Assessment of corrosion inhibitive behavior of 2-aminothiophenol derivatives on carbon steel in 1 M HCl

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JIEC-2152; No. of Pages 10 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Assessment of corrosion inhibitive behavior of 2-aminothiophenol derivatives on carbon steel in 1 M HCl Khalid I. Kabel a,*, Khaled Zakaria b, Mohammed A. Abbas a, E.A. Khamis a a b

Petroleum Applications Department, Egyptian Petroleum Research Institute, Nasr City, P.B. 11727, Cairo, Egypt Analysis and Evaluation Department, Egyptian Petroleum Research Institute, Nasr City, P.B. 11727, Cairo, Egypt

A R T I C L E I N F O

Article history: Received 15 April 2014 Received in revised form 20 July 2014 Accepted 22 July 2014 Available online xxx Keywords: Acid solutions Carbon steel Polarization EIS Acid inhibition

A B S T R A C T

The inhibition effect of three newly synthesized 2-aminothiophenol derivatives on the corrosion of carbon steel in 1 M HCl was investigated using electrochemical measurements and quantum chemical calculations. The experimental results suggest that these compounds are efficient corrosion inhibitors and the inhibition efficiencies increase with increasing their concentrations. The efficiencies obtained from EIS measurements were in good agreement with those obtained from the polarization measurements. Polarization measurements show that these inhibitors act as mixed-type inhibitors. The adsorption of these inhibitors was found to obey Langmuir adsorption model. The computed quantum chemical properties show good correlation with experimental inhibition efficiencies. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction The corrosion of metals is a common problem in the industry, and it has received a great attention. Carbon steel is cheap and high strength and is most applicable in many industrial sections such as marine applications, chemical processing, petroleum production and refining, construction and metal-processing equipment [1,2], despite it has a relatively high cost. These applications usually induce serious corrosive effects on equipment, tubes and pipelines made of iron and its alloys [3]. The study of the corrosion process of carbon steel in acidic media has become important, particularly because of the increased industrial applications of acid solutions. The applications of mineral acids as aggressive solutions are numerous. The most important areas of application are acid pickling, oil well acidizing, acid cleaning and acid de-scaling, etc. Among the acid solutions, hydrochloric acid is one of the most widely used agents. Because of the general aggressiveness of acids, using inhibitors is often used to control an attack of acid environment and to reduce the overall

* Corresponding author. Tel.: +20 2 22745902; fax: +20 2 22747433. E-mail addresses: [email protected] (K.I. Kabel), [email protected] (K. Zakaria), [email protected] (M.A. Abbas), [email protected] (E.A. Khamis).

corrosion current density [4]. Corrosion inhibitors are the chemicals that minimize or prevent corrosion if they are added at low concentrations in an aggressive environment. Most commercial acid inhibitors are organic compounds containing hetero atoms such as sulfur [5], oxygen [6], nitrogen [7], and phosphorous. Inhibitor molecules are adsorbed on the metal surface, thus resulting in film formation. The adsorbed film acts like a barrier, which separates the metal surface from the corrosive medium and so decreases the corrosion extent. In general, adsorption of inhibitor molecules on the metal surface depends on: the nature and the surface charge of the metal, the adsorption mode, chemical structure and type of electrolyte solution [8,9]. The inhibition efficiency of inhibitors increases in the order of: O < N < S < P [6,7,10]. Organic amine corrosion inhibitors have been widely used for corrosion protection of metal from aggressive environments due to their economic and effective corrosion retarding capability in various industrial fields. These inhibitors are added in small concentrations into the corrosive electrolyte to form a thin passive film on the surface of the metal substrate, retarding the access of the corrosive species to the metal. It is well-known that organic amines with low molecular mass and high-water solubility have enhanced adsorption and corrosion inhibition properties [11,12].

http://dx.doi.org/10.1016/j.jiec.2014.07.042 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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The effect of the molecular structure on the chemical reactivity has been an object of great interest in several disciplines of chemistry. In this respect, quantum chemical calculations have been widely used to investigate the molecule in its electronic structure level and to interpret the experimental results. The inhibition property of a compound has been often correlated with its molecular properties. Therefore, it is worthwhile to compute these features theoretically [13]. Invaluable quantum chemical parameters such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and dipole momentum (m) obtained by this method, help to understand the adsorption properties by considering the structure of every individual molecule [14]. The present study was undertaken to investigate the inhibition of carbon steel corrosion in 1 M HCl by three newly synthesized 2aminothiophenol derivatives namely, N-(2-carboxyethylthiophenyl)-b-Alanine (I), ethoxylated b-Alanine N-[o-[(2-carboxyethyl)thio]phenyl] (II) and ethoxylated b-Alanine N-[o-[(2amidoethyl)thio]phenyl] (III) which presented in Scheme 1. Our study was conducted by using polarization curves and electrochemical impedance spectroscopy (EIS) methods and the quantum chemical study. The nature of the inhibitor adsorption process was also studied and discussed.

Experimental Materials All the experiments were performed on carbon steel (CS) of the following composition (wt.%): 0.05% C, 0.28% Mn, 0.023% P, 0.019% S, 0.02% Si, and the remainder Fe. Carbon steel was used as working electrode. The electrode was embedded in (Araldite) epoxy resin in such a way that the flat surface was in contact with the aggressive solution (test solution) and with 1 cm2 exposed area. Before each test, specimens were abraded with different emery papers (grade 800, 1000 and 1200), and then washed in deionized water and acetone. 1 M HCl was prepared by Aldrich reagent and deionized water, 2-aminothiophenol (2-ATP), acrylic acid (AA), p-toluene sulphonic acid (PTSA) and dodecylamine (DA) were purchased from Aldrich, polyethylene glycol (PEG 1000) was purchased from Merck. Inhibitors synthesis and characterization A mixture of 2-aminothiophenol (5 g, 40 mmol) and acrylic acid (9.06 ml, 120 mmol) in toluene (10 ml) was maintained at room temperature for 72 h. The light-greenish crystalline precipitate O

H N

NH2

OH

+ 2 CH2=CHCOOH SH

OH

S

Acrylic acid

O

N-(2-carboxyethylthiophenyl)- -Alanine

2-ATP

(I)

O

H N

PEG 1000

H

O O

n OH

S O

ethoxylated -Alanine N-[o-[(2-carboxyethyl)thio]phenyl]

(II)

dodecylamine O

H N

H

O O R

S O

n

N H

ethoxylated -Alanine N-[o-[(2-amidoethyl)thio]phenyl]

(III) Scheme 1. The synthetic route and molecular structure of newly synthesized corrosion inhibitors I, II and III.

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was filtered off, washed with ether, and crystallized from 1:1 (isopropanol:water) to give N-(2-carboxyethylthiophenyl)-bAlanine (I), a method described elsewhere [15]. The monoesterification of dicarboxylic acid derivative will be maintained in a three-necked flask fitted with Dean and Stark, condenser and thermocouple. A mixture of compound (I) (2.69 g, 10 mmol) and polyethylene glycol 1000 (12 g, 12 mmol) in the presence of PTSA 0.2% and toluene (10 ml) was maintained at 120 8C. The reflux of the reaction mixture was continued until the theoretical amount of water was collected azeotropically by a Dean and Stark trap, the solvent was evaporated by rotary and the final product was washed by isopropanol three times and extracted by 5% sodium carbonate solution [16] to give ethoxylated b-Alanine N-[o-[(2-carboxyethyl)thio]phenyl] (II). Finally (1.26 g, 1 mole) of a compound (II) was reacting with (0.18 g, 1 mmole) of dodecylamine. The reaction was carried out at 110–140 8C in the presence of PTSA as a catalyst and toluene, with continuous stirring and nitrogen atmosphere. The reaction was continued until the theoretical amount of water was collected [17]. The final product was purified as mentioned above to produce product ethoxylated b-Alanine N-[o-[(2-amidoethyl)thio]phenyl] (III). The synthesized inhibitors were illustrated in Scheme 1, and the chemical structure was confirmed by FT-IR spectroscopic analysis using Nicolet iS10 FT-IR spectrometer Thermo Fisher Scientific (USA). The molecular weights were measured using Waters 600 E analytical gel-permeation chromatograph (GPC) equipped with a Waters model 4110 refractive index detector. Styragel columns were used at 408 8C with HPLC-grade toluene as mobile phase at a flow rate of 0.7 ml/min. Corrosive medium The aggressive solution of 1 M HCl was prepared by dilution of AR grade HCl (37%) with double distilled water. The concentration ranges of synthesized compounds were (1  105–50  105 M). Electrochemical measurements Potentiodynamic polarization measurements were carried out using VoltaLab PGZ-301 (France), and controlled by Tacussel corrosion analysis software model (Voltamaster 4) at under static condition. All experiments were conducted in a conventional three-electrode glass cell assembly with a platinum wire as auxiliary electrode, saturated calomel electrode (SCE) as reference electrode and carbon steel as working electrode. Potentiodynamic polarization curves were obtained by varying the potential automatically from 800 to 300 mV in relation to a steady-state open circuit potential (EOCP) with the scan rate 2 mV s1. The polarization curves were obtained after 1 h in the open-circuit potential. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a potentiostat (VoltaLab PGZ-301) attached to the Zsimpwin software program. The measurements were carried out using AC signal (10 mV) peak to peak at OCP in the frequency range of 100 kHz–50 mHz. In all experiments, the carbon steel electrode was also allowed to reach its stable opencircuit potential (OCP), which occurred after 1 h. EIS diagrams are given in the Nyquist representation. Quantum chemical study All required molecular parameters of the 2-ATP, I, II and III have been geometrically carried out based on MINDO3 semi-empirical method ever used for organic inhibitor’s calculation [18] at an Unrestricted Hartree Fock (UHF) level which are implemented in

3

Hyperchem 8.0. The molecule 2D sketch was obtained by ISIS Draw 2.1.4. Quantum chemical parameters such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) have been calculated. The calculation of other parameters such as log P (hydrophobic parameter), and polarizability was obtained by QSAR method from the optimized geometry.

Results and discussion Chemical structure confirmation of the synthesized inhibitors The structural characteristics of purified products were confirmed by FT-IR spectroscopy in the range 4000–500 cm1 as shown in Fig. 1. Product (I): The bands at 2500–3300 cm1 for OH acids overlapping with NH group and the band at 3376 cm1 for OH free group, the bands at 2655–2934 cm1for C–H, the band at 1709 cm1 attributed to C5 5O acid, the two bands at 1593 cm1 attributed to NH group, SC group band appears as a weak band at 950 cm1, the bands at 1000–1250 cm1 for C–N group. Product (II): The secondary amine band appears at 3400 cm1 and overlapping with OH group band. The strong band appears at 1107 cm1 for ethylene oxide group. The ester group was appearing at 1730 cm1. Product (III): The primary and secondary amine bands appear at 3400 cm1 and overlapping with OH group band. The strong band appears at 1107 cm1 for ethylene oxide group. The amide group was appearing at 1643 cm1, the C–N band was appearing at 1352 cm1. The determined mean molecular weights of all products are given in Table 1 have been found to be very near to that calculated theoretically. Electrochemical polarization studies The electrochemical polarization measurements of carbon steel in 1 M HCl with and without various concentrations (1  105– 50  105 M) of 2-ATP, I, II and III at 303 K is shown in Fig. 2(a)–(d). The values of electrochemical parameters such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (ba and bc) and corrosion current density (Icorr) were extracted by Tafel extraploting the anodic and cathodic lines and are listed in Table 2. The percentage of inhibition efficiency (h %) was calculated using the following equation [19]:

h¼

ðIo  IÞ  100% Io

(1)

where Io and I are the corrosion current densities in the absence and presence of inhibitors respectively. From the results in Table 2, it can be observed that the values of corrosion current density (Icorr) of carbon steel in the inhibited solutions are lower than that in the uninhibited solution. The inhibition efficiency obtained from polarization measurements is found to follow the order: III > II > I > 2-ATP, indicating the more beneficial effect of compound (III) on corrosion inhibition of carbon steel. As seen from Fig. 2(a)–(d), the anodic and cathodic Tafel slopes decrease with the increase of inhibitor concentration, suggesting that both anodic dissolution of carbon steel substrate and hydrogen evolution are suppressed. So, the addition of the synthesized inhibitors has an inhibiting effect on the both anodic and cathodic parts of the polarization curves and shifts both the anodic and cathodic curves to lower current densities. This may be ascribed to adsorption of the inhibitors over the metal surface.

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Fig. 1. FTIR charts of newly synthesized 2-aminothiophenol derivatives.

Therefore, the synthesized compounds act as mixed-type inhibitors in HCl solution [20]. Table 1 The theoretically and determined molecular weights of the prepared compounds. Samples

Theoretical

Determined

I II III

269 1251 1418

270 1258 1425

Electrochemical impedance spectroscopy EIS measurements provide a better understanding of the corrosion mechanism taking place at the electrode surface including the kinetics of the electrode processes and simultaneously about the surface properties of the investigated systems. The method is widely used for investigation of the corrosion

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1-1M HCl 2 -1 2-1x10-4 M -5 3-1x10 M -2 -4 4-5x10 M -3 5-5x10-5 M -750

-600 -450 E(mV)

-300

c

4

1

1 logI(mA.cm-2)

1

1 logI(mA.cm-2)

0

a

3 4 5

2

0

1. 1 M HCl -1 2. 1x10-5 M -5 3. 5x10 M -2 -4 4. 1x10 M -4 5. 5x10 M -3 -750

0

1. 1 M HCl -1 2. 1x10 -5 M -5 3. 5x10 M -2 -4 4. 1x10 M -4 -3 5. 5x10 M -600 -450 E(mV)

-1 -2

-4

-300

d

1 2

0

-3

-600 -450 E(mV)

b

1

5

3

32 1 54

-750

logI(mA.cm-2)

logI(mA.cm-2)

1

5

-300

3 4 5

1. 1 M HCl -5 2. 1x10 M -5 3. 5x10 M -4 4. 1x10 M -4 5. 5x10 M -750

-600 E(mV)

-450

-300

Fig. 2. Potentiodynamic polarization curves for carbon steel in 1 M HCl without and with various concentrations of inhibitors at 303 K: (a) 2-ATP, (b) I, (c) II and (d) III. Table 2 Electrochemical polarization parameters for the corrosion of carbon steel in 1 M HCl containing various concentrations of 2-ATP, I, II and III at 303 K. Inhibitors

Concn. (105 M)

R (mmpy)

Ecorr. (mV)

Icorr (mA cm2)

ba (mV dec1)

bc (mV dec1)

Rp (V cm2)

h (%)

u

2-ATP

Blank 1 5 10 50

13.78 6.410 4.910 3.966 3.220

516.9 573.7 565.4 561.3 557.3

1.180 0.548 0.420 0.339 0.275

202 73.4 104.9 95.0 119.5

199.0 130.7 146.2 136.9 141.4

53.00 65.90 89.02 125.8 153.6

– 53.5 64.4 71.3 76.7

– 0.535 0.644 0.712 0.767

I

1 5 10 50

6.527 3.994 2.827 2.198

570.6 557.6 560.6 558.8

0.558 0.342 0.242 0.188

89.0 97.9 129.5 120.3

138.2 145.8 150.8 123.1

84.61 113.2 175.2 205.7

52.7 71.0 75.2 84.1

0.527 0.710 0.752 0.841

II

1 5 10 50

4.304 3.823 2.433 1.755

583.0 561.7 582.1 566.6

0.368 0.327 0.208 0.150

109.9 132.8 73.40 116.7

133.8 163.8 121.2 133.9

181.7 191.0 201.3 210.3

68.8 72.3 82.4 87.3

0.688 0.723 0.824 0.873

III

1 5 10 50

2.347 2.204 1.140 0.071

549.1 546.6 540.3 547.0

0.201 0.189 0.098 0.060

97.80 94.50 115.8 168.7

139.9 113.6 138.6 150.5

227.9 239.2 294.3 772.8

83.0 84.0 91.7 94.9

0.830 0.840 0.917 0.949

inhibition processes [21]. The Nyquist plots of carbon steel in 1 M HCl solution in the absence and presence of different concentrations of the synthesized inhibitors given in Fig. 3(a)–(d). It is clear from the figure that all impedance spectra have a depressed semicircular shape in the complex impedance plane, with the center under the real axis. Deviations from a perfectly circular shape indicate the frequency dispersion of the interfacial impedance [22]. The measured impedance data were analyzed by fitting into equivalent circuit given in Fig. 4. The circuit consists of Rs (the resistance of solution between working electrode and counter electrode), and Rct. When a non-ideal frequency response is present, it is commonly accepted to use distributed circuit elements in an equivalent circuit. It is worth mentioning that the double layer capacitance (Cdl) value is affected by imperfections of the surface, and that this effect is simulated via a constant phase element (CPE).

CPE is substituted for the capacitive element to give a more accurate fit. The depression in Nyquist semicircles is a feature for solid electrodes and often referred to frequency dispersion and attributed to the surface heterogeneity resulting from surface roughness, impurities, dislocations, grain boundaries, adsorption of inhibitors, formation of porous layers [23] and other inhomogeneities of the solid electrode [24]. The CPE is a special element whose admittance value is a function of the angular frequency (v), and whose phase is independent of the frequency. The CPE impedance (ZCPE) is obtained by the following equation: Z CPE ¼ Q 1 ð jvÞn

(2)

where Q is the CPE coefficient, n the CPE exponent (phase shift), v the angular frequency (v = 2:f), where f is the AC frequency), and j here is the imaginary unit. When the value of n is 1, the CPE behaves like an ideal double layer capacitance (Cdl) [25]. The

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Fig. 3. Nyquist plots for carbon steel in 1 M HCl without and with various concentrations of inhibitors at 303 K: (a) 2-ATP, (b) I, (c) II and (d) III.

Fig. 4. Equivalent circuit model applied for fitting of the impedance spectra.

large Rct values are generally associated with a slower corroding system as the Rct value is a measure of electron transfer across the surface and is inversely proportional to the corrosion rate [28]. On the other hand, the values of Cdl decrease with an increase in inhibitor concentration. This situation was the result of an increase in the surface coverage by these inhibitors which led to an increase in the inhibition efficiency. Also, this effect leads to a protective film, inhibiting species or both on carbon steel surface. The thickness of the protective layer, dorg is related to Cdl by the following equation [29]:

correction of capacity to its real values is calculated from: n1

C d1 ¼ Q ðvmax Þ

(3)

where vmax is the frequency at which the imaginary part of impedance (Z00 ) has a maximum [26]. The degree of surface coverage (Q) is calculated from the EIS data by using the following equation:

u¼

1  Cd1 Cd1

(4)

where, Cdl and C*dl are the double-layer capacitances in the absence and presence of inhibitors, respectively. The values of percentage inhibition efficiency were calculated from the values of charge transfer resistance (Rct) according to the following equation [27]:

h% ¼

1  Rct  100 Rct

(5)

where Rct and R*ct are the charge transfer resistances with and without the inhibitors, respectively. Inhibition efficiencies and other calculated impedance parameters obtained from fitted spectra are given in Table 3. The equivalent circuit model represents the charge transfer and metal/solution interface features related to the corrosion process of carbon steel in hydrochloric acid solution. As seen from Table 3, the charge transfer resistance (Rct) values of inhibited substrates increase with the concentration of inhibitors. The most pronounced effect and the highest Rct are obtained by inhibitor (III). A

dorg ¼

eo et C d1

(6)

where eo is the dielectric constant and et is the relative dielectric constant. This decrease in the Cdl, which can result from a decrease in local dielectric constant and/or increase in thickness of the electrical double layer, suggested that the compounds act via adsorption at the metal/solution interface [30]. It could be assumed that the decrease of Cdl values is caused by the gradual replacement of water molecules by adsorption of organic molecules on the electrode surface, which decreases the extent of the metal dissolution [31]. The adsorption can occur either directly on the basis of donor–acceptor interaction between the unshared electron pairs and/or p-electrons of inhibitor molecule and the vacant d-orbitals of the metal surface or by interaction of the inhibitors with already adsorbed chloride ions [32]. These results confirm that all of the synthesized inhibitors exhibit good inhibiting performance of carbon steel in HCl solution, and the more efficient to inhibit the corrosion of carbon steel is compound (III). It is worth noting that the inhibition efficiencies calculated from EIS measurements are in good agreement with that obtained from potentiodynamic polarization measurements. Adsorption isotherm and thermodynamics calculations The action of an inhibitor in aggressive acid media is assumed to be due to its adsorption at the metal/solution interface. The

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Table 3 Electrochemical impedance parameters and inhibition efficiency for corrosion of carbon steel in 1 M HCl in the presence and absence of different concentrations of 2-ATP, I, II and III respectively. Inhibitors

Concn. (105 M)

Rs (V cm2)

n

Cdl (mF cm2)

Rct (V cm2)

h (%)

2-ATP

Blank 1 5 10 50

1.93 2.30 2.87 3.08 3.39

0.80 0.80 0.80 0.80 0.77

556 350 287 281 263

46.45 66.93 83.00 121.0 162.0

– 30.6 44.0 61.6 71.3

I

1 5 10 50

2.35 2.95 3.36 6.31

0.76 0.80 0.80 0.80

333 286 276 245

81.00 98.00 138.5 241.0

42.7 55.4 66.5 80.7

II

1 5 10 50

3.03 3.23 3.23 3.88

0.80 0.70 0.80 0.80

327 262 256 191

144.5 156 173 295

67.9 70.2 73.2 84.3

III

1 5 10 50

2.03 2.44 2.53 2.98

0.76 0.80 0.70 0.70

260 211 201 145

194 268 355 696

76.1 82.6 86.9 93.3

adsorption process depends on the electronic characteristics of the inhibitor, the nature of metal surface, temperature, steric effects and the varying degrees of surface-site activity [33]. In fact, the solvent H2O molecules could also be adsorbed at the metal/ solution interface. Therefore, the adsorption of organic inhibitor molecules from the aqueous solution can be considered as a quasisubstitution process between the organic compounds in the aqueous phase Org(sol) and water molecules at the electrode surface H2O(ads) [33]: OrgðsolÞ þ xH2 OðadsÞ $ OrgðadsÞ þ xH2 O

(8)

where x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor. The type of the adsorption isotherm can provide additional information about the properties of the tested compounds. In order to obtain the adsorption isotherm, the degree of surface coverage (u) of the inhibitor must be calculated. In this study, the degree of surface coverage values (u) for various concentrations of the inhibitors in acidic medium have been evaluated from Tafel polarization data and listed in Table 2. Attempts were made to fit the u values to various isotherms, including Langmuir, Temkin, Frumkin and Flory–Huggins. By far, the best fit is obtained with the Langmuir isotherm. Langmuir adsorption isotherm is described by the following equation [34]: C inh:

u

¼

1 þ C inh: K ads

(9)

free energy of adsorption, with the following equation: !   DG0ads 1 exp K ads ¼ 55:5 RT

(10)

where R is the gas constant (8.314 J K1 mol1), T is the absolute temperature (K), the value 55.5 is the concentration of water in solution expressed in M [33]. The calculated values of Kads and DG0ads are given in Table 4. Generally, the values of DG0ads up to 20 kJ mol1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than – 40 kJ mol1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [33]. In the present work, the calculated values of DG0ads at 303 K for carbon steel lies between 38.5 and 41.1 kJ/mol1, which indicate that the adsorption of inhibitor molecules on the metal surface involves complex interactions: both physical and chemical processes [38]. The large values of DG0ads and its negative sign are usually characteristic of a strong interaction and a high efficient adsorption. Despite the nearly similar DG0ads values for all inhibitors, compound (III) can be considered as a most stable compound from adsorption standpoint, having the highest Kads and DG0ads values. This can be simply explained by the presence of additional donor atoms, such as oxygen, sulphur and nitrogen, in the appended functional groups. Quantum chemical calculations

where Cinh is the inhibitor concentration, Kads is the adsorption equilibrium constant and u is the surface coverage. Fig. 5(a)–(d) shows the plots of Cinh/u versus Cinh and the expected linear relationship is obtained for different inhibitors. The strong correlations (R2 = 0.9995) confirm the validity of this approach. However, the slopes of the C/u versus C plots were close to 1 and showed a little deviation from unity which meant nonideal simulating [35] and unexpected from Langmuir adsorption isotherm. They might be the results of the interactions between the adsorbed species on the metal surface by mutual repulsion or attraction [36]. On the other hand, the relatively high value of adsorption equilibrium constant (Kads) reflects the high adsorption ability of the studied inhibitors on the carbon steel surface [37]. Kads values could be calculated from the intercepts of the straight lines on the C/u-axis, the Kads was related to the standard

The use of quantum chemical calculations is very important in studying the correlation between molecular structure and corrosion inhibition efficiency [39]. Moreover, a theoretical study permits the pre-selection of compounds with the necessary structural characteristics to act as organic corrosion inhibitors. Thus, several studies have incorporated these theoretical approaches [40]. In this study, the relationship between quantum chemical parameters and inhibition efficiency was investigated. The optimized geometry of all synthesized compounds showing the HOMO and LUMO is given in Fig. 6. Frontier orbital theory is useful in predicting adsorption centers of the inhibitor molecules responsible for the interaction with surface metal atoms [41]. The values of quantum chemical parameters necessary for a meaningful discussion of the current work are reported in Table 5 and

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Fig. 5. Langmuir adsorption isotherm (C/Q versus C) of the studied inhibitors in 1 M HCl at 303 K: (a) 2-ATP, (b) I, (c) II and (d) III.

Table 4 Equilibrium adsorption parameters for adsorption of 2-ATP and its derivatives on carbon steel surface in 1 M HCl solution. Inhibitors 2-ATP I II III

Slope 1.290 1.118 1.175 1.045

Regression coefficient (R2)

Kads

0.9999 0.9993 0.9998 0.9999

136,200 78,697.0 119,674 222,965

DG0ads ,

(kJ mol1)

39.9 38.5 39.6 41.1

include the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), the HOMO–LUMO energy gap (DEH–L), the dipole moment (m), log P (substituent constant-measure of differential solubility of a compound in two solvents and characterizes the hydrophobicity/hydrophilicity of a molecule) and molecular polarizability (pol.). However, it would be pertinent to mention here that the information about the molecular reactivity obtained from these quantum chemical descriptors cannot directly be translated into corrosion inhibition efficiency, because the adsorbability of an effective corrosion inhibitor involves more processes such as film formation and the nature of the metal surface. It is for this reason that in some cases a comparison between calculated quantum chemical parameters and inhibition efficiency does not yield a good correlation. It is well known that the value of EHOMO is often associated with the electron donating ability of a molecule, higher the values of EHOMO greater is the tendency of the molecules to donate electrons to appropriate acceptor molecules with low-energy, empty

molecular orbital such as 3d orbital of Fe atom. As the values of the EHOMO increases there will be greater adsorption (and therefore inhibition) of inhibitor molecules which is achieved by influencing the transport process through the adsorbed layer. Also the energy of the lowest unoccupied molecular orbital (ELUMO) refers to the suitability of the molecule to accept electrons. Thus the lower value of ELUMO, more is the probability of the molecule to accept electrons [42]. The calculations listed in Table 5 showed that the highest energy EHOMO is assigned for the compound (III), which is expected to have the highest corrosion inhibition among the synthesized compounds. Energy gap (DEH–L), is an important parameter as a function of reactivity of the inhibitor molecule toward the adsorption on the metallic surface. Lower values of the energy difference will render good inhibition efficiency, because the energy to remove an electron from the last occupied orbital will be low [43]. Reportedly, excellent corrosion inhibitors are usually organic compounds which not only offer electrons to the unoccupied orbital of the metal but also accept free electrons from the metal [44,45]. A molecule with a low energy gap is more polarizable and is generally associated with the high chemical activity and low kinetic stability and is termed soft molecule [46]. Regarding from DEH–L, it was found that the minimum value obtained by compound (III) which has the best inhibition efficiency. The dipole moment is another indicator of the electronic distribution in molecules, and it is a good indicator of the hydrophilic/hydrophobic character of a given system. A high dipole moment indicates polar character for the molecule, while a low dipole moment indicates nonpolar character for the molecule. Trends in the calculated dipole moments and observed inhibition

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Fig. 6. Molecular orbital of the synthesized compounds showing the HOMO and the LUMO. Table 5 Calculated quantum chemical parameters of the synthesized compounds. Inhibitor

EHOMO (eV)

ELUMO (eV)

DE (eV)

m (debye)

log P

Polarizability (A˚3)

I II III

7.77 7.77 7.76

0.31 0.28 0.08

7.46 7.49 7.68

3.83 6.04 7.77

1.16 3.85 5.11

27.2 35.9 54.3

efficiency are not always univocal [47,48]. It is generally agreed that the adsorption of polar compounds possessing high dipole moments on the metal surface should lead to better inhibition efficiency. Comparison of the results obtained from quantum chemical calculations with experimental inhibition efficiencies indicated that the inhibition efficiencies of the inhibitors increase with increasing value of the dipole moment. Another chemical parameter/quantity that is related to the hydrophilic/hydrophobic nature of the molecule is log P (substituent constant). Substituent constants are empirical quantities, which account for the variation of the structure and do not depend

on the parent structure but vary with the substituent [49] A large value of log P implies less solubility of the inhibitor in solution and therefore greater adsorbability of the inhibitor on the metal surface [50]. The results for the calculated inhibitors show that the log P value increases in the order I < II < III (Table 5), which suggests that compound III would have a greater tendency to adsorb on the metal surface than other compounds in acidic solution. Polarizability is the ratio of induced dipole moment to the intensity of the electric field. The induced dipole moment is proportional to polarizability [51]. Some attempts have been made to relate the polarizability of some corrosion inhibitors to their

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inhibition efficiency. According to Arslan et al. [52], the minimum polarizability principle (MPP) expects that the natural direction of evolution of any system is toward a state of minimum polarizability. From the results obtained from quantum chemical calculations, the trend for the increase in the inhibition efficiencies of the inhibitors with respect to increasing polarizability correlates well with the order of the experimental % (h) results (III > II > I). Conclusions The following conclusions can be drawn from the study: (1) The results showed that inhibitors (2-ATP, I, II and II) have excellent inhibition efficiency for the corrosion of carbon steel in 1 M HCl. The inhibition efficiency increases in the following order III > II > I > 2-ATP. (2) The potentiodynamic polarization curves indicated that 2-ATP, I, II, III act as a mixed-type of inhibitors by inhibiting both anodic metal dissolution and cathodic hydrogen evolution reactions. (3) 2-ATP, I, II, III molecules follows Langmuir adsorption isotherm for the adsorption on metal surface in 1 M HCl solution. (4) The impedance results indicate that the value of charge transfer resistance increased and double layer capacitance decreased. This result can be attributed to the increase of thickness of electrical double layer. (5) The quantum chemical approach is adequately sufficient to also forecast the inhibitor effectiveness using the theoretical approach. However, it may be used to find the optimal group of parameters that might predict the structure and molecule suitability to be an inhibitor.

Acknowledgment The authors acknowledge the Egyptian Petroleum Research Institute (EPRI) for the financial assistance and facilitation of our study. References [1] [2] [3] [4] [5] [6]

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