Experimental and Computational Studies of Vitellaria

Vol. 13, 2021, pp. 66~86

ISSN: 2605-6895

66

Experimental and Computational Studies of Vitellaria Paradoxa

Extract as Aluminium Corrosion Inhibitor in Acidic and

Alkaline Media

Abdullahi Muhammad Ayuba, Mustapha Ashiru Auta and Najib Usman Shehu Department of Pure and Industrial Chemistry, Faculty of Physical Sciences, Bayero University, Kano,

Nigeria

Article Info ABSTRACT

Article history:

Received Jun 20th, 2021

Revised Sep 20th, 2021

Accepted Nov 06th, 2021

The adsorptive and inhibitive action of Vitellaria paradoxa (VP)

towards general and pitting corrosion of aluminium (Al) was

investigated both theoretically in gas phase and using weight loss in

both HCl and NaOH solutions. The plant extract was found to inhibit

the corrosion of aluminium with better inhibition efficiency in the

acidic medium than in the alkaline medium. The inhibition efficiency

of VP in both media decreased with increase in temperature, period of

immersion and corrodent concentration, but increased with increasing

VP concentration. Adsorption characteristics of VP on aluminium

surface in HCl and NaOH solutions was more consistent obeying

Langmuir, Flory-Huggins and Temkin isotherm models and the

process obeys physisorption mechanism. Thermodynamic parameters

including; Ea, Qads, ΔSads, ΔHads and ΔGads for the adsorption predicted

a spontaneous process. Quantum studies of the adsorptive interaction

of VP phytochemicals on Al surface favoured the process of back

donation. Data obtained from molecular dynamic simulations

corresponds to a stable adsorption structure of the compounds with

ester being best adsorbed among the four selected molecules, while the

Fukui indices indicated that oxygen and carbon are the reactive atoms

of the molecules for electrophilic and nucleophilic attacks. Theoretical

adsorption energy data obtained from molecular dynamic simulations

confirmed the spontaneity and physical mechanism of the process with

figures less than the threshold values required for chemical adsorption

Keyword:

Aluminium

Corrosion

Vitellaria paradoxa

Inhibition

Computational

Corresponding Author: Abdullahi Muhammad Ayuba

Adress: Department of Pure and Industrial Chemistry, Faculty of Physical Sciences, Bayero University,

Kano, Nigeria.

Email: [email protected] Phone:+2348062771500

1. INTRODUCTION

mailto:[email protected]

A.M. Ayouba et al.. RHAZES: Green and Applied Chemistry, Vol. 13, 2021, pp.66-86

67

The use of aluminium (Al) and its alloys in industries have been subjected to corrosion

challenges because of the aggressive corrosive environment upon which they are put to use.

Even though, this metal is covered with a thin passive surface oxide film which do slowly

corrode in aggressive media. The exposed unprotected metal atoms become prone to attacks

by aqueous corrosive medium which subsequently leads to a sequence of electrochemical

reactions. Processes occurring at the bared metal surface which increases the rate of metal

dissolution resulting into the formation of soluble complexes are environmentally dependent

[1]. Al metal is known for its high technological value which normally results into numerous

industrial applications of which aerospace is a good example. For this reason, the corrosion

of Al and/or its alloys have been a challenge to material scientists and one major method of

minimizing this effect is the use of inhibitors [2]. Serious researches are underway with an

aim of developing effective corrosion inhibitors of plant origin for aluminium in various

aggressive environments. This is chanelled towards finding a replacement to toxic synthetic

inhibitors which have deleterious effects to the environment [3]. Adsorption characteristics

of these inhibitors depend on several factors including the nature and number of potential

adsorption sites present in the inhibitor molecule. Many researchers have tried to relate the

corrosion inhibitor efficiency with a number of electronic and structural parameters of these

molecules [4]. Many of these properties contribute simultaneously to the inhibition

efficiency, therefore sometimes very difficult to establish which one play a better role in

increasing inhibition efficiency of corrosion. This instigated the need for computational

studies to further reveal the mechanism of the inhibition [2].

The plant of interest in this research Vitellaria paradoxa (formerly Butyrosper

mumparkii), commonly known as shea tree or Vitellaria, is a tree of the Sapotaceae family.

It is the only species in genus Vitellaria, and is indigenous to Africa [5]. The Shea fruit

consists of a thin, tart, nutritious pulp that surrounds a relatively large, oil-rich seed from

which Shea butter is extracted. The shea tree is a traditional African food plant. It has been

claimed to have potential to improve nutrition, boost food supply in the "annual hungry

season", foster rural development, and support sustainable land care [6]. Shea butter has

many uses and may or may not be refined. In the West, it is mostly used for cosmetics as

emollient. Throughout Africa it is used extensively for food, is a major source of dietary fat,

and for medicinal purposes [7]. Fat extracts from the kernel of the plant is used extensively

in cosmetics and chocolate industries [8].

Fig 1: Picture of Vitellaria paradoxa plant

Several researches have been carried out on the inhibition of the corrosion of metals by some

plant extracts. Extracts of phyllanthus amarus [9], Opuntia [10], Menthapulegium [11],


68

Strychnosnux-vomica [12], Ammi visnaga [13] have been investigated for their corrosion

inhibition potentials. However, there is paucity of literature on the use of ethanol extract of

VP as an inhibitor for the corrosion of aluminium in HCl and NaOH media. Therefore, it is

within the scope of this research to use and establish the adsorptive, inhibitive, kinetic and

thermodynamic properties of VP on the corrosion of aluminium in HCl and NaOH media

using weight loss method of analysis. The interaction between three triterpenoid compounds;

betulinic acid [14], ursolic acid and oleanolic acid [15] and (E)-3-Phenylacrylic acid 17-(4-

ethyl-1,5-dimethyl-hexyl)-10,13-dimethyl-2,3,4,7,8,9,10,11,12,13,14,15,16,17-

tetradecahydro-1H-cyclopenta[a]phenanthren-3-yl ester [16] all isolated from VP and metal

surface was simulated using molecular dynamics whereas analysis of the electronic

properties and energetics of the inhibitor molecules were achieved using density functional

theory (DFT).

2. RESEARCH METHOD

Aluminium sheets (coupons) of composition (wt %); Al(98.70), Si(0.48), Cl(0.014),

K(0.04), Ca(0.01), Ti(0.005), V(0.016), Mn(0.012), Fe(0.50), Ni(0.013), Cu(0.048),

Ga(0.013), In(0.10), Te(0.010), Ba(0.009), Os(0.032), Ir(0.03) were used for this study. A

compact energy dispersive X-ray spectrometer (Mini pal) controlled by a computer was used

for the determination of the coupon composition. Sample of the Al pellet was loaded into

the sample chamber of the spectrometer, a voltage (30 kV max.) and a current (1mA max.)

was applied to produce the x-rays to excite the sample for 10 minutes. The result (spectrum)

of the analysed sample was used to determine the sample elements concentration (%). The

Al sheet was pressed-cut mechanically into coupons of 3 by 4 by 0.11 cm dimensions.

Coupons were polished, degreased in ethanol, dried with acetone and preserved in a

desiccator. Analar grade reagents and double distilled water was used for solution

preparations.

2.1 Extraction of the Plant

VP leaves samples were collected from Kadargo golf club Kainji, new Bussa, Niger

state, North-central of Nigeria. The samples were washed, air dried, ground, sieved and

soaked in a solution of ethanol for 1 week to percolate and then filtered. Thick syrup of the

extract was obtained using a rotar vapour (BUCHI R110, 400C) and was later air dried.

Different VP extract concentrations were prepared by dissolving 0.2, 0.4 and 0.6g of it

differently in 1dm3 solutions of 0.6, 0.8 and 1.0M HCl and NaOH respectively [17].

2.2 Chemical Analysis

Phytochemical analysis of the VP ethanol extract was carried out according to the

method reported by Ndukwe et al. [18] and Eddy et al. [17,19]. For the identification of

saponin, frothing and Na2CO3 tests were adopted, while bromine water and ferric chloride

tests were employed for the identification of tannin. Cardiac glycosides were identified using

Leberman’s and Salkowski’s test, while Dragendorf, Hagger and Meyer reagents were used

for the identification of alkaloids.

2.3 Weight Loss Method


69

Into an open beaker, a previously weighed Al coupon was completely immersed in

200cm3 of the test solution and was covered, inserted into a water bath maintained at 303 K.

Each test coupon was withdrawn from the test solution after every hour, washed in a solution

containing 50% NaOH and 100gL-1 of zinc dust (in order to remove the corrosion product).

The washed coupon was rinsed in acetone and dried in air and reweighed. The difference in

weight for a period of 6 hours was taken as total weight loss. The experiments were repeated

at 313 and 323 K respectively. The degree of surface coverage (θ), inhibition efficiency (%I)

of the inhibitor and corrosion rates (CR in gh-1cm-2) were calculated from weight loss results

using Equations (1) to (3) respectively:

θ = 1 – 𝑊1

𝑊2 (1)

% I = (1 - 𝑊1

𝑊2) x 100 (2)

CR (gh-1cm-2) = 𝛥𝑊

𝐴𝑡 (3)

Where W1 and W2 are the weight losses (g) for aluminium in the presence and

absence of the inhibitor in HCl and NaOH solutions, θ is the degree of surface coverage of

the inhibitor, A is the area of the aluminium coupon (in cm2), t is the period of immersion

(hours) and ΔW is the weight loss of aluminium after time, t [20].

2.4 Quantum Chemical Calculations

In order to study the effect of the electronic and molecular structure on inhibition

efficiency, quantum chemical calculations with complete geometry optimizations of the VP

phytochemicals as corrosion inhibitors was performed using DMol3 module of Materials

Studio software version 8.0 (BIOVIA, Accelrys). This module is an atomic orbital

implementation of density functional theory (DFT) in the local density approximation (LDA)

regime. Calculations were done in the Perdew-Wang Correlation (PWC) form, at the double

numerical quality plus d-functions (DND) atomic basis set level. Complete geometrical

optimizations of the structures of VP compounds from the literature, the electron density,

the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular

orbital (LUMO) among others were obtained [21].

2.5 Molecular Dynamic Simulations

The molecular dynamics construction of unit cells and optimizations were performed

using Forcite plus forcefield in the Accelrys Material Studio 8.0 software. The Al was

cleaved along the densely packed reflection (Al (110)) being the most stable reflection

compared to the open Al (111) and Al (100) planes. Forcite quench molecular dynamics was

applied to sample low energy configurations of adsorbed molecules on the Al (110) surface.

Calculations were carried out in 5000 steps within simulation time of 5 ps invoking a 6 × 5

supercell at 350 K [21].

3. RESULTS AND ANALYSIS

The study on the corrosion inhibition of Al in HCl and NaOH solutions using VP

extract as inhibitor was conducted using weight loss and quantum chemical methods. The

results of this study were presented in tabular format and discussed herewith. Weight loss


70

experiments were conducted by varying the temperatures (303, 313 and 323K), corrodent

(HCl and NaOH) concentration (0.6, 0.8 and 1.0M) and inhibitor (VP extract) concentrations

(0.2, 0.4 and 0.6g/L) to obtain the results presented under the following headings:

3.1 Effect of Corrodent, Inhibitor Concentration and Temperature Variation

Table 1 presents values of corrosion rate for the corrosion of Al in varying HCl and

NaOH concentrations with/without the inhibitor at 303K temperature. Accordingly, the

higher the corrodent concentration the higher the corrosion rates for both corrodents in the

absence of the inhibitor. The same experiment was repeated at 313 and 323K temperatures

with similar results obtained. A similar observation was reported by Ayuba et al. [20].

The effect of temperature on the corrosion system was tested at three different

temperatures (303, 313 and 323K) respectively at a given HCl or NaOH concentration in the

absence of the inhibitor. It can be observed from Table 1 that the highest corrosion rate of

Al was observed at 323K for 1.0M HCl. The results obtained for 0.60 and 0.80M HCl

solutions were similar to that of 1.0M HCl. Similarly as in the effect of corrodent

concentration, higher temperatures produced higher weight loss with time, thus higher rate

of reaction. Comparing the results of the three HCl concentrations, it is also evident that a

combination of higher temperatures and corrodent concentrations produced respectively

higher weight loss resulting into a higher corrosion rate. Therefore, the higher the

temperature the higher the rate of corrosion, suggesting a physical adsorption mechanism [2,

17]. The effect of temperature on the corrosion system was also tested at three different

temperatures (303, 313 and 323K) respectively in NaOH solutions. It can be observed that

the highest weight loss of Al was observed at 323K for 1.0M NaOH. The results obtained

for 0.60 and 0.8M HCl solutions were similar to that of NaOH. Comparing the results of the

three NaOH concentrations, it is also evident that a combination of lower temperatures and

corrodent concentrations produced respectively lower weight loss resulting into a lower

corrosion rate.

To test and establish whether the inhibitor has an effect on the rate of Al corrosion

in varying HCl solution, varying concentrations of the inhibitor was used. As presented in

Table 1, the results of the effect of VP extract on the corrosion of Al in varying HCl

concentration and temperatures. It reveals that the corrosion rate of Al in HCl increased with

increase in concentration and temperature but decreased with increase in the concentration

of VP extract. Similar results were also obtained for NaOH solutions. At higher

temperatures, weight loss were found to increase with increase in temperature even as the

VP extract concentrations got higher, indicating that the rate of corrosion of aluminium in

all HCl or NaOH concentrations increases with increase in temperature and that VP extract

is adsorbed on the surface of Al according to the mechanism of physical adsorption [17, 19,

22, 23].

3.2 Inhibition Efficiencies of the Plant Extract

To further establish the results of this study, values of percentage inhibition

efficiencies of Al in HCl and NaOH solutions with or without VP extract as inhibitor using

weight loss methods were calculated. The results were as presented in Table 2. From the

results obtained it is evident that the inhibition efficiency decreases with increase in


71

corrodent concentration and temperature but increases with increasing VP extract

concentrations in all systems (HCl or NaOH) tested. These results signify that the inhibitor

is an adsorption inhibitor and that the adsorption of the inhibitor on Al surface favours the

mechanism of physical adsorption [24].

Table 1: Corrosion rates (gh-1cm-2) of Al in HCl and NaOH solutions containing various

concentrations of VP extract at varying temperatures

Temperature 303K 313K 323K

Corrodent Inhibitor Corrodent Concentration

CONCENTRATION 0.6M 0.8M 1.0M 0.6M 0.8M 1.0M 0.6M 0.8M 1.0M

0.00g/L 0.625 1.014 1.167 0.681 0.901 0.931 0.708 0.903 0.972

HCl 0.20g/L 0.625 0.972 1.097 0.667 0.875 0.903 0.708 0.889 0.944

0.40g/L 0.625 0.944 1.083 0.653 0.861 0.889 0.694 0.861 0.917

0.60g/L 0.597 0.931 1.028 0.653 0.847 0.875 0.667 0.819 0.875

0.00g/L 0.681 0.986 1.375 1.014 1.139 1.167 1.083 1.236 1.264

NaOH 0.20g/L 0.486 0.972 1.361 1.069 1.083 1.111 0.986 1.222 1.250

0.40g/L 0.389 0.944 1.208 0.958 1.014 1.056 0.958 1.181 1.306

0.60g/L 0.333 0.431 1.000 0.917 0.972 1.222 0.931 1.139 1.267

Table 2: Percentage Inhibition Efficiencies of Al in HCl and NaOH solutions containing

various concentrations of VP extract at varying temperatures

Temperature 303K 313K 323K

Corrodent Inhibitor Corrodent Concentration

Concentration 0.6M 0.8M 1.0M 0.6M 0.8M 1.0M 0.6M 0.8M 1.0M

0.00g/L 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

HCl 0.20g/L 5.952 4.110 2.041 3.077 2.985 0.000 2.857 1.538 0.000

0.40g/L 7.143 6.849 4.082 5.714 4.615 1.961 4.615 4.478 0.000

0.60g/L 11.90 8.219 4.444 10.00 9.231 5.882 6.154 5.970 4.082

0.00g/L 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

NaOH 0.20g/L 28.57 1.408 1.01 2.740 1.878 1.076 1.282 1.124 1.099

0.40g/L 42.86 5.051 4.525 11.54 4.98 4.524 5.479 4.494 4.396

0.60g/L 56.34 51.02 11.11 15.38 14.63 10.29 8.219 7.865 7.692

Table 3: Phytochemicals in VP Extract

Phytochemicals Results

Carbohydrate +

Free Reducing Sugar +

Cardiac Glycoside -

Saponin +

Steroid +

Alkaloids +

Flavonoids -

Anthracene -

Ketoses +

+ : Detected - : Not detected

3.3 Phytochemical Composition of Ethanol Extracts of VP


72

The ethanol extract of VP was subjected to phytochemical screening in other to detect the

group of phytochemicals present in the extract. The results obtained are as presented in Table

3. It can be observed that carbohydrates, saponin, free reducing sugars, steroids, alkaloids

and ketoses were detected. It is expected that these phytochemicals may be responsible for

the inhibition potentials of the studied plant extract.

3.4 Kinetic Consideration

As established by many researchers, Eddy et al. [17, 19, 23, 24] reported that most

corrosion reactions obey the first order kinetic model. To further establish that with respect

to this study, equation (4) was used where k1 is the first order reaction rate constant and t is

the time in hours:

−𝒍𝒐𝒈 (𝒘𝒆𝒊𝒈𝒉𝒕 𝒍𝒐𝒔𝒔) = 𝒌𝟏𝒕

𝟐.𝟑𝟎𝟑 (4)

The plots of –log (weight loss) versus time (in the presence and absence of VP extract

were linear (with almost unity values for R2) confirming that a first order kinetic is applicable

to the corrosion of Al in the presence and absence of VP extract in HCl and NaOH solutions.

Also the half-life of a first order reaction is related to the rate constant according to equation

(5):

𝒕𝟏/𝟐 = 𝟎.𝟔𝟗𝟑

𝒌𝟏 (5)

Values of the half-life and rate constants obtained from the slopes of the kinetic plots

are presented in the Table 4. The results revealed that the half-lives of Al in the presence of

VP extract were higher than those without VP extract indicating that the inhibitors increased

the half-life of Al in HCl or NaOH solution. Secondly, it was also observed that the higher

the VP extract concentration the higher the half-life values. Thirdly, both temperature and

corrodent concentrations were found to affect the half-life values, as can be seen from the

Table 4.

3.5 Thermodynamic Considerations

3.5.1 Effect of Temperature

In order to study the effect of temperature on the corrosion of Al in varying

concentrations of HCl and NaOH containing various concentrations of VP extract, the

Arrhenius equation (6) was used:

𝑪𝑹 = 𝑨𝒆𝒙𝒑 (−𝑬𝒂

𝑹𝑻) (6)

Taking logarithm of both sides at a particular temperature gives equation (7), while

at two different temperatures gives equation (8)

𝒍𝒐𝒈(𝑪𝑹) = 𝒍𝒐𝒈𝑨 − 𝑬𝒂

𝟐.𝟑𝟎𝟑𝑹𝑻. (7)

𝒍𝒐𝒈 (𝑪𝑹𝟐

𝑪𝑹𝟏) =

𝑬𝒂

𝟐.𝟑𝟎𝟑𝑹(

𝟏

𝑻𝟏−

𝟏

𝑻𝟐) (8)

Where CR1 and CR2 are the corrosion rates of Al at temperatures T1 (303K) and T2

(323K) respectively, Ea is the activation energy and R is the gas constant. Calculated values

of activation energy using equation (8) are presented in Table 5. Effect of corrodent

concentration and inhibitor concentrations were taken into consideration. It was observed

that the values obtained in the presence of the inhibitor were all greater than those without

inhibitor, indicating that the inhibitor increases the activation energy and thereby decreasing


73

the rate of reaction. It was also observed that, as the concentration of corrodent increases the

activation energy decreases thereby increasing the rate of corrosion. It can also be observed

from Table 5 that the activation energies are less than the threshold value of 80kJ/mol which

is required for the mechanism of chemical adsorption. Therefore the adsorption of VP extract

on the surface of Al is consistent with the mechanism of physical adsorption [25].

The heat of adsorption Qads of VP extract on the surface of Al in HCl and NaOH

solution was calculated using equation (9):

𝑸𝒂𝒅𝒔 = 𝟐. 𝟑𝟎𝟑𝑹 [𝒍𝒐𝒈 (𝝑𝟐

𝟏−𝝑𝟐) − 𝒍𝒐𝒈 (

𝝑𝟏

𝟏− 𝝑𝟏)] 𝒙

𝑻𝟏− 𝑻𝟐

𝑻𝟐 − 𝑻𝟏

𝒌𝑱

𝒎𝒐𝒍𝒔 (9)

Where Qads is the heat of adsorption of the VP extract on the surface of Al, R is the

gas constant, θ1 and θ2 are the degrees of surface coverage of VP extract at 303K (T1) and

323K (T2) respectively. Calculated values of Qads are also presented in Table 5. These values

were all negative and tend to decrease with corrodent concentration and increase with VP

extract concentration indicating that the adsorption of VP extract on the surface of Al in HCl

and NaOH is in all cases tested exothermic.

Table 4: Kinetic parameters for the corrosion inhibition of Al in HCl and NaOH solutions

using VP extract as inhibitor

Corrodent Temp Inhibitor conc 0.00g/L 0.20g/L 0.40g/L 0.60g/L

Kinetic

parameter k1 t1/2 k1 t1/2 k1 t1/2 k1 t1/2

0.6M 0.133 5.206 0.133 5.206 0.133 5.206 0.141 4.926

303K 0.8M 0.052 13.21 0.059 11.66 0.091 7.632 0.103 6.747

1.0M 0.029 23.84 0.039 17.64 0.041 16.73 0.050 13.81

0.6M 0.119 5.828 0.133 5.206 0.133 5.206 0.133 5.206

HCl 313K 0.8M 0.067 10.38 0.072 9.650 0.072 9.650 0.080 8.697

1.0M 0.072 9.650 0.072 9.65 0.074 9.315 0.077 8.998

0.6M 0.112 6.174 0.112 6.174 0.133 5.206 0.141 4.926

323K 0.8M 0.072 9.650 0.085 8.138 0.091 7.632 0.097 7.17

1.0M 0.059 11.66 0.064 10.78 0.067 10.38 0.069 10.01

0.6M 0.119 3.96 0.238 3.266 0.212 2.913 0.175 5.828

303K 0.8M 0.057 11.66 0.195 10.78 0.064 3.550 0.059 12.14

1.0M 0.002 205.8 0.021 67.19 0.010 32.52 0.003 413.6

0.6M 0.041 15.91 0.064 11.20 0.062 10.78 0.044 16.73

NaOH 313K 0.8M 0.033 16.73 0.059 13.21 0.052 11.66 0.041 20.95

1.0M 0.029 18.63 0.055 15.15 0.046 12.66 0.037 23.84

0.6M 0.052 12.14 0.067 11.20 0.062 10.38 0.057 13.21

323K 0.8M 0.019 32.52 0.033 25.58 0.027 20.95 0.021 35.67

1.0M 0.016 39.46 0.029 29.85 0.023 23.84 0.018 44.08

Table 5: Some adsorption parameters for the inhibition of the corrosion of Al in varying

Corrodent Solution by VP extract


74

Inhibitor conc 0.00g/L 0.20g/L 0.40g/L 0.60g/L

Corrodent Cor conc Ea Qads Ea Qads Ea Qads Ea Qads

0.6M -20.2 0.00 -17.9 -57.03 -18.1 -249 -15.3 -305

NaOH 0.8M -6.77 0.00 -5.85 -27.12 -4.79 -36.56 -4.86 -54.4

1.0M 3.434 0.00 5.096 -7.015 6.724 -26.13 7.024 -35.5

0.6M -3.34 0.00 -10.5 -190.9 0.000 -190.9 3.858 51.11

HCl 0.8M 2.565 0.00 6.698 -25.81 9.595 -167.4 8.539 -116

1.0M -61.9 0.00 -37.8 -18.03 -38.7 -125.9 -39.1 -114

3.5.2 Enthalpy and Entropy of Adsorption

In order to calculate some other thermodynamic parameters for the adsorption (ΔSads

and ΔHads) of VP extract on Al surface, the transition state equation (10) was used:

𝒍𝒐𝒈 (𝑪𝑹

𝑻) = 𝒍𝒐𝒈 (

𝑹

𝑵𝒉) +

∆𝑺𝒂𝒅𝒔

𝟐.𝟑𝟎𝟑𝑹−

∆𝑯𝒂𝒅𝒔

𝟐.𝟑𝟎𝟑𝑹𝑻 (10)

Where CR is the corrosion rate, T is absolute temperature, R is gas constant, N is

Avogadros constant, h is planks constant, ΔSads and ΔHads are entropy and enthalpy of

adsorption. From this equation (10), a plot of log(CR/T) versus 1/T gave a straight line with

a slope and intercept equal to -ΔHads/2.303R and [log(R/Nh) + ΔSads/2.303R]. Values of

ΔSads and ΔHads calculated from equation (10) are recorded in Table 6. The negative values

of ΔHads indicated that the corrosion of aluminium inhibited by VP extract is exothermic and

increases with corrodent concentration increase, suggesting a feasible reaction.

3.5.3 Free energy of Adsorption

The Gibb’s free energy of adsorption of VP extract onto the surface of Al in HCl and

NaOH solutions was calculated at three different temperatures using the Gibbs-Helmholtz

equation (11):

ΔGads = ΔHads - TΔSads (11)

Where ΔGads is the free energy of VP extract adsorption onto Al surface, ΔHads is the

enthalpy of its adsorption and ΔSads is the entropy of adsorption and T is the temperature

(K). The calculated values of the free energy are as presented in Table 7. From the results

obtained the free energies were found to increase with temperature, decrease with inhibitor

concentration and increase with HCl concentration but decrease with NaOH concentration.

However all these values are lower in actual value than the threshold of -40kJ/mol required

for chemical adsorption. This indicates that the adsorption of VP extract onto the surface of

Al is spontaneous and also supports the mechanism of physical adsorption [25].

Table 6: Enthalpy and entropy of adsorption of VP extract on the surface of Al in test

solution

Inhibitor conc 0.00g/L 0.20g/L 0.40g/L 0.60g/L

Corrodent Corr conc ΔH ΔS ΔH ΔS ΔH ΔS ΔH ΔS

0.6M -3.446 0.227 -7.659 0.227 -7.659 0.229 -3.829 0.229


75

HCl 0.8M -4.595 0.226 -11.49 0.227 -11.49 0.227 -8.616 0.227

1.0M -7.659 0.225 -19.15 0.225 -19.15 0.225 -15.32 0.226

0.6M -36.38 0.218 -86.16 0.213 -72.76 0.215 -63.19 0.214

NaOH 0.8M -9.574 0.222 -70.84 0.215 -1.915 0.223 -7.659 0.223

1.0M -3.829 0.223 -9.574 0.223 -9.574 0.223 -7.659 0.222

Table 7: Gibb’s free energy of adsorption of VP extract onto Al surface in HCl and NaOH

solutions at varying temperatures

Corrodent HCl NaOH

Temperature

CRC INC 303K 313K 323K 303K 313K 323K

0.00g/L -73.51 -75.78 -78.06 -102.5 -104.7 -106.9

0.6M 0.20g/L -76.51 -78.78 -81.06 -150.8 -152.9 -155.0

0.40g/L -72.76 -75.05 -77.34 -137.8 -140.0 -142.1

0.60g/L -77.15 -79.44 -81.73 -128.1 -130.3 -132.4

0.00g/L -76.11 -78.37 -80.63 -76.82 -79.03 -81.25

0.8M 0.20g/L -77.35 -79.62 -81.89 -136.0 -138.1 -140.3

0.40g/L -80.34 -82.61 -84.89 -69.56 -71.79 -74.02

0.60g/L -80.40 -82.67 -84.95 -75.25 -77.48 -79.71

0.00g/L -71.93 -74.18 -76.43 -71.30 -73.53 -75.76

1.0M 0.20g/L -87.43 -89.68 -91.93 -77.16 -79.39 -81.62

0.40g/L -87.43 -89.68 -91.93 -77.05 -79.27 -81.50

0.60g/L -83.65 -85.91 -88.16 -75.07 -77.30 -79.52

Key: INC (Inhibitor Concentration) and CRC (Corrodent Concentration)

3.6 Adsorption Isotherm Models

Generally, four types of adsorptions may take place, involving organic molecules at

the metal solution interface, namely; the electrolytic attraction between charged molecules

and the charged metal, interaction of unshared electron pairs in the molecules with the metal,

interaction of s electrons with metal and combination of the above [17, 19]. These

mechanisms are usually described by adsorption isotherms. The adsorption behavior of VP

extract was studied by fitting data obtained from degree of surface coverage to different

adsorption isotherms including Langmuir, Temkin, Frumkin, Flory-Huggins and El-Awardy

adsorption isotherms. The tests reveal that the adsorption of VP extract on Al surface is best

described by Langmuir, Flory-huggins and Temkin values in NaOH solution whereas

Frumkin, Langmuir, Temkin, Flory-Hoggins at higher temperature (323K) in HCl solutions

adsorption isotherms in decreasing order based on plot correlation coefficient (R2).

3.6.1 Langmuir Adsorption Isotherm Model

The adsorption behavior of VP extract was studied by fitting the data obtained on the

expression of Langmuir adsorption isotherm model as in equation (12):

𝑪

𝝑=

𝟏

𝑲𝒂𝒅𝒔+ 𝑪 (12)


76

Taking logarithm of both sides yielded equation (13)

𝒍𝒐𝒈 (𝑪

𝝑) = 𝒍𝒐𝒈 𝑪 − 𝒍𝒐𝒈 𝑲𝒂𝒅𝒔 (13)

Where C is the concentration of the inhibitor in the solution, θ is the degree of surface

coverage of the inhibitor, and Kads is the equilibrium constant of adsorption. Using equation

(13), a plot of log(C/θ) versus logC was found to be linear for NaOH solution, indicating the

application of the Langmuir model to the adsorption of VP extracts on aluminium surface.

Values of Langmuir adsorption parameters deduced from the slopes and intercept of the plots

are presented in Table 8. From the results obtained, the slopes and values of R2 are very close

to unity indicating that there is a strong adherence of the inhibitor adsorption to the

assumption of Langmuir isotherm; monolayer adsorption and non-interaction of adsorbed

molecules as earlier reported by Eddy and Odiongenyi [25]. The plots of log(C/θ) versus

logC were not linear and have a very low R2 values in HCl solution, indicating that the

adsorption of the inhibitor was not consistent with the assumptions of Langmuir all

conditions tested except at 323K. Values of the adsorption parameters deduced from

Langmuir plots are recorded in Table 8.

3.6.2 Temkin Adsorption Isotherm Model

According to Temkin adsorption isotherm, the degree of surface coverage (θ) is

related to the concentration of the inhibitor (C) in the bulk electrolyte according to equation

(14):

𝑒𝑥𝑝 (– 2𝑎𝜗) = 𝐾𝐶 (14)

Upon taking the logarithm of both sides and re-arranging, equation (15) and (16) resulted:

𝜗 = −2.303𝑙𝑜𝑔𝐾

2𝑎−

2.303𝑙𝑜𝑔𝐶

2𝑎 (15)

𝜗 =−𝒍𝒏𝑲

𝟐𝒂−

𝒍𝒏𝑪

𝟐𝒂 (16)

Where K and a are the equilibrium constant of adsorption and Temkin interaction

parameter. Plot of θ versus logC was found to be linear for NaOH solution, indicating the

application of the Temkin model to the adsorption of VP extracts on Al surface. Values of

Temkin adsorption parameters deduced from the slopes and intercept of the plots are

presented in Table 8. From the results obtained, the slopes and values of R2 are very close to

unity indicating that there is a strong adherence of the inhibitor adsorption to the assumption

of Langmuir isotherm; mono layer adsorption and non-interaction of adsorbed molecule

[25].

The plots of θ versus logC were not linear and have a very low R2 values for HCl

solutions except at 323K, indicating that the adsorption of the inhibitor was not consistent

with the assumptions of Temkin except at higher temperature. Values of the adsorption

parameters deduced from Temkin plots are recorded in Table 8.

3.6.3 Frumkin Adsorption Isotherm Model


77

The adsorption of VP extract on Al surface was also tested using Frumkin adsorption

isotherm which can be expressed as in equation (17):

𝑙𝑜𝑔 (𝐶 𝑥 𝜗

1−𝜗) = 2.303𝑙𝑜𝑔𝐾 + 2𝛼𝜗 (17)

Where K is the adsorption-desorption equilibrium constant and α is the lateral

interaction term describing the molecular interaction in the adsorbed layer. A plot of log(C

x θ/1-θ) versus θ was made, where K and α values where obtained from the slope and

intercept respectively. Values of Frumkin adsorption isotherm are also presented in Table 8.

From the results, values of α where found to be positive which also indicate the attractive

behavior of the inhibitor on the surface of Al which was found to decrease with increase in

HCl concentration.

A plot of log(C x θ/1-θ) versus θ were not linear and have a very low R2 values for

NaOH solutions at all condition tested, indicating that the adsorption of the inhibitor was not

consistent with the assumptions of Frumkin. Values of the adsorption parameters deduced

from Frumkin plots are recorded in Table 8.

3.6.4 Flory-Huggins Adsorption Isotherm Model

The values of θ/C in the plots (not shown) were evaluated directly from equation (18)

determined from the weight loss data. It was found that the experimental data obtained within

the temperature range (303, 313 and 323K) fits Flory–Huggins adsorption isotherm which

is given by equation (18):

𝑙𝑜𝑔 (𝜗

𝐶) = 𝑙𝑜𝑔𝐾 + 𝑥𝑙𝑜𝑔 (1 − 𝜗) (18)

Where θ is the degree of surface coverage, x is the number of inhibitor molecules

occupying an active site (or the number of water molecules replaced by one molecule of

VP), K the equilibrium constant of adsorption and C is the different concentrations of the

systems studied.

The plot of log (θ/C) versus log(1−θ) for Flory–Huggins’ isotherm at 303K gave

straight lines for Al in all concentrations of NaOH in the presence of VP extract (inhibitor).

Similar trend was observed at 313 and 323K respectively. Results from Table 8 shows that

values of x decreases with increase in NaOH concentration and also temperature. This

clearly indicates that the rate of VP extract adsorption on the surface of the Al metal

decreases with increase in temperature and NaOH concentrations [26].

The plot of log (θ/C) versus log(1−θ) were not linear and have a very low R2 values

for HCl solutions except at higher temperature (323K), indicating that the adsorption of the

inhibitor was not consistent with the assumptions of Flory-huggins except at higher

temperature. Values of the adsorption parameters deduced from Flory-huggins plots are

recorded in Table 8.

Table 8: Adsorption isotherm model parameters for the adsorption of VP extract on Al

surface in HCl and NaOH solutions at varying temperatures

Adsorption Corrodent Corrodent 303K 313K 323K

Isotherm Type Conc R2 Slope K R2 Slope K R2 Slope K

Lu

ng

mu

ir

0.6M 0.61 2.11 1.31 1.00 1.00 1.09 0.92 1.28 0.93


78

HCl 0.8M 0.71 0.41 0.84 0.16 0.26 1.07 0.99 0.47 0.75

1.0M 0.73 -0.75 0.12 0.81 2.92 2.18 0.99 0.37 1.11

0.6M 0.98 -1.5 -0.6 0.81 -2.9 -2.1 0.98 -2.00 -1.7

NaOH 0.8M 1.00 -4.4 -2.7 0.96 -1.9 -1.4 0.94 -2.7 -2.2

1.0M 0.99 -3.07 -2.3 0.98 -2.0 -1.5 0.94 -2.7 -2.2

CONC R2 α K R2 Α K R2 α K

Fru

mk

in

0.6M 1.00 0.00 0.63 0.97 0.03 0.09 0.91 0.00 0.65

HCl 0.8M 0.90 -0.03 0.84 0.98 0.02 0.87 0.99 -0.1 1.3

1.0M 1.00 -0.05 1.12 0.96 0.00 0.62 0.99 0.00 1.07

0.6M 0.04 0.28 -0.1 0.64 0.01 0.28 0.02 0.02 0.02

NaOH 0.8M 1.00 0.00 0.46 0.10 0.03 0.06 0.63 0.01 0.15

1.0M 0.81 0.01 0.18 0.07 0.03 0.04 0.62 0.01 0.15

CONC R2 x K R2 X K R2 x K

F

lory

-Ho

gg

ins 0.6M 1.00 0.00 57.2 0.00 7.16 211 0.97 0.02 8.27

HCl 0.8M 0.82 -0.72 -3.3 0.07 -1.0 -3.5 0.95 0.17 7.39

1.0M 0.24 -0.51 3.81 0.67 -0.3 94.7 0.94 0.69 13.6

0.6M 0.98 -0.99 -4.4 0.99 -1.8 -26 1.00 -1.8 -38

NaOH 0.8M 0.92 -0.14 -5.1 1.00 -1.5 -20 0.99 -1.9 -43

1.0M 0.95 -1.8 -32 1.00 -1.5 -21 0.99 -1.9 -44

CONC R2 y K R2 Y K R2 y K

E

l-A

wa

rdy 0.6M 0.61 -1.54 -2.5 0.00 -1.1 0.00 0.75 -1.3 -1.7

HCl 0.8M 0.94 0.07 1.99 0.61 -1.0 0.78 0.99 -0.7 0.59

1.0M 0.83 -0.78 0.65 0.74 -2.6 -3.4 1.00 -1.1 0.65

0.6M 0.90 -0.53 -0.8 0.65 -2.1 -2.0 0.92 -1.7 -1.0

NaOH 0.8M 0.98 -2.87 -4.2 0.84 -1.4 -1.0 0.86 -2.2 -1.7

1.0M 0.89 -2.34 -2.2 0.92 -1.5 -1.0 0.86 -2.2 -1.7

CONC R2 a K R2 A K R2 a K

T

emk

in

0.6M 0.61 0.05 0.08 0.00 0.08 0.00 0.98 0.24 0.34

HCl 0.8M 0.98 0.38 0.47 0.61 0.08 0.08 0.98 0.16 0.13

1.0M 0.78 0.13 0.12 0.98 0.04 0.06 0.98 0.07 0.06

0.6M 0.91 0.21 -0.4 0.71 0.00 -0.2 0.98 -0.10 0.01

NaOH 0.8M 0.90 -0.24 -1.2 0.92 0.02 -0.2 0.98 -0.10 0.00

1.0M 1.00 -0.04 -0.2 0.98 0.01 -0.2 0.98 -0.10 0.00

3.6.5 El-Awady thermodynamic-kinetic model

The plot of log(θ/1−θ) against logC were not linear and have a very low R2 values

for all corrodent which showed that the results obtained from the study does not fit into the

El-Awady thermodynamic-kinetic model which is given by equation (19):

𝑙𝑜𝑔 (𝜗

1− 𝜗) = 𝑙𝑜𝑔 𝐾 + 𝑦𝑙𝑜𝑔 𝐶 (19)


79

Where θ is the degree of surface coverage of the inhibitor, K is the equilibrium

constant of adsorption, 1/y represents the active sites on Al surface covered by inhibitor

molecules [26]. The values of y are increasing or 1/y decreasing as temperature and HCl

concentration decreases. This also reveals that the number of active sites replaced by the

inhibitor molecules decreases with increase in temperature and HCl concentrations. From

the results obtained it is seen that the adsorption of the inhibitor was not consistent with the

assumptions of El-Awady thermodynamic-kinetic model. Values of the adsorption

parameters deduced from this isotherm model plots are recorded in Table 8.

3.7 Quantum Chemical Calculations

Quantum chemical calculations of the four compounds (betulinic, oleanolic, ursolic

and ester) which are all isolates of VP were performed to further elucidate the inhibition

mechanism at atomistic level. It is evident that higher EHOMO edge improves the ability of an

inhibitor to donate electrons to acceptor atom/molecule/group and vice-versa for ELUMO [27].

The frontier molecular orbital structures of the three compounds (betulinic, ursolic and

oleanolic acids) which have similar molecular weight (457g/mol), total number of electrons

(252), highest occupied molecular orbital (HOMO) number (126) and lowest unoccupied

molecular orbital number (LUMO) number (127) and that of ester with dissimilar properties

were presented in Figure 1.

The isolated compounds are highly electron donating molecules, susceptible to

electrostatic attraction on the aluminium surface due to their energy of HOMO (EHOMO),

energy of LUMO (ELUMO) and energy gap (∆E) values. Compounds with higher negative

value of EHOMO and lowest positive value of ∆E is expected to inhibit corrosion more due to

its high tendency of donating electrons [21]. It would be seen from Table 9 that higher EHOMO

value (-5.499) was assigned to betulinic acid while lowest ∆E value (0.172) was assigned to

an ester. Excellent corrosion inhibitors are usually those organic compounds who not only

offer electrons to unoccupied orbital of the metal but also accept free electrons from the

metal [28].Thus, ester is expecting to supersede betulinic due to its (ELUMO) value (-3.322).

Ionization potential (I), electron affinity (A), absolute global hardness and global

softness are fundamental descriptors of the chemical reactivity of the compounds at atomic

and molecular level. Hard molecules with high ionization energy are more stable (with

chemical inertness) due to attributed large energy gap which signifies the resistance towards

the deformation or polarization of the electron cloud of the atoms, ions or molecules under

small perturbation of chemical reaction while soft molecules with low ionization energy are

highly reactive [29].


80

Optimized geometry of

betulinic acid


ursolic acid


oleanolic acid


ester

Electron density of

betulinic acid

Electron density of

ursolic acid

Electron density of

oleanolic acid

Electron density of

ester

HOMO of betulinic acid

HOMO of ursolic acid

HOMO of oleanolic acid

HOMO of ester

LUMO of betulinic acid

LUMO of ursolic acid

LUMO of oleanolic acid LUMO of ester

Figure 1: Optimized geometrical structures and frontier molecular orbitals of the VP

compounds (Colour scheme: White = hydrogen, grey = carbon, red = oxygen).

Least ionization potential (3.494) and hardness (0.172) as well as high electron

affinity (3.322) and softness (5.814) values were observed in the ester molecule with highest

inhibition efficiency because adsorption of inhibitor onto Al surface occurs at the part of the

molecule which has the greatest softness and lowest hardness [30]. The reverse is the case

for betulinic acid. Ionization potential and electron affinities are related to the energy of

highest occupied molecular orbital and lowest unoccupied molecular orbital by Koopman’s

theorem [31, 32] as in equations (20 and 21):

𝐼 = −𝐸𝐻 (20)

𝐴 = −𝐸𝐿 (21)

Absolute global hardness, global softness (inverse of chemical hardness) and

absolute electronegativity were derived from ionization potential and electron affinity as

presented in equations (22-24) [27, 33, 34]:


81

η = (𝐼−𝐴

2) = −

𝐸𝐿𝑈𝑀𝑂−𝐸𝐻𝑂𝑀𝑂

2 (22)

σ=1

η=

2

𝐼−𝐴= −

2

𝐸𝐿𝑈𝑀𝑂−𝐸𝐻𝑂𝑀𝑂 (23)

χ=(𝐼+𝐴)

2 = −

𝐸𝐿𝑈𝑀𝑂+𝐸𝐻𝑂𝑀𝑂

2 (24)

Electrophilicity index (ω) indicates the ability of the compounds to accept electrons

whereas nucleophilicity (ε) which is the inverse of electrophilicity (1/ ω) represents the

propensity of the compounds to donate or share electrons. It is known that molecules with

large values of electrophilicity index are poor corrosion inhibitors while molecules with high

values of nucleophilicity are good corrosion inhibitors. Table 9 reported large values of

electrophilicity index of an ester (33.762) which is due to the quadratic dependence of LDA

functional on electronegativity, consequently these two parameters cannot be used to judge

the inhibition efficiency of the studied compounds as suggested by Guo et al. [35].

Table 9: Values of electronic/structural parameters of VP extract phytochemicals Electronic/structural property Betulinic acid Ursolic acid Oleanolic acid Ester

HOMO (at orbital number) 126 126 126 150

LUMO (at orbital number) 127 127 127 151

EHOMO (eV) -5.499 -5.412 -5.274 -3.494

ELUMO (eV) -0.475 -0.683 -0.767 -3.322

∆E (eV) 5.024 4.729 4.507 0.172

Molecular mass (g/mol) 457 457 457 529

Ionization potential (I) (eV) 5.499 5.412 5.274 3.494

Electron affinity (A) (eV) 0.475 0.683 0.767 3.322

Absolute electronegativity (χ) 2.987 3.048 3.021 3.408

Chemical Potential (μ) -2.987 -3.048 -3.021 -3.408

Absolute global hardness (η) 2.512 2.365 2.254 0.172

Global softness (σ) 0.398 0.423 0.444 5.814

Global electrophilicity index (ω) 1.776 1.964 2.024 33.762

Nucleophilicity (ε) 0.563 0.509 0.494 0.030

Total number of electrons 252 252 252 300

Fraction of electrons transferred

(∆N)

0.5201 0.5395 0.5721 6.3866

Energy of back donation (∆b-d) -0.628 -0.591 -0.564 -0.043

The supercilious nature of inhibition can also be related to the value of the fraction

of electrons transferred (ΔN) shown in equation (25). This important parameter is based on

absolute electronegativity of both the metal and the inhibitor and their absolute hardness. A

higher value of ΔN is an evidence for relatively stronger electron donation and a higher

tendency to interact with the aluminium [21]. 𝜒𝐴𝑙 and 𝜒𝑖𝑛ℎ represent absolute

electronegativity of aluminium and inhibitor (compounds) respectively, η𝐴𝑙 and η𝑖𝑛ℎ are the

hardness of Al and inhibitor compounds respectively. The theoretical value of

electronegativity and hardness of bulk Al are 5.6eV and 0 respectively by assuming that for

a metallic bulk I = A [36, 37] because they are softer than neutral metallic atoms.

Impressively from Table 9, ester exhibits the highest ΔN value (6.3866) due to its stronger

electron donation.

∆N = 𝜒𝐴𝑙− 𝜒𝑖𝑛ℎ

2(η𝐴𝑙+η𝑖𝑛ℎ) (25)

Energy of back donation (ΔE𝑏−𝑑) calculated using equation (26) is another important

parameter that describes the interaction of inhibitor molecules with metal surface as

proposed by Gomez et al. [38]. Bedair [39] reported that if the value of global hardness is

positive and ΔE𝑏−𝑑 is negative then the process of back donation is favoured. According to

the results obtained in Table 9 the values of global hardness are positive and that of back


82

donation are negative, to that end we can say that the interaction of the inhibitor molecules

with Al surface involves the transfer of charge from compounds to Al metal and vice versa.

According to energy of back donation, the inhibition efficiency of the compounds obeys the

trend Ester>Oleanolic>Ursolic>Betulinic which is in agreement with parameters earlier

discussed.

ΔE𝑏−𝑑 = −η

4 =

1

8(𝐸𝐻𝑂𝑀𝑂 − 𝐸𝐿𝑈𝑀𝑂) (26)

Presence of different functional groups on the studied compounds allowed the

investigation of the local reactivity of the molecules. The condensed Fukui indices calculated

using DMol3 Mulliken and Hirshfeld analysis of Material Studio 8 distinguished each part

of the compounds on the basis of its distinct chemical behavior due to different substituents

[21, 40]. Thus, the site for nucleophilic and electrophilic attack will be the position where

the value of F+ and F- are a maximum respectively as presented in Table 10. It can be

observed from Table 10 that the electrophilic and nucleophilic indices of betulinic acid are

oriented on oxygen O(17) and O(36) respectively for Mulliken and Hirshfeld while that of

ursolic and oleanolic acids are oriented around oxygen O(33 and 34) for Mulliken and

Hirshfeld electrophilic and carbon C(28 and 29) for Mulliken and Hirshfeld nucleophilic

attacks. Carbon C(17) remain the most reactive site on ester for both Mulliken and Hirshfeld

electrophilic and nucleophilic attacks with the exception of Mulliken nucleophilic where

oxygen O(4) shows more reactivity. This indicates that carbon and oxygen atoms at the

carboxylic and/or hydroxyl positions are the preferred adsorbed sites on the molecules.

Table 10: Calculated Fukui Indices for Inhibitor Molecules with the Al (110) Surface using

Forcite Quench Dynamics Electrophilic (F-) Nucleophilic (F+)

Mulliken Hirshfeld Mulliken Hirshfeld

Molecule Atom Value Atom Value Atom Value Atom Value

Betulinic acid

O(17) 0.151 O(17) 0.137 O(36) 0.128 O(36) 0.123 Ursolic acid

O(33) 0.138 O(33) 0.131 C(28) 0.134 C(28) 0.120 Oleanolic acid

O(34) 0.121 O(34) 0.115 C(29) 0.153 C(29) 0.138

Ester C(17) 0.065 C(17) 0.062 O(4) 0.063 C(17) 0.066

3.8 Molecular Dynamic Simulations

The equilibrium configuration of lowest energy of the four inhibitors molecules

adsorbed on Al (110) surface can be further investigated using Forcite quench molecular

dynamics in Materials Studio 8. Figure 2 shows the cross-sectional side views of the lowest

energy adsorption configurations for monomolecular compounds on Al (110) surface.

Theoretical adsorption energy, Eads and binding energy were calculated to quantitatively

estimate the interaction between the molecules and the Al (110) surface using equation (27)

[21]:

Eads = -Binding energy = Etotal – (Einhibitor + EAl surface) (27)

Where Einhibitor, EAlsurface and Etotal correspond to the total energies of the inhibitor molecule,

Al (110) plane and the adsorbed molecule coupled with gas phase Al (110), respectively. In

this study, the Al (110) surface energy was zeroed. Parameters calculated such as kinetic


83

energy, potential energy, the energy of the inhibitor molecule, energy of Al (110) surface

and binding energy, are presented in Table 11. Basically, the negative value of adsorption

energy and positive value for binding energies corresponds to a stable adsorption structure

indicating good performance of the compounds as inhibitors with ursolic acid having the

highest value (-42.633) slightly higher than oleanolic acid. It has been reported by Akalezi

et al. [41] that Eads value below 100 kcalmol-1(418.4kJmol-1) corresponds to physical

adsorption and vice-versa for chemical adsorption. This strong affinity of the compounds for

the Al (110) surface accounts for the remarkable corrosion inhibition efficacy of the extract

as observed experimentally [42, 43].

Table 11: Calculated Adsorption Parameters for the Interaction of the Compounds with the

Al (110) Surface using Forcite Quench Dynamics

Properties (kcalmol-1)

Inhibitor Molecules

Betulinic acid Ursolic acid Oleanolic acid Ester

Total Kinetic Energy 53.699 115.324 57.121 83.248

Total Potential Energy -83.826 -93.141 -102.927 -143.938

Energy of Molecule -52.202 -52.585 -60.294 -114.366

Energy of Al(110) Surface 0.000 0.000 0.000 0.000

Adsorption Energy -31.624+4.187×10-6 -42.633+2.199×10-5 -41.584+0.047 -29.572+16.286

Binding Energy 31.624 42.633 41.584 29.572

Overlaid betulinic acid

Overlaid ursolic acid

Overlaid oleanolic acid

Overlaid ester

Figure 2: Side view snap shot of monomolecular adsorbed molecules on Al (110) surface.

4. CONCLUSION

The adsorptive and inhibitive properties of VP extract was studied on Al surface

corrosion in HCl and NaOH solutions at varying temperatures, inhibitor and corrodent

concentrations using weight loss and computational methods. From the findings of this work,

the following conclusions can be drawn;

1. VP extract and its isolated compounds were found to inhibit Al corrosion in HCl and

NaOH solutions.

2. Increase in the concentration of VP extract decreases the rate of Al corrosion in HCl

thereby increases the inhibition efficiency.

3. Higher corrodent concentration, temperature reduced the inhibitive effect of VP extract

on the corrosion of Al in both corrodent solutions.

4. The corrosion reaction in the presence and absence of VP extract was found to follow first

order kinetics and its half-life increases with VP extract concentration, but decreases


84

with increase in temperature and corrodents concentration.

5. The theoretical adsorption energy (Eads) and thermodynamic parameters (Ea, Qads, ΔSads,

ΔHads, ΔGads) data for the adsorption of VP extract and its isolated compounds to the

surface of Al in test solutions was found to obey the mechanism of physical adsorption.

6. The adsorption of VP extract onto the surface of Al in NaOH solution was found to follow

the models of Langmuir, Flory-Huggins and Temkin adsorption isotherms, whereas in

HCl was found to follow the models of Langmuir, Flory-Hoggins and Frumkin

adsorption isotherms.

7. Comparison of the inhibition efficiency of the bulk extracts (used in weight loss) and

isolated compounds (used in computational methods) are closely related.

8. Ethanol extract of VP extract and its isolated compounds are good inhibitors for the

corrosion of Al in HCl and NaOH solutions. The inhibition efficiency of the extract may

be due to the presence of saponin, carbohydrate, steroid, free reducing sugar, ketoses and

alkaloids present there in.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the contribution of Dr. David Arthur of Baze University,

Abuja, Nigeria for the installation of Acceryls Materials Studio 8 software package.

REFERENCES [1] J. A. Bardwell, and B. MacDoughall, B. Electrochim. Acta, 34, 229. 1989.

[2] K. F. Khaled and M. M. Al-Qahtani. The inhibitive Effect of Some Tetrazole Derivatives Towards Al

Corrosion in Acid Solution: Chemical, Electrochemical and Theoretical Studies, Materials Chemistry

and Physics, 113, 150-158. 2009.

[3] J. Buchweishaija and G. S. Mhinzi. Natural products as a source of environmentally friendly corrosion

inhibitors: The case of natural tree gum exudates from Acacia seyalvarseyal,

PortugaliaeElectrochimicaActa, 26: 257-265. 2008.

[4] L. Kobotiatis; N. Pebere; P. G. Koutsoukos. Study of the Electrochemical Behaviour of the 7075

Aluminium Alloy in the Presence of Sodium Oxalate, Corrosion Science, 41, 941-957. 1999.

[5] P. Byakagaba. Population structure and regeneration status of Vitellaria paradoxa under different land

management regimes in Uganda. Agricultural Journal, 6(1) 14-22. 2011.

[6] J. A. Yidana, E. T. Masters and P. N. Lovett. Trade and sustainable forest management. FAO.org. Retrieved

14-03-2015. 2010.

[7] K. Neumann. Remains of woody plants from Saouga, a medieval west African village. Vegetation History

and Archaeobotany, 7:57-77. 1998.

[8] E. W. Nester, C. E. Robert, N. N. Pearsal, D. G. Anderson, M. T. Nester. Microbiology: A Human

Perspective 2nd Edition. McGraw Hill Inc. pp. 401-700. 1998.

[9] P.O. Okafor, M. E. Ikpi, I. E., Uwah, E. E., Ebenso, U. J. Ekpe, S. A. Umoren. Inhibitory action of

Phyllanthusams against amarus extracts on the corrosion of mild steel in acidic media, Corrosion

Science; 50(8):2310-2317. 2008.

[10] A. Y. El-Etre. Inhibition of aluminium corrosion using Opuntia extract. Corros. Sci., 45: 2485-2495. 2003.

[11] A. Bouyanzer; B. Hammouti; L. Majidi. Pennyroayl oil from Menthapulegium as corrosion inhibitor for

steel in 1 M HCl, Mater Lett, 60:2840-284325. 2006.

[12] R. P. Bothi, M. G. Sethuraman. Strychnosnux-vomica an eco-friendly corrosion inhibitor for mild steel in

1 M sulfuric acid medium. Mater Corros, 60:22-28. 2009.

[13] A. Y. El-Etre, and Z. El-Tantawy. Inhibition of metallic corrosion using Ficus extract. Port. Electrochim.

Acta24: 347-356. 2006.

[14] E. Talla, J. N. Nyemb, A. T. Tiabou, S. G. Z. Djou, P. Biyanzi, L. Sophie, L. V. Elst, and J. M. Tanyi.

Antioxidant Activity and a New Ursane-type Triterpene from Vitellariaparadoxa (Sapotaceae) Stem

Barks, European Journal of Medicinal Plants, 16(3), 1-20. 2016.

[15] L. Catteau, N. T. Reichmann, J. Olson, M. G. Pinho, V. Nizet, F. V. Bambeke. and J. Quetin-Leclercq.

Synergy between Ursolic abd Oleanolic Acids from Vitellariaparadoxa Leaf Extract and β-Lactams


85

against Methicilin-Resistant Staphylococcus aureus: In Vitro and In Vivo Activity and Underlying

Mechanisms, Molecules, 22, 2245. 2017.

[16] T. Buxton, S. Takahashi, A. D. Eddy, J. Baffoe-Ansah, E. O. Owusu and C. Kim. Insecticidal Activities

of Cinnamic Acid Esters Isolated from Ocimum gratissimum L. and Vitellaria paradoxa Gaertn

Leaves against Tribolium castaneum Hebst (Coleoptera:Tenebrionidae). Pest Management Science,

76(1). 2019.

[17] N. O. Eddy, A.O. Odiongenyi. Inhibitive, Adsorption and Synergistic Studies on Ethanol Extract of

Gnetum Africana as Green Corrosion Inhibitor for Mild Steel in H2SO4. Green Chem. Lett. Rev.(2),

111-119. 2009.

[18] I. G. Ndukwe, J. O. Amupitan, Y. Isah and K. S. Adegoke. Phytochemical and antimicrobial screening of

the crude extracts from the root, stem bark and leaves of Vitellaria paradoxa (GAERTN. F), African

Journal of Biotechnology, 6(16), 1905-1909. 2007.

[19] N. O. Eddy, S. A. Odoemelam, A. O. Odiongenyi. Joint Effect of Halides and Ethanol Extract of

Lasianthera Africana on the Inhibition of the Corrosion of Mild Steel in H2SO4. J. Appl. Electrochem.,

39 (6), 849-857. 2009.

[20] A. M. Ayuba, A. Uzairu, J. F. Iyun, H. Abba. The Use of Glutamic Acid as Corrosion Inhibitor for

Aluminium in HCl Solution. IOSR Journal of Applied Chemistry (IOSR-JAC): 7(1), 45-49. 2014.

[21] N. U. Shehu, U. I. Gaya and A. A. Muhammad. Influence of Side Chain on the Inhibition of Aluminium

Corrosion in HCl by α-Amino Acids, Applied Science and Engineering Progress, 12(3), 186-197.

2019.

[22] P. C. Okafor, E. E. Ebenso. Inhibitive Action of Carica papaya Extracts on the Corrosion of Mild Steel

in Acidic Media and their Adsorption Characteristics. Pigm. ResinTechnol., 36 (3), 134-140. 2007.

[23] N. O. Eddy, S. A. Odoemelam, A. J. Mbaba. Inhibition of the Corrosion of Mild Steel in HCl by

Sparfloxacin, African Journal of Pure and Applied Chemistry, 2(12), 132-138. 2008.

[24] N. O. Eddy, B. I. Ita. QSAR, DFT and Quantum Chemical Studies on the Inhibition Potentials of Some

Carbazones for the Corrosion of Mild Steel in HCl, J. Mol. Model, DOI 10.1007/s00894-010-0731-7.

2007.

[25] N. O. Eddy. Inhibitive and Adsorption Properties of Ethanol Extract of Colocasia esculenta Leaves for

the Corrosion of Mild Steel in H2SO4. Int. J. Phys. Sci., 4 (3), 1-7. 2009.

[26] E. E. Ebenso. Synergistic Effect of Halide Ion on the Corrosion Inhibition of Aluminium in H2SO4

using 2-acetylphenothiazine. Material Chemistry and Physics, 79: 58-70. 2003.

[27] A. M. Ayuba, A. Uzairu, H. Abba, G. A. Shallangwa. Theoretical study of aspartic and glutamic acids as

corrosion inhibitors on aluminium metal surface. Morrocan Journal of Chemistry, 6(1): 160-172.

2018.

[28] T. Arslan, F. Kandemirli, E. E. Ebenso, I. Love and H. Alemu. Quantum chemical studies on the corrosion

inhibition of some sulphonamides on mild steel in acidic medium. Corrosion Science, 51: 35-47.

2009.

[29] P. Udhayakala, T. V. Rajendiran and S. Gunasekaran. Theoretical approach to the corrosion inhibition

efficiency of some pyrimidine derivatives using DFT method. Journal of Computational Methods in

Molecular Design, 2(1): 1-15. 2012.

[30] D. Maryam, S. Mehdi, M. Sayyed and K. Habibi. Theoretical study of inhibition efficiencies of some

amino acids on corrosion of carbon steel in acidic media: green corrosion inhibitors. Journal of Amino

Acids, 48(1): 291-306. 2016.

[31] N. P. Bellafont, F. Illas and P. S. Bagus. Validation of Koopmans' theorem for density functional theory

binding energies. Phys. Chem. Chem. Phys. 17, 4015–4019. 2015.

[32] A. A. Khadom. Quantum chemical calculations of some amines corrosion inhibitors/copper Alloy

interaction in hydrochloric acid. Journal of Materials and Environmental Sciences, 8: 1153-1160,

2017.

[33] F. Jia-jun, L. Su-ning, W. Ying, C. Lin-hua and L. Lu-de. Computational and electrochemical studies of

some amino acid compounds as corrosion inhibitors for mild steel in hydrochloric acid solution.

Journal of Material Science, (45): 6255–6265. 2010.

[34] H. R. Lgaz, R. Salghi, A. Chaouiki, Shubhalaxmi, S. Jodeh, K. S. Bhat. Pyrazoline derivatives as possible

corrosion inhibitors for mild steel in acidic media: A combined experimental and theoretical approach.

Cogent Engineering, 5: 1441585. 2018.

[35] L. Guo, Z. S. Safi, S. Kaya, W. Shi, B. Tüzün, N. Altunay, C. Kaya. Anticorrosive effects of some

thiophene derivatives against the corrosion of iron: a computational study. Front. Chem. 6, 155. 2018.

[36] P. Zhao, Q. Liang and Y. Li. Electrochemical, SEM/EDS and quantum chemical study of phthalocyanines

as corrosion inhibitors for mild steel in 1 mol/L HCl. Applied Surface Science, 252: 1596-1607. 2005.

[37] M. K. Awad. Quantum chemical studies and molecular modeling of the effect of polyethylene glycol as

corrosion inhibitors of an aluminium surface. Canadian Journal Chemistry, 91: 283-291. 2013.


86

[38] B. Gómez, N. V. Likhanova, M. A. Domínguez-Aguilar, R. Martínez-Palou, A. Vela and J. L. Gazquez.

Quantum chemical study of the inhibitive properties of 2-pyridyl-azoles. J. Phys. Chem. B 110, 8928–

8934. 2006.

[39] M. A. Bedair. The effect of structure parameters on the corrosion inhibition effect of some heterocyclic

nitrogen organic compounds. J. Mol. Liq. 219, 128–141. 2016.

[40] S. John and A. Joseph. Quantum chemical and electrochemical studies on the corrosion inhibition of

aluminium in 1N HNO3 using 1, 2, 4-triazine. Materials and Corrosion, 64(7): 625-632. 2013.

[41] C. O. Akalezi, C. E. Ogukwe, C. K. Ennenebaku and E. E. Oguzie. Corrosion inhibition of aluminium

pigments in aqueous alkaline medium using plant extracts. Environment and Pollution, 1(2): 45-60.

2012.

[42] F. E. Awe, S. O. Idris, M. Abdulwahab and E. E. Oguzie. Theoretical and experimental inhibitive

properties of mild steel in HCl by ethanolic extract of Bosciasenegalensis. Cogent Chemistry, 1(1):

1112676. 2015.

[43] A. A. Muhammad. Theoretical and Experimental Studies on the Corrosion Inhibition of Aluminium in

Hydrochloric acid Using Amino and Hydroxycarboxylic Acids as inhibitors. Ph.D (Unpublished)

Thesis, Ahmadu Bello University, Zaria. 2016.


Documents

Experimental and Computational Studies of Vitellaria