Surface and Electrochemical Investigation of a Novel Isoxazolone Inhibitor for Carbon Steel Corrosion: Adsorption and Inhibition Mechanism

Abstract

A novel isoxazolone derivative, (E)-4-(4-(diethylamino)­benzylidene)-3-methylisoxazol-5­(4H)-one (DEAI), was synthesized via a one-pot, environmentally friendly protocol and characterized by spectroscopic techniques. Its corrosion inhibition performance toward carbon steel in 1 M HCl was investigated using electrochemical methods, including potentiodynamic polarization and electrochemical impedance spectroscopy, combined with surface analyses (AFM, XPS, UPS) and theoretical calculations. Electrochemical results indicate that DEAI acts as a mixed-type inhibitor with predominant cathodic behavior, achieving maximum inhibition efficiencies of 87.5% (303 K), 88.1% (308 K), 92.7% (313 K), 90.2% (318 K), and 86.7% (323 K) at 7.5 × 10^–5 M. Impedance measurements reveal that the corrosion process is governed by charge transfer and that inhibition efficiency increases with inhibitor adsorption. UV–vis analysis suggested that the functional groups of DEAI form a complex with iron cations on the carbon steel surface. Surface characterization confirms the formation of a protective interfacial film. Atomic force microscopy (AFM) shows a significant reduction in surface roughness, while X-ray photoelectron spectroscopy (XPS) provides evidence of strong interactions between inhibitor molecules and the steel surface through Fe–O and Fe–N bonds. Ultraviolet photoelectron spectroscopy (UPS) reveals modifications in the electronic structure at the metal–inhibitor interface, indicating enhanced surface passivation and strong electronic coupling. Adsorption follows a modified Langmuir isotherm and involves both physical and chemical interactions. Density functional theory and molecular dynamics simulations further support strong adsorption of DEAI on the Fe(110) surface. These findings provide comprehensive insight into the mechanism of inhibition and highlight the key role of interfacial interactions in corrosion protection.

1. Introduction

Metal corrosion remains one of the most pervasive and costly forms of degradation in modern industry, compromising the reliability and service life of metallic infrastructure, particularly carbon steel exposed to aggressive aqueous environments. − This electrochemical process converts the metal into oxide or hydroxide species, leading to wall thinning, leakage, and, in severe cases, catastrophic failure of critical components. , It also imposes a major economic burden, accounting for several percent of GDP in industrialized countries when both direct and indirect costs are considered. Therefore, in strategic sectors such as oil and gas production, petrochemistry, energy, and desalination, corrosion management is a key issue for sustainable operation and risk reduction. − In the oil and gas industry in particular, carbon steel pipelines, tubing, and process equipment are frequently subjected to severe service conditions, including high temperatures, elevated pressures, and the presence of corrosive species such as HCl, H₂S, CO₂, and chloride-rich brines. ,− Acidizing operations, well stimulation treatments, and scale removal procedures routinely employ concentrated hydrochloric acid solutions in contact with steel surfaces for extended periods of time, leading to accelerated metal dissolution if an appropriate protection strategy is not implemented. ,, Similarly, in pickling, descaling, and cleaning processes in the steel, chemical, and power industries, strong mineral acids (HCl, H₂SO₄) are used to remove oxide scales and surface contaminants, but they also induce an intense attack on carbon steel substrates. , In these contexts, optimizing corrosion control is not only an economic issue but also an essential requirement for ensuring process safety, environmental protection, and regulatory compliance. Among the various approaches used to mitigate steel corrosion in acidic media, such as material selection, alloying, coatings, cathodic protection, , and process modification, the addition of organic corrosion inhibitors directly to the corrosive solution is widely recognized as one of the most practical and cost-effective strategies. − Organic inhibitors are typically adsorbed onto the metal surface, forming a protective layer that separates the metallic substrate from the aggressive environment and slows both anodic metal dissolution and cathodic hydrogen evolution reactions. The performance of these inhibitors depends heavily on their molecular structure, which determines how they interact with metal surfaces via physisorption, chemisorption, or a combination of both. − Numerous studies have highlighted that molecules comprising heteroatoms, such as sulfur, nitrogen, and oxygen, as well as functional groups, like imine, hydroxyl, carbonyl, thiol, and π-conjugated systems in aromatic rings, demonstrate significant inhibition efficiencies in acidic environments by adsorbing at active sites and blocking the access of corrosive species. − Recent investigations have confirmed that heterocyclic organic compounds remain among the most efficient corrosion inhibitors for steel in acidic media because of their multiple adsorption centers and favorable electronic properties. , Newly developed nitrogen- and oxygen-containing heterocycles have exhibited excellent inhibition efficiencies, while combined electrochemical, surface characterization, and theoretical approaches have considerably improved the understanding of their adsorption mechanisms and protective performance. − Over the past few decades, the concept of “green” or environmentally friendly corrosion inhibitors has attracted growing interest, driven by increasingly stringent environmental regulations and the need to replace toxic or carcinogenic compounds such as chromates, nitrites, and certain aromatic amines. − Ideal environmentally friendly corrosion inhibitors should combine high efficiency, stability, low toxicity, and biodegradability. In this context, heterocyclic organic molecules are attractive candidates because their structural versatility enables fine-tuning of electronic properties and adsorption behavior. ,, Isoxazolone and oxazolone-based heterocycles have recently attracted considerable interest as promising scaffolds for corrosion inhibition in acidic media. Recent studies have further demonstrated that isoxazolone-derived and structurally related heterocyclic compounds exhibit remarkable inhibition efficiencies in acidic media owing to their high electron density, π-conjugated systems, and multiple adsorption centers. These investigations highlighted the decisive influence of substituent effects and molecular electronic properties on adsorption strength and corrosion inhibition performance. Furthermore, the integration of electrochemical techniques with density functional theory (DFT) and molecular dynamics (MD) simulations has significantly improved the understanding of inhibitor adsorption mechanisms and structure–activity relationships, thereby facilitating the rational design of high-performance heterocyclic corrosion inhibitors. − These compounds possess high electron density, extended π-conjugation, and multiple potential adsorption centers, typically including CO, CN, and N–O functionalities, which promote strong interactions with metallic surfaces. In addition, isoxazol-5­(4H)-one derivatives constitute an important subclass within the isoxazolone family, a group of heterocyclic compounds widely recognized for their diverse biological and pharmacological properties. Owing to the presence of a reactive heterocyclic core, these derivatives have attracted considerable interest in medicinal chemistry and materials science. , The nature of substituent radicals attached to the isoxazol-5­(4H)-one moiety plays a crucial role in modulating its structure–activity relationship. In particular, attaching electron-donating groups boosts the electron density of the heterocyclic ring, thereby enhancing its electron-donating ability. This promotes stronger coordination interactions between the inhibitor and the metal surface as electrons transfer to iron’s vacant d-orbitals. Consequently, the metal-inhibitor bond strengthens, resulting in improved adsorption and the formation of a more stable, protective film on the metal surface. Mehtab Parveen et al. reported the synthesis and characterization of a series of isoxazol-5­(4H)-one derivatives and demonstrated their structural stability through X-ray diffraction and density functional theory (DFT) calculations. They thus highlighted the value of combining experimental and theoretical approaches for the design of effective corrosion inhibitors. Although these recent studies demonstrated the promising performance of isoxazolone derivatives as corrosion inhibitors, most investigations mainly relied on electrochemical techniques supported by DFT calculations. Comprehensive studies combining electrochemical measurements with advanced surface-sensitive techniques, such as AFM, XPS, and UPS, together with multiscale atomistic simulations remain scarce. Therefore, further investigations are required to establish a deeper understanding of the adsorption mechanism and the physicochemical nature of the protective film formed on the steel surface. In parallel with synthetic and electrochemical investigations, computational chemistry has become an indispensable tool for understanding and predicting the behavior of corrosion inhibitors at the molecular scale. , DFT calculations reveal key molecular descriptors related to the reactivity, electron-donating/accepting ability, and adsorption behavior of corrosion inhibitors. , In general, a high E_HOMO value, low E_LUMO, and a smaller energy gap indicate stronger electron exchange with the metal surface and thus stronger inhibitor–metal interactions. , Molecular electrostatic potential (MEP) maps and charge distribution analyses further help identify nucleophilic and electrophilic regions of the molecule that may act as adsorption centers. , These simulations provide key information on interaction energy, binding strength, surface coverage, and adsorption behavior, helping assess film compactness and stability under operating conditions. When combined with electrochemical techniques such as potentiodynamic polarization (PDP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS), along with spectroscopic tools such as UV–Vis and FTIR, this approach provides a comprehensive understanding of corrosion inhibition and supports the rational design of next-generation inhibitors. Despite these significant advances, a comprehensive understanding of the relationship between molecular structure, electronic properties, adsorption mechanism, and protective film formation is still lacking for many isoxazol-5­(4H)-one derivatives. Therefore, further investigations combining electrochemical methods, advanced surface characterization, and multiscale computational approaches are required to clarify the inhibition mechanism at the metal/electrolyte interface. In this context, the present work investigates the corrosion behavior and mechanism of (E)-4-(4-(diethylamino)­benzylidene)-3-methylisoxazol-5­(4H)-one (DEAI) on carbon steel in acidic media 1 M HCl. Its performance is evaluated by conventional electrochemical techniques (PDP, LPR, and EIS) together with advanced surface and interfacial analyses. AFM is used to assess surface morphology and roughness changes, XPS to identify the chemical composition of the protective layer, and UPS to probe electronic interactions at the metal–inhibitor interface. These experimental results are supported by DFT, MD, and RDF simulations to clarify the adsorption behavior at the atomic scale. Overall, this combined experimental and theoretical approach provides deeper mechanistic insight into corrosion inhibition and highlights the value of surface-sensitive techniques in the development of next-generation inhibitors.

2. Materials and Methods

2.1. Materials Preparation

The materials used for inhibitor synthesis and corrosion inhibition analysis were commercially available. 1H and 13C NMR spectra were recorded at room temperature using a Bruker AC300 spectrometer. DMSO-d ₆ was employed as a solvent, and TMS was used as an internal standard. Chemical shifts (δ) are reported in parts per million (ppm), while coupling constants (J) are given in Hertz (Hz). Specimens used in this study were cut from a C35 carbon steel plate, with the following chemical composition (weight%): C 0.35–0.36%, Si 0.3%, Mn 0.7%, S 0.016%, Cr 0.077%, Ti 0.011%, Ni 0.059%, Co 0.009%, Cu 0.160%, and the remaining balance was iron Fe. Before starting the experiment, the carbon steel C35 specimens machined into 10 mm × 10 mm × 3 mm were mechanically pretreated by sequential abrasive grinding using sandpapers of grit sizes ranging from 80 to 1200. The specimens were then rinsed with distilled water, degreased with acetone, thoroughly washed again with distilled water, and finally dried in air. The corrosive solution (electrolyte) is a 1 M hydrochloric acid (HCl) solution, prepared by dilution of analytical grade 37% HCl in distilled water. The inhibited solutions with concentrations ranging from 7.5 × 10^–6 to 1 × 10^–4 (M) were made by adding a suitable amount of isoxazolone inhibitor to a 1 M HCl solution.

2.2. Synthesis and Characterization of Isoxazolone Inhibitor

An efficient and environmentally friendly process was employed for the synthesis of (E)-4-(4-(diethylamino)­benzylidene)-3-methylisoxazol-5­(4H)-one compound. The protocol was based on a one-pot, three-component reaction, involving an aromatic aldehyde, β-ketoester, and hydroxylamine hydrochloride in an ethanol–water solution, and using a small amount of sodium acetate as a safe catalyst. The heterocyclic isoxazolone compound was prepared as described in the literature (Figure (a)). To an aqueous-ethanolic solution of sodium acetate (1 mmol), hydroxylamine hydrochloride (10.2 mmol), ethyl acetoacetate (10 mmol), and 4-(dimethylamino)­benzaldehyde (10 mmol) were added. The mixture was stirred magnetically at room temperature, and the progress of the reaction was monitored by TLC using (n-hexane/ethyl acetate: 2:1 v/v). After completion of the reaction, the red precipitate was filtered, washed with cold water, and recrystallized from water–ethanol. Red solid. Yield: 71%. Melting point: 126–128 °C. ¹H NMR (400 MHz, DMSO-d ₆, δ, ppm) (Figure (b)): δ 1.16 (t, J = 7.0 Hz, 6H, CH₃), 2.21 (s, 3H, CH₃), 3.53 (q, J = 7.1 Hz, 4H, CH₂), 6.89–6.81 (m, 2H aromatic, CH₂), 7.59 (s, 1H CH) 8.45 (d, J = 8.8 Hz, 2H aromatic, CH₂). ¹³C NMR (101 MHz, DMSO-d ₆, δ, ppm) (Figure (c)): δ 11.79 (C₅), 12.95 (C₁₂), 44.80 (C₁₄), 108.97 (C₇), 111.86 (C₁), 121.21 (C₈), 138.44 (C₉), 150.63 (C₆), 152.95 (C₁₀), 162.56 (C₅), 170.38 (C₂). DEPT-135 (101 MHz, DMSO-d ₆, δ, ppm) (Figure (d)): δ 150.62 (C₆), 138.43 (C₉), 111.87 (C₁), 40.72 (C₁₄ negative peak), 12.95 (C₁₂), 11.78 (C₁₅). FT-IR (ν, cm^–1): ν 3018 (C–H_sp2), 2991 (C–H_sp3), 1714 (CO), 1590 (CN), 1194 (C–O).

1.

(a) Synthetic route to (E)-4-(4-(diethylamino)­benzylidene)-3-methylisoxazol-5­(4H)-one. (b) ¹H NMR, (c) ¹³C NMR, and (d) DEPT-135 NMR spectra of the synthesized compound.

2.3. Electrochemical Measurements

Electrochemical tests including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were carried out using an OrigaFlex 01A potentiostat/galvanostat and monitored by a personal computer via a GPIB Interface and OrigaMaster 5 software. The corrosion cell used for electrochemical tests was made of Pyrex glass, had a total volume of 250 mL, and was equipped with a conventional three-electrode system: A platinum electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Regarding the working electrode, a steel specimen with dimensions of 10 mm × 10 mm × 3 mm was embedded in epoxy resin, with a conductive-coated wire attached to ensure electrical contact. An exposed surface area of 1 cm² was left in contact with the electrolyte. To achieve steady-state potential (E _OCP), the carbon steel electrode was immersed in both inhibited (7.5 × 10^–6 to 1 × 10^–4 M) and inhibited-free media (1 M HCl) at different temperatures (303–323 K) for 1 h. Potetiodynamic polarization curves (PDP) were generated at a scan rate of 0.5 mV/s and current potential ranging from −800 mV to −200 mV vs SCE. The corrosion current density (i _corr) and other relevant parameters (E _corr b _a and b _c) were extrapolated from the linear parts of the Tafel curves. The percentage of corrosion inhibition efficiency (IE _PDP %) was calculated as follows:

$$I{E}_{PDP}(\%)=\frac{i_{\mathrm{corr}}^0-i_{\mathrm{corr}}}{i_{\mathrm{corr}}^0}\times 100$$ 1

Where $i_{\mathrm{corr}}^0$ and $i_{\mathrm{corr}}$ represent the corrosion current densities for free-inhibitor and for isoxazolone inhibitor, respectively.

OCP analysis offers limited and semiquantitative information on corrosion behavior. Based on the Stern–Geary method, the linear polarization resistance (LPR) technique was conducted to gain insight into the polarization resistance (R _p) and subsequently evaluate corrosion inhibition efficiency. This method states that the current response varies linearly with the applied potential close to the corrosion potential (E _corr), and the slope of the current–potential plot vs E _corr corresponds to the polarization resistance (R _p). Indeed, the R _p values were obtained from Tafel and LPR measurements by applying the Stern–Geary equation:

$$R_p=\frac{b_a{b}_c}{2.303(b_a+b_c)}\times \frac{1}{i_{corr}}=\frac{B}{i_{corr}}$$ 2

Where b _a and b _c are the anodic and cathodic Tafel slopes, respectively. B denotes the Stern–Geary constant. The corrosion rate (C.R) is directly proportional to the amount of metal dissolution and can be determined from the corrosion current density (i _corr) using Faraday’s law

$$C.R=\frac{i_{corr}kEw}{d}$$ 3

Where k = 0.13 represents a constant factor used for converting i _corr to C.R (MPY). EW denotes the equivalent weight of the steel alloy, and d refers to its density. The percentages of corrosion inhibition efficiency of both methods, C.R and LPR, were computed according to the following equations:

$$I{E}_{\mathrm{C.R}}(\%)=\frac{C.R_0-C.R_{inh}}{C.R_0}\times 100$$ 4

$$I{E}_{\mathrm{LPR}}(\%)=\frac{R_{pinh}-R_p}{R_{pinh}}\times 100$$ 5

Where C.R ₀ and C.R _inh represent the corrosion rate with and without various doses of isoxazolone inhibitor, while R _p and R _pinh denote the polarization resistance with and without various doses of isoxazolone inhibitor, respectively.

Electrochemical impedance spectroscopy (EIS) measurements were carried out using the same electrochemical system, which was connected to ZPlot 2.80 software to control and record each EIS experiment. The data were then fitted to a Nyquist graph using ZView 4.0h software. The impedance response of the electrochemical system was measured using an AC perturbation with a peak-to-peak amplitude of 10 mV and a data density of 10 points per decade, over a frequency range from 100 kHz to 100 mHz. The inhibition efficiency (IE _EIS%) was calculated from the charge transfer resistance (R _ct) as previously described.

$$I{E}_{\mathrm{EIS}}(\%)=\frac{R_{\mathrm{ct}}-R_{\mathrm{ct}}^0}{R_{\mathrm{ct}}}\times 100$$ 6

Where $R_{\mathrm{ct}}^0$ and R _ct denote the charge transfer resistance of carbon steel with and without various doses of isoxazolone inhibitor, respectively.

All electrochemical tests were performed in triplicate to ensure reproducible results.

2.4. UV–Visible Spectroscopy

In the present investigation, UV–visible spectroscopy was employed to get information on the complexation of iron cations and isoxazolone inhibitor and thereby elucidate its possible inhibition mechanism. The study based on the comparison between spectra of the pure inhibitor solution and solution after carbon steel immersion. The UV–vis measurements were recorded in 1 M HCl on Agilent Cary 3500 Multicell UV–vis spectrophotometer using 7.5 × 10^–5 (M) of isoxazolone inhibitor at 303 K, before and after 1 h and 15 days of immersion.

2.5. Atomic Force Microscopy

Surface morphology of the carbon steel samples was investigated using Atomic Force Microscopy (AFM) in tapping mode, which is well suited for heterogeneous and corrosion-affected metallic surfaces because it provides high-resolution imaging while minimizing surface damage during scanning. Both topographic and phase images were simultaneously acquired to evaluate surface relief and local mechanical heterogeneity. The topographic signal provides information on surface texture and roughness evolution, while the phase signal reflects variations in tip–surface interactions related to differences in composition, adhesion, or local surface properties. AFM measurements were performed at two scan sizes, 50 × 50 μm² and 10 × 10 μm², allowing examination of the surface morphology at both microscale and local scales. Three representative samples were analyzed: polished carbon steel (CS-P) as the reference substrate, carbon steel immersed in 1 M HCl without inhibitor (CS-HCl) representing the corroded surface, and carbon steel immersed in 1 M HCl with inhibitor (CS-HCl-INH) representing the protected surface. For preparation of the AFM specimens, CS-HCl and CS-HCl-INH were immersed for 1 h at 303 K in 1 M HCl without and with 7.5 × 10^–5 M DEAI, respectively. After immersion, the specimens were gently rinsed with distilled water, dried in air, and analyzed by AFM. Image processing and quantitative analysis were carried out using standard AFM procedures. The extracted parameters include the arithmetic mean roughness (Sa), phase roughness (Sφ), and the statistical height distribution obtained from the topographic maps. Gaussian fitting of the height histograms was used to determine the median height (Z_med) and the full width at half-maximum (fwhm), providing a quantitative measure of surface height dispersion and morphological heterogeneity. The phase signal is interpreted as a measure of variations in local tip–surface interaction and is not considered a direct chemical-composition map.

2.6. Insitu Characterization

The specimens analyzed by XPS and UPS were prepared under the same conditions as those used for AFM. The polished carbon-steel specimen (CS-P) was used as the reference, whereas CS-HCl and CS-HCl-INH were immersed for 1 h at 303 K in 1 M HCl without and with 7.5 × 10^–5 M DEAI, respectively. After immersion, the specimens were gently rinsed with distilled water, dried in air, and transferred to the ultrahigh-vacuum analysis chamber. The surfaces were first analyzed in their as-introduced state and subsequently after Ar⁺ sputtering to obtain comparative information on the outer surface layer and the underlying near-surface region.

X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a Scienta Omicron system equipped with a DAR-400 dual Mg/Al X-ray source and a hemispherical Argus analyzer (128 channels). Spectra were acquired using monochromatic Al Kα radiation (hν = 1486.7 eV), operated at an anode voltage of 15 kV and an emission current of 15 mA (225 W). Quantitative analysis and peak deconvolution were conducted using CasaXPS software with standard relative sensitivity factors (RSFs).

Ultraviolet Photoelectron Spectroscopy (UPS) measurements were performed using a Scienta Omicron HIS-13 discharge source with He I radiation (hν = 21.22 eV). A negative sample bias of −6 V was applied during measurements to clearly resolve the secondary electron cutoff, enabling accurate determination of the work function and occupied-state onset relative to the Fermi level.

Following acquisition of the as-introduced XPS and UPS spectra, selected surfaces were sputtered in the preparation chamber using a Scienta Omicron ISE-5 Ar⁺ ion source. The sputtering was conducted under an argon partial pressure of 2 × 10^–6 mbar. Ions were accelerated at an energy of 4 keV with an ion current of 25 μA for a duration of 10 min. This treatment attenuated the outer corrosion-product or inhibitor-rich layer and increased the contribution of the underlying metal/oxide region. Because energetic Ar⁺ bombardment may partially reduce iron oxides, preferentially remove certain elements, and fragment or redistribute organic species, the postsputtering spectra were interpreted comparatively rather than as an undisturbed chemical depth profile.

2.7. Computational Methods

Computational methods, such as density functional theory (DFT) and molecular dynamics simulations (MD), have proven to be powerful tools for understanding and predicting the adsorption mechanism of corrosion inhibitors on metal surfaces. DFT calculations was performed with Gaussian 09W software package and GaussView molecular visualization program. The optimization of the structure for isoxazolone derivative was conducted in the gas phase at the DFT/B3LYP level with 6-311++G­(d,p) basis set employing empirical dispersion correction (D3BJ). Key factors are derived from the output file of the optimized structure include the energies of frontier molecular orbital (FMO) density distributions i.e., the highest occupied molecular orbital energy (E _HOMO), and the lowest unoccupied molecular orbital energy (E _LUMO), the total energy (E _tot) and the dipole moment (μ). Additional pertinent descriptors including energy gap (ΔE _GAP = E _LUMO – E _HOMO), ionization potential (IP = −E _HOMO), electron affinity (EA = −E _LUMO), electron negativity (χ), hardness (η), softness (σ) and the fraction of electrons transferred (ΔN) were computed by expression:

$$\chi =\frac{IP+EA}{2}$$ 7

$$\eta =\frac{IP-EA}{2}$$ 8

$$\sigma =\frac{1}{\eta }$$ 9

$$\mathrm{\Delta }N=\frac{\varphi -{\chi }_{Inh}}{2({\eta }_{Fe}+{\eta }_{Inh})}$$ 10

Where ϕ (the work function of iron) = 4.82 eV and η_Fe = 0

Besides DFT method, a molecular dynamics (MD) simulation was run on the selected compound to determine the most stable adsorption configuration of the isoxazolone molecule on the Fe(110) surface. The MD simulation was carried out using Forcite module of BIOVIA Materials Studio software (Version 2023 v23.1.0). The simulation box, with dimensions of (24.82 × 24.82 × 38.10 ų) under periodic boundary conditions, and consisted of a five-layer of iron Fe(110) slab widened into a 10 × 10 Å supercell. To improved accuracy of the adsorption energy, the simulation was performed on a system composed of 400 (H₂O) molecules, 10 (H₃O⁺) and 10 ions of (Cl^–). The MD simulation of the adsorption system (comprising N, O, C, H, Cl^– and Fe atoms) was carried out using the COMPASS force field with a time step of 1 fs and a total simulation time of 2000 ps, under an NVT ensemble maintained at 303 K. The interaction and binding energy were calculated using the following formula:

$$E_{\mathit{int}eraction}=E_{total}-(E_{Fe(110)+H_{2}O+H_3{O}^{+}+C{l}^{-}}+E_{inhibitor})$$ 11

$$E_{binding}=-E_{interaction}$$ 12

Moreover, Radial Distribution Function (RDF) analysis was computed from MD trajectories to monitor the distance evolution between Fe(110) surface and group atoms of the compound.

3. Results and Discussion

3.1. Study of the Influence of Inhibitor Concentration

In studying corrosion science, potentiodynamic polarization analysis was employed to investigate the kinetics of iron dissolution and hydrogen evolution in corrosive environments. Figure shows the influence of inhibitor concentration on carbon steel corrosion in a 1 M HCl solution, both with and without various doses of the isoxazolone inhibitor at 303 K. All the electrochemical corrosion kinetic parameters including corrosion current density (i _corr), corrosion potential (E _corr), cathodic and anodic Tafel slopes (b _a and b _c), and corrosion inhibition efficiency (IE%) obtained from the three applied methods are listed in Table . It can be inferred from the data that increasing the concentration of isoxazolone inhibitor moves Ecorr slightly toward more negative values than that measured in the free acid solution, with the magnitude of the shift remaining below 85 mV relative to the blank solution. This behavior demonstrates the inhibitive effect of isoxazolone inhibitor on the cathodic reaction. The slight shift of corrosion potential toward the cathodic branch direction reveals that the isoxazolone inhibitor in 1 M hydrochloride solution can be categorized as a mixed-type corrosion inhibitor with a predominantly cathodic effect. Inspection data shows a gradual decrease of both corrosion current density (i _corr) and corrosion rate (C.R) with an increase in the concentration of the isoxazolone inhibitor. Meanwhile, the polarization resistance (R _p) increases with increasing inhibitor concentration. This trend suggests that isoxazolone molecules block the active sites on the metal surface, thereby limiting the access of corrosive species from the acidic medium to the iron surface. Besides, the anodic Tafel slope (b _a) of isoxazolone inhibitor has been observed to be relatively constant, suggesting that the molecules of isoxazolone inhibitor are initially adsorbed onto the metal surface, and thereby blocks the reaction sites without significantly influencing the anodic reaction mechanism. Similarly, no significant changes in the cathodic Tafel slope values are observed in the presence of isoxazolone inhibitor. This phenomenon is the result of a protective film forming on the steel electrode, which delays the transport of H⁺ ions to the cathodic sites, and consequently reduces the rate of H₂ gas evolution. As can be seen, the anticorrosion performance improves progressively with increasing inhibitor concentration as more inhibitor molecules adsorb onto the carbon steel surface, forming a compact and protective adsorbed film. However, beyond 7.5 × 10^–5 M, the inhibition efficiency decreases, likely due to surface saturation and/or the rearrangement of adsorbed molecules, which reduces the compactness and barrier effectiveness of the protective layer. The highest values of corrosion inhibition efficiencies of 87.50%, 87.70% and 86.70% were observed at 7.5 × 10^–5 M, which were obtained from corrosion current density, corrosion rate and linear polarization resistance methods, respectively. The corrosion inhibition efficiency (IE%) of the three applied methods are in very good agreement, supporting the reliability and accuracy of the results.

2.

Potentiodynamic polarization curves for carbon steel corrosion in 1 M HCl with and without various doses of isoxazolone inhibitor at 303 K.

1. Potentiodynamic Polarization Parameters and Corrosion Inhibition Efficiencies for Carbon Steel in 1 M HCl with and without Various Doses of Isoxazolone Inhibitor at 303 K.

	–E corr (mV)	–b c	b a	i corr (μA cm–2)	IEi corr (%)	C.R (mpy)	IE C.R (%)	R p (Ω)	IE LPR (%)
Blank	481	168	90	2.00 ± 0.06	----	23.50	----	9.30 ± 0.1	----
7.5 × 10–6	495	142	100	0.59 ± 0.03	70.50	6.90	70.60	36.90 ± 0.2	74.80
1 × 10–5	506	105	90	0.49 ± 0.002	75.50	5.70	75.70	39.60 ± 0.3	76.50
2.5 × 10–5	522	110	95	0.44 ± 0.005	78.00	5.10	78.30	46.10 ± 0.2	79.80
5 × 10–5	516	107	91	0.35 ± 0.003	82.50	4.70	80.00	54.20 ± 0.4	82.80
7.5 × 10–5	514	90	90	0.25 ± 0.004	87.50	2.90	87.70	70.00 ± 0.3	86.70
1 × 10–4	510	106	96	0.40 ± 0.005	80.00	4.60	80.40	55.00 ± 0.2	83.10

The influence of inhibitor concentration of isoxazolone molecule was also evaluated using electrochemical impedance spectroscopy (EIS). This technique makes it possible to analyze interfacial transport kinetics and provides direct insight into adsorption processes at the electrode–electrolyte interface. Figure a) illustrates the Nyquist diagram for carbon steel corrosion in a 1 M HCl solution, both with and without various doses of the isoxazolone inhibitor at 303 K. As evidence, the EIS spectra of both free and inhibited acid solutions exhibit identical shape of the semicircles with a single relaxation time, revealing that the corrosion behavior is mainly controlled by charge-transfer mechanism. The observed depressed shape of the capacitive loops is due to frequency dispersion of interfacial impedance arising from impurities, such as corrosion products, as well as surface roughness and heterogeneity of surface-active sites. An increase in semicircle size with a rise in inhibitor concentration indicates greater adsorption of isoxazolone molecules, leading to increased surface coverage (θ) and improved corrosion protection of carbon steel surface. Moreover, the experimental impedance data of the investigated inhibitor were modeled using an appropriate equivalent electrical circuit. The fitting quality of the proposed equivalent circuit was assessed using the chi-square (χ²) values listed in Table . Since the χ² values are low, an excellent agreement is observed between the experimental impedance data and the fitted results. This confirms that the proposed equivalent circuit adequately describes the electrochemical behavior of the studied system. The equivalent electrical circuit is composed of three key components, the solution resistance which is in series with constant phase element (CPE) and in parallel to the charge transfer resistance (R _ct) (Figure b). To provide a more accurate adjustment, a constant phase element (CPE) was substituted for an ideal double-layer capacitance (C_dl), since the interfacial double layer does not exhibit a perfect capacitive behavior. So, the impedance function (Z) of a CPE can be expressed mathematically as follow:

$$Z_{CPE}=Q^{-1}{(j\omega )}^{-n}$$ 13

3.

Nyquist diagram for carbon steel corrosion in a 1 M HCl solution (a) both with and without various doses of the isoxazolone inhibitor at 303 K. Electrical equivalent circuit (b) used to fit the EIS spectra.

2. Electrochemical Impedance Parameters and Corrosion Inhibition Efficiencies for Carbon Steel in 1 M HCl with and without Various Doses of Isoxazolone Inhibitor at 303 K.

	R s (Ω cm2)	R ct (Ω cm2)	Q × 103 (μF sn–1)	n	C dl (μF cm–2)	χ2 × 10–3	IE EIS (%)
Blank	0.5 ± 0.03	6.80 ± 0.05	8.20 ± 0.1	0.79	3800 ± 6	2.50	-----
7.5 × 10–6	1.5 ± 0.02	35.70 ± 0.2	1.22 ± 0.02	0.79	530.40 ± 3	3.40	80.90
1 × 10–5	1.9 ± 0.03	37.50 ± 0.4	1.47 ± 0.03	0.78	649.10 ± 4	5.50	81.90
2.5 × 10–5	1.7 ± 0.02	43.80 ± 0.3	0.89 ± 0.01	0.80	395.50 ± 3	6.10	84.50
5 × 10–5	1.8 ± 0.03	54.50 ± 0.2	0.85 ± 0.02	0.79	375.60 ± 4	3.20	87.50
7.5 × 10–5	1.4 ± 0.04	64.30 ± 0.3	0.80 ± 0.02	0.77	329.70 ± 5	4.20	89.40
1 × 10–4	1.4 ± 0.02	52.00 ± 0.2	1.22 ± 0.01	0.74	380.50 ± 4	2.20	86.90

Q, j, and ω are respectively, the CPE proportionality constant in μF/s^n–1 cm², the angular frequency in Rad s^–1 (ω = 2πf), the imaginary unit (j ² = −1). While n is the CPE exponent and denotes the heterogeneity of capacitance, ranging from 0 to 1. The above parameters can be employed to estimate the double-layer capacitance (C_dl) using the following equation:

$$C_{\mathrm{dl}}={(Q{R}_{\mathrm{ct}}^{1-n})}^{1-n}$$ 14

R _ct represents the charge transfer resistance.

The extracted and estimated electrochemical impedance parameters for inhibited and uninhibited carbon steel system are summarized in Table . Examining impedance data parameters, the CPE proportionality constant (Q) for uninhibited system is much greater than that for inhibited system due to the high corrosion rate at the carbon steel-electrolyte interface. Similar observations were recorded for the double-layer capacitance (C _dl), and its value is greater in the unprotected system than in the protected system, due to the accumulated iron oxide/hydroxide salts and other chemical corrosion products arising from the oxidation–reduction process of acidic corrosion. However, increased inhibitor concentration of isoxazolone from 1 × 10^–5 to 7.5 × 10^–5 (M) causes a decline in C _dl values from 649.10 to 329.70 (μF cm^–2), respectively. This trend is characterized by a reduction in the local dielectric constant and/or an enlargement of the thickness of the electrical double layer in the inhibited system. Furthermore, addition of different doses of inhibitor to the test solution shows a rising trend of the charge transfer resistance (R _ct) due to the gradual adsorption of isoxazolone molecules on the steel-electrolyte interface. Such adsorption hinders charge and mass transfer processes occurring at the inhibitor-metal interface. Another quantitative parameter derived from impedance data is the CPE exponent (n), which ranges between 0.74 and 0.80, exhibiting a moderate to good degree of surface inhomogeneity at the steel-solution interface. The best anticorrosion performance of about 90% was observed in the presence of 7.5 × 10^–5 M. It can be concluded that the corrosion inhibition efficiency consists with those estimated by means of corrosion current density, corrosion rate and polarization resistance.

3.2. Study of the Influence of Temperature

The effect of temperature on the corrosion inhibition of isoxazolone compound was systematically investigated using potentiodynamic polarization technique over a temperature range of 303–323 K, both in the absence and presence of various inhibitor concentrations (7.5 × 10^–6 to 7.5 × 10^–5 M). Figure displays potentiodynamic polarization curves and Table summarizes their electrochemical parameters. At any given temperature, the corrosion potential (E _corr) relatively shifts toward more negative values compared to the unprotected system, with a displacement of less than 85 mV. These findings confirm that isoxazolone compound displays dual-action (mixed-type) inhibition behavior across the entire temperature range investigated. The observed increase in corrosion current density with temperature (303–323 K) in the blank medium signals an accelerated corrosion rate, resulting from increased iron dissolution at the anodic sites and increased hydrogen evolution at the cathodic sites. However, at each investigated temperature, addition of different doses of isoxazolone inhibitor to a 1 M hydrochloride solution reduces the transport of hydrogen ions and iron oxidation on cathodic and anodic branches, respectively. At each temperature, the anodic and cathodic polarization branches remain almost parallel, indicating that isoxazolone inhibitor does not modify the fundamental corrosion mechanism, but rather influences the charge and electron transfer kinetics. The anticorrosion performance slightly improved, rising from 87.5% at 303 K to 88.1% at 308 K, and achieved a maximum value of 92.7% at 313 K (7.5 × 10^–5 M). The observed improvement in corrosion inhibition efficiency with rising temperature can be attributed to the enhanced adsorption of isoxazolone molecules onto the carbon steel surface, arising from increased molecular mobility and stronger interactions between the active sites of the inhibitor and the iron surface metal. Such behavior reveals a predominant contribution of both physisorption and chemisorption at moderate temperatures. However, a further increase in temperature beyond 313 K led to a gradual decrease in corrosion inhibition performance, achieving 90.2% at 318 K and 86.7% at 323 K. This phenomenon is likely due to weakening effect of heat, inducing partial repulsion of inhibitor molecules from the steel surface and/or structural rearrangement or degradation of the protective adsorbed film at elevated temperatures. − When isoxazolone molecules are pushed away from carbon steel surface, more area of the surface becomes uncovered and more susceptible to the acidic medium, resulting in reduced corrosion inhibition efficiency and increased corrosion rate. Consequently, the observed temperature dependence demonstrates that the adsorption behavior includes a mixed adsorption mode, comprising both physisorption and chemisorption at low to moderate temperatures, and predominantly chemisorption at elevated temperatures. ,− A comparison of the present corrosion inhibitor with previously reported small molecules containing an isoxazole moiety (Table ) reveals comparable or superior corrosion inhibition performance. More importantly, the present compound exhibits excellent inhibition efficiency at a considerably lower concentration than most reported inhibitors, demonstrating its high potency and practical advantages for corrosion protection.

4.

Potentiodynamic polarization curves for carbon steel corrosion with and without various doses of the isoxazolone inhibitor at temperatures ranging from 303 to 323 K.

3. Potentiodynamic Polarization Parameters for Various Inhibitor Concentrations at Temperatures Ranging from 303 to 323 K.

Temperature (K)	C (M)	E corr (mV)	i corr (μA cm–2)	–b c	b a	IE corr (%)	θ
303	Blank	481	2	168	90	----	----
	7.5 × 10–6	495	0.59	142	100	70.50	0.705
	1 × 10–5	506	0.49	105	90	75.50	0.755
	2.5 × 10–5	552	0.44	110	95	78.00	0.780
	5 × 10–5	516	0.35	107	91	82.50	0.825
	7.5 × 10–5	514	0.26	170	95	87.50	0.875
308	Blank	454	3.1	176	100	----	----
	7.5 × 10–6	470	0.76	160	102	75.50	0.755
	1 × 10–5	453	0.64	158	90	79.30	0.793
	2.5 × 10–5	493	0.59	165	110	81.00	0.810
	5 × 10–5	466	0.48	174	100	84.50	0.845
	7.5 × 10–5	450	0.26	170	95	88.10	0.881
313	Blank	452	4.1	170	105	----	----
	7.5 × 10–6	470	1.17	172	120	71.50	0.715
	1 × 10–5	479	0.87	170	122	78.80	0.788
	2.5 × 10–5	479	0.64	150	105	84.40	0.844
	5 × 10–5	463	0.58	160	95	85.80	0.858
	7.5 × 10–5	458	0.30	155	96	92.70	0.927
318	Blank	435	4.5	180	110	----	----
	7.5 × 10–6	457	1.46	160	100	67.50	0.675
	1 × 10–5	456	0.93	135	100	79.30	0.793
	2.5 × 10–5	482	0.84	130	100	81.30	0.813
	5 × 10–5	479	0.76	120	95	83.10	0.831
	7.5 × 10–5	464	0.44	170	102	90.20	0.902
323	Blank	454	4.95	185	135	----	----
	7.5 × 10–6	464	1.94	154	110	62.70	0.627
	1 × 10–5	462	1.72	166	120	66.90	0.669
	2.5 × 10–5	467	0.94	170	104	81.90	0.819
	5 × 10–5	463	0.88	160	110	83.10	0.831
	7.5 × 10–5	470	0.66	150	100	86.70	0.867

4. Comparative Performance Overview of Small Molecules Containing an Isoxazole Moiety Used as Corrosion inhibitors − .

3.3. Thermodynamic and Kinetic Parameters

The thermodynamic and kinetic parameters including corrosion activation energy (E _a) and activation enthalpy (ΔH _a) were determined from potentiodynamic polarization measurements using Arrhenius and transition-state equations:

$$\ln i_{corr}=\ln A-\frac{E_a}{RT}$$ 15

$$\ln \left(\frac{i_{corr}}{T}\right)=-\frac{\mathrm{\Delta }{H}_a}{RT}+\ln \left(\frac{R}{N_{a}h}\right)+\left(\frac{\mathrm{\Delta }{S}_a}{R}\right)$$ 16

Where, i _corr, A, R, h, N _a and T are the corrosion current density, the Arrhenius factor, the universal gas constant, Plank’s constant, Avogadro’s number and the absolute temperature, respectively.

Figure (a) illustrates a linear Arrhenius plot of (ln i _corr vs 1/T), from which corrosion activation energy (E _a) and logarithm Arrhenius constant (ln A) were determined from the slope and intercept, respectively. Table displays activation parameters of carbon steel with and without various doses of isoxazolone inhibitor at 303–323 K. As shown in table. The corrosion activation energy (E _a) is found to be influenced by the concentration of the isoxazolone inhibitor. At the lowest inhibitor concentration (7.5 × 10^–6 M), E _a (46.73 kJ mol^–1) is slightly higher than that of the uninhibited solution (42.53 kJ mol^–1), indicating that the initially adsorbed inhibitor molecules increase the energy barrier for the corrosion process, which is characteristic of a contribution from physical adsorption. As the inhibitor concentration increases, E _a gradually decreases to values below that of the blank solution, reaching 31.26 kJ mol^–1 at 7.5 × 10^–5 M. This decrease suggests the formation of a more stable and strongly adsorbed protective layer, indicating that chemisorption becomes the dominant adsorption mechanism at higher inhibitor concentrations. Additionally, Figure (b) displays the transition state plot (ln i _corr/T vs 1/T), from which activation enthalpy (ΔH _a) and activation entropy (ΔS_a) were determined from the slope and intercept, respectively. The positive activation enthalpy (ΔH _a > 0) values for carbon steel dissolution, both in the absence and presence of the isoxazolone inhibitor confirm the endothermic nature of the dissolution process, indicating that the anodic dissolution of steel requires an energy input to reach the activated state.

5.

Arrhenius (a) and transition-state (b) graphs for carbon steel dissolution with and without various doses of isoxazolone inhibitor. (c) Graph of Gibbs free energy of adsorption versus temperature for isoxazolone inhibitor.

5. Kinetic and Thermodynamic Activation Parameters for Carbon Steel Dissolution with and without Various Doses of Isoxazolone Inhibitor.

C (M)	E a (kJ mol–1)	ΔH a (kJ mol–1)	ln A	ΔS a (kJ mol–1)	E a – ΔH a
Blank	42.53	39.93	15.02	–128	2.60
7.5 × 10–6	46.73	44.12	19.07	–95.07	2.61
1 × 10–5	39.47	36.87	17.83	–105.29	2.60
2.5 × 10–5	33.39	30.79	11.32	–159.47	2.60
5 × 10–5	35.35	32.73	13.89	–138.11	2.62
7.5 × 10–5	31.26	28.65	14.08	–136.53	2.61

The quite negative value of the entropy of activation (ΔS _a) reveals that the formation of the activated complex is characterized by more randomness in the system. The nonmonotonic variation of ΔS _a with rising isoxazolone concentration may be due to surface restructuring and competitive adsorption processes.

3.4. Adsorption Isotherms and Thermodynamic Parameters

The interaction between isoxazolone inhibitor and carbon steel surface was evaluated using four adsorption isotherms: Temkin, Frumkin, Freundlich and Langmuir isotherms (Figure ). Consequently, Langmuir adsorption isotherm produced the best fit with regression coefficients R² values close to unity. The surface coverage (θ) was computed from the corrosion inhibition efficiency that was derived from potentiodynamic polarization measurements using the appropriate equation.

$$\theta =\frac{I{E}_{PDP}}{100}$$ 17

6.

Isotherm plots for adsorption of isoxazolone inhibitor for carbon steel corrosion at different temperatures ranges 303–323 K.

The linearized Langmuir adsorption isotherm can be described mathematically as follows:

$$\frac{C}{\theta }=\frac{1}{K_{ads}}+C$$ 18

Where K _ads represents the adsorption–desorption equilibrium constant, C refer to the molarity of isoxazolone inhibitor.

As seen in Figure , the plot of C/θ against C at temperatures range of 303–323 K exhibits slightly diverging slopes from unity, which can be explained by a modified Langmuir adsorption isotherm. This divergence was ascribed to the interactions between numerous inhibitor molecules adsorbed on the metal surface, each of which occupied more than one adsorption site. Thus, the modified Langmuir isotherm was expressed as follows:

$$\frac{C}{\theta }=\frac{n}{K_{ads}}+nC$$ 19

The adsorption–desorption equilibrium constant derived from the intercept of the linear plot was employed to calculate the standard free energy of adsorption by equation:

$$\mathrm{\Delta }{G}_{ads}^o=-RT\ln (55.5k_{ads})$$ 20

R, T and 55.5 represent the universal gas constant (8.314 J K^–1 mol^–1), the absolute temperature (Kelvin) and the molar concentration of the solvent (mol L^–1) respectively. The strongly negative values of the standard free energy of adsorption ΔG _ads imply a highly stable protective layer on the steel surface and a highly spontaneous adsorption process. Based on the literature, ΔG _ads values above −40 kJ mol^–1 typically denote chemisorption, whereas values below −20 kJ mol^–1 generally indicate physisorption. In the present investigation, the standard free energy of adsorption ΔG _ads for isoxazolone inhibitor was in the range of −42.56–44.79 kJ mol^–1 at temperature of 303–323 K, disclosing then a chemisorption process, respectively. Additional thermodynamic parameters of adsorption also were computed such as enthalpy (ΔH _ads) and entropy (ΔS _ads) of adsorption. These two parameters were derived from the graph of ΔG _ads vs T (Figure (c)), which was obtained using a thermodynamic formula:

$$\mathrm{\Delta }{G}_{ads}^o=\mathrm{\Delta }{H}_{ads}^o-T\mathrm{\Delta }{S}_{ads}^o$$ 21

The corresponding slope (ΔS _ads) and intercept (ΔH _ads) of isoxazolone inhibitor for carbon steel corrosion are listed in Table . The enthalpy value was found to be negative (ΔH _ads < 0), indicating an exothermic process. Some studies have suggested that a negative sign for this parameter reflects physical and chemical adsorption processes. In contrast, the quite positive value of entropy (ΔS _ads > 0) reflects a quasi-substitution process in which adsorbed water molecules are desorbed from the iron surface and replaced by isoxazolone inhibitor molecules.

6. Thermodynamic Parameters of Adsorption of Isoxazolone Inhibitor for Carbon Steel Corrosion at Temperatures Ranges 303–323 K.

T (K)	Slope	K ads × 105	ΔG ads (kJ mol–1)	ΔH ads (kJ mol–1)	ΔS ads (J mol–1 K–1)
303	1.12	3.92	42.56	–8.31	113
208	1.12	4.00	43.31
313	1.08	3.72	43.83
318	1.11	3.71	44.52
323	1.12	3.16	44.79

3.5. UV–Visible Spectroscopic Investigation of Inhibitor-Metal Interactions

UV–visible spectroscopy was employed to investigate the adsorption behavior of corrosion inhibition for isoxazolone compound and to evaluate its interaction with the metal surface. This technique can provide insight into possible complex formation. The UV–visible absorption spectra shown in Figure were recorded before and after immersing a steel substrate in 1 M HCl containing amount of 7.5 × 10^–5 (M) of isoxazolone inhibitor for immersion time of 1 h and 15 days, over a temperature of 303 K, respectively. The UV–visible spectrum recorded before immersion exhibits three characteristic absorption bands at 234, 289, and 333 nm, with corresponding absorbance values of 0.02, 0.13, and 0.08, respectively. These bands can be attributed to n→σ*, π→π*, and n→π* electronic transitions associated with the auxochromic and chromophoric groups present in the isoxazolone structure. − Compared with the reference spectrum obtained before immersion, the spectra recorded after immersion show noticeable changes in absorption intensity, clearly indicating interactions between isoxazolone molecules and the carbon steel surface. Notably, the increase in absorbance intensity with immersion time indicates a progressive increase in the concentration of dissolved iron species and inhibitor-related species in the solution, suggesting stronger interactions between dissolved iron ions and isoxazolone molecules during prolonged immersion, which may be associated with the possible formation of iron-inhibitor complexes at the interface. The observed phenomenon highlights the decisive role of the functional groups in the adsorption process through coordination with surface metal ions. Furthermore, the appearance of a new absorption band around 450 nm may be linked to ligand-to-metal charge transfer (LMCT) transitions and/or magnetic perturbation effects induced by metal-inhibitor interactions. ,

7.

UV–visible spectrum of isoxazolone solution before and after immersion of carbon steel.

3.6. Morphological Study of the Inhibitor Effect

Atomic Force Microscopy (AFM), particularly in tapping mode, provides nanoscale information on both surface topography and phase contrast, making it a powerful tool for evaluating surface degradation, roughness evolution, and local heterogeneity induced by corrosive attack and inhibitor adsorption. ,− In the present study, AFM was used to compare the morphology of polished carbon steel, carbon steel exposed to 1 M HCl without inhibitor, and carbon steel exposed to 1 M HCl containing the inhibitor. The combined analysis of topographic maps, phase images, and height distribution histograms provides direct insight into the extent of surface deterioration in the blank acidic medium and the degree of surface preservation in the inhibited solution. This multiscale AFM approach is particularly appropriate for the present objective because it combines direct three-dimensional morphology with quantitative roughness, height-dispersion, and phase-response parameters. Figure presents representative two-dimensional topographic and phase images together with corresponding three-dimensional reconstructions obtained for the three investigated samples at two scan sizes, 50 × 50 μm² and 10 × 10 μm². The polished carbon steel surface (CS-P) exhibits the typical morphology of a mechanically prepared substrate, characterized by oriented polishing lines and relatively regular surface features. At the smaller scan size, the topographic image remains comparatively smooth, with a low roughness value (S _a = 13.37 nm) and limited phase dispersion (S _φ = 1.50°), indicating a rather homogeneous surface at the local scale. At the larger scan size, the polishing grooves are more clearly visible, but the surface remains relatively uniform compared with the corroded samples, with S _a = 42.08 nm and S _φ = 2.91°. These observations confirm that the polished substrate provides an appropriate morphological reference for evaluating the subsequent effects of acid attack and inhibitor protection. After immersion in 1 M HCl without inhibitor (CS-HCl), the AFM images reveal a pronounced modification of the steel surface at both observation scales. At 10 × 10 μm², the surface becomes much more irregular and heterogeneous, with developed nodular features and clear height fluctuations, reflected by a strong increase in topographic roughness to S _a = 56.91 nm. The phase image also becomes more contrasted, with S _φ = 2.10°, indicating increased variation in local tip–surface interactions. − , At the larger scale (50 × 50 μm²), the surface degradation becomes even more evident, with severe roughening, large protrusions and depressions, and a marked amplification of the roughness parameter to S _a = 363.2 nm, while the phase roughness rises to S _φ = 5.64°. These features are characteristic of intense acid-induced attack and formation of a highly irregular corroded surface. In the presence of inhibitor (CS-HCl-INH), the AFM observations indicate a partial preservation of surface uniformity compared with the uninhibited acidic medium. At 10 × 10 μm², the surface still shows relief development, but the topography appears more compact and less disrupted than that of CS-HCl, with a reduced roughness value (S _a = 44.91 nm). The phase roughness also decreases to S _φ = 1.90°, suggesting a lower degree of local heterogeneity. At 50 × 50 μm², the inhibited sample remains rougher than the polished reference, but its morphology is clearly less deteriorated than that of the blank acid-exposed sample, with S _a = 229.2 nm and S _φ = 3.90°. Relative to CS-HCl, DEAI therefore reduces Sa by 36.9% at 50 × 50 μm² and by 21.1% at 10 × 10 μm². The corresponding phase-dispersion reductions are 30.9% and 9.5%, respectively. The reduction in both topographic and phase roughness compared with CS-HCl indicates that the inhibitor effectively mitigates acid attack and promotes the formation of a more homogeneous protective surface region. The magnitude of the roughness reduction depends on the scan area because the larger map captures corrosion features with a wider lateral and vertical size distribution, whereas the smaller map probes local surface texture. The different percentages obtained at the two scales are therefore complementary rather than contradictory. Overall, the AFM images show a clear morphological sequence from the relatively smooth polished substrate to the severely damaged surface in uninhibited acid, followed by a partially preserved and less heterogeneous surface in the inhibited medium. This evolution supports the protective action of the inhibitor and provides direct surface-scale evidence complementing the electrochemical results.

8.

Representative AFM topographic and phase images, together with corresponding 3D surface reconstructions, for polished carbon steel (CS-P), carbon steel immersed in 1 M HCl without inhibitor (CS-HCl), and carbon steel immersed in 1 M HCl with inhibitor (CS-HCl-INH), recorded at two scan sizes: 10 × 10 μm² and 50 × 50 μm². The phase images represent variations in local tip–surface interactions rather than direct chemical-composition maps.

Similar inhibitor-induced smoothing of carbon-steel surfaces in hydrochloric acid has been observed by Timoudan et al., who used two- and three-dimensional AFM maps and relative-height profiles to demonstrate attenuation of surface damage by benzimidazole derivatives. Abd El Maksoud et al. reported a decrease in the average roughness of carbon steel from 271.7 nm in 1 M HCl to 108.2 nm in the presence of furosemide, corresponding to a 60.2% reduction after 24 h. Although this reduction is larger than the 21.1–36.9% Sa decrease obtained here, direct comparison of absolute AFM values must be made cautiously because roughness depends strongly on inhibitor chemistry, immersion duration, scan area, surface preparation, and image-processing conditions. In situ AFM studies have also directly linked the development of adsorbed inhibitor aggregates on carbon steel with reduced electrochemical corrosion rates, supporting the mechanistic relevance of the present morphological changes.

Figure presents the statistical distributions of surface heights extracted from the AFM topographic maps for the three samples at both scan sizes. As in the support work, Gaussian fitting of the histograms can be used to quantify the dispersion of surface elevations through the fwhm parameter, where broader peaks indicate greater morphological heterogeneity and narrower peaks indicate a more uniform surface. ,, For the polished reference sample (CS-P), the height distributions are relatively narrow at both scales, with median height values of 322.3 and 116.0 nm, and corresponding fwhm values of 99.6 and 21.1 nm for the 50 × 50 μm² and 10 × 10 μm² scans, respectively. These low values confirm the comparatively smooth and homogeneous nature of the mechanically polished steel surface. For the acid-exposed sample without inhibitor (CS-HCl), the histograms become significantly broader, reflecting the strong development of surface irregularities induced by corrosive attack. The median height reaches 1996.5 nm at 50 × 50 μm² and 347.8 nm at 10 × 10 μm², while the corresponding fwhm values increase sharply to 691.9 and 146.4 nm, respectively. This marked broadening demonstrates the large dispersion of surface elevations and confirms the highly heterogeneous character of the corroded steel surface.

9.

Height distribution histograms derived from AFM topographic maps for CS-P, CS-HCl, and CS-HCl-INH at 50 × 50 μm² and 10 × 10 μm², together with Gaussian fitting curves used to estimate the center of the height distribution and the full width at half-maximum (fwhm).

In contrast, the inhibited sample (CS-HCl-INH) exhibits narrower height distributions than CS-HCl at both scales. The median height values decrease to 972.9 nm at 50 × 50 μm² and 311.9 nm at 10 × 10 μm², while the fwhm values decrease to 546.2 and 83.87 nm, respectively. These changes correspond to fwhm reductions of 21.1% at 50 × 50 μm² and 42.7% at 10 × 10 μm² relative to the uninhibited surface. Although the inhibited surface does not fully recover the smoothness of the polished substrate, the reduction in peak width relative to the blank acidic medium clearly indicates a lower dispersion of surface elevations and therefore a more uniform morphology. Because the absolute Z_med value depends on the vertical reference and image-leveling procedure, the comparison of morphological heterogeneity is based primarily on Sa and fwhm rather than on Z_med alone. These results confirm that the inhibitor reduces the severity of acid attack and contributes to the formation of a less damaged and more homogeneous surface layer. Taken together, the histogram analysis is in full agreement with the AFM topographic and phase images, confirming that exposure to uninhibited acid produces severe roughening and strong surface heterogeneity, whereas the inhibited medium substantially attenuates these effects.

Table summarizes the main statistical parameters extracted from the AFM topographic and phase images for the three investigated samples at both scan sizes (50 × 50 μm² and 10 × 10 μm²). The results show a pronounced increase in surface roughness, height dispersion, and phase heterogeneity after immersion of carbon steel in 1 M HCl without inhibitor, reflecting severe surface degradation due to acid attack. In contrast, the sample exposed to the inhibited solution exhibits lower roughness, narrower height distributions, and reduced phase dispersion compared with the uninhibited case. Specifically, DEAI decreases Sa by 36.9% and 21.1%, fwhm by 21.1% and 42.7%, and S _φ by 30.9% and 9.5% at the large and small scan sizes, respectively. These results indicate that the inhibitor effectively mitigates surface deterioration and promotes a more uniform and protective surface layer on the carbon steel substrate.

7. Summary of AFM Statistical Parameters Extracted from Topographic and Phase Images at Two Observation Scales for Polished Carbon Steel (CS-P), Carbon Steel Immersed in 1 M HCl without Inhibitor (CS-HCl), and Carbon Steel Immersed in 1 M HCl with Inhibitor (CS-HCl-INH).

Statistical quantities	Samples	Large-scan 50 × 50 μm2	Small-scan 10 × 10 μm2	Comments
Topographic mean roughness, Sa (nm)	CS-P	42.08	13.37	Reference polished surface
	CS-HCl	363.20	56.91	Severe corrosion-induced roughening
	CS-HCl-INH	229.20	44.91	Sa reduction vs CS-HCl: 36.9% (large) and 21.1% (small)
Height-distribution center, Zmed (nm)	CS-P	322.30	116.00	Absolute values depend on vertical referencing
	CS-HCl	1996.50	347.80	Large corrosion-induced vertical features
	CS-HCl-INH	972.90	311.90	Lower values than CS-HCl; interpreted with Sa and fwhm
fwhm of height distribution (nm)	CS-P	99.60	21.10	Narrow reference distributions
	CS-HCl	691.90	146.40	Strongly broadened height distributions
	CS-HCl-INH	546.20	83.87	fwhm reduction vs CS-HCl: 21.1% (large) and 42.7% (small)
Phase roughness, Sφ (deg)	CS-P	2.91	1.50	Reference phase dispersion
	CS-HCl	5.64	2.10	Greater local tip–surface interaction heterogeneity
	CS-HCl-INH	3.90	1.90	Sφ reduction vs CS-HCl: 30.9% (large) and 9.5% (small)

3.7. In-Situ Characterization of the Inhibitor Effect

3.7.1. XPS Results

X-ray Photoelectron Spectroscopy (XPS) was employed to investigate the surface chemical composition and chemical states of carbon steel exposed to 1 M HCl solution in the absence and presence of the inhibitor. This technique provides detailed information on the elemental composition of the upper surface layers (typically <10 nm), as well as the nature of species formed during corrosion and inhibition processes. To obtain comparative information about the outer adsorbed layer and the underlying interfacial region, analyses were performed in the as-introduced state and after Ar⁺ ion sputtering. , Because energetic Ar⁺ bombardment may partially reduce iron oxides and fragment organic species, the postsputtering spectra were interpreted comparatively rather than as an undisturbed chemical depth profile. Particular attention was given to the main detected elements (Fe, O, C, Cl, and N) in order to elucidate the corrosion mechanisms and assess the formation and persistence of the inhibitor-derived protective layer. Similar methodological caution was emphasized by Walczak et al., who showed that postimmersion oxidation can modify the chemical state of corrosion-inhibitor/metal interfaces measured by XPS.

Figure (a) shows the XPS survey spectra of the investigated samples, including polished carbon steel after sputtering (CS-P@Etched), carbon steel immersed in 1 M HCl without inhibitor (CS-HCl and CS-HCl@Etched), and carbon steel immersed in 1 M HCl in the presence of the inhibitor (CS-HCl-INH and CS-HCl-INH@Etched). The spectra reveal the presence of the main elements detected at the surface, namely Fe, O, C, Cl, and N, depending on the sample and treatment conditions. These elements are identified through their characteristic photoelectron peaks, such as Fe 2p, O 1s, C 1s, N 1s, and Cl 2p, as well as Auger transitions including Fe LMM, O KLL, and C KLL. The relative intensities and presence of these signals vary significantly between samples, reflecting differences in surface composition, corrosion products, and inhibitor adsorption. The quantitative surface compositions derived from the XPS survey spectra are summarized in Table . The results show significant variations in elemental distribution depending on the exposure conditions and surface state. For the nonsputtered samples, both CS-HCl and CS-HCl-INH exhibit low Fe content and high C and O contributions, indicating that the surface is predominantly covered by corrosion products and adsorbed species. The detection of 0.76 at. % N exclusively on CS-HCl-INH provides direct evidence for the presence of nitrogen-containing inhibitor-derived species. However, without high-resolution N 1s deconvolution, this result should not be considered unambiguous evidence for a specific Fe–N bond. In addition, the Cl concentration decreases from 1.49 at. % for CS-HCl to 0.36 at. % for CS-HCl-INH, corresponding to an approximately 76% reduction. This marked decrease indicates that the inhibitor-containing layer limits the adsorption or retention of aggressive chloride species at the carbon-steel surface. Similar XPS evidence was reported by Yousif et al. for Schiff-base inhibitors on carbon steel in 1.0 M HCl, where inhibitor-characteristic C, N, and O signals together with Fe and Cl confirmed the formation of an adsorbed organic protective layer.

10.

XPS analysis of carbon-steel surfaces exposed to 1 M HCl in the absence and presence of the inhibitor, before and after Ar⁺ ion sputtering. (a) Survey spectra showing the main elements detected at the surface. (b–d) High-resolution Fe 2p, O 1s, and C 1s spectra, respectively, together with their fitted components and cumulative envelopes.

8. Atomic Concentrations (at.%) of Fe, O, C, Cl, and N Obtained by XPS for Carbon-Steel Surfaces Exposed to 1 M HCl in the Absence and Presence of the Inhibitor, before and after Ar⁺ Ion Sputtering .

	Atomic percentage (at.%)
Surface state	Fe	O	C	Cl	N	O/Fe	Comment
CS-P@Etched	70.89	11.73	17.38	–	–	0.17	Metal-rich surface after attenuation of the native oxide layer
CS-HCl	6.59	27.68	64.24	1.49	–	4.20	Carbon- and oxygen-rich corrosion-affected outer surface
CS-HCl-INH	6.14	26.35	66.39	0.36	0.76	4.29	Inhibitor-containing outer surface with markedly reduced Cl concentration
CS-HCl@Etched	39.00	26.02	32.58	2.40	–	0.67	Sputtered corrosion-product/metal interfacial region
CS-HCl-INH@Etched	43.30	26.42	27.77	1.26	1.25	0.61	Sputtered inhibited region containing residual N and lower Cl and O/Fe ratios

^aThe O/Fe ratio is included as a semiquantitative indicator of relative oxygen enrichment and should not be regarded as a direct determination of the Fe oxidation state.

After Ar⁺ ion sputtering, a marked increase in Fe content and a decrease in carbon are observed, reflecting removal of the outer layer and exposure of the underlying interface. Notably, CS-HCl-INH@Etched exhibits a higher Fe content, a lower Cl concentration, and persistent nitrogen compared with CS-HCl@Etched. The Cl concentration remains lower in the inhibited sputtered region, decreasing from 2.40 to 1.26 at. %, while N remains detectable at 1.25 at. %. Because XPS atomic concentrations are normalized to the total signal, the apparent increase in N after sputtering should not be interpreted as absolute nitrogen enrichment. Instead, it indicates that inhibitor-derived nitrogen species remain detectable within the sputtered near-surface region. These results indicate that the inhibitor modifies the underlying interfacial region, limits chloride accumulation, and reduces continued surface degradation. Nevertheless, owing to possible sputter-induced modification of the organic layer, these observations do not constitute a direct molecular depth profile. The high-resolution Fe 2p spectra shown in Figure (b) were deconvoluted into contributions attributed to Fe⁰ (∼707.4 eV), Fe²⁺ (∼709.6 eV), and Fe³⁺ (∼711.6 eV), together with satellite features (∼713–718 eV). These assignments are consistent with reported values for iron and its oxides, although Fe 2p spectra are inherently complex due to multiplet splitting and many-body interactions, leading to overlapping contributions from Fe²⁺ and Fe³⁺ species. , Therefore, the fitted contributions are considered semiquantitative indicators of relative chemical-state changes rather than absolute concentrations of individual iron phases. This cautious interpretation is consistent with the reference-based analysis of Biesinger et al., who demonstrated that Fe 2p spectra of metallic Fe, oxides, and hydroxides are strongly affected by multiplet splitting, shakeup structure, peak asymmetry, and overlapping binding energies. The CS-P@Etched sample is dominated by Fe⁰, confirming exposure of the metallic substrate after removal of most of the native oxide layer. In contrast, the nonsputtered samples (CS-HCl and CS-HCl-INH) are dominated by oxidized iron species, indicating the formation of corrosion products and surface layers that attenuate the metallic signal. After Ar⁺ ion sputtering, both CS-HCl@Etched and CS-HCl-INH@Etched exhibit mixed Fe⁰/Fe²⁺/Fe³⁺ contributions, characteristic of the metal/oxide interface. Compared with CS-HCl@Etched, the inhibited sample shows a slightly higher Fe⁰ contribution, increasing from approximately 29.75% to 33.02%, while the Fe²⁺ and Fe³⁺ contributions decrease slightly. This trend is consistent with a lower progression of interfacial oxidation in the presence of the inhibitor. However, because Ar⁺ bombardment may partially reduce iron oxides, the interpretation is based on the comparative behavior of specimens treated under identical sputtering conditions. The satellite structure associated with Fe²⁺ and Fe³⁺ species is treated cautiously due to multiplet and shakeup effects. , In the present work, it is considered globally as a well-defined shoulder, since further differentiation does not significantly affect the overall interpretation. The O 1s spectra shown in Figure (c) were deconvoluted into up to four components at approximately 530.2, 531.4, 532.9, and 534.3 eV, corresponding to lattice oxygen (Fe–O), hydroxylated species (Fe–OH/FeOOH), oxygen in C–O/OH-containing species, and adsorbed H₂O/COx, respectively. , The low-binding-energy component (∼530.2 eV) corresponds predominantly to lattice oxygen in iron oxides, while the ∼531.4 eV component is attributed to hydroxylated corrosion products. , The higher-binding-energy contributions (∼532.9–534.3 eV) arise from oxygen-containing organic species and adsorbed water, consistent with the C 1s analysis. ,, For CS-HCl, the Fe–O, Fe–OH, C–O/OH, and COx/H₂O contributions represent approximately 31.47%, 33.74%, 21.86%, and 12.83%, respectively. In the presence of DEAI, these contributions change to approximately 28.14%, 32.86%, 23.88%, and 15.20%, respectively. The decrease in the oxide/hydroxide-related contributions, together with the increase in the higher-binding-energy components, is consistent with the additional contribution of oxygen-containing inhibitor functionalities and adsorbed interfacial species. However, because these components overlap with hydroxylated corrosion products and adsorbed water, O 1s alone cannot establish a unique Fe–O–DEAI coordination environment. After sputtering, the spectra become dominated by lower-binding-energy components, reflecting attenuation of the outer hydrated and organic-rich layer and increased contribution from the oxide/metal region. The Fe–O contribution reaches approximately 60.09% for CS-HCl@Etched and 63.12% for CS-HCl-INH@Etched, whereas the Fe–OH contribution decreases from approximately 29.06% to 28.05%. The inhibited sputtered region is therefore slightly more oxide-dominated and less hydroxylated than the corresponding uninhibited region. This indicates that the inhibitor modifies, rather than suppresses, oxide formation and the hydration state of the corrosion-product/metal interface. These percentages represent relative fractions within the O 1s envelope and should not be interpreted as direct measurements of the total surface oxidation. The C 1s spectra shown in Figure (d) were fitted with six components at approximately 282.9, 284.1, 285.0, 286.0, 287.5, and 289.4 eV, assigned to Fe–C (carbide-like), Fe–C-related interfacial carbon, C–C/C–H, C–O/C–N, CO, and O–CO species. ,,, The two low-binding-energy components (∼282.9 and ∼284.1 eV) are attributed to carbon interacting with iron, originating from intrinsic carbide phases and/or interfacial carbon associated with the substrate. ,, Their appearance primarily in sputtered samples is consistent with their subsurface or interfacial origin. Nonsputtered samples are dominated by hydrocarbon and oxygenated carbon species associated with corrosion products and adsorbed species. In CS-HCl-INH, the simultaneous presence of N, C–O/C–N, and CO contributions is chemically consistent with the nitrogen- and oxygen-containing molecular structure of DEAI and therefore supports inhibitor adsorption. After Ar⁺ ion sputtering, the decrease in organic-carbon contributions and the emergence of low-binding-energy carbon components indicate increased exposure of the buried region. The components at approximately 282.9 and 284.1 eV should not, however, be considered direct evidence for the formation of new Fe–C bonds with DEAI because they may also arise from intrinsic carbon in the C35 steel substrate or from sputter-induced fragmentation of organic species. Therefore, these components are attributed cautiously to carbide-like and Fe-associated interfacial carbon rather than to a specific carbide phase. The persistence of the N signal and C–O/C–N- and carbonyl-related components in CS-HCl-INH@Etched provides more reliable evidence that inhibitor-derived species remain detectable within the sputtered near-surface region. , Overall, the XPS results confirm that exposure to 1 M HCl leads to the formation of an oxidized, carbon-rich surface layer, whereas the inhibited system shows clear evidence of inhibitor adsorption through the presence of nitrogen and reduced chloride content. After Ar⁺ ion sputtering, both systems reveal a mixed metal/oxide interface; however, the inhibited sample exhibits a slightly higher metallic contribution, a lower Cl concentration, and a lower O/Fe ratio. The combined presence of N-containing species, oxygenated carbon groups, and reduced chloride accumulation indicates that DEAI forms an inhibitor-containing surface layer and modifies the underlying corrosion-product/metal region. These experimental observations are consistent with the predicted adsorption activity of the N- and O-containing groups and the conjugated π-system of DEAI. They also correlate with the electrochemical increase in charge-transfer resistance and decrease in corrosion-current density, indicating that inhibitor adsorption reduces the accessibility of active sites and hinders interfacial charge transfer.

3.7.2. UPS Results

Ultraviolet Photoelectron Spectroscopy (UPS) was used to investigate the electronic structure of carbon steel surfaces exposed to 1 M HCl in the absence and presence of the inhibitor. In contrast to XPS, which provides elemental and chemical-state information, UPS probes the valence electronic structure, allowing determination of the work function (WF), valence-band maximum (VBM), and ionization potential (IP) of the surface. ,,,,, Due to the lower photon energy employed, UPS is highly surface-sensitive (≈2–3 nm), making it particularly suitable for analyzing adsorbed layers and interfacial electronic modifications. ,, For the chemically heterogeneous steel/corrosion-product/organic surfaces examined here, the EF–EVBM value is treated as an operational occupied-state onset rather than as a bulk-semiconductor band edge; consequently, the derived IP values are considered apparent ionization potentials. Figure presents the UPS spectra, including the secondary electron cutoff, valence-band region, and near-Fermi edge. The work function was determined from the secondary electron cutoff, while the valence-band spectra were used to estimate the E_F – E_VBM separation. Further experimental details about bias and SE separation can be found in previous work. , The extracted parameters are summarized in Table , providing a comprehensive comparison of the electronic properties of the different surface states. A negative sample bias of −6 V was applied to separate the secondary-electron cutoff from the analyzer threshold and improve the reliability of the WF determination. The valence-band spectra provide insight into the occupied density of states (DOS) and its evolution with surface treatment. The polished reference (CS-P@Etched) exhibits a finite DOS extending up to the Fermi level, characteristic of a metallic surface. , In contrast, the nonetched samples (CS-HCl and CS-HCl-INH) show a depletion of states near the Fermi level and a redistribution of spectral weight toward higher binding energies, indicating the formation of electronically blocking surface layers associated with corrosion products and adsorbed species. In the inhibited sample, additional modifications of the DOS are observed, reflecting the contribution of molecular orbitals from the adsorbed inhibitor. After Ar⁺ ion etching, the partial recovery of states near the Fermi level indicates exposure of the underlying interface. However, the inhibited sample still exhibits reduced DOS intensity near E_F compared to the uninhibited surface, demonstrating a persistent modification of the interfacial electronic structure. Because the spectra contain overlapping contributions from metallic Fe, iron oxide/hydroxide corrosion products, carbonaceous species, and DEAI, the changes in spectral shape are interpreted as a redistribution of occupied-state intensity rather than as unique assignments of individual molecular orbitals. The polished reference (CS-P@Etched) exhibits a work function of 5.54 eV and a metallic-like valence-band structure, with the Fermi level intersecting the occupied states. In this case, the apparent value at ∼0.75 eV corresponds to the binding energy of the last density-of-states feature rather than a true EF–EVBM separation, confirming the metallic character of the exposed substrate. This observation is consistent with XPS results showing dominant Fe⁰ contribution and a low O/Fe ratio, indicative of a metal-rich surface. In contrast, the nonetched samples (CS-HCl and CS-HCl-INH) display large EF–EVBM values (∼3.9–4.0 eV), indicating that the outer surface is electronically screened from the metallic substrate by corrosion-product and adsorbate layers. Such behavior reflects the formation of electronically insulating or semiconducting surface layers, typically associated with corrosion products or adsorbed species. Notably, the inhibited sample (CS-HCl-INH) exhibits a significantly lower work function (3.42 eV) compared to CS-HCl (5.35 eV), demonstrating that the inhibitor forms an electronically distinct outer layer. UPS is particularly sensitive to such adsorbate-induced changes in surface electronic structure, including shifts in vacuum level and modification of frontier orbitals. ,, The 1.93 eV decrease in WF is accompanied by only a 0.07 eV change in E_F – E_VBM (3.91 to 3.98 eV), indicating that the dominant inhibitor-induced effect is a vacuum-level shift associated with an interfacial dipole rather than a simple displacement of the occupied-state onset. This pronounced decrease in work function indicates the formation of a surface dipole associated with the adsorbed inhibitor layer. Similar adsorbate-induced potential shifts have been reported for amine-, amide-, and carboxylic-acid-containing molecules adsorbed on iron oxide films, where the magnitude and direction of the interfacial dipole depended on both the organic functionality and the oxide surface chemistry. Molecular orientation and electron affinity have likewise been shown to strongly control work-function modification at organic/metal interfaces. This observation is fully consistent with XPS results showing the presence of nitrogen- and oxygen-containing species and reduced chloride adsorption. After Ar⁺ ion etching, both systems show a pronounced decrease in E_F – E_VBM, indicating removal of the outer electronically blocking layer and exposure of the buried interface. The CS-HCl@Etched sample exhibits the highest work function (5.95 eV) and a small E_F – E_VBM value (0.63 eV), suggesting a transition toward a more metallic-like but oxidized interface. This increase in work function reflects the exposure of an oxide-rich interfacial region with stronger electron-withdrawing character. This behavior correlates with XPS results, which reveal increased Fe content and the presence of mixed Fe²⁺/Fe³⁺ species characteristic of corrosion layers at the metal/oxide interface. Classical XPS/UPS investigations of oxygen adsorption and thin-oxide formation on iron similarly demonstrated that progressive oxidation produces pronounced changes in both the valence-state distribution and the electronic properties of the iron surface. Because Ar⁺ bombardment can partially reduce iron oxides and alter surface stoichiometry, the postetching values are interpreted comparatively for samples treated under identical conditions. Importantly, the CS-HCl-INH@Etched sample remains electronically distinct from the uninhibited etched surface. It exhibits a lower work function (5.26 eV) and a larger E_F – E_VBM value (1.18 eV), indicating that the inhibitor induces a persistent modification of the interfacial electronic structure. Relative to CS-HCl@Etched, the inhibited etched surface shows a 0.69 eV lower WF and a 0.55 eV larger E_F – E_VBM separation. The higher E_F – E_VBM value compared to CS-HCl@Etched suggests persistent attenuation of the near-E_F occupied-state intensity at the interface. This is further supported by XPS results showing the persistence of nitrogen and oxygen-containing species and reduced chlorine content after etching. The valence-band spectra also reveal redistribution of the occupied density of states, confirming that the inhibitor modifies not only the surface composition but also the electronic properties of the interface. These differences provide comparative evidence of a persistent inhibitor-induced electronic effect within the sputtered near-surface region, but they should not be taken as proof that an intact molecular layer survives the energetic ion treatment. In summary, the UPS results demonstrate that corrosion leads to the formation of an electronically blocking surface layer, while the inhibitor induces significant modifications of both the outer surface and the buried interface. The combined evolution of WF and E_F – E_VBM before and after etching clearly indicates a two-level effect involving both the outer adsorbed layer and the underlying interface. The outer DEAI-containing layer produces a pronounced 1.93 eV work-function decrease, consistent with interfacial-dipole formation, whereas the etched inhibited region retains distinct WF and occupied-state-onset values relative to the uninhibited interface. The persistence of these electronic changes after etching provides comparative evidence for inhibitor-induced modification of the near-surface interfacial region. This conclusion is in excellent agreement with XPS analysis, which shows retained inhibitor-derived species and reduced chloride adsorption, supporting a mechanism in which the inhibitor stabilizes the surface both chemically and electronically, thereby enhancing corrosion resistance. The UPS findings also agree with the increased charge-transfer resistance and reduced corrosion-current density, indicating that the chemically adsorbed barrier and its associated surface dipole hinder interfacial electron transfer and the electrochemical corrosion reactions.

11.

UPS analysis of carbon steel surfaces exposed to 1 M HCl in the absence and presence of DEAI, before and after Ar⁺ ion sputtering. (a) Secondary electron cutoff region used to determine the work function (WF). (b) Valence-band spectra showing the distribution of occupied electronic states. (c) Near-Fermi edge region used to estimate the E_F – E_VBM separation. For CS-P@Etched, the feature near 0.75 eV is an occupied-state feature rather than a true VBM.

9. Work Function (WF), E_F – E_VBM, and Apparent Ionization Potential (IP_app) of Carbon Steel Surfaces Determined by UPS, before and after Ar⁺ Ion Sputtering.

Sample	WF (eV)	(EF–EVBM) (eV)	IPapp (eV)	Comment
CS-P@Etched	5.54	∼0/no distinct onset	∼5.54	Metallic-like polished substrate; the Fermi level intersects the occupied states
CS-HCl	5.35	3.91	9.26	Corrosion-product-rich outer layer with strongly attenuated near-EF states
CS-HCl-INH	3.42	3.98	7.40	Inhibitor-rich outer layer with a pronounced adsorbate-induced work-function shift
CS-HCl@Etched	5.95	0.63	6.58	Exposed metal/oxide interfacial region with high WF and increased near-EF intensity
CS-HCl-INH@Etched	5.26	1.18	6.44	Modified interfacial region retaining inhibitor-induced electronic effects

3.8. Computational Studies

3.8.1. DFT Calculations

Figure displays the equilibrium geometry and frontier molecular orbitals (HOMO and LUMO) of the neutral form. Their relevant chemical indicators are listed in Table . According to the literature, a large value of E _HOMO indicates a molecule’s ability to donate electrons, while a small value of E _LUMO indicates its ability to accept electrons. The corresponding values of the isoxazolone molecule exhibits a strong electron exchange with the iron surface due to its high E _HOMO (−5.78 eV) and low E _LUMO (−2.53 eV) energies. The energy gap (ΔE _GAP) is a critical parameter for verifying the reactivity of corrosion inhibitor molecules. According to Koopman’s theorem, a small energy gap indicates low kinetic stability and high chemical reactivity. Consequently, the calculated value of the isoxazolone inhibitor (−3.25 eV) is in accordance with the theorem. Another significant quantum index is electron negativity (χ), a measure of a molecule’s capacity to attract electrons. Great value of this parameter (4.16 eV) implies a strong tendency of the inhibitor to attract electrons from the metal surface. A strong affinity for adsorption is suggested by the higher softness (σ) and lower hardness (η) values of the isoxazolone molecule. The fraction of electrons transferred (ΔN) is a useful quantum chemical descriptor for quantifying the extent of charge transfer between the inhibitor molecule and the metal surface. Generally, a larger positive ΔN value supports a greater tendency of the inhibitor to donate electrons to the vacant orbitals of the metal. In the present study, the positive (ΔN = 0.20 > 0) value obtained for the investigated inhibitor confirms its favorable electron-donating capability, promoting stronger adsorption and improved surface coverage of the carbon steel surface. Furthermore, electronic distribution of the isoxazolone compound can be visualized using a molecular electrostatic potential (MESP) map, which offers valuable insight into the regions susceptible to electrophilic and nucleophilic attack. In short, the red (partial negative charge) and blue (partial positive charge) regions are shown by nucleophilic and electrophilic centers, respectively. As can be seen from the Figure the regions favorable for nucleophilic attack are localized over oxygen and nitrogen heteroatoms of isoxazolone ring. While the regions favorable for electrophilic attack are associated around a phenyl moiety and (−N­(CH₃)₂) entity. The calculated quantum chemical descriptors and electronic properties are in good agreement with the experimentally observed high inhibition efficiency, demonstrating a strong correlation between the theoretical predictions and experimental findings and reinforcing the conclusion that the investigated inhibitor provides effective and stable corrosion protection.

12.

Equilibrium geometry, frontier molecular orbitals (HOMO; LUMO) and MEPS iso-surface for neutral form obtained from DFT calculations.

10. Calculated Quantum Chemical Parameters of Isoxazolone Molecule Obtained from DFT Simulation.

Inhibitor	μ (D)	E tot (Har)	E HOMO (eV)	E LUMO (eV)	ΔE GAP (eV)	χ (eV)	η (eV)	σ (eV)	ΔN
Isoxazolone	10.49	–842.64	–5.78	–2.53	3.25	4.16	1.63	0.61	0.20

3.8.2. MD and RDF Simulations

Molecular dynamics simulation is a powerful tool in corrosion sciences that is used to investigate the adsorption behavior of organic compounds on the metal surface. The orientation of the absorbed inhibitor of isoxazolone molecule on Fe (110) surface is depicted in Figure (a) and its calculated energy parameters are summarized in Table . As can be seen, the isoxazolone molecule is nearly parallel to the Fe(110) surface. This orientation provides robust interfacial interactions and favorable surface coverage of the iron substrate. A negative sign for the interaction energy indicates that the inhibitor molecule is attracted to the metal surface through spontaneous and exothermic processes, suggesting a strong affinity between the inhibitor molecule and the iron surface. The calculated interaction energy is highly negative (−214.235 kcal mol^–1), indicating strong and thermodynamically favorable adsorption of the isoxazolone molecule on the Fe(110) surface. Such a large negative interaction energy reflects a pronounced interaction between the inhibitor and the metal surface, resulting in the formation of a stable and compact adsorbed layer that effectively blocks active corrosion sites and restricts the penetration of aggressive ions to the metal surface. Radial distribution function (RDF) analysis provides valuable insight into the nature of the interactions between the inhibitor molecule and the Fe(110) surface by describing the probability of finding specific inhibitor atoms at a given distance from the metal surface. According to the literature, RDF peaks located within the range of 1.0–3.5 Å are generally attributed to chemisorption, reflecting the formation of chemical bonds between the inhibitor and the metal surface, whereas RDF peaks at distances greater than 3.5 Å are typically associated with physisorption, arising from electrostatic or van der Waals interactions.

13.

(a) Simulated stable adsorption configuration of isoxazolone inhibitor on Fe(110) surface (Side view and Top view) obtained from MD simulation at 303 K. (b) RDF peaks of N, O and C atoms for isoxazolone inhibitor on Fe(110) surface obtained from MD trajectories at 303 K.

11. Calculated Energy Parameters of Isoxazolone Inhibitor on Fe(110) Surface Obtained from MD Simulation at 303 K.

Inhibitor	E tot (kcal mol–1)	E surface+solution (kcal mol–1)	E inhibitor (kcal mol–1)	E interaction (kcal mol–1)	E binding (kcal mol–1)
Isoxazolone	–54223.914	–54077.108	67.430	–214.235	214.235

As shown in Figure (b), the RDF peaks corresponding to the nitrogen and carbon atoms of the isoxazolone inhibitor are located at 4.39 Å and 4.67 Å, respectively, indicating that these atoms interact with the Fe(110) surface predominantly through physical adsorption. In contrast, the oxygen atoms of the isoxazolone ring exhibit an RDF peak at 3.07 Å, which falls within the characteristic chemisorption region. This result suggests that the oxygen atoms actively participate in the adsorption process by donating their lone-pair electrons to the unoccupied d-orbitals of surface iron atoms, leading to the formation of coordination bonds. Therefore, the adsorption mechanism of the investigated inhibitor involves mixed adsorption, in which chemisorption occurs primarily through the oxygen atoms, while carbon and nitrogen atoms contribute mainly through physisorption. The RDF simulation aligns well with the experimental findings of thermodynamic parameters.

3.9. Mechanism of Adsorption and Corrosion Inhibition

A comprehensive adsorption and corrosion inhibition mechanism for DEAI can be proposed by integrating the experimental results with the theoretical analyses (Figure ). It is well established that DEAI inhibits the corrosion of carbon steel by adsorbing onto the carbon steel surface through a combination of physisorption and chemisorption. Physical adsorption occurs through electrostatic interactions between the charged isoxazolone molecules and the charged carbon steel surface. In the acidic medium (1 M HCl), protonated DEAI molecules also interact electrostatically with preadsorbed Cl^– ions on the positively charged carbon steel surface. The strength of these interactions decreases as the temperature increases. Chemical adsorption subsequently takes place through the donation of lone-pair electrons from the nitrogen and oxygen heteroatoms to the vacant 3d orbitals of Fe atoms, leading to the formation of coordination bonds. Besides, donor–acceptor (back-donation) interactions involving the π-electron system of the phenyl ring further strengthen the adsorption of DEAI on the steel surface. The synergistic contribution of these physical and chemical interactions results in the formation of a compact and protective adsorbed film, effectively isolating the metal surface from the aggressive acidic environment and thereby mitigating the corrosion process.

14.

Proposed adsorption and corrosion inhibition mechanism of DEAI on the carbon steel surface in 1 M HCl solution.

4. Conclusion

A novel isoxazolone-based derivative (DEAI) was successfully synthesized through a one-pot environmentally friendly procedure and demonstrated excellent corrosion inhibition performance for carbon steel in 1 M HCl over the temperature range of 303–323 K. Electrochemical measurements (PDP, EIS, and LPR) revealed that DEAI acts as a mixed-type inhibitor with cathodic predominance, achieving a maximum inhibition efficiency of 92.7% at 313 K and a concentration of 7.5 × 10^–5 M. Thermodynamic analysis indicated a mixed physisorption-chemisorption mechanism following a modified Langmuir adsorption isotherm. Surface characterization by AFM, XPS, and UPS confirmed the formation of a compact protective film and strong electronic interactions at the inhibitor–metal interface. These experimental findings were corroborated by DFT, molecular dynamics (MD), and radial distribution function (RDF) simulations, which demonstrated favorable electronic properties, strong adsorption on the Fe(110) surface, and stable interactions through heteroatoms and π-electrons. The novelty of this work lies in the comprehensive elucidation of the corrosion inhibition mechanism by integrating conventional electrochemical techniques with advanced surface-sensitive analyses (AFM, XPS, and UPS) and multiscale computational approaches (DFT, MD, and RDF). This integrated methodology provides deeper insight into the adsorption behavior and the electronic interactions governing inhibitor performance at the metal/electrolyte interface. The excellent inhibition efficiency, robust adsorption behavior, and stable protective film demonstrate that DEAI is a promising candidate for protecting carbon steel in aggressive acidic environments encountered in industrial applications such as acid pickling, oil-well acidizing, and chemical cleaning. Furthermore, this study provides valuable insights for the rational design and development of next-generation isoxazolone-based corrosion inhibitors with enhanced performance and industrial applicability.

Data will be made available upon request. The data used are available throughout the manuscript text.

C.R.: Writingreview and editing, Methodology, Formal analysis, Data curation, Conceptualization. O.B.: Writingreview and editing, Validation, Supervision, Resources, Project administration, Funding acquisition. A.S.: Writingreview and editing, Writingoriginal draft, Validation, Resources, Methodology, Investigation, Data curation, Formal analysis. M.G.: Writingoriginal draft, Visualization, Validation, Resources, Methodology, Formal analysis, Data curation, Conceptualization. F.B.: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Resources, Methodology, Formal analysis, Data curation, Conceptualization. F.Z.: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Resources, Methodology, Formal analysis, Data curation, Conceptualization. C.A.: Writingoriginal draft, Visualization, Validation, Resources, Methodology, Formal analysis, Data curation. I.A.: Writingoriginal draft, Visualization, Validation, Resources, Data curation, Conceptualization. C.E. G.: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Resources, Methodology, Formal analysis, Data curation, Conceptualization.

The authors declare no competing financial interest.