Experimental and theoretical evaluation of the anticorrosive proprieties of new 1,2,3-triazolyl-acridine derivatives

2.1 Chemical synthesis of the corrosion inhibitors

2.1.2

2.1.2 Methods

At first, in a round bottom flask under nitrogen atmosphere, containing a solution of 6,9-dichloro-2-methoxyacridine (2.0 mmol) in DMF (6.0 mL), sodium azide (6.0 mmol) was added. The reaction was magnetically stirred over 42 h at 65 °C protected from the light (Scheme 1). Thin layer chromatography (TLC) indicated the complete consumption of the reactant. The reaction mixture was poured directly into ice and vacuum filtered. 9-azido-6-chloro-2-methoxyacridine was obtained in quantitative yield as a green solid and used in the next reaction step without further purification. FTIR ν_max (cm⁻¹): 2108, 1633, 1556, 1518, 1475, 1248, 1215, 1029, 914, 819, 658.

Close

Scheme 1

Synthetic pathway for the synthesis of the new 1,2,3-triazoyl-acridine derivatives. (i) NaN₃, DMF, 65 °C, 42 h. (ii) respective alkyne, CuSO₄·5H₂O, AcOH (cat.), sodium ascorbate, t-BuOH/H₂O (1:1), 15–35 min, ultrasound irradiation.

Export to PPT

Then, to a round bottom flask, a mixture of 9-azido-6-chloro-2-methoxyacridine (1.00 mmol), respective alkyne (1.20 mmol), CuSO₄·5H₂O (0.072 mmol), an excess of sodium ascorbate (0.16 mmol), acetic acid (0.33 mmol, 30 mol% based on the azido compound) and a mixture of an equal amount of tert-butanol and water (1.00 mL) was subjected to ultrasound irradiation for 15–35 min protected from the light (Scheme 1). The reactions were monitored by TLC. After, a liquid–liquid extraction was performed with ethyl acetate and water. The organic layer was dried with anhydrous sodium sulphate, filtered off and the solvent removed by reduced pressure evaporation. The resultant mixture was purified by column chromatography using a gradient of hexanes/ethyl acetate or chloroform/ethyl acetate, depending on the polarity of each compound, obtaining compounds TTA, ATM and ATA.

2.1.2.1

2.1.2.1 6-chloro-2-methoxy-9-(4-(((tetrahydrofuran-3-il)oxy)methyl)-1H-1,2,3-triazol-1-il)acridine (TTA)

15 min. Yellow solid, 48 % m.p.: 170.2–171.4 °C. FTIR ν_max (cm⁻¹): 3087, 2940, 2869, 1631, 1474, 1447, 1423, 1222, 1202, 1120, 1063, 1031, 970, 935, 905, 896, 872, 824, 813, 791, 775, 571. ¹H NMR (300.00 MHz, DMSO‑d₆) δ (ppm): 1.46–1.59 (m, 4H), 1.60–1.90 (m, 2H), 3.53 (dd, J = 5.7 and 11.0 Hz, 1H), 3.80 (s, 3H), 4.81–4.98 (m, 4H), 6.52 (d, J = 2.3 Hz, 1H), 7.37 (d, J = 9.4 Hz, 1H), 7.69 (ddd, J = 2.3, 3.9 and 9.4, Hz, 2H), 8.23 (d, J = 9.4 Hz, 1H), 8.34 (d, J = 2.3 Hz, 1H), 8.81 (s, 1H). ¹³C NMR (APT, 75.0 MHz, DMSO‑d₆) δ (ppm): 18.9, 20.2, 24.9, 25.0, 30.1, 54.9, 55.5, 55.6, 59.5, 61.4, 62.5, 93.3, 97.3, 97.5, 97.7, 120.5, 122.9, 124.0, 126.7, 127.4, 127.8, 129.0, 129.1, 131.2, 134.6, 135.6, 144.9, 146.5, 146.8, 148.7, 158.7. HRMS (ESI) calc for C₂₂H₂₁ClN₄NaO₃ [M + Na]⁺: 447.119439 Found. 447.118728. Δ −1.6 ppm.

2.1.2.2

2.1.2.2 (1-(6-chloro-2-methoxyacridin-9-il)-1H-1,2,3-triazol-4-il)methanol (ATM)

35 min. Green solid, 48 %. m.p: 207.1–209.3 °C. FTIR ν_max (cm⁻¹): 3234, 3162, 1628, 1566, 1481, 1440, 1424, 1305, 1289, 1234, 1223, 1204, 1070, 1046, 1038, 1014, 938, 836, 818, 712. ¹H NMR (500.00 MHz, DMSO‑d₆) δ (ppm): 3.80 (s, 3H), 4.80 (d, J = 5.6 Hz, 2H), 5.35 (d, J = 5.6 Hz, 1H), 6.55 (d, J = 2.3 Hz, 1H), 7.34 (d, J = 9.3 Hz, 1H), 7.67–7.70 (m, 2H), 8.22 ppm (d, J = 9.3 Hz, 1H), 8.33 (d, J = 2.3 Hz, 1H), 8.67 (s, 1H). ¹³C NMR (APT, 125.00 MHz, DMSO‑d₆) δ (ppm): 55.52, 56.12, 98.20, 121.07, 123.48, 124.53, 127.17, 127.28, 127.98, 129.55, 131.67, 135.06, 136.30, 147.04, 147.29, 149.30, 159.2. HRMS (ESI) calc for C₁₇H₁₄ClN₄O₂ [M + H]⁺: 341.079980 Found. 341.080311. Δ + 1.0 ppm.

2.1.2.3

2.1.2.3 (1-(6-chloro-2-methoxyacridin-9-il)-1H-1,2,3-triazol-4-il)methyl acetate (ATA)

20 min. Green solid, 61 %. m.p.: 155.4–155.0 °C. FTIR ν_max (cm⁻¹): 3438, 3088, 3068, 2961, 1736, 1633, 1476, 1463, 1447, 1437, 1424, 1385, 1366, 1276, 1256. ¹H NMR (500.0 MHz, DMSO‑d₆) δ (ppm): 2.12 (s, 3H), 3.80 (s, 3H), 5.40 (s, 2H), 6.48 (d, J = 2.2 Hz, 1H), 7.36 (d, J = 9.4 Hz, 1H), 7.68 (td, J = 2.2 and 9.4 Hz, 2H), 8.22 (d, J = 9.4 Hz, 1H), 8.33 (d, J = 2.2 Hz, 1H), 8.86 (s, 1H). ¹³C NMR (APT, 125.00 MHz, DMSO‑d₆) δ (ppm): 20.55, 55.52, 56.95, 97.43, 120.35, 122.81, 123.91, 126.67, 127.44, 128.53, 129.13, 131.14, 134.54, 135.27, 142.89, 146.46, 146.70, 158.72, 170.06. HRMS (ESI) calc for C₁₉H₁₆ClN₄O₃ [M + H]⁺: 383.09054 Found. 383.091637. Δ −2.9 ppm.

2.2 Materials

Hydrochloric acid solution (1 mol/L) was produced via dilution of HCl 37 % (analytical grade) in ultrapure H₂O. 1020 Mild Steel with the following composition (Table 1) was used in this work:

The metal specimens used in the gravimetric experiments had an exposed around 3.5 cm² and the working electrode area was limited in 1 cm².

2.3 Gravimetric experiments

The Weight Loss Study (WLS) was conducted in accordance to the norms of the American Society for Testing Material (ASTM G1-03 (2017)) (Faydy et al., 2019). The immersion time of 5 h was determined when the corrosion rate of the black solution started to become stable (different immersion times were applied, from 0.5 h to 10 h). WLS was performed at three temperatures (298, 318 and 338 K). The data was used to determine the corrosion rate and the anticorrosive efficiencies of the tested compounds (de Sampaio et al., 2020).

2.4 Electrochemistry

Electrochemical measurements were performed in a three-electrode cell. Mild steel was the working electrode, a long platinum wire was the counter electrode, and for the reference electrode Ag|AgCl was used. The instrument was an AutoLab PGSTAT 128 N (Metrohm®). Open Circuit Potential was measured for 60 min before each sequence of experiments (Fernandes et al., 2022a, 2022b). They were:

• Electrochemical Frequency Modulation (EFM, with a base frequency of 0.1 Hz, applied multipliers of 2 and 5, and 10 mV amplitude) (Machado Fernandes et al., 2023);

• Electrochemical Impedance Spectroscopy (EIS, with a frequency range of 100000–0.1 Hz and 10 mV amplitude) (Fernandes et al., 2019a, 2019b, 2019c);

• Linear Polarization Resistance (LPR, potential range of ± 10 mV and 0.075 mV s⁻¹) (Machado Fernandes et al., 2020a, 2020b);

• Potentiodynamic Polarization (PP, potential range of ± 300 mV and 1 mV s⁻¹) (Fernandes et al., 2019a, 2019b, 2019c).

2.5 Atomic force Microscopy (AFM)

The topography of the mild steel surface was analyzed after the gravimetric experiment on corroded and inhibited scenarios, and also after the grounding process, with the help of a Nanosurf Easy Scan 2 Atomic Microscope.

2.6 First-principles DFT simulation

The first-principles DFT method is a wonderful theoretical tool in predicting structural and electronic properties. Herein, the inhibitor-iron interactions were fully optimized by spin polarized first-principles DFT, including dispersion interaction (DFT-D3) and ultra-soft pseudopotentials using the CASTEP code (Clark et al., 2005). The generalized gradient approximation (GGA) within its PBE formulation was used for the electron exchange and correlation (Perdew et al., 1996). The adsorption systems were fully optimized using a self-consistent field (SCF) tolerance of 10^-6 eV/atom, and plane-wave basis set with an energy cut-off of 30 Ry. All other convergence tolerance values were set according to fine quality. Meanwhile, the bulk lattice parameters optimization was carried out using (8x8x8) k-point grid, which was reduced to (2x2x1) k-point grid for adsorption models.

The optimization of bulk lattice parameters reproduced a value of 2.854 Å, which is close to the experimental one of 2.862 Å, confirming the accuracy of selected parameters. Inhibitor-iron adsorption models were generated by constructing Fe(1 1 0) iron surface consisting of a (5x5) supercell and a vacuum spacing of 20 Å along the z-direction separating periodic image in each direction. Then, inhibitor molecules were placed on the top side of the slab and all atoms were allowed to relax except the two bottom-most atomic layers. DFT optimization of standalone molecules was carried out by constructing a cubic box of 30 Å in size. The interaction energy was used as a main parameter to estimate the adsorption strength of molecules, which is calculated according to the following equation:

2.7 Molecular dynamics simulation

Molecular dynamics simulation can be used to get information about how strong the interaction of inhibitor molecules with the iron surface in presence of a simulated solution is (Obot et al., 2018). To this end, an iron surface was constructed from (1 1 0)-cleaved iron unit cell, extended to (7x7) supercell and then placed in contact with a solvent layer consisting of one inhibitor molecule, 491 water molecules, 9 H₃O⁺ and 9Cl^-. After initial optimization of constructed simulation boxes via Smart Minimizer protocol, MD equilibrium process was carried out for 5000 ps in canonical ensemble (NVT) using COMPASSIII force field (Sun, 1998). Furthermore, the summations techniques of Ewald and atom-based cut-off were used to treat non-bonded electrostatic and van der Waals interactions, respectively. The velocity Verlet integration scheme using a time step of 1 fs was used to solve the motion equation of Newton (Swope et al., 1982). The temperature was controlled by Nose-Hoover thermostat (Martyna et al., 1992). Because of its known overestimation of the interaction energy magnitudes, interaction energy values will not be analyzed from MD simulations.

3.1 Synthesis of the new corrosion inhibitors

The novel 1,2,3-triazoyl-acridine derivatives were obtained through a simple two step-approach, as outlined in the Scheme 1. The first step involved a nucleophilic aromatic substitution reaction between the commercially available 6,9-dichloro-2-methoxyacridine and sodium azide, providing the intermediate 9-azido-6-chloro-2-methoxyacridine, as a green solid, in quantitative yield. This azide intermediate was readily reacted with the corresponding commercially available alkyne under an ultrasound-assisted CuAAC methodology, as previously described for the isatine-triazole derivatives preparation (Silva et al., 2016), furnishing TTA, ATM, and ATA after 15–35 min, in 48–61 % yield range.

The insertion of the azide group in the intermediate 9-azido-6-chloro-2-methoxyacridine was confirmed through FT-IR spectroscopy. A strong absorption band at 2108 cm⁻¹ corresponding to the characteristic stretching N = N = N bond was observed (Figure S1, supplementary information). The structures of all 1,2,3-triazoloyl-acridine derivatives were fully characterized through spectroscopic and spectrometric analyses (Figures S2-S13, supplementary information). All ¹H NMR spectra showed the characteristic singlet of the hydrogen atom of triazolic ring CH at 8.81, 8.67, and 8.86 ppm for TTA, ATM, and ATA respectively, confirming the 1,2,3-triazolic nucleus formation. The other observed ¹H NMR signals and all ¹³C NMR signals were also in good agreement with the expected structures for the desired compounds. Moreover, HRMS unequivocally confirmed the molecular structures of the desired corrosion inhibitors.

3.2 Weight Loss Study

Gravimetric experiments are a reliable tool in the corrosion inhibitor field. WLS is applied to determine whether a particular compound acts attenuating the metallic in an aggressive environment, allowing the calculation of the corrosion rate (CR) through the mass variation of the specimens. The difference in the CR values permits obtaining anticorrosive efficiency and surface coverage value (Chaouiki et al., 2022). All data is shown in Table 2 for an immersion time of five hours:

The results show that all three molecules at the lowest evaluated concentration (0.0625 mmol/L) already act very satisfactorily as corrosion inhibitors, presenting from 64 to 71 % efficiency. That is a great result since usually, at this low concentration, organic compounds do not present relevant corrosive inhibitory activity (Cherrad et al., 2022) with exceptions being reported in the literature (Noor and Al-Moubaraki, 2008).

All three compounds have different substituents attached to the triazolic nucleus. TTA has a 2-methoxytetrahydro-2H-pyran (tetrahydropyranyl ether), ATM has a methanol (methyl ether) and ATA has a methyl acetate (an acetate). Those different structures clearly influence the anticorrosive efficiency of the 1,2,3-triazolyl-acridine derivatives. The presence of a 6-member saturated heterocycle leads to a result of 85.2 % corrosion mitigation for TTA, diminishing the corrosion rate (CR) from 1.713 to 0.254 mm year⁻¹. When we exchange this substituent for one with less steric effect as –CH₂-OH, the efficiency goes up to 90.4 %, lowering CR to 0.164 mm year⁻¹. Changing now for another one with a similar steric effect but with a higher electron density due to the presence of another O atom (acetate) and a π bond, a better anticorrosive efficiency is found, reaching 94.2 %, with the corrosion rate going from 1.713 to 0.100 mm year⁻¹. Those values were reached at 1.00 mmol/L and prove the three 1,2,3-triazolyl-acridine derivatives are great corrosion inhibitors for mild steel in acidic media. So, the tested compounds follow the following order of inhibition: ATA > ATM > TTA.

The anticorrosive efficiency of organic compounds is directly related to their capacity of interacting with metallic alloy and adsorb on the surface, producing a protective layer that will prevent the corrosion process to take place. Thus, the literature well characterizes the fact that this protection is precisely the surface area covered by organic compounds. With that in mind, evidencing that the different substituents present in the evaluated compounds directly influence the anticorrosive efficiency of the 1,2,3-triazolyl-acridine derivatives, we made use of theoretical studies (vide infra) to understand their influence on the electronic properties of the compounds and how they interact with the mild steel surface.

3.3 Adsorption Isotherm

Eleven different isotherms (Freundlich, Frumkin, Langmuir, Temkin, Dubinin–Radushkevic, Kiselev, Flory-Huggins, Hill-De Boer, Elovich, Halsey, and Fowler–Guggenheim) were tested using their mathematical equation to understand more about the adsorption process. Each on has its own theory and the evaluation criterion for choosing the best fitting via coefficient of determination (R²) (En-Nylly et al., 2023).

TTA, ATM, and ATA suited the Langmuir monolayer formation theory at all tested temperatures (298, 318, 338 K). This means that each molecule adsorbs on one specific active site of the mild steel surface and do not interact with neighboring compounds (Jiang et al., 2023). Fig. S14 shows the graphs for all three OCIs, making it possible to calculate the adsorption constant (K_ads) using the Langmuir equation:

The high K_ads values obtained (Table 3) for all three compounds display that the equilibrium is dislocated favoring the adsorption of TTA, ATM, and ATA on the mild steel surface, dislocating water molecules. The Gibbs adsorption energy (ΔG_ads) was then calculated according to the following equation (Liu et al., 2023):

It is well known in the literature that ΔG_ads values less negative than −20 kJ mol⁻¹ characterize physisorption processes, while ΔG_ads values more negative than −40 kJ mol⁻¹ characterize chemisorption processes. From the values exhibited in Table 3, it is possible to realize that TTA, ATM, and ATA experience both interaction types in the adsorption process on the mild steel surface (Hajjaji et al., 2021).

The variation of adsorption enthalpy was also calculated, using ΔGads vs. T(K). As shown in Table 3, it was found that for TTA, ATM, and ATA, those values are positive, characterizing an endothermic process, supporting the chemisorption phenomenon.

3.4 Electrochemical Impedance Spectroscopy

The technique Electrochemical Impedance Spectroscopy (EIS) allowed to better understand the phenomena occurring in the electrolyte/metal interface, giving details about the protection mechanism. The Nyquist plots for TTA, ATM, and ATA is shown in Fig. 2.

Close

Fig. 2

Nyquist plots for TTA, ATM, and ATA in 1.00 mol/L HCl at 298 K.

Export to PPT

Due to the square axes in the Nyquist plots, it is possible to observe from Fig. 2 that the results are depressed non-perfect semicircles, resulting in frequency dispersion. The graphs also exhibit a single loop form, indicating the presence of only one capacitive phase (Olasunkanmi et al., 2020).

To translate those results to numbers, an equivalent circuit was employed (Machado Fernandes et al., 2020a, 2020b, AlSalhi et al., 2023, Andrés Coy-Barrera et al., 2023). The system is composed by a solution resistance (R_s), a charge transfer resistance (R_ct) and a constant phase element (CPE), organized as presented in Fig. 3. It is important to mention that CPE was used instead of a pure capacitor due to surface roughness and inhomogeneity, leading to the depressed form observe in the Nyquist plots (Fernandes et al., 2022a, 2022b).

Close

Fig. 3

Equivalent circuit used to simulate Electrochemical Impedance Spectroscopy results.

Export to PPT

The obtained data is present in Table 4. The values of n prove that CPE was corrected used and the working electrode is not a pure capacitor (n = 1) and suffers from surface inhomogeneity. Also, Brug’s formula (Brug et al., 1984) (Eq. 3) was used to calculate the double layer capacitance (C_dl). This equation is used when slow charge transfer rules the corrosive process, which can be inferred by the single loop in the Nyquist plots in both blank and inhibited solutions.

Using the Helmholtz equation, the double layer capacitance can be defined as follow:

The term ε⁰ is always constant, as it represents the air permittivity. The other terms are local dielectric constant (ε), surface in contact with the electrolyte (S), and layer thickness (d). Thus, when introducing organic molecules into the aggressive medium and they adsorb on the metal surface to protect the metal from the corrosive process, the displacement of water molecules from the working electrode surface occurs, significantly decreasing the local dielectric constant and the exposed metallic surface and increasing the thickness of the protective layer, explaining the difference in C_dl values from blank and inhibited solutions (Rbaa and Lakhrissi, 2019).

To go deep and understand more about the change in C_dl values when varying the concentration of the three inhibitors, we made use of a crossing data between those results and the weight loss results, as first proposed by Fernandes et al (Fernandes et al., 2019a, 2019b, 2019c). So, a graph is suggested plotting the double layer capacitance versus what is called relative electroactive area, e.g., the surface not covered by organic molecules (the S from the Helmholtz equation but replaced for 1 – θ obtained via gravimetric experiments), as shown in Fig. 4.

Close

Fig. 4

Plots of double layer capacitance vs relative electroactive area for TTA, ATM, and ATA in 1 mol/L HCl.

Export to PPT

It can be seen from Fig. 4 and the presented coefficient of determination that an excellent linear fit is found for the three compounds, which suggests that the fraction ε/d is constant, corroborating the Langmuir theory seen in section 3.3. of a protective monolayer formation on the metallic surface (Machado Fernandes et al., 2022a, 2022b).

The Nyquist plots (Fig. 2) showed bigger semicircles as TTA, ATM, and ATA concentration in the electrolyte increased. Such diameters are associated with charge transfer resistance, illustrating the difficulty of occurring redox reactions at the working electrode (Damej et al., 2021, Al Jahdaly, 2023). The results exhibited in Table 4, after the simulation with the equivalent circuit, confirm that idea.

TTA reaches a R_ct of 509.3 Ω cm² at 1.00 mmol/L (in comparison to 75.0 Ω cm² from the blank solution) giving more than 85 % anticorrosive efficiency. ATM present 89.4 % efficiency mitigating corrosion (R_ct of 708.1 vs 75.0 Ω cm² from the blank solution) at the highest concentration tested. ATA prevents the 1020 mild steel corrosion in a range from 69 to 94 %, with Rct values varying from 244.7 to 1279.5 Ω cm². These results corroborate the gravimetric ones and prove that all three compounds are excellent corrosion inhibitors.

3.5 Linear Polarization resistance

Linear Polarization Resistance (LPR) experiments were used to acquire the polarization resistance (R_p) of the mild steel working electrode. By varying the potential (±10 mV vs OCP) and collecting the current resulting from this disturbance in the system, R_p is obtained through Ohm’s Law (Fernandes et al., 2020, Altunbaş Şahin, 2022).

From the results in Table 5, it is possible to realize that even at the lowest tested concentration (0.06 mmol/L), all three organic compounds prevent the oxidation from the working electrode at satisfactory levels (between 60 and 70 % efficiency). At 1.00 mmol L^-1, TTA, ATM, and ATA present 554.2, 788.0, and 1271.0 Ω cm², reaching 85.8, 90.0, and 93.8 % anticorrosive efficiency, respectively, in agreement with EIS data.

3.6 Potentiodynamic Polarization

The polarization curves were acquired after an extensive potential sweep of ± 300 mV and are exhibited in Fig. 5. It is possible to observe that in all three cases, the presence of the organic compound in the electrolyte affects both cathodic and anodic currents (Kaskah et al., 2017). And the decay in corrosion current density is more conspicuous as the concentration of inhibitor increases, directly influencing in the two redox reactions: hydrogen gas evolution and iron dissolution (Rbaa et al., 2020).

Close

Fig. 5

Potentiodynamic Polarization curves for TTA, ATM, and ATA in 1.00 mol/L HCl at 298 K.

Export to PPT

It is well defined in the literature that changes in corrosion potential (E_corr) inside the range of ± 85 mV characterize organic molecules as mixed-type, affecting both cathodic and anodic branches (Fernandes et al., 2019a, 2019b, 2019c, Nahlé et al., 2022, Zaher et al., 2022). The results of E_corr are shown in Table 5 and prove that TTA, ATM, and ATA are mixed-type corrosion inhibitors for 1020 mild steel in 1.00 mol/L HCl at 298 K.

A crucial observation to note is that when potentials within the range of 0.250 V (vs Ag|AgCl) are approached, the effectiveness of the three inhibitors against corrosion significantly diminishes. This phenomenon can be attributed to the region's characterization as the desorption potential. It is plausible to infer that in this range, the rate of desorption surpasses the rate of adsorption, leading to accelerated metal dissolution and subsequently heightened corrosion current density (Mourya et al., 2014, Zulkifli et al., 2017).

3.7 Electrochemical Frequency Modulation

Electrochemical Frequency Modulation (EFM) technique was employed to collect the values of corrosion current density (i_corr) without the need to previously know the Tafel constants. To measure the confidence of the experiment, causality factors 2 and 3 are used (Obot and Onyeachu, 2018). The obtained graphs are shown in Figs. S15-S17 and the data in Table 6.

From Table 6, the value of i_corr for the blank solution depicts a high rate of redox reactions from the corrosive process of carbon steel. As seen in other electrochemical experiments, in the lowest tested concentration (0.0625 mmol/L) all three organic compounds act as good corrosion inhibitors, lowering the corrosion current density from 120.1 to 45.4, 40.6, and 35.0 µA cm⁻² (Machado Fernandes et al., 2022a, 2022b).

At a concentration of 1.00 mmol/L, TTA, ATM, and ATA present anticorrosive efficiency of 85.5, 90.1, and 95.0, respectively, lowering i_corr from 120.1 (blank solution) to 17.5, 11.9, and 6.0 µA cm⁻². These results confirm that all three evaluated compounds act as excellent corrosion inhibitors for 1020 mild steel in hydrochloric acid and corroborate the others obtained results (Haruna et al., 2021).

Furthermore, as previously discussed in other sections, the introduction of the three corrosion inhibitors does not modify the reaction mechanism. This observation holds true in this context as well, as evidenced by the Tafel parameters (βa and -βc), which exhibit negligible alterations in their values between uninhibited and inhibited solutions. This consistency further supports the established fact that the presence of these inhibitors does not impact the underlying reaction mechanism.

3.8 Atomic force Microscopy (AFM)

It was well defined via gravimetric and electrochemical measurements that TTA, ATM, and ATA act as corrosion inhibitors via adsorption on the mild steel surface. Thus, AFM was employed to evaluate the mild steel surface topography on different moments, as shown in Fig. 6. The average roughness (R_a) of those surfaces was also calculated and is exhibited in Table 7.

Close

Fig. 6

AFM images for grounded (a) and corroded (b) mild steel, and in the presence of 1.00 mmol/L of TTA (c), ATM (d), and ATA (e).

Export to PPT

From Fig. 6a, the low R_a is explained with a very smooth surface, with grounding marks from the #600 SiC-emery paper. From Fig. 6b, one can see the extreme damage caused by the aggressive medium (1 mol/L HCl) to the metallic surface, thus explaining the high roughness value. From Fig. 6c-e, much more protected and preserved surfaces are observed, with marks similar to those seen in the grounded mild steel and reasonable R_a values, suggesting the adsorption of TTA, ATM, and ATA, impeding the corrosive process (Tiwari et al., 2021).

3.9 First-principles DFT simulations

3.9.1

3.9.1 Geometries and energies

Investigating the inhibitor-metal interactions from experiments would be difficult to give relevant information regarding the inhibitor active sites and adsorption configurations. First-principles DFT simulation is a very promising theoretical tool to simulate inhibitor molecules’ interactions with the metal surface, thus revealing insights into electronic and structural features relevant to inhibitor performance (Gustinčič and Kokalj, 2015, Kokalj, 2015, Kokalj et al., 2020).

The optimized adsorption geometries of TTA, ATM, and ATA molecules on Fe(1 1 0) surface are shown in Fig. 7. Interestingly, it can be observed that all molecules exhibit a parallel adsorption configuration on the iron surface. Although that was the initial adsorption configuration, molecules would always be in their most stable adsorption geometry. In confirmation of this interpretation, previous studies found that large-size inhibitor molecules tend to adopt a parallel disposition over metal surfaces (Guo et al., 2017, Guo et al., 2018, El-Haitout et al., 2022, El-Haitout et al., 2023). Looking now at each of the individually optimized adsorption geometry, one can notice that the TTA molecule forms only one bond with iron surface whereas ATM and ATA molecules form several bonds with iron atoms. The bond that is formed between the C atom of the TTA molecule and iron atoms has a distance length of 2.228 Å. Heteroatoms of the TTA molecule are all close to the upper layer of the iron surface, but without any bonding with iron atoms.

Close

Fig. 7

DFT-optimized adsorption geometries of (a) TTA, (b) ATM, and (c) ATA molecules on Fe(1 1 0) iron surface.

Export to PPT

Moving to the ATM molecule which has a methanol attached to its triazolic nucleus, one finds that both C and O atoms bond with iron atoms while no bond involves the N atoms. The Fe-C bonds have distances of 2.279, 2.313, and 2.305 Å while Fe-O has a distance of 2.033 Å. Nitrogen atoms of ATM molecule have also a close contact with iron atoms, but no bonding is noticed. The ATA’s molecule, on the other hand, ends up breaking the single bond between O and C atoms of the methyl acetate attached to the triazolic nucleus. There is formation of more bonds between ATA’s atoms and iron atoms compared to the previous two molecules. The acetate moiety forms two Fe—O bonds with distance lengths of 1.88 and 1.931 Å whereas C atoms form four Fe—C bonds with distances between 2.275 and 2.318 Å.

Without looking at interaction energies of optimized adsorption systems, it seems that number of formed bonds increases in the following trend: TTA < ATM < ATA. The triazolic nucleus is substituted by 2-methoxytetrahydro-2H-pyran in the case of TTA, however, its effect on the compound’s experimental inhibition performance is not significant, which is also noticeable from first-principles DFT simulations. The substitution of the triazolic nucleus by a methanol (ATM) and methyl acetate (ATA) seems to significantly change their electronic density distribution, and thus their interactive force with the iron surface, reaching a higher corrosion inhibition performance.

Before moving to the energetic analysis of optimized adsorption geometries, it would be beneficial to compare the bond lengths of interacting molecules with their respective sum of covalent radii to get an initial idea about the nature of the formed bonds. It has been reported that the sum of the covalent radii for Fe-C (r_C + r_Fe) and Fe-O (r_O + r_Fe) are 2.08 and 1.98 Å, respectively (Cordero et al., 2008). It can then be concluded that formed bonds between TTA, ATM, and ATA’s atoms and the iron surface are all covalent, a statement that can be confirmed or denied later by projected density of states analysis.

The energetic analysis reveals that TTA, ATM, and ATA have an interaction energy of −0.49, −2.53, and −2.77 eV, respectively. These values reveal no surprise results but confirm the geometrical conclusions made from optimized adsorption geometries. Once again, it can be said that substituting the triazolic nucleus with a methyl acetate or methanol is the most beneficial in terms of inhibitors’ reactivity and interactions with the metal surface. It highlights that the right substitution is not always related to the nature of the functional group, but other factors would influence the effectiveness of the entire compound such as the adsorption configuration and charge distribution, among others (Verma et al., 2018).

3.9.2

3.9.2 Projected density of states

The electronic projected density of states (PDOSs) is a useful tool to gain insights into the bonding nature of inhibitor molecules on Fe(1 1 0) surface. The plots of projected density of states of TTA, ATM, and ATA molecules in their isolated and adsorbed states are shown in Fig. 8. The substrate–adsorbate interactions can lead to considerable changes in the electronic density of states. By inspecting the PDOS plots of 3d iron orbitals and those of the isolated inhibitor molecules, one can notice that several molecules’ peaks exist within the energy range of iron 3d orbital’s peaks (Kumar et al., 2022). These molecular states are likely to hybridize upon adsorption of inhibitor molecules on the iron surface.

Close

Fig. 8

Projected density of states of (a)-(b) isolated and (a’)-(b’) adsorbed inhibitor’s molecules on Fe(1 1 0) surface.

Export to PPT

Close

Fig. 8

Projected density of states of (a)-(b) isolated and (a’)-(b’) adsorbed inhibitor’s molecules on Fe(1 1 0) surface.

Export to PPT

The PDOSs of adsorbed molecules show that several peaks disappear and some of them are slightly broadened and decrease in intensity. These changes are mainly a result of a strong hybridization of molecules’ p-orbitals with Fe-3d orbitals (Kumar et al., 2020, Lgaz and Lee, 2022). When inhibitor molecules approach the metal surface after first step physisorption, donor–acceptor interactions will occur between molecules’ active sites and iron atoms, which is nothing but a charge transfer, resulting in the formation of covalent bonds. These results confirm what has been concluded from section 3.9.1 that the formed bonds between inhibitor molecules and Fe(1 1 0) surface are covalent. Furthermore, given the fact that N atoms do not participate in any covalent bonding with iron atoms suggests that these atoms are primarily responsible for initial physical interactions with the iron surface as most of N atomic sites can easily be protonated in acidic mediums (Ebenso et al., 2021).

3.10 Molecular dynamics

Molecular dynamics simulation is one of the powerful theoretical tools that plays a growing role in various research fields. It is an attractive means for studying chemical phenomena in nanoscale; hence it is suitable for investigating the behavior of corrosion inhibitors when interacting with metal surface. Despite its known limitations in simulating bond-breaking and bond-formation along with overestimated adsorption energy (Kokalj, 2021), it is a useful way to get knowledge of adsorption configuration of molecules when interacting with metal surface in presence of a simulated solution. Herein, the best adsorption configurations of the inhibitor-Fe(1 1 0) are obtained from MD simulations as depicted in Fig. 9. From this Figure, it can be observed that all molecules tend to adopt a parallel disposition over the metal surface either completely or partially, suggesting their higher adsorption competitiveness in presence of water and corrosive particles.

Close

Fig. 9

The best adsorption configuration of (a) TTA, (b) ATM, and (c) ATA molecules on Fe(1 1 0) iron surface in presence of a simulated corrosive solution, obtained by molecular dynamics simulation.

Export to PPT

ATA molecule exhibits a nearly flat disposition over the metal surface, which is not surprising given its high affinity to iron atoms as demonstrated by DFT simulation and experimental findings. The ATM molecule, on the other side, exhibits a partially flat disposition over the iron surface through its triazolic nucleus and chlorophenyl part, which seems to be in line with DFTB results. Concerning the TTA molecule, it can be seen that it also exhibits a partial parallel adsorption on the metal surface through its triazolic nucleus, and the substituted group attached to it. These results highlight that, in the presence of a solution, the three molecules have a high ability to adsorb on the metal surface. This adsorption, which occurs in a nearly flat adsorption mode, can maximize the interaction of molecules’ reactive sites with iron atoms, forming a protective layer that prevents acid corrosion (Saha et al., 2018).