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. 2024 Mar 5;9(12):13746–13763. doi: 10.1021/acsomega.3c08282

Adsorption and Inhibition Mechanisms of New Pyrazole Derivatives for Carbon Steel Corrosion in Hydrochloric Acid Solutions Based on Experimental, Computational, and Theoretical Calculations

Loubna Adlani †, Nisrine Benzbiria ‡, Abderrahim Titi §, Nadia Timoudan ∥, Ismail Warad ⊥, Abeer AlObaid #, Basheer Mohammed Al-Maswari , Fouad Benhiba †,∥,*, Rachid Touzani §, Hassan Zarrok †, Fouad Bentiss ∇,○, Hassan Oudda †, Abdelkader Zarrouk ∥,*
PMCID: PMC10976362  PMID: 38560005

Abstract

graphic file with name ao3c08282_0017.jpg

The study aims to synthesize two green pyrazole compounds, N-((1H-pyrazol-1-yl)methyl)-4-nitroaniline (L4) and ethyl 5-methyl-1-(((4-nitrophenyl)amino)methyl)-1H-pyrazole-3-carboxylate (L6), and test their action as corrosion inhibitors for carbon steel (CS) in a 1 M HCl solution. Both chemical and electrochemical methods, namely, gravimetric measurements (WL), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS), were used to assess the efficiency of the investigated molecules. DFT calculations at B3LYP/6-31++G (d, p) and molecular dynamics simulation were used to carry out quantum chemical calculations in order to link their electronic characteristics with the findings of experiments. The organic products exhibited good anticorrosion ability, with maximum inhibition efficiencies (IE %) of 91.8 and 90.8% for 10–3 M L6 and L4, respectively. In accordance with PDP outcomes, L6 and L4 inhibitors act as mixed-type inhibitors. Assessment of the temperature influence evinces that both L4 and L6 are chemisorbed on CS. The adsorption of L4 and L6 on CS appears to follow the Langmuir isotherm. Scanning electron microscopy and UV–visible disclose the constitution of a barrier layer, limiting the accessibility of corrosive species to the CS surface. Theoretical studies were performed to support the results derived from experimental techniques (WL, PDP, and EIS).

1. Introduction

As a universal issue, corrosion leads to tremendous safety and economic casualties worldwide.1−3 Beyond that, corrosion also gives rise to environmental hazards as a result of the release of poisonous solvents and chemicals via the corrosion-ridden metallic components.4,5 Carbon steel (CS) and other iron alloys are commonly employed in various industrial processes, some of which involve the employment of overly concentrated acids.6,7 As a result, a significant portion of metallic components are lost owing to corrosion.8−10 Given the massive losses, a variety of corrosion prevention approaches have been devised to suit the budget, the nature of the metal, and the media.11−13 Regardless, the most prevalent and frequently used method is the application of organic products as inhibitors since they are efficient, easily synthesized, and show high inhibition performance coupled to cost effectiveness. In most cases, the mitigation of corrosion is achieved by developing a protective layer at the interface metal/medium.14−23 According to the literature, electron-rich centers as polar functional groups (−NO2, −NH2, −NHMe, −OH, −OMe, −CN, etc.) and double/triple links (>C=O, –N=O, –C≡C–, –C≡N, >C=S, –N=N–, etc.) of the aromatic ring (s) and/or side chain are considered as good adsorption centers.24−26 While a number of heterocycles were assessed as potent inhibitors, the main focus of current investigations is on developing cost-effective and eco-friendly compounds. Pyrazole derivatives constitute one such alternative as they are heterocyclic products used in several fields, nontoxic (used as dyes, pesticides, insecticides, fungicides, and herbicides), and relevant candidates to mitigate the corrosion of metals. Some of the research conducted on the prohibition activity of pyrazole derivatives against CS corrosion in 1 M HCl is reported in Table 1.22−25

Table 1. Inhibition Efficiencies of Some Tested Pyrazole Products as Corrosion Inhibitors of CS in 1 M HCl Medium27−30.

1.

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In the current investigation, the inhibitory capacity of two novel pyrazole compounds, namely, N-((1H-pyrazol-1-yl)methyl)-4-nitroaniline (L4) and ethyl 5-methyl-1-(((4-nitrophenyl)amino)methyl)-1H-pyrazole-3-carboxylate (L6), was investigated against the degradation of CS in molar HCl medium. The innovative aspect of our work lies in the fact that the selected compounds are employed for the first time as corrosion inhibitors. In addition, pyrazole derivatives have a variety of substituents; therefore, the outcomes of this work would be useful for researchers to devise, improve, and synthesize further efficient corrosion inhibitors.

A variety of both electrochemical and chemical methods were employed for this purpose. The morphology and nature of the protecting film developed on CS were inspected by using scanning electron microscopy (SEM). The UV–visible analysis was executed to gain insight into L4 and L6 inhibitor adsorption on the metallic surface. Outputs of theoretical investigations were examined and discussed to support the experimental results.

2. Experimental Methods

2.1. Materials

The material used to conduct this study is a CS bar with the following elemental compositions (wt %): C (0.370), Si (0.230), Mn (0.680), Cr (0.077), S (0.016), Ti (0.011), Ni (0.059), Co (0.059), Cu (0.16), and Fe (remainder). Before each experiment, the surface of the metal was polished mechanically in freshwater using sand paper with progressively finer particle sizes ranging from 180 to 1200. Then, the samples were washed with distilled water, degreased with acetone, and air-dried.

2.2. Chemicals

All of the chemicals provided by Sigma-Aldrich were used without additional modification. Figure 1 depicts the architecture of L4 and L6.

Figure 1.

Figure 1

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N-((1H-Pyrazol-1-yl)methyl)-4-nitroaniline (L4) and ethyl 5-methyl-1-(((4-nitrophenyl)amino)methyl)-1H-pyrazole-3-carboxylate (L6).

Analytical commercial-grade 37% HCl was diluted with twice-distilled water to create molar HCl aqueous solutions. The L4 and L6 inhibitor doses used for the corrosion trials ranged from 10–3 to 10–6 M in a 1 M HCl solution devoid of the inhibitors. The electrochemical tests were carried out in a room with regular air and no turbulence. It is to be highlighted that L6 exhibits stability in hydrolysis due to its aromatic ester nature. Additionally, the ester function strongly interacts with the pyrazole through a conjugated system. This conjugation further reinforces the molecular structure, providing additional protection against hydrolysis. Thus, the combination of aromatic esterification and the conjugated system imparts an enhanced resistance to the hydrolysis reaction.

2.3. Electrochemical Apparatus and Measurements

A standard three-electrode cell is used, consisting of a CS working electrode (WE), a wire of Pt acting as the counter electrode, and a saturated calomel electrode (SCE) acting as the reference electrode. The electrochemical cell, which contained 100 mL of diluted hydrochloric acid, completely submerged the electrodes. With the exception of the impact temperature research, all studies were carried out at room temperature without stirring.

Data on electrochemistry [electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP)] was gathered using a potentiostat made by Volta Lab (PGZ 100) and VoltaMaster software. The generated curves were used to investigate how well CS corroded in molar HCl in the absence of L4 or L6 and in its presence.

The WE was kept submerged in the electrolyte for 30 min in order to establish a steady open-circuit potential (EOCP). The electrochemical tests were then conducted. The polarization graphs were plotted at a rate of 5 × 10–4 V/s, scanning from −0.8 to 0 V/SCE. The linear polarization resistance was measured by drawing the electrode potential ±0.01 V vs EOCP starting at a greater negative potential. Prior to all testing, the voltage had been stable for more than 30 min.

The electrochemical measurements were triple-realized for each inhibitor concentration to ensure reproducibility.

The inhibitory efficiency was calculated using the following formula

2.3. 1

The corrosion current densities in the presence and absence of chemical inhibitors are thus represented by the symbols icorr and icorr (inhib).

The EIS measurements were drawn at Eocp using the same workstation as before, utilizing an AC signal with an amplitude of 10 mV and a frequency domain of 105 to 10–2 Hz. ZView software, version 3.4, used the Nyquist model to analyze the EIS charts for a selected equivalent electrical circuit.

The inhibitory efficacy utilizing EIS was calculated using the formula shown in ref (31)

2.3. 2

The values of polarization resistance in the presence and absence of the inhibitor are indicated by the symbols RP(inhib) and RP.

Since we operated in the same settings, we were able to use the results that had already been published by our team for the effects of temperature and concentration using stationary and transient polarization techniques in the absence of an inhibitor.32

2.4. Gravimetric Measurements

The NACE/ASTM 169/G31–12a: Standard Guide for Laboratory Immersion Corrosion Testing of Metals33 was followed in handling the WL trials of CS in the test medium. The test medium contained 100 mL of 1 M HCl at 303 K together with L4 or L6 at various concentrations (10–6, 10–5, 10–4, and 10–3 M). The CS panels (2.0 × 2.0 × 0.1 cm) were mechanically polished using sand papers with grade nos 400, 600, 800, 1000, and 1200 to achieve a mirror finish. They were then successively cleaned with deionized water and anhydrous ethanol. The CS panels were immersed in the test liquid at 303 K for 6 h after their masses were precisely weighed and recorded. In order to remove the corrosive compounds that had accumulated on the CS surface, the CS panels were then disassembled and cleaned with deionized water. After being dried by cool air, the cleaned CS panels were weighed once more to determine their exact masses after corrosion. Equation 3 was used to calculate the inhibition efficiencies (IEWL) of L4 and L6 for CS in the test medium34

2.4. 3

where C0R and CR stand for the weight loss of the CS panel in the test media without and with the inhibitory drug present, respectively.

The operating temperature for the temperature effect component was gradually increased from 303 to 333 K in a solution containing 10–3 M L4 or L6 inhibitors.

2.5. UV–Vis Study

The efficiency of the inhibitor against corrosion was examined using the absorption UV–visible spectroscopy method. It is based on how well they can absorb light of particular wavelengths. Measurements of solution absorbance were made with and without the presence of the C-steel sample for 72 h in the acidic medium at 10–3 M L4 or L6, in order to investigate how organic molecules interact with the metallic substrate in the molar HCl medium. Between 200 and 600 nm wavelengths were scanned. It has been done using a JASCO V-700 UV–visible spectrophotometer.

2.6. SEM Explorations

Surface examinations were conducted using the SEM method. It was decided upon the ideal concentration of 10–3 M corrosive solutions, both with and without inhibiting molecules. The samples were individually subjected to the aforementioned aggressive media for 24 h before being gently removed, rinsed with purified water, dried, and assessed for surface morphological examination by SEM. The surface morphology was investigated using the JEOL-JSM-IT-100 model. SEM images taken at 10,000× magnification showed the metal in question. Since we carried out the experiments under the same conditions, we were able to use the results already published by our team for the SEM–EDX analysis.35

2.7. DFT and MD Details

We made an effort to look into the mechanism of action of the pyrazole analogues on the Fe surface using the DFT technique in the aqueous phase.36 This theoretical innovation was also made to link chemical reactivity indices to the estimated experimental inhibitory efficacy of neutral and protonated pyrazole analogues.37 The molecular structure of the molecules under examination was optimized using the Gauss. 09 software program to the final geometry at the DFT/B3LYP/6-31++G (d, p).38ELUMO, EHOMO, ΔE, global electronegativity ≪χ≫, global hardness ≪η≫, and electrons transferred ≪ΔN110≫ from occupied organic molecule orbitals to vacant orbitals of metal surface. With Φ = χ(Fe110) = 4.82 eV, the work function (eq 7) explains the theoretical value of χ in Fe(110).39

2.7. 4
2.7. 5
2.7. 6
2.7. 7

Investigations of the interactions of molecules investigated with Fe(110) systems are done using MD simulations. This simulation was carried out using the Materials Studio/8 suite’s Forcite module, just like our earlier work.40 The interactions of all the systems under study were carried out using a simulation box (27.30 × 27.30 × 37.13 Å3) by the Forcite module in Materials Studio/8. 491H2O, 9H3O+, 9Cl–, and pyrazole analogues fill this need. With a simulation time of 1000 ps and 1.0 fs and the COMPASS force field, the Andersen thermostat controlled the modeled system’s temperature to 303 K in the NVT ensemble.41

3. Results and Discussion

3.1. Gravimetric Measurements

3.1.1. Effect of Concentration

The influence of L4 and L6 concentrations on the protection performance was evaluated gravimetrically for the CS electrode in 1 M HCl at 303 K. Table 2 displays corrosion rate values (CR) as well as inhibition activities for each inhibitor concentration (from 10–6 to 10–3 M). One may notice that the corrosion rate subsides with the increase in L4 and L6 concentration. This results in a gradual increase of the inhibition efficiency (IEWL %) reaching maximal values of 90.1 and 91.8% at an optimized concentration of 10–3 M L4 and L6, respectively. The difference noticed in the percentage of the IEWL can be attributed to the molecular structure of the L6 inhibitor as it contains additional oxygen atoms. Such heteroatoms have the capacity to form a donor–acceptor surface complexes and thus influence the inhibition efficiency. From these results, it can be suggested that L4 and L6 molecules mitigate the corrosive attack of CS surface subjected to the aggressive electrolyte through their adsorption.

Table 2. WL Descriptors for the CS Electrode in 1 M HCl Medium in the Absence and Presence of Diverse L4 and L6 Concentrations at T = 303 ± 2 K.
medium C (M) CR (mg/cm2 h) IEWL (%) θ
blank 1 2.880 ± 0.060    
  10–3 0.236 ± 0.003 91.8 0.918
L6 10–4 0.276 ± 0.008 90.4 0.904
  10–5 0.469 ± 0.010 83.7 0.837
  10–6 0.662 ± 0.020 77.0 0.770
  10–3 0.285 ± 0.006 90.1 0.901
L4 10–4 0.628 ± 0.020 78.2 0.782
  10–5 0.720 ± 0.030 75.0 0.750
  10–6 0.884 ± 0.020 69.3 0.693
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3.1.2. Effect of Temperature

It is acknowledged that temperature significantly alters the conduct of metals in aggressive media as well as the interaction between the metallic substrate and the inhibitory products. In order to assess the impact of temperature on the weight loss trend and its respective inhibition performance, gravimetric trials were conducted for the CS sheet immersed for 6 h with and without 10–3 M L4 or L6 compounds in 1 M HCl solution at temperatures in the range of 303–333 K. The WL descriptors are gathered in Table 3. One may be clearly noticed that the increment of temperature induces an increase in CR values, which negatively affects IEWL %. Earlier studies stated that the coarseness of metallic surfaces increases at higher temperatures, which in turn undermines the inhibitor adsorption on CS.42,43 Correspondingly, an increment in temperature affects in a negative way the interaction of both L4 and L6 with the CS surface. Several authors reported that the observed behavior might be the consequence of the low chemical performance of inhibitory molecules as temperature rises, which leads in turn to their desorption from CS surface.44

Table 3. Evolution of W and the Corresponding IEWL % vs Various Temperatures for CS Immersed in 1 M HCl with and without 10–3 M L4 and L6.
medium T (K) CR (mg/cm2 h) IEWL (%)
blank 303 02.880 ± 0.060    
  313 04.803 ± 0.100    
  323 07.187 ± 0.200    
  333 10.050 ± 0.200    
  303 0.236 ± 0.003 91.8 0.918
L6 313 0.499 ± 0.010 89.6 0.896
  323 1.049 ± 0.030 85.4 0.854
  333 2.231 ± 0.040 77.8 0.778
  303 0.285 ± 0.006 90.1 0.901
L4 313 0.581 ± 0.010 87.9 0.879
  323 1.171 ± 0.030 83.7 0.837
  333 2.402 ± 0.070 76.1 0.761
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Further details on the adsorption ability of the investigated inhibitors were given through the evaluation of various activation parameters such as Ea (activation energy), ΔHa* (activation enthalpy), and ΔSa* (activation entropy). These descriptors were assessed by examining the temperature influence as expressed by the Arrhenius law (eq 8) and transition state formula (eq 9)45

3.1.2. 8
3.1.2. 9

where A denotes the Arrhenius constant, R designates the gas constant, T represents the absolute temperature (K), N denotes Avogadro’s number, and h stands for Planck’s constant.

The, respective, graphs are shown in Figures 2 and 3. For both L4 and L6 inhibitors, the Arrhenius plotting depicts straight lines whose slopes are equal to −Ea/R. As depicted by Table 4, the derived values of Ea of CS in 1 M HCl containing both L4 and L6 are higher, with respect to the inhibitor-free solution. Hence, it may be postulated that adding L4 or L6 to the medium generates a rise in the energy barrier associated with the corroding process. To put it another way, the corrosion will become less potent on the CS surface and more arduous owing to the obstruction of CS-active centers by L4 or L6 molecules.46 Besides, since the activation energy for the L6-containing medium exceeds that for the L4-containing solution, one may assume that the corrosion reaction of CS will possibly be more challenging with the introduction of L6 to the hostile environment.

Figure 2.

Figure 2

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Arrhenius graphs for (CS/1 M HCl) without and with 10–3 M L6 and L4.

Figure 3.

Figure 3

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Transition state graphs for (CS/1 M HCl) without and with 10–3 M L6 and L4.

Table 4. Thermodynamic Parameters Were Generated for 10–3 M L6 and 10–3 M L4 in 1 M HCl.
C (M) R2 Ea (kJ/mol) ΔHa* (kJ/mol) ΔSa* (J/mol K)
blank 0.99775 34.90 32.27 –129.50
10–3 M L6 0.99955 62.72 60.09 –58.94
10–3 M L4 0.99954 59.48 56.84 –68.05
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The positive values of ΔHa* emphasize that the generation of the activated complex is accomplished through an endothermic process. Moreover, in the presence of L4 or L6, ΔHa* values are greater if compared with the ones estimated in the inhibitor-free solution, demonstrating that the corrosion of CS is mitigated through the addition of the investigated molecules. The negative values of ΔSa* indicate the presence of a direct order of the reaction owing to the transformation from reactants to activate complexes.45,46

3.1.3. Adsorption Isotherm

The interaction between organic molecules and metals is generally accomplished by their adsorption on the surface. Such adsorption may be viewed as a quasi-replacement process of H2O molecules by the organic products in the aqueous phase (Org(aq)), as expressed by the subsequent reaction

3.1.3. 10

where x stands for the number of water molecules expelled by the adsorbed inhibitors on the metallic surface.

In general, studies conducted on the adsorption behavior of organic molecules enable us to unravel the mode of action of the inhibitors. The identification of the nature of adsorption usually relies on plotting various adsorption isotherms (Langmuir, Temkin, and Frumkin) in order to ascertain the most appropriate one. As depicted in Figure 4, the Langmuir isotherm exhibits an almost unitary correlation for both L4 and L6 inhibitors. The formula expressing the Langmuir isotherm is denoted by eq 11(47)

3.1.3. 11

where Kads denotes the adsorption/desorption equilibrium constant and C stands for L4 or L6 concentration in the medium. The high values of Kads emphasize the potent adsorption of both of the tested inhibitors. Accordingly, on the basis of weight-loss measurements, one may deduce that L4 and L6 are monolayers adsorbed on CS.

Figure 4.

Figure 4

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Isotherm models for adsorption of L4 and L6 on the CS surface in 1 M HCl.

To gain further understanding of the L4 and L6 adsorption mechanisms on CS, the values of the standard adsorption free energy (ΔGads) were assessed through eq 12(48)

3.1.3. 12

where R designates the gas constant and T stands for the absolute temperature (K). The 55.5 value denotes the H2O concentration in the solution (mol/L).

As disclosed in Table 5, the negative values of ΔGads point out that the layers adsorbed on the CS surface are stable.48 For both L4 and L6, the derived values of ΔGads are close to −40 kJ/mol [ΔGads (L4) = −40.24 kJ/mol and ΔGads (L6) = −44.94 kJ/mol]. Therefore, the adsorption of both molecules occurs through electron transfer between L4 or L6 and the CS surface to create a covalent bond (chemical adsorption).49

Table 5. Thermodynamic Descriptors Resulting from the Langmuir Isotherm Plot for the {L4 and L6/CS/1 M HCl} System.
medium slope R2 Kads (L/mol) ΔGads* (kJ/mol)
L4 1.10 0.9999 156150.21 –40.24
L6 1.08 0.9999 1009018.6 –44.94
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3.2. Electrochemical Measurements

3.2.1. PDP Curves

Corrosion kinetics of CS in 1 M HCl solution with and without varying concentrations of L4 and L6 were evaluated through PDP plots drawn under stationary conditions after 30 min of immersion at corrosion potential (Ecorr) and T = 303 (±2) K. The acquired data are depicted in Figure 5, while the corresponding electrochemical kinetic descriptors, explicitly Ecorr, corrosion current density (icorr), the anodic Tafel slope (βa), and the cathodic Tafel slope (βc), derived from the extrapolation method, are illustrated in Table 6. From Figure 5, one may observe that the progressive introduction of both L4 and L6 to the medium gives rise to a prominent subsidence of the anodic and cathodic partial current densities. This reflects the attenuation of the CS anodic dissolution and the cathodic generation of H2. The cathodic branches exhibit parallel lines for both L4 and L6 molecules, emphasizing that the L4 and L6 presence in the solution does not adjust the process of hydrogen production. It can be then suggested that the reduction of hydrogen ions on CS is predominantly achieved via the electron-transfer mechanism (pure activation process).50 In the presence of each inhibitor (L4 and L6), the anodic Tafel segments above −300 mV exhibit approximately identical conductivities for each of the studied concentrations. However, the anodic current densities are still lower for each inhibitory concentration compared to the one drawn in a blank 1 M HCl solution. This is primarily driven by the significant dissolution of the CS-inducing desorption of the inhibitors. In this case, it might be suggested that the rate of desorption of L4 and L6 molecules is higher compared to their adsorption.

Figure 5.

Figure 5

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PDP plots for CS in 1 M HCl in the absence and presence of diverse concentrations of L4 and L6.

Table 6. PDP Descriptors for CS in 1 M HCl (Blank) and with Various L4 and L6 Concentrations at T = 303 K (±2).
system C (M) –Ecorr (mV/SCE) icorr (μA cm–2) βa (mV dec–1) –βc (mV dec–1) Rp (Ω cm2) IEPOL (%)
blank 1 456.3 ± 6 1104.1 ± 4.9 112.8 155.4 25.7  
L4 10–3 448.5 ± 3 158.5 ± 1.6 73.3 83.1 106.7 85.6
  10–4 453.7 ± 2 290.8 ± 2.1 77.3 114.8 69.0 73.6
  10–5 426.5 ± 4 324.7 ± 2.5 70.4 150.3 64.1 70.6
  10–6 452.4 ± 4 396.3 ± 3.0 79.8 98.9 48.4 64.1
  10–3 449.9 ± 3 143.2 ± 1.3 63.2 82.5 108.5 87.0
L6 10–4 428.3 ± 5 155.6 ± 1.4 70 106.7 118.0 85.9
  10–5 444.9 ± 5 234.6 ± 2.0 68.2 87.9 71.1 78.7
  10–6 448.7 ± 2 302.5 ± 2.9 85.4 114 70.1 72.6
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A review of Table 6 reveals a progressive subsidence of icorr values as L4 and L6 concentrations increase in the medium. Furthermore, a subtle displacement of Ecorr is depicted toward more positive values with the introduction of L4 and L6 in the solution. Riggs reported that organic products may be classified as anodic or cathodic inhibitors in case Ecorr shifts at least 85 mV regarding the potential found molar HCl.51 As indicated in Table 6, the differences of Ecorr values (ΔEcorr = 7.8 mV for the system {CS/1 M HCl/10–3 M L4} and 6.4 mV for {CS/1 M HCl/10–3 M L6}) are lower than 85 mV, confirming the mixed character of both L4 and L6.52,53 In addition, the reported variation in Tafel slopes (βa and βc) values upon the presence of L4 or L6 denotes that the tested inhibitors tend to minimize the corrosion-sensitive specific surface area and alter the corrosion pathway, presumably through the blockage of the reaction centers on CS surface by L4 and L6 molecules.54

Furthermore, the value of the Stern–Geary coefficient B [B = βa × βc/2.303 (βa + βc)] does not change remarkably. Note that this coefficient associates with the corrosion rate and the polarization resistance (Rp = B/icorr). Accordingly, it can be noticed that Rp values determined by the calculation of the Stern–Geary coefficient are very close to those assessed by EIS measurements (Section 3.2.2), thus reflecting the consistency of the results obtained by both electrochemical methods.

The inhibition activities IEPOL (%), assessed from PDP plots by eq 1, increase significantly with increasing L4 and L6 concentration, reaching maximal values of IEPOL (L4) = 85.6% and IEPOL (L6) = 87% for a concentration of 10–3 M. The slight increase of IE % achieved for L6 compared to that for L4 can be associated with the increment of electron transfer owing to the functionalization by the carboxylate group in the L6 compound. Further insights will be given by theoretical studies. The aforementioned outcomes further substantiate that the adsorption of L4 or L6 on the Fe-CS/1 M HCl complex can proficiently obstruct the active centers on CS and subsequently preclude corrosion. The PDP findings are consistent with the favorable outcomes derived from gravimetric trials.

3.2.2. EIS Data

The Nyquist spectra for CS at Ecorr in 1 M HCl at 303 K with and without L4 and L6 inhibitors are evinced in Figure 6a, whereas the related EIS descriptors are gathered in Table 7. For all concentrations, Nyquist curves show the presence of a unique flattened semicircle, which reveals that CS corrosion is a process controlled by a single charge transfer55 in the absence or presence of the inhibitors in the aggressive solution. A wider radius of the loops with the increment of L4 or L6 inhibitory concentration points out an increase of the electron-transfer mechanism,56,57 which reflects in turn the development of a dense anticorrosion film (mono- or multilayer) on the CS electrode surface, restraining hence the deterioration of the electrode effectively.58

Figure 6.

Figure 6

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(a) Nyquist diagrams of CS in inhibitor-free 1 M HCl solution and with the addition of various concentrations of L4 and L6 at 303 ± 2 K and (b) the corresponding electrical circuit employed for fitting the EIS spectra.

Table 7. Electrochemical Impedance Descriptors of CS in 1 M HCl Solution Comprising Varied Concentrations of L4 and L6 at 303 ± 2 K.
system C (M) Rs (Ω cm2) Rp (Ω cm2) 106 × Q (μF sn–1 cm–2) n Cdl (μF cm–2) IEEIS (%)
blank 1 0.83 ± 0.06 21.6 ± 0.6 293.9 ± 5.8 0.845 ± 0.001 116.2  
L6 10–3 1.32 ± 0.03 215.9 ± 4.3 072.2 ± 6 0.865 ± 0.002 033.7 89.9
  10–4 1.25 ± 0.04 177.3 ± 5.3 131.6 ± 6 0.858 ± 0.001 070.6 87.7
  10–5 1.12 ± 0.01 116.8 ± 2.3 188.7 ± 6 0.843 ± 0.005 092.7 81.4
  10–6 0.95 ± 0.02 086.0 ± 0.9 220.0 ± 6 0.834 ± 0.003 099.9 74.7
L4 10–3 1.14 ± 0.03 185.5 ± 3.7 075.2 ± 6 0.890 ± 0.002 044.3 88.2
  10–4 1.09 ± 0.02 093.7 ± 1.0 134.6 ± 6 0.873 ± 0.002 071.2 76.8
  10–5 0.98 ± 0.01 078.7 ± 0.8 191.7 ± 6 0.858 ± 0.005 095.7 72.4
  10–6 0.91 ± 0.01 065.2 ± 0.7 223.1 ± 6 0.843 ± 0.006 101.5 66.7
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Figure 6b depicts the equivalent circuit used to adjust EIS data recorded for the {1 M HCl/L4 or L6/CS} system, where Rs refers to the aqueous solution resistance, Rp designates the resistance of polarization, paralleling a constant phase element (CPE) referring to the capacitance of the electrical double layer (Cdl) at the interface CS/1 M HCl. The impedance function of a CPE (taking into account a nonideal capacitive response on a heterogeneous surface) is described by eq 13

3.2.2. 13

where Q, i, ω, and n denote, respectively, the CPE constant, imaginary number (i2 = −1), angular frequency (ω = 2πf), and phase shift. n designates the divergence with respect to the ideal conduct, comprised between 0 and 1. As claimed by MacDonald,59 the value of n gives an indication of the substrate heterogeneity. For instance, n is homologous to a resistance when n = 0, a capacitor for n = 1, and assigned to a diffusion process when n = 0.5.

From CPE parameters, the double capacitance values Cdl were assessed according to eq 14

3.2.2. 14

Table 7 discloses the derived EIS indices: Rp, n, Cdl, and the inhibition efficiencies IEEIS (%). The subtle increase of n values with the progressive addition of both L6 and L4 molecules, if compared to the inhibitor-free electrolyte, could be attributable to a lower level of surface coarseness of the electrode.60 This can be a consequence of the development of a barrier layer on the CS substrate. A review of Table 7 unveils that Cdl values consistently subside with the gradual addition of L4 or L6 molecules, which might be attributed to a lowering of local dielectric constant along with CS exposure and/or an apparent increment in the electrical double-layer thickness.61 One might be suggested that the observed trend might be allocated to the coverage of the metal surface by L4 or L6 molecules. This coverage may be induced by the adsorption of the organic pyrazole molecules on CS through the displacement of H2O from the surface, thus ensuring a significant decrease in CS dissolution.62

In this work, Rp values were estimated instead of those of Rct. Rp gives more accurate insights into the inhibition process as it takes into account both charge-transfer resistance Rct and film resistance Rf (Rp = Rf + Rct). According to Table 7, Rp values exhibit a remarkable increment as the L4 or L6 concentration increases. Thus, one can assume that the progressive introduction of L4 or L6 into 1 M HCl solution enhances Rct values. Indeed, higher Rct values reflect the delay of the charge-transfer process due to the development of protective films of L4 or L6 by adsorption on the CS substrate.

Based on the above findings, the inhibition activity IEEIS (%) values show an enhancement with increasing inhibitory concentration {IEEIS (L4) = 89.9% and IEEIS (L6) = 88.2% for a concentration of 10–3 M}. A small increase in IEEIS % evaluated for L6 is noticed when compared to the one assessed for the L4 inhibitor. Note that a similar trend was discerned through PDP measurements and weight-loss data. Therefore, it is possible to conclude that both tested inhibitors impart efficient protection against CS corrosion in the HCl environment.

3.3. SEM/EDX Analysis

CS specimens were inspected by SEM before immersion (Figure 7a) and maintained immersed for 24 h at 303 K in 1 M HCl without (Figure 7b) and with the introduction of 10–3 M L4 (Figure 7c) and 10–3 M L6 (Figure 7d). Except for the appearance of some polishing scratches, no discernible defects are visible on the SEM image of sanded CS. After immersion in 1 M HCl for 24 h, the CS surface undergoes severe corrosion, as witnessed by the formation of a rust layer with a flowery structure with fine plates. The related EDX spectrum evinces the occurrence of high peaks of Cl and O atoms, which may possibly correspond to the presence of FeCl2 and Fe3O4 as corrosion products.63 This emphasizes that the CS substrate undergoes general corrosion in 1 M HCl. Following the addition of 10–3 M L4 and L6 inhibitors, the surface is capped with a platelet-shaped compound, indicating the development of an adsorbed protective layer. In the presence of inhibitors, the EDX spectra depict peaks of the O atom (adsorption of L4 and L6), while the Cl peak contribution (mostly coming from the solution) is significantly diminished. On the basis of these observations, it can be suggested that both L4 and L6 have great protective abilities as they impede the electrolyte accessibility to CS specimens through the formation of a stable and adherent deposit.

Figure 7.

Figure 7

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SEM micrographs and their related EDX analysis of CS specimens (a) before immersion, (b) after 24 h immersion in 1 M HCl, (c) 1 M HCl + 10–3 M L4, and (d) 1 M HCl + 10–3 M L6 at 303 K.

3.4. UV–Visible Investigation

Further insights into the interaction of CS and the inhibitory molecules were gained through UV–visible spectroscopic measurements. The latter was conducted in 1 M HCl solution enclosing 10–3 M of either L4 or L6 before and after immersion in the CS for 72 h at 303 K (Figure 8). The absorption curves of the studied inhibitory compounds before CS immersion (black curve) depict two visible bands around 211 and 258 nm for L4 and a single band at 240 nm for L6. After 72 h of CS immersion, one may notice a shift to a higher value (247 nm) and the disappearance of the second band (located at 258 nm before CS immersion) in the case of L4 inhibitor, whereas a very slight displacement of the band to 238 nm/and an increase in its maximum (λmax) are shown for L6 inhibitor. This emphasizes the interaction (production of a complex) between the investigated pyrazole compounds and Fe2+ ions in the aggressive solution.64,65

Figure 8.

Figure 8

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UV–visible spectra of 10–3 M L4 and L6 compounds added to 1 M HCl with and without CS.

3.5. DFT Reactivity

To explore the corrosion inhibition efficiency and offer a potential corrosion inhibition mechanism, a set of the electronic characteristics, global reactivity descriptors, and local reactivity indices of the neutral (L4 and L6) and protonated (L4H+ and L6H+) forms were determined.66

To better understand the reactivity of L4 and L6 in the protonated state, we have presented the results of Marvin sketch software.67,68

Figure 9 presents the variation of protonation % vs pH of L4 and L6. As analyzed in this figure, the nitrogen atom hybridized sp2 and included in the pyrazole ring protons with a high percentage for two molecules, which makes the molecules positively charged as cations.

Figure 9.

Figure 9

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Protonation site defined by its percentage vs pH of L4 and L6.

The absence of imaginary frequencies with the use of the same degree of theory is required to ensure the compliance of the structure. The molecular forms’ structures, highest occupied molecular orbitals (HOMOs), and lowest unoccupied molecular orbitals (LUMOs), were traced using the GaussView/5 program.

Figure 10 depicts the energy-minimized structures, frontier molecular orbitals (FMOs), and molecular electrostatic potentials (MEPs) of two neutral and protonated forms, L4 and L6 and L4H+ and L6H+. The electronic representations of HOMO and LUMO show that the neutral forms L4 and L6 are occupied in the structural part of methyl-4-nitroaniline. In contrast, the FMOs for L4H+ and L6H+ have an almost total occupation except for a few atoms, implying that the reactivity is very high with other species and especially with the metal surface.

Figure 10.

Figure 10

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Energy-minimized structures, FMOs, and MEPs of the two neutral and protonated forms for L4 and L6 and L4H+ and L6H+.

EHOMO and ELUMO values serve to analyze the donor/acceptor ratio in the molecule/metal relationship.69 In this investigation of quantum computation, the behavior of the reactivity of the two considered neutral (L4 and L6) and charged (L4H+ and L6H+) forms is followed by the two descriptors. Table 8 shows the different values of the reactivity descriptors of these molecules.

Table 8. Descriptors for Reactivity of the L4, L6, L4H+, and L6H+.

molecules EHOMO (eV) ELUMO (eV) ΔE (eV) X (eV) Η (eV) ΔN110
L4 –6.223 –2.016 4.207 4.119 2.103 0.167
L6 –6.372 –2.095 4.277 4.233 2.138 0.137
L4H+ –9.763 –5.703 4.060 7.733 2.030 –0.717
L6H+ –9.779 –5.953 3.826 7.866 1.913 –0.796
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The data in Table 8 reveal that the L4 molecule is more nucleophilic (EHOMO = −6.223 eV) and the L6H+ molecule is more electrophilic (ELUMO = −5.953 eV).

As the least significant value of this descriptor, ΔE (ELUMO – EHOMO) enables us to quantify the reactivity of a molecule, and this particular reactivity is the greatest.

The description comparison between all values of this descriptor shows that the charged form L6H+ is more reactive, with a low value of 3.826 eV. Therefore, ΔE (L6) > ΔE (L4) implies that L4 is more reactive than L6, which does not support the experimental findings. However, the order of ΔE for the protonated forms confirms the experimental results.

The electronegativity (X) presents an attractive power of the electrons of a chemical species; the high value of X indicates a high interaction between the inhibitor and iron surface.70 The value of 7.866 eV of L6 posted in Table 8 shows this more reactive form.

The fraction of electrons transferred (ΔN110) values provides useful information on the tendency of the electrons to flow from the inhibitor molecule to the CS surface (ΔN > 0) or from the CS to the inhibitor (ΔN < 0).71

In this computation, the L4 molecule (ΔN110 = 0.176) is more likely to share its electron to form coordination bonds with Fe(110). In contrast, L4H+ and L6H+ do not show this attitude, which may be influenced by protonation of the nitrogen atom.

The chemical quantum descriptors of the neutral forms do not show a correlation with the experimental results, whereas these descriptors for the protonated forms are in good agreement with the inhibitory efficacy values of L4 and L6 ΔE(L4H+) > ΔE(L6H+), X(L6H+) > X(L4H+), η(L4H+) > η(L6H+), etc. The reactivity of L6H+ is higher than that of L4H+, possibly due to the ester group of the protonated L6 form. This shows that the protonated form is more likely to be dominant in the acidic environment.

The Fukui function analysis was utilized to estimate the nucleophilic (fk+) and electrophilic (fk–) assaults caused by the two forms’ most active centers in order to better understand how the L4, L6, L4H+, and L6H+ adsorb onto the CS (Figures 11 and 12).72

Figure 11.

Figure 11

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3D Isosurfaces of the Fukui functions (isosurface density) for the neutral (L4) and protonated (L4H+) inhibitors.

Figure 12.

Figure 12

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3D Isosurfaces of the Fukui functions (isosurface density) for the neutral (L6) and protonated (L6H+) inhibitors.

This approach was carried out using the Dmol3 method with the GGA correlation exchange functional and the DNP base, as reported in the work published by Benhiba et al.72 The calculation data for all forms are listed in Tables S1 and S2, as well as 3D isosurfaces of the Fukui functions (isosurface density) for the neutral (L4 and L6) and protonated (L4H+ and L6H+) inhibitors. It has been ascertained that the neutral form of L4 contains several electron donor and attractor sites that efficiently enhance the reactivity of this molecule. This shows that L4 is more effective against the corrosion of the studied steel and results in a greater number of available coordination bonds enhancing the adsorption of the L4 onto the Fe support. For charged forms, the local reactivity arises and increases by the attractor effect due to the protonation, i.e., the attractor sites are very substantial.

3.6. MD Simulation Study

The adsorption pattern of a molecule onto a metal can be predicted and understood with the aid of computational approaches such as MD simulations. The MD is an effective instrument for examining the behavior of molecular systems as well.73 The current research aims to assess and comprehend the behavior of the L4, L6, L4H+, and L6H+ molecules on the iron atomic support (Fe(110)). Furthermore, Figures 13 and 14 reproduce the best adsorption configuration of the two forms onto the Fe(110) surface. The optimal arrangement of the L4, L4H+, and L6H+ molecules occupies a sizable portion of the Fe(110), as shown in these two pictures. This demonstrates that the simulated molecule contains more reactive sites that are concentrated in the region of the molecule that is filled with FMO (HOMO _ LUMO) iron atoms and has a high potential to adsorb onto the metal due to the presence of coordination bond inhibitors, such as Fe(110). However, the L6 molecule carries a few motifs and atoms that do not interact with the first layer of Fe(110). This suggests that this inhibitor is less reactive than the other species.

Figure 13.

Figure 13

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Side (left) and top (right) views of the L4/Fe(110) and L6/Fe(110) systems.

Figure 14.

Figure 14

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Side (left) and top (right) views of the L4H+/Fe(110) and L6H+/Fe(110) systems.

The Einteraction (Einter) value is defined by the equation hereunder74

3.6. 15

The low value of Einter confirms interactions between all forms and the iron atoms.75Table 9 lists the values of this descriptor for all systems. Based on a comparison analysis, it seems that the largest negative value of L4/Fe(110) (−778.604 kJ mol–1) indicates a more significant interaction. As a result, the data from this simulation support the findings of the experimental investigation and DFT.

Table 9. Einter for the Systems L4/Fe(110), L6/Fe(110), L4H+ /Fe(110), and L6H+/Fe(110) (in kJ mol–1).

systems Einteraction
L4/Fe(110) –778.604
L6/Fe(110) –755.431
L4H+/Fe(110) –772.529
L6H+/Fe(110) –774.648
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Consequently, as shown in the DFT section, the protonated form of L6H+ exhibits high reactivity (interaction) with the Fe(110) surface, confirming the experimental results.

This method’s primary objective was to enter the radial distribution function (“RDF”) in order to evaluate the acceptability of the interatomic distances for L4-Fe, L6-Fe, L4H+-Fe, and L6H+-Fe for adsorption.76 The published literature confirmed that the likelihood of chemical adsorption was higher when the bond length was less than 3.5 Å. By contrast, physical adsorption is more probable.76 In Figure 15, the spectral data from this method is illustrated. The first peaks reveal that the bond lengths for L4-Fe, L6-Fe, L4H+-Fe, and L6H+-Fe are less than 3.5 Å.

Figure 15.

Figure 15

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RDFs for L4-Fe, L6-Fe, L4H+-Fe, and L6H+-Fe.

4. Conclusions

Two pyrazole derivatives, L4 and L6, were employed to alleviate the corrosion of CS in HCl medium. According to the electrochemical tests, the inhibitors act in a mixed-type process of CS corrosion. L6 exhibits a much better inhibitive effect, with a maximum IE % of 91.8% at 10–3 M and high corrosion inhibition stability. L4 and L6 are observed to adsorb on the CS surface, obeying the Langmuir adsorption isotherm. The L4 and L6 generate a protective organic/inorganic thin film to inhibit surface exposure to the corrosive electrolyte. The DFT parameters and experimental findings showed good agreement. The investigated inhibitors are almost parallel to the Fe(110) surface according to MD simulation.

Acknowledgments

The authors extend their appreciation to the Researchers Supporting Project number (RSP2024R381), King Saud University, Riyadh, Saudi Arabia.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c08282.

  • Fukui indices (fk+ and fk–) of the atoms for L4 and L4H+ molecules and Fukui indices (fk+ and fk–) of the atoms for L6 and L6H+ molecules (PDF)

Author Present Address

△ Engineering Laboratory of Organometallic, Molecular Materials and Environment (LIMOME) University Sidi Mohamed Ben Abdellah, Faculty of Sciences, Chemistry Department, P.O. Box 1796 (Atlas), 30000 Fez, Morocco

The authors declare no competing financial interest.

Supplementary Material

ao3c08282_si_001.pdf (81.8KB, pdf)

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