1 Introduction

Aluminium and its alloys have received a great attention mostly in industry due to their high corrosion resistance, lightweight, low density, ductility, and high electrical and thermal conductivity [1, 2]. An initial formation of a compact and adhesive passive thin oxide layer at surface of aluminium metal is responsible for its high corrosion resistance [3, 4]. However, the presence of insidious ions like chlorine and aggressive acidic medium can easily dissolve protective oxide layers due to electrochemical corrosion, and thus, it further propagates to aluminium dissolution [5,6,7]. Hence, it is of great challenge to protect aluminium and its alloys from acidic medium. Nevertheless, many techniques have been employed in order to protect the electrochemical reaction of aluminium upon acidic medium such as conversion and organic coating, the use of corrosion inhibitors, and anodizing processes [8,9,10,11,12,13,14,15,16]. Among them, inhibition methods are widely accepted and proved to be a cost-effective process for aluminium corrosion where the use of organic molecules, containing oxygen, sulphur, and nitrogen, forms a protective film on aluminium surface which effectively attached by different mechanisms such as physisorption and chemisorption [17, 18].

Inhibition ability of most of the organic molecules and polymer-based compounds has been extensively studied for aluminium corrosion [19,20,21,22,23,24]. Recently, attempts have been made to investigate the inhibition effectiveness of medicinal drugs due to their easy availability and non-toxic nature. Gece et al. have identified about 17 medicinal drugs that could be used as efficient corrosion inhibitors for many metals and their alloys in different corrosive media [23]. Moreover, expired and/or unused drugs have received much attention so as to use as corrosion inhibitors for metals in various media [25]. In this regard, some of the drugs, namely antibacterial drugs, antifungal drugs, ciprofloxacin, norfloxacin and ofloxacin, cefazolin, fluoroquinolones, ketoconazole, and sulpha drugs, have been extensively studied as corrosion inhibitors for various metals and alloys [26,27,28,29,30,31,32,33,34,35,36]. In the case of aluminium corrosion, a couple of the drug molecules have been reported as efficient corrosion inhibitors for aluminium in acidic media [37, 38].

Herein, we demonstrate that the expired antibacterial drugs of moxifloxacin and betnesol act as efficient inhibitors for aluminium corrosion in 1 M H2SO4 solution. From the experimental studies, it is clearly shown that the inhibition efficiency of the inhibitors and transfer resistance increase with increasing inhibitor concentration. Moreover, polarization curves illustrate the anodic-type inhibitor nature of moxifloxacin and betnesol. In addition, thermodynamical and adsorption isotherm calculations further revealed that drug molecules have both chemisorption and physisorption on aluminium metal surface and follow the Langmuir adsorption isotherm.

2 Materials and Methods

2.1 Sample Preparation

High-purity aluminium specimens (~ 98.5%) with dimensions of 2.5 cm × 1.0 cm × 0.2 cm were used for all the corrosion-based experimental studies, where surface area of 1 cm2 was exposed during electrochemical experiments and the rest were covered with the epoxy resin. Prior to corrosion test, the aluminium specimens were finely polished using a silicon carbide papers with the grade sizes starting from 600 to 1200. The polished samples were properly rinsed, degreased by using AR grade acetone, then dried, and subsequently stored in vacuum desiccators. For weight loss measurements and electrochemical studies, the test solution of 1 M H2SO4 was prepared by dilution of analytical grade using double-distilled water. The drugs of moxifloxacin and betnesol were procured in the tablets form and then used after their expiry date of 1 year. The molecular structures of given drugs are shown in Fig. 1. The concentration of the inhibitors used for this study was ranged from 100 to 400 ppm.

Fig. 1
figure 1

Molecular structure of a betnesol, b moxifloxacin

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2.2 Methods

2.2.1 Weight Loss Measurements

The weight loss measurements were taken for all the aluminium specimens in 100 ml test solution of 1 M H2SO4 in the absence and presence of the inhibitors for 24 h at various concentrations and different temperatures range of 303–333 K. Triplicate experiment was conducted for the each concentration of the inhibitors so as to confirm the reproducibility, and the average weight loss values were taken to calculate the corrosion rate and inhibition efficiency of the chosen inhibitors. The corrosion rate, inhibition efficiency, and surface coverage were calculated using the following equation [39],

$${\text{Corrosion rate}},\, C_{{{\text{R}} }} = \frac{ w}{ST}$$
(1)

where ΔW is the average weight loss of aluminium specimen in the test solution, S is the exposure area, and T is the time.

Additionally, inhibition efficiency, η, is calculated by the following equation [40]

$$\eta \left( \% \right) = \left[ {\frac{{C_{{R\left( {\text{uninh}} \right)}} - C_{{R\left( {\text{inh}} \right)}} }}{{C_{{{\text{R}}\left( {\text{uninh}} \right)}} }}} \right] \times 100$$
(2)

where C R(uninh) and C R(inh) stand for the corrosion rate in the absence and presence of inhibitor in 1 M H2SO4 solution, respectively. Surface coverage area was calculated by the following equation,

$$\theta = \left[ {\frac{{C_{{{\text{R}}\left( {\text{uninh}} \right)}} - C_{{{\text{R}}\left( {\text{inh}} \right)}} }}{{C_{{{\text{R}}\left( {\text{uninh}} \right)}} }}} \right]$$
(3)

2.2.2 Electrochemical Measurements

Electrochemical (EC) measurements were taken in a three-electrode cell using CHI760 electrochemical work station at 303 K. Here, aluminium specimens were used as working electrodes, while platinum and saturated calomel act as a counter and reference electrodes, respectively. Prior to experiment, the working electrode was immersed in the test solution for 30 min until it reaches steady-state potential. The aluminium specimen was exposed of about 1 cm to various concentrations of inhibitors in 100 ml of 1 M H2SO4 at room temperature. After establishing open-circuit potential, the potential dynamic polarization curves were recorded with scan rate of 0.16 mv s−1 in the potential range from – 900 to – 600 mv. Corrosion current density values (I corr) were obtained from the Tafel polarization curves.

From the studies, inhibition efficiency, η, is defined as [41],

$$\eta \% = \frac{{I^{\text{o}}_{\text{corr}} - I^{\prime}_{\text{corr}} }}{{I^{\text{o}}_{\text{corr}} }} \times 100$$
(4)

where I ocorr and I ′corr are the values of current density in the absence and presence of inhibitors, respectively.

Electrochemical impedance measurements were taken using the same electrochemical cell and work station in the frequency range from 0.01 to 100 kHz using amplitude of 10 mV peak to peak with alternative current signal at open-circuit potential. The impedance data are obtained from Nyquist plots. The charge transfer values were calculated from the difference between low and high frequencies. The inhibition efficiency, η, was calculated from the charge resistance values obtained from the impedance measurements by using the following expression [42],

$$\eta \% = \frac{{R_{\text{ct}} - R^{\text{o}}_{\text{ct}} }}{{R_{\text{ct}} }} \times 100$$
(5)

where R ct and R oct are charge transfer resistance in the presence and absence of inhibitor, respectively.

2.2.3 Surface Morphology

The surface morphologies and elemental analysis of the aluminium specimens, immersed in 1 M H2SO4 solution for 4 h, and in the presence of 400 ppm drugs of betnesol and moxifloxacin were examined by using a JEOL scanning electron microscope, JSM-5800 (MODELS-340N). Aluminium samples with the dimension of 2.5 cm × 1.0 cm × 0.2 cm were subjected for the surface morphology studies.

2.2.4 Study of Synergistic Effect

The synergistic effect of halide ions with the inhibitor on the corrosion inhibition was studied by potentiodynamic polarization studies. The synergism parameter was estimated using the following equation proposed by Aramki and Hackermann [43,44,45,46],

$$S_{I} = \frac{{1 - I_{1 + 2} }}{{1 - I^{\prime}_{1 + 2} }}$$
(6)

where I 1+2 = (I 1 + I 2); I 1 is the inhibition efficiency of the halide, I 2, the inhibition efficiency of inhibitors, and \(I^{\prime}_{1 + 2 }\) the inhibition efficiency for betnesol and moxifloxacin in combination with halide ion.

3 Results and Discussion

3.1 Effect of Inhibitor Concentration

A conventional weight loss measurement was used to estimate the corrosion rate (C R) and inhibition efficiency of aluminium in 1 M H2SO4 solution in the presence and absence of different concentrations of betnesol and moxifloxacin. Figure 2a, b shows the corrosion rate of aluminium as the function of betnesol and moxifloxacin concentrations, respectively, at different temperatures. From the figures, it is clearly seen that corrosion rate drastically increases with temperature and substantially decreases with increasing the drug’s concentration. These results further indicate that used drugs of betnesol and moxifloxacin act as the strong inhibitors for aluminium corrosion. Typically, the calculated corrosion rate of aluminium in 1 M H2SO4 is ~ 6.41 mg cm−2 h−1 at 333 K, which decreases to 1.15  and 1.30 mg cm−2 h−1 for the 100 ppm concentrations of betnesol and moxifloxacin, respectively. The minimum corrosion rate is observed for the drugs at 400 ppm concentration, as shown in Fig. 2a, b. In addition, the inhibition efficiency, η, of drugs on aluminium corrosion was also calculated with different temperatures. The inhibition efficiency significantly increases with an addition of drug concentrations, and it substantially decreases with the increase in temperature, as shown in Fig. 3a, b. The maximum inhibition efficiencies of 95.31 and 86.17% at 303 K are achieved for the 400 ppm concentration of betnesol and moxifloxacin, respectively. From these results, it is obvious that both the drugs act as efficient inhibitors for aluminium corrosion in 1 M H2SO4 solution, which is probably due to the presence of active compounds in the drugs that strongly attach on the surface of aluminium and promotes anticorrosion. The effects of solution temperature and betnesol and moxifloxacin concentration on corrosion rate (C R) of aluminium and inhibition efficiency η (%) are summarized in Table 1.

Fig. 2
figure 2

Variation of corrosion rate on aluminium in 1 M H2SO4 with a betnesol, b moxifloxacin concentration at different temperatures

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Fig. 3
figure 3

Corrosion inhibition of aluminium expressed as inhibition efficiency (η%) as a function of a betnesol, b moxifloxacin concentration in 1 M H2SO4 solution at different temperatures

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Table 1 Temperature effect on aluminium corrosion rate and corrosion inhibition efficiency in 1 M H2SO4 with different concentrations of betnesol and moxifloxacin
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3.2 Potentiodynamic Polarization Measurement

Potentiodynamic polarization measurements were taken for the aluminium corrosion in 1 M H2SO4 solution with and without drugs in order to understand the kinetics of metal dissolution (anodic reaction) and evolution of hydrogen (cathodic reaction). Figure 4a, b shows the polarization curves of aluminium corrosion with different concentrations of betnesol and moxifloxacin in 1 M H2SO4 solutions. In Fig. 4, it can be clearly observed that the nature of polarization curves in the absence and presence of drug inhibitors shows the same trend; however, the presence of drug molecules in acidic solution significantly decreases the corrosion rate which can be visualized via notable reduction in corrosion current density and from the changes in the anodic and cathodic branches of polarization curves. This reduction in corrosion current density can be attributed to blocking of aluminium surface by the adsorption of drug molecules through active sites.

Fig. 4
figure 4

Polarization curves for aluminium in 1 M H2SO4 containing various concentrations of a betnesol, b moxifloxacin

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In order to further understand the kinetic process of aluminium corrosion in 1 M H2SO4 solution with and without drug molecules, the electrochemical parameters, such as corrosion current density (I corr), current potential (E corr), cathodic Tafel slope (β c), anodic Tafel slope (β a), and the corresponding inhibition efficiency η (%), were deduced from the polarization curves, and the values are summarized in Table 2.

Table 2 Corrosion parameters obtained from potentiodynamic polarization curves for corrosion of aluminium in 1 M H2SO4 with different concentrations of betnesol and moxifloxacin at room temperature
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From the calculated parameters, it can be seen that corrosion current density significantly reduces from ~ 419 to 19 µA cm2 for betnesol, whereas it decreases to 62 µA cm2 for moxifloxacin, which clearly illustrates that the used drugs act as the good inhibitors for aluminium corrosion in 1 M H2SO4 solution. An addition of both the drug molecules in the acidic solution tries to move the corrosion potential values (E corr) towards more positive side (from Fig. 4a, b), which manifests that the utilized drugs act as mixed type inhibitors but predominantly anodic [47]. Moreover, the values of cathodic Tafel slope (β c) and anodic Tafel slope (β a) substantially decrease with the addition of drugs as compared to blank solution, which indicates that both anodic (dissolution of metal) and cathodic (evolution of hydrogen) reactions can be effectively suppressed by the used drugs. From all the experiments, it is concluded that inhibition efficiency substantially increases with the addition of drug concentration and achieved the maximum efficiency of 95.40 and 85.01% in both the drugs of betnesol and moxifloxacin, respectively, for the inhibitor concentration of 400 ppm (refer Table 2). When comparing both the polarization curves of the two drugs (Fig. 4a, b), inhibition efficiency of inhibitors increases with inhibitor concentration, and betnesol in fact possesses a better inhibition capability than that of moxifloxacin drug for aluminium corrosion in 1 M H2SO4 solution (see Table 2).

3.3 EIS Measurements

Weight loss measurements and potentiodynamic polarization curves showed that the inhibition efficiency increases with the increase in concentrations of betnesol and moxifloxacin. With an aim to explore the corrosion mechanism of these drugs and their inhibition efficiencies on aluminium surface in 1 M H2SO4 solution, the electrochemical impedance measurements were taken for all the samples over the frequency range 0.01 Hz to 100 kHz at open-circuit potential. The electrochemical impedance spectrum in the form of Nyquist plots is shown in Fig. 5. In the impedance diagram, all the blank and inhibitor mixed samples show only one single capacitive loop with slightly depressed charge transfer semicircles, which represents that corrosion of aluminium in 1 M H2SO4 solution is mainly dominated by charge transfer process, the formation of protective layer on top of aluminium surface [48, 49]. From the Nyquist plots, it is clear that the impedance of aluminium surface increases with the addition of drugs concentration. Moreover, the diameter of the semicircle capacitive loop increases with increasing the inhibitor concentration (see Fig. 5a, b), which ultimately means that adsorption of inhibition molecules strongly adsorbs the active surface of the aluminium specimens.

Fig. 5
figure 5

Impedance curves for aluminium in 1 M H2SO4 a betnesol, b moxifloxacin at various concentrations

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For all the obtained Nyquist plots, an appropriate equivalent circuit diagram was proposed and the impedance parameters were deduced from the diagram. Figure 6 shows the proposed equivalent diagram, where Rs is the solution resistance, and R ct is the charge transfer resistance. The impedance parameters, such as solution resistance R s, charge transfer resistance R ct, double-layer capacitance C dl, and inhibition efficiency η%, are presented in Table 3.

Fig. 6
figure 6

Equivalent circuit diagram used to fit impedance data for aluminium corrosion in 1 M H2SO4 in the absence and presence of inhibitors

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Table 3 Impedance parameters derived from spectra recorded for aluminium in 1 M H2SO4 with varying concentrations of betnesol and moxifloxacin at room temperature
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From Table 3, it is clearly seen that when inhibitor concentration increases, the charge transfer resistance increases, whereas C dl value drastically reduces. Typically, the value of C dl decreases from 115 to 89 µF cm2 for betnesol, whereas it further reduces to 76 µF cm2 for moxifloxacin. This huge reduction of C dl with the addition of inhibitor concentration is due to the increase in the thickness of the electrical double layer and/or decrease in local dielectric constant as per the following equation [50],

$$C_{\text{dl}} = \frac{{\varepsilon \varepsilon_{0} A}}{\delta }$$
(7)

where ε is a dielectric constant of the medium, ε0 is the vacuum permittivity, A is the electrode area, and δ is the thickness of the protective layer. This notable decrease in C dl values indicates that the drug molecules strongly adsorb at the interface of aluminium metal and solution, and forms a thin protective layer that strongly prevents the further dissolution of aluminium and promotes anticorrosion. The calculated inhibition efficiencies from EIS studies are ~ 94 and ~ 85% for betnesol and moxifloxacin, respectively. From the studies, it is noteworthy to say that the used drugs of betnesol and moxifloxacin with the concentrations of 400 ppm act as efficient inhibitors for aluminium corrosion in 1 M H2SO4 by the formation of protective thin layer at the interface of aluminium and solution. Furthermore, the results thus obtained from EIS method have a good agreement with the weight loss measurements.

3.4 Effect of Temperature

In order to study the effect of temperature on corrosion rate and the inhibition efficiency of betnesol and moxifloxacin inhibitors, the weight loss measurements were taken in the temperature range 303–333 K in the absence and the presence of inhibitors at various concentrations. The experimental results are summarized in Table 1. As seen from Fig. 2 and Table 1, as temperature increases, corrosion rate substantially increases and conversely inhibition efficiency slowly decreases for the blank and inhibited samples. Typically, CR increases from ~ 0.64 to ~ 6.41 mg cm−2 h−1 as temperature increases from 303 to 333 K for the blank sample, while C R increases from 0.06 to 0.36 mg cm−2 h−1 and 0.12 to 0.63 mg cm−2 h−1 for betnesol and moxifloxacin inhibitors at the concentration of 400 ppm in 1 M H2SO4 solution. This increase in C R is due to the evolution of hydrogen gas with higher dissolution of metal. Further, the activation energy, E a, of corrosion process can be determined from the following Arrhenius equation [51],

$$C_{\text{R}} = A\exp \left( { - \frac{{E_{\text{a}} }}{RT}} \right)$$
(8)

where C R is the corrosion rate, R the gas constant, T the absolute temperature, and A the pre-exponential factor. Figure 7a, b shows the logarithmic C R as a function of temperature (1/T), and the calculated activation energies (E a) at different concentrations of betnesol and moxifloxacin in 1 M H2SO4 are presented in Table 4. It is clearly seen that activation energy substantially decreases with the increase in inhibitor concentration, which signifies that corrosion of aluminium is suppressed by the used inhibitors with specific interactions of aluminium surface and inhibitor molecules. In particular, reduction of activation energy in the presence of drug inhibitors suggests that interactions of inhibitors on metal surfaces are chemisorption, meaning that corrosion of aluminium has been suppressed by electron transfer mechanism [52, 53].

Fig. 7
figure 7

Arrhenius plots for the corrosion rate of aluminium in 1 M H2SO4 solution in the absence and presence of a betnesol, b moxifloxacin at different concentrations

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Table 4 Calculated activation energy (Ea) values for aluminium corrosion rate in 1 M H2SO4 in the absence and presence of different concentrations of betnesol and moxifloxacin
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3.5 Adsorption Isotherm

In order to understand the interaction between aluminium metal surface and utilized drug inhibitors of betnesol and moxifloxacin, our experimental results were fitted with various adsorption isotherms such as Frumkin, Langmuir, Temkin, and Freundlich. The experimental data of dissolution of aluminium metal in 1 M H2SO4 with and without inhibitors were best fitted with the following Langmuir adsorption isotherm equation,

$$\frac{C}{\theta } = \frac{1}{{K_{\text{ads}} }} + C$$
(9)

where C is the concentration of inhibitor, K ads is equilibrium constant of adsorption, and θ is surface coverage. In all the experimental data, the correlation coefficient, R 2, was used to determine the adsorption isotherm, where higher values of R 2 > 0.99 suggest that adsorption of inhibitor on aluminium surface follows the Langmuir adsorption isotherm. Figure 8a, b shows the C/θ versus C plots, which strongly fits with Langmuir adsorption and its slope value of nearly unity further confirms the proposed isotherm. With this, it can be mentioned that used drug inhibitors form a very thin protective layer on top of aluminium active sites and promote anticorrosion of aluminium in 1 M H2SO4 solution.

Fig. 8
figure 8

Langmuir adsorption plot for aluminium in 1 M H2SO4 containing different concentrations of a betnesol, b moxifloxacin at different temperatures

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The equilibrium constant of adsorption process, K ads, was calculated from the intercept of C/θ axis of Fig. 8a, b, and the temperature-dependent values of K ads are listed in Table 5. A higher value of K ads indicates that the inhibitor is strongly adsorbed on the aluminium surface.

Table 5 Adsorption parameter derived from Langmuir adsorption isotherms for aluminium corrosion in 1 M H2SO4 at different temperatures
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The standard free energy change of adsorption (ΔG oads ) and the adsorption constant (K ads) is related by the following expression [54],

$$\Delta G_{\text{ads}}^{\text{o}} = - RT\ln \left( {55.5\, K_{\text{ads}} } \right)$$
(10)

where 55.5 is the molar concentration of water in the solution and T is the absolute temperature.

The calculated temperature-dependent ΔG°ads is presented in Table 5. In addition, as temperature increases, the values of ΔG°ads slightly reduce from − 19.88 to −23.26 kJ mol−1 and −20.95 kJ mol−1 to − 24.12 kJ for betnesol and moxifloxacin, respectively. This observed reduction in ΔG°ads and negative values suggest that the adsorption process of inhibitors is spontaneous and the inhibitor strongly adsorbs on metal surface of aluminium in 1 M H2SO4 solution. Moreover, all the calculated values of ΔG°ads vary from − 19 to −30 kJ mol−1 for the individual inhibitors of betnesol and moxifloxacin with the concentration of 400 ppm, which implies that the adsorption mechanism on aluminium surface might be due to both physisorption and chemisorption [55].

Further, to explore the mechanism of corrosion, other thermodynamic functions of enthalpy (ΔH) and entropy (ΔS) were calculated by the following expression,

$$\Delta G_{\text{ads}}^{\text{o}} = \Delta H_{\text{ads}}^{\text{o}} - \Delta S_{\text{ads}}^{\text{o}}$$
(11)

where ΔH oads and ΔS oads are the enthalpy and entropy of adsorption, respectively. The free energy of adsorption as a function of temperature is presented in Fig. 9. The slope and incept of the straight line give rise to the adsorption values of ΔH oads and ΔS oads , respectively. In particular, ΔH oads has the values of − 2.378 and − 3.5395 kJ mol−1 and ΔS oads has the values of 0.0264 and 0.0236 kJ mol−1 for betnesol and moxifloxacin inhibitors, respectively. The negative values of ΔH and positive values of ΔS indicate that the adsorption process is spontaneous in nature.

Fig. 9
figure 9

Plot of the standard free energy of adsorption versus T

T

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3.6 SEM and FTIR Analysis

Figure 10 presents the scanning electron micrographs of aluminium samples with and without the presence of inhibitors for the concentration of 400 ppm in 1 M H2SO4 solution. Figure 10a reflects a corroded surface of aluminium metal with deep pits, which was dipped in the aggressive corrosive solution of 1 M H2SO4 for 4 h, coined as blank (no inhibitor added). Figure 10b, c shows the surface morphologies of aluminium immersed in 1 M H2SO4 in the presence of the 400 ppm moxifloxacin and betnesol, respectively. The surface roughness of aluminium and deep pits created by aggressive H2SO4 has been remarkably reduced in the presence of inhibitors, because the strong interaction of inhibitor molecules is seen on top of aluminium surface. The micrographs of inhibited samples further confirm that drugs thus used as inhibitors form a thin protective layer by attaching on active sites of aluminium and thereby promote anticorrosion. As a comparison, betnesol offered the best inhibition effect on the aluminium surface in 1 M H2SO4 compared to moxifloxacin. Moreover, to further confirm the inhibition stability of utilized drug molecules, the morphologies of inhibited samples have repeated after a few months and their micrographs are shown in Fig. 10d, e. From SEM micrographs, one could conclude that the used drugs actively participate on anticorrosion even after a couple of months.

Fig. 10
figure 10

SEM images of aluminium sample a blank, b 400 ppm moxifloxacin, c 400 ppm betnesol, d after few months later 400 ppm moxifloxacin, e after few months later 400 ppm betnesol

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Further, in order to confirm the presence of functional groups in drugs of moxifloxacin and betnesol and to clarify the effect of corrosion inhibition within shelf life, FTIR studies have performed for the drugs with before and after the expired date. The FTIR spectrums of both drugs are shown in Fig. 11a, b. In particular, betnesol shows a characteristic O–H stretching, C=O stretching band, and C–O of phenol stretching at 3326, 1604, and 1272 cm−1, respectively [56]. The IR spectrum of the moxifloxacin is characterized by principal absorption peaks at 3527 cm−1 for O–H stretching, 2950 cm−1 for C–H symmetric and asymmetric stretching, 1708 cm−1 for C=O stretching, peaks at 1623, 1515, and 1454 cm−1 for C=C stretching, 1319 cm−1 for C–N stretching, 1184 cm−1 for monofluorobenzene stretching, 875 cm−1 for C–H bending of substituted benzene. These FTIR spectra of moxifloxacin have a good agreement with reported data [57, 58]. The FTIR spectrum of both moxifloxacin and betnesol shows no significant change in the drug molecules. This suggests that chosen drugs do not show any noticeable degradation even after expiry date.

Fig. 11
figure 11

FTIR spectrums of a betnesol, b moxifloxacin

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3.7 Synergism parameter

The synergistic effect of halide ions on the used inhibitor of betnesol and moxifloxacin in the corrosion inhibition was extensively studied by potentiodynamic polarization studies. In general, synergistic parameter, S > 1 indicates a synergistic inhibitive effect, whereas S < 1 represents an antagonistic effect. It is well known that the halide-based ions can effectively promote the adsorption of the organic cations by acting as an intermediate layer between the positively charged aluminium surface and the positive end of the utilized organic inhibitor [59,60,61,62,63,64,65,66]. The synergistic parameters were calculated using the polarization studies with the addition of 100 ppm concentration of KCl, KBr, and KI in the solution of 400 ppm inhibitor’s concentration, and the data are listed in Table 6. From the results, it is clearly seen that the addition of iodide ions with both the inhibitors (betnesol and moxifloxacin) shows synergistic effect, whereas chloride and bromide ions exhibit antagonistic effect. This is due to the fact that iodine has a large ionic radius, high hydrophobicity, and low electronegativity, compared to the other halide ions (Cl or Br), which ultimately helps to the adsorption of the inhibitor molecules onto the aluminium metal surface [67].

Table 6 Synergism parameter (S) for 400 ppm in combination with 100 ppm concentration of KCl, KBr, KI
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3.8 Mechanism of inhibition

From the electrochemical theory of corrosion, it has been clearly understood that the corrosion process of aluminium in acidic solution can be explained by anodic and/or cathodic process where anodic process refers the passage of metal ions from the surface of solid metal to solution and cathodic reaction can be described as the discharge of hydrogen ions in order to produce hydrogen molecules or can be stated as reduction of oxygen. This could be generally described as follows [24],

$$\begin{array}{*{20}l} {{\text{Al }} \to {\text{ Al}}^{3 + } + 3{\text{e}}^{ - } } \hfill \\ {{\text{Al}}\left( {\text{OH}} \right)_{{3({\text{aq}})}} + \, 3{\text{H}}^{ + } \to {\text{ Al}}_{{({\text{aq}})}}^{3 + } + 3{\text{H}}_{2} {\text{O}}} \hfill \\ \end{array}$$

However, corrosion inhibitors can significantly suppress these anodic or cathodic processes either by physisorption or by chemisorption, where interactions of metal and inhibitors may be due to electrostatic interaction or electron transfer process, respectively. In general, the organic molecules/compounds which consist of the elements of O, N, P, and S, the functional groups of C–O, C=O, –OH, and N–H, and O-heterocyclic rings, have been reported to be the efficient inhibitors for corrosion of aluminium and other metal alloys as those functional groups vigorously attach on the active sites of aluminium and form a thin corrosion protective layer.

In the present work, the utilized expired drugs of betnesol and moxifloxacin have the strong functional groups of C–O, C=O, –OH, –NH (refer Fig. 1), which synergistically occupy the active sites of aluminium and lead to the formation of a thin protective layer on surface of aluminium. This passive layer protects the further corrosion of aluminium against 1 M H2SO4 solution. Since every single functional group participates on the formation of thin protective layer due to the specifically attracted active sites of aluminium, this corrosion inhibition could be possible by synergetic role of mixed functional groups. Moreover, experimental data strongly obey Langmuir adsorption isotherm, which in fact supports that chosen drug inhibitors form a very thin protective layer on top of aluminium. Moreover, the observed negative value of ΔG°ads with the range from − 19 to − 30 kJ mol−1 further confirms the spontaneous adsorption of inhibitors on aluminium surface by both physisorption and chemisorption processes. All the experiments including weight loss method, potentiodynamic polarization curves, electrochemical impedance spectroscopy, and theoretical adsorption calculations have also shown that used drug molecules act as excellent inhibitors for aluminium corrosion. In particular, betnesol has shown better inhibition performance than that of moxifloxacin for aluminium corrosion in 1 M H2SO4 solution, which may be due to the presence of excess −OH groups with the aromatic ring.

4 Conclusion

The expired drugs of moxifloxacin and betnesol were extensively studied as the corrosion inhibitors for aluminium in 1 M H2SO4 solutions. The experimental results showed that the addition of moxifloxacin and betnesol inhibitors significantly helps to increase the inhibition efficiency of aluminium at various temperatures. Moreover, potentiodynamic polarization curves illustrated both the drugs of betnesol and moxifloxacin act as mixed type inhibitors but predominantly anodic. In summary, both the drugs promote the anticorrosion of aluminium in 1 M H2SO4 solution by both physisorption and chemisorption and followed the Langmuir isotherm model. In comparison, betnesol has shown slightly better inhibition efficiency against aluminium corrosion in 1 M H2SO4 than that of moxifloxacin drug.