Inhibition performance of a novel quinoxaline derivative for carbon steel corrosion in 1 M HCl

In this work, the effect of a new quinoxaline derivative, 2-phenyl-3-(prop-2-yn-1-yloxy) quinoxaline (PYQX), was evaluated as a corrosion inhibitor for carbon steel (CS) in 1 M HCl electrolyte. Weight loss measurement, atomic absorption spectroscopy, potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy with energy dispersive spectroscopy, and UV-vis spectroscopy were employed to assess the inhibitory activity. The electronic properties of the interaction between the inhibitor molecule and the CS substrate were studied using molecular dynamics (MD) simulation, density functional theory (DFT), and Fukui functions. According to AC impedance experiments, the inhibitor under consideration showed a maximum level of 98.1 % inhibition efficiency at 1 mM and 30 °C. The Langmuir adsorption isotherm model explains the adsorption of PYQX on the CS surface. A slope of 1 denotes a strong molecule-substrate interaction, suggesting that the binding occurs at specific surface locations. To understand


Introduction
Carbon steel (CS) is often used for structural supports and components in railway systems, bridges, and other public works because of its high tensile strength, abrasion resistance, and superior surface properties [1].However, exposure of steel to aggressive environments, such as acid-cleaning processes, can cause the metal to deteriorate and reduce its durability [2].Corrosion inhibitors are a cost-efficient and highly effective way to mitigate the corrosive effects of steel in acidic environments [3,4].
Quinoxaline compounds have been extensively studied as corrosion inhibitors, which are environmentally safe and applicable to multiple fields such as chemistry, mechanics, and materials.It is suggested that the functional groups, conjugated multiple bonds, and aromatic rings present in quinoxaline-based molecules contribute to their excellent inhibitory performance.
Numerous corrosive conditions have been used to thoroughly assess the inhibitory abilities of these derivatives.Zarrouk et al. used gravimetric and electrochemical techniques to explore the impact of several quinoxaline analogs on the deterioration of copper in HNO3 acid [5].Recently, several authors have worked on the inhibition of corrosion of mild steel in 1 M HCl corrosive media by certain quinoxaline compounds using weight loss (WL) and electrochemical techniques, showing the high potential of quinoxaline molecules to prevent corrosion of metals [6][7][8][9].The literature survey also showed that calculations using density functional theory (DFT) and molecular dynamic simulations (MDS) frequently provide good theoretical justifications for experimental findings in corrosion inhibition investigations [10].
In this study, the inhibitory effect of 2-phenyl-3-(prop-2-yn-1-yloxy) quinoxaline (PYQX) on CS corrosion in 1 M acidic electrolyte has been investigated.The adsorption mechanism of this quinoxaline derivative on the CS surface is further explained using quantum chemical calculations supported by DFT, Fukui functions, and molecular dynamics (MD) simulations.

Materials and solution
In this study, CS plates with dimensions 1.5×1.5×0.3 cm were employed for testing samples.Its composition (in weight percent) is the following: C-0.3700 %, Si-0.2300 %, Mn-0.6800 %, S-0.0160 %, Cr-0.0770 %, Ti-0.0110 %, Ni-0.0590 %, Co-0.0090 %, Cu-0.1600 %, and iron (Fe) as the remaining content.The working electrode (WE) area exposed for the electrochemical corrosion test was 1 cm 2 in size.Before each use, the samples were meticulously polished with abrasive paper of increasing grain size, dried, washed with distilled water and degreased with acetone.
The electrolyte medium, 1 M HCl, was obtained by diluting concentrated 37 % analytical-grade HCl with distilled water.The synthesized compound PYQX, with the structure shown in Figure 1, dissolves readily in 1 M HCl solution, allowing the preparation of four inhibitor assays that ranged from 10 -3 to 1 mM.

Figure 1. Molecular structure of PYQX Weight loss measurements
Polished CS plates were weighed and in triplicate, placed into 50 mL glass beakers using plastic forceps.These beakers contained 1 M HCl solutions with and without various inhibitor dosages.After six hours, the samples were removed, rinsed with distilled water, dried, and re-weighed.The initial and final weights of the plates were used to determine the weight loss (WL) as m / mg (before and after soaking).By applying the following calculations to the weight loss data, the inhibitory effectiveness WL / % and corrosion rate (CR) (mg cm -2 h -1 ) were determined by equations ( 1) and ( 2) [11]: where and CR u and CR i are corrosion rates in the uninhibited and inhibited electrolyte.

Electrochemical testing
All electrochemical tests were carried out using a PGZ 100 potentiostat under computer control with the Voltamaster 4 program.The test model was a standard three-electrode system composed of a saturated calomel reference electrode (SCE), a platinum counter electrode, and a CS plate with an exposed surface area of 1 cm² working electrode.After observing the evolution of the open circuit potential (OCP), which usually takes approximately half an hour to stabilize, electrochemical experiments were conducted.Tafel curves were scanned by sweeping the potential with a velocity of 0.5 mV/s around the OCP in the potential range of -800 to -100 mV.The amplitude of the 10 mV peak-to-peak AC signal at OCP was used to measure the EIS at frequencies ranging from 100 kHz to 10 mHz.Impedance data were collected and presented using Nyquist and Bode plots.
The equations ( 3) and ( 4) were employed to determine the inhibition efficiency (ƞ / %) [12]: where Rp and Rp 0 stand for the polarization resistance with and without inhibitor solution, respectively, while  represents the extent of surface coverage.where icorr 0 and icorr are corrosion current densities without and with inhibitor solution, respectively.
The results for the blank obtained by two electrochemical techniques (stationary and transient) in analysing the effects of concentration and temperature were taken from the previous article [12] since the experiments were carried out under the same conditions.

Atomic absorption spectroscopy
AAS was used to quantify the metallic elements in solutions.After electrochemical tests, this method was employed to determine the iron concentration in 1 M acidic media with and without varying amounts (10 -3 to 1 mM) of PYQX at 30 °C.
Using the atomic absorption data, the equation ( 5) was applied to determine the inhibition efficiency [13]: where CFe 0 and CFe represent Fe concentration in solutions without and with various PYQX amounts, respectively.

Surface characterization and electrolyte analysis
Using scanning electron microscopy with a magnification of 10000, the surface morphologies of the CS samples were examined following their submersion in acidic media for 24 hours at 30 °C, both without and with the optimum concentration of 1 mM PYQX.At the accelerating voltage of 20 kV, energy dispersive X-ray spectroscopy (EDS) coupled with SEM was utilized to get information about the chemical composition of the material.SEM-EDS results in the absence of the inhibitor were taken from a work already published by our research team [12].The electrolytes were carefully examined using a Jenway (series 730) UV-vis spectrophotometer before and after immersion of the CS for 72 hours at 30 °C in 1 M HCl solutions, including 1 mM of PYQX, to check for the potential formation of ferric ion-inhibitor complexes.

Computer-based research
Density functional theory (DFT) study DFT has been utilized for a long time to assess the reactivity of organic inhibitors and correlate it to the efficiency of corrosion prevention.The program Gaussian 09 was employed to determine the DFT-based quantum physics parameters.The geometric optimization of the quinoxaline derivative PYQX in its neutral and protonated forms in the aqueous phase was produced by DFT calculations using the function B3LYP and the 6-311++G(d,p) basis set [14].
The energy gap (ΔE) energies of the lowest unoccupied molecular orbital (ELUMO), highest occupied molecular orbital (EHOMO), as well as the other quantum descriptors were calculated with the equations ( 6) to (9): Global hardness (): Global electronegativity (): The fraction of electrons transferred: where Փ is the work function explains the theoretical value of  in Fe110 with Φ = (Fe110) = 4.82 eV.

Fukui indice calculations
The analysis of Fukui indices was applied to the inhibitor molecule PYQX to uncover the functional groups that enable it to interact with the iron site.This comprehensive analysis revealed that the local reactivity of PYQX was mainly attributed to the interactions between certain local reactive canters and the iron region.The dual numerical polarization basis set, consisting of the d and p orbitals, was employed in the calculations.The Fukui function is determined as the primary derivative of the electron density ( ( )) r  in relation to the electron number (N) in a constant external potential ( ( )) r  as in equation ( 10) [15,16]: The Fukui function displays the available position in the inhibitor where nucleophilic or electrophilic assaults are most likely [15,16].The following finite difference approximation approaches can be used to express Fukui indices for nucleophilic attack (fk + ) and electrophilic assault (fk -), equations (11) and (12): where, qk(N), qk(N + 1) and qk(N -1)) represent the neutral, cationic and anionic species, respectively.

Molecular dynamics simulations
MD was used in order to explore the interaction of the substances under study with Fe (110).Like in our previous work [16], this simulation was carried out at a similar level using the Forcite module found in the Materials Studio/8 using simulation box "2.730×2.730×3.713nm 3 " (27.30×27.30×37.13 Å 3 ).This vacuum of this box is filled by 491 H2O, 9 H3O + , 9 Cl -, and quinoxaline derivative.The Andersen thermostat was used to calibrate the system temperature to 303 K at 1000 ps simulation time and 1.0 fs utilizing the NVT ensemble, all while operating within the COMPASS force field.

Weight loss and atomic absorption spectroscopy analysis
WL measurements were carried out to assess the corrosion inhibition ability of PYQX, as well as the impact of inhibitor concentration on the corrosion rate.Table 1 shows that after 6 hours of submersion, the corrosion speed of CS in the acid media was higher than that of the inhibitor solution.Simultaneously, raising the PYQX concentration from 10 -3 to 1 mM lowered the corrosion rate from 0.321 to 0.072 mg cm -2 h -1 of CS, corresponding to an increase in inhibition effectiveness from 83.1 to 96.5 %.This demonstrates that the adsorption of inhibitor molecules protects the CS surface by blocking mass and charge transfer processes.Interestingly, the same outcomes were obtained using the atomic absorption technique (Table 1), where it was found that the addition of PYQX reduced the concentration of iron in the HCl medium.Hence, it seems that the inhibitor included in the mixture reduces steel degradation by improving the surface covering and retarding corrosion.

Electrochemical analysis
Open circuit potential plots Figure 2 illustrates how the open circuit potential (OCP) value changes depending on the duration of immersion of CS in 1 M HCl electrolyte medium, both with and without PYQX.It is obvious that 1800 s is long enough for the system to stabilize.It was also observed that the EOCP is displaced in a positive direction following the addition of PYQX.This can result from PYQX sticking to the steel electrode surface [17].It is noteworthy to say that there is no specific relationship between the potential shift of EOCP and PYQX concentration.

Potentiodynamic polarization analysis
Figure 3 illustrates the PDP curves for CS performed at 30 °C in the electrolyte without and with various doses of PYQX.It is evident that including PYQX in the corrosive medium induces an overall reduction in cathodic and anodic current densities, which becomes more pronounced with increasing amounts of PYQX.This result suggests that PYQX reduces anodic oxidation and retards the reduction of H + protons [17].The reduction trend of the cathodic segment was greater than that of the anodic segment, suggesting that PYQX has a greater protective influence on the cathodic reaction than on the anodic reaction.On the downside, the cathodic portions are linear, indicating that PYQX has little influence on the H + ion reduction mechanism and that the H + reduction reaction at the CS interface occurs mainly by a pure activation [17].
Table 2 presents the outcomes of inhibition effectiveness (P / %), as well as electrochemical corrosion factors such as corrosion current density (icorr.),corrosion potential (Ecorr), and Tafel slopes E / mV vs. SCE (cathodic (c) and anodic (a)) for different concentrations of PYQX, obtained by extrapolation of the linear cross-sections of the anodic and cathodic polarization plots at Ecorr [18].Analysing the corrosion potential allows the inspection of the inhibition mechanism of inhibitor molecules.The shift in Ecorr with PYQX injection was significantly less than 85 mV from baseline, a value considered a threshold for identifying the type of inhibitor.The slight shift in Ecorr following the addition of the inhibitor demonstrated that investigated molecules function as a mixed-type inhibitor by limiting CS oxidation and H + ion reduction.It follows that surface adsorption, leading to the formation of a coating layer covering the active sites, is the primary mechanism behind the inhibitory activity of PYQX [18].The P increases as the inhibitor PYQX concentration increases, peaking at 98.1 % at 1 mM.This result agrees well with the conclusions of the measures of atomic absorption and weight loss.

Electrochemical impedance spectroscopy analysis
Electrochemical impedance spectroscopy (EIS) is generally used to explore the mechanisms of charge transfer, surface passivation, and corrosion inhibition at the metal-electrolyte interface.Typically, Nyquist and Bode curves are employed to highlight these outcomes of EIS.The properties of corrosion prevention of quinoxaline derivative on CS have already been explored by EIS [19].Nyquist impedance plots and Bode impedance modulus and phase angle plots of CS in the uninhibited electrolyte and with a variety of PYQX doses at 30 °C are shown in Figures 4 and 5, respectively.
Nyquist plots (Zi vs. Zr) presented in Figure 4 show that just one capacitive loop has an identical form for all experienced inhibitor concentrations.These similar impedance responses indicate that log (i / mAcm -2 ) the presence of PYQX did not alter the corrosion mechanism significantly, and the charge transfer is the primary factor controlling the process of corrosion.Similarly, Figure 4 reveals that the capacitive loop diameter rises with a rising dose, thus indicating inhibition efficiency.The obvious rise of impedance modulus |Z| at low frequencies, as evident in the Bode graphs (Figure 5), implies the rise of charge transfer resistance with the rise of the inhibitor concentration.This could be a confirmation of better coverage with the increase of PYQX dose, related to the planar geometry of the quinoxaline derivative, which may boost the adsorption of this substance on the CS surface by creating a barrier coat that prevents corrosion [19].The phase angle diagrams present only one peak, revealing that the corrosion process requires only one stage and is governed by charge transfer.With increasing inhibitor concentration, the phase angles at corresponding peaks attained higher negative values due to considerable molecular adsorption, suggesting that the tested inhibitor PYQX may be involved in creating a protective layer on the metal substrate [20][21][22].
The experimental Nyquist and Bode diagrams for PYQX were adjusted using the equivalent electrical circuit depicted in Figure 6.The equivalent circuit in Figure 6 includes a solution resistance (Rs), a polarization resistance (Rp), and a constant phase element (CPE) describing a contribution of double-layer impedance.Equation ( 13) can be utilized to determine the expression for the impedance of the CPE [19]: where Q, i,  and n are CPE constant, imaginary number, angular frequency ( =2πf, where f is frequency), and CPE exponent, which can be used to measure the roughness or heterogeneity of the surface, respectively.
The double layer capacity (Cdl) of an adsorbed film is connected to the CPE settings in accordance with equation ( 14): The impedance parameter values retrieved and determined by fitting EEC in Figure 6 to EIS results in Figures 4 and 5 are presented in Table 3, together with calculated values for inhibitor efficacy using eq.( 4), and chi-square ( 2 ) values obtained after impedance data fitting procedure.Low values of  2 indicate a strong correlation between impedance data obtained by the chosen model and the measured outcomes.This suggests that there is a good concordance between the two sets of data.
Data in Table 3 generally show that the value of Rp rises as PYQX content does.This rise leads to the protection of the CS and reduces the number of active sites produced by binding the negatively charged Cl -ions to the surface of CS.The simultaneous reduction in Cdl values can be ascribed to a decline in the local dielectric constant and/or an augmentation in the thickness of the electrical double layer, indicating the adsorption of the studied molecules onto the metal surface [12].
The EIS study gave an inhibitory efficiency of about 97.1 % for a concentration of 10 mM PYQX at 30 °C.This confirms the previous results obtained by WL, AAS and PDP.

Adsorption isotherm
After adhering to the metal/solution contact, organic molecules restrict the dissolution of C▪S.Thus, it is needed to investigate the adhesion property of PYQX molecules on the C▪S contact.To achieve this goal, a variety of adsorption models were investigated in the present research.,including the Langmuir, Freundlich, El-Awady, Temkin, and Flory-Huggins isotherms.Finally, the results of this study indicate that the Langmuir model is the best-suited model for characterizing the adsorption of PYQX onto the CS contact, which can be expressed by equation (15): where Cinh, Kads, and  are the PYQX concentration, the equilibrium adsorption constant and the extent of surface coverage, respectively.The standard Gibbs free energy of adsorption Gads 0 is related to the Kads according to equation ( 16) [20]: where 55.5 is the concentration of water in solution, R and T are gas constant and absolute temperature (303 K), respectively.The Langmuir isotherm graph is illustrated in Figure 7.It has already been recognised that when Gads 0 > -20 kJ mol -1 , inhibitor molecules are adsorbed onto the CS surface by electrostatic interactions (physisorption), while when Gads 0 < -40 kJ mol -1 , inhibitor adsorption occurs through the formation of coordination bonds (chemisorption) between compound molecules and the CS surface.In cases where -40 kJ mol -1 < Gads 0 < -20 kJ mol -1 , inhibitor adsorption occurs through both physisorption and chemisorption [20]./ mol L -1 R 2 value of linear regression in Figure 7 is 1, indicating a close-to-perfect linear correlation.This affirms that the Langmuir adsorption model applies to the adsorption process of PYQX in the corrosion test.This data suggests that the adsorption process between PYQX and the corrosion system is highly efficient, allowing for consistent and reliable results.The negative value of Gads 0 and the high value of Kads, which takes on the value 1.1273 L mol -1 , demonstrate the durability of the bilayer taken up on the CS interface, resulting in a higher effectiveness of inhibition [21].Additionally, the calculated value of Gads 0 in this study is equal to -45.22 kJ mol -1, which reveals adsorption of 2-phenyl-3-(prop-2-yn-1-yloxy) quinoxaline on the CS surface is mainly through chemisorption.

Temperature effect
In pickling, inhibitors are employed to guard against an acidic assault on metal equipment.Pickling processes are often carried out at high temperatures.For application, a corrosion inhibitor must maintain its effectiveness in a corrosive system at specific working temperatures.The research has clearly established that when temperature increases, organic molecules are desorbed from surfaces, which then results in increased corrosion of metals.This phenomenon has been documented and observed in numerous studies.Therefore, it is doubtful that higher temperatures can cause enhanced metal corrosion [21].
In the interest of investigating how temperature affects the efficiency of PYQX, PDP tests were carried out in the uninhibited electrolyte and with 1 mM PYQX at temperatures fluctuating between 30 to 60 °C (Figure 8).The inhibitor efficiency values () and electrochemical variables calculated for this study are presented in Table 4.The results show that an elevation in temperature produces higher icorr values and this effect is greater in the uninhibited electrolyte.This means that the rise in temperature speeds up the disintegration of the CS by causing desorption of the PYQX molecules from the CS surface.
The p dropped from 98.1 to 91.6 %, demonstrating that PYQX underwent desorption from the CS surface, and consequently, the degree of surface coverage decreases as the adsorptiondesorption equilibrium shifts in favour of inhibitory desorption [21].

Activation parameters
The descriptors of the important indices, which are activation energy (Ea), enthalpy change (ΔHa), and entropy change (ΔSa), were determined in order to better understand how temperature affects the inhibition mechanism.The Ea value of the corrosion process at varying temperatures, both without and with 1 mM of PYQX was computed by applying the Arrhenius equation ( 17 where A, R, T, N, and h are the pre-exponential constant, gas constant, absolute temperature, Avogadro number, and Planck's constant, respectively.The Ea value for CS oxidation was calculated without and with 10 mM of PYQX using the slope (-Ea/R) of the plot presented in Figure 9, which shows the dependency of ln(icorr) on 1/T.Likewise, by tracing ln(icorr/T) as a function of 1/T under the same conditions (Figure 10), the slope calculates the values of ΔHa, while the y-intercept calculates the values of ΔSa.The activation energy and thermodynamic parameters resulting from the Arrhenius and the transition state plots are reported in Table 5.  T -1 / 10 -3 K -1

Figure 10. Transition-state plots for CS in uninhibited 1 M HCl electrolyte and with 1 mM of PYQX
It has been noted in Table 5, that when 10 -3 M of PYQX is present, the Ea value is greater than that of the uninhibited electrolyte, suggesting that there is an energy barrier between the CS surface and inhibitor molecules under study.The oxidation process of the CS substrate is endothermic, as indicated by the positive sign of the activation enthalpy change (ΔHa).In addition, the existence of PYQX increased the value of ΔHa, stating that the dissolution of CS is difficult due to the adsorption of PYQX molecules on the CS surface.On the other side, ΔSa value attains a high positive value compared to the uninhibited medium, indicating an increase in disorder of the system occurs throughout the adsorption process of the transformation from reactant molecules to activated complexes [23].

Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis
Surface examination by scanning electron microscopy (SEM) scanning technique was completed to generate evidence of the impact of inhibitor compounds on the microstructure of the CS surface and to confirm the creation of a protected layer by PYQX on the substrate.Figure 11 displays CS surface SEM micrographs after 24 h of soaking in the uninhibited medium 23 and with 1 mM of PYQX. Figure 11(a) shows that the polished CS surface has many scratches from polishing.It is clear from Figure 11(b) that the morphology of the entire CS area after immersion in the uninhibited medium is severely damaged, while Figure 11(c) shows that the surface morphology greatly improves when 1 m M PYQX is present in the solution.This shows that the inclusion of PYQX leads to a reduction in the number of active corrosion centres by adsorption of the molecules of PYQX on the surface of the CS substrate, thus creating a stable, protective film.The EDS spectrum for the polished CS presented in Figure 11(a) showed a strong iron peak and peaks characteristic for the chemical composition of the CS sample.After immersion of the CS in the HCl electrolyte, the EDS spectrum in Figure 11(b) shows strong Cl and O peaks, suggesting the existence of oxides and chlorides in the corrosion products.On the surface of the specimens exposed to the PYQX-inhibited acidic electrolyte, a remarkable reduction in Cl content is revealed in Figure 11(c), which confirms the adsorption of PYQX molecules on the area of CS.SEM images (a/b) and (c/d) are taken from [12] UV-visible spectra analysis UV-visible spectrometry was applied to see how CS and molecules of PYQX interact.The absorption spectra of corrosive media, including 1 mM of PYQX, before and after 72 h of CS soaking, are plotted in Figure 12.As in previous research [24], a change in the location of the absorbance peak or a change in the amplitude of the absorbance indicates the development of a complex between two species present in the acidic electrolyte.A careful examination of the absorption spectra showed a notable shift in the absorption bands from 252 to 254 nm and 321 to 327 nm.Furthermore, a remarkable increase in the absorption maximum (λmax) and the appearance of an additional peak at 226 nm reflects the interaction between the studied PYQX compounds and Fe 2+ ions in the solution.These findings demonstrate the development of a complex between quinoxaline molecules and Fe 2+ ions in the acidic media [25].

Density functional theory reactivity
A molecule with heteroatoms in its structure binds one or more H + protons in an acidic medium, which can modify its chemical reactivity.Figure 13 shows the percentage distribution of the different protonated forms vs. pH using MarvinView software, suggesting that in dependence on pH, the PYQX molecule likely protonates in two forms.The optimal protonation occurs at position 2, wherein the nitrogen atom (sp 2 -hybridized) within the foundational structure of quinoxaline is in closer proximity to the protons of the benzene ring, thereby inducing a change in the neutral state of PYQX.Therefore, the current analysis is intended to follow the reactivity behaviour of two forms, neutral and protonated.
To investigate the effectiveness of corrosion inhibition and evoke a plausible corrosion inhibition mechanism, the electronic properties, global activity descriptors and local activity indices of PYQX neutral and charged were calculated [26].To maintain structural conformity, the absence of imaginary frequencies must be ascertained using the same level of theory.To plot the structure, HOMO and LUMO of the compounds, GaussView/5 software [27] was applied.
Figure 14 depicts the energy-minimized structures and FMOs (HOMO and LUMO) of the PYQX neutral and charged.For the two forms in question, it is noticeable that the distribution of FMOs is total over the structure of both forms of the PYQX, implying that the reactivity is very high with other species and especially with the metal surface.EHOMO and ELUMO values serve to assess the donor/acceptor ratio in the molecule/metal relationship [26].In this study, the reactivity of the PYQX neutral and charged is followed by these two descriptors.Table 6 groups different values of the reactivity descriptors of both forms.It is revealed in Table 6 that the PYQX neutral is more nucleophilic (EHOMO=-6.103eV) and the PYQX charged is more electrophilic (ELUMO=-6.835eV).Therefore, ΔE (ELUMO-EHOMO) allows to measure the reactivity of a molecule, as the most minimal value of this descriptor means that the specific reactivity is maximal [26].The description comparison between the values for both forms of this descriptor (ΔE) shows that the charged PYQX is more reactive with a low value of 3.453 eV.
The electronegativity () presents an attractive power of the electrons of a chemical species; the relevance of the descriptor translates the reactivity of interaction [26].The value of 8.561 eV for the charged PYQX, as shown in Table 6, indicates that it is a more reactive inhibitor on surfaces with a negative charge.The reactivity and hardness () of a molecule are reciprocally proportional: a harder molecule implies lower reactivity [26].The data in Table 6 show that charged PYQX (1.726 eV) is more reactive.The fraction of electron transferred (N110) values provide useful information on the tendency of electrons to flow from the inhibitor molecule to the CS surface (N > 0), or from the carbon steel to the inhibitor (N < 0) [28].In this present computation, the neutral PYQX molecule (ΔN110 = 0.197) is more likely to share its electrons in order to form coordination bonds with Fe (110).The inhibitory efficacy of a molecule can be measured and assessed by the prevalence of active sites throughout its structure.In addition, the sites of high local reactivity support substantial inhibitory efficacy.These centres may bind with others located at the metal surface by chemical bonds [29].In Table 7, the various sites of PYQX molecule neutral and charged are collected, respectively, with the most suitable atoms presenting the highest reactivity, giving the acceptor (nucleophilic attack (f + )) and donor (electrophilic-attack (f -)).The condensed Fukui functions (f + and f -) for PYQX neutral indicated that C2, C3, C6, N10, C11, C16, and C18 are the most electrophilic attacks centres, whereas the C1, C2, C3, C6, N7, C8, C9, and N10 are highest nucleophilic attacks centres.These sites are suitable for donating and accepting electrons.After protonation, the electron-donating effect decreased noticeably while the electronattracting effect increased.

Molecular dynamics simulation
Molecular dynamics (MD) simulation is a valuable approach to describe better and understand the adsorption pattern of a molecular substance on a metallic material [30].In this work, the simulation deals with the reactivity of quinoxaline molecules (neutral and charged) and Fe (110).This research also investigated the reactivity model of two molecule forms on the effectiveness of surface protection against attack by corrosive species contained in the molar HCl solution.Figure 15 illustrates the adsorption of the species of interest (lateral and top representations) onto Fe (110).Further analysis of these two configurations reveals that both molecules adsorb wholly and largely occupy the iron atomic layer, resulting in a high content of anticorrosive protection.The interaction energy (Einter) value is established by the equation ( 19) [31]: The Einter with a low value indicates the big interactions for both systems Fe/ PYQX (neutral) and Fe/PYQX (charged).It appears from the comparative study that the most negative value of Fe/PYQX (charged) with -889.929kJ mol -1 reflects a big interaction.These simulation data clearly prove the agreement compared with the quantum computations.
The primary objective of this model is to assess the significance of adsorption by utilizing interatomic distances for Fe/PYQX (neutral), which is equal to -888.428 kJ mol -1 , and Fe/PYQX (charged), which is equal to -889.929 kJ mol -1 , through the introduction of the radial distribution function (RDF).The published literature confirms that the likelihood of chemical adsorption is higher when the bond length is less than 0.35 nm (3.5 Å).By contrast, physical adsorption is more probable [31].In Figure 16, the 1st peak displays the bond lengths for both systems are less than 0.35 nm.

Inhibitory mechanism
The adsorption process of inhibitor on a metal surface explains the mechanism of corrosion inhibition.Figure 17 shows a possible mechanism for the adsorption of PYQX on the C▪S surface in 1M HCl.Organic molecules adsorb on metal surfaces primarily by chemisorption, physisorption, or both.In this study, PYQX has many functional groups, such as O and N heteroatoms and heterocycles, that can help the inhibitor stick to the surface.Organic compounds present both neutral and protonated forms when their heteroatoms undergo protonation in acidic environments.A positive charge may form on the metal surface as a result of fast metal oxidation.A negatively charged surface may emerge as a result of the positive charge attracting chloride ions present in the solution.The protonated form then physically adheres to the newly formed negatively charged surface.This creates a protective layer that slows down the anodic corrosion process.Also, the positively charged ions that stick to the cathodic sites compete with H + for electrons, which lowers the production of H2 [32].The neutral form of PYQX chemisorbed to the metal surface through interactions between the empty d-orbital of Fe atoms and the free electron pairs of heteroatoms (N, O).The transfer of excess negative charge from the Fe d-orbital to the unoccupied π* orbital of inhibitor molecules can occur on the CS surface, leading to greater inhibitor adsorption on the steel surface.Moreover, between the aromatic cycles and the d-orbitals of the CS surface, electron retrodonation may occur [33].At last, a barrier is created on the steel surface by an adsorbed inhibitor film to prevent metal corrosion from the corrosive medium.

Conclusion
Corrosion inhibition of carbon steel was investigated in a molar hydrochloric acid environment using a novel quinoxaline derivative.The study used a combination of chemical and electrochemical techniques, as well as some theoretical simulations.The main results of this research can be resumed as follows: PYQX demonstrated exceptional inhibitory efficacy against CS corrosion in acidic electrolytes.As the concentration increases, its performance strengthens, reaching 98.1 % at 1 mM.
• PYQX is a mixed-type inhibitor with a cathodic inclination, as indicated by the PDP profiles, which show that it significantly inhibits cathodic hydrogen evolution processes and anodic metal dissolution.EIS analyses showed that when PYQX is present, Rp values go up and double-layer capacitance decreases.This shows that the inhibitor can slow down CS corrosion.• The chemisorption mechanism of PYQX adsorption on the CS interface is consistent with the Langmuir adsorption isotherm.• Surface and electrolyte investigations (SEM, EDS, and UV-visible) showed that a protective coating has formed over the surface of CS. • DFT and MD simulations showed high reactivity with CS (Fe (110)).

Figure 2 .
Time variation in EOCP for CS in uninhibited 1 M HCl electrolyte, and with a variety of PYQX doses

Figure 3 .
Figure 3. PDP curves for CS in uninhibited 1 M HCl electrolyte and with a variety of PYQX doses

Figure 11 .
SEM/EDS photographs obtained: immediately after polishing (a/b), 24 hours after soaking in 1 M HCl media (c/d), and 24 hours after soaking in 1 M HCl media with 1 mM PYQX (e/f).

Figure 12 .
Figure 12.UV-visible spectra of 1 M HCl including 1 mM of PYQX a -before and b -after 72 hours of CS immersion

Figure 13 .
Figure 13.Content of different protonated forms of PYOX as a function of pH

Figure 17 .
Figure 17.Schematic illustrating of PYQX adsorption mechanism on the C-S surface in chloride solution

Table 1 .
Weight loss and AAS data for CS in 1 M HCl without and with different concentrations of PYQX

Table 2 .
PDP factors and P for CS corrosion in the uninhibited 1 M HCl electrolyte and

Table 3 .
EIS variables and inhibitory effectiveness for CS in the uninhibited 1M HCl electrolyte and with a variety of PYQX doses C / mMRs / W cm 2 Rp / W cm 2 Q / 10 -6 W -1 s n cm -2 n Cdl / F cm -2

Table 4 .
PDP electrochemical variables of CS in 1 M HCl electrolyte without and with 1 mM of PYQX, at different temperatures SCE -c / mV dec -1 a / mV dec -1 icorr / µA cm -2 p / % SCEFigure 8. PDP graphs of CS in 1 M HCl electrolyte a -without and b -with 10 -3 µM of PYQX, at different temperatures

Table 5 .
Activation energy and thermodynamic parameters of CS in uninhibited 1 M HCl electrolyte and with 1 mM of PYQX

Table 6 .
Chemical descriptors (all in eV) of neutral and charged PYQX molecules

Table 7 .
Atoms of local reactivity for neutral and charged PYQX molecule