Improved corrosion resistance of mild steel in acidic solution by hydrazone derivatives: An experimental and computational study

2.1 Synthesis

All chemicals were obtained from Sigma (Merck) and used as received. Melting points were recorded using Stuart melting point apparatus SMP30 (Bibby Scientific). The IR spectra were measured using FTIR spectrometer (Nicolet 380, Thermo Electron Corporation, USA). The ¹H NMR and ¹³C NMR spectra were recorded with Bruker 400 MHz in DMSO. Scheme 1 represents the general procedure for synthesis of hydrazones (3a-c) (see Scheme 2).

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Scheme 1

Hydrazones synthesis scheme.

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Scheme 2

The protonation state of the hydrazone molecule in 1 M HCl.

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Synthesis and characterization of methyl 2-[(2,3-dimethylphenyl)amino]benzoate (1): A mixture of (2.0 mg, 8.28 mol) Mefenamic acid and (2.0 mg, 16.6 mmol) anhydrous potassium carbonate was added to dry acetone (80 ml) and stirred for 60 min. Then Excess of Methyl Iodide (3.6 mg, 24.8 mmol) was added. The resulting mixture was refluxed for 18 h. The reaction mixture was filtered when hot and the resultant cakes were washed with dry acetone (6 × 2 ml). The filtrate was evaporated under reduced pressure to afford the ester as an oily material, solidified after 4 days at room temperature with m.p = 100–102 °C. IR(KBr): (C⚌O ester 1725 cm⁻¹), (NH 3332 cm⁻¹), (C—H aliphatic 2999–2943 cm⁻¹), (⚌C—H aromatic 3012 cm⁻¹).

Synthesis and characterization of 2-[(2,3-dimethylphenyl)amino]benzohydrazide (2): A mixture of Mefenamic acid methyl ester (1) (1 mmol) and hydrazine hydrate (85%, 2.5 mmol) was irradiated, in closed vessel, under microwave irradiation at 300 W and 2500 °C for 7 min. The reaction mixture was cooled, the separated solid was washed with water, filtered, dried and recrystallized from ethanol to afford the hydrazide (2) with m.p = 119–121 °C. IR(KBr): (C⚌O amide 1654 cm⁻¹), (NH 3290 cm⁻¹), (NH₂,3290–3309 cm⁻¹), (C—H aliphatic 2949 cm⁻¹), (⚌C—H aromatic 3018 cm⁻¹). ¹H NMR (400 MHz, DMSO): δ = 2.1(s,3H, CH₃),2.2(s,3H, CH₃), 9.4(s,1H, NH), 9.8 (br.s,1H, NH amide), 3.5 (br.s, 2H,-NH₂), 6.7–7.9(m,7H,Ar-H). ¹³C NMR δ = 13.5(CH₃), 20.2(CH₃), 113.9, 115.9, 116.9, 119.6, 125.0, 125.8, 128.2, 129.3, 131.8, 137.7, 139.3, 145.9 (Aromatic carbons), 168.4 (C⚌O amide).

General procedure for synthesis of hydrazones(3a-c): A mixture of equimolar ratio of Mefenamic acid hydrazide and appropriate aldehydes with catalytic amount of glacial acetic acid was refluxed for 5 h. On cooling, the precipitate was separated then collected and recrystallized from ethanol to give 3a-c.

N′-[(E)-4-benzylidene]-2-(2,3-dimethylanilino)benzohydrazide (3a, C₂₃H₂₂N₂O): m.p = 193–195 °C; IR(KBr): (C⚌O amide 1639 cm⁻¹), (NH 3199 cm⁻¹) (NH 3338 cm⁻¹), (C—H aliphatic 2947 cm⁻¹), (C⚌N 1597 cm⁻¹). ¹H NMR (400 MHz, DMSO): δ = 2.1(s,3H,CH₃), 2.2(s,3H,CH₃), 9.2(s,1H,NH), 9.9(s,1H,NH amide), 6.6–7.8(m,12H,aromatic), 7.9(s,1H, —CH⚌N—).

¹³C NMRδ = 18(CH₃), 24(CH₃), 116, 118, 119.4, 119.5, 121, 125, 130.3, 130.7, 133.4, 134.3, 134.5, 134.6, 137, 142, 143, 150.6, 150.9, 151 (Aromatic carbons), 173(C⚌O amide), 162(C⚌N).

N′-[(E)-4-methylbenzylidene]-2-(2,3-dimethylanilino)benzohydrazide (3b, C₂₄H₂₄N₂O): m.p = 196–199 °C; IR(KBr): (C⚌O amide 1643 cm⁻¹), (NH 3142 cm⁻¹) (NH 3350 cm⁻¹), (C—H aliphatic 2939 cm⁻¹), (C⚌N 1571 cm⁻¹). ¹H NMR (400 MHz, DMSO): δ = 2.07 (2,3H, CH₃), 2.2(s, 3H, CH₃), 2.4(s,3H, CH₃), 9.2(s,1H, NH), 9.9(s,1H, NH), 6.7–7.7(m, 11H, aromatic), 7.9(s, 1H, —CH⚌N—). ¹³C NMRδ = 13(CH₃), 20.08(CH₃), 20.6(CH₃), 113, 114, 116, 120, 123, 125, 127, 128, 129.1, 129.7, 131, 132, 137, 138, 146, 148 (aromatic carbons), 164(C⚌N), 168(C⚌O).

N′-[(E)-4-Dimethylaminobenzylidene]-2-(2,3-dimethylanilino)benzohydrazide (3c, C₂₅H₂₇N₃O): m.p = 245–248 °C; IR(KBr): (C⚌O amide 1658 cm⁻¹), (NH 3277 cm⁻¹), (NH 3400 cm⁻¹), (C—H aliphatic 2933 cm⁻¹), (C⚌N 1606 cm⁻¹). ¹HNMR (400 MHz, DMSO): δ = 2.1(s, 3H, CH₃), 2.2(s, 3H, CH₃), 2.4(s, 6H, CH₃), 11.8 (s,1H, NH), 9.2(s,1H, NH), 8.3(s,1H, NH), 6.7–7.7(m,11H, aromatic). ¹³C NMR δ = 13(CH₃), 20(CH₃), 29(2CH₃), 111, 114, 117, 119, 120, 121, 122, 125, 127, 130, 131, 133, 137, 138, 142, 147 (aromatic carbon), 162 (C⚌N),165 (C⚌O).

2.2 Materials and corrosive solutions

The metal substrates used in this research were made from commercially available mild steel with a chemical composition of: 0.370% C, 0.230% Si, 0.680% Mn, 0.016% S, 0.077% Cr, 0.011% Ti, 0.059% Ni, 0.009% Co, 0.160% Cu, and balance Fe. Prior to each individual experiment, test coupons were mechanically abraded using a belt grinding polishing machine. To obtain a polished surface, mild steel surface was grounded with emery papers of 400–1600 grades and cleaned thoroughly. The test solution (1 M HCl) was prepared by diluting 37% analytical grade HCl with double distilled water.

2.3 Weight loss measurements

According to the ASTM standard (Scully and Baboian, 1995), a disk-shaped specimens (38 mm in diameter and 3 mm in thickness with a hole of 8 mm diameter) were prepared for each weight loss tests. The mild steel samples were immersed in uninhibited and inhibited solutions for 24 h at 303 K. The MS samples were prepared as mentioned previously and weighted using a precision balance with a sensitivity of 0.1 mg. After each test, the MS specimens were taken out and rinsed thoroughly with distilled water and acetone, dried and re-weighed accurately. The experiments were carried out in triplicates and only the average values of the weight losses were reported. The following equation (Eq. (1)) was used to determine the corrosion rate in millimeters per year (mm y⁻¹):

2.4 Electrochemical measurements

Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) were performed using a three-electrode setup and a Volta Lab (Tacussel- Radiometer PGZ 100) potentiostat. The three-electrode system consisted of a working electrode (mild steel) with the surface area of 1 cm² in contact with the electrolyte, Pt as an auxiliary electrode alongside a saturated calomel reference electrode. Before electrochemical measurements, the working electrode was first immersed in the test solution for 30 min in order to be in equilibrium with the environment. The electrochemical measurements were performed when the OCPs became stable (Fig. S1, Supplementary material). Measurement of the impedance spectra was obtained in the frequency range of 10 mHz to 100 kHz and peak to peak perturbations of 5 mV. The Tafel polarization measurements were recorded at a scanning range of 1 mV s⁻¹ in the range from −800 to −200 mV vs. SCE at steady open-circuit potential. The electrochemical parameters were extracted from Tafel plots at ± 50 mV around E_corr using Tafel extrapolation method (Tkacz et al., 2016). All electrochemical measurements were performed in triplicate in a cell containing 80 ml of the electrolyte at 303 K.

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.08.004.

Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) were performed using a three-electrode setup and a Volta Lab (Tacussel- Radiometer PGZ 100) potentiostat. The three-electrode system consisted of a working electrode (mild steel) with the surface area of 1 cm² in contact with the electrolyte, Pt as an auxiliary electrode alongside a saturated calomel reference electrode. Before electrochemical measurements, the working electrode was first immersed in the test solution for 30 min in order to be in equilibrium with the environment. The electrochemical measurements were performed when the OCPs became stable (Fig. S1, supplementary material). Measurement of the impedance spectra was obtained in the frequency range of 10 mHz to 100 kHz and peak to peak perturbations of 5 mV. The Tafel polarization measurements were recorded at a scanning range of 1 mV s⁻¹ in the range from −800 to −200 mV vs. SCE at steady open-circuit potential. The electrochemical parameters were extracted from Tafel plots at ± 50 mV around E_corr using Tafel extrapolation method (Tkacz et al., 2016). All electrochemical measurements were performed in triplicate in a cell containing 80 ml of the electrolyte at 303 K.

2.5 SEM and XPS studies

Scanning electron microscopy has been used to investigate the changes in surface of the mild steel samples, before and after addition of inhibitor in 1 M HCl. The MS samples were scanned after 6, 12 and 24 h of immersion in 1 M HCl with 5 × 10⁻³ M of HDZ-1. SEM imaging was performed by a Hitachi TM-1000 SEM at an accelerating voltage of 15 kV. The spectra of XPS were recorded by a Physical Electronic PHI 5700 spectrometer with a non-monochromatic Mg-Kα radiation (300 W, 15 kV, and 1253.6 eV). High-resolution spectra of the elements of interest were recorded by a hemispherical multichannel detector operating in the constant pass energy mode at 25.9 eV, and step size of 0.1 eV, using a 720 μm diameter analysis area. The XPS spectra were quantified using a non-linear least squares algorithm with a Shirley background and a Gaussian–Lorentzian combination. The X-ray photoelectron spectra obtained were analyzed using PHI SmartSoft software and processed using MultiPak 9.3 package.

2.6 Computational details

3.1 Weight loss measurements

Gravimetric tests based on mass loss measurements are still a reliable method for assessing corrosion of metals. Weight loss tests were undertaken for MS in 1 M HCl devoid of and in the presence of different concentrations of HDZ-1, HDZ-2, and HDZ-3 at 303 K. The expressions below were used to calculate the percentage inhibition efficiency, ${\eta }_{\mathit{WL}}(\%)$ and the surface coverage ( $\theta$ ) (Lgaz et al., 2017a):

From the data in this Table, it is apparent that the corrosion rate of the samples immersed in the solutions devoid of inhibitors is significantly high (130.4 mm y⁻¹) compared to the inhibited solutions. Moreover, it is clear from the data that the corrosion rate mitigated considerably with an increase in the concentration of hydrazone derivatives. The maximal inhibition efficiency was obtained with 5 × 10⁻³ M and no further increase was observed with higher concentration of the compounds. Meanwhile, the inhibition efficiency in presence of inhibitors was found to increase continuously with an increase in the concentration of inhibitors. This is a clear indication that by increasing inhibitor concentration, there will be more and more hydrazone molecules to be adsorbed on the mild steel surface and therefore the inhibition of all corrosion forms prevailing at the metal-electrolyte interface. The order of the inhibition efficiencies at the same concentration is HDZ-1 > HDZ-2 > HDZ-3. This difference can be interpreted in terms of an electron donating powers of the three investigated hydrazones. The dimethylamino is an important donor group with Hammett substituent constant (σ) equal to −0.83 which is more negative than those of methyl group (−0.170) and H (0.00). A strong electron donating power mainly favors the adsorption of corrosion inhibitor on the metallic surface (Singh et al., 2018). These findings are discussed in the following sections in detail.

3.2 Potentiodynamic polarization curves

Potentiodynamic polarization technique was used to obtain information about the kinetics of the mild steel corrosion in 1 M HCl solution at different concentrations of hydrazone derivatives (Chaitra et al., 2016). Fig. 1 depicts the current–potential relationships (cathodic and anodic) for mild steel in 1 M HCl devoid of and in presence of a range of inhibitor concentrations.

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Fig. 1

Potentiodynamic polarization curves for mild steel in 1 M HCl without and with various concentrations of hydrazone derivatives at 303 K.

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Table 2 lists the polarization parameters for the uninhibited and inhibited solutions and the corresponding inhibition efficiency values which were calculated using the below expression (Lgaz et al., 2017a):

Observation of Fig. 1 reveals that both the cathodic and anodic reactions inhibited significantly as evidenced in the shifting of the corrosion rate to lower values. Upon inspection of Fig. 1, it can be seen that when the potential goes towards positive values ∼−300 mV, the anodic branch is slightly changed. This phenomenon is well known as 'desorption potential' and may be explained as follows (El Mehdi et al., 2003): Under sufficiently positive potential conditions, the inhibitor molecules adsorbed on the steel surface commence to move away and no longer can sufficiently protect the mild steel at a higher potential than desorption potential. These results seem to be consistent with other research which found that an increase in anodic currents is mainly associated with desorption potential that alters significantly the inhibitor film (Bentiss et al., 2006). When we observed the influence of inhibitor molecules on the cathodic Tafel curves, we found that the corrosion rate decreased markedly to lower values in comparison to the blank and all curves give rise to parallel lines which indicates that the inhibitor molecules does not modify the hydrogen evolution mechanism and the transfer reaction controls the hydrogen ion reduction (HIR).

Looking at Table 2, it is apparent that the i_corr values were found to decrease continuously with increase in the concentration of hydrazone derivatives following the order of i_corr(HDZ-3) > i_corr(HDZ-2) > i_corr(HDZ-1). At the same time, there are no significant changes in corrosion potential values except a slight positive shift in the case of HDZ-3. Overall, these results could be explained by assigning hydrazone derivatives as a mixed type inhibitors (Hegazy et al., 2013).

The values of the inhibition efficiency are shown in Table 2. It is also evident that the inhibition efficiency increases with increasing concentrations of hydrazone derivatives. This is because the increase in the concentration of inhibitors leads to an increase in the surface blocking of surface active sites by inhibitors. Potentiodynamic results were in perfect conformity with the results obtained from weight loss study.

3.3 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy measurements were conducted on mild steel in 1 M HCl to clarify the kinetics of the electrode processes. Fig. 2. and S2 (supplementary material) depict EIS results in the Nyquist, Bode phase angle and Bode modulus formats. The EIS data recorded at the open-circuit potential illustrate a capacitive loop in form of depressed semicircles, which is seen in Bode plots only one time-constant. That is to say that the charge transfer process mainly controls the dissolution of mild steel in 1 M HCl solution (Lgaz et al., 2017b). By increasing concentration, a non-conducting protective layer on the mild steel surface was formed due to the adsorption of more inhibitor molecules on the metal surface replacing effectively the pre-adsorbed water molecules.

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Fig. 2

EIS curves of mild steel substrate immersed in 1 M HCl solution without and with different concentrations of hydrazone derivatives at 303 K.

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Electrochemical impedance spectroscopy measurements were conducted on mild steel in 1 M HCl to clarify the kinetics of the electrode processes. Fig. 2. and S2 (Supplementary material) depict EIS results in the Nyquist, Bode phase angle and Bode modulus formats. The EIS data recorded at the open-circuit potential illustrate a capacitive loop in form of depressed semicircles, which is seen in Bode plots only one time-constant. That is to say that the charge transfer process mainly controls the dissolution of mild steel in 1 M HCl solution (Lgaz et al., 2017b). By increasing concentration, a non-conducting protective layer on the mild steel surface was formed due to the adsorption of more inhibitor molecules on the metal surface replacing effectively the pre-adsorbed water molecules.

The EIS results of the mild steel in presence and absence of hydrazone derivatives were analyzed by fitting the experimental data to electrical equivalent circuit models using Zview software. In the proposed electrical equivalent circuit (Fig. S3, Supplementary material), R_s, represents the uncompensated solution resistance, CPE, represents the constant phase element and R_p, represents the polarization resistance which is the sum of different resistances operating at the mild steel/electrolyte interface (El Azhar et al., 2001). The CPE is defined by the expression (Lgaz et al., 2017b):

The EIS results of the mild steel in presence and absence of hydrazone derivatives were analyzed by fitting the experimental data to electrical equivalent circuit models using Zview software. In the proposed electrical equivalent circuit (Fig. S3, supplementary material), R_s, represents the uncompensated solution resistance, CPE, represents the constant phase element and R_p, represents the polarization resistance which is the sum of different resistances operating at the mild steel/electrolyte interface (El Azhar et al., 2001). The CPE is defined by the expression (Lgaz et al., 2017b):

where Y₀ is the CPE constant, ω is the angular frequency for which imaginary part of the impedance reaches its maximum (Eq. (16)) (Salarvand et al., 2017), j is the imaginary number and n is the exponential term of a CPE which gives details about the degree of surface inhomogeneity (Saha et al., 2016).

The interfacial capacitance C_p (sum of different capacitances operating at the mild steel/electrolyte interface) can be calculated as follows (Olasunkanmi et al., 2016):

The electrochemical impedance parameters such as R_s, C_p, R_p, CPE, and n as well as the ${\eta }_{\mathit{EIS}}(\%)$ values are listed in Table 3. The values of the inhibition efficiency were calculated by the following equation (Gholami et al., 2013):

As shown in Table 3, a significant increase in polarization resistance values was observed with accompanying decrease in $C_{\text{p}}$ values. The increase in R_p values implied that a protective layer was formed on the mild steel surface, thereby retarding the charge transfer process (Saha et al., 2016). Adsorbed inhibitor molecules at the mild steel/electrolyte interface replace the surface adsorbed water molecules which increase the thickness of the double layer, thus leading to the reduction of $C_{\text{p}}$ values and therefore efficiently minimized dissolution of steel (Ongun Yüce et al., 2014). Like potentiodynamic polarization and weight loss results, the inhibition performances of tested compounds follow the order of HDZ-1 > HDZ-2 > HDZ-3. As expected, the presence of a strong electron-donating group (like dimethylamino group) substantially increases the reactivity of HDZ-1 and, hence, results in high inhibition efficiency.

To investigate the effect of immersion time on the inhibitor performance, EIS test was carried out in the absence and presence of 5 × 10⁻³ M of HDZ-1 after 6, 12, and 24 h immersion times at 303 K (Fig. 3). The electrochemical parameters at different immersion times are listed in Table 4. The results revealed that the inhibitor showed excellent performance at all immersion times. The increase in immersion time had a positive effect on inhibition efficiency which reaches its maximum at 12 h while a small decrease of 1% was observed at 24 h compared to the initial immersion time of 0.5 h. These results indicate that the tested hydrazone is an effective corrosion inhibitor and this obvious effectiveness is mainly due to the stability of the inhibitor film on the steel surface.

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Fig. 3

EIS curves of mild steel substrate immersed in 1 M HCl solution without and with 5 × 10⁻³ M of HDZ-1 at varied times.

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All in all, the comparison of the electrochemical results with weight loss data for tested compounds showed that there is a substantial agreement between the inhibition efficiency values from the three tests. Further experimental and theoretical studies were performed to confirm and validate these findings (See sections below).

3.4 Adsorption isotherm

For getting better insights into the nature of adsorption of organic inhibitor molecules on mild steel surface in corrosive medium, the adsorption isotherms are usually used (Khaled and Abdel-Rehim, 2011; Khaled and El-Maghraby, 2014). Use of such adsorption isotherms can facilitate effective comparison between the inhibitor molecules by fitting the experimentally generated data with isotherm models. The degree of surface coverage ( $\theta$ ) values used for the construction of adsorption isotherm were extracted from weight loss measurements. This is especially important since the immersion time is higher enough to study the adsorption ability of inhibitors. The adsorption isotherm was determined by applying different adsorption isotherm models such as Langmuir, Temkin, Freundlich, and Frumkin to the same set of experimental data, where we found that Langmuir model (Fig. 4) serves better than all the other models. The Langmuir model Eq. (19) is of the following form (Olasunkanmi et al., 2016):

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Fig. 4

The isotherm plots for the adsorption of hydrazones on mild steel in 1.0 M HCl at 303 K: (a) Langmuir, (b) Temkin, (c) Frumkin and (d) Freundlich.

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The terms shown in the above equation are defined as follow: $K_{\text{ads}}$ is the adsorption-desorption equilibrium constant and $C_{\text{inh}}$ is the hydrazones concentration. The $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ values (free energy of adsorption) were calculated using the following equation (Olasunkanmi et al., 2016):

A correlation between C/ $\theta$ vs. C (Fig. 4(a)) for three compounds gave a straight line with correlation coefficient (R2) close to 1, suggesting that the adsorption of tested inhibitors on mild steel surface obeyed the equation of Langmuir's adsorption model.

In addition, it is evident from Table 5 that the slopes of the straight lines are close to 1 which is an indication that there are no interactions among the adsorbed inhibitor molecules.

In an attempt to mitigate the dissolution of mild steel, the inhibitor molecules got adsorbed onto the metal surface by replacing the pre-adsorbed water molecules. The adsorption of inhibitor molecules can be through physical, chemical or physicochemical adsorption. In this regard, the free energy of adsorption can give useful information about the adsorption mode involved in the interaction of hydrazone derivatives to the mild steel surface. According to many authors in the field (Chaitra et al., 2016; Gurudatt and Mohana, 2014), the value of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$  ∼ −40 kJ mol⁻¹ or more negative, is mainly counted to be chemisorption, whereas, if $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$  ∼ −20 kJ mol⁻¹ or less negative, the adsorption is mainly classified as physisorption. In the present study, $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ values are in between −20 kJ mol⁻¹ and −40 kJ mol⁻¹, which indicate a comprehensive adsorption (both physisorption and chemisorption) (Lgaz et al., 2017b). A better explanation of these results are included in the XPS and theoretical sections.

3.5 XPS studies

In recent years, X-ray photoelectron spectroscopy (XPS) was extensively used in wide range of fields, including material characterization research and surface chemistry of metals (Guzmán et al., 2017). In corrosion inhibition research, XPS is a useful method to ascertain the adsorption of inhibitor molecules on the metal surface (Finšgar and Merl, 2014). The XPS analysis were done on the steel surface after 24 h of immersion time in 1 M HCl solution containing 5 × 10⁻³ M of HDZ-1 (The three compounds have similar molecular structures and belong to the same family). Fig. 5 shows the obtained high-resolution X-ray photoelectron deconvoluted profiles of Fe 2p_3/2, C 1s, N 1s and O 1s core levels.

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Fig. 5

The XPS deconvoluted profiles of Fe 2p_3/2, C 1s, N 1s and O 1s acquired from mild steel after immersion in 1 M HCl containing 5 × 10⁻³ M of HDZ-1.

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The Fe 2p_3/2 peak for the HDZ-1-treated surface is usually decomposed into four peaks located at ∼707.0, ∼710.5, ∼712.2, and ∼713.7 eV representing four different environments. The peaks at ∼710.5 and ∼712.2 eV are mainly attributed to ferric oxides such as Fe₂O₃ (i.e., Fe³⁺ oxide) and/or Fe₃O₄ (i.e., Fe²⁺/Fe³⁺ mixed oxide) and to ferric hydroxide FeOOH (Bouanis et al., 2009; Pech-Canul and Bartolo-Perez, 2004), while that located at ∼707.0 eV is attributable to metallic iron (Di Castro and Ciampi, 1995). The last peak at ∼713.7 eV may be assigned to the satellite of Fe(III) (Galtayries et al., 2006).

The oxygen 1s X-ray photoelectron spectroscopy (XPS) spectra acquired from mild steel, subsequent to immersion in 1 M HCl solution with HDZ-1 may be fitted into one main peak at ∼531.8 eV indicating one chemical form of O atom present in the inhibited interface. The peak located at binding energy (∼531.8 eV) is due to the presence of the oxygen double bonded to carbon (C⚌O) (Boyd et al., 2001; Kannan et al., 2007).

The carbon atom based bonds are the main constituent of the inhibitor molecule, thus, the C 1s XPS spectra was recorded in order to get information about the possible chemical forms of C atoms adsorbed on the mild steel surface. The C 1s signal can be deconvoluted in three components located at ∼285, ∼286.4 and ∼289.2 eV binding energies. The binding energy at ∼285 eV is consistent with the formation of C—C, C⚌C, and C—H bonds related to the different groups of HDZ-1 (Outirite et al., 2010; Seah et al., 1983). It can also be assigned to contaminant hydrocarbons (Bouanis et al., 2016). The C 1s peaks at ∼286.4 eV could be attributed to the formation of C—N and C⚌O groups in hydrazone derivative (Kelemen et al., 2002). The binding energy at ∼289.2 eV is mainly attributed to the presence of imide group (N—C⚌O) and to the carbon atom of the C⁺—O (Briggs and Seah, 2003; Zarrouk et al., 2015) resulting from the protonation of the HDZ-1 molecule in 1 M HCl as shown in the following scheme.

Focusing now on the curve-fitting of the N 1s XPS spectra which can be deconvoluted by using two different components located at 398.8 and 400.2 eV. The binding energy value of 398.8 eV can be assigned to the unprotonated N atoms present in hydrazone group of HDZ-1 while the binding energy of 400.2 eV can be attributed to the coordination of the nitrogen atom with the mild steel surface ( $\text{N}-\text{Fe}$ ) (Tang et al., 2013; Weng et al., 1995).

The inhibition efficiency of HDZ-1 can be enhanced by several ways including chemical and physical interactions. The presence of the heteroatoms such as oxygen and nitrogen in the molecules of hydrazone derivatives facilitates the coordination interaction with iron atoms and leads to a high tendency for protonation in acidic medium. The high amount of π-bonds could definitely enhance the donation via the transfer of electrons from the occupied π orbitals of the inhibitor molecule to iron unoccupied orbitals and back-donation through the transfer of electrons from the d-orbitals of iron to the vacant π* orbital of the inhibitor molecule. These results are remarkable and highlight the utmost importance of both chemical and physical interactions in enhancing the adsorption of an inhibitor molecule. A fuller picture of these interactions will be developed by combining the experimental data with the results of the theoretical calculations.

3.6 SEM

SEM pictures were used to further investigate the surface morphology of mild steel before and after 6 h, 12 h and 24 h of immersion in the 1 M HCl solutions with and without 5 × 10⁻³ M HDZ-1 at 303 K. Fig. 6 shows the surface morphology by SEM for mild steel in absence and presence of tested inhibitor molecule. In absence of inhibitor, it is clearly observed that the surface of the mild steel (after 24 h immersion) is highly corroded and damaged due to rapid corrosion attack in acidic solution. In presence of inhibitor, significant improvements were observed at different immersion times with smoother surfaces. The remarkable improvement of mild steel surface reflects the high adsorption ability of inhibitors on the mild steel surface.

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Fig. 6

SEM micrographs acquired from mild steel samples subsequent to immersion for 6, 12 and 24 h in 1 M HCl at 303 K.

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3.7 Quantum chemical calculations

3.7.1

3.7.1 Global reactivity descriptors

In most of the previous corrosion research, Density functional theory (DFT) calculations played a major role in predicting the adsorption ability of inhibitors through the successful investigation of their electronic properties (Guo et al., 2017). In the light of this, and as a continuation of our efforts towards understanding factors that affect the effectiveness of tested hydrazone derivatives, quantum chemical calculations were performed using density functional theory (DFT) methodology. Fig. 7 shows the fully optimized geometries, HOMO called as the highest occupied molecular orbital, LUMO called as the lowest unoccupied molecular orbital and molecular electrostatic potential (MEP) of the three inhibitors molecules.

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Fig. 7

Optimized molecular structure, HOMO and LUMO orbitals, and molecular electrostatic potential of hydrazone derivatives calculated at DFT/6-311++G(d, p).

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The reactive sites of the molecules can be better studied by analyzing the HOMO and LUMO while the reactivity trend can be identified by studying the electronic parameters. The electron donation tendency of a compound is mainly associated with an increase in HOMO energy whereas the electron acceptance ability is correlated with a decrease of LUMO energy (Singh et al., 2018). Fig. 7 shows that the HOMO is partially distributed on the molecular structure of the HDZ-2 and HDZ-3 without any observed reactivity of substituted phenyl. It is also evident that both inhibitors have similar HOMO distribution while in HDZ-1, the electron density has extended on almost the entire molecular structure, thereby enhancing the donating ability of this compound. Besides, the insignificant HOMO densities on the C⚌O group is because oxygen atom has higher electronegativity and therefore it has greater tendency to hold onto its lone pair of electrons. On the other hand, the LUMO electron densities of HDZ-2 and HDZ-3 shifted toward the non-reactive parts in HOMO distributions. In return, like HOMO distribution, the LUMO electron density of HDZ-1 is localized almost on the entire molecular structure showing the tendency of the mentioned compound towards chemical and physical interactions, which further support the experimental investigations.

Table 6 reports the quantum chemical parameters, i.e. HOMO and LUMO energies, HOMO−LUMO gaps (ΔE = E_HOMO − E_LUMO) and fraction of electron transferred ΔN. The HOMO energy corresponds to the ability of an inhibitor molecule to offer electrons to another molecule having empty orbits (Saha et al., 2016). A high value of HOMO energy for a molecule is related to its tendency to donate electrons to the vacant orbital of the metal atoms while a decrease in LUMO energy reflects the ease of electron acceptation by this level (Saha et al., 2016). The energy gap is one of the key quantum chemical parameters in a comparison between inhibitor molecules having similar molecular structures since it characterizes the chemical reactivity, molecular chemical stability, and hardness of an inhibitor molecule. The adsorption ability, thus the inhibition performance increases when ΔE_gap value decreases, whereas a higher value of energy gap is associated with high stability of the compound, thus a low tendency to react (Singh et al., 2016). It is apparent from Table 6 that the inhibitor molecule possessing the highest E_HOMO value is HDZ-1, followed by HDZ-2 while the HDZ-3 has the lowest HOMO value. The extent to which the molecule can accept electrons is indicated by E_LUMO values. Thus, compared to HDZ-2 and HDZ-3, the inhibitor molecule HDZ-1 has a greater tendency towards electron donation to iron atoms. As follows from Table 6, the extent of increment for the LUMO energies follows the order of HDZ-2 < HDZ-3 < HDZ-1 which is not in agreement with the order of the inhibition efficiency. As reported in Table 6, the HDZ-1 possesses the lowest energy gap, followed by HDZ-2 while the HDZ-3 has the higher energy gap value following therefore the order of corresponding inhibition efficiency. Previous experimental and theoretical studies had noted that there is a good correlation between the amount of electron transferred from the inhibitor to metal atoms and the strength of the interactions between inhibitor and metal surface. It is noted that an inhibitor molecule transfer its electrons to metal if ΔN > 0 and vice versa if ΔN < 0 (Kokalj, 2010; Kovačević and Kokalj, 2011). Therefore, according to this criterion, the results in Table 6 show that the three compounds have the tendency to donate electrons to the metal surface. In addition, the ΔN values follow the same order of the HOMO energies values, which indicate that the electron donating ability of the inhibitor molecules plays the crucial role in the difference between tested compounds. The results emerged from the quantum chemical parameters somewhat match and confirm those obtained from XPS studies which show that the potential sites for electron donation are more dominant than those of electron acceptation.

Table 6 Calculated Quantum chemical parameters for hydrazone derivatives obtained from DFT/B3LYB/6-311++G (d, p).

Inhibitors	EHOMO (eV)	ELUMO (eV)	ΔEgap (eV)	ΔN110
HDZ-1	−4.410	−0.815	3.595	0.162
HDZ-2	−5.055	−1.433	3.622	0.104
HDZ-3	−5.219	−1.317	3.902	0.095

3.7.3

3.7.3 Fukui functions

From the results mentioned above, it can be concluded that the hydrazone derivatives possess higher susceptibility to accepting and donating electrons from/to iron atoms. Thus, the identification of the most reactive sites may be very useful in supporting experimental-based hypothesis. To this end, the condensed Fukui functions of the inhibitor molecule can be a useful parameters in assessing the local reactivity of the inhibitor molecules (Lgaz et al., 2017a).

The local reactivity analysis based on the Fukui function calculations provides information about the atomic site with the highest tendency to be involved in electrophilic and nucleophilic reactions. The atomic sites for electrophilic will be those bearing a positive charge ( $f^{+}$ ) and possessing a tendency to accept an electron while those for nucleophilic bearing a negative charge ( $f^{-}$ ) and possessing a tendency to donate an electron (Lgaz et al., 2017b). The greater the Fukui function value of a given atom is, the higher its local reactivity (Lgaz et al., 2017b). The most active sites for nucleophilic and electrophilic attacks are graphically represented in Fig. 8.

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Fig. 8

Graphical representation of the calculated Fukui indices of hydrazone derivatives.

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In the HDZ-1, most positive atoms of $f^{-}$ function are located on the N(34), C(37), and N(47); while most positive atoms of $f^{+}$ function are located on the C(23), O(31) and C(38). In HDZ-2, the most positive atoms of $f^{-}$ function are located on C(4), C(14) and C(26); while most positive atoms of $f^{+}$ function are located on the N(18), N(34) and C(35). In the case of HDZ-3, N(18), C(21) and C(26) are the most positive atoms of $f^{-}$ while C(30), C(34), and C(43) are the most electrophilic sites. Overall, these results are generally reassuring since there is a clear trend in increasing electro-donating tendency from HDZ-3 to HDZ-1. It can be also observed that the electron donation ability seems to be more important than that of electron acceptation, which well supports the above-mentioned results.

3.8 MD simulations

With increasing computational power and memory, the use of computational techniques holds a great promise to provide accurate simulations for more complex physicochemical systems. In recent years, there has been growing interest in the use of molecular dynamics (MD) simulations to study the movement of atoms directly (Ricke, 2018; Wypych, 2017). In corrosion research field, one of the recent themes is the understanding of the corrosion inhibition process at an atomic or molecular level (Weder et al., 2016). In the light of this, and as a continuation of our efforts towards a more comprehensive evaluation of the corrosion inhibition properties of tested organic compounds, MD simulations have been done on an inhibitor and its environment of solvent and corrosive particles like $H_3{O}^{+}$ and $C{l}^{-}$ ions.

Fig. 9 presents the best adorable adsorption configuration of the tested inhibitors on the Fe(1 1 0) surface. Careful inspection of this figure reveals that the studied molecules move to the Fe (1 1 0) surface to parallel or flat disposition. Without a doubt, the flat disposition of inhibitors can play an essential role during adsorption on the surface of the metal since corrosion inhibitors with parallel adsorption have better chances of covering a wide area of the metal surface, thus great extent of inhibition efficiency. The extent of interaction between the Fe surface and inhibitor molecules can unambiguously be demonstrated by estimating the interaction and binding energies. A large negative values of the interaction energies indicates that the interactions between inhibitor molecules and Fe(1 1 0) surface are strong (Zhou et al., 2008). As Table 7 shows, there is a significant difference between three simulated compounds, the HDZ-1 has the highest absolute value of interaction energy, followed by HDZ-2; and the lowest value is for HDZ-3. Therefore, It can be confirmed that the increase of the interaction energy depend on the nature of electron donating groups. Furthermore, it is noteworthy that high binding energy implies that the adsorption of inhibitor molecules is strong and stable on Fe(1 1 0) surface (Obot et al., 2013; Zhou et al., 2008). In this context, Anton Kokalj in his comments on the paper by Mendes et al. (Mendes et al., 2012) has written that higher magnitude of binding energy indicates that probably there is more than one bond to the surface per molecule (Kokalj, 2013). Even though the changes between inhibitor molecules were small from a molecular point of view, the inhibitor with –N(CH₃)₂ group has more tendency to interact with the metal surface, hence, increasing the inhibition efficiency due to high electron donating property of this group (Singh et al., 2018). Thus, a change in structure, and/or in the nature of the functional group, is expected to affect the compound's efficiency. Our results are in good agreement with experimental and DFT calculations and broadly supported the majority of the findings in the literature.

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Fig. 9

Equilibrium adsorption configurations of the hydrazone derivatives on the Fe (1 1 0) surface in solution obtained by molecular dynamics simulations.

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3.9 Radial distribution function

To further explore the interaction mechanism between inhibitor molecules and iron atoms, radial distribution functions were analyzed from MD trajectories. The RDF is an advantageous technique to determine the bond length and its peak distance indicate how much meaningful are interactions of inhibitor molecules with iron surface (Benzon et al., 2017). Particularly, the typical bond length of physisorption is greater than 3.5 Å, much longer than chemical interaction types, which fall in the range of 1 Å–3.5 Å (Xie et al., 2015). The total radial distribution functions of all inhibitor molecules are represented in Fig. 10.

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Fig. 10

RDF analysis of hydrazone derivatives adsorbed on the Fe (1 1 0) surface in solution.

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From the data in Fig. 10, it is apparent that all inhibitor molecules have interesting interaction with iron atoms. The first peaks for (HDZ-3)-Fe, (HDZ-2)-Fe and (HDZ-1)-Fe are all located at distances smaller than 3.5 Å. The functional groups that include heteroatoms and π-electrons provide key interactive force to inhibitor molecules. Like experimental and quantum chemical calculations results, the RDFs results show strikingly similar extent of adsorption where the HDZ-1 has the strongest interactions with iron atoms. The results of RDFs indicate that the interactive forces were also well correlated with the order of the binding energies. In summary, these results provide strong evidence for the effect of the functional groups, especially those with significant electro-donating properties, on the inhibitive performance of organic compounds.

3.10 Anticorrosion mechanism of hydrazone derivatives

Although there are a large number of published studies reporting various biological properties of hydrazone derivatives, the studies on the effectiveness and mechanism of these compounds in corrosion inhibition remain insufficient. Herein, the results showed that the hydrazone derivatives are effective corrosion inhibitors with great electronic properties. The Tafel polarization results showed that the three compounds are mixed type inhibitors controlling both anodic and cathodic reactions. The electrochemical and weight loss data showed that corrosion resistance of mild steel in presence of HDZ-1 is evidently superior to those with HDZ-2 and HDZ-3. In addition to experimental data, the computational calculations provided a great help in explaining the adsorption strength of inhibitors and identifying the active sites. It was expected that the investigated compounds would markedly reduce the corrosion rate of mild steel owing to their unique chemical reactivity. The presence of nitrogen and oxygen atoms allows the hydrazones to exhibit both hard and soft base characters and thus leads to a strong interaction with the steel surface. This statement was supported by XPS studies, which showed that the HDZ-1 is mainly chemically adsorbed on the steel surface along with, to some extent, the existing physical adsorption. Furthermore, the difference in inhibition efficiency between HDZ-1 and unsubstituted HDZ-3 was significant. The possible reason for this result was found to be due to the electron donating power of (-N(CH₃)₂). As the observed outcome of the present investigation, which represents a combination of a multitude of interactions between studied hydrazones and mild steel surface, we can suggest that the inhibition mechanism is mainly a result of a combination of a three kinds of interactions (Fig. 11):

1. The surface charge of the mild steel plays a crucial role during interaction with an inhibitor molecule. It is consistently seen in similar studies (Yıldız et al., 2014) that the mild steel surface carries a positive excess charge in HCl solution. Moreover, it is known that organic inhibitor molecules having heteroatoms in their molecular structure are expected to be protonated in an acidic solution in balance with the matching molecular structure (Yıldız et al., 2014). Therefore, the chlorine ions will be preferentially absorbed onto the steel surface, thus, permit positively charged HDZs to come into negatively charged metal surface through the formation of electrostatic interactions (Solmaz et al., 2008).

2. The physical adsorption is the first step and the base of chemical adsorption, then, the hydrazone derivatives can be chemically adsorbed through acceptor-donor interactions (Tang et al., 2006). The presence of π electrons of the phenyl rings and unshared electron pairs in heteroatoms can ensure the donation of the electron to the vacant d-orbitals of iron, which is chemical adsorption.

3. A greater amount of reactive sites might leads to an extra negative charge on steel surface due to a strong donation of electrons. This extra charge might be transferred to inhibitor molecules by means of back-donation interactions, which mean the transfer of electrons from the d-orbitals of iron to the vacant π* orbital of the inhibitor molecules.

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Fig. 11

Pictorial representation of adsorption of HDZ-1 on mild steel surface in 1 M HCl solution.

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