Three new reduced forms of synthesized Schiff bases as potent anti-corrosion inhibitors for carbon steel in artificial seawater

Abstract

As part of the development of a new organic entity, we synthesized three new reduced forms of Schiff bases named 2,2’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)disphenol (I1), 4,4’-(((2,2-dimethylpropane-1,3-diyl)bis (azanediyl)bis(methylene)bis(2-methoxyphenol) (I2), and 6,6’-(((2,2-dimethylpropane-1,3-diyl) bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (I3). In order to develop new organic ligands to inhibit steel corrosion in 1M HCl solution, various electrochemical methods, such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), along with surface visualization through atomic force microscopy (AFM), were employed. PDP results revealed excellent inhibition by compound I3 (71%) at a concentration of 1 mg/L. These findings were supported by the observation of a protective layer formation during prolonged immersion of steel in a corrosive solution, with or without inhibitors. In addition to gaining insights into the interaction mechanism and adsorption mode, density functional theory, Monte Carlo, and molecular dynamic simulations were conducted, revealing valuable information about the interaction of the inhibitors with the steel surface. Average surface roughness (R_a) values obtained for the artificial seawater in the absence and presence of inhibitor are 887 nm for blank, 195 nm for I1, 158 nm for I2, and 105 nm for I3.

Introduction

Carbon steel degradation is one of the important phenomena that is observed in all domestic and industrial sectors (Jafari et al. 2024a, 2024b). Material loss and economic loss are the direct consequence of it. It mainly occurs due to chemical, electrochemical, or biochemical interactions unintentionally causing corrosion (Mohsenifar et al. 2016; Jafari and Sayin 2016a, 2016b). Carbon steel used in industry is expected to have certain properties, the most important of which is corrosion resistance. Despite having the best corrosion resistance, commercial iron has poor machinability and weldability (Jafari and Sayin 2015a). The properties can be improved by alloying. Cu, Mn, Mg, Si, and other alloying elements are commonly used to give iron desirable properties (Ameri et al. 2022; Jafari and Akbarzade 2017; Jafari et al. 2013a). The API 5L series is the most popular engineering alloy group, with Mg and Si added as alloying elements. These are widely used in aerospace, aviation, defense, and other industries (Jafari et al. 2013a). The presence of alloying elements in them makes the API 5L have realistically good corrosion resistance properties. Pickling with hydrochloric acid, on the other hand, results in corrosion (Jafari et al. 2014). Pickling is usually carried out to remove the scales and rust formed on the surface of the material. During this process, the metal may undergo disintegration. API 5L grade B has a wide range of applications in defense, aviation, industries, construction, and domestic means (Mohsenifar et al. 2016; Jafari and Sayin 2016a). As a result, corrosion control on API 5L carbon steel must be addressed. In addition to these characteristics, API 5L carbon steels are prone to corrosion in a chloride ion-containing medium. This leads to the destruction of the material. Therefore, it is necessary to control corrosion. One of the methods to control the corrosion rate is to add inhibitors to the corrosive media. Due to the presence of the –C = N– group, an electron cloud on the aromatic ring, the electronegative N, O, and S atoms in the molecule, Schiff bases may be good corrosion inhibitors (Jafari et al. 2013b). Schiff bases have been previously reported as effective corrosion inhibitors for steel in acid mediums like hydrochloric acid. Schiff bases inhibit corrosion better than Amides and Aldehydes, which reduce the corrosion rate (Benmahammed et al. 2019). The inhibition efficiency of Schiff base compounds will increase by adding halogens to their composition due to the increase in electron density (Zhang et al. 2012, 2018; Saha et al. 2015). N₂O₂, − NO₂, − CH₃, and − Br⁻ are various substitutes in structers of Schiff bases which are the most effective because of their higher electron donation ability. The presence of these substitutes will improve the substrate’s physical connection and inhibition and efficiency (Jafari et al. 2024a, 2013b; Negm et al. 2011; Jafari and Ameri 2024). NO group in Schiff base compounds has various applications in catalyst industries because of their particular surface properties (Jafari and Akbarzade 2017; Jafari et al. 2014, 2013b; Jafari and Sayin 2015b). Dinitrogen tetroxide Schiff bases also protect steel against corrosion in H₂SO₄ media (Rezaeivala et al. 2022). In addition, these compounds inhibit corrosion on the steel surface against corrosive ions, especially in acidic solutions. By comparing the examined Schiff base with other inhibitors of steel in acidic media that have been reported, the comparison makes it evident that the compound we evaluated had high corrosion efficiency in the HCl environment.

In this work, the reduced Schiff bases form compounds have the formula2,2’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)disphenol (I1), 4,4’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (I2), and 6,6’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (I3) were investigated for the first time by Tafel, Electrochemical Impedance Spectroscopy (EIS), and AFM tests in the artificial seawater. The solutions containing concentrations of 1 mg/L of three inhibitors were used, and the effects of electronic structure and chemical structure of derivatives on inhibition efficiency and comparison of the effect of different substituents were evaluated. Quantum and molecular simulation methods have also been performed as part of this work.

Experimental procedure

The electrodes were prepared from API 5L B carbon steel specimens. The specimens of dimension 1 cm × 1 cm (exposed) (isolated with polyester resin) were used for polarization and electrochemical impedance methods. They were polished mechanically using different grade emery papers from 220 up to 2000 and washed thoroughly with triple distilled water and degreased with acetone before being immersed in the acid solution.

The testing solution was artificial seawater with 3% concentrations of Cl − (wt.%). The artificial seawater was prepared with ultra-pure water and A.R. reagents including NaCl, MgCl₂, and CaCl₂.

Three synthesized Schiff base named 2,2’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)disphenol (I1), 4,4’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (I2), and 6,6’-(((2,2-dimethylpropane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (I3) was prepared according to the described procedure (Benmahammed et al. 2019; Keypour et al. 2008, 2009, 2013). For I1, 2,2-dimethylpropane-1,3-diamine (0.20 g, 2 mmol) and salicylaldehyde (0.48 g, 4 mmol) were mixed in methanol (30 ml). The stirred mixture was refluxed for 6 h. The solution was filtered, and the filtrate volume was reduced to ca. 10 ml. The obtained compound was washed with methanol and dried in a desiccator. For I2, 2,2-dimethylpropane-1,3-diamine (0.20 g, 2 mmol) and 2-hydroxy-3-methoxybenzaldehyde (0.60 g, 4 mmol) were mixed in methanol (30 ml). The stirred mixture was refluxed for 6 h. The solution was filtered, and the filtrate volume was reduced to ca. 10 ml. The obtained compound was washed with ethanol and dried in a desiccator. For I3, 2,2-dimethylpropane-1,3-diamine (0.20 g, 2 mmol) and 4-hydroxy-3-methoxybenzaldehyde (0.60 g, 4 mmol) were mixed in ethanol (30 ml). The stirred mixture was refluxed for 2 h. The solution was filtered, and the filtrate volume was reduced to ca. 10 ml. The obtained compound was washed with ethanol and dried in vacuo.

The concentration of inhibitors employed was 1 mg/L. Figure 1 shows the Schiff base ligands of this study.

Fig. 1

The chemical structure of the inhibitors

Atolab device was used to perform polarization tests. The tests were performed on a standard cell containing platinum wire as an auxiliary electrode and calomel as a reference electrode. The experiments were performed at room temperature. At the beginning of each experiment, an interval of about 30 min was applied to stabilize the potential of the components in the solution from the potential range of about − 500 to 100 mV compared to the open circuit potential and scan rate of 1 mV/s.

An electrochemical test was performed for steel samples after 30 min of immersion in the solutions. For this purpose, a three-electrode electrochemical cell including counter electrode (graphite), reference electrode (calomel electrode), and working electrode (steel sample) is connected to a computer-controlled AutoLab potentiostat/galvanostat system (PGSTAT 302) and tested in open circuit potential in the frequency range of 100 kHz to 10 MHz with a peak amplitude of 10 mV A.C.

The Frontier molecular orbitals (FMOs) were studied using the Materials Studio software program’s DMol3 module (Oyeneyin et al. 2022; Quraishi et al. 2009; Timoudan et al. 2024; Zaroual et al. 2024). The dual scalar and polarization basis set (DNP) (Zhao and Truhlar 2008; Furtado et al. 2022) in combination with functional M06-L (M. Ben Hadj Ayed, T. Osmani, N. Issaoui, A. Berisha, B. Oujia, H. Ghalla 2019; Mardirossian and Head-Gordon 2017; Klamt 2005) was utilized to improve the geometrical features of all inhibitors in order to have a better understanding of the electronic correlation between them. In order for the field to converge in a self-consistent approach, there has to be a difference in energy of less than 10⁻⁷ Ha. For the purpose of including the solvent, water, in the density functional theory (DFT) computations, the conductor-like screening model (COSMO) was utilized (Klamt 2018; Berisha 2019a; Faydy et al. 2021).

To study the plausibility of interactions between the inhibitor molecules and the iron (110) surface atoms, the Adsorption Locator tool in Material Studio was utilized in conjunction with Molecular and Molecular Dynamics software. A cell with the dimensions 2.482 nm × 2.482 nm × 1.32 nm, in which a vacuum slab extends 3.5 nm on top of the cell’s Fe(110) surface.

The MC simulations were carried out by continually loading one inhibitor molecule (I1, I2, or I3) into a simulated cell, along with corrosive species such the following: H₂O (800), Na⁺ (5), Ca²⁺ (2), Mg²⁺ (2), and Cl⁻ (13) and simulating the effects of these corrosive species using the Adsorption locator module in Material Studio. Because it was believed that this layer would include reasonable sites for the inhibitor molecule to adsorb, the target atoms for the MC simulation were specifically selected from the very top layer of the Fe(110) slab. As a direct result of drawing the conclusion of the simulation, the equilibrium adsorption configuration that required the least amount of energy to maintain was investigated and characterized. The subsequent phase in our process was making use of the well-known COMPASS forcefield and running MD simulations in the Forcite module of Material Studio at a temperature of 298 K with all of the Fe(110) atoms, with the exception of the top two layers, frozen (with a time step of 1.0 fs). The simulation acquired 2 ns (Dagdag et al. 2021a; Jafari et al. 2022a). Using the trajectory of the MD simulation, a radial distribution function, or RDF, was made in order to make a classification of how the inhibitor molecule binds.

The solubility test of the prepared Schiff base ligand was determined in different solvents such as distilled water, ethanol, methanol, dimethylsulfoxide, dimethylformamide, acetone, and acetic acid by shaking a small amount of each of the compounds in a test tube containing 10 ml portions of each of the solvents.

The solubility of the Schiff base ligands was determined in different solvents. From the result of the solubility test, it can be observed that the ligands were soluble in hydrochloric acid, dimethylsulfoxide, and dimethylformamide; slightly soluble in ethanol and methanol; and insoluble in water and soluble in acetone.

The steel samples were exposed to 1 mgL⁻¹ of these compounds in the artificial seawater for 7 days of immersion at ambient temperature. Then, the changes that occurred on the surface were evaluated by atomic force microscope (AFM) NanoSurf easyscan2.

Results

Open-circuit potential (OCP)

The potential created between seawater and the samples, which is the steel surface under study, in relation to a SCE electrode that will be put into the solution near the working electrode, is known as the open circuit potential. The collected results showed that before the impedance and polarization experiments started, the solutions under investigation had acquired a satisfactory level of stability. The OCP value first varied for blank before stabilizing. However, when the samples were added to 1 mg/L of three ligands, I1, I2, and I3, there was a discernible fluctuation that shifted in a negative direction throughout the course of the time. The value of OCP remained stable and was trending upward. Three ligands, I1, I2, and I3, had steady and unstable adsorption on the steel sample’s surface. It could be the cause of behavioral abnormalities (Fig. 2).

Fig. 2

OCP curve

Potentiodynamic polarization results

To understand the interactions between inhibitor ligands and steel in an acidic solution, the PDP study was performed at different concentrations of I1, I2, and I3. Figure 2 shows the PDP curves for carbon steel without and with the addition of I1, I2, and I3 molecules in the corrosive medium at a temperature of 25℃ for 3 days. The electrochemical settings are summarized in Table 1.

Fig. 3

Potentiodynamic plots for corrosion of steel (with and without inhibitors) in Demineralised water

Table 1 Potentiodynamic polarization measurements for steel in water for 3 days

Examination of the electrochemical parameters related to the graphs in Fig. 3 reveals that the addition of organic molecules I is reflected by a displacement of the current intensity towards lower values compared to the value calculated in the control, and an efficiency more marked inhibitor. Furthermore, it is clearly observed that the addition of three ligands, I1, I2, and I3, in a corrosive environment shifts the corrosion potential towards more cathodic values compared to the control. This results in a reduction in anodic and cathodic current densities, thus slowing down the metal dissolution in the acidic solution (Dagdag et al. 2021a; Rahimi et al. 2022). Apparently, the incorporation of I1, I2, and I3 ligands in the acidic solution induces significant changes in the slopes of the anodic and cathodic Tafel curves, suggesting alterations in reaction mechanisms. However, unlike the cathodic behavior, the slope of the anodic Tafel curve decreases compared to the inhibitor-free environment, indicating that the organic molecules of I ligands significantly slow down the rate of metal degradation by modifying the anodic reaction mechanism (Bhardwaj et al. 2022). Similarly, the results in Table 1 indicate that the addition of these ligands in the corrosive environment has no significant effect on the anodic and cathodic slope values. The cathodic Tafel curves remain largely parallel in inhibited and uninhibited solutions. This reflects the remarkable influence of the ligands on inhibiting the hydrogen evolution reaction rate without altering the mechanism of this reaction through a simple blocking effect, making it difficult for H⁺ ions to reach the metal surface and undergo reduction (Ganjoo et al. 2022; Daoudi et al. 2022; Ouass et al. 2021a). Consequently, the reduction of H⁺ ions can only occur on the unprotected metal surface or if H⁺ ions reach the carbon steel surface through the partially blocked pores of the inhibitor film, which occurs through a charge transfer mechanism (Damej et al. 1254). On the other hand, Fig. 2 clearly shows that I1, I2, and I3 act as a mixed-type corrosion inhibitor. As a result, it can be stated that these ligands reduce both the H⁺ ion reduction reaction and the iron degradation reaction (Ould Abdelwedoud et al. 2022; Mehmeti 2022). However, the inhibitory effectiveness of the three synthesized organic compounds decreases in the order I3 > I2 > I1, with an excellent inhibitory efficiency reaching 71% for I3. This ranking can be attributed to the structure of the inhibitors, including the presence of functional groups, substitution groups, the number of heteroatoms, and π electrons. The presence of the methoxy (− OCH₃) and hydroxyl (− OH) groups enhances the protection provided by these ligands. Therefore, the adsorption of I3 on the steel surface is more pronounced compared to other ligand, which has a flat structure.

This suggests that a change in compound does not change the steps involved in the anodic and cathodic reactions. There is a + 2 to + 4 mV change in corrosion potential with adding inhibitors for steel in water. With the introduction of coating, the maximum displacement of lower than − 38 mV in corrosion potential was observed. These insights that the blocking effect of compounds and its filler is more at the cathode compared to the anode. The corrosion inhibition efficiencies of the ligands were reduced in the following order: I3 > I1 ≈ I2.

Electrochemical impedance spectroscopy

Figure 3 represents Bode and Nyquist plots. Nyquist plot obtained by taking an imaginary part of impedance on the Y-axis and the real part of impedance on the X-axis for the corrosion of steel with and without coating.

Fig. 4

a. Nyquist plot for corrosion of steel in water: (1) blank, (2) I1, (3) I2, (4) I3. b. Bode plot for corrosion of steel in water: (1) blank, (2) I1, (3) I2, (4) I3

As observed from Fig. 4, the shape of the curves is half-circle, which implies that the transfer of charges takes place between steel and water.

The semi-circles were diminished, this may be due to non-homogeneity on the metal surface causing the variation in frequency. The non-homogeneity may be due to the deposition of ferric oxide or other ions on the steel surface. The semi-circle diameter represents the resistance to the exchange of ions at the mild steel-fresh/demineralized water interface (R_ct). With adding the inhibitors, the diameter of semicircles increases, this observation may be due to the decrease in the mobility of ions of water and reactivity of the steel surface.

With additional inhibitors, C.E. increased that shows an increase in the adsorbing tendency of the compounds does not efficiently increase the charge transfer at the metal solution interface, or decreased charges at the double layer due to an increase in viscosity of the medium, or the number of reactive sites is decreased due to decrease in conductivity of steel. Due to the combined effect of all or any one to two effects may be dominating with adding inhibitors.

The Nyquist plots are evaluated by applying the impedance Excel data to the Zimpfin software to get the appropriate equivalent circuits. The satisfactory equivalent circuit is shown in Fig. 5.

Fig. 5

Equivalent circuit for impedance data fitted for steel in water

R_s is the resistance offered by the solution present between anode and cathode, constant phase element 1 (CPE 1), the electrical double layer dielectric constant, CPE 2 capacitance of the passive ferric oxide film, and R_f film resistance. Due to the irregular surface, the charged layer present on the steel surface and water boundary is not a perfect capacitor. Hence, it is replaced by CPE (Fakhry et al. 2024; Alamiery et al. 2021).

The CPE impedance is calculated using Eq. (1):

$$Z={Q}^{-1} (\text{iw}{)}^{-n}$$

(1)

The Q is the proportionality coefficient; w = 2 πfmax is the angular frequency; fmax is the frequency at which the imaginary component of the impedance is maximum, i is the imaginary number, and the power n is associated with the phase shift. If n = 1, then the charged double layer at the boundary behaves like an ideal capacitor. The correction in the capacitance to its real value is calculated using Eq. (2).

$${C}_{\text{dl}}=Q(w{)}^{n-1}$$

(2)

The C_dl at the steel water with and without coatings was evaluated using (3).

$${C}_{\text{dl}}=1/2 \pi \text{fmax}. {R}_{p}$$

(3)

The polarization resistance R_p at the steel water with and without coatings was calculated using (4).

$${R}_{p}={R}_{\text{ct}}+{R}_{f}+{R}_{s}$$

(4)

From Table 2 and Fig. 3, it is observed that R_s with and without inhibitors in water have the nearly same value. By introducing inhibitors on steel R_p value rises, and C_dl values suppress. It confirms the blocking of active centers present on steel surfaces by the molecules of Schiff bases. The inhibition effect of compounds was enhanced by adding I3. At the steel surface and water interface net exchange of charges decreases (resistance to charge transfer R_ct increases) and the thickness of the electrical double layer increases due to the presence of inhibitors of big Schiff-base molecules. Therefore, by introducing inhibitors, capacitance rises, and resistance to shifting equilibrium potential from the net corrosion current (R_p) rises at the steel and water interface increases.

Table 2 Impedance data for steel in water for 3 days

Density functional theory

In the preliminary step of the approach, a conformer search (Faydy et al. 2023; Akkermans et al. 2020) was performed utilizing the Boltzmann jump search method, using 2000 as the number of searched conformer structures and the COMPASS III forcefield (Molhi et al. 2021a). This was done in order to acquire the lowest feasible start energy for the molecule while also quickening the density functional theory (DFT) computations. DFT calculations began by picking the conformer with the lowest energy level, as illustrated in Fig. 6.

Fig. 6

The conformer search's energy profile and the inhibitors' corresponding lowest energy structures

The sigma-profile charge density curve is created using calculations based on the COSMO model. The electrostatic potential in COSMO is represented by the use of partly charged atomic nuclei (Faydy et al. 2021; Berisha 2019b; Rahimi et al. 2022). Figure 7 is an illustration that depicts how an inhibitor can perform either the role of an acceptor or a donor of H-bonds.

Fig. 7

COSMO profiles of the inhibitors

H-bond acceptor/donor interactions between water molecules occur when an inhibitor is dissolved in water. This property controls how soluble the inhibitor is (Bhardwaj et al. 2022; Ganjoo et al. 2022).

As seen in Fig. 8, HOMO in the inhibitor molecules is positioned on one side of the ring containing O atoms, whereas LUMO is located on the opposite side of the ring, implying that these parts of the molecule can are prone to electron transfer to/from the metal’s surface (Daoudi et al. 2022; Ouass et al. 2021a; Damej et al. 1254).

Fig. 8

HOMO, LUMO and ESP of the inhibitors

This exchange of electrons consequently results in the generation of a protective organic layer that coats the surface of the metal and preserves it from corrosion (Akkermans et al. 2020; Bhardwaj et al. 2022; Ganjoo et al. 2022; Ould Abdelwedoud et al. 2022; Mehmeti 2022). Electron acceptors, also called LUMOs, are parts of an inhibitor’s structure that are responsible for taking electrons from a surface with a lot of electron density, like the surface of a metal (Bhardwaj et al. 2022; Daoudi et al. 2022; Ouass et al. 2021a). As a result of the exchange of lone pair electrons between heteroatoms (N and O) and the vacant iron d-orbital, which consequences in a moderate rise in surface absorption potential, it is presumed that adsorption on the surface of the metal will be significantly greater. This is because the exchange of lone pair electrons results in a moderate rise in surface absorption potential (Bhardwaj et al. 2022; Mehmeti 2022; Jafari et al. 2022b).

Table 3 contains the most regularly used descriptors, which are organized by frequency of usage (the equations used to calculate them can be found in the following references) (Damej et al. 1254; B. Ould abdelwedoud, M. Damej, K. Tassaoui, Berisha, H. Tachallait, K. Bougrin, V. Mehmeti, M. Benmessaoud 2022; Dagdag et al. 2021b). DFT simulations of inhibitor adsorption can help understand inhibitor adsorption mechanisms. Numerous studies suggest that the inhibitors’ ability to exchange electrons with Fe(110) supports its adsorption. The inhibitors’ low electron affinity and large ionization potential sustenance this interpretation (Table 3). Chemical softness and hardness predict the inhibitor’s metal-surface adsorption affinity (Dagdag et al. 2021a; Berisha 2021; Kalai et al. 1191).

Table 3 Calculated theoretical chemical parameters for the inhibitors

High values of chemical softness are also expected to show the inhibitor’s affinity for the metal surface. Inhibitors have an ∆E value in the range of − 1, which reflects their aptitude to receive electrons from the Fe(110) surface.

The Mulliken atomic charges (MAC) are a key factor in determining which atoms, typically known as inhibitory sites, are involved in the process of metal adsorption (Bhardwaj et al. 2022; Mehmeti 2022; Molhi et al. 2021b).

Numerous studies have shown that it is more probable for Fe(111) O surface atoms and inhibitor molecules to interact when the inhibitor atoms have a negative atomic charge (MAC). This has been shown both experimentally and theoretically. The MAC values of the inhibitors are displayed in Fig. 9, and these are the atoms that we are interested in. The atoms of oxygen and nitrogen in the inhibitors have significant negative charges, which indicates that these centers contain the greatest electron density and are thus able to adhere to metal surfaces with the greatest degree of success. Figure 7 presents the molecular electrostatic potential, also known as MEP, of the inhibitors at varying concentrations (area in red) (Jafari et al. 2022a; Berisha 2019b; Haldhar et al. 2021; Guo et al. 2014).

Fig. 9

Mulliken atomic charge (MAC) values for heteroatoms in the selected inhibitor atoms

Monte Carlo and molecular dynamic simulations

Throughout this case, the adsorption energy may be easily calculated by beginning with the Fe(110) surface. Adsorption energy may be calculated using the following equation: (Eads) (Berisha 2021; Hsissou et al. 2019; Dagdag et al. 2019, 2020; Hsissou et al. 2019; Abbout et al. 2020):

$${E}_{\text{adsorption}}={E}_{\text{Fe}{\left(110\right)}_{||}\text{inhibitor} }-\left({\text{Fe}}_{\left(110\right)}+{E}_{\text{inhibitor}}\right)$$

(5)

where ${E}_{\text{Fe}{\left(110\right)}_{||}\text{inhibitor}}$ is the total energy of the simulated system, E_Fe, and ${E}_{\text{inhibitor}}$ is the total energy of the Fe(110) surface and the corresponding free inhibitor molecules.

As a last step in verifying the accuracy of the MC calculations, the inhibitor’s adsorption geometry was examined in detail. Checking whether equilibrium can be reached in the MC simulation using the steady-state energy levels. As the simulation progressed, the system eventually settled into its lowest-energy condition. Inhibitor arrangement on a possible Fe(110) plane is shown in Fig. 6. The backbone of an inhibitor molecule adhering to the surface atoms of the Fe(110) plane may be responsible for the observed adsorption pattern (directed by heteroatoms, mainly O) (Ouass et al. 2021b; Jessima et al. 2020).

Adsorption is caused when molecules exhibit their heteroatoms and electron rings at the surface, which is what gives them the capacity to absorb. Adsorption is what gives molecules their ability to absorb (Akkermans et al. 2020; Ganjoo et al. 2022). The adsorption of inhibitors results in the formation of massive E_ads (Fig. 10) on the surface of the metal. These inhibitor compounds have a significant adsorption interaction with the metal due to the exceptionally high adsorption energies that they possess. By making use of this contact, a protective layer may be formed, which serves to shield the metal surface from the effects of corrosion. Based on the E_ads values that were found through MC, the order of corrosion inhibition performance of the inhibitors is I3 > I1 ≈ I2. The MD model of adsorption dynamics is generally acknowledged to be the more accurate of the two (Dagdag et al. 2021a; Faydy et al. 2023; Akkermans et al. 2020; Berisha 2019b; Ouass et al. 2021a). After several hundreds of ps of NVT simulation, it is clear that the inhibitors in Fig. 10 adopt a horizontal structure on one side of the molecule rings onto the metal surface and are substantially adsorbed onto the Fe surface (Fig. 11).

Fig. 10

The "image" of the examined inhibitor molecules on Fe(110) with the lowest energy, as calculated by MC and MD

Fig. 11

Distribution of the adsorption energies for the inhibitors obtained via MC

Adsorption processes are generally portrayed on the RDF graph of metal surfaces if peaks form at a specific distance from the metal surface (Jessima et al. 2020). When the heights are in the zone of 1–3.5 Å in range, it is thought to indicate a chemisorbable process. On the other hand, for physical adsorption, RDF peaks are expected to be present at distances larger than 3.5 Å (Bhardwaj et al. 2022; Ganjoo et al. 2022; Daoudi et al. 2022; Ouass et al. 2021a; Damej et al. 1254).

Fe surface and inhibitor O and N atoms had RDF peak values at distances of less than 3.5 Å—except the I1 molecule, where it seems that the N atoms have insignificant adsorption contribution to this molecule (Fig. 12) (Akkermans et al. 2020; Bhardwaj et al. 2022; Mehmeti 2022). As seen by its comparatively high negative energy value and RDF peaks, in this instance, the inhibitors seem to be significantly interacting with the surface of the metal through the O/N atoms.

Fig. 12

The MD trajectory analysis was used to determine the RDF of heteroatoms (O and N) for the inhibitors that were adsorbed on the Fe (110) surface

Surface analysis

AFM has been widely utilized in the corrosion field to assess the 3D corrosion morphology of steel surfaces in recent research. The three-dimensional AFM images of polished steel surfaces, and specimens immersed in the artificial seawater with and without I1, I2, and I3, are shown in Fig. 13a, b, c, and d, respectively. The images obtained in the presence of I3 exhibited a smoother surface. Average surface roughness (R_a) values obtained for the artificial seawater in the absence and presence of inhibitor are 887 nm for blank, 195 nm for I1, 158 nm for I2, and 105 nm for I3. The decrease in the R_a values clearly demonstrates the adsorption of inhibitor molecules on the steel surface (Jafari and Sayin 2015b).

Fig. 13

Surface of steel electrode by AFM microscope at artificial seawater: (a) without, (b) with 1 mg/L I1 (c) with 1 mg/L I2 and (d) with 1 mg/L I3

Conclusion

The inhibition efficiency of corrosion protection of API 5L B in seawater was increased with an increase in the addition of inhibitors. Three compounds have symmetrical two parts, and there are two benzene rings and two imine groups in each ligand. The adsorption of aromatic rings and heteroatom centers on the steel surface and the interaction of aliphatic moieties with the steel surface caused these compounds to have a good performance in protecting the steel against corrosion. The inhibiting effect of these compounds due to their adsorption operates via the formation of a barrier film on the metal surface. I3 has the best adsorption effect of these compounds; an important structural factor is a linear structure because it raises electrostatic interaction between Schiff bases and API 5L surface. Regarding this point, I3 is capable of forming a strong bond with the steel surface. Furthermore, electronegative O atoms helped the adsorption of this ligand on the steel surface. I1 and I2 are only different from the molecular structures of the substituents near the nitrogen atom. According to the results of the MD calculations, the inhibitor is mostly flat-adsorbed. The outcomes of the experiment provide the hypothesis that the inhibitor possesses large adsorption energies of the negative variety.