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. 2023 Jun 5;8(24):21571–21584. doi: 10.1021/acsomega.3c00741

Corrosion Inhibition of Azo Compounds Derived from Schiff Bases on Mild Steel (XC70) in (HCl, 1 M DMSO) Medium: An Experimental and Theoretical Study

Ammar Zobeidi †,‡, Salah Neghmouche Nacer †, Salem Atia ‡, Latifa Kribaa ‡, Aicha Kerassa §, Abasse Kamarchou ‡, Mousa AlNoaimi ∥,⊥, Djamel Ghernaout #,∇, Mohamed A Ali ○, Abdelmajeed Adam Lagum ◆, Noureddine Elboughdiri #,¶,*
PMCID: PMC10285953  PMID: 37360453

Abstract

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The inhibitory activity of three prepared azo compounds derived from Schiff bases, namely, bis[5-(phenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C1), bis[5-(4-methylphenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C2), and bis[5-(4-bromophenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C3), against corrosion of steel type XC70 in (HCl, 1 M DMSO) medium was investigated experimentally by electrochemical measurements and theoretically using density functional theory (DFT). The correlation between corrosion inhibition and concentration is direct. The maximum inhibition efficiency at 6 × 10–5 M for the three azo compounds derived from Schiff bases was 64.37, 87.27, and 55.47% for C1, C2, and C3, respectively. The Tafel curves indicate that the inhibitors follow a mixed but predominantly anodic inhibitor system and have a Langmuir isothermal adsorption process. The observed inhibitory behavior of compounds was supported by DFT calculation. It was also found that there was a strong correspondence between the theoretical and experimental results.

1. Introduction

Organic inhibitors are among the most critical methods for inhibiting corrosion in acidic environments.1,2 Acid solutions are used in many industrial processes, and organic inhibitors protect several minerals against acid.3,4 Compounds containing heteroatoms (N, P, S) have proven a good inhibitor efficiency. Indeed, several heterocyclic compounds are corrosion protection inhibitors with high performance in acidic medium. Furthermore, these organic molecules can be adsorbed on the surface of the metal, which lowers the corrosion rate in an acidic environment.5−7

Schiff bases and their derivatives have recently become widely used as corrosion inhibitors thanks to their low cost and good inhibiting effect.8−13 Corrosion inhibitors mainly depend on the adsorption phenomenon and how they inhibit corrosion. The presence of double bonds and heterogeneous atoms such as sulfur, phosphorus, nitrogen, and oxygen make these compounds suitable inhibitors.14−17

In this work, three azo compounds derived from Schiff bases, namely, bis[5-(phenylazo)-2-hydroxybenzaldehyde]-4, 4′-diaminophenylmethane (C1), bis[5-(4-methylphenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C2), and bis[5-(4-bromophenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C3) (Figure 1), were prepared. An investigation was carried out using electrochemical methods to determine the influence of the donating groups of these azo compounds (bromo and methyl) on the corrosion inhibition provided towards iron (HCl, 1 M DMSO). Calculations based on density functional theory (DFT) were utilized to explain the action mechanism of the inhibitors, besides their electrical characteristics and structural influences on how effectively they inhibit.

Figure 1.

Figure 1

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Structure of the studied inhibitors (C1, C2, and C3).

In addition, the circumstances that led to the production of two different ZnO deposits on an indium tin oxide substrate were investigated. These conditions changed the proportions of zinc oxide salt. The initial deposits were acquired from an aqueous mixed bath containing the salt at a lower concentration and temperature. Meanwhile, the second deposits were produced by annealing simonkolleite (Zn5(OH)8Cl2H2O) for 1 h at 400 °C. In addition, the changes in structural and morphological analytical results for both deposits were compared to those of the simonkolleite phase. These discrepancies were observed in both deposits. Impedance spectroscopic curves and the photoelectrochemical behavior of three films are also explored in this study. This research provides information on the electrical conductivity of the deposits as well as their photosensitivity.

2. Experimental Section

2.1. Solution and Specimen Preparation

The chemicals utilized in the current study were obtained from Merck and were of reagent-grade quality. Three azo compounds, C1, C2, and C3, were synthesized using published methods.18,19 The physical and analytical data for the Schiff bases are given in Table 1. The structures of the synthesized Schiff bases were identified using spectroscopic techniques such as IR, UV–vis, 1H NMR, and 13C NMR. The Schiff base derivatives used in this study are presented in Figure 1.

Table 1. Physical and Elemental Analysis Data of the C1, C2, and C3 Schiff Bases.

            found (calculated)
compound compound formula formula weight (g mol–1) yield (%) color λmax in DMF (nm) %C %H %N
C1 C39H34O2N6 618.62 88.44 yellow 385 75.71 (75.45) 5.54 (5.83) 13.58 (13.35)
C2 C41H38O2N6 646.67 75.89 light brown 360 76.14 (75.76) 5.92 (5.95) 12.99 (13.13)
C3 C39H32O2N6Br2 776.42 69.20 sepia 363 60.32 (59.83) 4.15 (4.04) 10.82 (10.97)
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The considered mild steel (XC70) is commercially available in the following composition (wt %): 0,065% C; 0.01% S; 0.245% Si; 1.685% Mn; 0.002% P; 0.042% Cr; 0.062% N; 0.010% Cu; 0.042% Al; 0.067% Nb; 0.014% V; 0.019% Ti; 0.005% Mo; and the rest is Fe. The steel was cut into cylindrical disks and was the working electrode in all electrochemical studies. The acidic medium (HCl, 1 M) was prepared from distilled water and HCl of 37% purity. The azo compounds (inhibitors) were dissolved in DMSO, so the corrosive medium was (HCl, 1 M DMSO). The concentrations of C1, C2, and C3 used as inhibitors ranged from 10–6 to 6 × 10–5 M.

2.2. Measurements Using Electrochemistry

The electrochemical investigation was carried out with the assistance of a Voltalab40 (PGZ301) equipped with a Volta-master4 type program and a 500 mL cylindrical glass cell with five holes (one for the XC70 metal working electrode (WE), the second for the platinum wire auxiliary electrode, and the third for the reference electrode (RE), i.e., saturated calomel electrode (SCE)). The WE was scraped before the start of each test with emery paper (80, 200, 1200, and 4000), then washed with distilled water and acetone, and finally well dried. Cathode and anode branches were acquired in the field −600 to −250 mV/ECS at a 30 mV/sec scan rate. The corrosion potential (Ecorr) and current density (icorr) were determined by extrapolating data from Tafel curves. The inhibition efficiency (IEp, in %) was determined using eq 1(20)

2.2. 1

where icorr(inh) and icorr are the corrosion current densities with an inhibitor and without an inhibitor, respectively.

Electrochemical impedance spectroscopy (EIS) studies were all done at 25 °C. The inhibition efficiency (IEEIS, in %) was determined using eq 2(21)

2.2. 2

where Rt,inh and Rt,corr are the charge transfer resistances with and without an inhibitor, respectively.

2.3. Computational Details

DFT calculations were based on B3LYP,22 which is functional with 6-311G(d,p) and 6-31G(d,p) basis sets,23 and implemented in GAUSSIAN 09 software.24−28 On an Intel Core i3-3120M laptop with 2.50 GHz and 6.00 Go RAM, frequency analysis was used to ensure that the optimized geometry was not a saddle point.

3. Results and Discussion

3.1. Tafel Curves

Figure 2 shows the Tafel curves of steel (XC70) in the solution consisting of HCl and 1 M DMSO, both with and without the presence of inhibitors of varying concentrations. Table 2 lists the electrochemical properties of icorr and Ecorr vs SCE, anodic and cathodic Tafel slopes (ba, bc), and IE. Electrochemical polarization measurements show that both cathodic and anodic current densities decreased with increasing inhibitor concentrations. This indicates that inhibitors reduce both the cathodic reduction reaction (eq 3) and metal dissolution (eq 4).

Figure 2.

Figure 2

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Tafel polarization curves of three inhibitors (C1, C2, and C3) at 25 °C.

Table 2. Electrochemical Parameters Determined by the Tafel Method of the Inhibitors (C1, C2, and C3) at 25 °C.

compounds Cinh (M) –Ecorr (mV/SCE) βa (mV/dec) –βc (mV/dec) icorr (μA/cm2) IEp (%)
C1 blank 504.3 82.5 53.6 101.80  
1 × 10–6 484.7 46.5 35.8 75.97 25.37
2 × 10–6 480.2 40.3 33.2 72.28 28.99
1 × 10–5 480.1 37.5 31.6 49.61 51.26
6 × 10–5 477.9 43.2 33.6 35.95 64.37
SD 253.41 27.00 17.67 31.75 25.31
Pearson χ2 –0.72 0.11 –0.10 –0.87 0.87
C2 blank 504.3 44 55 101.80  
1 × 10–6 477.8 47.6 41.6 52.41 48.51
2 × 10–6 475.1 37.8 31.9 38.43 62.21
1 × 10–5 480.1 83.4 55.2 28.39 72.11
6 × 10–5 462.4 44 55 12.95 87.27
SD 251.19 27.09 23.21 30.78 26.28
Pearson χ2 –0.93 –0.17 0.63 –0.87 0.87
C3 blank 504.3 49.9 39.8 101.80  
1 × 10–6 475.2 50.9 89.39 84.18 17.37
2 × 10–6 490.2 98.5 83.5 53.64 47.37
1 × 10–5 487 45 36.9 46.75 54.07
6 × 10–5 476.5 49.9 39.8 45.40 55.47
SD 254.91 30.83 30.36 32.54 17.36
Pearson χ2 –0.45 –0.36 –0.65 –0.53 0.53
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In addition, increasing the inhibitor concentration slightly modifies the cathodic slope, indicating that the cathodic reaction mechanism has not been altered. The presence of inhibitors changes the corrosion potential toward the nobler potential compared to the result obtained without inhibitors. In contrast, the maximum change in Ecorr values was −41.9 mV, less than ±85 mV. This indicates that these inhibitors act as a mixed type, even if the anodic inhibition is predominant.29−31 Furthermore, the inhibitory activity increased with increasing inhibitor concentration and reached values of 64.37, 87.27, and 55.47% for C1, C2, and C3, respectively, at the optimum concentration (6 × 10–5 M).

As illustrated by such findings, the inhibitory activity of C2 is greater than that of other azo compounds. Indeed, the methyl group (−CH3), which has an inducing donor effect, makes the inhibitors form a stronger coordination bond with iron than the electron withdrawing bromine (−Br) group.32

3.1. 3
3.1. 4

3.2. Impedance

A standard Randle’s circuit obtained from a capacitive semicircle in impedance spectra for cases of corrosion of steels in an acidic medium has been described in the literature.33,34 The Nyquist plots for iron in the (HCl, 1 M DMSO) solution are shown in Figure 3, both without any inhibitors and with a range of different concentrations of C1, C2, and C3 inhibitors. The impedance plots exhibit a nonideal semicircle in the presence and absence of an inhibitor. This is probably due to the mild steel’s surface heterogeneity, which is usually attributed to frequency dispersion and surface homogeneity. The deviation is explained by the roughness and impurities resulting from the formation of porous layers of inhibitors on the surfaces.35,36 The diameter of the capacitive ring is directly proportional to the concentration due to the inhibitors’ adsorption on the metal surface. This adsorption results from the interaction between the inhibitors’ nitrogen and oxygen and the steel’s character, which allows forming a film on the surface. The aromatic system can also contribute as π-electrons enhance the absorption of inhibitors on the surface and increase their efficiency.37,38 An increase in absolute impedance at lower frequencies, generally shown by Bode plots, results in more excellent protection at greater inhibitor concentrations. The observation of a single-phase peak in the center frequency range indicates only one stable contact of the electric double layer, as shown in Figure 3.

Figure 3.

Figure 3

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Mild steel corrosion Nyquist diagrams in (HCl, 1 M DMSO) media at 25 °C without and with different concentrations of three inhibitors (C1, C2, and C3).

Table 3 lists the electrochemical parameters such as pseudo-capacitance (Cdl), transfer resistance (Rct), and IE that have been calculated for different concentrations of inhibitors C1, C2, and C3 in the medium (HCl, 1 M DMSO) for XC70 steel type.

Table 3. Impedance Parameters for Corrosion of Mild Steel (XC70) in (HCl, 1 M DMSO) without and with Different Concentrations of the Inhibitors (C1, C2, and C3) at 25 °C.

compounds Cinh (M) Rct (Ω cm2) Cdl (μF/cm2) n IE (%) θ
C1 blank 73 153.5 0.8772    
1 × 10–6 103.3 61.77 0.7281 29.33 0.2933
2 × 10–6 111.4 65.29 0.7591 34.47 0.3447
1 × 10–5 147.8 70.79 0.7761 50.60 0.5060
6 × 10–5 186.7 54.14 0.7884 61.78 0.6178
SD 43.83 40.92 0.055 23.55 0.24
Pearson χ2 0.93 –0.76 1.00 0.87 0.87
C2 blank 73 153.5 0.8762    
1 × 10–6 97.98 57.88 0.7781 48.32 0.4832
2 × 10–6 185.1 67.16 0.7876 60.5 0.605
1 × 10–5 195 81.54 0.7894 62.56 0.6256
6 × 10–5 371.4 84.03 0.7937 80.18 0.8018
SD 117.22 37.70 0.040 30.34 0.30
Pearson χ2 0.95 0.72 1.00 0.92 0.92
C3 blank 73 153.5 0.8736    
1 × 10–6 97.98 66.66 0.7491 25.49 0.2549
2 × 10–6 138.8 101.9 0.7631 47.40 0.4740
1 × 10–5 151 86.46 0.7781 51.56 0.5156
6 × 10–5 154.4 42.03 0.7957 52.72 0.5272
SD 35.87 41.93 0.048 22.67 0.23
Pearson χ2 0.57 –0.81 1.00 0.53 0.53
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On the one hand, Rct values have been calculated at higher and lower frequencies from the differences in impedance. On the other hand, Cdl values were obtained at f(−zimax) frequency, at which the imaginary component of the impedance is maximal, using eq 3(39,40)

3.2. 5

where Cdl is the pseudo-capacitance, Rct is the transfer resistance, and f(−zimax) is the maximal frequency.

Also, the Rct of the double-layer decreased (Cdl) and IE increased with increasing inhibitor C1, C2, and C3 concentrations. By increasing the interactions, the number of inhibitors adsorbed on the metal surface increased, reducing the number of active sites exposed to corrosion.40 The inhibitor (C2) was found to have the highest percentage of inhibition efficiency (80.18%) compared to C1 (61.78%) and C3 (52.72%).

On the other hand, the variation in phase shift values (n) is not significant, thus confirming that the mechanism of dissolution of XC70 in HCl, 1 M DMSO in both the absence and presence of inhibitors is controlled by charge transfer. Moreover, in the presence of inhibitors C1, C2, and C3, n is found to be between 0.72 and 0.79, and these values are lower than those obtained in a blank solution (0.87). This is due to an increase in surface inhomogeneity as a result of inhibitor adsorption.20

The equivalent electrical circuit represents the mild steel/solution interface in the absence and presence of the inhibitor (Figure 4). The circuit consists of the electrolytic resistance (Rs), the charge transfer resistance (Rct), and the constant phase element (CPE). The CPE parameter replaces the double-layer capacitance (Cdl) to fit the experimental results better.39 It is recommended to use the CPE instead of the pure capacitor to model the frequency dispersion, generally related to surface heterogeneity caused by corrosion in an acidic medium.

Figure 4.

Figure 4

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Electrochemical equivalent circuit used for simulating the impedance spectra.

3.3. Adsorption Study

An adsorption isotherm is required to investigate corrosion in the presence of organic chemicals to determine their inhibitory mechanism. The Langmuir, Timken, and Franken isotherms40−42 have been utilized.

3.3. 6
3.3. 7
3.3. 8

where Cinh is the inhibitor concentration, Θ is the surface coverage, and Kads is the adsorption balance constant.

The inhibitors’ adsorption on the steel surface can explain the corrosion inhibition mechanism by preparing organic azo compounds. There are two types of adsorption, namely, chemical and physical. The first occurs on the (lone pair) inhibitor (d orbital) coordination bond. To investigate which isotherms are best suited for adsorption, Franken, Langmuir, and Temkin isotherms were tested, and the obtained results are presented in Table 4 and Figure 5. The linear correlation value closest to unity was obtained with the Langmuir isotherm, and the value of the slope of this isotherm is close to unity. This means that the adsorption process on the metal surface of the compound (C1, C2, and C3) follows the Langmuir isotherm.42 From the Langmuir isotherm (eq 6), the adsorption constant (Kads, ΔGads°) can be calculated using eqs 9 and 10, respectively41

3.3. 9
3.3. 10

Table 4. Linear Correlation of the Different Adsorption Isotherms of Inhibitors (C1, C2, and C3) on Mild Steel (XC70) in (HCl, 1 M DMSO) at 25 °C.

compounds Langmuir Franken Temkin
C1 0.998 0.923 0.854
C2 0.999 0.901 0.944
C3 0.999 0.182 0.679
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Figure 5.

Figure 5

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Different adsorption isotherms of inhibitors (C1, C2, and C3) on the mild steel surface in (HCl, 1 M DMSO) media at 25 °C.

Table 5 presents the free enthalpy ( ΔGads°) values of −42.58, −43.29, and −40.92 kJ/mol for azo compounds C1, C2, and C3, respectively. Again, the inhibitors’ spontaneous adsorption onto the metal’s surface was observed. The high values for the equilibrium constant of adsorption are a consequence of the powerful adsorption of C1, C2, and C3 on the surface of the mild steel (XC70) used in the experiment (HCl, 1 M DMSO).43

Table 5. Linear Correlation of the Langmuir Isotherm and Thermodynamic Parameters of Adsorption of Inhibitors (C1, C2, and C3) on Mild Steel (XC70) in (HCl, 1 M DMSO) at 25 °C.

compounds Cinh (M) Kads × 105 (M–1) KL R2 θ –ΔGads° (kJ/mol)
C1 1 × 10–6 5.288207 0.6541 0.9990 25.37 ± 18.53 42.58
2 × 10–6 0.4859 28.99 ± 18.53
1 × 10–5 0.1590 51.26 ± 18.53
6 × 10–5 0.1361 64.37 ± 18.53
C2 1 × 10–6 7.062146 0.5860 0.9994 48.51 ± 16.33 43.29
2 × 10–6 0.4145 62.21 ± 16.33
1 × 10–5 0.1240 72.11
6 × 10–5 0.1055 87.27
C3 1 × 10–6 2.712232 0.7866 0.9995 17.37 40.92
2 × 10–6 0.6483 47.37
1 × 10–5 0.2693 54.07
6 × 10–5 0.2350 55.47
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For the three inhibitors, ΔG° values were more significant than −40 kJ/mol, suggesting chemical adsorption.44 Moreover, the absolute value of standard free energy (ΔG°) decreases in the following order: C2 > C1 > C3. Hence, the corrosion inhibition efficacy rises with increasing negative values (ΔGads°). This result agrees with the damping efficiency values obtained from impedance and Tafel polarization curves.

3.4. Scanning Electron Microscopy

Figure 6 shows micrographs of mild steel under pre- and post-immersion conditions, where the immersion period was assumed to be 24 h at 25 °C in the absence and presence of inhibitors (C1, C2, and C3). Figure 6 demonstrates that in the absence of an inhibitor, the surface of mild steel is rougher and more porous due to the dissolution of mild steel (XC70) in (HCl, 1 M DMSO) medium. However, in the presence of inhibitors (C1, C2, and C3), the surface looks smooth, and practically all cavities have disappeared.

Figure 6.

Figure 6

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Surface morphology of the mild steel samples before and after immersion in uninhibited and inhibited (HCl, 1 M DMSO) at 25 °C: (a) free, (b) with inhibitor C1, (c) with inhibitor C2, and (d) with inhibitor C3 over 24 h.

3.5. Contact Angle Measurement

Figure 7a–d shows the contact angle measurements in the absence (a) and the presence of the inhibitors C1 (b), C2 (c), and C3 (d). It can be seen that the angle in the absence of the inhibitor is smaller than that in its presence, indicating the occurrence of spacing between the water and the metal surface in the presence of the inhibitors C1, C2, and C3. On the other hand, the contact angle increased with increasing molecular weight and reached values of 68°, 71°, and 75° for C1, C2, and C3, respectively. This is because a larger molecular weight inhibitor has a greater tendency to remain as discrete droplets on the surface rather than spreading out, which results in a higher contact angle.45 All of this proves that the inhibitors C1, C2, and C3 were adsorbed on the surface of mild steel.20

Figure 7.

Figure 7

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Measurement of the contact angle of a mild steel surface after a 24 h immersion in blank (a) and (HCl, 1 M DMSO) loaded with 10–5 M of three inhibitors C1 (b), C2 (c), and C3 (d).

3.6. Density Functional Theory (DFT) Calculations

3.6.1. Molecular Structure

Bond lengths and angles for the studied three inhibitors are listed in Table 6 and shown in Figure 8. Also, Figure 9 shows that the studied molecules consist of two symmetrical parts. Indeed, the symmetry groups of all azo compounds are C1, and all dihedral angles in the selected moieties are equal to 0° or 180° due to the resonance effect, supporting the inhibitors’ planarity. Electrons may be, however, exchanged between inhibitors’ atoms and functional groups with the electrode surface. This phenomenon is influenced by structural and electronic properties such as chain length, molecule planarity, molecular volume, availability of the lone pair of electrons that are present on the heteroatoms (O, N, and S), polarity, dipole moment, the energy of frontier molecular orbitals, and instauration or aromatic ring, are generally considered to be effective corrosion inhibitors.46,47

Table 6. Bond Length and the Bond Angle of the Studied Inhibitors (C1, C2, and C3).
  compounds C1 C2 C3
bond length C1–C2 1.519 1.519 1.519
C2–C3 1.519 1.519 1.519
C4–N5 1.399 1.394 1.402
N5–C6 1.459 1.456 1.462
C6–C7 1.520 1.520 1.519
C8–N9 1.412 1.412 1.409
N9–N10 1.255 1.256 1.256
N10–C11 1.418 1.415 1.416
bond angle C1–C2–C3 114.092 115.181 112.900
C4–N5–C6–C7 67.733 70.113 66.609
C8–N9–N10–C11 180 180 180
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Figure 8.

Figure 8

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Optimized structure of the three inhibitors (C1, C2, and C3) obtained at the DFT/B3LYP/6-311G (d, p) level.

Figure 9.

Figure 9

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Selected planar moieties of the inhibitors (C1, C2, and C3).

3.6.2. Frontier Orbital Energies and Global Reactivity Parameters

The highest occupied molecular orbital, abbreviated HOMO, and the lowest unoccupied molecular orbital, abbreviated LUMO, are the two types of boundary molecular orbitals found in molecules (LUMO). Low kinetic stability is associated with molecules with a small HOMO–LUMO gap and high chemical reactivity.48,49 Applying Koopman’s theory led to estimating molecular properties associated with reactivity and selectivity based on the DFT-Koopman’s theory.50−52 To get an approximation of the electron affinity (A) and the ionization potential (I), the negative values of (EHOMO) and (ELUMO) are taken, respectively. This is done in quantitative terms.52 Furthermore, the relationships used in all of these calculations are as follows53,54

3.6.2. 11
3.6.2. 12
3.6.2. 13
3.6.2. 14
3.6.2. 15
3.6.2. 16
3.6.2. 17
3.6.2. 18
3.6.2. 19
3.6.2. 20
3.6.2. 21

knowing that χFe: electronegativity of iron (7.0 eV); χinh: electronegativity of the inhibitors; ηFe: chemical hardness of iron (0 eV); and ηinb: chemical hardness of the inhibitors.

As shown in Table 7, calculations establish that the highest EHOMO is observed in compound C2 (−4.949 eV), which may be attributed to the additional inductive effect (electron-donating) of lateral methyl groups. Inhibitors with a high EHOMO can remarkably push electrons into the low vacancy orbital of the corrosive environment, thereby delaying the anodic processes.55−57 In addition, the stability of LUMO supports the electron-receiving potential of the corrosion inhibitor, facilitating the interaction between them. This reaction improves the isolation of the corrosive material from the corrosive environment. Further, the lowest ELUMO is observed in compound C3 (−2.481 eV), which may be attributed to the other electron-withdrawing group (i.e., bromo).

Table 7. Global Reactivity Descriptors of the Studied Inhibitors According to Density Functional Theory (DFT) (C1, C2, and C3).
comp EHOMO (eV) ELUMO (eV) Eg (eV) η (eV) δ (eV–1) χ (eV) μ (eV) ω ΔN Δψ Eb–d (eV)
C1 a –5.113 –2.308 2.8058 1.403 0.356 3.710 –3.710 4.907 1.1724 0.204 –0.351
b –4.905 –2.065 2.8393 1.420 0.352 3.485 –3.485 4.278 1.240 0.619 –0.355
C2 a –4.949 –2.192 2.7568 1.378 0.363 3.571 –3.571 4.625 1.2439 0.216 –0.345
b –4.737 –1.954 2.7838 1.392 0.359 3.345 –3.345 4.020 2.784 0.656 –0.348
C3 a –5.184 –2.481 2.7030 1.351 0.370 3.832 –3.832 5.433 1.1720 0.184 –0.338
b –4.963 –2.223 2.7400 1.370 0.365 3.593 –3.593 4.711 1.240 0.622 –0.342
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a

6-311G (d, p) basis set.

b

6-31G (d, p) basis set.

The low energy gap has also been associated with molecular ductility, chemical reaction, kinetic instability, and good anticorrosion potential.58,59 As illustrated in Table 7, the values of Eg are very close and may be classified in the following order: C3 > C2 > C1, indicating that compound C3 would offer adequate anticorrosion protection. Therefore, the inhibitors studied generally have a low energy band, meaning that they can be anticorrosion materials. Also, as depicted in Table 7, all azo compounds’ chemical potential values are negative, implying that the azo compounds are stable.

Besides, the low potential value (−3.832 eV) and the significant electrophilicity value (5.433) for compound C3 favor its electrophilic behavior. On the contrary, the high potential value (−3.571 eV) and the low electrophilicity value (4.625) for compound C2 promote its nucleophilic behavior.60 This is a result that corresponds to the HOMO and LUMO energies values.61 Some electrons were transferred (ΔN); this parameter is generally used to indicate the ability of a molecule to accept or transfer electrons to or from a metal.62−65 The inhibitor can donate its electron to the metal if ΔN > 0, and the reverse occurs in the case of ΔN < 0. Thus, the positive values of ΔN for all of the inhibitors studied imply that the electron donation is from the inhibitor to the metal surface and follows the order C2 > C1 > C3, along with molecule–metal interaction energy (Δψ) that follows the same trend for the presented property of the three molecules. These results are compatible with experimental inhibition efficiency. Moreover, the inverse grant can be obtained from Eb–d. Indeed, a negative sign of Eb–d indicates that back donation from the metal to the inhibitor is energetically favorable. All of the inhibitors have ∼(−0.34 eV). The back donation and donation processes strengthen the inhibitors’ adsorption on the iron surface.

Figure 10 illustrates the HOMO map of C1, C2, and C3 inhibitors. The HOMO map is essentially delocalized over the middle moiety (i.e., two benzene rings attached by the CH2 group and two primary amino groups −NH–CH2−), indicating that this part is rich in electrons and is the one that can donate to the metal. Similarly, the LUMO diagram for inhibitors clearly shows the delocalization of vacant lower molecular orbitals on a single planar moiety of azo compounds (two benzene rings attached by the −N=N– group), except the carbon atom of the lateral methyl group in compound C2. This phenomenon indicates that the LUMO of the inhibitors is available for back donation.

Figure 10.

Figure 10

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Schematic representation of HOMO and LUMO of inhibitors (C1, C2, and C3) obtained by DFT at the B3LYP/6-311G (d, p) level.

3.6.3. Corrosion Inhibition and the Molecular Electrostatic Potential (MEP)

The molecular electrostatic potential, often known as the MEP, is used to predict the reactivity of inhibitor molecules and the overall charge distribution.66 For all of the researched azo compounds, the color coding for these maps runs from −0.07363 au (dark red) to 0.07363 au (dark blue). The MEP map and contour plot of C1, C2, and C3 were made in the optimal geometry and were carried out using the Gauss 05 view. These results are displayed at the top of the page (Figure 11).

Figure 11.

Figure 11

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Electrostatic properties of inhibitors (C1, C2, and C3): 3D MEP contour map at the top of the figure and 2D MEP in the middle, and MEP maps for the dipole and the Mullikan charge populations are displayed at the bottom (the region that is rich in electrons appears red, whereas the area that lacks electrons appears blue).

The MEP map, which can be found in Figure 11, makes it abundantly evident that the regions surrounding the hydrogen atoms of the amine group and hydroxyl groups’ hydrogen atoms have a significant lack of electrons (blue color).

In addition, there are the hydrogen atoms within the benzene rings, the hydrogen atoms within the two methyl groups that make up the C2 inhibitor, and its binding site for electrophiles. The second presentation of the MEP and 2D contour map (Figure 11) shows that the electron-rich region around all nitrogen, oxygen atoms, and benzene rings are also evident. The space around the two Br atoms for C3 is the electronegative region due to its inductive effect (i.e., charge transfer from the benzene ring to the bromo atoms) and bonding sites to nucleophiles.

As an excellent way to explain the differences in electromagnetism of atoms in a molecule, the charge population and direction of the dipole moment, depicted at the bottom of Figure 1, are often used to support MEP mapping.62 For example, the hydrogen atoms that make up the amino and hydroxyl groups have the most significant positive charge (green). The MEP map depicts them as having the darkest shade of blue possible.

The Mulliken charge populations and the dipole moment direction are projected onto the molecular plane for the three Schiff bases. The charge population and direction of the dipole moment can be comprehended by examining the electrostatic potential (bottom panels of 1), which identifies electron density-rich regions centered on O atoms in the hydroxyl group, N atoms in the −N=N– group, C atoms of the carbon bone chain, and some carbon atoms in benzene rings. Electrophiles often attack the locations with the highest electron density. Therefore, the active centers are O, N, and C atoms, with the most significant capacity to attach to the metal surface. In contrast, the HOMO (Figure 10) was mainly spread across the atoms, as mentioned earlier. Therefore, these regions are likely the major bonding sites.63,64

4. Conclusions

This work was dedicated to investigating the inhibitory activity of three prepared azo compounds derived from Schiff bases, namely, bis[5-(phenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C1), bis[5-(4-methylphenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C2), and bis[5-(4-bromophenylazo)-2-hydroxybenzaldehyde]-4,4′-diaminophenylmethane (C3), against corrosion of steel type XC70 in (HCl, 1 M DMSO) medium experimentally using electrochemical measurements and theoretically using density functional theory (DFT). From this research, the main conclusions drawn are listed below:

  • Derivatives of Schiff bases azo amines C1, C2, and C3 act as corrosion inhibitors for mild steel type XC70 in solution (HCl, 1 M DMSO). Efficiency increases by increasing concentration. The maximum inhibition efficiency was also observed around C1, i.e., 64.37%; C2, i.e., 87.27%; and C3, i.e., 55.47% at the concentration (6 × 10–5 M) at ambient temperature.

  • The corrosion rate of XC70 decreases with mounting concentrations of C1, C2, and C3.

  • Inhibitors C1, C2, and C3 act as mixed but mainly anodic inhibitors, i.e., they inhibit cathodic and anodic reactions during corrosion.

  • The Langmuir adsorption isotherm is the best way to describe the inhibitors’ adsorption (XC70).

  • These values were found to be −42.58, −43.29, and −40.92 kJ/mol for compounds C1, C2, and C3, respectively, indicating that the adsorption of the inhibitors onto the metal surface was a spontaneous reaction.

  • The predominant adsorption of the C1, C2, and C3 inhibitors on extrusive steel XC70 is chemical.

  • The order of inhibitors in terms of efficacy is C2 > C1 > C3.

  • The theoretical calculations are in good agreement with the experimental results, where (ΔN) and (Δψ) obey the following order: C2 > C1 > C3.

  • Compound C2 favors its nucleophilic character because of the trend of HOMO energy: C2 > C1 > C3, which may be attributed to an inductive effect (electron-donating) of lateral methyl groups. On the contrary, compound C3 favors its electrophilic character, which may be attributed to electron-withdrawing groups (bromo), which do decrease the LUMO energy. Therefore, this compound has the most significant value of the back-donation energy (Eb–d).

  • The negative sign of (Eb–d) indicates that back donation to the inhibitor is energetically favorable.

  • The molecular electrostatic potential (MEP) and Mulliken charge populations indicate that inhibitors adsorb through the N and O atoms and π-electron active centers.

  • The theoretical calculations and experimental findings concur that three were produced.

Acknowledgments

The authors are highly thankful to the Algerian General Directorate for Scientific Research and Technological Development, and the authors of this piece would like to express their gratitude to the Pollution & Waste Treatment Laboratory (PWTL) at the University of Ouargla, Algeria.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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