1 Introduction

Because of its excellent mechanical properties and low cost, carbon steel is mostly used in the construction and industrial fields (Ref 1, 2). Everyone is aware that pickling is typically used after steel has been used to prevent further oxidation and corrosion. In pickling, strong ions can cause the carbon steel surface to corrode. Thus, it is essential to prevent corrosion in the steel (Ref 3). The way that is most convincing is by applying corrosion inhibitors to metals, which lessen acid assault and have proven to be quite successful in practice (Ref 4, 5).

Corrosion inhibitors are frequently synthesized from moderately cheap and promptly accessible crude materials such as carbonyl compounds, thiols, and amines (Ref 6,7,8). The majority of these inhibitors include π- electrons and functional groups such as –C=C–, –OR, –C=O, –S=O, –N=C–, and heteroatoms such as S, O, and N (Ref 9,10,11,12,13,14). These organic molecules elaborate a protective coating to minimize metal corrosion, which safeguards the metal from directly contacting corrosive media and consequently diminishes the corrosion rate. They act on the metal surface through an adsorption mechanism (Ref 15). However, this adsorption relies on many factors, including the molecular structure, type of corrosive medium, and nature of the metal surface (Ref 16).

There are plenty of organic compounds that have been proven to inhibit metallic corrosion. However, some of these organic inhibitors are toxic, and their usage is restricted to safeguard ecosystems. Benzimidazoles and 8-quinolinols were cited as the best N-containing heterocyclic organic compounds, as they possess suitable properties for corrosion mitigation. However, the presence of N, O, and aromatic rings in quinoline and benzimidazole molecules makes them perfect candidates to be employed as corrosion inhibitors (Ref 17,18,19). They are planar compounds with two rings possessing heteroatoms and an aromatic system between the two rings, which can form a uniform film on the surfaces of metal, thus preventing the corrosive attack on the metal (Ref 20,21,22). It is also regarded as a promising group of bioactive compounds, displaying a variety of pharmaceutical activities and having little toxicity (Ref 23,24,25). Recently, benzimidazole analogs and 8-quinolinol-based compounds have received significant interest as efficient corrosion inhibitors for alloys and metals in acidic electrolytes (Ref 26). But no reports on the corrosion mitigation of 8-quinolinol directly bearing a benzimidazole moiety have been found.

The primary novelty of our study lies in investigating the performance of two distinct types of 8-quinolinol directly bearing benzimidazole nuclei (Fig. 1), i.e., 5-(5-nitrobenzimidazol-2-yl)8-quinolinol (NBLQ) and 5-(5-methylbenzimidazol-2-yl)8-quinolinol (MBLQ), as corrosion inhibitors for C–S in HCl electrolyte. The corrosion mitigation of MBLQ and NBLQ was evaluated via the measures of weight reduction and electrochemical. Quantum chemical calculations were also used to highlight the relationship between the molecular structures and the protective effectiveness of 8-quinolinol directly bearing a benzimidazole moiety.

Fig. 1
figure 1

Chemical structures of the investigated 8-quinolinol directly bearing benzimidazole

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2 Experimental

2.1 Materials

The employed electrode was carbon steel (C–S), whose composition (wt.%) was: 0.370 C, 0.230 Si, 0.680 Mn, 0.016 S, 0.077 Cr, 0.011 Ti, 0.059 Ni, 0.009 Co, 0.160 Cu, and the rest including Fe. The C–S samples were painstakingly made ahead of any experiment. They were sanded with diverse grades of emery paper, degreased with alcohol, and air-dried. The reference electrolyte (1 M HCl) was prepared by dilution of 37% hydrochloric acid fuming (Merck) with bi-distilled water. For the weight reduction tests, the dimensions of the C–S substrates are 4.1 × 0.9 × 0.2 cm3, while only 1 cm2 was employed for electrochemical assays. Regarding solubility, MBLQ and NBLQ are completely soluble in the aggressive electrolyte.

2.2 Preparation of Inhibitors

MBLQ and NBLQ were synthesized in our laboratory according to the previously published experimental procedures (Ref 27) as follows: 4-methyl-o-phenylenediamine or 4-nitro-o-phenylenediamine was added to a stirred solution of (2 mmol), sodium metabisulfite (10 mL), and 5-formyl-8-quinolinol (2 mmol) in 30 mL of N, N-dimethylformamide. The mixture was then stirred and heated to 120 °C. After 7 h, 10 mL of water was added to the reaction mixture, and the precipitate obtained was filtered, washed, and purified by column chromatography.

The elemental analysis, Fourier-transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance spectroscopy (NMR) results of the synthesized 8-quinolinol-benzimidazoles derivatives: (1) NBLQ: Brick red powder; Yield 41%; m. p > 260 °C; IR (KBr), υ (cm−1): 3449 (OH); 1370(C–N);1H NMR (300 MHz, DMSO-d6, δppm): 11.399 (s, 1H, N–H), 8.889 (d, 1H, Ar–H), 8.699 (d, 1H, Ar–H), 8.009 (d, 1H, Ar–H), 7.914 (t, 1H, Ar–H), 7.769 (s, 1H, Ar–H), 7.673 (d, 1H, Ar–H), 8.034 (d, 1H, Ar–H), 7.043 (d, 1H, Ar–H); 13C NMR (75.4 MHz, DMSO–d6, δppm): 111.062, 117.554, 119.298, 122.053, 128.014, 129.047, 129.204, 148.170 (Ar–CH), 125.053, 131.853, 131.981, 132.407, 132.443, 133.085, 150.324, 152.976 (Ar–C); Elemental analysis Calcd. for (C16H10N4O3, %): C, 62.74; H, 3.29; N, 18.79 %. Found for (C16H10N4O3, %): C, 62.72; H, 3.34; N, 18.96.

The elemental analysis, IR, and NMR results of the synthesized 8-quinolinol-benzimidazoles derivatives: (2) MBLQ: Brown powder; Yield 52 %; m. p: 213-215 °C; IR (KBr), υ (cm−1): 3482 (OH), 1370(C–N); 1H NMR (300 MHz, DMSO-d6, δppm): 11.146(s, 1H, N–H), 8.889 (d, 1H, quinoline-H), 8.438 (d, 1H, Ar–H), 7.725 (d, 1H, Ar–H), 7.176 (t, 1H, Ar–H), 7.292 (s, 1H, Ar–H), 7.552 (d, 1H, Ar–H), 6.913 (d, 1H, Ar–H), 6.573 (d, 1H, Ar–H), 2.250 (s, 3H, –CH3); 13C NMR (75.4 MHz, DMSO-d6, δppm): 21.238 (CH3), 114.593, 123.665, 127.845, 128.540, 128.607, 145.076, 146.042, 146.485 (Ar–CH). 119.908, 125.920, 128.752, 132.770, 134.072, 139.989, 152.264, 154.023 (Ar–C); Elemental analysis Calcd. for (C17H13N3O, %): C, 74.17; H, 4.76; N, 15.26 %. Found for (C17H13N3O, %): C, 74.10; H, 4.56; N, 15.34.

2.3 Weight Reduction Tests

Tests for weight loss were carried out in line with ASTM standards (Ref 28). Triplicate C–S substrates are dipped in 1 M HCl for 6 hours at 303 K with and without 8-quinolinols. The \(w_{{{\text{cr}}}}\) and \(\eta_{{\text{w}}} \left( \% \right)\) of 8-quinolinols are determined by the Eq 1 and 2 (Ref 29):

$$w_{{{\text{cr}}}} = \frac{\Delta m}{{S \times t}}$$
(1)
$$\eta_{{\text{w}}} \left( \% \right) = \frac{\theta }{100} = \left( {\frac{{w_{{{\text{cr}}}}^{^\circ } - w_{{{\text{cr}}}} }}{{w_{{{\text{cr}}}}^{^\circ } }}} \right) \times 100$$
(2)

where \(\Delta m\) refers to the median mass loss, \(S\) denotes the C–S surface area of the substrates;\(t\) corresponds to the dipping time, \(w_{{{\text{cr}}}}^{^\circ }\) and \(w_{{{\text{cr}}}}\) are the corrosion rates in the reference and inhibited electrolyte, respectively.

2.4 Electrochemical Examination

Potentiodynamic polarization (PDP) and electrochemical impedance (EIS) examinations were realized via the PGZ100 Electrochemical Workstation connected with three-electrode equipment, which features a platinum electrode (counter electrode), working electrode (C–S), and Hg/Hg2Cl2/KCl electrode (reference electrode). The C–S was first dipped in the uninhibited solution at open circuit potential (OCP) during the 1800 s toward reaching a steady state. The EIS measurements were recorded between 105 and 0.01 Hz, and the amplitude of the perturbation signal at the OCP is 0.005 V. In addition, the EIS data were analyzed via Zview 2 software. However, the measurements of PDP were conducted from − 0.8 to 0.1 V/OCP with a sweep rate of 0.5 mV/s. All measures were repeated three times to get reliable data. The diverse parameters of PDP were set from the Tafel zone by the extrapolation technique, comprising corrosion potential (Ecorr), anodic /cathodic Tafel slopes (\(\beta_{{\text{a}}}\),\(\beta_{{\text{c}}}\)), and corrosion current (\(i_{{{\text{cr}}}}^{{}}\)). The protective efficacy (\(\eta\)) was measured according to the following Eq 3 and 4 (Ref 29):

$$\eta_{{{\text{PDP}}}} \left( \% \right) = \left( {\frac{{i_{{{\text{cr}}}}^{^\circ } - i_{{{\text{cr}}}}^{{}} }}{{i_{{{\text{cr}}}}^{^\circ } }}} \right) \times 100$$
(3)
$$\eta_{{{\text{EIS}}}} \left( \% \right) = \left( {\frac{{R_{{{\text{Pi}}}} - R_{{{\text{P}}^{^\circ } }} }}{{R_{{{\text{Pi}}}} }}} \right) \times 100$$
(4)

where \(R_{{{\text{P}}^{^\circ } }}\) and \(R_{{{\text{Pi}}}}\) stand for the reference electrolyte's polarization resistance and its inhibited equivalent, respectively. The acronyms \(i_{{{\text{cr}}}}^{{}}\) and \(i_{{{\text{cr}}}}^{^\circ }\) represent, respectively, the corrosion current density in the reference electrolyte and its inhibited counterpart.

Since we operated under the same settings, we used the data that had already been published (Ref 30) for the effect of concentration and temperature on the reference electrolyte in the electrochemical investigation.

2.5 Surface Studies

The surface of C–S substrates in the reference electrolyte and its inhibited counterpart for 24 h were photographed using a JEOL-JSM-IT-100 instrument. In the absence of inhibitors, we used SEM-EDS analysis in work submitted simultaneously.

2.6 Computational and MD Simulation Details

DFT was used to optimize the geometries of the two envisaged forms of the investigated inhibitors (MBLQ and NBLQ). B3LYP hybrid functional with the 6 − 311 +  + G(d,p) basis set in aqueous solution used the polarizable continuum model (PCM) solvation model, as implemented in the Gauusian-09 series of program packages (Ref 31). The minima of the potential energy surfaces of these substrates were evidenced by the absence of imaginary frequencies used to perform all the calculations. The energy gaps of the investigated forms, ΔEg, were calculated using (ΔEg = ELUMO–EHOMO), where EHOMO and ELUMO are the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).

The most important global reactivity descriptors, such as global hardness (η), electronegativity (χ), and the fraction of electrons transferred (ΔN110), were estimated in terms of the vertical ionization potential (I) and vertical electron affinity (EA) (Ref 32). The calculation of these chemical quantum descriptors is done using the following Eq 5, 6, and 7 (Ref 32):

$$\chi = \frac{1}{2}\left( {\mathop E\nolimits_{{{\text{HOMO}}}} + \mathop E\nolimits_{{{\text{LUMO}}}} } \right)$$
(5)
$$\eta = \frac{1}{2}\left( {E_{{\text{HOMO}}} - E_{{{\text{LUMO}}}} } \right)$$
(6)
$$\mathop {\Delta N}\nolimits_{{{110}}} = \frac{{\mathop \chi \nolimits_{{\mathop {{\text{Fe}}}\nolimits_{{{110}}} }} - \mathop \chi \nolimits_{{{\text{inh}}}} }}{{2\left( {\mathop \eta \nolimits_{{\mathop {{\text{Fe}}}\nolimits_{{{110}}} }} + \mathop \eta \nolimits_{{{\text{inh}}}} } \right)}} = \frac{{\Phi - \mathop \chi \nolimits_{{{\text{inh}}}} }}{{2\mathop \eta \nolimits_{{{\text{inh}}}} }}$$
(7)

The built systems in Materials Studio/8 software are subjected to a dynamic simulation in order to obtain the adsorption pattern of the neutral and protonated forms of the target molecules (MBLQ and NBLQ) on the iron surface (Fe (1 1 0)), the latter remaining energetically more stable in relation to Fe (1 1 1) and Fe (1 0 0) (Ref 33, 34). The simulations are all performed using Forcite with periodic systems through the action of the COMPASS force field. The 33 Å3 vacuum of the vacuum layer is occupied by chemical species present in the simulation cell as 5Cl−5H3O+, 500 H2O, and neutral and protonated forms. This simulation is run in the attendance of the NVT ensemble with time steps of “1 fs,” and “1000 ps” as simulation time. The system was configured at a temperature of 303 K and controlled by the Andersen thermostat. The simulation box takes on dimensions of 31.49*31.49*37.00 Å3 at periodic boundary conditions (Ref 35).

3 Results and Discussion

3.1 Measurements of Weight Reduction

Table 1 summarizes the weight reduction results of C–S substrates immersed in the reference electrolyte and with various concentrations of 8-quinolinol-containing benzimidazole analogs over the course of six hours. With increase in MBLQ and NBLQ doses, the C–S corrosion rate in the reference electrolyte significantly decreased, while the \(\eta_{{\text{w}}} \left( \% \right)\) rose, up to a ceiling of 93.4% for MBLQ and 90.1% for NBLQ at 0.001 M. This may be a result of the 8-quinolinol-containing benzimidazole analogs adsorbing on the C–S through the lone pairs of nitrogen and oxygen atoms as well as the electrons of the quinoline and benzimidazole rings (Ref 36). In addition, MBLQ offers superior inhibitory performance compared to NBLQ, which can be explained by the influence of the methyl group in MBLQ. The methyl group is an electron donor and assists in enriching the electron density at the adsorption sites of the MBLQ and hence raising adsorption susceptibility as opposed to the nitro (NO2) substituent (one electron-attracting group) in the NBLQ, which perhaps diminishes the adsorption susceptibility of the molecule owing to a lower electron density at the adsorption sites. Regardless of the inductive effect, the protection performance is closely related to the Hammett substituent constant (\(\sigma_{{\text{p}}}\)). The negative \(\sigma_{{\text{p}}}\) value of inhibitors is linked to greater adsorption flair in comparison with the positive \(\sigma_{{\text{p}}}\) value. The \(\sigma_{{\text{p}}}\) value of methyl (− 0.17) is more negative as opposed to that of nitro ( + 0.78), revealing that the methyl group has a higher electron-donating capability than the nitro group, which fosters the composition of metal–inhibitor bonds and gives more C–S protection.

Table 1 Weight reduction outcomes of C–S substrates containing diverse amounts of MBLQ and NBLQ at 303 K
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3.2 Open circuit Potential (OCP) and the Measurements of PDP

Figure 2 exhibits the OCP diagram of the C–S electrode in the reference electrolyte and with 8-quinolinols. Figure 2 reveals that the whole OCP curve stabilizes in the first 300 s, indicating that the 1800 s are more than sufficient for the C–S to have reached a steady state. However, it is noticed that after adding the 8-quinolinols, the OCP turns into a nobler trend. This manifests that the metal dissolution kinetics of C–S were influenced more vigorously in the attendance of MBLQ and NBLQ. This is due to the composition of the stable protective film by the 8-quinolinols at C–S, which protects it from the attack of the aggressive electrolyte (Ref 37).

Fig. 2
figure 2

OCP plots for C–S in the reference electrolyte only and with the attendance of 8-quinolinols

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Figure 3 groups the PDP diagrams for C–S in the reference electrolyte and with different amounts of 8-quinolinols. Table 2 summarizes the relevant electrochemical parameters. Figure 3 elucidates that the PDP curve shapes were similar despite the change in amounts for MBLQ and NBLQ, which indicates that the reaction mechanism of the corrosion protection process for MBLQ and NBLQ was unchanged. It also appears that the anodic and cathodic Tafel branches are oriented toward the low-current densities, manifesting that the two anodic and cathodic reactions of C–S corrosion were suppressed in the attendance of MBLQ and NBLQ (Ref 38). The current density of MBLQ and NBLQ declines as the level of their amounts gets higher, indicating that as the inhibitor concentration rises, the inhibitory performance gets better. From − 200 to 0 mV, the anodic current density rapidly increases with rising potential, manifesting that the MBLQ and NBLQ molecules begin to desorb if the potential is above a specific potential, which is generally identified as the desorption potential (Ref 39). This phenomenon is probably associated with metal oxidation, resulting in the desorption of the MBLQ and NBLQ molecules from the C–S surface.

Fig. 3
figure 3

PDP plots for C–S in the reference electrolyte only and with the attendance of 8-quinolinols

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Table 2 Inhibition performance and electrochemical variables for C–S corrosion in the reference electrolyte only and with the attendance of 8-quinolinols
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Analysis of Table 2 reveals that the movement in the values of Ecorr among the uninhibited and inhibited systems is strictly smaller than 85 mV, demonstrating that the 8-quinolinols act as mixed-type inhibitors (Ref 40). Besides, the \(i_{{{\text{cr}}}}\) values are significantly lower in the existence of MBLQ and NBLQ molecules than in their absence. This finding demonstrates that MBLQ and NBLQ molecules more tightly adsorb on the C–S surface via the quinoline and benzimidazole rings and thus inhibit the exposed sites of the C–S surface in charge of the corrosion. The addition of 8-quinolinols (MBLQ and NBLQ) causes alterations in the \(\beta_{{\text{c}}}\) values, suggesting a change in the process of cathodic hydrogen evolution. This implies that 8-quinolinols strongly prevent the corrosion process of C–S, and their inhibitory efficacy rises with concentration. The inhibition of the cathodic process is related to the creation of a surface monolayer caused by inhibitor molecule adsorption (Ref 41). Furthermore, the presence of 8-quinolinols considerably changes the values of the \(\beta_{{\text{a}}}\) compared to the reference electrolyte, suggesting that 8-quinolinols can impact the anodic process kinetics. The presence of heteroatoms in the quinoline and benzimidazole rings, such as nitrogen (N) and oxygen (O), offers lone pairs of electrons that aid in the synthesis of iron (II)-MBLQ and/or NBLQ complex compounds. This changes the dissolving process of C–S (Ref 42). The ηPDP (%) values for both 8-quinolinols rise with concentration, reaching 96.7% for MBLQ and 93.9% for NBLQ when the concentration reaches 0.001 M, respectively. Rising efficiency with dose may be attributed to the composition of the barrier film, which prohibits acid electrolyte from attacking the metal surface owing to the adsorption of adsorbed species of MBLQ and NBLQ on the C–S surface involving interactions among the π-electrons of quinoline and benzimidazole rings and the vacant d-orbitals of iron atoms (Ref 43).

For comparing the inhibition performance obtained by MBLQ and NBLQ with other inhibitors comprising the 8-quinolinol scaffold (Table 3) (Ref 44,45,46,47,48). 8-quinolinols functionalized by electron donor groups or substituted with other aromatic heterocyclic entities were found to significantly improve inhibition performance. The benzimidazole ring, which in our case contains two nitrogen atoms and an π-electron, sharply enhances the electron density of the quinoline moiety, confirming the better inhibitory efficacy of NBLQ and MBLQ compared to the literature inhibitors.

Table 3 Comparative inhibition performance of MBLQ and NBLQ with similar literature inhibitors
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3.3 Measurements of EIS

The Nyquist plots of C–S in the HCl electrolyte alone and in combination with varying concentrations of 8-quinolinols are shown in Fig. 4(A). The impedance charts do not appear to be perfect semicircles, it seems. Solid electrodes that are regularly subjected to the frequency dispersion caused by the heterogeneity and roughness of the C–S surface exhibit the depressing nature of EIS plots (Ref 49). In these diagrams, we can observe that there is only one capacitive loop, which suggests that a process of charge transfer regulates the C–S dissolution mechanism (Ref 50). It is crystal clear from Fig. 4(A) that the loop increases as the dose of 8-quinolinols grows. This behavior might be caused by the formation of a protective layer after the 8-quinolinol molecules adsorb to the surface of the C–S.

Fig. 4
figure 4

Nyquist (A) and Bode diagrams (B) for C–S in the reference electrolyte and with the attendance of 8-quinolinols at 303 K

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To establish the EIS parameters, the equivalent circuit outlined in Fig. 5 is applied to fit the EIS data obtained via the Zview 2 software. In the circuit, \(R_{{\text{e}}}\) refers to the electrolyte resistance, \(R_{{\text{p}}}\) denotes the polarization resistance, and CPE refers to the constant phase component. The CPE was utilized in lieu of the double-layer capacitor (\(C_{{{\text{dl}}}}\)) (Ref 36, 51), which is influenced by defects in the surface. The CPE is expressed by the Eq 8 (Ref 52):

$$Z_{{{\text{CPE}}}} = \left( {\frac{1}{{Q \times \left( {i \times w} \right)^{n} }}} \right)$$
(8)

where \(Q\) stands for the CPE modulus; \(i\) denotes the imaginary number; \(w\) stands for the angular frequency; \(n\) is the deviation index (− 1 ≤ \(n\) ≤ 1), which is related to the heterogeneity of the electrode surface. However, the \(C_{{{\text{dl}}}}\) magnitudes are determined via the Eq 9 (Ref 52)

Fig. 5
figure 5

The employed equivalent circuit

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.

$$C_{{{\text{dl}}}} = \left( {Q \times \left( {R_{{\text{P}}} } \right)^{1 - n} } \right)^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 n}}\right.\kern-0pt} \!\lower0.7ex\hbox{$n$}}}}$$
(9)

All EIS magnitudes registered in Table 4 were obtained with \(\chi^{2}\) values less than or equal to 10−3, revealing that the error among the experimental and fitted values is very small. Based on Table 4, the \(R_{{\text{p}}}\) values in the attendance of NBLQ and MBLQ are larger than the reference electrolyte. The rise in \(R_{{\text{p}}}\) values is attributable to the composition of an isolating protective layer at the metal–electrolyte interface. These outcomes confirm that the NBLQ and MBLQ have been functioning through adsorption at the C–S interface. The \(C_{{{\text{dl}}}}\) decreases, \(R_{{\text{p}}}\) value rises with rising inhibitor concentration, which may be due to an improvement in the thickness of the electrical double layer (Ref 53). Table 4 shows that the attendance of 8-quinolinols causes a reduction in magnitude due to the eventual replacement of the water-adsorbed molecules at the C–S interface by MBLQ and NBLQ molecules with a lower dielectric constant (Ref 15, 36, 54, 55). As indicated in Table 4, the MBLQ performs as the strongest corrosion inhibitor with the ηEIS(%) value = 94.7%, while the NBLQ performs as the weaker corrosion inhibitor with the ηEIS(%) value = 93.5%. These outcomes affirm the results achieved from weight loss and PDP tests and exhibit the same trends. It is common knowledge that methyl is an electron-donating group that can raise the electron density on the benzimidazole ring, enhancing the adsorption capacity of MBLQ on the C–S, thereby, leading to increased inhibition performance. On another, the nitro in NBLQ inhibitor is an electron attractive group that can diminish the electron density on the benzimidazole ring, consequently, reducing the adsorption capacity of NBLQ on the C–S surface, which minimizes the inhibition performance.

Table 4 Electrochemical impedance variables for C–S in the reference electrolyte only and with the attendance of 8-quinolinols
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Bode-phase diagrams for C–S in 1 M HCl alone and in combination with various amounts of 8-quinolinols are displayed in Fig. 4(B). Phase plots for both NBLQ and MBLQ exhibited a single peak, meaning that the process of charge transfer is the only process that happens at the C–S electrolyte interface (Ref 50, 56). When there are molecules of 8-quinolinols at the contact, the peak of the phase-angle rises, causing a more capacitive response (Ref 57). Also, the increase in absolute impedance at low frequencies in the bode diagram supports the great performance with the increase in the dose of 8-quinolinols.

3.4 Temperature Influence

Temperature influence on the C–S corrosion in 1 M HCl alone and in combination with 10−3 M of

8-quinolinols were examined utilizing PDP assays at a temperature of between 303 and 333 K, as manifested in Fig. 6. The accompanying parameters and inhibition performance for 8-quinolinols are compiled in Table 5. Apparently, the growth of \(i_{{{\text{cr}}}}\) is more marked with elevation of temperature in the reference and inhibited systems, and the inhibition performance of NBLQ and MBLQ declines in the concerned temperatures. Therefore, the inhibition performances of the examined 8-quinolinols are mightily temperature dependent. This behavior might be explained by the knowledge that increases in temperature favor shifting the equilibrium of adsorption-desorption toward desorption of the molecules of 8-quinolinols on the C–S (Ref 58). In any case, MBLQ has the best performance compared to NBLQ.

Fig. 6
figure 6

PDP plots of C–S in the reference electrolyte alone and with the attendance of 10−3 M of 8-quinolinols at diverse temperatures

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Table 5 The electrochemical variables for C–S in the reference electrolyte alone and with the attendance 8-quinolinols at several temperatures
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The enthalpy (\(\Delta H_{{\text{a}}}\)), activation energy (\(E_{{\text{a}}}\)), and entropy (\(\Delta S_{{\text{a}}}\)) of activation were computed by the next Eq 10 and 11 (Ref 59, 60).

$$\ln i_{cr} = \ln A - \frac{{E_{a} }}{RT}$$
(10)
$$\ln \left( {{\raise0.7ex\hbox{${i_{cr} }$} \!\mathord{\left/ {\vphantom {{i_{cr} } T}}\right.\kern-0pt} \!\lower0.7ex\hbox{$T$}}} \right) = \ln \left( {R/\left( {Nh} \right)} \right) + \frac{{\Delta S_{a} }}{R} - \frac{{\Delta H_{a} }}{RT}$$
(11)

where the pre-exponential constant, Plank's constant, and Avogadro's number, respectively, are denoted by the letters A, h, and N.

The \(E_{{\text{a}}}\) values for the reference electrolyte alone and with 0.001 M of NBLQ and MBLQ are estimated through the linear fit of \(\ln \,i_{cr}\) vs. \(\frac{1}{T}\) lines (Fig. 7a). Across the slope and intercept of the straight lines obtained from the \(\ln \left( {{\raise0.7ex\hbox{${i_{cr} }$} \!\mathord{\left/ {\vphantom {{i_{cr} } T}}\right.\kern-0pt} \!\lower0.7ex\hbox{$T$}}} \right)\) vs.\(\frac{1}{T}\) plot (Fig. 7b), the \(\Delta H_{{\text{a}}}\) and \(\Delta S_{{\text{a}}}\) magnitudes were acquired and tabulated in Table 6.

Fig. 7
figure 7

The lines of Arrhenius (a) Alternative Arrhenius (b) of C–S in the reference electrolyte alone and with the attendance of 8-quinolinols

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Table 6 Activation variables for C–S in the reference electrolyte alone and with the attendance of 8-quinolinols
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Analysis of Table 6 outcomes exhibits that \(E_{{\text{a}}}\) values for the electrolyte containing MBLQ and NBLQ are greater than those in the reference electrolyte, which attributes to their physisorption on the C–S surface and also discloses that further energy barriers are needed for C–S corrosion than in their absence (Ref 61). In addition, both 8-quinolinols pursue the sequence \(E_{{\text{a}}}\)(MBLQ) > \(E_{{\text{a}}}\)(NBLQ), testifying that the MBLQ can be regarded as the best inhibitor for C–S. The positive sign of \(\Delta H_{{\text{a}}}\) mirrors the endothermic nature of the C–S dissolution reaction and points to the C–S dissolution in the reference electrolyte being tough (Ref 62). The \(\Delta S_{{\text{a}}}\) value was greater and positive in the inhibited electrolyte in comparison with the negative value in the reference electrolyte, manifesting a reduction in the randomness upon the conversion from reactants to activated complex owing to the NBLQ and MBLQ adsorption from the acid electrolyte (Ref 63).

3.5 Adsorption Isotherm

After simulating some model adsorption isotherms (Langmuir, Temkin, and Frumkin isotherms), it turned out that the Langmuir isotherm is better described as “adsorption conduct,” which is expressed by the Eq 12 (Ref 64):

$$C \times \theta^{ - 1} = K_{{{\text{ads}}}}^{ - 1} + C$$
(12)

where θ denotes the coverage of the surface \(\left( {\frac{{\eta_{{\text{w}}} }}{100}} \right)\) and \(K_{{{\text{ads}}}}\) stands for the adsorption equilibrium constant.

The fitted lines of 8-quinolinols based on the weight loss outcomes are outlined in Fig. 8. The values of the linear correlation coefficients (R2) equal one, revealing the best fitting of the Langmuir isotherm. The free energy of adsorption (\(\Delta G_{{{\text{ads}}}}^{^\circ }\)) can be expressed by the Eq 13 (Ref 64):

$$55.5 \times K_{{{\text{ads}}}} = e^{{\left( {{\raise0.7ex\hbox{${ - \Delta G_{{{\text{ads}}}}^{^\circ } }$} \!\mathord{\left/ {\vphantom {{ - \Delta G_{{{\text{ads}}}}^{^\circ } } {RT}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${RT}$}}} \right)}}$$
(13)
Fig. 8
figure 8

Langmuir adsorption line for MBLQ and NBLQ

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The values of adsorptive parameters are listed in Table 7. High values of \(K_{{{\text{ads}}}}\) signal that the MBLQ and NBLQ are readily and forcefully adsorbed on the C–S, resulting in higher levels of performance. Moreover, the adsorption nature was ranked as chemisorption when the \(\Delta G_{{{\text{ads}}}}^{^\circ }\) value was superior to − 40 kJ/mol, while inhibitors behaved as physisorption when the \(\Delta G_{{{\text{ads}}}}^{^\circ }\) value was inferior to − 20 kJ/mol (Ref 65). In our case, the \(\Delta G_{{{\text{ads}}}}^{^\circ }\) of MBLQ and NBLQ is closer to − 40 kJ/mol, implying that the adsorption of MBLQ and NBLQ is by nature chemisorption.

Table 7 Adsorption parameters for C–S in the reference electrolyte with diverse amounts of MBLQ and NBLQ
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3.6 Surface Analysis

Figure 9(a), (b), (c), and (d) illustrates the micrographs of the C–S substrates exposed in the reference electrolyte for 6 h only and with 0.001 M of MBLQ and NBLQ. The C–S substrate, before submerging, shows a clean surface with some distinctive lines that resulted from the mechanical preparation (Fig. 9a). However, as seen in Fig. 9(b), the C–S in the reference electrolyte is severely corroded, numerous cracks appear covered with many corrosion products. NBLQ and MBLQ have significantly enhanced the surface morphologies of substrates when added in comparison with the reference electrolyte, indicating that the layer composition caused by the 8-quinolinols has reduced contact between C–S and the hydrochloric electrolyte (Fig. 9c, d) (Ref 66).

Fig. 9
figure 9

SEM-EDS micrographs for C–S: (a) directly after polishing, (b) after 6 h of submerging in the reference electrolyte, (c) in 1 M HCl + 0.001 of NBLQ, d) in 1 M HCl + 0.001 of MBLQ

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In contrast, Fig. 9 illustrates the EDS spectra of C–S in different situations. The polished C–S surface displays minor peaks related to C–S composition, along with a substantial iron peak (Fig. 9a). When the C–S is submerged in an acidic electrolyte, the EDS spectrum of Fig. 9(b) shows strong characteristic peaks of Cl and O, which represent the HCl electrolyte and Fe oxide components, respectively. These peaks indicate that corrosion products have accumulated on the C–S surface. However, the peaks related to corrosion products and Cl almost vanish when 8-quinolinols are present, as shown in Fig. 9(c) and (d). In summary, based on the SEM-EDS data, it can be concluded that 8-quinolinols effectively inhibit C–S corrosion in an acidic environment.

3.7 FMO Analysis

Reactivity both locally and globally Quantum descriptors give an approximation of a chemical species' chemical reactivity in its most stable geometry and are frequently employed to describe the chemical species' inhibitory potency (Ref 67, 68). NBLQ and MBLQ are able to protonate in the acidic medium HCl, and with the Marvin Sketch software, Fig. 10 indicates the possibility of the occurrence of double protonation at two nitrogen atoms, N3 and N12, for both compounds in the neutral state.

Fig. 10
figure 10

Protonation distribution (%) vs. pH of the MBLQ and NBLQ

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The optimized geometrical structures and frontier molecular orbitals (HOMO and LUMO) of the neutral (MBLQ and NBLQ) and protonated (MBLQ2P and NBLQ2P) molecules are shown in Fig. 11 and 12.

Fig. 11
figure 11

Optimized structures, FMO (HOMO and LUMO) repartitions on the structures of MBLQ and NBLQ neutral

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Fig. 12
figure 12

Optimized structures, FMO (HOMO and LUMO) repartitions on the MBLQ2P and NBLQ2P

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According to an examination of Fig. 11, the MBLQ and NBLQ's entire atomic structure is covered by HOMO and LUMO distributions. This enables us to demonstrate the involvement of numerous atomic centers in the donation and/or acceptance of electrons; the donation takes place through the HOMO orbitals, while the acceptance takes place through the vacant LUMO orbitals (Ref 68). This perspective can therefore improve the responsiveness of MBLQ and NBLQ to the surface support being studied in the HCl electrolyte as well as the adsorption of neutral forms.

According to Fig. 11, MBLQ and NBLQ generate a new FMO distribution as a result of double protonation, with the HOMO daughter occupying the remaining space for the MBLQ2P and NBLQ2P while the LUMO daughter occupies the quinoline's core structure.

The many global responsiveness descriptors that were employed to rate the test drugs' inhibitory potency are listed in Table 8. It was discovered that a molecule's high EHOMO value indicates a strong potential for electron release. On the other hand, a high ELUMO value indicates a low potential for receiving electrons (Ref 69). As a result, the compound's high chemical activity is shown by the low value of the band gap energy (ΔEg) (Ref 70). According to the numbers listed in Table 8 for the two forms, MBLQ's EHOMO energy (− 5.418 eV) is greater than NBLQ's (EHOMO = − 5.946 eV). This demonstrates that MBLQ has a high capacity to provide electrons with the highest reactivity, along with a surface that is well prepared to accept them. Thus, this property has an even stronger effect on the ΔEg, which becomes smaller for MBLQ. According to Table 8, the computations imply that MBLQ (3.740 eV) has a higher degree of reactivity than MBLQ (3.754 eV).

Table 8 Descriptors of the reactivity of the neutral and no-neutral forms
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The occurrence of CH3 in MBLQ as an electron donor group may promote the important reactivity of this molecule in comparison with NBLQ, which bears in its structure an attractor group (NO2) that may negatively influence this behavior. A molecule is hard if electron transfer is difficult. Therefore, electron resistance is measured by hardness (η) (Ref 71). A minimum value of η (eV) implies high inhibitory efficiency (Ref 72). As indicated in Table 8, it is quite evident that the NBLQ (1.877 eV) is much harder than the MBLQ (1.870 eV).

With regard to the electronegativity descriptor (χ), a high χ value of a molecule suggests a high electron attraction effect toward it (Ref 73). Conversely, a total donation effect exists in which a minimum value of χ indicates a strong power to give away electrons more easily. For our purposes, the MBLQ has a smaller χ value (3.548 eV) than that computed for the NBLQ (χ = 4.069 eV). This implies that MBLQ, which has the highest electronegativity value, interferes more positively with the metallic surface.

The largest ΔN value was linked to a high inhibitor efficiency based on prior research, which revealed that ΔN values represent a tendency within a collection of molecules (Ref 72). Our recent research suggests that MBLQ functions as a superb electron donor. Once MBLQ and NBLQ were double protonated, the electron donation was reduced due to falling EHOMO and ΔN110 values, whereas the acceptor effect increased due to falling ELUMO values.

To achieve comparable outcomes to those found with the FMO cast, Fukui indices are a good approach to finding the appropriate target sites for electrophilic (\(f_{k}^{ - }\)) and nucleophilic (\(f_{k}^{ + }\)) attacks. Yang and Mortier described the condensed state of Fukui functions in a molecule with N electrons according to the next Eq 14 and 15:

$$f_{k}^{ + } = \left[ {q_{k} \left( {N + 1} \right) \, - \, q_{k} \left( N \right)} \right]$$
(14)
$$f_{k}^{ - } = \left[ {q_{k} \left( N \right) \, - \, q_{k} \left( {N - 1} \right)} \right]$$
(15)

More details of the calculations are already given by Benhiba et al. (Ref 73). The higher the value of the condensed Fukui function, the more active the atomic center of the molecule (Ref 73).

The data of the \(f_{{\text{k}}}^{ + }\) and \(f_{{\text{k}}}^{ - }\) for the MBLQ and NBLQ in the no-neutral and charged states are concentrated onto the atoms of Fig. 13. As seen in this figure, the distribution of Fukui functions for nucleophilic and electrophilic attacks is nearly completely distributed over all atoms in the MBLQ structure, indicating that these atoms are good sites for receiving and donating electrons. Moreover, the double protonation of the investigated compounds results in an increase in active electron acceptors and a decrease in active electron donor sites.

Fig. 13
figure 13

Condensed Fukui functions of the most active atoms in the MBLQ and NBLQ neutral and protonated forms for nucleophilic (color red) and electrophilic (color red) attacks (Color figure online)

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3.8 MD Simulation

To fully understand the adsorption of the investigated quinoline molecule derivatives in neutral and protonated forms onto Fe (110) surfaces, MD simulations were carried out. Crucially, the inhibitory performance of these compounds was evaluated from a strictly theoretical point of view (Ref 74, 75). Figure 14 and 15 depicts snapshots of lateral, upper, and coverage density adsorption of the subject forms adsorbed on the iron atomic support, achieved upon the MD simulations to reach the equilibrium state. These two figures reproduce the most powerful adsorption configurations of the forms obtained on the Fe (110) surface. As given in these representations, all systems adsorb through the whole structure that spreads over the major part of Fe (110). It appears that these forms have several active donor/acceptor centers that contribute to the increase in inhibition efficiency of the compound of interest.

Fig. 14
figure 14

Images of the side and top views for MBLQ/Fe(110) ((a) adsorption and (b) coverage densities) and NBLQ/Fe(110) ((c) adsorption configuration and (d) coverage densities).

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Fig. 15
figure 15

Snapshots of lateral, upper for MBLQ2P/Fe (110) ((a) adsorption and (b) coverage densities) and NBLQ2P/Fe (110) ((c) adsorption configuration and (d) coverage densities)

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The Einteraction and Ebinding are calculated using the next Eq 16 and 17 (Ref 76):

$$E_{{{\text{Interaction}}}} = E_{{{\text{total}}}} - (E_{{{\text{surface}} + {\text{ solution }} + }} E_{{{8} - {\text{quinolinols}}}} )$$
(16)
$$E_{{{\text{binding}}}} = - E_{{{\text{interaction}}}}$$
(17)

where Etotal is the total energy of all systems, Ei is the total energy of 8-quinolinols.

The interactions of the three studied forms with iron atoms are evaluated by the small value of Einteraction, while the high values of Ebinding show strong adsorption (Ref 77, 78). The energy values of these two descriptors for the four interaction systems are gathered in Table 9. Regarding the calculated values gathered in this table, it appears from the comparative analysis that the most negative value of the MBLQ/Fe(110) system (− 1002.394 kJ mol−1) reflects a better interaction between the neutral form and the Fe (110), while the high value of Ebinding (1002.394 kJ mol−1) of this system shows that the neutral form adsorbs significantly on the atomic layer under study. This suggests that the neutral version of the inhibitor (MBLQ) utilized allows for the Fe to be protected on both energy indices and for the construction of a more effective energy barrier. Therefore, the degree of adsorption is presented in the following way: MBLQ/Fe (110) > MBLQ2P /Fe (110) > NBLQ/Fe (110) > NBLQ2P/Fe (110). This classification clearly confirms the inhibitory efficacy values.

Table 9 Two energy indices values of Fe (110)/8-quinolinols.
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To appreciate the nature of the bonds that are formed between the investigated inhibitor and the iron atoms, the radial distribution function (RDF) approach is invoked. Calculating the interatomic distance between the atoms and heteroatoms of the various forms (MBLQ, NBLQ, MBLQ2, and NBLQ2P), as well as those of Fe, is critical using the RDF analysis (110) (Ref 79). Literature data confirmed that in the case of bond length values below 3.5 Å, the probability of chemical adsorption is higher. In contrast, physical adsorption is more likely (Ref 80). Figure 16 and 17 provides an illustration of the spectral data from this method. According to the first peak that emerged, the bond lengths between the two studied forms and the Fe atoms are less than 3.5 Å. It can be seen that these forms adsorb in the same way despite the effect of protonation, which justifies the values of Einteraction (Ebinding) that are closed between them.

Fig. 16
figure 16

RDF of MBLQ and NBLQ onto Fe (110).

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Fig. 17
figure 17

RDF of MBLQ2P and NBLQ2P onto Fe (110)

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3.9 Adsorption Mechanism of 8-Quinolinols

The manner in which 8-quinolinols adsorb to C–S can occur in various ways. Firstly, it involves the electrostatic interaction of protonated 8-quinolinol molecules (MBLQ2H+ and NBLQ2H+) with pre-existing Cl−ions that are already adsorbed on the C–S (physisorption). Secondly, it involves the interaction among the lone pairs of electrons on the heteroatoms (O and N) and the empty d-atomic orbital of iron (chemisorption). Finally, it involves donor-acceptor interactions between the π-electrons of the quinoline ring and the empty d-atomic orbital of iron. MBLQ2H+ and NBLQ2H+ can bond or adsorb with the C–S through a synergistic impact with the adsorbing Cl− The MBLQ2H+ and NBLQ2H+ will then compete with H+ for electrons on the C–S and eventually release hydrogen gas, returning the 8-quinolinol molecules to their neutral state with free lone pairs of electrons available for empty d-orbitals of iron atoms. The HOMO orbital for both MBLQ and NBLQ is mostly found in the quinolinol and benzimidazole nuclei, suggesting that these rings can donate electrons to iron atoms' unoccupied d-orbitals to form coordinate-covalent bonds via nucleophilic interaction. The negative charges that accumulate on the C–S can be transferred from the d-orbitals of iron to the unoccupied π* of 8-quinolinols (retro-donation) (Fig. 18). All these ways of adsorption contribute to the strong adsorption of 8-quinolinols on the C–S. Ultimately, a layer of adsorbed 8-quinolinol film forms on the C–S, acting as a barrier between the C–S and the corrosive medium, preventing the corrosion of the metal (Ref 80, 81).

Fig. 18
figure 18

A depiction of how the 8-quinolinols adsorb on the C–S

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4 Conclusion

The inhibition properties of 8-quinolinols, namely MBLQ and NBLQ, on C–S in 1 M HCl have been investigated using electrochemical, weight reduction, surface scanning, and quantum chemical calculations. Based on the data, the following conclusions may be drawn:

  • Because of the electron-releasing action of the methyl group, MBLQ has a better inhibitive performance than NBLQ.

  • The examined 8-quinolinols, MBLQ and NBLQ, act as potent corrosion inhibitors for C–S in a 1 M HCl electrolyte, and their effectiveness improves with the inhibitor concentration in the following order: MBLQ (96.7%) > NBLQ (93.9%).

  • PDP curves show that 8-quinolinols significantly inhibit anodic metal dissolution and cathodic hydrogen evolution processes, indicating that they behave as mixed-type inhibitors. EIS tests show that the presence of MBLQ and NBLQ improves charge transfer resistance (RP) while decreasing the constant phase element of the double layer (CPE), hence validating the inhibitors' inhibitory impact on C–S corrosion.

  • The chemisorption of 8-quinolinols on the C–S surface is compatible with the Langmuir adsorption isotherm.

  • The conclusions of computational approaches are congruent with the results of experimental research.