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Original article
01 2021
:15;
103491
doi:
10.1016/j.arabjc.2021.103491

Green synthesis, electrochemical, and DFT studies on the corrosion inhibition of steel by some novel triazole Schiff base derivatives in hydrochloric acid solution

, , , , , , , ,
a Egyptian Organization for Standardization and Quality, Egypt
b Department of Chemistry, Faculty of Science (Men’s Campus), Al-Azhar University, Nasr City 11884, Cairo, Egypt
c College of Science and Arts, University of Bisha, Al-Namas 61977, P.O. Box 101, Saudi Arabia
d Cairo Oil Refining Company (CORC), Cairo, Egypt
e Department of Green Chemistry, National Research Centre, Dokki, Cairo 12622, Egypt
f Faculty of Engineering, Kafrelshikh Univrsity, Kafrelshikh 33516, Egypt
g National Research Institute of Astronomy and Geophysics (NRIAG), Helwan 11421, Cairo, Egypt

⁎Corresponding authors at: Department of Chemistry, Faculty of Science (Men’s Campus), Al-Azhar University, Nasr City 11884, Cairo, Egypt (M.A. Bedair). moatazmohamed@azhar.edu.eg (Moataz M. Abdelsalam), m_bedier@azhar.edu.eg (Mahmoud A. Bedair), mbedair@ub.edu.sa (Mahmoud A. Bedair), younischem@gmail.com (Ahmed Younis)

Received: , Accepted: ,
© The Author(s) 2021
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

In this work, three triazole Schiff base derivatives 1-((1H-1,2,4-triazol-3-ylimino)methyl)naphthalen-2-ol (TMN), N-(furan-2-ylmethylene)-1H-1,2,4-triazol-3-amine (FTA) and N-(thiophen-2-ylmethylene)-1H-1,2,4-triazol-3-amine (TTA) were synthesized under ultrasonic irradiation and were investigated as corrosion inhibitors. FT-IR, NMR, and elemental analysis were used to elucidate the chemical structure of the synthesized inhibitors. The inhibitive characteristics of these inhibitors on C-steel corrosion in 1.0 M HCl were studied using three different techniques. The applied techniques were Electrochemical frequency modulation, electrochemical impedance spectroscopy, and potentiodynamic polarization curves. The acquired data from the experimental methods showed that the optimal concentration for TMN, FTA, and TTA is 1.0 × 10−3 mol/L, and the inhibition efficiency reached up to 91.68%, 88.44%, and 87.29% for TMN, TTA, and FTA, respectively. Experiments also indicated that these compounds act as mixed-type inhibitors, and their concentration affects the inhibition efficiency in a directly proportional manner. Quantum paraments obtained from density functional theory (DFT) showed good agreement between experimental and computational results.

Keywords

Ultrasonic irradiation
Polarization
Corrosion inhibitors
Electrochemical Impedance Spectroscopy
DFT
Molecular Dynamics
1

1 Introduction

Corrosion is a natural phenomenon that occurs naturally towards refined metals to convert them into more stable forms like sulfides, oxides, and hydroxides. Thus, corrosion is defined as the natural deterioration caused by the environmental attack towards the material (Bedair et al., 2017; Perumal et al., 2017). C-steel represents the backbone material in various industries because of its excellent physical characteristics and high economic value (Adawy et al., 2016; Espinoza-Vázquez et al., 2017; Fouda et al., 2014; Masroor, 2017). However, the corrosion process is still a significant issue causing severe losses of the C-steel production worldwide. Industrially, corrosion occurs to C-steel due to several reasons, like the use of the acidic solution, especially hydrochloric acid, as a cleaning agent for the removal of undesirable oxide and rusted layer (Badr et al., 2018; Bedair et al., 2019a; Tan et al., 2021a; Tourabi et al., 2017). Commonly, the corrosion process can't be eliminated, but it may be controlled using many effective methods. Luckily, one of the simplest, reliable, and cheapest method used in this track is the addition of corrosion inhibitors to the acidic solutions. These inhibitors play a significant role in protecting the metal surface during the cleaning or pickling process. The effectiveness of the corrosion inhibitor itself depends on its chemical composition, especially those containing some heteroatoms like nitrogen (N), oxygen (O), and sulfur (S), and conjugated groups. These heteroatoms can create coordination bonds by donating their lone pair of electrons to the d-orbital of the iron atom leading to adsorb on the metal surface effectively (Bİlgİç and Çaliskan, 2001; Ergun and Emregül, 2014; Tan et al., 2019). The azomethines or Schiff bases can be used as corrosion inhibitors in the acidic medium because of their high inhibition efficiency (Bedair et al., 2020; Bedair et al., 2019b). Schiff base was first reported by Hugo Schiff, a scientist born in Germany in 1864, by condensation of aldehyde and amine (Fabbrizzi, 2020). Lately, different routes employed to prepare Schiff bases, such as implementing green chemistry approaches using the ultrasonic irradiation method, which has low effects on the environment and surroundings (environmentally friendly) (Hassan et al., 2021; Hassan et al., 2019; Younis & Awad, 2020). Corrosion inhibitors derived from Schiff bases containing triazoles and their derivatives have taken a lot of considerations in the last years (Gopi et al., 2010; Phadke Swathi et al., 2017; Shetty, 2020). These compounds are categorized as environment-friendly inhibitors due to their unique chemical activities and limited toxicity, in addition to their particular tendency towards metal surfaces, enabling them to replace water molecules localized on the metal surface. Besides, they possess unshared electron pairs and considerable π-electrons on the nitrogen atoms that can interact with d-orbitals of iron metal to provide such a protective film preserving the metal from exposure to the corrosive medium (Bentiss et al., 1999).

This work aimed to investigate the newly synthesized triazole Schiff bases as corrosion inhibitors by applying electrochemical measurements on C-steel coupons in 1.0 M HCl solution. It also incorporates the study of the structural morphology of the C-steel coupons after inhibition. Quantum chemical calculations of the prepared inhibitors were discussed, while mass loss measurements were conducted elsewhere (Hassan et al., 2019).

2

2 Materials and experimental methods

2.1

2.1 Materials and measurements

All chemicals consumed for the synthesis of Schiff base inhibitors and the preparation of the 1.0 M HCl solution were of analytical grade, purchased from Sigma Aldrich, and were used as received. Chemicals used are distilled water, 37% HCl, 3-amino-1,2,4-triazole as primary amine, 2-hydroxy-1-naphthaldehyde, furan-2-carboxaldehyde, and thiophene-2-carboxaldehyde as three different aldehydes. Melting points were measured by electrothermal apparatus and without correction. FT-IR spectra were recorded on a Vertex 70 Analyzer, Bruker, USA, from 4000 to 400 cm−1 in KBr pellets. The elemental analysis C, H, N, S analyzer was conducted using a Flash 2000 organic Elemental Analyzer, Thermo, USA. The 1HNMR spectra were recorded on an Agilent Technologies model spectrometer NMR400-mercury 400. 1H spectra were measured at 300 MHz in dimethylsulphoxide (DMSO‑d6), and by the utilization of Tetramethyl-silane (TMS) as an internal reference and chemical shifts are pointed out in δ (ppm). Reactions carried out under ultrasonic irradiation were performed by digital ultrasonic cleaner - wuc-d10h - Daihan scientific (with a frequency of 40 kHz and HF-peak out of 407 W). The three inhibitors’ names 1-((1H-1,2,4-triazol-3-ylimino) methyl) naphthalen-2-ol, N-(furan-2-ylmethylene)-1H-1,2,4-triazol-3-amine and N-(thiophen-2-ylmethylene)-1H-1,2,4-triazol-3-amine were abbreviated as TMN, FTA and TTA respectively.

2.2

2.2 Synthesis of inhibitors under sonication

A mixture of 3-amino-1,2,4-triazole (2.44 g, 0.029 mol) with three different aldehydes 2-hydroxy-1-naphthaldehyde (5.00 g, 0.029 mol), furan-2-carboxaldehyde (2.4 mL, 0.029 mol), and thiophene-2-carboxaldehyde (2.7 mL, 0.029 mol) in 5 mL ethanol or distilled water/acetic acid was placed in 50 mL erlenmeyer flask and subjected to ultrasound irradiation waves at room temperature for the appropriate time until completion of the reaction (monitored by TLC). The resulting solid was collected by filtration and purified by crystallization from the suitable solvent to afford the pure products TMN, FTA, and TTA, respectively.

2.3

2.3 Electrochemical measurements

Electrochemical frequency modulation (EFM), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization measurements were also employed to study the corrosion behaviour on C-steel coupons immersed in 1.0 M HCl solution with and without various concentration ranges of the three inhibitors TMN, FTA, and TTA at room temperature (30 °C). Experiments were done using three conventional electrodes consists of a platinum counter electrode (C.E.) and a saturated calomel electrode (SCE) as a reference electrode. The working electrode (WE) was a rod of C-steel embedded in polyvinyl chloride (PVC) holder using epoxy resin; thus, the flat surface (area 1 cm2) was the exposed part for corrosion. All measurements were done using Gamry PCI4G750 Potentiostat/Galvanostat/ZRA analyzer, with a Gamry framework system based on ESA400 connected to a personal computer. Gamry applications include EFM140, EIS300, and dc105 for EFM, EIS, and dc corrosion measurements, respectively, with a computer for collecting data. Gamry Echem Analyst 5.5 software was used for data analysis. Before each experiment, the working electrode (WE) was immersed in the test solution for 30 min to achieve the steady-state position of open circuit potential (OCP). Potentiodynamic polarization measurements were obtained by changing the electrode potential automatically from −0.5 to 0.5 V vs.SCE at open circuit potential with a scan rate of 5 mV s−1 at 30 °C. For electrochemical impedance spectroscopy (EIS) measurements, a small alternating voltage perturbation (10 mV) was enjoined on the cell over the frequency range of 100 kHz: 0.2 Hz at 30 °C. Electrochemical frequency modulation (EFM) was performed by using two frequencies equal to 2.0 and 5.0 Hz. At the same time, the base frequency was equivalent to 0.1 Hz. We used a perturbation signal with an amplitude of 10 mV for both perturbation frequencies of 2.0 and 5.0 Hz.

2.4

2.4 C-steel coupons

Surface morphology tests were performed on C-steel specimens of the following elemental composition: (0.11 %C, 0.45% Mn, 0.04% P, 0.05% S, 0.25% Si, and the remnant is Fe) (Hassan et al., 2019).

2.5

2.5 Concentration range of inhibitors and acid

The concentrations range of the prepared inhibitors were 0.50 × 10−4 M, 0.75 × 10−4M, 1.00 × 10−4 M, 5.00 × 10−4 M, and 10.00 × 10−4 M. All solutions were prepared using distilled water.

2.6

2.6 Surface morphology

SEM and EDX analyzed C-steel coupon surfaces after 24 h immersion in 1.0 M HCl solution with and without the optimal inhibitor concentration. Forthwith, after the corrosion tests, samples were subjected to SEM and EDX studies to obtain the surface morphology. SEM Jeol JSM-5400 was used for the experiments.

2.7

2.7 Computational calculations

The density functional theory (DFT) method was employed to calculate the quantum parameters of our investigated inhibitors. Optimizing the geometrical structure of the prepared inhibitors was done using Becks three-parameter exchange functional B3 with Lee-Yang-Parr (LYP) non-local correlation functional with 6-311+G (d, p) as the basis set. The molecules were created using Gauss View, 6.0 executed in Gaussian 09 program package (Frisch et al., 2009; Lee et al., 1988). Key parameters like the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE) between ELUMO and EHOMO, electronegativity (χ), hardness (η), softness (σ) fraction of transferred electrons (ΔN) and Total Negative Charge (TNC) were also calculated by the following formulas (Bedair et al., 2021; Tran et al., 2021):

(1)
Ionization Potential (IP) = −EHOMO
(2)
Electron affinity (EA) = −ELUMO
(3)
Electronegativity (χ) = (IP + EA)/2
(4)
Electronic hardness (η) = (IP − EA)/2
(5)
Chemical softness (σ) = 1/η
(6)
F r a c t i o n o f t r a n s f e r r e d e l e c t r o n s ( Δ N ) = ( χ C - χ D ) 2 ( η C + η D ) where χC and χD are the electronegativity of the metal and inhibitor, respectively, and ηC and ηD are the corresponding chemical hardness. The calculations were conducted for both neutral and protonated molecules in aqueous and gas phases. Natural bonding orbitals (NBO) were also calculated.

Molecular dynamic simulation (MDS) studies were conducted to inspect the interaction of the synthesized inhibitors on iron surfaces, using Materials studio software from Accelrys Inc (Tan et al., 2020). The software used to build inhibitor molecules and Fe surfaces using the drawing tools in Materials Visualizer. We choose Fe (1 1 0) surface as the most stable surface to simulate the adsorption process (Pan et al., 2021). First, an optimized geometry model of molecules was prepared, which will adsorb on the surface. The MD simulation of the interaction between the inhibitor molecules and the Fe surface (1 1 0) was carried out in a simulation box (19.85 Å × 19.85 Å × 25.13 Å) with periodic boundary conditions to model a representative part of an interface free from any arbitrary boundary effects. The interaction energy EFe-inhibitor of the Fe surface with the inhibitor was calculated according to the following equation (Kaur et al., 2021):

(7)
EFe-inhibitor = Ecomplex − (EFe + Einhibitor) where Ecomplex is the total energy of the Fe crystal and the adsorbed inhibitor molecule, EFe and Einhibitor are the total energy of Fe crystal and free inhibitor molecules, respectively. The binding energy is the negative sign value of interaction energy (Moustafa et al., 2021).
(8)
Ebinding =  − EFe-inhibitor

3

3 Result and discussion

The TMN, FTA, and TTA inhibitors were synthesized by reacting 3-amino-1,2,4-triazole with appropriate aldehyde under ultrasonic irradiation as green chemistry approaches (Scheme 1). In order to optimize the reaction conditions, the synthesis of FTA was taken as a representative example and carried out by using different solvents (Table 1). It was found that using ethanol or water/acetic acid as solvents are optimal conditions for achieving the 95% yield after only 30 min. TMN and TTA were synthesized under the same experimental conditions (Table 2).

Synthetic route of the investigated three Schiff base inhibitors.
Scheme 1
Synthetic route of the investigated three Schiff base inhibitors.
Table 1 Optimization of reaction conditions of FTA by using different solvents.
Entry Solvent Reaction time (min.) Yield%
1 water 180 No reaction
2 Water/acetic acid (20:1) 30 95
3 Acetic acid 60 80
4 ethanol 30 95
5 Methanol 180 62
Table 2 Optimization of reaction conditions of TMN, TTA and FTA by using different solvents
Product Solvent Reaction time Yield
FTA Water/acetic acid (20:1) 30 95
ethanol 30 95
TMN Water/acetic acid (20:1) 50 94
ethanol 50 90
TTA Water/acetic acid (20:1) 50 70
ethanol 50 85

3.1

3.1 Characterization of the synthesized inhibitors

The chemical structure of the inhibitors TMN, FTA and TTA were characterized by FT-IR and 1HNMR as illustrated in Figs. 1 and 2. FT-IR spectrum of (TMN) showed bands at 1088 cm−1 for triazole (N—N), 1370 cm−1 for (C—O), 1525 cm−1 for (C⚌C), 1575 cm−1 for triazole (C⚌N), 1619 cm−1 for azomethine (C⚌N), 2885 cm−1 for aliphatic (C—H), 3029 cm−1 for Aromatic (C—H), 3107 cm−1 for triazole (N—H), 3441 cm−1 for phenolic (O—H). FT-IR spectrum of (FTA) showed some bands at 588–627 cm−1 for the furan ring, 1088 cm−1 for triazole (N—N), 1388 cm−1 for (C—O), 1526 cm−1 for (C⚌C), 1577 cm−1 for triazole (C⚌N), 1610 cm−1 for azomethine (C⚌N), 2900 cm−1 for aliphatic (C—H), 3112 cm−1 for triazole (N—H). FT-IR spectrum of (TTA) also showed some bands at 969 cm−1 for (C—S), 1087 cm−1 for triazole (N—N), 1515 cm−1 for (C⚌C), 1553 cm−1 for triazole (C⚌N), 1601 cm−1 for azomethine (C⚌N), 2888 cm−1 for aliphatic (C—H), 3101 cm1 for triazole (N—H).

FT-IR spectra of the synthesized Schiff base inhibitors.
Fig. 1
FT-IR spectra of the synthesized Schiff base inhibitors.
H1NMR spectra of the synthesized Schiff base inhibitors.
Fig. 2
H1NMR spectra of the synthesized Schiff base inhibitors.

Elemental analysis of (TMN); C % (found = 66.43, calc. = 65.54), H % (found = 4.42, calc. = 4.23), N % (found = 22.11, calc. = 23.52). Elemental analysis of (FTA); C % (found = 66.43, calc. = 51.85), H % (found = 3.88, calc. = 3.73), N % (found = 33.95, calc. = 34.55). Elemental analysis of (TTA); C % (found = 48.34, calc. = 47.18), H % (found = 3.63, calc. = 3.39), N % (found = 31.27, calc. = 31.44), S % (found = 14.64, calc. = 17.99).

The 1HNMR (DMSO) spectra of (TMN) showed peaks at 7.15 ppm (d, 1H, naphthalene C8-H), 7.38 ppm (t, 1H, naphthalene C2-H), 7.57 ppm (t, 1H, naphthalene C1-H), 7.84 ppm (d, 1H naphthalene C3-H), 8.0 ppt (d, 1H, naphthalene C7-H), 8.36 ppt (d, 1 J, naphthalene C6-H), 8.49 ppm (s, 1H, triazole C-H), 10.04 ppm (s, 1H, azomethine C-H), 14.62 ppm (s, 1H, O.H.), and 10.79 ppm (s, 1H, triazole N.H.). The 1HNMR (DMSO) spectra of (FTA) also showed peaks at 6.72 ppm (dd, 1H, furan C9-H), 7.33 ppm (d, 1H, furan C10-H), 7.99 ppm (d, 1H, furan C8-H), 8.97 ppm (s, 1H, triazole C-H), 8.25 ppm (s, 1H, azomethine C-H), 13.66 ppm (s, 1H, triazole N.H.). The 1HNMR (DMSO) spectra of (TTA) showed peaks at 7.23 ppm (dd, 1H, thiophene C9-H), 7.89 ppm (d, 1H, thiophene C10-H), 8.26 ppm (d, 1H, thiophene C8-H), 9.32 ppm (s, 1H, azomethine C-H), 9.75 ppm (s, 1H, triazole C-H), 13.97 ppm (s, 1H, triazole N.H.).

3.2

3.2 Electrochemical measurements

3.2.1

3.2.1 Electrochemical frequency modulation, EFM

Electrochemical frequency modulation (EFM) is a rapid, powerful, and non-destructive corrosion rate measurement method that instantly gives corrosion current values without former awareness of Tafel constants. Furthermore, EFM in a single data set can measure Tafel parameters, corrosion rate, and causality factors. To make it more straightforward, a given example of the EFM spectrum of TMN, which is known as the “intermodulation spectrum”, is illustrated in Fig. 3.

Intermodulation spectrum for steel in 1.0 M HCl in the absence and presence of different concentrations from TMN at 30 °C.
Fig. 3
Intermodulation spectrum for steel in 1.0 M HCl in the absence and presence of different concentrations from TMN at 30 °C.

The link between the current density in µA cm−2 and frequency in Hz is represented by is the two prominent peaks (the higher in amplitude). These peaks are the response to the two applied frequencies, 2 and 5 Hz (excitation frequencies), while other peaks represent the summits, harmonics, and differences. Causality factors serve as an internal check on the genuineness of the EFM measurement. We can validate the experimental EFM data (Abdel-Rehim et al., 2006; Al-Amiery et al., 2016; Al-Mobarak et al., 2011; Bedair et al., 2017). Table 3 shows the EFM parameters for the studied corrosion medium with and without the prepared inhibitors under investigation. From Table 3, it appears that icorr is reduced with raising inhibitor concentration, while the inhibitor's efficiency is also increased by increasing inhibitor concentration. TMN inhibitor has the lowest current density equal to 54.56 μA cm−2 and the highest inhibition efficiency.

Table 3 Electrochemical kinetic parameters obtained by EFM technique for steel in the absence and presence of various concentrations of the synthesized Schiff bases in 1 M HCl at 30 °C
Inhibitor name Conc (M). Icorr (μA cm−2) βa (mV dec−1) βc (mV dec−1) CF-2 CF-3 k (mpy) θ ηEFM%
blank 0.0 × 10−4 656.40 82.93 90.64 2.30 2.54 304.03 – –
TMN 10 × 10−4 54.56 58.88 63.74 1.99 4.98 25.23 0.916 91.68
5.0 × 10−4 90.67 99.69 152.30 1.90 3.59 41.43 0.861 86.18
1.0 × 10−4 132.60 99.32 156.20 1.78 2.91 60.59 0.797 79.79
0.75 × 10−4 163.70 92.49 96.75 1.86 4.22 75.82 0.751 75.06
0.5 × 10−4 198.20 92.38 104.7 1.76 2.76 91.79 0.698 69.80
FTA 10 × 10−4 83.42 101.40 158.10 2.36 2.49 38.12 0.872 87.29
5.0 × 10−4 127.20 87.53 114.70 1.70 3.17 58.11 0.806 80.62
1.0 × 10−4 142.90 100.10 148.30 1.98 2.65 65.31 0.782 78.22
0.75 × 10−4 189.60 85.83 91.48 1.37 4.71 87.83 0.711 71.11
0.5 × 10−4 237.83 88.13 101.30 1.64 2.93 110.17 0.637 63.76
TTA 10 × 10−4 75.84 88.49 102.40 1.98 4.41 34.65 0.884 88.44
5.0 × 10−4 99.24 92.42 113.20 2.11 4.53 45.35 0.848 84.88
1.0 × 10−4 139.70 103.00 124.90 2.62 3.78 63.83 0.787 78.71
0.75 × 10−4 178.20 67.30 71.11 2.38 3.01 82.54 0.728 72.85
0.5 × 10−4 226.40 86.49 90.97 1.58 4.28 104.86 0.655 65.50

In contrast, FTA has the highest current density equal to 101.40 μA cm−2 and the lowest inhibition efficiency at the same concentration value. By knowing that standard values for CF-2 and CF-3 are equivalent to 2.0 and 3.0, respectively, any deviation in the values of the causality factor from the theoretical ones may be a consequence of small perturbation amplitude, insufficient resolution in the spectrum frequency, or an inhibitor that is not functioning properly. The casualty factors values shown in Table 3 are approaching the standard values, suggesting that the obtained data are satisfactory, validating both Tafel slopes and icorr. The inhibition efficiency (ηEFM%) was also calculated by applying equation (9) as follows (Khaled, 2008, 2009):

(9)
η EFM % = 1 - i corr i ° corr × 100 where icorr and i°corr represent the densities of the corrosion current with and without the studied inhibitors, respectively. From the given data in Table 3, we can infer that increasing inhibitor dosage into the acidic solution decreases corrosion current density, prompting the inhibitors inhibition efficiency (Abuelela et al., 2021). As a result, and based on the percentage of the obtained efficiencies, TMN has the highest inhibition efficiency (91.68%) compared to TTA (88.44 %) and FTA (787.29%), which has the lowest inhibition efficiency.

3.2.2

3.2.2 Polarization measurements

The corrosion behaviour of C-steel in an acidic medium was studied by implementing the polarization curve method, as shown in Fig. 4. The evaluated electrochemical parameters including the corrosion potential (Ecorr), Tafel constants (βa and βc), the current density (Icorr), the efficiency percentage (ηp), and the surface coverage (Θ) are tabulated in Table 4. The inhibition efficiency (%ηp) was evaluated from the measured corrosion current density values (icorr) using equation (10) (Abuelela et al., 2021):

(10)
η p % = i ° corr - i corr i / i ° corr × 100 where the corrosion current density without and with inhibitors iocorr and iicorr represents respectively, from Table 4, the results approved that the inhibitor addition reduces both cathodic hydrogen evolution and anodic metal dissolution. By comparing the current density with inhibitor concentration, we can notice that the current density (Icorr) decreases with the increase of inhibitor dose. Besides, the exitance of the inhibitor in the solution led the corrosion potential (Ecorr) towards a positive direction of less than 80 mV compared to the acid blank. Therefore, the investigated inhibitors may be classified as an inhibitor of a relatively mixed-type inhibitor (Bedair et al., 2019a; Elabbasy and Gadow, 2021; Tan et al., 2021b; Zhang et al., 2021). Furthermore, a protective film on the electrode surface was formed due to the decreases of the corresponding current densities and the increase in the degree of surface coverage (θ) with increasing inhibitor concentration, which approves that the prepared inhibitors act as corrosion inhibitors. Based on the data obtained from the polarization method, the order of inhibition efficiency of the three inhibitors increases in the following order:

  • FTA < TTA < TMN.

Potentiodynamic polarization curves for the corrosion of steel in 1.0 M HCl in the absence and presence of different concentrations of Schiff base inhibitors (TMN, FTA, and TTA) at 30 °C.
Fig. 4
Potentiodynamic polarization curves for the corrosion of steel in 1.0 M HCl in the absence and presence of different concentrations of Schiff base inhibitors (TMN, FTA, and TTA) at 30 °C.
Table 4 Electrochemical parameters for steel dissolution in 1.0 M HCl solution containing different concentrations of the Schiff bases obtained from polarization measurements at 30 °C.
Inhibitor name Conc (M). Ecorr vs. SCE (mV) Icorr (µA cm−2) βa (mV dec−1) βc (mV dec−1) k (mpy) θ ηp%
blank 0.0 × 10−4 −471 410 148.3 166.7 187.11 – –
TMN 10 × 10−4 −449 45.6 124.7 151.3 20.82 0.888 88.87
5.0 × 10−4 −473 81.4 133.7 152.3 37.21 0.801 80.14
1.0 × 10−4 −511 122 146.3 145.3 55.88 0.702 70.24
0.75 × 10−4 −464 142 139.5 164.1 64.87 0.653 65.36
0.5 × 10−4 −505 165 150.6 151.9 75.26 0.597 59.75
FTA 10 × 10−4 −462 80.1 129.3 161.6 36.60 0.804 80.46
5.0 × 10−4 −464 109 135.8 158.8 49.63 0.734 73.41
1.0 × 10−4 −510 131 148.6 145.6 56.88 0.680 68.04
0.75 × 10−4 −471 147 139.9 162.9 67.30 0.641 64.14
0.5 × 10−4 −508 179 153.3 152.7 81.88 0.563 56.34
TTA 10 × 10−4 −455 79.9 91.5 154.1 36.49 0.805 80.51
5.0 × 10−4 −470 101 137.0 156.3 46.29 0.753 75.36
1.0 × 10−4 −513 130 152.3 152.9 59.27 0.682 68.29
0.75 × 10−4 −475 145 144.2 161.5 66.11 0.646 64.63
0.5 × 10−4 −504 166 150.6 180.9 75.96 0.595 59.51

3.2.3

3.2.3 Electrochemical impedance spectroscopy (EIS)

One of the most important and usable electrochemical techniques is electrochemical impedance spectroscopy (EIS) which can evaluate the corrosion behaviour of C-steel in an acidic medium. EIS experiment was applied at 30 °C. Fig. 5 and Fig. 6 are respectively show the Nyquist and Bode plots of C-steel in 1.0 M HCl in the existence and disappearance of inhibitors. Nyquist plots show a semicircle in each curve that stands for a time constant because of the charge transfer resistance. Increasing concentration leads to enlarging the capacitive loop diameter, which means the inhibition effect is getting improved. Formula (9) was applied to study the inhibition efficiency based on impedance measures (Saxena et al., 2018):

(11)
η % = ( R ct - R ct 0 / R ct ) × 100 where Rct and R0ct represent transfer resistance in case of the existence and disappearance of inhibitors in the acidic solution, respectively, the inhibition efficiency is improved by increasing inhibitor concentration, getting the highest value of 91.12 %, 86.19 %, and 84.65 % at 10 × 10−4 M for TMN, TTA, and FTA, respectively.
Nyquist plots for steel in 1.0 M HCl in the absence and presence of different concentrations of surfactants (TMN, FTA, and TTA) at 30 °C and the equivalent circuit model used to fit the EIS data.
Fig. 5
Nyquist plots for steel in 1.0 M HCl in the absence and presence of different concentrations of surfactants (TMN, FTA, and TTA) at 30 °C and the equivalent circuit model used to fit the EIS data.
Bode and phase angle plots of impedance spectra for steel in 1.0 M HCl in the absence and presence of different concentrations of Schiff base inhibitors (TMN, FTA, and TTA) at 30 °C.
Fig. 6
Bode and phase angle plots of impedance spectra for steel in 1.0 M HCl in the absence and presence of different concentrations of Schiff base inhibitors (TMN, FTA, and TTA) at 30 °C.

The impedance curves were illustrated for Nyquist plots more clarifications by applying the experimental data to an equivalent circuit model shown in Fig. 6. Rs refers to the solution resistance, Rct the charge transfer resistance, and the constant phase element of double-layer CPE in the circuit. CPE is utilized as a capacitor in an electrochemical process to deal with the non-ideal capacitance response. The impedance of the CPE is expressed as follows (Rugmini Ammal et al., 2018):

(12)
Z CPE = Y 0 - 1 j ω - n where Y0is the CPE constant, n is the C.P. Exponent, j = (−1)1/2is an imaginary number, and ω is the angular frequency in rad s−1 (ω = 2πf), where f is the frequency in Hz. Double-layer capacitance(Cdl) were obtained by the following equation (Dutta et al., 2017):
(13)
C dl = Y 0 R ct 1 - n 1 / n

all obtained EIS parameters are given in Table 5. which reveals that Cdl, the double-layer capacitance decreases with the increase of inhibitor concentration which also proved that an adsorption process is taking place between inhibitors and the metal surface through replacing water molecules with inhibitor molecules or by increasing the double layer thickness, while Rct, the charge transfer resistance decreases with the decreases of inhibitor concentration. It is also apparent from Table 5 that the order of the efficiency percentage of the inhibitor is as follows

  • FTA < TTA < TMN

Table 5 EIS parameters, slopes of the Bode impedance magnitude at intermediate frequencies (S), and maximum phase angle values (α°) for carbon steel corrosion in 1.0 M HCl in the absence and presence of different concentrations of Schiff bases at 30 °C.
Inhibitor name Conc (M). Rs (Ru) (Ω cm2) Rct (Rp) (Ω cm2) Cdl (μF cm−2) S α° (°) θ ηz %
blank 0.0 × 10−4 37.01 1.635 52.47 −0.005 −57 – –
TMN 10 × 10−4 417.1 1.611 58.65 −0.163 −70.17 0.911 91.12
5.0 × 10−4 227.9 1.533 50.77 −0.064 −72.02 0.837 83.76
1.0 × 10−4 152.3 1.346 74.35 −0.062 −70.56 0.756 75.69
0.75 × 10−4 126.5 1.290 69.29 −0.053 −70.45 0.707 70.74
0.5 × 10−4 80.69 1.100 188.10 −0.012 −57.49 0.541 54.13
FTA 10 × 10−4 241.2 1.292 69.68 −0.122 −71.33 0.846 84.65
5.0 × 10−4 165.2 1.295 87.93 −0.082 −69.32 0.775 77.59
1.0 × 10−4 140.4 1.378 100.50 −0.077 −68.25 0.736 73.63
0.75 × 10−4 84.4 1.058 191.30 −0.042 −56.68 0.561 56.14
0.5 × 10−4 69.59 1.079 204.60 −0.021 −57.08 0.468 46.81
TTA 10 × 10−4 268 1.341 66.35 −0.158 −72.7 0.861 86.19
5.0 × 10−4 190.5 1.339 74.38 −0.082 −71 722 80.57
1.0 × 10-4 152.3 1.276 77.26 −0.050 −69.78 0.756 75.69
0.75 × 10−4 85.63 0.852 146.10 −0.037 −59.54 0.567 56.77
0.5 × 10−4 72.93 1.312 170.00 −0.032 −52.42 0.492 49.25

With the aid of the tabulated data in Table 5 and the bode modulus and phase angle (α) plotted in Fig. 6, the synthesized inhibitor forms a protective film on the surface of the metal, preventing its dissolution to the acid due to increased impedance values with the increase of inhibitor dosage. Furthermore, the obtained data from slops of bode impedance (S) and at intermediate frequencies and maximum phase angle shows significant shifting towards −1 and − 90° in the presence of inhibitors that agree on the ideal capacitive behaviour. In conclusion, an action of inhibition took place on the metal surface, preventing it from dissolution during the existence of the inhibitor.

3.3

3.3 SEM and EDX

The surface of the tested specimens was studied after emersion in 1.0 M HCl solution for 24 h with and without the investigated inhibitors. The study was conducted using an electron scanning microscope (SEM), a magnification tool cupelled with energy-dispersive X-ray spectroscopy (EDX). Images obtained from SEM are presented in Fig. 7, which indicates that the surface is severely damaged in the blank sample. While in the case of the inhibited samples that contain the studied inhibitors TMN, FTA, and TTA, it shows an excellent protective film formed on the metal surface. Furthermore, the data provided by EDX in Fig. 7 and Table 6 shows peaks of iron and chlorine in the case of the blank sample. In contrast, samples with inhibitors show iron, oxygen, sulfur, and nitrogen peaks without any chlorine peaks.

SEM and EDX of the steel surface after 24 h in the absence and presence of 10 × 10−4 M of different Schiff base inhibitor (TMN, FTA, and TTA) at 30 °C.
Fig. 7
SEM and EDX of the steel surface after 24 h in the absence and presence of 10 × 10−4 M of different Schiff base inhibitor (TMN, FTA, and TTA) at 30 °C.
Table 6  Quantitative analysis for steel surface after 24 hrs immersion in 1.0 M HCl in the presence and absence of the prepared compounds (TMN, FTA and TTA) from EDX.
Element Blank (HCl- Fe) TMN - Fe FTA - Fe TTA - Fe
Mass % Atom % Mass % Atom % Mass % Atom % Mass % Atom %
Cl 2.73 3.58 – – – – – –
C 1.58 6.14 1.67 6.91 1.52 6.35 1.37 5.36
N – – 2.03 2.85 1.39 1.97 1.23 1.67
O 5.07 14.75 1.92 6.00 1.92 6.04 3.72 10.90
S – – – – – – 1.24 1.95
Fe 90.62 75.53 94.38 84.24 95.17 85.64 92.44 80.12
Total 100 100 100 100 100 100 100 100

3.4

3.4 Theory and computational details

3.4.1

3.4.1 Quantum chemical calculations

3.4.1.1
3.4.1.1 Non-protonated inhibitor

Generally, Inhibitors are selected from those compounds, especially hetero atoms containing organic compounds that offer electrons to the unoccupied orbitals of the metal and gain free electrons from the metal. Fig. 8 shows inhibitors optimized structures, HOMO (the highest occupied molecular orbital), LUMO (the lowest unoccupied molecular orbital), and molecular electrostatic potentials (ESP) of the prepared inhibitors (TMN, FTA, and TTA). In addition to experimental methods, theoretical calculations were applied to evaluate the mechanism of corrosion inhibition by applying the density functional theory (DFT). Molecular optimization was achieved by using the density functional theory (DFT)/B3LYP with the basis set 6-311+G (d,p). The highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), energy gap (ΔE), electronegativity (χ), Global Hardness (η), Global Softness (σ), the fraction of transferred electrons (ΔN), and Total Negative Charge (TNC) were calculated and presented in Table 7. It is acknowledged that the molecule with the highest EHOMO is considered to have the highest tendency to donate electrons to an electron-poor species. Moreover, ELUMO shows the tendency of a molecule to accept electrons; therefore, it's evident from data in Table 7 that TMN has the highest EHOMO and the lowest ELUMO, which translated its high inhibition efficiency among FTA and TTA (Abd El-Lateef, 2015; Momeni et al., 2015; Mourya et al., 2015; Xie et al., 2015). The energy gap ΔE (ELUMO - EHOMO) was presumed to be an essential indicator of the energy of the intermediate excited state, which gives reliable information for the capability of a corrosion inhibitor to be adsorbed on the metal surface effectively (Zuo et al., 2021). The smaller value of ΔE, the greater the reactivity of an inhibitor molecule, so it is evident from Table 7 that TMN has the lowest value of ΔE equal to 3.673 eV while FTA has the highest value of ΔE equal to 4.249. Thus, we can order the synthesized inhibitors as TMN˃TTA˃FTA, which aligns with previous results obtained from the experimental analysis. The calculation also revealed that TMN possesses the highest total negative charge (-3.263 – neutral form – aqueous phase) compared to TTA (-2.127) and FTA (-2.083) in terms of electrostatic attraction between the metal substrate and the inhibitors. This high negative charge on the TMN molecule aids its high inhibition efficiency. The fraction of electrons transferred (ΔN) values give information about the ability of a tested inhibitor to transfer its electrons to the metal. Positive values of (ΔN) prove that electrons can follow from inhibitor molecules to the metal and vice versa. From Table 7, it is clear all inhibitors can transfer their electrons toward the metal as they possess a ΔN value greater than 0. It was found that the TMN inhibitor has the most significant values amongst all three inhibitors. The (ΔE back donation) is an important parameter that explains the interaction between inhibitors and the metal surface. If (ΔE back donation) ˃ 0, the inhibition efficiency will increase due to increased stabilization energy gained from the interaction between inhibitor molecules and the metal surface. From (ΔE back donation) data, the inhibitor's efficiency can be ordered as TMN˃TTA˃FTA. Molecular volume (M.V) plays a crucial role during the inhibition process. This parameter indicates how these inhibitors can cover more surface area of the C-steel substrate during their contact. Thus, inhibition efficiency increases as the volume of the molecules increase due to the increase of the contact area between molecule and surface, favouring more surface coverage of carbon steel by the inhibitors. It was found that TMN has the most prominent molecular volume (192.513 cm3/mol) compared to the other two inhibitors TTA and FTA.

(a) The optimized molecular structure for the inhibitor molecules using DFT/B3LYP/6-311+G (d,p) (b) HOMO, LUMO and ESP of the inhibitor molecules using DFT/B3LYP/6-311+G (d,p).
Fig. 8
(a) The optimized molecular structure for the inhibitor molecules using DFT/B3LYP/6-311+G (d,p) (b) HOMO, LUMO and ESP of the inhibitor molecules using DFT/B3LYP/6-311+G (d,p).
Table 7 The calculated quantum chemical parameters in eV for the investigated inhibitors at DFT/B3LYP/6-31G+(d,p) in the gas and aqueous phases.
Molecule EHOMO ELUMO ΔE ΔE back donation DM T.E. M.V. cm3/mol TNC η σ ω X ΔN ΔN max ηEFM*
(eV) (eV) (eV) (eV) (D) (eV) (e) (eV) (eV−1) (eV) (eV) (e) (e) (%)
Neutral inhibitors
gas phase TMN −5.850 −2.177 3.673 −0.459 3.3792 −21634.6 192.513 −2.648 1.836 0.544 4.386 4.013 0.812 2.185 91.68
FTA −6.159 −1.910 4.249 −0.531 2.3814 −15350.6 105.538 −1.632 2.124 0.470 3.832 4.035 0.697 1.899 87.29
TTA −6.213 −2.061 4.151 −0.518 2.3864 −24132.4 94.254 −1.846 2.075 0.481 4.123 4.137 0.689 1.993 88.44
aqueous phase TMN −6.041 −2.373 3.667 −0.458 4.4450 −21635.1 120.973 −3.263 1.833 0.545 4.827 4.207 0.761 2.294 91.68
FTA −6.361 −2.194 4.167 −0.520 3.2563 −15351.1 108.702 −2.006 2.083 0.479 4.391 4.277 0.653 2.053 87.29
TTA −6.407 −2.305 4.102 −0.512 3.1524 −24132.9 116.548 −2.127 2.051 0.487 4.626 4.356 0.644 2.124 88.44
Protonated inhibitors
gas phase TMN-H+ −9.246 −6.702 2.543 −0.317 12.766 −21642.9 157.345 −2.851 1.271 0.786 25.001 7.974 −0.383 6.270 91.68
FTA-H+ −10.200 −6.883 3.316 −0.414 8.4008 −15358.8 114.223 −1.529 1.658 0.602 21.997 8.541 −0.464 5.150 87.29
TTA-H+ −10.156 −6.857 3.298 −0.412 9.6897 −24140.6 103.958 −1.779 1.649 0.606 21.940 8.507 −0.456 5.158 88.44
aqueous phase TMN-H+ −6.546 −3.431 3.115 −0.389 18.495 −21645.3 159.089 −3.060 1.557 0.642 7.988 4.988 0.645 3.202 91.68
FTA-H+ −7.150 −3.421 3.729 −0.466 12.044 −15361.3 116.858 −1.767 1.864 0.536 7.492 5.286 0.459 1.417 87.29
TTA-H+ −7.169 −3.436 3.733 −0.467 13.679 −24143.1 131.740 −1.888 1.866 0.535 7.533 5.303 0.454 1.420 88.44

*Experimental inhibition efficiency obtained from electrochemical frequency modulation technique.

Softness (σ) is known as the mutual of hardness (η). The principles of chemical hardness and softness are tools for the estimation of the stability of chemical compounds. Hardness and softness are employed extensively to make predictions about chemical reactivity. It was informed that a molecule with a low energy gap is more polarizable, generally correlated with high chemical activity and low kinetic stability, and is designated a soft molecule. Table 7 indicates that TMN has a chemical softness (σ) equal to 0.544 eV, higher than 0.481 eV and 0.470 eV for TTA and FTA, respectively. Contrarily, TMN has a chemical hardness (η) equal to 1.836 eV, lower than 2.075 eV and 2.124 eV for TTA and FTA, respectively. These indicators reflect that TMN has the highest reactivity among the other inhibitors, more ability to adsorb on the metal surface, aligned with the experimental results (Tran et al., 2021).

Active centres within the three prepared Schiff base inhibitors were also calculated using Mulliken population analysis. The existence of more than one active centre is always considered an advantage because the more negative the atomic partial charges of the adsorbed centre are, the more smoothly the atoms donate their electrons to the unoccupied d-orbital of the steel metal. Quantum measurements were carried out for both protonated and non-protonated inhibitors in gas and aqueous phases.

In the aqueous phase, it was found that the highest negative charge was localized on N1, and it was equal to −0.446, −0.438, and −0.410 for FTA, TTA, and TMN, respectively. And it is also found that N6 has a high negative charge equal to −0.392, −0.359, and −0.273 for TMN, FTA, and TTA, respectively. For TTA, it was also found that N3, N4, C7, C21, andC22 have a high negative charge equal to −0.250, −0.243, − 0.397, − 0.374, and −0.153, respectively. For FTA, it was found that N3, N4, C7 C19, C21, and O23 have high negative charge values equal to −0.239, −0.258, −0.114, −0.128, −0.196, and −0.267, respectively. TMN has high negative charges equal to −0.258, −0.237, −0.336, −0.290, −0.363, −0.173, −0.058, −0.150 and −0.597 on N3, N4, C10, C11, C12, C14, C16, C16, and O18 respectively.

Bond length calculation for non-protonated inhibitors in the aqueous phase showed that the longest bond length was between C19 and S24 for TTA inhibitor, equal to 1.75302 Å. In contrast, the second-longest bond was found between C8 and N9 for TMN, and it was equal to 1.45057 Å, and the last longest bond was between C7 and N19 for FTA, and it was equal to 1.43499. The shortest bond length within all three inhibitors was between N6 and C7, and it was equal to 1.29205 Å, 1.29229 Å, and 1.30383 Å for TTA, FTA, and TMN, respectively.

In the gas phase, it was found that the highest negative charge was localized on N1, and it was equal to −0.375, −0.366, and −0.337 for FTA, TTA, and TMN, respectively. And it is also found that N6 has a high negative charge equal to −0.392, −0.282, and −0.206 for TMN, FTA, and TTA, respectively. For TTA, it was also found that N3, N4, C7, C21, and C22 have a high negative charge equal to −0.256, −0.177, − 0.347, − 0.355, and −0.140, respectively. For FTA, it was found that N3, N4, C7 C19, C21, and O23 have high negative charge values equal to −0.244, −0.191, −0.028, −0.115, −0.170, and −0.228, respectively. TMN has high negative charges equal to −0.264, −0.175, −0.336, −0.256, −0.343, −0.153, −0.052, −0.104 and −0.553 on N3, N4, C10, C11, C12, C14, C16, C16 and O18 respectively.

Bond length calculation for non-protonated inhibitors in the gas phase showed that the longest bond length was between C19 and S24 for TTA inhibitor, equal to 1.74997 Å. In contrast, the second-longest bond was found between C8 and N9 for TMN, and it was equal to 1.44995 Å, and the last longest bond was between C7 and N19 for FTA was equal to 1.43937. The shortest bond length within all three inhibitors was between N6 and C7, and it was equivalent to 1.28787 Å, 1.28856 Å, and 1.30323 Å for FTA, TTA, and TMN, respectively

Natural bonding orbital (NBO) analysis was also carried out to obtain more information about molecular orbital electron distribution and electron delocalization within the molecule. Regarding NBO analysis, charge distribution was calculated for TMN, FTA, and TTA, and it was found that O18 has the highest negative charge value, and it was equal to −0.703. In contrast, C9 has the minimum negative charge value, and it was equal to −0.012 both within TMN inhibitor. It was also found that N1, N3, N4, N6, C8, C10, C11, C12, C13, C14, C15, and C16 have negative charge values equal to −0.538, −0.399, −0.299, −0.547, −0.197, −0.233, 0.-227, −0.258, −0.203, −0.089, −0.169, and −0.286 respectively. For FTA, it was found that N1, N3, N4, N6, C20, and C21 have the following negative charge values −0.538, −0.401, −0.287, −0.446, −0.275, and −0.347, respectively. For TTA, negative charge values equal to −0.538, −0.401, −0.289, −0.465, −0.295, −0.235, −0.297 and −0.431 for N1, N3, N4, N6, C19, C20, C21, and C22 respectively.

3.4.1.2
3.4.1.2 Protonated inhibitor

Heteroatom plays a crucial role in protonated inhibitors in terms of increasing the molecule's tendency toward protonation in the aqueous acidic medium (Bedair, 2016); thus, the protonated forms of the synthesized inhibitors were studied. The calculations showed the remarkable stability of protonated inhibitors. For TMN, FTA, and TTA, the localized lone pair of electrons on nitrogen atoms can be readily protonated.

Quantum chemical parameters were calculated for the protonated inhibitors in the gas phase, or the aqueous phase showed no considerable change in behaviour than the non-protonated inhibitors.

In the aqueous phase, it was found that the highest negative charge was localized on N3, and it was equal to −0.601, −0.581, and −0.57 for TMN, TTA, and FTA, respectively, and N6 has a high negative charge equal to −0.393, −0.345 and −0.258 for TMN, FTA, and TTA respectively. For TTA, it was also found that N1, C7, C21, and C22 have a high negative charge equal to −0.177, −0.372, − 0.370, and −0.116, respectively. For FTA, it was found that N1, C19, C21, and O23 have high negative charge values equal to −0.197, −0.155, −0.188, and −0.241, respectively. TMN has high negative charges equal to −0.141, −0.196, −0.371, −0.263, −0.359, and −0.56 on N1, C12, C14, C15, C16, and O18 respectively.

The bond length was also calculated for protonated inhibitors in the aqueous phase using (DFT)/B3LYP with the basis set 6–311 + G (d, p) t for the three prepared inhibitors. The bond length of the synthesized inhibitors varied from 1.26140 Å to 1.75517 Å. The longest bond was between C19 and S24 for TTA inhibitor in and it was equal to 1.75517 Å, while the second-longest bond was found between C2 and N3, and it was equivalent to 1.48976 Å and 1.48960 Å within FTA and TMN, respectively. The shortest bond length within all three inhibitors was between N1 and C2, and it was equal to 1.26140 Å, 1.173 Å, and 1.26213 Å for FTA, TTA, and TMN, respectively

In the gas phase, the highest negative charge was localized on N3, and it was equal to −0.630, −0.617, and −0.606 for TMN, TTA, and FTA, respectively, and N6 has a high negative charge equal to −0.402, −0.275 and −0.205 for TMN, FTA, and TTA respectively. For TTA, it was also found that N1, C7, and C21 have high negative charges equal to −0.146, −0.386, and − 0.335, respectively. For FTA, it was found that N1, C19, C21, and O23 have high negative charge values equal to −0.165, −0.132, −0.152, and −0.197, respectively. TMN has high negative charges equal to −0.111, −0.322, −0.227, −0.342, −0.213,108 and −0.497 on N1, C10, C11, C12, C14, C16 and O18, respectively.

Bond length calculation for protonated inhibitors in the gas phase showed that the longest bond length was between C19 and S24 for TTA inhibitor, equal to 1.75704 Å. In comparison, the second-longest bond was found between C2 and N3, and it was equal to 1.50745 Å and 1.50700 Å within FTA and TMN, respectively. The shortest bond length within all three inhibitors was between N1 and C2, and it was equal to 1.25645 Å, 1.25654 Å, and 1.25725 Å for FTA, TTA, and TMN, respectively.

3.4.2

3.4.2 Fukui indices

Fukui function analysis was used to better recognize the studied inhibitor's adsorption process on the C-steel surface and enhance the experimental results. Moreover, using Fukui function analysis helped to evaluate the nucleophilic and electrophilic attacks caused by the most active sites of the inhibitor molecules. Calculation of Fukui indices f +and f – were obtained using the following equations (Saha et al., 2016):

(14)
f k + = q k N + 1 - q k ( N ) ( F o r n u c l e o p h i l i c a t t a c k )
(15)
f k - = q k N - q k ( N + 1 ) ( F o r e l e c t r o p h i l i c a t t a c k ) where qk was the gross charge of k atom, i.e., the electronic density at a point r in space around the molecule, the qk (N − 1), qk (N + 1), qk (N) and are the charges of the cationic, anionic, and neutral species respectively. High values of f k + refers to a highly nucleophilic atom that can easily accept electrons from the metal surface in the adsorption process, the highest values of f k + for TMN are located on O (18), N (4), and C (8), the highest values of f k + for FTA are located on N (4), N (6), and C (22), and the highest values of f k + for TTA are located on S (24), C (7), N (6), and N (4), where C (19) has the lowest value. Furthermore, the highest values of f k - refers to the high tendency of an atom in an inhibitor molecule to donate an electron to surface the metal substrate. The highest values of f k - for TMN are located on C (7), C (11), and N (4), the highest values of f k - for FTA are located on C (7), N (6), and N (4), and the highest values of f k - for TTA are located on S (24), N (4), and N (6) where C (19). The obtained conclusion from Fukui analysis complies with HOMO and LUMO orbitals distribution. From Fukui analysis and the frontier molecular orbitals results, it is suggested that the best demonstration of the adsorption process of the studied inhibitor and the metal substrate is by donor–acceptor interaction.

3.4.3

3.4.3 Molecular dynamic simulations

Molecular dynamic (M.D.) simulation was applied to study the adsorption interaction between the synthesized inhibitor molecules and the C-steel surface. Fig. 9 reveals the top and the side view of the adsorbed molecules of the studied inhibitors (TMN, FTA, and TTA). From Fig. 9, it is determined that the best adsorption configuration of TMN, FTA, and TTA over the metal surface is when molecules are parallel to the Fe (1 1 0) surface. This adsorption configuration indicates that the prepared inhibitors may have the ability to donate and accept electrons. The donation process will take the direction from the inhibitor molecules to the unoccupied orbitals of iron. In contrast, the acceptance process will occur through back-bonding, and it will take the opposite direction from d-orbitals of iron to the inhibitor molecules. Table 8 shows the total energy, in kcal mol−1, of the substrate–adsorbate configuration, which is defined as the sum of rigid adsorption energy, adsorbate components, and deformation energy. In our study, adsorption energy in kcal mol−1 reports energy emitted (or demanded) when the adsorption of the relaxed adsorbate molecules took place on the surface of the C-steel coupon. The deformation energy also implies the adsorption energy concerns the released energy in kcal mol−1 when the adsorption of the relaxed adsorbate molecules is adsorbed on the metal surface. Furthermore, the rigid adsorption energy in kcal mol−1 implies the same principle for the adsorption energy. The difference between deformation and rigid energies refers to the energy emitted (demanded) when the unrelaxed adsorbate molecules are adsorbed on the metal surface (Azzaoui et al., 2017; Bedair et al., 2017; Bedair et al., 2016). Table 8 shows the values of (dEads/dNi), which states that the energy, in kcal mol−1, of substrate–adsorbate configurations where one of the adsorbate molecules has been removed. Table 8 also shows the binding energy (Ebind) values equal to the adsorption energy values but in the opposite sign. The binding energies of TMN, TTA, and FTA are 134.164 kcal mol−1, 90.001 kcal mol−1 and 88.839 kcal mol−1, respectively. These results show that these inhibitors have a solid adsorption capability onto the Fe (1 1 0) surface; it also indicates that the inhibition efficiency of the three synthesis inhibitors can be arranged onto FTA < TTA < TMN, this arrangement agrees with the previous results gained by experimental work.

Molecular simulations for the most favourable modes of adsorption obtained for the investigated inhibitors on Fe (1 1 0) surface, side, and top view.
Fig. 9
Molecular simulations for the most favourable modes of adsorption obtained for the investigated inhibitors on Fe (1 1 0) surface, side, and top view.
Table 8 Outputs and descriptors calculated by the simulation for the adsorption of the inhibitors on Fe (1 1 0) surfaces.
Inhibitor Total energy Adsorption energy Rigid adsorption energy Deformation energy (dEads/dNi) Binding energy (ηEFM*)
%
TMN −33.055 −134.164 −134.784 0.619 −134.164 134.164 91.68
FTA −79.655 −88.839 −89.310 0.471 −88.839 88.839 87.29
TTA −63.656 −90.001 −90.659 0.658 −90.001 90.001 88.44

*Experimental inhibition efficiency obtained from electrochemical frequency modulation technique.

3.4.4

3.4.4 Adsorption mechanism

Molecular dynamic simulations revealed that TMN molecules could be adsorbed parallel to the C-steel surface owing to combined physical and chemical interactions forming a protective thin film of TMN molecules on that surface. Such adsorption occurs through the inhibitor’s active sites, including the π-electrons of the aromatic ring, azomethine group, and the lone pair electrons of O and N atoms to the d-orbitals of iron. Back-donation from iron d-electrons to TMN molecules is an additional probability in which the adsorption process became likely “synergetic”. Nevertheless, the protonated TMN molecules can be adsorbed at the cathodic- and anodic- sites in the presence of chloride ions via physical columbic interaction. This adsorbed layer can curb the attack of aggressive ions like H3O+, leading to more protection to carbon steel against corrosion. Fig. 10 illustrates the mechanism of the adsorption process on the carbon steel surface, in addition to the inhibition behaviour of the TMN molecule.

Adsorption mechanism of TMN on carbon steel surface in 1.0 M HCl.
Fig. 10
Adsorption mechanism of TMN on carbon steel surface in 1.0 M HCl.

4

4 Conclusion

This work evaluates three triazole Schiff base derivatives as corrosion inhibitors for carbon steel in 1.0 M HCl solution. Studies of the three inhibitors were conducted using three different experimental techniques in addition to computational studies. Thus, it is feasible to conclude the following results:

  • In this work, three Schiff base inhibitors were prepared using the ultrasonic irradiation method, which is eco-friendly, reduces the reaction time and increases the reaction yield.

  • The structure of the prepared inhibitors was spectroscopically elucidated by FT-IR, NMR, and elemental analysis techniques.

  • Electrochemical measurements approved that the adsorption process of three inhibitors TMN, FTA, and TTA, at the metal/acid solution interface, efficiently occurs where TMN has the most outstanding inhibition efficiency.

  • Potentiodynamic polarization outcomes revealed that all three inhibitors under investigation could diminish the corrosion rate and are mixed-type inhibitors for hydrochloric acid in carbon steel.

  • The three synthesized Schiff base inhibitors replace the water molecules on the carbon steel substrate to form a protective film preserving the metal surface from the corrosive solution, which the SEM analysis confirmed.

  • Quantum chemical properties like ΔE, EHOMO, and ELUMO directly correlated to the experimentally achieved inhibition efficiencies. It has been noticed for this group of three inhibitors that inhibition efficiency increased with the higher EHOMO and the lower ELUMO and ΔE. Electrochemical measurements gave consistent results from which we can imply that the efficiency of three compounds follows the order: FTA < TTA < TMN.

  • Molecular dynamic simulation proved the adsorption of the three inhibitors on the metal surface in a parallel position. It also indicated that TMN has the highest binding energy than TTA and FTA.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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