Reusing an Expired Drug as a Sustainable Corrosion Inhibitor for Bronze in 3.5% NaCl and Simulated Acid Rain Solutions

Abstract

In recent years, the concept of reusing expired pharmaceuticals as corrosion inhibitors has attracted considerable attention due to the increasing demand for sustainable and eco-friendly solutions. This paper investigates the potential of an expired drug, called Fluimucil, containing N-acetylcysteine (NAC, 300 mg/3 mL), as a green corrosion inhibitor of bronze exposed to 3.5 wt.% NaCl solution and simulated acid rain (pH = 3.4). Potentiodynamic polarization measurements revealed that the drug acted mainly as a cathodic-type inhibitor in both electrolytes. Inhibition efficiency increased with drug concentration, reaching the maximum values of 86.7% in the presence of 36 mM NAC in the saline solution and 90.2% in the presence of 6 mM NAC in simulated acid rain. The anticorrosive effect of the drug was likely due to the adsorption of NAC on the bronze surface, which hindered to some extent the charge transfer reaction and corrosion product formation, thereby offering enhanced protection. Disregarding the nature of the corrosive electrolyte, NAC adsorption on the bronze followed the Langmuir isotherm model, involving a combination of physisorption and chemisorption processes. Surface examination by SEM-EDX confirmed that expired Fluimucil significantly mitigated the surface degradation and the corrosion products on the bronze.

1. Introduction

Copper–tin alloys, widely known as bronzes, are among the earliest alloys developed by humans, which have been endlessly used over the centuries in art, architecture, and various industrial and marine applications. Bronzes are relatively resistant to corrosion because they spontaneously develop thin layers of corrosion products on their surface, designated as ‘patinas’, which tend to be extremely stable [1,2], protecting the metals against corrosion and offering an aesthetically pleasant appearance [3]. However, bronze can undergo significant corrosion when exposed to aggressive humid environments, containing chlorides, carbonates, nitrates, or sulphate ions, such as seawater in marine atmosphere or acid rainwater encountered in industrial urban areas affected by increased air pollution [4,5].

The application of corrosion inhibitors is one of the most feasible approaches for protecting metals against corrosion [6]. Various organic compounds containing heteroatoms (N, S, P and O), polar functional groups, and/or π electrons in conjugated multiple bonds have been long recognized as efficient corrosion inhibitors of metals in many aggressive media [7,8]. The anticorrosion effectiveness of these organic compounds depends on their ability to adsorb on the metal surface and form protective layers that further prevent the corrosion process. Despite their extensive use, most of the synthetic chemicals exploited as corrosion inhibitors are expensive, non-biodegradable, and often hazardous to humans and the environment. For instance, the most effective corrosion inhibitor for bronzes—benzotriazole (BTA)—is suspected to be potentially carcinogenic and has toxic effects on the aquatic organisms [9].

In recent years, in view of increasing ecological awareness, the research goals in the corrosion inhibitors field were directed toward the development and exploration of new inexpensive and effective molecules with high biodegradability, non-bioaccumulative behavior, and negligible environmental impact. Such examples of environmentally friendly corrosion inhibitors are amino acids [10,11,12], plant extracts [13,14], natural polymers [15], ionic liquids [16], and carbohydrates [17].

Various pharmaceutical medicines are also regarded as greener approaches for replacing the traditional harmful corrosion inhibitors [18,19,20], as many drugs are generally less toxic and more biodegradable compared to synthetic chemicals. However, most of the drugs are expensive, and their application as corrosion inhibitors is appropriate only when they become pharmaceutical wastes due to expiration, unuse, recall, damage, or contamination [21]. Currently, pharmaceutical wastes can be found in the municipal sewage effluents, surface waters, ground water, and even in drinking water [22,23], representing a major environmental concern that is likely to be exacerbated in the future given the expected increase in drug usage. The primary sources of pharmaceutical pollution include wastewater from human and veterinary consumption and excretion, the improper disposal of unused or expired medications, and effluents from pharmaceutical industries [24,25]. Numerous studies [26,27,28] have reported that the most common pharmaceutical contaminants in aquatic systems are antibiotics; analgesics; non-steroidal anti-inflammatory drugs; anticancer, antidiabetic, and antifungal drugs; antihistamines; antidiuretics; beta-blockers; opioid pain medications; and hormones. A comprehensive study [29] monitoring 61 pharmaceutical products, sampled from 1052 sites in 258 rivers across 104 countries, revealed that over 25% of the analyzed sites were contaminated with at least one active pharmaceutical ingredient at harmful levels. The highest contamination levels were observed in low- to middle-income countries from sub-Saharan Africa, South Asia, and South America. Pharmaceutically active contaminants might pose serious threats to ecosystems and wildlife, since they can induce antimicrobial resistance, act as endocrine disruptors, and impair reproduction, particularly in fish populations [30]. Serious declines in scavenger species, such as vulture populations in South Asia due to their consumption of diclofenac-treated livestock, have also been reported [31,32]. Likewise, humans are exposed to pharmaceuticals through contaminated drinking water and by ingesting the residues that accumulate in fish, meat, dairy products, and plant crops [33].

In this context, the idea of the re-valorization of unused or expired drugs for corrosion control can offer a potential solution to several major environmental and economic challenges, i.e., reducing environmental pollution with pharmaceutically active compounds and lowering the disposal costs of pharmaceuticals wastes [34]. A perusal of the literature revealed that numerous drugs have been investigated as corrosion inhibitors [18,35,36,37] for various metallic materials based on iron [38,39,40,41,42,43,44], aluminum [45,46,47], copper [34,48,49,50,51,52,53,54], or zinc [54,55,56] in different environments, while only limited research is available on copper alloys [57,58]. Additional information on the effectiveness of several pharmaceuticals reported as corrosion inhibitors of different metallic materials is provided in Table S1 in the Supplementary Materials.

The aim of the present paper is to evaluate the properties of an expired drug, named Fluimucil, as a green corrosion inhibitor for bronze in two corrosive environments, namely 3.5 wt.% NaCl solution and simulated acid rain of a pH = 3.4 typical of a highly polluted urban atmosphere. The active ingredient in Fluimucil is N-acetyl cysteine (NAC), which is a safe and inexpensive drug [59], approved by the U.S. Food and Drug Administration, and recognized by the World Health Organization as an essential medicine. N-acetyl cysteine is widely used as primary antidote for acetaminophen (paracetamol) overdose and, in recent years, as mucolytic agent in respiratory diseases [60]. It also has antioxidant, anti-inflammatory, antibacterial, anticancer, and neuroprotective properties [60,61].

According to our recent search, there are no reported data available in the literature on the use of NAC-containing drugs as corrosion inhibitors for metals. However, it should be mentioned that synthetic N-acetyl cysteine has previously been reported as an effective inhibitor for the corrosion of steel in 15% HCl solution [62], C-steel in 1 M HCl [63], and copper [50] and Cu–10Al–5Ni [64] in NaCl solution.

The corrosion inhibition mechanism of the proposed drug for bronze in the two corrosive environments was examined using electrochemical techniques (potentiodynamic polarization and electrochemical impedance spectroscopy) and surface analysis through scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). Various adsorption isotherms were also tested to determine their potential relevance in explaining the adsorption behavior of the drug on the bronze surface.

2. Materials and Methods

2.1. Materials

The working electrode was a bronze rod embedded in epoxy resin with an active surface area of 0.28 cm². The chemical composition of the bronze working electrode is presented in

Table 1. Chemical composition (at. %) of the bronze electrode.

Cu	Sn	Pb	Zn	Fe	S	Si
93.58	3.96	0.20	1.44	0.26	0.41	0.15

.

Prior to each corrosion experiment, the bronze surface was mechanically grounded with different SiC abrasive papers (P800, P1000, P2000, and P4000) to obtain a smooth surface. Then, the bronze was thoroughly rinsed with distilled water and immediately immersed in the corrosive solution. The counter and reference electrodes were a large platinum foil and a calomel saturated electrode (SCE), respectively.

The corrosion measurements were carried out in two electrolytes, namely a solution of 3.5 wt.% NaCl and a synthetic acid rain solution containing 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ + 0.2 g L⁻¹ NaNO₃, acidified to a pH = 3.4 by the addition of diluted H₂SO₄ to simulate the conditions encountered in a polluted urban environment [65]. All the solutions were prepared using analytical-grade reagents (Merck, Darmstadt, Germany) and distilled water.

The drug used as corrosion inhibitor was Fluimucil solution at 300 mg/3 mL produced by Zambon^TM (Bresso, Italy), which contains N-acetyl cysteine (NAC) as the active ingredient.

The reuse of the expired Fluimucil as a potential sustainable corrosion inhibitor for bronze was proposed considering that its active ingredient (NAC) is non-toxic, biodegradable, and highly soluble in water [66] and contains a thiol group with strong affinity for copper. The medicine used in this study was expired by five months. The concentrations of NAC in the two corrosive electrolytes ranged from 0.6 mM to 36 mM. The molecular formula of N-acetyl cysteine is illustrated in Figure 1.

2.2. Electrochemical Methods

Electrochemical experiments were performed using a Gill AC potentiostat (ACM Instruments, Grange-over-Sands, UK) controlled by a computer. Before each experiment, the bronze electrode was left for 1 h in the test solution to attain a steady-state condition. Polarization curves were recorded at a sweep rate of 10 mV/min, in the potential range of ±250 mV with respect to the open-circuit potential. Electrochemical impedance spectroscopy (EIS) measurements were performed at the open-circuit potential, in the frequency range from 10 kHz to 10 mHz, at 5 points per decade, and at an AC voltage amplitude of ±10 mV. The impedance parameters were calculated by fitting experimental results to equivalent electrical circuits using ZSimpwin 3.2 software.

For each experiment, 100 mL of electrolyte was used. The measurements were performed at the room temperature (25 °C), and at least two replicates were conducted for each experimental condition.

The corrosion inhibition efficiency values were estimated from the EIS and polarization measurements, according to the following equations:

$${\mathsf{\eta }}_{\mathrm{i}\mathrm{m}\mathrm{p}}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{R}}_{\mathrm{p}}-{\mathrm{R}}_{\mathrm{p}}^0}{{\mathrm{R}}_{\mathrm{p}}}}·100$$

(1)

$${\mathsf{\eta }}_{\mathrm{p}\mathrm{o}\mathrm{l}}(\%)={\displaystyle \frac{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^0-{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^0}}$$

(2)

where ${\mathrm{R}}_{\mathrm{p}}$ and ${\mathrm{R}}_{\mathrm{p}}^0$ represent the polarization resistances in solutions with and without an inhibitor, respectively. Similarly, ${\mathrm{i}}_{\mathrm{corr}}^0$ and ${\mathrm{i}}_{\mathrm{corr}}$ are the values of the corrosion current densities, calculated in the absence and in the presence of the drug, respectively.

2.3. SEM-EDX Investigations

For morphological studies, the bronze surface was prepared by keeping the bronze electrode for 48 h in the two electrolytes, without and with the optimum concentrations of NAC. After exposure, the metallic specimens were gently washed with distilled water, dried at room temperature, and analyzed without any further treatment. For this purpose, a scanning electron microscope model, the TM4000Plus II microscope (Hitachi, Tokyo, Japan), coupled to an energy-dispersive X-ray spectrometer (Oxford instrument, Abingdon, UK, SDD detector, WD ~10 mm, accelerating voltage of 15 kV) was used.

3. Results and Discussion

3.1. Electrochemical Impedance Spectroscopy

Nyquist diagrams of bronze corrosion recorded after immersion for 1 h in 3.5% NaCl and simulated acid rain (pH = 3.4) solutions, in the absence and in the presence of various concentrations of NAC, are shown in Figure 2. The experimental data, represented by symbols, are displayed together with the fitted spectra depicted as lines with crosses.

As shown in Figure 2, all impedance spectra were characterized by a capacitive behavior with two depressed loops, although their separation was not clearly resolved. A straight line in the low-frequency region could also be noticed for the bronze exposed to both uninhibited corrosive electrolytes, indicating the presence of the Warburg impedance. This suggests that the mechanism of bronze corrosion in the uninhibited solutions was mixed-controlled by the charge transfer and the diffusion of dissolved oxygen towards the electrode [67] or of some soluble metallic species from the surface in the bulk solutions. The significant increase in the capacitive loops in the presence of the drug correlated with the inhibition of the bronze corrosion process in both tested electrolytes.

The experimental impedance data were further fitted using the equivalent electrical circuits from Figure 3, in which R_e is the solution resistance, the high-frequency elements R_ct − Q_dl are ascribed to the charge transfer process at the bronze/electrolyte interface, the R_F − Q_F parameters are related to an oxidation–reduction process involving a reaction intermediate (i.e., Cu⁺ ions accumulated probably as Cu₂O during the corrosion process), according to a mechanism previously reported [68], and W stands for the Warburg impedance. The coefficients n_dl and n_F were used to reproduce the depressed features of the impedance spectra, as shown in the Nyquist diagrams (Figure 2).

In the equivalent circuits shown in Figure 3, the ideal capacitive elements were replaced by constant phase elements (CPEs), represented by the terms Q and n, considering the frequency dispersion effect. The impedance of the CPEs has the following form [69]:

$${\mathrm{Z}}_{\mathrm{C}\mathrm{P}\mathrm{E}\left(\mathsf{\omega }\right)}=\mathrm{Q}={\left[\mathrm{C}{\left(\mathrm{j}\mathsf{\omega }\right)}^{\mathrm{n}}\right]}^{-1}$$

(3)

where Q represents a pre-exponential factor, with the dimensions of Ω⁻¹ cm⁻² sⁿ; ω = 2πf is angular frequency in rad·s⁻¹ and n is the exponent, which defines the character of frequency dependence (−1 ≤ n ≤ 1). The values of n are associated with the non-uniform distribution of current at the solid surface, mainly caused by its roughness, porous layer formation, degree of polycrystallinity, inhibitor adsorption, etc. [70].

From the CPE parameters, the values of the pseudo-capacitances (C) were calculated using the equation below:

$$\mathrm{C}={\left({\mathrm{R}}^{1-\mathrm{n}}\mathrm{Q}\right)}^{1/\mathrm{n}}$$

(4)

The fitting parameters for the bronze corrosion in the absence and in the presence of varying concentrations of NAC are listed in

Table 2. Impedance parameters values for the bronze immersed in the two corrosive electrolytes without and with NAC, at different concentrations.

NAC conc. (mM)	Re (kΩ cm2)	Rct (kΩ cm2)	Qdl (μF sn−1cm−2)	ndl	Cdl (μF cm−2)	RF (kΩ cm2)	QF (mF sn−1cm−2)	nF	CF (mF cm−2)	Rp (kΩ cm2)	W (S s1/2 cm−2)	ηimp (%)
3.5% NaCl solution
0	3.4	0.51	326.9	0.755	182.6	0.27	1.38	0.676	0.86	0.78	0.0095	-
0.6	3.5	1.08	182.5	0.781	115.6	1.58	0.66	0.500	0.68	2.66		70.7
3	4.8	1.37	178.1	0.797	124.4	1.77	0.62	0.526	0.60	3.14		75.2
6	5.3	1.51	134.0	0.809	91.9	1.81	0.51	0.500	0.46	3.32		76.5
12	4.7	1.96	140.4	0.791	99.8	1.90	0.64	0.500	0.77	3.86		79.8
18	6.5	2.93	119.2	0.796	91.1	2.06	0.52	0.500	0.57	4.99		84.4
36	8.0	3.49	86.2	0.810	65.0	2.39	0.41	0.500	0.39	5.88		86.7
0.2 g L−1 NaHCO3 + 0.2 g L−1 Na2SO4 + 0.2 g L−1 NaNO3, pH = 3.4 solution
0	0.2	0.22	653.1	0.600	178.2	1.01	5.51	0.738	10.2	1.23	0.0043	-
0.6	1.8	4.64	220.4	0.705	222.5	1.52	1.85	0.650	3.24	6.16		80.0
3	1.8	7.09	156.4	0.716	162.9	3.08	4.49	0.928	5.51	10.17		87.9
6	1.3	9.20	95.9	0.744	91.8	3.35	1.30	0.959	1.39	12.55		90.2
12	1.0	5.38	113.8	0.794	100.2	3.29	0.94	0.500	2.89	8.67		85.8
18	1.1	3.87	176.0	0.740	153.8	1.19	3.30	0.65	6.87	5.06		77.9
36	1.2	3.02	148.4	0.771	116.8	1.61	1.59	0.500	4.04	4.63		75.8

.

The polarization resistance values, R_p, calculated as R_ct + R_F, are also depicted in Table 2. As previously reported [71], the low-frequency limit of the impedance, R_p, correlates better with the corrosion rate, when a redox process involving the corrosion products occurs at the interface. Accordingly, the R_p values were further used to estimate the inhibition efficiency, η_imp, using Equation (1).

The addition of NAC to both tested electrolytes resulted in an increase in the charge transfer resistance, R_ct values, and this effect was enhanced as the drug concentration increased up to certain values, i.e., 36 mM in the 3.5% NaCl solution and 6 mM in 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ + 0.2 g L⁻¹ NaNO₃ (pH = 3.4) electrolyte, respectively. These concentrations of NAC in Fluimucil appeared to be the optimal ones in the investigated experimental conditions. A slight decrease in the R_ct values was observed at higher concentrations of NAC in the simulated acid rain solution, but they remained above the one of the uninhibited electrolyte (Table 2). These results indicate that NAC was able to hinder the charge transfer process in both tested electrolytes, most likely through its adsorption on the metal surface.

The adsorption of NAC on bronze also changes its interfacial capacitance, according to the Helmholtz model [72]:

$${\mathrm{C}}_{\mathrm{d}\mathrm{l}}={\displaystyle \frac{\mathsf{\epsilon }·{\mathsf{\epsilon }}_0}{\mathrm{d}}}·\mathrm{A}$$

(5)

where ε₀ and ε are the vacuum and local dielectric constants, respectively; A is the electrode area; and d is the thickness of the double-layer. Thus, the pre-existing water molecules from the bronze surface are replaced by the adsorbed drug molecules with lower permittivity, and this reduces the local dielectric constant, while increasing the thickness of the double layer [72]. Furthermore, the adsorption of NAC on the bronze reduced the exposed surface area in contact with the corrosive solutions. These effects accounted for the decrease in the double layer capacitances, C_dl, generally observed in the drug-containing electrolytes (Table 2). As expected, the lowest C_dl values were obtained in the presence of the NAC concentrations that also gave the highest R_ct values.

The faradaic resistance, R_F, increased with NAC concentration up to its optimal values (36 mM in the saline solution and 6 mM in simulated acid rain), while the faradaic capacitance, C_F, simultaneously decreased. This behavior allowed the assumption that the adsorption of NAC on the bronze surface inhibited to some extent the development of the corrosion products (most likely Cu₂O), which became less active in the redox processes, thereby providing improved corrosion protection. However, by increasing the NAC concentration above 6 mM in the simulated acid rain solution, a slight acceleration of the faradaic processes occurring on the bronze surface could take place, resulting in higher C_F values (Table 2). This may have been due to the partial desorption or reorganization of the inhibitor molecules on the bronze surface [34].

The maximum inhibition efficiency values, η_imp, reached 86.1% in the 3.5% NaCl solution and 90.2% in the simulated acid rain (pH = 3.4). These results confirm that the expired Fluimucil solution had good inhibiting properties on bronze corrosion exposed to both tested electrolytes.

3.2. Potentiodynamic Polarization Measurements

The polarization curves of bronze exposed to 3.5% NaCl and simulated acid rain solutions, without and with different concentrations of NAC, are shown in Figure 4.

Electrochemical corrosion parameters, including corrosion current densities (i_corr), corrosion potentials (E_corr), and cathodic (β_c) and anodic (β_a) Tafel slopes were estimated by Tafel extrapolation and are listed in

Table 3. Electrochemical corrosion parameters of bronze in 3.5% NaCl and simulated acid rain (pH = 3.4) solutions, without and with different concentrations of NAC.

NAC (mM)	Ecorr (mV vs. ESC)	icorr (μA cm−2)	|βc| (mV dec−1)	βa (mV dec−1)	ηpol (%)
3.5 wt.% NaCl solution
0	−229.4	221.3	70.0	67.4	-
0.6	−279.8	101.9	186.9	129.5	54.0
3	−293.8	69.5	142.9	70.6	68.1
6	−286.8	58.5	143.0	73.2	73.6
12	−300.7	54.3	131.9	78.5	75.5
18	−291.8	43.8	156.1	95.4	80.2
36	−328.7	34.6	202.3	121.7	83.2
0.2 g L−1 NaHCO3 + 0.2 g L−1 Na2SO4 + 0.2 g L−1 NaNO3, pH = 3.4 solution
0	−52.2	24.3	83.9	46.9	-
0.6	−88.8	11.4	82.1	58.2	53.1
3	−145.8	4.4	74.5	92.3	81.9
6	−185.5	2.4	118.0	93.9	90.1
12	−214.1	3.8	157.7	80.8	85.8
18	−249.4	4.4	223.3	139.6	81.9

. The i_corr values were further used to calculate the inhibition efficiency, η_pol, using Equation (2), and the obtained values are also included in Table 3.

As expected, the corrosion current density estimated for the uninhibited saline environment was almost ten times higher compared to the i_corr value of the uninhibited simulated acid rain of a pH = 3.4. The greater corrosive aggressiveness of the saline environment was associated with the presence of chloride ions, which promote the preferential dissolution of the Cu-rich phase in bronze exposed to high-oxygen saline conditions [73], leading to the formation of loosely adherent corrosion products [74]. Patina flaking, commonly observed on Cu-alloys after immersion in marine environments, was associated with the gradual transformation of nantokite (CuCl) into paratacamite (Cu₂(OH)₃Cl) of a larger volume [75] that induced internal stress within the patina [76].

As illustrated in Table 3, some important shifts in E_corr towards more negative values were noticed in the presence of increasing concentrations of NAC in both tested electrolytes. The utmost changes in the E_corr due to the addition of NAC exceeded −85 mV, allowing us to suppose that NAC acted mainly as a cathodic-type inhibitor for the bronze corrosion in the two electrolytes.

The i_corr values decreased considerably in the presence of the drug in both tested electrolytes, and this decreasing trend was more pronounced as the concentration of NAC increased up to 36 mM in the 3.5% NaCl solution and 6 mM in the simulated acid rain, respectively. At these concentrations, the inhibiting efficiencies of NAC on bronze corrosion reached the highest values of 83.2% in the 3.5% NaCl solution and 90.1% in the simulated acid rain of pH = 3.4 (Table 3). The inhibited efficiency values obtained from the potentiodynamic polarization curves and the EIS measurements were in reasonable agreement.

The changes in the cathodic (β_c) and anodic (β_a) Tafel slopes observed in the presence of the drug suggest that the oxygen reduction and the bronze dissolution were affected by the inhibitor addition in both corrosive solutions, but no clear trend was observed.

3.3. Adsorption Mechanism

In order to gain further insight into the nature of the inhibitor adsorption on the bronze surface, the polarization data obtained from the two corrosive solutions were used. The surface coverage, θ, was calculated as a function of the NAC concentration, based on the following equation:

$$\mathsf{\theta }={\displaystyle \frac{{\mathsf{\eta }}_{\mathrm{p}\mathrm{o}\mathrm{l}}}{100}}$$

(6)

Several widely studied adsorption isotherms, such as Langmuir, Temkin, Frumkin, and Freundlich were applied to the experimental data, according to the following equations [77]:

$$\mathrm{Langmuir} \mathrm{isotherm}: {\displaystyle \frac{{\mathrm{c}}_{\mathrm{I}\mathrm{n}\mathrm{h}}}{\mathsf{\theta }}}={\displaystyle \frac{1}{\mathrm{K}}}+{\mathrm{c}}_{\mathrm{I}\mathrm{n}\mathrm{h}}$$

(7)

$$\mathrm{Temkin} \mathrm{isotherm}: {\mathrm{e}\mathrm{x}\mathrm{p}}^{(-2\mathrm{a}\mathsf{\theta })}=\mathrm{K}{\mathrm{c}}_{\mathrm{I}\mathrm{n}\mathrm{h}}$$

(8)

$$\mathrm{Frumkin} \mathrm{isotherm}: \mathrm{l}\mathrm{n}\left[{\displaystyle \frac{\mathsf{\theta }}{\left(1-\mathsf{\theta }\right)}}{\mathrm{c}}_{\mathrm{I}\mathrm{n}\mathrm{h}}\right]=\mathrm{l}\mathrm{n}\mathrm{K}+2\mathsf{\alpha }\mathsf{\theta }$$

(9)

$$\mathrm{Freundlich} \mathrm{isotherm}: \log \mathsf{\theta }=\log K+{\displaystyle \frac{1}{n}}\log {\mathrm{c}}_{\mathrm{I}\mathrm{n}\mathrm{h}}$$

(10)

where c_Inh is the concentration of NAC, K is the adsorption equilibrium constant, and a is the molecular interaction parameter.

Figure 5 shows the fitting plots of the four isotherm models used to study the adsorption of NAC-containing drug on the bronze surface in the tested electrolytes. The correlation coefficient (R²) values were used to select the isotherm that best fitted the experimental data.

The results in Figure 5 reveal that the Langmuir model is the most suitable to describe the adsorption of NAC on bronze in both corrosive environments [78]. It is known that the Langmuir isotherm approach assumes that the inhibitor is adsorbed in the form of a monolayer on a fixed number of equivalent adsorption sites (one molecule occupies a single surface site), and the adjacent adsorbed molecules do not interact with each other on a homogenous solid surface [34]. However, in real experimental conditions, the functional groups grafted on the adsorbed organic molecules could interact through common repulsive or attractive forces, and these might explain the small deviations from unity in the slopes of the Langmuir plots shown in

Table 4. Thermodynamic parameters for the adsorption of NAC on the bronze surface in the two tested electrolytes at 298 K.

Electrolyte	Slope	K (L mol−1)	$\mathsf{\Delta }{\mathbf{G}}_{\mathbf{ads}}^0$ (kJ mol−1)
3.5 wt. % NaCl	1.18	1044.9	−27.2
0.2 g L−1 NaHCO3 + 0.2 g L−1 Na2SO4 + 0.2 g L−1 NaNO3, pH = 3.4	1.19	25,839.8	−35.1

.

The standard free energy of adsorption $\mathsf{\Delta }{\mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ was calculated from the adsorption equilibrium constant, K, according to Equation (11), and the results are presented in Table 4 [79].

$$K={\displaystyle \frac{1}{55.5}}\exp \left({\displaystyle \frac{-\mathsf{\Delta }{\mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0}{\mathrm{R}\mathrm{T}}}\right)$$

(11)

As shown in Table 4, the calculated values of $\mathsf{\Delta }{\mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ for NAC adsorption laid in the range from −20 to −40 kJ mol⁻¹ in both corrosive environments, suggesting that it acts by mixed adsorption on bronze, including physical and chemical processes. The large negative values of $\mathsf{\Delta }{\mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^0$ also show the spontaneity of the adsorption process on the bronze surface.

As we previously mentioned, the inhibition behavior of NAC on bronze corrosion primarily arises from its adsorption on the metal surface. This process is pH-dependent and controlled by the ionization states of NAC’s functional groups [63,80].

It is known [61] that the pKa values of NAC are 3.14 for the carboxyl group and 9.27 for the thiol group. In the simulated acid rain of a pH = 3.4, NAC existed mainly in a protonated form. In this state, the adsorption of NAC occurs predominantly through the sulphur atoms of the thiol (-SH) group [50,64], which could coordinate with the vacant d-orbitals of the copper atoms to form stable covalent Cu–S bonds. This results in the formation of a chemisorbed layer that effectively blocks the active sites on the bronze, thereby reducing the corrosion rate. In addition to chemisorption, the partial deprotonation of the carboxyl group might allow some physical interactions [80] that further stabilize the adsorbed inhibitor film and improve the surface coverage.

In neutral saline environments, NAC exists as a zwitterionic form, and it bonds to the bronze surface through the thiol and deprotonated carboxyl groups [50]. However, this increased electron density on the carboxylate group could enhance its nucleophilicity, which might facilitate the coordination with the copper ions from solution [81,82]. As previously reported [83], the formation of a soluble complex between the inhibitor molecule and hydrated Cu²⁺ ions would help increase the metal dissolution. This might explain the slightly lower corrosion resistance experimentally observed in the saline environments compared to in the simulated acid rain.

3.4. SEM-EDX Analysis

To confirm the adsorption of the drug molecules on metal, SEM and EDX experiments were further carried out. The microscopic morphologies of the bronze surface before and after 48 h of immersion in the two corrosive electrolytes, in the absence and in the presence of the optimal concentrations of NAC, are presented in Figure 6, together with the corresponding EDX profile analyses.

Figure 6a, corresponding to the unexposed bronze, shows that the alloy was alpha-bronze, with local Sn enrichment; it also reveals some Pb globules, which appear as white spots in the BSE image. The optical micrographs included in Figure S1 from the Supplementary Materials also reveal the presence of Pb inclusions.

After 48 h of immersion in the uninhibited corrosive electrolytes, the bronze showed significant surface damage, including visible pitting and corrosion products (Figure 6b,d). The high levels of oxygen, i.e., 36.70 at.%, and 29.64 at.%, identified on the bronze surface exposed to the saline and acid rain solutions confirmed that the corrosion products were mainly oxides (most likely Cu₂O). As expected, the chloride (1.63 at.%) was also present on the sample exposed to the 3.5% NaCl solution (Figure 6b’).

The addition of the drug in the two electrolytes resulted in rather smooth and uniform surfaces, significantly preventing the occurrence of corrosion pits and reducing the metal roughness, most probably due to the adsorption of NAC on the bronze. As expected, the oxygen and chloride contents decreased markedly within the surface layer (Figure 6c’,e’) confirming that the NAC adsorption prevented corrosion product formation on the metal surface.

SEM-EDX analysis aligned with the electrochemical findings, confirming the effectiveness of the Fluimucil drug in mitigating the corrosion of bronze exposed to both 3.5% NaCl and simulated acid rain electrolytes.

4. Conclusions

The present study investigated the ability of a sustainable pharmaceutical drug, named Fluimucil, which contains N-acetylcysteine (NAC, 300 mg/3 mL) to protect a bronze substrate against corrosion in two aggressive electrolytes, i.e., 3.5 wt.% NaCl solution and simulated acid rain (pH = 3.4).

The results of the electrochemical and surface analysis led to the following conclusions:

(1)

The drug acted as a good cathodic inhibitor of bronze corrosion in both tested electrolytes; the maximum values of the inhibition efficiency were 86.7% in the presence of 36 mM NAC in the saline solution and 90.2% obtained by the addition of 6 mM NAC in simulated acid rain. Generally, the inhibition performance of the drug was higher in the simulated acid rain solution than in the saline environment.

(2)

EIS results showed that the adsorption of NAC on bronze surface hindered the charge transfer reaction and impeded the formation of corrosion products, which became less prone to participate in redox reactions.

(3)

The inhibition mechanism was attributed to the spontaneous adsorption of NAC on the bronze surface via physical and chemical interactions. In both tested electrolytes, the adsorption process followed the Langmuir isotherm.

(4)

SEM and EDX analysis confirmed that the drug was able to effectively suppress corrosion product formation on the bronze surface exposed to both corrosive environments.

In conclusion, the reuse of expired Fluimucil could offer a green solution for mitigating bronze corrosion, which respects the principles of environmentally conscious corrosion control and the circular economy by extending the life cycle of pharmaceutical products, while simultaneously reducing pharmaceutical waste and its associated disposal costs. However, further research is needed to investigate the long-term stability and effectiveness of the drug, under different conditions, for cultural heritage and industrial applications.