ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2016, Vol. 52, No. 4, pp. 721–730. © Pleiades Publishing, Ltd., 2016.

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

Adsorption and Inhibition Effect of Novel Cationic Surfactant for Pipelines Carbon Steel in Acidic Solution1 M. Abdallaha, b, Hatem M. Eltassa, M. A. Hegazyc, and H. Ahmedb aChem.

Dept., Fac. of Appl. Sci., Umm Al-Qura University, Makkah Al-Mukarama, Saudi Arabia bChem. Dept., Fac. of Sci, Benha University, Benha, Egypt c Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt e-mail: [email protected] Received August 16, 2015

Abstract—A new cationic surfactant was prepared and examined as an inhibitor for the corrosion of carbon steel in 1.0 M HCl solution using weight loss measurements, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The chemical structure of the prepared cationic surfactants has been proven by FTIR spectra. The inhibiting effect of the cationic surfactant as a result of the formation of the protective layer adsorbed on the carbon steel surface The adsorption of the inhibitor was discussed accordingly to the Langmuir isotherm. Polarization data indicated that the cationic surfactant is a mixed-type inhibitor.. The effect of temperature on the corrosion rate of carbon steel in1. 0 M HCl solution devoid of and containing the novel cationic surfactant was examined and thermodynamic parameters were computed. Some surface parameters were calculated and explained. DOI: 10.1134/S207020511604002X

1. INTRODUCTION Hydrochloric acid solutions are used in oil well stimulation to get rid of undesirable scale and rust from the carbon steel surface. It may cause dangerous corrosion problems in production tubing, downhole tools and coating. The addition of corrosion inhibitors is necessary to diminish the corrosion attack of the acid on tubing and casing materials (carbon steel), inhibitors is added to the acid solution during the acidifying process [1]. Most corrosion inhibitors are organic compounds containing one or more reaction center to facilitate the adsorption process [2]. Specific types of organic inhibitors are the surface active agents or surfactant molecules and exhibit peerless characteristic due to their amphiphilic molecule. So used in the corrosion inhibition of metals and alloys against corrosion attack. Several ionic and nonionic surfactant molecules are used previously as corrosion inhibitors [3–13]. The heteroatom such as nitrogen, sulfur, oxygen and even selenium, and phosphorus present in the chemical structure of the organic compounds plays a serious role in the adsorption operation. The adsorption bond strength is mainly dependent on the electron density and the polarizability of the reaction center. It is observed that the strength of the adsorption of surfactant compounds increases with the increase of molecular weight and the 1 The article is published in the original.

dipole moment. The inhibition efficiency of these compounds increases in the order of: O < N < S < P [14]. The goal of this work is to study the inhibiting effect of the synthesized novel cationic surfactant molecules toward the corrosion of C-steel in 1.0 M HCl solution using chemical technique e. g. weight loss and electrochemical chemical measurements, e.g. potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The effect of increasing temperature on the corrosion of carbon steel in 1.0M HCl solution devoid of and containing different concentrations of cationic surfactant were examined and some thermodynamic parameters were computed and explained. 2. MATERIALS AND EXPERIMENTAL TECHNIQUES 2.1. Synthesis of Inhibitor The cationic surfactant molecule used as a corrosion inhibitor in this study, was synthesized through three steps. In the first step, the quaternization reaction of 1-bromododecane with an appropriate amount of 2-(dimethylamino) ethanol in the molar ratio of 1 : 1 to produce N-(2-hydroxyethyl)-N,Ndimethyldodecan-1-ammonium bromide, [5]. The reactants were allowed to ref lux in ethanol for 12 h. Then the reaction mixture was left to cool at room temperature. The precipitate was further purified by diethyl ether, then crystallized from ethanol. In the

721

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O H2 C C O O H2 CHCH2O C C C OH

H3C C12H25 N H3 C

O 9

H

CH2

Br O

C

O

carbon steel coupons were immersed in the test solution for a period reached to 24 h. The coupons washed with twice distilled water, and dried using filter paper, finally weighed. The experiment was repeated and the average weight was taken.

O 9H

N-dodecyl-1,30-dihydroxy-30-(29-hydroxy-2-oxo3,6,9,12,15,18,21,24,27-nonaoxanonacosyl)N,N-dimethyl-28-oxo-3,6,9,12,15,18,21,24,27,31decaoxatritriacontan-33-aminium bromide Fig. 1. Chemical structure of the prepared novel cationic surfactant.

second step, the esterification reaction of the synthesized quaternary ammonium salt with citric acid in the presence of toluene as a solvent and p-toluene sulfonic acid as a dehydrating agent [15] in the molar ratio of 1 : 1 to produce N-(2-((1,3-dicarboxy-2(carboxymethyl) Propan-2-yl)Oxy)ethyl)-N, Ndimethyldodecan-1-ammonium bromide. The reaction was completed when the water was removed from the reaction system. The reaction mixture was distilled under vacuum to completely remove the solvent. In the third step, the product of the previous step and polyethylene glycol were esterified as described in the second step in the molar ratio of 1 : 2 to N-dodecyl-1,30-dihydroxy-30-(29-hydroxy-2Oxo-3,6,9,12,15,18,21,24,27-nonaoxanonacosyl)-N,Ndimethyl-28-Oxo-3,6,9,12,15,18,21,24,27,31-decaoxatritriacontan-33-ammonium bromide. Figure 1 shows the chemical structure of the prepared compound and its characterized by FTIR and 1HNMR spectroscopic analyses. 2.2. Carbon Steel The chemical composition of carbon steel specimens used in this study is (wt %): 0.191 C, 0.052 Si, 0.941 Mn, 0.009 P, 0.008 Cr, 0.005 S, 0.013 Ni, 0.034 Al, 0.016 V, 0.003 Ti, 0.022 Cu, and the remainder is Fe. 2.3. Weight Loss Measurements The carbon steel coupons having dimensions 5 cm × 3 cm × 0.5 cm and abraded with a series of grades of emery papers ranged from 300 to 1400 and then are washed with double distilled water. After weighing carefully, the coupons are immersed in a closed beaker containing 100 mL of 1.0 M HCl solution devoid of and containing different concentrations of synthesized inhibitors at various temperatures in the range 20– 80°C, the selected temperature was adjusted by water bath provided with a thermostat control ±1°C. The

2.4. Electrochemical Measurements Potentiodynamic polarization techniques were done using a electrochemical cell contained three electrodes, platinum counter electrode (CE), saturated calomel electrode (SCE) as a reference electrode and the carbon steel electrode used as working electrode (WE). Before each measurement, the C-steel electrode was immersed in a test solution at open circuit potential (OCP) for 30 min, until a corrosion potential was attained. AVoltalab 40 Potentiostat PGZ 301 and a personal computer with Voltamaster 4 software at 20°C were used for potentiodynamic polarization measurements. The electrode potential was changed automatically from –800 to –300 mV vs. SCE at open circuit potential with a scan rate 2 mV s–1 at 20°C for these measurements. Electrochemical impedance spectroscopy (EIS) measurements were carried out previously elsewhere [16]. A small alternating voltage perturbation (5 mV) was imposed on the cell over the frequency range of 100 kHz–30 mHz at 20oC. 2.5. Surface Tension Measurements The surface tension for different concentrations of the cationic surfactant was measured as described previously using Du Nouy Tensiometer (Kruss Type 6) at 25°C [16]. The synthesized cationic surfactant was dissolved in bi distilled water. 3. RESULTS AND DISCUSSIONS 3.1. Chemical Structure Confirmation of the Synthesized Surfactants FTIR spectrum showed that the characteristic bands for the alkyl part were at 2923.27 and 2876.21 cm–1 for asymmetric and symmetric stretching (CH), respectively. Whereas, they were observed at 1352.98 cm–1 for symmetric bending (CH3), at 1464.90 cm–1 for symmetric bending (CH2), and at 727.81 cm–1 for –(CH2)n–rock. C–O stretching band 1250.37 cm–1, C–N+ at 1050.09 cm–1, C=O at 1735.42 cm–1, 3400.76 cm–1 was due to stretching OH. FTIR spectra approved the prospective functional groups in the surfactant compound as shown in Fig. 2.

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Fig. 2. FTIR of cationic surfactant. 1H

NMR Spectra

3.2. Weight Loss Measurement 3.2.1. Effect of inhibitor concentration. Some electrochemical parameters such as, the corrosion rate (k), the degree of surface coverage (θ) and the percentage inhibition efficiency (ηw) were calculated from weight loss measurements using the following equations [17] and given in Table 1.

k = ΔW , At

ηw =

(1)

( k free − k inh ) k free

( k free − k inh ) k free

1.027

1.15

0.934

4.387

19.426

8

7 6 5 4 3 Chemical shift, ppm

2

1

0

Fig. 3. 1HNMR of prepared cationic surfactant.

The 1H NMR spectrum of the prepared surfactant compound displayed different bands at δ = 0.7889– 0.9763 ppm (t, 3H, NCH2CH2(CH2)nCH3); δ = 1.1942 ppm (m, 36H, NCH2CH2(CH2)nCH3); δ = 1.6128 ppm (m, 4H, COOCH2CH2O(CH2CH2O)nH, NCH2CH2(CH2)nCH3, CH3NCH2CH2); δ = 4.5871 ppm (m, 12H, NCH2CH2OCO, COOCH2CH2O (CH2CH2O)n has shown in Fig. 3.

θ=

9

1.349

1050.09 727.81

1464.90 1352.98 1250.37

1000

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 11 10

723

1.349 0.683

3400.76

2923.27 2876.21

1735.42

Transmittance, %

100 90 80 70 60 50 40 30 20 10 4000 3500 3000 2500 2000 1500 Wavenumber, cm–1

Signal intensity, %

ADSORPTION AND INHIBITION EFFECT

(2)

,

× 100,

(3)

where, (ΔW) is the difference in the weight loss of carbon steel sheets before and after immersing in the test solution, kfree and kinh represents the corrosion rate of the carbon steel in 1.0 M HCl solution devoid of containing different concentrations of the synthetic cationic surfactant, respectively, (t) is the time in hours and (A) is the surface area in cm2. The corrosion rate of carbon steel in 1.0 M HCl solution containing the cationic surfactant lowered by increasing the inhibitor concentration and consequently the inhibition efficiency increases as shown in Table 1. These results indicate that the cationic surfactant acted as a good inhibitor for carbon steel in 1 M HCl solution. From Fig. 4, it is observed that the inhibition efficiencies, increases with increasing the inhibitor concentration. This mention that the cationic surfactant is adsorbed onto the C-steel surface by the blocking of the active sites and separated the surface of C-steel from the corrosive action of the chloride ions present in the acid solution.

3.2. Adsorption Isotherm The adsorption isotherm was obtained by supposing that the inhibiting impact is resulting from the adsorption of cationic surfactant in steel/solution interface. The adsorption of the cationic surfactant is considered as substitutional processes between the adsorbed water molecule at the carbon steel surface

Table 1. Corrosion parameters obtained from the weight loss measurements at different temperatures 20°C Conc. of inhibitor, k, θ M mg cm–2 h–1 0.00

40°C ηw, %

k, mg cm–2 h–1

θ

60°C ηw, %

k, mg cm–2 h–1

θ

80°C ηw, %

k, mg cm–2 h–1

θ

ηw, %

–5

5 × 10

0.4192 0.0696

0.00 0.00 0.83 83.40

1.6996 0.2855

– – 0.83 83.20

5.7325 0.9688

– – 0.83 83.10

16.0965 2.6559

– – 0.84 83.50

1 × 10–4

0.0608

0.86 85.50

0.2464

0.86 85.50

0.8427

0.85 85.30

2.3018

0.86 85.70

–4

0.0448

0.89 89.30

0.1808

0.89 89.36

0.6248

0.89 89.10

1.6724

0.90

–3

0.0401

0.90 90.43

0.1609

0.91 90.54

0.5450

0.90 90.49

1.4726

0.91 90.85

–3

0.0284

0.93 93.22

0.1156

0.93 93.20

0.4020

0.93 92.99

1.0686

0.93 93.36

5 × 10

1 × 10 5 × 10

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95 93

0.005 20°C 60°C

40°C 80°C

0.004 C/θ, M

ηw, %

91 89 87 85 83 –4.4

0.003 0.002 0.001

–3.9

–3.4 log C [M]

–2.9

–2.4

Fig. 4. The relation between the percentage inhibition efficiency of the cationic surfactant and the logarithmic concentration of inhibitors at different temperatures.

(H2O)ads and the cationic surfactant in the aqueous solution (Surfactant)aq according the subsequent equation: (Surfactant)aq + n(H2O)ads = (Surfactant)ads + (H2O)aq.(4) where, n is the size ratio, the number of water molecules replaced by the cationic surfactant molecule. Trials were made to fit θ values to disparate isotherms including Bockris–Swinkels, Flory–Huggins, Temkin, Frumkin Freundlich and Langmuir isotherms. However, the best result was agreed with the Langmuir adsorption isotherm. According to Langmuir isotherm, the values of θ is related to the concentration of inhibitors, according to the following equation [18]:

C = 1 + C, (5) θ K ads where, C is the inhibitor concentration, and Kads the equilibrium constant of adsorption, Fig. 5 displays the plotting of C vs C/θ. A straight line with unit slope was gained and the values of regression coefficients (R2) confirmed the validity of this approach and the slope clothed to unity. Langmuir adsorption isotherm supposes that the adsorbed species occupy only one surface site and there is no interaction between the adsorbed species. The value of Kads obtained from the reciprocal of the intercept of Langmuir plot as shown in Fig. 5. The value of Kads indicates a strong adsorption of cationic surfactant on the surface of C-steel [3]. A dimensionless separation factor (RL) is the fundamental characteristic of Langmuir isotherm, was calculated using the following equation [19]:

1 (6) . 1 + K adsC The smallest value of RL (RL < 1) signed a highly adsorption of the inhibitor at the steel surface If RL > 1 the adsorption is unfavorable. RL =

0

0.001

0.002 0.003 C, M

20°C

40°C

60°C

80°C

0.004

0.005

Fig. 5. Relation between C/θ and C according to Langmuir adsorption isotherm.

The data obtained from Table 2 show that the value RL is less than unity indicating that the adsorption of cationic surfactant on the steel surface is favorable.

° was The standard free energy of adsorption, G ads computed from the value of the equilibrium constant of adsorption using the following equation:

° = −RT ln(55.5K ads ), (7) Δ G ads where, T is the absolute temperature, R is the molar gas constant, and the value 55.5 is the concentration of water in solution expressed in M.

 ≤ –20 kJ mol–1 Generally, when the values of Δ G ads indicated that the adsorption is physically owing to the electrostatic interaction between the charged molecules and the charged metal. While the values of  ≥ –40 kJ mol–1 indicated that the adsorption is Δ G ads chemically involve charge transfer or involvement from the surfactant molecules to the steel surface to form a coordinate bond [5]. The calculated values of  proved that the adsorption of novel cationic surΔ G ads factant on the carbon steel surface in 1.0 M HCl solution is mixed physical and chemical adsorption but chemical adsorption is more favor than physical.  and Furthermore, the values of the enthalpy Δ H ads  of adsorption were calculated using the entropy Δ S ads following equation [20]:  = ΔH  − T ΔS  . (8) Δ G ads ads ads Figure 6 represents the relation between the values  versus T of carbon steel devoid of and conof Δ G ads taining various concentrations of cationic surfactant in 1 M HCl solution. A straight line relationship is  and an intercept of obtained with a slope of – Δ S ads  . Δ H ads

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The adsorption thermodynamic parameter values for of the inhibitor are given in Table 3. It is evident that the value of the enthalpy of adsorption is positive  ) indicating an endothermic adsorption pro( Δ H ads  indicates the substitucess. The positive sign of Δ S ads tion process, which can be attributed to the increase in the solvent entropy and more positive water desorption entropy [21].

Table 2. The calculated values of dimensionless separation factor, RL for carbon steel surface in 1 M HCl solution containing different concentrations of inhibitor at various temperatures Temperature, oC

Conc. of inhibitor, M

RL

5 × 10–5

0.2688

1×

10–4

0.1553

5×

10–4

0.0355

1×

10–3

0.0180

5 × 10–3

0.0037

5 × 10–5

0.2641

10–4

0.1521

5 × 10–4

0.0346

1 × 10–3

0.0176

5 × 10–3

0.0036

5 × 10–5

0.2604

1 × 10–4

0.1497

5 × 10–4

0.0340

1 × 10–3

0.0173

10–3

0.0035

5 × 10–5

0.2567

1 × 10–4

0.1472

5×

10–4

0.0334

1×

10–3

0.0170

5×

10–3

0.0034

20

3.3. Electrochemical Impedance Spectroscopy Figures 7 and 8 represent the Nyquist and Bode plots for carbon steel in1. 0 M HCl solution devoid of and containing the novel cationic surfactant. It’s obvious that, there is only one single depressed semicircle. As the concentration of cationic surfactant increases, the diameter of semicircle increases indicating that the corrosion of C-steel is mainly controlled by a charge transfer process Moreover, Nyquist diagrams are not ideal semicircles in 1 M HCl solution due to the frequency dispersion effect as a result of the roughness and inhomogeneous of the steel surface. The presence of inhibitor increase the diameter of the capacitive loop and this diameter increases with the surfactant concentration [22]. The equivalent circuit represented in Fig. 9 is used to analyze the electrochemical impedance spectra, where Rs represents the solution resistance, Rct is the charge-transfer resistance, and the constant phase element (CPE), instead of a pure capacitor represents the interface capacitance. The impedance of CPE is described by the following equation: −n

Z CPE = Q −1 (i ωmax ) ,

725

1× 40

60

5×

80

(9)

where, Q is the CPE constant, ωmax is the angular frequency in red. S–1, ω = 2πfmax, (where f is the frequency at which the imaginary component of the impedance is maximum), is the imaginary number and n is a CPE exponent which can be used as a gauge of the heterogeneity or roughness of the surface. The values of n are generally in the presence of the inhibitor is lower than in the absence of inhibitor solutions. This indicated that the inhibitor contributes to increase surface heterogeneity due to its adsorption at the steel/solution interfaces. Double layer capacitance values (Cdl) obtained from the con-

stant phase element (CPE) parameters according to the following equation [23]:

C dl = Q ( ωmax )

n −1

(10)

.

The inhibition efficiency was obtained from the charge transfer resistance using the following equation: η I=

Rct − Rct × 100, Rct

(11)

where, Rct and Rct are the values of charge-transfer resistance devoid of and containing the inhibitor, respectively.

Table 3. The adsorption thermodynamic parameters at different temperatures Temperature, oC

Kads × 10–4, M–1

c Δ G ads , kJ mol–1

20 40 60 80

5.44 5.57 5.68 5.79

–36.35 –38.89 –41.43 –43.97

 , kJ mol–1 Δ H ads

 , J mol–1 K–1 Δ S ads

0.89

127.08 127.09 127.07 127.09

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300 1M HCl

–40 –42

0.0001 M

0.0005 M

0.001 M

0.005 M

200 150 100

–44

50

–46 290

300

310

320 330 T, K

340

350

360

0

 and T of C-steel in Fig. 6. The relation between Δ G ads 1 M HCl solution devoid of and containing various concentrations of cationic surfactant. 1M HCl 0.00005 M 0.0001 M 0.0005 M 0.001 M 0.005 M

4 3 2 1 0 –2

–1

0

1 2 3 log f [Hz]

4

5

20 10 0 –10 –20 –30 –40 –50 –60 –70 –80

100

200

300 400 Zr, Ω cm2

600

500

700

Fig. 7. The impedance Nyquist diagram for the carbon steel in 1.0 M HCl solution devoid of and containing various concentrations the novel cationic surfactant.

Φ, deg

5 log Z [Ω cm2]

0.00005 M

250 –Zi, Ω cm2

ΔGabs, kJ mol–1

–38

R.E.

CPE n

Rs

W.E.

Rct

Fig. 8. Impedance Bode diagram for C-steel in 1.0 M HCl solution devoid of and containing various concentrations of cationic surfactant.

Fig. 9. The electrical equivalent circuit used for modeling the interface steel/1.0 M HCl solution in the absence and presence of the cationic surfactant.

The computed values of the electrochemical impedance parameters are given in Table 4. Inspection of this table, as the concentration of the novel cationic surfactant increases, the values of the percentage inhibition efficiency increases, the values of Cdl creases and Rct increase. The observed results due to the reduction of local dielectric constant and increase the thickness of electrical double layer and the water molecule is

replaced by the surfactant molecule. From the above data it is proposed that this compound acts through the adsorption at the C-steel/solution interface [6]. 3.4. Potentiodynamic Polarization Figure 10 displays the potentiodynamic anodic and cathodic polarization curves of carbon steel in

Table 4. Electrochemical impedance parameters for corrosion of C-steel in 1 M HCl solution devoid of and containing various concentrations of cationic surfactant Conc. of inhibitor M

Rs, Ω cm2

Qdl, −1 mΩ sn cm−2

n

Rct, Ω cm2

Cdl, μF cm–2

ηI, %

0.00 –5

5 × 10

2.5 2.9

0.1606 0.0275

0.91 0.76

42.5 259.8

102.2 19.85

– 83.64

1 × 10–4

2.8

0.0242

0.83

301.0

17.54

85.88

–4

2.9

0.0155

0.76

450.1

10.53

90.56

–3

2.5

0.0131

0.74

503.5

9.78

91.56

–3

3.5

0.0112

0.78

639.6

7.44

93.36

5 × 10 1 × 10 5 × 10

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The percentage inhibition efficiency ηp was estimated from the values of icorr using the subsequent equation:

i −i η p = corr inh × 100. icorr

(12)

Where, iinh and icorr are the corrosion current densities for C-steel electrode in the existence and the lack of inhibitor, respectively. Inspection of Table 5, it is evident that, the presence of synthesized cationic surfactant depresses the corrosion current density and there is a slight shift in the values of corrosion potential. The values of βc and βa are nearly constant confirming that the cationic surfactant is mixed inhibitors. The values of icorr decreased and the values of ηp increases by increasing the concentration of the inhibitor indicating the inhibiting effect of cationic surfactant. Due to the presence of some active sites in the chemical structure of the cationic surfactant illustrate the inhibiting effect of this compound acted by its adsorption on the steel surface The corrosion inhibition efficiencies are directly proportional to the amount of adsorbed inhibitor. The functional groups in the chemical structure of the inhibitor play an essential role during the adsorption process.

–1.5 –2.0 –2.5 –3.0 log i, [A cm2]

1.0 M HCl solution devoid of and containing various concentrations of the novel cationic surfactant at a scan rate 2 mV s–1.It is clear that both the anodic dissolution reaction of steel and cathodic hydrogen evolution reactions were suppressed with the increment of an inhibitor. There is a positive shift for the anodic overvoltage and negative shift for the cathodic overvoltage compared to the blank solution. This proves that the synthesized cationic surfactant is of mixed inhibitor. [24]. Some corrosion kinetics parameters, such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βc & βa),corrosion current density (icorr) achieved from the extrapolation of the anodic and cathodic polarization curves, and inhibition efficiency (ηp) were computed and presented in Table 5.

727

–3.5 –4.0 –4.5 1M HCl 0.00005 M

–5.0 –5.5

0.0001 M

–6.0 –6.5 –850

–750

–650 –550 –450 E vs. SCE, mV

–350

Fig. 10. Potentiodynamic polarization curves of C-steel in 1.0 M HCl solution devoid of and containing various concentrations of cationic surfactant at a scanning rate 2 mV s–1.

3.5. Effect of Temperature The dissolution of C-steel in 1.0 M HCl solution is generally accompanied by the evolution of hydrogen gas, the amount of which is directly proportional to the temperature leading to higher corrosion rate at higher temperatures. Synergistically, the reactant energy to form the activated complex, which in its turn dissociates to form the corrosion products increases with increasing temperature. In our study, it is clear that variation in temperature did not cause any significant change in inhibition efficiency. This inference proposing that a strong adsorption of inhibitor on the C-steel surface which forming adsorbed film [25]. So, the inhibiting effect of the cationic surfactant on the steel surface was explained through a chemical adsorption 3.6. Activation Energy Some activated thermodynamic functions like the activation energy (Ea), the enthalpy of activation

Table 5. Corrosion kinetics parameters obtained from the anodic and cathodic polarization curves for C-steel corrosion in 1.0 M HCl solution devoid of and containing various concentrations of inhibitor Conc. of inhibitor, M

Ecorr, mV (SCE)

icorr, mA cm–2

βa, mV dec–1

βc, mV dec–1

ηp, %

0.00 –5

5 × 10

–523 –535

0.4241 0.0711

186 208

–155 –148

– 83.24

1 × 10–4

–521

0.0629

192

–148

85.17

5 × 10–4

–523

0.0617

199

–158

85.46

1 × 10–3

–521

0.0397

201

–162

90.64

5 × 10–3

–526

0.0307

195

–158

92.77

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3

1M HCl

ln k, [mg cm–2 h–1]

2

0.0001 M

0.00005 M 0.0005 M

0.001 M

0.005 M

1 0 –1 –2 –3 –4 2.8

2.9

3.0

3.1 3.2 1000/T, K–1

3.3

3.4

Fig. 11. Arrehenius plots curves for carbon steel dissolution in 1 M HCl solution devoid of and containing various concentrations of cationic surfactant.

(ΔH*), and the entropy of activation (ΔS*) have a major role in understanding the inhibitors mechanism of organic inhibitors. The values of the activation energy (Ea) were computed from the Arrhenius equation as follows [26].

−E a (13) + ln A, RT where is the rate of carbon steel dissolution, A is the frequency factor, and R is the universal gas constant. The relation between k and 1/T for corrosion of carbon steel in 1 M HCl solution devoid of and containing various concentrations of the cationic surfactants is presented in Fig. 11. Linear plots were obtained with slope equal (–Ea/R). The computed values of Ea are given in Table 6. It is clearly seen that the values of Ea in the presence of the inhibitor are lower or unchanged relative to the free solution. The lower or unchanged value of Ea in the presence of inhibitor is referred to its chemical adsorption, while the adverse is physical adsorption [27]. Popova et al. [27] arranged the corrosion inhibitors into three types according to the effect of temperature with the inhibition efficiency and the activation energy: ln k =

1. As the temperature is rising, the inhibition efficiency decreases. The value of the Ea in the presence of inhibitor is greater than that in the free solution. 2. As the temperature is rising, the inhibition efficiency does not change with rising temperature. The value of Ea does not change with the absence and presence or inhibitors. 3. As the temperature is rising, the inhibition efficiency increases. The value of Ea for the corrosion process is smaller than that obtained in the free solution. It is obvious that there is no observed change in the percentage inhibition efficiency as the temperature is varied. This explained due to the specific interaction between the C-steel surface and the cationic surfactant [28]. However, unchanged or lower values of Ea in the presence of the cationic surfactant compared to the blank solution indicating that the adsorption of the inhibitor is chemisorbed. The activation thermodynamic function such as the enthalpy (ΔH*) and the entropy (ΔS*) of activation of the corrosion process was calculated from the alternative Arrhenius equation, or transition state equation [29]:

()

( )

⎛ ⎛ ⎞ ⎞ (14) ln k = ⎜ ln ⎜ R ⎟ + Δ S * ⎟ − Δ H * , T R ⎠ RT ⎝ ⎝ N A h⎠ where, h, N, T and R are the Planck’s constant, the Avogadro’s number, the absolute temperature and the universal gas constant, respectively. Figure 12 displays the relationship between ln(k/T) and 1/T for carbon steel dissolution in 1 M HCl solution devoid of and containing various concentrations of inhibitor. A straight line relationships are gained with a slope of (–ΔH*/R) and an intercept of ln ((R/Nh) + ΔS*/R). The calculated values of ΔH* and ΔS* are given in Table 6. The values of ΔH* are positive in the absence and presence of inhibitor denoting the endothermic effect of the steel dissolution process and it indicates that the dissolution of steel is difficult [30]. It is obvious that from Table 6 the values of Ea and ΔH* vary in the same manner. This result permit to verify the known thermodynamic reaction between the Ea and ΔH* Eq. (15) [30]. It is clear that the values of ΔH* are lower than that of Ea indicating that the

Table 6. Some thermodynamic activation parameters for corrosion of C-steel in 1 M HCl solution devoid of and containing various concentrations of cationic surfactant Conc. of inhibitor, M

Ea, kJ mol–1

ΔH*, kJ mol–1

ΔS*, J mol–1 K–1

Ea – ΔH*, kJ mol–1

0.00 5 × 10–5 1 × 10–4 5 × 10–4 1 × 10–3 5 × 10–3

52.36 52.32 52.25 52.11 51.81 52.25

49.70 49.67 49.59 49.45 49.16 49.59

–156.08 –250.23 –259.03 –277.92 –290.10 –298.52

2.66 2.66 2.66 2.66 2.66 2.66

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corrosion process must involve a gaseous reaction, simply a hydrogen evolution reaction associated with a decrease in total reaction volume [31, 32]. (15) Δ H a = E a − RT . It is clear that from Table 6, the negative values of ΔS* in the absence and presence of the inhibitor indicating that the activated complex in the rate determining step represents an association rather than dissociation meaning that a decrease in disordering takes place on going from reactants to activate complex. Similar results have been reported in the literature for carbon steel dissolution in the absence and presence of inhibitors in HCl solution [30, 33]. Also the ΔS* values tend to more negative values as the synthesized cationic surfactant concentration increases showing more ordered behavior leading to increased inhibition efficiency.

log k/T, [mg cm–2 h–1 K–1]

ADSORPTION AND INHIBITION EFFECT

729

–1.5 –2.0

1M HCl

0.005 M

0.00005 M

0.0001 M

0.0005 M

0.001 M

–2.5 –3.0 –3.5 –4.0 –4.5 2.8

2.9

3.0 3.2 3.1 1000/T, K–1

3.3

3.4

Fig. 12. The relationship between logk/T and 1/T of carbon steel in 1.0 M HCl solution devoid of and containing various concentrations of cationic surfactant.

3.7. The Surface Properties of the Investigated Surfactant Figure 13 shows the relationship between the surface tension (γ) and the logarithm of the molar concentration of the tested cationic surfactant (logC). It is clear that from this figure, the values of γ decrease at lower concentrations of the surfactant molecule and this lowering increases at high concentrations of inhibitor. However, at certain concentration is reached the surfactant molecules form micelles, which are in equilibrium with the free surfactant molecules. The critical micelle concentration (CMC) was calculated from the intersection of the two straight lines. Some surface active properties of the surfactant such as, effectiveness (πcmc), maximum surface excess (Γmax) and minimum area per molecule (Amin) were computed from the subsequent equation:

π cmc = γ o − γ cmc,

Γ max =

( )(

)

dγ −1 , nRT d ln C 14

62 57 52 47 42 37 32 27 22 –6.1

–5.1

–4.1 –3.1 log C, [M]

–2.1

–1.1

Fig. 13. Variation of the surface tension with the logarithm of the molar concentrations of the cationic surfactant.

(16) (17)

10 (18) , N A Γ max where γo, γcmc and NA are the surface tension of pure water, the surface tension at critical micelle concentration and the Avogadro’s number, respectively. The surface active parameters are presented in Table 7. The values of CMC are related to the effectiveness of surfactants as corrosion inhibitors [16]. Below the CMC value, the increase of the concentration of cationic surfactant led to more adsorbed of surfactant on the steel surface. On the other hand, increasing the concentration of surfactant above the CMC value does not affect the surface tension, as shown in Fig. 13, and not affect the value of inhibition efficiency. This could be attributed to the fact that above CMC the surface of C-steel is covered with a monolayer of surfactant molAmin =

Surface tension, mN cm–1

67

ecules and the additional molecules combine to form micelles in the bulk of the solution. It is, clear that the surfactant molecules considered as an excellent corrosion inhibitor at lower values of CMC, since, the inhibition effectiveness decreases as the CMC value increases.

3.8. Mechanism of Inhibition The inhibiting effect of the organic molecules toward the corrosion of the carbon steel in acidic solutions was interpreted by its adsorption onto the steel/solution interface. Adsorption of organic inhibitor on the steel surface may occur surface in one or more of the following ways: (a) electrostatic interaction between the charged molecules and the charged metal, (b) interaction of unshared electron pairs in the molecule with the metal, (c) interaction of p-electrons in the metal and/or, (d) a combination of types (a–c) [4].

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ABDALLAH et al.

Table 7. Surface active properties of the cationic surfactant CMC × 103, mol dm–3

γCMC, mN m–1

ΠCMC, mN m–1

Γmax × 1011, mol cm–2

Amin, nm2

7.6

29

43

1.21

1.38

The studded cationic surfactants have quaternary nitrogen atom (N+) and counter ion (Br–) are adsorbed on the cathodic sites of the C-steel and decrease the hydrogen evolution reaction. The adsorption on anodic site occurs through a lone pair of electrons of oxygen atoms in cationic surfactants which decrease the anodic dissolution of the carbon steel. The high protection efficiency of the cationic surfactant is attributed to the presence of many adsorption centers in the chemical structure, larger molecular size of the compounds. This adsorption cannot be considered only as purely physical or as a purely chemical adsorption phenome . Physical non, it’s suggested by the value of Δ G ads adsorption is the result of electrostatic attractive forces between the cationic form of inhibitor and the electrically charged steel surface. Chemisorption process involves charge sharing or charge transfer from the lone pairs of electrons in hetero-atoms (N and O) to the vacant d-orbital in the carbon surface to form a coordinate bond. Weight loss measurements suggested the corrosion inhibition efficiency, increased with increasing the inhibitor concentration. This behavior was due to the adsorption and the coverage of inhibitor on carbon steel surface. These inhibition actions occur via blocking the active sites by replacement of water molecules and form a compact barrier film on the steel surface. 4. CONCLUSIONS 1. The novel cationic surfactant is a good inhibitor for the corrosion of carbon steel in 1.0 M HCl solution. The inhibitory efficiency of this molecule depends on its concentration. 2. The novel cationic surfactant acted as a mixed— type inhibitor. 3. The high inhibition efficiency of the cationic surfactant was explained by its adsorption on the steel surface and a protective film formation. 4. The adsorption of cationic surfactant on the carbon steel follows Langmuir adsorption isotherm. REFERENCES 1. Rajeev, P., Surendranathan, A.O., and Murthy, Ch.S.N., J. Mater. Environ. Sci., 2012, vol. 3, p. 856. 2. Abdallah, M., Atwa, S.T., Salem, M.M., and Fouda, A.S., Int. J. Electrochem. Sci., 2013, vol. 8, p. 10001. 3. Yıldız, R., Doner, A., Doğan, T., and Dehri, I., Corros. Sci., 2014, vol. 82, p. 125. 4. Negm, N.A., Nadia, G., Kandile, B., et al., Corros. Sci., 2012, vol. 65, p. 94.

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