Electrochemical and computational estimations of cephalosporin drugs as eco-friendly and efficient corrosion inhibitors for aluminum in alkaline solution

In this study, the anionic state of Ceftriaxone sodium (Cefx) and Ceftazidime (Cefz) medication corrosion inhibition capabilities for Al in 0.1 M NaOH solution are explored using various electrochemical analyses. Furthermore, the morphological structure and surface chemical composition of the impact of these drugs on the Al substrate in NaOH are investigated. For the prediction and analysis of interactions between molecule structure and inhibition efficiency, quantum chemical calculations (QC), Monte Carlo simulations (MC), and molecular dynamics (MD) simulations (MD) are performed. The electrochemical findings reveal that the inhibitory effectiveness increases with increasing drug concentrations and declines with rising temperature, reaching a maximum value of 78.4% for 300 ppm Cefx while 59.5% for 300 ppm Cefz at 293 K, implying that Cefx outperforms for Cefz. In addition, the studied drugs act as cathodic inhibitors, and their adsorption is spontaneous and mixed type adsorption in its nature that obeys Freundlich isotherm for Cefz while Temkin isotherm is the best-fitted one for Cefx. Surface analysis and wettability measurements imply that Cefx and Cefz shield the Al against corrosion by surface adsorption and generating a protective hydrophobic film. Thermodynamic activation parameters in the absence and presence of 300 ppm of the studied drugs are calculated and discussed. The energies of the border molecular orbitals and computed molecular parameters for the investigated drugs revealed that anionic Cefx is more readily adsorbed on the Al surface than Cefz. This finding is validated further using MC and MD simulations. Overall, the proposed cephalosporin drugs delivered a cost-effective and facile approach for boosting the efficiency of corrosion inhibitors for Al under aggressive conditions.

Electrochemical. Electrochemical  www.nature.com/scientificreports/ as working, Ag/AgCl and Pt wire as reference and counter electrodes, respectively. The obtained electrochemical results were recorded and analyzed using OrigaMaster-5 software. The potentiodynamic polarization curves are measured at a fixed potential of ± 100 mV around E OCP at 293 K with a fixed sweep rate of 1 mV s −1 . The electrochemical impedance spectroscopy (EIS) tests are performed at E OCP using 10 mV amplitude, and the frequency range was 10 kHz to 100 mHz. The ZSimpWin software was utilized to fit the obtained results.
Surface characterization. The morphological characteristics of Al substrates (1 cm × 1 cm × 0.1 cm) in the absence and existence of the examined drugs are investigated using FESEM (Zeiss Sigma 500 VP Analytical FE-SEM, Germany). In addition, the elemental composition of the surface is analyzed using XPS and elemental mappings coupled with the FESEM. XPS analyses are performed using (K-ALPHA, Thermo Fisher, USA). Al samples are polished, cleaned with acetone, rinsed with distilled water, and dried before being submerged in pure 0.1 M NaOH media and the presence of 300 ppm of each drug for 2 h at 293 K.
Water contact angle measurements. The impact of Cefx and Cefz drugs on the surface wettability characteristics of Al surfaces are examined using the water contact angle (WCA) analysis. The Goniometer (model 250, Ramé-Hart, USA) was employed to estimate a sessile drop's static contact angle. Typically, 5-8 μL of aqueous solutions are dropped onto a polished and balanced Al surface using a microsyringe at room temperature. The resulted images are processed using DROP-image software to evaluate the contact angle (θ).

Computational methodology. Hartree Fock calculations (HF). Quantum chemical calculations (QC)
depending on HF are carried out utilizing Gaussian 09 program 34 . The input files of the studied drugs are prepared with GaussView6.0. 16. The structures of Cefx and Cefz are optimized by 6-311G++ (d, p) basis set. The energies of the boundary molecular orbitals (HOMO and LUMO) for the tested drugs are acquired and utilized to compute additional molecular properties such as ionization energy (I), global hardness (η), electron affinity (A), electronegativity (χ), and softness (σ) using the following equations 35,36 : The value of χ and η can be used to compute the proportion of electrons transported (ΔN) from the inhibitor molecule to the Al surface using Eq. (6) 35,36 . where χ Al = 3.23 eV/mol and η Al = 0 eV/mol, based on Pearson's electronegativity scale, supposing the I = A for the Al bulk structure owing to they are softer than neutral Al atoms 37 .
Monte Carlo simulation (MC). MC simulation of the configurational space of a single molecule of the Cefx and Cefz with Al (111) surface is employed to determine low-energy adsorption sites as the temperature gradually reduces. The simulated annealing process used the Metropolis method. The MC simulation is carried out by the Adsorption Locator module that existed in the BIOVIA Materials Studio 2017 package. A simulation box (28.63 Å × 28.63 Å × 37.01 Å) with a 35 Å-thick vacuum slab above the surface is created from a supper cell of (5 × 5) using a unit cell of (4.05 Å × 4.05 Å × 4.05 Å) Al with a cell formula of Al4. The simulations are carried out using the COMPASS forcefield and its charges are used. The summation method for the electrostatic interaction is Ewald 38 . For the van der Waals interactions, a cut-off distance of 15.5 Å is used with a cubic spline switching function, and the summing approach is atom-based. (MD). The MD simulations are conducted employing the FORCITE module implemented in the BIOVIA Materials Studio. Because corrosion occurs in aqueous solution, the lowest-energy structures of the inhibitor-Al complexes derived from the MC simulation are filled with water (950 molecules) to model the influence of solvent. The forcefield and its charges, as well as the summation method, are the same as in the MC simulations. The MD simulations are run at 298 K (controlled by the Nose thermostat) utilizing a canonical ensemble (NVT) with a time step of 1.0 fs, and a simulation time of 1000 ps (1 × 10 6 steps).

Results and discussions
Open circuit potential (E OCP ) measurements. The change in the E OCP values of the Al electrode with immersion time in the pure 0.1 M NaOH media and existence of different concentrations of Cefx and Cefz is studied to emphasize the electrochemical properties in alkaline solutions at 293 K. Figure 2 depicts the E OCP of the Al substrate as a function of the immersion duration in various concentrations of Cefx (Fig. 2a) and Cefz (Fig. 2b) ranged from 0 to 300 ppm in 0.1 M NaOH solution. The detailed E OCP values, on the other hand, are listed in Table 1. As represented in the Fig. 2 that the E OCP value in a blank solution shows a positive shift at the . Then, when the immersion period is increased, it enters a stable state. The addition of Cefx and Cefz causes the E OCP to move negatively compared to the blank solution, suggesting that the cathodic corrosion process is significantly slowed.
The positive shift in the beginning of the curve for the inhibited solutions occurs till 200 s period indicating that the studied drugs cover the Al surface and hence form protective film on Al surface 40,41 . The inhibition of the cathodic reaction causes accumulation of the electrons generated from the anodic reaction, leading to more negative potential values for inhibited solutions compared to blank solution 40 .
The E OCP are also analyzed at 303, 313 and 323 K in the absence and existence of 300 ppm of the investigated drugs and their values are listed in Table 2. It is clearly shown from Fig. 3a-c that the positive shift in the beginning of the curves at high temperature is less than that at 293 K, indicating that the time taken for the formation of passive layer before reaching the steady-state value declines and therefore the inhibition efficiency decreases. In addition, the E OCP curves at high temperatures are shifted to positive direction compared to that at 293 K either for blank or both studied drugs. Figure 3d represents the OCP curves of blank and 300 ppm of the studied drugs at 323 k. The figure indicates that the Curves in presence of both Cefx and Cefz drugs are remained in negative direction relative to blank one at the same temperature indicating that increasing the temperature doesn't alter the mechanism of inhibition or type of used inhibitor.
Potentiodynamic polarization (PDP) measurements and the mechanism. PDP tests are performed on Al in 0.1 M NaOH medium by automatically altering the potential ± 100 mV against its E OCP value in  where i o corr and i corr are the corrosion current densities for blank and inhibited solutions, respectively. Figure 4 represents the PDP profiles of the Al in the pure 0.1 M NaOH and existence of different concentrations of Cefx (Fig. 4a) and Cefz (Fig. 4b). The results demonstrate that the greater value of I corr and the more positive value of the E corr (212.29 µA/cm 2 , − 1463.7 mV vs. Ag/AgCl) are observed in the inhibitor-free solution compared to (45.78 µA/cm 2 , − 1533.4 mV vs. Ag/AgCl) and (85.99 µA/cm 2 , − 1512.2 mV vs. Ag/AgCl) in the presence of 300 ppm from Cefx and Cefz, respectively. Furthermore, the values of E corr for all studied concentrations of both drugs are slightly shifted to negative direction compared to inhibitor-free solution, this means that the used drugs are cathodic ones, that mainly inhibit the cathodic reduction reaction of H 2 O through the retardation of the cathodic polarization and decreasing the corrosion current density without any effect on the anodic polarization 44 . The multistep electrochemical process of anodic dissolution of Al in the alkaline solution has been summarized as follows 40,45 :  On the surface of aluminum, the cathodic process includes the reduction of water molecules 45,46 .   www.nature.com/scientificreports/ maximum inhibition efficiency (78.4%) is detected at 300 ppm of Cefx, concerning 59.5% at 300 ppm of Cefz. The analyses show that increasing the concentrations of Cefx and Cefz improves the Al surface coverage and hence the inhibition efficiency in the sequence Cefx > Cefz. Due to their adsorption via their hetero-active sites on the Al surface, these drugs produce a barrier protective film that inhibits the active sites for reduction reactions 47 . PDP measurements are also accomplished at 303,313 and 323 K in the absence and presence 300 ppm of Cefx and Cefz drugs. The electrochemical polarization parameters such as i corr , E corr , βc and βa are given in Table 2. Figure 6 represents PDP of the Al in pure 0.1 M NaOH and presence of 300 ppm of Cefx and Cefz drugs at various temperatures from 293 to 323 K. As demonstrated in the Fig. 6, the E corr at high temperatures is shifted to positive direction relative to that at 293 K which implies that the studied drugs act as cathodic inhibitors even at high temperatures. Increasing the temperatures results in the desorption of Cefx and Cefz anions from Al surface so the inhibition of cathodic reaction decreases, accumulation of electrons at the anode reduces and thus the potential moves to positive direction. Table 2 strongly suggests that raising the temperature increases the corrosion current (i corr ), lowering the inhibitory efficiency of both Cefx and Cefz drugs.
Electrochemical impedance spectroscopy (EIS) measurements. Aluminum/electrolyte interface relationship during the corrosion process is evaluated using EIS technique at different concentrations of both Cefx and Cefz at E OCP . Figure 7a depicts the Nyquist plots of Al in blank 0.1 M NaOH media and existence of 300 ppm of both Cefx and Cefz at 293 K. The impedance profile displays the presence of two capacitive loops at high and low-frequency zones, which are ascribed to charge transfer reactions through the Al/electrolyte interface, as well as the presence of one inductive loop at the intermediate frequency region for the adsorption of the corrosion product intermediates on Al surface 46,48 . However, in the presence of both drugs, the capacitive loops' width grows without altering the impedance diagrams' forms, indicating that the corrosion process is unchanged in the absence and presence of the examined drugs 49 .This observation agrees well with PDP measurements which can be indicated from the parallel cathodic polarization curves 44 . At high frequencies, the large capacitive semicircle is referred to as the redox Al to Al + reaction (Eq. 8) which is the rate-determining phase in the charge transfer reaction throughout the corrosion process 48,49 . At low frequencies, the second capacitive www.nature.com/scientificreports/ semicircle can be attributed to the fast-complementary redox Al + to Al 3+ reactions (Eqs. 9 and 10). Meanwhile, the inductive loop in the middle frequencies is assigned to the presence of the adsorbed Al(OH) 1-3 intermediates on Al surface during its dissolution 48,49 . The corresponding phase angle and Bode impedance magnitude plots for Al in pristine 0.1 M NaOH solution and existence of 300 ppm of the Cefx and Cefz at 293 K are depicted in Fig. 7b. The bode graphs indicate the presence of two-time constants for charge transfer events during Al dissolution. Furthermore, the area under the curves is ordered as follows: Cefx > Cefz > blank. This discovery may be illustrated by the adsorption of the examined medicines on the Al surface, which protects it against corrosion. Furthermore, the higher area in the presence of Cefx compared to Cefz suggesting the Al surface coverage in case of Cefx is higher than that of Cefz, consequently its higher inhibition efficiency. Figure 7c depicts a model of the experimental data fitted to an electrochemical equivalent circuit, which is consistent with recent work by Yong Liu et al. 49 .
According to equivalent circuit measurements, the electrochemical parameters including, resistance to charge transfer (R ct1 , R ct2 ), bulk resistance (R s ), and double-layer (C dl ) in pure NaOH electrolyte and in existence of varying concentrations of Cefx and Cefz are summarized in Table 3. Data show that the presence of Cefx and Cefz changes the structure of the Al electrolyte interface compared to inhibitor free solution. This behaviour is resulted from increasing R s , R ct1 and R ct2 values with decreasing C dl and CPE values in the presence of the studied drugs compared to the inhibitor free solution. These findings can be explained to the replacement of water molecules by the adsorbed barrier protective insulating layers from Cefx and Cefz on Al surface, which decreasing the local dielectric constant and/or increasing the thickness of the adsorbed barrier protective layer, that impedes the charge transfer reaction through Al/electrolyte interface 47 . The inhibition efficiencies (η%) are calculated from total charge transfer resistance R T (R T = R ct1 + R ct2 ) and listed in Table 3 using the following equation 48 : where θ is the surface coverage, R T o and R T are total charge transfer resistances in the absence and presence of the studied drugs, respectively. Data show that, the values of η% increase with increasing the concentrations of both drugs in the following order: Cefz < Cefx and reach the maximum (69.6%) at 300 ppm for Cefx which agreed well with PDP measurements.
EIS measurements are also performed at 303,313 and 323 K in the absence and presence 300 ppm of Cefx and Cefz drugs at E OCP . Data obtained at 303 K are fitted to the same equivalent circuit which fits 293 K data (14) η% = θ × 100 40  www.nature.com/scientificreports/ (assume EC 1 ) while this EC 1 doesn't fit the data obtained at 313 and 323 K (assume EC 2 ), therefore C dl in EC 1 is replaced by CPE in EC 2 . The electrochemical parameters derived from EC 2 (Fig. S1), including constant phase element (CPE1 and CPE2), charge transfer resistances (R ct1 , R ct2 ), solution resistance (R s ), calculated R T and η% are given in Table 4. It is clearly shown from Fig. 8 that the shape of high frequency capacitive loop changes which is proved by the replacement of the C dl in EC 1 (Fig. 7c) for Al samples at 293 and 303 K by CPE in EC 2 (Fig. S1a) at 313 and 323 K which is elucidated to the desorption of Cefx and Cefz anions from Al surface at high temperatures and thus increase the corrosion rate and consequently the inhomogeneity of Al surface. It is clear from Table 4 that the addition of the studied drugs to sodium hydroxide solution at any given temperature rises the value of R T but reduces the value of C dl compared to blank solution at the same temperature. The decrease in R T values of Cefx and Cefz drugs with increasing the temperature confirms the desorption of the studied drugs from Al surface, decreasing the Al surface coverage with these drugs, increasing the corrosion rate and therefore decrease the inhibition efficiencies. Bode plots are shown in Fig. S1b-d. It is clearly seen that the impedance (Z) of Al in the absence and presence of Cefx and Cefz drugs at 293 and 303 K is more at lower frequency and it declines small with a rise in frequency and then rises again up to a precise frequency after this frequency, the impedance decreases steeply signifying the presence of two time constants while at 313 and 323 K the change in the impedance values before the sharp decrease is not clear due to the change in the shape of Nyquist plots. Moreover, the impedance of all samples decreases with the increase in temperature at low frequency region. Furthermore, the inhibition efficiency of Cefx and Cefz are compared to those of other organic compounds described in the literature as inhibitors for aluminum and its alloys and summarized in Table 5 [50][51][52][53][54][55] . According to Table 5, the proposed drugs have close efficiency or relativity higher than some expired, extracted, synthetic drugs recently reported.   Fig. 9. Furthermore, the homogeneity of Al  www.nature.com/scientificreports/ surface in the existence of Cefx is greater than in the presence of Cefz, indicating that Cefx is a more effective inhibitor than Cefz. The improvement of the homogeneity of the Al surface is attributed to the adsorption of these medicines on the Al surface, resulting in the formation of a protective film that shields the metal from the aggressive NaOH solution 5 . Furthermore, elemental mappings are taken to confirm the distribution and existence of different elements on Al surface after immersion in pure 0.1 M NaOH for 2 h at 293 K in and presence of 300 ppm Cefx and Cefz. Figure 9 reveals the presence of oxygen in the absence of the studied drugs, indicates the presence of Al oxide/ hydroxide due to the aggressive attack by OH − ions. While the existence of C, N and S elements following immersion in solutions containing Cefx and Cefz confirmed drug adsorption on the Al surface, the production of the protective adsorbed films that suppressed the aggressive attack between Al surface and OH − ions.  Table 6 explores the survey spectra inspects the surface composition of Al substrates subjected to the pure NaOH solution and presence of 300 ppm of the examined Cefx and Cefz. The pristine sample has a 24.36 at.% Al and 39.75 at.% O related to the presence of Al oxide/hydroxide. The adsorption of Cefx and Cefz is confirmed by decreasing Al and O contents with increasing the C content, as well as the appearance of new peaks for N and S. High-resolution XPS spectra in the binding energy (BE) range of the Al 2p, C 1s, N 1s, S 2p, and O 1s peaks are presented in Figs. 10, S2, S3, and S4. The deconvolution of the Al 2p (Fig. 10a-c) included two peaks at 73.1 eV and 75 eV for blank, Cefx and Cefz, which are assigned to the Al metal and Al-O respectively 50,56 . The intensity of the Al 2p peak falls somewhat in the presence of the examined drugs as they form a layer on the Al substrate, as shown by the presence of N 1s, S 2p, and an increase in the carbon content on the Al surface. The resolved C 1s spectra of Al for blank solution is deconvoluted into two peaks as shown in Fig. 10d. The first peak at 285.2 eV signified to C-C/C-O/C-H, while the second peak at 289 eV attributed to O-C=O group 57 . The addition of the studied drugs leads to the deconvolution of C 1s peak into five peaks, as shown in Fig. 10e,f. The peak at 284.6 eV was attributed to C-C/C=C/C-H bonds that may have originated from the examined medications, but also to adventitious carbon adsorbed on the Al surface as a result of air exposure 58 . The peaks at 285.5 eV and 286.3 eV are attributed to C-O and C-N/C-S/C=N 59,60 . The two remaining peaks, at 287.5 eV and 289.1 eV, directly correlate to carbonyl and carboxyl functional groups respectively 58 . The XPS spectrum related to the O1s signal is shown as supplementary Fig. S2. The oxygen O1s peak is deconvoluted into two peak singlets. The predominant component at 532.7 eV is ascribed to surface hydroxyl groups in blank samples and oxygen in carbonyl or ester groups inside the adsorbed layer in inhibited solutions while the peak at 531.5 eV is assigned to C-O/Al-O-C 60 . The XPS spectrum related to N 1s (Fig. S3) shows five characteristic peaks at 398.4, 399.5, 400.4, 401.2 eV and 402 eV, corresponding to the C-N, Al-N, C=N, N-C=O and N-N/N-O bonds respectively 61,62 . The S 2p peak of Cefx is deconvoluted into four peaks at binding energies of 162.1 eV, 163.3 eV, 164. eV, and 164.2 eV, assigned to endocyclic (2p3/2, 2p1/2) for thiazine ring, exocyclic C-S-C bond, and C-S-C of thiazole ring, respectively. On the other hand, The figure (Fig. S4) presents the S 2p peak of Cefz is deconvoluted into three peaks due to the absence of an exocyclic S atom [63][64][65][66] . The appearance of spectra of N 1s and S 2p for inhibited Al samples reveals that Cefx and Cefz can adsorb on Al surface and increase its corrosion resistance. The atomic percent of elements (Table 6) indicate that the adsorption of the examined medicines on the Al surface occurs in the following order: Cefx > Cefz.   Fig. 11. The hydrophobicity nature of Al surface in the presence of the studied drugs can be correlated to the adsorption of the hydrophobic groups in their molecular structures. Therefore, it decreases the adhesion contact with Al surface and enhances its corrosion resistance [67][68][69][70][71] . This increase in hydrophobicity is consistent with the predicted inhibitory efficiencies, which are listed in the following order: Cefx > Cefz.
Adsorption isotherms. Adsorption isotherms give significant information regarding the interaction between the Al/electrolyte interface and the researched medicines, depending on the charges dispersed throughout their chemical structures 72 . As depicted in Fig. 12, various isotherms are investigated using experimental data from EIS, and PDP measurements at 293 K. Temkin adsorption isotherm is found to be the best fit one for Cefx while Freundlich adsorption isotherm fits Cefz well as 73,74 . Table 7 shows the computed values of K ads and ΔG o ads for Cefx and Cefz based on the best fitting adsorption isotherms. The results indicate that the adsorption of Cefx and Cefz on the Al surface is spontaneous and mixed, with physisorption dominating both drugs. This is due to the electrostatic interaction between the charged Al surface and charged drug molecules which is characterized by the values of ΔG o ads 72,75 . The greater K ads and more negative ΔG o ads values for Cefx explain the drug's www.nature.com/scientificreports/ intense and impulsive adsorption on the Al surface compared to Cefz, resulting in a better protection efficiency that fits with earlier findings. The higher adsorption of Cefx can be explained to the presence of dioxo-methyltriazin-sulfanyl moiety in its structure which is confirmed by theoretical calculations.
Thermodynamic activation parameters. Thermodynamic activation parameters are determined and summarised in Table 2 for the pure electrolyte, and the existence of 300 ppm of Cefx and Cefz are calculated and tabulated in Table 2 using Arrhenius and transition state equations as follows 76 : where A is the Arrhenius constant, R represents the universal gas constant, T donates the absolute temperature, n is Avogadro's number and h refers to plank's constant. ΔH* and ΔS* are the dissolution process's activation enthalpy and entropy. The corrosion rate (CR) is directly proportional to the corrosion current density (I corr ), and E a denotes the activation energy. Figure 13a represents the linear relationships of Ln (I corr ) with 1/T for Al in pure 0.1 M NaOH and presence of 300 ppm of Cefx and Cefz. The values of E a are calculated and listed in Table 2. Data show that the values of E a in the presence of studied drugs are higher than that in blank solution which indicate that rate of Al dissolution decreases in presence of Cefx and Cefz. Moreover, the values of E a increase in the following order Cefz < Cefx which indicate that the rate of Al dissolution in the presence of Cefx is lower than that in the presence of Cefz and hence Cefx is more efficient inhibitor than Cefz. Figure 13b represents the relation between Ln (I corr /T) and 1/T, from which the values of ΔH* and ΔS* are calculated and summarized in Table 2. The obtained results display that ΔH* values are positive indicating the endothermic nature of Al dissolution process. Higher values of ΔH* in the presence of Cefx and Cefz compared to inhibited free solution enhance the corrosion resistance of Al in 0.1 M NaOH 76 , as a result of their adsorption on its surface forming an energy barrier which is higher for Cefx in comparison with Cefz. Furthermore, negative values of ΔS* suggest that association rather than dissociation is the rate-determining step to produce activated complex, resulting in a reduction in disturbance 76 . Furthermore, the higher ΔS* values in the presence of Cefx and Cefz may be ascribed to an increase in randomness at the Al/solution interface after their adsorption, which can be      83 . The inhibitory efficacy of inhibitors rises as ΔN values increase owing to an increase in electron donating between the examined medicines and the Al surface. It is clear from Fig. 14 that HOMO of the studied drugs is focused on oxygen, nitrogen, and Sulphur of cephalosporin group for Cefz and dioxo-methyl-triazine sulfanyl group for Cefx. The higher inhibition efficiency of Cefx compared to Cefz is correlated to its higher values of E HOMO, σ and ΔN with lower values of ΔE, I, A, χ and η as shown from Table 8. The above results can be illustrated to the presence of dioxo-methyl-triazine sulfanyl group in Cefx molecular structure.
Molecular electrostatic potential (ESP) maps are very important parameters as they provide a visual method to know the parts of molecules on which the electron density is higher than other parts to determine the reactive parts in the molecules 82 . Different colors reflect different ESP values. The blue indicates the area with the highest positive ESP, the red indicates the region with the most negative ESP, and the green indicates the region with zero ESP. It can be shown from Fig. 14 that Cefz has red regions less than that of Cefx. In additions the figure shows that the electron rich centers are referred to the oxygen of dioxo-methyl-triazine sulfanyl group and other functional groups in Cefx while oxygen of carboxylate and amide groups in Cefz. It can be concluded that the presence of dioxo-methyl-triazine sulfanyl group in Cefx drug increases its electron density, so increases the electrostatic attraction with Al surface and consequently its inhibition efficiency. These findings agree well with experimental data.

Monte Carlo simulations. MC simulations explore the interaction between the studied inhibitors and Al
surface. The side and top views of the stable adsorption configuration of a single molecule of Cefx and Cefz with Al (111) surface are shown in Fig. 15. It is observed from the figure that the surface adsorption of Cefx molecule preferably happened by flat type molecular orientation and parallel to Al surface. This orientation allows its active centers to react effectively with the Al surface leading to higher protection efficiency than Cefz molecule. On the hand, the adsorption energies of Cefx and Cefz are − 239.81 and − 156.55 kcal/mol, respectively. Cefx's largest negative adsorption energy value reflects its strongest spontaneous adsorption on Al when compared to Cefz, and hence its better inhibitory effectiveness 81,83 .  Fig. 16. This image clearly shows that the examined drugs are situated extremely close to the metal surface, indicating that the affinity of Cefx and Cefz for the Al surface is strong even in the presence of solvent molecules. This validates the adsorption of these medicines on the Al surface, protecting it against corrosion.
Radial distribution function (RDF) is applied to calculate the distances between the atoms of the studied drugs and the Al surface to highlight the type of the adsorption process. If the value of the interatomic distance (r) is within the range of 1-3.5 Å, then the chemical bonds are favorably present, while physical interactions have been observed at r > 3.5 Å 84 . Figure S5 shows RDF for oxygen atoms of the active functional groups for both Cefx and Cefz drugs and top layer of Al (111) surface. It is observed from the figure that, the distance found for O 13

Conclusion
In this work, various doses of anionic states of Cefx and Cefz medicines are tested as corrosion inhibitors for Al in 0.1 M NaOH solution at 293 K. The electrochemical results show that Cefx and Cefz are effective Cathodic inhibitors. Furthermore, the inhibitory efficiencies rise with increasing concentrations, with the greatest percent (78.4 percent) recorded at 300 ppm of Cefx compared to 59.5 percent at 300 ppm of Cefz. The activation energy barrier of Al dissolving in 0.1 M NaOH is low compared to that in the presence of Cefx, which is greater than that of Cefz, suggesting a reduction in corrosion rate in the presence of the examined medications. Surface analysis and wettability studies show the existence of adsorbed hydrophobic layers of Cefx and Cefz on the Al surface. Cefz obeys the Freundlich isotherm with spontaneous and mixed type adsorption, while Cefx obeys the Temkin isotherm with spontaneous and mixed type adsorption. MC/MD simulations demonstrate that Cefx has an adsorbed flat type molecular orientation parallel to the Al surface, indicating that it has a greater protective effect than Cefz medication. www.nature.com/scientificreports/