The hydrochloric acid (HCl) 1 M solution was prepared by dilution of HCl 37% of analytical quality with bidistilled water. The material used in this study is 1 cm long aluminum rod. Firstly, the aluminum rod was mechanically abraded with abrasive papers in various grain sizes (from grade 150 to 800); rinsed with bidistilled water, degreased in acetone, washed once again with bidistilled water and dried before their immersion in experimental solution. Concentration range of inhibitor used in the tests was from around 10⁻⁶ to 10⁻³ mol/L.

The aggressive solution (1.0 M HCl) was prepared by dilution of analytical grade 37% HCl with double-distilled water. Corrosion inhibitor solution is prepared by dissolving the desired heterocyclic weight of inhibitor in 1 M HCl. Four different concentrations, namely, 10⁻³, 10⁻⁴, 10⁻⁵ and 10⁻⁶ mol/L by weight are used for the evaluation of corrosion inhibition. Structural formulae of the examined inhibitors are shown in Figure 1 .

The weight-loss of aluminum rod specimens, 1 cm in 1.0 M HCl, with and without the addition of inhibitors, was determined after 1 h period of immersion at a temperature of 25˚C in air atmosphere without bubbling. Triplicate experiments were performed in each case and the mean value of the weight-loss has been reported.

For convenience reasons, the calculations of isolated molecules were performed with the local Gaussian-type-orbital basis set and Gaussian 09 program especially because the evaluation of vertical ionization potentials and electron affinities involve calculations of charged species. All calculations were done with DFT at B3LYP level of theory which yields good descriptions of organic molecules. The electronic properties of these structures were studied based on the nature bond orbital analysis (NBO). The calculations of the local and global reactivity indicators of the such as the inhibitor molecules localization of frontier molecular orbitals, E_HOMO (Energy of the highest occupied molecular orbital), E_LUMO (Energy of the lowest occupied molecular orbital), ∆E_gap (Energy gap), I (ionization energy), A (electron affinity), χ (Absolute electronegativity), η (Global hardness), S (Global softness), ω (Global electrophilicity index), ∆N (Fraction of electrons transferred from single inhibitor molecule to metallic surface) and total energy (E_T) were used to explain the electron transfer mechanism between the neutral and protonated forms of inhibitor molecules (NCF & NCCM) and the aluminum surface in acid medium [37] [38] .

$I=-E_{\text{HOMO}}$(1)

$A=-E_{\text{LUMO}}$(2)

$\text{Δ}{E}_{L-H}=E_{\text{LUMO}}-E_{\text{HOMO}}$(3)

$\chi =\frac{I+A}{2}=\frac{1}{2}\left(E_{\text{HOMO}}+E_{\text{LUMO}}\right)$(4)

$\eta =\frac{I-A}{2}=-\frac{1}{2}\left(E_{\text{HOMO}}-E_{\text{LUMO}}\right)$(5)

$S=\frac{1}{\chi }=-2/\left(E_{\text{HOMO}}-E_{\text{LUMO}}\right)$(6)

$\omega \left(\frac{{\mu }^2}{2}\right)S=\frac{I+A}{8}$(7)

$\text{Δ}N=\frac{{\chi }_{Al}-{\chi }_{inh}}{2\left({\eta }_{Al}+{\eta }_{inh}\right)}$(8)

where ${\chi }^{Al}$ and ${\chi }^{inh}$ represent the absolute electronegativity of Al and the inhibitor molecule respectively, ${\eta }_{Al}$ and ${\eta }_{inh}$ represent the absolute hardness of Al and the inhibitor molecule, respectively. By assuming that for a metallic I = A; because they are softer than the neutral metallic atoms [39] ; theorical values for the electronegativity ${\chi }^{Al}=4.28\text{eV}$ and ${\eta }_{Al}=0$ the global hardness were hence used.

Weight-loss tests or Gravimetric tests were carried out to analyze the influence of concentration and temperature on corrosion rate and inhibition efficiency. Weight-losses are expressed in mg per hour per cm² of the mild surface area, which were determined in the absence and in the presence of the additives at 25˚C after 1 h of hold time immersion. The highest concentration is sometimes limited by the solubility of the compound. Higher concentrations were not tested even at two consecutive concentrations no further increase is observed in terms of inhibiting efficiency.

The corrosion rates (W), the degree of surface coverage (θ) and the inhibition efficiency IE (%) were calculated using the following expressions:

$W=\frac{m_0-m_1}{St}$ (9)

$\theta =\frac{W_0-W}{W_0}$ (10)

$\text{IE}\left(\%\right)=\frac{W_0-W}{W_0}\times 100$ (11)

-m₀ is the initial mass of the sample before testing.

-m₁ is the final mass of the sample after corrosion.

-S is the total surface area of the sample.

-t is the total corrosion time.

-W₀ is the corrosion rate in the blank solution.

-W is the corrosion rate in the solution containing MPIP.

Figure 2 analysis reveals that the corrosion rate (W) increases with temperature, while it decreases with higher concentrations of the inhibitor. These results highlight the strong dependence of corrosion kinetics on both the temperature of the corrosive medium and the MPIP concentration. Moreover, the data indicate that the thickness of the protective layer formed on the metal surface increases proportionally with the amount of MPIP present.

The protective film formed by the complexation of MPIP with the metal progressively deposits onto the aluminum surface, thereby mitigating its dissolution. However, the thickness of this film decreases with increasing temperature. These findings underscore the critical influence of temperature and concentration on the corrosion kinetics of aluminum, as well as on the adsorption behavior and inhibitory performance of MPIP at the metal interface, as reported by several studies [40] .

Figure 3 illustrates the combined influence of temperature (T) and inhibitor concentration (C_inh) on the inhibition efficiency IE (%) of MPIP. Analysis of the data reveals that the increase in inhibition efficiency with rising MPIP concentration is attributed to the adsorption of the inhibitor onto the aluminum surface. This adsorption is facilitated by the availability of non-bonding electron pairs on heteroatoms (N and O) and the π-electrons of the aromatic rings, enabling chemisorptive interactions with the metal surface. Such interactions result in the formation of a dense and stable adsorbed film, which acts as a physical barrier that limits the access of aggressive species to the metal surface, thereby reducing the anodic dissolution process.

However, a decrease in inhibition efficiency is observed with increasing temperature. This behavior is ascribed to the thermal desorption of MPIP, which occurs due to increased thermal agitation and reduced stability of the inhibitor–metal interactions. The acceleration of desorption at elevated temperatures compromises the integrity of the protective film, thereby exposing the metal surface to corrosive attack. This temperature-dependent behavior aligns with findings reported by [41] [42] , and suggests a predominantly physico-chemical adsorption mechanism that is sensitive to the thermdynamic conditions of the system.

A thermodynamic study was carried out to determine the nature and mechanism of adsorption of the inhibitor on the aluminum surface.

It is widely acknowledged that adsorption isotherms provide useful insights into the mechanism of corrosion inhibition. In order to obtain the isotherm type model, one supposes that inhibitor acts via a simple adsorption mode. Thus, the apparent corrosion rate of the inhibited aluminum sample is proportional to the ratio of the surface covered $\theta$ and that not covered $1-\theta$ by the inhibitor. Fractional coverage values $\theta$ have been evaluated for different concentrations of the compound under study from corrosion rates in uninhibited and inhibited solutions by means of Equation (2).

Five isotherm models have been explored to describe phenomena at the metal/solution interface. The aim of this approach is to assess the compound’s adsorption capacity, elucidate the inhibition mechanism involved and optimize the process. So we have retained Langmuir, Temkin, Frumkin, El-Awady and Adejo-Ekwenchi isotherms [43] - [47] . The Equations that define these isotherms are expressed in Table 1 .

$C_{inh}$ is MPIP’s concentration; $K_{ads}$ is the equilibrium constant of the adsorption process; $f$ is a factor energetic inhomogeneity in the surface; $\theta$ is surface coverage; $K_{ads}={K^{\prime }}^{1/y}$; $1/y$ is active sites occupied by an inhibitor molecule; $K_{AE},b$ are d’Adejo-Ekwenchi isotherm parameters.

Figures 4-8 illustrate the best fit obtained from the plot of fractional surface coverage $\theta$ versus $C_{inh}$. All the tested isotherms yield straight lines as shown in Figures 4-8 . Table 2 gives the different parameters of studied isotherms.

Langmuir isotherm exhibited the highest determination coefficients (values close to 1), indicating excellent agreement with the experimental results. This model assumes monolayer adsorption on a homogeneous surface without interactions between adsorbed molecules. The slopes of the Langmuir plots were close to unity, confirming the applicability of this model and suggesting that the MPIP molecules adsorb uniformly across the aluminum surface, with negligible lateral interactions [48] .

Temkin isotherm accounted for weak interactions between adsorbed species and suggested slight heterogeneity in adsorption energies. Although its fit was slightly less accurate than Langmuir’s, it remains relevant, especially at higher inhibitor concentrations [49] .

While El-Awady model proposed heterogeneous adsorption sites and potential molecular interactions. Its R² values were close to unity but still lower than those of the Langmuir model, indicating minor surface heterogeneity or intermolecular effects [46] .

The Frumkin isotherm, an extension of the Langmuir model, incorporates attractive or repulsive interactions between adsorbed molecules. The model showed a good fit (R² ≈ 1) and suggested the presence of weak electrostatic forces. This supports the idea of physical adsorption contributing to the inhibitory process [50] .

Finally, the Adejo-Ekwenchi isotherm also fitted the experimental data well. The decreasing trend of the slope parameter b with increasing temperature, as observed in Table 2 , points toward a physisorption mechanism. According to [42] [48] [51] , such behavior is characteristic of van der Waals-type interactions, which do not involve the formation of chemical bonds and are easily reversed at elevated temperatures.

Overall, the Langmuir model remains the most suitable under the tested conditions. However, the inclusion of other isotherms provides complementary insight into the nature of the adsorption process, including molecular interactions, surface heterogeneity, and thermally sensitive physical adsorption mechanisms.

The study of adsorption isotherms serves as a fundamental tool for elucidating the molecular-level mechanisms governing corrosion inhibition. A better understanding of the adsorption behaviour of an inhibitor can be achieved by investigating the thermodynamics of the adsorption process. Thus, the thermodynamic properties are based on the analysis of adsorption thermodynamic parameters. The standard free energy of adsorption ( $\Delta G_{ads}^0$) is calculated using the following Equation [50] :

$\Delta G_{ads}^0=-RT\ln \left(55.5K_{ads}\right)$(17)

where:

Indeed, the adsorption of MPIP on the aluminum surface follows the Langmuir isotherm model most appropriately.

Furthermore, the standard enthalpy change of adsorption ( $\Delta H_{ads}^0$) and the standard entropy change of adsorption ( $\Delta S_{ads}^0$) are determined from the Gibbs-Helmholtz Equation:

$\Delta G_{ads}^0=\Delta H_{ads}^0-T\Delta S_{ads}^0$(18)

In fact, plotting $\Delta G_{ads}^0$ as a function of temperature ( Figure 9 ) allows for the determination of the thermodynamic quantities $\Delta H_{ads}^0$ and $\Delta S_{ads}^0$. The corresponding values derived from this analysis are summarized in Table 3 .

The negative values of the standard Gibbs free energy of adsorption ( $\Delta G_{ads}^0$) indicate that the adsorption of MPIP onto the aluminum surface is a spontaneous process, accompanied by the formation of a stable adsorbed layer [52] . The measured values, ranging from −35 to −37 kJ·mol⁻¹, suggest that a mixed adsorption mode, so the mechanism likely involves both physisorption and some degree of chemisorption [51] .

Furthermore, the standard enthalpy change of adsorption ( $\Delta H_{ads}^0$) is negative, confirming that the process is exothermic [40] . This exothermic nature favors physisorption, although it is often associated with a decrease in inhibition efficiency at elevated temperatures.

Finally, the positive value of the standard entropy change of adsorption ( $\Delta S_{ads}^0$) reflects an increase in disorder at the metal/solution interface during MPIP adsorption. This increase is primarily attributed to the displacement of water molecules initially adsorbed on the aluminum surface, which are replaced by inhibitor molecules [30] - [33] [40] .

The effect of temperature on corrosion and its inhibition process for aluminum in 1 M HCl in absence and presence of different concentrations of MPIP at different temperatures ranging from 298 K to 323 K was evaluated. The dependence of corrosion rate on the temperature can be regarded as an Arrhenius-type process, the rate of which is given by Arrhenius:

$\log W=\log A-\frac{E_a}{2.3RT}$(19)

where W is the corrosion rate in the presence of inhibitor, E_a the apparent activation energy, R the universal gas constant, A the frequency factor.

Corrosion rates were used to additionally achieve data on the apparent activation enthalpy ( $\Delta H_a^{*}$) and activation entropy ( $\Delta S_a^{*}$) changes for the formation of the activated complex in the transition state using the relation (20):

$\log \left(\frac{W}{T}\right)=\log \left(\frac{R}{\aleph h}\right)+\frac{\Delta S_a^{*}}{2.303R}-\frac{\Delta H_a^{*}}{2.303RT}$ (20)

where h is Planck’s constant and N is Avogadro number.

The plots of logW and logW/T versus 1/T for aluminum in corrosive environment in the absence and in the presence of various concentrations of the MPIP inhibtor are given by Figure 10 and Figure 11 .

The slopes $\frac{E_a}{2.3RT}$ and $\frac{\Delta H_a^{*}}{2.303RT}$ of the straight lines were used to evaluate (E_a) and ( $\Delta H_a^{*}$) , whereas the intercept $\log \left(\frac{R}{\aleph h}\right)+\frac{\Delta S_a^{*}}{2.303R}$ was used to evaluate ( $\Delta S_a^{*}$). All the results are presented in Table 4 .

The value of activation energy (E_a) for hydrochloric acid solution without MPIP is 98.52 kJ/mol, while in the presence of MPIP as a corrosion inhibitor is 111.54 kJ/mol. The high activation energy value of MPIP is often interpreted as suggesting the formation of a physical/electrostatic adsorption film [48] .

The positive $\Delta H_a^{*}$ value refers to an endothermic dissolution process for surface of aluminum at room temperature in the presence of MPIP molecules [53] . $\Delta H_a^{*}$ increased with a temperature increase, which supports the endothermic nature of aluminum surface dissolution and the decrease of the inhibition performance of MPIP molecules. The apparent entropy activation ( $\Delta S_a^{*}$) values at room temperature

were positive with increasing temperature. In the absence of MPIP molecules, the apparent entropy activation values were also positive at higher concentrations of MPIP. ( $\Delta S_a^{*}$) with considerable and positive values implies that the activated complex in the rate-determining step means a dissociation rather than an association stage, suggesting that an increase in disordering takes place on proceeding from reactants to the activated complex [54] .

Quantum chemical principles have been widely used to study corrosion inhibition including structure optimization calculations, semi-empirical, ab initio and DFT calculations. To corroborate the experimental results and the electronic parameters which are very important to explain the molecule reactivity, a computational analysis based on density functional theory (DFT) was carried out.

The quantum chemical parameters for the neutral form ( Figure 12 ) of the investigated inhibitor calculated by DFT/B3LYP with the 6-31G (d, p) are listed in Table 5 . The values of the frontier molecular orbital energies (i.e. energy of the highest occupied molecular orbital (E_HOMO), energy of the lowest unoccupied molecular orbital (E_LUMO), the energy gap (ΔE_L–H), total energy (E_T), Hardness (η), Electrophilicity (ω), Ionization Potential (I), and dipole moment (μ) were also calculated [46] .

Generally, the energy gap of a molecule is a quantum chemical parameter that indicates hardness or softness of molecular species. Hard molecules are characterized with larger value of energy gap and are less reactive than soft molecules, which are characterize by small energy gap [55] . The quantum chemical parameters of MPIP provide consistent evidence for its strong inhibitory performance on aluminum. The relatively high HOMO energy indicates that MPIP can readily donate electrons to the metal surface, while its low LUMO energy suggests a simultaneous ability to accept electrons, thereby favoring donor–acceptor interactions [42] [56] - [58] . This dual behavior, combined with the small energy gap (ΔE), reflects high chemical reactivity and facilitates electron transfer during adsorption. The moderate dipole moment (μ = 1.8126 D) confirms sufficient molecular polarity to promote surface interactions. In addition, the high softness (σ) and low hardness (η) values classify MPIP as a “soft” molecule, further enhancing its adsorption affinity. The elevated electrophilicity index (ω = 3.6942) highlights its strong electron-accepting tendency, particularly toward reactive species such as H⁺ in acidic media, supporting the formation of a protective barrier layer on the aluminum surface. Moreover, the lower electronegativity of MPIP (χ_inh = 3.7542 eV) compared to aluminum (χ_Al = 4.28 eV), along with the positive fraction of electrons transferred (ΔN > 0), confirms electron migration from the metal to the inhibitor [58] . Taken together, these descriptors underline the capability of MPIP to strongly adsorb onto aluminum through synergistic donor–acceptor and electrostatic interactions, thereby ensuring effective corrosion inhibition.

Reactive sites correspond to molecular regions that are most likely to donate or accept electrons in interactions with the metal. In the case of MPIP, the molecule tends to accept electrons through the LUMO and to donate electrons via the HOMO ( Figure 13 ). The identification of these sites is based on the analysis of Fukui functions ( $f_k^{+}$, $f_k^{-}$) and the dual descriptor (Δf_k (r)) [59] . According to the literature, the atom with the highest values of $f_k^{+}$ and Δf_k (r) is considered the most favorable site for nucleophilic attack, generally associated with the LUMO of MPIP. Conversely, the atom displaying the highest $f_k^{-}$ value and the lowest Δf_k (r) is the predominant site for electrophilic attack, which is linked to the HOMO [57] [60] .

The Fukui function analysis in Table 6 provides further insight into the reactive sites of MPIP and their role in its adsorption on aluminum. Carbon C (1) was identified as the most favorable site for electrophilic attack, while oxygen O (37) emerged as the primary nucleophilic center.

These findings indicate that electron transfer to the metal occurs predominantly through C (1), whereas O (37) facilitates nucleophilic interactions by generating local electron deficiencies ( Figure 14 ). Such donor–acceptor exchanges reinforce the inhibitor–metal bonding and contribute to the formation of a stable Al–MPIP protective layer. This electronic distribution strongly supports the adsorption capacity of MPIP and its effectiveness in preventing aluminum dissolution in aggressive media, in agreement with previous studies [57] [60] .