Introduction

In the oil and gas industry, corrosion control and surface technology play a significant role. Corrosion poses a potential hazard in production engineering and significantly impacts all stages of production, from small components to large-scale energy generation1,2. Generally, most equipment tends to corrode when exposed to an acidic environment. Whereas acidic solutions find widespread use in various industries, serving purposes such as cleaning metallic parts of machines, tanks, and pipelines, the eventual result accompanying these processes is corrosion, which manifests as damage to the metal. Addressing corrosion, deemed as economically challenging, has paved the way for the explorations of various technologies. In this regard, researchers have proposed various strategies, including modifications to material compositions3,4, adjustments to solution formulations5, enhancements in operational procedures6, and the adoption of corrosion inhibitors7,8,9,10,11 and coatings12,13 as practical approaches to mitigate corrosion. Organic inhibitors are usually used to prevent this unwanted issue by sticking to the metal’s surface. The way they stick is affected by several factors, including how the metal surface behaves, the electrical potentials at the edges, and the properties of the inhibitor itself, such as its structure, how many places it can attach to the particle, the size of the particle, its charge density, the method of adsorption, the formation of metal complexes, and how much area the inhibitor covers on the metal’s surface, all of which affect how well the inhibitor works. The process of adsorption thereby involved is highly influenced by various factors, which includes metal surface’s behaviour, the electrochemical potentials at the edge, and the inhibitor’s intrinsic characteristics, which include the inhibitor’s structure, the adsorption site’s amount within the particle, the particle’s size, charge density, the adsorption method, metal formation, and the inhibitor’s projected area on the metal’s surface, which in turn determine the inhibitor’s efficiency14,15,16,17,18. Thus, in this regard, organic particles including hetero molecules like sulphur (S), oxygen (O), nitrogen (N), or π-electrons, are demonstrated to play a significant role in inhibiting corrosion effectively due to the existence of high-density electrons to interact with the surface of the metal and get adsorbed effectively19.As per the reported literature, Schiff bases have emerged as effective corrosion inhibitors, especially in the acidic medium owing to the occurrence of the azomethine group (-C = N) in their structures. R. Kooliyat et al. investigated the corrosion inhibition potency of a Schiff base 2,2’-(5,5-dimethylcyclohexane-1,3-diylidene)bis(azan-1-yl-1-ylidene) diphenol on mild steel in 1 M HCl and 0.5 M H2SO420. The research observed that the Schiff base acts as a varied type and an excellent corrosion inhibitor for mild steel in both the acid solutions. Further, the inhibitory properties of the two Schiff bases 2-(4-hydroxy-3-methoxybenzylidineamino)-4-nitrophenol and 5-((4-chloro-3-nitrophenylimino)methyl)-2-methoxyphenol were estimated for the mild steel’s corrosion in 1 M solution of HCl. The studies show that they are effective for preventing the mild steel from corroding in polluted or acidic environments21. In addition, the inhibition efficiencies of six new SBs, specifically, (((E)-(p-Tolylimino)methyl)phenol (1), ((E)-(Phenylimino)methyl)phenol (2)), (2 (E)-2-(Hydroxybenzylideneamino)benzoic acid (2-((E)-(2 Mercaptophenylimino)methyl)phenol (4)), 2-((E)-(2 Hydroxyphenylimino)methyl) phenol (5), and (E)-2-(Hydroxybenzylideneamino)benzoic acid (6) on the stainless steel (304SS) corrosion in the solution of hydrochloric acid were studied by cyclic polarization, polarization resistance, and weight loss processes. The findings demonstrated that the Schiff bases (1–6) were effective corrosion-inhibiting agents for stainless steel (304SS) in acidic environments. The investigated Schiff bases exhibit an improvement in inhibitory efficiencies in the order 1 < 4 < 2 < 3 < 5 < 6. The six Schiff bases exhibit varying inhibitory efficiency, which could be attributed to variations in their molecular structure, functional group type, and the adsorption manner22. In lieu of these reports, our present study aims to synthesise a novel Schiff base (Z)-2-((3-nitrobenzylidene)amino)phenol (NBAP) and assess its performance as an efficient and proficient inhibitor towards the XC70 steel’s deterioration in the 1 M of hydrochloric acid. The synthesized NBAP was successfully characterized by various spectral processes like 13 C and 1 H nuclear magnetic resonance (NMR), ultraviolet-visible spectroscopy (UV-Vis), elemental and FTIR (Fourier Transform) analysis. The inhibition performance of NBAP was investigated using electrochemical impedance measurements, potentio dynamic polarisation, and surface studies via scanning electronic microscopy measurements. We also performed quantum chemical computations using the functional theory of density processes, which provided insights into the corrosion inhibition mechanism. The detailed synthesis of NBAP with chloric acid has cleared the door for a thorough investigation of its characteristics and performance. NMR, UV-Vis, elemental analysis, and FTIR spectroscopy have all revealed structural nuances and spectral properties that are critical for understanding this compound’s behavior. Furthermore, the analysis of its inhibitory properties using impedance, polarization, and scanning microscopy reveals its potential applications in corrosion prevention. These findings, together with strong quantum chemical computations based on density functional theory, establish a coherent narrative that highlights NBAP’s promise and lays the groundwork for future study in this field. The combination of synthesis, characterization, and performance evaluation confirms NBAP as a substantial contributor to advances in material science and engineering.

Experimentation

Instruments and materials

All chemicals and reagents were procured from Merck, Fluka or Sigma-Aldrich and utilized without any further purification. The UV-visible spectrum was observed by utilizing a UV-Vis spectrometer UV-1800 Shimadzu. The Schiff base NBAPFTIR spectrum was observed using the Fourier transform infrared spectrophotometer, Shimadzu, using the conventional KBr pellet technique 400–4000 cm⁻¹ range. The NBAP melting point was resolved on a 7779 Kofler Bank instrument. 13 C-NMR and 1 H-NMR spectra were observed on Bruker’s spectrometer (at 100 MHz and 400 MHz working frequencies). The LECO TruSpec Micro CHNS elemental micro analyser was used to determine the elemental composition and weight%. The working electrode for electrochemical measurements was an XC70 mild steel’s ranking with the subsequent compound compositions (wt%): 0.245% Si, 0.065% C, 0.001% S, 0.042% Al, 1.685% Mn, 0.002% P, 0.010% Cu, 0.042% Cr, 0.265Ni, 0.005% Mo, 0.010% Cu, 0.014% V, 97.55% Fe, and Nb. The corrosive 1 M solution of HCl was arranged by using double deionised water and the 37% Merck HCl solution.

Synthesis of the schiff base

The reaction of 3-nitro benzaldehyde and 2-aminophenol in methanol under nitrogen conducted to the formation of (Z)-2-((3-nitrobenzylidene) amino) phenol (NBAP) in a 58.05% yield (Fig. 1)23.

Fig. 1
figure 1

Schematic of reaction for Schiff base NBAP synthesis.

Full size image

A solution of 3-nitro benzaldehyde (0.76 g, 2 mmol) in absolute methanol (8 ml) was added in a dropwise manner to a solution of 2-aminophenol (0.57 g; 2 mmol), dissolved in methanol (8 mL) (Fig. 1). The obtained mixture was refluxed, and stirred under dinitrogen atmosphere for 24 h. The brown precipitate was filtered and washed several times with cold methanol and finally with diethylether.

The synthesized NBAP compound’s solubility was high in dimethyl sulfoxide (DMSO), methanol, dichloromethane and CHN studies verified its purity.

M.P: 122 °C. 1H-NMR (DMSO-d6, 400 MHz; ppm): 8.741 (1s, -OH); 8.706 (-CH = N-, 1s); 6.71–8.42 (aromatic protons, m, 8 H); 13C-NMR (DMSO-d6,100 MHz, (δ) ppm): 155.532 (-CH = N-); 115.629 to 151.829 (aromatic carbons, m, 12 C). IR (KBr, cm− 1): 3512 υ (OH), 3077 − 3044 υ (CH), 1584 υ (C = N), 1476 υ (C = C), 1508,1334 υ (NO2). UV-Vis (nm, λmax, MeOH as solvent): 236, 268. Anal. Calc. for C13H10N2O3: C, 64.46; H,4.16; N, 11.56%. Found: C, 64.87; H,4.07; N, 11.31.

Electrochemical study

The corrosion behavior of XC70 steel was investigated using electrochemical techniques, including Electrochemical Impedance Spectroscopy (EIS) and Potentio dynamic Polarization (PDP). Electrochemical measurements were performed with a Potentiostat-Galvanostat PGZ 301 and a 40 radiometer VOLTALAB, integrated with Voltamaster 4 software for data acquisition and analysis. The electrochemical tests were conducted in a thermostatic cell maintained at a constant temperature (± 1 °C) to simulate real-world conditions.

For the electrochemical setup, a silver chloride–silver (Ag/AgCl) reference electrode (1 M KCl saturated) was used to measure the potential, while a platinum plate served as the counter electrode (CE). The mild steel XC70 was used as the working electrode (WE), with its exposed surface area of approximately 1 cm². Before each measurement, the steel specimen was polished with 600-grit emery paper, followed by cleaning with distilled water and ethanol, and then immersed in the test solution. To achieve a stable open circuit potential (OCP), the specimen was immersed in the corrosive solution for 30 min, both in hydrodynamic and stagnant conditions, ensuring the stabilization of the system.

For the PDP measurements, the potential was scanned from − 700 mV to -300 mV (vs. Ag/AgCl) at a scan rate of 1 mV/s, in order to determine the corrosion current density (icorr), corrosion potential (Ecorr), and Tafel slopes (βa and βc) under varying inhibitor concentrations and temperatures. These measurements were carried out in 1 M HCl solution at different temperatures (25 °C, 35 °C, and 45 °C) to assess the effect of temperature on corrosion inhibition.

For the EIS measurements, the impedance spectra were recorded at the corrosion potential over a frequency range from 100 kHz to 0.01 Hz, using an AC amplitude of 10 mV. The charge transfer resistance (Rct) and double layer capacitance (Cdl) were obtained from the Nyquist plots, and the inhibition efficiency (EIEIS) was calculated based on the changes in the impedance data with and without the inhibitor. The corrosion potential (Ecorr), current densities (icorr), and the cathodic and anodic (βc, βa) slopes were ascertained using the Tafel plot’s extrapolation. The efficiency (EIp) of corrosion inhibition was evaluated by utilizing the subsequent equation (Eq. 1)24,25:

$$E{I_p}\left( \% \right)=\left( {\frac{{\left( {i_{{corr}}^{o} - {i_{corr}}} \right)}}{{i_{{corr}}^{o}}}} \right) \times 100~$$
(1)

Whereas, \({i_{corr}}\) and \(i_{{corr}}^{o}\) are the current densities (corrosion) value after the XC70 immersion in the acidic medium with and without the inhibitor correspondingly and \(E{I_p}\) is the corrosion inhibition efficiency.

Spectroscopy dimensions of electrochemical impedancewere passed out following the immersion of 30 min of the carbon steel, at constant potential\(\left( {{E_{OCP}}} \right)\), in a 100 kHz to 10 MHz frequency range with 10 mV amplitude excitation signal. The corrosion inhibition efficiency (EIEIS(%)) was evaluated from the findings of impedance using Eq. (2)24,25:

$${\text{E}}{{\text{I}}_{{\text{EIS}}}}\left( {\text{\% }} \right)=\frac{{{{\text{R}}_{{\text{ct}}\left( {{\text{inh}}} \right)}} - {{\text{R}}_{{\text{ct}}}}}}{{{{\text{R}}_{{\text{ct}}\left( {{\text{inh}}} \right)}}}} \times 100$$
(2)

Where \({{\text{R}}_{{\text{ct~}}\left( {{\text{inh}}} \right)}}\) and \({{\text{R}}_{{\text{ct~}}}}\) are the resistance values (charge transfer) in the presence and absence of XC70 inhibitor of mild steel in the 1 M HCl respectively.

SEM morphological characterization

The polished mild steel XC70 was absorbed in the corrosive medium for about 50 days, and the morphological analysis was performed in the presence and absence of the inhibitor (10–4 M) via SEM imaging. The SEM micrograph was recorded by using the field emission scanning electron microscopy (FESEM) JEOL Neoscope JCM-5000 of the unit of Thin Films Development and Applications, Research Centre in Industrial Technologies CRTI, Algeria.

Theoretical calculations

The quantum chemical calculations were performed using Density Functional Theory (DFT) method. The detailed calculation of the molecular structure of NBAP in both the gaseous and aqueous phase is effectuated with 09 program of Gaussian26, using functional correlation B3LYPand basis set of 6-31G27. The electron affinity (A), absolute electronegativity (χ), ionization potential (I), electrophilicity index (ω), energy gap (ΔE), dipole moment (µ), global hardness (η) and softness (σ), and the number of electron’s transferred (ΔN) for the NBAP were calculated using the following Eqs. (3) and (4):

$$\chi =I+\frac{A}{2}~$$
(3)
$$\eta =I - A/2$$
(4)

where, a/c to Koopmans’ theory32, A and I are associated with the orbital energies frontier a/c to Eqs. (5) and (6):

$$I= - {E_{HOMO}}$$
(5)
$$A= - {E_{LUMO}}~$$
(6)

The η and χ values for the NBAP studied were evaluated using A and I values attained from the evaluations of chemical’s quantum.

ΔNis the transferred electron’s amount was evaluated by utilizing the below relation (Eq. 7):

$$\Delta N=\frac{{\left( {{\chi _{Fe}} - {\chi _{Inh}}} \right)}}{{2\left( {{\eta _{Fe}}+{\eta _{Inh}}} \right)}}$$
(7)

Where, χInh and χFe is the absolute electronegativity of inhibitor and iron respectively, and ηInh and ηFe is the absolute hardness of inhibitor and iron respectively. The hypothetical χ (7.0 eV mol− 1) value and η (0 eV mol− 1) value for iron were obtained from the previous studies28.

The value of electrophilicity index (ω) and softness (σ) were evaluated by utilizing the relations below (Eqs. 8 and 9):

$$\sigma =\frac{1}{\eta }$$
(8)
$$\omega =\frac{{{\chi ^2}}}{{2\eta }}$$
(9)

Results and discussion

Spectroscopic studies

1H-NMR and 13C-NMR analysis

The structural features of the as-synthesized NBAP were established from13C-NMR and 1H-NMR spectra as presented in Fig. 2a. In the 1H-NMR spectra, the O–H proton and the azomethine proton are responsible for the singlet peaks at 8.741 ppm and 8.706 ppm, respectively29,30. The presence of the singlet peak of the azomethine validates the Schiff base NBAP formation. The NBAP aromatic protons are examined as multiplets in 6.71–8.42 region30,31. In the 13C-NMR spectra of NBAP (Fig. 2b, the peak at155.532 ppm is attributed to imine carbon atom while those for aromatic carbons, the peaks are observed in the region of 115.629 to 151.829 ppm.

Fig. 2
figure 2

(a) 1H-NMR and (b) 13C-NMR spectra of the Schiff base NBAP.

Full size image

FT-IR analysis

To confirm the presence of functionalities within the as-synthesized Schiff base NBAP, FTIR spectra was recorded as depicted in Fig. 3. The FTIR spectrum illustrated a characteristic band at 3512 cm− 1 which is assigned to the OH group of stretching vibrations. A sharp peak at 1584 cm− 1 is ascribed to the occurrence of azomethine group (C = N)32,33.The band observed at 1476 cm− 1 is allocated to the stretching vibrations (C = C) of aromatic rings32.Further, the peaks at 1508 and 1334 cm− 1 are assigned to the asymmetric and symmetric NO2group stretching vibrations34. All these spectral features in FTIR clearly confirm the successful formation of NBAP.

Fig. 3
figure 3

FT-IR spectra of the Schiff base NBAP.

Full size image

UV-Visible study

The UV-Visible spectrum of NBAP was recorded at an ambient temperature in methanol at the spectral range of 250–800 nm and is depicted in Fig. 4. The as-synthesized Schiff base clearly shows the presence of two bands at 236 nm and 268 nm which are allocated to π → π* transitions of aromatic system and n → π* transitions of azomethine (-C = N) group respectively35.

Fig. 4
figure 4

UV-Vis spectrum of the as-synthesized Sciff base NBAP.

Full size image

Measurements of potentiodynamic polarization

Figure 5 shows the XC70 steel Eocp vs. time curves in the absence and presence of NBAP. The values of the Eocp in the presence of different concentrations of NBAP move to negative values compared with that of steel in the absence of inhibitor, thus increasing the adsorption of inhibitor molecules on the metal surface. These molecules form a corrosion-resistant barrier to the metal. TheXC70 steel potentiodynamic curves of polarization in the 1 M solution of HCl in the absence and presence of NBAP inhibitor at different concentrations are depicted in Fig. 6. The important corrosion parameters obtained from the polarization studies viz.: corrosion potential (Ecorr), the cathodic and anodic Tafel slopes (βc, βa), corresponding inhibition efficiency (\(\:{EI}_{p}\)%), and corrosion current densities (icorr) at diverse concentrations of NBAP are displayed in Table 1. From the potentio dynamic polarization studies, the following speculations were made.

Fig. 5
figure 5

Eocp vs. time curves for XC70 steel in 1 M HCl solutions with.

various inhibitor concentrations at 293 K.

Full size image
Fig. 6
figure 6

Polarization curves for mild steel XC70 at 293 K in 1 M HCl with and without varying concentrations of Schiff base NBAP.

Full size image
Table 1 Electrochemical polarization parameters for XC70 steel in 1 M HCl without and with different concentrations of NBAP.
Full size table

(i) The corrosion potential (Ecorr) values obtained in presence of NBAP were found to shift toward anodic values. (ii) Subsequent decrease in the corrosion current density (icorr) was observed in the presence of the inhibitor NBAP. This was ascribed to the adsorption of the NBAP inhibitor on the surface of steel. (iii) The inhibitory efficiency of Schiff base NBAP was found to raise with rise in the concentration still it reached a highest efficiency value of 89% at an optimum concentration of 10− 4 M. The high inhibition efficiency exhibited by NBAP clearly indicated its strong adsorption on XC70’s steel surface and the excellent protective ability of inhibitor against the corrosion. The excellent performance of NBAP is due to the occurrence of nitrogen and oxygen donor atoms which have better coordination ability with Fe2+ based metal sites on the steel surface which results in strong adsorption and thereby high inhibition efficiency36,37. (iv) The addition of NBAP Schiff base inhibitor was found to affect both anodic as well as cathodic Tafel slopes (βa, βc). Therefore, NBAP can be measured as a varied type (anodic/cathodic) inhibitor38.

All the inferences made from potentiodynamic polarization studies clearly demonstrated the Schiff base NBAP efficiency as an inhibitor of corrosion.

Electrochemical impedance measurements (EIS)

Electrochemical Impedance Spectroscopy data of the mild steel’s XC70 in 1 M of HCl was recorded within and without the diverse concentrations of NBAP Schiff base in order to provide the capacitive and resistive behavior of the solution/metal interface. The corresponding XC70 mild steel’s Nyquist plots in the presence and absence of different concentrations of NBAP obtained are depicted in Fig. 7.The Nyquist diagrams show single capacitive loop with non perfect semicircles because of frequency dispersion ensuing from the roughness and non-homogeneity of the surface of steel24. Moreover, the diameters corresponding to the resistance (Rct) of charge transfer are affected by the varying concentration of the as-synthesized Schiff base. The study of the inhibitory efficiency of the compound NBAP as a concentration function shows that the semicircle’s diameter rises with rise in concentration. This clearly implies to the fact that the charge transfer resistance to corrosion process increases. This can ascribe to the Schiff base NBAP adsorption on the mild steel surface of XC70 which subsequently decreases the rate of corrosion of the XC70 steel39,40. These results reveal that the corrosion of mild steel in 1 M solution of HCl media is controlled by the process of charge transfer21,38. Besides, the adsorption of the inhibitor results in the formation of porous layers on the steel surface and adds to the heterogeneity of the surface layer of XC70 steel. The comparable circuit is utilized to illustrate the impedance spectra obtained in our case is depicted in Fig. 8.

Fig. 7
figure 7

Plots ofNyquist for steel XC70 in 1 M HCl without and with diverse concentrations of NBAP at 293 K.

Full size image
Fig. 8
figure 8

The comparable circuit utilized for simulation of the spectra of electrochemical impedance for the corrosion inhibition study of XC70 mild steel by Schiff base NBAP.

Full size image

Figure 9 represents the Bode plots (phase angle and Magnitude of Bode impedance) observed for the steel electrode of XC70absorbed in 1 M of HCl without and with the NBAP diverse concentrations. From the Bode plots (Fig. 9a), it could be seen that the overall impedance rises with rising inhibitor concentration (log Zi vs. log f). Furthermore, the Bode plots show a steady increase in the phase angle shift, which is clearly correlated with the rise of inhibitor adsorption on the surfaces of carbon steel (Fig. 9b).

Fig. 9
figure 9

Bode (a) and phase angle (b) plots for XC70 mild steel in 1 M HCl in the absence and presence of different concentrations of NBAP at 293 K.

Full size image

The various electrochemical parameters and the efficiency of inhibition obtained from the measurements of EIS (EIEIS%) for the XC70 corrosion at varied concentrations of the NBAP are tabularize in Table 2. In contrast to the unfettered system, Table 2 shows that the inhibitory system’s charge transfer resistance (Rct) has increased. As the inhibitor concentration increased, the Rct values increased while the capacitance values (CPE) tends to decrease and the CPE values decreased from 446.2 to 69.80 µF/cm2. The decrease in the values of CPE strongly supports the inhibitor’s adsorption on the surface of steel. The decrease in CPE is explained by a decrease in the local dielectric constant and/or an increase in the thickness of the double electrical layer, resulting from the adsorption of inhibitory molecules following the replacement of water molecules by the inhibitor molecules on the electrode surface41,42,43.Further, the Rct values increases from 49.92 to 255.3 Ohm.cm2as the inhibitor reaches the optimal concentration. The inhibition’s efficiency also ascends with the rise in inhibitor’s concentration and attains an amount of the order of 80% at 10− 4 M (optimum). These findings unambiguously signify that the molecule’s inhibitor gets chemically adsorbed on steel’s surface and form thicker or better dispersed inhibitory films that protect the surface of the mild steel XC70 from corrosion.

Table 2 Electrochemical impedance parameters for XC70steel in 1 M HCl without and with different concentrations of NBAP.
Full size table

Adsorption isotherm

The organic molecules interaction having polar groups or atoms adsorbed on the cathode or anode sites on the metal surface acts an important part in the phenomenon of adsorption. In order to understand the inhibition effect of NBAP due to adsorption, the coverage values of fractional surface (θ) as a inhibitor concentration’s function was attained from the following equation (Eq. 10):

$$\frac{C}{\theta }=\frac{1}{{{K_{ads}}}}+C$$
(10)

where θ is the surface coverage, Kads is equilibrium adsorption constant, and C is NBAP concentration (M).

To facilitate the most significant isotherm adsorption that fits well to the NBAP adsorption on the XC70 mild steel, numerous isotherms adsorption viz. Temkin, Frumkin, and Langmuir were evaluated to fit the recovery rate values (θ = EI (%) /100) to the isotherm standard. All these isothermal models were compared according to the coefficient of the correlation R2. After testing the different isotherms (Fig. 10), the fines t explanation of the behavior of adsorption to the Schiff base NBAP studied was provided by Langmuir Adsorption isotherm44,45 with a coefficient of regression up to 0.999 (Fig. 10a). The constant of equilibrium (Kads= 1.42 × 106) of adsorption was resolute from the straight-line intercepts obtained from the plot of Cinh/θ versus C. The Kads value is directly correlated to the inhibition efficiency i.e. higher the value of Kads, stronger is the adsorption and hence more effective inhibition46,47. The high value of Kads obtained (1.42 × 106) for the tested inhibitor viz. The NBAP signifies that the inhibitor strongly adheres to the XC70 mild steel surface and exhibit high inhibition efficiency. This is in agreements with our results obtained from electrochemical measurements.

Fig. 10
figure 10

Different Adsorption isotherms (a) of Langmuir, (b) Fremkin and (c) Temkin for Schiff base NBAPin1 M HCl at 293 K.

Full size image

Besides Kads, the free energy of adsorption (ΔG0ads) was obtained from the adsorption constant value by using the following equation:

$$\Delta G_{{ads}}^{^\circ }= - RTln\left( {55.5 \times {K_{ads}}} \right)$$
(11)

Where R and T are gas constant and system temperature respectively while as 55.5 is the water’s molar concentration48.The free energy values hence obtained was found to be equal to -44.3 kJ/mol. The negative value of ΔG°ads confirmed the spontaneity of the adsorption of NBAP and the stability of the adsorbed layer on the surface of mild steel. It is worth to mention that the ΔG°ads value reflects the behavior of adsorption procedure. If the ΔG°ads is − 20 kJ/mol or fewer, then it indicates a physisorption process while if itis − 40 kJ/mol or extra negative then it indicates that the process is chemisorption and involves formation of covalent or coordinate bond among the metal surface and inhibitor molecules49,50,51,52. As the calculated value of ΔG°ads for the studied inhibitor is − 44.3 kJ/mol, it is evident that tested inhibitor is chemisorbed on the steel’s surface.

Temperature’s effect

Temperature is a significant factor in determining the efficiency of adsorption process. Depending on the type of process involved and the molecular nature, it can modify the interactions among the metal surface and inhibitor molecules. It is known that increasing the temperature generally accelerates the release of hydrogen from the metal surface in an acidic media, which leads to a higher dissolution rate of the steel. A study using different temperatures among 293 and 333 K was conducted to verify the impact of this parameter on NBAP compound’s inhibitory power. Figure 11 denotes the XC70 steel’s polarization curves in 1 M solution of HCl in the presence and absence of NBAP at the optimum concentration of 10− 4 M and at diverse temperatures after 30 min of immersion time. Table 3 provides a summary of the electrochemical parameter and inhibitory efficiency values related to steel corrosion in 1 M HCl with and without 10− 4 M of NBAP at different temperatures. From the Fig. 10, it is quite clear that there is a rise in the corrosion current density (icorr) with a rise in temperature53. The anodic and cathodic Tafel slopes are parallel, indicating the same mechanism for the steel’s disintegration and proton reduction at its surface. It is known that the rise in the efficiency of inhibition at high temperature is associated to the chemisorption of the adsorbed species24,54. The inhibition rate slightly increases with temperature. However, the NBAP compound has a notable about 92% inhibitory efficiency at T = 333 K. This can be attributed to the occurrence of oxygen atom, nitrogen atom, and the π electrons of the aryl rings in the NBAP compound.

Fig. 11
figure 11

Tafel graphs showing the corrosion of XC70 steel in 1 M HCl at various temperatures with and without 10− 4 M of NBAP.

Full size image
Table 3 Electrochemical characteristics of corrosion of XC70 steel in 1 M HCl as a function of temperature in absence and presence of inhibitor NBAP.
Full size table

The corrosion rate’s dependence on the temperature is articulated by the Arrhenius relation:

$${i_{corr}}=A{\text{exp}}\left( { - \frac{{{E_a}}}{{RT}}~} \right)~~$$
(12)

where,

Ea= activation energy. R = Universal gas constant. T = Temperature. A = Arrhenius pre-exponential factor.

A plot of ln icorr as a temperature function(1/T) is depicted in Fig. 12. A linear variation is exhibited by the mild steel in the 1 M solution of HCl in the presence and absence of inhibitor. Subsequently the values of activation energy (Ea) are determined using Eq. 12. The value of Ea for the inhibited system was found to be higher in acid solution. The higher value of Ea is ascribed to the NBAP chemisorption on the steel surface54,55. This suggests that the NBAP provides a higher barrier of energy for the reaction of corrosion by covering more quantity of sites on the surface of steel in existence of acidic medium, thereby leading to lower corrosion rate and hence demonstrates superior inhibition efficiency.

Fig. 12
figure 12

Arrhenius graphs of XC70 steel in 1 M of HClwith and without 10− 4 M of NBAP.

Full size image

The kinetic model was utilized to evaluate the corrosion reaction’s thermodynamic characteristics like: ΔSa and ΔHa to determine the interaction of NBAP particles with the surface of steel. The parameters were calculated for the activation complex formation in the transition state relation using the transition state expression as presented in Eq. (13):

$${i_{corr}}=~\frac{{RT}}{{Nh}}\exp \left( {\frac{{\Delta {S_a}}}{R}} \right)\exp \left( { - \frac{{\Delta {H_a}}}{{RT}}} \right)$$
(13)

Where, ‘N’ denotes Avogadro’s number and ‘h’ is the Plank’s constant.

Figure 13 depicts the deviation of lni/T as a temperature function (1/T)in the linear plots form with slope equal to -ΔHa / R and the line’s extrapolation give the intercept’s [ln R / Nh + ΔSa / R] values from which theΔSa and ΔHa values are calculated. The corresponding values of thermodynamic parameters are summarized in Table 4.

Fig. 13
figure 13

Statetransitioncurves for XC70steel in 1 M HCl with and without 10− 4 M of NBAP.

Full size image
Table 4 Activation characteristics for the adsorption of NBAP on XC70 steel surface.
Full size table

The positive value of the activation enthalpy (23.43 kJ / mol) suggests the NBAP adsorption on the XC70 mild steel is an endothermic process. Further, it also signifies that the chemisorption process is predominant as reported by54,55,56,57. Additionally, the activation entropy’s (ΔSa) negative sign in both uninhibited and inhibited solutions symbolizes an association before dissociation in the complex of activation produced in the rate-determine step of the corrosion process. Further, the more negative value of ΔS in the existence of the inhibitor in compare of the uninhibited sample can be explained in terms of blocking of the reaction of cathodic and anodic sites by the inhibitory molecules indicating a decrease in the disorder reactive to the activated complex54. We can notice a fine conformity of our outcomes with the literature58,59.

Morphological analysis

The morphological analysis of surface within and with no inhibitor in 1 M of hydrochloric acid was carried out by SEM (Scanning Electron Microscope). Figure 14 represents the SEM micrographs ofXC70 in the 1 M solution of HCl within and without of 10− 4 M of NBAP after seven weeks of immersion. As seen in Fig. 14a, cracks and pits could be observed on the mild steel’s surface. This indicated severe corrosion of the surface of steel in 1 M solution of HCl in the nonexistence of the inhibitor. Figure 14b depicts the surface characteristic of steel’s surface in presence of the inhibitor and it is clearly seen that it shows a surface covered with a protective layer. This is due to the formation of a complex between NBAP and Fe2+ on the surface of metal preventing the steel’s dissolution. The occurrence of this barrier layer is because of the inhibitory molecule’s adsorption on the surface, which substantially decreases the rate of corrosion of the metal and thereby protects it from losing out to corrosion.

Fig. 14
figure 14

SEM images of steel surface after immersion for 7 weeks at 293 K in 1 M HClin absence of inhibitor (a); with inhibitor NBAP[10− 4 M](b).

Full size image

Quantum chemical calculations

Density Functional Theory (DFT) is a modeling process widely utilized to study molecular and electronic structure of a molecule. We computed the Quantum structure-activity relationships (QSAR) to find out the molecular structure’s effect of tested inhibitor on the efficiency of corrosion inhibition. This is performed by calculating various parameters of quantum chemical and correlating with the experimental attained corrosion inhibition efficiency60,61. EHOMO is the Highest Occupied Molecular Orbital’s energy (HOMO) while as ELUMO is the Lowest Unoccupied Molecular Orbital’s energy (LUMO). Generally, HOMO shows a tendency to donate its electron while LUMO possesses the electron acceptance capability. In general, a high value of EHOMO represents the ability of the inhibitor to provide electrons to the transition metal’s uninhabited d-orbital while as lower ELUMO involves the capacity to admit electrons from the surface of metal60,61,62.The LUMO, optimized structure, and HOMO of the as-synthesized compound NBAP are presented in Fig. 15. It is evidently seen in Fig. 15 that the HOMO site’s density of electron is situated almost over the whole chemical structures of the NBAP inhibitor. On the other hand, the uninhabited molecular orbital’s (LUMO) electron density is distributed only to the phenyl linked to the group nitro51. This implies that not only the π orbitals (HOMO) of the Schiff base NBAP transfer the electron to the vacant orbitals of Fe2+ but the π* orbitals (LUMO) of the base also accept the electron density from the metallic iron. To determine the adsorptive nature of the inhibitor, the energy difference among ELUMO and EHOMO i.e. the energy gap (ΔE) is required. An indicator of an inhibitor’s responsiveness to adsorption on the metallic surface is usually the energy gap among HOMO and LUMO. A narrower gap is thought to make adsorption easier, which raises the effectiveness of inhibition, and vice versa63,64,65.The deliberated quantum parameter of NBAP in the aqueous solution and gas phase are recapitulating in the Table 5. The data arranged in Table 5 demonstrate that the NBAP inhibitor has a large EHOMO (-6.00 eV) energy and a low ELUMO energy (-2.65 eV). Furthermore, the gap between HOMO-LUMO in NBAP is reduced after solvation from − 3.67 eV to -3.35 eV. This clearly confirms that the tested inhibitor undergoes chemisorption on the metal surface29,66.

Fig. 15
figure 15

The optimized geometry, LUMO and HOMO orbitals of NBAP given by theB3LYP/6–31(d.p)obtained from DFT method. Gaussian 09 W D.01 × 86 [2013, ENG] :: RuTracker.org. GaussView 6.0.16 GNU\WIN x64 [2016, ENG] :: RuTracker.org.

Full size image
Table 5 The calculated quantum chemical parameters for the studied NBAP inhibitor in the gas and aqueous phase obtained using the DFT method at the B3LYP/6-31G(d, p) basis set.
Full size table

The softness parameter (σ) is also an indicator of inhibitor reactivity. The higher value of softness parameter indicates more reactivity, since the molecule will be more polarizable and it will easily change its electronic cloud to interact with the surface of metal67.Because the smoothness value (σ = 0.54 eV− 1) increases and the stiffness value (η = 1.83 eV) decreases, the NBAP inhibitor exhibits better chemical stability with the metal surface63,66. The electrophilic index (ω) is another parameter that boosts the efficiency of inhibition. The low value of ω, the greater is the efficiency of inhibition. The NBAP exhibits a low value of ω(5.6 eV), which validates its higher inhibition efficiency. Moreover, the electronegativity (χ) values are also significant. The high quantum parameter (χ = 4.33 eV) value is approving for the better inhibition efficiency24. The µ (dipole moment) is a significant additional parameter that determines the electrostatic contact among the metal surface and inhibitor. The greater the dipole moment, the greater the reactivity is. In this research, the high dipole moment value of NBAP is equal to 6.22 in the phase of aqueous and 5.32 Debyes in the gaseous phase clearly confirms its strong adsorption and high reactivity on the surface of metal62.

This study also computed the ΔN, or electron’s fraction was transported to the surface of metal from the molecule’s inhibitor. The Lukovits investigation68 states that an inhibitory efficiency is deemed good if ΔN value is less than 3.6. In this instance, the rate of charge transfer (ΔN) was found to be 0.79 eV, which is less than the Lukovits limit value. This demonstrates unequivocally that the NBAP inhibitor has a strong inhibitory impact on the mild steel’s surface corrosion, and that chemisorption is the method of inhibitions62,69.A helpful metric for identifying the locations of nucleophilic and electrophilic reactions is molecular electrostatic potential (MEP), and is correlated with electron density. The distributions of charge have been shown by diverse colors; MEP’s negative regions(yellow and red) were associated to the electrophilic reactivity whereas in the blue region’s electron density (positive) was present70,71. As depicted in Fig. 16, the regions with negative electron density, indicated by yellow are mostly centered on the C = N bond and NO2 group, signifying a massive electron deficiency and consequently a probable electrophilic region’s active center. Conversely, the positive electron density region is localized around the hydroxyl group’s oxygen and therefore acts as active nucleophilic sites.

Fig. 16
figure 16

Counter and MEP plot of the inhibitor studies (a) Front view and (b) Rearview. Gaussian 09 W D.01 × 86 [2013, ENG] :: RuTracker.org. GaussView 6.0.16 GNU\WIN x64 [2016, ENG] :: RuTracker.org.

Full size image

Molecular dynamics simulation (MDS)

Molecular Dynamic simulations were conceded out in order to determine the behavior of interactions between the metal surface and the inhibitor. The interaction among the inhibitor and the Fe (110) surface with the subsequent sizes (17.38 × 17.38 × 27.16 Å) was executed using the force fields of COMPASS to emend all the arrangements of the measured system. Additional replication facts can be established from the previous studies63,72. MDS could be an excellent way to recover scientific knowledge and gain insights in the field. We studied the adsorption of inhibitory molecules on an iron surface in the absence and presence of water in order to evaluate the experimental and theoretical results and to explain the inhibition process. Side (a) and top (b) views and the mainly stable configuration of low energy for NBAP adsorption on the interface of Fe (110) obtained using MDS are depicted in Fig. 17. It is obvious from the figure that the inhibitor is oriented parallel to surface of Fe (110). This results in the enhanced recovery rate of the inhibitory molecules (θ = 0.89) on the metal surface. In most cases, the interaction’s type can be illustrated by the distinctive bond length created among inhibitor atoms and iron. Lgaz et al.73 investigated that the space among the atom’s molecule and the surface of metal between 1 and 3.5 Å designates a shorter bond length, which is associated to chemisorption whereas physisorption is ascribed to the distances greater than 3.5 Å. In this research the distance among iron and NPAP is fewer than 3.5 Å, which suggests that chemisorption may be the important part in the route of inhibition. This is in clear agreement with our experimentally obtained kinetic and thermodynamic results64,74.

Fig. 17
figure 17

Side and top views of the final adsorption of the adsorption of NBAPonthe Fe (110) surface in solution: (a) side view; (b) top view. DS BIOVIA Materials Studio 2023 23.1.0.3829 × 86 × 64 [2022, ENG] :: RuTracker.org.

Full size image

Inhibition mechanism analysis

The adsorption of NBAP can occur through a chemical mechanism, initially involving an electron sharing between the nitrogen atom of the imine (-C = N-) and iron, leading to the formation of a strong coordination bond with Fe. Additionally, the oxygen of the hydroxyl group (-OH) can also establish a coordination bond with iron.

In contrast, the oxygens of the nitro group (-NO2), carrying a partial negative charge, interact with Fe through weak electrostatic forces.

Furthermore, the double bonds present in the structure of NBAP enable a reciprocal electron transfer, where the d-electrons of iron can be donated to the π orbital of the inhibitor, forming a back-donation bond, facilitated by the various orientations of the iron d orbitals (Fig. 18).

Fig. 18
figure 18

Schematic representation of the inhibition mechanism of NBAP.

Full size image

Conclusion

In this study, the Schiff base (Z)-2-((3-nitrobenzylidene)amino)phenol (NBAP) was successfully synthesized and evaluated as a corrosion inhibitor for XC70 steel in a 1 M HCl solution. The NBAP demonstrated excellent corrosion inhibition properties, with its efficiency increasing as both temperature and inhibitor concentration were raised. Potentiodynamic polarization studies revealed that NBAP acts as a mixed-type inhibitor, effectively protecting the steel surface from corrosion by adsorbing onto it and forming strong covalent and coordinate bonds between the donor atoms (N and O) of the inhibitor and the Fe²⁺ ions present on the steel surface. The adsorption of NBAP followed the Langmuir adsorption isotherm, and thermodynamic parameters, such as activation energy (Ea) and the free energy of adsorption (ΔG°ads), confirmed that the adsorption process is chemisorptive. Scanning electron microscopy (SEM) analysis further supported the formation of a protective inhibitive layer on the steel surface. Additionally, density functional theory (DFT) and molecular dynamics simulations (MDS) were employed to complement the experimental results and validate the adsorption mechanism. Beyond corrosion protection, Schiff bases like NBAP are gaining attention in various fields such as pipeline protection, electrocatalysis, and biomedical applications, including anti-inflammatory, antioxidant, and therapeutic properties. As research continues to explore their potential, Schiff bases are expected to play an increasingly important role in both industrial and medicinal advancements, paving the way for more sustainable and effective materials across diverse sectors.