF2 and 1-butyl-3 methylimidazolium bis(trifluoromethyl-sulfonyl) imide ([Bmim]NTF2). The results showed that [PDmim]NTF2 has the strongest capability of electron donor, and [Pmim]NTF2 has capability of electron acceptor. The active centers are located in the ring of imidazole cation and the N and O atoms of anions. Sasikumar et al. [28] studied the behavior of 1-decyl-3-methylimidazolium tetrafluoride ([Dmim] BF4), 1-ethyl-3-methylimidazolium boron tetrafluoride ([Emim]BF4) and 1-butyl-2,3-dimethylimidazolium tetrafluoride ([BDmim]BF4) inhibition on low carbon steel by density functional theory in hydrochloric acid solution. The results showed that the variations of the *E*HOMO, hardness (*η*) and dipole moment (*μ*) are in agreement with the electrochemical experiments. The order of inhibition efficiency is [Dmim]BF4 > [BDmim]BF4 > [Emim]BF4, which agrees well with experiments. The corrosion inhibition behavior of 1-hexyl-3-methylimidazolium iodide ([Hmim] I), 1-hexyl-3-methylimidazolium phosphorus hexafluoride ([Hmim]PF6), 1-hexyl-3-methylimidazolium trifluoromethanesulfonate ([Hmim]TFO) and 1-hexyl-3 methylimidazolium boron tetrafluoride ([Hmim]BF4) on carbon steel was studied by Mashuga et al. [29] with density functional theory. It was found that the variation order of *E*HOMO, hardness (*η*), softness (*S*) and the dipole moment (*μ*) is [Hmim]PF6 < [Hmim]BF4 < [Hmim]I < [Hmim]TFO. The results agree well with the experimental inhibition efficiency measured by electrochemical methods. The inhibition performance of two 1-ethyl-3-methylimidazolium-based ionic liquids formed with ethyl sulfate ([Emim]EtSO4) and acetate ([Emim]Ac) and three 1 butyl-3-methylimidazolium-based ionic liquids with acetate ([Bmim]Ac), thiocyanate ([Bmim]SCN) and dichloride ([Bmim]DCA) on carbon steel in acid medium was studied with density functional theory by Yesudass et al. [30]. The results show that the inhibition efficiency follows [Emim]Ac < [Bmim]SCN < [Bmim]Ac < [Bmim]DCA < [Emim]EtSO4, which is consistent with the experimental results. As mentioned above, although some progress has been made in the study of the corrosion mechanism and properties of low carbon steel by ionic liquid corrosion inhibitors in the corrosion medium, there are still some problems. Experimental methods (weight-loss method, electrochemical experiment method and surface morphology analysis method) are used to study the ionic liquid, mainly focusing on the evaluation of the inhibition performance and efficiency of ionic liquid, but it is difficult to know the inhibition mechanism of ionic liquid on the iron surface. However, the structure parameters calculated by quantum chemistry are not systematically analyzed. For example, for the imidazole system with different cations, how do the structure parameters and property change with the increase of the cationic alkyl chain length? For the system with the same cations and different anions, how do the structure parameters change? There is no systematic analysis of the properties of different surfaces of iron, but it simply selects a surface as the adsorption surface of ionic liquid. At the same time, different surface of iron may have different electric charge and different electric charge. There are some relations between iron and HOMO and LUMO of ionic liquid molecules, but there are few

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor*

*DOI: http://dx.doi.org/10.5772/intechopen.92768*

Molecular simulation is only a simple calculation of adsorption energy between the metal surface and ionic liquid inhibitors, but there are few studies on the quantitative description of the molecular metal atom combination of ionic liquid, the determination of the functional groups of ionic liquid, and the acquisition of the process information of adsorption and film formation, which cannot guide the development of ionic liquid corrosion inhibitors. In this paper, the equilibrium geometry, frontier orbit, global and local reactivity of [Cnmim]Cl system, [Cnmim]Ac system and

related researches.

**39**

In recent years, ionic liquids (ILs) as a new green solvent, which is formed by organic cations with various inorganic or organic anions, have attracted more and more attention [4–6]. Due to the heterocyclic structure, heteroatoms (such as N, O and S) and multiple bonds in ionic liquid molecules, they have become effective inhibitors and are commonly used in the field of corrosion protection [6–9]. The ionic liquid common inhibitors are imidazole ionic liquid, pyridine ionic liquid and quaternary ammonium ionic liquid. Compared with traditional corrosion inhibitors, ionic liquid corrosion inhibitors are always renewable, inexpensive, ecologically acceptable, readily available and environmentally friendly and biodegradable. Various ionic liquids such as imidazoline, pyridine, ammonium, pyridazine and benzimidazole were widely used as corrosion inhibitors for carbon steel. The experimental methods such as surface analysis techniques, electrochemical tests and electrochemical impedance spectroscopy are used extensively to study the performance of ionic liquid inhibitors [9–25].

However, experimental research on ionic liquid inhibitors mainly focuses on the evaluation of their corrosion inhibition performance. The research on the corrosion inhibition mechanism of ionic liquid inhibitors is less, which can not reveal some details of the corrosion inhibition process, such as the quantitative description of the molecular metal atom combination of inhibitors, the determination of the functional groups of inhibitors and the information on the process of ion adsorption and film formation of inhibitors [9–11]. It is impossible to guide the development of ionic liquid inhibitors by obtaining and diffusing the inhibitors on the metal surface. Therefore, it is necessary to study the mechanism of ionic liquid inhibitors by theoretical calculation, so as to provide theoretical guidance for the development and application of new and efficient ionic liquid inhibitors.

With the rapid improvement and advancement of computer science and technology, and the gradual improvement of related theories, quantum chemical calculation as an important theoretical calculation method has been proved to be very effective in determining the electronic structure of molecules and clarifying the reaction activity of molecules. The inhibition mechanism and performance of inhibitors are often the same as the electronic structure and reaction activity of inhibitors. Therefore, it is very important to study the electronic structure and reaction activity of ionic liquid inhibitors for the design of new ionic liquid inhibitors [26]. Murulana et al. [27] studied 1-octyl-3-methylimidazolium bis

#### *Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor DOI: http://dx.doi.org/10.5772/intechopen.92768*

(trifluoromethyl-sulfonyl) imide ([Hmim]NTF2), 1-propyl-3-methylimidazoliumbis(trifluoromethyl-sulfonyl) imide ([Pmim]NTF2), 1-propyl-2,3-dimethylimidazolium bis(trifluoromethyl-sulfonyl) imide [PDmim]NTF2 and 1-butyl-3 methylimidazolium bis(trifluoromethyl-sulfonyl) imide ([Bmim]NTF2). The results showed that [PDmim]NTF2 has the strongest capability of electron donor, and [Pmim]NTF2 has capability of electron acceptor. The active centers are located in the ring of imidazole cation and the N and O atoms of anions. Sasikumar et al. [28] studied the behavior of 1-decyl-3-methylimidazolium tetrafluoride ([Dmim] BF4), 1-ethyl-3-methylimidazolium boron tetrafluoride ([Emim]BF4) and 1-butyl-2,3-dimethylimidazolium tetrafluoride ([BDmim]BF4) inhibition on low carbon steel by density functional theory in hydrochloric acid solution. The results showed that the variations of the *E*HOMO, hardness (*η*) and dipole moment (*μ*) are in agreement with the electrochemical experiments. The order of inhibition efficiency is [Dmim]BF4 > [BDmim]BF4 > [Emim]BF4, which agrees well with experiments. The corrosion inhibition behavior of 1-hexyl-3-methylimidazolium iodide ([Hmim] I), 1-hexyl-3-methylimidazolium phosphorus hexafluoride ([Hmim]PF6), 1-hexyl-3-methylimidazolium trifluoromethanesulfonate ([Hmim]TFO) and 1-hexyl-3 methylimidazolium boron tetrafluoride ([Hmim]BF4) on carbon steel was studied by Mashuga et al. [29] with density functional theory. It was found that the variation order of *E*HOMO, hardness (*η*), softness (*S*) and the dipole moment (*μ*) is [Hmim]PF6 < [Hmim]BF4 < [Hmim]I < [Hmim]TFO. The results agree well with the experimental inhibition efficiency measured by electrochemical methods. The inhibition performance of two 1-ethyl-3-methylimidazolium-based ionic liquids formed with ethyl sulfate ([Emim]EtSO4) and acetate ([Emim]Ac) and three 1 butyl-3-methylimidazolium-based ionic liquids with acetate ([Bmim]Ac), thiocyanate ([Bmim]SCN) and dichloride ([Bmim]DCA) on carbon steel in acid medium was studied with density functional theory by Yesudass et al. [30]. The results show that the inhibition efficiency follows [Emim]Ac < [Bmim]SCN < [Bmim]Ac < [Bmim]DCA < [Emim]EtSO4, which is consistent with the experimental results.

As mentioned above, although some progress has been made in the study of the corrosion mechanism and properties of low carbon steel by ionic liquid corrosion inhibitors in the corrosion medium, there are still some problems. Experimental methods (weight-loss method, electrochemical experiment method and surface morphology analysis method) are used to study the ionic liquid, mainly focusing on the evaluation of the inhibition performance and efficiency of ionic liquid, but it is difficult to know the inhibition mechanism of ionic liquid on the iron surface. However, the structure parameters calculated by quantum chemistry are not systematically analyzed. For example, for the imidazole system with different cations, how do the structure parameters and property change with the increase of the cationic alkyl chain length? For the system with the same cations and different anions, how do the structure parameters change? There is no systematic analysis of the properties of different surfaces of iron, but it simply selects a surface as the adsorption surface of ionic liquid. At the same time, different surface of iron may have different electric charge and different electric charge. There are some relations between iron and HOMO and LUMO of ionic liquid molecules, but there are few related researches.

Molecular simulation is only a simple calculation of adsorption energy between the metal surface and ionic liquid inhibitors, but there are few studies on the quantitative description of the molecular metal atom combination of ionic liquid, the determination of the functional groups of ionic liquid, and the acquisition of the process information of adsorption and film formation, which cannot guide the development of ionic liquid corrosion inhibitors. In this paper, the equilibrium geometry, frontier orbit, global and local reactivity of [Cnmim]Cl system, [Cnmim]Ac system and

Due to its low cost and excellent physical, chemical and mechanical properties, iron and its alloys are widely used in many fields, and most of them are in contact with corrosive environments, such as tanks, pipelines, boilers, oil and gas production units and refineries [1]. Therefore, it is necessary to take certain protective measures, and the addition of an inhibitor can significantly reduce the corrosion rate of iron and ferroalloy, and the amount of inhibitor is small, which has become an economic and effective method [2]. Traditional inhibitors, such as chromate, nitrite, silicate and molybdate, can react with metals to constitute a passive film or a dense metal salt protective film on the metal surface, preventing the corrosion of metals and alloys [3]. However, this kind of inhibitor is usually used in large amount and high cost. In addition, when the amount of inhibitor is insufficient, it will lead to serious local corrosion of metals and alloys. In addition, these inhibitors have certain toxicity and poor biodegradation ability. When they are accustomed, there are serious environmental and ecological problems and they are increasingly restricted. Therefore, it is very important to develop environment-friendly inhibitors. As an environment-friendly inhibitor, imidazolines, amides and other inhibi-

tors have the characteristics of high efficiency, low toxicity and easy

imidazole were widely used as corrosion inhibitors for carbon steel. The

performance of ionic liquid inhibitors [9–25].

and application of new and efficient ionic liquid inhibitors.

experimental methods such as surface analysis techniques, electrochemical tests and electrochemical impedance spectroscopy are used extensively to study the

However, experimental research on ionic liquid inhibitors mainly focuses on the evaluation of their corrosion inhibition performance. The research on the corrosion inhibition mechanism of ionic liquid inhibitors is less, which can not reveal some details of the corrosion inhibition process, such as the quantitative description of the molecular metal atom combination of inhibitors, the determination of the functional groups of inhibitors and the information on the process of ion adsorption and film formation of inhibitors [9–11]. It is impossible to guide the development of ionic liquid inhibitors by obtaining and diffusing the inhibitors on the metal surface. Therefore, it is necessary to study the mechanism of ionic liquid inhibitors by theoretical calculation, so as to provide theoretical guidance for the development

With the rapid improvement and advancement of computer science and technology, and the gradual improvement of related theories, quantum chemical calculation as an important theoretical calculation method has been proved to be very effective in determining the electronic structure of molecules and clarifying the reaction activity of molecules. The inhibition mechanism and performance of inhibitors are often the same as the electronic structure and reaction activity of inhibitors. Therefore, it is very important to study the electronic structure and reaction activity of ionic liquid inhibitors for the design of new ionic liquid inhibitors [26]. Murulana et al. [27] studied 1-octyl-3-methylimidazolium bis

alloy [1–3].

*Density Functional Theory Calculations*

**38**

biodegradation, which are commonly used in the corrosion inhibition of iron and

In recent years, ionic liquids (ILs) as a new green solvent, which is formed by organic cations with various inorganic or organic anions, have attracted more and more attention [4–6]. Due to the heterocyclic structure, heteroatoms (such as N, O and S) and multiple bonds in ionic liquid molecules, they have become effective inhibitors and are commonly used in the field of corrosion protection [6–9]. The ionic liquid common inhibitors are imidazole ionic liquid, pyridine ionic liquid and quaternary ammonium ionic liquid. Compared with traditional corrosion inhibitors, ionic liquid corrosion inhibitors are always renewable, inexpensive, ecologically acceptable, readily available and environmentally friendly and biodegradable. Various ionic liquids such as imidazoline, pyridine, ammonium, pyridazine and benz[Omim]Y (n = 2, 4, 6, 8, Y = Cl, BF4, HSO4, Ac and TFO) are studied by quantum chemical calculation. The active region, inhibition efficiency of possible interaction between ionic liquid molecules and iron surface are preliminaries analyzed.

the bonding ability of a molecule with iron. The smaller Δ<sup>E</sup> is, the stronger bonding

In the concept density functional theory, according to Koopmans theorem [36], ionization potential *I* under vacuum condition can be approximated to the negative value of *E*HOMO *I* ¼ �*E*HOMO. Similarly, the electron affinity a is approximately negative for *E*LUMO *A* ¼ �*E*LUMO. The dipole moment (*μ*), electronegativity (*μ*), softness (*S*) and polarizability (*α*) of a molecule are generally referred to as the global reactivity of a molecule [37, 38]. Molecular polarity is always described by dipole moment (*μ*), which is defined as the product of atomic distance *R* and atomic charge

ability with iron is, and the easier adsorption on the iron surface [35].

*DOI: http://dx.doi.org/10.5772/intechopen.92768*

*Theoretical Study of the Structure and Property of Ionic Liquids as Corrosion Inhibitor*

*q* as *μ* ¼ *qR* [39]. Generally, the higher *μ* will make the molecules more easily adsorbed on the surface of carbon steel [40], and the efficiency of the molecular inhibitor will increase accordingly. However, the dipole moment reflects the global polarity of molecules rather than the polarity of the bond in the molecule. According to the concept density functional theory, the electronegativity (*χ*) of the system with

n electrons is defined as follows when the external field *ν*(*r*) is fixed [38]:

*<sup>η</sup>* <sup>¼</sup> <sup>1</sup> 2

follows [42]:

following [37]:

**41**

Using the finite difference approximation, the global hardness (*η*) and electronegativity (*χ*) are related to ionization energy *I* and affinity energy *A* as

*<sup>χ</sup>* <sup>¼</sup> *<sup>I</sup>* <sup>þ</sup> *<sup>A</sup>*

*<sup>η</sup>* <sup>¼</sup> *<sup>I</sup>* � *<sup>A</sup>*

According to Koopmans theorem [36], the electronegativity (*χ*) and global

*<sup>χ</sup>* ¼ � *<sup>E</sup>*HOMO <sup>þ</sup> *<sup>E</sup>*LUMO

*<sup>η</sup>* <sup>¼</sup> *<sup>E</sup>*LUMO � *<sup>E</sup>*HOMO

Global softness (*s*) is usually defined as the reciprocal of global hardness [37].

*<sup>S</sup>* <sup>¼</sup> <sup>1</sup> *η*

Electronegativity (*χ*) is a scale of the ability of atoms in molecules to attract electrons. The larger its value is, the easier it is to attract electrons [38] reflecting a better inhibitor effect. The smaller the global hardness (*η*) or the larger the global softness (*S*) of the molecule means that the stronger the interaction between metal surface and a molecule [43], and a higher corrosion inhibition efficiency. Parr introduced the concept of electrophilic index (*ω*), which is defined as

hardness (*η*) can be calculated from *E*HOMO and *E*LUMO as

*<sup>χ</sup>* ¼ � *<sup>∂</sup><sup>E</sup> ∂N* 

νð Þ*r*

νð Þ*r*

Hardness (*η*) is defined as the second derivative of the system energy *E* to *N* [41]

*∂*2 *E ∂N*<sup>2</sup>  (1)

*:* (2)

<sup>2</sup> , (3)

<sup>2</sup> (4)

<sup>2</sup> , (5)

<sup>2</sup> *:* (6)

*:* (7)
