# Computational and Experimental Evaluation of Inhibition Potential of a New Ecologically Friendly Inhibitor Leaves of Date Palm (Phoenix dactylifera L.) for Aluminium Corrosion in an Acidic Media

Authors: Oraas Adnan Hatem
Journal: International Journal of Corrosion
 (2022)

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## Abstract

PDL (Phoenix dactylifera leaves) is widely spread in Iraq and is known to be rich in phytochemicals as flavonoids, saponins, tannins, glycosides, oils, and lipids. The effect of PDL extract in reducing the corrosion of Aluminium in 1 M HCl solution using a weight loss technique, and computational chemistry calculations were investigated in this study. The study carried out at different temperatures (20, 30, 40, and 50) in the presence of plant extract and the absence of extract. A number of parameters were included to be detected in this study according to the density functional theory (DFT)/P3LYP/6-311G, including the highest occupied molecular orbital EHOMO, the lowest unoccupied molecular orbital ELUMO, energy gapΔE, softness S, hardness η, dipole moment μ, electronegativity χ, electrophilicity ω, inhibitor-metal interaction energy ∆ψ, and electrons transferred fraction ΔN. Two adsorption isotherms were used to explain inhibitor adsorption behavior. Two adsorption isotherms were used to explain inhibitor adsorption behavior, the Freundlich adsorption isotherm and the Langmuir adsorption isotherm, the Freundlich adsorption isotherm was discovered to be followed by the inhibitor with correlation coefficient values ranging from 0.98 to 0.94 with temperature increased from 20 to 50 degrees Celsius. The adsorption mechanism includes a physical adsorption process. The results showed that with the increment of the inhibitors concentration, there was an improvement of the inhibition efficiency. The most outstanding inhibitor efficiency was 97.7% at 10 mL/L inhibitor concentration.


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## Body

## 1. Introduction

Numerous studies have been conducted on Aluminium and its alloys due to its high technological value and wide range of industrial applications, notably in the aerospace and domestic industries. Aluminium and its alloys, on the other hand, are reactive and prone to corrosion. Aluminium is dependent on the creation of a compact, firmly adhering, and continuous passive oxide coating when exposed to air or aqueous solutions. This is what gives Aluminium its corrosion resistance in most situations. When the metal is exposed to strong acids or bases, this amphoteric surface layer drastically deteriorates [1]. Aluminium metal has the ability to form a thin, stable oxide layer on its surface, but when aggressive media, such HCl, are present, this stable layer is disrupted, and the metal starts to corrode [2]. In the industry, numerous compounds are used as corrosion inhibitors to eliminate or minimize metal corrosion in acidic environments. Because of their toxicity and high cost, it is critical to producing inhibitors that are both environmentally friendly and less expensive [3]. Plants, for example, include a wide range of chemical compounds (alkaloids, steroids, amino acids, tannins, and flavonoids), the majority of which are known to have inhibitory properties. The corrosion attitude of aluminium has been explored in acidic media. Iron corrosion has been successfully inhibited by adding halide salt to an H2SO4 solution containing an organic inhibitor [4, 5]. Concerns regarding chemical inhibitors’ toxicity, which may hurt living things and contaminate the environment, have been attributed. Plant extracts have become popular as corrosion inhibitors because they are friendly to the environment, widely available, and environmentally sustainable sources of various inhibitors. A significant amount of naturally produced compounds is found in plant extracts that could be extracted using simple, inexpensive methods [6].Plant extracts were employed in order to avoid metal corrosion in acidic and alkaline settings. These extracts degrade biologically and clear heavy metals and other potentially hazardous materials [7–9].This study used weight loss measurements to explore the inhibitive performance of Phoenix dactylifera leaves extract in preventing aluminium corrosion in 1 M hydrochloric acid. The goal of this study is to learn more about the function of the functional groups of the inhibitors used in the adsorption process and the effect of the medium’s nature on the corrosion mechanism.


## 2. Methodology

### 2.1. Specimen Preparation

The purity of the aluminium which is used in the experiment is 99.85 percent. The aluminium sheets were cut into 3 × 3 × 1 cm for each coupon (couponexposedarea=4.65squareinch). A 3-mm diameter hole was bored on the upper edge to suspend each coupon in the corrosive environment. The coupons were cleaned and degreased, after being washed in 100 percent ethanol, sprayed in acetone, and left to dry. The dehydrated coupons were maintained in a dry desiccators until they were needed. This experiment employed analytical grade chemicals.The composition of the Aluminium has been determined by (optical emission spectroscopy OES).The result is depicted in Table1.Table 1
Aluminium coupon composition as evaluated by OES.
ElementAlSiFeCuMnMgCrNiZnTiGaVWeight %99.850.0490.0680.0040.0020.0010.0030.0020.0050.0020.0080.006


### 2.2. Preparation of Corrosive Solution

1 M HCl solutions were produced by diluting the (37% HCl) solution with deionized water. Before each experience, the test solutions were made fresh by mixing the extract immediately into the corrosion solution.


### 2.3. Preparation of Phoenix dactylifera Leaves PDL Extract

Phoenix dactylifera leaves are a common, inexpensive, and readily available plant in Iraq. To avoid any decrement in the concentration of corrosive solution after adding a particular amount of inhibitor solution, PDL was extracted in an acidic solution with the same concentration as the corrosive solution. PDL leaves were collected in the month of October, and they were rinsed with distilled water, cut into small pieces, and sun-dried. The dried samples were crushed with an electric blender. The extract is made by refluxing 10 gm of powder in 100 mL of 1 M HCl for 4 hours at 40°C and then filtering carefully as shown in Figure1.Figure 1
Phoenix dactylifera leaves and powder.


### 2.4. Weight Loss Measurement

For weight loss measures, aluminium specimens were employed. After being cleaned and dried, the specimens were plunged in a 1 M HCl solution with or without an inhibitor for 3 hr and then washed, dried, and weighed. Experiments were performed at different concentrations of inhibitor (2, 4, 6, 8, and 10 mL/L) and at varying temperatures (20, 30, 40, and 50°C) to calculate the amount of weight lost. The corrosion rate was determined using the following relationship [10] based on the change in weight of specimens.
(1)ρ=KWDAT,where ρ is the corrosion rate in mpy, K is the rate constant equal to 534 mpy, W is the weight loss, D is the material density, T is the time of the exposure, and A is the coupon exposed area.


## 2.1. Specimen Preparation

The purity of the aluminium which is used in the experiment is 99.85 percent. The aluminium sheets were cut into 3 × 3 × 1 cm for each coupon (couponexposedarea=4.65squareinch). A 3-mm diameter hole was bored on the upper edge to suspend each coupon in the corrosive environment. The coupons were cleaned and degreased, after being washed in 100 percent ethanol, sprayed in acetone, and left to dry. The dehydrated coupons were maintained in a dry desiccators until they were needed. This experiment employed analytical grade chemicals.The composition of the Aluminium has been determined by (optical emission spectroscopy OES).The result is depicted in Table1.Table 1
Aluminium coupon composition as evaluated by OES.
ElementAlSiFeCuMnMgCrNiZnTiGaVWeight %99.850.0490.0680.0040.0020.0010.0030.0020.0050.0020.0080.006


## 2.2. Preparation of Corrosive Solution

1 M HCl solutions were produced by diluting the (37% HCl) solution with deionized water. Before each experience, the test solutions were made fresh by mixing the extract immediately into the corrosion solution.


## 2.3. Preparation of Phoenix dactylifera Leaves PDL Extract

Phoenix dactylifera leaves are a common, inexpensive, and readily available plant in Iraq. To avoid any decrement in the concentration of corrosive solution after adding a particular amount of inhibitor solution, PDL was extracted in an acidic solution with the same concentration as the corrosive solution. PDL leaves were collected in the month of October, and they were rinsed with distilled water, cut into small pieces, and sun-dried. The dried samples were crushed with an electric blender. The extract is made by refluxing 10 gm of powder in 100 mL of 1 M HCl for 4 hours at 40°C and then filtering carefully as shown in Figure1.Figure 1
Phoenix dactylifera leaves and powder.


## 2.4. Weight Loss Measurement

For weight loss measures, aluminium specimens were employed. After being cleaned and dried, the specimens were plunged in a 1 M HCl solution with or without an inhibitor for 3 hr and then washed, dried, and weighed. Experiments were performed at different concentrations of inhibitor (2, 4, 6, 8, and 10 mL/L) and at varying temperatures (20, 30, 40, and 50°C) to calculate the amount of weight lost. The corrosion rate was determined using the following relationship [10] based on the change in weight of specimens.
(1)ρ=KWDAT,where ρ is the corrosion rate in mpy, K is the rate constant equal to 534 mpy, W is the weight loss, D is the material density, T is the time of the exposure, and A is the coupon exposed area.


## 3. Result and Discussion

### 3.1. The Inhibition Efficiency: The Inhibitor Efficiency Was Determined Using the Formula below [11]

(2)IE=1−ρinhρplank×100%,where ρplank is the weight reduction in the absence of inhibitor and ρinh is the weight reduction in the presence of inhibitor. The surface coverage degree θfor different inhibitor concentrations is obtained with the given equation
(3)θ=IE100,where θ is the surface coverage degree and IE is the inhibitor efficiency. At various temperatures and inhibitor concentrations, the corrosion and inhibitor efficiency rates are evaluated, and the results are compiled in Table 2 and Figures 2 and 3. The rate of corrosion tends to increase as the temperature increases and reduces with the inhibitor concentration increment.Table 2
Effect of inhibitor concentration and temperature on the corrosion behavior of aluminium in hydrochloric acid in the absence and presence of PDL extract as a corrosion inhibitor.
EXPConcentration of inhibitor/(mL/L)Temperature/C°CR/mpyθ(surface coverage)% IE10201.861002305.1680034017.3650045024.9940052201.4470.22222.2226303.9280.24024.00074011.3700.34534.52485013.1890.47247.22994201.0340.44444.44410302.2740.56056.00011404.3410.75075.00012506.0220.75975.906136200.6200.66666.66714301.4470.72072.00015403.3080.80980.95216504.4650.82182.134178200.2070.88988.88918300.5580.89289.20019401.2400.92992.85720501.4740.94194.1032110200.1450.92292.22222300.3100.94094.00023400.8270.95295.23824500.7240.97197.105Figure 2
Temperature’s effect on corrosion rate of aluminium after immersion in 1 M hydrochloric acid for 3 hr at different concentrations of inhibitor.Figure 3
The impact of temperature on inhibitive efficiency of PDL extract for aluminium corrosion in 1 M hydrochloric acid.


### 3.2. Temperature’s Effect: Thermodynamic Studies

Temperature affects the rate of nearly all chemical reactions and increases the speed of corrosion and effectiveness of inhibition. The corrosion rate of aluminium in an acidic medium increases with increasing temperature. The Arrhenius equation, in which the rate of corrosion reaction is connected with temperature, can be used to express this effect:(4)lnCR=lnA−EaRT,where Ea is the activation energy, A is the frequency factor, R is the universal gas constant, and T is the temperature in Kelvin.
(5)LnCRT=LnRNh+∆SR−∆HRT.We may plotlnCR/T versus 1/T using Equation (5), the straight line slope is (−ΔH/R), and the intercept is (ΔS/R+lnR/Nh), from which ΔH and ΔS can be determined (Figures 4 and 5 and Table 3).Figure 4
Arrhenius plot of aluminium corrosion in 1 M hydrochloric acid, including varying temperatures and concentrations of PDL extract.Figure 5
Langmuir adsorption isotherm of PDL extract for aluminium corrosion in 1 M hydrochloric acid at different temperatures.Table 3
Enthalpy, entropy, and activation energy of the Aluminium corrosion reaction with different concentrations of PDL extract.
Inhibitor concentration (mL/L)EaR2−∆HKJ/mol−∆SKJ/mol01181.8610.96968.5820.63321065.6730.94058.3630.5924743.0820.97844.3310.5476852.5300.97250.7900.5688945.8720.94150.4760.55210909.9140.97543.5740.524Table3 shows that the enthalpy of activation values for PDL extract were all negative, indicating that the aluminium corrosion process is exothermic. Additionally, the activation entropies were negative, showing that the activation complex represented association steps and that the reaction was both spontaneous and available.


### 3.3. Adsorption of Inhibitor and Isotherm

#### 3.3.1. Langmuir Adsorption Isotherm

Many examples of strong adsorption do not suit the Langmuir isotherm model, which is a set of uniform adsorption sites. This isotherm is expressed mathematically as [12]:
(6)Ciθ=1KL+Ci,where KL is the equilibrium constant (L/mL), KL represents the adsorption degree (a higher value of KL shows that the inhibitor is heavily adsorbed on the surface of the metal), and Ci represents the inhibitor concentration (mL/L) (Figure 6 and Table 4).
(7)KL=155.5exp−∆G°adsRT,where ΔGo is the standard adsorption free energy (kJ mol-1). Ci/θ versus Ci can be plotted according to Equation (5). The values of KL are constructed using the intercept. With increasing temperature, the calculated behavior of the equilibrium constant of adsorption (KL) decreased.Figure 6
Freundlich adsorption isotherm of PDL extract for aluminium corrosion in (1 M HCl) at different temperatures.Table 4
Langmuir equilibrium constant, standard adsorption free energy and correlation coefficient for Langmuir adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKL∆G°adsKJ/molR2200.120-4.6200.495300.148-5.3020.635400.268-7.0250.902500.397-8.3040.991In general, negative values ofΔGads larger than 40 kJ mol1 suggest chemical adsorption, whereas negative values of less than 20 kJ mol1 imply physical adsorption [13]. According to the data result, ΔGads values are negative and are less than -20 kJmol-1.This suggests that the inhibitor’s spontaneous adsorption on the metal surface supports the physical adsorption process.


#### 3.3.2. Freundlich Adsorption Isotherm

The equation can represent this isotherm is:(8)θ=KFCin,where KF (Freundlich constant) and n are constants for a particular system at a specific temperature.This isotherm can be expressed as follows [14]:
(9)Lnθ=LnKFCi1/n,(10)Lnθ=LnKF+1/nLnCi.We can plot Lnθ against Ln Ci using Equation (10), where the intercept and slope reveal the values of KF and 1/n, respectively. The corrosion inhibition behavior of PDL tends to be more compatible with Freundlich isotherm (Figure 6 and Table 5).Table 5
Freundlich parameters for Freundlich adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKF1/nR220-2.110.9250.98630-1.920.8560.95940-1.360.6190.88850-0.9970.4440.946The value of1/n is employed to express the ease of adsorption. Usually, when 0<1/n<1, adsorption is believed to be easy, and it considers to be moderated when 1/n=1 and difficult if 1/n>1 [15]. According to the result in Table 5, the adsorption of PDL inhibitor on the surface of aluminium is thought to be an easy process.The phytochemical analysis DPL leaves extract resulted in the identification of three main components, oleanolic acid, vanillyl alcohol, andβ sitosterol 3-O-β-D-glucoside in addition to minor constituents, fatty acids, lutein, and sucrose [16] which is expected to be adsorbed on the surface of aluminium resulting in inhibition of corrosion and reduction in the corrosion rate.Vanillyl alcohol was chosen as a model for theoretical study due to its high percentage in the PDL extract and the simplicity of its chemical composition.


### 3.4. Computational Study

The theoretical characteristics of the inhibitor compound were calculated employing the density functional theory DFT/B3LYP. The base set 6-311G(d,p) was employed in all computations. A computational study has been conducted with Gaussian 07 software. EHOMO, ELUMO (Figure7), energy gap ΔE, μ, S, η, ω, χ, ∆ψ inhibitor-metal interaction energy, and electrons transferred fraction (∆N) have been calculated (Table 6) employing Equations (11)–(16):
(11)IEIonizationpotential=EN−1−EN,(12)EAElectronaffinity=EN−EN+1.Figure 7
(a) Optimized structure of vanillyl alcohol according to DFT, (b) HOMO, and (c) LUMO.
(a)(b)(c)Table 6
Quantum chemical parameters for Vanillyl Alcohol obtained by DFT/B3LYP/6-311G (d,p) level.
Inhibitor moleculeEHOMOELUMO∆EIEEAηχμSω∆N∆ψPDL-11.26-0.2910.9711.260.295.495.783.710.091.25-0.140.1At the optimized geometry, the neutral molecule has the optimal energy EN, and EN+1 and EN-1 are single point (SP) energies of the neutral, anionic, and cationic forms of the multiple electron system (N).
(13)χ=−μ=−∂E∂Nνr=12IP+EA,(14)η=12∂2E∂N2νr=12∂μ∂Nνr=12IP−EA,(15)S=12η,(16)ω=μ22η,where the relative values of χ and η have been utilized, and the electron transfer fraction (N) from the inhibitor to the surface of the metal has been calculated as seen as [17]:
(17)∆N=χmetal−χinhibitor2ηmetal+ηinhibitor,where χmetal, χinhibitor, and ηmetal and ηinhibitor indicate the species’ absolute electronegativity and hardness; differences in electronegativity drive electron transport. The summation of the hardness parameters serves as resistance [18]. In this research, ηAl=0, whereas the theoretical aluminium work function, Alφ=4.28eV, was utilized instead χ of Al [19]. So, Equation (10) can be rewritten as:
(18)∆N=ϕAl−χinhibitor2ηinhibitorAnother significant factor measured, the initial inhibitor-metal interaction energy (∆ψ), developed by Kokalj and Kovacevic, is used in this work [20]. The initial interaction energy of inhibitor-metal ∆ψ has been computed as follows:
(19)∆ψ=χmetal−χinhibitor24ηmetal+ηinhibitor,which can be written as:
(20)∆ψ=χmetal−χinhibitor24ηinhibitor.The lower the value ofχ for inhibitor, the less the molecule electron grip and the greater the tendency to release electrons during intermolecular interactions [21]. The corrosion inhibitor had a negative ∆N value, indicating that it supplied electrons to the surface of the metal [22]. The increase in the number of electrons donated to the surface of the metal increases the efficiency of inhibition [23–25]. The inhibitor compounds’ capacity to absorb on the surface of Aluminium is linked to HOMO and LUMO [26, 27]. HOMO refers to the capacity to donate electrons, whereas the capability to accept electrons is referred to as LUMO. An energy gap with low values is a crucial indicator of the significant efficacy of the inhibitor due to the low energy required to remove electrons from an occupied orbital [28]. The dipole moment (μ) was another quantum parameter that turned out to be essential for corrosion inhibition effectiveness. More specifically, bond polarity and electron distribution are important [29]. EHOMO and ELUMO are related to the softness and hardness, respectively [28]. Vanillyl alcohol has the lowest hardness value of any inhibitor, 5.49. (eV).A corrosion inhibitor with a low hardness value is expected to have a high inhibitory effectiveness [16]. Adsorption occurs where a molecule has a high softness value for easy electron transport [30].


## 3.1. The Inhibition Efficiency: The Inhibitor Efficiency Was Determined Using the Formula below [11]

(2)IE=1−ρinhρplank×100%,where ρplank is the weight reduction in the absence of inhibitor and ρinh is the weight reduction in the presence of inhibitor. The surface coverage degree θfor different inhibitor concentrations is obtained with the given equation
(3)θ=IE100,where θ is the surface coverage degree and IE is the inhibitor efficiency. At various temperatures and inhibitor concentrations, the corrosion and inhibitor efficiency rates are evaluated, and the results are compiled in Table 2 and Figures 2 and 3. The rate of corrosion tends to increase as the temperature increases and reduces with the inhibitor concentration increment.Table 2
Effect of inhibitor concentration and temperature on the corrosion behavior of aluminium in hydrochloric acid in the absence and presence of PDL extract as a corrosion inhibitor.
EXPConcentration of inhibitor/(mL/L)Temperature/C°CR/mpyθ(surface coverage)% IE10201.861002305.1680034017.3650045024.9940052201.4470.22222.2226303.9280.24024.00074011.3700.34534.52485013.1890.47247.22994201.0340.44444.44410302.2740.56056.00011404.3410.75075.00012506.0220.75975.906136200.6200.66666.66714301.4470.72072.00015403.3080.80980.95216504.4650.82182.134178200.2070.88988.88918300.5580.89289.20019401.2400.92992.85720501.4740.94194.1032110200.1450.92292.22222300.3100.94094.00023400.8270.95295.23824500.7240.97197.105Figure 2
Temperature’s effect on corrosion rate of aluminium after immersion in 1 M hydrochloric acid for 3 hr at different concentrations of inhibitor.Figure 3
The impact of temperature on inhibitive efficiency of PDL extract for aluminium corrosion in 1 M hydrochloric acid.


## 3.2. Temperature’s Effect: Thermodynamic Studies

Temperature affects the rate of nearly all chemical reactions and increases the speed of corrosion and effectiveness of inhibition. The corrosion rate of aluminium in an acidic medium increases with increasing temperature. The Arrhenius equation, in which the rate of corrosion reaction is connected with temperature, can be used to express this effect:(4)lnCR=lnA−EaRT,where Ea is the activation energy, A is the frequency factor, R is the universal gas constant, and T is the temperature in Kelvin.
(5)LnCRT=LnRNh+∆SR−∆HRT.We may plotlnCR/T versus 1/T using Equation (5), the straight line slope is (−ΔH/R), and the intercept is (ΔS/R+lnR/Nh), from which ΔH and ΔS can be determined (Figures 4 and 5 and Table 3).Figure 4
Arrhenius plot of aluminium corrosion in 1 M hydrochloric acid, including varying temperatures and concentrations of PDL extract.Figure 5
Langmuir adsorption isotherm of PDL extract for aluminium corrosion in 1 M hydrochloric acid at different temperatures.Table 3
Enthalpy, entropy, and activation energy of the Aluminium corrosion reaction with different concentrations of PDL extract.
Inhibitor concentration (mL/L)EaR2−∆HKJ/mol−∆SKJ/mol01181.8610.96968.5820.63321065.6730.94058.3630.5924743.0820.97844.3310.5476852.5300.97250.7900.5688945.8720.94150.4760.55210909.9140.97543.5740.524Table3 shows that the enthalpy of activation values for PDL extract were all negative, indicating that the aluminium corrosion process is exothermic. Additionally, the activation entropies were negative, showing that the activation complex represented association steps and that the reaction was both spontaneous and available.


## 3.3. Adsorption of Inhibitor and Isotherm

### 3.3.1. Langmuir Adsorption Isotherm

Many examples of strong adsorption do not suit the Langmuir isotherm model, which is a set of uniform adsorption sites. This isotherm is expressed mathematically as [12]:
(6)Ciθ=1KL+Ci,where KL is the equilibrium constant (L/mL), KL represents the adsorption degree (a higher value of KL shows that the inhibitor is heavily adsorbed on the surface of the metal), and Ci represents the inhibitor concentration (mL/L) (Figure 6 and Table 4).
(7)KL=155.5exp−∆G°adsRT,where ΔGo is the standard adsorption free energy (kJ mol-1). Ci/θ versus Ci can be plotted according to Equation (5). The values of KL are constructed using the intercept. With increasing temperature, the calculated behavior of the equilibrium constant of adsorption (KL) decreased.Figure 6
Freundlich adsorption isotherm of PDL extract for aluminium corrosion in (1 M HCl) at different temperatures.Table 4
Langmuir equilibrium constant, standard adsorption free energy and correlation coefficient for Langmuir adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKL∆G°adsKJ/molR2200.120-4.6200.495300.148-5.3020.635400.268-7.0250.902500.397-8.3040.991In general, negative values ofΔGads larger than 40 kJ mol1 suggest chemical adsorption, whereas negative values of less than 20 kJ mol1 imply physical adsorption [13]. According to the data result, ΔGads values are negative and are less than -20 kJmol-1.This suggests that the inhibitor’s spontaneous adsorption on the metal surface supports the physical adsorption process.


### 3.3.2. Freundlich Adsorption Isotherm

The equation can represent this isotherm is:(8)θ=KFCin,where KF (Freundlich constant) and n are constants for a particular system at a specific temperature.This isotherm can be expressed as follows [14]:
(9)Lnθ=LnKFCi1/n,(10)Lnθ=LnKF+1/nLnCi.We can plot Lnθ against Ln Ci using Equation (10), where the intercept and slope reveal the values of KF and 1/n, respectively. The corrosion inhibition behavior of PDL tends to be more compatible with Freundlich isotherm (Figure 6 and Table 5).Table 5
Freundlich parameters for Freundlich adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKF1/nR220-2.110.9250.98630-1.920.8560.95940-1.360.6190.88850-0.9970.4440.946The value of1/n is employed to express the ease of adsorption. Usually, when 0<1/n<1, adsorption is believed to be easy, and it considers to be moderated when 1/n=1 and difficult if 1/n>1 [15]. According to the result in Table 5, the adsorption of PDL inhibitor on the surface of aluminium is thought to be an easy process.The phytochemical analysis DPL leaves extract resulted in the identification of three main components, oleanolic acid, vanillyl alcohol, andβ sitosterol 3-O-β-D-glucoside in addition to minor constituents, fatty acids, lutein, and sucrose [16] which is expected to be adsorbed on the surface of aluminium resulting in inhibition of corrosion and reduction in the corrosion rate.Vanillyl alcohol was chosen as a model for theoretical study due to its high percentage in the PDL extract and the simplicity of its chemical composition.


## 3.3.1. Langmuir Adsorption Isotherm

Many examples of strong adsorption do not suit the Langmuir isotherm model, which is a set of uniform adsorption sites. This isotherm is expressed mathematically as [12]:
(6)Ciθ=1KL+Ci,where KL is the equilibrium constant (L/mL), KL represents the adsorption degree (a higher value of KL shows that the inhibitor is heavily adsorbed on the surface of the metal), and Ci represents the inhibitor concentration (mL/L) (Figure 6 and Table 4).
(7)KL=155.5exp−∆G°adsRT,where ΔGo is the standard adsorption free energy (kJ mol-1). Ci/θ versus Ci can be plotted according to Equation (5). The values of KL are constructed using the intercept. With increasing temperature, the calculated behavior of the equilibrium constant of adsorption (KL) decreased.Figure 6
Freundlich adsorption isotherm of PDL extract for aluminium corrosion in (1 M HCl) at different temperatures.Table 4
Langmuir equilibrium constant, standard adsorption free energy and correlation coefficient for Langmuir adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKL∆G°adsKJ/molR2200.120-4.6200.495300.148-5.3020.635400.268-7.0250.902500.397-8.3040.991In general, negative values ofΔGads larger than 40 kJ mol1 suggest chemical adsorption, whereas negative values of less than 20 kJ mol1 imply physical adsorption [13]. According to the data result, ΔGads values are negative and are less than -20 kJmol-1.This suggests that the inhibitor’s spontaneous adsorption on the metal surface supports the physical adsorption process.


## 3.3.2. Freundlich Adsorption Isotherm

The equation can represent this isotherm is:(8)θ=KFCin,where KF (Freundlich constant) and n are constants for a particular system at a specific temperature.This isotherm can be expressed as follows [14]:
(9)Lnθ=LnKFCi1/n,(10)Lnθ=LnKF+1/nLnCi.We can plot Lnθ against Ln Ci using Equation (10), where the intercept and slope reveal the values of KF and 1/n, respectively. The corrosion inhibition behavior of PDL tends to be more compatible with Freundlich isotherm (Figure 6 and Table 5).Table 5
Freundlich parameters for Freundlich adsorption isotherm of the PDL extract inhibitor for aluminium corrosion at different temperatures.
Temperature °CKF1/nR220-2.110.9250.98630-1.920.8560.95940-1.360.6190.88850-0.9970.4440.946The value of1/n is employed to express the ease of adsorption. Usually, when 0<1/n<1, adsorption is believed to be easy, and it considers to be moderated when 1/n=1 and difficult if 1/n>1 [15]. According to the result in Table 5, the adsorption of PDL inhibitor on the surface of aluminium is thought to be an easy process.The phytochemical analysis DPL leaves extract resulted in the identification of three main components, oleanolic acid, vanillyl alcohol, andβ sitosterol 3-O-β-D-glucoside in addition to minor constituents, fatty acids, lutein, and sucrose [16] which is expected to be adsorbed on the surface of aluminium resulting in inhibition of corrosion and reduction in the corrosion rate.Vanillyl alcohol was chosen as a model for theoretical study due to its high percentage in the PDL extract and the simplicity of its chemical composition.


## 3.4. Computational Study

The theoretical characteristics of the inhibitor compound were calculated employing the density functional theory DFT/B3LYP. The base set 6-311G(d,p) was employed in all computations. A computational study has been conducted with Gaussian 07 software. EHOMO, ELUMO (Figure7), energy gap ΔE, μ, S, η, ω, χ, ∆ψ inhibitor-metal interaction energy, and electrons transferred fraction (∆N) have been calculated (Table 6) employing Equations (11)–(16):
(11)IEIonizationpotential=EN−1−EN,(12)EAElectronaffinity=EN−EN+1.Figure 7
(a) Optimized structure of vanillyl alcohol according to DFT, (b) HOMO, and (c) LUMO.
(a)(b)(c)Table 6
Quantum chemical parameters for Vanillyl Alcohol obtained by DFT/B3LYP/6-311G (d,p) level.
Inhibitor moleculeEHOMOELUMO∆EIEEAηχμSω∆N∆ψPDL-11.26-0.2910.9711.260.295.495.783.710.091.25-0.140.1At the optimized geometry, the neutral molecule has the optimal energy EN, and EN+1 and EN-1 are single point (SP) energies of the neutral, anionic, and cationic forms of the multiple electron system (N).
(13)χ=−μ=−∂E∂Nνr=12IP+EA,(14)η=12∂2E∂N2νr=12∂μ∂Nνr=12IP−EA,(15)S=12η,(16)ω=μ22η,where the relative values of χ and η have been utilized, and the electron transfer fraction (N) from the inhibitor to the surface of the metal has been calculated as seen as [17]:
(17)∆N=χmetal−χinhibitor2ηmetal+ηinhibitor,where χmetal, χinhibitor, and ηmetal and ηinhibitor indicate the species’ absolute electronegativity and hardness; differences in electronegativity drive electron transport. The summation of the hardness parameters serves as resistance [18]. In this research, ηAl=0, whereas the theoretical aluminium work function, Alφ=4.28eV, was utilized instead χ of Al [19]. So, Equation (10) can be rewritten as:
(18)∆N=ϕAl−χinhibitor2ηinhibitorAnother significant factor measured, the initial inhibitor-metal interaction energy (∆ψ), developed by Kokalj and Kovacevic, is used in this work [20]. The initial interaction energy of inhibitor-metal ∆ψ has been computed as follows:
(19)∆ψ=χmetal−χinhibitor24ηmetal+ηinhibitor,which can be written as:
(20)∆ψ=χmetal−χinhibitor24ηinhibitor.The lower the value ofχ for inhibitor, the less the molecule electron grip and the greater the tendency to release electrons during intermolecular interactions [21]. The corrosion inhibitor had a negative ∆N value, indicating that it supplied electrons to the surface of the metal [22]. The increase in the number of electrons donated to the surface of the metal increases the efficiency of inhibition [23–25]. The inhibitor compounds’ capacity to absorb on the surface of Aluminium is linked to HOMO and LUMO [26, 27]. HOMO refers to the capacity to donate electrons, whereas the capability to accept electrons is referred to as LUMO. An energy gap with low values is a crucial indicator of the significant efficacy of the inhibitor due to the low energy required to remove electrons from an occupied orbital [28]. The dipole moment (μ) was another quantum parameter that turned out to be essential for corrosion inhibition effectiveness. More specifically, bond polarity and electron distribution are important [29]. EHOMO and ELUMO are related to the softness and hardness, respectively [28]. Vanillyl alcohol has the lowest hardness value of any inhibitor, 5.49. (eV).A corrosion inhibitor with a low hardness value is expected to have a high inhibitory effectiveness [16]. Adsorption occurs where a molecule has a high softness value for easy electron transport [30].


## 4. Conclusion

The result shows that PDL acid extract is an excellent and efficient inhibitor of Aluminium corrosion in hydrochloric acid media, and as extract concentration increases, the efficiency of the inhibition increases. The adsorption of different concentrations of plant extract on the surface of Aluminium in 1 M HCl acid obeyed the Freundlich adsorption isotherm. The values of standard free energy of adsorption show that the inhibitor adsorption on the Aluminium surface proceeded via physisorption.The fact that the free energy of adsorption has a negative sign suggests that the inhibitors’ adsorption on the Aluminium surface occurred in a spontaneous process.



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*Source: 5953561-2022-08-04.xml*