Effectiveness of Some Novel Ionic Liquids on Mild Steel Corrosion Protection in Acidic Environment: Experimental and Theoretical Inspections

Three ionic liquids (ILs)—1-butyl-1-methyl-pyrrolidinium Imidazolate (BMPyrIM), 1-butyl-3-methyl-imidazolium Imidazolate (BMImIM), and bis(1-butyl-3-methyl-imidazolium Imidazolate) (BBMImIM)—were synthesized and examined experimentally and theoretically as potential inhibitors for mild steel corrosion in HCl (1.0 M) solution. To our knowledge, two of the ILs successfully synthesized in our laboratory named BMPyrIM and BBMImIM are novel. Different electrochemical (potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS)), surface and structural (scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), Atomic force microscopy (AFM) and Fourier Transform Infrared Spectroscopy (FTIR)) and theoretical (Density functional theory (DFT)) techniques were utilized to confirm their use as efficient environmentally safe inhibitors. These ionic liquids were designed to study the cation effect (imidazolium and pyrrolidinium) and the dimeric effect of the imidazolium-based IL. A pronounced inhibiting effect was recorded using the optimum concentration (5 × 10−3 M) of BBMImIM with protection efficiency of 98.6% compared to 94.3% and 92.4% for BMImIM and BMPyrIM, respectively. The investigated ILs act as a mixed-type corrosion inhibitors and their protection obeys Langmuir adsorption isotherm. The results obtained by SEM, EDS and AFM confirmed the mild steel protection by the formation of protective film of the ILs on the steel surface resulted in less damaged surfaces compared with the blank solution. Furthermore, quantum chemical calculations illustrated the electronic structure of the investigated ILs and their optimized adsorptiοn configurations on mild steel surface. The findings from the different techniques helped to provide a supported interpretation of the inhibition mechanism.


Introduction
Corrosion plays a tremendous and significant role in various industries and, therefore, it causes huge economic losses worldwide [1]. Steel corrosion is commonly acknowledged to have a harmful effect on the structure durability and the mechanical operations. Acidic solutions such as hydrochloric acid (HCl) are extensively used for mild-steel pickling, cleaning, and descaling. Corrosion of mild steel is one of the most significant issues in industry. Corrosion inhibitors can effectively reduce corrosion which suppresses the high rate of metal dissolution [2,3]. These inhibitors form a protective film on metal surfaces to shield the metal from the corrosive solutions and, thus, inhibit corrosion [4]. A good inhibitor should have a great affinity to be adsorbed on the metal surfaces and hence inhibit it through strong binding with the metallic d-orbitals [5,6]. It is well known that π electrons; aromatic rings; heteroatoms, such as N, O, and S; and polar functional units such as -OH, -NH 2 , and -SH could significantly improve corrosion inhibition efficiency [7,8]. One of the easiest, most effective and economical protocols is the use of organic inhibitors; however, it water, and air dried. The experiments were carried out in 1.0 M HCl using (HCl 32%) bought from MERCK CHEMICALS and diluted using bi-distilled water.

BBMImIM, BMImIM and BMPyrIM Ionic Liquids
The investigated ionic liquids (BBMImIM, BMImIM and BMPyrIM) were prepared according to a previously described and established procedure [29,30]. For the synthesis of BMImIM and BMPyrIM, 4.12 moles of 1-methylimidazole or 1-methyl-pyrrolidine were dissolved in 295 mL of dry acetonitrile in a two-necked round-bottomed dry flask. To this flask, 1-bromobutane (4.13 moles) was dropped wisely with stirring for 1 h under argon at 0 • C. The reaction mixtures were then kept at room temperature for further 30 min, followed by raising the temperature to 70 • C for 24 h. After cooling, the reaction mixture was concentrated under vacuum till dryness followed by washing with dry cold acetone and ethyl acetate and concentration in vacuo till dryness until BMImBr or BMPyrBr was obtained. A methanol solution of NaOH was placed in a flask and stirred followed by adding an equimolar amount of imidazole at room temperature. After 1 hr, an equal mole of BMImBr or BMPyrBr was added to the mixture. Then, an appropriate amount of anhydrous ether was added three hours later with vigorous stirring at room temperature for 48 h. The reaction mixture was then filtered, washed with anhydrous ether and the filtrates were concentrated in vacuo until dry to yield the desired BMImIM and BMPyrIM ionic liquids. Mover, for synthesis of BBMImIM, 4.12 moles of 1-methylimidazole and excess 1,4-dibromobutane (4.2 moles) were stirred with 200 mL acetonitrile in round 500 mL round-flask at 0 • C for 24 h. The excess of solvent was then removed using a vacuum, and the obtained solid was then washed three times by acetone, followed by vacuum-dried to yield bis(1-butyl-3-methyl-imidazolium bromide) salt (BBMImBr). Then, 1.2 moles of BBMImBr in a 250 mL single-neck flask was dissolved in 50 mL methanol. To this solution, 2.4 moles of imidazole was added dropwise with stirring at room temperature for 48 h. The reaction mixture was then filtered, washed with anhydrous ether and the filtrates were concentrated in vacuo until dry to afford the desired BBMImIM IL.
The name, molecular formula, and molar mass of the examined ILs are included in Table 1.

Electrochemical Measurements
The electrochemical cell is made up of three electrodes: 1 cm 2 Mild-steel, saturated calomel electrode (SCE), and 1 cm 2 platinum foil, which serve as working, reference, and counter electrodes, respectively. PDP plots were automatically recorded at a scan rate (1 mVs −1 ) in the potential range of (−0.25 to 0.25 V SCE ) of the open circuit potential (OCP) and the Stern-Geary method [31] was used to calculate the corrosion current density (i corr ). EIS tests were carried out in a three-electrode cell at 25 • C in the frequency range (100 kHz to 0.1 Hz) and peak to peak amplitude of 10 mV using alternating current (AC) signals and the spectra were documented after 20 min of immersing in the solution under study. All the electrochemical measurements were performed by 2273 PARSTAT electrochemical workstation Potentiostat (PowerSuite, Ametek Scientific Instrument, Elancourt, France).

Quantum Chemical Calculation
Quantum chemical calculations were achieved using Materials Studio Modeling 7.0 software Accelrys Co. The ionic liquids structures were fully adjusted by DMol 3 module (version 2016) permitting the solvation effect management via COSMO controls. We used an ab initio quantum approach using double numeric plus polarization (DNP) and becke one parameter (BOP) basis set.

Potentiodynamic Polarization Measurements (PDP)
The PDP plots of Mild-steel alloy in aerated 1.0 M HCl solution in the absence and with different concentrations of BBMImIM are shown in Figure 1. Similar curves in the presence of BMImIM and BMPyrIM were also recorded but not shown. The measurements were carried out after 20 min to reach a steady state potential at a scan rate of 1.00 mVs −1 . Results of corrosion potential (E corr ) and i corr obtained by extrapolating the cathodic (β c ) and anodic (β a ) Tafel slopes are presented in Table 2. The obtained results demonstrate that in the existence of 1M HCl solution free from ILs, the corrosion current density is relatively high in the cathodic part due to hydrogen evolution reaction. In case of using BBMImIM as an inhibitor, the anodic and cathodic polarization plots were markedly altered to more noble potentials compared to bare solution. The shift in corrosion potential is also increased by increasing the BBMImIM concentration. Moreover, the i corr is decreased by raising the IL concentration as presented in Table 2.  The inhibition efficacy of the examined ILs to control the mild steel dissolution was estimated from the polarization curves by the following equation [32]: where j corr and j corr are the corrosion current density (µA cm −2 ) with and without the examined ILs, respectively. Using Equation (1) and according to the data in Table 2, the inhibition efficacy raises upon increasing the IL concentration. The greatest inhibition efficacies obtained for each of the studied inhibitors were 98.62%, 94.33% and 92.32% in presence of 5 × 10 −3 M of BBMImIM, BMImIM and BMPyrIM, respectively. The results confirmed that the investigated ILs have an impact on both anodic steel dissolving and cathodic hydrogenevolution processes which confirm that the explored ILs are effective inhibitors for steel dissolution in the acidic solutions.

Adsorption Isotherm
To define the adsorption type of the synthesized ionic liquids on the mild steel interface, several isotherms (Frumkin, Freundlich, Langmuir, and Temkin) were used. In our work, Langmuir isotherm has exhibited an outstanding match based on regression coefficient values that were near unity. The θ results estimated from the PDP methodology were evaluated using the Langmuir isotherm [33], which could be expressed as: where C denotes the IL concentration while K ads is the constant of adsorption-desorption. Figure 2 displays a linear plot of C/θ vs. C according to the Langmuir-isotherm with a regression constant (R 2 ) and slope close to unity ( Table 3). The intercept of the plot was used to obtain the K ads values for the examined ILs, whereas the standard free energy (G ads ) values were produced as follows [34]: where 55.5 denotes H 2 O concentration in solution, T is the absolute temperature and R is the gas constant. The greater values of K ads given in Table 3 for the examined ILs manifest the significant interaction of the ILs molecules with the mild steel interface. Compared to the other ILs, BBMImIM showed the highest K ads value that confirms its best inhibition effectiveness. Furthermore, the negative ∆G ads values presented in Table 3 reveal the spontaneous development of the studied ILs on the Mild steel interface [35]. All the estimated ∆G ads values were between −20 kJ mol −1 and −40 kJ mol −1 indicating that the interaction among the ILs and the steel interface included both physical and chemical adsorption. The physical adsorption depends on the electrostatic interaction among the charged surface of mild steel with the charged molecules of ILs [36]. While the chemical adsorption was accompanied to the removal of H 2 O molecules from the metallic interface [37].

EIS Study
EIS test was utilized to validate the electrochemical data gathered using the PDP approach. Figure 3 represents the Nyquist curves of mild steel corrosion in 1.0 M HCl medium without and in the existence of diverse concentrations of BBMImIM (comparable plots for BMImIM and BMPyrIM are not shown). It is obvious from these figures that the Nyquist plots depict depressed semicircle which may attributed to the roughness produced from the alloy surface and the adsorption of ILs resulted in the formation of porous layers [38]. When Nyquist plots were analyzed electrochemically, the difference among real impedance at higher and lower frequencies generates charge transfer. Furthermore, following the addition of the ILs, the diameters of the semicircles of Nyquist plots showed a significant increase than in the absence of the ILs, and the diameters were steadily enlarged by raising the BBMImIM concentration.  These findings show that the dissolution rate of mild steel surfaces in the existence of ILs is pronouncedly reduced when the corrosive action of acidic solution is controlled. This is owing to the formation of a protecting layer on the steel interface which effectively suppressed its corrosion [39].
The designed circuit for fitting the EIS results is shown in Figure 4, that was successfully utilized to assess EIS data for mild steel in HCl solution [40]. The R s , R ct and CPE represent the solution resistance, charge transfer resistance and the double layer capacitance, respectively. It is essential to use CPE as a replacement for capacitance due to metal/inhibitor interactions and considering inhomogeneity and roughness. The CPE impedance was obtained as follows [41]: where Y 0 is the magnitude of CPE and n signifies its exponent, j is the imaginary number and ω is the angular frequency.  The values of n could be used to determine the topology of the deteriorated metal surfaces in corrosive medium, with a larger value indicating better surface smoothness. The value of n ranges from 0.0 to 1.0. When this number near unity, it indicates that the capacitor is in an ideal condition and the metal interface is highly homogenous. If n value is 0, the CPE signifies a resistive behavior but, as n = 1/2 and Y 0 = W, this indicates Warburg impedance.
The inhibition efficiency (η EIS ) for the corrosion of mild steel in HCl can be obtained from the R ct values as follow [42]: where R ct and R ct(inh) represent the charge-transfer resistances in the uninhibited and the inhibited solutions, respectively. The recorded EIS data are displayed in Table 4. It is evident that the charge transfer resistance values have clearly improved as the BBMImIM concentration is raised, whereas the C dl values decreased. This shows that the adsorbed IL molecules are gradually increased on the mild steel surface, resulting in a decrease in the active sites necessary for the dissolving reaction. The rise in R ct indicates a significant limitation of charge transfer through the protective layer with the adsorbed ILs on the metallic surface, reflecting the high resistance to acidic attack and, as a result, inhibiting the dissolution process. Moreover, the C dl values were diminished owing to a drop in the dielectric constant or the improvement in the electrical double layer thickness attributed to the ILs adsorption on the steel surface [43].
The Bode plots for mild steel in 1.0 M HCl solution with and without various concentrations of BBMImIM are presented in Figure 5. From this figure, it is obvious that the existence of the studied ILs resulted in raising the phase angle in comparison with the blank solution. The higher value of the phase angle in the presence of BBMImIM confirmed the low roughness of the steel-surface as well as the effective adsorption of the IL on the surface with protective layer growth. In addition, the presence of BBMImIM showed higher resistance at the low frequency region and it was increasing with the rise of BBMImIM concentration. In summary, the findings recorded from PDP method agreed with the EIS results, approving the validity of both approaches for assessing the tested ILs.  Figure 6A) was degraded, rough and porous with the evidence of dense film of corrosion products due to the steel degradation in the corrosive medium. In contrast, the addition of the examined BMPyrIM, BMImIM and BBMImIM ILs in the acidic media, considerably inhibited the mild steel surface and resulted in a smoother surface as shown in Figure 6B-D). The steel surface was smoother in the existence of BBMImIM compared to BMImIM and BMPyrIM, respectively. Consequently, BBMImIM, BMPyrIM and BMImIM can obviously suppress the aggressive attack of the HCl solution along with the development of a protective layer on the mild steel surface that resulted in a pronouncedly high inhibition from dissolution.

Atomic Force Microscopy Analysis
AFM analysis is regarded as a crucial characterization technology for investigating the morphology of the metallic surface and evaluating the dissolution process that happens at the metal/electrolyte contact. Figure 8A-D display the 2d and 3d AFM images of mild steel dipped in 1.0 M HCl for 24 h in the absence and existence of the optimum concentration (5 × 10 −3 M) of the investigated ILs (BMPyrIM, BMImIM and BBMImIM). Apparently, the AFM images collected in the absence of the investigated ILs revealed a damaged and rough surface with various pits caused by the aggressive ions action ( Figure 8A). In addition, reasonably smoother surfaces were achieved when the studied BMPyrIM, BMImIM and BBMImIM ILs were used, as displayed in Figure 8B-D, respectively. These observations obviously reveal that mild steel degradation might be controlled and reduced. Furthermore, the addition of the examined ILs showed the creation of protective thin layer on the steel surface with the following inhibition trend: BBMImIM > BMImIM > BMPyrIM.

ATR-FTIR Study
The ATR-FTIR spectra of pure BBMImIM, BMImIM and BMPyrIM ionic liquids and the adsorbed ILs on mild steel surface in the range from 4000 to 400 cm −1 are presented in Figure 9. The peak around 3600-3200 cm −1 is assigned to the vibrational −OH from water, whereas the peaks between 3200 and 3100 cm −1 are assigned to the C-H stretching vibrations of the imidazole and pyrrolydinium rings [44]. The bands obtained among 3000 and 2800 cm −1 are matched to the C-H stretching vibrations of the alkyl groups attached to the imidazolium and pyrrolydinium rings. The peaks at 1650-1500 cm −1 and 1451 cm −1 are characteristics to the skeletal vibration of the rings [45], while the peak at about 1253 cm −1 assigned to symmetric C-H vibration of imidazolium and pyrrolydinium rings. Furthermore, the peaks at about 1168 and 827 cm −1 are corresponded to in-plane C-H bending vibration peaks of imidazolium ring whereas the peak at 623 cm −1 is the characteristic of the CH 2 plane rocking vibration of the alkyl chain.  From the analysis of the FTIR spectra, it is clear that there are nearly similar peaks in the same region of spectrums in case of the adsorbed layer on the mild steel surface compared with the pure IR spectrum of the corresponding IL. There is a little shift and decease in the intensity of the bands owing to the chemical interaction among the ILs and the steel surface. These findings suggest the adsorption of the examined ILs on the steel surface confirming their pronounced protection efficiency.

Quantum Chemical Study
As per Frontier molecular orbital theory (FMO), the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) can be readily associated to the inhibitor's protective efficacy [46]. Therefore, using inhibitors with high E HOMO and low E LUMO values enhances the corrosion protection performance. Because E HOMO is connected to the inhibitor's potential for donating electrons, raising E HOMO values makes it simpler for electron donation to the metal's vacant d-orbital. While E LUMO is related to a molecule's capacity to receive electrons. Consequently, the smaller E LUMO value, the higher probability a molecule is to receive electrons. As a result, an inhibitor's inhibition efficacy typically rises as E HOMO values increase and E LUMO values diminish. Additionally, a reduced energy difference among LUMO and HOMO energy (∆E) for an ionic liquid suggests its high deterioration protection efficacy.
The BBMImIM ionic liquid showed the largest value of E HOMO in comparison with the BMImIM and BMPyrIM as revealed in Table 5. As presented in Figure 10, the HOMO level for the BBMImIM IL was clearly positioned along the imidazolyl ring, indicating that the nitrogen atoms are the desired place for electrophilic attacks on the steel interface. This will endorse the BBMImIM potential to be adsorbed on the metallic surface and boost the protection effectiveness, which is consistent with the experimental findings. Furthermore, the smaller E LUMO value for BBMImIM fits the results well, as shown in Table 5. The energy gap (∆E) is vital to validate the theoretical capability of the IL structure for the corrosion prevention that rises as the ∆E is decreased [47]. From Table 5, BBMImIM showed the lowest ∆E value compared to BMImIM and BMPyrIM, which confirms that BBMImIM is more likely to be adsorbed on the steel interface. The findings also reveal a strong association among both the molecular structure of the examined ILs and their propensity for protecting the steel surface in the aggressive solution. As the molecular weight of the inhibitor increase, the efficacy of inhibition enhances since the contact area between the ILs molecules and the steel surface rises. Therefore, BBMImIM has the greatest surface area and, hence, greater protecting effectiveness. Additionally, the obtained dipole moment (µ) values were used to distinguish the ILs under consideration. It is known that IL with a high µ value can facilitate the adsorption action and hence the inhibition process owing to a strong dipole-dipole interaction among the IL and the steel surface [29]. According to the results in Table 5, BBMImIM showed the highest µ value when compared to the other ionic liquids, which is consistent with the other obtained results.

Corrosion Inhibition Mechanism
The corrosion inhibition mechanism of mild steel in the HCl solution in the existence of the examined ILs can be explained as follows: Generally, in the acidic solutions and the absence of inhibitors, the mild-steel surface has been shown to be positively charged and can easily be corroded owing to the aggressive chloride ions present in the solution. The use of organic inhibitors can suppress metallic dissolution by being adsorbed on the metal surface through the functional groups and the π electrons that exist on these inhibitors. According to the literature, ionic liquids, like most of the corrosion inhibitors, inhibit the metals and alloys corrosion via blocking the cathodic and anodic reaction sites.
In this study, the existence of imidazolium and pyrrolidinium-based ionic liquids resulted in slowing down the anodic dissolution reactions rate by the growth of a protecting layer capable of isolating the metallic surface away from the acidic medium as follows [48]: The inhibiting effect of the examined ILs resulted from covering the steel surface with the aromatic-ring and the butyl chains to form an efficient layer at the steel/solution boundary covering the steel surface's active centers. The imidazole's nitrogen and oxygen atoms have the ability to donate lone-pairs of electrons to the vacant d-orbital of steel producing a coordination bond and also, the imidazole π-electrons are bonded to the steel surface's vacant d-orbital to facilitate their protection. Furthermore, the possibilities of hydrogen bonding among the imidazolium ring hydrogen and the counter ions in addition to H 2 O molecules assist the inhibition action.
In the absence of ILs, the reactions at the cathodic part were involved in the hydrogen gas evolution as in the following equation: While in the existence of the investigated ILs, the cationic moiety of the ILs (imidazolium or pyrrolydinium moiety) were adsorbed at the cathodic site and thus competing with the H + ions to suppress the cathodic processes as follows: Moreover, the imidazolyl anion can act as counter ion among the steel surface and the imidazolium or pyrrolydinium cations, which assist the physical adsorption of the ILs on the steel surface via the electrostatic interactions [25]. Since the inhibiting efficiency depends on both the cathodic and anodic parts of the ionic liquid molecule, BBMImIM ionic liquid showed the highest level of protection due to the higher surface coverage owing to its dimeric structure and the higher molar mass which resulted in superior protection.
In order to explain the cation effect, the lower protection efficiency obtained using the pyrrolydinium cation compared with the imidazolium cation can be interpreted by the positive charge localization on the nitrogen atom of the pyrrolydinium cation that led to the repulsion with the positively charged steel surface in the acidic solution resulting in a lower protection in comparison with the imidazolium cation. The experimental and the theoretical results support each other confirming the order of protection of the examined ILs as follows: BBMImIM > BMImIM > BMPyrIM. Finally, according to the fascinating physiochemical properties of ILs, including non-toxicity, high conductivity, non-flammability, less volatility, and high thermal and chemical stability, they can be used instead of traditional volatile (toxic) compounds as corrosion inhibitors in several industries.

Conclusions
Different experimental, surface analysis and theoretical techniques were applied to investigate the protection efficiency of mild steel dissolution in 1.0 M HCl using ionic liquids namely BBMImIM, BMImIM and BMPyrIM. The results demonstrate that the examined ILs were effectively able to protect the mild steel surface via their physisorption and chemisorption on the steel surface obeying Langmuir adsorption isotherm. The examined ILs are acting as mixed-type inhibitors controlling the anodic and cathodic corrosion mechanisms. The inhibition efficiency of the ILs was increased as the concentration increased up to 5 × 10 −3 M resulted in the highest protection of 98.6% using BBMImIM followed by 94.3% and 92.4% in the presence of BMImIM and BMPyrIM, respectively. SEM, AFM and ATR-FTIR graphs proved the passive film formation on the steel surface which mitigates the steel corrosion in presence of ILs and these results were supported using the EDS spectra which verified the existence of the carbon and the nitrogen of the ILs on the steel surface. The electrochemical, surface analysis and theoretical results match each other, suggesting that the investigated ILs have excellent corrosion inhibition performance for the mild steel surface in acidic solution.