Pyridinium-based ionic liquids as novel eco-friendly corrosion inhibitors for mild steel in molar hydrochloric acid: Experimental & computational approach

Surfaces and Interfaces 22 (2021) 100881

Available online 25 December 2020

2468-0230/© 2020 Elsevier B.V. All rights reserved.

Pyridinium-based ionic liquids as novel eco-friendly corrosion inhibitors for mild steel in molar hydrochloric acid: Experimental &

computational approach

F. EL Hajjaji

^a,*

, R. Salim

^a,*

, M. Taleb

^a

, F. Benhiba

^b

, N. Rezki

^c

, Dheeraj Singh Chauhan

^d

, M.

A. Quraishi

^d

aEngineering Laboratory of Organometallic, Molecular Materials and Environment, Faculty of Science, USMBA, Fez, Morocco

bLaboratory of separation processes, Faculty of Sciences, Ibn Tofail University, P.O. Box. 133, 14000, Kenitra, Morocco

cDepartment of Chemistry, Faculty of Science, Taibah University, Al-Madinah Al-Munawarah 30002, Saudi Arabia

dCenter of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

A R T I C L E I N F O Keywords:

Inhibition performance Adsorption

Ionic liquids DFT calculations Mild steel

A B S T R A C T

Two ionic liquids (ILs), (E)-4-(2-(4-fluorobenzylidene)hydrazinecarbonyl)-1-propylpyridin-1-ium iodide (Ipyr- C3H7) and (E)-4-(2-(4-fluorobenzylidene) hydrazinecarbonyl)-1-pentylpyridin-1-ium iodide (Ipyr-C5H11) were evaluated as novel inhibitors for mild steel corrosion in 1 M HCl using electrochemical techniques and quantum chemical calculation. The results revealed that Ipyr-C3H7 and Ipyr-C5H11 acted as mixed-type inhibitors with anodic predominance and achievd an inhibition efficiency of around 88%. The adsorption of the ILs on the metal surface followed the Langmuir kinetic-thermodynamic isotherm. The theoretical approach was performed using DFT calculations at B3LYP, 6–311++G(d,p) to correlate the experimental results. Most descriptors showed a good correlation with the inhibition performance achieved experimentally. Molecular dynamics simulations show that the selected molecules adsorb parallel to the substrate surface.

1. Introduction

Ionic liquids (ILs) are structurally defined as ligation of organic cations such as N-alkylimidazolium, alkylphosphonium, alkylpyr- idinium, or alkylammonium and an inorganic anion such as iodide, chloride, bromide anion. The synthesis of ionic liquids (ILs) has been an emerging field of interest for many investigators due to their tunable properties, e.g., the non-flammability, almost negligible vapor pressure, non-volatility, high electrical conductivity, and excellent thermal sta- bility [1,2]. It has also been reported that such class of ILs and their analogs were used as media for metal electrodeposition, catalysts, effective corrosion inhibitors, and in food science [3,4].

On the other side, mild steel is considered one of the most important metallic materials in the world. It is largely used in different industrial types [5], such as cleaning and descaling using aggressive solutions, especially hydrochloric acid [6–8]. To prevent the industrial equipment from the corrosion process after several treatments, adding inhibitors remains the best solution, in this case, especially the heteroatoms such as sulfur, nitrogen, oxygen, phosphorus, or multi-bonded rings [9].

Usually, the heterocyclic molecules categories such as azoles, pyridines, and pyrimidines are the most used organic corrosion inhibitors [10].

However, the main lacuna of these molecules is the participation of synthesis steps as well as the solubility matters and the intrinsic toxicity of produced organic compounds. The research and development in the area of corrosion inhibitors has received a great attention on the application of eco-friendly substances like natural extracts [11,12], pharmaceutical products [13], biological polymers [14], and ionic liq- uids, etc. Recently, ILs have emerged as a major area of research and development in modern chemical science. Many research works were oriented towards the application of ILs as corrosion inhibitors[15,16].

Previous studies have verified that imidazolium [17], ammonium [18], pyridazinium [16]shows a good performance toward the corrosion behavior of metals in aggressive media.

The ILs have received large attention as natural inhibitors to decrease the corrosion process of metals, especially in the aggressive medium [15,19]. Besides, the existence of the large cation as well as the anionic groups, leads to an intermolecular synergism phenomenon that can enhance the adsorption of these ILs on the metallic substrate.

* Corresponding authors.

E-mail address: dheeraj.chauhan.rs.apc@itbhu.ac.in (D.S. Chauhan).

Contents lists available at ScienceDirect

Surfaces and Interfaces

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https://doi.org/10.1016/j.surfin.2020.100881

Received 15 September 2020; Received in revised form 10 November 2020; Accepted 6 December 2020

Several previous papers have reported that pyridinium-ILs show a high inhibition property against the corrosion of numerous metals [20,21].

For example, recently, in 2020, Priya Kumari Paul et al. investigated the adsorption of two carbohydrazide-pyrazole compounds on mild steel in 15% HCl solution. They reported that the studied inhibitors showed a high inhibition efficiency of around 98% based on various techniques.

According to Langmuir isotherm, a monolayer barrier was formed on the steel surface confirmed by AFM and XPS analysis [20].

In this paper, (E)-4-(2-(4-fluorobenzylidene)hydrazinecarbonyl)-1- propylpyridin-1-ium iodide (Ipyr-C3H7) and (E)-4-(2-(4-fluo- robenzylidene)hydrazinecarbonyl)-1-pentylpyridin-1-ium iodide (Ipyr- C5H11) were synthesized as the reported literature [22] (Table 1). The synthesized ionic liquids were investigated as corrosion inhibitors for mild steel in 1 M HCl by polarization curves (PDP) and electrochemical impedance spectroscopy (EIS). The synthesized molecules had LD50

(oral rat, mg/kg) of 643 for Ipyr-C3H7 and 836 for Ipyr-C5H11, sup- porting their environmentally benign nature. The various kinetic/- thermodynamic indices of adsorption are evaluated and discussed. On the other side, DFT calculations were executed using Gaussian 09 based on B3LYP/6–311++G (d,p) to correlate their adsorption mode with the various descriptors of molecular structures. Molecular dynamics (MD) simulation was also used to evaluate the adsorption performance of the two inhibitory molecules on the metal surface.

2. Experimental details

2.1. Chemical synthesis of ionic liquid compounds

The general methodology for the preparation of targeted ionic liquids

Ø Conventional synthesis

Pyridine hydrazine SI (1 mmol) and propyl iodide and/or pentyl iodide (1.2 mmol) in acetonitrile (30 mL) were heated under reflux for 72 h. Upon cooling, under the reduced pressure, the excess solvent was removed, and the solid product was recovered following filtration and acetonitrile washing to obtain the desired pyridinium ionic liquids Ipyr- C₃H₇and Ipyr-C₅H₁₁.

Ø Ultrasound induced preparation

Pyridine hydrazine SI (1 mmol) and propyl iodide and/or pentyl iodide (1.2 mmol) in acetonitrile (30 mL) were irradiated under an ul- trasonic processor for 12–14. The reaction mixture was treated as mentioned above in the conventional method to afford the same ionic liquids SI3 and SI5.

Ø Characterization of 4-(2-(4-fluorobenzylidene) hydrazinecarbonyl)-1-propylpyridin-1-ium iodide (Ipyr-C3H7)

Yellow crystals; mp: 186–187 ^◦C. IR (KBr, cm⁻¹): υ=3065 (Har), 2955, 2890 (Hal), 1689 (C=O), 1615 (C=N). ¹H NMR (400 MHz, DMSO- d6): δ_H =12.53 (0.25H, s, NH), 12.49 (0.75H, s, NH), 9.34 (1.5H, d, J = 8 Hz, Har), 9.25 (0.5H, d, J =4 Hz, Har), 8.55 (d, 1.5H, J =8 Hz, Har), 8.51 (s, 0.75H, HC=N), 8.41 (0.5H, d, J =4 Hz, Ar-H), 8.16 (0.25H, s, H- C=N), 7.89 (1.5H, dd, J =4 Hz, 8 Hz, Har), 7.62 (0.5H, dd, J =4 Hz, 8 Hz, Har), 7.38 (1.5H, dd, J =8 Hz, 12 Hz, Har), 7.27 (0.5H, dd, J =8 Hz, 12 Hz, Har), 4.68 (2H, dd, J =4 Hz, 8 Hz, CH2N), 1.95–2.04 (2H, m, NCH2CH2), 0.90–0.95 (3H, m, CH3). ¹³C NMR (100 MHz, DMSO-d6): δ_C

=164.30, 163.82, 161.35, 157.85 (C=N, C=O); 148.72, 148.39, 146.41, 144.78, 144.14, 129.29, 129.26, 129.11, 128.90, 128.81, 128.52, 128.43, 126.18, 125.22, 115.27, 115.06, 114.86 (Car); 61.38, 61.30 (NCH₂); 23.19, 23.11 (NCH₂CH₂); 9.30, 9.27 (CH₃). ¹⁹F NMR (377 MHz, DMSO-d6): δF =(-109.41 to -109.33), (-109.89 to -109.83) (1F, 2 m, Far). MS (ESI, m/z)=413.35 [M⁺].

ØCharacterization of 4-(2-(4-fluorobenzylidene)hydrazine- carbonyl)-1-pentylpyridin-1-ium iodide (Ipyr-C5H11)

Yellow crystals; mp: 216–217 ^◦C. IR (KBr, cm⁻¹): υ =3070 (Har), 2970, 2891 (Al-H), 1682 (C=O), 1622 (C=N). ¹H NMR (400 MHz, DMSO-d6): δ_H =12.52 (0.25H, s, NH), 12.47 (0.75H, s, NH), 9.34 (1.5H, d, J =8 Hz, Har), 9.25 (0.5H, d, J =8 Hz, Har), 8.53 (1.5H, d, J =8 Hz, Har), 8.50 (0.75H, s, HC=N), 8.40 (0.5H, d, J =8 Hz, Har), 8.15 (0.25H, s, HC=N), 7.89 (1.5H, dd, J =4 Hz, 8 Hz, Har), 7.62 (0.5H, dd, J =4 Hz, 8 Hz, Har), 7.37 (1.5H, dd, J =8 Hz, 12 Hz, Har), 7.23 (0.5H, t, J =8 Hz, Har), 4.68 (2H, t, J = 8 Hz, CH2N), 1.93–2.02 (2H, m, NCH2CH2), 1.25–1.37 (4H, m, 2 ×CH2), 0.86–0.90 (3H, m, CH3). ¹³C NMR (100 MHz, DMSO-d₆): δ_C =164.75, 162.28, 158.76 (C=N, C=O), 149.63, 149.33, 147.33, 145.68, 145.08, 130.23, 130.20, 129.82, 129.73, Table 1

Abbreviations, Structures and IUPAC Name for the studied inhibitors.

Abbreviations Structures IUPAC Name

Ipyr-C3H7 (E)-4-(2-(4-fluorobenzylidene)hydrazinecarbonyl)-1-propylpyridin-1-ium iodide

Ipyr-C5H11 (E)-4-(2-(4-fluorobenzylidene)hydrazinecarbonyl)-1-pentylpyridin-1-ium iodide

Fig. 1.Ultrasound versus conventional synthesis of pyridinium based ionic liquids tagged hydrazones Ipyr-C3H7 and Ipyr-C5H11.

129.44, 129.35, 127.11, 126.15, 116.18, 115.97, 115.76 (Car), 60.99, 60.91 (NCH2), 30.31, 30.20 (NCH2CH2), 27.55, 21.51, 27.47 (2 ×CH2), 13.71, 13.70 (CH3). ¹⁹F NMR (377 MHz, DMSO-d6): δ_F =(-109.41 to -109.34), (-109.89 to -109.81), (1F, 2 m, Ar-F).

The synthesis of the targeted pyridinium ionic liquids (ILs) was carried out under both ultrasound and conventional conditions, as illustrated in Fig. 1. Thus, the thermal alkylation of N-(4-fluo- robenzylidene)isonicotinohydrazide SI with propyl iodide and/or pentyl iodide, in refluxing acetonitrile for 72 h, afforded the corresponding pyridinium ionic liquids tagged as Schiff base Ipyr-C₃H₇ and Ipyr-C₅H₁₁ in 83% and 91% yield, respectively. Under green ultrasound conditions, the quaternization reaction required 12–14 h to afford the same ILs in 93–94% yield. It should be noted that the Schiff base precursor SI has been prepared according to a previously reported procedure [22]

through the condensation isonicotinohydrazide with p-fluorobenzaldehyde.

The achievement of reaction of quaternization has been obviously proved by various analysis: mass spectra, IR,¹H NMR, and ¹³C NMR of studied ILs Ipyr-C3H7 and Ipyr-C5H11. Their ¹H NMR spectra revealed the presence of two characteristic signals (m, CH3) and (dd, NCH2) at 0.88 and 4.70 ppm, respectively, which obviously evidences the achievement of the procedure of alkylation. The remaining methylene

protons were also recorded in the aliphatic region. The imine proton (HC=N) resonated as two distinct singlets between 8.15 to 8.51 ppm, with a ratio of 1:3. The amine proton (NH) appeared with the same isomeric distribution as two singlets between 12.47 to 12.55 ppm. In the aromatic region, eight aromatic protons were also recorded. The pres- ence of such pairing of signals could occur presumably due to the exis- tence of a diastereomeric mixture (i.e., E/cis and E/trans) for each of the imine-amide units.

The ¹³C NMR measurement supported the presence of the diaste- reomeric mixture via the appearance of two sets of signals for each peak.

The CH3 and NCH2 carbon atoms resonated in the form of two sets of signals at 9.27–13.71 and 60.91–61.38 ppm, respectively. In the downfield region (157.85–165.22 ppm), the C=N and C=O groups were recorded as double signals due to the E/cis and E/trans diastereomers.

2.2. Materials

The mild steel specimens had the following composition (% by weight): Fe (99.30%), S (0.05%), P (0.09%), Si (0.38%), Al (0.01%), Mn (0.05%), C (0.21%). These specimens were polished with SiC Emery paper grade (180–1500), acetone degreasing, and washing with water (ELGA PURELAB flex), and dried. The hydrochloric acid molar medium was prepared by dilution of commercial acid (37%HCl: Fischer Scien- tific) using distilled water.

2.3. Electrochemical measurements

Electrochemical experiments were performed using Radio- meteranalytical (VoltaLab-PGZ 100), controlled with Voltamaster 4 suite. The experiments were conducted using a cell with three-electrode system: mild steel with a surface area of 1 cm² as a working electrode, platinum as a counter electrode, and Ag/AgCl as reference. The impedance spectroscopy (EIS) experiments were carried out using AC signals of 10 mV amplitude within the frequency window: 100 kHz–100mHz. The Nyquist plots were plotted and analyzed using a suitable equivalent circuit. The inhibition efficiency was calculated by Eq. (1) [23]:

η_imp%= [

R^′_p− Rp

/ Rp

]

×100 (1)

R^′_p and Rp are the polarization resistance in the presence and the absence of inhibitor, respectively.

The polarization measurements were measured by shifting the electrode potential automatically from − 800 to − 100 mV with a

Fig. 3. Polarization curves of mild steel in 1 M HCl medium before and after addition of various concentrations of Ipyr-C3H7 & Ipyr-C5H11. Fig. 2.Evolution of open circuit potential (OCP) versus time for mild steel in 1

M HCl at optimum concentration of Ipyr-C3H7 & Ipyr-C5H11 at 298 K.

scanning rate of 1 mV/s. In this case, the inhibition efficiency (η_pp%) was calculated using the corrosion currents according to equation Eq. (2) [9]:

η_pp%=[

icorr− i^′_corr/ icorr

]×100 (2)

icorr and i^′_corr are the values of corrosion current densities in the presence and absence of inhibitors, respectively.

2.4. Density functional theory calculations

The quantum approach was very useful nowadays in order to correlate the descriptors of molecular structures with their inhibition performance. Therefore, the Density functional theory (DFT) method was carried out with basis set B3LYP/6–311G++ (d,p) using the Gaussian 09 program. The theoretical descriptors such as the energies of the lowest unoccupied and the highest occupied molecular orbital

(EHOMO & ELUMO), the energy gap (ΔE), the dipole moment (μ), the softness (σ), the electronegativity (χ), and the fraction of electrons transferred (ΔN110) were extracted and discussed.

2.5. MD simulation

Molecular dynamics (MD) simulation was used to evaluate the interaction and adsorption performance of Ipyr-C3H7 and Ipyr-C5H11on the surface of Fe (110) using the Forcite module, which is implemented in Materials Studio 8 [24]. This technique was performed with periodic boundary conditions in a simulation system of 32.270*32.270*34.134ų. The studied system contained chemical ele- ments (inhibitory monomer, Br⁻ +5H3O⁺+5Cl⁻ +500H2O) and a slab of Fe (110). The COMPASS force field has been used [25]. All systems surveyed were carried out in the presence of Andersen thermostat at 298 and 328 K, NVT ensemble, with a simulation time of 400 ps and a time step of 1.0 fs [26].

2.6. Surface morphology analysis

The scanning electron microscopy (SEM) can be classified as a great technique for surface morphology analysis. To obtain more information about the adsorption of the present ILs inhibitors, the SEM technique was performed after 6 h of immersion in the absence and the presence of optimum concentration (10⁻³ M) of Ipyr-C3H7 and Ipyr-C5H11

inhibitors.

Table 3

Impedance results of mild steel in 1 M HCl with and without inhibitors at different concentrations at 298 K.

Medium Conc (M) Rs(Ω.cm²) Rp(Ω.cm²) Q(µF.S^n-1) ndl Cdl(µF. cm⁻²) ƞ_imp% Ɵ

HCl 1 1.12 34.7 419.0 0.773 121.0 – –

Ipyr-C3H7 5.10⁻⁵ 1.84 55.11 269.5 0.755 69.16 37.0 0.37

10⁻⁴ 1.64 63.15 212.1 0.782 63.37 45.0 0.45

5.10⁻⁴ 2.5 214.9 106.2 0.791 39.19 83.8 0.838

10⁻³ 1.07 306.8 62.38 0.798 22.92 88.6 0.886

Ipyr-C5H11 5.10⁻⁵ 1.63 50.1 301.6 0.740 69.6 30.7 0.307

10⁻⁴ 1.64 62.7 224.0 0.772 63.88 44.6 0.446

5.10⁻⁴ 1.31 182.2 110.7 0.787 38.51 80.9 0.809

10⁻³ 1.52 285.5 78.54 0.795 29.62 87.8 0.878

Fig. 4.Nyquist plots for Ipyr-C3H7&Ipyr-C5H11 at various concentrations.

Table 2

Electrochemical parameters obtained from polarization curves.

Medium Conc.

M -Ecorr (mV/

Ag/AgCl) icorr(µA.

cm⁻²) -βc(mV.

dec⁻¹) βa(mV.

dec⁻¹) η_PP%

HCl 1 437 983 140 150 -

Ipyr- C3H7

5.10⁻⁵ 390 606 121 94 38.3

10⁻⁴ 388 448 143 93 54.4

5.10⁻⁴ 387 121 142 75 87.6

10⁻³ 383 97 124 67 90.1

Ipyr- C5H11

5.10⁻⁵ 371 674 118 69 31.4

10⁻⁴ 393 526 116 93 46.4

5.10⁻⁴ 377 118 140 63 87.9

10⁻³ 372 105 135 70 89.3

3. Results and discussion

3.1. Concentration effect of the studied ionic liquids 3.1.1. Open circuit potential

The variation of the mild steel potential versus the elapsed time during 30 min for the uninhibited and the inhibited (10⁻³ M of Ipyr- C3H7 and Ipyr-C5H11) solution was presented in Fig. 2.

It is clear that the addition of the studied molecules induces a shift in OCP (i.e., the corrosion potential Ecorr). Based on the plots presented in Fig. 2, it can be observed that the mild steel samples could achieve a quasi-stable open circuit potential under 30 min. Therefore, 30 min OCP measurement was assumed prior to performing all electrochemical measurements in this work.

3.1.2. Polarization curves

The polarization curves of mild steel in 1 M HCl without and after adding various concentrations of ILs inhibitors at 298 K are illustrated in Fig. 3. The electrochemical parameters such as Icorr, Ecoor, and Tafel slopes (ßc, ßa) were extracted using the extrapolation of Tafel branches and regrouped in Table 2.

It is observed from Tafel curves that the current densities (i_corr) ob- tained in the presence of studied ionic liquids decreased slightly with the rise of inhibitor concentrations until attaining 97µA.cm⁻² for Ipyr-C3H7

and 105 µA.cm⁻² for Ipyr-C₅H₁₁ at the optimum concentration (10⁻³ M). On the other hand, the inhibition efficiency achieves percentage values of 88% for Ipyr-C3H7 and 87% for Ipyr-C5H11. Generally, if the displacement in the E_corr is greater than 85 mV in the inhibited solution with respect to the Ecorr value for the uninhibited solution, the inhibitor molecules can be classified as anodic or cathodic types; otherwise, the inhibitors can be categorized as mixed type [27,28]. Herein, the studied ILs inhibitors at different concentrations bring about a slight variation in the corrosion potential values, which doesn’t exceed 85 mV with respect

to the uninhibited solution. Therefore, Ipyr-C3H7 & Ipyr-C5H11 com- pounds can be categorized as mixed type having anodic predominance.

Also, it is observed from polarization curves obtained that a small de- viation of current densities in the anodic area explaining the desorption effect in the potential more than -250 mV, especially in the two optimum concentration. This potential can be distinct as desorption potential Edes

indicating, therefore, the desorption of the studied molecules from the steel surface [28]. In addition, it can be seen that a small variation in the values of the cathodic slope indicates that no modification was observed in the cathodic mechanism.

These results suggest the formation of a protective layer reducing, therefore, the corrosion rate and blocking the anodic and cathodic sites.

The polarization technique remains insufficient since it can’t explain the elementary steps. Thus, the electrochemical impedance spectroscopy (EIS) was performed [27].

3.1.3. Electrochemical impedance spectroscopy

The inhibition effect of ILs inhibitors was also evaluated by elec- trochemical impedance spectroscopy technique. The Nyquist plots for mild steel with and without inhibitors obtained at 298 K are represented in Fig. 4. The Electrochemical impedance parameters obtained were extracted after a good fitting and regrouped in Table 3.

All concentrations of the studied inhibitors (Ipyr-C3H7 & Ipyr-C5H11) were tested using different circuits, but showing a good simulation is presented as the Nyquist plots using the equivalent circuit in Fig. 5. This equivalent circuit usually shows a good simulation for the ionic liquid compounds investigated in the hydrochloric acid medium [29,30]. It has a capacitance phase element Q in parallel with the polarization resis- tance Rp, and both are in series with the solution resistance Rs.

From Fig. 4, it is revealed that the semicircles obtained are not perfect, which may be due to heterogeneity of the electrode surface.

Many researchers explain that heterogeneity can arise from impurities, roughness, dislocations, inhibitor adsorption, etc. [31,32]. On the other side, these capacitive loops increase with the rise of inhibitor concen- tration, indicating, therefore, the inhibition performance of our ionic liquid compounds.

The analysis of electrochemical parameters regrouped in Table 3 shows that polarization resistance (Rp) increases with inhibitor con- centration, while the constant phase element (Q) decreases significantly.

Therefore, the double-layer capacitance Cdl decreases slightly, with the rise of inhibitors concentration from 22.92 to 69.16 µF. cm⁻² for Ipyr- C3H7 and from 29.62 to 69.6 µF. cm⁻² for Ipyr-C5H11. This diminution in Cdl values indicates the adsorption of our molecules and therefore reducing the corrosion process of mild steel in the hydrochloric acid solution [33]. Also, this performance was confirmed with the inhibition efficiencies obtained, which shows a value of 88.6% in the presence of

Fig. 6.Bode plots with and without different concentrations of Ipyr-C3H7 & Ipyr-C5H11 inhibitors.

Fig. 5. Equivalent circuit used in experimental impedance fit.

Fig. 7. Isotherm models tested of Ipyr-C3H7 & Ipyr-C5H11 at 298 K.

Table 4

Linear equations of different isotherms tested.

Isotherms Linear equations Description

Langmuir

Cinh

θ = 1

K+Cinh (i)

Θ: the degree of surface coverage.

Cinh: the inhibitor concentration.

K: the equilibrium constant of the adsorption/desorption process.

n: Freundlich constant indicating adsorption capacity and adsorption intensity.

a: the parameter for lateral molecular interactions: a value of a>0 indicates attraction, while a<0 means repulsion.

Freundlich

lnθ=lnK+1

nlnCinh (ii) Frumkin

ln

(Cinh(1− θ) θ

)

= −lnK− 2aθ (iii)

Temkin

θ= −1 2aln(K) − 1

2aln(Cinh) (iv)

Ipyr-C3H7 and 87.8% in the presence of Ipyr-C5H11 inhibitor at the op- timum concentration 10⁻³ M. This small difference may be due to the long carbon chain in Ipyr-C5H11inhibitor. Furthermore, it can be observed that the inhibition efficiencies obtained from EIS analyses are in good agreement with that attained by the polarization curves tech- nique confirming that the studied ionic liquids act as good inhibitors against corrosion of mild steel in hydrochloric acid medium. The effective capacity Cdl was calculated as Eq. (3):

Cdl= (

Q×R¹⁻_p ⁿ )_1/n

(3) Where Q is the constant phase angle (CPE), Rp is the polarization resistance, and n is the heterogeneity values of surface (0<n<1) [34].

The impedance diagrams without and with the different inhibitor concentrations was also depicted in the Bode plane with their simula- tions. These diagrams display the variation of the logarithm of the impedance modulus |Z(ω)| and the phase shift (φ) versus the logarithm of the frequency (log f). It can be seen from Fig. 6 that a single peak at intermediate frequencies signifying the existence of one time constant.

The analysis of the absolute impedance log |Z| and the phase versus log(f) diagrams shows a reasonable order with the inhibitor concentra- tion. It can be seen from log/Z/versus log (f) that a resistive behavior was shown at low frequencies since they tend toward zero [35]. On the other side, a linear relationship of log/Z/vs. log f, which gives a phase angle less than -90^◦and a slope value approaching -1, can be seen at the intermediate frequencies. This deviation from the ideal capacitor, which

has a slope of -1 and a phase -90^◦ justified the equivalent circuit ob- tained for our results [36].

3.1.4. Adsorption isotherm of studied ILs

In the present study, different isotherms were tested to find the appropriate one and the information about the adsorption mechanism of Ipyr-C3H7 & Ipyr-C5H11 inhibitors. Therefore, the isotherms models were tested (Langmuir, Temkin, Frumkin, Freundlich) using the elec- trochemical spectroscopy impedance results (Fig. 7). The linear equa- tions of various isotherms were regrouped in Table 4.

The analysis results of the tested isotherms models show that Ipyr-

C3H7 & Ipyr-C5H11 inhibitors obey the Langmuir model since the slope

and the regression coefficient are close to unity [37]. Therefore, sug- gesting that one active site can replace one adsorbed water molecule.

From the Freundlich isotherm, it can be seen that the constant K has no significance even with the regression coefficient close to 1. On the other hand, the regression coefficient obtained from Frumkin isotherm is too small, which leads us to suggest that the studied inhibitors disobey this isotherm model [38]. It can be observed that the experimental data are too far from the fitting curve, which confirms the same observation.

However, the regression coefficient obtained from Temkin isotherm is close to unity. Also, the negative value of the parameter (a) reflects the repulsive interaction in the adsorbed film [39]. The values of the stan- dard free adsorption energies ΔG⁰ads can be accessed using the equation of van’t Hoff Eq. (4):

ΔG^∘_ads= − RTln(55.5Kads) (4)

Fig. 8. Nyquist plots for mild steel with and without Ipyr-C3H7 inhibitor at different immersion time.

Table 5

Parameters obtained from different tested isotherm models.

Isotherms Inhibitors R² Parameters K ∆G^◦ads (kJ/

mol) Langmuir Ipyr-C3H7 0.999 Slope 1.027 1.03

10⁴ -32.86 Ipyr-

C5H11

0.999 1.022 8.72

10³ -32.45 Freundlich Ipyr-C3H7 0.987 n 3.19 8.22 -15.18

Ipyr- C5H11

0.985 2.81 11.00 -15.91

Frumkin Ipyr-C3H7 0.399 a -0.14 1.13

10⁴ -33.10 Ipyr-

C5H11

0.657 -0.10 9.25

10³ -32.59 Temkin Ipyr-C3H7 0.987 a -2.66 1.33

10⁵ -39.20 Ipyr-

C5H11

0.994 -2.51 9.67

10⁴ -38.42

Table 6

Electrochemical EIS parameters with and without Ipyr-C3H7 inhibitor at different immersion times.

Milieu Time (h) Rs(Ω.

cm²) Rct (Ω. cm²) Q(µF.

S^n-1) ndl Cdl(µF.

cm⁻²⁾ IE%

1 M HCl ½ 1,12 34,7 419,0 0,773 121,0 -

1 1,18 27,3 622,7 0,735 143,0 -

2 1,01 22,5 706,7 0,784 226,7 -

4 1,02 19,8 840,4 0,794 292,0 -

6 1,00 19,4 965,3 0,796 349,0 -

12 1,20 10,3 949,2 0,764 419,5 -

Ipyr- C3H7

½ 1.07 306.8 62.3 0.798 22.9 88.6

1 2.93 273.3 232. 7 0.770 102.2 90.0

2 1.50 262.2 152.5 0.809 71.6 91.4

4 1.85 227.7 340.2 0.814 190.1 91.3

6 1.74 220.9 390.0 0.810 219.9 91.2

12 1.47 128.0 316.6 0.833 166.7 91.9

Where R and T have the usual denotations, 55.5 is the concentration of water in M.

According to the literature, an electrostatic interaction took place between the charged molecules and charged metal surface (physical adsorption) when the ΔG⁰ads is around -20 kJ. mol⁻¹. While those values around -40 kJ mol⁻¹ or more involve the sharing or transfer of electrons from inhibiting molecules to the metal surface, forming a coordinated bond (chemisorption) [40,27]. On the other side, Chaitra et al. [41]

evaluated the inhibition efficiency of some triazole Schiff bases for mild

steel in an acidic medium and found that the ΔG⁰ads values were be- tween − 30.47 kJ mol⁻¹ and -34.05 kJ mol⁻¹. Therefore, it was inter- preted that metal-inhibitor interaction could be attributed to being a mixture of physical and chemical adsorption. In our case, the obtained values of ΔG⁰ads are -32.8 kJ/mol for Ipyr-C3H7 and -32.4 kJ/mol for Ipyr-C5H11, according to Langmuir isotherm (Table 5). Furthermore, Temkin isotherm shows the same trend as the Langmuir model, con- firming, therefore, that our inhibitors have mixed adsorption (physical and chemical adsorption).

3.1.5. Immersion time of studied ILs

The immersion time technique was also carried out to evaluate the electrochemical phenomena taking place at the interface of metal/so- lution at varying immersion times from 0.5 h to 12 h. This technique was studied using the electrochemical impedance spectroscopy for Ipyr-C₃H₇ inhibitor since it shows the best performance in the concentration effect.

The Nyquist plots obtained without and with the optimum concentration of tested ionic liquid at the varying time of immersion in 1 M HCl so- lution were presented in Fig. 8, and their parameters were regrouped in Table 6.

The analysis of the immersion time of Ipyr-C3H7 inhibitor shows that polarization resistance values Rp decrease with the immersion time of studied ILs from 306.8 Ω cm² in 0.5 h to 128 Ωcm² in immersion time of 12 h. However, the double-layer capacitance started to increase at 1 h and then decreased at 2 h to attain a value of 71.6 µF.cm⁻² and started increasing after this. These results can be attributed to the desorption of the molecules from the metal surface [42].

Fig. 9. Nyquist plots for mild steel with and without Ipyr-C3H7 & Ipyr-C5H11 inhibitors at different temperatures.

Table 7

Electrochemical EIS parameters of mild steel in 1 M HCl at temperature range 298K-328 K.

Medium Temp.

(K) Rs(Ω cm²) Rp(Ω

cm²) Q(Ω⁻¹ Sⁿ cm⁻²)

ndl Cdl(µF.

cm⁻²)

ƞ_imp%

1 M HCl 298 1.12 34.7 419 0.773 121.0 -

308 1.81 20.8 431 0.766 125.2 -

318 2.10 11.3 537 0.722 139.1 -

328 1.16 6.8 629 0.797 158.5 -

Ipyr- C3H7

298 1.07 306.8 62.38 0.798 22.9 88.6

308 1.27 147.6 147.2 0.775 48.4 85.9

318 1.77 80.0 137.0 0.804 45.5 85.8

328 1.23 44.7 165.6 0.792 45.7 84.8

Ipyr- C5H11

298 1.52 285.5 78.5 0.795 29.6 87.8

308 1.23 111.4 117.0 0.774 32.9 81.3

318 1.09 49.6 192.6 0.772 48.8 77.2

328 1.10 19.4 247.8 0.797 63.9 64.9

On the other side, the inhibitory efficiency increased after 2 h to attain a maximum value of 91.4% and remained stable around this ef- ficiency, indicating the protective inhibitor film formation on the steel surface due to the adsorption of the studied inhibitors.

3.2. Temperature effect of studied ILs

The temperature is a parameter that can modify the interactions that happen between a metallic surface and the inhibited & uninhibited corrosive electrolyte [40]. Therefore, it’s interesting to investigate the inhibition efficiency of these ILs at different temperatures. This inves- tigation was performed using electrochemical impedance spectroscopy (EIS). The Nyquist plots of mild steel in 1 M HCl in the absence and containing the optimum concentration of Ipyr-C3H7 & Ipyr-C5H11 in- hibitors from 303 K to 333 K are shown in Fig. 9. On the other hand, the different electrochemical parameters were calculated and summarized in Table 7.

Firstly, the Nyquist diagrams obtained for Ipyr-C3H7 & Ipyr-C5H11 at different temperatures were simulated using the equivalent circuit given in Fig. 5. Moreover, it can be observed from these plots that the semi- circles obtained decreased with the rise of temperature, indicating a decrease in the polarization resistance, which is also confirmed with values regrouped in Table 7. This decrease in the polarization resistance goes apparently with an increase in the double layer capacitance C_dl [43]. Consequently, the inhibition efficiency values at optimum con- centration decreased from 88.6% to 84.8% for Ipyr-C3H7 and from 87.8% to 64.9% for Ipyr-C₅H₁₁. These results confirm that Ipyr-C₃H₇ inhibitor provides greater protection from corrosion at higher temper- atures compared to Ipyr-C5H11, which contains a long carbon chain.

Fig. 10.Polarization curves for steel surface with and without ILs inhibitor at various temperatures.

Table 8

Electrochemical parameters of mild steel in 1 M HCl at temperature range 298K- 328 K.

Medium TemperatureK -EcorrmV/

Ag/AgCl icorrµA

cm⁻² -βcmV

dec⁻¹ β_amV dec⁻¹

η_PP%

1 M HCl 298 437 983 140 150 **

308 456 1470 136 138 **

318 455 2200 116 130 **

328 454 3200 115 122 **

Ipyr- C3H7

298 383 97 124 67 90.1

308 381 175 131 80 88.0

318 389 282 136 77 87.1

328 390 430 138 93 86.5

Ipyr- C5H11

298 372 105 135 70 89.3

308 376 250 136 78 82.3

318 371 494 130 67 77.5

328 370 1074 138 98 65.8

Therefore, it can be understood that the long chain of Ipyr-C₅H₁₁ ionic liquid does not provide sufficient coverage of the metal surface at elevated temperatures [44].

On the other side, the temperature effect of the optimum concen- tration of Ipyr-C3H7 and Ipyr-C5H11 has also been investigated in the same ranging temperature using the polarization curves technique. The polarization curves at the optimum concentration (10⁻³ M) are

presented in Fig. 10, and the various electrochemical parameters are listed in Table 8.

From the temperature analysis, it can be seen that the icorr values in the presence of the studied inhibitors are less than those obtained in the blank solution signifying that these compounds have considerably inhibited the corrosion reaction of mild steel. In addition, both com- pounds showed a good inhibition performance at various temperatures confirming the efficiency obtained with EIS measurements. These effi- ciencies slightly decreased with the rise of temperature, but this decrease is smaller in the presence of Ipyr-C3H7 despite having a less percentage at the temperature of 298 K compared to Ipyr-C5H11

inhibitor.

The Arrhenius plots of ln(icorr) vs. 1000/T and ln(icorr /T) vs. 1000/T of mild steel in 1 M HCl solution containing Ipyr-C3H7 and Ipyr-C5H11

are calculated from Arrhenius Eq. (5) and transition state Eq. (6) and

Fig. 12.Optimized structures, HOMO & LUMO and ESP maps for the studied compounds in neutral form at B3LYP/6–311G++(d,p).

Fig. 11.Arrhenius & transition state plots for mild steel in 1 M HCl solution with and without optimum concentration of studied inhibitors.

Table 9

Thermodynamic activation parameters for Ipyr-C3H7 and Ipyr-C5H11. Activation parameters 1 M HCl Ipyr-C3H7 Ipyr-C5H11

Ea (kJ/mol) 33.8 40.2 62.2

∆∆Ha*kJ/mol 29.4 37.6 59.6

∆∆Sa* (J/mol. K) -88.9 -80.3 -5.99

presented in Fig. 11. The activation parameters for MS in 1 M HCl with and without the studied ILs derivative are presented in Table 9.

icorr=A e (

−Ea RT

)

(5)

icorr= RT Nh e

(

ΔS∗ R

) e

(

−ΔH∗ RT

)

(6) Where N is the Avogadro number, T is the absolute temperature, R is the gas constant, and h is the Plank’s constant.

From activation parameters analysis, it can be observed that E_a values in the presence of studied inhibitors are higher compared with the value of the blank solution, indicating therefore that the studied inhibitors adsorb on the steel surface by physical adsorption [27]. The positive values for ∆Ha* reflect the endothermic nature of the mild steel dissolution process [9]. On the other hand, the negative sign of entropy (∆Sa*) indicates a diminution in the degree of randomness, which occurred when the reactants are transformed into activated complexes.

3.3. DFT calculations

3.3.1. Global molecular reactivity of ILs inhibitors

The density functional theory (DFT) is a powerful quantum chemical method used in corrosion inhibition study to correlate the molecular electronic parameters with their performance to prevent the corrosion of the mild steel surface. Moreover, this can allow an understanding of the mechanism that could happen between these inhibitors and the steel surface [44,45]. The optimized structures, HOMO & LUMO electron density distribution, and electrostatic potential surfaces (ESP) for the studied ionic liquid molecules are shown in Fig. 12. The various de- scriptors were given in Table 10.

From Fig. 12, it is revealed that Ipyr-C3H7 and Ipyr-C5H11 have nearly the same HOMO density distribution, which is centered basically on the C=O bond, N-N bond, and benzene ring attached to these mol- ecules. On the other side, the distribution of LUMO density is particu- larly localized on the pyridazine motif for both inhibitors. Moreover, the electrostatic potential surfaces (ESP) is another form that can give us an idea about the electrophilic active sites of molecular structure. From these ESP maps, it can be suggested that the electrophilic active site is localized around atoms of pyridazine and the carbon linked to the ni- trogen atom of this motif for both compounds since it shows a dark blue color in the ESP maps [46].

It is known that the energy of frontier orbitals such as the LUMO and the HOMO are important descriptors that can explain the adsorption capacity of the studied molecules. Therefore, the smaller value of ΔE_gap Eq. (7) can be explained by small values of energy required to remove an electron from the HOMO of the electron-donating species to the LUMO of the electron-acceptor and make the adsorption of these molecules

easier [46]. In our case, Ipyr-C3H7 compound shows a smaller energy gap than Ipyr-C5H11, indicating the high reactivity of this one and therefore showing a higher inhibition performance compared to Ipyr-C3H7.

ΔEgap=ELUMO− EHOMO (7)

Moreover, the dipole moment (μ) can affect the corrosion inhibition process [47]. As many authors reported, the most effective compound has a high value, which is the same results that was found. Furthermore, the Ƞ & σ descriptors can be calculated using the Eq. (8) & Eq. (9) and explain, therefore, the stability and reactivity of the studied structures. It can be seen from these values that Ipyr-C₃H₇ is more reactive than Ipyr-C5H11 since it has a small value of hardness and a high value of softness.

η= − 1

2(EHOMO− ELUMO) (8)

σ⁼1/η (9)

It can also be observed that the absolute electronegativity (χ), which indicates the tendency of an atom or group of atoms to attract the shared pair of electrons to itself, can be calculated by Eq. (10) [48]. This Table 11

Most active sites of the studied Ionic liquid inhibitors using DFT at B3LYP/

6–311++G in gas phase.

Inhibitors Atoms P(N) P (N-1) P (N+1) F⁺K F⁻k

Ipyr-C3H7 C 2 6.2259 6.1612 6.2347 0.0088 0.0646 C 3 6.1934 6.1285 6.2101 0.0166 0.0649 C 6 5.5963 5.4870 5.6292 0.0328 0.1093 C 8 5.9886 5.8287 6.0721 0.0835 0.1598 N 9 7.2959 7.1875 7.2952 -0.0007 0.1084 C 12 6.0410 6.0575 6.1720 0.1309 -0.0164 C 15 5.8931 5.8684 5.9767 0.0836 0.0247 C 16 5.8890 5.8831 6.0028 0.1137 0.0058 N 17 7.3199 7.3005 7.4122 0.0923 0.0193 O 21 8.5606 8.4730 8.6455 0.0848 0.0876 Ipyr-C5H11 C 2 6.2298 6.1633 6.2372 0.0074 0.0665 C 3 6.1985 6.1318 6.2131 0.0145 0.0667 C 6 5.6000 5.4886 5.6311 0.0311 0.1114 C 8 5.9762 5.8224 6.0544 0.0782 0.1537 N 9 7.2971 7.1837 7.2963 -0.0008 0.1134 C 12 6.0409 6.0613 6.1812 0.1402 -0.0203 C 15 5.8922 5.8710 5.9876 0.0953 0.0212 C 16 5.8928 5.8877 5.9919 0.0991 0.0050 N 17 7.3174 7.2984 7.4113 0.0939 0.0189 O 21 8.5602 8.4734 8.6437 0.0835 0.0868

Fig. 13.Temperature fluctuation equilibrium of the systems studied.

Table 10

Quantum chemical descriptors for Ipyr-C3H7&Ipyr-C5H11obtained with DFT, B3LYP/6–31G++(d,p)level in gas and aqueous phases.

Parameters Ipyr-C3H7 Ipyr-C5H11

Gas Aqueous Gas Aqueous

EHOMO (eV) -9.3968 -6.9564 -8.3241 -6.9796

ELUMO (eV) -6.5831 -3.1876 -5.4247 -3.1329

ΔEgap (eV) 2.8137 3.7688 2.8994 3.8466

σ _(eV^¡1_{) 0.7108 0.5306 0.6897 0.5199}

Ƞ (eV) 1.4068 1.8844 1.4497 1.9233

χ _{(eV) 7.9899 5.0720 6.8744 5.0562}

µ (D) 12.9569 15.5535 9.4741 11.7618

ΔN -1.1266 -0.0668 -0.7085 -0.0614

Ω 22.6887 6.8258 16.2989 6.6461

Ξ 0.0440 0.1465 0.0613 0.1504

Total energy (u.a) -959.2723 -959.3477 -1037.9218 -1037.9958

descriptor also showed the same tendency as the parameters discussed before.

χ=1

2(EHOMO+ELUMO) (10)

It can be clear that the quantum descriptors obtained in the aqueous phase don’t have a big difference compared to those extracted in the gas phase. Furthermore, the fraction of electrons transferred (ΔN110) from the ILs to the Fe (110) surface was calculated using the Eq. (11), which considers that the Fe (110) plane was the most stable plane [42]:

ΔN110= χ_Fe110− χ_inh 2(

η_Fe110+η_inh⁾⁼ Φ− χ_inh

2η_inh ⁽¹¹⁾

the work function Φ presents the theoretical value of electronegativity in the plane (110) of iron (Φ=χ(Fe110) =4.82 eV), and the global hardness corresponds to the metallic bulk (η (Fe) =0 eV). In our case, this index also goes with the same trend as the other descriptors studied before indicating that Ipyr-C3H7 is more reactive compared to Ipyr-C5H11, which is also confirmed with the electrophilicity index since it has a high value. Finally, it can be seen that all descriptors indicate that Ipyr-C3H7

has a good performance with a small difference, which is maybe due to the carbon chain. The increase of the carbon chain in the molecule Ipyr- C5H11 leads to suggest that small molecules adsorb onto the steel surface compared to Ipyr-C3H7, which has a smaller carbon chain.

3.3.2. Local molecular reactivity of Ipyr-C3H7 & Ipyr-C5H11 inhibitors In order to determine the reactive sites (electrophilic and nucleo- philic attack), the Fukui functions have been calculated using the nat- ural populations for atoms in different states anionic, cationic, and neutral species. These indices were calculated from the following equations Eq. (12) & Eq. (13) [49]:

Nucleophilic attack f⁺_k =Pk(N+1) − Pk(N) (12) Electrophilic attack f⁻_k =Pk(N) − Pk(N− 1) (13) Where PK(N), PK(N+1), and PK(N-1) are the natural populations for the atom k in the neutral, anionic, and cationic species, respectively.

According to Table 11, it can be noticed that the C12, C16, C15, N17,

and O21 atoms are the most favorable sites for nucleophilic attack since it has a high value of f^þK for both the studied compounds. On the other hand, the most active sites of electrophilic attack localized in C6, C8, and N9 atoms since recorded the high values of f^¡k for both compounds.

Moreover, it can be seen that the atoms of the carbon chain, which is the difference between the studied compounds, don’t show as the most active sites. These results confirm the observation shown in the distri- bution of the molecular orbital and explain the adsorption behavior of these ILs onto the steel surface.

3.3.3. MD simulation

To obtain more knowledge on the reaction mechanism between the investigated ILs and the metal surface, we employed the MD simulation approach, as well as the influence factors such as temperature on the interaction and/or on the adsorption configuration and adsorption Fig. 14.Best adsorption configuration of molecules Ipyr-C3H7 & Ipyr-C5H11 on the Fe (110) surface at 298 and 328 K.

Table 12

Einteraction and Ebinding energies of molecules Ipyr-C3H7 & Ipyr-C5H11 on the Fe (110) surface at 298 and 328 K.

Temperatures Ipyr-C3H7/Fe (110) Ipyr-C5H11/Fe (110) E interaction E binding E interaction E binding

298 K -759.456 759.456 -734.459 734.459

328 K -698.847 698.847 -681.384 681.384