Polymeric Ionic Liquids Based on Benzimidazole Derivatives as Corrosion Inhibitors for X-65 Carbon Steel Deterioration in Acidic Aqueous Medium: Hydrogen Evolution and Adsorption Studies

Ionic liquids have significantly enhanced ecofriendly benefits compared to the traditional inhibitors. In the present work, new four polymeric ionic liquids based on benzoimidazole derivatives were synthesized through the reaction of 2-styryl-1H-benzo[d]imidazole with alkyl halide to form PIL1. Then, Cl– anions were exchanged with different anions through the neutralization reaction to form other investigated polymers. Their structures were chemically elucidated using Fourier transform infrared spectroscopy, 1H NMR, and 13C NMR. Their influence on carbon steel (CS) as corrosion inhibitors has been checked with dielectric spectroscopy in addition to potentiodynamic polarization curves. It was found that the percentage of inhibition efficiency increases as inhibitor’s concentrations increase, suggesting a decrease in the rate of CS corrosion. Additionally, the hydrogen evolution rate controlled by the four polymers was monitored. Addition of the prepared polymers lessened the rate of generation of hydrogen as the inhibitor’s concentrations augmented. Scanning electric electron microscopy in addition to energy-dispersive X-ray diffraction has proved the morphology of the CS surface as well as the formed protective film.


INTRODUCTION:
The utilization of carbon steel (CS) is extensively prevalent in the production as well as the transportation of both crude oil and natural gas because of its nominal price, simple production, and significant mechanical merits. Nevertheless, CS shows a high rate of corrosion. 1−6 Many of the corrosion inhibitors used in acidic media for CS corrosion control are particularly toxic and harmful to the environment. 7−9 As a consequence, in recent years, ionic liquids (ILs) have been designed and synthesized.
Ionic liquids' application has been considered as a novel green approach owing to their numerous interesting properties, for instance, low melting point (lower than 100°C), high polarization, low toxicity, insignificant vapor pressure (which means they do not evaporate and not pollute the environment), and thermal and chemical stability. Consequently, ILs reduced the harmful impact on the environment, and this makes them a perfect substitute for extremely volatile, traditional, harmful corrosion inhibitors. 10,11 Ionic liquids (ILs) are melted organic salts formed from both organic cations and several inorganic anions with countless functional groups. ILs own a large number of physicochemical properties, 12−15 essentially, nonflammability and enhanced ionic conductivity, electrical conductivity, and solvent transport, besides outstanding thermal and chemical steadiness. Generally, ILs have N, S, and P as the essential atoms of cations. Besides, most of these IL salts are established on imidazolium and pyridinium moieties as cations, whereas the characteristic anions are sulfonates, tetrafluoroborates, phosphates, and bis(triflouromethane-sulfonyl) imide. 16−18 Imidazolium compounds are stated to show anticorrosion performance on various metals and alloys such as aluminum, copper, mild steel, etc. Parveen et al. studied the corrosion inhibitive action of imidazolium-based ionic liquids in 1 M H 2 SO 4 on mild steel. 19 Likhanova and co-workers studied different imidazolium-type ionic liquids containing hexaflourophosphate as an anion in 1 M H 2 SO 4 , which have shown good efficiency as corrosion inhibitors of carbon steel. 20 Atta et al. studied the effect of different types of ammonium tosylate ionic liquids as corrosion inhibitors on a carbon steel surface in 1 M HCl, which show good anticorrosion properties. 21 The unique characteristics of ILs are the key for applying ILs in innovative and new applications. In general, ILs are considered as effective anticorrosion compounds for different metal surfaces owing to their elevated activity in acidic corrodent media. 22−24 The anticorrosion potential of polymeric ionic liquid (PIL) nanoparticles via thiol-ene photo-polymerization within a miniemulsion was disclosed by Taghavikish et al. 25 Atta et al. 26 have also investigated the boosted anticorrosion performance of a hyperbranched PIL. Furthermore, in our previous work, PILs based on chitosan derivatives 27 and acrylamides 28 have also been reported.
The present research was aimed to synthesize four polymeric ionic liquids based on benzimidazole derivatives. Their anticorrosion performance was tested for CS surface with lower concentrations in HCl (hydrochloric acid, 1 M concentration). The polymers under analysis were prepared through the reaction of 2-styryl-1H-benzo[d]imidazole with alkyl halide to form PIL1. Then, the Cl − anion was exchanged with different anions through the neutralization reaction to form other investigated polymers.
Their influence on carbon steel (CS) as corrosion inhibitors has been checked with dielectric spectroscopy (EIS), "Nyquist as well as bode plots", in addition to potentiodynamic polarization curves. Furthermore, the hydrogen evolution rate regulated by the prepared polymers was monitored. Scanning electron microscopy [SEM] in addition to energy-dispersive Xray diffraction (EDX) was used to examine the CS surface morphology to verify the defensive films formed.

RESULTS AND DISCUSSION
Polyionic liquid (PIL1) was synthesized through the reaction of 2-styryl-1H-benzo[d]imidazole (Sb1) with alkyl halide to form a monomer (IL1). Thereupon, a radical polymerization reaction transformed this monomer into an ionic liquid polymer (PIL1). The chloride ("Cl − ") anion of monomer IL1 was exchanged with different anions to form the monomers IL2, IL3, and IL4. The monomers were transformed to ionic liquid polymers (PIL2, PIL3, and PIL4) by the radical polymerization reaction. As a result, an anticorrosive film protecting carbon steel surfaces was formed. Scheme 1 describes the chemically elucidated structure of the polymeric ionic benzimidazole liquids.
2.1. PILs' Characterization. The Fourier transform infrared (FTIR) spectrum of PIL1 is shown in Figure 1a. The signals at 3057.23 and 3022.47 cm −1 are assigned to the stretching of aromatic hydrogens. The signals at 2922.64 and 2852.58 cm −1 are allocated to asymmetric and symmetric C− H stretching, respectively.
The bands at 1636.55, 1595.61, 1551.11, and 1383.18 cm −1 are assigned to the CN stretching band, aromatic ring's "CC" stretching, aromatic ring's C-C stretching, and C-N vibrational stretching, respectively. However, the strong peak at 711 cm −1 is assigned to the C-H bending of the (CH 2 ) n skeleton. The polymerization reaction has been ascertained through the disappearing of the characteristic vinyl band (C-H, out-of-plane bending) at 985 cm −1 . Figure 1b displays the 1 H NMR spectrum of PIL1. Lack of CCH signals within the range of 5−6.5 ascertains the occurrence of polymerization. Furthermore, signals were displayed at chemical shifts of 0.86 ppm (t, 6H, (CH 2 ) 11 -CH 3 ), 1.25 (t, 44H, (CH 2 ) 11 -CH 3 ), 1.71 (t, 4H, N-CH 2 -CH 2 ), and 3.2 (1H, NC-CH-CH-Ph) of polymerized hydrogens, 3.6 (t, 4H, N-CH 2 -CH 2 ), and 7.23−7.87 (m, 9H, Ar H). Figure 1c displays the 13 C NMR spectrum of PIL1. It indicates The FTIR spectrum of PIL2 is shown in S1(a). The bands at 2921.77 and 2852.72 cm −1 are allocated to asymmetrical and symmetrical C−H stretching, respectively. The bands at 1711, 1639.74, 1596.86, 1547.73, 1391.90, and 1306.31 cm −1 are assigned to the carbonyl CO ester stretching, CN stretching, aromatic ring's "CC" stretching, aromatic ring's C-C stretching, C-N vibrational stretching, and C-O vibrational stretching, respectively. However, the strong band at 718.63 cm −1 is assigned to the C-H bending of skeletal (CH 2 ) n . The absence of the 985 cm −1 band for the vinyl group (CH out-of-plane bending) ascertains the occurrence of polymerization. S1(b) illustrates the 1 H NMR spectrum of PIL2. Lack of CCH signals within the range of 5 to 6.5 ascertains the occurrence of polymerization. Moreover, chemical shifts' signals are obvious at 0.86 ppm (t, 6H, (CH 2 ) 11 -CH 3 ), 1.25 (t, 44H, (CH 2 ) 11 -CH 3 ), 1.71 (t, 4H, N-CH 2 -CH 2 ), 3.2 (t, 1H, NC-CH-CH-Ph) of polymerized hydrogens, 3.6 (t, 4H, N-CH 2 -CH 2 ), and 7.08−7.86 (m, 14H, Ar H). S1(c) displays the 13  2.2. Hydrogen Evolution Reaction (HER) Quantification. Figures 2 and S4 illustrate the volume of H 2 gas generated from CS corrosion in a 1 M HCl solution with time when PIL inhibitors are absent and also in their presence. A significant increase in H 2 was obvious as the immersion period increased. The hydrogen generation rate (H r ) was calculated using eq 1.
In addition, adding several concentrations of PIL1, PIL2, PIL3, and PIL4 disturbs the hydrogen generation rates (H r ) as

ACS Omega
http://pubs.acs.org/journal/acsodf Article can be observed in Figure 3, since as PILs' concentrations increase, H r decreases.
Equation 2 was used to assess the efficiency (I H %) of PILs for regulating H 2 evolution. Figure 4 demonstrates the inhibitors' efficiency (I H %) plotted versus logarithmic PILs' concentrations. It has been concluded that the effect of PILs on inhibition increases as their concentrations increase. In fact, PIL1 > PIL2 > PIL3 > PIL4 was the order of the inhibitors' anticorrosion effectiveness.
These PIL inhibitors inhibit the CS dissociation in HCl and, thus, delay and obstruct the cathodic H 2 generation reaction through adsorption at the metal/acidic solution interface. The strength of the prepared PILs to inhibit the hydrogen evolution is significantly influenced by the inhibitors' chemical structures. 30 Inhibitors of PIL type are capable of creating a thin film on the CS surface. The heteroatoms (predominantly N and O) transfer their electrons (charges) to the metal's dorbitals. Then, they form a strong shielding cover on the metal through forming coordinate bonds (called the chemisorption mechanism). Also, the existence of homoatomic ">CC<" or heteroatomic ">CO, > CN−" multiple bonds enriches the ability of the inhibitor molecules to be adsorbed by improving the electron donating tendency resulting from extensive conjugation.
Throughout the metal−inhibitor interactions, the negatively charged metallic surface (due to adsorption of counterions of the electrolytes) attracted the positively charged ( + N) PIL inhibitor molecules via electrostatic attractions. 1 These electrostatic attractions showed that, in an acidic electrolyte, interaction of inhibitor molecules (having heteroatoms) with metal surfaces includes a physisorption mechanism. Then, it was followed by a chemisorption mechanism in the final interaction stage.
There is a synergistic effect between the cation and the anion of the corrosion inhibitors. PIL1 offers high corrosion resistance as compared to other synthesized PILs because of the presence of chloride ions. The presence of halide ions assists the adsorption of organic inhibitors through forming intermediate bridges between positively charged inhibitor molecules and the carbon steel surface. Consequently, corrosion inhibition synergism results from increasing surface coverage as a result of ion-pair interactions between the organic cation and the anion. 31 2.3. Potentiodynamic Polarization (PDP) Measurements. Steel electrochemical polarization curves attained in 1 M HCl solution with and without different PIL inhibitor concentrations are shown in Figures 5 and S5. Dwindling was observed in anodic along with cathodic currents in the case when the inhibitor was present. The decline became more apparent at higher inhibitor concentrations. The arrangement of a protective overlay to protect the steel surface against the corrosion medium may be the logical reason for the apparent decline, since inhibitor's adsorption on the steel surface minimizes the hydrogen evolution [cathode's reaction] as well as minifies iron metal deterioration [the reaction at the anode].
Parameters such as the corrosion potential (E corr ) and corrosion current density (I corr ) in addition to Tafel slopes of the cathode (βc) and anode (βa) have been extracted from the     Table 1.
The degree of surface coverage (θ) and the inhibition efficacy (IE%) are computed through eqs 1 and 2, respectively. 32−34 in which I cor(1) and I cor (2) are the corrosion current densities in the absence and presence of the inhibitor, respectively. Table 1 lists the IE percentages with increasing inhibitor levels. It has been shown that IE percentages increased as the inhibitor's concentrations were augmented, due to an increase in the amount of inhibitors' accumulation along with adsorption on the steel surface. This process thus leads to a high inhibition of corrosion. At 250 ppm concentration of PIL1, the utmost efficient inhibition was approximately 99.3%.
The use of different techniques may be the logical reason for the dissimilar values of the IE% obtained from the potentiodynamic polarization technique and the hydrogen evolution technique. Furthermore, the inhibitor may be named anodic or cathodic if the value of E corr surpasses 85 mV. Table   1 shows the various values of E corr with an utmost E corr value of less than 85 mV indicating the mixed corrosion mode (disrupting both the anodic and the cathodic reactions together). 35     . On the whole, corrosion is related to the behavior of the double layer. Therefore, action of inhibitors includes their arrangement as well as adsorption through replacing aquatic molecules from the interface between steel and the corroding medium. 39 As the inhibitor concentrations augmented, both R ct and IE % values increased, as observed in Figures 6 and S7 and also in Table 2. The effectiveness of the inhibition (IE%) can be computed through utilizing eq 3. 40−42 in which R ct 1 and R ct 2 are the charge transfer resistance in the uninhibited and inhibited solutions, respectively.
In impedances, R ct values were determined through variation at lesser and higher frequencies. The use of different techniques may be the logical reason for the dissimilar values of the IE% obtained from the potentiodynamic impedance technique and the hydrogen evolution technique. At a concentration of 250 ppm of PIL1, extreme efficacy (96.85%) inhibition was achieved. The utmost inhibition efficacy of 96.85% was achieved at 250 ppm of the PIL1 inhibitor.
The Bode as well as phase angle graphs for CS in hydrochloric acid whose concentration is 1 M alone as well as in the presence of PILs are shown in Figures 7 and S8. A spectrum of frequency was implemented for the Bode phase plot to explain and also clarify the enhanced phenomena occurring at the interfaces. A phase angle has been employed at higher frequencies to give an overall indication of the inhibition efficiency. The phase angle at high frequencies was applied to get an overall indication for the inhibitory efficiency. The phase angle of −90°is well known to have a perfect capacitive action.
The regular accretion in the phase angle shift near the effective capacitive action as the inhibitor concentrations increased is obvious in Figures 7 and S8. 43,44 The absolute impedance increased in the Bode plot at lower frequencies. This emphasizes that the formed protective overlay with the increase in the amount of the inhibitor is accompanied by the inhibitors' adsorption impact on the CS surface.
2.5. Surface Morphology Examination. To observe the morphology of the CS surface, scanning electron microscopy (SEM) has been performed. Additionally, energy-dispersive Xray (EDX) spectrometry was carried out to identify the composition of the CS surface elements prior to and after immersion of the inhibitor in the corroding medium. Figure 8a depicts the EDX bands for the adsorbed elements on the CS surface in the case of blank solution. Signals of O and Fe prove that iron oxide is present in the solution, resulting from metal dissolution on the anodic reaction. Additionally, after dipping into HCl solution (1 M) with no inhibitor (blank), the SEM picture is also illustrated in Figure 8a. A coarse and heavily corroded surface has been observed with total destruction on the CS surface.
In Figure 8b, on adding 250 ppm PIL1 inhibitor, the EDX spectrum displays additional signals, approving the existence of C and N atoms in the PIL1 inhibitor. Moreover, the signals of Fe are significantly inhibited, compared to those of the samples in Figure 8a, because of formation of a defensive inhibitor film. The SEM picture in Figure 8b displays a perfect reduction in the rusted zones produced by the inhibitor molecules being adsorbed on the CS surface. In this way, a shielding film was formed on the CS surface but not on the CS surface dipped into the aggressive corroding media lacking the inhibitors. EDX and SEM examinations ascertain the growth of an inhibitive film on the CS surface and, hence, the Fe dissolution was inhibited and the hydrogen gas evolved, resulting from corrosion, was hindered.  ACS Omega http://pubs.acs.org/journal/acsodf Article 2.6. Corrosion Inhibition Mechanism. The pictorial diagram for the inhibition mechanism on carbon steel surface in 1 M HCl can be seen in Figure 9. The anion improves the adsorption ability of the organic cation by forming a connecting bridge between the negatively charged metal surface and the organic inhibitor and synergistically increases the corrosion inhibition ability of organic compounds significantly. 31 The adsorption of anions makes the CS surface negatively charged, as a result making it easier for organic cations to be adsorbed on the CS surface by electron interactions. This is the physical adsorption.
Chemically, the inhibitor molecule directly reacts with the CS surface to form a coordination bond, and this process is called chemical adsorption. As a result, a defensive film is In conclusion, ionic liquids offer a potential opportunity for pioneering applications for green chemistry. Unlimited growth in this field is expected due to the outstandingly superior, ecofriendly, and sustainable benefits of these compounds compared to conventional and known inhibitors. The mixture was refluxed for 6 h to produce IL2. Benzoyl peroxide (0.5 wt % monomer) was then introduced to the refluxed mixture, and subsequently, the temperature was increased to 70°C and maintained for about 10 h. Afterward, the reaction mixture was cooled and acetone (250 mL) was added. The precipitate thus formed was filtered off, washed with absolute methanol as well as diethyl ether, and finally dried at 20°C under vacuum. The formed polymer was labeled as PIL2. The same procedure was used to synthesize PIL3 and PIL4 using sodium formate and m-amino sodium benzoate, respectively.
4.3. Spectroscopic Assessments. The molecular structure of the prepared ionic liquid polymers was assessed through analyses like infrared (IR) as well as 1 H NMR, in addition to 13 C NMR spectra. Infrared (IR) analysis was performed utilizing a Fourier transform infrared spectrophotometer, FTIR, Bruker. The spectrometer [Bruker Advance DRX-400] along with the solvent DMSO-d 6 was utilized for 13 C NMR and 1 H NMR analyses. The spectrometer has a resonance frequency of 400 MHz.
4.4. Hydrogen Evolution Reaction (HER) Quantification. Water replacement is the technique used to estimate the rate of hydrogen evolution. This method aimed to quantify hydrogen evolution is parallel to that described earlier. 45,46 First, the corroding medium (1 M HCl), 100 mL, was put into a glass container. A CS coupon, having dimensions of 3.5 cm × 2.5 cm × 2 mm, was immersed within the corroding medium. This vessel was immediately locked up to avoid the leakage of H 2 gas. Thereupon, the amount of H 2 gas produced was recorded at almost fixed times during the corrosion reaction. The hydrogen gas volume was measured on the basis of the fact that the gas volume (in cm 3 ) replaces the water level in the burette.
The following expression 47 was used to evaluate the hydrogen generation rate (H r ) where v 2 and v 1 are the volumes of H 2 gas produced at t 2 and t 1 time intervals, respectively. Additionally, the efficacy of PILs (I H %) for governing hydrogen gas production was revealed using the following equation in which H ro and H r are the rates of evolution of hydrogen in the absence and presence of the readily prepared PILs, respectively. 4.5. Electrochemical Measurements. Potentiostat PGZ 402 [Voltalab 80 Tacussel Radiometer] was run to perform the electrochemical measurements. The Voltamaster-4 program was used to perform these measurements. A 100 mL electrochemical glass cell with 3 electrodes' spaces has been used. The electrochemical cell was filled with the corroding medium (100 mL of 1 M HCl). The carbon steel was the working electrode (WE).
A saturated calomel electrode [SCE] as the reference electrode and a platinum [Pt] electrode as an auxiliary electrode were utilized.
Moreover, the SCE was connected to a Luggin capillary. The capillary slope was made adjacent to the WE surface to lessen the potential drop (IR drop). All potential values were quantified versus SCE. Prior to all tests, the CS electrode's surface was hand-glazed with various specific emery sheets, subsequently washed with distilled water, and ultimately dried. After keeping for an hour in the test solution, the electrode potential was stabilized to maintain a steady-state "open-circuit potential". An electrode area of 1 cm 2 was exposed to the devastating media. The whole steps function at ambient temperature and exposed the electrochemical cell to air. 4.6. Surface Morphology Studies. For this research, a scanning electron microscopy (SEM) instrument of model Quanta 250 FEG and an EDX (energy-dispersive X-ray diffraction) instrument were employed.
The applied accelerating voltage was 30 kV, and the magnification force was X = 2000. The surface morphological properties were tested through dipping the carbon steel (CS) coupon in the blank solution as well as in the inhibitor solution containing certain concentration of the prepared inhibitor.