Effectiveness of corrosion protection for 6061 aluminum with orchid leaf extract inhibitor in 1M HCl solution

2.1. Sample preparation

Aluminium Alloy 6061 (AA6061) samples with dimensions of 1 cm × 1 cm × 0.2 cm were soldered to copper wires and then encapsulated using epoxy resin. The exposed aluminium surface (1 cm²) was polished sequentially with 180-, 400-, 800-, and 1200-grit sandpapers. The alloy chemical composition was Mg (0.829%), Fe (0.187%), Cr (0.123%), Cu (0.231%), Zn (0.093%), Ti (0.027%), Si (0.821%), Mn (0.071%), and Al balanced [15,16]. For inhibitor preparation, leaves of the Larat orchid (Dendrobium discolor Lindl.) were collected from ornamental plant suppliers in West Lombok, Indonesia. The leaves were washed, air-dried, and ground into fine powder; 10 to 15 g of powder were extracted with 250 mL of analytical-grade ethanol using a Soxhlet apparatus (IKA, Germany) for 4 h, and the solvent was evaporated to obtain a concentrated extract. Phytochemical screening confirmed the presence of polyphenols and flavonoids as major constituents. Six concentrations of inhibitor: Blank, 1000, 2000, 3000, 4000, and 5000 mg L^-1, were tested in 1 M HCl solution.

2.2. Electrochemical testing

Electrochemical measurements, including OCP, PDP, and EIS, were performed using Autolab Metrohm PGSTAT 204N (Herissau, Switzerland) in a standard three-electrode cell. The working electrode was the aluminium specimen, the counter electrode was a platinum wire, and the reference electrode was Ag/AgCl (3 M KCl). The samples were stabilized in 1 M HCl for 15–30 min before testing. OCP was recorded for at least 30 min to ensure potential stabilization, followed by PDP scans from −0.25 to +0.25 V at a rate of 0.5 mV s^-1. The inhibition efficiency (IE) was calculated using Eq. (1) [17]:

EIS measurements were performed within the frequency range of 10⁻²–10⁵ Hz to capture both high- and low-frequency charge-transfer processes, using a 10 mV AC amplitude at OCP. The extended frequency range improves the fitting accuracy of the equivalent circuit and enables better evaluation of diffusion effects. The inhibition efficiency was determined using Eq. (2) [18,19].

Cathodic and anodic interaction coefficients (f_c, f_a) of the orchid extract inhibitor are given in eq. (3) and (4) [20]:

2.3. Surface morphology study

The post-corrosion surface morphology and chemical characteristics of the aluminium samples were analyzed using scanning electron microscope (SEM, FEI Inspect S-50, Tokyo, Japan), FTIR (IRSpirit, Shimadzu, Japan), and UV-Vis Spectrophotometer (BIOBASE BK-UV1900, Jinan, China) following the procedures described in our previous studies [21]. Instrument origin details were added for clarity and reproducibility.

2.4. Density functional theory

To investigate the interaction between aluminium and inhibitor molecules, DFT calculations were conducted using Gaussian 09W (Gaussian Inc., USA). The Becke three-parameter Lee–Yang–Parr (B3LYP) functional with the 6-31G(d,p) basis set in the liquid phase was applied. The selected molecular structures (A₁ and A₂) correspond to the major phenolic and flavonoid constituents reported in orchid species. The optimized geometries showed no imaginary vibrational frequencies, confirming stability. The calculated parameters—E_HOMO, E_LUMO, and related descriptors- were obtained as follows [22]: The calculated parameters—E_HOMO, E_LUMO, and related descriptors—were shown in Equations (5-9)

Here, ${\chi }_{\text{Al}}$ and ${\eta }_{\text{Al}}$ represent the work function and absolute hardness of Aluminium, with values of 4.82 eV and 0.0 eV [23], respectively. Meanwhile, ${\chi }_{\text{inh}}$ and ${\eta }_{\text{inh}}$ indicate the electronegativity and hardness value of the inhibitor.

2.5. Monte Carlo simulation

MC simulations were performed using the Adsorption Locator module in Dassault systèmes BIOVIA Materials Studio 2017 to evaluate the adsorption of inhibitor molecules on the Al(111) surface [24,25]. The inhibitor compounds and solvent molecules were pre-optimized in the Forcite module using the COMPASSII force field. The aluminium surface was modeled as an Al(111) slab (12 Å × 12 Å × 25 Å, 6 atomic layers) [26,27]. The system contained each inhibitor molecule and 100 water molecules to mimic the aqueous environment. Although H⁺ and Cl⁻ ions were not included due to computational limitations, their influence on adsorption behavior is discussed in Section 3.9. Future simulations incorporating ionic species are planned for enhanced realism.

2.6. Isothermal adsorption

The adsorption behavior of the inhibitor molecules on the aluminium surface was evaluated using four classical isotherm models: Langmuir, Freundlich, Temkin, and Frumkin, based on the surface coverage (θ) derived from PDP data [28,29]. The adsorption isotherm equations were shown in Equations (10-13)

where C represents the extract concentration, θ indicates surface coverage, and K_ads is the adsorption equilibrium constant.

The Langmuir model was selected for further analysis due to its high correlation coefficient (R² = 0.94) and physical interpretability, indicating a predominant monolayer adsorption mechanism. Slight deviation from linearity suggests partial multilayer formation, which is discussed in Section 3.6.

3.1. Potentiodynamic polarization

The good inhibition of the orchid extract inhibitor was analyzed based on its performance in 1 M HCl solution against the aluminium surface and its response to anodic and cathodic reactions, with the results of the PDP and EIS curves shown in Figure 1. Furthermore, Figure 1(a) shows the PDP curves with 6 inhibitor variations. The results of adding the inhibitor show a shift in the polarization curve, accompanied by a decrease in the corrosion current density (i_corr). The movement of the curve was also observed to be more identical towards the anodic direction than the cathodic, which essentially indicates that this orchid extract inhibitor is a mixed inhibitor and primarily inhibits the metal surface oxidation reaction in its anodic region [30,31]. This is further supported by the fact that if the shift in e_corr value for the inhibitor has not yet exceeded 85 mV, it is still classified as a mixed inhibitor [32-35]. Additionally, the similarity in trend between anodic and cathodic branches further confirms that the inhibitor suppresses both reactions simultaneously, which is typical for organic molecules containing multiple adsorption centers such as hydroxyl, carbonyl, and aromatic groups.

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Figure 1.

Electrochemical test results of the inhibitor: (a) PDP curves, (b) Nyquist plots, (c) Bode and phase plots, and (d) corrosion rate on the PDP curve.

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Table 1 shows a shift in e_corr from low potential to higher potential, observed between 417.98 mV and 441.99 mV. The corrosion current density (i_corr) shows a decrease with the addition of the inhibitor, and the higher the concentration of inhibitor added, the greater the decrease. However, the peak is at 3000 mg L^-1 with an efficiency of 90.11% and then there is a further decrease. This is because the inhibitor has reached its optimal performance. Next, this mixed inhibitor inhibits the dissolution of the anodic metal and the cathodic hydrogen precipitation reaction. Where βa is more reduced than βc, indicating that anodic inhibition dominates, see Table 1. The decrease in inhibition efficiency at concentrations above 3000 mg L^-1 may be attributed to aggregation or oversaturation of inhibitor molecules on the aluminum surface, leading to a loosely packed or multilayer film that partially hinders effective adsorption. This phenomenon has been widely reported for natural organic inhibitors and is considered a typical desorption–reorientation effect at high inhibitor concentrations [32-35].

Due to the complex nature of the inhibition process, we added two indicators to explore the inhibition process in this study further: the anodic reaction coefficient/index (fa) and the cathodic reaction index (fc), as proposed by Professor Cao, as shown in equations (3) and (4) [20,36]. As seen in Table 2, the values of coefficients fa and fc were obtained, which did not exceed 1. This indicates that the influence of orchid extract in inhibiting corrosion is caused by the adsorption of the inhibitor on the aluminum surface, which can occupy active sites or occur due to the effect of geometric blocking [27,37]. The decrease in the values of fa and fc indicates that the adsorption of the inhibitor on the aluminum surface can be effective in improving performance to minimize electrolyte ion attack. Additionally, a value of fa greater than fc indicates that the inhibitor inhibits anodic corrosion. This finding reinforces that the inhibition mechanism is dominated by surface adsorption rather than modification of electrochemical kinetics, suggesting that the organic molecules mainly function by forming a compact surface barrier that limits both anodic dissolution and cathodic hydrogen evolution.

3.2. Electrochemical impedance spectroscopy

The Nyquist plot in Figure 1(b) shows the effect of all inhibitor concentrations on a 1M HCl solution at 27°C. This impedance arc, which shows a non-uniform semi-circular curve, is caused by the adsorption of inhibitors on the surface of the working electrode with varying inhibitor concentrations [37]. The differences in the curves indicate the effect of the inhibitor on the corrosion mechanism. The semi-circular arc on the Nyquist plot indicates the charge transfer resistance (RCT). RCT represents the resistance to the electrochemical reaction at the electrode surface, meaning the larger the semi-circular arc, the lower the corrosion rate because it becomes more difficult for the charge transfer of the corrosion reaction on the aluminum surface due to the orchid extract inhibitor [30,37]. An increasing diameter indicates good inhibitor performance. Furthermore, orchid extract inhibitors can form a protective layer to isolate the aluminum surface from the corrosive solution of the metal substrate and inhibit charge transfer. The increased RCT value also indicates that the protective layer is becoming denser [35]. However, a slight decrease in RCT at higher inhibitor concentrations (4000 and 5000 mg L^-1) can be observed, which supports the hypothesis of surface saturation or multilayer adsorption as discussed in the PDP results. This reduction may arise from partial desorption or irregular molecular arrangement, which reduces the uniformity of the protective film. The consistency between PDP and EIS data thus strengthens the interpretation that 3000 mg L^-1 represents the optimal balance between adsorption coverage and molecular stability.

The modulus and phase angle plots in Figure 1(c) serve as the foundation for reinforcing the findings from the Nyquist plot, where the inhibitory performance of the orchid extract can be confirmed by the increase in modulus and phase angle values [38]. The phase angle peak increases with the addition of the inhibitor, indicating that the inhibitor has good corrosion protection in HCl solution. Furthermore, the phase angle peak can be seen to be less than 90°, suggesting that the inhibitor in the solution is influenced by diffusion [37,38]. This non-ideal capacitive behavior, indicated by a phase angle below 90°, reveals that the corrosion process involves both charge transfer and diffusion-controlled steps. The presence of the inhibitor modifies the interfacial double layer, producing a mixed adsorption–diffusion control mechanism, typical for organic inhibitors interacting through π-electron and lone-pair donation.

3.3. Open circuit potential

OCP plot for aluminum in a 1M HCl solution with and without 3000 mg L^-1 inhibitor can be seen in Figure 2. From the figure, it can be observed that the OCP measurements were taken over 120 s against variations in the OCP values. Observations show that the potential is more stable with 3000 mg L^-1 inhibitor compared to without an inhibitor. Then, the potential of the inhibitor was seen to shift towards a more positive value compared to that without the inhibitor, indicating that the inhibitor molecules can protect the metal substrate from the solution [39]. It was also observed that the potential remained stable throughout the testing duration with the inhibitor, demonstrating its ability to protect against corrosion reactions [40]. To improve the accuracy of potential stabilization analysis, it is recommended to extend the OCP measurement duration to at least 1 h. A longer observation period would allow the identification of slow adsorption and steady-state equilibrium typical for organic inhibitor systems. This gradual stabilization reflects the time-dependent adsorption of orchid extract molecules on the aluminum surface, leading to a more accurate representation of surface coverage and film formation kinetics.

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Figure 2.

Observation without and with inhibitor (a) OCP curve (b) measured OCP value.

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3.4. FT-IR analysis

To ensure the adsorption of the orchid extract inhibitor on the aluminum surface, FT-IR testing was conducted, and the results can be seen in Figure 3(a) with variations without inhibitor and with 3000 mg L^-1 inhibitor. The characteristic absorption bands around 3400-3500 cm^-1 are confirmed to originate from hydroxyl groups or hydrogen interaction bonds with the surface, resulting from O–H stretching. From the blank to 3000 mg L^-1 variation, a shift from the absorption band at 3432 cm^-1 to 3455 cm^-1 is observed, which can be attributed to the active adsorption of the inhibitor on the metal surface. In the absorption band region around 2900-3100 cm^-1, the stretching of the complex amino acid C–H groups appears in the inhibitor, although it has weak wave bands and is identified as asymmetric methyl stretching 2 and 1 [41-46]. The C–H stretching vibrations naturally occur in the range of 3100-3000 cm^-1 [47-50]. For the inhibitor, it was observed at 3092 cm^-1. The C–H stretching vibration is observed in the absorption band between 1300 and 1000 cm^-1, with the peak at 1511 cm^-1 attributed to the stretching vibration of the aromatic C–H functional group. In addition to the C–H stretching vibrations of benzene derivatives, which are seen in the range of 1000-600 cm^-1 and appear at 720 cm^-1. Next, the stretching vibrations of the carbon rings are in the range of 1650-1200 cm^-1 [43,46]. The C–C stretching vibrations are observed at 1560 and 1613 cm^-1, respectively. The C–C functional group vibrations in the aromatic rings occur at 753 and 465 cm^-1, respectively. The characteristics and intensity of the C=O functional group can enhance hydrogen bonding/conjugation, with carbonyl compounds appearing in the absorption band region of 1730-1665 cm^-1 [51,52]. C–O stretching is typically found in the 1260-1000 cm^-1 region [42,53]. The peak at 1028 cm^-1 was identified as the C–O stretching vibration. The peak observed at 720 cm^-1 in the IR spectrum was recognized as the C–H vibration in alkanes. The absorption band at 2260-2100 cm^-1 was identified from the -C=C stretching vibration in alkynes.

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Figure 3.

Results of characterization testing without and with inhibitors on (a) FTIR spectrum and (b) UV-Vis.

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The observed changes in band position and intensity confirm chemical interaction between inhibitor functional groups and the aluminum surface. Specifically, the shift of O–H and C=O peaks indicates participation of hydroxyl and carbonyl groups in chemisorption, while the persistent aromatic C=C vibrations suggest that π-electron interaction contributes to physisorption. Therefore, FTIR analysis demonstrates that both physisorption and chemisorption mechanisms coexist, as also supported by DFT results discussed in Section 3.8.

3.5. UV-Vis analysis

The UV-Vis spectrum for the aluminum surface in 1M HCl solution, tested with and without an inhibitor, and with the addition of 3000 mg L^-1 inhibitor, is shown in Figure 3(b). The observation of the bond formed between the inhibitor and Al can be shown through an absorbance graph. The absorption band around 240 nm is identical to the benzene ring π-π* transition, a photochemical process from the orchid extract conjugated with C=C bonds and involving intramolecular charge transfer [54,55]. The absorption band at 340 nm is associated with the n–π* transition, which is primarily related to C (C-C) groups, carbonyl groups, and C double bonds (C=O) [55]. Next, the wavelength shifts from 200 nm to 240 nm and 320 nm to 340 nm increased without the inhibitor, indicating an interaction between the metal and the molecules, possibly involving the formation of a complex between iron and the phytochemicals from the orchid extract, which is linked to the adsorption of inhibitor molecules occurring on the outer surface of the metal [36,56-58]. The appearance of distinct π–π and n–π transitions indicates the presence of conjugated aromatic systems and lone-pair donor atoms in the inhibitor molecules. The red shift of absorption bands after adsorption suggests the formation of coordination complexes between aluminum atoms and oxygen-bearing groups in the inhibitor. This supports the FTIR and DFT results, confirming the occurrence of electron donation from inhibitor molecules to the metal surface and validating the adsorption–complexation mechanism.

3.6. Isothermal adsorption

To help understand the inhibitory mechanism of organic materials that can affect/reduce corrosion reactions by adsorbing onto the metal surface, several models can be linked to explain the interactions that occur. Where the models, such as the Langmuir, Freundlich, Temkin, and Frumkin equations, can be seen in eq. (10,11,12,13) [58-62]. Furthermore, linear regression analysis (R²) indicates that the adsorption model follows the Langmuir isothermal adsorption equation when fitted using PDP data, as the R² value is closest to 1 compared to the other models. Therefore, the Langmuir adsorption equation analysis was used to identify the interactions occurring between the orchid extract inhibitor and the aluminum surface (Figure 4). Calculating the standard free energy of adsorption for organic extracts is still not widely done because the molecular masses of organic compounds are not yet known, but many studies report and investigate these parameters [62]. To identify the adsorption of orchid extract inhibitors onto the aluminum surface, eq. (14) below is used to calculate the standard free energy of adsorption.

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Figure 4.

Isothermal adsorption curves (a) Langmuir, (b) Freundlich, (c) Temkin, (d) Frumkin.

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where ∆G_ads represents the standard free energy of adsorption, T is the absolute temperature, R is the molar gas constant, and K_ads is the adsorption equilibrium constant.

Although the Langmuir model (R² = 0.94) provides the best fit, the deviation from ideal linearity indicates that multilayer or non-ideal adsorption behavior may also occur. This suggests possible lateral interactions among adsorbed molecules, leading to partial multilayer coverage. Nevertheless, the predominance of Langmuir behavior supports the assumption of monolayer adsorption as the main mechanism. The combined interpretation of adsorption data, ΔGads value (−27.31 kJ mol^-1), and FTIR/DFT results confirms that the inhibition mechanism involves both physisorption via electrostatic attraction and chemisorption through coordination of oxygen and π-electron donor sites with aluminium atoms.

Table 3 shows that the Kads values for all equation models exceed 1000 L mol^-1, indicating that the inhibitor can produce high efficiency [58,60,62]. If the value of ΔG_ads is -20 kJ mol^-1 or higher, this is called physisorption, where the process does not involve the breaking or forming of chemical bonds. Here, an interaction occurs between the inhibitor molecules and the metal surface, caused by electrostatic forces. When the value of ΔG_ads is -40 kJ mol^-1 or lower, it is called chemisorption, which is a process accompanied by bond formation. Here, the inhibitor can share and transfer charge to the aluminum surface [63,64]. Therefore, the value of ΔG_ads -27 kJ mol^-1 obtained from the model indicates a type of physisorption with chemisorption, suggesting that the inhibitor has a very complex mechanism in the protection process.

Further analysis is linked to the electrochemical data from the PDP in Figure 1(a) above. The slopes of the anodic and cathodic Tafel curves indicate that the inhibitor molecules not only form passive physical protection but also interact with more active electrochemical sites on the aluminum surface. In the FTIR test of the orchid extract, hydroxyl and carbonyl functional groups were found, indicating that these are the cause of high adsorption in forming coordinate bonds with Al atoms on the surface. It is estimated that this process occurs at anodic and cathodic sites. Where there is inhibition of metal dissolution and a reduction in hydrogen evolution to suppress corrosion kinetics. Generally, in the initial stage, physisorption interaction occurs with the surface, followed by the more dominant chemical bonding. This involves coordination with or protonation of organic extract molecules related to the opposite charge on the metal surface [65-67]. At optimal concentrations, the inhibitor completely covers the surface of Al because the molecules are adsorbed and deposited across the entire surface, essentially forming a layer and acting as a shield to isolate the surface from the corrosive environment. Ultimately, this reduces the corrosion rate by preventing corrosive ions from interacting with the aluminum surface [59]. This combination of adsorption types is typical for organic inhibitors containing oxygenated and aromatic groups. The balanced interaction between physical and chemical adsorption ensures film stability and high inhibition efficiency, especially at the optimal concentration of 3000 mg L^-1.

3.7. SEM observation

SEM was used to observe the morphology in a 1M HCl solution on the surface of aluminum with or without the orchid extract inhibitor. In Figure 5(a), significant/extensive surface degradation was observed, with a high degree occurring at several points marked by holes/cracks, accompanied by a rough texture [67]. The findings from the blank variation observations indicate extensive metal dissolution caused by the aggressive attack of corrosive ions in the HCl solution. This corrosive environment is intense in the metal oxidation process and damages the surface structure. This morphological observation is consistent with the results of the PDP and EIS tests, where high corrosion rates and low resistance were observed in the blank variation. The severe pitting and rough morphology observed in the uninhibited sample confirm strong chloride ion penetration and aluminum dissolution.

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Figure 5.

SEM test results of specimens with (a) no inhibitor, (b) addition of 3000 mg L^-1 inhibitor.

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In Figure 5(b), it is observed that with the addition of the inhibitor, the surface appears smoother and more uniform. Therefore, these inhibitors can affect the interaction between the solution and the metal surface, where the inhibitor molecules are adsorbed onto the surface and form a film layer to protect against anodic and cathodic attack. Thus reducing surface damage and maintaining the integrity of the metal. This is consistent with high efficiency at 3000 mg L^-1 for PDP and EIS. SEM image observations provide clear visual evidence of the inhibitor’s performance, where severe damage occurs on almost all surface areas without the inhibitor, while its presence can reduce the damage. This indicates that a protective layer is formed on the metal surface due to the inhibitor’s property of being easily adsorbed onto it. The compact and homogeneous surface morphology observed with 3000 mg L^-1 inhibitor indicates the formation of a continuous organic film, consistent with the high inhibition efficiency recorded electrochemically. This morphological evidence directly supports the presence of an adsorbed inhibitor layer that acts as a physical barrier preventing chloride penetration.

3.8. Density functional theory calculations

In analyzing the nature of corrosion inhibition, it is closely related to the molecular structure of the inhibitor; therefore, it is necessary to understand quantum chemical calculations more deeply. The molecular structure for this quantum chemical calculation is derived from the identified major components of the orchid extract as shown in Figure 6. Here, the benzene-containing species from the inhibitor are focused on for analysis, although we are aware that organic compounds contain very complex mixtures; this is due to computational limitations. In Figure 7, the optimized structure of the orchid extract inhibitor compound, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and electrostatic potential (ESP) can be seen. The representative compounds (A₁ and A₂) were modeled based on phenolic and flavonoid components detected in orchid species, which contain π-bonds and oxygen donor atoms suitable for adsorption. These structural features provide multiple active sites capable of coordinating with the aluminum surface, validating the experimental observation of dual adsorption modes.

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Figure 6.

Main structures of orchid extract compounds.

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Figure 7.

DFT calculation results (structure optimization, HOMO, LUMO, and ESP).

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Generally, in the process of electron donation and acceptance in compounds, it can be associated with HOMO (electron donor/contributor) and LUMO (electron acceptor/receiver). In Figure 7, which shows the HOMO and LUMO of the compound structure, the red areas are identified as negative charges (-) and the green areas as positive charges (+). Furthermore, the distribution of HOMO and LUMO is observed to be uniform, covering the entire structure. This indicates that this structure has high absorption in the HOMO-LUMO region. The HOMO is focused on the region containing benzene and hydroxyl groups (-OH), while the LUMO is focused on the alkane (C–H) and carbonyl (C=O) bonds located on the opposite side. This typical distribution of HOMO with LUMO can minimize the anodic metal oxidation and the rate of the cathodic reaction, where one or more of these molecules interact at the positive polarization of the anodic reaction site, while the others interact by attracting electrons to the cathodic site of the metal surface [28]. The smaller ΔEgap of compound A₁ supports its higher reactivity and adsorption tendency, while the larger ΔEgap of A₂ indicates stronger stability in the corrosive medium, implying a synergistic effect between the two molecular species. These DFT findings confirm that the inhibitors can donate electrons to vacant orbitals on Al atoms while accepting back-donation, promoting strong chemisorptive interaction. ESP shows points with the highest to lowest positive ESP and plays a role in interactions within molecules with surfaces, which can describe areas where electrophilic and nucleophilic attacks are likely to occur and how this can impact systems such as corrosive environments with organic inhibitors [68].

In Table 4, you can see the parameters obtained in the quantum chemical study calculations. Where the E_HOMO value of compound A₁ is greater than A₂ and the E_LUMO of A₁ is greater than A₂, this indicates that A₁ is more likely to donate and accept electrons on the metal surface. Next, chemical reactivity and stability are indicated by the value of ∆E_gap, which is the difference between E_HOMO and E_LUMO [69]. A lower ∆Egap value indicates easier adsorption of the inhibitor on the metal surface. Therefore, molecule inhibitor A₁ has better potential in influencing the corrosion reaction. Ionization potential (I) is identically known as the standard minimum energy required for a molecule to release an electron [62,70]. Therefore, A_l is easier to donate electrons with empty d orbitals on the metal surface. At the same time, electron affinity (A) is identical to the energy released when an electron is added to a neutral molecule [62,71]. Therefore, a higher A₂ will tend to accept electrons more easily, impacting the performance of cathodic inhibition. Chemical hardness (η), which is identical to the electronic chemical potential, and a high molecular softness level will be defined as a smaller ∆E_gab [72,73]. Where the low molecular violence of A₁ indicates reactivity and a greater tendency to be adsorbed onto the metal surface. Next, the molecule’s ability to attract electrons is indicated by its electronegativity value (χ) [74]. The high electronegativity value of A₂ suggests that the molecule can function well in aggressive environments, such as acids with chloride ions, Cl. To analyze the number of electrons that an inhibitor molecule can transfer, it can be related to the fraction of charge transferred (ΔN). Where ΔN A₁, which is higher than A₂, indicates that the reactivity and the number of electrons transferred/donated are greater on the metal surface, resulting in better effectiveness of the molecule’s protection in inhibiting corrosion reactions [75]. The low dipole moment also indicates the compound’s ability to accumulate on metal surfaces easily [76]. Then, electrophilic and nucleophilic parameters are highly desirable because they can react with and neutralize electrophilic species that promote corrosion [62]. ESP shows points with the highest to lowest positive ESP. Where the nucleophilicity of compound A₂ is greater than A₁, this indicates conjugation and electron density distribution around the compound, leading to the neutralization of electrophilic and nucleophilic species that can cause corrosion [77]. From all the parameters of the quantum chemical calculation results for these two organic compounds, it can be explained that these compounds work well to inhibit the anodic and cathodic reactions that can damage the metal surface.

3.9. Monte carlo simulation

In understanding how the nature and processes of interaction of organic inhibitor compounds in solution on metal surfaces work, the use of MC simulation analysis is very appropriate, based on the involvement of optimal geometric energy. Thus, the 2 compounds that were configured in the validation test under optimal energy conditions can be seen in Figure 8. In both compounds, the top and side views show the compounds close to the metal substrate, with the adsorption orientation aligned and parallel to the surface, identical to the formation of a protective layer. This confirms that the configuration of compounds A₁ and A₂ near the Al (111) substrate can be adsorbed onto the metal structure and prevent electron exchange between the solution/substrate, which triggers the corrosion reaction [77].

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Figure 8.

Simulation on the Al (111) surface and 100 water molecules with compounds (a) A₁ dan (b) A₂.

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Table 5 shows the negative adsorption energy values for both compounds, indicating a boundary between physical and chemical adsorption and the addition of water molecules. The parallel orientation of the compounds on the Al (111) surface can also maximize physical and chemical bonding due to the contribution of p-electrons from the carbon rings and lone pairs from the oxygen atoms to the metal. Next, the attractive force of inhibitor molecules in the electrolyte solution to the surface is due to strong and weak bonds in certain regions of the molecule and the surface. In Table 5, where the adsorption energies of both compounds (-61.67 and -66.64 Kcal mol^-1) are higher than that of water (-33.83 Kcal mol^-1), this is likely because progressive substitution of water molecules by inhibitor molecules A₁ and A₂ occurs on the surface, leading to the formation of a protective film [47,77].

The negative adsorption energy values obtained (−61.67 and −66.64 kcal mol^-1) indicate spontaneous adsorption and strong interaction between the inhibitor molecules and Al(111) surface. The parallel orientation of A₁ and A₂ observed in simulation images suggests optimal molecular alignment for film formation. Although H⁺ and Cl⁻ ions were not explicitly included, their potential influence on adsorption stability is acknowledged, as discussed in Section 2.5. The consistency between MC and DFT results validates the predicted adsorption configuration and energy trends.

3.10. Inhibitor mechanism

Organic inhibitor compounds can be easily adsorbed onto metal surfaces due to their unique properties, thus forming a protective layer. This is influenced by the structure and charge of the inhibitor, the acidic/basic environmental conditions, and the characteristics of the metal surface itself. Several experimental and theoretical techniques have been proven, one of which is electrochemical testing. This can show the inhibitory results of aluminum through two different mechanisms, such as adsorption and the formation of a protective layer by orchid extract inhibitors. This adsorption stems from the general adsorption properties of inhibitors, such as physisorption and chemisorption. The high electron density in the aromatic/benzene ring can donate its π electrons to the empty p orbitals of aluminum, limiting the corrosion interaction of Cl and causing the metal surface to dissolve. Additionally, electrons can also be donated through aluminum because, in some conditions, parts of the molecule can be electron-deficient.

Therefore, the good performance of the orchid extract inhibitor is due to its chemical and physical adsorption properties on the metal surface, which are consistent with experimental and theoretical results indicating electron transfer and interaction between different charges. The presence of electrostatic attraction by the inhibitor molecules on the metal surface due to this charge difference provides the impetus for positively charged molecules to form a protective layer, thus preventing corrosion-causing interactions. The interaction of the inhibitor with synergistic Al ions also provides a better effect on the surface. The mechanism of the inhibitor with the Al surface is based on Figure 9, which illustrates how this inhibitor interacts.

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Figure 9.

Inhibitor mechanism.

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The combined experimental and theoretical results indicate that the inhibition mechanism involves both electrostatic (physisorption) and covalent-type (chemisorption) interactions. Hydroxyl and carbonyl groups act as electron donors to the metal surface, while aromatic π-systems facilitate physical adsorption through π–d orbital overlap. This dual mechanism results in the formation of a compact, adherent film that limits both anodic dissolution and cathodic hydrogen evolution. The overall mechanism illustrated in Figure 9 is therefore supported by convergent findings from FTIR, UV-Vis, DFT, and MC analyses.