Corrosion inhibition by imidazoline and imidazoline derivatives: a review

2.1 Background

Imidazoline or dihydroimidazole (Ji Ram et al. 2019) was first used as a cationic surfactant for dye levelling agents in textile industry in the 1930s (Levinson 1999; Tyagi et al. 2007). In the 1950s, it was first used in fabric softeners as a household product for aqueous dispersion (Tyagi et al. 2007). In the 1960s, it was launched as a household product in Europe and Japan (Tyagi et al. 2007). It is unclear when it was first exploited as a corrosion inhibitor. However, Ferm and Riebsomer (1954) mentioned that imidazoline derivatives were reported to inhibit corrosion against brines, weak inorganic acids, organic acids, carbon dioxide and hydrogen sulfide in 1950–1952. In 1985, an article entitled “The Existence of Imidazoline Corrosion Inhibitors” by Martin and Valone mentioned that the first successful use of hydrocarbon solutions of imidazolines and their salts as corrosion inhibitors was carried out in 1946 at the Richard King lease, Crane County, Texas (Martin and Valone 1985). They indicated that imidazolines were easy-to-use and inexpensive substances that were readily applicable for commercialisation (Martin and Valone 1985). Additionally, they also highlighted that imidazoline salts were more resistant to hydrolysis than the conjugate base (deprotonated) form, enabling an improved steric barrier to corrodent penetration. Thus, it is highly probable that imidazolines were first utilised in the field of corrosion inhibition in the 1940s. Since then, the use of imidazoline inhibitors has been considered a new trend in electromigration inhibitor research. They have been reported as effective organic corrosion inhibitors with several advantages such as low toxicity and high efficacy (Pan et al. 2020).

Recently, several research studies focused on the role of various substituents, namely mercapto, benzene and methyl groups, on promoting the corrosion inhibition of imidazoline molecules. It was reported that these groups have remarkable influence on adsorption of organic molecules on metal surfaces (Solomon et al. 2019a). Guo et al. (2021) suggested that the –PH₂O₃ functional group could be introduced into the structure of imidazoline derivatives and form coordination compounds that exhibit great adsorption on metal surfaces. Wang et al. (2022) prepared complex inhibitors called SMIF: (E)-1-(2-(1-butyl-2-(hexadec-7-en-1-yl)-1-iodo-2,5-dihydro-1H-1λ5-imidazol-1-yl)ethyl)-3-phenylthiourea and benzimidazole (BMZ) that helped reduce the cost of inhibitors. Migahed et al. (2017) synthesised 1-(2-aminoethyl)-1-dodecyl-2-hexadecyl-4,5-dihydro-1H-imidazol-1-ium (PQI) and 1-(2-aminoethyl)-1-dodecyl-2-heptadecyl-4,5-dihydro-1H-imidazol-1-ium (SQI) with improved inhibition efficacy for carbon steel in oil well formation water. Zhang et al. (2007) prepared an imidazoline derivative by addition of thiourea substituents in its pendant side chain. Obike et al. (2018) reported synthesis and inhibition behaviours of imidazoline derivatives in various structures of their pendant side chains.

2.2 Structure and properties

Generally, imidazoline compounds consist of three parts, an imidazoline head group, a pendant group and a hydrocarbon tail (Bajpai and Tyagi 2006; Yoo et al. 2012) as shown in Figure 1. The head group is the main structure of the imidazoline molecule. It consists of a 5-membered heterocyclic ring (C₃N₂H₄) with two nitrogen atoms as active sites used to adsorb on metal surfaces (Obike et al. 2018). The imidazoline ring can be divided into three different isomeric forms, 2 imidazoline (4,5-dihydroimidazole), 3-imidazoline (2,5-dihydroimidazole) and 4 imidazoline (2,3-dihydroimidazole) (Ferm and Riebsomer 1954; Ji Ram et al. 2019; Sharma and Ghuge 2018; Solomon et al. 2019a), as shown in Figure 2. The 2 imidazoline and 3-imidazoline structures contain an imine centre, whereas 4-imidazoline contains an alkene substructure (Ji Ram et al. 2019). The 2-imidazoline isomer was most commonly found in some natural products and drugs (Liu and Du 2009; Solomon et al. 2019a). The pendant side chain acts as an anchor that can help maintain adsorption on metal surfaces (Obike et al. 2018). The characteristics of pendent group are dependent on the amine types used in imidazoline preparation. The imidazolines were synthesised through a reaction between natural fatty acids and various amines, such as diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA) (Muktiarti et al. 2020), ethylenediamine (EDA) and aminoethylethanolamine (AEEA) (Obike et al. 2018). The inhibition efficacy relies strongly on the solubility of imidazoline (Muktiarti et al. 2020) and the presence of polycentric adsorption sites (Obike et al. 2018).

Figure 1:

Molecular structure of imidazoline using diethylenetriamine and fatty acid as starting materials.

Figure 2:

Chemical structure of (a) 2-imidazoline, (b) 3-imidazoline and (c) 4-imidazoline.

The pendant side chain is the main part for structural modification to enhance inhibition. For example, the pendant side of an imidazoline structure was modified using thiourea (Zhang et al. 2007) and sulfhydryl substituents (Zheng et al. 2022). Solomon et al. (2019a) prepared an imidazoline derivative, namely N-(2-(2-tridecyl-4,5-dihydro-1H-imidazol-1-yl)ethyl)tetradecanamide (NTETD), which consists of 13 carbon atoms linked to an imidazoline moiety on one of the nitrogen atoms. In addition, the sulfhydryl group could serve as a powerful active site for adsorption and exhibited a great ability to bond with metal surfaces through coordination bonds, leading to an enhanced inhibition (Zhang et al. 2007; Zheng et al. 2022). Moreover, an effective imidazoline-based corrosion inhibitor should have a pendant chain length longer than 12 carbon atoms that is directly attached on the ring N atoms (Solomon et al. 2019a). Alternatively, it could have a relatively small organic radical attached to one of the N atoms of the imidazoline ring. Solomon et al. (Solomon et al. 2019a) found that the partitioning cum inhibiting properties of imidazoline could be improved in corrosive media by introduction of suitable pendant groups with appropriate lengths and anchoring positions.

The part of the long hydrophobic hydrocarbon chain was designed as an initial structure of a tall oil fatty acid chain (Martínez-Palou et al. 2004; Wahyuningrum et al. 2008; Yoo et al. 2012). The influence of length and the number of double bonds in the hydrocarbon tail was studied by Yoo et al. (2012). Biodiesel-based 2-(2-alkyl-4,5-dihydro-1H-imidazol-1-yl) ethanol derivatives (A-DIEs) were prepared from various sources such as Palm BD, Soy-BD and castor oil with a controlled number of double bonds, i.e., C12(0)-DIE, C18(1)-DIE, C18(2)-DIE. Their inhibition efficiency depended strongly on both the degree of unsaturation and the alkyl chain length.

Most commonly used corrosion inhibitors are either based on a long-chain aliphatic diamine or long carbon chain imidazolines. Imidazoline products are solid forms with high thermal stability in organic nitrogenous bases and easily form quaternary salts. Accordingly, the final products of imidazolines with substituents at position 2 are generally present in solid form or a heavy viscous liquid probably caused by hydrogen bonding. N1 substituents are normally liquid with lower solubility in polar solvents (Ji Ram et al. 2019). 2-Alkyl-2-imidazoline is highly soluble in water, alcohol, acetone and chloroform compared to its 1 alkyl-2-imidazoline analogue. In contrast, its solubility is relatively low in benzene, petroleum ether and carbon tetrachloride (Ji Ram et al. 2019).

To improve solubility and other related properties, the imidazoline structure needs to be modified. Several approaches are possible (Guo et al. 2021; Migahed et al. 2017; Wang et al. 2019, 2022; Zhang et al. 2015c; Zhao and Zou 2013). Guo et al. (2021) found that imidazoline derivatives functionalised by a –PH₂O₃ group rendered the effect of hydrogen bonding between –NH₂ and water molecules, which helped to increase the adsorption capacity by complexation of –PH₂O₃ with iron ions. Zhang et al. (2015c) prepared a bromide-substituted imidazoline (3-IM) with a good inhibition efficacy in a hydrochloric acid environment. Composite inhibitor molecules could be stable on the surfaces of carbon steel with a complex inhibitor, e.g., 0.1 g/L SMIF: (E)-1-(2-(1-butyl-2-(hexadec-7-en-1-yl)-1-iodo-2,5-dihydro-1H-1λ5-imidazol-1-yl)ethyl)-3-phenylthiourea and 0.9 g/L BMZ (Wang et al. 2022). Migahed et al. (2017) modified the structure of 2-(2-trifluoromethyl-4,5-dihydro-imidazol-1-yl)-ethylamine(1-IM) and 2-(2-trichloromethyl 4,5-dihydro-imidazol-1-yl)-ethylamine (2-IM) through a quaternization process using dodecyl chloride. This resulted in formation of the following final products: 1-(2-aminoethyl)-1-dodecyl-2-hexadecyl-4,5-dihydro-1H-[imidazol-1-ium (PQI) and 1-(2-aminoethyl)-1-dodecyl-2-heptadecyl-4,5-dihydro-1H-imidazol-1-ium (SQI). A long-chain fatty acid amide (LFA) could be employed as a corrosion inhibitor that provides a stable adsorption film on steel surfaces via chemical bonds between the heteroatoms (in the acyl, amine and phosphate groups of an LFA molecule) and Fe atoms, yielding multiple adsorption centres (Wang et al. 2019).

2.3 Synthesis processes

In 1888, Hofmann first synthesised 2-methyl-imidazoline by heating N,N′-diacetylethylenediamine in dry HCl (Mehedi and Tepe 2020). Since then, a variety of synthesis methods have been developed to produce imidazolines from various starting materials. The methods can be categorised according to functional groups such as isocyanide, amidine, imine, aziridine, cyanide, amide and others (Mehedi and Tepe 2020) for various prospective applications. However, this review focuses only on the synthesis processes of imidazoline corrosion inhibitors. In general, the optimum conditions used in imidazoline synthesis are relatively simple and involve addition of fatty acids and polyamine compounds under solvent-free conditions (Mehedi and Tepe 2020). The fatty acids or tall oil fatty acids commonly used are stearic acid, palmitic acid, lauric acid, oleic acid and some oils. Examples of the polyamines are ethylenediamine (EDA), diethylenetriamine (DETA), aminoethyl ethanolamine, aminoethylethanolamine (AEEA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) (Bajpai and Tyagi 2008). Tall oil fatty acids are considered as low-cost raw materials for the synthesis of imidazoline inhibitors as they are common by-products of coniferous wood in the pulping industry (Demirbas 2008). Thus, the estimated price of imidazoline compounds is approximately 0.02–30 USD/m³ (Zhong et al. 2020).

The synthesis process is a facile method that is controllable by means of modulating reaction temperature and pressure (Bajpai and Tyagi 2008; Martínez-Palou et al. 2003; Tyagi et al. 2007). Formation of imidazoline compounds can be carried out in two steps (Figure 3). First, the carboxylic acid group(s) in the fatty acids undergo an amidation reaction with the amine group(s) in polyamine compounds. Second, an intramolecular cyclisation reaction yields the corresponding imidazoline ring (Tyagi et al. 2007; Zhao et al. 2021). Ditama et al. (2020) prepared imidazolines based on a reaction between methyl oleic fatty acid and N-(2-aminoetil)-3-aminopropiltrimetoksisilan (DAMO) under an inert atmosphere with a reaction time that was over 17 h. Similarly, Wahyuningrum et al. (2008) used a total time of approximately 13 h to prepare imidazoline derivative compounds.

Figure 3:

The synthesis pathway for imidazoline, starting from diethylenetriamine and fatty acid.

Geng et al. (2022) prepared rosin imidazoline by slowly adding TETA in a mixture of rosin and xylene at 120 °C for 4 h. Then, the temperature was increased to 220 °C for 6 h. Guo et al. (2021) synthesised 2-[2-(7-isopropyl-1,4-dimethyl-9,10-octahydro-phenanthren-1-yl)-4,5-dihydro-1-yl]-ethylamino}-methyl)-phosphonicimidazole (GSIM) via an amidation reaction at 140 °C for 2 h, followed by a cyclization reaction at 220 °C for 2 h. Moreover, Zheng et al. (2022) reported that an imidazoline derivative, mercapto-oleic imidazoline (MOI), was produced from oleic imidazoline (OI) and mercaptopropionic acid under agitation and reflux for 4 h.

All of the above-mentioned processes are conventional synthesis or traditional methods with limitations that include long reaction times, low yields and tedious work-up (Martínez-Palou et al. 2003). Several literature reports proposed a one-step preparation of imidazolines and their derivatives by microwave irradiation (Bajpai and Tyagi 2008; Martínez-Palou et al. 2003; Usyatinsky and Khmelnitsky 2000; Wahyuningrum et al. 2008; Wolkenberg et al. 2004). Martínez-Palou et al. (2003) showed that long chain 2-alkyl-1-(2-hydroxyethyl)-2-imidazolines and their amide precursors can be prepared by both domestic microwave (DMW) and monomode microwave (MMW) ovens. The reactions were fully completed within a short period of time, yielding high purity products (>95%) (Martínez-Palou et al. 2003). Similarly, Bajpai and Tyagi (2008) presented the production of long chain dialkyldiamido imidazolines using DETA and several fatty acids under non-solvent microwave irradiation. It was suggested that this method could provide higher yields and faster reaction times compared to conventional methods. Solomon et al. employed a microwave oven to promote an acid–base reaction for 7 min. This resulted in formation of a novel palmitic imidazoline compound, N-(2-(2-pentadecyl-4,5-dihydro-1H-imidazol-1-yl)ethyl)palmitamide (NIMP) (Solomon et al. 2019a), and an imidazoline derivative, N-(2-(2-tridecyl-4,5-dihydro-1H-imidazol-1-yl)ethyl)tetradecanamide (NTETD) (Solomon et al. 2019b). In the same way, microwave irradiation is an efficient and quick method for preparation of long chain 2-(2-heptadecyl-4, 5-dihydro-1H-imidazole-1-yl) ethanol (HDIE) (Munis et al. 2020). In some cases, high chemical yields of 91–94% can be obtained within 7–10 min of reaction time (Wahyuningrum et al. 2008). Accordingly, this novel synthesis route is an alternative to the conventional thermal condensation method, especially when using a solvent-free reaction.

It is possible to do an additional step in the chemical modification after imidazoline synthesis. For example, Wang et al. (2022) mixed 1-iodobutane with n-phenylthiourea at 140 °C for 1.5 h after the end process of oleic imidazoline synthesis for production of novel complex inhibitors (SMIF + BMZ). Moreover, two imidazoline derivatives, including SMIF and SMID, were attained from two precursors, n-phenylthiourea and n-methylthiourea (Xiong et al. 2021). Substituents of thiourea on the pendant side of imidazoline ring were also studied by Zhang et al. (2007).

2.4 Characterisation

FT-IR and NMR (¹H NMR and ¹³C NMR) techniques are commonly used to assess chemical structure of imidazoline and imidazoline derivatives. Table 1 summarises the FT-IR analysis of imidazoline and imidazoline derivatives including wavenumbers and their corresponding assignments. The characteristic peaks of imidazoline compounds can be found at approx. 1633–1655 cm⁻¹ corresponding to C=N stretching in the imidazoline ring (Bajpai and Tyagi 2008; Geng et al. 2022; Migahed et al. 2017; Obike et al. 2018; Rahayu et al. 2021; Xiong et al. 2021; Zheng et al. 2022). Also, imidazoline ring C–N stretching is located at around 1550 cm⁻¹ (Kong et al. 2016; Obike et al. 2018; Zheng et al. 2022), and the characteristic peak of N–H stretching is noted in the range between 3275 and 3380 cm⁻¹ (Kousar et al. 2020; Migahed et al. 2017; Munis et al. 2020; Rahayu et al. 2021; Solomon et al. 2019a; Tripathy and Mishra 2017; Zhang et al. 2015d; Zheng et al. 2022). Additionally, other absorption peaks can be found depending on substituents and modified groups.

Structural characterisation of imidazolines and their derivatives should also be carried out using an NMR technique. NMR peaks can be used to identify the chemical structure of imidazolines and their derivatives through ¹H NMR and ¹³C NMR spectra. For example, the ¹H NMR spectra of CH₂ attached to the nitrogen of an imidazoline ring could be observed at δ values of 2.66, 2.75 and 2.80 ppm (Bajpai and Tyagi 2008). CH₂ attached to the carbon of an imidazoline ring is at δ values of 2.36, 2.37 and 2.39 ppm (Bajpai and Tyagi 2008). An equivalent methylene group in the imidazoline ring appears at δ values of 3.591, 3.613 ppm (Migahed et al. 2017). C sp² from C=C alkene in an unsaturated hydrocarbon chain that can be found at ¹³C NMR spectra at 130.1, 129.9 ppm. C=N is at 164.6 ppm. C sp³ is bonded to the nitrogen atom in both the imidazoline ring and pendant group at 56.7–52.9 ppm as well as C sp³ in the hydrocarbon chain at 32.0–14.3 ppm (Budiana et al. 2020). Additionally, the NMR spectra of imidazoline derivatives can be varied depending on the preparation process, the structure of the substituent group and the structure of final product. A complete list of both the ¹H NMR and ¹³C NMR spectra found in imidazolines and imidazoline derivatives is presented in Table 2.

3.1 Conditions of corrosion inhibition evaluation

3.2 Electrochemical measurements: OCP studies

OCP monitoring during corrosion testing is a useful tool to study corrosion resistance behaviour. The evolution of OCP curves as a function of immersion time can be used to determine the effect of a corrosive medium and inhibitor addition on steel surfaces. Zhang et al. (2015c) and Umoren (2016) suggested that an immersion time of 1 h was suitable for the electrochemical experiments since the E_corr curves reached a stable state in this timeframe.

In an inhibitor free system, the corrosion potential (E_corr) moves toward a more negative state and potentially has the lowest values due to the breakdown of the oxide film on carbon steel surfaces (Li et al. 2021; Migahed et al. 2017; Xiong et al. 2021). Munis et al. (Munis et al. 2020; Wang et al. 2022) proposed that the negative value of E_corr curves was the corrosive nature of the blank solution because of the absorption of negative chloride ions on the working electrode (WE) in a NH₄Cl medium. In an efficiently inhibited system, E_corr should move to more positive values (Li et al. 2021; Migahed et al. 2017; Umoren 2016; Wang et al. 2022) as shown in Figure 4. The difference in OCP values in the absence and presence of a particular inhibitor indicates that the carbon surface was covered and protected by the inhibitor film (Umoren 2016). Additionally, the E_corr shift in the inhibited system could also be used to predict inhibition type, e.g., anodic or cathodic, if the difference in displacement of E_corr with and without inhibitor is greater than 85 mV (Solomon et al. 2019b; Umoren 2016).

Figure 4:

OCP curves of corrosion inhibitors at different concentrations of the imidazoline derivative and benzimidazole (polypropylene glycol, PPG) (Umoren 2016). Reprinted with permission from Elsevier.

3.3 PDP and LPR studies

PDP and LPR measurements can be used to gain insights into the kinetics of anodic (metal dissolution) and cathodic reactions (hydrogen evolution) (Alhaffar et al. 2018). PDP studies are some of the most commonly used DC electrochemical methods in corrosion measurements (Telegdi et al. 2018). In PDP, a wide range of voltage is applied to a test electrode and the current response is monitored and recorded according to the oxidation or reduction reactions on electrode surface. As shown in Figure 5, the data presentation is called a polarisation curve, which is a plot of the applied potential and current density (I or log I) (Telegdi et al. 2018, Zhang et al. 2015d). This curve can be used to determine the values of various parameters such as E_corr, anodic and cathodic Tafel slopes (b_a, b_c), corrosion current density (I_corr) and inhibition efficiency (η%) (Pavithra et al. 2010; Wang et al. 2019). The I_corr value is usually reduced when imidazolines or imidazoline derivatives are added. Alteration of the peak position and Tafel slope should also be determined. With an inhibitor in the system, the polarisation curve is remarkably shifted to a more negative potential in comparison to the blank, which verifies its inhibition efficacy in rendering anodic and cathodic reactions (Zhang et al. 2012, 2007). In some cases, the shift in potential is more pronounced on the cathodic branch than the anodic branch. Here, the polarisation curve indicates that a particular inhibitor is predominantly under cathodic control (Solomon et al. 2019b).

Figure 5:

Potentiodynamic polarisation curves of pipeline steel in 15% HCl solution with the absence and presence of an inhibitor with various concentrations of polypropylene glycol (PPG) additives (Umoren 2016). Reprinted with permission from Elsevier.

The shape of Tafel curve is associated with b_a and b_c values. In the inhibited system, the shape of PDP curve significantly changes, implying that corrosion processes on steel surfaces are greatly affected by a given inhibitor (Zhang et al. 2015d). The change of b_a and b_c values can provide information on the corrosion inhibition mechanism(s) of imidazolines or imidazoline derivatives in protecting carbon steel surfaces (Munis et al. 2020; Zhang et al. 2015b). For example, the difference of b_a values obtained from the inhibited and uninhibited systems confirmed that 1-IM and 2-IM corrosion inhibitors underwent the kinetics processes of an anodic reaction (Zhang et al. 2015b). Solomon et al. (2019b) showed the change of b_c and b_a values in inhibited and uninhibited systems based on the utilisation of NTETD inhibitor. They found that the difference in b_c values was greater than that of b_a values as NTETD acted as a mixed type corrosion inhibitor with a predominant effect on cathodic-corrosion half reactions. When adding Gemini surfactant, the b_a value increased approximately two times due to an alteration of the anodic reaction progress (Zhang et al. 2012). Alternatively, Zhang et al. (2015b) reported that the b_c value could be unchanged if an inhibitor does not involve hydrogen production and the cathodic process in an electrochemical system.

The E_corr value in an inhibited system can be shifted to either a more cathodic side (toward negative value) or a more anodic side (less negative value) with respect to that of the blank solution. Similar to OCP studies, PDP curves can also be used to determine a cathodic or anodic inhibition when the shift in corrosion potential is over 85 mV (Munis et al. 2020; Zhang et al. 2015c). For instance, Munis et al. (2020) found that the difference between E_corr of blank and inhibited systems was 97 mV using an HDIE inhibitor. This implies that the HDIE functioned as a mixed-type inhibitor predominantly of the anodic reaction. Solomon et al. (2019a) reported that E_corr shifted toward more positive values using NIMP inhibitor. Consequently, NIMP had greater inhibitive influence on the cathodic corrosion half reactions than anodic corrosion half reactions. However, most imidazolines and imidazoline derivatives act as mixed-type corrosion inhibitors (Alhaffar et al. 2018; Migahed et al. 2017; Obike et al. 2018; Pavithra et al. 2010; Zhang et al. 2007, 2015d, Zheng et al. 2022).

The LPR technique provides a rapid corrosion analysis in which the WE is polarised by a very small potential perturbation of less than ±30 mV versus OCP. The change in the applied potential induces a change in current response between the counter and working electrodes. The polarisation resistance (R_p) is defined by the slope of the linear portion of a voltage versus current curve near the E_corr (Arya and Joseph 2021). This R_p value can be used to analyse the i_corr value of the metal through the Stern and Geary method (Angst and Büchler 2016; Law et al. 2000) Eq. (1):

The value of η (%) is obtained by the following equation Eq. (2):

The b_a and b_c coefficients can be obtained from a Tafel plot or estimated from experience with the testing system. In commercial instruments, the b values are an assumed value of about 18 mV (Agrawal 2001). The R_p value should increase in an inhibited solution compared to the blank test, implying formation of protective layer on carbon steel surfaces (Migahed et al. 2017; Solomon et al. 2019a). The trend of the R_p value as a function of the inhibition efficacy is the same as for PDP analysis (Alhaffar et al. 2018; Umoren 2016).

3.4 EIS studies

EIS is a useful technique to study the formation and destruction of inhibitor films and their corrosion protection mechanism(s) (Tan et al. 1996). Prior to measurements, the WE is immersed in a solution with and without inhibitors to insure a steady OCP. EIS information can be analysed using Nyquist and Bode plots. Figure 6 shows EIS plots including (a) Nyquist, (b) Bode modulus and (c) phase angle plots.

Figure 6:

Electrochemical impedance spectra including (a) Nyquist, (b) Bode modulus and (c) phase angle plots (Umoren 2016). Reprinted with permission from Elsevier.

As shown in Figure 6(a), a Nyquist plot displays a correlation between the real and imaginary parts of the impedance over a wide frequency range (Guo et al. 2021; He et al. 2014; Huang et al. 2016; Zhang et al. 2015d). A semi-circular plot is obtained in the high-frequency side of the impedance spectra, followed by a steep curve at the low-frequency side, corresponding to electrically conductive properties and electrode polarisation effect, respectively (Hadjichristov et al. 2021). An ideal Nyquist plot according to the principles of EIS should present a perfect semicircle. A real Nyquist plot of an inhibited system is commonly a distorted semicircle (Yoo et al. 2012). This distortion is a characteristic of solid electrodes due to physical phenomena such as surface roughness and inhomogeneities of solid electrodes as a result of corrosion processes (Alhaffar et al. 2018; Umoren 2016). The shape of Nyquist plots in the blank and the inhibited system are relatively similar because addition of inhibitors to a corrosive environment does not influence the corrosion mechanism (Alhaffar et al. 2018; Umoren 2016; Zhang et al. 2015d). However, the size of Nyquist impedance for the inhibited system is usually larger than that of the blank system as the charge transfer process can be retarded in the presence of an inhibitor film on carbon steel surfaces (Guo et al. 2021; Solomon et al. 2019b; Zhang et al. 2015d; Zheng et al. 2022).

The lack of frequency information in a Nyquist plot leads to a Bode plot (Poursaee 2014), which is an effective technique for EIS data presentation. A Bode plot is a graph of log frequency on the abscissa (Figure 6(b)), and a combination of the magnitude (log of the absolute value of the impedance (|Z|)) and the phase shift of the frequency response (Figure 6(c)). The performance of a corrosion inhibitor can be analysed from the Bode magnitude plot at low frequencies (He et al. 2014; Zhang et al. 2015d). A large impedance modulus obtained in an inhibited system directly relates to high corrosion protection (He et al. 2014; Zhang et al. 2015b). A Bode phase plot presents the phase shift (θ) on the ordinate versus log of frequency. This is a quick way to assess the formation of an inhibitor film on metal surfaces.

The presence of a time-constant (or frequency) distribution is usually modelled with a constant phase element (CPE). The impedance response for electrochemical systems typically reflects a distribution of reactivity that is commonly represented as a CPE in equivalent electrical circuits (Nkomo and Masia 2021). The number of CPEs can be determined from the number of phase peaks over a wide range of frequencies (Zhang et al. 2015b). For instance, Zhang et al. (2015c) showed phase angle plots obtained from a system containing imidazoline inhibitor in 0.5 M HCl. All the phase angle plots exhibited only one phase peak at a middle frequency.

Moreover, the change in phase angle indicates elimination of diffusion control (Alhaffar et al. 2018). Saviour et al. (Umoren 2016) found a wide phase angle peak showing the presence of more than two time constants in the absence and presence of inhibitor. Zhang et al. (2007) also found two time constants at two frequencies, about 10 and 1000 Hz, corresponding to the influence of the double-layer capacitance/charge transfer resistance and the relaxation process of an adsorbed inhibitor film, respectively. However, Solomon et al. (2019b) reported that one time constant is present in the blank system while double peaks or two time constants appear in inhibited systems due to the influence of film resistance as an additional element. Also, the magnitude of phase angles increases with the addition of imidazolines or imidazoline derivatives because of the surface coverage by the inhibitors molecules (He et al. 2014). This indicates the non-ideal behaviour of a capacitor with an increase in capacitance by the presence of interference, i.e., the adsorbed inhibitor molecules at the interface (Porcayo-Calderon et al. 2015). The imperfections within the capacitor’s material with the existing protective film to prevent corrosion of metal surface create resistance causing the capacitor to dissipate energy. As a result, the polarisation resistance (R_p) values should be increased while the double-layer capacitance (C_dl) values should be decreased with increasing the inhibitor dosage. This also leads to a decrease in the corrosion rate (Ouakki et al. 2019).

Interpretation of the electrochemical behaviour from EIS spectra requires an appropriate physical model that represents the electrochemical process and system. Solomon et al. (2019b) proposed equivalent circuit diagrams that can be perfectly fitted with impedance data in both the blank and inhibited systems. The blank system is fitted with a single time constant model consisting of the solution resistance (R_s), charge transfer resistance (R_ct) and constant phase element (CPE). The inhibited system is usually described by the two time constant model having a film resistance (R_f) as additional element. A CPE, a non-integer power that is dependent on frequency, is employed to describe the impedance behaviour of the electric double layer and is also used to calculate the value of double-layer capacitance (C_dl). The R_t and C_dl values can be analysed through data fitting of the impedance spectra (Zhang et al. 2007). The R_t values increase with addition of inhibitors, which leads to the film formation of imidazolines or imidazoline derivatives. This can be used to assess corrosion inhibition efficacy (He et al. 2014; Umoren 2016; Zhang et al. 2007). Alternatively, C_dl values are reduced as the surface coverage of inhibitor molecules replaces that of water molecules. Hence, the local dielectric constant and the thickness of adsorbed inhibitor film at the electrode interface becomes smaller (Zhang et al. 2007; Umoren 2016).

3.5 Weight loss testing

A weight loss technique is a simple and inexpensive method for determining the corrosion rate (CR) of carbon steel and corrosion inhibition efficacy. Weight loss measurements take much time (usually more than a week) to provide accurate results. However, multiple samples can be tested simultaneously (Pearson and Cousins 2016). Rahayu et al. (2021) suggested that long-term measurements weaken the interaction between metal surface and inhibitor molecules leading to an increased corrosion rate. Also, the behaviour and reaction type of inhibitor molecules cannot be obtained using this method. Normally, the weight loss of carbon steel sheet and the corresponding corrosion rate decrease with the addition of effective inhibitors. Furthermore, inhibition efficacy is improved with increased inhibitor dosage (Munis et al. 2020). Additionally, the corrosion inhibition efficacy depends strongly on film formation and surface coverage by inhibitor molecules (He et al. 2014).

The procedure involves an immersion test followed by weight loss measurement done to calculate the average CR value of a specimen in a well-defined environment over an extended period of time as expressed by the following equation (Vedalakshmi et al. 2009) Eq. (3):

where weight loss is in g, K = a conversion factor, i.e., 8.76 × 10⁴ (in mm/y), ρ = density of sample (g/cm³) (density of steel = 7.87 g/cm³), A = area of specimen (cm²) and t = time of exposure (h).

The results obtained from weight loss studies can be used to determine the relative corrosiveness of different operating parameters such as inhibitor type and dosage, steel type, vapour versus liquid phase and type of corrosive media (Pearson and Cousins 2016). Not only those operating variables but the CR value is also strongly influenced by the operating temperature. In principle, the CR value increases with temperature (Jevremović et al. 2013; Solomon et al. 2020; Zhang et al. 2013) due to a higher rate of corrosion reaction by corrosive species, e.g., CO₂. The operating temperature also causes an alteration of adsorption and desorption behaviours of inhibitor molecules to reach a dynamic equilibrium (Zhang et al. 2013). Solomon et al. (2020) published the CR results obtained under various temperatures together with the calculated corrosion kinetic parameters. For weight loss measurements, the duration of immersion was ranged between 72 and 168 h at various temperatures of 25–60 °C (Ansari et al. 2016; Geng et al. 2022; He et al. 2014; Munis et al. 2020; Solomon et al. 2019a,b; Xiong et al. 2021; Zhang et al. 2012, 2015d). A variety of weight loss data using imidazoline inhibitors was reported depending on the inhibitor dosage and temperature, for example, 0.76 mmpy (400 ppm, at 25 °C) (Solomon et al. 2019b), 0.016 mmpy (1.0 mM/L, at 25 °C) (Munis et al. 2020), 0.51 mmpy (500 ppm, at 25 °C) and 14.35 mmpy (500 ppm, at 60 °C) (Ansari et al. 2016).

3.6 Adsorption studies and corrosion inhibition mechanisms

The analysis of different adsorption isotherms (Guo et al. 2021; Migahed et al. 2017; Solomon et al. 2019a,b; Xiong et al. 2021; Zhang et al. 2015c,d) and computational simulations (Abd El-Lateef et al. 2021; Munis et al. 2020; Rodríguez-Valdez et al. 2006; Xiong et al. 2021; Zhang et al. 2015b) were studied in order to understand the adsorption behaviour of imidazolines and their derivatives on metal surface. Adsorption isotherms can be used to evaluate the strength of interaction between inhibitors and metal surface (Solomon et al. 2019b). The surface coverage (θ) value can be fitted into a Langmuir adsorption isotherm model through the general equation (Guo et al. 2021; Xiong et al. 2021; Zhang et al. 2015d) Eq. (4):

where θ is surface coverage inhibition efficiency (η) obtained from weight loss measurement (=η100), K_ads is an adsorption–desorption equilibrium constant and C_inh is the concentration of inhibitors. The Langmuir adsorption isotherm model is exploited to explain the adsorption characteristics of imidazolines and their derivatives (Guo et al. 2021; Xiong et al. 2021; Zhang et al. 2015c,d). The K_ads value indicates the bond strength between inhibitors and metal surface. High K_ads value means strong interaction (Solomon et al. 2019a) and high rate of inhibitor adsorption on the metal surface (Munis et al. 2020). The K_ads value also relates to the free energy of adsorption (ΔGads0) as shown by the following expression Eq. (5):

where R is an ideal gas constant (J mol⁻¹ K⁻¹), T is the thermodynamic temperature (K) and 55.5 is the molar concentration of water in solution (mol dm⁻³) (Xiong et al. 2021; Zhang et al. 2015b,c). The ΔGads0 value can be taken as a benchmark for comprehensive understanding of the adsorption mechanism(s) of imidazolines and their derivatives on metal surface. Normally, the ΔGads0 value is less negative than −20 kJ mol⁻¹ indicating that the physisorption mechanism or electrostatic interaction occurs between the inhibitor molecules and the metal surface. When the ΔGads0 value is more negative than −40 kJ mol⁻¹, the inhibitors are adsorbed on the metal surface by chemisorption mechanism. The adsorption mechanisms of imidazolines and their derivatives can be both via chemisorption and physisorption (Guo et al. 2021; Munis et al. 2020; Xiong et al. 2021; Zheng et al. 2022) and this infers spontaneous adsorption (He et al. 2014; Solomon et al. 2019a,b, Zhang et al. 2015c).

To understand the adsorption mechanisms of the imidazolines and their derivatives on the metal surface, quantum chemical was employed to describe fractional transfer of electrons from the inhibitor molecules to metal surface and the molecular orbital energies. The frontier orbital model (FOM) could be used to explain the electron transfer behaviour of inhibitor molecules through the lowest unoccupied molecular orbital (LUMO) energy level and the highest occupied molecular orbital (HOMO), indicating the capacity of a particular inhibitor molecule to accept and provide electrons, respectively. The higher HOMO energy (E_HUMO) value means that the imidazoline molecule donates electrons to the empty d-orbital of the metal more easily and this links to a strong susceptibility for nucleophilic attack. On the other hand, the lower LUMO energy (E_LUMO) value relates to the easier transfer of electrons from the metal surface to the imidazoline molecule or the electron-accepting properties of the inhibitor molecule (Zhang et al. 2015b). Zheng et al. (2022) discussed computational simulations of the optimised molecular structures of various inhibitors and the electronic distributions of HOMO and LUMO. It was suggested that the HOMO and the LUMO of imidazoline molecule distributed around the imidazoline ring indicating that the imidazoline molecule can accept electrons from the Fe atoms and can also transfer electrons to the Fe atoms. The low energy band gap (ΔE) is related to the stability of transition complex that determines the interaction between the inhibitor molecules and the metal surface. The low ΔE value is synonymous with high chemical reactivity and also high inhibition efficiency (Xiong et al. 2021). The imidazoline molecules have low ΔE value of around 2–4 eV (Munis et al. 2020; Zhang et al. 2015b,c) with additional studies on electrostatic potential (ESP) regions (Zheng et al. 2022) to fully explain the existing electrostatic interaction. Figure 7 illustrates ESP distributions of various imidazoline molecules in negative and positive regions as blue and red areas, respectively. Basically, the more positive ESP and more negative ESP regions refer to attraction of nucleophilic and electrophilic species, respectively. The results in Figure 7 show that several extreme points undergoing electrophilic (or nucleophilic) reactions with the Fe atoms are located around the imidazoline ring and the sulfhydryl (for imidazoline derivatives) regions (Zheng et al. 2022). Similarly, Geng et al. (2022) confirmed that two N atoms of the imidazoline ring were highly reactive towards electrophilic reactions (i.e., accept electrons from the Fe atoms).

Figure 7:

Electrostatic potential of (a) imidazoline and (b) imidazoline derivatives (Zheng et al. 2022). Reprinted with permission from Elsevier.

There are also various authors presenting the mechanisms of corrosion inhibition by imidazoline inhibitors. The corrosion inhibition mechanisms of imidazolines and their derivatives on the metal surface can be explained by physisorption and chemisorption. This is also related to the steel surface, which can appear as (i) normal metal surface and (ii) steel surface hydrated with chloride ions and excess negative charges. The imidazolines and their derivatives have three adsorption centres including N, O and the imidazoline ring (Solomon et al. 2019a). The O and N heteroatoms are protonated and then interact with the hydrated steel surface through physisorption mechanism (Solomon et al. 2019a). On the contrary, the electron pair from the imidazoline ring can be donated to the empty d-orbital of Fe resulting in the formation of covalent kind of bonding via chemisorption mechanism (Solomon et al. 2019a,b; Zheng et al. 2022). Zhang et al. (2015b) reported that the hydrated chloride ions were adsorbed on the electropositive carbon steel surface leading to excess negative charges raised from chloride-containing environment and hence enhanced the adsorption of cations. For example, imidazoline molecules could react with H⁺ to form N-onium ions (He et al. 2014) and then were electrostatically attracted to the negatively charged steel surface hydrated with chloride ions (Solomon et al. 2019b). This enabled the formation of a protective film of the protonated imidazoline molecules on the solid/liquid interface. In addition, the free N atoms existing on the imidazoline ring can directly bond on the Fe surface yielding a chemical adsorption monolayer (He et al. 2014). The long-chain alkyl group of imidazoline molecule forms a dense hydrophobic layer on the steel surface, which hinders the passage of corrosive species to the metal surface (Qian and Cheng 2019; Solomon et al. 2019b). Also, Munis et al. (2020) reported that the hydrophobic alkyl chain shaded the movement of Cl⁻ ions and electrons from reaching the metal surface. Consequently, the more compact and intact the imidazoline film is formed on the steel surface, the better inhibition efficacy it can be achieved through a combination of chemisorption and physisorption.

3.7 Surface characterisation

After corrosion testing, surface examination and surface chemical composition analysis are done using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and contact angle techniques. SEM is employed to confirm the formation of an inhibitor film on carbon steel surfaces by examining surface morphology at a microscopic level. Rough surface morphology is usually attained on carbon steel surfaces in a blank test because the surface is damaged in an acid solution, yielding deposition of corrosion products on the surface (Solomon et al. 2019b; Umoren 2016; Wang et al. 2019; Zhang et al. 2012). In an inhibited system, carbon steel exhibited fewer corroded surfaces (Alhaffar et al. 2018; Solomon et al. 2019b; Zhang et al. 2012) as shown in Figure 8. The EDX spectrum of a polished sample showed high peak intensity of Fe, which is the main constituent of carbon steel (Migahed et al. 2017). When soaking in a chloride media with no inhibitor, Munis et al. (2020) reported that the surface was rich in chloride ions because they reacted with Fe and produced soluble FeCl₃ as a corrosion product. Umoren (2016) found an increase in O atoms caused by the formation of an oxide layer on carbon steel surfaces. In contrast, the presence of inhibitors in the system causes a reduction of O atoms on a weight percent basis, assessed by EDX (Guo et al. 2021; Solomon et al. 2019a,b). Solomon et al. (2019b) believed that the inhibitor molecules were probably adsorbed on the surfaces and effectively protected them from further dissolution. They also observed a higher content of carbon atoms (in wt%) caused by adsorption of NTETD molecules on carbon steel surfaces. However, the N atom in the imidazoline ring could only be detected at some points over the entire metal surface as a specific deposit film spot (Solomon et al. 2019b). The inhibited system exhibits an increase in water contact angle that reveals the hydrophobicity of the surface. Moreover, the surface roughness decreases with the adsorption of inhibitor molecules (He et al. 2014; Zheng et al. 2022).

Figure 8:

SEM images of carbon steel samples tested in 1 M HCl solution with an absence or a presence of particular inhibitor: (a) the metal surface before corrosion test, (b) the metal surface after immersion with no inhibitor and (c) the metal surface after immersion containing 500 mg/L inhibitor (Zhang et al. 2012). Reprinted with permission from Wiley Online Library.

Many models that have been proposed to explain the corrosion inhibition mechanism(s) of imidazolines and imidazoline derivatives based on electrochemical results and surface characterisation. It is often not possible to define a single general mechanism of inhibition to an inhibitor because the mechanism may change with test conditions. Thus, the predominant mechanism(s) of imidazolines or imidazoline derivatives may vary with these parameters. Table 4 summarises all studies related to imidazoline inhibitors that cover preparation processes, corrosion inhibition measurements, metal types, corrosive media, inhibitor dosage and inhibition efficacy.