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Corrosion inhibition behavior of new synthesized nonionic surfactants based on amino acid on carbon steel in acid media
Journal of Molecular Liquids 219 (2016) 1078–1088
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Corrosion inhibition behavior of new synthesized nonionic surfactants based on amino acid on carbon steel in acid media A.M. Al-Sabagh, N.M. Nasser, Olfat E. El-Azabawy, Amira E. El- Tabey ⁎ Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt
a r t i c l e
i n f o
Article history: Received 18 October 2015 Accepted 17 March 2016 Available online 29 April 2016 Keywords: Corrosion inhibition Amino acid Carbon steel HCl EIS
a b s t r a c t Three structurally different amino acids (Leucine, phenyl alanine and methionine) were reacted with oleic acid and polyethylene glycol (600) to give three nonionic surfactants (S1, S2 and S3). The chemical structures of the prepared surfactants were confirmed by FT-IR and 1H, 13CNMR. The surface parameters were calculated by surface tension measurements. The inhibition efficiencies of the prepared inhibitors on carbon steel in 1 M HCl were studied using polarization, and electrochemical impedance spectroscopy. Correlation between the quantum chemical calculations and inhibition efficiency was discussed. From the obtained results, the inhibitor based on L-phenyl alanine has high efficiency (94.14% at 1 × 10−3 mol L−1) more than the other inhibitors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Corrosion is worth investigating in oilfield applications, because corrosion problems represent a large portion of the total costs for oil producing companies every year worldwide. Corrosion in oilfields occurs at all stages from downhole to surface equipment and processing facilities. It appears as leaks in tanks, casings, tubing, pipelines, and other equipment [1]. Corrosion problems are usually connected with operating problems and equipment maintenance, leading to recurrent partial and even total process shutdown, resulting in severe economic losses. Acids are injected into the well during the acidizing stimulation process and cause serious corrosion issues [2–4]. The use of corrosion inhibitors is considered as the most effective method for the protection of many metals and alloys against such acid attack [5–8]. Compounds containing functional groups with high electron density have been found to be very efficient inhibitors against metal corrosion in many environments, and include molecule with heteroatom functional groups (e.g., \\C_O, N_N and \\NR2, SH groups), conjugated bonds and aromatic systems [9–15]. The efficiency of a corrosion inhibitor depends strongly on its adsorption on the metal surface. Adsorption depends on the nature and the state of the metal surface, on the type of corrosive medium and on the chemical structure of the inhibitor [16,17]. Recently, the application of surfactants as effective corrosion inhibitors has attracted the attention of many researchers. The surfactant inhibitors have many advantages such as high inhibition efficiency, low price, low toxicity and easy production [18–23]. The adsorption of the surfactant on the metal surface can markedly change ⁎ Corresponding author. E-mail address: [email protected] (A.E.E.- Tabey).
http://dx.doi.org/10.1016/j.molliq.2016.03.048 0167-7322/© 2016 Elsevier B.V. All rights reserved.
the corrosion-resisting property of the metal [24]. The aim of the present study was to investigate the corrosion efficiency of some new nonionic surfactants based on the amino acid in acidic media using electrochemical polarization measurements. The quantum parameters calculated, we can mention the energies of HOMO, LUMO and energy gap (ΔE = ELUMO − EHOMO). 2. Experimental 2.1. Synthesis of the nonionic inhibitors 2.1.1. Synthesis of the amide compounds from amino acids 0.15 mol of different amino acid dissolved in xylene were treated by 1 mol of NaOH with continuous stirring, heating and refluxing for 15 min. Then 0.1 mol of oleic acid was added gradually with 0.1% amount of ptoluene sulfonic acid as a catalyst. The reaction was completed when the water removed from reaction system and it was 0.1 mol. The solvent was distilled off and then the product was dissolved in diethyl ether to remove unreacted of amino acid. 0.1 mol of conc. HCl was added to the amide products in the diethyl ether and the mixture was stirred for 5 min and then, the solvent was distilled off to obtain three amide compounds. 2.1.2. Synthesis of the esters compounds from amide of amino acids Equal molar ration of the produced amide compounds and polyethylene glycol (600) were charged into two-neck flask in presences of xylene as solvent and p-toluene sulfonic acid as a catalyst. The reaction mixture was then heated at 150 °C until the theoretical amount of water was collected in the Dean-Stark tube. The solvent was distilled off, and then the product was purified by washing with the super saturated NaCl solution, then the solvent was distilled off. The chemical structures
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Fig. 1. FT-IR spectra of amide of L-phenyl Alanine.
of the final products were confirmed by FT-IR, elemental analysis and 1 H, 13CNMR spectra. 2.2. Solutions
water and in 1 M HCl were done at 25 °C. All solutions were prepared in bidistilled water with a surface tension equal to 72 mN·m-1 at 25 °C.
2.4. Electrochemical measurements
The aggressive solution, 1 M HCl was prepared by diluting analytical grade HCl (37% wt) with bidistilled water. The concentration range of the synthesized inhibitor varied from 1 × 10−5 to 5 × 10−3 M for corrosion measurements. Bidistilled water was used for preparing the test solutions in all measurements. 2.3. Surface tension measurement Surface parameters such as the CMC, surface tension at the CMC (γCMC), effectiveness (ΠCMC), the maximum surface excess concentration (Γmax) and the minimum surface area per molecule (Amin) at the air/solution interface were determined by surface tension measurements. Surface tension measurements were obtained using Du-Nouy tensiometer (Kruss-K6 type) applying a platinum ring technique. Different concentrations of the synthesized surfactant in double distilled
The potentiodynamic polarization experiments were carried out using a conventional three-electrode electrochemical cell. A carbon steel cylinder which has chemical composition (wt%): 0.21 C, 0.035 Si, 0.025 Mn, 0.082 P, and the remainder is Fe pressed into a Teflon holder acted as a working electrode (WE). The exposed electrode area to the corrosive solution was 0.7 cm2. A saturated calomel electrode (SCE) connected through a salt bridge was used as a reference electrode, while a platinum foil was used as a counter electrode. For each experiment the WE was wet polished with emery paper up to 1200 grade of emery paper. Before each measurement, the electrode was inserted immediately into the glass cell that contained 100 mL of the blank and an investigated inhibitors solutions at open circuit potential (OCP) for 45 min, until a steady state was attained. All polarization curves were recorded by a Voltalab 40 Potentiostat PGZ 301 and a personal computer was used with VoltaMaster 4 software at 25 °C. The potentiodynamic
Fig. 2. FT-IR spectra of Ester of S2.
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Fig. 3. 1HNMR spectra of S1.
polarization measurements were obtained by changing the electrode potential automatically from − 800 to − 300 mV versus SCE with a scan rate of 2 mV s−1 at 25 °C. EIS measurements were carried out in a frequency range of 100 kHz to 10 mHz with a 4 mV sine wave as the excitation signal at open circuit potential.
the following quantum chemical indices such as the energy of highest occupied molecular orbital (EHOMO), the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE = EHOMO − ELUMO) and dipole moment (μ) were considered.
2.5. Quantum chemical study
2.6.1. Confirmation the chemical structures of the prepared compound The chemical structures of prepared surfactants were confirmed by FT-IR, elemental analysis and 13C, 1HNMR. Fig. 1 show the FT-IR of the amide of L-phenyl alanine as representative sample, it is clear that a new beak appears at 1674.54 due to amide group in addition to beak
The molecular structures of the prepared surfactants had been fully geometric optimize by MINDO3 semi-empirical method for organic inhibitors calculation which are implemented in Hyperchem 8.0, [25]
2.6. Result and discussion
Fig. 4. 1HNMR spectra of S2.
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Fig. 5. 1HNMR spectra of S3.
at 2856 and 2940 cm−1due to symmetric and asymmetric methylene group. The FT-IR of ester of amide of amino acid (L-phenyl alanine) shown in Fig. 2, it's clear from this figure anew beak appeared at 1735.23 cm−1 for carbonyl group of the ester linkage and abroad beak at 3423 cm−1 due to hydroxyl group of polyethylene glycol. The 1 HNMR of the three prepared surfactants were reported in Figs. 3, 4 and 5. From the Figs. 3, 4 and 5, the appearance of bands at δ = 8.1 (d) due to proton of amide linkage (\\NH) for S1, S2 and S3 respectively.
According to these figures characteristic bands appeared at δ = 0.80, 0.811 and 0.810 (a) due to terminal (\\CH3) and band at δ = 1.2 (b) for (\\CH2\\)n, of the prepared surfactants (S1, S2 and S3) respectively. In addition to these bands the characteristic band for the double bond of oleic acid appeared at δ = 5.3 (c) for three prepared surfactants, the appearance of these bands justified the formation amide compounds. For confirmation of formation the ester compound the proton which Neighboring to ester group appeared at δ = 4.1(e) for three
Fig. 6. 13CNMR spectra of S1.
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Fig. 7. 13CNMR spectra of S2.
prepared surfactants as shown in Figs.3, 4 and 5. The proton of ethereal linkage (\\CH2C_OCH2\\) and at δ = 3.5, 3.7 and 3.6 (f) for S1, S2 and S3 and 1H of the hydroxyl group at δ = 2.5 (g) for the prepared surfactants. The 13CNMR of the three prepared surfactants (S1, S2 and S3) shown in Figs. 6, 7 and 8, bands at δ = 14.32 and 28.93 (a and b) ppm for 13C of \\CH3 and (\\CH2\\)n of alkyl chain of oleic acid. The 13C of
amide linkage appeared at δ = 173.2 ppm for the three prepared surfactants (S1, S2 and S3). From the spectra it can be also observed that, appearance band at δ = 130.00 and 72.7 ppm (c and f) for 13C of (\\CH_CH\\) of oleic acid and (\\CH2\\O\\CH2\\).The 13C of (\\CH2\\) which attached by hydroxyl group appeared at δ = 63.7 ppm for the three prepared surfactants.
Fig. 8. 13CNMR spectra of S3.
A.M. Al-Sabagh et al. / Journal of Molecular Liquids 219 (2016) 1078–1088 Table 1 The elemental composition of the prepared surfactants. Surfactant code
S1 S2 S3
C%
H%
N%
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Table 3 Thermodynamic parameters of the micellization, the adsorption, and the structural effect for the prepared surfactants.
S%
Cal.
Found
Cal.
Found
Cal.
Found
Cal.
Found
61.41 62.90 59.21
60.59 61.98 58.77
9.72 9.19 9.36
8.55 9.00 8.97
1.43 1.38 1.40
1.23 1.02 1.30
– – 3.22
– – 2.99
Code
ΔGmic (Kj mol−1)
ΔGad. (Kj mol−1)
ΔGmic − ΔGad. (Kj mol−1)
S1 S2 S3
−25.30 −30.69 −27.99
−25.66 −31.16 −28.40
3.6 4.7 4.0
in Table 2. It was found that, effectiveness of the S2 large than the other surfactants. 2.9. Maximum surface excess (Γmax) Maximum surface excess (Γmax) is defined as the effectiveness of adsorption at an interface. The maximum surface excess concentration of surfactant ions, Γmax, was calculated from the slope of the straight line in the surface tension plot (dγ/d lnC) below CMC, using appropriate form of Gibbs adsorption equation [28]: Γ max ¼ −ð1=nRTÞðdγ=d lnCÞ Fig. 9. Variation of the surface tension with the prepared surfactants concentrations in water at 25 °C.
An elemental analysis (N, S, C and H) for the prepared surfactants (S1, S2 and S3) was carried out and the data are listed in Table 1. The results are comparable to theoretical values, which gives another evidence for the formation of the amide and esters. 2.7. Surface activity of the prepared compounds
ð2Þ
where Γmax is the maximum surface excess concentration of surfactant ions, R is the gas constant, T is the absolute temperature, C is the concentration of surfactant, γ is the surface tension at given concentration and n is the number of species ions in solution. The values of maximum surface excess concentration were calculated from Fig. 9 and listed in Table 2, it was found that the lowest maximum surface excess concentration achieved by S2 due to hydrophobic effect of carbon chain. 2.10. Minimum area per molecule (Amin)
2.7.1. The surface tension (γ) and critical micelle concentration (CMC) The surface tension (γ) of surfactants was measured for a range of concentrations above and below the critical micelle concentration (CMC). A representative plot of γ versus concentration for S1, S2 and S3 is shown in Fig. 9. Linear decrease in surface tension was observed with the increase of surfactant concentrations. This is a common behavior shown by surfactants in solution and is used to determine their purity and CMC's [26]. The CMC values were obtained from the break point in γ–log C plots are listed in Table (2). For the prepared surfactant the S2 achieved the lowest surface tension. A comparison of CMC for three surfactants demonstrates that, the S2 has the lowest CMC value. 2.8. Effectiveness (ΠCMC) The effectiveness of the surfactant solution (ΠCMC) is defined as the difference between the surface tension at the critical micelle concentration (γCMC) and that for the bi-distilled water (γo) at constant temperature [27]. ΠCMC ¼ γ0 −γCMC
ð1Þ
where γo and γCMC are the surface tensions of pure water and surface tension at CMC, respectively. ΠCMC values were calculated and listed
The minimum surface area per adsorbed molecule, Amin, can be obtained as follows [29]: A min ¼ 1016 =Γ max NA
ð3Þ
Where NA is the Avogadro's number and Γmax (mol m− 2) is the maximum surface excess of adsorbed surfactant molecules at the interface. The values of area per molecule for the prepared surfactants were calculated and listed in Table 2. it was clear, the PEG group constitutes the polar head of these surfactants but the hydrophobic part different according this reason the S2 have the high value of Amin. The thermodynamic parameters values (The free energy of micellization (Δ Gmic), the free energy of adsorption (Δ Gads) and the structural effect (ΔGmic − ΔGads)) were calculated and listed in Table 3 The ΔGmic may be calculated by the equation [30]: ΔGmic ¼ 2:3RTð1−α Þ log CMC
ð4Þ
Where R is the universal gas constant, T is the absolute temperature, α is the fraction of counter ions bound by micelle in case of ionic surfactants (α = 0 for nonionic surfactants) and CMC is the critical micelle concentration in mol L−1. Many investigations, deal with the thermodynamics of surfactant adsorption at the interface [31]. The
Table 2 Surface active properties for the prepared surfactants from surface tension measurements at 25 °C. Surfactants
CMC × 102 (mol L−1)
γCMC (mN·m−1)
ПCMC (mN·m−1)
Гmax × 1010 (mol cm−2)
Amin (A°2)
S1 S2 S3
1.2 0.3 0.6
42 38 40
30 34 32
8.2 7.1 7.8
20.04 23.28 21.21
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(Icorr), cathodic and anodic Tafel slopes (βc, βa) were derived from polarization curve and listed in Table 4 The protection efficiency can also be calculated from the electrochemical relation [34,35]:
Fig. 10. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S1 inhibitor at 25 °C.
thermodynamic parameters value of adsorption ΔGads were calculated via the following equation: ΔGads ¼ ΔGmic −0:6023 ΠCMC A min
ð5Þ
From Table (3) it can be found that, the free energy of micellization (Δ Gmic) and the free energy of adsorption (Δ Gads) less than zero (ΔGmic b 0), (ΔGads b 0) which means that, the micellization and adsorption are a spontaneous processes. It is also found from Table 3 that, the negativity values of ΔGmic and ΔGads for S2 are higher than the other surfactants. Also the ΔGads values are more negative than the ΔGmic values. This indicates that, the adsorption at the interface is associated with a decrease in the free energy of the system i.e. the adsorption process is more spontaneous. Also, this indicates that, the studied surfactants favor adsorption than micellization [32]. The values of the structural effect (ΔGmic − ΔGads) for the prepared surfactants are also shown in Table 3. These positive values reflect that, the prepared surfactants are more readily adsorbed at air/aqueous solution interface. This in turn could account for investigating these surfactants as corrosion inhibitors [33]. 2.11. Evaluation the prepared surfactants as corrosion inhibitors 2.11.1. Electrochemical measurements Figs. 10–12 show typical polarization curves for carbon steel in 1 M HCl solution in the absence and presence of different concentrations of the prepared inhibitors. The electrochemical parameters of the corrosion process, i.e. corrosion potential (Ecorr), corrosion current density
Fig. 11. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S2 inhibitor at 25 °C.
ηp % ¼ ½fIcorr −Icorr ðinhÞg=Icorr x 100
ð6Þ
θ ¼ fIcorr −Icorr ðinhÞg=Icorr
ð7Þ
where Icorr and Icorr(inh) are the corrosion current density values without and with inhibitor, respectively, determined by extrapolation of cathodic and anodic Tafel lines to the corrosion potentials. From Figs. 10-12 and Table 4 it can observed that, Both anodic and cathodic reactions of carbon steel electrode in 1.0 M HCl were inhibited after addition different concentrations of the prepared inhibitors and the inhibition efficiencies increase with increasing the inhibitors concentration, This reveals that the addition of inhibitor reduces anodic dissolution in addition to the retardation of the hydrogen ions reduction (i.e., hydrogen evolution) [36]. Because prepared inhibitors exhibited obvious cathodic and anodic inhibition effects, it could be concluded that the prepared inhibitors mainly acted as a mixed-type inhibitor for steel in 1.0 M HCl. From the data in Table 4, it can observed that the inhibitor which based on phenyl alanine exhibited high efficiency more than the other inhibitors this may due to presences of phenyl ring in its structure. The high efficiencies is attributed to the formed protective film of the inhibitors, which tends to be more complete and stable on carbon steel surface at the higher inhibitor concentrations (i.e., 1 × 10−3). The corrosion behavior of carbon steel in 1.0 M HCl solution was been also investigated in the absence and presence of different concentration of the prepared inhibitors after immersion 1 h by the electrochemical impedance spectroscopy (EIS). Nyquist plots of carbon steel in uninhibited and inhibited acidic solutions (1.0 M HCl) containing various concentrations of the prepared inhibitors were given in Figs. 13-15. The characteristic parameters associated to the impedance diagram (Rt and Cdl) and (ηI %) are given in Table 5. In the case of impedance study, ηI % is calculated using Rt as following equation [37]: ηI % ¼ ½fRct ðinhÞ−Rct g=RctðinhÞ x 100
ð8Þ
Where Rct and Rct (inh) are the charge transfer resistance values without and with the inhibitor, respectively. From the Figs.13–15 and Table 5, it can observed that, addition of synthesized inhibitors provided lower Cdl values, probably as a consequence of replacement of water molecules by inhibitor molecule at the electrode surface. Cdl values decrease with increasing the concentration this may due to a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules acted
Fig. 12. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S3 inhibitor at 25 °C.
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Table 4 Electrochemical parameters for carbon steel electrode in 1 M HCl in the absence and presence of the nonionic inhibitors at 25 °C. Inhibitors
C (mol L−1)
Ecorr (mV)
Icorr. (mA cm−2)
βa (mV dec−1)
βc (mV dec−1)
θ
ηp (%)
Blank
0.00 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
−525.8 −499.0 −518.0 −517.9 −579.9 −513.5 −514.3 −509.0 −518.1 −524.9 −489.0 −502.0 −552.4 −506.5 −502.5 −512.0
0.258 0.111 0.0971 0.0744 0.0684 0.0580 0.0652 0.0424 0.0360 0.0210 0.0151 0.0811 0.0645 0.0472 0.0396 0.0270
169.3 124.5 129.0 131.3 157.0 155.9 266.7 171.1 194.2 140.3 97.00 120.0 136.0 121.9 123.4 195.9
−212.0 −138.4 −133.0 −223.7 −122.7 −191.0 −161.6 −127.8 −140.2 −141.3 −112.0 −135.0 −129.8 −183.6 −185.3 −123.5
– 0.5671 0.6247 0.7121 0.7353 0.7756 0.7477 0.8359 0.8607 0.9187 0.9419 0.6905 0.7504 0.8174 0.8468 0.8955
– 56.71 62.47 71.21 73.53 77.56 74.77 83.59 86.07 91.87 94.19 69.05 75.04 81.74 84.68 89.55
S1
S2
S3
by adsorption at the metal/solution interface. From Table 5 it can also observed the charge transfer resistance values increase with increasing of the concentration of the prepared inhibitors [38–40]. This indicates that, these compounds were acting as adsorption inhibitor, phenyl alanine was achieved high efficiency (94%) compared by another inhibitors this may due to presence the phenyl group. 2.12. Adsorption isotherm The adsorption isotherm can be determined by assuming that inhibition effect is due mainly to the adsorption at metal/solution interface. Basic information on the adsorption of inhibitors on the metal surface can be provided by adsorption isotherm. In order to obtain the isotherm, the fractional surface coverage values (θ) as a function of inhibitor concentration must be obtained. The values of (θ) can be easily determined from the polarization measurements by the ratio ηp %/100, where ηp % is inhibition efficiency obtained by electrochemical method. So it is necessary to determine empirically which isotherm fits best to the adsorption of inhibitors on the carbon steel surface. Several adsorption isotherms (viz., Frumkin, Langmuir, Temkin, and Freundlich) were tested and the Langmuir adsorption isotherm was found to provide the best description of the adsorption behavior of the studied inhibitors. The Langmuir isotherm is given by following equation [41]: C=θ ¼ 1=Kads þ C
ð9Þ
Fig. 13. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S1 at 25 °C.
where K is the equilibrium constant of the adsorption process and C is the molar concentration of inhibitor. The plot of log (C/θ v C) versus log C gave a straight line as shown in Fig. 16. The linear regression coefficients (R2) are almost equal to 0.999 near to unity, confirming that the adsorption of studied inhibitors in 1.0 M HCl solution follows the Langmuir's adsorption isotherm. The free energy of adsorption (ΔGads) is related to the adsorption constant (K) with following equation [42,43]: ΔGads ¼ ‐RT ln ð55:5Kads Þ
ð10Þ
where the value 55.5 is the concentration of water in solution expressed in mol L−1. The values of K and (ΔGads) were calculated at 25 °C and are listed in Table 6. The large negative values of (ΔGads) confirmed the spontaneity of the adsorption process and stability of the adsorbed layer on the carbon steel surface. Moreover, the high value of (ΔGads) obtained for inhibitor based on L-phenyl alanine (S3) indicated that this compound is more strongly adsorbed on the carbon steel surface in 1.0 M HCl than the other inhibitors. This is in good agreement with the range of the inhibition efficiency values obtained from electrochemical techniques. It is well known that values of (ΔGads) of the order of −20 kJ mol−1 or lower indicate a physisorption while those more negative than −40 kJ mol−1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption). As can be seen in Table 6, the (ΔGads) value for S2 is − 29.07 kJ mol−1 clearly indicates its physical adsorption on the carbon steel surface [44].
Fig. 14. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S2 at 25 °C.
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Fig. 15. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S2 at 25 °C.
2.13. The Relation between corrosion inhibition and surface properties of the prepared surfactants Corrosion inhibition mechanism of the surfactants depends on their ability to adsorb on the corroding surface, forming a protective layer. So, the CMC considers a key factor in determining the effectiveness of surfactants as corrosion inhibitors. This could be attributed to the fact that above CMC the surface of steel is covered with a monolayer of surfactant molecules and the additional molecules combine to form micelles in the bulk of solution. On the basis of this view, among the studied surfactant (S2) which shows the lowest CMC (0.3 × 10−2) value and hence it considers the most effective corrosion inhibitor for steel. For the prepared nonionic surfactants, the greatest reduction of surface tension (effectiveness, ΠCMC) was achieved by S2 compared with that obtained by the other two surfactants. This is in good agreement with the inhibition efficiency results achieved by S2 Tables 2 and 3. It seems that the synthesized surfactants favor adsorption rather than micellization. The fact that Δ Gads is more negative compared with the corresponding Δ Gmic may be taken as a strong evidence for the more feasibility of the adsorption of the synthesized surfactants. It is noticed that Гmax of both S1 and S3 is higher than Гmax of S2. On the other hand, Amin values of both S1 and S3 are lower than that of S2. Considering these facts one can explains why S2 is more effective than the other inhibitors. The high value of Amin for S2 and low value of Гmax indicates that less numbers of molecules are adsorbed. This implies close packing of the adsorbed molecules associated with a high area on the metal surface for each molecule thus leading to more electrostatic Table 5 EIS parameters for the corrosion of carbon steel in 1 M HCl in the absence and presences of different concentrations of the prepared cationic inhibitors at 25 °C. Inhibitors
C (mol L−1)
Rs (ohm cm2)
Rct (ohm cm2)
Cdl (μF cm−2)
ηI (%)
Blank
0.00 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
1.85 0.534 0.284 0.746 0.732 0.831 0.850 0.740 0.350 0.190 0.441 0.746 0.881 0.600 0.742 1.16
114.2 294.9 307.0 394.0 461.6 486.5 463.0 755.7 1.221 1434 2274 375.0 465.1 570.1 799.7 1135
69.66 34.10 25.91 20.17 17.27 16.35 17.12 9.450 6.660 5.545 3.490 20.17 17.12 13.95 9.450 7.00
– 61.27 62.8 71.03 75.2 76.52 75.33 84.88 88.8 92 94.4 69.54 75.5 79.97 85.72 89.93
S1
S2
S3
Fig. 16. Langmuir isotherm adsorption model of inhibitors (S1, S2 and S3) on the carbon steel surface in 1 M HCl at 25 °C.
interaction of the well packed adsorbed layer and more homogenous adsorbed film. All these parameters explain why S2 is the most effective inhibitor. 2.14. Quantum chemical calculation The fully optimized minimum energy geometrical configuration of the prepared inhibitors is shown in Figs. 17a, b and c using optimize by MINDO3 semi-empirical method for organic inhibitors calculation which are implemented in Hyperchem 8.0,. Table (7) presented the calculated energy levels of the HOMO, LUMO, ΔE = ELUMO − EHOMO, and the dipole moment. The higher the HOMO energy of the inhibitor (less negative values), the greater the trend of offering electrons to the unoccupied d orbital of the metal and the higher the corrosion inhibition efficiency for iron in HCl acid solutions [45,46]. From Table 7 and Fig. 17 it clear that the inhibitor (S2) have the higher EHOMO value (− 7.9) than the other inhibitors (S1 and S3). In addition, the lower the LUMO energy, the easier the acceptance of electrons from the metal surface. In other words, the inhibition efficiency increases if the compound can donate electrons from its HOMO to the LUMO of the metal, whereby chelation on the metal surface occurs. Also, as the energy gap (ΔE) decreases, the efficiency of the inhibitor is improved. The order of decreasing the ELUMO values and the energy gap (ΔE) with increasing the inhibition efficiency for the three prepared surfactants is (0.21 b 0.24 b 0.25), (8.11 b 9.05 b 9.15) for inhibition efficiency (94.4 ˃ 89.93 ˃ 76.5%). The dipole moment (μ) is another important parameter of the electronic distribution in a molecule. It is related to the hydrophobic character of the molecules [47]. The low values of the dipole moment will favor the accumulation of inhibitor molecules on the metallic surface thus increasing the inhibition efficiency [48]. From Table 7 it can be observed that, the inhibitor (S2) have the lowest value of dipole moment. 3. Conclusion 1. Three nonionic surfactants (S1, S2 and S3) based on amino acids (Leucine, phenyl alanine and methionine) were prepared. 2. The chemical structures of the prepared surfactants were confirmed by FT-IR, elemental analysis and 1H, 13C NMR. Table 6 Adsorption parameters of the prepared inhibitors for carbon steel in 1 M HCl at 25 °C. Inhibitors S1 S2 S3
R2 0.9994 0.9999 0.9993
Kads (mol L−1) −5
2 × 10 8 × 10−6 1 × 10−5
ΔGads (kJ mol−1) −26.80 −29.07 −28.52
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Fig. 17. Molecular structure and HOMO–LUMO for prepared inhibitors. Table 7 Quantum chemical parameters of the investigated inhibitors. Inhibitors
EHOMO, (eV)
ELUMO, (eV)
ΔE = (ELUMO − EHOMO), (eV)
Dipole momentum (Debye)
Number of transferred electrons, ΔN
S1 S2 S3
−8.9 −7.9 −8.8
0.25 0.21 0.24
9.15 8.11 9.04
3.5 2.75 3.15
0.29 0.38 0.30
3. The surface tension measurements will be carried at 25 °C. 4. The corrosion behavior of the prepared surfactants will be studied. 5. The surfactants based on L-phenyl alanine have high corrosion efficiency. 6. The corrosion efficiency increases with increasing concentrations of surfactants. References [1] A.M. Al-Sabagh, N.M. Nasser, N.Gh. Kandile, M.A. Migahed, J.Dispers. Sci. Technol. 29 (2008) (2008) 161. [2] H.B. Fan, C.Y. Fu, H.L. Wang, X.P. Guo, J.S. Zheng, Br. Corros. J. 37 (2002) 122. [3] M.A. Migahed, I.F. Nassar, Electrochim. Acta 53 (2008) 2877. [4] J. Aljourani, K. Raeissi, M.A. Golozar, Corros. Sci. 51 (2009) 1836. [5] M. Mihit, S. El Issami, M. Bouklah, L. Bazzi, B. Hammouti, E. Ait Addi, R. Salghi, S. Kertit, Appl. Surf. Sci. 252 (2006) 2389. [6] M. Mihit, R. Salghi, S. El Issami, L. Bazzi, B. Hammouti, E. Ait Addi, S. Kertit, Pigm. Resin Technol 35 (2006) 151.
[7] A. Dafali, B. Hammouti, R. Mokhlisse, S. Kertit, Corros. Sci. 45 (2003) 1619. [8] S. Kertit, R. Salghi, L. Bazzi, B. Hammouti, A. Bouchtart, Ann. Chim. Sci. Mater. 25 (2000) 187. [9] M.G. Hosseini, M. Ehteshamzadeh, T. Shahrabi, Electrochim. Acta 52 (2007) 3680. [10] B. Obat, N.O. Obi-Egbedi, Corros. Sci. 52 (2010) 198. [11] L. Wang, Corros. Sci. 48 (2006) 608. [12] M. Bouklah, B. Hammouti, M. Lagrenee, F. Bentiss, Corros. Sci. 48 (2006) 2831. [13] M. Bouklah, A. Ouassini, B. Hammouti, A. ElIdrissi, Appl. Surf. Sci. 252 (2006) 2178. [14] I. Dehri, M. Ozcan, Mater. Chem. Phys. 98 (2006) 316. [15] N.O. Eddy, S.R. Stoyanov, E.E. Ebenso, Int. J. Electrochem. Sci. 5 (2010) 1127. [16] E. Sadeghi Meresht, T. Shahrabi Farahani, J. Neshati, Corros. Sci. 54 (2012) 36. [17] M. Gopiraman, N. Selvakumaran, D. Kesavan, R. Karvembu, Prog. Org. Coat. 73 (2012) 104. [18] D.N. Singh, A.K. Dey, Corrosion 49 (1993) 594. [19] G. Banerjee, S.N. Malhotra, Corrosion 48 (1992) 10. [20] S.T. Arab, E.A. Noor, Corrosion 49 (1993) 122. [21] L.A. Raspi, Nickel Corros. 49 (1993) 821. [22] Y. Chen, Y. Wang, G. Zhang, Daily Chem. Ind. 2 (1986) 56. [23] L. Shi, H. Song, Daily Chem. Ind. 1 (1987) 9. [24] M.A. Hegazy, A.S. El-Tabei, A.H. Bedair, M.A. Sadeq, Corros. Sci. 54 (2012) 219. [25] Shuduan Deng, Xianghong Li, Corros. Sci. 55 (2012) 407.
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[26] D.M. Small, The Physical Chemistry of Lipids: From Alkanes to Phospholipids, 4Plenum Press, New York, 1986. [27] S.M.I. Morsy, Egypt. J. Petrol. 14 (2005) 9. [28] F.K.G. Santos, E.L.B. Neto, M.C.P. Moura, T.N.C. Dantas, A.A.D. Neto, J. Colloids and Surfaces A: Physicochem. Eng. Aspects 333 (2009) 156. [29] S.M. Hamid, D.C. Sherrington, J. Brit. Polym. 16 (1984) 39. [30] Austin Samakande, Radhouane Chaghi, Gaelle Derrien, Clarence Charnay, Patrice C. Hartmann, J. Colloid Interface Sci. 320 (2008) 315. [31] A.M. Al-Sabagh, J. Poly. Adv. Technol. 11 (2000) 465. [32] A.M. AL-Sabagh, M. Elsabee, O.E. Elazabawy, Amira.E. Eltabey, J. Dispers. Sci. Technol. 31 (2010) 5. [33] A.M. Al-Sabagh, D.R.K. Harding, N.G. Kandile, A.M. Badawi, Amira E. El- Tabey, J. Dispers. Sci. Technol. 30 (2009) 472. [34] M.A. Ameer, E. Khamis, G. Al-Senani, J. Appl. Electrochem. 32 (2002) 149. [35] M. Bouklah, N. Benchat, A. Aouniti, B. Hammouti, M. Benkdour, M. Lagrenee, H. Vezin, F. Bentiss, J. Prog. Org. Coat. 42 (2004) 123.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Corrosion inhibition behavior of new synthesized nonionic surfactants based on amino acid on carbon steel in acid media A.M. Al-Sabagh, N.M. Nasser, Olfat E. El-Azabawy, Amira E. El- Tabey ⁎ Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt
a r t i c l e
i n f o
Article history: Received 18 October 2015 Accepted 17 March 2016 Available online 29 April 2016 Keywords: Corrosion inhibition Amino acid Carbon steel HCl EIS
a b s t r a c t Three structurally different amino acids (Leucine, phenyl alanine and methionine) were reacted with oleic acid and polyethylene glycol (600) to give three nonionic surfactants (S1, S2 and S3). The chemical structures of the prepared surfactants were confirmed by FT-IR and 1H, 13CNMR. The surface parameters were calculated by surface tension measurements. The inhibition efficiencies of the prepared inhibitors on carbon steel in 1 M HCl were studied using polarization, and electrochemical impedance spectroscopy. Correlation between the quantum chemical calculations and inhibition efficiency was discussed. From the obtained results, the inhibitor based on L-phenyl alanine has high efficiency (94.14% at 1 × 10−3 mol L−1) more than the other inhibitors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Corrosion is worth investigating in oilfield applications, because corrosion problems represent a large portion of the total costs for oil producing companies every year worldwide. Corrosion in oilfields occurs at all stages from downhole to surface equipment and processing facilities. It appears as leaks in tanks, casings, tubing, pipelines, and other equipment [1]. Corrosion problems are usually connected with operating problems and equipment maintenance, leading to recurrent partial and even total process shutdown, resulting in severe economic losses. Acids are injected into the well during the acidizing stimulation process and cause serious corrosion issues [2–4]. The use of corrosion inhibitors is considered as the most effective method for the protection of many metals and alloys against such acid attack [5–8]. Compounds containing functional groups with high electron density have been found to be very efficient inhibitors against metal corrosion in many environments, and include molecule with heteroatom functional groups (e.g., \\C_O, N_N and \\NR2, SH groups), conjugated bonds and aromatic systems [9–15]. The efficiency of a corrosion inhibitor depends strongly on its adsorption on the metal surface. Adsorption depends on the nature and the state of the metal surface, on the type of corrosive medium and on the chemical structure of the inhibitor [16,17]. Recently, the application of surfactants as effective corrosion inhibitors has attracted the attention of many researchers. The surfactant inhibitors have many advantages such as high inhibition efficiency, low price, low toxicity and easy production [18–23]. The adsorption of the surfactant on the metal surface can markedly change ⁎ Corresponding author. E-mail address: [email protected] (A.E.E.- Tabey).
http://dx.doi.org/10.1016/j.molliq.2016.03.048 0167-7322/© 2016 Elsevier B.V. All rights reserved.
the corrosion-resisting property of the metal [24]. The aim of the present study was to investigate the corrosion efficiency of some new nonionic surfactants based on the amino acid in acidic media using electrochemical polarization measurements. The quantum parameters calculated, we can mention the energies of HOMO, LUMO and energy gap (ΔE = ELUMO − EHOMO). 2. Experimental 2.1. Synthesis of the nonionic inhibitors 2.1.1. Synthesis of the amide compounds from amino acids 0.15 mol of different amino acid dissolved in xylene were treated by 1 mol of NaOH with continuous stirring, heating and refluxing for 15 min. Then 0.1 mol of oleic acid was added gradually with 0.1% amount of ptoluene sulfonic acid as a catalyst. The reaction was completed when the water removed from reaction system and it was 0.1 mol. The solvent was distilled off and then the product was dissolved in diethyl ether to remove unreacted of amino acid. 0.1 mol of conc. HCl was added to the amide products in the diethyl ether and the mixture was stirred for 5 min and then, the solvent was distilled off to obtain three amide compounds. 2.1.2. Synthesis of the esters compounds from amide of amino acids Equal molar ration of the produced amide compounds and polyethylene glycol (600) were charged into two-neck flask in presences of xylene as solvent and p-toluene sulfonic acid as a catalyst. The reaction mixture was then heated at 150 °C until the theoretical amount of water was collected in the Dean-Stark tube. The solvent was distilled off, and then the product was purified by washing with the super saturated NaCl solution, then the solvent was distilled off. The chemical structures
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Fig. 1. FT-IR spectra of amide of L-phenyl Alanine.
of the final products were confirmed by FT-IR, elemental analysis and 1 H, 13CNMR spectra. 2.2. Solutions
water and in 1 M HCl were done at 25 °C. All solutions were prepared in bidistilled water with a surface tension equal to 72 mN·m-1 at 25 °C.
2.4. Electrochemical measurements
The aggressive solution, 1 M HCl was prepared by diluting analytical grade HCl (37% wt) with bidistilled water. The concentration range of the synthesized inhibitor varied from 1 × 10−5 to 5 × 10−3 M for corrosion measurements. Bidistilled water was used for preparing the test solutions in all measurements. 2.3. Surface tension measurement Surface parameters such as the CMC, surface tension at the CMC (γCMC), effectiveness (ΠCMC), the maximum surface excess concentration (Γmax) and the minimum surface area per molecule (Amin) at the air/solution interface were determined by surface tension measurements. Surface tension measurements were obtained using Du-Nouy tensiometer (Kruss-K6 type) applying a platinum ring technique. Different concentrations of the synthesized surfactant in double distilled
The potentiodynamic polarization experiments were carried out using a conventional three-electrode electrochemical cell. A carbon steel cylinder which has chemical composition (wt%): 0.21 C, 0.035 Si, 0.025 Mn, 0.082 P, and the remainder is Fe pressed into a Teflon holder acted as a working electrode (WE). The exposed electrode area to the corrosive solution was 0.7 cm2. A saturated calomel electrode (SCE) connected through a salt bridge was used as a reference electrode, while a platinum foil was used as a counter electrode. For each experiment the WE was wet polished with emery paper up to 1200 grade of emery paper. Before each measurement, the electrode was inserted immediately into the glass cell that contained 100 mL of the blank and an investigated inhibitors solutions at open circuit potential (OCP) for 45 min, until a steady state was attained. All polarization curves were recorded by a Voltalab 40 Potentiostat PGZ 301 and a personal computer was used with VoltaMaster 4 software at 25 °C. The potentiodynamic
Fig. 2. FT-IR spectra of Ester of S2.
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Fig. 3. 1HNMR spectra of S1.
polarization measurements were obtained by changing the electrode potential automatically from − 800 to − 300 mV versus SCE with a scan rate of 2 mV s−1 at 25 °C. EIS measurements were carried out in a frequency range of 100 kHz to 10 mHz with a 4 mV sine wave as the excitation signal at open circuit potential.
the following quantum chemical indices such as the energy of highest occupied molecular orbital (EHOMO), the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE = EHOMO − ELUMO) and dipole moment (μ) were considered.
2.5. Quantum chemical study
2.6.1. Confirmation the chemical structures of the prepared compound The chemical structures of prepared surfactants were confirmed by FT-IR, elemental analysis and 13C, 1HNMR. Fig. 1 show the FT-IR of the amide of L-phenyl alanine as representative sample, it is clear that a new beak appears at 1674.54 due to amide group in addition to beak
The molecular structures of the prepared surfactants had been fully geometric optimize by MINDO3 semi-empirical method for organic inhibitors calculation which are implemented in Hyperchem 8.0, [25]
2.6. Result and discussion
Fig. 4. 1HNMR spectra of S2.
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Fig. 5. 1HNMR spectra of S3.
at 2856 and 2940 cm−1due to symmetric and asymmetric methylene group. The FT-IR of ester of amide of amino acid (L-phenyl alanine) shown in Fig. 2, it's clear from this figure anew beak appeared at 1735.23 cm−1 for carbonyl group of the ester linkage and abroad beak at 3423 cm−1 due to hydroxyl group of polyethylene glycol. The 1 HNMR of the three prepared surfactants were reported in Figs. 3, 4 and 5. From the Figs. 3, 4 and 5, the appearance of bands at δ = 8.1 (d) due to proton of amide linkage (\\NH) for S1, S2 and S3 respectively.
According to these figures characteristic bands appeared at δ = 0.80, 0.811 and 0.810 (a) due to terminal (\\CH3) and band at δ = 1.2 (b) for (\\CH2\\)n, of the prepared surfactants (S1, S2 and S3) respectively. In addition to these bands the characteristic band for the double bond of oleic acid appeared at δ = 5.3 (c) for three prepared surfactants, the appearance of these bands justified the formation amide compounds. For confirmation of formation the ester compound the proton which Neighboring to ester group appeared at δ = 4.1(e) for three
Fig. 6. 13CNMR spectra of S1.
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Fig. 7. 13CNMR spectra of S2.
prepared surfactants as shown in Figs.3, 4 and 5. The proton of ethereal linkage (\\CH2C_OCH2\\) and at δ = 3.5, 3.7 and 3.6 (f) for S1, S2 and S3 and 1H of the hydroxyl group at δ = 2.5 (g) for the prepared surfactants. The 13CNMR of the three prepared surfactants (S1, S2 and S3) shown in Figs. 6, 7 and 8, bands at δ = 14.32 and 28.93 (a and b) ppm for 13C of \\CH3 and (\\CH2\\)n of alkyl chain of oleic acid. The 13C of
amide linkage appeared at δ = 173.2 ppm for the three prepared surfactants (S1, S2 and S3). From the spectra it can be also observed that, appearance band at δ = 130.00 and 72.7 ppm (c and f) for 13C of (\\CH_CH\\) of oleic acid and (\\CH2\\O\\CH2\\).The 13C of (\\CH2\\) which attached by hydroxyl group appeared at δ = 63.7 ppm for the three prepared surfactants.
Fig. 8. 13CNMR spectra of S3.
A.M. Al-Sabagh et al. / Journal of Molecular Liquids 219 (2016) 1078–1088 Table 1 The elemental composition of the prepared surfactants. Surfactant code
S1 S2 S3
C%
H%
N%
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Table 3 Thermodynamic parameters of the micellization, the adsorption, and the structural effect for the prepared surfactants.
S%
Cal.
Found
Cal.
Found
Cal.
Found
Cal.
Found
61.41 62.90 59.21
60.59 61.98 58.77
9.72 9.19 9.36
8.55 9.00 8.97
1.43 1.38 1.40
1.23 1.02 1.30
– – 3.22
– – 2.99
Code
ΔGmic (Kj mol−1)
ΔGad. (Kj mol−1)
ΔGmic − ΔGad. (Kj mol−1)
S1 S2 S3
−25.30 −30.69 −27.99
−25.66 −31.16 −28.40
3.6 4.7 4.0
in Table 2. It was found that, effectiveness of the S2 large than the other surfactants. 2.9. Maximum surface excess (Γmax) Maximum surface excess (Γmax) is defined as the effectiveness of adsorption at an interface. The maximum surface excess concentration of surfactant ions, Γmax, was calculated from the slope of the straight line in the surface tension plot (dγ/d lnC) below CMC, using appropriate form of Gibbs adsorption equation [28]: Γ max ¼ −ð1=nRTÞðdγ=d lnCÞ Fig. 9. Variation of the surface tension with the prepared surfactants concentrations in water at 25 °C.
An elemental analysis (N, S, C and H) for the prepared surfactants (S1, S2 and S3) was carried out and the data are listed in Table 1. The results are comparable to theoretical values, which gives another evidence for the formation of the amide and esters. 2.7. Surface activity of the prepared compounds
ð2Þ
where Γmax is the maximum surface excess concentration of surfactant ions, R is the gas constant, T is the absolute temperature, C is the concentration of surfactant, γ is the surface tension at given concentration and n is the number of species ions in solution. The values of maximum surface excess concentration were calculated from Fig. 9 and listed in Table 2, it was found that the lowest maximum surface excess concentration achieved by S2 due to hydrophobic effect of carbon chain. 2.10. Minimum area per molecule (Amin)
2.7.1. The surface tension (γ) and critical micelle concentration (CMC) The surface tension (γ) of surfactants was measured for a range of concentrations above and below the critical micelle concentration (CMC). A representative plot of γ versus concentration for S1, S2 and S3 is shown in Fig. 9. Linear decrease in surface tension was observed with the increase of surfactant concentrations. This is a common behavior shown by surfactants in solution and is used to determine their purity and CMC's [26]. The CMC values were obtained from the break point in γ–log C plots are listed in Table (2). For the prepared surfactant the S2 achieved the lowest surface tension. A comparison of CMC for three surfactants demonstrates that, the S2 has the lowest CMC value. 2.8. Effectiveness (ΠCMC) The effectiveness of the surfactant solution (ΠCMC) is defined as the difference between the surface tension at the critical micelle concentration (γCMC) and that for the bi-distilled water (γo) at constant temperature [27]. ΠCMC ¼ γ0 −γCMC
ð1Þ
where γo and γCMC are the surface tensions of pure water and surface tension at CMC, respectively. ΠCMC values were calculated and listed
The minimum surface area per adsorbed molecule, Amin, can be obtained as follows [29]: A min ¼ 1016 =Γ max NA
ð3Þ
Where NA is the Avogadro's number and Γmax (mol m− 2) is the maximum surface excess of adsorbed surfactant molecules at the interface. The values of area per molecule for the prepared surfactants were calculated and listed in Table 2. it was clear, the PEG group constitutes the polar head of these surfactants but the hydrophobic part different according this reason the S2 have the high value of Amin. The thermodynamic parameters values (The free energy of micellization (Δ Gmic), the free energy of adsorption (Δ Gads) and the structural effect (ΔGmic − ΔGads)) were calculated and listed in Table 3 The ΔGmic may be calculated by the equation [30]: ΔGmic ¼ 2:3RTð1−α Þ log CMC
ð4Þ
Where R is the universal gas constant, T is the absolute temperature, α is the fraction of counter ions bound by micelle in case of ionic surfactants (α = 0 for nonionic surfactants) and CMC is the critical micelle concentration in mol L−1. Many investigations, deal with the thermodynamics of surfactant adsorption at the interface [31]. The
Table 2 Surface active properties for the prepared surfactants from surface tension measurements at 25 °C. Surfactants
CMC × 102 (mol L−1)
γCMC (mN·m−1)
ПCMC (mN·m−1)
Гmax × 1010 (mol cm−2)
Amin (A°2)
S1 S2 S3
1.2 0.3 0.6
42 38 40
30 34 32
8.2 7.1 7.8
20.04 23.28 21.21
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(Icorr), cathodic and anodic Tafel slopes (βc, βa) were derived from polarization curve and listed in Table 4 The protection efficiency can also be calculated from the electrochemical relation [34,35]:
Fig. 10. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S1 inhibitor at 25 °C.
thermodynamic parameters value of adsorption ΔGads were calculated via the following equation: ΔGads ¼ ΔGmic −0:6023 ΠCMC A min
ð5Þ
From Table (3) it can be found that, the free energy of micellization (Δ Gmic) and the free energy of adsorption (Δ Gads) less than zero (ΔGmic b 0), (ΔGads b 0) which means that, the micellization and adsorption are a spontaneous processes. It is also found from Table 3 that, the negativity values of ΔGmic and ΔGads for S2 are higher than the other surfactants. Also the ΔGads values are more negative than the ΔGmic values. This indicates that, the adsorption at the interface is associated with a decrease in the free energy of the system i.e. the adsorption process is more spontaneous. Also, this indicates that, the studied surfactants favor adsorption than micellization [32]. The values of the structural effect (ΔGmic − ΔGads) for the prepared surfactants are also shown in Table 3. These positive values reflect that, the prepared surfactants are more readily adsorbed at air/aqueous solution interface. This in turn could account for investigating these surfactants as corrosion inhibitors [33]. 2.11. Evaluation the prepared surfactants as corrosion inhibitors 2.11.1. Electrochemical measurements Figs. 10–12 show typical polarization curves for carbon steel in 1 M HCl solution in the absence and presence of different concentrations of the prepared inhibitors. The electrochemical parameters of the corrosion process, i.e. corrosion potential (Ecorr), corrosion current density
Fig. 11. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S2 inhibitor at 25 °C.
ηp % ¼ ½fIcorr −Icorr ðinhÞg=Icorr x 100
ð6Þ
θ ¼ fIcorr −Icorr ðinhÞg=Icorr
ð7Þ
where Icorr and Icorr(inh) are the corrosion current density values without and with inhibitor, respectively, determined by extrapolation of cathodic and anodic Tafel lines to the corrosion potentials. From Figs. 10-12 and Table 4 it can observed that, Both anodic and cathodic reactions of carbon steel electrode in 1.0 M HCl were inhibited after addition different concentrations of the prepared inhibitors and the inhibition efficiencies increase with increasing the inhibitors concentration, This reveals that the addition of inhibitor reduces anodic dissolution in addition to the retardation of the hydrogen ions reduction (i.e., hydrogen evolution) [36]. Because prepared inhibitors exhibited obvious cathodic and anodic inhibition effects, it could be concluded that the prepared inhibitors mainly acted as a mixed-type inhibitor for steel in 1.0 M HCl. From the data in Table 4, it can observed that the inhibitor which based on phenyl alanine exhibited high efficiency more than the other inhibitors this may due to presences of phenyl ring in its structure. The high efficiencies is attributed to the formed protective film of the inhibitors, which tends to be more complete and stable on carbon steel surface at the higher inhibitor concentrations (i.e., 1 × 10−3). The corrosion behavior of carbon steel in 1.0 M HCl solution was been also investigated in the absence and presence of different concentration of the prepared inhibitors after immersion 1 h by the electrochemical impedance spectroscopy (EIS). Nyquist plots of carbon steel in uninhibited and inhibited acidic solutions (1.0 M HCl) containing various concentrations of the prepared inhibitors were given in Figs. 13-15. The characteristic parameters associated to the impedance diagram (Rt and Cdl) and (ηI %) are given in Table 5. In the case of impedance study, ηI % is calculated using Rt as following equation [37]: ηI % ¼ ½fRct ðinhÞ−Rct g=RctðinhÞ x 100
ð8Þ
Where Rct and Rct (inh) are the charge transfer resistance values without and with the inhibitor, respectively. From the Figs.13–15 and Table 5, it can observed that, addition of synthesized inhibitors provided lower Cdl values, probably as a consequence of replacement of water molecules by inhibitor molecule at the electrode surface. Cdl values decrease with increasing the concentration this may due to a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules acted
Fig. 12. Polarization curves for the carbon steel in 1 M HCl in the absence and presence of different concentrations of the S3 inhibitor at 25 °C.
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Table 4 Electrochemical parameters for carbon steel electrode in 1 M HCl in the absence and presence of the nonionic inhibitors at 25 °C. Inhibitors
C (mol L−1)
Ecorr (mV)
Icorr. (mA cm−2)
βa (mV dec−1)
βc (mV dec−1)
θ
ηp (%)
Blank
0.00 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
−525.8 −499.0 −518.0 −517.9 −579.9 −513.5 −514.3 −509.0 −518.1 −524.9 −489.0 −502.0 −552.4 −506.5 −502.5 −512.0
0.258 0.111 0.0971 0.0744 0.0684 0.0580 0.0652 0.0424 0.0360 0.0210 0.0151 0.0811 0.0645 0.0472 0.0396 0.0270
169.3 124.5 129.0 131.3 157.0 155.9 266.7 171.1 194.2 140.3 97.00 120.0 136.0 121.9 123.4 195.9
−212.0 −138.4 −133.0 −223.7 −122.7 −191.0 −161.6 −127.8 −140.2 −141.3 −112.0 −135.0 −129.8 −183.6 −185.3 −123.5
– 0.5671 0.6247 0.7121 0.7353 0.7756 0.7477 0.8359 0.8607 0.9187 0.9419 0.6905 0.7504 0.8174 0.8468 0.8955
– 56.71 62.47 71.21 73.53 77.56 74.77 83.59 86.07 91.87 94.19 69.05 75.04 81.74 84.68 89.55
S1
S2
S3
by adsorption at the metal/solution interface. From Table 5 it can also observed the charge transfer resistance values increase with increasing of the concentration of the prepared inhibitors [38–40]. This indicates that, these compounds were acting as adsorption inhibitor, phenyl alanine was achieved high efficiency (94%) compared by another inhibitors this may due to presence the phenyl group. 2.12. Adsorption isotherm The adsorption isotherm can be determined by assuming that inhibition effect is due mainly to the adsorption at metal/solution interface. Basic information on the adsorption of inhibitors on the metal surface can be provided by adsorption isotherm. In order to obtain the isotherm, the fractional surface coverage values (θ) as a function of inhibitor concentration must be obtained. The values of (θ) can be easily determined from the polarization measurements by the ratio ηp %/100, where ηp % is inhibition efficiency obtained by electrochemical method. So it is necessary to determine empirically which isotherm fits best to the adsorption of inhibitors on the carbon steel surface. Several adsorption isotherms (viz., Frumkin, Langmuir, Temkin, and Freundlich) were tested and the Langmuir adsorption isotherm was found to provide the best description of the adsorption behavior of the studied inhibitors. The Langmuir isotherm is given by following equation [41]: C=θ ¼ 1=Kads þ C
ð9Þ
Fig. 13. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S1 at 25 °C.
where K is the equilibrium constant of the adsorption process and C is the molar concentration of inhibitor. The plot of log (C/θ v C) versus log C gave a straight line as shown in Fig. 16. The linear regression coefficients (R2) are almost equal to 0.999 near to unity, confirming that the adsorption of studied inhibitors in 1.0 M HCl solution follows the Langmuir's adsorption isotherm. The free energy of adsorption (ΔGads) is related to the adsorption constant (K) with following equation [42,43]: ΔGads ¼ ‐RT ln ð55:5Kads Þ
ð10Þ
where the value 55.5 is the concentration of water in solution expressed in mol L−1. The values of K and (ΔGads) were calculated at 25 °C and are listed in Table 6. The large negative values of (ΔGads) confirmed the spontaneity of the adsorption process and stability of the adsorbed layer on the carbon steel surface. Moreover, the high value of (ΔGads) obtained for inhibitor based on L-phenyl alanine (S3) indicated that this compound is more strongly adsorbed on the carbon steel surface in 1.0 M HCl than the other inhibitors. This is in good agreement with the range of the inhibition efficiency values obtained from electrochemical techniques. It is well known that values of (ΔGads) of the order of −20 kJ mol−1 or lower indicate a physisorption while those more negative than −40 kJ mol−1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption). As can be seen in Table 6, the (ΔGads) value for S2 is − 29.07 kJ mol−1 clearly indicates its physical adsorption on the carbon steel surface [44].
Fig. 14. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S2 at 25 °C.
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Fig. 15. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of different concentrations of inhibitor S2 at 25 °C.
2.13. The Relation between corrosion inhibition and surface properties of the prepared surfactants Corrosion inhibition mechanism of the surfactants depends on their ability to adsorb on the corroding surface, forming a protective layer. So, the CMC considers a key factor in determining the effectiveness of surfactants as corrosion inhibitors. This could be attributed to the fact that above CMC the surface of steel is covered with a monolayer of surfactant molecules and the additional molecules combine to form micelles in the bulk of solution. On the basis of this view, among the studied surfactant (S2) which shows the lowest CMC (0.3 × 10−2) value and hence it considers the most effective corrosion inhibitor for steel. For the prepared nonionic surfactants, the greatest reduction of surface tension (effectiveness, ΠCMC) was achieved by S2 compared with that obtained by the other two surfactants. This is in good agreement with the inhibition efficiency results achieved by S2 Tables 2 and 3. It seems that the synthesized surfactants favor adsorption rather than micellization. The fact that Δ Gads is more negative compared with the corresponding Δ Gmic may be taken as a strong evidence for the more feasibility of the adsorption of the synthesized surfactants. It is noticed that Гmax of both S1 and S3 is higher than Гmax of S2. On the other hand, Amin values of both S1 and S3 are lower than that of S2. Considering these facts one can explains why S2 is more effective than the other inhibitors. The high value of Amin for S2 and low value of Гmax indicates that less numbers of molecules are adsorbed. This implies close packing of the adsorbed molecules associated with a high area on the metal surface for each molecule thus leading to more electrostatic Table 5 EIS parameters for the corrosion of carbon steel in 1 M HCl in the absence and presences of different concentrations of the prepared cationic inhibitors at 25 °C. Inhibitors
C (mol L−1)
Rs (ohm cm2)
Rct (ohm cm2)
Cdl (μF cm−2)
ηI (%)
Blank
0.00 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3 1 × 10−5 5 × 10−5 1 × 10−4 5 × 10−4 1 × 10−3
1.85 0.534 0.284 0.746 0.732 0.831 0.850 0.740 0.350 0.190 0.441 0.746 0.881 0.600 0.742 1.16
114.2 294.9 307.0 394.0 461.6 486.5 463.0 755.7 1.221 1434 2274 375.0 465.1 570.1 799.7 1135
69.66 34.10 25.91 20.17 17.27 16.35 17.12 9.450 6.660 5.545 3.490 20.17 17.12 13.95 9.450 7.00
– 61.27 62.8 71.03 75.2 76.52 75.33 84.88 88.8 92 94.4 69.54 75.5 79.97 85.72 89.93
S1
S2
S3
Fig. 16. Langmuir isotherm adsorption model of inhibitors (S1, S2 and S3) on the carbon steel surface in 1 M HCl at 25 °C.
interaction of the well packed adsorbed layer and more homogenous adsorbed film. All these parameters explain why S2 is the most effective inhibitor. 2.14. Quantum chemical calculation The fully optimized minimum energy geometrical configuration of the prepared inhibitors is shown in Figs. 17a, b and c using optimize by MINDO3 semi-empirical method for organic inhibitors calculation which are implemented in Hyperchem 8.0,. Table (7) presented the calculated energy levels of the HOMO, LUMO, ΔE = ELUMO − EHOMO, and the dipole moment. The higher the HOMO energy of the inhibitor (less negative values), the greater the trend of offering electrons to the unoccupied d orbital of the metal and the higher the corrosion inhibition efficiency for iron in HCl acid solutions [45,46]. From Table 7 and Fig. 17 it clear that the inhibitor (S2) have the higher EHOMO value (− 7.9) than the other inhibitors (S1 and S3). In addition, the lower the LUMO energy, the easier the acceptance of electrons from the metal surface. In other words, the inhibition efficiency increases if the compound can donate electrons from its HOMO to the LUMO of the metal, whereby chelation on the metal surface occurs. Also, as the energy gap (ΔE) decreases, the efficiency of the inhibitor is improved. The order of decreasing the ELUMO values and the energy gap (ΔE) with increasing the inhibition efficiency for the three prepared surfactants is (0.21 b 0.24 b 0.25), (8.11 b 9.05 b 9.15) for inhibition efficiency (94.4 ˃ 89.93 ˃ 76.5%). The dipole moment (μ) is another important parameter of the electronic distribution in a molecule. It is related to the hydrophobic character of the molecules [47]. The low values of the dipole moment will favor the accumulation of inhibitor molecules on the metallic surface thus increasing the inhibition efficiency [48]. From Table 7 it can be observed that, the inhibitor (S2) have the lowest value of dipole moment. 3. Conclusion 1. Three nonionic surfactants (S1, S2 and S3) based on amino acids (Leucine, phenyl alanine and methionine) were prepared. 2. The chemical structures of the prepared surfactants were confirmed by FT-IR, elemental analysis and 1H, 13C NMR. Table 6 Adsorption parameters of the prepared inhibitors for carbon steel in 1 M HCl at 25 °C. Inhibitors S1 S2 S3
R2 0.9994 0.9999 0.9993
Kads (mol L−1) −5
2 × 10 8 × 10−6 1 × 10−5
ΔGads (kJ mol−1) −26.80 −29.07 −28.52
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Fig. 17. Molecular structure and HOMO–LUMO for prepared inhibitors. Table 7 Quantum chemical parameters of the investigated inhibitors. Inhibitors
EHOMO, (eV)
ELUMO, (eV)
ΔE = (ELUMO − EHOMO), (eV)
Dipole momentum (Debye)
Number of transferred electrons, ΔN
S1 S2 S3
−8.9 −7.9 −8.8
0.25 0.21 0.24
9.15 8.11 9.04
3.5 2.75 3.15
0.29 0.38 0.30
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