Inhibition Effect of Opuntia Stem Extract on Corrosion ... - Springer Link

ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2017, Vol. 53, No. 3, pp. 560–572. © Pleiades Publishing, Ltd., 2017.

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

Inhibition Effect of Opuntia Stem Extract on Corrosion of Mild Steel: a Quantum Computational Assisted Electrochemical Study to Determine the Most Effective Components in Inhibition1 Ebrahim Honarmanda, *, Hossein Mostaanzadeha, Mohammad Hassan Motaghedifardb, *, Mojtaba Hadia, and Maryam Khayadkashanic aDepartment

of Chemistry, Faculty of Science, University of Qom, Qom, I.R. Iran Young Researchers and Elites Club, Qom Branch, Islamic Azad University, Qom, Iran c Department of Analytical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, I.R. Iran *e-mail: [email protected]; [email protected] b

Received May 30, 2016

Abstract⎯The use of opuntia ficus indica stem extract as a mild steel corrosion inhibitor in 2.0 M HCl solution was investigated by weight loss measurements, potentiodynamic polarization and Electrochmical impedance spectroscopy (EIS) methods. The weight loss results showed that opuntia stem is an excellent corrosion inhibitor for mild steel immersed in 2.0 M HCl. EIS measurements showed an increase in the transfer resistance with the increase of inhibitor concentration. In addition, the inhibition action of the extract was discussed in view of Langmuir adsorption isotherm. It was found that the presence of extract increases the activation energy of the corrosion reaction. In order to explain which components of opuntia stem extract is the most efficient inhibitor, quantum chemical calculations of 8 constituent molecules of extract were applied. The correlation between inhibitive effect and molecular structures was investigated. Some parameters, such as EHOMO, ELUMO, gap energy (∆E), electronegativity (χ), global hardness (η) and the fraction of electrons transferred from the inhibitor molecule to the metallic atom (∆N) were calculated. The direction of dipole moment μ→, location of HOMO orbitals and NBO charges of atoms can determine how to connect the molecules on the surface of the metal. Keywords: green corrosion inhibitor, opuntia stem extract, density functional theory, langmuir adsorption isotherm, HOMO and LUMO orbitals DOI: 10.1134/S207020511703008X

1. INTRODUCTION Acid solutions are commonly used for the removal of undesirable scale and rust in metal finishing industries, cleaning of boilers and heat exchangers. To prevent unexpected metal dissolution and excess acid consumption in the process of cleaning, therefore, inhibitors will be inevitable to be put into use [1]. Most of the well-known acid inhibitors are organic compounds that contain nitrogen, sulfur, oxygen and multiple bonds in the molecules and that are adsorbed on the metal surface and these compounds have continued to provoke research interests [2–12]. The inorganic compounds such as chromate, dichromate and nitrate also are used as inhibitors [13]. At the same time, the biology toxicity of these products, especially chromate and organic phosphate, are documented about their environmental harmful characteristics [14, 15]. Recently, good results were obtained when lanthanide salts, which have a low toxicity, were 1 The article is published in the original.

employed as corrosion inhibitors [16, 17]. Unfortunately, these compounds are very expensive. So the development of novel corrosion inhibitors of natural source and non-toxic type, which do not contain heavy metals, has been considered more important and desirable [18–24]. The present study is an attempt to find a cheap and environmentally safe inhibitor for mild steel corrosion in the acidic solution i.e. HCl, where the extract of opuntia plant stems is tested. This plant is cultivated in Chile for fruit production and also in South Korea for use in the manufacture of health foods such as jam and juice [25, 26]. Opuntia belongs to the family cactaceae and has the scientific name (opuntia ficus indica). The opuntia stem extract contains mainly polysaccharide which is a mixture of mucilage and pectin [27, 28]. The mucilage isolated from the modified stems of opuntia ficus indica contains D-galactose, L-arabinose, D-xylose, L-rhamnose and D-galacturonic acid [29]. The high percentage of pectins and fibers can increase fecal mass and intestinal motility, which in turn affect cholesterol and

560

INHIBITION EFFECT OF OPUNTIA STEM EXTRACT H

561

OH OH

H

HO H H

OH H

O H

HO

O

H

O H

OH

OH

H

H

OH

H OH

OH

(I)

CH3 H O OH H OH OH H OH H

(II)

(III) HO

HO

H OH

OH

H

O

OH H

O H

O

H3 C O OH

OH HO

H

O

(IV)

OH (V)

O HO

H

HO

H

O

H

HO

HO H

H

HO

H

O H

OH

(VI)

HO

O

O

H OH

HO H

OH OH

H

OH (VII)

O

(VIII)

Fig. 1. Chemical structures of opuntia stem extract components.

glucose plasma levels [25]. It also contains non-volatile acids including malic and citric acids [30]. Opuntia extracts were reported to contain several compounds among which the main compounds are shown in Fig. 1. In the literature, few studies have been carried out recently in various conditions about corrosion inhibiting properties of Opuntia extract. Hammouch et al. [31] showed an important inhibition efficiency about 86% of the Opuntia extract, toward corrosion of iron in 3% sodium chloride. In an interesting study investigated by El-Etre, [32] the inhibition action of the mucilage extracted from the modified stems of prickly pears (Opuntia) toward acid corrosion of aluminium, acts a good corrosion inhibitor for aluminium corrosion in 2.0 M HCl solution. The inhibition efficiency attained 96%. More recently, Salghi et al. [33] showed that a high efficiency 90% was obtained at 5 g/L of a Prickly pear seed oil extract tested as corrosion inhibitor for mild steel in 1 M HCl. The study carried by Chtaini et al. [34] showed that the pigment extracted from reddish prickly pear juices act as a good inhibitor for 316L stainless steel in 30% H3PO4 solution. The inhibition efficiency of pigment extracts increases with the concentration to attain a maximum value 97% at 1 mL of pigment extract. Recently, Hammouti et al. has studied the inhibitive action of the fruit juice of opuntia ficus indica (OFI) on corrosion behavior of mild steel. The maximum inhibiting efficiency attained 93.37% [35].

According to the best of our knowledge, no research has been conducted on the inhibition effect of opuntia ficus stem extract on the mild steel as well as details of which constituent is most effective. So, In this work we study the inhibitive action of the modified stems of opuntia ficus indica on corrosion behavior of mild steel in 2 M HCl. Weight loss measurements, potentiodynamic polarization and electrochemical impedance spectroscopy was used in the present work to calculate the inhibition efficiency of the opuntia stem extract. Also, the effect of temperature on the corrosion reaction rate in free and inhibited acid solutions was interpreted by means of the langmuir isotherm adsorption. Then, the activation energy of corrosion reaction in absence and presence of inhibitors was calculated. In addition, for the first time, quantum chemical calculations were under taken to explain the adsorption and inhibition behavior of these molecules on the mild steel surface. To explain which molecules of extract has higher inhibition efficiency on mild steel we calculated the electronegativity (χ), global hardness (η) and proportion of electrons transferred (∆N) of extract constituents compounds. 2. EXPERIMENTAL 2.1. Electrochemical Experiments Polarizations and Impedance measurements were carried out using an Autolab Potentiostat / Galvanos-

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tat PGSTAT30 (Eco Chemie, Utrecht, The Netherlands) controlled by General Purpose Electrochemical Systems (GPES) and Frequency Response Analyser (FRA) 4.9 software and adopted by a conventional three-electrode system. In electrochemical cell, the mild steel as working electrode, a platinium counter electrode and silver-silver chloride (Ag/AgCl/KClsat) electrode as reference electrode were used. Before polarization and EIS measurements, the working electrodes were immersed into the test solution and left for 30 min at the open circuit potential (OCP). Scan rate of potential was 5.0 mV/s and potential was scanned in the range of –0.5 to +0.5 V relative to the corrosion potential. Polarization data was analyzed using GPES electrochemical software and corrosion current density values were obtained by Tafel extrapolation method. The temperature was adjusted to 35– 65°C using TAMSON model T1000 thermostat. The frequency range for EIS measurements was 100 kHz to 10 mHz with an applied potential signal amplitude of 5.0 mV around the rest potential. Using this method, the polarization resistance Rp, solution resistance Rs and double-layer capacitor Cdl were calculated for mild steel in free and inhibited acid solutions. 2.2. Quantum Chemistry Analysis The use of quantum chemical calculations has become popular for screening new potential corrosion inhibitors [36]. In most cases, such screening consists of calculating several electronic structural parameters of isolated molecules either in gas (G) or in aqueous (A) phase (the solvent is usually treated by some variant of conductor polarized continuum model (CPCM) [37]. Theoretical calculations were carried out at density functional theory (DFT) level using the 6-311++G(d,p) basis set for all atoms with Gaussian 03 program package [38]. The electronic properties such as highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy and frontier molecular orbital coefficients have been calculated. The molecular sketches of all compounds were drawn using Gauss View 03 [39]. The natural bond orbital (NBO) analysis which suggested by Reedet al. [40, 41] was applied to determine the atomic charges. 2.3. Plant Material and Extraction The stems of opuntia ficus indica were purchased from Barijessence Pharmaceutical, Iranian herbal drug and opuntia stem extract were abstracted from stems according to the literature [25].

Table 1. Chemical composition of mild steel samples (wt %) Element wt %

C

Si

Mn

P

S

Al

Cu

Fe

0.027 0.0027 0.34 0.009 0.007 0.068 0.007 Balk

weight loss experiment were coupons with a surface area of 1.0 cm2 and specimens used in the electrochemical experiments were coupons sealed by epoxy resin with exposure surface of 1.0 cm2. Prior to all measurements, mild steel specimens were polished with emery papers 240, 800, 1200 and 2200 grit, respectively, degreased in ethanol, washed in distilled water, dried in room temperature. 2.5. Test Solutions Preparation The aggressive solution of 2.0 M HCl was prepared by dilution of analytical grade HCl (35%) purchased from Merck and doubly distilled water was used for preparing test solutions for all experiments. The concentration range of opuntia employed was 200 to 1000 ppm, and the solution in the absence of opuntia extract was taken as blank for comparison. 2.6. Weight Loss Experiments For weight loss measurements, the coupons initial weight was recorded using an analytic balance (precision: ±0.1 mg) before immersion in 150 mL open beakers containing 100 mL of corroding solution (2.0 M HCl) without and with different concentrations of plant extract. The specimens were taken out, washed, dried and reweighed accurately. The average weight loss of the three parallel mild steel sheets could be obtained. In this way, the corrosion rates of mild steel were determined using Eq. (1) for 24 h immersion period at 25 ± 0.2°C from mass loss: (1) w = Δm . St Where Δm is the mass loss, S is the area, and t is the immersion period. The percentage protection efficiency IE, % was calculated according the relationship Eq. (2):

IE% =

w0 − w × 100. w0

(2)

Where W0 and W are the corrosion rates of mild steel in absence and presence of inhibitor, respectively. 3. RESULTS AND DISCUSSION 3.1. Weight Loss Measurements

2.4. Specimen Preparation The chemical composition of mild steel samples is shown in Table 1. Specimens which were used in the

The weight loss data are represented in Table 2. It can be found that the inhibition efficiency increases with increasing in concentration of opuntia extract.


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INHIBITION EFFECT OF OPUNTIA STEM EXTRACT Table 2. Weight loss results of mild steel corrosion in 2 M HCl with addition of various concentrations of opuntia stem extract W, Concentration, ppm mg cm–2 h–1 2 M HCl 200 400 600 800 1000

1.580 0.205 0.114 0.095 0.082 0.076

θ

IE, %

– 0.87 0.928 0.94 0.948 0.952

– 87 92.8 94 94.8 95.2

The result indicated that opuntia extract could act as a good hydrochloric acid inhibitor on mild steel. 3.2. Electrochemical Experiments 3.2.1. Polarization measurements. The currentpotential relationships (cathodic and anodic) for mild steel in 2.0 M HCl with different additions of opuntia extract are shown in Fig. 2. Also, values of associated electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), cathodic Tafel slope (bc), anodic Tafel slope (ba) and IE% are presented in Table 3. In this case, the inhibition efficiency is defined as follows: b inh ⎛ I corr0 ⎞ − I corr IE P (%) = ⎜ ⎟⎟ × 100 . b ⎜ I corr0 ⎝ ⎠

(3)

b inh Where I corr and I corr are the corrosion current density values in absence and presence of inhibitor, respectively. Inspection of Table 3 reveals that the addition of opuntia extract decreases markedly the corrosion current. This behavior reflects its ability to

0

log I [A cm–2]

–1 –2 –3 blank 200 ppm 400 ppm 600 ppm 800 ppm 1000 ppm

–4 –5 –6 –7 –0.55 –0.60

–0.45 –0.35 –0.25 –0.50 –0.40 –0.30 –0.20 E/V vs. Ag/AgCl

Fig. 2. Polorization curves of mild steel in 2.0 M HCl containing various concentration of opuntia stem extracts.

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inhibit the corrosion of mild steel in HCl solution. The IE increases as the extract concentration is increased. The fact that addition of opuntia extract cause no change in the anodic and cathodic Tafel slopes, indicating that the inhibitors are first adsorbed onto steel surface and therefore impedes by merely blocking the reaction sites of iron surface without affecting the anodic and cathodic reaction mechanism [42]. 3.2.2. Electrochemical impedance spectroscopy measurements. Figure 3 shows Nyquist plots obtained from ac impedance measurements for mild steel in 2.0 M HCl in the presence of different concentrations of opuntia extract. The diameter of Nyquist plots increased accordance with increasing the concentration of opuntia extract indicating strengthening of inhibitive film. The above impedance diagrams (Nyquist) contains a depressed semicircle with the center under the real axis, such behaviour is characteristic for solid electrode which is attributed to surface roughness and inhomogeneities of metal electrodes. In these cases, the parallel network polarization resistance and double layer capacitance (Rct – Cdl) is usually a poor approximation especially for system where an efficient inhibitor is used. For the description of a frequency independent phase shift between an applied alternating potential and its current response, a constant phase element (CPE) is used instead of capacitance (C). The CPE is defined by the mathematical expression [43, 44]:

Z CPE = 1 Y 0( j ω) n .

(4)

Where ZCPE, impedance of CPE; Y0, a proportional factor; ω, Angular frequency; j, (–1)1/2; n, surface irregularity. The CPE, which is considered a surface irregularity of the electrode, causes a greater depression in Nyquist semicircle diagram, where the metal solution interface acts as a capacitor with irregular surface. If the electrode surface is homogeneous and plane, the exponential value (n) becomes equal to 1 and the metal solution interface acts as a capacitor with regular surface, i.e. when n = 1, Y0 = capacitance [45]. Simulation of Nyquist plots with Randle’s model containing constant phase element (CPE) instead of capacitance and charge transfer resistance (Rct) showed excellent agreement with experimental data. The main parameters deduced from the analysis of Nyquist diagram for 2.0 M HCl containing various concentrations of opuntia extract are given in Table 4. On increasing opuntia extract concentration, the charge transfer resistance (Rct) increased and capacitance (Y0) decreased indicating that increasing opuntia extract concentration decreased corrosion rate. Decrease in the capacitance was caused by reduction in local dielectric constant and/or by increase in the thickness of the electrical double layer. This fact suggests that the inhibitor molecules acted by adsorption at the metal/solution interface [46]. The lower value of n for 2.0 M HCl medium indicated surface inhomoge-


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Table 3. Polarization parameters for mild steel in 2.0 M HCl in the presence and absence of opuntia entract Concentration, ppm

–Ecorr, mV, vs. Ag/AgCl

0 200 400 600 800 1000

394 411 416 417 418 419

–bc, mV

ba,

dec–1

mV

80 84 83 85 86 83

Icorr,

dec–1

IE, %

μA cm–2

75 73 75 78 79 76

563.2 71.7 40.2 33.0 31.4 27.3

– 87.3 92.9 94.1 95.2 95.4

Table 4. Impedance data of mild steel in 2.0 M HCl with and without opuntia stem extract Concentration, ppm

Rs, Ω

Rct, Ω

Y0, μF cm–2

n

IE, %

0 200 400 600 800 1000

0.24 0.31 0.32 0.31 0.33 0.32

32.3 230.7 409.3 438.2 502.3 536.0

47.2 21.8 16.4 12.6 9.3 7.4

0.76 0.78 0.86 0.87 0.89 0.90

– 86.0 92.1 92.6 93.6 94.0

neity resulted from roughening of metal surface due to corrosion. Addition of plant extract (1000 ppm) increased n value from 0.76 to 0.90 indicating reduction of surface inhomogeneity due to the adsorption of plant extract molecules. The inhibition efficiency obtained from the charge-transfer resistance is calculated by the following relation [35]:

R − Rct IE% = ct × 100 . Rct

in free acid solutions. Consequently, the IE of the extract decreases with increasing temperature. These results suggest a physical adsorption of the extract compounds on the mild steel surface. The apparent activation energy, Ea of the corrosion reaction was determined using Arrhenius plots. The Arrhenius equation could be written as:

−E I cor = A exp ⎛⎜ a ⎞⎟ . ⎝ RT ⎠

0

(5)

Rct0

Where Rct and are the charge-transfer resistances in the absence and presence of the plant extract. The values of IE% inhibitor are in quite good agreement with the results obtained previously from polarization measurements.

(6)

Where Icor is the corrosion current density, Ea as the apparent activation energy of the corrosion reaction 250

blank 200 ppm 400 ppm 600 ppm 800 ppm 1000 ppm

3.3. Effect of Temperature and Activation Energy The effect of temperature on the corrosion of mild steel in free acid solutions and in presence of 1000 ppm inhibitor was studied in the range of 30–60°C, using polarization measurements. Figure 4 (a and b) presents polarization curves for mild steel electrode in 2.0 M HCl, in presence and absence of 1000 ppm opontia extract at 30–60°C. The respective kinetic parameters are given in Table 5. It was found that the rates of mild steel corrosion, in free and inhibited acid solutions increases with a increasing in temperature. The mild steel corrosion rate in inhibited solution is more affected by increasing of temperature than that

Z '', Ω cm2

200 150

100 50

0

100

200

300 400 Z ', Ω cm2

500

600

Fig. 3. Nyquist plots for mild steel in 2.0 M HCl containing opuntia stem extract with different concentration.


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INHIBITION EFFECT OF OPUNTIA STEM EXTRACT

0

–5

(a)

blank 1000 ppm opontia extract

–6

–2 –3 30°C 35°C 45°C 50°C 55°C 60°C

–4 –5 –6

–7 –0.60 –0.55 –0.50 –0.45 –0.40 –0.35 –0.30 –0.25 E/V vs. Ag/AgCl (b)

–8 –9

–10 –11 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 1000/T, K–1

–1 log I [A cm–2]

–7

ln Icorr, [A cm–2]

log I [A cm–2]

–1

0

565

–2

Fig. 5. Arrhenius plots of mild steel in 2.0 M HCl in absence and presence of 1000 ppm of opuntia stem extract.

–3

50°C 45°C 35°C 30°C 55°C 60°C

–4 –5

–11 blank

–6 –0.60 –0.55 –0.50 –0.45 –0.40 –0.35 –0.30 –0.25 E/V vs. Ag/AgCl ln Icorr, [A cm–2]

Fig. 4. Effect of temperature on polorization curves of mild steel corrosion rate in (a) free acid solutions and (b) 1000 ppm inhibited acid solutions.

and A as the Arrhenius pre-exponential factor. The apparent activation energy of the corrosion reaction in presence and absence of the inhibitor could be determined by plotting lnIcorr with 1/T which gives a straight line (Fig. 5) with a slope permitting the determination of Ea. Figure 5 shows those plots in absence and presence of 1000 ppm of opuntia extract. Maximum protection efficiency was obtained at this concentration using polarization and impedance techniques as shown in Tables 2 and 3. The values of Ea were calculated and found to be 42.57 and 53.58 kJ mol–1 for corrosion reactions in free and inhibited acid solutions, respectively. It is clear that the activation energy increases in presence of opuntia stem extract and consequently the rate of corrosion decreases. An alternative formulation of Arrhenius equation is [47]:

( ) (

)

I corr = RT exp Δ S * exp − Δ H * . N Ah R RT

1000 ppm opontia extract

–12

(7)

Where Icorr is the corrosion current density obtained from polarization measurements, h as Planck’s constant, NA as Avogadro’s number, R as the ideal gas

–13 –14 –15 –16 –17 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 1000/T, K–1

Fig. 6. Arrhenius plots of ln (Icorr/T) versus 1/T in the absence and presence of 1000 ppm of opuntia stem extract.

constant, ∆H* as the enthalpy of the activation and ∆S* as the entropy of activation. Figure 6 shows a plot of ln(Icorr/T) against 1/T. Straight lines are obtained with a slope of (–∆H*/R) and an intercept of (ln(R/Nh) + (∆S*/R)) from which the values of ∆H* and ∆S* are calculated and listed in Table 6. The positive signs of ∆H* reflect the endothermic nature of the mild steel dissolution process. The analysis of results of Table 6 shows that the values of Ea and ∆H* enhance with the inhibitor suggesting that the energy barrier of corrosion reaction increases with presence of opuntia extract. This means that the corrosion reaction will further be pushed to surface sites that are


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Table 5. Polarization parameters and corresponding inhibition efficiency for the corrosion of the mild steel in 2.0 M HCl without and with addition of 1000 ppm of opuntia extract at different temperatures

2.0 M HCl

1000 ppm

–bc

Icorr,

ba

Temperature, °C

–Ecorr, mV

30

404

572

89

77

–

35

401

710

88

66

–

45

399

1258.4

84

65

–

50

389

1746

86

66

–

55

398

2180

82

67

–

60

392

2370

89

66

–

30

423

36.9

59

53

93.6

35

421

50.3

115

72

91.5

45

417

101.7

93

63

83.6

50

413

135.9

84

37

78.7

55

407

167.6

119

43

74.8

60

404

263.3

155

34

61.8

IE, %

μA cm–2

characterized by progressively higher values of Ea in the presence of the inhibitor [48]. 3.4. Adsorption Isotherm The adsorption isotherm can be determined if the inhibitor effect is due mainly to the adsorption on the metal surface (i.e. to its blocking). The type of the adsorption isotherm can provide additional information about the properties of the tested compounds. Surface coverage (θ, i.e. fractional inhibition efficiency) values for opuntia extract as determined by the weight loss measurements for various concentrations of the inhibitors are reported in the Table 2. The θ values were used to find the best adsorption isotherm between those more frequently used, i.e. Temkin, Langmuir, Frumkin, Freundluich. Data were tested graphically by fitting to various isotherms. A fitted straight line is obtained for the plot of C/θ versus C with slopes close to 1 as seen in Fig. 7. This means that the adsorption of opuntia extract on the mild steel surface obeys the Langmuir isotherm. The isotherm is given by [49]:

C = 1 +C. (8) θ K Where K is the binding constant representing the interaction of additives with a metal surface and C is the concentration of the additives. The strong correlation (R2 > 0.99) suggests that the adsorption of inhibitor on the mild steel surface obeyed this isotherm. This isotherm assumes that the adsorbed molecules occupy only one site and there are no interactions with other adsorbed species [50].

mV dec–1

It is considerable to mention here that, the θ values obtained from the other employed techniques also obey the langmuir adsorption isotherm. The energy of adsorption, however, could not have been calculated due to the unknown molecular mass of the extract.

3.5. Mechanism of Inhibition The effectiveness of a corrosion inhibitor can be related to its molecular spatial structure and molecular electronic structure in addition to its hydrophobicity and solubility [51–53]. In a corrosion system containing an inhibitor, the inhibitor and the metal act as a Lewis base and a Lewis acid, respectively. The stability of the adsorption bond is related to the Pearson’s HSAB principle [54]. Bulk metals are soft acids, thus soft base inhibitors are most effective for metals corroding in acid solutions. Owing to the complex chemical composition of the opuntia stem extract, it is quite difficult to assign the inhibitive effect to a particular Table 6. Activation parameters of dissolution reaction of mild steel in 2.0 M HCl solution without and with 1000 ppm opuntia extract Concentration, ppm

Ea, kJ

mol–1

ΔH*, kJ mol–1

ΔS*, J mol–1K–1)

Blank

42.57

39.93

–175.3

1000

53.58

50.94

–161.8


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using 6-311++G(d,p) basis set. Molecular orbitals are designated with respect either to the HOMO or LUMO orbital [56–59]. The hard soft acid-base (HSAB) parameters can be derived by using eigen values of HOMO and LUMO, –EHOMO (the energy of the highest occupied molecular orbital) and –ELUMO (the energy of the lowest unoccupied molecular orbital), for the ionization potential and electron affinity [60, 61]. The electro negativity, χ, is the negative of chemical potential and hence given by:

1200 1000

C/θ

800

y = 1.02696x + 22.52861 R2 = 0.99998

600 400 200 0

200

400

600 800 C, ppm

567

1000

1200

Fig. 7. Variation of θ with concentration of different opunita extracts fitted to langmuir isotherm.

constituent. However, eight constituents of opuntia stem extract were selected and for each constituent, quantum chemical calculations were performed. The optimized molecular structures of the eight constituents of opuntia stem extract are shown in Fig. 1 and frontier molecule orbital density distribution, highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of eight constituents are shown in Fig. 8. In the most of constituents, HOMO orbitals are localized over the benzene rings or oxygen atoms. The adsorption of these compounds on the metal surface reduces the surface area available for corrosion. The degree of protection increases with an increase in extract concentration due to higher degree of surface. 3.6. Quantum Chemical Calculations Adsorption is known to be the key mechanism of inhibition action, and it might be suggested that the inhibitor molecules are adhered to the metal surface, which decreases the surface area at which cathodic and anodic reactions take place [55]. Quantum chemical methods enable the definition of a large number of molecular quantities characterizing the reactivity, shape, and binding properties of a complete molecule as well as of molecular fragments and substituents. Although it is very difficult to assign the inhibitive effect to a particular opuntia ficus indica constituent but we consider these constituents including I to VIII. The molecular sketches of these constituents were drawn using Gauss View 3.0. All the quantum chemical calculations were performed with complete geometry optimization using a standard Gaussian 03 software package. Geometry optimization and quantum chemical calculations performed using density functional theory (DFT). The Becke three-parameter hybrid functional was combined with the Lee, Yang and Parr (LYP) correlation functional and denoted as B3LYP and was employed in the DFT calculations

χ = –μ ≈ –1/2(EHOMO + ELUMO).

(9)

While chemical hardness, η is approximated by: η ≈ 1/2(EHOMO – ELUMO).

(10)

The work function Φ of the metal surface is taken as electro negativity; whereas chemical hardness is neglected, because η of the bulk metal is inverses of their density state at the Fermi level which is an exceedingly small number. The number of electrons transferred from the molecule to metal ΔN, will be given by below equation: ΔN= χFe – χinh/2(ηFe+ ηinh).

(11)

Where Fe is considered as a Lewis acid according to the HSAB concept [62]. The difference in electro negativity drives the electron transfer and the sum of the hardness parameters act as a resistance. In order to calculate the fraction of electrons transferred, a theoretical value for the electro negativity of bulk iron was used χFe ≈ 7 eV, and a global hardness of ηFe ≈ 0, by assuming that for a metallic bulk I = A (where I is the ionization energy and A is the electron affinity) [63, 64] because they are softer than the neutral metallic atoms. However, as comparison of inhibition efficiency of eight constituent opuntia ficus indica extract was aim of this study, it seems that quantum chemical calculations can be used to explain the difference in behavior of these eight constituent. The molecular structures of these constituents were first optimized and then the frontier molecule orbital density distributions of the molecules are given in Fig. 8. Some of the calculated electronic parameters such as the highest occupied (EHOMO) and lowest unoccupied (ELUMO) molecular orbital energies, the energy gap (∆E = ELUMO – EHOMO), and the dipole moment are presented in Table 7. EHOMO is often associated with the electron donating ability of inhibitor molecules to the unoccupied orbital of a metal. Thus, the less negative values of EHOMO and, similarly, the lower value of the gap energy should both increase the effectiveness of inhibition; it is well visible on the compounds IV and V. The results of Table 7 show that the EHOMO for compounds IV and V are higher than other. Thus, these compounds have the lowest energy gap (∆E = ELUMO – EHOMO); this


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HOMO

LUMO

I

II

III

IV

V

VI

VII

VIII

Fig. 8. Optimization geometry of the major constituents of opuntia extract with HOMO and LUMO density.

indicate that IV and V could have better performance as a corrosion inhibitor. In addition, the lower LUMO energy, the easier acceptance of electrons from metal surface, as the energy gap (∆E = ELUMO – EHOMO) decreased and the efficiency of inhibitor improved [65]. On the other

hand, Table 7 shows the dipole moments for IV and V are nearly higher in comparison to other compounds of opuntia stem extract. Some authors showed that an increase of the dipole moment leads to decrease of inhibition and vice versa, suggesting that lower values of the dipole moment will favor accumulation of the


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Table 7. Orbital energies for HOMO, LUMO, HOMO – LUMO gap energy (∆E) and dipole moments (μ) of Compounds in the gaseous (G) and aqueous (A) phases* Compound

Phase

I

G A G A G A G A G A G A G A G A

II III IV V VI VII VIII

EHOMO, eV –7.423 –7.721 –7.574 –7.577 –7.494 –7.525 –6.246 –6.403 –5.633 –5.846 –7.604 –7.497 –7.767 –8.011 –7.744 –8.945

ELUMO, eV –1.156 –0.962 –0.776 –0.340 –0.572 –0.308 –0.789 –0.809 –1.955 –2.175 –1.669 –1.609 –1.496 –1.122 –1.402 –0.882

∆E, eV

μ, D

6.267 6.759 6.798 7.237 6.922 7.217 5.457 5.594 3.678 3.671 5.935 5.888 6.271 6.889 6.342 8.063

3.2076 4.4056 4.0833 5.4272 2.4670 3.0561 5.2308 6.2520 6.8374 9.8511 1.8155 1.3964 6.2734 5.7203 6.5273 8.1364

* All quantum chemical parameters calculated at DFT level using the 6-311++G(d,p) basis set.

inhibitor in the surface layer. In contrast, the increase of the dipole moment can lead to increase of inhibition and vice versa, which could be related to the dipoledipole interaction of molecules and metal surface [66]. We suggest that increase of dipole moment leads to Table 8. Electronegativity (χ), global hardness (η) and proportion of electrons transferred (∆N) of compounds Compound

Phase

χ

η

∆N

I

G A G A G A G A G A G A G A G A

4.289 4.341 4.175 3.958 4.033 3.916 3.517 3.606 3.794 4.010 4.636 4.553 4.631 4.566 4.573 4.913

3.133 3.379 3.399 3.618 3.461 3.608 2.728 2.797 1.839 1.835 2.967 2.944 3.135 3.444 3.171 4.031

0.433 0.393 0.416 0.420 0.429 0.427 0.638 0.607 0.872 0.815 0.398 0.416 0.378 0.353 0.383 0.259

II III IV V VI VII VIII

increase the ability of corrosion inhibition when the center of negative local charge overlaps with maximum charge density of HOMO orbitals. This subject is truly seen for compounds IV and V but nerveless large dipole moment in compound VIII as of no overlap between negative local charge and maximum charge density HOMO orbitals we see the little inhibitor corrosion. The electro negativity (χ), global hardness (η) and proportion of electrons transferred (∆N) of compounds are summarized in Table 8. From Table 8 it is evident that the fraction of electrons transferred ΔN, will be higher for IV and V than others. On the other hand, we think that inhibition efficiency of V is better than IV since compound V has less electro negativity (χ), less global hardness (η), higher proportion of electrons transferred (∆N) and higher surface coverage in flat orientation. The muliken and natural charges (e) for oxygen atoms of 8 constituents are represented in Table 9. The electrostatic interaction between positive charge (surface of metal) and local negative charges on oxygen atoms support the physical adsorption of these molecules on the steel. The regions of highest electron density (HOMO) are the sites at which electrophiles attack and represent the active centers, with the utmost ability to bond to the metal surface, whereas the LUMO orbital can accept the electrons in the d-orbital of the metal (Fe) using antibonding orbitals to form feedback bonds


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Table 9. Muliken and Natural Charges (e) for atoms I

Muliken

Natural

II

Muliken

Natural

III

Muliken

Natural

IV

Muliken

Natural

10O 11O 13O 15O 21O 18O 20O

–0.019 –0.188 –0.195 –0.217 –0.205 –0.152 –0.280

–0.604 –0.731 –0.746 –0.769 –0.730 –0.658 –0.619

10O 11O 13O 15O 20O 23O

–0.118 –0.194 –0.241 –0.243 –0.321 –0.195

–0.627 –0.730 –0.751 –0.765 –0.749 –0.700

11O 12O 14O 16O 18O

–0.065 –0.213 –0.176 –0.255 –0.248

–0.614 –0.745 –0.741 –0.758 –0.754

22O 23O 27O 25O 32O 34O 11O

–0.081 –0.161 –0.225 –0.228 –0.321 –0.237 0.019

–0.622 –0.733 –0.752 –0.766 –0.761 –0.676 –0.563

V

Muliken

Natural

VI

Muliken

Natural

VII

Muliken

Natural

VIII

Muliken

Natural

23O 31O 21O 25O 27O 29O 24O

0.045 –0.261 –0.230 –0.157 –0.196 –0.247 –0.339

–0.496 –0.572 –0.682 –0.627 –0.660 –0.666 –0.612

17O 19O 21O 23O 14O 16O

–0.217 –0.249 –0.209 –0.230 –0.259 –0.233

–0.739 –0.753 –0.760 –0.755 –0.732 –0.544

3O 9O 13O 20O 2O 12O 19O

–0.073 –0.197 –0.181 –0.107 –0.204 –0.243 –0.251

–0.691 –0.770 –0.688 –0.664 –0.571 –0.576 –0.624

3O 10O 14O 2O 13O

–0.165 –0.272 –0.223 –0.261 –0.256

–0.690 –0.758 –0.703 –0.567 –0.572

[67]. According to dipole moment, negative local charge and density of HOMO orbitals, it should be expect benzene ring and nearly flat-lying adsorption orientation for compounds IV and V, respectively (see Fig. 9). The OH, O–Me and carbonyl groups of compound V, can be help to have planar adsorption orientation on the surface of mild steel. It seems that the interaction of pi molecular orbitals of benzene rings in compounds IV and V with d-orbitals of Fe atoms in mild steel increases adsorption of theme on the surface of mildsteel. As can be seen from Fig. 8 there is no planer structure for other six constituents, thus their flat orientation on the metal surface are impossible. It seems that these conditions also help to increase the

inhibition efficiency of compounds IV and V related to the other constituent compounds of opuntia stem extract. 4. CONCLUSION The opuntia ficus indica stem extract acts as a good inhibitor for corrosion of mild steel in 2.0 M HCl solution. The IE increases with increasing extract concentration. The inhibition action is performed via adsorption of the extract compounds on the mild steel surface. The adsorption process is spontaneous and follows Langmuir adsorption isotherm. Raising the temperature increases desorption of molecules from

270

310 230 290

110

250 210

240 340

IV

V

Fig. 9. The schematic representation of the adsorption behavior of IV and V on mild steel in acidic solution. PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES

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the surface of the stainless steel and thereby reducing the IE of the extract. The presence of inhibitor at the constant temperature increases the activation energy of the corrosion reaction. The adsorption process may be physical in nature. The computational parameters such as EHOMO, ELUMO, gap energy (∆E), electronegativity (χ) and global hardness (η) were calculated. The results of quantum calculations of 8 constituents of extract revealed that compounds IV (Arbutin) and V (Isorhamnetin) have higher inhibition corrosion in opuntia stem extract. REFERENCES 1. Sykes, J.M., Br. Corros. J., 1990, vol. 25, p. 175. 2. Maayta, A.K. and Al-Rawashdeh, N.A.F., Corros. Sci., 2004, vol. 46, p. 1129. 3. Hui-Long, W., Rui-Bin, L., and Jian, X., Corros. Sci., 2004, vol. 46, p. 2455. 4. Morales-Gil, P., Negron-Silve, G., Romero-Romo, M., et al., Electrochim. Acta, 2004, vol. 49, p. 4733. 5. Ozcan, M., Dehri, I., and Erbil, M., Appl. Surf. Sci., 2004, vol. 236, p. 155. 6. Ravichandran, R., Nanjundan, S., and Rajendran, N., Appl. Surf. Sci., 2004, vol. 236, p. 241. 7. Ramesh, S.V. and Adhikari, A.V., Mater. Chem. Phys., 2009, vol. 115, p. 618. 8. Behpour, M., Ghoreishi, S.M., Gandomi-Niasar, A., et al., J. Mater. Sci., 2009, vol. 44, p. 2444. 9. Hosseini, M., Mertens, S.F.L., Ghorbani, M., and Arshadi, M.R., Mater. Chem. Phys., 2003, vol. 78, p. 800. 10. Soltani, N., Behpour, M., Ghoreishi, S.M., and Naeimi, H., Corros. Sci., 2010, vol. 52, p. 1351. 11. Granero, M.F.L., Matai, P.H.L.S., Aoki, I.V., and Guedes, I.C., J. Appl. Electrochem., 2009, vol. 39, p. 1199. 12. Shukla, S.K. and Quraishi, M.A., Corros. Sci., 2009, vol. 51, p. 1007. 13. Fontana, M.G., Corrosion Engineering, Singapore: McGraw-Hill, 1986. 14. Sinko, J., Prog. Org. Coat., 2001, vol. 42, p. 267. 15. Manahan, S.E., Environmental Chemistry, Boca Raton: CRC Press/Lewis Publ., 1996. 16. Mansfeld, F., Lin, S., Kim, S., and Shih, H., Corros. Sci., 1986, vol. 27, p. 162. 17. Macdonald, D.D., J. Electrochem. Soc., 1993, vol. 138, p. 127. 18. Abdel-Gaber, A.M., Abd-El-Nabey, B.A., and Saadawy, M., Corros. Sci., 2009, vol. 51, p. 1038. 19. Oguzie, E.E., Corros. Sci., 2008, vol. 50, p. 2993. 20. Li, Y., Zhao, P., Qiang, L., and Hou, B., Appl. Surf. Sci., 2005, vol. 252, p. 1245. 21. Behpour, M., Ghoreishi, S.M., Khayat Kashani, M., and Soltani, N., Mater. Corros., 2009, vol. 60, p. 895. 22. Valek, L. and Martinez, M., Mater. Lett., 2007, vol. 61, p. 148. 23. El-Etre, A.Y., Corros. Sci., 2003, vol. 45, p. 2485.

571

24. Abdel-Gaber, A.M., Abd-El-Nabey, B.A., Sidahmad, I.M., et al., Corros. Sci., 2005, vol. 48, p. 2765. 25. Rahim, A.A., Rocca, E., Steinmetz, J., et al., Corros. Sci., 2007, vol. 49, p. 402. 26. Saleem, M., Kim, J.H., Han, C.K., et al., Phytochemistry, 2006, vol. 67, p. 1390. 27. Matsuhiro, B., Lillo, L.E., Saenz, C., et al., Carbohydr. Polym., 2006, vol. 63, p. 263. 28. Rizk, A.M., and Al-Nowaihi, A., The Phytochemistry of the Horticultural Plants of Qatar, Oxford: Alden Press, 1989. 29. Karawya, M.S., Wassel, G.M., Baghdadi, H., and Ammar, N.M., Planta Med., 1980, vol. 46, p. 68. 30. McGarvie, D. and Parolis, H., Carbohydr. Res., 1981, vol. 88, p. 305. 31. Hammouch, H., Bennghmouch, L., Srhiri, A., and Hajjaji, N., Cactusnet, 2007, no. 11, p. 57. 32. Belbachir, C., Aouniti, A., Khamri, M., et al., J. Chem. Pharm. Res., 2013, vol. 12, p. 1307. 33. Laqhaili, A., Hakiki, A., Mossaddak, M., et al., J. Chem. Pharm. Res., 2013, vol. 12, p. 1297. 34. El Gharras, H., Hnini, K., Chtaini, A., et al., An Indian J., 2009, vol. 5, p. 1. 35. Hammouti, B., Hakiki, A., Mossaddak, M., and Boudalia, M., J. Chem. Pharm. Res., 2014, vol. 6, p. 1417. 36. Kokalj, A., Electrochim. Acta, 2010, vol. 56, p. 745. 37. Barone, V. and Cossi, M., J. Phys. Chem. A, 1998, vol. 102, p. 1995. 38. Frisch, M.J., Trucks, G.W., Schlegel, H.B., et al., Gaussian 03, Revision C.02, Wallingford, CT: Gaussian, 2004. 39. Gaussian View, V. 3.0, Pittsburgh, PA: Gaussian, 2003. 40. Reed, A.E. and Weinhold, F., Chem. Rev., 1998, vol. 88, p. 899. 41. Schlegel, H.B., in Advances in Chemical Physics, vol. 67: Ab Initio Methods in Quantum Chemistry I, Lawley, K.P., Ed., New York: John Wiley & Sons, 1987, p. 249. 42. Aljourani, J., Raeissi, K., and Golozar, M.A., Corros. Sci., 2009, vol. 51, p. 1836. 43. Ashassi-Sorkhabi, H., Seifzadeh, D., and Hosseini, M.G., Corros. Sci., 2008, vol. 50, p. 3363. 44. Roeper, D.F., Chidambaram, D., Clayton, C.R., and Halada, G.P., Electrochim. Acta, 2008, vol. 53, p. 2130. 45. Behpour, M., Ghoreishi, S.M., Soltani, N., et al., Corros. Sci., 2008, vol. 50, p. 2172. 46. Sorkhabi, H.A., Shaabani, B., and Seifzadeh, D., Electrochim. Acta, 2005, vol. 50, p. 3446. 47. Bouklah, M., Benchat, N., Hammouti, B., et al., Mater. Lett., 2006, vol. 60, p. 1901. 48. Larabi, L., Benali, O., and Harek, Y., Mater. Lett., 2007, vol. 61, p. 3287. 49. Zhao, P., Zhong, Ch., Hunag, L., et al., Corros. Sci., 2008, vol. 50, p. 2166. 50. Ali, S.A., Saeed, M.T., and Rahman, S.U., Corros. Sci., 2003, vol. 45, p. 253. 51. Pearson, R.G., Inorg. Chem., 1988, vol. 27, p. 734. 52. Sastri, V.S. and Perumareddi, J.R., Corrosion, 1994, vol. 50, p. 432.


Vol. 53

No. 3

2017

572

HONARMAND et al.

53. Sastri, V.S. and Perumareddi, J.R., Corrosion, 1999, vol. 53, p. 671. 54. Lukovits, I., Kalman, E., and Zucchi, F., Corrosion, 2001, vol. 57, p. 3. 55. Bard, A.J. and Faulkner, L.R., Electrochemical Methods: Fundamentals and Applications, New York: John Wiley & Sons, 1980, p. 517. 56. Awad, M.K., Issa, R.M., and Atlam, F.M., Mater. Corros., 2009, vol. 60, p. 813. 57. Li, W., Zhao, X., Liu, F., et al., Mater. Corros., 2009, vol. 60, p. 287. 58. Hasanov, R., Sadikoglu, M., and Bilgic, S., Appl. Surf. Sci., 2007, vol. 253, p. 3913. 59. Amin, M.A., Khaled, K.F., and Fadl-Allah, S.A., Corros. Sci., 2010, vol. 52, p. 140.

60. Pearson, R.G., Inorg. Chem., 1988, vol. 27, p. 734. 61. Pearson, R.G., J. Am. Chem. Soc., 1963, vol. 85, p. 3533. 62. Khaled, K.F. and Amin, M.A., Corros. Sci., 2009, vol. 51, p. 1964. 63. Babic-Samardzija, K., Lupu, C., Hackerman, N., and Barron, A.R., J. Mater. Chem., 2005, vol. 15. p. 1908. 64. Zhao, P., Liang, Q., and Li, Y., Appl. Surf. Sci., 2005, vol. 252, p. 1596. 65. Musa, A.Y., Kadhum, A.A.H., Mohamad, A.B., et al., J. Mol. Struct., 2010, vol. 969, p. 233. 66. Oguzie, E.E., Enenebeaku, C.K., Akalezi, C.O., et al., J. Colloid Interface Sci., 2010, vol. 349, p. 283. 67. Herrag, L., Hammouti, B., Elkadiri, S., et al., Corros. Sci., 2010, vol. 52, p. 3042.


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