The study utilized a rectangular silicon-aluminum alloy coupon (2 cm × 4 cm × 0.05 cm) with the following chemical composition (wt.%): 2.23% Si, 0.57% Fe, 0.1% Cr, 0.07% Cu, 0.03% Ti, 0.02% Zn, 0.02% Pb, and the balance aluminum, as determined at the CSIR-Central Electrochemical Research Institute (CSIR-CECRI), India.

The aggressive medium was a 0.5 M HCl solution, prepared by diluting a commercial 35 % analytical-grade reagent (RANKEM) with bidistilled water. Inhibitor solutions containing MELE and MEFE plant extracts were tested at concentrations between 0.1 and 0.35 g∙L⁻¹. To prepare the two extracts, leaves and flowers of Mussaendaerythrophylla, a species commonly cultivated in Indian gardens and parks, were collected from CSIR-CECRI. The fresh plant material was rinsed, shade-dried at ambient temperature, and ground to a homogeneous powder (200 g). Soxhlet extraction of the powdered material was conducted with an ethanol-water mixture (80:20, v/v) using a 500 mL Borosil apparatus; the process was deemed complete when the solvent in the siphon tube appeared colourless. The crude extract was concentrated to roughly 100 mL, defatted with petroleum ether, and the alcoholic phase was separated, further concentrated to approximately 50 mL, and dried in a vacuum oven at 60˚C for three days. The resultant dark brown solid (30 g) was kept in a desiccator until required. All solvents were freshly distilled before use, and all experiments were performed using glass vessels to minimize the risk of contamination from plastic materials.

Corrosion inhibition was evaluated using the mass loss technique, a widely adopted method due to its simplicity and reliability [23][24]. Before testing, the specimens were progressively abraded with emery paper to achieve a mirror-like finish, followed by rinsing with distilled water, degreasing with acetone, and air-drying. Triplicate samples were accurately weighed before being immersed in the hydrochloric acid solution using a glass hook, both in the absence and presence of the extracts. The temperature was maintained between 25˚C and 45˚C using a thermostatically controlled bath. After a 1-hour immersion period, the three specimens were retrieved, thoroughly cleaned, dried, and weighed again. The corrosion rate (CR) was then determined according to the following relationship:

where m 1 and m 2 are the mass (in g) before and after immersion in the test solution, respectively, S is the total surface of the sample (in cm²), and t is the immersion time (in h). The inhibition efficiency IE (%) was then calculated using the following relation:

In this equation, w 0 and w are the corrosion rates of the aluminium in the absence and presence of the investigated extract, respectively.

Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were conducted using a standard three-electrode cell setup. The electrochemical cell consisted of a platinum foil (1 cm²) as the counter electrode (CE), a saturated calomel electrode (SCE) as the reference electrode, and a silicon-aluminum alloy sample as the working electrode (WE). The aluminum samples were embedded in epoxy resin, exposing only a 1 cm² surface area. The exposed surface was prepared following the procedure outlined in the previous section. All experiments were performed using a Gill AC Model 1566 electrochemical instrument. Before measurements, the open-circuit potential (OCP) was established by monitoring the potential over time, both to verify attainment of a steady state and to determine the necessary immersion time for stabilization. Polarization curves were then recorded within a range of −200 mV to +200 mV relative to the OCP, with a scan rate of 1 mV/s. EIS measurements were carried out across a frequency range of 100 kHz to 100 mHz, with an AC amplitude of 10 mV. Each electrochemical experiment was repeated at least three times at room temperature to check the reproducibility in aerated condition.

The volatile compounds in the plant extracts (MELE and MEFE) were analyzed by gas chromatography-mass spectrometry (GC-MS). For analysis, 10 mg of each extract was dissolved in 10 mL of ethanol. The resulting solution was filtered, centrifuged at 10,000 rpm for 20 minutes, and filtered again. A 1 µL aliquot of the purified solution was injected into the GC-MS system. The analysis was performed using an Agilent Technologies 5975C Inert MSD with a Triple-Axis detector. Separation was achieved on a DB-5 fused silica capillary column (30 m × 0.25 mm ID × 0.25 µm film thickness) with helium as the carrier gas at a constant flow rate of 1.0 mL/min. The injector temperature was set to 275˚C, and samples were injected in split mode with a 10:1 ratio.

The oven temperature program was as follows: initial hold at 110˚C for 2 minutes, ramped to 200˚C at 10˚C/min, held at 200˚C for 9 minutes, then ramped to 280˚C at 5˚C/min, with a final hold at 280˚C for 2 minutes. The mass spectrometer transfer line was maintained at 280˚C. Electron ionization (EI) at 70 eV was used, and mass spectra were acquired in scan mode over a mass range of 20 - 600 *m/z* with a scan rate of 0.5 seconds per scan. Compound identification was conducted by comparing the acquired mass spectra against those in the National Institute of Standards and Technology (NIST) mass spectral library.

The surface morphology of the silicon-aluminum alloy samples after immersion in the test solutions was characterized by scanning electron microscopy (SEM) using a TESCAN microscope. Micrographs were acquired at 1000× magnification using the secondary electron detector, with an accelerating voltage of 15 kV, a working distance of 17 mm, and a 127 μm aperture.

The molecular properties of the extract’s main constituents were determined using density functional theory (DFT). Initial molecular structures were constructed and pre-optimized using GaussView 6.0.16. Subsequent quantum chemical calculations were performed using Gaussian 09 W software without imposing symmetry constraints. The B3LYP hybrid functional (Becke’s three-parameter exchange with Lee-Yang-Parr correlation) [25][26] was employed with the 6 - 311 G basis set for all computations [27].

The corrosion rate of aluminium in 0.5 M HCl was evaluated using mass loss data, both with and without plant extracts (MELE and MEFE). As shown in Figure 1, the corrosion rate at 25˚C decreases significantly with the addition of the extracts and exhibits a concentration-dependent decline. This trend confirms that both MELE and MEFE act as effective corrosion inhibitors for aluminium in acidic media.

(a) (b)

Figure 1. (a) Corrosion rate and (b) Inhibition efficiency with increasing concentration of Mussaendaerythrophylla extracts (MELE and MEFE) for silicon-aluminum alloy immersed in 0.5 M HCl at 25˚C.

The inhibition efficiency (Figure 1) increases with inhibitor concentration for both extracts, reaching values between 60% and 85% as the concentration rises from 0.1 g∙L⁻¹ to 0.35 g∙L⁻¹. This trend suggests enhanced surface coverage due to greater inhibitor adsorption on the metal surface [3][8]. Despite their different chemical compositions, MELE and MEFE exhibit comparable performance, a similarity that can be traced to their shared functional features, namely aromatic rings, extended carbon chains, and oxygenated moieties, which govern their adsorption behavior on the metal surface.

The inhibitory effectiveness of an organic compound is primarily determined by its ability to adsorb onto the substrate, forming a protective layer. The adsorption process is commonly described as a substitution reaction where inhibitor molecules in the aqueous solution, Inh_(sol), displace water molecules, H₂O_(ads) previously adsorbed on the metal surface [28]:

In this equation, x corresponds to the number of water molecules replaced by a single inhibitor molecule. To analyze the adsorption behavior of the extracted inhibitors on the aluminum alloy surface, several adsorption isotherm models were applied to fit the experimental data. Among the models tested, the Langmuir adsorption isotherm provided the best correlation. The standard Langmuir isotherm is expressed as follows, where K a d s represents the equilibrium constant for the adsorption process.

However, the Langmuir plot exhibited a slope deviating from unity (Table 1), indicating deviations from ideal monolayer adsorption behavior. This suggests phenomena such as multilayer formation or repulsive interactions between adsorbed molecules [24][29]. The adsorption equilibrium constant ( K a d s ) was subsequently determined using the modified Langmuir isotherm proposed by Villamil et al., as shown in Equation (5).

Table 1. Parameters derived from the straight line of the Langmuir isotherm.

The standard Gibbs free energy of adsorption ( Δ G a d s ) is deduced from the following expression:

whereR is the universal gas constant, T is the absolute temperature, and C e is the concentration of water, taken as 1000 g·L⁻¹ (55.5 mol∙L⁻¹). The calculated Δ G a d s value is negative, confirming the spontaneity of the adsorption process. According to established literature [24][29][30], the magnitude of Δ G a d s provides insight into the adsorption mechanism. Values lower than −40 kJ∙mol⁻¹ involve charge sharing; those higher than −20 kJ∙mol⁻¹ are consistent with electrostatic interaction; and those between these two values indicate both physisorption and chemisorption. Based on this framework, the calculated value close to −20 kJ∙mol⁻¹ could express the predominance of physisorption.

To elucidate the mode of adsorption, the corrosion rate of the silicon-aluminum alloy in 0.5 M HCl was investigated as a function of temperature (25˚C - 45˚C), both in the absence and presence of the plant extracts (0.35 g∙L⁻¹). The apparent activation energy (E_a) was determined from the Arrhenius plot (Equation (7)), while the enthalpy ( Δ H a * ) and entropy ( Δ S a * ) of activation were calculated from the transition state plot (Equation (8)) [31][32]. The linearity of the plots (R² > 0.9) confirms the applicability of these models. The calculated thermodynamic parameters are summarized in Table 2.

A key finding is the higher activation energy for the inhibited systems compared to the blank acid. This is a typical indicator of physisorption, where the inhibitor creates an additional energy barrier for the corrosion process [33]. The positive values of Δ H a * confirm the endothermic nature of the silicon-aluminum alloy dissolution, which is inherently energetically challenging [30]. The further increase in Δ H a * in the presence of the inhibitors reflects their enhanced protective efficiency. Furthermore, the increase in the entropy of activation ( Δ S a * ) suggests a more disordered state in the activated complex relative to the reactants. This is consistent with the desorption of water molecules upon inhibitor adsorption, leading to a net increase in the system’s disorder, which also supports a physical adsorption mechanism [30][34].

Table 2. Activation parameters for aluminium dissolution in 0.5 M HCl, without and with MELE and MEFE inhibitors across a temperature range of 25˚C to 45˚C.

Electrochemical measurements of the plant extract’s inhibitory properties have elucidated the mechanisms of the silicon-aluminum alloy corrosion and inhibitor adsorption. Figure 2 presents the open-circuit potential (OCP) plots and potentiodynamic polarization curves. In the blank acid solution, the OCP rose rapidly during the first 1000 seconds as the protective Al₂O₃ layer dissolved, then continued to increase gradually before stabilizing after approximately 8100 seconds. In contrast, the inhibited solutions reached a steady state much earlier, at 4800 seconds, demonstrating the protective effect of the extracts. The Tafel curves, recorded after a fixed immersion time of 9000 seconds to ensure consistency, are also shown in Figure 2. The linear segment of the cathodic branch indicates that the hydrogen evolution reaction is controlled by a pure activation mechanism [35]. The unchanged shape of the curves in the inhibited solution implies that adding 0.35 g∙L⁻¹ of the plant extract does not alter the corrosion mechanism but significantly reduces the corrosion current density (i_corr). Table 3 lists the parameters derived from the polarization curves. The observed shift in corrosion potential (E_corr) was less than ±85 mV, which is insufficient to classify the inhibitors as purely anodic or cathodic. According to the literature [2][24], this indicates a mixed-type inhibitory behavior. The inhibitory efficiencies, calculated using Equation (9), corroborate the gravimetric results. In this relation,i_corr and i_corr(inh) are the corrosion densities in blank and inhibited solutions, respectively.

Table 3. Electrochemical parameters for silicon-aluminum alloy in 0.5 M HCl, without and with plant extracts.

Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 100 kHz to 0.1 Hz. The resulting Nyquist plots (Figure 2) are characterized by a broad capacitive arc and a similarly sized inductive loop. The capacitive arc at high-to-intermediate frequencies represents the charge transfer process at the metal/electrolyte interface, associated with the dissolution of the silicon-aluminum alloy oxide layer and the formation of the electrochemical double layer [2][17]. The addition of 0.35 g∙L⁻¹ of the plant extract significantly increased the diameter of this capacitive arc, confirming the inhibitory effect of MELE and MEFE. The protective barrier formed by adsorbed inhibitor molecules effectively shields the aluminum alloy surface from corrosion. The inductive half-loop, a characteristic feature of aluminum in hydrochloric acid, is often attributed to the adsorption of species (e.g., H⁺, Cl⁻ ions, or inhibitor molecules) [20][36], their relaxation on the surface [37], or the redissolution of the passivated metal [38]. At low frequencies, the appearance of diffusion-related processes indicates that the adsorption is constrained by the diffusion of species from the solution. The overall similarity of the Nyquist spectra suggests that the corrosion mechanism is unchanged by the inhibitors [39]. However, the capacitive loops deviate from an ideal semicircle, a phenomenon known as frequency dispersion, which is attributed to the non-uniform distribution of reaction sites across the metal surface [40]. The corrosion processes are modeled by the equivalent circuit in Figure 2, where Re represents the electrolyte resistance. The double-layer capacitance is represented by a constant phase element (CPE) in parallel with the polarization resistance (Rp), which characterizes the charge transfer resistance. The inductive behavior is modeled by an inductance (L) in series with an inductive resistance (Rind). The electrochemical parameters obtained from fitting the experimental data to this circuit are summarized in Table 3.

(a) (b)

(c) (d)

Figure 2. (a) Open-circuit potential (OCP) and (b) potentiodynamic polarization (Tafel) curves for a silicon-aluminum alloy in 0.5 M HCl solution in the absence and presence of plant extract inhibitors; (c) Nyquist diagrams and (d) corresponding equivalent circuit models for the blank and extract-containing solutions.

The surface analysis by the SEM tool is shown in Figure 3. The aluminum alloy was deeply corroded after its contact with the blank acid solution at 25˚C. This is pitting corrosion over virtually the entire surface of the substrate. However, the corrosion attack was significantly reduced by adding 0.35 g∙L⁻¹ of plant extracts. The attack was limited in some areas of the metal, and certain regions remained unaffected by the aggressive environment.

(a)

(b) (c)

Figure 3. SEM micrographs of the silicon-aluminum alloy surface after immersion in 0.5 M HCl solutions without and with Mussaendaerythrophylla extract. (a) Blank (b) Leaf Extract (c) Flower extract.

GC-MS analysis, referenced against the NIST database, identified a series of organic compounds bearing O-H, C=O, and C-O functional groups in the leaf (MELE) and flower (MEFE) extracts of Mussaendaerythrophylla (Table 4). The leaf extract (MELE) was found to be primarily composed of di-n-octyl phthalate (C₂₄H₃₈O₄) and 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl (C₁₇H₃₀O) (Figure 4). In contrast, the dominant constituent in the flower extract (MEFE) was phenol, 2,6-bis(1,1-dimethylethyl) (C₁₄H₂₂O).

Figure 4. Major phytochemical constituents identified in the leaf (MELE) and flower (MEFE) extracts of Mussaendaerythrophylla. (a) phenol, 2,6-bis(1,1-dimethylethyl); (b) 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl; (c) di-n-octyl phthalate.

Table 4. Chemical constituents identified by GC-MS analysis in the leaf (MELE) and flower (MEFE) extracts of Mussaendaerythrophylla. Compound identifications are based on the NIST Mass Spectral Database.

Density Functional Theory (DFT) has emerged in recent years as a critical computational tool for probing the molecular-level mechanisms of organic corrosion inhibitors [41]-[43]. In this study, we applied DFT to evaluate three main metabolites from Mussaendaerythrophylla, di-n-octyl phthalate, 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl, and phenol, 2,6-bis(1,1-dimethylethyl), by calculating their global chemical reactivity descriptors. A comparative analysis of their frontier molecular orbitals (HOMO and LUMO) reveals distinct electronic distributions across the three compounds, as illustrated in Figure 5 and summarized in Table 5.

For di-n-octyl phthalate, the HOMO and LUMO are both centered on the phthalate core but differ in reactivity. The LUMO is strongly delocalized over the aromatic ring, with significant electron density on its carbon and oxygen atoms, identifying this region as the primary electrophilic site. In contrast, the HOMO, indicative of nucleophilic character, is also associated with the phthalate group but exhibits notably weaker density on the oxygen atoms. In phenol, 2,6-bis(1,1-dimethylethyl), the HOMO and LUMO show remarkably similar spatial distributions. Electrophilic density localizes on specific carbons of the benzene ring and methyl groups, whereas nucleophilic character is broadly delocalized throughout the phenolic system. The most extensive delocalization occurs in 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl, where frontier orbitals spread across the conjugated carbon backbone. This distribution is non-uniform, with heightened electron density on methyl-substituted and terminal carbon atoms.

Figure 5. Frontier molecular orbitals (HOMO and LUMO) for the major compounds identified in the leaf (MELE) and flower (MEFE) extracts of Mussaendaerythrophylla. (a) LUMO (left) and HOMO (right) orbitals of di-n-octyl phthalate; (b) LUMO (left) and HOMO (right) orbitals of 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl; (c) LUMO (left) and HOMO (right) orbitals of phenol, 2,6-bis(1,1-dimethylethyl).

Table 5 presents the calculated quantum chemical descriptors for these molecules. The LUMO energy, which reflects electron-accepting ability, is notably low for the two aromatic compounds, consistent with their high electron affinity. In contrast, HOMO energies, which indicate electron-donating tendency, are similar across all three, suggesting each can donate electrons to vacant metal orbitals [25][44]. The energy gap, ΔE (Equation (10)), a key indicator of reactivity, aligns with literature values and supports the corrosion inhibition potential of these molecules [43][45]. Additional global reactivity descriptors, including electronegativity (χ, Equation (11)), further elucidate the inhibitor-metal interaction [41].

Table 5. Global reactivity descriptors of the leaf (MELE) and flower (MEFE) extracts of Mussaendaerythrophylla by DFT calculation.

Di-n-octyl phthalate exhibits a higher electronegativity, whereas the other two compounds have lower electronegativities relative to the work function of aluminium ( Φ A l = 4.28 eV) [46]. This difference leads to distinct electron-transfer behaviors, quantified by the fraction of transferred electrons, Δ N (Equation (12)).

Here, Φ A l = 4.28 eV and η A l = 0 for aluminum [30][46]. A positive Δ N (>0) implies electron donation from inhibitor to metal, observed for the 4,8,12-tetradecatrien-1-ol, 5,9,13-trimethyl and the phenolic compound. Conversely, the negative Δ N (<0) for di-n-octyl phthalate suggests electron back-donation from the metal to the inhibitor. The electrophilicity index (ω) measures stabilization energy upon electron acquisition [43]. The di-n-octyl phthalate acts as the strongest electrophile (ω = 3.850 eV), while the aliphatic alcohol behaves as a moderate electrophile (ω = 1.306 eV). Finally, hardness (η; Equation (14)) and softness (σ; Equation (15)) provide insight into molecular reactivity [45]. The aromatic compounds are softer (lower η), indicating greater reactivity than the harder tetradecatrienol.

In summary, the combination of a low Δ E gap, complementary electron-transfer mechanisms ( Δ N ), strong electrophilicity, and favorable softness values demonstrates that all three molecules can participate effectively in forming a protective barrier on the silicon-aluminum alloy surface through synergistic interactions with the metal substrate.