Enhanced anticorrosion performance of antipyrine derivatives on mild steel in an acidic environment: an experimental and theoretical analysis

Abstract

Efficient and structurally diverse organic corrosion inhibitors remain a major challenge for mild steel protection in acidic environments. In this study, three novel adamantyl amide derivatives of 4-aminoantipyrine, M1, M2, and M3, were synthesized and evaluated to investigate the influence of steric effects and accessibility of donor sites on corrosion inhibition performance. The corrosion inhibition performance was examined by weight loss measurements, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and surface characterization techniques, such as SEM, EDX, FESEM, and XPS, along with density functional theory (DFT) calculations. Among the investigated compounds, M1 exhibited superior inhibition efficiency, achieving up to 98.92% at 0.5 mM in 1.0 M HCl, outperforming its sterically hindered analogs M2 and M3. The inhibition efficiency increased with concentration and immersion time but decreased with temperature, indicating a predominantly physisorption-driven process. The thermodynamic parameters (E_a ≈ 44–55 kJ mol⁻¹; ΔG ≈ −10 to −16 kJ mol⁻¹) further support a mixed adsorption mechanism with dominant electrostatic interactions. The electrochemical results revealed a significant reduction in the corrosion current density and enhanced polarization resistance, confirming the formation of a protective inhibitor film. Surface analyses demonstrated the formation of a compact and adherent layer, particularly for M1, which correlated with its superior performance. DFT analysis enabled molecular-level understanding of the process, which showed that adsorption was favored by low energy gaps, high softness values, and high electron transfer ability. Compared to conventional antipyrine-based Schiff bases, the presence of an adamantyl amide moiety offers dual functionality for surface shielding and steric modulation for electron donation. The present investigation has established the importance of steric effects around the adsorption active sites for the effective design of high-performance organic inhibitors for corrosion protection.

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1. Introduction

The problem of mild steel corrosion in acidic environments has been a major challenge in many industrial processes, such as pickling, descaling, and oil well acidizing, where hydrochloric acid is commonly used as an aggressive medium. Material degradation becomes very fast under such conditions, causing operational inefficiencies and huge economic losses globally.^1–3 Organic corrosion inhibitors are one of the most effective and practical anticorrosion methods among various corrosion mitigation strategies. This is mainly because these inhibitors work by adsorbing onto metal surfaces and forming protective films that inhibit electrochemical reactions.^4–6 The molecular structure of these compounds, especially the presence of heteroatoms (N, O, S), π-electron systems, and functional groups that can facilitate adsorption and surface interaction, greatly determines their inhibition efficiency.^7–9 4-Aminoantipyrine derivatives have emerged as corrosion inhibitors because of their structural flexibility, easy functionalization, and multiple adsorptive active sites.^10–12 Recent studies have revealed that 4-aminoantipyrine based Schiff bases have good inhibition power, mainly because their electron-donating capacity and conjugation effect are boosted, thereby favoring metal surface adsorption.^13–16 Although many 4-aminoantipyrine-based inhibitors have been described, previous studies were based on the Schiff base series, for which the optimization of the inhibitory properties was achieved mainly through aromatic substitution and alteration of electron density.^12,17–19

More importantly, previous research does not provide a thorough understanding of the role steric hindrance around adsorption-active centers plays in inhibitor performance. The addition of bulky groups can produce a hydrophobic surface and better barrier properties; however, very crowded conditions with large groups may physically block the key atoms (N and O) from reaching the metal surface, thereby decreasing the adsorption strength.^20,21 The balance between hydrophobic improvement and steric restriction, which is largely unexplored, is an important deficiency in the scientific design of effective corrosion inhibitors.^22–24

The adamantyl amide-functionalized 4-aminoantipyrine derivatives could provide the solution to these issues. The adamantyl moiety, which has a unique cage-like skeleton and is very hydrophobic, is anticipated to improve surface coverage and lead to the formation of a dense and stable protective layer.^25,26 Meanwhile, its addition through amide bonds results in a well-structured means to systematically assess the effect of steric hindrance on adsorption efficiency.²⁷ Unlike classic antipyrine or Schiff base inhibitors, this concept allows for a direct comparison of molecules with different steric environments surrounding the adsorption-active sites.²⁸ Therefore, hydrophobic surface coverage to steric accessibility of electron-donating centers determines the corrosion inhibition performance. More precisely, it is proposed that limiting steric hindrance around N and O atoms results in stronger adsorption and higher inhibition efficiency, while greater steric congestion prevents effective interaction with the metal surface; however, the electronic properties are favorable.^29,30

Herein, three adamantyl derivatives of 4-aminoantipyrine inhibitors (M1, M2 and M3) were synthesized, and their performance was assessed through weight loss determination, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). Using surface characterization methods (SEM, EDX, FESEM, and XPS), adsorption and protective film formation were validated, and molecular-level insights into electronic structure and reactivity were revealed through density functional theory (DFT) computations.^31–33 In the present study, for the first time, the application of adamantane amide functionalization as an alternative approach for the optimization of antipyrine-based inhibitors was demonstrated, which allows for the optimization of not only the inhibitory properties but also the mechanistic investigation of the effects of steric hindrance on electron donation and adsorption ability. Compared with the previously investigated antipyrine-based inhibitors, the new compounds provide a better understanding of the structure–property relationships by allowing for the investigation of the effects of the distance and steric hindrance between the electron-donating sites and the hydrophobic cage unit. In this regard, M1 exhibited superior performance because the donor centers were less sterically hindered, allowing stronger interactions with the mild steel surface and more effective protective film formation.

2. Experimental section

2.1. Materials and methods

The entire chemical used in the synthesis of organic inhibitors (M1, M2 and M3) were of analytical grade and used as received without any further purification. These chemicals were purchased from Sigma-Aldrich, Merck, Fischer Chemicals and Acros Organics. The progress of chemical reactions and purity of synthesized products was checked by thin layer chromatography (TLC) aluminum sheets coated with Kiesel gel 60, F₂₅₄ (Merck). The melting points were measured by open capillary tubes in DMP-300 A&E lab, UK. The functional groups were identifying through IR spectra recorded on Bruker Alpha 4 using attenuated total reflectance (ATR) on platinum. The proton (¹H-NMR), carbon-13 (¹³C-NMR), and correlated nuclear magnetic resonance spectroscopy (COSY), heteronuclear single quantum coherence spectroscopy (HSQC) measurements were performed on Bruker Ascend^TM 400 and Bruker Ascend^TM 500 MHz spectrometer. The mass spectra were recorded on the Agilent Technologies mass spectrometer micro time of flight electrospray ionization (TOF-ESI) positive and negative targeted modes. The tafel, bode, phase angle and nyquist plots were recorded on the Gamray Potentiostat.

2.2. Synthesis

2.2.1. 4-Amino-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (2). The ester derivative of 4-aminoantipyrine (1) (12.5 mmol) was dissolved in dichloromethane DCM (6.25 mL) and 98% sulfuric acid (6.25 mmol) under stirring for 6 h. The progress of the formation of 4-aminoantipyrine (2) was continuously checked by TLC. Upon cessation of product production, extraction of (2) was conducted using 2.0 M sodium hydroxide (4.5 mL) in combination with an aqueous phase rendered acidic through the addition of 37% HCl (20 mL), followed by extraction with diethyl ether (4.5 mL). The organic phase was then washed with 10% aqueous Na₂SO₄ and dried with anhydrous Na₂SO₄ to obtain a pure product.³⁴ Yield 98%, melting point 106–110 °C, ¹H-NMR Bruker 400 MHz (DMSO-d₆, ppm): [s, 3H, CH₃], 2.09 [s, 3H, N–CH₃], 2.73 [s, 2H, NH₂], 3.84 [m, 1H, Ar-H], 7.23 [m, 4H, Ar-H], 7.44. ¹³C-NMR (DMSO-d₆, ppm) [10.38, 38.79, 120.58, 122.31, 125.58, 129.39, 135.95, 136.09, 161.89], HRMS (Bruker Micro TOF) m/z [M + H, 204.11, M + Na, 226.09].

2.2.2. 4-((1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino)-4-oxobutanoic acid (4). 4-Aminoantipyrine (1 mmol) (2) solution in ethyl acetate was added dropwise to a stirred solution of succinic anhydride (3) (1 mmol) in ethyl acetate (20.0 mL) in an ice bath and under a N₂ purge environment. The reaction mixture was stirred for 8.0 hours at room temperature after 30 min. The light yellow colored precipitates separated from the solvent through vacuum filtration and recrystallized the product (4) in chloroform.³⁵ Yield 98.6%, melting point 199–202 °C, IR cm⁻¹: [3283 (NH amide), 2929 (OH of carboxylic acid), 1741 (C=O), 1666 (C=O), 1615 (C=C) and 1600 (C=O)], ¹H-NMR Bruker 400 MHz (DMSO-d₆, ppm): [s, 3H, CH₃], 2.09 [m, 4H, CH₂–CH₂], 2.51 [s, 3H, N–CH₃], 3.03 [m, 3H, Ar-H], 7.30 [m, 2H, Ar-H], 7.50 [s, 1H, NH] 9.11, [s, 1H, COOH] 12.10. ¹³C-NMR (DMSO-d₆, ppm) [11.65, 29.54, 30.34, 36.54, 108.16, 123.86, 126.61, 129.54, 135.56, 152.89, 162.31, 171.09, 174.26], HRMS (Bruker Micro TOF) m/z [M + H, 304.12, M + Na, 326.11, M − H, 302.11].

2.2.3. Synthesis of N1-(adamantan-1-yl)-N4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)succinamide (M1). N1-((S)-1-((3S,5S,7S)-Adamantan-1-yl)ethyl)-N4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)succinamide (M2), N1-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-N4-((2S,4R)-1,7,7-acid) (4) (1 eq.) was added to a solution of amines (0.1 mol), DMAP (0.1 eq.), and EDCl (4 eq.) in DCM (20 mL) under stirring at room temperature and an inert (N₂ purge) environment. The resulting mixture was stirred at room temperature for 48 h. Subsequently, the product was extracted with water and saturated with aqueous sodium hydrogen carbonate. The organic layer was concentrated using a rotary evaporator, and the product was separated on a silica column using a suitable solvent system. The NMR spectra of products (M1–M3) were recorded in deuterated DMSO-d₆.³⁶
2.2.3.1. (M1). N1-(Adamantan-1-yl)-N4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)succinamide. Yield 96%, melting point 229–232 °C, solubility DMSO/DMF, IR cm⁻¹: [3358 N–H, 2904 N–H, 2848 C–H aromatic, 1643 C=O], ¹H-NMR Bruker 400 MHz (DMSO-d₆, ppm): [m, 6H, 3CH₂], 1.60 [m, 6H, 3CH₂], 1.92 [m, 3H, 3CH] 2.00 [s, 3H, CH₃], 2.09 [m, 2H, CH₂], 2.32 [m, 2H, CH₂], 2.45 [s, 3H, N–CH₃], 3.03 [s, 1H, NH], 3.18 [m, 1H, Ar-H], 7.31 [m, 2H, Ar-H], 7.36 [t, 2H, Ar-H], 7.51 [s, 1H, NH] 9.04, ¹³C-NMR (DMSO-d₆, ppm) [11.63, 29.28, 31.18, 31.94, 36.55, 41.55, 49.07, 50.99, 108.21, 123.83, 126.58, 129.53, 135.58, 152.95, 162.34, 171.94, 171.63], HRMS (Bruker Micro TOF) m/z [M + H, 437.25, M + Na, 459.24, M − H, 435.24].
2.2.3.2. (M2). N1-((S)-1-((3S,5S,7S)-Adamantan-1-yl)ethyl)-N4-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)succinamide. Yield 97.5%, melting point 223–228 °C, solubility DMSO/DMF, IR cm⁻¹: [3302.06 N–H, 2896.68 N–H, 2847.62 C–H aromatic, 1655.69 C=O, 1639.20 C=O, 1541.09, 1308.83, 1221.42], ¹H-NMR Bruker 400 MHz (DMSO-d₆, ppm): [d, 3H, CH₃–CH], 0.93 [m, 6H, 3CH₂], 1.46 [m, 6H, 3CH₂], 1.64 [m, 3H, 3CH] 1.93 [s, 3H, CH₃], 2.09 [m, 2H, CH₂], 2.40 [m, 2H, CH₂], 2.50 [s, 3H, N–CH₃], 3.04 [m, 1H, CH], 3.54 [m, 3H, Ar-H], 7.36 [s, 1H, NH], 7.49 [m, 2H, Ar-H], 7.51 [s, 1H, NH] 9.05, ¹³C-NMR (DMSO-d₆, ppm) [11.63, 14.69 28.27, 31.18, 31.37, 36.05, 36.56, 37.15, 38.41, 52.38, 108.24, 123.82, 126.57, 129.53, 135.58, 152.88, 162.32, 170.97, 171.64], HRMS (Bruker Micro TOF) m/z [M + H, 465.28, M + Na, 486.26, M − H, 463.27].
2.2.3.3. (M3). N1-(1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-N4-((2S,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl)succinamide. Yield 95.8%, melting point 204–206 °C, solubility DMSO/DMF, IR cm⁻¹: [3356.88 N–H, 2951.58 C–H aromatic, 1674.05 C=O, 1630.65, 1580.77, 1529.52], ¹H-NMR Bruker 400 MHz (DMSO-d₆, ppm): [s, 3H, CH₃], 0.69 [s, 3H, CH₃], 0.83 [m, 2H, CH₂], 0.86 [m, 3H, CH₃] 0.89 [m, 2H, CH₂], 1.23 [m, 2H, CH₂], 2.40 [m, 1H, CH], 1.59 [m, 2H, CH₂], 1.70 [s, 3H, CH₃], 2.09 [m,2H, CH₂], 2.43 [m, 2H, CH₂], 2.49 [s, 3H, N–CH₃], 3.03 [m, 1H, CH] 4.10 [m, 3H, Ar-H], 7.34 [m, 2H, Ar-H], 7.50 [m, 1H, NH], 7.71 [s, 1H, NH] 9.04, ¹³C-NMR (DMSO-d₆, ppm) [11.64, 14.33, 18.95, 20.24, 28.05, 28.23, 31.35, 36.10, 36.54, 44.86, 48.21, 49.78, 53.17, 108.20, 123.84, 126.59, 129.54, 135.57, 152.92, 162.33, 171.54, 171.70], HRMS (Bruker Micro TOF) m/z [M + H, 439.27, M + Na, 461.25, M − H, 437.25].

¹H-NMR, ¹³C-NMR, COSY, HSQC, IR and HRMS spectra of all the synthesized compounds are available in SI (Fig. S1–S32).

2.3. Anticorrosion evaluation

2.3.1. Solution. The standard corrosion solution of 1.0 M HCl was made by diluting AR grade HCl (37%) with distilled water. This standard solution was used as a baseline for comparison. M1–M3 were dissolved in 1.0 M HCl solutions to achieve the required concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mM).³⁷

2.3.2. Weight loss analysis. An analytical balance with a precision of ±0.001 mg was used for the weight loss experiment. Prior to beginning each experiment, the mild steel coupon surface was cleaned using emery papers of various grades, such as 320, 660, and 1200. Acetone was used to degrease the coupons, and acetone and double-distilled water were used to rinse the coupons. Each coupon's weight was precisely determined after it had dried in warm air flow. The coupons were submerged in 30 mL of 1.0 M HCl solution in the absence and presence of various concentrations (0.2–1.0 mM) of the inhibitors (M1–M3) for varying time periods (1, 5, 12, 24, and 48 h) at controlled temperatures (30, 40, 50, and 60 °C) using an oven. After the experiment, the coupons were cleaned chemically to remove the corrosion products, involving immersion in 1.0 M HCl, rinsing with distilled water, propanone, and ethyl alcohol, drying with fast warm airflow, and precise re-weighing.^38,39 The chemical treatment referred to in the weight-loss section, involving a short immersion in 1.0 M HCl following ASTM/G1 practice, is only performed after gravimetric determinations for the removal of the bulk corrosion products and does not alter the surface-analysis data.^40,41

2.3.3. Electrochemical analysis. All electrochemical measurements were performed on a Gamry electrochemical instrument using a three-electrode system. The Ag/AgCl electrode was used as the reference electrode, a graphite rod was used as the counter electrode while 4.0 cm² mild steel strips were used as the working electrode in all the electrochemical measurements. Before every experiment, the mild steel strips were pretreated similarly as in the weight loss experiment, including cleaning, degreasing, rinsing with water, and hot air drying. The working electrode strips were immersed in 1.0 M HCl for 30 min to stabilize the open circuit potential (OCP) before each experiment.^39,42–45
2.3.3.1. Potentiodynamic polarization (PDP). Potentiodynamic polarization (PDP) studies were conducted automatically across a potential range of −250 to +250 mV at open circuit potential (OCP) with a scan rate of 0.5 mV s⁻¹. All electrochemical analyses were performed at a constant temperature of 298 K. Tafel curves were obtained from the installed program of the gamry potentiostat.^43,46
2.3.3.2. Electrochemical impedance spectroscopy (EIS). EIS experiments were recorded via excitation signals of 5 mV peak to peak amplitude at the corrosion potential by using the frequency range of 10⁶ Hz to 0.1 Hz at the corresponding OCP in the Gamry Echem Analyst tool. Resistance parameters obtained from the Bode and Nyquist plots.^46,47

2.3.4. Computational details.
2.3.4.1. Structures preparation and optimization. Molecular geometries for the M1, M2, and M3 inhibitors were generated in three dimensions using Avogadro software⁴⁸ and were subjected to initial geometric optimization using molecular mechanics to generate acceptable initial coordinates. Final electronic structure calculations were performed using density functional theory (DFT) with the ORCA (version 6.1) package.⁴⁹ To balance computational accuracy with efficiency for these large organic systems, the B3LYP/def2-SVP level of theory was selected.⁵⁰ Specifically, the def2-SVP basis set includes necessary polarization functions for heteroatoms, while the use of RIJCOSX approximations with the def2/J auxiliary basis allowed for rigorous geometry optimization in an aqueous medium (CPCM) without prohibitive computational costs.⁵¹ The solvent effects of the acidic aqueous medium were incorporated to accurately represent the experimental conditions using the Conductor-Like Polarizable Continuum Model (CPCM) at 298.15 K (dielectric constant of water = 78.39).^52,53 After geometry optimization of all compounds, harmonic vibrational frequency analysis was performed at the same level of theory. The absence of any imaginary (negative) frequencies confirmed that the optimized structures represented the true energy minima on the potential energy surface. To ensure high-precision results for conceptual DFT reactivity descriptors, single-point energy calculations were performed at the more robust B3LYP/def2-TZVP level. This triple-zeta basis set was chosen to minimize basis set superposition error (BSSE) and provide a converged electronic density, which is critical for adsorption-related predictions. Grimme's D3BJ dispersion corrections were included to accurately account for the non-covalent interactions typical of inhibitor-metal interfaces. Following the vertical approximation, the energies (i) the neutral molecule [E(N)], (ii) the cation obtained by removing one electron [E(N − 1)], and (iii) the anion obtained by adding one electron [E(N + 1)]. The cation and anion calculations were performed as single-point calculations at the optimized geometry of the neutral molecule. Atomic partial charges were further evaluated using Mulliken population analysis to identify potential nucleophilic and electrophilic adsorption sites.^54,55
2.3.4.2. Quantum chemical descriptors calculations. The quantum chemical descriptor in terms of the vertical ionization potential (I_v) and 0 vertical electron affinity (A_v) was determined according to the following equations.⁵⁶

I_v ≈ E(N − 1) − E(N) (1)

A_v ≈ E(N) + E(N + 1) (2)

Here, E(N) is the total energy of the optimized structures, and E(N − 1) and E(N + 1) stand for the cation and anion energies in the optimized neutral geometries, respectively. Using the I_v and A_v values, the energy gas (ΔE_g = I_v − A_v) value was calculated.⁵⁷ The HOMO and LUMO energies were computed and used to measure the energy gap (ΔE_gap) of the compounds as follows:

ΔE_gap = E_LUMOE_HOMO (3)

The electronegativity (χ), hardness (η), softness (S), chemical potential (Π) and nucleophilicity (ω) were evaluated using the following expressions, eqn (4)–(9):^58,59

χ = (I + A)/2 (4)

η = (I − A)/2 (5)

S = η − 1 (6)

Π = −1/2(I_v + A_v) (7)

ω = χ²/2η (8)

ε = 1/ω (9)

To determine the fractional number of electrons transferred (ΔN), we utilized the following eqn (10) from the ref. 60.

ΔN = (χ_Fe − χ_inh)/(2(χ_Fe + χ_inh)) (10)

eqn (11)^61,62 was developed based on the findings obtained from the determination of iron's electronegativity (7 eV) and hardness (0 eV).

ΔN = (7 − χ_inh)/(2(χ_inh)) (11)

Additionally, Gomez et al.⁶³ defined the electronic back-donation energy as eqn (12):

ΔE_b–d = −η_inh/4 (12)

In eqn (12), if ΔE_b–d ≤ 0, then the process is energetically favored.
2.3.4.3. Fukui indices and dual Fukui descriptors calculations. The local sites' reactivity responsible for interactions with the Fe-surface were investigated using the local Fukui indices condensed local softness (σ^a_k), and local electrophilicity (ω^a_k), according eqn (13):where q^(N)_k, q^(N+1)_k, and q^(N−1)_k are the charges values of atom k for neutral, cation and anion, respectively. The α = + and − denotes local softness quantities for nucleophilic, electrophilic, and radical attacks, respectively. More specifically, the dual Fukui descriptor or second order Fukui functions (f_k²), their associated dual local softness (Δσ_k) and dual local philicity (Δω_k) was used to provide a simplistic and instinctive method to local chemical reactivity. The definitions of these dual descriptors are followed as seen below in eqn (14)–(16)

f_k² = f⁺_k − f⁻_k (14)

Δσ_k = σ⁺_k − σ⁻_k (15)

Δω_k = ω⁺_k − (ω⁻_k) (16)

2.3.5. Surface analysis. To perform SEM, EDX, FESEM, and XPS analyses, mild-steel coupons were dipped into 1.0 M HCl with or without a 1.0 mM inhibitor for 5.0 hours.^53,64–66 The coupons were then gently rinsed with distilled water and absolute ethanol to remove loose corrosion products or excess electrolyte, thereby keeping the adsorbed inhibitor film intact. The coupons were immediately dried under a hot-air stream. In the methodology, the 1.0 M HCl cleaning step was used only for the gravimetric/weight-loss coupons, where removal of corrosion products before final weighing is required.⁶⁷ In contrast, the coupons used for surface characterization (SEM/FESEM/XPS) were handled separately: after immersion, they were washed with distilled water and ethanol and then dried before analysis, rather than being acid-cleaned.⁶⁸

3. Results and discussion

3.1. Spectroscopic characterization of synthesized compounds

The 4-aminoantipyrine (2) was synthesized with efficient yield (98%) from (1) through the acid hydrolysis. The ¹H-NMR spectrum (Fig. S1) of as-synthesized (2) shows the 1H multiplet and 4H multiplets in the aromatic region (7.23 ppm and 7.44 ppm respectively) which confirm the five aromatic protons. The two 3H singlets of methyl groups at 2.09 ppm (C–CH₃) and 2.73 ppm (N–CH₃) confirm the pyrazolone ring in the structure. The 2H singlet at 3.84 ppm indicate the NH₂ group on the pyrazolone ring of 4-aminoantipyrine (2). The ¹³C-NMR spectrum (Fig. S2) shows the nine non-equivalent carbons peaks at 10.38, 38.79, 120.58, 122.31, 125.58, 129.39, 135.95, 136.09, 161.89 ppm. Here the peaks at 10.38 and 38.79 ppm indicate the two methyl groups C–CH₃ and N–CH₃ respectively on the pyrazolone ring. The peaks at 161.89 ppm indicate the C=O group, 135.95 ppm CH₃–C=C and 120.58 H₂N–C=C of pyrazolone ring. The remaining peaks at 122.58, 125.58, 129.39 and 136.09 indicate the four non-equivalent aromatic carbons in the structure of 2. The COSY experiment (Fig. S3) shows H–H couplings and HSQC experiment (Fig. S4) C–H correlation in the structure. Through COSY and HSQC experiments the proton–proton and proton–carbon connectivities were confirmed. Furthermore, the high-resolution mass spectrometry (HRMS) spectrum (Fig. S5) shows M + H and M + Na peaks at m/z 204.11 and 226.09, respectively also proving the synthesis of 4-aminoantipyrine (2).

The compound 4 was synthesized from 2 and 3 as shown in Scheme 1. The FT-IR spectrum of 4 (Fig. S6) shows bands (cm⁻¹) at 3283 (NH amide), 2929 (OH of carboxylic acid broad), 1741 (C=O), 1666 (C=O), 1615 (C=C) and 1600 (C=O) which indicate the presence of carboxylic acid, amide, alkene and ketonic linkages in the structure. The ¹H-NMR spectrum of 4 (Fig. S7) shows the elimination of the NH₂ peak of 2H singlet at 3.84 ppm of compound 2, with an additional NH (amide) peak of 1H singlet at 9.11 ppm, indicating the formation of an amide linkage. Furthermore, this spectrum also shows 4H multiplets of CH₂–CH₂ at 2.51 ppm and a 1H broad singlet of COOH at 12.10 ppm, indicating the synthesis of the acid derivative of 4-aminoantipyrine (4). The ¹³C-NMR spectrum (Fig. S8) also shows thirteen non-equivalent carbons at 11.65, 29.54, 30.34, 36.54, 108.16, 123.86, 126.61, 129.54, 135.56, 152.89, 162.31, 171.09, 174.26 ppm. This spectrum shows two additional aliphatic carbons (CH₂–CH₂) at 29.54, 30.34 ppm, NH–C=O at 171.09 ppm, and COOH at 174.26 ppm peaks to compound 2. The H–H and H–C correlations are also confirmed by COSY (Fig. S9) and HSQC (Fig. S10) spectra respectively. The HRMS spectrum (Fig. S11 and S12) shows M + Na and M − H peaks at m/z 326.11 and 302.11, respectively, also justify the synthesis of the acid derivative of 4-aminoantipyrine (4).

Scheme 1 Schematic representation of the synthetic pathway for corrosion inhibitor molecules of 4-aminoantipyrine derivatives M1, M2 and M3.

The new amide derivatives M1–M3 were synthesized from 4. FT-IR was performed to analyze the formation of M1 as shown in Fig. S13. Formation of the amide function group in M1 appears at 2904 cm⁻¹ (NH amidic sharp) while the broad band of acid at 2929 cm⁻¹ in 4 was eliminated. Similarly, M2 and M3 (Fig. S14 and S15) shows new vibration bands (NH amidic sharp) at 2896 cm⁻¹ and 2851 cm⁻¹, respectively. The appearance of the NH band indicates the successful formation of amide linkage in M1, M2, and M3. The stronger band Likewise, in M1, M2, and M3, sharp vibration bands (C=O amidic) at 1643 cm⁻¹, 1639 cm⁻¹, and 1630 cm⁻¹, respectively, indicate the formation of amide linkages. As the presence of electron-donating groups and greater steric hindrance increases the electron density around the bond, it results in weaken bonds and decrease in the vibrational frequency, therefore, the amidic vibration bands (NH and C=O) of M2 and M3 appear at lower wavenumbers with high intensity compared to M1. As expected for amide formation, the N–H stretching band appears at 3358 cm⁻¹ (M1), 3302 cm⁻¹ (M2), and 3357 cm⁻¹ (M3). The red shift of 56 cm⁻¹ is observed for M2 can be explained by higher electron density at the amide nitrogen as a result of the α-methyl substitution on the adamantyl cage, weakening the N–H bond through steric crowding and hyperconjugation (the typical Δν NH range for these effects: 30–60 cm⁻¹) combination.⁶⁹ The C=O stretching frequencies are 1643 cm⁻¹ (M1), 1656/1639 cm⁻¹ (M2), and 1674/1631 cm⁻¹ (M3) correspondingly. The components at lower frequency in M2 and M3 (Δν C=O ≈ −4 to −12 cm⁻¹ relative to M1) are the result of a slight decrease in carbonyl bond order as the alkyl substituents on the adamantyl moiety acting as electron donor increase the resonance between the nitrogen lone pair. These little, yet consistent with the literature shifts indicate that large conformational changes are not revealed by the sterically modulated amides.^70,71

To further confirm the formation of as-synthesized M1, M2 and M3, the ¹H-NMR spectra for M1, M2 and M3 (Fig. S16–S18) were recorded. The elimination of the COOH broad peak of 1H singlet at 12.10 ppm of compound 4 with additional NH (amide) peaks of 1H singlet at 3.03 ppm, 7.49 ppm, and 7.71 ppm, respectively, indicates the formation of new amide linkages. The steric factor in the M2 and M3 explains the different peak positions in the spectrum. The aliphatic region of the ¹H-NMR spectrum of M1 contains additional 6H multiplets of 3CH₂ at 1.60 ppm, 6H multiplets of 3CH₂ at 1.92 ppm, and 3H multiplets of 3CH at 2.00 ppm indicate the adamantyl group in the structure. The aliphatic region of ¹H-NMR spectrum of M2 shows 3H doublet of CH₃–CH at 0.93 ppm, 6H muliplets of 3CH₂ at 1.46 ppm, 6H multiplets of 3CH₂ at 1.64 ppm, 3H mutiplets of 3CH at 1.93 ppm and 1H multiplet of CH₃–CH at 3.54 ppm which indicate adamantyl derivative group in the structure. The spectrum of M2 shows the additional two peaks of 3H doublet of CH₃–CH at 0.93 ppm and 1H multiplet of CH₃–CH at 3.54 ppm compared to M1 molecule which justify the structure of M2. The aliphatic region of ¹H-NMR spectrum of M3 also shows 3H singlet of CH₃–C at 0.69 ppm, 3H singlet of CH₃–C at 0.83 ppm, 2H muliplet of CH₂ at 0.86 ppm, 3H multiplet of CH₃ at 0.89 ppm, 2H mutiplets of CH₂ at 1.23 ppm, 2H mutiplets of CH₂ at 1.40 ppm, 1H mutiplets of CH at 1.59 ppm and 1H multiplet of CH at 1.70 ppm which indicate alkylated adamantyl group in the structure. The structure of M3 contains more three methyl groups on the adamantyl as compared to M1. The spectrum of M3 shows 3H singlet of CH₃–C at 0.69, 3H singlet of CH₃–C at 0.83 and 3H multiplet of CH₃ at 0.89 pm of these three methyl groups on the adamantly, justifying the structure of M3.

The ¹³C-NMR spectrum of M1 (Fig. S19) shows thirteen non-equivalent carbons at 11.63, 29.28, 31.18, 31.94, 36.55, 41.55, 49.07, 50.99, 108.21, 123.83, 126.58, 129.53, 135.58, 152.95, 162.34, 171.94 and 171.63 ppm. This spectrum bears four aliphatic additional carbons at 31.94, 41.55, 49.07 and 50.99 ppm confirming the adamentyl group in the structure. The ¹³C-NMR spectrum of M2 (Fig. S20) also shows thirteen non-equivalent carbons at 11.63, 14.69 28.27, 31.18, 31.37, 36.05, 36.56, 37.15, 38.41, 52.38, 108.24, 123.82, 126.57, 129.53, 135.58, 152.88, 162.32, 170.97 and 171.64 ppm. This spectrum has six aliphatic additional carbons at 14.69, 31.37, 36.05, 37.15, 38.41 and 52.38 ppm confirming the adamantly group in the structure. The ¹³C-NMR spectrum (Fig. S21) of M3 also shows twenty-two non-equivalent carbons at 11.64, 14.33, 18.95, 20.24, 28.05, 28.23, 31.35, 36.10, 36.54, 44.86, 48.21, 49.78, 53.17, 108.20, 123.84, 126.59, 129.54, 135.57, 152.92, 162.33, 171.54 and 171.70 ppm. This spectrum bears nine aliphatic additional carbons at 14.33, 18.95, 20.24, 28.05, 36.10, 44.86, 48.21, 49.78 and 53.17 ppm confirming the alkylated adamantly in the structure. The aliphatic signals of M1, M2 and M3 spectrums create main difference and confirm their structures. The ¹³C spectra of M1, M2 and M3 having 4, 6 and 9 non-equivalent carbons indicate the structural differences and confirm the structure of the synthesized compounds.

The H–H and H–C correlations of M1–M3 are also confirmed by COSY (Fig. S22 and S23) and HSQC (Fig. S24–S26) spectrums. Furthermore, The HRMS spectrum of M1, M2 and M3 (Fig. S27–S32) shows M + H, M + Na and M − H peaks at m/z 437.25, 459.24 and 435.24 respectively M1, 465.28, 486.26 and 463.27 respectively M2, and 439.27, 461.25 and 437.25 respectively M3 also proving the formation of M1, M2 and M3.

3.2. Anticorrosion measurements

The as-synthesized inhibitors were undergoes the anticorrosion analysis by weight loss and electrochemical measurements. The weight loss method was used to determine the effect of time, concentration and temperature on the corrosion inhibition properties of inhibitors. In the electrochemical method, the potentiodynamic polarization (PDP) curves via tafel plots and electrochemical impendence spectroscopy (EIS) via nyquist, bode and phase angle plots were used for anticorrosion evaluations. For these analyses, the required concentration inhibitors weight was dissolved in the 1.0 M HCl solution. All the inhibitors dissolved partially in pure water while clearly dissolved in acidic medium at room temperature due to protonation of nitrogen in the structure of inhibitors. Solubility tests showed that all three compounds were fully dissolving (>1.0 mM) when stirring at 298 K, so heating or sonication are not required at these low concentrations. Subsequent filtration revealed no evidence of precipitation, confirming the stability of the prepared solutions.^24,72

3.2.1. Weight loss evaluation.
3.2.1.1. Effect of concentration. These experiments were conducted to analyze the potential of the as-synthesized compounds in preventing the corrosion. The rate of corrosion (C_R), loss of weight with time, coverage of surface with inhibitors (θ) and efficiency of corrosion inhibition (IE%) for M1, M2 and M3 are tabulated in Table S1. Findings from these experiments facilitate the identification of the optimal inhibitor concentration for corrosion prevention and enable a comparative analysis of the synthesized inhibitors (M1–M3) with respect to their corrosion parameters.

The as-synthesized compounds were adsorbed on the exposed surface of MS coupons and block the reactive sites of the metal from corrosion that leads to a slow in the C_R. The unoccupied sites on the surface of iron coupons are susceptible to corrosion, and the sites blocked by inhibitors will not undergo corrosion.^73,74 The C_R of MS coupons were calculated through these analyses after 5.0 h at 303 K by using eqn (17).

C_R is the rate of corrosion of the MS strips, W is the decrease of weight of MS strips (g), ρ is the density (g cm⁻³), α is the exposed surface area (cm²) and t is the submerged time (h).

The corrosion rate C_R(o) of the iron coupons in the absence of inhibitors is 2.94 × 10⁻³ cm h⁻¹ shown indicating that unblocked iron sites are susceptible to corrosion under 1.0 M HCl environment. However, the addition of 0.2 mM of M1 as an inhibitor decreases the corrosion rate to 5.02 × 10⁻⁴ ± 1.0 × 10⁻⁰⁵ cm h⁻¹, as shown in Fig. 1a. It was observed that by increasing concentration to 1.0 mM, the C_R decrease to 6.02 × 10⁻⁵ ± 1.2 × 10⁻⁰⁶ cm h⁻¹. The C_R slows down with rise of concentration reveals the M1 molecule interacts with the iron sites to block them from corrosion. It can be assumed that by the rise in concentration of M1–M3 will enhances the adsorption of organic compounds on the surface of MS coupons.

Fig. 1 Effect of concentration (mM) on IE% and C_R of MS coupons immersed in (a) M1, (b) M2 and (c) M3 for 5.0 hours.

The C_R of M2 and M3 was determined under the same conditions. The C_R of iron coupons in 0.2 mM solutions of M2 (Fig. 1b) and M3 (Fig. 1c) were 1.03 × 10⁻³ ± 2.1 × 10⁻⁰⁵ cm h⁻¹ and 7.31 × 10⁻⁴ ± 1.5 × 10⁻⁰⁵ cm h⁻¹, respectively. The C_R of M2 and M3 were further decreased by increasing their concentrations. The C_R of iron coupons in 1.0 mM solutions of M2 and M3 were 4.44 × 10⁻⁴ ± 8.9 × 10⁻⁰⁶ cm h⁻¹ and 1.47 × 10⁻⁴ ± 2.9 × 10⁻⁰⁶ cm h⁻¹ respectively. It was showed by the experimental data that C_R decreases by raising the concentration of inhibitors (M1–M3). Among M1–M3, M1 exhibits the lowest C_R owing to its molecular structure, which presents minimal steric hindrance to the adamantly group. This suggests that M1 covers the surface of the iron coupon more effectively. The presence of alkyl groups on the adamantly moiety in M2 and M3 induces steric hindrance, thereby limiting the electron-donating capacity of the heteroatoms (N and O) within their structures. To further evaluate the performances of M1, M2, and M3, the θ of the inhibitors was measured using eqn (18).³⁷

Here, C_R(i) shows the corrosion rate with inhibitors, and C_R(o) shows the corrosion rate without inhibitor. The θ of inhibitors can also be correlated with their inhibition efficiency (IE%) in eqn (19).

IE% = θ × 100 (19)

Fig. 1a–c presents the IE% and Table S1 summarize the results of θ and IE% of inhibitors (M1–M3) related to their concentrations (C_inh). At lower concentrations, the C_R decreases rapidly and stabilizes after some time, whereas the IE% enhances before the formation of smooth layer of the inhibitors on the metallic surface. These trends are attributed to the adsorption of M1–M3, which forms a protective film that mitigates corrosion. Notably, all three compounds exhibit good inhibitive performance, even at low concentrations (0.2 mM), and maintain their performance across the entire concentration range. They owe their effectiveness to the presence of extensive amide linkages in the structure, which facilitate strong interactions between the as-synthesized compounds and the surface of MS strips, as further evidenced by surface analysis (SEM, FESEM, EDX, and XPS). The M1 (0.2 mM) shows θ and IE% of 0.830 ± 0.02 and 82.93 ± 1.6, respectively. Further increase in M1 concentration increases surface coverage (θ) and IE%. At 1.0 mM concentration of M1, the θ value increases to 0.980 ± 0.02, and IE% increases to 97.89 ± 1.9. Likewise, the θ and IE% of M2 at 0.2 mM are 0.650 ± 0.01 and 64.85 ± 1.3 respectively, whereas, at 1.0 mM concentration, θ and IE% further increase up to 0.850 ± 0.02 and 84.88 ± 1.7 respectively at a concentration of 1.0 mM. The θ and IE% of M3 at 0.2 mM are 0.750 ± 0.01 and 75.13 ± 1.5, while at 1.0 mM they are 0.950 ± 0.02 and 95.02 ± 1.9. Among these synthesized inhibitors, M1 shows the highest θ and IE compared to M2 and M3. The M3 has a higher θ and IE% than M2 but lower than M3. The lower values of C_R, higher values of θ and IE% of M1 is due to the lower steric hindrance of adamantly group for heteroatoms (O and N) than in case of M2 and M3. The non-substituted adamantly group of M1 enables the lone pairs of the heteroatoms (O and N) to be more donate able for adsorption on mild steel than to the steric hindered heteroatoms (O and N) of M2 and M3.

3.2.1.2. Effect of immersion time. The C_R, θ and IE% of MS strips adsorbed by organic inhibitors M1–M3 in the acidic medium were also affected by immersion time. The experiments were performed for 48.0 h immersion time at constant temperature of 303 K with M1–M3 concentrations ranges from 0.2 to 1.0 mM.

Results are shown in Fig. 2a–f and Table S2, revealing the C_R of M1–M3 significantly reduced by increasing their concentrations 0.2 to 1.0 mM. Higher concentrations of M1–M3 prove more effective in mitigating corrosion. The θ and IE% increases as the immersion time increases at constant inhibitor concentration (M1–M3). C_R of MS coupons in blank 1.0 M HCl is 4.62 × 10⁻⁴ cm h⁻¹ after 1.0 h immersion time. C_R of steel coupons in 0.2 mM M1 solution after 1.0 h immersion time is 1.71 × 10⁻⁴ ± 3.4 × 10⁻⁰⁶ cm h⁻¹ as shown in Fig. 2a, b and Table S2. The corrosion rates further decreases to 9.23 × 10⁻⁵ ± 1.8 × 10⁻⁰⁶ cm h⁻¹ in the same immersion time as the concentration of M1 inhibitor enhanced up to 1.0 mM. Likewise, IE% of M1 increases from 59.80 ± 1.2 to 78.35 ± 1.6 by increasing its concentration from 0.2 mM to 1.0 mM respectively, after 1.0 h exposure time. The other two inhibitors (M2 and M3) show the same trends of C_R and IE% upon increasing the concentration up to 1.0 mM, as summarized in Fig. 2c–f and Table S2. For M2, C_R decreases 2.92 × 10⁻⁴ ± 5.8 × 10⁻⁰⁶ cm h⁻¹ to 1.24 × 10⁻⁴ ± 2.5 × 10⁻⁰⁶ cm h⁻¹ and IE% increases 31.60 ± 0.6 to 70.91 ± 1.4 as the concentration rose 0.2 to 1.0 mM after 1.0 h exposure time. Similarly, C_R and IE% of M3 were 2.37 × 10⁻⁴ ± 4.7 × 10⁻⁰⁶ cm h⁻¹ and 44.53 ± 0.9 at 0.2 mM while 1.14 × 10⁻⁴ ± 2.3 × 10⁻⁰⁶ cm h⁻¹ and 73.18 ± 1.5 respectively at 1.0 mM concentration for 1.0 h immersion time. Notably, the IE% of M1–M3 also exhibits a time-dependent pattern. Increasing the immersion time, results in the decrease in C_R and increase in IE%. The present analysis evaluates C_R, θ and IE% of the mild steel up to 48 h. C_R in M1 decreases from 1.71 × 10⁻⁴ ± 3.3 × 10⁻⁰⁶ cm h⁻¹ to 1.83 × 10⁻⁶ ± 3.7 × 10⁻⁰⁸ cm h⁻¹ and IE% raises from 59.80 ± 1.2 to 96.43 ± 1.9 during 1.0 to 48 h immersion time, respectively, in 0.2 mM solution. Similarly, C_R of steel coupons in similar concentration of M2 decreases from 2.92 × 10⁻⁴ ± 5.8 × 10⁻⁰⁶ cm h⁻¹ to 1.07 × 10⁻⁵ ± 2.1 × 10⁻⁰⁷ and IE% increases 31.60 ± 0.6 to 79.20 ± 1.6 in 1.0 to 48 h, respectively. The third inhibitor M3 also follows the same trend of C_R (2.37 × 10⁻⁴ ± 4.7 × 10⁻⁰⁶ cm h⁻¹ to 5.07 × 10⁻⁶ ± 1.0 × 10⁻⁰⁷ cm h⁻¹), θ (0.450 ± 0.01 to 0.900 ± 0.02) and IE% (44.53 ± 0.9 to 90.11 ± 1.8) during 1.0 to 48.0 h in 0.2 mM solution. As the immersion time increases from 1.0 to 48 h, the corrosion rate consistently decreases, suggesting that extended exposure to M1–M3 enhances their inhibition effect. For example, at concentration of 1.0 mM of M1–M3, M1 exhibits 100 ± 2.0 IE% after 12 h of immersion, whereas M2 and M3 achieve 80.54 ± 1.6 and 97.48 ± 1.9 IE% after 12 h. The most significant effect is observed at the highest concentration (1.0 mM), resulting in IE% of 100 ± 2.0 for M1, 96.12 ± 1.9 for M2, and 99.33 ± 1.9 for M3 after 48 h. In contrast, at a concentration of 0.2 mM, the IE% are 96.43 ± 1.9 for M1, 79.20 ± 1.6 for M2, and 90.11 ± 1.8 for M3 after 48 h. A significant observation is made after 24 hours of immersion, when the IE% of M1–M3 begins to decline gradually due greater surface coverage with inhibitors. The IE% of M1–M3 also depends on the pH of medium, concentration of solution and temperature.⁷⁵

Fig. 2 Effect of time on C_R and IE% of MS coupons adsorbed by inhibitors (a) and (b) M1, (c) and (d) M2 and (e) and (f) M3.

3.2.1.3. Effect of immersion temperature. These experiments were designed to assess the thermal stability and IE% at elevated temperatures of the synthesized inhibitors. The corrosion rates and inhibition efficiencies of M1–M3 were evaluated using the mass reduction technique after a 5.0 h exposure time and temperatures changes 30 °C to 60 °C.

Increasing temperature results in an elevated corrosion rate and a reduced efficiency of M1–M3 inhibitors. At all concentrations, the IE% of M1–M3 reduced by raising the temperature, indicative of a physisorption process.^76,77 Fig. 3a–f and Table S3 illustrates the change of temperature effects on the C_R and IE% of MS coupons in acidic medium. At a temperature of 303 K, the bare MS coupon shows the C_R value of 1.36 × 10⁻³ cm h⁻¹. However, 0.2 mM of M1 addition decreases C_R to 4.96 × 10⁻⁴ ± 9.9 × 10⁻⁰⁶ cm h⁻¹ and IE% 63.65 ± 1.3. On the other hand, the C_R of M1 further increase by raising the temperature increases the C_R at the same concentration of 0.2 mM. Likewise, the same trend is observed for all the concentrations of M1. At 1.0 mM concentration of M1, the corrosion rate decreases to 1.59 × 10⁻⁴ ± 3.2 × 10⁻⁰⁶ cm h⁻¹, and IE% increases to 88.4 ± 1.8 at 303 K, as shown in Fig. 3a and b. However, upon further increase in temperature to 333 K, C_R of 1.0 mM solution of M1 increases to 1.14 × 10⁻³ ± 2.3 × 10⁻⁰⁵ cm h⁻¹, while the IE% decreases to 76 ± 1.5. The same trend appears with M1 and M2 as shown in Fig. 3c–f. The 1.0 mM solution of M2 demonstrates an IE% of 73.1 ± 1.5 at 303 K, decreasing to 57.4 ± 1.1 at 333 K, as shown in Table S3, Fig. 3c and d. M3 shows an IE% of 79.5 ± 1.6 at 303 K, which declines to 65.6 ± 1.3 at 333 K when the inhibitor concentration is 1.0 mM, as shown in Table S3, Fig. 3e and f.

Fig. 3 Effect of temperature on C_R and IE% of MS coupons adsorbed by inhibitors in acidic medium for 5.0 h exposure time (a) and (b) M1, (c) and (d) M2 and (e) and (f) M3.

To understand the adsorption of inhibitors (M1–M3) on MS strips surface in the acidic medium at different temperature (303 K to 333 K), the other thermodynamic parameters like activation energy (E_a), enthalpy change (ΔH), entropy change (ΔS) and Gibb's free change (ΔG) were determined by using eqn (20)–(23). Arrhenius equation (eqn (20)) used to calculate the activation energy (E_a) of the inhibitors. Where T = temperature (K), R = gas constant (8.314 J K⁻¹ mol⁻¹), and A is the Arrhenius constant. By plotting the graph between ln C_R vs. 1/T, the slope = −E_a/R and the intercept = ln A. The E_a can easily calculate from the slope.³⁷

By using the above eqn (20), the E_a of inhibitors (M1–M3) calculated were summarized in Table 1, Table S4 and Fig. 4a, c, e. All the Arrhenius curves gives the R² values near to unity justify the good fittings.

Table 1 Activation energy (E_a) values at various concentration of inhibitors (M1–M3) along with fitting parameters

Concentration (mM)	Slope	Intercept	R^2	Ea (kJ mol^−1)
M1
0.2	−6297.45	13.09 ± 1.5	0.99	52.36
0.4	−6095.47	12.17 ± 0.6	0.99	50.68
0.6	−5881.72	11.29 ± 1.2	0.99	48.90
0.8	−5442.33	9.71 ± 1.3	0.98	45.25
1.0	−5316.29	9.13 ± 1.3	0.98	44.20
M2
0.2	−6594.57	14.44 ± 1.0	0.99	54.83
0.4	−6506.76	13.96 ± 0.1	0.99	54.10
0.6	−6332.27	13.21 ± 0.8	0.99	52.65
0.8	−6266.21	12.88 ± 1.0	0.99	52.10
1.0	−6002.64	11.88 ± 1.7	0.98	49.91
M3
0.2	−6169.48	12.82 ± 0.9	0.99	51.29
0.4	−5945.32	11.79 ± 0.6	0.99	49.43
0.6	−5836.7	11.21 ± 1.4	0.98	48.53
0.8	−5777.46	10.90 ± 1.9	0.97	48.03
1.0	−5637.06	10.33 ± 2.2	0.97	46.87

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Fig. 4 Arrhenius (a), (c) and (e) and transition state (b), (d) and (f) plots for MS strips in acidic medium with and without M1–M3 inhibitors (a) and (b) M1, (c) and (d) M2 and (e) and (f) M3.

The M1, M2 and M3 give the E_a values of 52.36 kJ mol⁻¹, 54.83 kJ mol⁻¹ and 51.29 kJ mol⁻¹ respectively at their 0.2 mM solutions. The E_a values of all the inhibitors decreases by raising the concentration of as-synthesized inhibitors show the ease of adsorption of inhibitors on the surface MS strips at elevated concentration. The E_a of M1, M2 and M3 decreases to 44.20 kJ mol⁻¹, 49.91 kJ mol⁻¹ and 46.87 kJ mol⁻¹ respectively at 1.0 mM solutions. Our inhibitors represent the E_a values less than 80 kJ mol⁻¹ threshold, support the physisorption adsorption mechanism.⁷⁸ The high efficiency of M1 can be explained by the fact that it adsorbs strongly on the metal surface, and this may be attributed to less steric hindrances on the electron rich sites such as heteroatoms like nitrogen and oxygen. The presence of such sites enables the donation of an electron to the metal surface, thus forming a stable and compact film that prevents the corrosive substances from reaching the metal surface. The moderate inhibition efficiency of M3 indicates that the compound adsorbs on the metal surface but with low adsorption strength compared to that of compound M1. This may be attributed to the fact that the compound has fewer adsorption sites due to presence of alkyl substitution on the adamantyl group causes steric hindrance and an unfavorable orientation for adsorption, thus forming a less compact film that allows the corrosive substances to reach the metal surface. On the contrary, the least inhibition efficiency of M2 may be attributed to the fact that the compound adsorbs weakly on the metal surface. This may be attributed to the fact that the compound has structural limitations such as fewer electron-donating sites due to more steric hinderance, thus forming a less stable film that allows the corrosive substances to reach the metal surface and react with it.

The ΔH and ΔS were calculated by using eqn (21) shown in Table 2 along with fitting parameters.^79–81 In this context, ΔS and ΔH denote the changes in entropy and enthalpy of activation respectively, where h is Planck's constant and N is the Avogadro's constant.³⁷

Fig. 4b, d, f, and Table S4 summarize the results for MS coupons in acidic medium in presence and absence of M1–M3 from 0.2 to 1.0 mM concentration at different temperatures. The positive enthalpy (ΔH) suggests that steel dissolution is endothermic shows the stronger interaction between the as-synthesized inhibitors and surface of MS strips. By adsorption onto the metal surface, inhibitors reduce the disorder and randomness of the system. Higher concentrations result in a more compact inhibitor layer, decreasing the entropy.⁸² ΔH becomes less positive by increasing the inhibitor concentration. At higher concentration, the inhibitor molecules may occupy most of the available surface sites, leading to a decrease in the enthalpy change.⁸³ Moreover, higher concentrations increase steric hindrance, reducing the enthalpy change.⁸⁴ ΔH of M1 at 0.2 mM is 220.80 kJ mol⁻¹, which decreases to 202.30 kJ mol⁻¹ (shown in Table 2) by increasing its concentration to 1.0 mM. Similarly, ΔH of M2 decreases from 225.78 kJ mol⁻¹ to 215.96 kJ mol⁻¹ upon increasing concentration 0.2 to 1.0 mM. ΔH of M3 is 216.63 kJ mol⁻¹ and 208.89 kJ mol⁻¹ for 0.2 and 1.0 mM concentrations, respectively. As the inhibitor concentration increases, the entropy decreases. This is due to a more structured arrangement, decreased randomness, development of a thick protective layer, and robust interactions between the inhibitor and the mild steel surface at elevated concentrations.⁸⁵ Entropy change (ΔS) of M1 decreases −197.03 kJ mol⁻¹ to −197.04 kJ mol⁻¹ (shown in Table 2) as the concentration rises 0.2 to 1.0 mM. A similar trend is observed for M2 and M3, with entropies of −197.00 kJ mol⁻¹ and −197.04 kJ mol⁻¹ at 0.2 mM, respectively, and −197.05 kJ mol⁻¹ and −197.08 kJ mol⁻¹ at 1.0 mM. The experimental data also compare the adsorption capabilities of M1–M3 using thermodynamic parameters, specifically the enthalpy and entropy changes. These parameters decrease with increasing concentrations of M1–M3, indicating their strong interactions on the MS coupon surface. Experimental data suggest the lowest ΔH and ΔS for M1 compared to M2 and M3 is attributed to the lowest steric hindrance caused by the adamantyl group on the electron rich heteroatoms N and O. But in cases of M2 and M3, the heteroatoms feel more steric hindrance of the alkylated adamantyl groups on the electron donor heteroatoms reduces their adsorption capacity on the mild steel.

Table 2 ΔH (kJ mol⁻¹), and ΔS (kJ mol⁻¹) for MS coupons in acidic medium adsorbed by M1–M3 with fitting parameters

Concentration (mM)	Slope	Intercept	R^2	ΔH (kJ mol^−1)	ΔS (kJ mol^−1)
M1
0.2	−26.56 ± 1.2	0.0623 ± 0.004	0.99	220.80	−197.03
0.4	−26.24 ± 0.3	0.0604 ± 0.001	0.99	218.14	−197.04
0.6	−25.74 ± 0.8	0.0582 ± 0.002	0.99	214.05	−197.06
0.8	−24.55 ± 1.1	0.0538 ± 0.003	0.99	204.13	−197.10
1.0	−24.33 ± 1.0	0.0526 ± 0.003	0.99	202.30	−197.11
M2
0.2	−27.16 ± 1.3	0.0655 ± 0.004	0.99	225.77	−197.00
0.4	−27.06 ± 0.5	0.0646 ± 0.002	0.99	224.98	−197.01
0.6	−26.71 ± 0.9	0.0628 ± 0.003	0.99	222.10	−197.02
0.8	−26.62 ± 1.0	0.0622 ± 0.003	0.99	221.28	−197.03
1.0	−25.98 ± 1.7	0.0596 ± 0.005	0.99	215.96	−197.05
M3
0.2	−26.06 ± 0.8	0.0612 ± 0.002	0.99	216.63	−197.04
0.4	−25.69 ± 0.6	0.0590 ± 0.002	0.99	213.59	−197.06
0.6	−25.53 ± 1.0	0.0578 ± 0.003	0.99	212.24	−197.07
0.8	−25.44 ± 1.6	0.0571 ± 0.005	0.99	211.55	−197.07
1.0	−25.13 ± 1.8	0.0557 ± 0.006	0.98	208.89	−197.08

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3.2.1.4. Adsorption isotherms. The adsorption equilibrium data were systematically studied by applying the Temkin, Freundlich, Flory–Huggins, and Langmuir isotherm models. A detailed comparison of the correlation coefficients (R²) for all the models clearly establishes the superiority of the Langmuir model as shown in Table S11 and Fig. S34–S36. In the case of M1, the Langmuir model maintains high R² values at all temperatures, whereas the R² values for the Temkin model gradually reduce from 0.99 to 0.95. The R² values for the Freundlich model fluctuate between 0.96 and 0.98. The R² values for the Flory–Huggins model also show significant reductions at higher temperatures, as low as 0.91. In the case of M2, although the R² values for the Freundlich model are relatively higher at all temperatures, ranging from 0.97 to 0.98, the Langmuir model appears to be more reliable, as the R² values remain between 0.96 and 0.98 at all temperatures, whereas the R² values for the Temkin model are as low as between 0.88 and 0.95. For M3, the Langmuir model is again the best, as shown by the consistency of the R² values, ranging from 0.97 to 0.99, as opposed to the Freundlich model, whose consistency is slightly lower (0.95 to 0.99). The Temkin model, as before, is lower at 0.93 to 0.96. A more interesting observation is that the Flory–Huggins model shows a drastic drop in the R² value to 0.58 at higher temperatures, indicating its failure to properly describe the adsorption mechanism under such conditions. As observed, the Langmuir isotherm not only shows the best R² values but also shows the least variation with respect to temperature, indicating its superiority and reliability. Thus, it is highly indicative that the adsorption mechanism is best explained as one involving monolayer coverage on a homogeneous surface with energetically equivalent active sites and no significant intermolecular interaction, validating the Langmuir model as the best representation of the adsorption mechanism. The interaction between the surface if MS coupon and the inhibitor molecules were determined by calculating the surface coverage (θ) and Langmuir isotherms data fitting by using the eqn (22). C_inh represents the inhibitor concentration (mM) and K_ads is the equilibrium constant for adsorption process.⁸⁶The graph plots between C_inh/θ and C_inh shown in Fig. 5 provides a straight line K_ads at different temperature (303–333 K) based on the inverse of intercept.The easily calculated from eqn (23) by using the value of K_ads, the value 55.5 represents the [H₂O] in mol dm⁻³ adsorbed on the MS strip surface.

Fig. 5 Langmuir adsorption isotherms of inhibitors (M1 (a), M2 (b) and M3 (c)) on the surface mild steel in 1.0 M HCl.

The negative values of for the compounds show spontaneous adsorption on the metal surface. The more negative values of Gibb's free energy favor the chemisorption. The Gibb's free energy value less than −20 kJ mol⁻¹ represents physisorption adsorption mechanism.⁸⁷ In our study, M1, M2 and M3 shows −15.22 kJ mol⁻¹, −14.89 kJ mol⁻¹ and −15.97 kJ mol⁻¹ Gibb's free energies respectively at 303 K shown in Table 3. The values become less negative by increasing the temperature so the M1, M2 and M3 shows −13.68 kJ mol⁻¹, −10.72 kJ mol⁻¹ and −13.18 kJ mol⁻¹ Gibb's free energies respectively at 333 K. At low temperature, the Gibb's free energy data more likely to show chemisorption while high temperature favor the physisorption. Among the M1–M3, the M1 shows comparatively more negative values than the M2 and M3 exhibits their more affinity to adsorbed on the surface of mild steel. This exceptional behavior of M1 is explained by less steric hindrance by the adamantyl group on the electron donor N and O than the other inhibitors with more bulky adamantyl group.

Table 3 K_ads and ΔG (kJ mol⁻¹) values for MS strips adsorbed by M1–M3 with fitting data

Temperature (K)	Slope	Intercept	Kads	R^2	ΔG (kJ mol^−1)
M1
303	1.018 ± 0.04	0.132 ± 0.13	7.57	0.99	−15.22
313	1.033 ± 0.01	0.199 ± 0.01	5.01	0.99	−14.65
323	1.025 ± 0.01	0.261 ± 0.01	3.82	0.99	−14.38
333	0.923 ± 0.02	0.396 ± 0.01	2.52	0.99	−13.68
M2
303	1.242 ± 0.04	0.150 ± 0.03	6.66	0.96	−14.89
313	0.846 ± 0.05	0.609 ± 0.04	1.64	0.98	−11.74
323	0.827 ± 0.14	0.992 ± 0.09	1.01	0.93	−10.81
333	0.704 ± 0.08	1.153 ± 0.05	0.87	0.96	−10.72
M3
303	1.167 ± 0.01	0.098 ± 0.01	10.21	0.99	−15.97
313	1.045 ± 0.03	0.231 ± 0.02	4.32	0.99	−14.26
323	1.033 ± 0.02	0.296 ± 0.01	3.38	0.99	−14.06
333	0.971 ± 0.10	0.474 ± 0.07	2.11	0.97	−13.18

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Temperature variations in corrosion studies reveal the nature of adsorption of inhibitors on mild steel. Computed activation energy (E_a) for M1, M2, and M3 lie in the range of ∼44, 55 kJ mol⁻¹ and are lower than the value of 80 kJ mol⁻¹ which is considered as a signature of physisorption.⁸⁸ Therefore, it can be inferred that the adsorption of the inhibitors mainly occurs via a physisorption mechanism. Generally, physisorption features weak electrostatic interactions and is reflected by low activation energy values (E_a < 80 kJ mol⁻¹), whereas chemisorption is a result of strong coordinate bonding and is accompanied by elevated activation energies.⁸⁹ Additionally, the positive enthalpy values (ΔH) are an indication of the endothermic nature of the metal dissolution process. At the same time, the small decline in ΔH with raising inhibitor concentration shows a better interaction between inhibitor molecules and metal surface.⁸³ The negative entropy change values (ΔS) point out a loss of disorder in the activated complex formation, which is due to the regular adsorption of inhibitor molecules on the mild steel surface.⁹⁰ Although Gibbs free energy of adsorption (ΔG) values of around, 20 kJ mol⁻¹ or less negative are generally related to electrostatic interactions, the ΔG values of adsorption (around, 10 to 16 kJ mol⁻¹) reported strengthen the case of physisorption being the major mechanism.⁹¹ The presence of heteroatoms (N and O) that can donate electrons and the moderate magnitude of ΔG values suggest weak chemisorption contributions could have occurred. Hence, the overall adsorption mechanism may be regarded as a mixed-mode interaction where physisorption is the major one and chemisorption is the minor one. These observations are consistent with previously reported studies on organic corrosion inhibitors, where adsorption mechanisms often involve a combination of electrostatic interactions and partial charge transfer, especially in acidic media.⁹² The reduction of the corrosion inhibition effectiveness as the temperature rises strongly indicates a physisorption-type mode of action, as physisorbed molecules are more likely to detach when exposed to higher temperatures.⁹³

3.3. Electrochemical measurements

3.3.1. Open circuit potential (OCP) and Potentiodynamic polarization (PDP) measurements. The open circuit potential (OCP) were carried out between 0 to 1800s in presence and absence of inhibitors (M1–M3) in different concentration to verify that the inhibitors reached a stable potential.^94,95 The OCP ensure the equilibrium potential in the open circuit with no electrical interruption. This equilibrium forms between the interface of electrolyte and surface of electrodes. The OCP deviates in case of any interruption on the surface reaction. Fig. 6 and Table S6 represents the OCP curves of inhibitors different concentration of inhibitors and 1.0 M hydrochloric acid solution. All the OCP curves show that the inhibitors achieved the stable equilibrium potential in the concentrations of inhibitors and the results shows that the 800s is the sufficient waiting time for system to attain stable OCP curves. The blank 1.0 M HCl solution shows the OCP near −0.74 V while the OCP potential becomes least negative i.e. −0.54 V in case of 0.5 mM concentration of M1 proves the higher concentration of inhibitors facilitate the surface electrochemical equilibrium due to presence of thick layer of inhibitors on the mild steel. The EIS and PDP measurements were performed after attaining the stable OCP curves in every experiment.

Fig. 6 OCP curves of MS coupons adsorbed by (a) M1, (b) M2 and (c) M3.

Potentiodynamic polarization (PDP) measurements were performed to determine the effect of different concentrations of M1–M3 on the corrosion inhibition process of MS strips in acidic medium. The corrosion current density (i_corr), anodic tafel curve (β_a), cathodic tafel curve (β_c) and corrosion potential (E_corr) obtained from PDP measurements. Furthermore, the IE% and polarization resistance (R_P) were determined by using the eqn (24) and (25) respectively, as follow:^96–98

The i_corr were calculated in mA cm⁻² by extrapolating the tafel curves to the corrosion potential in presence and absence of the inhibitors. The PDP curves provides a visual representation of the corrosion behavior, were shown in Fig. 7 and Table 4, Table S7.

Fig. 7 PDP curves of MS coupons adsorbed by (a) M1, (b) M2 and (c) M3.

Table 4 PDP parameters of MS coupons in presence and absence inhibitors (M1–M3)

Concentration (mM)	Ecorr (V)	βc (V dec^−1)	βa (V dec^−1)	icorr (mA cm^−2)	RP (Ω cm^−2)	IE (%)
M1
0	−0.38	0.12	0.20	0.85	0.01	0
0.1	−0.44	0.02	0.13	0.36	0.01	56.99
0.2	−0.41	0.08	0.10	0.33	0.04	60.41
0.3	−0.41	0.13	0.08	0.22	0.05	73.75
0.4	−0.41	0.13	0.06	0.03	0.06	96.63
0.5	−0.40	0.24	0.06	0.01	0.10	98.92
M2
0.1	−0.42	0.14	0.11	0.60	0.01	29.02
0.2	−0.41	0.15	0.11	0.54	0.01	36.57
0.3	−0.39	0.16	0.09	0.46	0.01	45.08
0.4	−0.38	0.18	0.08	0.24	0.03	71.60
0.5	−0.37	0.19	0.08	0.02	0.42	98.22
M3
0.1	−0.44	0.15	0.14	0.47	0.02	43.87
0.2	−0.40	0.16	0.13	0.43	0.02	49.66
0.3	−0.39	0.18	0.11	0.34	0.02	59.40
0.4	−0.38	0.19	0.11	0.25	0.03	70.44
0.5	−0.38	0.22	0.10	0.04	0.22	94.76

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An effective anti-corrosion coating is characterized by reduced i_corr and an elevated E_corr.⁹⁹ When the mild steel is dipped in the solution of 1.0 M HCl, i_corr of 0.85 mA cm⁻² and E_corr of −0.38 V were obtained. The i_corr and E_corr were decreased to 0.36 mA cm⁻² and −0.44 V upon addition of 0.1 mM of M1 in the acidic medium. The i_corr and E_corr of M1 inhibitor shows 0.01 mA cm⁻² and −0.40 V respectively by increasing the concentration to 0.5 mM. Similarly, the M2 and M3 exhibits i_corr of 0.60 mA cm⁻² and 0.47 mA cm⁻² respectively in 0.1 mM concentration. While the E_corr of M2 and M3 were −0.42 V and −0.44 V, respectively in 0.1 mM concentration. By increasing concentration of M2 and M3 up to 0.5 mM, the i_corr becomes less negative of −0.37 mA cm⁻² and −0.38 mA cm⁻² respectively. The E_corr of M2 and M3 at 0.5 mM concentration were decreased to 0.02 V and 0.04 V respectively. The findings also indicate that increasing the concentrations of inhibitors decrease the i_corr in anodic and cathodic tafel lines gives more promising effects at 0.5 mM concentration. This effect reduces the metal dissolution on anode, thereby demonstrating the efficiency of M1–M3 as corrosion inhibitors. In M1, i_corr decreases 0.001 mA cm⁻² while for M2 and M3, i_corr decrease to 0.15 mA cm⁻² and 0.04 mA cm⁻², respectively at 0.5 mM concentration. Similarly, E_corr increase to 0.40 V for M2 and 0.42 V for M3 at higher concentration of 0.5 mM. M1 yields lower current density and higher voltage than M2 and M3, indicating that the M1 effectively protects the iron coupons from corrosion.^100,101 Similarly, IE% of M1–M3 also increases with increased concentrations (0.1–0.5 mM) as shown in Table 4. IE% of M1 increases from 56.99 to 98.92 by increasing the concentration 0.1 to 0.5 mM. The IE% of M2 increases from 29.02 to 98.22 and M3 from 43.86 to 94.76, by raising the concentration from 0.1 to 0.5 mM. The polarization resistances (R_P) of M1–M3 were also calculated by using the eqn (9) presented in Table 4. The R_P of mild steel in blank solution is 0.01 Ω cm⁻². The R_P values of M1 increase 0.01 Ω cm⁻² to 0.10 Ω cm⁻² by enhancing the concentration from 0.1 to 0.5 mM, exhibits the anticorrosion potential improves with raising the concentration of inhibitors. The inhibitors M2 (0.01 Ω cm⁻² to 0.42 Ω cm⁻²) and M3 (0.02 Ω cm⁻² to 0.22 Ω cm⁻²) shows the same trend of increasing R_P with increasing concentration reveals their exceptional anticorrosion potential. M1 inhibitor exhibits larger decrease in current density and increase in current voltage and IE% than the other inhibitors (M2 and M3) because of less steric hindrance in its structure. In cases of M2 and M3, the presence of more substituted alkyl groups decreases the electron-donating ability of heteroatoms due to steric hindrance.

3.3.2. Electrochemical impedance spectroscopy (EIS). EIS provides valuable information to deduce the corrosion mechanisms through the electrode and solution interface reactions. The bode, phase angle, and nyquist plots were generated during EIS measurements. The bode plot of the mild steel coating with a higher impedance value at low frequency shows maximum corrosion resistance.¹⁰² In the case of phase angle, the coating exhibits a large absolute value of phase angle at the higher frequencies, with the platform showing maximum corrosion resistance.¹⁰³ The larger capacitor arc radius of the nyquist plot reveals better anti-corrosion properties.⁹⁹ Fig. 8a–c and 9a–f give the nyquist and bode plots that presents the impedance characteristics of MS coupon submerged in acidic medium, highlighting the influence of varying concentrations (0.1–0.5 mM) of inhibitors (M1–M3) on its corrosion behavior.

Fig. 8 Nyquist plots of MS coupons adsorbed by of inhibitors (a) M1, (b) M2 and (c) M3 in acidic medium.

Fig. 9 Bode plots (a), (c) and (e) and phase angle plots (b), (d) and (f) of MS coupons adsorbed by organic inhibitors (M1 = a, b, M2 = c, d and M3 = e, f) in acidic medium.

The nyquist plots (Fig. 8a–c and Table S8) of MS coupons in the blank acid solution give the minimum capacitor arc radius, showing the maximum corrosion rate taken as reference for the inhibitors. The arc radius becomes wider in all the nyquist plots with increasing concentrations of the inhibitors, showing their anti-corrosion potential increase with concentration. All the inhibitors show maximum corrosion protection at higher concentration (0.5 mM). The maximum arc radius is observed in the M1 showing the better anticorrosion performance of M1 than M2, and M3.

Bode and phase angle plots at different concentrations (0.1–0.5 mM) of inhibitors (M1–M3) were shown in Fig. 9a–f and Table S9. All bode plots (9a, 9c, 9e) exhibit increase of impedance values with increase of concentration. All the inhibitors show highest value of impedance at low frequency. M1 and M3 show around 0.2 Ω cm⁻² impedance (Z) at 0.01 Hz, while M2 shows around −0.28 Ω cm⁻² at 0.01 Hz. Higher impedance values at low frequency reveal effective anti-corrosion potential of inhibitors for mild steel. M1 and M3 show better anti-corrosion performance than M2 due to higher impedance values at low frequency. On the other hand, the phase angle plots (9b, 9d, and 9f) move towards the positive at high frequency, elaborating the formation of effective protective layer of the inhibitors. The phase angle plots of the inhibitors presents a raised intensity in the phase angle curve with the raising the concentration of inhibitors. The maximum concentration of all the inhibitors shows that the capacitive rise of the phase angle curve indicates thicker protective layer formation on MS coupon surface. The shifting of phase angle towards the positive values at high frequencies for all inhibitors elaborates the improved corrosion protection. Among the inhibitors (M1–M3), M2 shows the minimum impedance and capacitor arc radius in the nyquist plot, while lower impedance values are observed in the bode plot than for the other inhibitors. M1 and M3 exhibit the maximum impedance and maximum anti-corrosion properties in bode plot and a high capacitor arc radius. The M1 and M3 molecules could be adsorbed strongly on the MS coupons surface due to presence of lone pairs of heteroatoms in their structure are readily available for donation due to less steric hindrance in the structure, so they show the maximum anti-corrosion potential in the nyquist plot. On the other hand, M2 has the maximum steric repulsion due to the presence of methyl groups on the adamantly fragment in its structure. The steric hindrance restricts the donor ability of lone pairs of heteroatoms, reducing the adsorption properties of M2, so it shows the minimum anti-corrosion tendency.

Fig. 10 shows the constant phase element (CPE) circuit used to resolve the impedance data, comprising a solution resistance (R_S) in series combined with polarization resistance (R_P) in parallel manner. Using this equivalent circuit, the impedance parameters, R_P and double layer capacitance (C_dl), were calculated by using eqn (26) as follow:

C_dl = (Y₀R_P¹⁻ⁿ)^1/n (26)

eqn (9) gives the C_dl based on the EIS data summarized in Table 5. This table includes the polarization resistance, denoted as R_P, both with and without the inhibitors (M1–M3). IE% increased due to enhancing the R_P at higher concentration of inhibitors indicates that effective adsorption on the surface of MS coupons.¹⁰⁴ The R_P values generated by EIS (shown in Table 5) and also determisned from tafel plots in PDP measurements (shown in Table 4) shows the same trend of increasing R_P values by increasing the concentration of the inhibitors reveals their reproducible and promising anticorrosion properties. The reduction C_dl could be explained by the formation of thick protective layer of inhibitors on metallic surface.¹⁰⁵ Collectively, these findings confirm that M1–M3 form a barrier on the MS coupon surface, effectively resist from corrosive environment and significantly inhibiting the corrosion reaction, thus providing a robust protective mechanism.^106–108 In conclusion, the results summarized in Table 5 and Fig. 8, 9 show that the system without inhibitors has the maximum C_dl (100.29 µF cm⁻²) and Y₀ (29.16 µS s^α cm⁻²), while the systems with inhibitors show lower values of these factors, indicating the adsorption of inhibitors on the surface of mild steel. Furthermore, the decrease in Y₀ and C_dl with increasing inhibitor concentrations indicates a more effective protective layer formation on the steel surfaces. M1 show 25.12 µF cm⁻² C_dl at 0.2 mM which decreases to 18.11 µF cm⁻² at 1.0 mM show promising anticorrosion activity at higher concentration. Similarly, Y₀ decreases 6.82 µS s^α cm⁻² to 5.03 µS s^α cm⁻² by increasing the concentration 0.1 mM to 0.5 mM. C_dl of M2 and M3 at 0.2 mM are 28.24 µF cm⁻² and 27.75 µF cm⁻² while at 0.5 mM 25.34 µF cm⁻² and 25.12 µF cm⁻² respectively. Y₀ of M2 and M3 at 0.2 mM are 27.66 µS s^α cm⁻² and 24.81 µS s^α cm⁻² while at 0.5 mM 8.70 µS s^α cm⁻² and 7.32 µS s^α cm⁻² respectively. Among M1–M3, the M1 inhibitor again shows the lowest values of C_dl (18.10 µF cm⁻²) and Y₀ (5.03 µS s^α cm⁻²) at high concentration (0.5 mM), indicating the largest anticorrosion potential. The less steric hindered structure of M1 enables the binding ability of heteroatoms (O and N) in the structure to adsorb more effectively than the other inhibitors.

Fig. 10 Equivalent EIS circuit model.

Table 5 EIS parameters of MS coupon adsorbed by the inhibitors (M1–M3) in acidic medium

Cinh (mM)	RS (Ω cm^−2)	RP (Ω cm^−2)	Cdl (µF cm^−2)	N	Y0 (µS s^α cm^−2)
M1
0	0.02	0.38	100.29	1.27	29.17
0.1	0.02	0.58	25.12	1.53	6.82
0.2	0.03	1.08	25.11	1.53	6.08
0.3	0.06	1.23	25.11	1.53	6.05
0.4	0.06	1.26	25.05	1.54	5.45
0.5	0.08	1.38	18.11	1.50	5.03
M2
0.1	0.01	0.40	28.24	1.24	27.67
0.2	0.01	0.43	27.60	1.30	25.13
0.3	0.01	0.43	27.36	1.32	21.36
0.4	0.02	0.48	24.92	1.54	21.30
0.5	0.02	0.50	25.34	1.52	8.71
M3
0.1	0.01	0.50	27.75	1.29	24.81
0.2	0.01	00.50	27.73	1.29	24.37
0.3	0.06	1.01	27.93	1.27	21.21
0.4	0.07	1.18	25.11	1.52	7.25
0.5	0.07	1.20	25.12	1.53	7.22

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One should note that inhibition efficiencies (IE%) derived from weight-loss and electrochemical techniques, namely PDP and EIS, are usually not the same. That is quite a frequently mentioned phenomenon in corrosion researches as the sensitivity of the different methodologies and the exposure duration vary.¹⁰⁹ Weight-loss test gives the average corrosion rate over long immersion periods (up to 48 h), reflecting the whole material degradation.⁴⁰ On the other hand, electrochemical methods provide instantaneous corrosion rates after short stabilization time which is a kind of inhibitor performance at metal/solution interface. The higher IE% values gotten from electrochemical measurements compared to other methods might be explained by the fact that inhibitor molecules quickly adsorb and a protective film is formed during the first moments of immersion, which very effectively hinders the charge transfer reactions. Whereas, in weight-loss experiments where exposure time is very long, factors like small desorption of inhibitor molecules, surface heterogeneity, and destruction of the protective film over time, can somewhat decrease the average inhibition efficiencies. A similar variation between gravimetric and electrochemical methods for organic corrosion inhibitors has been very well documented in the literature.^1,44,110 Electrochemical methods, by their very nature, are more precise in detecting very minute interfacial processes like charge transfer resistance and double-layer capacitance. Conversely, weight-loss methods measure the whole material loss, which could also involve localized corrosion effects that electrochemical means might not be able to capture completely. Therefore, the differences seen in the IE% values by different methods are not only natural but also highlight the complementary nature of these methods in providing a comprehensive understanding of the corrosion inhibition effectiveness.^111,112

Fig. 11 represents some reported compounds in literature derived from 4-aminoantipyrine with their IE% measured through electrochemical analysis. Govindaraju et al. report some Schiff bases derived from 4-aminoantipyrine (AAP) were 4-anisalidine amino antipyrine (AAAP), 4-benzalidine amino antipyrine (BAAP) and 4-salisilidine amino antipyrine (SAAP) undergoes PDP analysis shows promising anticorrosion potential. The IE% of 8.0 mM AAP was 82.48 in 1.0 M HCl solutions. In the BAAP, π electrons rich phenyl group was added to the AAP which increases the adsorption properties on the surface of mild steel and showed the IE% of 85.53. In cases of SAAP and AAAP, the PDP IE% were 90.83 and 87.40 due to presence of electron donor groups OCH₃ and OH respectively on the phenyl groups linked to the AAP through imine linkage. The electron donating OCH₃ and OH groups increases the electron density of at nitrogen of imine linkage and on benzene ring, increases their adsorption properties. On the other hand, Eldesoky et al. replaced the electron donating OCH₃ with electron withdrawing NO₂ group in case of 1,5-dimethyl-4-[(4-nitro benzylidene)-amino]-2-phenyl 1,2-dihydro-pyrazol-3-one (C3). The EIS data of NO₂ containing Schiff base of AAP show the IE% of 25.4 at 0.011 mM concentration in 2.0 M HCl. This abrupt decrease of adsorption ability of NO₂ group containing was due to electron withdrawing nature decreases the electron density of phenyl group and at imine linkage. Nnamdi C. Okey et al. reported a new Schiff base 4-[3-(4-methoxy-phenyl)-allylideneamino]-1,5-dimethyl-2-phenyl-1,2 dihydro-pyrazol-3-one (TMCATP) derived from 4-aminoantipyrine shows PDP IE% of 97.02 at 1.0 mM concentration in 1.0 M HCl. The TMCATP contain the phenyl group attached with one double bond and methoxy group linked with 4-aminoantipyrine. This extended conjugation and electron donating effect methoxy group make the inhibitor more adsorptive on the surface of mild steel. Ankita et al. also reported (E)-4-((4-chlorobenzylidene)amino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (L5) and (E)-4-((4-(dimethylamino)benzylidene)amino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (L7) derived from the 4-aminoantipyrine. The reported EIS IE% were 67.44 (L5) and 64.20 (L7) at 1.0 mM concentration in 1.0 N HCl. The L5 bears the Cl group while L7 have N(CH₃)₂ group on the phenyl ring attached with 4-aminoantipyrine. These electron donating groups were responsible for their large inhibition efficiencies.^113–117 In present study, the 4-aminoantipyrine (AAP) were linked with (electron donating groups) adamantyl and alkyl substituted adamantyl groups through two amide linkages containing O and N heteroatoms to enhanced their adsorption properties. The M1 inhibitor give IE% of 98.92 while in case of M2 and M3 inhibitors gives the IE% of 98.22 and 94.76 at 0.5 mM concentration in 1.0 M HCl in PDP experiments. The highest PDP IE% values of M1, M2 and M3 than previous reported 4-aminoantipyrine derivatives reveal their promising anticorrosion potential.

Fig. 11 Structure of 4-aminoantipyrine derivatives with their inhibition efficiencies.

Table 6 illustrates that the tested inhibitors match or even outperform in terms of inhibition efficiency those antipyrine derivatives already known in the literature, but at significantly lower concentrations. What is more, the design of the present compounds makes it possible to go beyond mere substituent effects when explaining their action mechanism, and this way it has been shown that functionalizing with adamantyl amide can enhance the corrosion inhibition by the joint effects of the hydrophobic film forming and the steric control of the adsorption-active donor atoms.

Table 6 Comparison of previously reported 4-aminoantipyrine-based corrosion inhibitors with the present adamantyl amide derivatives

Inhibitor	Structure type	Medium	Method	Conc. (mM)	IE (%)	Mechanistic insight	Ref.
AAP	Parent antipyrine	1.0 M HCl	PDP	8.0	82.48	Baseline adsorption	113 and 114
BAAP	Schiff base (phenyl)	1.0 M HCl	PDP	8.0	85.53	π-conjugation enhances adsorption	113 and 118
SAAP	Schiff base (OH)	1.0 M HCl	PDP	8.0	90.83	Electron-donating substituent	113
AAAP	Schiff base (OCH3)	1.0 M HCl	PDP	8.0	87.40	Electron-donating aromatic group	113 and 114
C3	NO2-Schiff base	2.0 M HCl	EIS	0.011	25.4	Electron-withdrawing reduces adsorption	114
TMCATP	Extended conjugation	1.0 M HCl	PDP	1.0	97.02	Conjugation + donation	115
L5	Schiff base (Cl)	1.0 N HCl	EIS	1.0	67.44	Substituent effect	13
L7	Schiff base (N(CH3)2)	1.0 N HCl	EIS	1.0	64.20	Limited donor efficiency	13
M1	Adamantyl amide	1.0 M HCl	PDP	0.5	98.92	Hydrophobic + low steric hindrance	This work
M2	Substituted adamantyl	1.0 M HCl	PDP	0.5	98.22	Steric hindrance reduces activity	This work
M3	Bulky amide derivative	1.0 M HCl	PDP	0.5	94.76	Hydrophobic but hindered adsorption	This work

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3.4. Surface analysis by SEM, EDX, FESEM and XPS

Fig. 12(a)–(d) represents the SEM images at 10 µm magnification power of MS coupons for 5.0 h of exposure to the 1.0 M HCl solution in presence and absence of 1 mM organic inhibitors (M1–M3). Fig. 12a illustrates the surface of MS coupons placed in blank acidic solution, clearly shows the severe corrosion with a highly porous, rough, and cracked structure. However, the surface of MS coupons exposed to acidic medium along with organic inhibitors M1, M2, and M3, as shown in Fig. 12b–d became smooth, less porous, less cracked, and rigid due to the formation of a thick layer by as-synthesized inhibitors (M1–M3). The compact layer of organic inhibitors protects the MS coupons from corrosion in an extremely acidic solution. Fig. 12b shows that the M1 forms a more dense layer compared to M2 (Fig. 12c) and M3 (Fig. 12d). The formation of a dense layer by the M1 was due to the less steric hindrance in the structure, which may provide more donating ability to form a more stable structure. These SEM results are similar to the outcomes of weight loss and electrochemical studies.

Fig. 12 SEM micrographs of mild steel coupon immersed (a) 1.0 M HCl blank, (b) M1, (c) M2 and (d) M3 for 5.0 h immersion time at 1 mM concentration in 1.0 M HCl.

Fig. 13(a)–(c) and Table 7 elaborates the results of EDX analysis performed on the MS strips exposed in 1.0 mM/1.0 M HCl solutions of inhibitors M1 (13a), M2 (13b) and M3 (13c). The M1 inhibitor adsorbed on the MS strip M1 inhibitor shows the atomic percentage compositions of carbon 35.18, nitrogen 1.70, oxygen 6.29 and iron 39.04. On the other hand the MS strips of M2 show carbon 37.90, nitrogen 1.86, oxygen 18.76 and iron 25.09 and the M3 have carbon 52.90, nitrogen 1.59, oxygen 33.73 and iron 2.43.

Fig. 13 EDX spectra of (a) M1, (b) M2, and (c) M3 after 5.0 h immersion time in acidic medium.

Table 7 Results of EDX analyses of MS coupon surface adsorbed by inhibitors (M1–M3) in acidic medium

Elements (symbol)	Percentage atomic composition
	M1	M2	M3
C	35.18	37.90	52.90
N	1.70	1.86	1.59
O	12.68	18.76	33.73
Fe	39.04	25.09	2.43

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All the EDX spectra clearly show the peaks of heteroatoms (nitrogen and oxygen) along with the carbon in sufficient proportion indicates the successful adsorption of inhibitors on the surface of mild steel coupons through heteroatoms and act as strong anticorrosion protective layer.

The surface of best performing inhibitor M1 was also analyzed by the FESEM shown in Fig. 14.

Fig. 14 FESEM images of M1 inhibitor at magnification power of (a) 20 µm, (b) 10 µm, (c) 2 µm and (d) 500 nm after 5 hours immersion time at 1 mM concentration in 1.0 M HCl.

The FESEM images were taken in different magnification (14a at 20 µm, 14b at 10 µm, 14c at 2 µm and 14d at 500 nm). All the FESEM images show the complete surface coverage, minimum cracks and strong adsorption of M1 on the MS surface to protect it completely from corrosion.

XPS analysis was performed on the MS coupon exposed in acidic medium with 1 mM M1 (Best performing inhibitor) for 5.0 h immersion time. The XPS survey peaks confirms the adsorption of M1 on the MS surface by exhibiting the presence of C 1s, O 1s, N 1s, Fe 2p, C KLL, N KLL, O KLL, Cu 2p, Al 1s, KLL and Al 1s peak as shown in Fig. 15a. The surveys graph includes the peaks of heteroatoms (O and N) reveals the adsorption of inhibitor.

Fig. 15 XPS surface analysis: (a) the XPS survey spectrum and the XPS spectrum of (b) C 1s, (c) N 1s, (d) O 1s, (e) Fe 2p_3/2, (f) Fe 2p_1/2 immersed in 1 M HCl containing 1 mM M1 inhibitor.

The deconvolutions of high resolution XPS peaks present the well-defined results to understand the adsorption mechanism shown in Fig. 15b–f and Table 8. The deconvoluted high resolution graph of C 1s generates the 3 peaks with binding energies 288.1 eV, 288.8 eV and 289.9 eV which confirms the metallic surface bears C–O, C–N, C=O, O–C–O and O–C=O groups.^119,120 The deconvoluted high resolution graph of N 1s exhibits two XPS strong peaks with binding energies 400.7 eV and 401.2 eV. The peaks confirm the presence of amidic nitrogen, pyrazolone ring nitrogen, protonated amide and Fe–N groups on Mild steel surface.¹²¹ The high resolution graph of O 1s deconvoluted to produce peaks at 527.95 eV, 530.6 eV, 531.7 eV and 532.2 eV. These peaks correspond to OH, Fe–O, C=O, H₂O Oxygen of M1 and chemisorbed oxygen on the MS surface.¹²² The deconvoluted high resolution spectra also covers the Fe 2p region and gives peak at 709.5 eV, 711.1 eV and 724.6 eV. These peaks show the presence of Fe, Fe₂O₃, Fe(II), Fe(III), Fe₃O₄ and Fe–O on the MS steel strips.¹²³ In conclusion the thermodynamic and different surface analysis supports physioorption and chemisorption of inhibitors on the MS surface. The heteroatoms (N and O) and carbon chains present on inhibitor structure play vital rule for adsorption on metallic surface and shows exceptional corrosion inhibition efficiency. All the results of XPS analysis are shown in Fig. 15a–f in the form of survey and high resolution graphs. The binding energies (eV) data and predicted assignments are summarized in Table 8.

Table 8 XPS results corresponding to the M1 layer adsorbed onto the steel surface after 5 hours immersed in 1.0 M HCl

Element	Binding energy (eV)	Assignment
C 1s (13b)	288.1	C–O, C–N
	288.8	C–N, C=O, O–C–O
	289.9	O–C=O
N 1s (13c)	400.7	Amide, NH2
	401.2	CONH2, protonated amide
O 1s (13d)	530.6	OH, metal oxides
	531.7	OH, C=O
	532.2	C=O, H2O
Fe 2p3/2 (13e)	709.5	Fe(0), Fe(II), Fe2O3
	711.1	Fe(III), Fe3O4
Fe 2p1/2 (13f)	724.6	Fe(III), Fe3O4

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3.5. DFT calculations

3.5.1. Frontier molecular orbitals (FMO), Mulliken charges and global reactivity descriptors. The geometrical shapes of the as-synthesized inhibitors were refined at the B3LYP/def2-SVP level of theory. To confirm the all refined geometries have no negative frequency and minimum potential energy surface, the harmonic frequency calculations were used at the same level. Upon the refined geometries gained, the Mullikan charges of inhibitors were determined and the frontier molecule orbitals, FMOs, HOMO and LUMO were calculated directly. Fig. 16 shows 3D refined geometries and special configuration of the HOMO and LUMO spread over the structure of the as-synthesized organic inhibitors.

Fig. 16 HOMO (blue), and LUMO (green) energy levels and optimized 3D structures (bottom) of the synthesized molecules M1, M2 and M3.

Mullikan charges are often used to identify the special centers of molecules which have ability to adsorbed.¹²⁴ The Mullikan charges calculated are given in Table 9, showing that the heteroatoms (oxygen and nitrogen) of the molecules have the maximum negative charge justifying their ability to donate electrons for adsorption on the metallic surface. The nitrogen and oxygen along with some carbons in the structure contain maximum negative charge could be perfect sites for making interaction on to the metallic surface due of their greater electron density. Moreover, the presence of the aryl group favors adsorbate-surface complex formation.¹²⁵ Mulliken charge distribution analysis provides the significant information regarding the adsorption behavior of the tested inhibitors (M1, M2, and M3) as shown in Table 9. A detailed comparison of the important charge values for the molecules indicates that these molecules are characterized by multiple electron-rich heteroatoms (N and O) that act as the main adsorption centers for the molecules, interacting with the vacant d-orbital of iron of mild steel through donor–acceptor interactions. In M1, the nitrogen atoms are characterized by significant negative charge values, i.e., N1 = −0.1679, N10 = −0.1153, while the oxygen atoms, i.e., O21 = −0.2633 and O22 = −0.2669, also have significant electron density, thus showing high electron-donating ability. On the contrary, M2 has comparatively less favorable characteristics, whereby, although the oxygen atoms possess a substantial negative charge (O24 ≈ −0.2680, O26 ≈ −0.2785), the replacement of adsorption centers by positively charged carbon atoms (C25 ≈ +0.1573) and a reduction in the electron density of the nitrogen atoms (N1 ≈ −0.1703, N27 ≈ −0.1412) could compromise its adsorption capability on the metal surface. Furthermore, M3 has intermediate adsorption characteristics, whereby highly negative charges on the oxygen atoms (O21 ≈ −0.2985, O24 ≈ −0.2763) indicate a substantial localized electron density, although a relatively lower contribution from the nitrogen atoms (N1 ≈ −0.1625, N10 ≈ −0.0968) and a non-uniform distribution of charges compromise its adsorption capability relative to M1. Furthermore, the presence of positively polarized carbon atoms in all the adsorbate molecules (C23 ≈ +0.1629, M1; +0.1573, M2; +0.1640, M3) facilitates electrostatic interactions with the metal surface, thereby confirming a mixed adsorption process that combines chemisorption and physisorption mechanisms. This is supported by theoretical principles that a higher negative charge density on the heteroatoms improves adsorption and inhibition efficiency.^6,126–128 The 3D optimized structure of the molecules M1, M2 and M3 with atoms numbers are given in Fig. S33 (SI). The electronic structure analysis of the synthesized molecules M1, M2, and M3 revealed distinct variations in their frontier orbital energies and band gaps, both in neutral and protonated states (Fig. 16(a) and (b)).

Table 9 Mullikan charges for synthesized molecules (M1, M2, M3)

M1						M2						M3
Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge	Atom	Charge
0 C	0.1159	24 O	−0.2625	48 H	−0.0148	0 C	0.122954	24 O	−0.26801	48 H	−0.01926	0 C	0.120314	24 O	−0.27629	48 H	−0.01503
1 N	−0.1679	25 N	−0.1448	49 H	−0.0129	1 N	−0.17034	25 C	0.157261	49 H	−0.01497	1 N	−0.16247	25 N	−0.14298	49 H	−0.01525
2 C	−0.0439	26 C	0.21693	50 H	−0.0152	2 C	−0.03636	26 O	−0.27852	50 H	−0.01557	2 C	−0.02003	26 C	0.203604	50 H	−0.01827
3 C	0.08376	27 C	−0.0388	51 H	−0.0164	3 C	0.092286	27 N	−0.14119	51 H	−0.01852	3 C	0.046717	27 C	−0.02954	51 H	−0.01124
4 C	−0.061	28 C	0.02137	52 H	−0.0194	4 C	−0.04844	28 C	0.20744	52 H	−0.01456	4 C	−0.06917	28 C	0.012986	52 H	−0.00159
5 N	−0.0701	29 C	0.03458	53 H	−0.0047	5 N	−0.06542	29 C	−0.03062	53 H	−0.01996	5 N	−0.06572	29 C	0.038738	53 H	0.01452
6 C	0.04384	30 C	0.01454	54 H	−0.0142	6 C	0.041079	30 C	0.014636	54 H	−0.01536	6 C	0.05335	30 C	0.012233	54 H	−0.01203
7 C	0.04802	31 C	0.01577	55 H	0.00196	7 C	0.094719	31 C	0.03423	55 H	−0.02094	7 C	0.076392	31 C	0.011939	55 H	−0.00321
8 C	0.05165	32 H	0.03983	56 H	−0.0143	8 C	0.030205	32 C	0.015239	56 H	−0.01814	8 C	−0.00783	32 H	0.044217	56 H	0.003726
9 C	0.03555	33 H	0.0541	57 H	−0.0211	9 C	0.039754	33 C	0.004984	57 H	−0.02029	9 C	0.054081	33 H	0.054159	57 H	0.003789
10 N	−0.1153	34 H	0.05971	58 H	0.00143	10 N	−0.14476	34 H	0.037432	58 H	−0.0136	10 N	−0.09677	34 H	0.059248	58 H	0.019002
11 C	−0.1557	35 H	0.04214	59 H	−0.0581	11 C	0.096832	35 H	0.053574	59 H	−0.00829	11 C	0.05408	35 H	0.015638	59 H	−0.00762
12 C	0.10851	36 H	0.04346	60 H	−0.0217	12 C	−0.25322	36 H	0.056611	60 H	0.004716	12 C	0.177899	36 H	0.050453	60 H	0.014151
13 C	0.12314	37 H	0.01896	61 H	−0.0213	13 C	0.104773	37 H	0.044861	61 H	0.003326	13 C	−0.05867	37 H	0.059907	61 H	−0.05112
14 C	−0.0934	38 H	0.12242	62 H	−0.0126	14 C	0.117646	38 H	0.038521	62 H	0.009516	14 C	0.101432	38 H	0.126413	62 H	−0.01684
15 C	0.10377	39 H	0.00568	63 H	−0.0005	15 C	−0.07806	39 H	0.023916	63 H	0.013068	15 C	0.08147	39 H	0.005176	63 H	−0.01817
16 C	0.12421	40 H	0.05345			16 C	0.10302	40 H	0.123097	64 H	0.00705	16 C	−0.21486	40 H	0.043934	64 H	−0.0108
17 C	−0.1026	41 H	0.01691			17 C	0.105322	41 H	−0.0164	65 H	−0.05646	17 C	0.103589	41 H	0.041144	65 H	−0.00139
18 C	0.10387	42 H	−0.0026			18 C	−0.08145	42 H	0.044735	66 H	−0.0174	18 C	−0.26524	42 H	0.038769
19 C	−0.0907	43 H	0.12228			19 C	0.106683	43 H	0.011003	67 H	−0.02139	19 C	0.093159	43 H	0.120604
20 C	0.14225	44 H	−0.0034			20 C	−0.08115	44 H	0.024401	68 H	−0.01265	20 C	0.097984	44 H	0.007596
21 O	−0.2633	45 H	0.02382			21 C	0.133992	45 H	0.127189	69 H	−0.00031	21 O	−0.29851	45 H	−0.01485
22 O	−0.2669	46 H	−0.0183			22 C	0.043432	46 H	0.001705			22 O	−0.26741	46 H	−0.02284
23 C	0.16286	47 H	−0.0082			23 O	−0.27654	47 H	−0.01308			23 C	0.164042	47 H	−0.03073

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The E_HOMO value of a molecule is indicative of its electron-donating ability, with higher E_HOMO values indicating easier donation of electrons to acceptor molecules with low energy, empty molecular orbitals. The E_LUMO value, on the other hand, represents the ability of molecule to accept electrons, with lower E_LUMO values indicating greater ease of electron acceptance. The absolute hardness (η) reflects the stability of a compound, while the global softness (S) quantifies the electron cloud polarization and is the opposite of hardness. A higher energy gap (ΔE_gap) indicates lower polarity, making the adsorption of the molecule on a metal surface more challenging. An effective corrosion inhibitor typically exhibits low ΔE and η values but a high S value. The calculated quantum chemical parameters from FMOs for the molecules (M1, M2 and M3) both in neutral and protonated states are given in Table 10. According to the values in Table 10, all molecules (M1, M2 and M3) demonstrate excellent corrosion inhibition properties, as their calculated quantum chemical parameters. In the neutral Form, M1 showed the highest value of E_HOMO (−5.042 eV) and the smallest energy gap (ΔE_gap = 4.321 eV) in comparison to M2 and M3. The high value of E_HOMO indicates a better ability of the molecule to donate electrons to the vacant d orbitals of the metal surface, thus forming a coordinate bond. With protonation, major changes in the electronic configuration were observed. The values of E_HOMO for all the inhibitors decreased, indicating a more negative value. However, the trend for E_HOMO was similar to that in the neutral state. The energy gap ΔE_gap is a major criterion for chemical reactivity and kinetic stability. In both the neutral and protonated states, the values of ΔE_gap for all three inhibitors are in the order M1 < M3 < M2. The energy gap for M1 is lower, indicating higher molecular reactivity and lower excitation energy, thus increasing the adsorption potential on the mild steel surface. On the other hand, M2 showed a higher energy gap in both states, indicating higher kinetic stability and a lower adsorption potential. The general notion is that the greater the dipole moment, the greater its ability to have protective action upon the metal surface. As shown from the computed results (Table 11), the dipole moment of the inhibitors studied were observed to be larger than that of the water molecule (μ = 1.84 Debye). The results provided suggest that the adsorption process of the studied inhibitors can be compared with that of replacing water on the metal surface while in solution. The vertical ionization potential I_v is defined as the amount of energy needed to remove an electron. The least I_v is observed for the neutral state of M1 (5.151 eV). Hence, it is the most capable of donating electrons to the metal. However, after protonation, the I_v of all the molecules is high (6.769 eV for M2). Hence, it is clear that after protonation, it is more difficult to remove electrons because the electrons are more stable. On the other hand, the electron affinity A_v is defined as the amount of energy that is required to add an electron to the molecule. The highest A_v is observed for the neutral state of M1 as compared to other molecules M2 and M3. The increase in A_v in protonation form (1.523 eV for M2 and M3) shows that it is easier for the protonated molecules to accept electrons from the metal. The electronegativity (χ) of the molecule represents the ability of the molecule to attract electrons towards itself. The neutral M3 has the highest electronegativity of 3.088 eV. It represents the ability of the molecule to pull electrons towards itself. The chemical potential (Π) of the molecule represents the negative of electronegativity. The more negative the chemical potential of the protonated molecules, the more stable the electronic system of the molecules, although highly reactive at the metal interface.

Table 10 Energy (E in Hartree), dipole moment (μ in Debye), molecular volume (V in e bohr⁻³) and the calculated quantum parameters (in eV and eV⁻¹) for M1, M2 and M3 inhibitors

	Neutral			Protonated
	M1	M2	M3	M1	M2	M3
ETotal: electronic energy; E(N + 1)/E(N − 1): energies of cation/anion; μ: dipole moment; volume: molecular volume; Iv: ionization potential; Av: electron affinity; Eg: band gap; EHOMO/ELUMO: orbital energies; ΔEgap: HOMO–LUMO gap; χ: electronegativity; Π = −χ: chemical potential. η: hardness; S: softness; ω: electrophilicity index; ε: nucleophilicity index; ΔN: electron transfer fraction; ΔE1, ΔE2: interaction energies; Eb-d: back-donation energy.
ETotal (N)	−1416.288	−1494.886	−1417.456	−1416.932	−1495.541	−1418.114
E(N + 1)	−1416.329	−1494.926	−1417.497	−1416.984	−1495.597	−1418.170
E(N − 1)	−1416.099	−1494.697	−1417.267	−1416.692	−1495.293	−1417.869
μ (D)	9.932	7.796	10.945	13.160	25.183	22.798
Volume	3462.222	3757.517	3574.804	3478.3639	3793.3213	3633.8774
EHOMO	−5.042	−5.074	−5.273	−6.63	−6.78	−6.88
ELUMO	−0.721	−0.448	−0.903	−1.22	−1.31	−1.31
ΔEgap	4.321	4.625	4.3701	5.41	5.47	5.57
Iv	5.151	5.166	5.175	6.510	6.769	6.667
Av	1.094	1.087	1.086	1.422	1.523	1.523
Eg	4.057	4.079	4.089	5.087	5.246	5.144
χ	2.882	2.761	3.088	3.925	4.045	4.095
η	2.161	2.313	2.185	2.705	2.735	2.785
S	0.463	0.432	0.458	0.370	0.366	0.359
Π	−2.8815	−2.761	−3.088	−3.925	−4.045	−4.095
ω	1.922	1.648	2.182	2.848	2.991	3.011
ε	0.520	0.607	0.458	0.351	0.334	0.332
ΔN	0.953	0.916	0.895	0.568	0.540	0.522
ΔE1	−0.570	−0.297	−0.752	−1.069	−1.159	−1.159
ΔE2	4.891	4.923	5.122	6.479	6.629	6.729
Eb-d	−0.540	−0.578	−0.546	−0.676	−0.684	−0.696

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Table 11 Comparison of ΔN (theoretical) and IE% (experimental)

Molecule	ΔN	IE%
M1	0.95	98.92
M2	0.92	98.22
M3	0.90	94.70

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According to the HSAB theory, a “soft” inhibitor is more efficient because it more readily interacts with “soft” iron atoms on the metal surface. M1 and M3 are “softer” in their neutral state compared to M2. The lower value of hardness and higher value of softness (S) for M1 indicate that its cloud is more easily polarizable, thus showing easier transition during the adsorption process on mild steel. The electrophilicity index (ω) is a measure of the total electrophilic nature of the molecule. From the results, it can be observed that the electrophilicity index increases upon protonation of the molecule. This indicates that the inhibitors possess a higher electrophilic nature in the acidic medium and can accept the electron density from the d-orbitals of the metal, thereby stabilizing the protective. Finally, the calculation of number of electron transfers calculated by the DFT studies shows the M1 highest ΔN value compared to M2 and M3, indicating that the M1 has more ability of the electron donation to make interaction with the mild steel surfaces. Hence, M1 is expected to possess the highest inhibition efficiency because it is better at donating electrons to the mild steel surface. Inspection of Table 10 shows that the ΔE_b-d derived for the M1, M2 and M3 inhibitors are <0 indicating that transfer of electron density to a molecule followed by back donation of electron density from the molecule to the Fe-surface is energetically favored situation. The negative values and small deviation between the ΔE_b-d values for the molecules simply confirms that these inhibitors behave like good corrosion inhibitors. Studies showed that there is a better relationship between inhibitor polarity and inhibitive ability.¹²⁹ In conclusion, from the above results, we can see that for the three molecules (M1, M2, and M3) that we studied, they all exhibit the inhibition ability to protect the Fe surface with M1, on very slight preference over M2 and M3. The mechanism of interaction, which involves predominantly chemisorption on a metal surface (soft-center), is well described by the principles of chemical softness and electron transfer.

In order to give a quantitative evidence to the correlation of frontier molecular orbital (FMO) descriptors with the experimental inhibition efficiency (IE%), a regression analysis was carried out using main quantum chemical parameters. Out of the explored descriptors, the fraction of electron transfer (ΔN) showed the highest positive correlation with experimentally obtained IE% values, giving the coefficient of determination (R² ≈ 0.79) shown in Table 11. In this relation the IE% of M1, M2 and M3 from electrochemical analysis of 98.92, 98.22 and 94.70 respectively related with their ΔN values of 0.95, 0.92 and 0.90 with good R² of 0.79. This high correlation means that the capability of the inhibitor molecules to donate electrons is the most important factor for their adsorption on the mild steel surface and, therefore, their corrosion inhibition enhancement. Especially, the higher ΔN value of M1 is well in line with its better inhibition efficiency, whereas M2 and M3 come at the next steps. In contrast, other descriptors such as the energy gap (ΔE_gap) and global softness (S) showed relatively weak correlations with IE% (R² ≈ 0.046 and 0.02, respectively), suggesting that these parameters alone cannot adequately describe the inhibition behavior. This observation highlights that corrosion inhibition is governed by a synergistic effect of multiple electronic factors rather than a single descriptor. Overall, the results confirm that the inhibition efficiency follows the order: M1 > M2 > M3.

Therefore, the present analysis demonstrates that ΔN serves as a reliable quantitative descriptor for correlating theoretical calculations with experimental inhibition efficiency.

3.5.2. Inhibitor-metal interaction type. In order to predict the type of interaction between the Fe and the inhibitors, and to propose a potential mechanism for the adsorption process, the energy gaps between the inhibitors and Fe-metal (ΔE₁ and ΔE₂) were estimated according to eqn (27) and (28).

ΔE₁ = E_LUMO(inhibitor) − E_LUMO(Fe) (27)

ΔE₂ = E_LUMO(Fe) − E_HOMO(inhibitor) (28)

where E_HOMO and E_LUMO of Fe are −7.9024 eV, and −0.151 eV, respectively.¹³⁰ ΔE₁ corresponds to the donated electrons from the d-orbitals of Fe surface, acting as a Lewis base, to the LUMO of the inhibitor as the Lewis acid. Meanwhile ΔE₂ indicates electrons donation from the HOMO of the inhibitor, acting as a Lewis base, to the vacant d-orbitals of Fe as Lewis acid. The ΔE₁ and ΔE₂ values are also calculated in Table 10, ΔE₁ is greater than ΔE₂ indicating that the compounds explored would preferentially behave as Lewis acids, with an affinity to donate electrons to the vacant d-orbitals of Fe.

3.5.3. Fukui functions and the local dual reactivity indices. Table S10 provides the calculated values of the local indices and their dual descriptors of the M1, M2 and M3 inhibitors, respectively. For the M1, the highest nucleophilic attack (f⁺_k) are O24, N1 and C4, while the highest electrophilic attacks (f⁻_k) are C26, C27 and C29. In the case of the M2 (neutral), the highest nucleophilic attack (f⁺_k) are O26, N1 and C4, while the highest electrophilic attacks (f⁻_k) are C28, C29, and C31. For the M3, the highest nucleophilic attack (f⁺_k) are O24, N1 and C4, while the highest electrophilic attacks (f⁻_k) are C26, C27 and C29. The estimated values of dual descriptors, which are shown in Table S10 of the SI, allow us to attain instantaneously the preferable sites for nucleophilic attacks (f_k², Δσ_k and Δω_k > 0) and the preferable sites for electrophilic attacks (f_k², Δσ_k and Δω_k < 0) over the system at point r.¹³¹

Fig. 17 displays the dual descriptors: f_k², Δσ_k and Δω_k of the neutral forms of M1, M2 and M3 compounds. As seen for the condensed Fukui functions and the local softness and philicity, the M1, M2 and M3 compounds have several active sites, and most of the active sites have a dual descriptor value lower than zero, besides some carbon atoms that have positive values. According to Fig. 17, it can be concluded that the most active sites that can interact with the iron surface in M1, M2 and M3 compounds are the oxygen and nitrogen atoms and some carbon atoms. However, in M1, M2 and M3 compounds, the different carbon atom of benzene ring is the site to accept electron of the Fe surface to form back-bonding interactions. These results are consistent with the results obtained via the HOMO, LUMO evaluations.

Fig. 17 (Left) 3D structure with atoms numbering of M1, M2 and M3, (middle) 3D-plots of iso-surface for dual Fukui function descriptor (red lobes) correspond to positive region and the blue lobes correspond to the negative region, (right) the condensed local dual descriptor f_k², Δσ_k, and Δω_k based on Fukui functions.

3.6. Corrosion inhibition mechanism

The organic inhibitor inhibits metal corrosion through an adsorption process.¹³² In the adsorption process, the water molecules and other corrosion species on the metal surface are replaced by the inhibitor molecules. The state of organic molecules and the metal surface charge influence the adsorption process in the acidic environment.¹³³ The mild steel in HCl carries a net positive charge, which causes the adsorption of chloride ions on the surface. Moreover, excess electrons are generated which facilitates the adsorption of the cationic species on the surface of mild steel. In the present study, the M1, M2, M3 have many sites available for interaction with mild steel surface. The interaction with mild steel surface through the amide linkage from antipyrine ring side and amide linkage from the adamantyl group side can be possible. The DFT studies show that the amide linkage from the antipyrine side, nitrogen of antipyrine ring are mainly involved to form interactions with steel surface. Whereas, in HCl solution, the M1, M2, and M3 may be protonated to from the electrostatic interaction with the chloride ions on the surface of mild steel leading to the physiosorption of the inhibitor molecules. The proposed mechanism is shown in Fig. 18. The M1 show the donation of electrons through the O, and N sites from the amide group attached to the antipyrine ring and nitrogen and oxygen from the antipyrine ring. Moreover, the oxygen of amide attached to the adamantyl group also involved in inhibiting the surface. The bonding interaction of the M1 with the mild steel surface is though the vacant d orbitals of mild steel.

Fig. 18 Proposed mechanism of inhibition of the mild steel surfaces using M1.

4. Conclusion

In conclusion, the study demonstrates the effectiveness of newly synthesized compounds M1, M2, and M3 as corrosion inhibitors for mild steel in an acidic environment by using both experimental and theoretical methods. The key findings and observations are summarized as follows:

1. The study shows that the M1, M2 and M3 are effective corrosion inhibitors for mild steel in acidic solution. It was observed that the corrosion rate decreases and IE% increases both by increasing the immersion time and concentration. Elevated temperature decreases the IE% and increases the corrosion rate.

2. Different thermodynamic parameters like activation energy (E_a), Enthalpy changes ΔH, Entropy change ΔS and Gibb's free energy change ΔG elaborates the adsorption of inhibitors on the surface of mild steel to mitigate the corrosion. This adsorption creates a protective layer on the mild steel that acts as a barrier to prevent corrosion entities from attacking on the metals. These thermodynamic parameters collectively indicated that adsorption procedure of these inhibitors is the mixture of physisorption and chemisorption depends of various conditions like temperature.

3. Electrochemical analysis, both PDP and EIS methods were used to assess the anticorrosion properties of the inhibitors. Various parameters calculated from these analyses provide the valuable information about the kinetics of corrosion process which reveals the extraordinary inhibitory properties of the inhibitors.

4. The literature study shows that these inhibitors comprise of nontoxic, ecofriendly and readily available structural fragments. The reagents and instrumentations need for the synthesis of these inhibitors are very cheap which make it more economic.

5. The quantum chemical and DFT measurements support the experimental data and proves the exceptional inhibitory efficiency of inhibitors.

In summary, these compounds show increased inhibition efficiency with higher concentrations and longer exposure times. The anti-corrosion potential of the compounds decreases at higher temperatures. Among these inhibitors (M1–M3), M1 exhibits the highest anticorrosion potential due to the lower steric hindrance on the adamantyl group in its structure. The research combines experimental and theoretical approaches, providing comprehensive insights into the inhibition mechanisms and highlighting the potential of these compounds for applications in corrosion protection.

Conflicts of interest

The authors declare no conflict of interests.

Data availability

The supporting data has been provided as part of the supplementary information (SI). it contains following files SI-1: it contains the characterization data NMR, FTIR, MS etc. and Table S11. SI-2: it contains Tables S1–S9 and S10. See DOI: https://doi.org/10.1039/d6ma00114a.

Acknowledgements

This research work is supported by the Ministry of Higher Education (MOHE) under the 2023 Translational Research Program for the Energy Sustainability Focus Area (Project ID: MMUE/240001), the 2024 ASEAN IVO (Project ID: 2024-02), and Multimedia University, Malaysia. Adil Afaq thanks the Higher Education Commission of Pakistan for IRSIP funding. Md. Akhtaruzzam acknowledges the Deanship of Graduate Studies and Scientific Research at the Islamic University of Madinah for the research support Program.

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