Design of New Ecofriendly Schiff Base Inhibitors for Carbon Steel Corrosion Protection in Acidic Solutions: Electrochemical, Surface, and Theoretical Studies

Abstract

Corrosion poses a significant problem for several industrial sectors, inducing continuous research and development of corrosion inhibitors for use across a wide range of industrial applications. Here, we report the effectiveness of three newly developed Schiff bases derived from amino acids and 4-aminoacetophenone, namely, AIP, AMB, and AImP, as environmentally friendly corrosion inhibitors for Q235 steel in hydrochloric acid using electrochemical and surface analyses, in addition to theoretical techniques. The electrochemical findings of potentiodynamic polarization (PDP) demonstrated that the explored compounds serve as mixed-type inhibitors and can effectively suppress steel corrosion, with maximal protection efficiencies of 93.15, 96.01, and 77.03% in the presence of AIP, AMB, and AImP, respectively, at a concentration of 10 mM. The electrochemical impedance spectroscopy (EIS) and polarization results confirmed the growth of a durable protective barrier on the steel surface in the existence of the inhibitors, which is responsible for decreasing the metallic dissolution. Results were further supported by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), UV–vis, and Fourier transform infrared (FTIR), which ascribed the development of inhibitor–adsorption films on the steel surface. The results of EDS and XPS analyses demonstrated the existence of the distinctive elements of the inhibitors on the metallic surface. Furthermore, density functional theory (DFT) calculations and Monte Carlo (MC) simulations showed the electronic structure of the examined inhibitors and their optimized adsorption configurations on the steel surface, which helped in explaining the anticorrosion mechanism. Finally, the theoretical and experimental findings exhibit a high degree of consistency.

1. Introduction

Corrosion affects the vast majority of manufacturing industries and can have annual costs in the billions of dollars. Acids and bases are frequently encountered in industrial processes because of their proximity to these environments.¹ They have a damaging effect on the environment. On the other hand, acids play a crucial role in the industrial sector, particularly in the cleaning systems used in the food industry, industrial cleaning, acid scaling, refining crude oil, acid pickling, and petrochemical processes.² In factories, acid treatments are often used to eliminate rust and scale. Steel and other metal types are often cleaned with hydrochloric acid. Most of the time, inhibitors are frequently used in these procedures to regulate how much metal dissolves. Economic, safety, and environmental concerns all point to improved methods for preventing metal corrosion.³ Strategies that try to quench, delay, and halt anodic, cathodic, or both processes can help reduce it. Metal corrosion can be significantly reduced by applying several strategies, such as material modernization, alloying, environmental cleansing, various coatings, and inhibitors.⁴ Corrosion inhibitors must be used to protect underlying materials from corrosion by providing adsorption and excellent anticorrosive properties.⁵ Inhibitors are commonly utilized in these procedures to regulate metal solubility.⁶ Acid corrosion inhibitors typically comprise organic molecules’ nitrogen, sulfur, and oxygen atoms. Organic compounds rich in nitrogen have long been recognized as effective corrosion inhibitors in dilute hydrochloric acid solutions.⁷ Corrosion inhibitors, like heterocycle compounds, slow the corrosion process when applied to a corrosive medium in small amounts; they are considered an effective and cost-efficient technique. Many recent studies have used this strategy to develop corrosion control methods.⁸ An inhibitor’s effectiveness depends on several elements, such as its solubility, reaction to the corrosive medium, and effective centers that can interact with the metallic surface. Because of their abundance of active regions, functional groups, and π-electrons, aromatic heterocyclics are among the most frequently employed materials as anticorrosion compounds.⁹ This is because they readily interact with the metallic surface forming a protective barrier through chemical adsorption, physical adsorption, or both.¹⁰

Schiff bases have gained attraction as corrosion inhibitors due to their superior adsorption and film-forming capabilities. The coordination of the imine group with the ions on the metallic surface is responsible for this strong behavior. Generally, Schiff bases are created from low-cost precursors by combining an aldehyde or ketone with the condensation-NH₂ moiety.¹¹ In addition, Schiff bases derived from amino acids have attracted the attention of researchers owing to their use in numerous pharmacological applications and other fields.¹²⁻¹⁴ Furthermore, they are considered suitable anticorrosion materials due to their efficacy against corrosion problems in multiple environments. The relationship among the molecular structure of these derivatives and their protection efficiency is strong.¹⁵ In their planar structures, N and S atoms contain electron pairs.¹⁶ Due to the presence of π-electrons in their chemical structures, these inhibitors readily adsorb on the metal surface.¹⁷ Temperatures and the acidity of a solution, among others, influence the corrosion rate of carbon steel; along with numerous industrial and environmental factors, these factors strongly correlate with the economic costs of various manufacturing disciplines. Therefore, researchers have increased their efforts to discover effective strategies to mitigate the corrosion of these facilities, particularly oil factories that are susceptible to these conditions due to their work. Anticorrosion chemicals are harmful,¹⁸ so chemists produce corrosive inhibitors with no toxic units and high inhibitory efficiency. Thus, biodegradable and nontoxic inhibitors are increasingly important. Schiff bases derived from amino acids were extensively recognized as green corrosion inhibitors mainly owing to their nontoxic nature and biodegradable characteristics. There have been many studies on the use of Schiff bases as corrosion inhibitors.¹⁹⁻²² The outstanding performance of amino acid derivatives as anticorrosion agents in various media motivates our laboratory team to seek the materials with the highest inhibition efficiencies to supplement our previous research in this field. In this context, the current study details the synthesis of three Schiff bases derived from amino acids and 4-aminoacetophenone, namely, (S,E)-2-(1-(4-aminophenyl)ethylideneamino)-4-(methylthio)butanoic acid (AIP), (S,Z)-2-((1-(4-aminophenyl)ethylidene)amino)-3-(1H-indol-3-yl)propanoic acid (AMB), and (S)-2-((1-(4-aminophenyl)ethylidene)amino)-3-(1H-imidazol-4-yl)propanoic acid (AImP). Synthesized AIP, AMB, and AImP inhibitors have lone pair electrons with nitrogen, oxygen, and sulfur and unsaturated π-electrons and carbon chains (producing a hydrophobic effect). This meets the requirements for a compound to bind to the surface of steel and suppress corrosion. Electrochemical measurement techniques were used to study and test the prepared compounds’ ability to inhibit the carbon steel corrosion used to make Iraqi oil tanks. Several spectroscopic methods were used to confirm the electrochemical results. These included scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), ultraviolet–visible (UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Also, density functional theory (DFT) and Monte Carlo (MC) simulations were conducted to delve deeper into corrosion protection mechanisms.

2. Experimental Techniques

2.1. Synthesis of 4-Aminoacetophenone and Amino Acid Derivative Inhibitors

1 mmol of 4-aminoacetophenone was dissolved in 10 mL of methanol solvent, with continuous stirring until complete dissolution; then, two drops of glacial acetic acid was added as a catalyst. Then, 1 mmol of the amino acids (methionine, tryptophan, and histidine) was dissolved in 15 mL of methanol solvent. The amino acid solution was added slowly to the aromatic ketone, and the mixture was refluxed for 8–10 h at a temperature of 60 °C in a water bath. All products were washed, dried, and recrystallized. The product’s amino acid derivatives were solid powders. Thin-layer chromatography followed the reaction using the mobile phase at 4:1 ethyl alcohol to benzene. Scheme 1 shows the preparation scheme for synthesized Schiff base derivatives.

Scheme 1. Scheme for Preparation of Schiff Base Derivatives AIP, AMB, and AImP.

2.2. Electrodes and Electrolytes

The dimensions of the Q235 steel sample used in the electrochemical measurements were 1 × 1 × 0.5 cm², with a total surface area of 1 cm². The composition was C = 0.174, Mn = 1.361, Si = 0.367, S = 0.003, P = 0.0116, Ni = 0.033, Cu = 0.012, and Fe = 98.037 by weight. For removing the undesirable layer from the steel specimen’s surface, grades of emery paper ranging from 80 to 2000 were utilized for cleansing and polishing. Then, demineralized purified water and acetone were utilized for washing followed by dehydrating the samples before the test. The necessary hydrochloric acid solution with a concentration of 1.0 M was made by diluting concentrated HCl with demineralized pure H₂O. Merck supplied concentrated hydrochloric acid (37%). Subsequently, multiple molar concentrations of each amino acid derivative were prepared by using diluted hydrochloric acid at a concentration of 1.0 M. The molecular weight of each amino acid derivative determined these concentrations. The studied Schiff base derivatives have exhibited excellent solubility in acidic water after dissolving in a very little amount of ethanol. Moreover, because the chosen corrosive electrolyte undergoes a process of spontaneous corrosion, no external stimulation or shaking is required for the process to progress.

2.3. Electrochemical Technique

A Q235 steel electrode with a cm² active surface area is used in an electrochemical cell. An epoxy holder supported the Q235 steel sample as a working electrode (WE). It employed a saturated calomel electrode (SCE) as the reference electrode and used platinum metal for the counter electrode (CE). Various concentrations of the explored compounds in the acidic electrolyte were applied to the working electrode in a volume of 200 mL. The Gamry framework software (version 7.10) was used to acquire data from the potentiostat, galvanostat, and ZRA analyzer used in electrochemical analysis. Gamry Echem Analyst (version 6.23) was used to determine the output data’s fitting and graphing. Before conducting every electrochemical test, the Q235 steel electrode was immersed in an acid electrolyte for 1800 s to attain a stable, steady state for the open circuit potential (OCP). The potential values for potentiodynamic polarization (PDP) ranged from −0.4 to 0.6 V at a scan rate of 0.2 mV s^–1 at 25 °C. In addition, the amended values for electrochemical impedance spectroscopy (EIS) were extremely small voltage (±10 mV) with a frequency range of 1 Hz to 100 kHz and 10 points per decade at 25 °C.

2.4. UV–Visible, FTIR, XPS, SEM, and EDS Mapping Spectral Surface Analyses

The complex formation among the explored amino acid derivatives and Q235 steel cations was demonstrated by monitoring the variations in wavelength values after 48 h of exposure to a corrosive media at an ambient temperature (using a UV–vis spectrophotometer; specifically, a double-beam UV–vis spectrophotometer, 190–1100 nm, Shimadzu brand). In addition, the FTIR (Bruker brand at a range of 400–4000 cm^–1) technique was used to analyze the inhibitors and the film produced on the Q235 steel surface after immersion in a 1.0 M HCl solution including the inhibitors to illustrate the adsorption process of the examined compounds under study (AIP, AMB, and AImP). XPS test was performed by an ESCA LAB 250Xi (Thermo Scientific, U.K.). Al Kα radiation (1486.6 eV) with a spot size of 850 μm was used as the X-ray irradiation source. The spectral acquisitions were collected and deconvoluted with Avantage software, version v5.956. SEM and EDS mapping capabilities (JEOL, IT200 SEM) were utilized for surface morphological examination. Coupons of Q235 steel with dimensions of 2 cm × 2 cm × 0.5 cm were abraded and cleansed with abrasive paper, with grades ranging from 1000 to 2000 prior to being exposed to the corrosive media for 24 h and subsequent examination. Before the investigation, the Q235 steel samples were washed with demineralized H₂O and dehydrated after 24 h. The steel coupons were affixed to the sample holder, and SEM testing was conducted at high magnification to acquire an excellently detailed image of the Q235 sample under examination. A mapping-enhanced EDS was operated to determine the organic elements absorbed to the surface of the steel. This was done to show that the complexation of the synthesized derivatives and Q235 steel cations protects the surface.

2.5. Theoretical Computations

DFT calculations executed via B₃LYP-functional with the DNP 4.4 basis set in the Dmol³ module of BIOVIA Materials Studio 2017 software were applied to investigate the energy minimization of AIP, AMB, and AImP molecules in aqueous conditions.²³ The DFT parameters, involving E_HOMO, E_LUMO, the energy gap (ΔE), electronegativity (χ), hardness (η), global softness (σ), electrophilicity index (ω) and the number of electrons transported (ΔN), ΔE_{back-donation}, and dipole moment (μ), have been calculated using the following formulas²⁴

1

2

3

4

where φ is the work function of Fe(110), χ_inh is the inhibitor electronegativity, η_Fe is the hardness of Fe (0 eV), and η_inh is the chemical hardness of the inhibitor.

For MC simulations, the adsorption locator module in BIOVIA Materials Studio 2017 software was utilized to discover the optimum adsorption arrangements of the AIP, AMB, and AImP on the Fe(110) surface.²⁵ The optimization of the adsorbate molecules was performed using the COMPASS force field.²⁶ Then, in a simulation box (37.24 Å × 37.24 Å × 59.81 Å), the adsorption of the investigated compounds, chloride ions, hydronium ions, and H₂O molecules on the Fe(110) surface was accomplished.²⁷

3. Results and Discussion

3.1. Confirmation of Amino Acid Derivative Synthesis

The physicochemical parameters of the AIP, AMB, and AImP have been determined, including color, melting point, and elemental percentages. The resulting AIP compound is a light brown powder that is a solid. With a melting point of 205–207 °C, the yield was 73%. The AMB and AImP yield and melting point values were 64% (200–202 °C) and 85% (204–206 °C), respectively. The brown powder represented the AMB component, whereas the yellow powder represented the AImP derivative. It is worth noting that ¹H and ¹³C NMR investigations revealed the molecular structures of AIP, AMB, and AImP. The ¹H and ¹³C NMR spectra are depicted in Figures 1 and 2, respectively. The ¹H NMR (D₂O) spectra of AIP, AMB, and AImP revealed distinct signals (Figure 1) at δ(ppm) = 1.9 (s, C–H3 aliphatic), 2 (s, C–H6 aliphatic), 2.4 (m, C–H3, 14 aliphatic), 3.4 (m, C–H11 aliphatic), 6.7 (m, C–H14, 16 aromatic), and 4.7 (m, C–H17, 18, 13, 2 aromatic) for AIP. The δ(ppm) values for the AMB are 2.5 (s, C–H17 aliphatic), 4.7 (d, C–H24, 23, 19, 7, 3 aliphatic), 7.6 (m, C–H10, 12, 9, 6 aromatic), 6.6 (m, C–H11, 20, 22 aromatic), and 3.4 (m, C–H4 aromatic). Furthermore, the singles for AImP are δ(ppm) = 2.7 (s, C–H13 aliphatic), 2.9 (m, C–H2 aliphatic), 4.6 (d, C–H4, 8, 9, 12 aromatic), 6.6 (C–H5, 7 aromatic), 7 (d, C–H15 aromatic), and 7.5 (d, C–H17, 16 aromatic). It is evident that molecular conformational differences account for the observed signal disparity. In addition, the ¹³C NMR (D₂O) of AIP, AMB, and AImP exhibited distinct signals, as depicted in Figure 2 at δ(ppm) = 14 (C6, C11), 32 (C3), 54 (C2), 114 (C14, C6), 131 (C12, C13, C17), 29 (C4), and 181 (C1, C13, C17) for AIP and 14 (C17), 32 (C4), 114 (C10, C11, C12, C9, C20, C22), 131 (C8, C6, C5, C13, C23, C19, C18), 117 (C4, C7), and 180 (C101) for the AMB structure. The chemical shift δ(ppm) = 25 (C2), 30.6 (C13), 113 (C5, C6), 115 (C15), 127 (C4, C8 aromatic), 133 (C14), 135 (C17), and 136 (C6) for the AImP.

Figure 1.

¹H NMR spectra of AIP, AMB, and AImP.

Figure 2.

¹³C NMR spectra of AIP, AMB, and AImP.

3.2. Open Circuit Potential (OCP) Measurements

To investigate the chemical characteristics and corrosion performance of the Q235 steel alloy, the OCP technique was evaluated as a function of immersion time. The OCP of Q235 steel as a function of the immersion time in 1.0 M HCl solution alone and with the explored inhibitors was measured and referred relative to the SCE electrode within a period of 30 min as presented in Figure 3. In the presence of the explored inhibitors, it is noticeable that the potential sharply shifted toward the more positive side (noble shift), indicating the buildup of a protecting film of the studied compounds on the metallic surface that suppresses anodic corrosion reaction and increases corrosion resistance. The potential values recorded for 1.0 M HCl alone (−0.556) and with 10 mM of AIP, AMB, and AImP are −0.539, −0.519, and −0.544 V_SCE, respectively. Accordingly, the effect of the inhibitors on OCP values is noticeable in all explored media, with less negative potentials obtained using the inhibitors compared to their absence, confirming their inhibiting action and the growth of a protective film at the steel/solution interface.²⁸

Figure 3.

OCP curves measured for Q235 steel in a 1.0 M HCl solution alone and with 10 mM AIP, AMB, and AImP.

3.3. Potentiodynamic Polarization

The PDP technique was utilized to investigate the ability of the synthesized inhibitors to hinder the corrosion of Q235 steel in 1.0 M HCl. The PDP curves of the Q235 steel alloy in 1.0 M HCl alone and in the existence of the examined compounds (AIP, AMB, and AImP) are depicted in Figure 4. The tests were conducted in aerated solutions after 30 min of electrode submerged in the studied solution to ensure reaching a steady state. The important electrochemical results involving corrosion potential (E_corr), and corrosion current density (i_corr) were achieved by extrapolation of the cathodic (β_c) and anodic (β_a) Tafel slopes and are summarized in Table 1. The results reveal noticeable changes in the corrosion potential and current values, according to the examined media. The results obviously demonstrate that the addition of the explored compounds resulted in a significant shift in the anodic and cathodic curves toward smaller current densities, implying that the anodic corrosion of Q235 steel and hydrogen ion reduction are retarded or slowed.

Figure 4.

Tafel plots for Q235 steel in 1.0 M HCl in the absence and existence of numerous concentrations of the examined inhibitors AIP (A), AMB (B), and AImP (C) at 298 K.

Table 1. Electrochemical Parameters for Q235 Steel in 1.0 M HCl in the Existence and Absence of Various Concentrations of AIP, AMB, and AImP Were Achieved by the PDP Technique at 25 °C.

inhibitors	conc. (mM)	icorr. (μA)	–Ecorr. (mV)	βc × 10–3 V decade–1	βa × 10–3 V decade–1	corrosion rate (mpy–1)	θ	ηPDP (%)
blank		590.0	567.0	116.4	82.50	269.7
AIP	3	88.90	523.0	130.5	81.90	38.44	0.8493	84.93
	5	65.32	522.0	158.0	83.70	27.07	0.8893	88.93
	10	40.44	519.0	143.3	102.2	18.69	0.9315	93.15
AMB	3	62.91	515.0	165.5	95.60	24.75	0.8934	89.34
	5	45.53	519.0	167.9	86.20	19.97	0.9228	92.28
	10	23.52	518.0	107.6	89.50	10.56	0.9601	96.01
AImP	3	193.2	510.0	144.2	86.10	90.41	0.6725	67.25
	5	162.1	517.0	114.9	74.40	72.12	0.7253	72.53
	10	135.5	530.0	157.1	85.30	61.2	0.7703	77.03

The inhibitive effectiveness of the inhibitors to suppress the alloy dissolution was obtained using the polarization curves with the following formula²⁹

5

where i_corr and i_corr(inh) are the corrosion current density in the absence and existence of the examined compounds, respectively.

The existence of the explored compounds increases the protection of steel alloy in HCl media, and as the investigated compound concentration was increased, a marked decrease in corrosion current was observed, with the best inhibition achieved using AMB inhibitor compared to AIP and AImP owing to the blockage of the active sites on the Q235 steel surface. Moreover, the investigated inhibitors could be categorized as mixed-type inhibitors because their presence causes the corrosion potential values to shift to less negative values with amounts less than 85 mV compared to when they are absent.³⁰

3.4. Electrochemical Impedance Spectroscopy (EIS)

The EIS technique is employed to find clearer insight into the mechanism and kinetics of electrochemical reactions that occur through the corrosion and inhibition processes, as well as to validate the PDP results.³¹ The impedance spectra for bare Q235 steel alone and in the presence of the examined inhibitors (AIP, AMB, and AImP) in 1.0 M HCl are displayed as Nyquist plots in Figure 5. The dotted lines in this figure characterize the recorded experimental results, and the solid lines correspond to the fitted results. The spectral representation of Nyquist plots shows comparable shapes in the absence and existence of the inhibitors, indicating that these inhibitors do not cause a morphological alteration in the corrosion mechanism. Furthermore, the Nyquist plots in the absence and existence of the inhibitors displayed a single imperfect semicircle, demonstrating that the corrosion process of Q235 steel involves a single charge-transfer operation.

Figure 5.

Nyquist plots for bare Q235 steel alone and in the existence of various amounts of the examined compounds AIP (A), AMB (B), and AImP (C) recorded in 1.0 M HCl at 298 K.

The Nyquist curve of the bare Q235 steel without the examined inhibitors is characterized by a depressing shape of semicircles in the high-frequency range, which signifies the roughness of steel alloy.³² When the investigated inhibitors were added, the semicircle shape of Nyquist spectra increased in comparison to the inhibitor’s free media, revealing their adsorption on the steel surface producing a compact protective film with higher impedance for the charge-transfer process, leading to a reduction in the corrosion process. Impedance curves grew in diameter when inhibitors were added and increased more with increasing their concentration; the AMB impedance curve exhibited the largest increase in diameter, followed by AIP and then AImP.

Figure 6 depicts a schematic representation of the equivalent circuit employed to fit the EIS results. The circuit consists of solution resistance (R_s), charge-transfer resistance (R_ct), and double-layer capacitance (C_dl). To accomplish a precise fit for the results, R_ct was changed with a constant phase element (CPE).

Figure 6.

Equivalent circuit was utilized to fit the impedance results of the bare steel alloy.

The impedance of CPE (Z_CPE) is illustrated in the following equation³³

6

where Y⁰ is the CPE constant, j is the imaginary number, ω is the angular frequency (measured in rad s^–1), and n is the shift in phase.

The resulting values could be used to clarify the morphological characteristics of metal surfaces deteriorated in corrosive solution. A larger value typically denotes significant smoothness of the surface. Additionally, when n = 0 and Y⁰ = R, the CPE denotes a resistive behavior. However, if n = 1 and Y⁰ = C, the CPE indicates capacitance. Meanwhile, if n = 0 and Y⁰ = 1/L, the CPE will denote an inductive behavior. Finally, when n = 1/2 and Y⁰ = W, the CPE describes a Warburg impedance.

The protection efficiency (η_EIS %) for Q235 steel corrosion was calculated using the R_ct values as shown below³⁴

7

where R_ct and R_ct(inh) are the charge-transfer resistances estimated in the absence and existence of the explored inhibitors, respectively.

The R_ct values recorded in Table 2 were determined from Nyquist plots by subtracting the impedance at the smaller and greater frequencies. A significant increase in the R_ct value was monitored in the presence of the investigated inhibitors in comparison with the uninhibited steel alloy. From Table 2 and Figure 5, it is apparent that with the increase of the investigated compound concentration, the R_ct values were increased, while the Y⁰ values corresponding to CPE_dl declined. This suggests that the adsorbed molecules of the investigated compounds on the metallic surface gradually increased and thus diminished the active sites that initiate the dissolution reaction. Furthermore, increasing R_ct demonstrated a significant inhibition of charge transfer in the protecting film caused by the adsorbed inhibitor on the steel surface, demonstrating excellent resistance to acidic attack and, as a result, controlling the extent of dissolution. Additionally, the values of C_dl have decreased as a result of the dielectric constant decline and/or the increase in the electric double-layer thickness caused by the adsorption of the examined compounds at the steel/electrolyte interface.^35,36

Table 2. Electrochemical Data Accomplished by the EIS Method for the Corrosion of Bare Q235 Steel Alloy alone and Inhibited with Numerous Concentrations of AIP, AMB, and AImP in 1.0 M HCl^a.

inhibitor	conc. (mM)	Rs (Ω cm2)	Rct (Ω cm2)	n	Cdl (μF cm–2)	θ	ηPDP (%)	φ2
blank		1.83	33	0.79	119.3
AIP	3	1.98	227	0.85	53.4	0.8546	85.46	0.00074
	5	2.12	281	0.88	44.3	0.8862	88.62	0.00065
	10	2.35	453	0.90	35.2	0.9271	92.71	0.00096
AMB	3	2.41	261	0.96	41.3	0.8736	87.36	0.00078
	5	1.80	525	0.92	32.1	0.9371	93.71	0.00054
	10	1.94	775	0.91	28.6	0.9574	95.74	0.00051
AImP	3	2.13	108	0.81	73.6	0.6944	69.44	0.00056
	5	2.45	124	0.80	69.4	0.7339	73.39	0.00078
	10	2.22	171	0.83	58.2	0.8070	80.70	0.00089

^aφ² is the goodness of the fit.

The Bode plots (impedance and phase angle vs frequency) for Q235 steel in 1.0 M HCl in the absence and existence of numerous amounts of AIP, AMB, and AImP are presented in Figure 7. The diagrams were used to understand the behavior of the adsorbed inhibiting film at the Q235 steel surface. The addition of the explored inhibitors showed a higher impedance value because of the difficulties of reacting at the interface. Moreover, a decrease in the phase angle value is less than 90°, demonstrating the roughness of the steel surface and its porous state because of the efficient adsorption of the explored inhibitors on the steel surface. The maximum impedance was recorded in the existence of the AMP inhibitor at the low-frequency region and increased with increasing its concentration, which granted the greatest corrosion inhibition to the steel alloy.

Figure 7.

Bode (A–C) and bode phase (A1–C1) plots for bare Q235 steel alone and in the existence of numerous amounts of AIP, AMB, and AImP, respectively, recorded in a 1.0 M HCl solution at 298 K.

In conclusion, the EIS and PDP results are in harmony, proving the pronounced effect of the explored inhibitors that can greatly enhance the anticorrosion performance of the Q235 steel alloy in acidic media.

3.5. SEM and EDS Analyses

The surface morphologies of Q235 steel samples were evaluated via SEM measurements after dipping in the corrosive medium (1.0 M HCl) alone and containing the investigated inhibitors (AIP, AMB, and AImP), and the SEM micrographs of the surfaces are presented in Figure 8. The metallic surface of steel after dipping in the aggressive media in the absence of a corrosion inhibitor is shown in Figure 8A in which a highly corroded surface was observed because of the destructive attack of the acidic solution on the steel specimen. Moreover, the surface was severely damaged and appeared to have apparent imperfections due to the corrosion process. In the case of steel samples submerged in HCl with 10 mM of the explored inhibitors, the surface morphology is obviously different, as depicted in Figure 8B–D. In the existence of AIP and AMB, the surfaces became smoother as well as less damaged compared with the surface immersed in HCl alone. This is caused by the formation of an inhibiting layer absorbed on the steel surface, which maintains the original surface state and thus inhibits corrosion. In the presence of AImP (Figure 8D), the cracks observed in the formed layer are probably caused by internal stresses that manifest as the thickness increases. Generally, it is evident according to the aforementioned images that their order of inhibition ability is AMB > AIP > AImP, which is compatible with the electrochemical findings.

Figure 8.

SEM images for the Q235 steel surface after dipping for 48 h in a 1.0 M HCl solution alone (A) and with 10 mM AIP (B), 10 mM AMB (C), and 10 mM AImP (D).

Figure 9 shows EDS spectra for Q235 steel after 48 h of immersion in 1.0 M HCl containing the optimum concentration (10 mM) of the investigated inhibitors (AIP, AMB, and AImP) at room temperature. The spectrum after immersion in 1.0 M HCl alone (Figure 9A) shows signals for Fe, C, and O related to the elements of the oxide film formed owing to the corrosion product buildup on the metallic surface. In the existence of the examined inhibitors (Figure 9B–D), it was obvious that the signals in the EDS spectrum associated with the corrosion products approximately diminished, and the appearance of peaks related to the inhibitors confirmed their effective adsorption on the metallic surface and suppression of steel corrosion in the corrosive solution. The intensity of the signal height of elements related to the inhibitors is higher in the presence of AMB than in the case of AIP and finally AImP, demonstrating the formation of a more intense film in the existence of AMB, which agrees with the SEM observations.

Figure 9.

EDS spectra for the Q235 steel surface after dipping for 48 h in a 1.0 M HCl solution alone (A) and with 10 mM AIP (B), 10 mM AMB (C), and 10 mM AImP (D).

In addition, EDS mapping analyses of the steel surface after 48 h of immersion in 1.0 M HCl in the absence and existence of the explored compounds are displayed in Figure 10. The figure depicts a uniform distribution of inhibitor elements on the alloy surface in the existence of the inhibitors. In general, the EDS spectrum and mapping display clear evidence of decreased corrosion products and the development of the inhibited film on the steel surface in the presence of the explored inhibitors.

Figure 10.

EDS mapping of the Q235 steel surface after dipping for 48 h in 1.0 M HCl alone (blank) and with 10 mM AIP, 10 mM AMB, and 10 mM AImP.

3.6. Fourier-Infrared Spectroscopic Studies

To illustrate the adsorption process of the investigated compounds on the Q235 steel surface, the FTIR technique was used to examine the inhibitors and the film growth on the steel surface after dipping in 1.0 M HCl containing the inhibitors as depicted in Figure 11. The FTIR spectra of pure AIP shown in Figure 11A showed an absorption band at 3402 cm^–1 owing to the N–H and a wide band at 3224 cm^–1 related to the CO–OH bond of the carboxylic acid. Also, an absorption band at 1690 cm^–1 refers to the stretching of the imine group (C=N), and the band at 1843 cm^–1 refers to the stretching of the C=O bond of the carboxylic acid. Also, for the bending band at 1650 cm^–1, absorption bands at 1404 and 1449 cm^–1 have been allocated to the N–H, O–H, and C–S, respectively. In the case of the film grown on the alloy surface immersed in AIP, the spectrum showed the most important band at 3419 cm^–1 that was allocated to the N–H amino bond, the band at 1633 cm^–1 related to the bending N–H amino bond, and the band at 1449 cm^–1 refers to the bending of the O–H of the carboxylic acid, and at 1383 cm^–1 due to the stretching of the C–N amino bond, and at 670 cm^–1 matches to the bending of the C–H of the 1,4-disubstituted aromatic ring.

Figure 11.

FTIR spectra of the pure inhibitors (red) and the film formed on the Q235 steel surface (blue) after dipping for 48 h in 1.0 M HCl containing AIP (A), AMB (B), and AImP (C) at 298 K.

The FTIR spectrum for ABM (Figure 11B) displays characteristic peaks to the N–H stretching of the amino group at (3402 cm^–1), the aromatic N–H stretching at (3670 cm^–1), the CO–OH stretching at (3224 cm^–1), the stretching of the imine group C=N at (1635 cm^–1), the C = O stretching for the carboxylic acid at (1875 cm^–1), the aromatic C–H bending at (2052 and 840 cm^–1), the N–H bending at (1650 cm^–1), and the O–H bending of the carboxylic acid at (1437 cm^–1). For the film that developed at the alloy surface immersed in ABM, the spectrum shows a broad band due to O–H stretching at (3421 cm^–1), C=N stretching at (1732 cm^–1), and aromatic C–N stretching at (1332 cm^–1).

The FTIR spectrum shown in Figure 11C for AImP presented absorption bands representative of the aromatic N–H at (3749 cm^–1), N–H stretching of the amino group at (3394 cm^–1), CO–OH bond stretch of the carboxylic acid at (3232 cm^–1), stretching the imine group C=N at (1635 cm^–1), stretching C=O of carboxylic acid at (1870 cm^–1), bending N–H at (1650 cm^–1), and O–H bending at (1434 cm^–1). Moreover, the FTIR spectrum for the film of AImP shows a broad band for the O–H stretching of the H atom at (3393), the C=N stretching at (1731), and the bending of the N–H bond at (1635).

Comparing the FTIR spectra in Figure 11 proves the adsorption of the explored compounds on the alloy surface, and the shift in some peaks demonstrates that the functional groups in the investigated inhibitors are responsible for this adsorption.

3.7. UV–Visible Spectroscopy

UV–visible spectroscopy was used to get a better understanding of the interactions among the steel alloy and the AIP, AMB, and AImP molecules. The absorption spectra of the steel surface prior to and after 48 h of steel immersion in 1.0 M HCl in the absence and existence of 10 mM of the examined compounds are displayed in Figure 12. As shown in Figure 12A, the UV spectrum for AIP before immersion had the highest absorbance band at 229 nm and another band at 268 nm owing to the electronic transitions n−π* and π–π* of the imine group (C=N) and the aromatic ring, respectively. After the immersion stage, the highest value of the absorbance was at 228 nm, and the other peak was at 282 nm. The increase in the intensity of the absorbance resulted from the hyperchromic effect. Figure 12B shows the absorption spectra of the AMB inhibitor before the steel immersion in which the highest absorbance peak is at 232 nm and other peaks are at 256 and 271 nm owing to the electronic transitions n−π* and π–π* of the imine group (C=N) and the transitions of the aromatic rings. After the immersion, the highest value of the absorbance was at 232 nm, while the other peaks were at 279 and 236 nm, with the absorption shift toward a higher wavelength (red shift), which was assigned to the π–π* transition of the C=C bonds existing in the examined inhibitor. In the case of AImP (Figure 12C), the UV spectrum before immersion showed the highest absorbance at 229 nm and another band at 252 nm because of the electronic transitions n−π* and π–π* of the imine group (C=N) and the aromatic ring, respectively. After immersion, the highest absorbance was recorded at 248 nm and the other one at 281 nm. The increase in the intensity of the absorbance resulted due to the hyperchromic effect, with a red shift in the absorption because of the π–π* transition resulting from the double bonds in the aromatic rings of AImP. According to the results recorded for the three inhibitors, the displacement (red shift) in the spectra with the decrease in the absorbance values suggests complex formation among the explored inhibitors and the steel surface, which is the indication of the sorption mechanism of inhibitors on the steel resulting in the corrosion mitigation.^37,38

Figure 12.

Results of UV–visible tests of the examined inhibitors AIP (A), AMB (B), and AImP (C) before and after immersion in HCl solution.

3.8. XPS Analysis

The XPS technique was utilized to distinguish the chemical composition and declare the bonding state of the studied inhibitor AMB. The XPS survey spectra of Q235 steel were recorded after immersion in a 1.0 M HCl solution containing 10 mM of AMB for 24 h as depicted in Figure 13. The presence of C, N, O, Cl, and Fe elements in XPS results with using the inhibitor provided evidence for the adsorption of AMB on the steel surface and the formation of a protective film. To get a more in-depth understanding of the interaction among AMB and steel surface and to explore the existence form of these elements, the XPS spectra of the recorded elements and their fitting results are presented in Figure 13.

Figure 13.

XPS spectra of Q235 steel in a 1.0 M HCl solution containing 10 mM of AMB: C 1s, O 1s, Fe 2p, Cl 2p, and N 1s.

Figure 13 shows the deconvoluted C 1s spectrum, which revealed three distinct peaks at 284.5 eV (C–C, C–H, and C=C) and 286.2 eV (C–N and C=N), and the third peak at 288.4 eV is attributed to the formation of a C–N⁺ species, which probably resulted from the N atom protonation and/or the N coordination with iron (N–Fe).³⁹ The O 1s spectrum is deconvoluted into two peaks at 530.07 and 531.5 eV, corresponding to FeO/Fe₂O₃ (at 530.07) and FeOOH (at 531.5 eV) as shown in Figure 13. The Fe 2p_3/2 spectrum (Figure 13) shows six peaks at 711.06 eV (Fe 2p_3/2 of Fe²⁺), 714.54 eV (Fe 2p_3/2 of Fe³⁺), 719.05 eV (Fe 2p_3/2 satellites of Fe²⁺), 724.04 eV (Fe 2p_1/2 of Fe²⁺), 727.9 eV (Fe 2p_1/2 level of Fe³⁺), and 732.4 eV (Fe 2p_3/2 satellites of Fe³⁺). The deconvolution of the Cl 2p spectrum reveals two peaks at 198.6 eV (NaCl) and 200.2 eV (FeCl₂, FeCl₃), which have been assigned to Cl 2p_3/2 and Cl 2p_1/2, respectively (Figure 13). The peak observed at 200.2 eV is attributed to the existence of ferrous/ferric chlorides, which suggest the chemisorption of chloride on the steel surface.⁴⁰ Moreover, the N 1s spectrum deconvoluted into three peaks at 399.7, 400.1, and 401.2 eV belonging to both neutral and protonated N components in the adsorbed film (Figure 13). The peak at 399.7 eV corresponds to a C–N bond, while the peak at 400.1 eV is related to N–Fe. The presence of the N–Fe bond suggests the chemical adsorption of AMB on the Q235 steel surface. The third peak identified at 401.2 eV is attributed to the existence of the positively charged N in the form of (=N + H−), which indicate the physical adsorption of AMB on the surface of Q235 steel.⁴¹ Finally, the results suggest that the investigated inhibitor AMB is able to form a protective layer on the steel surface via both chemical and physical adsorption mechanisms, which can efficiently control steel corrosion.

3.9. DFT Calculations

Molecular structures, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO) distributions, and the theoretical parameters for AIP, AMB, and AImP are exhibited in Figure 14 and Table 3, respectively. According to the Frontier molecular orbital (FMO) theory, the ability of donor or acceptor interactions occurring on the surface of an additive/metal is illustrated by its LUMO and HOMO energy levels.^26,42,43 Hence, an inhibitor molecule with high E_HOMO and small E_LUMO values is superior to inhibiting corrosion. From Table 3, the AMB molecule has the greatest E_HOMO value of −4.69 eV compared to those of AIP and AImP molecules (−4.76, −4.88 eV). Figure 14 reveals that the HOMO level for the inhibitor molecules is located on the amino, imine groups, indole, imidazole moieties, and phenyl rings, revealing that these locations are more susceptible to electrophilic attacks on the Q235 steel surface. These explanations corroborate the experimental findings that the molecules of the explored compounds are capable of being adsorbed on the surface of Q235 steel, hence increasing the protection efficiency. Nevertheless, Table 3 shows that the AMP molecule has a lower E_LUMO value (−1.98 eV) than the AIP and AImP molecules (−1.85, −1.79 eV). The low E_LUMO value for the AMB molecule confirms previous findings, suggesting that it has excellent protective properties.

Figure 14.

Optimized structure and HOMO and LUMO orbital occupation for the explored AIP, AMB, and AImP compounds utilized by the DFT approach.

Table 3. DFT Parameters of the Investigated AIP, AMB, and AImP Molecules.

parameters	AIP	AMB	AImP
EHOMO (eV)	–4.76	–4.69	–4.88
ELUMO (eV)	–1.85	–1.98	–1.79
ΔE = ELUMO – EHOMO (eV)	2.91	2.71	3.09
electronegativity (χ)	3.31	3.34	3.34
global hardness (η)	1.45	1.36	1.55
global softness (σ)	0.69	0.74	0.65
the number of electrons transferred (ΔN)	1.27	1.35	1.18
ΔEback-donation	–0.36	–0.34	–0.39
dipole moments (μ) (debye)	8.72	9.74	7.88
molecular surface area (Å2)	301.43	348.91	297.18

Also, lowering the energy gap (ΔE) is important since it increases the inhibitor additive’s corrosion–prevention properties.²⁷ According to Table 3, the ΔE value for the AMB molecule (2.71 eV) is lesser than that of the AIP and AImP molecules (2.91 and 3.09 eV), making it more probable that the AMB molecule will be adsorbed on the Q235 steel interface.

Commonly, the inhibitors’ capability to deliver electrons to the Q235 steel surface is often shown by relatively low electronegativity (χ) values.⁴⁴ In contrast, inhibitor molecules with large electronegativity have a better chance of receiving an electron from the Q235 steel interface (this process is known as “back-donation”) and forming a stronger connection with the Q235 steel surface.⁴⁵Table 3 demonstrates that AIP, AMB, and AImP molecules have somewhat high electronegativity, suggesting that they have the back-donation possibility to form a more robust bond with Q235 steel’s interface.

Furthermore, softness (σ) and hardness (η) are measures of the stability and reactivity of inhibitor molecules. The soft molecules exhibit a stronger protective capability compared to those of hard molecules because of the smooth transport of electrons to the Q235 steel contacted through the adsorption, making them powerful corrosion inhibitors.⁴⁶ As represented in Table 3, the AMB molecule exhibits superior inhibitory characteristics due to its higher σ value and lower η value when compared to those of the AIP and AImP molecules.

Moreover, the inhibitor’s capacity for electron donation or acceptance is assessed by the fraction of electron transfer (ΔN) and ΔE_{back-donation}. This means that if the values of ΔN are more than zero, it is likely that electrons will transfer from the inhibitor molecule to the Q235 steel interface atoms and vice versa if the values of ΔN are lower than zero (i.e., back-donation).⁴⁷Table 3 displays that the ΔN values for the investigated molecules are all higher than zero, indicating that AIP, AMB, and AImP molecules are proficient for providing electrons to the Q235 steel surface, besides the AMB molecule having a better donation ability than either AIP or AImP. Furthermore, when ΔE_{back-donation} is less than zero, the withdrawn electron from the Q235 steel surface atoms will be pursued by a back-donation from the molecule to the Q235 steel interface atoms, and this is the dynamically favored state.⁴⁸Table 3 exhibits that the E_{back-donation} values for AIP, AMB, and AImP molecules are all negative, indicating that back-donation is desirable for these molecules in order to build a strong connection with the Q235 steel interface.⁴⁹

The dipole moment is also a crucial parameter that is used in the corrosion protection prognosis approach.⁵⁰ The molecule’s adsorption on the metallic surface is enhanced, and the distortion energy is augmented by the increase in dipole moment. Hence, an enhancement in dipole moment results in better corrosion inhibition.⁵¹Table 3 displays that the high dipole moment value of the AMB molecule (9.74 debye) is compared to that of the AIP and AImP molecules (8.72 and 7.88 debye values), which gives the AMB molecule a greater tendency to be adsorbed on the Q235 steel surface and increases the corrosion protection.

In addition, the propensity of AIP, AMB, and AImP molecules to protect Q235 steel in corrosive media is directly correlated to their molecular surface area. The increase in the contact area among the inhibitor molecules and the metal surface led to greater efficacy in inhibition. As shown in Table 3, the molecular surface area of the AMB molecule (348.91 Ų) is significantly higher than that of the AIP and AImP molecules, making it a more effective inhibitor.

Additionally, the Dmol³ module assesses molecular electrostatic potential (MEP) mapping, which might discover the active sides of inhibitor species. The MEP mapping is a 3D visual indicator applied to recognize the total charge distribution’s influence totally on a molecule.⁵² The high electron density region, where the MEP is extremely negative (nucleophilic reaction), is described by the red color in the MEP maps displayed in Figure 15. On the other hand, the region with the most positivity is represented by the blue colors (electrophilic interaction).⁵³ The optical analysis of Figure 15 confirms that the nitrogen atoms in the amino and imine groups have a higher electron density than the oxygen atoms in the carboxylic groups. The areas with higher electron densities in inhibitor species (i.e., the red area) might be the most appropriate for attracting steel interfaces and producing robust adsorbed protective layers.

Figure 15.

Graphical presentation of the MEP of the investigated AIP, AMB, and AImP compounds utilized by the DFT approach.

3.10. MC Simulations

A clear idea of the adsorption mechanism was also suggested by MC simulations, which were devoted to identifying the affinities of the inhibitor species with the steel surface. Therefore, the adsorption locator module accomplishes the greatest appropriate adsorption configurations for the AIP, AMB, and AImP molecules on the Q235 steel interface in HCl media (Figure 16), that is, appearing in a roughly flat disposition, designating an enhancement in the adsorption and greatest surface coverage.⁵⁴ Furthermore, Table 4 displays the adsorption energies determined using the MC simulations. From the table, it was observed that the AMB molecule (−3286.05 kcal mol^–1) has greater negative adsorption energy in comparison with the AIP and AImP molecules (−3212.23, −3174.46 kcal mol^–1). These results are in accord with the experimental results and presume that the energetic adsorption of the AMB on the Q235 steel interface gives rise to the construction of a stable adsorbed film that inhibits the Q235 steel from corrosion.⁵⁵ Moreover, the adsorption energy values for the AMB molecule in both the unrelaxed (−3100.53 kcal mol^–1) and relaxed (−185.52 kcal mol^–1) states are more negative compared to the AIP and AImP molecules as shown in Table 4, corroborating that the AMB molecule has a larger protective performance than the AIP and AImP molecules.

Figure 16.

Highest proper adsorption arrangement for the investigated compounds AIP, AMB, and AImP on Fe(110) accomplished using an adsorption locator module.

Table 4. Data and Descriptors Calculated Using MC Simulations for the Adsorption of the Investigated Compounds AIP, AMB, and AImP on Fe(110).

corrosion systems	adsorption energy (kcal mol–1)	rigid adsorption energy (kcal mol–1)	deformation energy (kcal mol–1)	dEads/dNi: inhibitor (kcal mol–1)	dEads/dNi: Cl– ions (kcal mol–1)	dEads/dNi: hydronium (kcal mol–1)	dEads/dNi: water (kcal mol–1)
Fe(110)	–3212.23	–3035.63	–176.60	–284.47	–105.62	–63.64	–15.18
AIP
water
hydronium
Cl– ions
Fe(110)	–3286.05	–3100.53	–185.52	–306.61	–106.86	–65.42	–15.68
AMB
water
hydronium
Cl– ions
Fe(110)	–3174.46	–3005.09	–169.37	–261.27	–104.12	–64.06	–15.41
AImP
water
hydronium
Cl– ions

The dE_ads/dN_i values assist in clarifying the metal/adsorbate arrangement energy if the adsorbed inhibitor molecule or the molecules of other adsorbates were eliminated.⁵⁶Table 4 reveals that the dE_ads/dN_i values of AMB molecules (−306.61 kcal mol^–1) are greater than those of AIP and AImP molecules (−284.47, −261.27 kcal mol^–1), indicating that AMB molecules exhibit superior adsorption. In addition, the dE_ads/dN_i values are around −15.42 for water molecules, −64.37 for hydronium ions, and −105.53 for chloride ions. These values are much smaller compared to those of AIP, AMB, and AImP molecules, implying that inhibitor molecules are strongly adsorbed and easily outcompete H₂O molecules, hydronium ions, and chloride ions. Thus, both experimental and theoretical research agree that the molecules of AIP, AMB, and AImP are effectively adsorbed on the surface of the Q235 steel, creating a potent adsorbed layer that shields the alloy from corrosion in a corrosive medium.

3.11. Mechanism of Corrosion Inhibition

It is critical to understand the mechanism of corrosion protection provided by the explored organic inhibitors on the Q235 steel surface under the corrosive acidic conditions studied. The corrosion process of steel in the acidic solution involves an anodic process (metallic dissolution) and cathodic process (hydrogen evolution or oxygen reduction).⁵⁷ The main important role of employing the organic inhibitors is to suppress the oxidation of the metallic surface and prevent oxygen and water from diffusing onto the metal surface, and this will be achieved by the adoption of the inhibitors on the metallic surface.⁵⁸ The anticipated corrosion protection mechanism of the explored Schiff bases for Q235 steel in HCl solution in this study could be proposed as follows.

In a corrosive acidic medium, the steel surface will be positively charged due to metallic oxidation. Moreover, the negatively charged chloride ions of HCl will be attracted to the positively charged steel surface, promoting the metal dissolution. The heteroatoms of the explored inhibitor molecules in acidic media are protonated, which makes it easy to attract to the oppositely charged counterions via electrostatic force and adsorbed on the metallic surface.⁵⁹ After adsorption, the explored inhibitors can return to their neutral forms and then move their nonbonding electrons to d-orbitals of steel via a coordination bond through the chemisorption process. Consequently, the adsorption process of these compounds on the steel surface involved both physical and chemical adsorption processes.⁶⁰ The investigated inhibitors contain various heteroatoms such as oxygen, nitrogen, and sulfur, which can donate lone pairs of electrons to empty d-orbitals of the iron of steel, facilitating their adsorption. Furthermore, the presence of an aromatic ring, indole, and imidazole groups can be oriented toward the cathodic sites and facilitate the formation of an adsorbed layer that serves as a barrier among the alloy surface and corrosive solution. The presence of both neutral and protonated C and N elements in the interfacial film on the steel surface implies that the adsorbed constituents of AMB involved both protonated and nonprotonated (molecular) species. This would involve the electrostatic interaction of protonated species and the charged Fe surface (physisorption), possibly mediated by adsorbed anions (Cl^–), as well as covalent interactions between the unprotonated heteroatoms and vacant d-orbitals of Fe (chemisorption). The different characterization techniques like XPS, SEM, EDS, FTIR, and UV–vis approve the adsorption of the examined organic compounds on the steel surface, which is responsible for the pronounced surface protection compared with the bare steel surface, with the best protection recorded in the existence of the ABM inhibitor owing to the formation of a protecting film on the steel surface, which hinders the steel dissolution and penetration of the corrosive ions to the surface.

3.12. Assessing the Inhibition Efficiency of the Investigated Schiff Base Derivatives

The inhibition efficiency of the examined Schiff base was compared to that of the other published Schiff bases, as depicted in Table 5. Based on the results presented in this table, the investigated newly designed Schiff bases demonstrated superior corrosion inhibition performance compared to previously reported Schiff bases as inhibitors for Q235 steel alloy in an acidic HCl solution. The outstanding outcomes of this study imply that the newly designed Schiff bases possess unique structural elements and/or functional groups that play a part in their improved ability to inhibit corrosion. These characteristics could include aromatic rings, electron-donating groups, or other chemical moieties that are known to support corrosion inhibition.

Table 5. Comparing the Inhibition Efficiency of Recently Published Schiff Bases as Corrosion Inhibitors with Those of the Investigated Schiff Bases Used in This Study.

inhibitor code	steel alloy	corrosive medium	optimum concentration	maximum inhibition %	reference
PAMP	mild steel	1.0 M HCl	800 ppm	87.2	(61)
PPM				92.9
OQ	mild steel	1.0 M HCl	5.0 × 10–3 M	93.8	(62)
EQ				92.7
MQ				91.8
PMTTA	mild steel	1.0 M HCl	175 ppm	92.7	(20)
PATT				91.1
PMTA				87.8
PTA				80.5
CBTA	mild steel	1.0 M HCl	6 ppm	82.4	(63)
PATA				90.4
MBTA				92.8
MTMP				96.7
IICH	mild steel	1.0 M HCl	200 ppm	97.9	(14)
ICCH				95.3
IBCH				92.2
IMCH				99.3
L400	carbon steel	1.0 M HCl	1 × 10–4 M	92.0	(64)
L600				94.0
Cpd I	carbon steel	1.0 M HCl	250 ppm	77.9	(65)
Cpd II				81.1
Cpd III				85.4
S1	carbon steel	1.0 M HCl	100 ppm	89.5	(66)
S2				91.8
azoS8	carbon steel	1.0 M HCl	500 ppm	91.3	(67)
BTTM	mild steel	1.0 M H2SO4	10.2 × 10–4 M	95.6	(68)
Chi-Cn1	mild steel	0.5 M H2SO4	350 ppm	83.1	(69)
Chi-Cn2				90.3
Chi-Cn3				92.6
Chi-Cn4				93.6
Chi-Cn5				96.2
INB	carbon steel	0.5 M H2SO4	1.0 × 10–3 M	90.2	(70)
IMB				91.5
AIP	carbon steel	1.0 M HCl	10 mM	93.2	this work
AMP				96.01
AImp				77.03

4. Conclusions

In the present study, the newly synthesized compounds (AIP, AMB, and AImP) significantly protect Q235 steel from corrosion in an acidic solution (1.0 M HCl) and the investigation eventually results in the following conclusions:

• PDP and EIS results indicated the inhibition efficacy of the investigated inhibitors, which increases with increasing the inhibitor concentration.

• The PDP measurements designate that the examined inhibitors exhibit features of mixed-type inhibitors with high protection efficiencies of 93.15, 96.01, and 77.03% in the presence of AIP, AMB, and AImP, respectively.

• EIS measurements confirm the formation of an adsorbed protective layer via decreasing in C_dl values and increasing in R_ct values.

• The surface analysis (SEM and EDS) as well as XPS, UV–vis, and FTIR confirmed the inhibiting film formed on the steel surface by the examined inhibitors, with significant inhibition recorded in the case of AMB.

• Mechanism of the inhibition was investigated theoretically using the DFT calculations and MC simulations, which validated the experimental findings and confirmed that the adsorption of the examined inhibitor molecules on the steel surface occurs via active sites like −O, −N, and −S atoms and the π-bonds of the aromatic rings.

• The results recorded from the electrochemical, theoretical, and surface analyses are consistent.

The authors declare no competing financial interest.