MD explorations

Journal of Molecular Liquids 293 (2019) 111559

Contents lists available at ScienceDirect

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

Highly effective mild steel corrosion inhibition in 1 M HCl solution by novel green aqueous Mustard seed extract: Experimental, electronic-scale DFT and atomic-scale MC/MD explorations Ghasem Bahlakeh a,⁎, Ali Dehghani a, Bahram Ramezanzadeh b, Mohammad Ramezanzadeh b a b

Department of Chemical Engineering, Faculty of Engineering, Golestan University, Aliabad Katoul, Iran Department of Surface Coatings and Corrosion, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 1 April 2019 Received in revised form 29 July 2019 Accepted 10 August 2019 Available online 19 August 2019 Keywords: Green Mustard seed extract DFT/Monte Carlo/Molecular dynamics EIS Potentiodynamic polarization FT-IR Weight loss

a b s t r a c t The inhibitory behavior of Mustard seed extract for mild steel (MS) in HCl solution was explored. The extract structures were studied by FT-IR analysis connected with UV–Vis spectroscopy. The surface studies were accomplished using contact angle and SEM/AFM tests. The corrosion studies were scrutinized utilizing electrochemical techniques of weight loss, impedance spectroscopy (EIS), and potentiodynamic polarization at different temperatures (25, 35, 45 and 55 °C) and extract concentrations. To corroborate experiments, detailed theoretical studies were conducted. The EIS examination proved the maximum inhibition efficiency of 94% for 200 mg·L−1 of the extract. By enhancing the solution temperature, the steel resistance against corrosion decreased, and the efficiencies of 93, 90, 91, and 93% were obtained at 25, 35, 45, and 55 °C, respectively. Potentiodynamic polarization results demonstrated that the steel corrosion happened under control of anodic and cathodic reactions even at higher temperatures. The weight loss analysis proved 97% inhibition efficiency for the acid solution containing 200 mg·L−1 extract. The study of the adsorption isotherms showed that the adsorption of Mustard seed extract molecules was in the line of Langmuir isotherm. Electronic/atomic simulations proved the adsorption of Mustard seed extract molecules onto the steel substrate. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Corrosion is an electrochemical interaction between the environment and the metal which causes economic waste in different industrial processes [1]. The usage of different acidic solutions in drilling operations, exploration of gas, pickling, and acid cleaning could increase the unwanted corrosion of metal, leading to the loss of material [1]. For controlling the corrosion of metallic substrates, different approaches such as coatings and linings, cathodic protection and corrosion inhibitors have been frequently applied in a variety of industrial applications [2]. Among these methods, corrosion inhibitors are the most popular method to reduce the unwanted metal dissolution rate. The organic compounds with inhibitive activity in acid solutions contain atomic sites with high electron density which can donate charges to proper sites of metal adsorbent [2]. Additionally, the existence of polar functional groups in the structure of the inhibitors (including \\OH, C_O, \\CN, and\\NH2) can accelerate the adsorption of inhibitor molecules. The toxic properties of these organic inhibitors lead to the undesired

⁎ Corresponding author. E-mail address: [email protected] (G. Bahlakeh).

https://doi.org/10.1016/j.molliq.2019.111559 0167-7322/© 2019 Elsevier B.V. All rights reserved.

environmental effects, and for this reason, they have been replaced with green inhibitors. Green corrosion inhibitors are good sources of heteroatoms and polar functional groups which protect the metal surface against acidic media with lower environmental risks [3]. Ionic liquids [4], surfactants [5], biopolymers and plant extracts [6] are extensively employed as non-toxic alternatives for conventional inhibitors. However, compared to the biopolymers and ion liquids, the extracts of plants are more cost-effective and readily available sources for green corrosion inhibitors. Most of the parts of plants involve bioactive compounds with high anti-oxidant activity. Abdullah et al. applied 500 mg·L−1 of Curcumin, Parsley and Cassia bark extracts for corrosion prevention of mild steel in 1 M HCl and observed inhibition levels of 82.36, 84.57 and 87.57%, respectively [7]. Krishnan and Shibli examined the corrosion inhibition of Sesbania grandiflora leaf extract and found that 1000 mg·L−1 of green inhibitor prevented the steel corrosion with 96% efficiency [8]. In another report, Kalaiselvi et al. studied the corrosion prevention ability of Coreopsis tinctoria extract. Their study demonstrated that 500 mg·L−1 of Coreopsis tinctoria extract in 0.5 M H2SO4 solution on mild steel resulted in 81% inhibition [9]. In another similar work, Ikeuba et al. showed that saponins and crude extracts of Gongronema latifolium are suitable sources for green inhibitors. These authors reported the efficiencies of 94 and 97% in 5 M H2SO4, applying saponins and crude extracts, respectively [10].

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G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

The corrosion suppression action of Kiwi juice was investigated by Khadom et al. [11]. It has been proved that the optimum concentration of this green inhibitor protected the mild steel substrate in 1 M HCl with maximum efficiency of 96%. Based on the investigation given by Bader et al. [12], the methanolic extract of Acacia hamulosa (600 mg·L−1) controlled the corrosion of metal surface dipped in the corrosive media (1 M HCl) with 94% performance. Additionally, Nasr et al. observed that Matricaria recutitachamomile extract prevented the corrosion of steel in 0.5 M NaCl media with 99% effectiveness [13]. Also, Dehghani et al. explored the Tamarindus indiaca extract inhibition property against 1 M HCl steel corrosion. EIS results proved 93% degree of protection after 5 h MS immersion in 1 M HCl solution by applying 800 mg·L−1 of extract [14]. There are different kinds of Mustard plants like green, white, and yellow. The yellow Mustard includes some proteins, fiber, and vitamins such as vitamin C and many of the B-complex vitamins [15]. Previous researches reported that Mustard seeds have high biological characteristics. For example, results from Human & Experimental Toxicology, proved that Mustard seeds have high chemo-preventative potential and sufficient protection against the carcinogens toxicity [15]. The Mustard seed extract includes many active components, making it a good candidate as a corrosion inhibitor which has not been studied yet. In the present effort, the corrosion prevention capability of a novel green inhibitor based on Mustard seed extract for MS surface against HCl solution (1 M) was evaluated by experimental (including electrochemical and surface morphological) and theoretical (Monte Carlo, MC/molecular dynamics, MD and density functional theory, DFT) techniques. The characteristic groups of Mustard seed extract were investigated by UV–Vis and FT-IR analyses. The atomic-level classical simulations employing MC in connection with MD were particularly conducted to theoretically explore the interfacial adsorption affinity of Mustard seed extract species on the steel-based surface. Moreover, the electronic scale theoretical examinations using DFT were performed for detailed understandings of the active sites of extract molecules. 2. Experimental 2.1. Extraction procedure Mustard seed powder was prepared from Tehran-Iran stores. After weighting, one lit of distilled water was refluxed to 30 g of the powder. The resulting solution was heated up to 70 °C and stirred for 3 h. Thereafter, the aqueous solution passed from the filter to separate the unwanted impurities. In the next step, the filtrations were centrifuged for 5 min at 4000 rpm. The clear solution was collected and dried for 24 h at a steady temperature in an oven. Then, a specific amount of Mustard seed extract was dissolved in 1 M HCl for usage in corrosion examinations. The major constituents of Mustard seed extract are presented in Fig. S1. 2.2. Preparation of materials and samples Before all experiments, the MS specimens with elemental composition (wt%) of 93.9% Fe, 1.2% P, 1.1% Mn, 1.0% Si, 0.7% Cr and 1.7% Ni were employed for different electrochemical and surface morphological studies. After abrading with different grades of sand papers (400, 600, 800, and 1000), the samples were thoroughly washed and degreased by acetone and distilled water. Lastly, the spices were dried at room temperature. The HCl solution (1 M) was prepared by HCl 37% (Merck Co.) and distilled water. 2.3. Electrochemical measurements EIS and potentiodynamic polarization tests were performed with ACM electrochemical workstation, and the results were extracted by

ACM software. Electrochemical experiments were conducted by threeelectrode cells setup. The MS coupons, Ag/AgCl Sat. and platinum wire were respectively served as work, reference and counter electrodes. The EIS analysis was done through the frequency domain of 0.01 to 10 kHz with 10 mV constant amplitude. Also, the potentiodynamic polarization potential was accelerated from −250 mV to +250 mV with 0.2 mV s−1 constant scan rate. The EIS measurements were performed for different immersion times (0.5, 2.5, and 5 h) and temperatures (25, 35, 45, and 55 °C). Also, the polarization test was carried out at various temperatures. 2.4. Surface morphology To analyze the microstructure of MS substance, the SEM and AFM tests were considered. The MS samples were retrieved after 5 h immersion in HCl solution, and then the coupons were washed by distillated water and dried. Subsequently, the micrographs of the samples were monitored by Dual scope DS 95–200 Denmark and Philips XL30 for AFM and SEM tests, respectively. 2.5. UV–vis and FT-IR spectroscopies In FT-IR spectroscopy test, the functional groups in the range of 400–4000 cm−1 wavenumber were recorded at room temperature with Thermo Fisher Nicolet iS10 instrument. The UV–Vis test was conducted in homogenous 1 M HCl solution with Cecil ce9200 ranged from 200 to 800 nm wavelength. 2.6. Weight loss measurement The inhibition capacity of Mustard seed extract was further evaluated at distinct operational temperatures utilizing weight loss approach. Prior to each experiment 1 cm × 1 cm MS coupons were polished by 400–1200 grids of sand papers. Then, the spices were weighed carefully. The samples were subsequently immersed in electrolytes (1 M HCl + 0, 50, 100, 150 and 200 mg·L−1 Mustard seed extract). After specific immersion times (1, 2, 3, 4, 5, 12, 24, 48 and 72 h), the samples were taken out and washed employing deionized water. The metallic samples were weighed again. This analysis was also done at different temperatures. The methods chosen for calculating the average weight loss (ΔW), corrosion rate (CR), and corrosion inhibition efficiency (η%) were reported elsewhere [14]. 2.7. Details of theoretical investigations 2.7.1. Equilibrating neutral/protonated molecules For the analysis of electronic properties, the structures of organic molecules in Mustard seed extract were equilibrated applying DFT tools. The major constituents of Mustard seed extract, i.e., allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose are displayed in Fig. S1. Besides the neutral form, the protonated species were taken into account for DFT as the atomic sites with sufficient electrons are likely subject to protonation in the acidic medium [16]. The protonated Mustard compounds were built with linking of a proton to the center with the highest chance of protonation. To determine the atom which has the highest chance for accepting the proton, first, all possible mono-protonated forms of allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose and xylose shown in Fig. S1 (Supporting Information) were designed and equilibrated. Thereafter, the protonation affinity and basicity (termed as PA and B, respectively) of equilibrated compounds were determined [16] and recorded in Table S1. Based on this table, the allyl isothiocyanate, arabinose, galactose and rhamnose molecules protonated from their nitrogen, O3 (hydroxyl oxygen), O3 (ring oxygen), O5 atom (ring oxygen) atoms led to highest PA and B values, respectively. Hence, these sites were found to be the best sites for

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

protonation. In case of diallyl trisulfide species despite the greatest values resulted from C6 atom protonation the C1 site was taken into account for diallyl trisulfide mono-protonation because of the bond breakage. Similarly, due to the bond breakage of the protonated O3 atom of the xylose molecule, the protonation of this molecule was done through O5 atom (i.e., methoxy oxygen). The constructed neutral and chosen protonated forms of allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose and xylose molecules were equilibrated by DFT modeling. The B3LYP functional linked with basis function of 6-31G** [17] was adopted for running DFT equilibration. The molecular optimization was performed under humid situations through PCM model in combination with SCRF theory [18]. The fully relaxed geometries were affirmed by no imaginary frequencies observed in Frequency calculations. Both DFT equilibration and Frequency calculation of Mustard compounds were done using Gaussian 09 package [19]. For the relaxed molecules of Mustard seed extract, the graphics as well as energetics of highest occupied molecular orbital (HOMO and EHOMO) and those of lowest unoccupied molecular orbital (LUMO and ELUMO), the difference of these energies (i.e., ΔE L-H = ELUMO- E HOMO), the partial charges, ionization potential (I), electron affinity (A), and Fukui indices were studied. The atomic charges of Mustard seed extract compounds were examined by the ChelpG scheme. Furthermore, the electronic-scale feature of the amount of electron sharing (ΔN) was quantified by hardness (η) and electronegativity (χ) of green inhibitor and iron work function (Φ) [20,21].

2.7.2. Molecular simulations Interactions of Mustard seed extract molecules with the metallic substrate (i.e., steel) were theoretically assessed by classical simulations. The substrate of steel was described by facet (110) of pure iron metal [16]. Based on the literature [22], this iron surface has more stability compared with other surfaces like iron (100) and iron (111). To this purpose, an iron supercell with thickness 1.5 nm, vacuum region 4 nm and area (14 × 14) replication was prepared through cleaving the unit cell of iron. The surface of (110) is the most energetically stable one among various iron planes [20]. The DFT-equilibrated Mustard seed extract molecules (i.e., allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose) and the resulting supercell were utilized for executing the MC simulation with Adsorption Locator module of Materials Studio software [23]. In case of MD simulations, in addition to single extract molecules, water layer was also added to the constructed cell of Fe (110). The explicit addition of solvent molecules was done so as to continue the dynamics simulations in solution phase. Solution phase was prepared through a cell built at density of water, which composed of water molecules (491 H2O), and corrosive molecules (i.e., 9 Cl− and its counter ion of 9 H3O+) [24]. The number of these molecules was based on 1 M acidic medium [24]. In order to better equilibrate the adsorption model constructed for MD, cell optimization was done by minimizing the potential energy for 50,000 steps. Subsequently, the optimized models were further equilibrated by running MD simulations for the time period of 2 ns. This calculation was performed in the canonical ensemble (NVT) with 1 fs time step at room temperature. Both cell optimization and dynamics modeling were conducted using Forcite module. The potential energy within molecular simulations was expressed via the frequently used force field (FF) of COMPASS implemented in Materials Studio software [23]. The charges of Mustard seed extract molecules derived from the ChelpG method together with Ewald algorithm were adopted for assessment of columbic interactions. Additionally, with the aid of atom-based cutoff (connected with 12.5 Å radius) scheme, the van der Waals interactions were taken into consideration. All iron atoms were kept at fixed position, and the temperature was manipulated through Nose-Hoover thermostat.

3

3. Results and discussion 3.1. FT-IR spectroscopy results Since functional groups play an essential role in the inhibitor/surface interactions, the major characteristic groups of Mustard seed extract compounds were probed by FT-IR spectroscopy. The FT-IR results are presented in Fig. 1. According to this figure, the O\\H bond is noted to center around 3408 cm−1 wavenumber. Bending vibration of C\\H can be seen at 2916 cm−1. At 1655 cm−1, a C_C bond peak has appeared. A medium peak corresponding to the C_N functional group was indicated at a wavenumber of 1530 cm−1. Also, the weak adsorption bond at 1045 and 1052 cm−1 are associated with the C_S and C\\O stretching vibrations, respectively [25]. These observations prove the presence of active groups in the geometry of Mustard seed extract species (e.g., rhamnose, arabinose, and xylose). 3.2. UV–vis spectroscopy of Mustard seed extract To evaluate the interactions between the iron cations (i.e., Fe2+ and Fe 3+ ) and inhibitor molecules, the UV–Vis analysis was performed. The UV–Vis curves were recorded before and after MS coupons subjection to HCl solution. The results of this analysis are depicted in Fig. 1. The resulting curve obtained before metal (i.e., steel) immersion yielded the spectra with two peaks, a distinct sharp peak located at 212 nm wavelength and a weak one which emerged at 325 nm wavelength. These peaks are associated with ππ* (C_C adsorption) and n-π* (C_O adsorption) electrotransitions, respectively. According to Fig. 1, after 24 h immersion of MS in acid solution, the peaks of adsorption with lower intensity happened at 214 nm and 323 nm wavelengths. 3.3. Open circuit potential results Prior to the commencement of the electrochemical test, the potential of MS samples was plotted versus Ag/AgCl reference electrode as a function of immersion time. The plots presented in Fig. 2 show that 60 min exposure of metal samples to 1 M HCl is sufficient for stabilization of all samples. It is entirely evident that a similar trend exists for all samples, and the measured values of OCP continuously lowered toward negative regions. This action confirmed that the samples were preferentially affected by cathodic (hydrogen reduction) reactions [9]. However, the potential variation of the blank sample affirmed the oxide dissolution formation over the mild steel surface. 3.4. Potentiodynamic polarization test The inhibition performance of the inhibitor was explored using polarization test and the results were depicted in Fig. 3. It is perceivable that by the gradual addition of Mustard seed extract to 1 M HCl medium, the cathodic part remained unaltered [9]. The implication of this outcome is that the cathodic reactions by hydrogen evolution mechanism influenced the corrosion of mild steel surface and the prevention mechanism stayed without change at cathodic regions of steel [9]. It is further observed from Fig. 3 that by increasing the Mustard seed extract concentration corrosion current density (icorr) and corrosion potential (Ecorr) slightly decreased, disclosing the slight impact of cathodic corrosion inhibition. Through extrapolation of Tafel curves, a number of potentiodynamic polarization parameters such as Tafel slopes (i.e., anodic, βa and cathodic, βc), Ecorr and icorr were computed, and the extracted data are presented in Table 1. Higher values of βc as compared with βa reveal the fact that the Mustard seed extract inhibitor is predominantly cathodic inhibitor. Also, the shift in anodic Tafel slope values can be related to the adsorption of chloride ions or Mustard seed extract molecules on the steel substrate. The values of Ecorr demonstrate that the maximum displacement of Ecorr between the blank sample and the one protected

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G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

120

a

60

40

1052 & 1045

1655 1530

2916

80

3408

Transmittance (%)

100

C=N C-H

C=C

C-O & C=S

20

O-H 0 3900

3400

2900

2400

1900

1400

900

400

Wavenumber (cm-1) 3

b

before mild steel immersion 212 nm C=C adsorbance

2.5

after mild steel immersion

Absorbance

- * transition 2 214 nm 1.5

1

325 nm 323 nm

0.5

C=O adsorbance

after mild steel immersion

n- * transition

0

200

300

400

500

600

700

800

Wavelength (nm) Fig. 1. (a) FT-IR and (b) UV–Vis spectra of Mustard seed extract.

by 50 mg·L−1 of Mustard seed extract is 20 mV. Such value evidences that the Mustard seed extract could be categorized as an inhibitor with mixed type behavior [12]. Moreover, the decrement of icorr and

corrosion rate of steel against Mustard seed extract concentration (shown in Fig. 3) implies the improvement of passive layer coverage on the metal surface.

-50

blank 50 ppm

OCP vs. Ag/AgCl (mV)

-100

100 ppm 150 ppm

200 ppm

-150

-200

-250

-300

-350 0

60

120

180

240

Immersion time (min) Fig. 2. Open circuit potential versus time for mild steel sample immersed in 1 M HCl solution with different concentrations of Mustard seed extract.

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

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Fig. 3. (a) Potentiodynamic polarization diagram for mild steel immersed in 1 M HCl solution with different concentrations of Mustard seed extract; Potentiodynamic polarization diagram for mild steel immersed in 1 M HCl solution in the presence of (b) 200 mg·L−1 and (c) 0 mg·L−1 Mustard seed extract at different temperatures.

Table 1 Potentiodynamic polarization parameters after 5 h immersion of mild steel in HCl solution with different concentrations of Mustard seed extract and at different temperatures. “±” shows the standard deviation of three measurements. Concentration (mg·L−1) 0 50 100 150 200 Concentration (mg·L−1) 0

200

Temperature (°C)

Ecorr vs. Ag/AgCl (mV)

icorr (mA·cm−2)

CR (mm·year−1)

-βc (mV·dec−1)

βa (mV·dec−1)

25 25 25 25 25

−542 ± 9 −562 ± 12 −558 ± 10 −563 ± 14 −559 ± 8

0.99 ± 0.15 0.35 ± 0.09 0.22 ± 0.05 0.24 ± 0.02 0.21 ± 0.02

10.8 ± 1.12 5.2 ± 0.60 2.6 ± 0.30 2.8 ± 0.30 2.2 ± 0.20

182 ± 12 138 ± 15 156 ± 13 145 ± 8 136 ± 10

85 ± 11 94 ± 9 101 ± 5 95 ± 10 84 ± 9

Temperature (°C)

Ecorr vs. Ag/AgCl (mV)

icorr (mA·cm−2)

CR (mm·year−1)

−βc (mV·dec−1)

βa (mV.dec−1)

25 35 45 55 25 35 45 55

−542 ± 9 −439 ± 16 −458 ± 24 −493 ± 20 −559 ± 8 −461 ± 13 −471 ± 11 −491 ± 11

0.99 ± 0.15 0.87 ± 0.13 1.05 ± 0.14 1.23 ± 0.15 0.21 ± 0.02 0.24 ± 0.04 0.31 ± 0.04 0.34 ± 0.03

10.8 ± 1.12 10.1 ± 1.10 12.6 ± 1.16 14.2 ± 1.19 2.2 ± 0.2 2.7 ± 0.3 3.1 ± 0.3 3.8 ± 0.2

182 ± 12 180 ± 15 178 ± 11 178 ± 13 136 ± 10 130 ± 12 129 ± 11 138 ± 10

85 ± 11 88 ± 9 84 ± 6 89 ± 8 84 ± 9 88 ± 9 89 ± 8 81 ± 7

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The analysis of the polarization test was also performed at different electrolyte temperatures and extract concentrations of 0 and 200 mg·L−1 in 1 M HCl solution (Fig. 3-b and c). The computed Tafel parameters were presented in Table 1. The unchanged polarization curves indicate the simple protection mechanism at high temperatures. The slight differences in Ecorr values at more elevated temperatures reflect that the Mustard seed extract acted as cathodic/anodic inhibitor even at high temperatures [26]. Moreover, it is seen that by increasing the temperature, the icorr values increased significantly, which is ascribed to the desorption of the adsorbed inhibitor molecules from the metal sites, resulting in the steel corrosion intensification. The enhancement of the icorr values of the bare sample points to the disappearance of iron oxide/hydroxide particles from the metallic substrate at higher temperatures, declaring the deep corrosion occurrence [27]. 3.5. EIS test results The Nyquist and Bode plots related to the bare sample and also the steel samples exposed to HCl solutions with different concentrations of Mustard seed extract are presented in Figs. 4 and S1, respectively. Nyquist diagrams demonstrated a single semicircle shape, but the loops are not perfect semicircle. This feature may be attributed to the surface roughness, impurities, discontinuity in the electrode, inhibitor molecules adsorption, and inhomogeneity of the electrode surface [9].

a1

CPE

Unchanged shapes of Nyquist plots clarify that the metal steel corrosion was controlled without changing the corrosion mechanism [9]. Increment in the Nyquist diagrams diameter after addition of Mustard seed extract into acidic solution is ascribed to the Mustard seed extract molecules localization on the active sites of the metal surface, leading to passive layer appearance on the surface. Evidences from Fig. 4 displays a capacitive loop appearance in the high frequencies, which is associated with the charge transfer resistance and/or electric double layer. On the other hand, an inductive loop was shown in lower frequency resulted from adsorption of Cl−, H+, or protonated molecules of Mustard seed extract [28]. The EIS parameters were obtained by fitting the experimental results using one constant equivalent circuit model, and the results obtained from fitting are recorded in Table 2. The quantities of Rp, Rs, CPE, and Cdl in this table, respectively, represent the polarization resistance, the resistance of solution, constant phase element, and double layer capacitance. The mathematical expressions related to the inhibitor efficiency (η) and Cdl were mentioned elsewhere [14]. The data listed reflect that the Rs values in the inhibited samples are lower than those in the un-inhibited one, an indication of solution conductivity increment [12]. The enhancement in Mustard seed extract concentration gives rise to the maximum increase in Rp values, revealing the fact that the inhibitor molecules approached the metal/solution interface. The reduction in Y0 values as a consequence of immersion time and inhibitor concentration suggests the

a2

Rs Rp

b1

CPE

b2

Rs Rp

Fig. 4. Nyquist and Bode plots for mild steel sample during 5 h immersion in 1 M HCl solution with (a1,a2) 0 and (b1,b2) 200 mg·L−1 of Mustard seed extract.

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

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Table 2 Impedance parameters for mild steel during 5 h immersion in HCl solution with different concentrations of Mustard seed extract and at different temperatures. “±” shows the standard deviation of three measurements. Concentration (mg·L−1) 0

50

100

150

200

Concentration (mg·L−1) 0

200

Immersion time (h)

Rs (Ω·cm2)

Rp (Ω·cm2)

CPE Y0 (μS cm−2 Sn2)

0.5 2.5 5 0.5 2.5 5 0.5 2.5 5 0.5 2.5 5 0.5 2.5 5 Temperature (°C)

2.1 ± 0.8 2.3 ± 0.5 2.4 ± 0.4 1.9 ± 0.2 1.4 ± 0.4 2.1 ± 0.6 2.0 ± 0.2 2.5 ± 0.3 2.6 ± 0.4 2.0 ± 0.5 1.9 ± 0.2 2.4 ± 0.2 2.1 ± 0.4 1.8 ± 0.1 1.8 ± 0.1 Rs (Ω·cm2)

32.7 ± 2.1 36.4 ± 2.4 42.6 ± 3.1 210.7 ± 15.4 453.8 ± 14.3 440.8 ± 16.1 369.8 ± 5.1 588.3 ± 14.8 526.0 ± 16.2 237.2 ± 15.2 593.0 ± 12.1 490.4 ± 13.2 291.1 ± 12.5 685.9 ± 9.8 653.1 ± 10.1 Rp (Ω·cm2)

135 ± 3 155 ± 3 200 ± 3 35 ± 8 29 ± 7 35 ± 8 41 ± 3 45 ± 5 59 ± 7 37 ± 7 27 ± 5 36 ± 4 50 ± 4 53 ± 3 49 ± 3

2.4 ± 0.4 2.6 ± 0.2 2.2 ± 0.2 2.1 ± 0.3 1.8 ± 0.1 1.5 ± 0.1 1.9 ± 0.2 1.5 ± 0.2

42.6 ± 3.1 44.4 ± 4.9 35.7 ± 3.8 22.8 ± 2.9 653.1 ± 10.1 453.8 ± 31.8 423.7 ± 24.6 353.0 ± 35.1

solvent (water) molecules replacement with the Mustard seed extract molecules [12]. Using 200 mg·L−1 of Mustard seed extract the maximum efficiency of 94% was achieved after 2.5 h MS immersion in acid solution. Also, a higher magnitude of “n” signifies that upon introduction of Mustard seed extract into corrosive medium the surface homogeneity and smoothness intensified [12]. This is an indication of metal surface damage reduction as well as inhibitor molecules adsorption. Decrement of Cdl values is an implication of reduced local electric constant and/or the enhanced electric double layer width values. The surface coverage (θ) property of various samples was calculated as: θ = (C 0dl − Cidl)/C 0dl . The C0dl and Cidl respectively correspond to the double layer capacitance of the un-inhibited and inhibited samples [14]. The calculated values are given in Table 3. It is clearly evident from Table 3 results that the surface coverage is high enough for inhibiting the samples from corrosion. It is obvious that the Nyquist diameter size of the inhibited sample decreased by increasing the solution temperature (Fig. 5). The Nyquist diagram related to the bare sample did not change at 35 °C, which is likely because of the Fe oxide/hydroxide presence on the metallic substrate. However, after enhancing the temperature, the corrosion products were removed, and the sample resistance decreased drastically. The Nyquist simple shapes proved the simple corrosion inhibition mechanism (i.e., charge transfer). It is evident from Table 2 that the Rs values for the case of pure tempered 1 M HCl (i.e., inhibitor-free) changed slightly. However, the inhibited sample solution resistance diminished to lower values by enhancing the solution temperature, indicating the direct solution conductivity relation with temperature. The decrement of Rp values of the inhibited sample is related to the inhibitor molecules desorption from the steel surface. In other words, the desorption of inhibitor molecules at high temperatures resulted in the reduction of steel corrosion resistance [29]. The presence of corrosion products (i.e., Fe oxide/hydroxide) can provide some degree of protection for steel surface at lower temperatures. However, by increasing the solution temperature, the corrosion products can be removed from the metallic surface, causing to the deep corrosion of the metal surface.

0.86 ± 0.2 0.87 ± 0.3 0.86 ± 0.2 0.91 ± 0.2 0.92 ± 0.1 0.90 ± 0.2 0.91 ± 0.1 0.91 ± 0.1 0.87 ± 0.2 0.89 ± 0.2 0.93 ± 0.1 0.86 ± 0.3 0.90 ± 0.1 0.91 ± 0.1 0.91 ± 0.3

334.47 369.44 541.95 52.93 39.99 56.34 63.32 71.72 125.14 62.89 36.81 74.15 83.72 39.26 76.2

CPE Y0 (μS cm−2 Sn2)

25 35 45 55 25 35 45 55

Cdl (μF·cm−2)

n

200 ± 3 526 ± 11 373 ± 15 453 ± 20 49 ± 3 53 ± 8 45 ± 8 50 ± 5

Cdl (μF·cm−2)

n 0.86 ± 0.2 0.87 ± 0.1 0.86 ± 0.1 0.86 ± 0.9 0.91 ± 0.3 0.91 ± 0.2 0.91 ± 0.1 0.91 ± 0.1

541.95 605.11 419.79 503.54 76.20 55.15 47.66 53.51

θ (°)

84 89 89 84 80 76 81 90 86 75 89 86 θ (°)

86 90 88 89

η (%)

84 ± 0.1 92 ± 0.1 90 ± 0.1 90 ± 0.1 93 ± 0.1 91 ± 0.1 86 ± 0.1 93 ± 0.1 91 ± 0.1 88 ± 0.1 94 ± 0.1 93 ± 0.1 η (%)

93 ± 0.1 90 ± 0.2 91 ± 0.2 93 ± 0.1

Table 3 Langmuir adsorption isotherm parameters against immersion time for mild steel in 1 M HCl solution with different concentrations of Mustard seed extract and at different temperatures. Concentration (mg·L−1) 50

100

150

200

Concentration (mg·L−1) 50

100

150

200

Immersion time (h)

Kads (M−1)

(kJ·mol−1)

0.2 2.5 5 0.5 2.5 5 0.5 2.5 5 0.5 2.5 5

10,500 23,000 18,000 9000 13,285 10,111 4095 8857 4095 3666 7833 6642

−32.89 −34.83 −34.22 −32.50 −33.47 −32.79 −30.55 −32.46 −30.55 −30.28 −32.16 −31.75

Temperature (°C)

Kads (M−1)

(kJ·mol−1)

25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55

14,666 12,737 16,942 9180 8116 6393 5690 4938 6746 5424 4109 3531 6667 6693 3082 3084

−33.71 −34.48 −36.36 −34.74 −32.25 −32.72 −33.47 −33.10 −31.79 −32.30 −32.61 −32.21 −31.76 −32.84 −31.85 −31.85

8

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a1

CPE

a2

Rs Rp

b1

CPE

b2

Rs Rp

Fig. 5. Nyquist and Bode plots after 5 h immersion of mild steel in 1 M HCl solution with (a1,a2) 0 mg·L−1 and (b1,b2) 200 mg·L−1 of Mustard seed extract at different temperatures.

3.6. SEM and AFM morphological tests SEM and AFM are reliable tests to provide information about inhibitor capability in the electrode surface prevention from severe damage and passive layer generation. Comparisons between the inhibitor-free and inhibitor-containing samples from Fig. 6 corroborated that on the blank coupon surface extensive corrosion, remaining severely roughed surface morphology, occurred. After increasing the Mustard seed extract concentration in 1 M HCl solution, the inhibiting molecules of Mustard seed extract deposited on the metal substrate and a better corrosionresistive layer was visualized on the surface. The emergence of such layer remarkably reduced the corrosion products at the interface. To quantitatively explore the morphological features of the surface, AFM topographic images were presented in Fig. 7. It is visible that addition of 200 mg·L−1 of Mustard seed extract gave rise to reduced average roughness (Sa) values (i.e., Sa = 119 nm for the blank sample and Sa = 42 nm for the inhibited sample). This means that the adsorption of Mustard seed extract molecules on steel surface could significantly retarded the steel surface corrosion, leading to smoother surface with lower pits and damaged areas. 3.7. Inhibition mechanism of Mustard seed extract In acidic (i.e., low pH) environments the heteroatoms of organic molecules can accept proton (i.e., H+), leading to the formation of

protonated or positively-charged molecules [16]. Consequently, in the acidic solutions, the organic compounds of corrosion inhibitors likely coexist in neutral (i.e., non-protonated or neutrally-charged) and charged (i.e., protonated including mono-protonated, di-protonated and etc.) states. It is well documented from literature that the adsorption of organic molecules on the metal surface or metal/solution interface takes place through one of the following mechanisms or combination of them [22,30]: (a) attractive electrostatic interactions of charged molecules with charged metal (called as physical adsorption or physisorption) and (b) interaction of electron-rich sites (e.g., unshared electron pairs in heteroatoms of S, N, O and P and π electrons in multiple (double and triple) and conjugated bonds) with metal atoms (vacant or unfilled orbitals) which gives rise to the formation of coordinate covalent bond (called chemical adsorption or chemisorption). Accordingly, the lone pair electrons in S, N, and hydroxyl/carbonyl O heteroatoms and also the π electrons in various double bonds such as C_C, C_S, C_N and C_O (in allyl isothiocyanate, diallyl trisulfide and arabinose) can be shared with the unoccupied orbitals of iron atoms. Such an electron sharing leads to the chemical adsorption of these extract molecules on the iron surface through their coordination bond with metal atoms. Based on the literature [31,32] it is believed that in HCl solution (1 M) the steel surface possesses net positive charge and thus the corrosive anions like Cl− (with a net negative charge) in the acid solution can adsorb onto the steel surface through electrostatic attractions. The adsorption of halide anions could also happen through

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

a

9

b Un-inhibited

c

d

e Inhibited

Fig. 6. SEM images of mild steel surface after 5 h immersion in 1 M HCl solution with (a) 0, (b) 50, (c) 100, (d) 150 and (e) 200 mg·L−1 of Mustard seed extract.

the creation of oriented dipoles [22]. Both of these situations assist the subsequent adsorption of the organic cations (that is the aforementioned protonated or positively-charged organic extract molecules) [22]. As a consequence, the charged or cationic molecules of Mustard seed extract could have electrostatic attractions with the chloride anions adsorbed on the metal surface, resulting in the physical adsorption of charged forms of organic molecules. 3.8. Isotherms of adsorption Inhibitor molecules and water molecules compete with each other to react with the active sites of the metal surface. By overcoming the interaction energy between the inhibitor molecules and steel ions, the adsorption of Mustard seed extract molecules could happen. This adsorption could be described by different kinds of isotherms (i.e., Langmuir, Frumkin, Temkin, and Freundlich). The relations were discussed in previous works [33]. Fig. 8 shows the Langmuir isotherm fit with experimental data (with about 0.96 regression coefficient). Langmuir based adsorption isotherm ensures the formations of the single corrosion-inhibitive monolayer at solution/metal interface [1]. The

variation of Gibes free energy is related to equilibrium adsorption constant [34]. The predicted values of ΔG∘ads for different concentrations were tabulated in Table 3. Generally, much more negative values of ΔG∘ads are interpreted as spontaneous inhibitor molecules adsorption [34]. The ΔG∘ads values of equal to or higher than −20 kJ·mol−1 evidences the physisorption, while the values of −40 kJ/mol or more negative calculated for this parameter declares the chemisorption process [34]. Using the above equation the values of Gibbs free energy variation was computed and recorded in Table 3. It is seen that the variation of the ΔG∘ads quantity is noted to happen between −30 and −34 kJ·mol−1. This result signifies that the active inhibiting species exist in Mustard seed extract could attach to steel cations through both adsorption mechanisms (i.e., physical and chemical reactions). The extract components adsorption was additionally inspected at higher temperatures of 25, 35, 45, and 55 °C using Langmuir isotherm (Fig. 8-b), and the resulting parameters were presented in Table 3. It is obviously seen that the Kads values are high even at higher temperatures. Also, the magnitudes of Gibbs free energy (ΔG∘ads) varied between −31 and −36 kJ. mol−1, revealing the occurrence of both physisorption and chemisorption at increased temperatures [35].

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2D

3D

a

Height disterbution

blank

b

Fig. 7. AFM images of corroded mild steel surface after 5 h immersion in 1 M HCl solution with (a) 0 and (b) 200 mg·L−1 of Mustard seed extract.

3.9. Weight loss measurement results Table 4 reports the ΔW values for metal samples subjected to HCl electrolyte containing different concentrations of Mustard seed extract (0, 50, 100, 150 and 200 mg·L−1) at constant temperature (25 °C). According to listed data it is found that the ΔW of the bare sample is much higher than the inhibited samples. The enlargement of the immersion time resulted in the acceleration of steel corrosion. By adding more Mustard seed extract to electrolyte, the ΔW values diminished

a

substantially. This reduction is directly connected to the extract molecules adsorption on MS substance. Additionally, as illustrated in Table 4 by enhancing the Mustard seed extract concentration, the values of ΔW and CR were lowered, implying more vacant steel sites can be covered via green extract components. The high amounts of ΔW and CR for the un-inhibited spice demonstrate that deep corrosion occurred on the MS substance. However, these values are too low for the sample immersed in acidic media with 200 mg·L−1 Mustard seed extract. For this sample, 97% efficiency was provided after 3 h subjection.

b

Fig. 8. Langmuir adsorption isotherm for mild steel sample against (a) immersion time and (b) temperature variations in 1 M HCl solution with different concentrations of Mustard seed extract.

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559 Table 4 Calculated values of corrosion rate and inhibition efficiency during 72 h mild steel immersion in 1 M HCl solution with different concentrations of Mustard seed extract. Concentration (mg·L−1) 0

50

100

150

200

Immersion time (h) 1 2 3 4 5 12 24 48 72 1 2 3 4 5 12 24 48 72 1 2 3 4 5 12 24 48 72 1 2 3 4 5 12 24 48 72 1 2 3 4 5 12 24 48 72

ΔW (mg·cm−2) 0.09 ± 0.009 0.24 ± 0.012 0.56 ± 0.045 0.98 ± 0.09 1.12 ± 0.10 3.42 ± 0.290 6.31 ± 0.890 11.90 ± 1.31 18.64 ± 1.53 0.02 ± 0.002 0.03 ± 0.004 0.07 ± 0.008 0.11 ± 0.021 0.13 ± 0.022 0.64 ± 0.024 1.48 ± 0.056 3.31 ± 0.291 5.25 ± 0.491 0.01 ± 0.004 0.02 ± 0.006 0.05 ± 0.008 0.09 ± 0.009 0.12 ± 0.013 0.37 ± 0.015 0.88 ± 0.034 1.87 ± 0.151 2.95 ± 0.189 0.01 ± 0.001 0.02 ± 0.001 0.03 ± 0.002 0.09 ± 0.008 0.1 ± 0.009 0.73 ± 0.016 0.81 ± 0.028 1.72 ± 0.184 2.80 ± 0.214 0.008 ± 0.001 0.01 ± 0.001 0.02 ± 0.002 0.07 ± 0.005 0.09 ± 0.004 0.30 ± 0.009 0.62 ± 0.017 1.34 ± 0.112 2.16 ± 0.156

CR (mg·cm−2·h−1)

η (%)

0.090 ± 0.009 0.120 ± 0.006 0.186 ± 0.015 0.245 ± 0.021 0.224 ± 0.019 0.285 ± 0.024 0.262 ± 0.037 0.248 ± 0.021 0.259 ± 0.026 0.020 ± 0.002 0.015 ± 0.002 0.023 ± 0.002 0.027 ± 0.002 0.026 ± 0.002 0.054 ± 0.002 0.062 ± 0.001 0.069 ± 0.003 0.073 ± 0.006 0.010 ± 0.004 0.010 ± 0.003 0.016 ± 0.002 0.022 ± 0.001 0.024 ± 0.001 0.031 ± 0.001 0.036 ± 0.001 0.039 ± 0.003 0.041 ± 0.003 0.010 ± 0.001 0.010 ± 0.001 0.010 ± 0.001 0.022 ± 0.001 0.020 ± 0.002 0.031 ± 0.002 0.034 ± 0.002 0.036 ± 0.003 0.039 ± 0.002 0.008 ± 0.001

77 87 88 89 88 81 76 72 72 89 92 91 91 89 89 86 84 84 89 92 95 91 91 89 87 85 85 91

0.005 ± 0.001 0.006 ± 0.001 0.017 ± 0.001 0.018 ± 0.001 0.025 ± 0.002 0.026 ± 0.001 0.028 ± 0.002 0.030 ± 0.002

96 97 93 92 91 90 89 88

The computed CR values were compared for the samples exposed to the acidic media with temperatures of 25, 35, 45, and 55 °C and the resulting data are presented in Table S2. As shown, for each sample, after enhancing the solution temperature, a considerable enhancement can be seen especially for the bare sample. The CR of the un-inhibited sample is more affected than other samples. As discussed previously, the corrosion products covered the bare sample after acidic solution attack. However, after increasing the environment temperature, they can be easily dissolved and removed from the MS substance surface. This detachment leads to further steel surface access to aggressive ions, and so more aggression could occur at higher temperatures. However, after increasing the Mustard seed extract concentration, the CR became constant rarely even at high temperatures. These rare variations clarified the suitable adsorption of Mustard seed extract molecules on MS surface. 3.10. Thermodynamic parameters The adsorption of Mustard seed extract molecules was additionally evaluated against temperature. The recorded thermodynamic

11

parameters, i.e., activation energy, enthalpy, and entropy (shown as Ea, H* and S*, respectively) are provided in Table 5. The Arrhenius relation was employed to correlate the CR value with the temperature of medium. The evidences from Fig. 9 manifested that the corrosion rate is directly related to the media temperature. Also, the tabulated amounts in Table 5 represent that via gradual enhancement of Mustard seed extract quantities in electrolyte the Ea parameter raised to higher values. Such an increment ensures that the extract compounds adsorbed by electrostatic type reactions (physisorption) [36]. The enthalpy and entropy of activation values can be related to each other, as discussed previously. As provided in Table 5, the computed H* is directed to higher values disclosing endothermic interactions on the MS surface [36]. It is also recognized that at enhanced extract concentrations, the S* transferred to positive values, an observation ensuring the extract molecules adsorption on MS active sites.

3.11. Molecular simulations Theoretical examinations using molecular simulations were performed for obtaining insights at atomic levels regarding the propensity of Mustard seed extract molecules for adsorption on the metal. Fig. 10 exhibits the equilibrated cells of major Mustard seed extract compounds, which were elucidated at the end of the MC simulations. It is visible from all of these cells that the allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose compounds (both neutral and protonated) of Mustard seed extract inhibitor have an affinity for attaching to the steel-based metallic substrate. Through a closer evaluation, it is understood that all considered Mustard seed extract species equilibrated neighboring the surface with the adoption of a flat orientation. The values of different energetic quantities extracted from MC calculations are gathered in Table S3. Based on tabulated results, it is noted that the adsorption energies related to Mustard seed extract compounds are negative. The negative sign of adsorption energy of extract components proposes the stable molecular adsorption on the metallic surface [22]. The ability of neutral and protonated Mustard seed extract compounds for localization in vicinity of the crystalline plane of pure iron metal (i.e., Fe (110)) was also inspected employing MD modeling conducted in the aqueous environment. The snapshots which were obtained after 2 ns simulation of all six Mustard compounds are given in Fig. 11. The presented snapshots obviously illustrate that all six inhibiting species exist in Mustard extract equilibrated close to the uppermost atomic layer of iron adsorbent under humid situations of the surface. This graphical evidence also describes the adsorption tendency of considered Mustard extract molecules on the metallic substrate constructed from iron (110). For the quantitative investigation of these simulated snapshots, the magnitudes of the energy of adsorption (termed by ΔEads) were assessed according to the following relation: ΔEads = Etotal – (Esurface+solution + Einhibitor) [1,37]. The parameter Etotal expresses the potential energy related to the whole cell, the Esurface+solution proves the energy of cell in the absence of Mustard extract molecules (i.e., inhibitor), and the last term of Einhibitor is expressive of the energy

Table 5 Corrosion kinetic parameters for mild steel in 1 M HCl solution with different concentrations of Mustard seed extract. Concentration (mg·L−1) 0 50 100 150 200

Ea (kJ·mol−1)

S* (J·K−1·mol−1)

13 25 25 28 35

−54 −43 −43 −32 −24

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G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

a

b

Fig. 9. (a) Arrhenius plots and (b) transition-state plots for the corrosion rate (CR) of mild steel sample in 1 M HCl solution with different concentrations Mustard seed extract.

Allyl isothiocyanate

Galactose

Diallyl trisulfide

Rhamnose

Arabinose

Xylose

Fig. 10. The MC simulated snapshots of neutral (top row) and protonated (bottom rom) allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose and xylose compounds over Fe (110) surface.

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

of single or isolated inhibitor molecule. The derived ΔEads values (in kcal/mol) are equal to −60.61, −106.87, −141.05, −139.15, −122.66 and −144.81 dedicated to neutral allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose, respectively. In the protonated state, the allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose molecules adsorbed with energy magnitudes of −60.32, −168.83, −175.89, −190.59, −191.32, and −209.66, respectively. It is found that the adsorption energies of all neutral and protonated extract molecules calculated from MD simulations are negative, which further demonstrate the stable adsorption of extract molecules on the iron surface [22]. All these graphical and quantitative observations are in close agreement with experimental evidences. 3.12. DFT results The local sites which are reactive and likely contribute in adsorption of Mustard seed extract molecules on the metal were probed conducting

Allyl isothiocyanate

Galactose

13

first-principles approaches based on DFT. These potential centers through donor-acceptor interactions lead to localization of inhibiting species nearby the substrate. The frontier orbitals (including HOMO and LUMO) of a specific species determine the capacity of its active atomic regions for donating and receiving electrons in such adsorption mechanism. The HOMO suggests the sites which are the source of electrons and can act as a potential nucleophile, while the LUMO reflects the electron-deficient centers serving as suitable regions with electronaccepting tendency, i.e., electrophile. The equilibrium geometries of neutral and positively-charged (i.e., mono-protonated) allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose, and xylose molecules are summarized in Fig. S4. The frontier orbitals belonging to the neutral and mono-protonated geometries of Mustard seed extract substances are plotted in Figs. 12 and S5, respectively. In the neutral allyl isothiocyanate, it is seen that the double bonds of carbon atoms with sulfur, nitrogen and carbon atoms are sites for electron density surface of HOMO, while these three atoms formed the electron density surface

Diallyl trisulfide

Arabinose

Rhamnose

Xylose

Fig. 11. The MD simulated snapshots of neutral (top row) and protonated (bottom row) allyl isothiocyanate, diallyl trisulfide, arabinose, galactose, rhamnose and xylose compounds over Fe (110) surface.

14

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

HOMO

LUMO

Electrophilic attack

Nucleophilic attack

Fig. 12. The HOMO/LUMO and Fukui indices of neutral allyl isothiocyanate (first row), diallyl trisulfide (second row), arabinose (third row), galactose (fourth row), rhamnose (fifth row) and xylose (last row) in Mustard seed extract compounds.

of LUMO. Thus, the C_S, C_N, and C_C bonds have a better chance to supply electrons (i.e., π electrons) when exposed to an electrophile with poor electrons. In the second neutral molecule, diallyl trisulfide, the resulting HOMO plot demonstrates that the sulfur atoms together with one C_C bond emerged in electron density surface of this orbital, while LUMO plots distributed over the sulfur atoms and the C ones of mentioned double bond. The implication of such orbitals distribution is that electron pair of sulfur atoms and π electrons of double bonds could be shared with charge-poor metal sites (that is, vacant orbitals) within a donor-acceptor interaction. In the other neutral Mustard seed extract substances, which are linear and cyclic oxygenated species including arabinose, galactose, rhamnose, and xylose, the HOMO electron density isosurface localized mainly over all oxygen heteroatoms of the neutral molecule. This observation rationalizes that all O atoms of arabinose, galactose, rhamnose and xylose such as hydroxyl, ether, and carbonyl have a potentiality for involving in electron providing (i.e., lone pair) to the iron atoms. As shown, the electron density isosurface of LUMO substantially distributed on some C atoms, an evidence disclosing their strong donor-acceptor interaction affinity. According to the

graphics of electron density surface of frontier orbitals in positivelycharged Mustard seed extract compounds shown in Fig. S5, it is obviously seen that the mono-protonation of a specific site essentially affected the distribution of the orbitals. Through a closer inspection, it can be observed that the atomic center, which behaved as the possible HOMO of the neutrally-charged molecule, appeared as a primary reactive atom for LUMO when exposed to protonation. Consequently, the net positive charge bearing regions possibly assist the adsorption by taking the metallic atoms electrons. Besides the above-mentioned graphical results, the reactivity was also probed by analyzing the electronic-structure properties, for instance, energetic magnitudes of orbitals (and their dependent parameters of ionization potential, electron tendency or affinity, hardness, and electronegativity), and the value of charge which was shared between each Mustard seed extract compound and iron (110) species. The DFTcalculated values corresponding to these electronic quantities are recorded in Table 6 for non-protonated and protonated forms. The summarized data declare that the non-protonated as well as protonated galactose, rhamnose and xylose molecules possessed relatively similar

G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

15

Table 6 The HOMO and LUMO energies (eV), ELUMO - EHOMO energy gap (ΔEL-H), electron affinity (A), ionization potential (I), electronegativity (χ), hardness (η), and fraction of electrons transferred (ΔN) for neutral and protonated compounds. Adsorbate

EHOMO

ELUMO

ΔEL-H

A

I

χ

η

ΔN

Neutral inhibitors Allyl isothiocyanate Diallyl trisulfide Arabinose Galactose Rhamnose Xylose

−7.003 −6.862 −7.191 −7.184 −7.049 −7.151

−0.129 −1.788 −1.133 0.757 0.785 0.935

6.874 5.074 6.058 7.941 7.834 8.086

0.129 1.788 1.133 −0.757 −0.785 −0.935

7.003 6.862 7.191 7.184 7.049 7.151

3.566 4.325 4.162 3.213 3.132 3.108

3.437 2.537 3.029 3.970 3.917 4.043

0.1824 0.0975 0.1086 0.2023 0.2154 0.2117

Protonated inhibitors Allyl isothiocyanate Diallyl trisulfide Arabinose Galactose Rhamnose Xylose

−7.891 −7.542 −7.937 −8.158 −7.900 −7.574

−3.915 −3.216 −1.712 −0.356 −0.374 −0.633

3.976 4.326 6.225 7.802 7.526 6.941

3.915 3.216 1.712 0.356 0.374 0.633

7.891 7.542 7.937 8.158 7.900 7.574

5.903 5.379 4.824 4.257 4.137 4.103

1.988 2.163 3.112 3.901 3.763 3.470

−0.2723 −0.1292 −0.0006 0.0721 0.0907 0.1033

ΔN value, likely due to their similar molecular structures. Moreover, it is recognized that compared with neutral species, the computed value of ΔN in mono-protonated molecules is negative or near zero. Such outcome is an implication of the strengthened tendency of proton containing species for accepting electrons supplied by a nucleophile (i.e., the filled orbitals in steel or iron atoms). By examining the Fukui functions or indices (that is, f− and f+ reflecting the electrophilic and the nucleophilic character, respectively) the reactivity of non-protonated and protonated Mustard seed extract species was further graphically and quantitatively explored. Figs. 12 and S5 present the graphical Fukui functions results in neutral and proton containing molecules, respectively. The related quantitative results are tabulated in Tables S4 and S5. It is seen from Fig. 12 that the carbon double bonds shared with sulfur and nitrogen centers in neutral allyl isothiocyanate are most favorable for electrophilic attacks, which is also ensured by a large value found for f− function in N4 (0.132), C5 (0.115) and S6 (0.543) atoms. These graphics and related values manifest that allyl isothiocyanate is likely to use the double bonds in charge donation. The nucleophilic behavior of allyl isothiocyanate, on the other hand, concentrated on all backbone centers, which in accordance with LUMO plots again highlights the affinity toward taking charges. The electron density surface of both Fukui functions in non-protonated diallyl trisulfide substantially distributed over the middle three sulfur atoms. This graphical observation is also ensured by the greater amounts of f − function (i.e., S1 (0.261), S2 (0.126) and S3 (0.290)) and f+ one (i.e., S1 (0.190), S2 (0.245) and S3 (0.223)). Hence, the electron supplying to an appropriate electrophile and at the same time the electron receiving from a potential nucleophile in neutral diallyl trisulfide likely happen via sulfur atoms. As depicted in Fig. S5, in case of oxygenated compounds (i.e., arabinose, galactose, rhamnose and xylose) the electron density isosurface relevant for function of f− emerged primarily on a certain O atom (carbonyl O in arabinose (value of 0.239), ether O in galactose (value of 0.187) and rhamnose (value of 0.167) and two hydroxyl O centers in xylose (values of 0.140 and 0.145)). Therefore, it may be inferred that the mentioned heteroatoms when vulnerable to iron like electrophiles probably give the pair of electrons, which gives rise to chemisorption on steel. The atomic regions showing nucleophilic attacks are noted to locate chiefly on one heteroatom and carbon atom. Furthermore, the plots in Fig. S5 and recorded data in Tables S4 and S5 evidence that the Fukui functions distribution in proton containing species is somewhat different from that in non-protonated materials. It is identified that the atomic centers to which a proton connected are likely to have attacks of nucleophilic type, and thus have greater chances of accepting electrons which could be supplied by a proper nucleophile.

4. Conclusions The corrosion mitigation of Mustard seed extract was investigated for mild steel in HCl solution and the main results are listed below: 1. FT-IR and UV–Vis characterizations proved the presence of many characteristic groups in the structure of extract components. 2. The electrochemical studies clarified the mixed type action of Mustard seed extract inhibitor. Maximum efficiency of 94% was reached for the acid solution containing 200 mg·L−1 extract. 3. The adsorption behavior of Mustard seed extract molecules was according to Langmuir isotherm. 4. Weight loss analysis showed 97% inhibition performance for the acid solution containing 200 mg·L−1 Mustard seed extract. 5. Molecular simulations suggested the stable adsorption of extract molecules onto the iron surface.

Acknowledgment The present study was supported by Golestan University (grant 971711), Gorgan, Iran. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111559. References [1] D.S. Chauhan, K. Ansari, A. Sorour, M. Quraishi, H. Lgaz, R. Salghi, Thiosemicarbazide and thiocarbohydrazide functionalized chitosan as ecofriendly corrosion inhibitors for carbon steel in hydrochloric acid solution, Int. J. Biol. Macromol. 107 (2018) 1747–1757. [2] C. Verma, L.O. Olasunkanmi, T.W. Quadri, E.-S.M. Sherif, E.E. Ebenso, Gravimetric, electrochemical, surface morphology, DFT, and Monte Carlo simulation studies on three N-substituted 2-Aminopyridine derivatives as corrosion inhibitors of mild steel in acidic medium, J. Phys. Chem. C 122 (2018) 11870–11882. [3] A. Saxena, D. Prasad, R. Haldhar, G. Singh, A. Kumar, Use of Saraca ashoka extract as green corrosion inhibitor for mild steel in 0.5 M H2SO4, J. Mol. Liq. 258 (2018) 89–97. [4] P. Arellanes-Lozada, O. Olivares-Xometl, N.V. Likhanova, I.V. Lijanova, J.R. VargasGarcía, R.E. Hernández-Ramírez, Adsorption and performance of ammoniumbased ionic liquids as corrosion inhibitors of steel, J. Mol. Liq. 265 (2018) 151–163. [5] M. Mobin, R. Aslam, Experimental and theoretical study on corrosion inhibition performance of environmentally benign non-ionic surfactants for mild steel in 3.5% NaCl solution, Process. Saf. Environ. Prot. 114 (2018) 279–295. [6] M. Tabatabaei majd, G. Bahlakeh, A. Dehghani, B. Ramezanzadeh, M. Ramezanzadeh, A green complex film based on the extract of Persian Echium amoenum and zinc nitrate for mild steel protection in saline solution; electrochemical and surface explorations besides dynamic simulation, J. Mol. Liq. 291 (2019) 1–18.

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G. Bahlakeh et al. / Journal of Molecular Liquids 293 (2019) 111559

[7] M. Abdallah, H.M. Altass, B.A.L. Jahdaly, M. Salem, Some natural aqueous extracts of plants as green inhibitor for carbon steel corrosion in 0.5 M sulfuric acid, Green Chemistry Letters and Reviews 11 (2018) 189–196. [8] A. Krishnan, S. Shibli, Optimization of an efficient, economic and eco-friendly inhibitor based on Sesbania grandiflora leaf extract for the mild steel corrosion in aggressive HCl environment, Anti-Corrosion Methods and Materials 65 (2018) 210–216. [9] K. Kalaiselvi, I.-M. Chung, S.-H. Kim, M. Prabakaran, Corrosion resistance of mild steel in sulphuric acid solution by Coreopsis tinctoria extract: electrochemical and surface studies, Anti-Corrosion Methods and Materials 65 (2018) 408–416. [10] A.I. Ikeuba, P.C. Okafor, Green corrosion protection for mild steel in acidic media: saponins and crude extracts of Gongronema latifolium, Pigm. Resin Technol. 48 (2019) 57–64. [11] A. Khadom, K. Rashid, Adsorption and kinetics behavior of kiwi juice as a friendly corrosion inhibitor of steel in acidic media, World Journal of Engineering 15 (2018) 388–401. [12] A. Bader, U. Shaheen, M. Aborehab, Y. El Ouadi, A. Bouyanzer, B. Hammouti, T.B. Hadda, Inhibitory effect of Acacia hamulosa methanolic extract on the corrosion of mild steel in 1 M hydrochloric acid, Bull. Chem. Soc. Ethiop. 32 (2018) 323–335. [13] K. Nasr, M. Fedel, K. Essalah, F. Deflorian, N. Souissi, Experimental and theoretical study of Matricaria recutita chamomile extract as corrosion inhibitor for steel in neutral chloride media, Anti-Corrosion Methods and Materials 65 (2018) 292–309. [14] A. Dehghani, G. Bahlakeh, B. Ramezanzadeh, M. Ramezanzadeh, Electronic/atomic level fundamental theoretical evaluations combined with electrochemical/surface examinations of Tamarindus indiaca aqueous extract as a new green inhibitor for mild steel in acidic solution (HCl 1 M), J. Taiwan Inst. Chem. Eng. 102 (2019) 349–377. [15] R. Naderi, H. Ghadiri, Competition of different densities of wild mustard (Brassica kaber) and rapeseed (Brassica napus) in greenhouse, Desert 14 (2009) 151–155. [16] L.O. Olasunkanmi, I.B. Obot, M.M. Kabanda, E.E. Ebenso, Some Quinoxalin-6-yl derivatives as corrosion inhibitors for mild steel in hydrochloric acid: experimental and theoretical studies, J. Phys. Chem. C 119 (2015) 16004–16019. [17] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. 140 (1965) A1133–A1138. [18] J. Tomasi, B. Mennucci, R. Cammi, Quantum mechanical continuum solvation models, Chem. Rev. 105 (2005) 2999–3094. [19] M. Frisch, G. Trucks, H.B. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. Petersson, Gaussian 09, Revision D. 01, Gaussian, Inc., Wallingford CT, 2009. [20] S.K. Saha, P. Ghosh, A. Hens, N.C. Murmu, P. Banerjee, Density functional theory and molecular dynamics simulation study on corrosion inhibition performance of mild steel by mercapto-quinoline Schiff base corrosion inhibitor, Phys. E. 66 (2015) 332–341. [21] S.K. Saha, M. Murmu, N.C. Murmu, P. Banerjee, Evaluating electronic structure of quinazolinone and pyrimidinone molecules for its corrosion inhibition effectiveness on target specific mild steel in the acidic medium: a combined DFT and MD simulation study, J. Mol. Liq. 224 (2016) 629–638. [22] K. Anupama, A. Joseph, Experimental and theoretical studies on Cinnamomum verum leaf extract and one of its major components, eugenol as environmentally benign corrosion inhibitors for mild steel in acid media, Journal of Bio-and TriboCorrosion 4 (2018) 1–14.

[23] S.D. Accelrys Software Inc., 2009. [24] Z. Salarvand, M. Amirnasr, M. Talebian, K. Raeissi, S. Meghdadi, Enhanced corrosion resistance of mild steel in 1M HCl solution by trace amount of 2-phenylbenzothiazole derivatives: experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies, Corros. Sci. 114 (2017) 133–145. [25] M.T. Majd, T. Shahrabi, B. Ramezanzadeh, Production of an eco-friendly anticorrosion ceramic base nanostructured hybrid-film based on Nd (III)-C7H6N2 on the mild steel surface; electrochemical and surface studies, Constr. Build. Mater. 221 (2019) 456–468. [26] K.K. Anupama, K. Ramya, A. Joseph, Electrochemical and computational aspects of surface interaction and corrosion inhibition of mild steel in hydrochloric acid by Phyllanthus amarus leaf extract (PAE), J. Mol. Liq. 216 (2016) 146–155. [27] K.K. Anupama, K. Ramya, K.M. Shainy, A. Joseph, Adsorption and electrochemical studies of Pimenta dioica leaf extracts as corrosion inhibitor for mild steel in hydrochloric acid, Mater. Chem. Phys. 167 (2015) 28–41. [28] M. Ramezanzadeh, G. Bahlakeh, Z. Sanaei, B. Ramezanzadeh, Corrosion inhibition of mild steel in 1 M HCl solution by ethanolic extract of eco-friendly Mangifera indica (mango) leaves: electrochemical, molecular dynamics, Monte Carlo and ab initio study, Appl. Surf. Sci. 463 (2019) 1058–1077. [29] K. Ramya, R. Mohan, K.K. Anupama, A. Joseph, Electrochemical and theoretical studies on the synergistic interaction and corrosion inhibition of alkyl benzimidazoles and thiosemicarbazide pair on mild steel in hydrochloric acid, Mater. Chem. Phys. 149-150 (2015) 632–647. [30] K. Ramya, A. Joseph, Synergistic effects and hydrogen bonded interaction of alkyl benzimadazoles and thiourea pair on mild steel in hydrochloric acid, J. Taiwan Inst. Chem. Eng. 52 (2015) 127–139. [31] A.O. Yüce, G. Kardaş, Adsorption and inhibition effect of 2-thiohydantoin on mild steel corrosion in 0.1 M HCl, Corros. Sci. 58 (2012) 86–94. [32] D.K. Yadav, B. Maiti, M. Quraishi, Electrochemical and quantum chemical studies of 3, 4-dihydropyrimidin-2 (1H)-ones as corrosion inhibitors for mild steel in hydrochloric acid solution, Corros. Sci. 52 (2010) 3586–3598. [33] G. Bahlakeh, B. Ramezanzadeh, A. Dehghani, M. Ramezanzadeh, Novel cost-effective and high-performance green inhibitor based on aqueous Peganum harmala seed extract for mild steel corrosion in HCl solution: detailed experimental and electronic/ atomic level computational explorations, J. Mol. Liq. 283 (2019) 174–195. [34] A. Dehghani, G. Bahlakeh, B. Ramezanzadeh, M. Ramezanzadeh, Detailed macro−/ micro-scale exploration of the excellent active corrosion inhibition of a novel environmentally friendly green inhibitor for carbon steel in acidic environments, J. Taiwan Inst. Chem. Eng. 100 (2019) 239–261. [35] K.K. Anupama, K. Ramya, A. Joseph, Electrochemical measurements and theoretical calculations on the inhibitive interaction of Plectranthus amboinicus leaf extract with mild steel in hydrochloric acid, Measurement 95 (2017) 297–305. [36] M.A. Quraishi, A. Singh, V.K. Singh, D.K. Yadav, A.K. Singh, Green approach to corrosion inhibition of mild steel in hydrochloric acid and sulphuric acid solutions by the extract of Murraya koenigii leaves, Mater. Chem. Phys. 122 (2010) 114–122. [37] S.K. Saha, A. Dutta, P. Ghosh, D. Sukul, P. Banerjee, Novel Schiff-base molecules as efficient corrosion inhibitors for mild steel surface in 1 M HCl medium: experimental and theoretical approach, Phys. Chem. Chem. Phys. 18 (2016) 17898–17911.