Experimental, density functional theory and molecular

Journal of Molecular Liquids 273 (2019) 1–15

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Experimental, density functional theory and molecular dynamics supported adsorption behavior of environmental benign imidazolium based ionic liquids on mild steel surface in acidic medium Chandrabhan Verma a,b, Lukman O. Olasunkanmi a,b,c, Indra Bahadur a,b, H. Lgaz d,e, M.A. Quraishi f,⁎, J. Haque g, El-Sayed M. Sherif h,i, Eno E. Ebenso a,b,⁎⁎ a

Department of Chemistry, School of Chemical and Physical Sciences, Faculty of Natural and Agricultural Sciences, North-West University, Private BagX2046, Mmabatho 2735, South Africa Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Private Bag X2046, Mmabatho 2735, South Africa c Department of Chemistry, Faculty of Science, Obafemi Awolowo University, 220005 Ile-Ife, Nigeria d Laboratory of Applied Chemistry and Environment, ENSA, Universite Ibn Zohr, PO Box 1136, 80000 Agadir, Morocco e Laboratory of Separation Methods, Faculty of Science, Ibn Tofail University, PO Box 242, Kenitra, Morocco f Center of Research Excellence in Corrosion, Research Institute, KFUPM, Dhahran, Saudi Arabia g Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), 221005, India h Center of Excellence for Research in Engineering Materials (CEREM), King Saud University, P.O. Box 800, Al-Riyadh 11421, Saudi Arabia i Electrochemistry and Corrosion Laboratory, Department of Physical Chemistry, National Research Centre, El-Behoth St. 33, Dokki, Cairo 12622, Egypt b

a r t i c l e

i n f o

Article history: Received 17 May 2018 Received in revised form 21 September 2018 Accepted 27 September 2018 Available online 02 October 2018 Keywords: Ionic liquids Green corrosion inhibitors Acid solution Adsorption isotherm Computational simulations Mixed type inhibitors

a b s t r a c t This study describes the adsorption behavior of three 1-butyl-3-methyl-imidazolium ionic liquids namely, 1butyl-3-methylimidazolium chloride ([bmim][Cl]), 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) and 1butyl-3-methylimidazolium trifluoromethanesulphonate ([bmim][CF3SO3]) at mild steel/1 M HCl interface using experimental and computational studies. Electrochemical and gravimetric measurements revealed that the inhibition efficiency of the studied ionic liquids increases with increase in concentrations, such that [bmim] [Cl], ([bmim][CF3SO3] and [bmim][Ac]) gave the maximum inhibition efficiencies of 93.18%, 96.02% and 97.15%, respectively at a concentration as low as 8.67 × 10−4 mol L−1. The studied ionic liquids act as interfacial corrosion inhibitors and their adsorption at mild steel/electrolyte interface obeyed the Temkin adsorption isotherm. Polarization study suggested that the ionic liquids are mixed-type corrosion inhibitors. SEM and AFM analyses were undertaken to support the adsorption of the ionic liquid molecules on mild steel surface in 1 M HCl solution. Orientation and adsorption mode of 1-butyl-3-methyl-imidazolium molecules on mild steel surface was investigated using molecular dynamic (MD) simulations, which revealed that 1-butyl-3-methylimidazolium molecules adsorb on mild steel surface in flat orientations. Quantum chemical parameters for the neutral and protonated forms of the ionic liquids as well as the binding energies and radial distribution function indices obtained from molecular dynamic simulations corroborate the experimental results. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Modern researches in science and engineering are often directed towards the design and development of sustainable techniques. This is due to the increasing awareness on sustainability of eco-system and placement of stricter sanctions on subverting practices [1–3]. Ionic

⁎ Corresponding author. ⁎⁎ Correspondence to: E. E. Ebenso, Department of Chemistry, School of Chemical and Physical Sciences, Faculty of Natural and Agricultural Sciences, North-West University, Private BagX2046, Mmabatho 2735, South Africa. E-mail addresses: [email protected] (M.A. Quraishi), [email protected] (E.E. Ebenso).

https://doi.org/10.1016/j.molliq.2018.09.139 0167-7322/© 2018 Elsevier B.V. All rights reserved.

liquids can be regarded as greener chemicals due to their negligible vapor pressure, which accounts for their low tendency of contaminating the environment. This property of the compounds has promoted the investigation of more ionic liquids as potential corrosion inhibitors [4–7]. Ionic liquids can also be regarded as designer chemicals, as they can be suitably modified and used for countless applications by varying the nature of anions and cations thereby tuning their physiochemical properties [8,9]. Apart from their low vapor pressure, other unique properties of ionic liquids include high range of pH stability, non-toxicity, nonflammability and high chemical and thermal stability, which all contribute to their recent considerations as green corrosion inhibitors [10–12]. Furthermore, the limitation in the use of conventional organic and polymeric compounds as corrosion inhibitors due to their low solubility in

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C. Verma et al. / Journal of Molecular Liquids 273 (2019) 1–15

polar electrolytic media is readily overcame by using ionic liquids. This is because ionic liquids are characterized by wide range of solubility in polar aqueous systems [1,5–7]. Just like other organic compounds, the presence of π-electrons and unsaturated functional groups such as NC_Cb, NC_O,\\N_N\\NC_S, \\N_O,\\C`C\\and\\C`N, as well as non-bonding electrons of heteroatoms (N, O, S and P) in ionic liquids enhance their interactions with metallic atoms and hence, adsorption of their molecules on metallic surface [1,5–7,13]. Generally, the heteroatoms exist in polar functional groups such as hydroxyl (\\OH), methoxy (\\OCH3), amino (\\NH2), nitro (\\NO2), nitrile (\\CN), ether (\\O\\), ester (\\COOR), amide (\\CONH2), lactone (cyclic ester), lactam (cyclic amide), carboxyl (\\COOH), halogens (\\Cl, \\Br, \\I) etc. [1,13,14]. These functional groups do not only enhance bonding between inhibitors and metallic surfaces but also increase the solubility of the inhibitor molecules in polar electrolytes like 1 M HCl and water. Among the various families of ionic liquids, imidazolium based ionic liquids are being frequently studied and utilized in various applications. Tuken and coworkers had reported the inhibitive characteristics of 1ethyl-3-methylimidazolium dicyanamide (EMID) on mild steel corrosive dissolution in 0.1 M H2SO4 using several experimental methods [15]. EMID acted by adsorbing at metal/electrolyte interfaces and its adsorption followed the Langmuir adsorption isotherm. The inhibitive property of 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM] [HSO4]) and 1-butyl-3-methylimidazolium chlorides (BMIC) towards mild steel corrosion was investigated using chemical and electrochemical methods. Both the tested ionic liquids acted as mixed-type inhibitors whose adsorption obeyed the Langmuir adsorption isotherm and their inhibition efficiencies followed the order: [BMIM][HSO4] N BMIC [16]. In other words, the inhibition efficiencies vary with the type of anion. Similarly, a study reported by Zheng et al. [17] on two imidazolium ionic liquids namely, 1-allyl-3-octylimidazolium bromide [AOIM][Br] and 1-octyl-3-methylimidazolium bromide ([OMIM]Br) for mild steel corrosion in 0.5 M H2SO4 using computational and experimental methods showed that the two compounds are cathodic type inhibitors. Several other imidazolium based ionic liquids have been reported as effective inhibitors for the corrosion of mild steel and other metals and alloys in several electrolytic media [18–23]. In line with the continuous search for more eco-friendly, costeffective and efficient corrosion inhibitors, and the established satisfactory performances of ionic liquids, especially imidazolium based ionic liquids in this regard, the present study is designed to investigate the inhibitive effects of three imidazolium based ionic liquids (not previously reported) using experimental (gravimetric, electrochemical, SEM, AFM) and computational (DFT, molecular dynamics simulations) methods. We have discussed the mechanism of corrosion inhibition of mild steel surface using the findings of both experimental and theoretical studies. Coupled with the fact that imidazolium based ionic liquids have potential applications in many fields that are sustainable. Imidazolium based ionic liquids are regarded as the most important class of ionic liquids as they are non-toxic, biodegradable, inexpensive and water soluble. This is with the view of providing additional scientific evidence on the dependence of corrosion inhibition activities of imidazolium ionic liquids on the nature of anions. In this study, three imidazolium based ionic liquids namely, 1-butyl-3-methylimidazolium chloride ([bmim]Cl), 1-butyl-3-methylimidazolium acetate ([bmim] [Ac]) and 1-butyl-3-methylimidazolium trifluoromethanesulphonate ([bmim][CF3SO3]) were investigated for their inhibition performances on mild steel corrosion in 1 M HCl. The study also proposed the mechanism by which the studied ionic liquids inhibit mild steel corrosion in hydrochloric acid medium. Quantum chemical parameters for the neutral and protonated forms of the ionic liquids as well as the binding energies and radial distribution function indices obtained from molecular dynamic simulations corroborate the experimental results. Therefore, the study provides an advancement on the identification and design of biodegradable, environmentally

benign and efficient corrosion inhibitors using combined experimental and theoretical techniques. 2. Experimental sections 2.1. Materials 1-Butyl-3-methylimidazolium chloride ([bmim][Cl]), 1-butyl-3methylimidazolium acetate ([bmim][Ac]) and 1-butyl-3methylimidazolium trifluoromethanesulphonate ([bmim][CF3SO3]) used in the present study were purchased from Sigma-Aldrich and used for corrosion inhibition study without any purification. The investigated ionic liquids are further characterized by their FT-IR, 1H and 13C NMR spectra. The FT-IR, 1H and 13C NMR spectra of [bmim]Cl, [bmim] [CF3SO3] and [bmim][Ac] are presented in Fig. S1. IUPAC name, chemical structure, molecular weight, molecular formula and other relevant information of the ionic liquids are presented in Table S1 (Supporting information). Perkin–Elmer 100 FT–IR spectrophotometer instrument was employed for FT-IR analysis while 1H NMR (at 500 MHz) and 13C NMR (at 126 MHz) spectra of the ionic liquids was measured with Bruker Avance spectrometers at 25 °C using CDCl3 as solvent and tetramethylsilane as internal reference. The mild steel having percentage elemental composition of C (0.17%), P (0.019%), Si (0.26%), Mn (0.46%), S (0.017%) and Fe (balance; 99.704%) was employed as an alloy for chemical (gravimetric), surface morphology (SEM and AFM) and electrochemical studies. Before experimental works, mild steel specimens were polished with the emery papers of several grades (600–1200). The electrolyte medium of 1 M HCl was prepared by diluting the 37% HCl purchased from MERCK with double distilled water. 2.2. Methods 2.2.1. Gravimetric method The inhibitive effects of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] were determined using gravimetric method due to its simplicity, preciseness, accuracy and ease of measurement. During gravimetric study, working specimens of above stated composition was cut into 0.025 cm × 2.0 cm × 2.5 cm dimension and dipped into 100 mL of 1 M HCl in the absence and presence of different concentrations of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] at for 3 h. After the elapsed time, specimens were taken out, dried and weighted precisely. The difference in the initial and final weight (in mg) gives the value of exact weight loss in 3 h through which corrosion rate (CR), surface coverage (θ) and inhibition efficiency (η%) respectively were calculated according to the equations [24–26]: η% ¼

θ¼

w0 −wi 100 w0

η% 100

ð1Þ

ð2Þ

In the above equations, w0 and wi represent the initial and final (after 3 h immersion time) weight of the mild steel specimens, respectively. 2.2.2. Electrochemical methods For electrochemical studies, AUTOLAB galvanostat/potentiostat (PGSTAT) model 302 N purchased from Metrohm was used. The instrument was configured to use three electrodes system with mild steel as working electrode, graphite (rod) as counter electrode and Ag/AgCl as reference electrode. One sided 1 cm2 surface area of the working electrode was exposed to 1 M HCl solution without and with [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac]. The working electrode was subjected to unperturbed corrosion for 30 min in order to stabilize the open circuit


potential (OCP) before electrochemical signals were supplied to the system. The potentiodynamic polarization behavior of the mild steel working electrode in the electrolyte systems was studied by sweeping its potential from −250 mV to +250 mV with respect to the OCP at the scan rate of 1 mV/s. The polarization curves were extrapolated to find current density (icorr) through which inhibition efficiency was calculated according to the equation [24–26]: η% ¼

i0corr −iicorr i0corr

100

ð3Þ

icorr0 and icorri represent the corrosion current

where densities under noninhibited and inhibited conditions respectively. The impedance spectra of mild steel corrosion in 1 M HCl in the absence and presence of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] were obtained at the OCP by employing an AC signal in the frequency range of 100 kHz to 0.1 Hz and 5 mV amplitude. The charge transfer resistances (Rct) derived from Nyquist plots through were used to estimate the inhibition efficiency of the inhibitors according to the equation [24–26]: η% ¼

Rict −R0ct Rict

100

ð4Þ

where Rcti and Rct0 represent the charge transfer resistances for the uninhibited and inhibited conditions. 2.2.3. Surface morphology methods The surface morphologies of mild steel specimens that were made to corrode for 3 h in the absence and presence of maximum concentration of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] were examined using atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses. SEM images of inhibited and non-inhibited steel specimens were recorded at 2000× using SEM model Zeiss Evo 50 XVP model at the accelerating voltage of 20 kV. The AFM micrographs of the protected and non-protected surfaces of 5 mm × 5 mm scanning were recorded using NT-MDT multimode AFM, Russia. 2.2.4. Computational methods Relative reactivity of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] was determined with the view of relating it with the adsorption and corrosion inhibition tendencies of the ionic liquid molecules. Density functional theory (DFT) calculations were carried out on the studied molecules using the B3LYP/6-31G(d) implemented in Gaussian 09 (version D.01) [24–28]. Reactivity indices such as energy of the highest occupied molecular orbital (EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO) and energy gap (ΔE) were computed for both the neutral and protonated forms of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules. Calculations on the protonated forms were informed by the possibility of the ionic liquids to undergo proton addition in aqueous acid medium used for the corrosion studies. Other reactivity parameters such as ionization potential (I), electron affinity (A), global harness (η), softness (σ) and electronegativity (χ) were computed according to the respective Eqs. (6)–(10). The interactions between ionic liquids (inhibitors) and metallic surface can be regarded as a donor-acceptor type in which inhibitors molecules are considered as electron donor and metallic surfaces are considered as electron acceptor. The fraction of electron transferred (ΔN) from donor molecules ([bmim][Cl], [bmim][CF3SO3] and [bmim][Ac]) to the acceptor (Fe atom in the mild steel) was calculated estimated according to Eq. (11) [24–28]: I ¼ −E HOMO

ð5Þ

3

A ¼ −E LUMO

ð6Þ

η¼

1 1 ðI−AÞ ¼ ð−EHOMO þ ELUMO Þ 2 2

ð7Þ

σ¼

1 η

ð8Þ

χ¼

1 1 ðI þ AÞ ¼ ð−EHOMO −ELUMO Þ 2 2

ð9Þ

χ −χ inh ΔN ¼ Fe 2 ηFe þ ηinh

ð10Þ

where χFe and ηFe are electronegativity and hardness of Fe respectively, and literature values of 7.0 eV and 0.0 respectively were adopted for the two constants [24–28]. In furtherance of theoretical explanations for the inhibitive actions of the studied ionic liquids, the interactions between single molecule of each of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] and mild steel surface were simulated with molecular dynamics simulations method, using Materials Studio 6.0 (from Accelrys Inc.) [29]. The details of MD simulations as explained elsewhere [30,31] were used. The interaction (Einteraction) and binding (Ebinding) energies for metal-inhibitors interactions were calculated using Eqs. (12) and (13) respectively [32,33]: Einteraction ¼ Etotal −ðEsurfaceþsolution þ Einhibitor Þ

ð11Þ

EBinding ¼ −Einteraction

ð12Þ

where Etotal is the total energy of the entire system, Esurface+solution referred to the total energy of Fe(110) surface and solution without the inhibitor and Einhibitor represent the total energy of inhibitor. 3. Results and discussion 3.1. Gravimetric analyses: effect of ionic liquids concentration The effects of ionic liquids concentration on the dissolution of mild steel in 1 M HCl were considered based on weight loss analyses

Table 1 The weight-loss parameters obtained for mild steel in 1 M HCl at different concentrations of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac]. Surface coverage (θ)

Inhibitors

Conc. (mol L−1)

Weight loss (mg)

Inhibition efficiency (η%)

Blank

–

–

[bmim][Cl]

0.867 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 8.67 × 10−4 0.867 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 8.67 × 10−4 0.867 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 8.67 × 10−4

176 (±1.414) 68 (±1.414)

61.36

0.6136

49 (±0.707) 28 (±0.707) 19 (±0.707) 13 (±0.707) 12 (±0.707) 58 (±1.414)

72.15 84.09 89.20 92.61 93.18 67.04

0.7215 0.8409 0.8920 0.9261 0.9318 0.6704

43 (±1.414) 23 (±0.707) 14 (±0.707) 8 (±0.707) 7 (±0.707) 52 (±1.414)

75.56 86.93 92.04 95.45 96.02 70.45

0.7556 0.8693 0.9204 0.9545 0.9602 0.7045

36 (±0.707) 19 (±0.707) 11 (±0.707) 6 (±0.707) 5 (±0.707)

79.54 89.20 93.75 96.59 97.15

0.7954 0.892 0.9375 0.9659 0.9715

[bmim] [CF3SO3]

[bmim][Ac]

4


Table 2 Variation of CR and η% with temperature in absence and presence of maximum concentration of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules in 1 M HCl. Temperature (K)

308 318 328 338

Corrosion rate (CR) (mg cm−2 h−1) and Inhibition efficiency (η%)

Blank

[bmim][CF3SO3]

[bmim]Cl

[bmim][Ac]

CR

η%

CR

η%

CR

η%

CR

η%

5.86 (±0.059) 8.20 (±0.070) 12.4 (±0.094) 16.6 (±0.188)

– – – –

0.43 (±0.023) 1.40 (±0.047) 3.46 (±0.047) 6.60 (±0.094)

92.61 82.92 72.04 60.24

0.26 (±0.023) 0.93 (±0.047) 2.46 (±0.070) 5.86 (±0.094)

95.45 88.61 80.10 64.65

0.20 (±0.023) 0.73 (±0.047) 1.96 (±0.070) 4.50 (±0.094)

96.59 91.05 84.13 72.89

at different concentrations of [bmim][Cl], [bmim][CF 3 SO 3] and [bmim][Ac] and the results are presented in Table 1 and Fig. S2 (a) (Supporting information). The results showed that the protection ability of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] for mild steel in 1 M HCl increases with increasing concentration. Highest protection efficiency values of 93.18%, 96.02% and 97.15% were recorded for [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] respectively at 8.67 × 10−4 mol L−1. In other words, the order of inhibition efficiencies is [bmim][Ac] N [bmim][CF3SO3 ] N [bmim][Cl]. The difference between the inhibition efficiency values obtained at inhibitor concentrations of 6.94 × 10 −4 mol L −1 and 8.67 × 10 −4 mol L −1 are not significant, which implies that 6.94 × 10−4 mol L−1 is the optimum concentration of the studied ionic liquids. It is well documented in literature that increase in inhibitor concentration causes successive increase in surface coverage as the effectiveness of adsorption increases in the same order. However, after certain concentration, when maximum surface coverage has been reached, further increase in inhibitor concentrations does not make any significant effect on the protection ability of the inhibitors molecule. Furthermore, at lower concentrations, inhibitors molecules adsorb in a flat or nearly flat orientations that offer maximum surface coverage because at lower concentration intermolecular attractive force is dominated. Whereas, inhibitor molecule at concentration higher that optimum, adsorb by their vertical orientation that offer minimum surface coverage because at high concentration the molecules experience intermolecular repulsion. The investigated ionic liquids have the same cation, 1-butyl-3methylimidazolium ion but different anions. Therefore, the difference in their corrosion inhibition efficiencies could be traced to their different anions. The presence of acetate ion in [bmim][Ac] results in its highest protection ability among the investigated ionic liquids, while the lowest inhibition tendency of [bmim]Cl is attributed to the presence of chloride

ions. The effects of temperature on the protection ability of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] were investigated and the results are contained in Table 2 and Fig. S2(b) (Supporting information). The results revealed that inhibition efficiencies of all the studied ionic liquids decrease with increase in temperature. The decrease in protection

Fig. 1. Arrhenius plots for mild steel corrosion in 1 M HCl in the absence and presence of [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules.

Fig. 2. (a) Langmuir, (b) Temkin and (c) Freundlich adsorption isotherms for the adsorption of [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules on mild steel surface.

C. Verma et al. / Journal of Molecular Liquids 273 (2019) 1–15 Table 3 Kads and ΔG°ads for [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] on mild steel in 1 M HCl. Inhibitor

Kads (104 M−1)

Temperature (K) [bmim]Cl [bmim][CF3SO3] [bmim][Ac]

308 1.50 2.51 3.40

318 0.58 0.93 1.22

−ΔG°ads (kJ mol−1) 328 0.30 0.48 0.63

338 0.18 0.21 0.32

308 34.92 36.24 37.01

318 33.55 34.79 35.50

32.87 33.18 34.09 34.84

32.38 33.08 32.91 34.00

ability of ionic liquids upon increase in environmental temperature can be attributed to desorption of [bmim][Cl], [bmim][CF3SO3] and [bmim] [Ac] molecules from the surface. At elevated temperatures, inhibitor molecules acquire high kinetic energy that results into reduction in the attractive force between inhibitor molecules and metallic surface. Furthermore, increased in temperature can also results into acid catalyzed etching, rearrangement and fragmentation of inhibitor molecules, which ultimately lower inhibition efficiency. The relationship between inhibition efficiency and temperature can be explained with the help of Arrhenius equation [24–26,28]: logðC R Þ ¼

−Ea þ logA 2:303RT

ð13Þ

where Ea represents the activation energy (kJ mol−1), CR is the rate constant (mg cm−2 h−1), R, T and A represent the gas constant, absolute temperature and pre-exponential factor, respectively. Arrhenius plots for mild steel corrosion in 1 M HCl under inhibited and uninhibited conditions are shown in Fig. 1, while the values of slope, intercept, regression coefficients (R2) and activation energies are listed in Table S2

5

(Supporting information). The results showed that the dissolution of mild steel in the presence of [bmim][Cl], [bmim][CF3SO3] and [bmim] [Ac] molecules requires 78.58 kJ mol−1, 88.59 kJ mol−1 and 89.34 kJ mol−1 activation energy, respectively. The Ea values in the presence of ionic liquids are much higher than that of the uninhibited acid solution (30.50 kJ mol−1), which indicates that the presence of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules in the electrolytic medium creates a barrier for corrosion [34,35]. This observation suggests that the [bmim][Cl], [bmim]Cl, [bmim][CF3SO3] and [bmim] [Ac] molecules adsorb and create protective film on mild steel surface. Obviously, in acidic medium metallic surface becomes negatively charged owing to the adsorption of counter ions (chloride) of electrolyte and therefore cationic 1-butyl-3-methyl-imidazolium attracted strongly by the oppositely charged metallic surface through electrostatic force of attraction. Therefore, it is expected that the 1-butyl-3methyl-imidazolium cationic moieties of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] exert major influence on the their inhibition behavior and anionic moieties contributed to the inhibition only slightly. The mechanism of adsorption of the ionic liquid molecules on mild steel surface can be explained with the aid of adsorption isotherms. Different adsorption isotherms were explored based on the respective model relationships between surface coverage and inhibitor concentration. Temkin, Freundlich and Langmuir adsorption isotherm models were tested and the values of relevant adsorption parameters are listed in Table S3 (Supporting information), while isotherm plots are shown in Fig. 2. The regression coefficients are mostly close to unity for the Temkin isotherms. Therefore, the adsorption of [bmim][Cl], [bmim] [CF3SO3] and [bmim][Ac] molecules on mild steel surface in 1 M HCl is best described by the Temkin adsorption isotherm. The relationship

Fig. 3. Variation of EOCP vs. with time for mild steel corrosion in 1 M HCl with and without [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules.

6


Fig. 4. Polarization curves for mild steel corrosion in 1 M HCl with and without [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules.

between surface coverage and Kads can be represented as follows [28,35]: K ads C ¼

θ 1−θ

ð14Þ

In Eq. (15), C represent the constant for adsorption-desorption process of the [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules on

metallic surface. The Gibb's free energy (ΔGads) was calculated according to the equation [28,35]: ΔG0ads ¼ −RT ln ð55:5K ads Þ

ð15Þ

where 55.5 is the molar concentration of water in the acidic electrolyte solution, while all other symbols have their usual meaning.

Table 4 Potentiodynamic polarization parameters for mild steel corrosion in the absence and presence of different concentrations of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules. Inhibitor

Conc (mol L−1)

Ecorr (mV)

icorr (μA/cm2)

βa (mV/dec)

−βc (mV/dec)

η%

Θ

Blank [bmim][Cl]

– 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4

−423.56 −429.46 −442.84 −436.03 −448.96 −439.11 −450.81 −464.88 −447.74 −421.10 −446.47 −448.96 −448.31

408.54 117.46 76.13 48.72 36.11 92.15 52.17 42.65 28.24 72.43 46.08 34.63 18.23

103.24 77.90 67.38 99.34 113.55 92.72 117.44 138.76 154.38 54.41 68.69 116.42 171.92

62.26 89.31 98.68 92.18 106.13 79.58 100.14 96.84 95.46 108.51 84.37 110.31 101.27

– 71.24 81.36 88.07 91.16 77.44 87.23 89.56 93.08 82.27 88.72 91.52 95.53

– 0.7124 0.8136 0.8807 0.9116 0.7744 0.8723 0.8956 0.9308 0.8227 0.8872 0.9152 0.9553

[bmim][CF3SO3]

[bmim][Ac]


The calculated values of Kads and ΔGads for [bmim][Cl], [bmim] [CF3SO3] and [bmim][Ac] are listed in Table 3. It is known that a high value of Kads refers to strong adsorption tendency of the inhibitor

7

molecule, while a low value indicates weak adsorption ability. In the present study, the values of Kads are very high, indicating that [bmim] [Cl], [bmim][CF3SO3] and [bmim][Ac] have strong adsorption tendency

Fig. 5. (a–c): Nyquist (left side) and Bode frequency (right side) plots for mild steel corrosion in 1 M HCl in the absence and presence of [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules; (d): Equivalent circuit used for the analysis of the EIS data.

8


at mild steel/1 M HCl interface [28,35]. The values of ΔGads for the studied compounds range from −33.08 kJ mol−1 to −37.01 kJ mol−1. The negative values of ΔGads suggest that adsorption of the [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] on metal-electrolyte interfaces is an spontaneous process [28,34,35]. 3.2. Electrochemical studies The inhibitive strengths of the studied ionic liquids were also determined by electrochemical studies in order to corroborate the results from gravimetric measurements. The evolution of the open circuit potential (EOCP) with time was recorded as shown in Fig. 3. EOCP of the working electrode was recorded with respect to the reference electrode (Ag/AgCl) for the period of 2000 s. The EOCP-time profile in Fig. 3 shows that the EOCP is highly unstable within the first 400 s and assumes relatively straight line afterwards. This suggests that the mild steel/electrolyte interface assumed a stable after about 400 s and the 1200 immersion time is enough to achieve EOCP stabilization before further electrochemical measurements. In the case of inhibitor-containing systems, the initial variation and subsequent stabilization of the EOCP might indicate initial dissolution of surface oxide layers (Fe2O3 and Fe3O4) and ensuing formation of protective films of the inhibitors molecules [33,36]. Furthermore, the studied ionic liquids shifted the EOCP to more positive values relative to that of the blank. This suggest that the studied ionic liquids behaved as anodic type inhibitors, that is, they adsorb on the anodic sites on the mild steel surface and inhibit the anodic dissolution reaction [37,38]. Polarization curves for the acid driven dissolution of mild steel with and without [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules are shown in Fig. 4. The parameters extracted from the extrapolation of the

linear regions of the curves to the corrosion potential are listed in Table 4. The results showed that both the anodic and cathodic Tafel curves are affected by the addition of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac], even though the outlook of the polarization profiles in the absence and presence of the inhibitors is similar. This observation suggests that the studied ionic liquids affect the corrosion of mild steel through the surface-blocking by adsorbing on mild steel surface. The corrosion potential (Ecorr) is displaced by the ionic liquids when compared to that the Ecorr of the blank. The little changes in the Ecorr values as the concentration of the inhibitors vary could imply that the inhibitive effects of the studied compounds is based on geometric blocking of active sites on the steel surface [39–41]. Furthermore, the values of both anodic (βa) and cathodic (βc) Tafel slopes are affected by the inhibitor molecules. The βa values of are much more affected than the βc values, which further revealed the prime effects of the inhibitors on anodic reactions. Hence, [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] are mixed-type inhibitors with prevalent anodic influence [37,38]. The inhibition efficiencies of the ionic liquids follow the order [bmim][Ac] N [bmim][CF3SO3] N [bmim][Cl], which is consistent with what was observed from the gravimetric measurements. Nyquist and Bode frequency plots of acidic dissolution of mild steel with and without [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules are shown in Fig. 5. The Nyquist plots revealed a single semicircle in the inhibited and non-inhibited media, indicating that the acidic dissolution of mild steel involves single charge transfer mechanism. The concept of single charge transfer mechanism for acidic mild steel dissolution was also supported by single maxima of the Bode phase angle plots (Fig. 6). The diameters of the Nyquist curves increase with increasing concentration of the inhibitors, which suggests that the charge transfer process from metal (mild steel) to electrolyte (1 M HCl)

Fig. 6. Bode phase angle plots for mild steel corrosion in 1 M HCl in the absence and presence of [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules.


9

Table 5 Electrochemical impedance parameters for mild steel corrosion in the absence and presence of optimum concentration of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules. Inhibitors Blank [bmim][Cl]

[bmim][CF3SO3]

[bmim][Ac]

Conc (mol L−1)

Rs (Ω cm2)

Rct (Ω cm2)

n

Cdl (μF cm−2)

η%

θ

1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4 1.73 × 10−4 3.47 × 10−4 5.20 × 10−4 6.94 × 10−4

1.291 0.883 1.63 1.63 1.32 0.851 1.39 1.17 1.75 1.57 9.27 1.76 1.72

15.50 58.81 98.07 148.37 230.68 76.64 199.61 217.83 313.25 87.83 121.73 230.24 337.28

0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997

222.68 112.80 195.59 110.98 81.41 191.27 92.00 87.09 73.82 123.59 59.07 51.55 48.09

– 73.64 84.19 89.55 93.28 79.77 92.23 92.88 95.05 82.35 87.26 93.26 95.40

– 0.7364 0.8419 0.8955 0.9328 0.7977 0.9223 0.9288 0.9505 0.8235 0.8726 0.9326 0.9540

becomes more difficult in with increase in inhibitor concentration. The effectiveness of adsorption and protection ability of inhibitor molecules increase with increase in concentration, which retard the rapid charge transfer (oxidation of metal) from metal to electrolyte. On this basis, it can be concluded that the [bmim][Cl], [bmim][CF3SO3] and [bmim]

[Ac] molecules adsorb at mild steel/electrolyte interfaces and act as interfacial corrosion inhibitors. EIS indices were obtained by fitting the EIS results with the aid of an appropriate equivalent circuit (Fig. 5d). The circuit has three basic elements namely, charge transfer resistance (Rct), solution resistance (Rs) and a constant phase element (CPE). The

Fig. 7. SEM micrographs of mild steel corroded for 3 h in 1 M HCl (a), 1 M HCl + [bmim]Cl (b), 1 M HCl + [bmim][CF3SO3] (c) and 1 M HCl + [bmim][Ac] (d).

10


CPE was used in lieu of pure capacitor in order to ensure adequate description of the electrical structure of the electrode/electrolyte interface. The CPE impedance can be expressed as [39–41]: −1 1 Z CPE ¼ ðjωÞn Y0

ð16Þ

where, n is the phase shift and it is a gauge of the nature of CPE such that when n = 1, Y0 = C, indicating a capacitive CPE, n = 0, Y0 = R, meaning a resistive CPE, n = 0, Y0 = 1/L, implying an inductive CPE, and n = 1/2, Y0 = W for a Warburg impedance. More so, the value of n can also be used as a gauge of surface inhomogeneity, such that a large value of n is associated with low surface roughness and vice versa. In the present study n values are constant and very close to unity for inhibited and non-inhibited conditions indicating that the CPE is pseudo-capacitive in nature [26,28]. A high value of phase angle usually an indication of low surface roughness. The Bode phase angle plots presented in Fig. 6 revealed that the phase angle values in the presence of the inhibitors are higher than that of the blank. This observation suggests that the mild steel surface is smoother in the presence of the inhibitors, due to the formation of protective film of [bmim]Cl, [bmim][CF3SO3] and [bmim][Ac] molecules. The [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules adsorb on mild steel/electrolyte interfaces resulting in the formation of electric double layer, which has some capacitive characteristics and having the double layer capacitance (Cdl). The Cdl value can be estimated as [39–41]: C dl ¼ Y 0 ðωmax Þn−1

ð16Þ

where ωmax represents the frequency at which the imaginary impedance assumes the highest value (in rad s−1). The calculated values of Rct and Cdl along with other EIS indices are listed in Table 5. The results showed high values of Rct and low values of Cdl for mild steel in 1 M HCl in the presence of the studied ionic liquids compared to the values without the additives. These observations are attributed to the effective adsorption of the inhibitor molecules at the mild steel/electrolyte interfaces. The adsorbed film then acts as barrier for charge transfer and decreases the value of dielectric constant [39–41]. The inhibition efficiencies of the ionic liquids derived from the impedance measurements also follow the order [bmim][Ac] N [bmim][CF3SO3] N [bmim] [Cl] in accordance with the results obtained from gravimetric and polarization studies. 3.3. Surface morphology studies Figs. 7 and 8 show the SEM and AFM images of mild steel surfaces after dipping in 1 M HCl (in the absence and presence of the studied inhibitors) for 3 h. It can be seen that the SEM and AFM micrographs are highly scratched and corroded with obvious pits and cracks in the absence of the inhibitors. AFM micrograph of mild steel surface dipped in 1 M HCl without the inhibitor showed average surface roughness of 372 nm, while the surfaces of mild steel in the inhibitor-containing acid solution have average surface roughness values of 187 nm, 134 nm and 114 nm for the media containing [bmim][Cl], [bmim] [CF3SO3] and [bmim][Ac] respectively. The surface roughness of the polished mild steel was 85 nm. Thus, SEM and AFM micrographs showed relatively smoother surfaces for mild steel dipped in the inhibitor-containing corrosive media. The improvement in the surface of mild steel subjected to the inhibitor-containing electrolytes is as a

Fig. 8. AFM micrographs of mild steel corroded for 3 h in 1 M HCl (a), 1 M HCl + [bmim]Cl (b), 1 M HCl + [bmim][CF3SO3] (c) and 1 M HCl + [bmim][Ac] (d).


11

Fig. 9. Optimized molecular structures, HOMO and LUMO frontier molecular orbital images of neutral and protonated forms of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules.

result of the protective film of [bmim][Cl], [bmim][CF3SO3] and [bmim] [Ac] molecules, which serves as a barrier between the steel and the corrosive acid ions. 3.4. Computational studies The optimized structures and the graphical images of the frontier molecular orbitals of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] in both the neutral and protonated forms are shown in Fig. 9. Obviously, the highest occupied molecular orbital, HOMO depicts the electron rich region(s) of a molecule and it is mostly disposed to inhibitor-tometal electron transfer for covalent bonding formation between the metal and the inhibitor. The lowest unoccupied molecular orbital, LUMO reveals the electron deficient sites in the molecule and it is susceptible to electron acceptance during metal-inhibitors interactions. The HOMO is mainly located on the anionic part of ionic liquids, especially for the neutral forms of the compounds. This suggests that the

anions of the studied ionic liquids are most likely involved in forward donation of electrons to mild steel during the adsorption process. The electron density field of the anions is observed to extend slightly to the methyl (\\CH3) substituent on the cationic moiety. The two nitrogen atoms in the imidazolium ring might not be directly involved in electron donation, which is due to the electropositive character of the imidazolium ring. The trifluoromethanesulphonate anionic moiety of [bmim][CF3SO3] does not participate in the HOMO of the neutral molecule due to the high electronegativity of fluorine atoms. The three fluorine atoms might also make the sulphonate functional group to be relatively less electron donating than the acetate anion of [bmim][Ac]. It can also be seen that the LUMOs are mainly located on the imidazolium ring, leaving the methyl and butyl substituents of the ring. Reactivity parameters computed for the neutral and protonated forms of the studied ionic liquids are listed in Table 6. HOMO and LUMO energies, EHOMO and ELUMO respectively, are measures of electron donating and accepting

Table 6 DFT parameters computed for the neutral and protonated forms of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules using B3LYP/6-31G(d) model. Inhibitor

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

χ (eV)

η

σ

ΔN

μ

[bmim][Cl] [bmim][CF3SO3] [bmim][Ac] [bmim][Cl]-H+ [bmim][CF3SO3]-H+ [bmim][Ac]-H+

−4.3380 −4.5905 −2.1676 −4.1018 −4.5132 −3.3331

−0.1253 −0.9251 −0.8541 0.3491 −0.4375 −0.0356

4.2126 3.6653 1.3135 4.4509 4.0757 3.2974

2.2317 2.7578 1.5109 1.8763 2.4754 1.6843

2.1063 1.8326 0.6567 2.2254 2.0378 1.6487

0.4747 0.5456 1.5226 0.4493 0.4907 0.6065

1.1318 1.1573 4.1788 1.1511 1.1101 1.6120

10.3177 17.1894 17.7375 11.4300 18.6206 16.0367

12


Fig. 10. Side and top views of the final adsorption of the [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules on the Fe(110) surface in solution.

tendencies of the inhibitor molecules [39–41]. The results showed that the values of EHOMO and ELUMO for both the neutral and protonated inhibitors did not show any regular trend in relation to the inhibition efficiencies of the studied ionic liquids. The energy gap, ΔE is a measure of relative reactivity of a molecule. A low value of ΔE connotes high reactivity and vice versa [39–44]. It can be seen from the results that values of ΔE increase in the order [bmim][Ac] b ([bmim][CF3SO3]) b [bmim] [Cl], which suggests that [bmim][Ac] is the most reactive of the three ionic liquids, in agreement with the observed trend of corrosion inhibition efficiencies. Electronegativity (χ) is another reactivity parameter being frequently used as a measure of the readiness of a molecule to release its electrons to an accepting specie. A high value of χ implies high electron holding ability of a molecule. The values of χ in Table 6 revealed that although values χ did not showed any predicted trends, however, [bmim] [Ac] showed lowest value of χ (1.5109 eV) among the investigated ionic liquids, suggesting that it has highest electron donating tendency. Global harness (η) and softness (σ) are also reactivity parameters whose magnitude depends upon the values of EHOMO and ELUMO. A high value of softness and low value of hardness indicate high chemical reactivity and inhibition ability. The extent of metal-inhibitor interactions can be measured by the value of electron transfer (ΔN) from inhibitor to the metal. The calculated values of the fraction of electron transfer for the studied ionic liquids increase in the order [bmim][Cl] b [bmim][CF3SO3] b [bmim][Ac], which is in agreement with the order of inhibition efficiencies of the molecules. The computed values of dipole moment (μ) for the neutral molecules are also increase in the same order as the inhibition efficiencies. That is, [bmim][Ac] has the highest dipole moment, suggest that it is the most polarizable among the studied ionic liquids. Molecular dynamics (MD) is a physics-based modeling method that provides detailed information on the fluctuations and conformational

Table 7 Selected energy parameters obtained from MD simulations for adsorption of investigated [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules on Fe(110) surface. System

EInteraction (kJ/mol)

EBinding (kJ/mol)

Fe(110) + [bmim][Cl] Fe(110) + [bmim][CF3SO3] Fe(110) + [bmim][Ac]

−331.77 −568.06 −601.41

331.77 568.06 601.41

changes of atoms and molecules in materials [45]. In recent times, molecular dynamics simulations have also been used to explore specific interactions of corrosion inhibitor molecules with metal surface in a range of environments [46–48]. In the present investigation, MD simulations study was performed on the investigated ionic liquid molecules under the influence of corrosive ions such as H3O+ and Cl−. The most favourable adsorption alignments for the inhibitor molecules on Fe (110) are shown in Fig. 10. In the stable configurations, the inhibitor molecules are nearly parallel to the Fe(110) plane. In all cases, the inhibitor molecules are laying close to the surface, suggesting that the inhibitor molecules can strongly adsorb on iron surface to achieve high inhibition effectiveness. It can be observed from Fig. 10 that the anions of [bmim][CF3SO3] and [bmim][Ac] participate actively in the adsorption process, while the anion of [bmim][Cl], stays farther away from the Fe(110) surface. The interaction (Einteraction) and binding (Ebinding) energies for the three inhibitors were calculated and listed in Table 7. The huge negative values of the Einteraction for the investigated ionic liquids suggest that they strongly and spontaneously adsorbed on Fe(110) surface, which is an indication of their adsorption on mild steel surface, resulting into formation of stable inhibitive films [49–51]. The high values of Ebinding for [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules indicate that these molecules strongly adsorbed on metallic surface via more than one interaction (bindings). On the other hand, the high magnitude of the binding energies suggesting that the adsorption system is more stable and that there is more than one bond to the iron surface per ionic liquid molecules [52,53]. It is also important to mention that very high value of Ebinding (300–600 kJ mol−1) for [bmim][Cl], [bmim] [CF3SO3] and [bmim][Ac] suggest that they interact with metallic surface mainly through charge or electron sharing (chemical interactions). This observation further supports the results derived from weight loss method that the [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] mainly adsorb via chemisorption mechanism. As expected, the trend of the interaction energy corresponded to the trend of percentage inhibition efficiency, i.e. [bmim][Cl] b [bmim][CF3SO3] b [bmim][Ac]. This suggests that there are more reactive sites in [bmim][Ac] that act as an additional interaction centers with the metal surface. To gain more information about the interaction of the tested ionic liquids with mild steel surface, the radial distribution functions (RDFs) were calculated from the MD simulations trajectories [54]. The RDF, expressed as gAB(r), is defined as the probability of finding particle “B” within the range r + dr around particle A. It is one of the most important


tools for structural characterization of materials. The mathematical expression of gAB(r) is [55]: g AB ðr Þ ¼

NA X NB δ rij −r 1 1 X hρB ilocal NA i∈A j∈B 4πr 2

ð18Þ

13

where 〈ρB〉local represents the particle density of B averaged over all shells around particle A. The RDF is a useful method of judging the type of molecule–metal interaction [54]. The RDFs of the three inhibitor molecules are displayed in Fig. 11. The bond length of Fe-anion and Fe-cation are all less than 3.5 Å except of Fe-anion in [bmim][Cl]. This suggests that significant interactions could exist between the π-electrons and heteroatoms of ILs and metal surface [56]. Furthermore, the results indicate that the interactive forces of the cation and the anion on the iron surface were similar and very important in the case of [bmim][CF3SO3] and [bmim][Ac] compared to [bmim][Cl]. This implies that the relationship between the extent of adsorption and inhibition efficiency of the ionic liquids is a reflection of the interactive forces that dominate at the reactive centers of the inhibitor molecules. Evidently, the adsorption mode of [bmim] [CF3SO3] and [bmim][Ac] can block more active corrosion sites on iron surface at the same concentration, explaining the better inhibition effectiveness of these compounds than [bmim]Cl. These results are in good agreement with the results of quantum chemical calculations where the trend of molecular reactivity was found to correlate with the inhibitive strengths of the ionic liquids. 4. Mechanism of inhibition Similar to most of the traditional volatile corrosion inhibitors, ionic liquids also inhibit metallic corrosion by blocking anodic as well as cathodic reaction sites. Based on literature reports and the results of the present study, the mechanism of anodic and cathodic reactions and their inhibition can be represented as follows [57–59]: Anodic reactions: Fe þ nH2 O→FeðH2 OÞn

ð19Þ

− − FeðH2 OÞn þ 2Cl → FeðH2 OÞn 2Cl

ð20Þ

− þ − þ FeðH2 OÞn 2Cl þ ½bmim → FeðH2 OÞn 2Cl ½bmim

ð21Þ

− − þ − FeðH2 OÞn 2Cl ½bmim þ e− → FeðH2 OÞn 2Cl bmim

ð22Þ

The anodic dissolution of mild steel takes place according to the Eqs. (20) and (21). However, in the presence of imidazolium based ionic liquids, the rate of anodic dissolution reactions become slower owing to the formation of protective film by [bmim][Cl], [bmim] [CF3SO3] and [bmim][Ac] as shown in Eqs. (22) and (23) which separate metallic surface from aggressive acidic solution. Cathodic reactions: 1 H þ þ e− → H 2 2 þ

ð23Þ þ

Fe þ ½bmim →ðFe½bmimÞ þ

ðFe½bmimÞ þ e− →ðFe½bmimÞ

Fig. 11. RDFs of investigated [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] molecules on the Fe(110) surface in 1 M HCl solution.

ð24Þ ð25Þ

In the absence of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac], major cathodic reactions involve elimination of hydrogen gas according to Eq. (23). However, in the presence of the ionic liquids, the positively charged imidazolium moiety adsorb at cathodic site and inhibit cathodic reactions as shown in Eqs. (24) and (25). During metal-ionic liquid interactions, it can be assumed that negatively charged counter ions of electrolyte (1 M HCl) adsorb on the metallic surface and attract the positively charged imidazolium moiety of [bmim][Cl], [bmim][CF3SO3] and [bmim] [Ac] through electrostatic force of attraction (coulomb force) [57–59]. Moreover, chloride ion of [bmim]Cl, trifluoromethanesulphonate ion of [bmim][CF3SO3] and acetate ion [bmim][Ac] can also act as counter ions that facilitate the adsorption of imidazolium moiety through electrostatic interactions which results into multilayer adsorption i.e. physisorption

14


Fig. 12. Schematic illustration of anodic and cathodic reactions inhibition of mild steel corrosion using investigated imidazolium based ionic liquids.

[60–62]. However, the adsorbed imidazolium moieties of ionic liquids can accept electrons derived from further oxidation of iron atoms and get neutralized. The neutral form of the imidazolium nitrogen can donate their non-bonding electrons to the d-orbitals of surface iron atoms and form co-ordinate bonds (chemisorption) and results into mono-layer adsorption [57,60–62]. Furthermore, this type of charge donation causes inter-electronic repulsion in the metal, which encourages transfer of charge from the occupied orbital of the metal to the anti-bonding molecular orbitals of ionic liquids (retro-donation). The adsorption behavior of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] and mechanism of anodic and cathodic reactions inhibition is shown in Fig. 12.

iii. iv.

v.

vi.

5. Conclusions vii. Three 1-butyl-3-methylimidazolium based ionic liquids have been investigated as sustainable “green” corrosion inhibitors using various experimental and theoretical methods. The following conclusions have been derived from the studies: i. The investigated ionic liquids act as effective corrosion inhibitors and their inhibition efficiencies follow the order: [bmim][Cl] b [bmim][CF3SO3] b [bmim][Ac]. ii. The effectiveness of the imidazolium based ionic liquids as corrosion inhibitors depends on the nature of the anion. The acetate

viii.

ion enhanced the inhibition efficiency of [bmim][Ac] compared to the contribution of chloride ion [bmim][Cl]. Polarization study revealed that the investigated ionic liquids act as mixed type corrosion inhibitors with prevalent anodic effect. The adsorption of [bmim][Cl], [bmim][CF3SO3] and [bmim][Ac] at the mild steel/1 M HCl interface obeyed the Temkin adsorption isotherm. EIS study revealed that [bmim][Cl], [bmim][CF3SO3] and [bmim] [Ac] act as interfacial corrosion inhibitors and formed pseudocapacity double layer film on the steel surface. AFM and SEM revealed smoother mild steel surface in the presence of ionic liquids, thereby confirming the protective activity of the inhibitors for the mild steel. DFT computational study shows that the inhibitive strengths of the ionic liquids correlate with their relative reactivity and the presence of highly electronegative atoms in the anion of the ionic liquid weakens the inhibitive activity. MD simulations study revealed that the three ionic liquids considered in this study adsorbed on Fe(110) surface in a nearly flat orientation, resulting into an enhanced surface coverage and high inhibition efficiency. The values of adsorption energy (E ads) are consistent with the order of inhibition efficiencies obtained from experimental measurements.


Acknowledgment C. Verma and L. O. Olasunkanmi thankfully acknowledge North West University (NWU), South Africa for providing financial support under the Post-Doctoral Fellowship scheme. The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. RGP-160. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2018.09.139. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

[23]

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