Sustainable Use of Expired Metoprolol as Corrosion Inhibitor for Carbon Steel in Saline Solution

Abstract

The current paper examines the sustainable possibility for recycling unused or expired Metoprolol (MET), a benzodiazepine derivative, as an effective corrosion inhibitor for carbon steel in saline solutions. Repurposing expired medicinal drugs aligns with green chemistry concepts and supports circular economy initiatives by reducing pharmaceutical waste and averting the production of new synthetic inhibitors. The technical benefit of recycling expired MET drugs pertains to the elimination of costs associated with organic inhibitor manufacturing and the decrease in disposal expenses for the expired medication. A combination of electrochemical techniques (potentiodynamic polarization and electrochemical impedance spectroscopy) and quantum chemical calculations was employed to evaluate the inhibitory mechanism and efficacy of MET. At a concentration of 10⁻³ M, MET reduced the corrosion current density from 19.38 to 5.97 μA cm⁻², achieving a maximum IE of 69.1%. Adsorption Gibbs free energy, determined using different adsorption isotherms, revealed that interactions between metal atoms and MET adsorbed molecules have a chemical character with a ∆G^o_ads value of −50.7 kJ·mol⁻¹. Furthermore, quantum chemistry calculations indicate that the investigated drug, owing to its molecular structure (E_HOMO = −9.12 eV, E_LUMO = 0.21 eV, µ = 3.95 D), possesses the capacity to establish an adsorption layer on the metal surface, thereby impeding the diffusion of molecules and ions involved in the overall corrosion process. The results obtained using the different techniques were in good agreement and highlighted the effectiveness of MET in the corrosion inhibition of carbon steel.

1. Introduction

Metal corrosion is a destructive electrochemical phenomenon that impacts numerous industries, including petrochemicals, pharmaceuticals, and construction materials. The worldwide expenses associated with corrosion are substantial, and the advancement of efficient and eco-friendly protective approaches is essential [1]. In this context, the application of corrosion inhibitors constitutes a pragmatic and cost-effective approach to diminish the rate of metal deterioration [2]. Among these inhibitors, organic compounds are notably effective owing to their capacity to form a protective coating on the metal surface, hence diminishing the interaction with the corrosive environment [3].

Alongside corrosion inhibitors, alternative anti-corrosion technologies such as metallic coatings and cathodic protection have been proposed. The first of them involves the application of organic paints or metal layers, which serve as a physical deterrent to corrosion; nonetheless, they incur significant maintenance expenses and add considerable weight, especially on complex metal geometries [4]. The second method, cathodic protection, is employed for subterranean pipes and substantial metal structures; nevertheless, it entails significant initial costs and intricate implementation procedures [5]. In contrast to the aforementioned methods, the application of corrosion inhibitors, particularly expired pharmaceuticals, offers advantages such as ease of implementation, adaptability to complex metal geometries, and, importantly, contributes to sustainability by mitigating environmental contamination [6].

In recent years, a promising research direction has been aimed at reusing expired and non-compliant drugs as corrosion inhibitors [1]. This strategy not only offers an economical alternative to traditional inhibitors but also contributes to the reduction of pharmaceutical waste, an increasingly important aspect from a sustainability perspective [7]. Expired drugs contain complex chemical structures, often with heteroatoms such as oxygen, nitrogen, or sulfur, which favor adsorption on metal surfaces and reduce electrochemical reactions involved in corrosion [8].

The utilization of expired drugs as corrosion inhibitors presents problems concerning the stability of the active constituents and potential environmental impacts [3]. Despite the stability of the chemical structure of several medications post-expiration, a comprehensive analysis of their degradation is essential to validate their long-term efficacy [7]. The ecological consequences of these compounds also need to be analyzed, especially regarding the possible release of metabolites into aquatic environments [9]. Based on these considerations, future research should focus on optimizing the conditions of use of expired drugs as corrosion inhibitors, assessing the ecological impact, and exploring the possibilities of their recycling on an industrial scale [10]. Incorporating these chemicals into a sustainable corrosion prevention system might substantially aid in minimizing pharmaceutical waste and fostering ecological and economic solutions for the metal materials sector [11].

Beta-blockers have demonstrated significant promise as corrosion inhibitors among the many groups of medicines studied [7]. These pharmaceuticals possess functional groups capable of interacting with the metallic surface, forming a stable and protective coating [12]. Existing studies have shown that beta-blockers are highly effective in preventing steel corrosion in acidic environments [11]. The action mechanism of these compounds involves both types of adsorption, physical and chemical, which gives them mixed inhibitory properties, reducing both anodic and cathodic electrochemical processes [13].

Inadequate management of these expired medications can lead to environmental contamination. A study published in Environmental Pollution identified the presence of beta-blockers, such as metoprolol, in wastewater and surface water, underscoring the related ecological hazards [14,15]. Other studies show that these substances can have ecotoxic effects on aquatic organisms, including changes in behavior and metabolism [16,17]. In addition, research on how medications enter the ecosystem suggests that their disposal through sewage or landfills is a major source of pollution [18]. Exposure of aquatic species to beta-blockers may result in sublethal consequences, including alterations in behavior and physiology, which can impact biodiversity and the functioning of aquatic ecosystems. The environmental persistence of metoprolol is due to its chemical stability and resistance to biodegradation. This stability, although advantageous as a corrosion inhibitor, poses environmental problems.

A crucial determinant in the use of metoprolol as a corrosion inhibitor is its chemical stability in corrosive solutions. Studies indicate that metoprolol retains its structural stability in mildly acidic solutions but may undergo degradation at elevated temperatures or in the presence of strong oxidizing agents [19,20]. MET contains amine and ether groups that may be susceptible to hydrolysis or oxidation under certain conditions, which may affect its effectiveness as a corrosion inhibitor in the long term [21]. However, research suggests that, at concentrations used for metal protection, the degradation of MET is slow enough not to significantly affect its effectiveness in industrial applications [22].

Beta-blockers can be used as corrosion inhibitors; examples include atenolol, propranolol, nadolol, and metoprolol.

The inhibition effect of atenolol on mild steel in 1 M hydrochloric acid solution was examined by weight loss and electrochemical techniques [23]. The corrosion of aluminum in 0.1 M HCl solution in the absence and presence of β-blocker inhibitors (atenolol, propranolol, timolol, and nadolol) was examined utilizing weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques [24]. The inhibition behavior of metoprolol tablets on steel alloy (ST37) in 1 M hydrochloric acid and 0.5 M sulfuric acid solutions was investigated using three methods: potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning electron microscopy (SEM) [12].

Recent experimental studies have demonstrated the efficacy of metoprolol as a corrosion inhibitor. In the study published by Mohammadinejad et al. (2020) [12], high corrosion efficiencies were obtained (exceeding 90% for concentrations of 300 ppm inhibitor for mild steels in HCl 1 M media and 83% for 300 ppm MET in H₂SO₄ 0.5 M, by the potentiodynamic polarization method). This study identifies the adsorption process of the medication in both settings as physical, utilizing the Langmuir isotherm for analysis. MET is applicable in industrial contexts for acid stripping and the cleaning of metal surfaces [12].

Consequently, this study examined the impact of MET on the corrosion resistance of carbon steel in saline environments. This medicine is selected as a corrosion inhibitor due to its low toxicity and high solubility in acidic and saline environments. A significant quantity of this medication is now present in the “domestic area.” Currently, Metoprolol is among the most frequently prescribed beta-blockers. This information also corroborates the substantial quantity of expired or non-compliant medicines.

Techniques including linear polarization (Tafel plots) and EIS (electrochemical impedance spectroscopy) were utilized to assess the corrosion rate of the carbon steel samples and the inhibition efficiency of the drug.

Quantum chemical calculations have been utilized to evaluate the MET drug adsorption capability on the metal surface [25]. The characteristics of metal–inhibitor interactions in the metal-corrosion inhibition process were delineated by the use of adsorption isotherms [2,26].

This study aims to examine the efficacy of metoprolol, a commonly utilized beta-blocker, as a corrosion inhibitor for carbon steel in sodium chloride environments. Practical advantages include reduced environmental contamination from pharmaceutical waste and lower costs for disposal and inhibitor fabrication. Electrochemical techniques and chemical quantum calculations were utilized to enhance the understanding of metoprolol’s inhibitory efficacy, indicating a prospective solution for several sectors confronting corrosion issues.

2. Materials and Methods

2.1. Materials

Metoprolol has the chemical formula C₁₅H₂₅NO₃ and the IUPAC name (RS)-1-[4-(2-Methoxyethyl)phenoxy]-3-[(propan-2-yl)amino]propan-2-ol [27], and it is available under several trade names, such as Metoprolol Tartrate (Seloken^®, Lopressor^®) with short action and Metoprolol Succinate (Betaloc ZOK^®, Toprol-XL^®) with extended-release [28]. Figure 1 illustrates the structure of metoprolol.

Metoprolol exhibits moderate stability in saline solutions at physiological pH (7.4), while it can undergo hydrolysis and oxidation reactions under extreme acidic or basic pH conditions. Furthermore, deterioration may be affected by ultraviolet radiation or the presence of metal ions in solution [28].

The measurements were performed using the BioLogic SP150 galvanometric and potentiometric measurement apparatus (BioLogic Science Instruments, Seyssinet-Pariset, France) within a cell featuring a three-electrode configuration, comprising a carbon steel/platinum working electrode with an active surface area of 1 cm², two graphite rods serving as counter electrodes, and an Ag/AgCl reference electrode (Metrohm AG, Herisau, Switzerland). The chemical composition of the carbon steel working electrode is presented in

Table 1. The composition of the carbon steel working electrode (OLC45).

Fe (%)	C (%)	Cu (%)	Ni (%)	P (%)	S (%)	Mn (%)	Cr (%)	Si (%)
97.465	0.43	0.30	0.30	0.040	0.045	0.75	0.30	0.37

.

The corrosive media used for the experimental determinations consisted of 3.5% sodium chloride solutions (support electrolyte solution), respectively, with additions of expired Metoprolol (MET) of different concentrations, namely 10⁻⁶ mol L⁻¹, 10⁻⁵ mol L⁻¹, 10⁻⁴ mol L⁻¹, and 10⁻³ mol L⁻¹. The reactants utilized were sodium chloride a.p. (Merck KGaA, Darmstadt, Germany), and the corrosion inhibitor was Metoprolol in the form of metoprolol tartrate 50 mg/tablet manufactured by Labromed Pharma, Bucharest, Romania.

2.2. Electrochemical Methods

All electrochemical measurements were conducted with a BioLogic SP-150 Potentiostat/Galvanostat (BioLogic Science Instruments, Seyssinet-Pariset, France), controlled by EC-Lab v5.6 software (BioLogic Science Instruments, Seyssinet-Pariset, France). Cyclic voltammetry tests utilized a platinum electrode as the working electrode at polarization rates of 100, 10, and 5 mV s⁻¹, respectively, within a potential range of −1.5 to 2 V.

Linear voltammetry, chronoamperometry, chronopotentiometry, and electrochemical impedance spectroscopy were performed to investigate the corrosion rate, inhibitory efficiency, and stability of the electrode over time, therefore explaining the corrosion mechanism. The electrode used in these experimental phases was carbon steel, with the components of the electrochemical cell as previously specified. Before starting the experiments, the working electrode underwent grinding with Si-C abrasive paper of grits 80, 240, 800, 1200, and 2400, followed by ultrasonication, washing with acetone, and drying. To stabilize the electrode in the working environment, it was subjected to the chronopotentiometry procedure for one hour before the start of each experimental procedure.

The Tafel slope method was employed to assess the corrosion rate of carbon steel in a 3.5% NaCl corrosive environment. This involved recording a potentiodynamic curve at a rate of 1 mV s⁻¹, followed by its representation on a logarithmic scale and the determination of anodic and cathodic slopes. Their intersection yielded information about the corrosion current i_corr, and the corrosion potential E_corr, respectively. The results obtained were analyzed using the EC-Lab software; data for calculating corrosion rate and inhibitory efficiency are derived from the Stern–Geary relationship (Equation (1)) [29].

$$R_p={\displaystyle \frac{b_a·b_c}{i_{corr}·2.303·(b_a+b_c)}}$$

(1)

where:

$b_a,b_c$—represent the values of the anodic and cathodic slopes (mV dec⁻¹);

$R_p$—represent the values of the polarization resistance of the electrodes (Ω);

$i_{corr}$—corrosion current [A m⁻²].

From the values of the corrosion currents, or of the polarization resistance in the presence or absence of inhibitors, the inhibitory efficiency can be obtained, as shown in relation (2) [30].

$$IE={\displaystyle \frac{i_{corr}-i_{corr\left(inh\right)}}{i_{corr}}}·100$$

(2)

where:

IE—inhibitory efficiency [%];

$i_{corr}$, $i_{corr\left(inh\right)}$—the corrosion current in the absence or presence of the inhibitor (A m⁻²);

The corrosion mechanism at the interface of OLC45/saline electrolyte was investigated by electrochemical impedance spectroscopy (EIS) measurements. The tests were conducted at the OCP (open circuit potential) value throughout a frequency range of 10 mHz to 100 kHz, with an AC voltage amplitude of 10 mV rms. Sixty points were gathered for each recorded spectrum, distributed logarithmically with ten points every decade. Experimental data were fitted using an equivalent electrical circuit (EEC) according to a complex non-linear least squares Levenberg–Marquardt procedure, a technique inserted in the ZView v3.0 software (Scribner Associates, Inc., Southern Pines, NC, USA) [31].

2.3. Molecular Modelling

To complement the electrochemical measurements, it is necessary to perform theoretical molecular modeling calculations. Quantum calculations were conducted using Hyperchem 8.0 software (Hypercube Inc., Gainesville, FL, USA) [32]. The organic molecule of metoprolol was then modeled and optimized in both water and vacuum conditions using the Density Functional Theory (DFT) method to a B3LYP-type function with a 6-31G* basis set [33]. The graphic representation of the Highest Energy Occupied Molecular Orbital (HOMO), the Lowest Energy Unoccupied Molecular Orbital (LUMO), and the determination of their energy levels were established. The HOMO–LUMO energy difference (∆E), dipole moment (µ), absolute electronegativity (χ), chemical hardness (η), and softness (σ) were calculated based on these data.

The relations underlying the determination of the molecular descriptors mentioned above start from the correlation of the energies of the frontier orbitals with the ionization energy and the affinity for electrons from Equations (3) and (4), respectively [34].

$$\mathrm{I}=-\mathrm{E}\mathrm{HOMO}$$

(3)

$$\mathrm{A}=-\mathrm{E}\mathrm{LUMO}$$

(4)

The ionization potential, which indicates electron acceptance potential, allows for the calculation of absolute electronegativity and chemical hardness (η), reflecting the MET molecule’s resistance to charge transfer. Additionally, softness (σ) represents the MET molecule’s capacity to exchange electrons with the metal surface. These parameters, derived from Equations (5)–(7), elucidate the inhibitor’s interaction with the metal surface, contributing to corrosion inhibition [35].

$$\mathsf{\chi }={\displaystyle \frac{I+A}{2}}$$

(5)

$$\eta ={\displaystyle \frac{I-A}{2}}$$

(6)

$$\sigma ={\displaystyle \frac{1}{\eta }}$$

(7)

Other relevant properties for the appreciation of the way in which the metoprolol molecule blocks the metal surface from the corrosive attack are the molecular volume and the molecular surface, respectively, obtained by using the “QSAR Properties” module available in Hyperchem. Theoretical calculations are essential to establish a correlation between chemical interactions (donor–acceptor) and potential electrostatic interactions, providing a foundation for elucidating the inhibitory mechanism [36].

3. Results

3.1. Cyclic Voltammetry

To obtain information on how MET influences anode and cathode processes and its resistance to oxidation and reduction processes, cyclic voltammetry studies were conducted. Figure 2a,b, and c illustrate the results of cyclic voltammetry conducted in the 3.5% NaCl support electrolyte solution (BS). When the potential reaches above 1.25 V per anode branch, peaks associated with chlorine oxidation are observed, succeeded by oxygen release. With increasing concentration of MET, the values of oxidation currents of chlorine ions to molecular chlorine also decrease. At increased values of polarization rate, on the anode branch, cathodic reduction of positive hydrogen ions is observed, followed by the process of oxidation of hydrogen adsorbed on the electrode surface in the anode branch. A cathode peak of lower intensity can be observed around 0 V from E_Ag/AgCl and is due to the development of a surface layer of platinum oxides at the surface of the working electrode, followed by another peak at −0.4 V from E_Ag/AgCl for the process of reducing the surface area of the oxide layer during the oxidation step. Another peak corresponding to a lower current can be observed when sweeping the potential in the reverse direction, towards the OCP value, which is the starting point. The process of chlorine and oxygen formation on the surface of the working electrode is inhibited by adding metoprolol (MET) in neutral medium at concentrations of 10⁻⁵ mol L⁻¹ and 10⁻³ mol L⁻¹ MET, respectively, by shifting the potentials corresponding to these processes to the right.

The voltammograms shown in Figure 2a,b indicate that at high scan rates (100 mV s⁻¹), the system does not have sufficient time to achieve equilibrium at the electrode–electrolyte interface. Consequently, capacitive overload and diffusion limitations of the electroactive species arise, resulting in sharper and more pronounced peaks. In contrast, at low scan rates (5 mV s⁻¹), the process approaches equilibrium, with the current primarily influenced by steady-state or diffusion-controlled behavior. Consequently, the curves exhibit a flatter profile, showing significantly reduced or even absent peaks.

Cyclic voltammograms were plotted separately for the cathode and anode processes at a polarization rate of 5 mV s⁻¹ to trace the influence of MET addition (Figure 3). In the cathodic ray range (a), the current density decreases with respect to chlorine release from 350 to 100 A m⁻² and for oxygen from 300 to 200 A m⁻². In the anodic range (b), an increase in the modulus of current density from 100 to 400 A m⁻² is observed.

The excess associated with chlorine release is less than that for partial processes resulting in oxygen release, specifically the oxidation of oxygen ions and the formation and subsequent release of molecular oxygen. Metoprolol exhibits electrochemical stability across the potential range between the release potentials of hydrogen and oxygen, remaining unaffected by electrochemical oxidation or reduction reactions.

3.2. Linear Sweep Voltammetry

The diagrams were represented after purging the system with nitrogen in order to remove oxygen, thereby preventing secondary reactions that could take place at the electrode surface. This was followed by system stabilization through chronopotentiometry and potential monitoring over time.

Figure 4a,b represent the Tafel polarization curves recorded on OLC45 steel in the presence and absence of different concentrations of MET at polarization rates of 1 mV s⁻¹ (a) and 0.166 mV s⁻¹ (b), respectively.

Tafel fitting was performed for each polarization curve to determine the corrosion parameters of the OLC45 steel sample in 3.5% NaCl media; i_corr corrosion current, E_corr corrosion potential, v_corr corrosion rate, and polarization resistance R_p are shown in

Table 2. Tafel parameters characteristic of the carbon steel corrosion process in test solutions.

Metal	Metoprolol conc.	icorr	Ecorr	−bc	ba	Rp	vcorr	IE	θ
		(μA cm−2)	(mV)	(V dec−1)	(V dec−1)	(Ω)	(mm y−1)	(%)
OLC45	BS (NaCl 3.5%)	19.38	−548	216	126	1.78	0.282	-	-
	10−6 M	15.35	−578	144	158	2.13	0.222	20.7	0.21
	10−5 M	13.12	−601	146	142	2.38	0.191	32.3	0.32
	10−4 M	8.28	−624	119	116	3.09	0.120	57.2	0.57
	10−3 M	5.97	−636	108	95	3.68	0.086	69.1	0.69

.

The results in Table 2 indicate that an increase in MET concentration correlates with a decrease in corrosion current value, leading to a reduction in the corrosion rate of OLC45 steel. The maximum inhibitory efficiency value for the concentration of 10⁻³ mol L⁻¹ MET is 69.1%; the polarization resistance three times higher compared to the non-inhibitory variant. The presence of MET leads to a decrease in anode and cathode slope levels, indicating that Metoprolol acts as a mixed inhibitor.

3.3. Chronoamperometry Studies

The chronoamperometric method operates on the principle of monitoring current variation over time while keeping the electrode potential constant.

Figure 5 presents the graphical representation of time stamp curves with and without MET. Measurements were conducted for 30 min at a potential of 25 mV/E_OCP and for 15 min at 250 mV/E_OCP to study the evolution of the current value over time with varying concentrations of MET. Prior to conducting chronoamperometric experiments, the evolution of E_OCP in working solutions was measured (Figure 6). The resulting graph indicated that the current density stabilized quickly, approximately after 5 min, due to the formation and growth of a passivating layer characterized by evenly distributed films. The overall shape of the curves shows a uniform oxidation process that is influenced by the concentration of the corrosion inhibitor.

The values of current densities compared to concentrations at the two working potentials and the values of E_OCP are shown in

Table 3. Chronoamperometric and chronopotentiometric data characteristic of the carbon steel corrosion process in test solutions.

Metal	Metoprolol Concentration	Ecorr	Eox
			25 mv/EOCP	250 mv/EOCP
			icorr
		(mV)/Ref	(A m−2)
OLC45	BS (NaCl 3.5%)	−545	0.63	144
	10−6 M	−576	0.53	130
	10−5 M	−595	0.45	115
	10−4 M	−619	0.31	80
	10−3 M	−630	0.25	61.5

.

A partial protective layer composed of corrosion products develops on the electrode’s surface in drug-free solutions. Increasing the concentration of MET results in a reduction in corrosion current, attributable to the formation of a protective layer by MET molecules that inhibits further assaults by chloride ions or water molecules on the OLC45 steel surface. This phenomenon is corroborated by the Temkin adsorption isotherm, impedance spectra, and molecular descriptor values derived from theoretical calculations on the MET molecule.

3.4. Electrochemical Impedance Spectroscopy Studies

Figure 7a,b present EIS spectra represented as Nyquist and Bode plots illustrating OLC45 corrosion in saline test solutions in the absence and presence of different MET concentrations used in experimental studies. The continuous lines were produced by fitting using a Randles circuit from Figure 7, whereas open symbols represent experimental data. The Nyquist plots in Figure 7a exhibit a semicircular shape, representing the charge-transfer resistance (R_ct) in parallel with the double-layer capacitance (C_dl) [37].

The equivalent electrical circuit (EEC) depicted in Figure 8 consists of a solution resistance R_s in series with a parallel connection of a constant phase element (CPE), which represents the double-layer capacitance, and a charge transfer resistance R_ct. The resistance R_s component represents the uncompensated solution resistance. In the EEC used for simulating the OLC45 corrosion process, the ideal capacitor characterized by double-layer capacity (C_dl) is replaced by a constant phase element (CPE) because it represents more precisely the real electrochemical systems behavior, particularly that of depressed loops in EIS diagrams [38,39].

The impedance of the CPE element is described by Equation (8).

$$Z_{CPE}=1/T{\left(j\omega \right)}^n$$

(8)

where T is a parameter proportional to double-layer capacity, and n is an exponent that describes the CPE angle with values between 0 and 1 [38].

Double-layer capacity values (C_dl) were calculated using Equation (9).

$$C_{dl}=T^{1/n}{\left({\displaystyle \frac{1}{R_s}}-{\displaystyle \frac{1}{R_{ct}}}\right)}^{{\displaystyle \frac{n-1}{n}}}$$

(9)

The inhibition efficiency values of MET were determined using the charge transfer resistance (R_ct) values from

Table 4. Impedance parameters for carbon steel electrode in 3.5% NaCl in the absence and presence of varying concentrations of MET calculated by fitting the experimental data.

MET Conc. (M)	RS (Ω)	CPE-T (F cm−2 sn−1)	n	Rct (Ω cm2)	Chi2 · 103	Cdl · 104 (µF cm−2)	E (%)	θ
BS	8.77 (0.25%)	5.88·10−3 (0.53%)	0.67 (0.27%)	424 (1.05%)	1.55	13.5	–	–
10−6	8.55 (0.22%)	4.46·10−3 (0.40%)	0.63 (0.20%)	525 (0.69%)	0.69	6.94	19.2	0.19
10−5	8.43 (0.28%)	4.20·10−3 (0.48%)	0.61 (0.24%)	639 (0.91%)	0.92	5.00	33.6	0.34
10−4	8.36 (0.35%)	2.98·10−3 (0.55%)	0.60 (0.25%)	1020 (1.06%)	1.07	2.55	58.4	0.58
10−3	8.17 (0.24%)	2.21·10−3 (0.36%)	0.59 (0.18%)	1313 (1.01%)	0.87	1.36	67.7	0.68

, following Equation (10), where R⁰_ct and Rⁱ_ct are the charge transfer resistance in saline test solution without and with different MET concentrations, respectively.

$$IE\left(\%\right)={\displaystyle \frac{\left(\frac{1}{R_{ct}^0}\right)-\left(\frac{1}{R_{ct}^i}\right)}{\left(\frac{1}{R_{ct}^0}\right)}}·100$$

(10)

The semicircle diameter correlates to the R_ct value, which noticeably increases with the concentration of MET, suggesting that charge transfer is inhibited in its presence. This is also corroborated by the change in the characteristic frequency of the charge-transfer process toward lower values as MET concentration increases. The Bode plots in Figure 7b clearly show that the impedance magnitude at low frequency increases by about one order of magnitude at the highest MET concentration compared to the saline blank solution. Similarly, the phase angle increases in the presence of MET, while the frequency of its maximum shifts to lower values, reinforcing the inhibitory effectiveness of MET.

For a single time-constant process, as described by the EEC in Figure 7, the R_ct value is directly proportional to the corrosion resistance of OLC45 samples in saline media.

The fitting data from Table 4 clearly indicate that R_ct increases with higher concentrations of MET in saline test solutions, confirming the findings from linear polarization experiments. As R_ct values rise, C_dl values decrease accordingly. This reduction in C_dl aligns with the Helmholtz model, which states that double-layer capacitance is inversely proportional to surface charge. The decrease in C_dl may result from a lower local dielectric constant or an increase in the thickness of the electric double layer, suggesting that drug molecules are adsorbed at the carbon steel electrode/test solution interface. This implies that MET molecules gradually replace water on the electrode surface, forming a protective adherent film on the OLC45 electrode, thereby reducing the electrochemical dissolution rate of carbon steel in saline media [37,40]. Furthermore, the chi-square values on the order of 10⁻³ indicate an excellent correlation between the EIS experimental data and the values obtained using the EEC model.

3.5. Molecular Modelling

Theoretical molecular modeling simulations were conducted to enhance the understanding of the behavior of the Metoprolol molecule at the metal/electrolyte solution interface.

Figure 9 illustrates the optimized structure of the Metoprolol molecule, employed as a corrosion inhibitor in experimental investigations. The orientation of the dipole moment is represented from − to +. Oxygen atoms are depicted in red, carbon atoms in light blue, nitrogen atoms in dark blue, and hydrogen atoms in white. The data on the molecular descriptors for the Metoprolol molecule, obtained by using the Hyperchem program [41], namely the energy level of the highest occupied molecular orbital E_HOMO, the energy level of the lowest unoccupied molecular orbital E_LUMO, the energy gap between them ΔE, as well as the values of absolute electronegativity µ, chemical hardness η and softness σ, molecular volume V, and the surface of the S molecule, are presented in

Table 5. Molecular descriptors and their values for the Metoprolol molecule.

Molecular Descriptor	Value
EHOMO (eV)	−9.12
ELUMO (eV)	0.21
ΔE (eV)	9.33
µ (Debye)	3.95
χ (eV)	4.45
η (eV)	4.56
σ (eV−1)	0.22
V (Å3)	855.05
S (Å2)	529.18
V/S (Å)	1.62

.

To comprehend the electronic properties and align with the theory of molecular orbitals, the energy level occupied by electrons in a stable state of the molecule (Figure 10) and the energy level available for electrons upon excitation (Figure 11) were graphically represented, obtaining the values of their energies [42]. Based on the descriptor values, an E_HOMO of −9.12 eV was determined, indicating a high ionization potential for the Metoprolol molecule. The E_LUMO value is roughly 0.21 eV, indicating mostly chemical interactions, corroborated by the ∆G^o _ads value of −50.7 kJ/mol. From the descriptor values, a value of −9.12 eV was obtained for E_HOMO, which indicates a high ionization potential of the Metoprolol molecule.

The HOMO–LUMO energy differential ΔE = 9.33 eV indicates the stability of the Metoprolol molecule through its formation of chemical bonds at the metal–electrolyte solution interface level [43]. At the opposite pole, in addition to the chemical interactions described by the quantities mentioned above, there is still the possibility of electrostatic interactions between Metoprolol molecules and the surface of the steel electrode, which is appreciated by high values of the dipole moment (3.95 D) compared to the dipole moment of water. Its high value reflects a high inhibitory efficiency through the high ability of the organic molecule to interact with the steel surface. The high value of the absolute electronegativity χ (4.45 eV) is closely related to the transfer of electrolytes from the Metoprolol molecule to the steel, namely from the molecule with the highest chemical potential to the one with the lowest chemical potential, which is reinforced by the Gibbs adsorption energy ∆G_ads [44].

Alongside the dipole moment and the absolute electronegativity values, the average chemical hardness η (4.56 eV) and the softness σ of the molecule (0.22 eV⁻¹) suggest that it is a moderately weak molecule, exhibiting a slight propensity for electron transfer, attributable to the concurrent presence of physical and chemical interactions [45].

The volume and surface area of the molecule are intricately linked to the other calculated quantum parameters. The high values of the molecular volume (855.05 ų) and of the surface area (529.18 Ų), and the superunit ratio between them (1.62 Å), explain the principle of increasing the inhibitory efficiency with increasing volume due to the increase in the active surface of the metal occupied by the inhibitor molecule [46]. The quantum parameters that were evaluated indicate the ability of the Metoprolol organic molecule to adsorb on the steel surface through the transfer of non-participating electrons of the heteroatoms of the organic molecule to the vacant d orbitals, low in energy, of iron.

4. Adsorption Isotherms

To assess the interaction strength between the inhibitor and carbon steel, nine adsorption isotherms were evaluated: Langmuir, Freundlich, Frumkin, Temkin, Flory–Huggins, El–Awady, Dubinin–Radushkevic, Volmer, and Hill de Boer [44]. The analysis of the obtained results led to the conclusion that the most suitable for the adsorption of MET on steel in saline solution is the Temkin isotherm.

The Langmuir adsorption model, represented in its linearized form by Equation (11), is not applicable since, under experimental conditions, the slope of the isotherm dependence has the value 1.4338, significantly deviating from the theoretical value of 1, indicating that the assumption of ideal monolayer adsorption is not fulfilled [47,48].

$${\displaystyle \frac{c_{inh}}{\theta }}={\displaystyle \frac{1}{K_{ads}}}+c_{inh}$$

(11)

where c_inh is the concentration of the inhibitor, θ is the degree of surface coverage by the inhibitor, and K_ads is the adsorption equilibrium constant.

The Temkin model may be applied, with the isotherm represented by relationship (12), which is valid for heterogeneous surfaces such as carbon steel at degrees of coverage levels between 0.2 and 0.8 [43,44].

$$\theta =-{\displaystyle \frac{1}{f}}ln{K}_{ads}-{\displaystyle \frac{1}{f}}ln{c}_{inh}$$

(12)

where f is a parameter that characterizes the heterogeneity of the surface.

From the graphical representation of the Temkin isotherm (Figure 12), the adsorption constant K_ads was determined, and based on relationship (13), the Gibbs adsorption energy ∆G^o_ads was calculated [47].

$$∆G_{ads}^o=-RTln\left(55.56K_{ads}\right)$$

(13)

The results provided by the Temkin isotherm are shown in

Table 6. Parameters of the Temkin isotherm.

T [K]	a	b	r2	Kads	∆Go (kJ/mol)	f
298	1.21	0.0734	0.9749	1.4 ∙ 107	−50.7	13.6

.

According to the criteria 20/40 [38], if the standard Gibbs energy of adsorption ΔG^o_ads is above −20 kJ mol⁻¹, the bond between the metal and the inhibitor is purely physical, and if ΔG^o_ads is below −40 kJ mol⁻¹, the bond is purely chemical. MET molecules are adsorbing on the surface of carbon steel through strong chemical bonds.

5. Conclusions

The study assessed the expired Metoprolol (MET) inhibitory efficiency on carbon steel corrosion OLC45 in a 3.5% NaCl saline environment, emphasizing its potential role in sustainable corrosion protection. MET proved to be electrochemically stable and does not undergo further reactions (oxidation or reduction) in the potential range between −1.5 and 2 V. As the MET concentration increases, the anodic and cathodic slopes in the Tafel diagrams decrease significantly (from 216 mV dec⁻¹ and 126 mV dec⁻¹ in blank solution to 108 mV dec⁻¹ and 95 mV dec⁻¹ at 10⁻³ mol L⁻¹ MET), while the charge transfer resistance (R_ct) increases (from 424 Ω cm² in blank to 1313 Ω cm² at the highest concentration), as illustrated in the electrochemical impedance spectra. These results confirm that MET effectively inhibits corrosion by forming an adsorbed protective layer, which increases charge transfer resistance and reduces double-layer capacitance (C_dl decreasing from 13.5 ∙ 10⁻⁴ µF cm⁻² to 1.36 ∙ 10⁻⁴ µF cm⁻²), as supported by the EIS model fit. The chronoamperograms indicate a significant decrease in oxidation current as inhibitor concentration rises. The inhibitory effectiveness reaches around 69.1% at the maximal concentration assessed (10⁻³ mol L⁻¹ MET).

The molecular descriptors obtained from molecular modeling calculations indicate that the MET molecule can adsorb onto the metal surface by a mixed mechanism, involving both physical interactions and the transfer of non-participating electrons from the amino, ether, and hydroxyl functional groups. The Langmuir, Freundlich, Frumkin, Temkin, Flory–Huggins, El–Awady, Dubinin–Radushkevic, Volmer, and Hill de Boer isotherm models have been traced and Temkin provided the best description of the adsorption mechanism. From the value of ∆G^o_ads (−50.7 kJ mol⁻¹), the adsorption is chemical, and the energy parameters determined by the models traced indicate the formation of a stable monomolecular layer on the metal surface, with multiple occupancy of the active sites (parameter y = 0.313 in the El–Awady model), due to the presence of multiple functional groups of the molecule, as well as the formation of large complexes at the metal interface. For practical applications, MET can be added as a compound dissolved in a corrosive saline environment, such as water-cooling systems or storage tanks, especially where carbon steels are used. Owing to its inhibitory efficacy in this environment, it is optimally applied in situations where moderate inhibitions are sufficient or in combination with other green corrosion inhibitors to enhance corrosion protection via a synergetic effect.