# Corrosion Inhibition of 3003 Aluminum Alloy in Molar Hydrochloric Acid Solution by Olive Oil Mill Liquid By-Product

Authors: M. Chadili; M. M. Rguiti; B. El Ibrahimi; R. Oukhrib; A. Jmiai; M. Beelkhaouda; L. Bammou; M. Hilali; L. Bazzi
Journal: International Journal of Corrosion
 (2021)

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## Abstract

According to the literature, the works on the inhibition of aluminum alloy corrosion using naturally occurring compounds are limited. For this, the inhibiting effect of oil mill liquid by-product (OMW) on the corrosion of 3003 aluminum alloy (AA3003) in molar hydrochloric acid solution was evaluated using electrochemical techniques. In parallel, a computational approach based on DFT/B3LYP andMonte Carlo methods was used to understand the inhibition process under electronic and atomic scales, respectively. The experimental results reveal that OMW has a good inhibiting effect on the corrosion of AA3003 alloy in the tested solution and acts as a cathodic inhibitor. The inhibitory efficiency increases by increasing OMW concentration to attain 89% at 6.0 ppm. The effect of temperature shows that the inhibition efficiency of OMW decreases with temperature rising. Nevertheless, a good prevention capacity of 83% is obtained at 338 K. Such interesting achieved protection property was attributed to the adsorption of OMW constituents onto the alloy surface via a mixed physichemisorption process. This process is found to obey the Langmuir adsorption isotherm. Furthermore, the activation thermodynamic parameters of the corrosion process of AA3003 alloy were also determined and discussed. The computational outcomes outlined the ability of the OMW components to interact favorably with the metal surface, hence the formation of a protective layer, which justified the observed inhibition behaviors. Conferring to the present study, OMW can be used as a good green corrosion inhibitor for AA3003 alloy in the acidic medium.


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## Body

## 1. Introduction

The corrosion affects the industrial sector and can cost billions of dollars each year [1]. Besides, it causes a significant loss of energy and contributes to environmental degradation [2]. In the case of industrial processes, metals and metal alloys are attacked by acid solutions, which act as aggressive agents. In general, acids are heavily used in industry, mainly in industrial cleaning, petroleum refining, and petrochemical processes [3, 4]. Aluminum and its alloys are widely used in various fields because of their physicochemical properties compared to other materials [5]. The corrosive medium of hydrochloric acid is commonly used for chemical or electrochemical cleaning and acid stripping of aluminum. In aqueous solutions, a passive oxide film will be formed which is compact and adheres to the aluminum surface [6]. The film (amphoteric) formed is substantially dissolved in an acidic or basic medium and undergoes the corrosion phenomena [7, 8]. To combat this scourge, several techniques are used: alloy selection, anodic protection, and the use of inhibitors [9, 10]. The introduction of corrosion inhibitors into the aggressive solution is an effective method to stop or delay corrosion by using organic or inorganic compounds [11, 12].Until now, organic compounds containing N, O, P, or S heteroatoms are considered to be good corrosion inhibitors for aluminum in the acid environment, especially aromatic amines [13], carbonyl compounds [14], amino acids [15], phenols [16], polymers [17], and aliphatic amines [18, 19]. However, among those compounds, they have high toxicity to humans and the environment [20–22]. In recent decades, various studies have focused on the use of so-called ecological or green herbal inhibitors, namely, extracts and essential oils of aromatic and medicinal plants [23]. The choice of these ecological inhibitors essentially resides in their cost, their biodegradability, and a high efficiency generally exceeding 95% at low concentrations [23–26]. On the other hand, they are known as reducers of the corrosion of metals and their alloys in the acidic environment, to replace the toxic products currently used [27]. The inhibitory effect of these green inhibitors is due to the presence of high variability and richness of organic compounds that can be adsorbed on the metal surface, which leads to the formation of a protective film and therefore causes the limitation of the corrosion phenomenon [28, 29].Each year, several industrial processes in the Mediterranean region, such as Morocco, release large quantities of liquid waste during the olive oil production season [29]. These effluents, so-called olive oil mill wastewaters (OMW), can be considered a source of ecological inhibitors. OMW contains a variety of phenolic compounds with heterocyclic structures comprising nitrogen and oxygen atoms, which are responsible for inhibiting the corrosion process [30]. Furthermore, these effluents are not too expensive from an economic point of view compared to conventional synthetic inhibitors. Several authors have evaluated the property of OMW to act as a corrosion inhibitor. They have found better corrosion inhibition performances for steel [31] and iron [32]. In this regard, exploiting OMW as a corrosion inhibitor on an industrial scale can minimize also the damage to the environment. According to the available literature, there is no work yet on the use of OMW as anticorrosion compounds for aluminum alloys. For this, the current study will be focused on this subject.According to the above-mentioned view, this work is aimed at examining the protective effect of OMW toward the corrosion of 3003 aluminum alloy in 1 M hydrochloric acid. To evaluate the protective power of this inhibitor, potentiodynamic polarization and electrochemical impedance spectroscopy techniques were used. In this regard, the influence of OMW concentration and temperature on the observed inhibition behaviors was investigated.In recent years, the computational demarche has been widely served to investigate the inhibition process of metallic corrosion at electronic and atomic scales by using DFT and Monte Carlo methods, respectively [31, 32]. Besides, it is recognized that OMW is a mixture of several phenolic-based compounds [33]. In the present study, we have limited our electronic and atomic inspections on tyrosol L (Tyr) and hydrotyrosol L (HydroTyr) molecules, which are its major constituents [34]. In this context, we limited the use of the DFT/B3LYP method to investigate the reactivity of Tyr and HydroTyr molecule and their implicit interfacial interaction with the aluminum surface as well [31, 35], while the explicit interactions were discussed using the Monte Carlo/simulated annealing method as reported widely in the literature [36].


## 2. Materials and Methods

### 2.1. Materials and Solutions

All used reagents in this work were analar grade, and the distilled water was used to prepare solutions. The used aggressive solution (1 M HCl) was prepared by diluting a commercial hydrochloric acid solution (37%), provided by Fuka, with distilled water. A 3003 aluminum alloy (AA3003 notation will use subsequent) plate was used to conduct the current study. The composition of AA3003 alloy is summarized in Table1.Table 1
Nominal composition of used AA3003 alloy.
ElementFeSiCuZnMnNiTiMgCrAlContent (%)0.190.44<0.03<0.0020.04<0.003<0.0020.53<0.002BalanceThe OMW samples used in this study were taken from the third phase olive mill located in the region of Essaouira, Morocco (Mejji olive mill). The sample of OMW has been obtained from olives well-ripened and stored at 4°C. The effluent was subjected to several successive filtrations to remove the maximum suspended matter. The physicochemical characterizations of OMW were carried out according to the standard analytical methods for the examination of water and wastewater [37, 38]. The physicochemical characteristics of OMW are listed in Table 2.Table 2
Physicochemical characterization of EOMW used in the corrosion tests.
Parameter (unit)ValueTurbidity (NTU)2805pH5.22Conductivity (ms cm-1)10.52BOD5 (g L-1)62COD (g L-1)180K (g L-1)2.91Na (g L-1)2.71Cl (g L-1)3.37Density (g mL-1)1.03In order to study the inhibition capacity of OMW against the corrosion of AA3003, 10 mL of OMW is added to one liter of 1 M HCl solution, which is stirred for 30 min. The resulting aqueous solution was stored as the stock solution in the dark and at 4°C [39]. OMW concentrations ranging from 0.5 to 6.0 ppm were prepared by diluting the stock solution using 1 M HCl solution.


### 2.2. Electrochemical Methods

The AA3003 plate was used to prepare the working electrode for the electrochemical study, which was cut to obtain a rod and then mounted in glass tubes with a two-component epoxy resin leaving a contact area of 0.16 cm2. For the electrochemical tests, we used a thermostatic triple-walled glass cell with a platinum counterelectrode and a saturated calomel reference electrode. Prior to each experiment, the surface of the working electrode was mechanically polished using 1200-grade emery paper, washed with distilled water, and rapidly transferred to the electrochemical cell containing the test solution.The electrochemical measurements were performed using a VersaStat3 potentiostat/galvanostat and VersaStudio as control software. Prior to each experiment, the open circuit potential (OCP) of the AA3003 working electrode was measured as a function of time for 30 min, which was the appropriate time to reach a quasistationary state. The electrochemical impedance spectra have been recorded at the steady OCP value with 10 mV as the amplitude of the superimposed alternating signal, and the applied frequency varied from 100 kHz to 0.01 Hz. The anodic and cathodic polarization experiments were carried out at a scanning rate of 1 mV s-1 between -1200 and -500 mV/SCE. To achieve reproducibility, each experiment was repeated at least three times. The EIS data is analyzed with software based on a simplex regression of the parameters (ZSimpWin 3.1 software). All experiments were performed in a stagnant natural aerated solution at the chosen temperature using a water thermostat.


### 2.3. DFT Calculations

The electronic structure of the two main constituents of OMW (i.e., Tyr and HydroTyr) was studied using the DFT/B3LYP/6-311G(d,p) level of theory as implemented in Gaussian (version 09) software [40]. The aqueous phase was treated using the IEFPCM solvation model and setting water as solvent [41]. After the geometry optimization stage, some relevant electronic descriptors, i.e., gap energy (ΔE), chemical hardness (η), fraction of electrons transferred (ΔN), and total negative charge (TNC), of the molecules were calculated. The latest descriptors have been fully defined elsewhere, e.g., see [42]. To investigate the favorable centers of adsorption within these molecules, the frontier molecular orbital repartitions and the molecular electrostatic potential (ESP) map were plotted using GaussView (version 05) software and then discussed [43].


### 2.4. Monte Carlo Simulations

To investigate the adsorption process of Tyr and HydroTyr on the aluminum surface, the Monte Carlo simulations were carried out in the aqueous phase (100 H2O). The metallic substrate was modeled by five layers of Al(111) surface with 17.2 Å ×17.2 Å dimensions [44]. To avoid the possible intersupercell interactions that can be caused by the periodic boundary condition, a sufficient vacuum region of 50 Å is used [45]. Van der Waals and electrostatic interactions were treated using atom-based and Ewald summation methods, respectively. As a force field, the COMPASS is employed [46]. For the simulated annealing process, ten cycles of heat are used with 100 000 steps per each cycle. To get accurate results, the smart algorithm is employed and the convergence tolerances were fixed at 2×10−5 kcal mol-1, 10-3 kcal mol-1 Å-1, and 10-5 Å for energy, force, and displacement, respectively. The simulations were carried out using Material studio (version 06) software.


## 2.1. Materials and Solutions

All used reagents in this work were analar grade, and the distilled water was used to prepare solutions. The used aggressive solution (1 M HCl) was prepared by diluting a commercial hydrochloric acid solution (37%), provided by Fuka, with distilled water. A 3003 aluminum alloy (AA3003 notation will use subsequent) plate was used to conduct the current study. The composition of AA3003 alloy is summarized in Table1.Table 1
Nominal composition of used AA3003 alloy.
ElementFeSiCuZnMnNiTiMgCrAlContent (%)0.190.44<0.03<0.0020.04<0.003<0.0020.53<0.002BalanceThe OMW samples used in this study were taken from the third phase olive mill located in the region of Essaouira, Morocco (Mejji olive mill). The sample of OMW has been obtained from olives well-ripened and stored at 4°C. The effluent was subjected to several successive filtrations to remove the maximum suspended matter. The physicochemical characterizations of OMW were carried out according to the standard analytical methods for the examination of water and wastewater [37, 38]. The physicochemical characteristics of OMW are listed in Table 2.Table 2
Physicochemical characterization of EOMW used in the corrosion tests.
Parameter (unit)ValueTurbidity (NTU)2805pH5.22Conductivity (ms cm-1)10.52BOD5 (g L-1)62COD (g L-1)180K (g L-1)2.91Na (g L-1)2.71Cl (g L-1)3.37Density (g mL-1)1.03In order to study the inhibition capacity of OMW against the corrosion of AA3003, 10 mL of OMW is added to one liter of 1 M HCl solution, which is stirred for 30 min. The resulting aqueous solution was stored as the stock solution in the dark and at 4°C [39]. OMW concentrations ranging from 0.5 to 6.0 ppm were prepared by diluting the stock solution using 1 M HCl solution.


## 2.2. Electrochemical Methods

The AA3003 plate was used to prepare the working electrode for the electrochemical study, which was cut to obtain a rod and then mounted in glass tubes with a two-component epoxy resin leaving a contact area of 0.16 cm2. For the electrochemical tests, we used a thermostatic triple-walled glass cell with a platinum counterelectrode and a saturated calomel reference electrode. Prior to each experiment, the surface of the working electrode was mechanically polished using 1200-grade emery paper, washed with distilled water, and rapidly transferred to the electrochemical cell containing the test solution.The electrochemical measurements were performed using a VersaStat3 potentiostat/galvanostat and VersaStudio as control software. Prior to each experiment, the open circuit potential (OCP) of the AA3003 working electrode was measured as a function of time for 30 min, which was the appropriate time to reach a quasistationary state. The electrochemical impedance spectra have been recorded at the steady OCP value with 10 mV as the amplitude of the superimposed alternating signal, and the applied frequency varied from 100 kHz to 0.01 Hz. The anodic and cathodic polarization experiments were carried out at a scanning rate of 1 mV s-1 between -1200 and -500 mV/SCE. To achieve reproducibility, each experiment was repeated at least three times. The EIS data is analyzed with software based on a simplex regression of the parameters (ZSimpWin 3.1 software). All experiments were performed in a stagnant natural aerated solution at the chosen temperature using a water thermostat.


## 2.3. DFT Calculations

The electronic structure of the two main constituents of OMW (i.e., Tyr and HydroTyr) was studied using the DFT/B3LYP/6-311G(d,p) level of theory as implemented in Gaussian (version 09) software [40]. The aqueous phase was treated using the IEFPCM solvation model and setting water as solvent [41]. After the geometry optimization stage, some relevant electronic descriptors, i.e., gap energy (ΔE), chemical hardness (η), fraction of electrons transferred (ΔN), and total negative charge (TNC), of the molecules were calculated. The latest descriptors have been fully defined elsewhere, e.g., see [42]. To investigate the favorable centers of adsorption within these molecules, the frontier molecular orbital repartitions and the molecular electrostatic potential (ESP) map were plotted using GaussView (version 05) software and then discussed [43].


## 2.4. Monte Carlo Simulations

To investigate the adsorption process of Tyr and HydroTyr on the aluminum surface, the Monte Carlo simulations were carried out in the aqueous phase (100 H2O). The metallic substrate was modeled by five layers of Al(111) surface with 17.2 Å ×17.2 Å dimensions [44]. To avoid the possible intersupercell interactions that can be caused by the periodic boundary condition, a sufficient vacuum region of 50 Å is used [45]. Van der Waals and electrostatic interactions were treated using atom-based and Ewald summation methods, respectively. As a force field, the COMPASS is employed [46]. For the simulated annealing process, ten cycles of heat are used with 100 000 steps per each cycle. To get accurate results, the smart algorithm is employed and the convergence tolerances were fixed at 2×10−5 kcal mol-1, 10-3 kcal mol-1 Å-1, and 10-5 Å for energy, force, and displacement, respectively. The simulations were carried out using Material studio (version 06) software.


## 3. Results and Discussion

### 3.1. Potentiodynamic Polarization Results

The effect of the concentration of OMW on the corrosion behavior of AA3003 alloy in the 1 M HCl solution was studied by using anodic and cathodic polarization measurements after 30 min of the immersion time. Figure1 illustrates the potentiodynamic polarization curves for the alloy understudy in the absence and presence of OMW with different concentrations at 298 K.Figure 1
Polarization curves of AA3003 alloy in 1 M HCl solution without and with addition of OMW at different concentrations (at 298 K).As can be seen in the anodic branch (Figure1), we can point out the difficulty of recognizing the linear region of the Tafel law. Hence, the current densities are determined by extrapolating only the Tafel cathodic linear region to the corrosion potential. This method of adjustment was used by several authors for aluminum-based materials in the hydrochloric acid medium [25, 47]. The relevant determined electrochemical parameters, namely, current density (Icorr), corrosion potential (Ecorr), and cathodic slope (βc), as well as the inhibitory efficiency (IE%, Equation (1)) as a function of the OMW concentration, are tabulated in Table 3:
(1)IE%=1−Icorr′Icorr×100,where Icorr and Icorr′ are, respectively, the uninhibited and inhibited corrosion current densities of AA3003 alloy.Table 3
Derived polarization electrochemical parameters of AA3003 alloy in 1 M HCl medium in the absence and presence of OMW at different concentrations (at 298 K).
Conc. (ppm)IcormAcm−2−EcormV/SCE−βcmV/decIE%θBlank11.866811134——0.57.61982214335.790.35791.05.14081213156.680.56682.04.40381912562.890.62893.04.09081213673.940.73944.02.12781513282.070.82075.01.89481913884.000.84006.01.41981611588.040.8804It is clear from Table3 that the Tafel cathodic slope (βc) changes slightly in the presence of the OMW, which outlines no significant effect of OMW on the hydrogen reduction mechanism. Such a conclusion can be revealed from Figure 1 from which the cathodic polarization curves are almost parallel. On the other hand, it is found that the value of Icorr decreases with the addition of the OMW, which induces an increase of the inhibition efficiency as a function of the OMW concentration. This is due to the increase of the blocked fraction of the alloy surface by the adsorption of the active components present in OMW, as well as their synergistic effect [48]. The prevention effectiveness reaches 88% with a low concentration of OMW (6.0 ppm), indicating that OMW is an excellent inhibitor of AA3003 alloy in the 1 M HCl medium. Conferring to Figure 1, the OMW can be classified as a cathodic inhibitor due to the reduction of cathodic currents by its addition with respect to the uninhibited solution [49].


### 3.2. Electrochemical Impedance Spectroscopy Results

The Nyquist diagrams for studied aluminum alloy in 1 M HCl medium at 298 K, in the absence and presence of different OMW concentrations, were plotted inEOCP potential after 30 min of immersion as depicted in Figure 2. It is evident from this figure that the impedance diagrams consist of a large capacitive loop at high frequency followed by an inductive loop at low frequency. The first loop is due to the formation of an oxide film on the aluminum surface, while the second one is attributed to the relaxation of the charged intermediates adsorbed on the surface tested [50, 51]. In addition, the inductive semicircle diameter is considerably less than that observed in high frequencies. On the other hand, the impedance spectrum shows an approximately elliptical shape that is attributed to the frequency dispersion due to the roughness and inhomogeneity of the metal surface [48]. In the acidic media, similar impedance spectra have been reported in the corrosion literature for aluminum and its alloys [23, 25, 26, 28–30, 49–64]. The addition of the OMW does not affect the shape of the loops, indicating that no change in the corrosion mechanism of AA3003 alloy has occurred [65]. The impedance data are simulated by using the equivalent circuit illustrated in Figure 3 [66]. The components of this circuit are the electrolyte resistance (Rs), charge transfer resistance (Rt), and constant phase element (CPE) for the used electrode, and L and RL represent the inductance and the inductive resistance associated with the inductive loop, respectively. Accordingly, the calculated impedance parameters are given in Table 4. The inhibitory efficiency derived from the charge transfer resistance is obtained by the following relationship [49, 58, 65]:
(2)IE%=1−Rt0Rt×100,where Rt0 and Rt are the charge transfer resistances of AA3003 alloy without and with the OMW inhibitor, respectively.Figure 2
Nyquist plots of AA3003 alloy in 1 M HCl solution without and with addition of OMW at different concentrations (at 298 K).Figure 3
Equivalent electrical circuit used to model the experimental impedance data illustrated.Table 4
Mathematical expression of evaluated adsorption isotherms and its corresponding calculatedR2 values.
Adsorption isothermEquationR2Flory-HugginslnθC=lnKads+xln1−θ0.92Freundlichlnθ=lnKads+nlnC0.94El-Awadylnθ1−θ=lnKads+γlnC0.96Temkinθ=−12alnKads−12alnC0.97FrumkinlnC×θ1−θ=lnKads+2αθ0.98LangmuirCθ=1Kads+C0.99As can be noted in Figure2 and from the values of χ2 in Table 5, a good fit by using the chosen equivalent circuit was obtained. This means that the adjusted data have good agreement with the experimental data. According to Table 5, the Rt values increase with increasing OMW concentration, resulting in an increase in IE% attributed to the increase in organic inhibitory molecules (i.e., polyphenols) adsorbed on the surface of aluminum alloy [26, 58]. IE% attains a maximum value equal to 89%, which confirms that OMW has a good protection ability for AA3003 alloy in 1 M HCl medium. The IE% values computed from the EIS measurements are in good agreement with those found by the potentiodynamic technique [67]. Besides, conferring to the obtained Q values in the presence of the tested inhibitor, it can be assumed that the OMW active constituents were adsorbed onto the alloy surface to form a protective layer [68].Table 5
Electrochemical impedance parameters of AA3003 alloy in 1 M HCl medium without and with addition of OMW inhibitor at different concentrations (at 298 K).
Conc. (ppm)χ2RSΩcm2RtΩcm2RLΩcm2QSsncm−2nLHcm2IE%θBlank1.73×10−20.755.954.583330.85731.613—0.51.85×10−20.748.784.6490.80.98972.01932.20.3221.01.90×10−20.7314.745.6461.70.98474.77659.60.5962.01.81×10−20.7319.776.5094.40.98825.07469.90.6993.01.98×10−20.7126.858.66123.00.97876.82277.80.7784.02.03×10−20.7230.728.2772.60.955711.1580.60.8065.02.13×10−20.7234.7843.6862.60.968014.7182.90.8296.01.63×10−20.7754.4261.9784.00.948638.0689.10.891


### 3.3. Adsorption Isotherm

The adsorption process depends on several electronic properties of the inhibitor tested, the nature of the metal surface, the temperature, etc. [69]. The purpose of the use of adsorption isotherms is to describe the mechanism of interaction between the metal surface and the inhibitor (i.e., OMW). To understand the type of adsorption of OMW on the surface of the aluminum alloy in the 1 M HCl solution, it is important to study the relationship between the inhibitory power and the surface coverage (θ, Equation (3)) for different concentrations of OMW. Different adsorption isotherms were evaluated to find the best fit for the experimental data driven from potentiodynamic polarization technique. Table 4 summarizes the linear determination coefficients (R2) with equation expressions for each examined isotherm, namely, Langmuir, Freundlich, Temkin, Frumkin, Flory Hoggins, and El-Awady [55, 70–72]. According to the obtained R2 values, it is apparent that the adsorption of the OMW inhibitor on the surface of the alloy favorably obeys the Langmuir adsorption isotherm (Figure 4) [60]. In the equation of this isotherm, C is the OMW concentration and Kads represents the adsorption equilibrium constant:
(3)θ=1−Icorr′Icorr.Figure 4
Langmuir isotherm line of AA3003 alloy in 1 M HCl medium with different concentrations of OMW at 298 K.To explain the nature of the interaction between the AA3003 surface and the adsorbed OMW molecules, the thermodynamic adsorption energy,ΔGads0 (Gibbs free energy), was determined using the following relationship [49, 73]:
(4)ΔGads0=−RTln1×106Kads,whereR denotes the universal gas constant (8.314 J mol-1 K-1), T is the absolute temperature (K), and 1×106 value is the concentration of water CH2O calculated in mg L-1.The calculation of the value ofΔGads0 at 298 K gives −34.32 kJ mol-1. This negative value indicates that the adsorption of the inhibitor is spontaneous [74], and the interaction of adsorbed OMW molecules on the AA3003 surface is strong [49]. Depending on the value of ΔGads0, one can distinguish the type of adsorption of an inhibitor onto the metal surface. According to Chu and Sukava and Sangeetha et al. [75, 76], the energy values (ΔGads0) up to -20 kJ mol-1 indicate the presence of electrostatic interactions (physisorption) between the metal surface and the inhibitor. While if the values of ΔGads0 are lower than -40 kJ mol-1, the inhibitory molecules form more stable chemical bonds with the material (chemical adsorption). In our study, the calculated value of ΔGads0 (-34.32 kJ mol-1) outlines that the adsorption mechanism of OMW molecules onto the AA3003 surface is a mixed type, i.e., involves both physical and chemical adsorption processes [76].


### 3.4. Effect of Temperature

Temperature is one of the parameters that affect the corrosion phenomenon of metallic materials. The aim to study the effect of temperature is to examine the stability of the tested inhibitor. The influence of temperature on the corrosion of AA3003 in 1 M HCl solution in the absence and presence of 6.0 ppm OMW was performed in the temperature range of 298 to 328 K. Under these conditions, Figure5 illustrates the obtained polarization curves, and their corresponding extracted parameters are summarized in Table 6.Figure 5
Polarization curves of AA3003 alloy in 1 M HCl medium (a) without and (b) with addition of 6.0 ppm OMW at different temperatures.
(a)(b)Table 6
Derived polarization electrochemical parameters of AA3003 alloy in 1 M HCl medium in the absence and presence of 6.0 ppm OMW at different temperatures.
TKIcorrmAcm−2−EcorrmV/SCE−bcmV/decIE%θBlank29811.866811134——30824.790817151——31837.067829153——32841.559841156——6.0 ppm2981.41982211588.040.88043083.16282412487.560.87563185.01181712187.300.87303287.13581814383.140.8314From the obtained results, it can be revealed that the increase in temperature has no significant effect on the shape of the polarization curves (Figure5), hence the mechanism of AA3003 alloy corrosion. In addition, the values of current densities (Icorr) in the presence and absence of OMW increase clearly with increasing temperature (Table 6). In these circumstances, the inhibitory efficiency decreases with the rise in temperature of the electrolytic medium. According to several authors, the latter behavior is due essentially to the desorption of inhibitor molecules, which is to say, the reduction of the inhibitor capacity to be adsorbed on the metallic surface at the elevated temperatures [55, 77–79].In order to explain the adsorption mechanism of an inhibitor, it is necessary to determine some thermodynamic parameters of activation for the studied system, namely,Ea, ΔH∗, and ΔS∗). Figure 6 shows the variation of the logarithm of the corrosion current density (lnIcorr) of AA3003 as a function of (1000/T) in 1 M HCl solution in the absence and presence of OMW at 6.0 ppm. The found correlation coefficients for the obtained straight lines are 0.9. The slopes of these lines are used to determine the activation energies (Ea) according to the Arrhenius equation:
(5)Icorr=Aexp−EaRT,where T denotes the absolute temperature (K), R is the universal gas constant (J mol-1 K-1), and A is the Arrhenius constant.Figure 6
Arrhenius plots of AA3003 alloy in 1 M HCl in the absence (black line) and presence (red line) of OMW at 6.0 ppm.The activation energies (i.e.,ΔH∗ and ΔS∗) are determined using the following Arrhenius transition state equation:
(6)Icorr=RTNhexpΔS∗Rexp−ΔH∗RT,where N represents the Avogadro number (N=6.02252×1023mol−1), h is the Planck constant (h=6.626176×10−23Js), and ΔS∗ (entropy) and ΔH∗ (enthalpy) are the activation parameters. Figure 7 demonstrates the shape of lnIcorr/T as a function of 1000/T for the systems under study. Straight lines are obtained with an equal slope (−ΔH∗/R) and extrapolation of lines (lnR/Nh+ΔS∗/R) [68]. The values of Ea, ΔH∗, and ΔS∗ are listed in Table 7 for AA3003 in 1 M HCl medium without and with OMW inhibitor at 6.0 ppm.Figure 7
Arrhenius transition state plots ofln(Icorr/T) as a function of 1000/T for the systems under study.Table 7
Determined activation energies for AA3003 corrosion in 1 M HCl medium without and with OMW inhibitor at 6.0 ppm.
MediumEa (kJ mol-1)ΔH∗ (kJ mol-1)ΔS∗ (J mol-1 K-1)Blank36.2433.64-111.04+6.0 ppm of OMW46.8244.22-93.11According to the obtained data, the calculated activation energy (Ea) values for the uninhibited solution without and with OMW are 36.24 and 46.82 kJ mol-1, respectively. It is noted that the activation energy increases in the presence of OMW, which indicates that the AA3003 alloy corrosion process has been modified; this may be attributed to the influence of the molecules existing in OMW on this interfacial process [6]. Moreover, the tabulated values of ΔH∗ are positives that reflect the endothermic nature of the AA3003 dissolution process. It is evident that the activation energy increases sharply in the presence of the inhibitor. This is due to the adsorption of the inhibitory species on the surface of the alloy [71, 72, 80]. Besides, the entropy ΔS∗ increases more positively with the presence of the OMW inhibitor, in which the obtained values are -111.04 and -93.11 J mol-1 K-1 for the 1 M HCl medium without and with the addition of the inhibitor, respectively. These results imply the formation of a stable and ordered layer of this inhibitor on the AA3003 surface [81], and the negative sign of ΔS∗ indicates that the activated complex represents an association rather than a step of dissociation, which means that a decrease in the disorder occurs by passing reagents to the activated complex [6, 82].


### 3.5. DFT Results

The relevant electronic structure parameters of Tyr and HydroTyr molecules in the aqueous phase are summarized in Table8. Agreeing with Fukui’s theory, the molecule reactivity depends on its frontier molecular orbitals [83]. Under this concept, the energy gap (ΔE) was largely used to expect the reactivity of numerous inhibitor molecules. According to many reports, the value of ΔE has been correlated with the observed inhibition efficiency, in which the decrease of ΔE leads to good corrosion prevention. The calculated value of ΔE proposes that the HydroTyr molecule is more reactive than the Tyr one. Subsequently, the interaction of HydroTyr with the aluminum surface is more chemical than that of Tyr. On the other hand, the HydroTyr molecule can provide an enhanced prevention capacity as compared to Tyr.Table 8
Some relevant electronic structure parameters of Tyr and HydroTyr molecules.
MoleculeΔE (eV)η (eV)TNC (e)ΔN (e)Tyr6.3963.198-2.0110.146HydroTyr5.6122.806-1.9700.251The chemical reactivity can be further expected via the chemical hardness (η), which describes the polarization or the deformation abilities of the molecule [84]. In other words, the molecule with lower hardness easily tends to react and adsorb onto the metal surface, which results in its higher inhibition efficiency [85]. In our case, the lowest η value is obtained for HydroTyr as compared to Tyr. Therefore, we can conclude that HydroTyr interacts more effectively with the aluminum surface; thus, it can provide the highest prevention efficiency.It is well known that almost all metal surfaces are charged positively in the acidic media such as aluminum in our case. Consequently, the electrostatic interactions between the inhibitor molecules and metal surface will be great as the total negative charge (TNC) of thses molecules increased [86]. In the current study, the Tyr molecule displays more TNC negative value indicating its supplement tendency to interact electrostatically with aluminum surface regarding HydroTyr.To elucidate the implicit interfacial interactions between the considered molecules and the aluminum surface, the fraction of transferred electrons (ΔN) was calculated using the work function of Al(111) face [87]. According to the tabulated values (Table 8), both molecules exhibit the tendency to denote electrons to the aluminum surface, which is higher for the HydroTyr molecule (0.251) as compared to Tyr (0.146). This finding outlines the affinity of HydroTyr to form chemical bonds with the metal surface than Tyr.To explore the promising sites of adsorption within the Tyr and HydroTyr molecular skeleton, the distribution of frontier molecular orbitals and molecular ESP maps were plotted. Figure8 displays the obtained molecular plots. As can be seen from this figure, both frontier molecular orbital densities are vastly located on the carbon atoms within the benzene ring, while small densities are placed in some extrabenzene ring atoms. These observations underline the role of the benzene moiety in the chemical process during the inhibitor/aluminum interfacial interactions, especially for the HydroTyr molecule [88]. On the other hand, as shown in Figure 8, the electron-rich regions characterized by negative potentials (i.e., red regions) are mainly located on the alcohol functions for both considered molecules. The intense negative potential can be noted in the Tyr molecule as compared to the HydroTyr one. This indicates the ability of these major OMW-containing molecules to interact electrostatically with electropositive sites upon aluminum through these extrabenzene functions [89].Figure 8
Isosurface of frontier molecular orbitals (i.e., HOMO and LUMO) and ESP map (red (blue) color refers to negative (positive) potential) of Tyr and HydroTyr molecules.


### 3.6. Monte Carlo Simulations

In order to study, in detail, the adsorption process of the two chosen OMW constituents (i.e., Tyr and HydroTyr) on the aluminum surface, Monte Carlo simulations associated with the simulated annealing algorithm were carried out. Figure9 illustrates the energetic profile of a considered OMW molecule (i.e., HydroTyr) during the simulation in the aqueous phase.Figure 9
Energetic profile of HydroTyr molecule during the Monte Carlo simulation under explicit solvation condition.Figure10 represents the most stable adsorption configuration of the selected OMW components on the Al(111) surface. According to the obtained configurations, it is clear that Tyr and HydroTyr are nearly placed on the metal surface. This outlines the affinity of these compounds to be adsorbed onto the aluminum surface, subsequently the formation of a protective layer that covers the metal surface [90]. The energetic aspect of the adsorption process shows that the binding energies are 66.847 and 68.004 kcal mol-1 for HydroTyr and Tyr, respectively. These values reflect the capacity of the considered OMW components to interact spontaneously and effectively with the Al(111) surface [91]. Furthermore, the observed energy ranking, i.e., HydroTyr < Tyr, indicates the enhanced ability of Tyr to adsorb on the metal surface through nonbonding interactions (i.e., electrostatic and Van de Waals interactions) [92]. On the other hand, it was found that the binding energy of water molecules is about 15 kcal mol-1, which is lower than the corresponding ones of Tyr and HydroTyr molecules. Such finding underlines the affinity of OMW components to replace preadsorbed water molecules on the aluminum surface, which leads to the formation of a protective layer upon the metal surface [93, 94].Figure 10
Top and side views of the equilibrium adsorption mode of Tyr and HydroTyr molecules onto Al(111) surface.Given the above-mentioned statements, HydroTyr shows more tendency to interact with the aluminum surface through a chemical process rather than a physical (nonbonding interactions) one, whereas the opposite behaviors can be outlined for the Tyr molecule. Such finding explains the experimentally obtained value ofΔGads0 (-34.32 kJ mol-1), which indicates the mixed physicochemical character of the adsorption process of OMW components onto the metal surface.


## 3.1. Potentiodynamic Polarization Results

The effect of the concentration of OMW on the corrosion behavior of AA3003 alloy in the 1 M HCl solution was studied by using anodic and cathodic polarization measurements after 30 min of the immersion time. Figure1 illustrates the potentiodynamic polarization curves for the alloy understudy in the absence and presence of OMW with different concentrations at 298 K.Figure 1
Polarization curves of AA3003 alloy in 1 M HCl solution without and with addition of OMW at different concentrations (at 298 K).As can be seen in the anodic branch (Figure1), we can point out the difficulty of recognizing the linear region of the Tafel law. Hence, the current densities are determined by extrapolating only the Tafel cathodic linear region to the corrosion potential. This method of adjustment was used by several authors for aluminum-based materials in the hydrochloric acid medium [25, 47]. The relevant determined electrochemical parameters, namely, current density (Icorr), corrosion potential (Ecorr), and cathodic slope (βc), as well as the inhibitory efficiency (IE%, Equation (1)) as a function of the OMW concentration, are tabulated in Table 3:
(1)IE%=1−Icorr′Icorr×100,where Icorr and Icorr′ are, respectively, the uninhibited and inhibited corrosion current densities of AA3003 alloy.Table 3
Derived polarization electrochemical parameters of AA3003 alloy in 1 M HCl medium in the absence and presence of OMW at different concentrations (at 298 K).
Conc. (ppm)IcormAcm−2−EcormV/SCE−βcmV/decIE%θBlank11.866811134——0.57.61982214335.790.35791.05.14081213156.680.56682.04.40381912562.890.62893.04.09081213673.940.73944.02.12781513282.070.82075.01.89481913884.000.84006.01.41981611588.040.8804It is clear from Table3 that the Tafel cathodic slope (βc) changes slightly in the presence of the OMW, which outlines no significant effect of OMW on the hydrogen reduction mechanism. Such a conclusion can be revealed from Figure 1 from which the cathodic polarization curves are almost parallel. On the other hand, it is found that the value of Icorr decreases with the addition of the OMW, which induces an increase of the inhibition efficiency as a function of the OMW concentration. This is due to the increase of the blocked fraction of the alloy surface by the adsorption of the active components present in OMW, as well as their synergistic effect [48]. The prevention effectiveness reaches 88% with a low concentration of OMW (6.0 ppm), indicating that OMW is an excellent inhibitor of AA3003 alloy in the 1 M HCl medium. Conferring to Figure 1, the OMW can be classified as a cathodic inhibitor due to the reduction of cathodic currents by its addition with respect to the uninhibited solution [49].


## 3.2. Electrochemical Impedance Spectroscopy Results

The Nyquist diagrams for studied aluminum alloy in 1 M HCl medium at 298 K, in the absence and presence of different OMW concentrations, were plotted inEOCP potential after 30 min of immersion as depicted in Figure 2. It is evident from this figure that the impedance diagrams consist of a large capacitive loop at high frequency followed by an inductive loop at low frequency. The first loop is due to the formation of an oxide film on the aluminum surface, while the second one is attributed to the relaxation of the charged intermediates adsorbed on the surface tested [50, 51]. In addition, the inductive semicircle diameter is considerably less than that observed in high frequencies. On the other hand, the impedance spectrum shows an approximately elliptical shape that is attributed to the frequency dispersion due to the roughness and inhomogeneity of the metal surface [48]. In the acidic media, similar impedance spectra have been reported in the corrosion literature for aluminum and its alloys [23, 25, 26, 28–30, 49–64]. The addition of the OMW does not affect the shape of the loops, indicating that no change in the corrosion mechanism of AA3003 alloy has occurred [65]. The impedance data are simulated by using the equivalent circuit illustrated in Figure 3 [66]. The components of this circuit are the electrolyte resistance (Rs), charge transfer resistance (Rt), and constant phase element (CPE) for the used electrode, and L and RL represent the inductance and the inductive resistance associated with the inductive loop, respectively. Accordingly, the calculated impedance parameters are given in Table 4. The inhibitory efficiency derived from the charge transfer resistance is obtained by the following relationship [49, 58, 65]:
(2)IE%=1−Rt0Rt×100,where Rt0 and Rt are the charge transfer resistances of AA3003 alloy without and with the OMW inhibitor, respectively.Figure 2
Nyquist plots of AA3003 alloy in 1 M HCl solution without and with addition of OMW at different concentrations (at 298 K).Figure 3
Equivalent electrical circuit used to model the experimental impedance data illustrated.Table 4
Mathematical expression of evaluated adsorption isotherms and its corresponding calculatedR2 values.
Adsorption isothermEquationR2Flory-HugginslnθC=lnKads+xln1−θ0.92Freundlichlnθ=lnKads+nlnC0.94El-Awadylnθ1−θ=lnKads+γlnC0.96Temkinθ=−12alnKads−12alnC0.97FrumkinlnC×θ1−θ=lnKads+2αθ0.98LangmuirCθ=1Kads+C0.99As can be noted in Figure2 and from the values of χ2 in Table 5, a good fit by using the chosen equivalent circuit was obtained. This means that the adjusted data have good agreement with the experimental data. According to Table 5, the Rt values increase with increasing OMW concentration, resulting in an increase in IE% attributed to the increase in organic inhibitory molecules (i.e., polyphenols) adsorbed on the surface of aluminum alloy [26, 58]. IE% attains a maximum value equal to 89%, which confirms that OMW has a good protection ability for AA3003 alloy in 1 M HCl medium. The IE% values computed from the EIS measurements are in good agreement with those found by the potentiodynamic technique [67]. Besides, conferring to the obtained Q values in the presence of the tested inhibitor, it can be assumed that the OMW active constituents were adsorbed onto the alloy surface to form a protective layer [68].Table 5
Electrochemical impedance parameters of AA3003 alloy in 1 M HCl medium without and with addition of OMW inhibitor at different concentrations (at 298 K).
Conc. (ppm)χ2RSΩcm2RtΩcm2RLΩcm2QSsncm−2nLHcm2IE%θBlank1.73×10−20.755.954.583330.85731.613—0.51.85×10−20.748.784.6490.80.98972.01932.20.3221.01.90×10−20.7314.745.6461.70.98474.77659.60.5962.01.81×10−20.7319.776.5094.40.98825.07469.90.6993.01.98×10−20.7126.858.66123.00.97876.82277.80.7784.02.03×10−20.7230.728.2772.60.955711.1580.60.8065.02.13×10−20.7234.7843.6862.60.968014.7182.90.8296.01.63×10−20.7754.4261.9784.00.948638.0689.10.891


## 3.3. Adsorption Isotherm

The adsorption process depends on several electronic properties of the inhibitor tested, the nature of the metal surface, the temperature, etc. [69]. The purpose of the use of adsorption isotherms is to describe the mechanism of interaction between the metal surface and the inhibitor (i.e., OMW). To understand the type of adsorption of OMW on the surface of the aluminum alloy in the 1 M HCl solution, it is important to study the relationship between the inhibitory power and the surface coverage (θ, Equation (3)) for different concentrations of OMW. Different adsorption isotherms were evaluated to find the best fit for the experimental data driven from potentiodynamic polarization technique. Table 4 summarizes the linear determination coefficients (R2) with equation expressions for each examined isotherm, namely, Langmuir, Freundlich, Temkin, Frumkin, Flory Hoggins, and El-Awady [55, 70–72]. According to the obtained R2 values, it is apparent that the adsorption of the OMW inhibitor on the surface of the alloy favorably obeys the Langmuir adsorption isotherm (Figure 4) [60]. In the equation of this isotherm, C is the OMW concentration and Kads represents the adsorption equilibrium constant:
(3)θ=1−Icorr′Icorr.Figure 4
Langmuir isotherm line of AA3003 alloy in 1 M HCl medium with different concentrations of OMW at 298 K.To explain the nature of the interaction between the AA3003 surface and the adsorbed OMW molecules, the thermodynamic adsorption energy,ΔGads0 (Gibbs free energy), was determined using the following relationship [49, 73]:
(4)ΔGads0=−RTln1×106Kads,whereR denotes the universal gas constant (8.314 J mol-1 K-1), T is the absolute temperature (K), and 1×106 value is the concentration of water CH2O calculated in mg L-1.The calculation of the value ofΔGads0 at 298 K gives −34.32 kJ mol-1. This negative value indicates that the adsorption of the inhibitor is spontaneous [74], and the interaction of adsorbed OMW molecules on the AA3003 surface is strong [49]. Depending on the value of ΔGads0, one can distinguish the type of adsorption of an inhibitor onto the metal surface. According to Chu and Sukava and Sangeetha et al. [75, 76], the energy values (ΔGads0) up to -20 kJ mol-1 indicate the presence of electrostatic interactions (physisorption) between the metal surface and the inhibitor. While if the values of ΔGads0 are lower than -40 kJ mol-1, the inhibitory molecules form more stable chemical bonds with the material (chemical adsorption). In our study, the calculated value of ΔGads0 (-34.32 kJ mol-1) outlines that the adsorption mechanism of OMW molecules onto the AA3003 surface is a mixed type, i.e., involves both physical and chemical adsorption processes [76].


## 3.4. Effect of Temperature

Temperature is one of the parameters that affect the corrosion phenomenon of metallic materials. The aim to study the effect of temperature is to examine the stability of the tested inhibitor. The influence of temperature on the corrosion of AA3003 in 1 M HCl solution in the absence and presence of 6.0 ppm OMW was performed in the temperature range of 298 to 328 K. Under these conditions, Figure5 illustrates the obtained polarization curves, and their corresponding extracted parameters are summarized in Table 6.Figure 5
Polarization curves of AA3003 alloy in 1 M HCl medium (a) without and (b) with addition of 6.0 ppm OMW at different temperatures.
(a)(b)Table 6
Derived polarization electrochemical parameters of AA3003 alloy in 1 M HCl medium in the absence and presence of 6.0 ppm OMW at different temperatures.
TKIcorrmAcm−2−EcorrmV/SCE−bcmV/decIE%θBlank29811.866811134——30824.790817151——31837.067829153——32841.559841156——6.0 ppm2981.41982211588.040.88043083.16282412487.560.87563185.01181712187.300.87303287.13581814383.140.8314From the obtained results, it can be revealed that the increase in temperature has no significant effect on the shape of the polarization curves (Figure5), hence the mechanism of AA3003 alloy corrosion. In addition, the values of current densities (Icorr) in the presence and absence of OMW increase clearly with increasing temperature (Table 6). In these circumstances, the inhibitory efficiency decreases with the rise in temperature of the electrolytic medium. According to several authors, the latter behavior is due essentially to the desorption of inhibitor molecules, which is to say, the reduction of the inhibitor capacity to be adsorbed on the metallic surface at the elevated temperatures [55, 77–79].In order to explain the adsorption mechanism of an inhibitor, it is necessary to determine some thermodynamic parameters of activation for the studied system, namely,Ea, ΔH∗, and ΔS∗). Figure 6 shows the variation of the logarithm of the corrosion current density (lnIcorr) of AA3003 as a function of (1000/T) in 1 M HCl solution in the absence and presence of OMW at 6.0 ppm. The found correlation coefficients for the obtained straight lines are 0.9. The slopes of these lines are used to determine the activation energies (Ea) according to the Arrhenius equation:
(5)Icorr=Aexp−EaRT,where T denotes the absolute temperature (K), R is the universal gas constant (J mol-1 K-1), and A is the Arrhenius constant.Figure 6
Arrhenius plots of AA3003 alloy in 1 M HCl in the absence (black line) and presence (red line) of OMW at 6.0 ppm.The activation energies (i.e.,ΔH∗ and ΔS∗) are determined using the following Arrhenius transition state equation:
(6)Icorr=RTNhexpΔS∗Rexp−ΔH∗RT,where N represents the Avogadro number (N=6.02252×1023mol−1), h is the Planck constant (h=6.626176×10−23Js), and ΔS∗ (entropy) and ΔH∗ (enthalpy) are the activation parameters. Figure 7 demonstrates the shape of lnIcorr/T as a function of 1000/T for the systems under study. Straight lines are obtained with an equal slope (−ΔH∗/R) and extrapolation of lines (lnR/Nh+ΔS∗/R) [68]. The values of Ea, ΔH∗, and ΔS∗ are listed in Table 7 for AA3003 in 1 M HCl medium without and with OMW inhibitor at 6.0 ppm.Figure 7
Arrhenius transition state plots ofln(Icorr/T) as a function of 1000/T for the systems under study.Table 7
Determined activation energies for AA3003 corrosion in 1 M HCl medium without and with OMW inhibitor at 6.0 ppm.
MediumEa (kJ mol-1)ΔH∗ (kJ mol-1)ΔS∗ (J mol-1 K-1)Blank36.2433.64-111.04+6.0 ppm of OMW46.8244.22-93.11According to the obtained data, the calculated activation energy (Ea) values for the uninhibited solution without and with OMW are 36.24 and 46.82 kJ mol-1, respectively. It is noted that the activation energy increases in the presence of OMW, which indicates that the AA3003 alloy corrosion process has been modified; this may be attributed to the influence of the molecules existing in OMW on this interfacial process [6]. Moreover, the tabulated values of ΔH∗ are positives that reflect the endothermic nature of the AA3003 dissolution process. It is evident that the activation energy increases sharply in the presence of the inhibitor. This is due to the adsorption of the inhibitory species on the surface of the alloy [71, 72, 80]. Besides, the entropy ΔS∗ increases more positively with the presence of the OMW inhibitor, in which the obtained values are -111.04 and -93.11 J mol-1 K-1 for the 1 M HCl medium without and with the addition of the inhibitor, respectively. These results imply the formation of a stable and ordered layer of this inhibitor on the AA3003 surface [81], and the negative sign of ΔS∗ indicates that the activated complex represents an association rather than a step of dissociation, which means that a decrease in the disorder occurs by passing reagents to the activated complex [6, 82].


## 3.5. DFT Results

The relevant electronic structure parameters of Tyr and HydroTyr molecules in the aqueous phase are summarized in Table8. Agreeing with Fukui’s theory, the molecule reactivity depends on its frontier molecular orbitals [83]. Under this concept, the energy gap (ΔE) was largely used to expect the reactivity of numerous inhibitor molecules. According to many reports, the value of ΔE has been correlated with the observed inhibition efficiency, in which the decrease of ΔE leads to good corrosion prevention. The calculated value of ΔE proposes that the HydroTyr molecule is more reactive than the Tyr one. Subsequently, the interaction of HydroTyr with the aluminum surface is more chemical than that of Tyr. On the other hand, the HydroTyr molecule can provide an enhanced prevention capacity as compared to Tyr.Table 8
Some relevant electronic structure parameters of Tyr and HydroTyr molecules.
MoleculeΔE (eV)η (eV)TNC (e)ΔN (e)Tyr6.3963.198-2.0110.146HydroTyr5.6122.806-1.9700.251The chemical reactivity can be further expected via the chemical hardness (η), which describes the polarization or the deformation abilities of the molecule [84]. In other words, the molecule with lower hardness easily tends to react and adsorb onto the metal surface, which results in its higher inhibition efficiency [85]. In our case, the lowest η value is obtained for HydroTyr as compared to Tyr. Therefore, we can conclude that HydroTyr interacts more effectively with the aluminum surface; thus, it can provide the highest prevention efficiency.It is well known that almost all metal surfaces are charged positively in the acidic media such as aluminum in our case. Consequently, the electrostatic interactions between the inhibitor molecules and metal surface will be great as the total negative charge (TNC) of thses molecules increased [86]. In the current study, the Tyr molecule displays more TNC negative value indicating its supplement tendency to interact electrostatically with aluminum surface regarding HydroTyr.To elucidate the implicit interfacial interactions between the considered molecules and the aluminum surface, the fraction of transferred electrons (ΔN) was calculated using the work function of Al(111) face [87]. According to the tabulated values (Table 8), both molecules exhibit the tendency to denote electrons to the aluminum surface, which is higher for the HydroTyr molecule (0.251) as compared to Tyr (0.146). This finding outlines the affinity of HydroTyr to form chemical bonds with the metal surface than Tyr.To explore the promising sites of adsorption within the Tyr and HydroTyr molecular skeleton, the distribution of frontier molecular orbitals and molecular ESP maps were plotted. Figure8 displays the obtained molecular plots. As can be seen from this figure, both frontier molecular orbital densities are vastly located on the carbon atoms within the benzene ring, while small densities are placed in some extrabenzene ring atoms. These observations underline the role of the benzene moiety in the chemical process during the inhibitor/aluminum interfacial interactions, especially for the HydroTyr molecule [88]. On the other hand, as shown in Figure 8, the electron-rich regions characterized by negative potentials (i.e., red regions) are mainly located on the alcohol functions for both considered molecules. The intense negative potential can be noted in the Tyr molecule as compared to the HydroTyr one. This indicates the ability of these major OMW-containing molecules to interact electrostatically with electropositive sites upon aluminum through these extrabenzene functions [89].Figure 8
Isosurface of frontier molecular orbitals (i.e., HOMO and LUMO) and ESP map (red (blue) color refers to negative (positive) potential) of Tyr and HydroTyr molecules.


## 3.6. Monte Carlo Simulations

In order to study, in detail, the adsorption process of the two chosen OMW constituents (i.e., Tyr and HydroTyr) on the aluminum surface, Monte Carlo simulations associated with the simulated annealing algorithm were carried out. Figure9 illustrates the energetic profile of a considered OMW molecule (i.e., HydroTyr) during the simulation in the aqueous phase.Figure 9
Energetic profile of HydroTyr molecule during the Monte Carlo simulation under explicit solvation condition.Figure10 represents the most stable adsorption configuration of the selected OMW components on the Al(111) surface. According to the obtained configurations, it is clear that Tyr and HydroTyr are nearly placed on the metal surface. This outlines the affinity of these compounds to be adsorbed onto the aluminum surface, subsequently the formation of a protective layer that covers the metal surface [90]. The energetic aspect of the adsorption process shows that the binding energies are 66.847 and 68.004 kcal mol-1 for HydroTyr and Tyr, respectively. These values reflect the capacity of the considered OMW components to interact spontaneously and effectively with the Al(111) surface [91]. Furthermore, the observed energy ranking, i.e., HydroTyr < Tyr, indicates the enhanced ability of Tyr to adsorb on the metal surface through nonbonding interactions (i.e., electrostatic and Van de Waals interactions) [92]. On the other hand, it was found that the binding energy of water molecules is about 15 kcal mol-1, which is lower than the corresponding ones of Tyr and HydroTyr molecules. Such finding underlines the affinity of OMW components to replace preadsorbed water molecules on the aluminum surface, which leads to the formation of a protective layer upon the metal surface [93, 94].Figure 10
Top and side views of the equilibrium adsorption mode of Tyr and HydroTyr molecules onto Al(111) surface.Given the above-mentioned statements, HydroTyr shows more tendency to interact with the aluminum surface through a chemical process rather than a physical (nonbonding interactions) one, whereas the opposite behaviors can be outlined for the Tyr molecule. Such finding explains the experimentally obtained value ofΔGads0 (-34.32 kJ mol-1), which indicates the mixed physicochemical character of the adsorption process of OMW components onto the metal surface.


## 4. Conclusion

The effect of OMW on the corrosion behavior of AA3003 alloy in molar hydrochloride solution was investigated through electrochemical measurements complemented by thermodynamic studies and theoretical calculations. Based on the obtained results, OMW acts as a good corrosion inhibitor for AA3003 alloy at lower concentrations with interesting protection efficiencies even at elevated temperatures. According to the thermodynamic parameters, the adsorption process of OMW constituents onto the alloy surface is a mixed physichemisorption type. On the other hand, computational simulations outlined the affinity of the major OMW components to interact with the aluminum surface, which leads to the formation of a protective layer. OMW can be used instead of conventional toxic compounds to prevent acidic corrosion of AA3003 alloy.



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*Source: 6662395-2021-01-28.xml*