1. Introduction Metallic materials and their alloys underpin virtually every sector of modern infrastructure and manufacturing [ 1, 2, 3]. Despite their widespread utility, these materials are inherently susceptible to electrochemical degradation when exposed to media rich in aggressive species such as chloride and sulfate ions [ 4, 5]. The economic and structural toll of such degradation, ranging from equipment failure to unplanned shutdowns, has spurred sustained efforts to develop protective strategies, among which the addition of organic inhibitors to the corrosive medium stands out for its simplicity, affordability, and demonstrated effectiveness [ 6, 7, 8]. The protective action of organic inhibitors stems from their ability to anchor onto metallic substrates and form a molecular barrier between the metal and its environment [ 9, 10, 11]. Efficient anchoring typically requires a conjugated backbone capable of π electron overlap with surface orbitals, together with heteroatoms (N, O, S) whose lone-pair electrons can coordinate directly with the partially vacant d-band of the metal [ 12, 13, 14]. Within this broad family, nitrogen-bearing heterocycles and their derivatives, including azines, amino acids, imines, Schiff bases, and hydrazides, have attracted particular attention over the past decade [ 15, 16, 17]. Recent literature has showcased the versatility of these compounds; for instance, Naveenkumar et al. (2026) [ 18] demonstrated that Schiff bases can achieve high inhibition efficiency using potentiodynamic polarization and EIS. Similarly, the work of Ahmed et al. (2026) [ 19] and El Hamri et al. (2026) [ 20] emphasized the role of functionalized nitrogen-bearing groups in enhancing the adsorption stability on carbon steel surfaces in 1 M HCl. Furthermore, Elaraby et al. (2025) [ 21] highlighted the importance of multi-scale quantum insights to validate these protective effects, while Al-Amiery et al. (2024) [ 22] underscored the shift toward more sustainable, environmentally responsible corrosion control strategies. However, many of these studies still rely predominantly on static quantum-chemical calculations, often overlooking the complex dynamic behavior of the inhibitor–metal interface under real service conditions. Hydrazones, defined by the characteristic C=N–N linkage, occupy a noteworthy position in this landscape [ 23]. The –NH–N=CH– fragment imparts an unusual dual character: the carbon center is simultaneously electrophilic and nucleophilic, while both nitrogen atoms function as nucleophilic donors. This combination confers mixed hard–soft Lewis base behavior that favors strong, multisite coordination with iron and steel surfaces [ 24, 25, 26]. Despite these promising features, a significant research gap remains: the precise electronic influence of the furanyl moiety when coupled with a hydrazone bridge, particularly the impact of protonation in highly acidic (1.0 M HCl) environments, is not yet fully understood. Most existing computational models fail to account for how the transition from neutral to cationic forms, which is typical during industrial acid pickling and cleaning processes, alters the binding geometry and orbital interactions at the corroding interface. While gravimetric and electrochemical measurements remain indispensable for quantifying protection levels, they offer limited visibility into the atomic-scale events that dictate how an organic molecule settles onto and interacts with a corroding interface. Computational chemistry has filled this gap. In particular, quantum-mechanical modeling at the DFT level can map the electronic landscape of an inhibitor, frontier orbitals, charge distribution, and local reactivity, while MD simulations capture the time-dependent behavior of the adsorbed layer under conditions that approximate the real service environment. Against this backdrop, the present study introduces the synthesis and multiscale evaluation of a new furanyl hydrazone derivative, FNH, as a high-performance inhibitor for XC70 carbon steel. The novelty of this work lies in its integrated approach; we bridge the gap between macroscopic electrochemical observables (Tafel and EIS) and microscopic insights (DFT and MD) to elucidate the bidirectional electron exchange mechanism. By focusing on both neutral and protonated states, this research provides a mechanistic blueprint that explains how FNH achieves exceptional inhibition efficiency, offering a strategic framework for the future design of heteroatom-rich inhibitors in aggressive acidic media. 2. Methods and Materials 2.1. N′-[(E)-Phenylmethylidene] Furan-2-Carbohydrazide (FNH) All chemicals used for the synthesis of FNH were of analytical grade, purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. 2-furoic acid hydrazine (0.01 mol) was dissolved in 15 mL of heated ethanol. Benzaldehyde (0.01 mol) was then added, and the resulting mixture was refluxed for three hours. Thin layer chromatography with a CH 2Cl 2: MeOH (5:1) eluent system was used to track the progress of the reaction. After cooling, the precipitate was filtered out, cleaned with cold ethanol and purified by recrystallisation from a mixture of ethanol and water ( Scheme 1). Analytical and spectroscopic data for the synthesized compound are presented below: Yield: 1.85 g (86.3%); m.p: 226.8 °C. Anal. Calcd. for C 12H 10N 2O 2 (%): C, 67.28; H, 4.71; N, 13.08. Found (%): C, 67.12; H, 4.65; N, 13.19. UV-Vis (EtOH) λ max (nm): 219, 302. FT-IR (KBr, cm −1): ν N − H (hydrazine: CO–NH–N=) 3175, ν C − H (arom) 3103, ν C = O (CO–NH–) 1630, ν C = N (–NH–N=CH–) 1584, ν C − N (CO–N–H) 1282, δ C − H (arom. mono-substituted) 756 and 691. 1H NMR (DMSO-d 6, δppm): 11.92 (s, 1H, NH), 8.50 (s, 1H, CH=N), 7.98–7.95 (m, 2H, Ar-H), 7.74–7.70 (m, 2H, Ar-H), 7.47–7.43 (m, 3H, Ar-H), 6.73 (dd, J = 3.5, 1.8 Hz, 1H, furyl-H); 13C NMR decoupled (DMSO-d 6, δ (ppm): 148.3 (CH=N), 146.3 (CH-furyl), 130.6, 129.3, 127.5 (CH-phenyl), 115.4, 112.6 (CH-furyl). The SMILES code is O=C(N/N=C/c1ccccc1)c2ccco2. 2.2. Steel Coupons and Solutions XC70 carbon steel, supplied by the mold-making unit of Setif, Algeria, was selected as the metallic substrate for all corrosion experiments. Its nominal composition, determined by optical emission spectrometry and expressed in weight percent, is: C 0.065, Si 0.245, Mn 1.685, Cr 0.042, Al 0.042, Ni 0.026, V 0.014, Cu 0.010, Mo 0.005, P 0.002, and S 0.001, with iron accounting for the remainder. The aggressive electrolyte consisted of 1 M hydrochloric acid, prepared by volumetric dilution of a 37% HCl stock solution (Sigma-Aldrich, St. Louis, MO, USA) with double-distilled water; the exact molarity was checked by acid–base titration before each experimental campaign. Inhibitor-containing media were prepared by dissolving weighed amounts of the synthesized FNH hydrazone directly in the 1 M HCl to yield a concentration series spanning four orders of magnitude, from 1 × 10 −6 ଗ୍ଧକ୍ଟ ୧ ୍ଠ ୧୦ −4 M. 2.3. Weight Loss Experiment The effect of inhibitor concentration on corrosion protection performance was assessed through gravimetric measurements. XC70 carbon steel coupons of cylindrical geometry, presenting a total exposed area of 4 cm 2, were abraded using successive grades of emery paper and subsequently degreased with acetone. Each coupon was then suspended in 50 mL of 1 M HCl maintained at ambient temperature for an exposure duration of 24 h, in the absence and presence of FNH at different concentrations. The mass of each coupon was determined before and after immersion on an analytical balance with a readability of 0.001 g. All trials were performed in triplicate, and the averaged values are reported. 2.4. Electrochemical Measurements Electrochemical characterization was performed with a Bio-Logic SP-150 potentiostat/galvanostat controlled by EC-Lab software (version V11.42; Bio-Logic Science Instruments, Seyssinet-Pariset, France). The cell—a 100 mL double-walled Pyrex vessel connected to a thermostatic bath—housed three electrodes: a 2 cm 2 platinum plate serving as the auxiliary electrode, a saturated Ag/AgCl half-cell as the reference, and the XC70 carbon steel specimen (exposed area 0.2 cm 2) as the working electrode. Prior to data acquisition, the steel electrode was held in the test solution for 60 min until the open-circuit potential (E OCP) reached a stable value. EIS measurements followed: a 10 mV peak-to-peak sinusoidal signal was applied around E OCP while the excitation frequency was logarithmically swept from 100 kHz to 10 mHz. Once the impedance acquisition was complete, the electrode potential was ramped linearly at 1 mV s −1 across a ±250 mV interval centered on E OCP to record the potentiodynamic polarization response. Every measurement sequence was carried out three times on freshly prepared electrodes; the curves shown are representative of the triplicate set. 2.5. Surface Analysis Using AFM and SEM 2.6. DFT and MD Computations The microspecies distribution of FNH as a function of pH was calculated using the MarvinSketch software package (ChemAxon, Budapest, Hungary, version 25.3.5), which predicts protonation equilibria based on calculated pKa values and the Henderson–Hasselbalch equilibrium formalism. The software determines the relative abundance of each protonation state through thermodynamic acid–base equilibria according to PH = PK a + log c o n j u g a t e b a s e w e a k a c i d (1) The interfacial behavior of FNH on Fe (110) was examined using an MD simulation performed with the Forcite module. The simulation was carried out in a simulation box (24.82 × 24.82 × 44.20 Å 3) with periodic boundary conditions, and the COMPASSIII force field (Condensed Phase Optimized Molecular Potentials for Atomistic Simulation Studies) was used to optimize the structures of all components of the system [ 35]. The Fe (110) surface was selected to simulate the adsorption process, since it is the most stable low-Miller-index iron surface and therefore the most abundant. First, the Fe crystal was imported and cleaved along the (110) plane, and a 9.93 Å slab thickness was employed. The Fe (110) surface was relaxed by minimizing its energy using the smart minimizer method, and then enlarged to a (10 × 10) supercell to provide a large surface area for inhibitor interactions. A vacuum slab with zero thickness was built. Two independent cells were built in this way—one with neutral FNH and one with the protonated FNH-H + cation—to compare their adsorption. Each cell includes water (490 molecules), H 3O + (5 molecules), and Cl − (5 molecules) for a more reliable system, and placed above the iron surface layer. All atoms in the Fe (110) surface were kept frozen during the simulation process; only the FNH and water molecules were allowed to contact the iron surface freely. Nonbonding, van der Waals, and electrostatic interactions were set as atom-based summations using the Ewald summation method, with a cutoff radius, spline width, and buffer width set to 15.5 Å, 1 Å, and 0.5 Å, respectively. The Andersen algorithm was employed to control the simulation temperature at different levels. The simulation was performed at 298 K in the NVT ensemble, with a time step of 1 fs and a simulation time of 100 ps. The dynamics were run until the entire system reached equilibrium, at which point both the temperature and the energy of the system were balanced. The strength of the inhibitor–surface association was quantified through the interaction energy and the corresponding binding energy, calculated according to Equations (2) and (3) [ 36]: E i n t e r a c t i o n = E t o t a l − ( E s u r f a c e + s o l u t i o n + E i n h i b i t o r ) (2) E b i n d i n g = − E i n t e r a c t i o n (3) In these expressions, E total denotes the energy of the complete system, E s u r f a c e + s o l u t i o n represents the combined energy of the Fe slab and the water phase, and E i n h i b i t o r is the energy of the free inhibitor molecule. 3. Results and Discussion 3.1. Gravimetric Measurements Gravimetric data recorded after 24 h of immersion were used to quantify the protective action of FNH toward mild steel in 1 M HCl at ambient temperature. The corrosion rate, W (g cm −2 h −1), and the corresponding inhibition efficiency, IE W (%), were derived from Equations (4) and (5), respectively: W = m 1 − m 2 S . t (4) I E W = W 0 − W W 0 ୍ଠ 100 (5) Here, m 1 and m 2 (g) are the masses of the steel coupon before and after exposure, S (cm 2) is the area in contact with the solution, and t (h) is the total immersion period. W 0 and W denote the corrosion rates obtained in the uninhibited and inhibited media, respectively. The resulting corrosion parameters are compiled in Table 1. A clear concentration-dependent trend is observed: IE W rises steadily with FNH concentration and attains 94.94 ± 1.2 at 1 × 10 −4 mol L −1. Such pronounced protection originates from the adsorption of FNH molecules onto the steel substrate, where the nitrogen and oxygen heteroatoms supply lone-pair electrons to the partially filled d-orbitals of surface iron atoms. This electron donation facilitates the formation of a dense, adherent barrier film that effectively suppresses the corrosion process [ 37, 38]. 3.2. Potentiodynamic Polarization Curves The influence of FNH on the anodic and cathodic half-reactions occurring at the steel surface was examined by potentiodynamic polarization. Prior to each scan, the carbon steel electrode was allowed to stabilize for 60 min in 1 M HCl containing FNH at concentrations between 1 × 10 −6 and 1 × 10 −4 M; a blank experiment without inhibitor was run under identical conditions. The recorded polarization curves are displayed in Figure 1. Inspection of the curves shows that the addition of FNH over the entire concentration range produces a pronounced suppression of the corrosion current density relative to the uninhibited acid. Both the anodic and cathodic branches shift toward lower current values, whereas the corrosion potential undergoes only a modest displacement with respect to the blank. This dual retardation of metal dissolution and hydrogen evolution is characteristic of a mixed-type inhibitor. The gradual decline in current density at higher FNH concentrations reflects a progressive buildup of adsorbed molecules on the electrode surface, generating a protective film that restricts the access of chloride ions and protons to the underlying metal and, consequently, diminishes the overall dissolution rate [ 39]. The related electrochemical parameters, such as corrosion potential E corr, corrosion current density i corr, and Tafel slopes β a, β c, determined by extrapolation of the Tafel regions, surface coverage (θ) and the inhibitory effectiveness (IEp (%)) are listed in Table 2. The inhibitory effectiveness was calculated using the following equation: I E p = i c o r r ° − i c o r r i n h i c o r r ° (6) where i c o r r ° and i c o r r i n h , are the corrosion current density without and with inhibitor, respectively. Quantitatively, i corr falls from 2.64 ± 0.05 mA cm −2 in the blank solution to 0.06 ± 0.0071 mA cm −2 ବଗ୍ଧ ୧ ୍ଠ ୧୦ −4 M, confirming the strong corrosion-mitigating capacity of FNH in 1 M HCl. In parallel, IEp (%) (%) and the fractional surface coverage θ increase with concentration, reaching 97.59 ± 0.22% and 0.9759 at the highest dosage examined, which points to the establishment of a dense and well-organized adsorbed layer over the steel surface. The net shift in E corr does not exceed 85 mV (ranging from −403 to −444 mV), further corroborating the mixed inhibition character of FNH, with concurrent action on both the anodic and cathodic partial reactions [ 40]. The changes recorded in the anodic (β a) and cathodic (β c) Tafel slopes warrant a more detailed mechanistic interpretation. Two conceptually distinct inhibition mechanisms can be identified from the behavior of the Tafel slopes: (i) geometric site-blocking, in which the inhibitor physically occupies active surface sites without altering the intrinsic electrochemical reaction pathway, in this case, β a and β c remain essentially invariant with inhibitor concentration, and the suppression of i corr solely reflects a reduction in the effective electroactive area (θ → 1); and (ii) modification of the charge-transfer mechanism, in which the adsorbed inhibitor participates in the electrode reaction by modifying the intermediate steps of the iron dissolution sequence (Fe → Fe* → Fe 2+ + 2e −) or the hydrogen evolution mechanism (H 3O + + e − → Hads → H 2), resulting in a systematic variation in the Tafel slopes with inhibitor concentration [ 41]. Applying this framework to the FNH data in Table 2, the values of β a vary from 153.3 mV dec −1 (blank) to 40.4–91.9 mV dec −1 (inhibited), and β c varies from 101.1 mV dec −1 to 48.6–131.6 mV dec −1. The non-systematic variation in both Tafel slopes across the concentration series—rather than a clean invariance or a monotonic trend—indicates a mixed situation: FNH simultaneously reduces the accessible electroactive area (dominant effect, responsible for the large icorr suppression) and exerts a moderate reorganizing influence on the charge-transfer kinetics at the Fe/solution interface. The occasional increase in β c above the blank value is attributed to the partial blocking of cathodic hydrogen evolution sites by adsorbed FNH molecules, which modifies local proton availability and alters the apparent cathodic transfer coefficient. This behavior is consistent with the strong chemisorptive interaction of FNH with surface Fe atoms via its N and O donor centers. Consequently, FNH inhibits corrosion via a combination of geometric blocking of active sites and partial modification of the charge-transfer kinetics, with the former being the dominant mechanism [ 42]. The inhibition efficiency of FNH (IE p = 97.59% at 1 × 10 −4 M) represents a significant advancement relative to contemporary benchmarks. Recent studies by Danyaro et al. [ 43] on benzoic acid-based Schiff bases reported an efficiency of 89.9%, while Satarkar and Dubey [ 44] achieved 91.2% with 5-aryl-furan derivatives. The comprehensive review by Koushik et al. [ 45] highlights that many green inhibitors plateau around 85%–88% in aggressive acidic media. Even compared with the eco-friendly systems reported by Chaouiki et al. [ 46], which reached 93%, and the furfural-derived Schiff bases studied by Ginting et al. [ 47] (78.6%), FNH provides a more robust protective layer. This superior performance underscores the critical role of the electronic synergy between the furanyl moiety and the hydrazone bridge, which ensures stable interfacial anchoring and superior surface coverage relative to these recently reported scaffolds. 3.3. Electrochemical Impedance Spectroscopy (EIS) EIS was employed to probe the interfacial processes governing the corrosion of the steel electrode in 1 M HCl, both in the uninhibited acid and in the presence of FNH at concentrations spanning 1 × 10 −6 ଗ୍ଧକ୍ଟ ୧ ୍ଠ ୧୦ −4 M. The resulting Nyquist and Bode representations are collected in Figure 2. The Nyquist diagrams ( Figure 2a) consist of a single, depressed capacitive arc for every solution examined, pointing to a corrosion process governed predominantly by charge-transfer kinetics [ 48]. In the inhibitor-free acid, the arc diameter is comparatively small, consistent with a low charge-transfer resistance (R ct) and rapid metal dissolution. Upon successive introduction of FNH, the semicircle diameter expands considerably, with the enlargement becoming increasingly pronounced at higher concentrations [ 49]. This progressive growth in R ct provides direct evidence for the effective protective action of the compound under study. The departure of the experimental arcs from perfect semicircular geometry is a well-known consequence of frequency dispersion originating from surface roughness, microscopic heterogeneities, grain boundaries, dislocations, and the fractal character of real electrode surfaces [ 50]. The Bode modulus representation ( Figure 2b) offers complementary insight: the low-frequency impedance modulus |Z| increases systematically with rising FNH concentration, corroborating the progressive buildup of an insulating adsorbed layer on the steel substrate [ 51]. In the corresponding phase-angle plots ( Figure 2c), a single broad maximum is resolved for all systems, signifying a single relaxation time constant—fully consistent with the single capacitive loop in the Nyquist plane. The broadening and upward shift in the phase angle upon inhibitor addition reflect the displacement of water molecules from the metal–solution interface by adsorbed FNH species [ 52]. The collective impedance evidence leaves little doubt that FNH adsorption onto the steel substrate is the origin of the observed corrosion suppression. By occupying electrochemically active sites, the organic film raises the energetic cost of the interfacial charge-transfer step, which manifests experimentally as growing R ct and rising low-frequency |Z| values across the entire concentration series. Quantitative extraction of the kinetic parameters required fitting the measured Nyquist responses to an appropriate electrical analog. The equivalent circuit retained for this purpose ( Figure 2d) couples R s—representing the ohmic drop through the bulk electrolyte—in series with a parallel R ct/CPE element that captures the faradaic and capacitive contributions of the corroding interface. A constant phase element was chosen over an ideal capacitor because real electrode surfaces, with their inherent roughness, grain-boundary network, and compositional gradients, invariably produce a frequency-dependent capacitive response that a single C dl value cannot reproduce. The CPE impedance takes the form: Z C P E = 1 Q ( j ω ) n (7) Here Q carries units of s n Ω −1 cm −2 and sets the magnitude of the pseudo-capacitive response, j = √(−1), ω is the angular frequency, and the exponent n—bounded between 0 and 1—serves as a diagnostic of surface ideality. When n approaches unity the element collapses to a classical parallel-plate capacitor; progressive departure toward lower n values signals increasing dispersion of relaxation times, a fingerprint of microscopic heterogeneity across the electrode area. The parameters extracted from the EIS fitting are listed in Table 3. The effective double-layer capacitance was evaluated from the CPE parameters using [ 53]: C d l = Q R c t 1 − n 1 n (8) The inhibition efficiency was then determined according to [ 54]: I E E I S % = R c t i n h − R t c ° R c t i n h ୍ଠ 100 (9) where R c t ° and R c t i n h are the charge-transfer resistances recorded in the absence and in the presence of FNH, respectively. The quality of the equivalent circuit fitting was assessed through the normalized chi-squared values (χ 2/ Z ) reported in Table 3. For all inhibited solutions, χ 2/ Z values are satisfactorily low, ranging from 4.50 × 10 −3 to 1.11 × 10 −1, confirming the excellent agreement between the experimental EIS spectra and the proposed R s(R ct/CPE) equivalent circuit model and validating the reliability of the extracted impedance parameters. The slightly elevated χ 2/ Z value observed for the blank solution (0.17) is attributed to the intrinsic complexity of the bare steel/HCl interface in the absence of an organic inhibitor film, where additional interfacial processes—including active pit initiation and oxide layer dissolution—contribute to deviations from the ideal circuit response. This behavior is well documented for bare carbon steel in aggressive acidic media [ 55] and does not affect the validity of the inhibited system analyses. As seen in Table 3, R ct undergoes a substantial increase upon inhibitor addition, rising nearly 19-fold from the blank solution to 1 × 1 × 10 −4 M FNH. This trend signals the gradual development of a surface film that increasingly impedes the interfacial charge-transfer step. In parallel, C dl drops markedly from 205 ± 04 to 1.89 ± 0.05 µF cm −2, a decrease that can be rationalized within the Helmholtz model (Equation (10)) by the substitution of adsorbed water—possessing a high dielectric constant—by organic inhibitor molecules of lower permittivity, and/or by an increase in the effective thickness of the electrical double layer [ 56]. C d l = ε 0 ε d S (10) In this expression, d represents the film thickness, S the electrode area, ε 0 the vacuum permittivity, and ε the local dielectric constant of the adsorbed layer. The CPE exponent n provides additional insight into surface heterogeneity. When n = 1, the interface behaves as a perfect capacitor on a flat, homogeneous surface; values of n 40 kJ mol −1) is consistent with spontaneous chemisorption involving charge transfer between FNH and surface Fe atoms. Full details, equations, and graphical representations (Arrhenius and Eyring plots, Langmuir isotherm) are provided in the Supplementary Materials (Table S1, Figures S1–S3). 3.5. Surface Characterization 3.5.1. Atomic Force Microscopy AFM was used to probe how FNH modifies the nanoscale landscape of the steel surface. XC70 coupons were retrieved after 24 h of contact with 1 M HCl at room temperature—one set from the plain acid, the other from a solution containing 1 × 10 −4 M FNH—and scanned alongside a freshly polished reference specimen. The three-dimensional height maps are collected in Figure 5a–c; Table 6 lists the corresponding mean roughness data. The coupon withdrawn from the inhibitor-free acid ( Figure 5b) exhibits a cratered terrain densely populated with pits of irregular shape and depth, a morphology that reflects the sustained corrosive action of the HCl medium on the bare metal. A strikingly different picture emerges when FNH is present ( Figure 5c): the topography is considerably flattened and the pit population is largely suppressed, pointing to effective mitigation of iron dissolution [ 62, 63]. This leveling effect is consistent with the self-assembly of an organic barrier derived from adsorbed FNH molecules that shields the underlying substrate from direct acid contact. On a quantitative basis, the mean roughness drops from 419.981 nm for the unprotected surface to 356.186 nm under FNH coverage, numerically corroborating the visual evidence of surface preservation. The interpretation of the residual roughness of 356 nm should be considered carefully. This value must be compared in relative, not absolute, terms: the uninhibited surface undergoes approximately a 10.7-fold increase in roughness relative to the polished reference (39.19 nm), while the inhibited surface shows only a 9.1-fold increase. Moreover, a 97.59% reduction in corrosion rate still permits 2.41% of the original dissolution rate to continue over 24 h in a highly aggressive 1 M HCl medium, which is sufficient to generate measurable surface damage. Additionally, AFM topography detects all surface features, including the texture of the deposited organic film itself, which contributes to the measured RMS value and is not a manifestation of corrosion damage. 3.5.2. Scanning Electron Microscopy SEM micrographs of XC70 carbon steel after 24 h of exposure at ambient temperature to 1 M HCl, without and with 1 × 10 −4 M FNH, are shown in Figure 6a–c. The coupon retrieved from the blank acid ( Figure 6b) presents a heavily corroded surface characterized by deep attack features resulting from severe dissolution. In contrast, the specimen exposed to the inhibited solution ( Figure 6c) is covered by a relatively continuous deposit exhibiting a platelet-like morphology, which is attributed to the chemisorption of hydrazone FNH molecules onto the active metallic sites. The resulting compact and adherent organic film restricts the transport of aggressive Cl − and H + ions toward the underlying steel, thereby suppressing both anodic dissolution and cathodic hydrogen evolution and confirming the protective action of FNH [ 27]. 3.6. Computational Investigation 3.6.1. Prediction of the Major Micro Species in an Acidic Medium 3.6.2. DFT Descriptors and Frontier Molecular Orbitals In both forms the HOMO is delocalized over the conjugated backbone with appreciable amplitude on the furan ring, the carbonyl group and the hydrazone bridge, identifying these electron-rich regions as the primary donor sites for coordination with vacant Fe d-orbitals [ 66]. The LUMO topology, by contrast, differs between the two species: in neutral FNH it is broadly delocalized across the carbonyl, hydrazone and aromatic fragments, whereas in FNH-H + it contracts onto the azomethine bridge (C7=N8) and the adjacent phenyl ring. These differences in acceptor topology are expected to affect how each form interacts with the metal surface. Protonation produces a moderate narrowing of the HOMO–LUMO gap (from 4.293 eV in FNH to 3.855 eV in FNH-H +) which, together with a pronounced rise in electrophilicity, signals heightened electron-accepting reactivity and, by extension, stronger adsorption affinity for the protonated species. Both forms exhibit relatively low global hardness (η = 2.147 and 1.928 eV) and comparatively high softness, consistent with facile charge redistribution and favorable inhibitor–metal interaction [ 67]. The electrophilicity index follows the order ω(FNH-H +) > ω(FNH) (6.728 vs. 3.968 eV), confirming the superior electron-accepting character of the protonated form [ 68]. Positive ΔN values for both species (0.669 and 0.495) indicate a net electron-donation capability toward the Fe surface, while the negative back-donation energy (ΔE b-d) shows that synergistic charge transfer from the metal to the inhibitor is thermodynamically favorable, reinforcing the stability of the adsorbed complex. 3.6.3. Fukui Function Analysis Condensed Fukui indices were computed for both FNH and FNH-H+ to pinpoint the atomic centers most actively involved in inhibitor–metal electron exchange. The local indices were evaluated within the finite-difference approximation as: f k + = q k N + 1 − q k N (11) f k − = q k N − q k N − 1 (12) where q k (N + 1),q k (N), and q k (N − 1)are the atomic charges of the anionic, neutral, and cationic species, respectively. The charge analysis was carried out using the Hirshfeld scheme. Local reactivity was resolved through the condensed Fukui functions. The electrophilic Fukui index ( f k − ), which maps the electron-donating propensity of individual atoms, attains its largest values at the hydrazone nitrogens N8 and N9 together with the aromatic carbon C4 in neutral FNH, identifying this set as the dominant donor centers responsible for coordination with vacant Fe d-orbitals during surface attachment [ 69]. Protonation reshapes this landscape: while f k − at the nitrogens drops sharply, the furan-ring carbons—C11, C13 and C14 in particular—gain appreciable donor character, reflecting a migration of the reactive electron density onto the heterocyclic ring. The net effect is a relocation of the molecular donor footprint available for metal binding, which is expected to sustain the anchoring of the adsorbed layer. A complementary picture emerges from the nucleophilic Fukui index ( f k + ), which gauges the capacity of each atom to accommodate incoming electrons. In the neutral molecule the highest f k + values reside on the azomethine carbon C7 and the carbonyl group (C10, O16), confirming their role as acceptor sites through which metal-to-ligand back-donation can occur. After protonation, the azomethine carbon C7 becomes by far the strongest acceptor, while the protonated nitrogen N8 and the phenyl-ring carbons (C2, C4, C6) acquire substantially elevated f k + magnitudes relative to the neutral molecule, signaling a markedly enhanced electron-accepting ability redistributed toward the imine bridge and the aromatic framework. Overall, the Fukui analysis shows that FNH already possesses multiple donor and acceptor centers suited to cooperative interaction with the steel surface, but protonation spatially reorganizes local reactivity—shifting electron-donating density toward the furan ring while concentrating electron-accepting density on the azomethine bridge and phenyl ring. This donor–acceptor partitioning within a single molecule is fully coherent with the frontier-orbital results discussed above, and rationalizes the superior adsorption strength and inhibition efficiency observed experimentally for the protonated form under acidic conditions ( Figure 10). 3.6.4. MD Simulation MD simulations were carried out to examine the adsorption of FNH and its protonated form FNH-H+ on the Fe(110) surface within a simulated corrosive aqueous environment at 298 K. Each inhibitor was initially positioned above the metal slab immersed in a solution box containing water molecules and chloride ions, and the system was allowed to evolve until the lowest-energy adsorption configuration was located. Equilibration was monitored through the temporal evolution of the total energy and temperature. As shown in Figure 11, both the neutral and protonated forms adopt an almost flat, parallel orientation relative to the Fe(110) plane. This near-coplanar arrangement maximizes the contact area between the conjugated backbone—the furan ring, carbonyl, hydrazone bridge and phenyl ring—and the surface iron atoms, allowing the heteroatom lone pairs and the π-system to interact simultaneously with vacant Fe d-orbitals. Such a flat-lying geometry is characteristic of strong adsorption, since it enables a single molecule to shield the largest possible number of surface active sites from the aggressive electrolyte [ 36]. Small but discernible geometric differences distinguish the two species. In the protonated form the molecule lies slightly closer and flatter against the surface, with the protonated hydrazone segment tilting toward the metal, whereas the neutral form retains a marginally greater stand-off distance. These subtle reorientations illustrate how protonation modifies the local charge distribution and, in turn, the molecule–surface registry, without disrupting the favorable parallel adsorption mode. The corresponding energetics reinforce the geometric picture. Both forms display large negative interaction energies ( Table 8), confirming spontaneous and strong adsorption on Fe(110). The protonated species exhibits the more exothermic interaction energy and, equivalently, the larger binding energy, indicating a firmer attachment to the metal in the acidic medium. The magnitude of these energies is consistent with combined electrostatic and coordinative (donor–acceptor) bonding rather than weak physisorption alone, and supports the conclusion that the almost parallel adsorption of both molecules is energetically preferred and able to occupy a maximal fraction of surface sites [ 36]. 4. Conclusions This study establishes FNH as a highly effective and structurally robust inhibitor, bridging the gap between molecular design and industrial applicability for carbon steel protection in aggressive HCl environments. This work examined the anticorrosive properties of FNH, a hydrazone Schiff base whose structural integrity and chemical purity were verified to ensure the reliability of the subsequent multi-scale analysis. Beyond quantifying its high efficiency, the major contribution of this work lies in the synergistic integration of experimental and computational scales, providing a definitive blueprint for the adsorption mechanism. Electrochemical testing and impedance measurements revealed a superior protective capacity that remains resilient across the thermal range explored, reflecting an increasingly effective barrier at the metal–electrolyte boundary. A fundamental insight gained from this research is the critical role of the protonated species in acidic media. Contrary to traditional views, our theoretical analysis demonstrates that acid-induced protonation actually enhances the molecule’s electronic communication with the iron surface through a bidirectional electron exchange. This, combined with a confirmed ‘flat-lying’ geometry, ensures maximum surface coverage and explains the exceptional stability of the organic deposit observed by surface imaging. Ultimately, FNH represents more than just a potent inhibitor; it serves as a mechanistic model for the development of future eco-friendly, heteroatom-rich inhibitors. This study provides a strategic framework for designing sustainable corrosion mitigation solutions for infrastructure and manufacturing exposed to aggressive conditions. We note, however, that SEM/AFM offer only morphological insight into the protective film, and that the MD simulations employ an ideal Fe(110) slab with single-molecule adsorption, so the mechanistic picture should be regarded as a well-supported but still simplified representation of the real, heterogeneous steel–acid interface. Supplementary Materials The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/coatings16060678/s1, Figure S1: Arrhenius plots of the corrosion current density for X70 carbon steel in 1 M HCl solution in the absence and presence of FNH at different temperatures; Figure S2: Plot of ln(i corr/T) versus the reciprocal of temperature (1/T) for X70 carbon steel in the absence and presence of 10 −4 M FNH; Figure S3: Adsorption isotherm plots of (a) Temkin, (b) Frumkin, and (c) Langmuir models for XC70 carbon steel in 1 M HCl in the presence of the FNH inhibitor, obtained using the three methods at 293 °C; Table S1: Activation parameters of the corrosion process for carbon steel in 1 M HCl solution in the absence and presence of the inhibitor FNH at its optimal concentration (1 × 10 −4 M). References [ 70, 71, 72, 73, 74, 75, 76, 77, 78, 79] are cited in the Supplementary Materials. Author Contributions Conceptualization, writing—original draft preparation, formal analysis, writing—review and editing, N.B., L.T., and I.S.; validation, investigation, methodology, software, data curation, visualization, supervision, project administration, H.L., A.D., and H.-S.L.; investigation, formal analysis, writing—review and editing, I.B., M.F., S.B., and K.D. All authors have read and agreed to the published version of the manuscript. Funding This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023–00217322). Institutional Review Board Statement Not applicable. Informed Consent Statement Not applicable. Data Availability Statement The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023–00217322). Conflicts of Interest The authors declare no conflicts of interest. References Scheme 1. General procedure of the synthesis of N’-[( E)-phenylmethylidene]furan-2-carbohydrazide. Scheme 1. General procedure of the synthesis of N’-[( E)-phenylmethylidene]furan-2-carbohydrazide. Figure 1. Polarization curves of XC70 carbon steel in 1 M HCl in the absence and presence of various concentrations of FNH inhibitor at 293 K. Figure 1. Polarization curves of XC70 carbon steel in 1 M HCl in the absence and presence of various concentrations of FNH inhibitor at 293 K. Figure 2. EIS diagrams for XC70 carbon steel in 1 M HCl solution containing different concentrations of FNH: ( a) Nyquist plots, ( b) Bode modulus plots, ( c) Bode phase angle plots, and ( d) the equivalent electrical circuit (EEC) used to fit the impedance data. Figure 2. EIS diagrams for XC70 carbon steel in 1 M HCl solution containing different concentrations of FNH: ( a) Nyquist plots, ( b) Bode modulus plots, ( c) Bode phase angle plots, and ( d) the equivalent electrical circuit (EEC) used to fit the impedance data. Figure 3. Temperature-dependent potentiodynamic polarization (PDP) profiles for XC70 carbon steel in 1 M HCl: (HCl) uninhibited solution and (FNH) solution containing 1 × 10 −4 M FNH. Figure 3. Temperature-dependent potentiodynamic polarization (PDP) profiles for XC70 carbon steel in 1 M HCl: (HCl) uninhibited solution and (FNH) solution containing 1 × 10 −4 M FNH. Figure 4. Nyquist plots for XC70 mild steel in 1 M HCl solution in the absence (HCl) and presence of FNH inhibitor at different temperatures. Figure 4. Nyquist plots for XC70 mild steel in 1 M HCl solution in the absence (HCl) and presence of FNH inhibitor at different temperatures. Figure 5. Three-dimensional AFM topographies of XC70 carbon steel: ( a) as-polished reference surface, ( b) after 24 h immersion in 1 M HCl, and ( c) after 24 h immersion in 1 M HCl with 1 × 10 −4 M FNH inhibitor. Figure 5. Three-dimensional AFM topographies of XC70 carbon steel: ( a) as-polished reference surface, ( b) after 24 h immersion in 1 M HCl, and ( c) after 24 h immersion in 1 M HCl with 1 × 10 −4 M FNH inhibitor. Figure 6. SEM micrographs of XC70 carbon steel surfaces: ( a) polished condition, ( b) after 24 h exposure to 1 M HCl, and ( c) after 24 h exposure to 1 M HCl in the presence of 1 × 10 −4 M FNH. Figure 6. SEM micrographs of XC70 carbon steel surfaces: ( a) polished condition, ( b) after 24 h exposure to 1 M HCl, and ( c) after 24 h exposure to 1 M HCl in the presence of 1 × 10 −4 M FNH. Figure 7. The percentage of microspecies of the investigated inhibitor at pH = 0 in acidic solution. Figure 7. The percentage of microspecies of the investigated inhibitor at pH = 0 in acidic solution. Figure 8. Identified microspecies of FNH in the 0–14 pH range. Figure 8. Identified microspecies of FNH in the 0–14 pH range. Figure 9. Optimized structure, HOMO-LUMO distributions in ( a) neutral and ( b) protonated forms of FNH (isovalue = 0.03 a.u.). Figure 9. Optimized structure, HOMO-LUMO distributions in ( a) neutral and ( b) protonated forms of FNH (isovalue = 0.03 a.u.). Figure 10. Condensed Fukui indices (f + and f −) illustrating the ( a) neutral and ( b) protonated states of FNH. Figure 10. Condensed Fukui indices (f + and f −) illustrating the ( a) neutral and ( b) protonated states of FNH. Figure 11. MD-Equilibrium adsorption configurations of the studied inhibitor ( a) FNH and ( b) FNH-H+ on Fe (110) surface. Figure 11. MD-Equilibrium adsorption configurations of the studied inhibitor ( a) FNH and ( b) FNH-H+ on Fe (110) surface. Table 1. Corrosion parameters obtained from weight loss measurements for XC70 steel in 1 M HCl after 24 h at room temperature, in the absence and presence of various concentrations of FNH. Table 1. Corrosion parameters obtained from weight loss measurements for XC70 steel in 1 M HCl after 24 h at room temperature, in the absence and presence of various concentrations of FNH. C (M) ∆m (g) W c o r r (g cm −2 h −1) θ IE (%) ୧ ୍ଠ ୧୦ −6୦.୦୦୩୦ ବ୍ଦ ୦.୦୦୦୧ ୦.୦୧୪୬ ବ୍ଦ ୦.୦୦୦୫ ୦.୬୯୭ ବ୍ଦ ୦.୦୨ ୬୯.୭୦ ବ୍ଦ ୨.୧ ୧ ୍ଠ ୧୦ −5୦.୦୦୧୪ ବ୍ଦ ୦.୦୦୦୧ ୦.୦୦୬୮ ବ୍ଦ ୦.୦୦୦୩ ୦.୮୫୮ ବ୍ଦ ୦.୦୧ ୮୫.୮୬ ବ୍ଦ ୧.୫ ୫ ୍ଠ ୧୦ −5୦.୦୦୦୯ ବ୍ଦ ୦.୦୦୦୧ ୦.୦୦୪୪ ବ୍ଦ ୦.୦୦୦୨ ୦.୯୦୯ ବ୍ଦ ୦.୦୧ ୯୦.୯୧ ବ୍ଦ ୧.୫ ୧ ୍ଠ ୧୦ −4୦.୦୦୦୫ ବ୍ଦ ୦.୦୦୦୦୫ ୦.୦୦୨୪ ବ୍ଦ ୦.୦୦୦୨ ୦.୯୪୯ ବ୍ଦ ୦.୦୧ ୯୪.୯୪ ବ୍ଦ ୧.୨ Table 2. Electrochemical parameters obtained from Tafel plots at 293 K in 1 M HCl, in the absence and presence of various concentrations of FNH. Table 2. Electrochemical parameters obtained from Tafel plots at 293 K in 1 M HCl, in the absence and presence of various concentrations of FNH. C (M) −E corr (mV/Ag/AgCl) i corr (mA/cm 2) ß a (mV/dec) −ß c (mV/dec) R pθ IEp (%) Blank ୪୨୬.୧ ବ୍ଦ ୦.୮ ୨.୬୪ ବ୍ଦ ୦.୦୫ 153.3 101.1 10.24 / / ୧ ୍ଠ ୧୦ −6୪୩୬.୫୪ ବ୍ଦ ୦.୪ ୦.୭୫ ବ୍ଦ ୦.୦୦୮ 143.7 48.58 21.00 0.7127 ୭୧.୨୭ ବ୍ଦ ୦.୮୫ ୧ ୍ଠ ୧୦ −5୪୪୦.୯୯ ବ୍ଦ ୦.୧୨ ୦.୨୭ ବ୍ଦ ୦.୦୦୪ 91.90 131.6 86.92 0.8976 ୮୯.୭୬ ବ୍ଦ ୦.୫୨ ୫ ୍ଠ ୧୦ −5୪୦୨.୬୪ ବ୍ଦ ୦.୪୫ ୦.୧୨ ବ୍ଦ ୦.୦୦୫ 40.40 83.60 100.16 0.9529 ୯୫.୨୯ ବ୍ଦ ୦.୩୫ ୧ ୍ଠ ୧୦ −4୪୪୪.୨୪ ବ୍ଦ ୦.୧୫ ୦.୦୬ ବ୍ଦ ୦.୦୦୭୧ 48.60 55.20 187.03 0.9759 ୯୭.୫୯ ବ୍ଦ ୦.୨୨ Table 3. EIS fitting parameters and corresponding inhibition efficiencies for XC70 steel in acidic medium at 293 K. Table 3. EIS fitting parameters and corresponding inhibition efficiencies for XC70 steel in acidic medium at 293 K. C(M) Rs(Ω.cm 2) Rct(Ω.cm 2) Qμ F . s ( n − 1 ) n C dl(µF.cm −2) χ 2 / Z IE EIS (%) Blank ୦.୪୧୪୦ ବ୍ଦ ୦.୦୦୫ ୭.୭୪୦ ବ୍ଦ ୦.୦୮ ୧୦୪୦ ବ୍ଦ ୧୫ ୦.୭୩୩୯ ବ୍ଦ ୧.୦୫ ୨୦୫ ବ୍ଦ ୦୪ 0.17 / ୧ ୍ଠ ୧୦ −6୧.୪୧୨୪ ବ୍ଦ ୦.୦୧୫ ୨୬.୨୬ ବ୍ଦ ୦.୨୫ ୫୪୦ ବ୍ଦ ୧୦ ୦.୭୪୬୪ ବ୍ଦ ୧.୧୦ ୮୩.୫ ବ୍ଦ ୦୨ 0.004497 ୭୦.୫୨ ବ୍ଦ ୦.୯୫ ୧ ୍ଠ ୧୦ −5୦.୫୨୨୪ ବ୍ଦ ୦.୦୦୮ ୫୭.୩୨ ବ୍ଦ ୦.୪୫ ୩୪୫ ବ୍ଦ ୦୮ ୦.୬୬୪୫ ବ୍ଦ ୦.୯୫ ୮.୯୬ ବ୍ଦ ୦.୧୫ 0.07282 ୮୬.୪୯ ବ୍ଦ ୦.୭୨ ୫ ୍ଠ ୧୦ −5୦.୬୮୨୦ ବ୍ଦ ୦.୦୧୦ ୯୧.୨୬ ବ୍ଦ ୦.୬୨ ୩୨୬ ବ୍ଦ ୦୭ ୦.୬୫୦୬ ବ୍ଦ ୦.୯୦ ୪.୮୦ ବ୍ଦ ୦.୦୯ 0.1105 ୯୧.୫୧ ବ୍ଦ ୦.୫୫ ୧ ୍ଠ ୧୦ −4୦.୫୬୬୮ ବ୍ଦ ୦.୦୦୯ ୧୪୩.୬୬ ବ୍ଦ ୦.୯୫ ୧୨୪ ବ୍ଦ ୦୫ ୦.୬୭୪୦ ବ୍ଦ ୦.୯୨ ୧.୮୯ ବ୍ଦ ୦.୦୫ 0.05304 ୯୪.୬୧ ବ୍ଦ ୦.୩୮ Table 4. Tafel polarization–derived electrochemical parameters and corresponding inhibition efficiencies (IE%) for XC70 carbon steel in 1 M HCl at different temperatures, evaluated at the optimal FNH concentration (1 × 10 −4 M). T
