Influence of the alkyl chain length of 2 amino 5 alkyl 1,3,4 thiadiazole compounds on the corrosion inhibition of steel immersed in sulfuric acid solutions

Corrosion Science 54 (2012) 231–243

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Influence of the alkyl chain length of 2 amino 5 alkyl 1,3,4 thiadiazole compounds on the corrosion inhibition of steel immersed in sulfuric acid solutions M. Palomar-Pardavé a,⇑, M. Romero-Romo a, H. Herrera-Hernández a, M.A. Abreu-Quijano a, Natalya V. Likhanova b, J. Uruchurtu c, J.M. Juárez-García d a

Universidad Autónoma Metropolitana-Azcapotzalco, Departamento de Materiales, Av. San Pablo 180, Col. Reynosa Tamaulipas, C.P. 02200, México D.F., Mexico Instituto Mexicano del Petróleo, Programa de Investigación y Posgrado, Eje Central Lázaro Cárdenas No. 152, San Bartolo Atepehuacán, C.P. 07730, México, D.F., Mexico Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Mor., Mexico d Laboratorio de Microanálisis, Centro Nacional de Metrología, km 4.5 Carretera a los Cues, municipio de El Marqués, Querétaro, Mexico b c

a r t i c l e

i n f o

Article history: Received 16 May 2011 Accepted 14 September 2011 Available online 21 September 2011 Keywords: A. Steel A. H2SO4 A. Thiadiazoles B. EIS B. SEM C. Acid inhibition

a b s t r a c t The corrosion inhibition efficiency of 2 amino 5 alkyl 1,3,4 thiadiazole compounds with different alkyl chain lengths, namely: 2 ethyl, 3 n propyl, 5 n penthyl, 7 hepthyl, 11 undecyl and 13 tridecyl, was evaluated in the system steel/1 M H2SO4. These compounds were synthesized, characterized by FT-IR and NMR spectroscopy analysis and evaluated using electrochemical impedance spectroscopy and SEM analysis. The results showed that the inhibition mechanism involves blockage of the steel surface by the inhibitor molecules by a Langmuir-type adsorption process and that the alkyl chain length plays an important role in the inhibition efficiency of the synthesized inhibitors. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The study of inhibiting effects of organic molecules on the corrosion of steels commonly used in the petroleum and petrochemical industry is presently a central field of research [1–5]. However, it is an area which is rather difficult to tackle due to the numerous variables that may intervene, namely: the chemical nature of the corrosive media, pH, temperature and the molecular structure of the inhibitor, among others. In this respect, our group has shown that different heterocyclic organic molecules [6–10] and amphiphilic compounds [11,12] have proved to possess significant corrosion inhibition properties for pipeline steel grade API 5L X52 immersed in aggressive media, namely: H2SO4 or HCl. There are also, among others, fatty amides [13], 4,5 diphenyl 1 vinylimidazole derivative compounds [14], bis-imidazoline and imidazoline compounds [15], thiadiazoles derivatives [16–23], which have also been evaluated as corrosion inhibitors for steel [11,12,14–16], copper [13] and bronze [17]. In particular thiadiazoles’ derivatives have been assessed from the experimental point of view, in 20% formic acid and 20% acetic acid [16], in 3.0% NaCl solutions [18], in 0.5 M H2SO4 [20], in HCl [21], as well as from theoretical calculations [23]. However, the influence of the alkyl chain length of 2 amino 5 alkyl 1,3,4 thiadiazole compounds on the corrosion inhibition ⇑ Corresponding author. E-mail address: [email protected] (M. Palomar-Pardavé). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.020

ability for steel immersed in sulfuric and in particular, the kinetics of such corrosion inhibition process has received much less attention. It is important to mention that, to the best of our knowledge, the information about corrosion inhibition of 2 amino 5 alkyl 1,3,4 thiadiazole considered mostly studies on the 5 methyl, 5 ethyl, 5 n propyl, or 5 hepthyl 2 amino 1,3,4 thiadiazoles [16,18,23]. Notwithstanding, the authors did not correlate the hydrophobic alkyl chain length with corrosion inhibition effect. Therefore, in this work, we aboard this last aspect of the thiadiazoles’ derivatives by studying six compounds, three of which (IC-5, IC-11 and IC-13) were evaluated for the first time as corrosion inhibitors. 2. Experimental procedure 2.1. Materials The reagents: thiosemicarbazide (99%), sulfuric acid (95–98%), propionic acid (99.5%), butyric acid (99%), hexanoic acid (99%), caprylic acid (98%), lauric acid (98%) and myristic acid (99–100%) were purchased from Aldrich and used as received. All solvents were HPLC grade. 2.2. Synthesis of corrosion inhibitors The synthesis of 2 amino 5 alkyl 1,3,4 thiadiazoles (IV) from thiosemicarbazide (I) and the corresponding acids (II) (Scheme 1) by a

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crude product was filtered and washed with water. It was recrystallized from an alcohol–water or DMFA–water and dried in vacuo.

conventional method is a common practice [24]. The first stage of this reaction consists in the formation of the alkyl amide of thiosemicarbazide (III), which can exist in two tautomers (III A and IIIB). The reaction was carried out in the presence of an acidic catalyst and dehydration agent (sulfuric or phosphoric acids), as the use of basic catalysts such as sodium etoxide or piperidine leads to formation of 1, 2, 4 triazoles [25]. However, the preparation of these compounds was performed more efficiently by microwave (MW) radiation, because the reaction rate is accelerated. Kidwai et al. [26], have proposed a new method to prepare 2 amino 5 alkyl 1,3,4 thiadiazoles using microwave energy and alumina, yet this method is not reproducible and melting temperature data and 1H NMR spectra do not agree with the reported compounds, which means that this work reports for the first time characterization data of 2 amino 5 undecyl 1,3,4 thiadiazole and 2 amino 5 tridecyl 1,3,4 thiadiazole. The molecular structure of the compounds (Table 1) was characterized by NMR and IR. The results of the analyses indicated that composition and structures were in agreement with the synthesized molecules, which are stable in air, water and in common organic solvents. 2 Amino 5 alkyl 1,3,4 thiadiazoles were synthesized in accordance to a published procedure [12], but the novelty was the use of microwave radiation. A flask containing 105 mM mixture of the corresponding acid, 10 mL of concentrated sulfuric acid and 100 mM of thiosemicarbazide were introduced in the microwave reactor (closed vessel) and irradiated for 1 h (20 W) under magnetic stirring. The temperature was raised to 80–90 °C. After reaction completion the mixture was cooled, poured into ice water and neutralized with concentrated ammonia solution. The precipitated

2.3. Inhibitors characterization All compounds were characterized by 1H and 13C NMR and IR spectroscopies. Melting points were measured in a Fisher Scientific apparatus equipped with a 300 °C thermometer. 1H NMR (300 MHz) and 13C NMR (75.4 MHz) spectra were obtained with a JEOL Eclipse-300 equipment in CDCl3 and DMSO-d6 at room temperature. Chemical shifts (d) were reported in ppm with respect to a scale calibrated to tetramethylsilane (TMS), which was used as an internal standard. All the microwave-assisted reactions were performed using a controllable single-mode microwave reactor, CEM Discovery, designed for synthetic use. The reactor was equipped with a magnetic stirrer and pressure, temperature (on-line IR detection) controls. 2 Amino 5 ethyl 1,3,4 thiadiazole (IC-2) obtained as a white powder with a yield of 83%. M.p. 200–203 °C (ethanol–water). 1H NMR (DMSO-d6) 1.18 (t, J = 7.4 Hz, 3H), 2.78 (quat, J = 7.4 Hz, 2H), 6.99 (s, 2H) ppm. 13C NMR (DMSO-d6) 13.92, 23.25, 160.11, 168.34 ppm. IR (500–4000 cm1, KBr pellet): v 3289, 3113, 3102, 2980, 2941, 1638, 1526, 1508, 1498, 1027, 529 cm1. 2 Amino 5 n propyl 1,3,4 thiadiazole (IC-3) obtained as a white–yellow powder with a yield of 85%. M.p. 204–206 °C (ethanol–water). 1H NMR (DMSO-d6) 0.92 (t, J = 7.4 Hz, 3H), 1.63 (sx, J = 7.4 Hz, 2H), 2.76 (t, J = 7.2 Hz, 2H), 6.93 (s, 2H) ppm. 13C NMR (DMSO-d6) 13.17, 22.29, 31.27, 158.16, 168.01 ppm. IR (500–4000 cm1, KBr pellet): v 3289, 3113, 3102, 2980, 1638, 1526, 1498, 1027 cm1.

O NH2 NH

R COOH

S NH2

R

NH

NH

II

NH2

OH

N

N N

N SH

S

H2O

I

R

NH2

III a

H2O

NH2

III b

S

R

IV

where R= C2H5, C5H11, C7H15, C11H23, C13H27. Scheme 1. Chemical reaction to synthesize 2 amino 5 alkyl 1,3,4 thiadiazoles.

Table 1 Name, structure and molecular weights of the compounds synthesized. Entry

Structure

IC-2

N N NH2

IC-3

S

NH2

S

NH2

S

NH2

S

NH2

S

S

143.21

2 Amino 5 n penthyl 1,3,4 thiadiazole

171.26

2 Amino 5 hepthyl 1,3,4 thiadiazole

199.31

2 Amino 5 undecyl 1,3,4 thiadiazole

255.42

2 Amino 5 tridecyl 1,3,4 thiadiazole

283.48

C11H23

N N NH2

2 Amino 5 n propyl 1,3,4 thiadiazole

C7H15

N N

IC-13

129.18

C5H11

N N

IC-11

2 Amino 5 ethyl 1,3,4 thiadiazole

C 3H 7

N N

IC-7

Relative mol. weight

C2H5

N N

IC-5

Name

C13H27

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Blank

125

150

(b)

165 Hz 652 Hz

41.2 Hz

125

100

19.6 Hz

(a)

50

100

-Zim / Ω - cm2

-Zim / Ω - cm2

5.02 Hz

101 kHz

75

131 Hz

75

15.9 Hz 50

25 25

0

0

160 kHz

96.8 mHz

321 kHz

-25

100 mHz

-25 0

25

50

75

100

125

Zre / Ω - cm2 150

0

25

50 5

75

0

100

125 1

2

150

5

Zre / Ω - cm2

200

(c)

(d) 175

125

150 100

-Zim / Ω - cm2

-Zim / Ω - cm2

125

3.75 Hz

75

100

50

9.08 Hz

75

50 25

25 0

0

101 kHz -25 0

25

50

75

100

125

-25

150

0

25

50

75

100

125

150

175

200

Zre / Ω - cm2

Zre / Ω - cm2 500

100 mHz

204 kHz

31.5 mHz

150

(e)

(f) 125

400

100

-Zim / Ω - cm2

-Zim / Ω - cm2

300

8.81 Hz 200

100

75

200 mHz 50

25

0

0

503 kHz

200 mHz

200 mHz

200 mHz

8

-25

-100 0

100

200 2

0

0

Zre / Ω -

300 cm2

400

500

0

25

50

75

Zre / Ω -

100 1

0

0

125

150

cm2

Fig. 1. Nyquist plots recorded in the system API 5L X52/1 M H2SO4 with different compounds: (a) IC-2, (b) IC-3, (c) IC-5, (d) IC-7, (e) IC-11 and (f) IC-13 at different concentrations indicated in a.

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2 Amino 5 n penthyl 1,3,4 thiadiazole (IC-5) obtained as a white powder with a yield of 87%. M.p. 193–195 °C (ethanol– water). 1H NMR (DMSO-d6) 0.87 (t, J = 7.1 Hz, 3H), 1.30 (m, 4H), 1.61 (qi, J = 7.1 Hz, 2H), 2.77 (t, J = 7.1 Hz, 2H), 6.90 (s, 2H) ppm. 13 C NMR (DMSO-d6) 13.62, 21.56, 28.55, 29.24, 30.37, 158.31, 167.96 ppm. IR (500–4000 cm1, KBr pellet): v 3289, 2952, 2918, 2848, 1637, 1528, 1480, 1029 cm1. 2 Amino 5 hepthyl 1,3,4 thiadiazole (IC-7) obtained as a white powder with a yield of 78%. M.p. 183–185 °C (ethanol–water). 1H RMN (DMSO-d6) 0.86 (t, J = 7.2 Hz, 3H), 1.27 (m, 8H), 1.61 (qi, J = 7.2 Hz, 2H), 2.77 (t, J = 7.4 Hz, 2H), 6.85 (s, 2H) ppm. 13C NMR (DMSO-d6) 13.57, 21.73, 27.99 (2C), 28.75, 29.18, 30.86, 158.25, 167.83 ppm. IR (500–4000 cm1, KBr pellet): 3290, 2951, 2917, 2847, 1604, 1568, 1528, 1478, 1029 cm1. 2 Amino 5 undecyl 1,3,4 thiadiazole (IC-11) obtained as white–yellow waxy-like solid with a yield of 75%. M.p. 62–64 °C (ethanol). 1H NMR (CDCl3) 0.86 (t, J = 6.6 Hz, 3H), 1.25 (m, 16H), 1.77 (qi, J = 6.6 Hz, 2H), 2.77 (t, J = 7.4 Hz, 2H), 6.89 (s, 2H) ppm. 13 C NMR (CDCl3) 14.30, 22.88, 25.55, 26.71, 29.21, 29.32, 29.53, 29.62, 29.79 (2C), 32.11 158.22, 167.86 ppm. IR (500–4000 cm1, KBr pellet): v 3280, 2954, 2918, 2850, 1604, 1568, 1471, 1167, 715 cm1. 2 Amino 5 tridecyl 1,3,4 thiadiazole (IC-13) obtained as a white waxy-like solid with a yield of 73%. M.p. 59–61 °C (DMFA– water). 1H NMR (CDCl3) 0.88 (t, J = 6.8 Hz, 3H), 1.26 (m, 20H), 1.76 (qi, J = 6.8 Hz, 2H), 2.80 (t, J = 7.4 Hz, 2H), 6.90 (s, 2H) ppm. 13 C NMR (CDCl3) 14.29, 22.86, 25.50, 26.67, 29.18, 29.30, 29.53, 29.59, 29.75, 29.82 (3C), 32.10 158.14, 167.12 ppm. IR (500– 4000 cm1, KBr pellet): v 3269, 2951, 2917, 2847, 1604, 1569, 1472, 1165, 1029, 715 cm1. 2.4. Electrochemical measurements The inhibition efficiency of the thiadiazole derivatives shown in Table 1, as corrosion inhibitors for mild steels in 1 M sulfuric acid (H2SO4) solution was determined by Electrochemical Impedance Spectroscopy (EIS). All of then were dissolved in an ethanol solution to obtain a 1000 ppm solution. From this solution, different solutions were prepared with concentrations ranging from 5 to 100 ppm (mg L1), in 1 M H2SO4 support electrolyte. This concentration range was chosen on the basis of our previous experience [7,8] to perform the corrosion/inhibition experiments. The blank solution for comparison was devoid of thiadiazole inhibitors. For some selected illustrative cases the evaluation of inhibition efficiency with time was also carried out. A conventional electrochemical cell with a three-electrode set-up was used. The working electrode was prepared from rectangular API 5L X52 pipeline steel coupons. After electrical connection through a copper wire, the steel specimens were encapsulated in an epoxy resin at room temperature as to expose approximately 1 cm2 surface area to the corrosive electrolyte. Prior to each experiment, the surface electrode was prepared using conventional metallographic methods. Subsequent to cleaning the electrodes ultrasonically with ethanol for 10 min and rinsing with acetone, they were finally dried in warm air. The reference electrode was saturated calomel (SCE) while a graphite rod served as counter-electrode. The working electrode was immersed in the test electrolyte during 15 min until a steady-state open circuit potential (Eocp) was measured. Electrochemical impedance measurements, EIS, were performed under laboratory air conditions using the Zahner IM6 electrochemical workstation, with a small sinusoidal perturbation signal of ±10 mV around Ecorr, within the 106 to 102 Hz frequency range recording 10 points per decade. The experimental impedance data obtained were represented using Nyquist plots. The charge-transfer resistance, Rct and the double-layer capacitance (Cdl) values, were calculated by fitting an appropriate equiv-

alent electrical circuit (EEC) to the experimental data points of the impedance plots. Finally, the resulting surface corrosion morphologies associated to some of the experiments of inhibiting efficiency were obtained by means of SEM coupled with WDS/EDS microanalysis detectors (JEOL JXA-8200 microscope).

3. Results and discussion Fig. 1 shows the corrosion data of pipeline steel exposed to 1 M H2SO4 solution containing different concentrations of IC-2, IC-3, IC-5, IC-7, IC-11 and IC-13 thiadiazole derivatives inhibitors as recorded by EIS; the response in the absence of inhibitor (namely Blank) is also shown. For the blank the impedance spectra exhibit one single depressed semicircle, which indicates that the corrosion of steel is mainly controlled by a charge transfer process. In contrast, when inhibiting molecules are present, the impedance spectra are characterized, in general, by two time constants consisting of a large capacitive loop at high to medium frequency and an inductive loop at low frequency. The capacitive loop at high frequencies represents the phenomenon associated with the electrical double layer. It arises from the time constant of the electrical double layer and the charge transfer occurring in the corrosion process [27], at this frequency region, for most of the cases, a diffusion (Warburg) impedance is clearly present. The large capacitive loop makes an angle with the real axis and its intersection gives the solutions’ resistance between the working electrode and the counter electrode (Rs). On the other hand, the low frequency inductive loop may be attributed to the relaxation process resulting from adsorption of species like FeSO4 or inhibitor species on the electrode surface. Such systems can be described in terms of electrical circuits that involve inductance elements [28]. It is important to mention that similar Nyquist plots have been obtained by Li et al. [4], during the corrosion of cold rolled steel in 0.5 M H2SO4 with different concentrations of the corrosion inhibitor Triazolyl blue tetrazolium bromide, by Umoren et al. [27], for iron in 0.5 M H2SO4 in the presence of polyacrylamide and by Amin et al. [29], for iron in 1.0 M HCl solutions containing different concentrations of non-ionic surfactants of the TRITON-X series. Considering the characteristic depicted by the impedance plots

Fig. 2. The electrochemical equivalent circuits used for simulation of impedance spectra for (Circuit A) in the absence (blank) and (Circuit B) in the presence of inhibitors. Circuit C was used solely for IC-11 inhibitor when present at concentration values greater than 20 ppm, in this case Rmol is associated with the resistance of the adsorbed inhibitor layer.

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(a)

(b) 100

-Zim / Ω - cm2

75

165 Hz

25.2 Hz 50

25

IC-2 [ 5 ppm ] measured fitted

0

201 kHz

200 mHz

5.02 Hz

101 kHz

-25 0

25

50

75

100

125

Zre / Ω - cm2 100

75

(c)

(d)

80 50

-Zim / Ω - cm2

-Zim / Ω - cm2

60

44.5 Hz

40

8.81 Hz 25

20 0

IIC-3 -3 [ 5 ppm ] C

IC-5 [ 5 ppm ]

101 kHz

measured fitted

0

403 kHz

me sured measured fitted

10 mHz

a

215 mHz -20

-25

0

20

40

60

80

100

0

25

Zre / Ω - cm2

50

75

Zre / Ω - cm2

100

60

(e)

(f)

75

50

-Zim / Ω - cm2

-Zim / Ω - cm2

40

34.9 Hz

25

IC-7 C-7 [ 10 ppm ] measured fitted

506 kHz

25

50

Zre / Ω - cm2

measured fitted

201 kHz

139 mHz

215 mHz

-25 0

IC-11 [ 5 ppm ]

0

I

0

66 Hz 20

75

100

-20 0

20

40

60

Zre / Ω - cm2

Fig. 3. Comparison of experimental EIS data (points) measured for pipeline steel samples immersed in 1 M H2SO4 and the simulated (line) for some of the data shown in Fig. 1. (a) The blank, (b) IC-2 at 5 ppm, (c) IC-3 at 5 ppm, (d) IC-5 at 5 ppm, (e) IC-7 at 10 ppm, (f) IC-11 at 5 ppm, (g) IC-11 at 30 ppm and (h) IC-13 at 5 ppm. The equivalent electric circuit used to obtain the impedance parameters is shown in Tables 2–7 is indicated in each case.

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75

(g)

(h)

400

300

-Zim / Ω - cm2

-Zim / Ω - cm2

50

8.81 Hz 200

71.2 Hz 25

IC-13 [ 5 ppm ]

0

204 kHz

IC-11 [ 100 ppm ]

100

measured fitted

measured fitted 0

215 mHz

-25 0

100

503 kHz

200

300

400

500

0

25

200 mHz

Zre / Ω - cm2

50

75

Zre / Ω - cm2

Fig. 3 (continued)

recorded in the absence and in the presence of inhibitors, it was decided to use the equivalent electrical circuits (ECC) shown in Fig. 2, in order to simulate the experimentally generated impedance diagrams. Fig. 3 shows some examples of a simulated spectrum, using the ZView impedance fitting program, and experimentally generated impedance diagrams for the blank and in the presence of the inhibitors. A good fit was obtained with the model used for all experimental data. From Fig. 3, it is clear that the measured impedance plot is in accordance with the equivalent circuit shown in Fig. 2. The circuit consists of the solution resistance Rs, the charge transfer resistance, Rct, the constant phase element, CPE and the inductive elements, RL and L. For the description of a frequency independent phase shift between an applied AC potential and its current response, a constant phase element (CPE) is used, which is defined in impedance representation as:

n Z CPE ¼ Y 1 0 ðjxÞ

ð1Þ

where Y0 is the CPE constant, n is the CPE exponent which can be used as a gauge of the heterogeneity or roughness of the surface, j2 = 1 is an imaginary number and x is the angular frequency in rad s1. Depending on n, CPE can represent a resistance (ZCPE = R, n = 0); capacitance (ZCPE = C, n = 1), Warburg impedance (ZCPE = W, n = 0.5) or inductance (ZCPE = L, n = 1). The correct equation to convert Y0 into Cdl is given by [30].

C dl ¼ Y 0 ðx00m Þn

ð2Þ 00

where Cdl is the double layer capacitance and x is the angular frequency at which Zre is maximum. The values of the electrochemical parameters obtained from Nyquist plots are given in Tables 2–7. The results in the Tables show that Rct increases with increasing inhibitor concentration,

Table 2 Electrochemical parameters obtained during the fitting procedure to the data shown in Fig. 1a (inhibitor IC-2) with the equivalent electrical circuit ‘‘B’’ shown in Fig. 2. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-2 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

n

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 50 80 100

485 440 434 436 439 442 445 461

1.6 2.0 2.0 2.0 2.0 2.0 2.1 2.2

25 119 93 98 97 64 56 49

219 64 79 87 93 113 123 171

0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7

– 163 111 140 99 78 59 26

– 519 513 506 408 347 293 190

– 79 73 75 74 62 55 49

Table 3 Electrochemical parameters obtained during the fitting procedure to the data shown in Fig. 1b (inhibitor IC-3) with the equivalent electrical circuit ‘‘B’’ shown in Fig. 2. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-3 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

n

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 80 100

485 420 426 425 433 431 433

1.6 3.0 2.8 2.8 2.9 2.9 3.0

25 90 119 125 126 130 133

219 110 100 99 92 91 90

0.8 0.7 0.7 0.7 0.7 0.7 0.7

– 115 119 125 132 158 163

– 479 504 509 512 524 601

– 72 79 80 80 81 81

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Table 4 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 1c (inhibitor IC-5) with the equivalent electrical circuit ‘‘B’’ shown in Fig. 2. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-5 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

n

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 50 100

485 413 417 417 425 423 425

1.6 – 1.9 – – – 2.3

25 69 84 88 98 104 145

219 155 112 100 83 79 64

0.8 0.6 0.5 0.6 0.5 0.5 0.6

–

– 175 215 347 499 561 756

– 64 71 72 75 76 83

60 84 109 133 159 255

Table 5 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 1d (inhibitor IC-7) with the equivalent electrical circuit ‘‘B’’ shown in Fig. 2. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-7 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

n

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 50 80 100

485 434 416 431 429 424 428 426

1.6 1.8 1.8 1.8 1.8 1.9 1.9 2.1

25 54 82 99 140 142 157 182

219 141 122 116 94 90 73 59

0.8 0.7 0.7 0.6 0.6 0.7 0.6 0.7

– 67 182 225 396 408 482 658

– 160 297 385 579 696 726 876

– 54 70 75 82 82 84 86

Table 6 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 1e (inhibitor IC-11) with different equivalent electrical circuits, for inhibitor concentrations lower that 30 ppm circuit ‘‘B’’ shown in Fig. 2 was used, whereas for concentrations equal or greater than 30 ppm, circuit ‘‘C’’ was used. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-11 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Cmol (lF/cm2)

Rmol (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 50 100

485 437 435 429 424 425 416

1.6 1.7 2.0 1.7 1.9 2.2 2.3

– – – – 26 10 2

– – – – 16 17 42

25 58 115 177 265 395 468

219 112 74 53 46 35 25

– 44 107 304 – – –

– 297 463 555 – – –

– 57 78 86 91 94 95

Table 7 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 1f (inhibitor IC-13) with the equivalent electrical circuit ‘‘B’’ shown in Fig. 2. For the blank circuit ‘‘A’’ in Fig. 2 was used. IC-7 (ppm)

Ecorr (mV vs. SCE)

Rs (X cm2)

Rct (X cm2)

Cdl (lF/cm2)

n

L (H cm2)

RL (X cm2)

Inhibition efficiency (%)

0 5 10 20 30 50 80 100

485 421 434 437 430 432 429 424

1.6 2.5 2.4 2.3 3.5 2.7 2.3 2.3

25 70 95 109 116 119 122 140

219 134 111 95 89 77 69 58

0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.6

– 77 149 177 223 244 340 472

– 293 340 432 518 588 605 671

– 65 74 77 79 79 80 82

corresponding with the increase in the diameter of the semicircles. As can be noted from Fig. 1, the diameter of the semicircles increases with increasing inhibitors’ concentration except for the IC-2 compound, see Fig. 1a. Further, for a given inhibitor concentration, for example 30 ppm, Rct increases with increasing alkyl chain length of the 2 amino 5 alkyl 1,3,4 thiadiazole compounds except for IC-13. The largest diameter semicircle, about 475 X cm2 (Z 0real ) was recorded after adding 100 ppm IC-11, see Fig. 1e, whereas the shortest diameter, 50 X cm2, corresponds to IC-2 at the same concentration. Also note that for IC-13 the diameter of the semicircle was approximately 130 X cm2. Note that, compared to the blank’s semicircle diameter (ca. 25 X cm2), all

molecules tested displayed a different capacity toward the steel’s corrosion inhibition. According with the value obtained for n, it is possible to say that in all cases, the CPE used during the fitting process of the experimental impedances data is associated with the Warburg impedance. The increase in Rct values with increasing inhibitor concentration could be interpreted as a formation of an insulating adsorption layer. It is also observed that Cdl decreases with increasing concentration. This can be attributed to adsorption of inhibitor onto the steel surface leading to formation of a film which isolates the metal from the dissolution and charge transfer. In addition, the more the inhibitor is adsorbed, the greater the thickness increase

M. Palomar-Pardavé et al. / Corrosion Science 54 (2012) 231–243

of the barrier layer, which increases according to the expression of the layer capacitance presented in the Helmholtz model given in Eq. (3) [8]:

C dl ¼

ee0 A

ð3Þ

d

where d is the thickness of the protective layer, e is the dielectric constant of the medium, e0 is the vacuum permittivity and A is the effective area of the electrode. The inhibition efficiency of the thiadiazole derivatives was calculated from the charge-transfer resistance (Rct) values; see Tables 2–7, according to the following equation [2]:

RctðinhÞ  RctðblankÞ Inhibition efficiencyð%Þ ¼  100 RctðinhÞ

ð4Þ

where Rct(inh) and Rct(blank) are the charge-transfer resistance values with and without inhibitor, respectively. Rct can be interpreted as an electrical term that is inversely proportional to the corrosion rate. As can be noted from Tables 2–7, the estimated values for the charge transfer resistance (Rct) and the capacitance of the double layer in the acid solution without inhibitor were 24.8 X cm2 and 219.2 lF/cm2 respectively, whereas in the presence of the inhibitor IC-11 at a 100 ppm concentration, Rct and Cdl the corresponding values measured were 468 X cm2 and 25.2 lF/cm2 respectively, with a maximum inhibition efficiency of about 95%. Generally, it was observed that as the thiadiazole derivatives inhibitors were added to the acid solution in concentrations between 5 and 100 ppm it resulted in increased Rct values, thus increasing the percentage inhibition efficiency, however, it becomes clear that Cdl values tended to decrease. It could be assumed that such Cdl decrease is due to a gradual replacement of water molecules located at the steel interface by adsorption of organic molecules on surface electrode’s sites. This also causes a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer [31]. The addition of thiadiazole derivatives shifts the corrosion potential (Ecorr) toward more positive values, except for the case when the thiadiazole alkyl chain contains a small number of carbons, which indicates that these compounds influence the kinetics of the anodic and cathodic processes. Therefore, these results strongly suggest that the thiadiazole derivative compounds investigated here can be classified as a mixed-type inhibitors acting via molecular adsorption process on the metal surface. The inhibition efficiencies of these compounds valued from the EIS technique using Eq. (4), and shown in Fig. 4, vary with respect to their concentration as follows: as the inhibitor concentration increases its inhibition efficiency also increases, except for the

inhibition efficiency of IC-2 that diminishes. For a given inhibitor concentration, namely 50 ppm, the inhibition efficiency of these compounds is rated as: IC-2 < IC-3 < IC-13 < IC-5 < IC-7 < IC-11 therefore, in terms of inhibition capacity of these compounds, the optimum carbon atoms number for the alkyl chain is 11. The inhibition efficiency of the molecule 2 Amino 5 undecyl 1, 3, 4 thiadiazole (IC-11) reaches a 90% value at 30 ppm and 95% at 100 ppm. Based on these results, it was decided to choose 100 ppm as the optimum inhibitor concentration in order to evaluate inhibition efficiency at different immersion times. 3.1. Effect of the size in the alkyl chain group Fig. 5 shows the impedance behavior through a Nyquist representation for some thiadiazole compounds as a function of the size of their alkyl chain group. The impedance plots were recorded at 100 ppm of organic molecules added in the corrosive solution (1 M H2SO4).

Blank 500 IC-2 IC-3 IC-5 IC-7 IC-11 IC-13

165 Hz

400 652 Hz

300

-Zim / Ω - cm 2

238

41.2 Hz

5.02 Hz

101 kHz

8.81 Hz

200

100

3.75 Hz

0

500.3 kHz 10 mHz -100

0

100

200 mHz

200

300

400

Fig. 5. Electrochemical impedance diagrams (Nyquist plots) for pipeline steel in 1 M H2SO4 containing 100 ppm concentration of various thiadiazole compounds as indicated in the figure, the blank response is also shown for comparison.

inhibition efficiency / %

100

Table 8 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 7 with the equivalent electrical circuit ‘‘A’’ shown in Fig. 2.

80

IC-2 IC-3 IC-5 IC-7 IC-11 IC-13

60

40

0

20

40

60

80

100

Inhibitor Concentration / ppm Fig. 4. Variation of the inhibition efficiency of the IC-2, IC-3, IC-5, IC-7, IC-11 and IC13 thiadiazole inhibitors as a function of their concentration.

500

Zre / Ω - cm 2

Time (h)

Rct (X cm2)

Cdl (lF/cm2)

1 4 24 72 144 192 240 336 360 504 720

27 29 30 19 18 16 15 14 12 11 10

159 146 133 208 314 544 845 1128 1313 1761 2997

239

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The impedance diagrams in Fig. 5 still show depressed capacitive semicircles, whose diameter increases with the size of the chain alkyl group indicating the molecule’s ability to protect the steel against the corrosion process. However, a decrease in corrosion protection was only observed for the compound having the longest chain (IC-13). The analysis of the impedance diagrams was performed according to the equivalent electrical circuits proposed in Fig. 2, applying the least squares fitting. The calculated impedance parameters given in Table 8 show that the organic com-

μ

(a)

pound with an 11 alkyl chain group carbon atoms displayed the best inhibition efficiency of about 95% and a small double layer capacitance of 25.2 lF/cm2. As the results show clearly from Fig. 6a, all the inhibitors possess the capacity to diminish the double layer capacitance, as compared with that of the blank, due to variation in the value of the thickness of the protective layer (d), see Eq. (3). As the inhibitor’s alkyl chain increases from IC-2 to IC-11, the double layer capacitance diminishes. However, when the alkyl chain increases to IC-13, the Cdl increases to values obtained with IC-7, which could mean that the alkyl chain of IC13 is no longer linear but bent into a loop-like structure. In order to further support this hypothesis, both theoretical and experimental additional studies will be required. Moreover, this structural change in IC-13 can also explain why the inhibition efficiency, see Fig. 6b, reaches a maximum value when the number of carbon atoms in the alkyl chain of the thiadiazole compounds was 11.

3.2. Effect of the exposure time in the corrosive media It has been shown [3] that the inhibiting ability of organic molecules strictly depends on exposure time in the aggressive solutions. In order to examine the effect of exposure time on the corrosion

(b)

500

Circuit “C”

Fig. 6. Variation of (a) the double layer capacitance and (b) the inhibition efficiency as a function of the number of carbon atoms in the alkyl chain of the thiadiazole compounds. The data were obtained from EIS measurements for pipeline steel immersed in 1 M H2SO4 containing 100 ppm concentrations of the respective thiadiazole compound.

-Zim / Ω - cm2

400

300

200 362 mHz 1d

612 Hz 51.7 mHz

100 260 kHz

10d 30d

0 0

100

200

300

400

500

Zre / Ω - cm2 Fig. 8. Experimental (points) and simulated (lines) electrochemical impedance diagrams, Nyquist plots, for pipeline mild steel immersed in 1 M H2SO4 in the presence of 100 ppm IC-11 during different immersion times indicated in the figure in days. The equivalent electric circuit used to obtain the impedance parameters shown in Table 9 is indicated in the figure.

Table 9 Electrochemical parameters obtained during the fitting procedure of data shown in Fig. 8 with the equivalent electrical circuit ‘‘C’’ shown in Fig. 2.

Fig. 7. Electrochemical impedance diagram plots for pipeline mild steel immersed in 1 M H2SO4 during different times: hours (h) or days (d), indicated in the figure.

Time (h)

Rct (X cm2)

Cinh (lF/cm2)

n

Inhibition efficiency (%)

24 72 144 192 240 336 360 504 720

566 312 267 152 157 156 183 257 135

26 36 45 46 54 62 69 77 98

0.7 0.8 0.7 0.7 0.7 0.7 0.7 0.6 0.5

96 92 91 84 84 84 86 90 81

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behavior of the pipeline steel samples in 1 M H2SO4 solution with and without the addition of 100 ppm IC-11, the EIS technique was used. 3.2.1. On the Blank corrosion kinetics Fig. 7 shows the experimental impedance plots for the mild steel pipeline samples immersed in 1 M H2SO4 during different immersion times. The related electrochemical data are shown in Table 8. As can be noticed from Fig. 7 and Table 8, the charge transfer resistance of the mild steel in 1 M H2SO4 solution decreased from around 27 to ca. 10 X cm2 after its exposure for 720 h to the acid media, indicating that it becomes more susceptible to corrosion as a function of time.

Fig. 9. Variation of the inhibition efficiency of the IC-11 thiadiazole inhibitors as a function of time.

3.2.2. On the steel corrosion inhibition kinetics in the presence of IC-11 Fig. 8 shows experimental and simulated, impedance plots for the mild steel immersed in 1 M H2SO4 in the presence of 100 ppm of IC-11 for different immersion times. The related electrochemical data are shown in Table 9. As can be noticed from Fig. 8 and Table 9, the polarization resistance of the mild steel in

Fig. 10. Secondary electron images and their corresponding EDX analyses, of the API 5L X 52 surfaces: (a and b) prior to exposure to the medium, (c and d) after immersion for 720 h, in the aqueous 1 M H2SO4 solution, and (e and f) after immersion for 720 h, exposure in 1 M H2SO4 solution plus 100 ppm addition of the IC-11 inhibitor.

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1 M H2SO4 solution decreased from 566 to ca. 100 X cm2 after its exposure for 720 h in the acid media, however, the charge transfer resistance of mild steel in these conditions is large enough as compared with that of the blank at the same immersion time. From Fig. 9 it is possible to note that inhibition efficiency diminishes linearly (inhibition efficiency = 0.0565 t (h) + 96.76) up to 250 h, though after this time it remains practically constant up to 800 h. Also the change in the n value related to the depression angle reflects the roughness of the metal surface and/or the inhibitor structure due to the corrosion process, diminish its value as a function of time, hence as the metal is corroded.

Temkin’s model:

K ad C ¼ ef h

ð6Þ

Langmuir’s model:

C 1 ¼ þC h K ad

ð7Þ

Freundlich’s model: 1

K ad C n ¼ h

ð8Þ

3.3. SEM analysis

Frumkin’s model:

Fig. 10 shows the results of the SEM analysis on the topography of the samples examined. Fig. 10a reveals some relevant surface characteristics of the steel sample before exposure to the corrosive environment, while Fig. 10b presents the microanalysis that resulted from energy dispersive analysis, EDX: it becomes clear that it corresponds to the basic elemental composition of the steel. Fig. 10c presents the micrograph obtained after immersion in the 1 M H2SO4 solution, and Fig. 10d shows the corresponding elemental microanalysis. From these results, it is straightforward that in this case, either pits or corrosion products having a sulfate-rich composition were formed. Fig. 10e reveals the surface of the steel sample after exposure to the 1 M H2SO4 containing the IC-11 inhibitor at 100 ppm, the concentration that displayed the largest charge transfer resistance value and the greatest inhibition efficiency: Fig. 10f shows the corresponding EDX microanalysis. It is important to stress that when the IC-11 compound was present in solution, the morphology of the steel surface turns out to be quite different from both the previous ones and that the IC-11 molecules was still present on the substrate surface, possibly forming some kind of iron complex. Furthermore, corrosion neither originated pits nor products having a sulfate-rich composition.

K ad C ¼



 h ef h 1h

ð9Þ

where Kad is the adsorption equilibrium constant, C the concentration, and f is the molecular interaction constant. Once a suitable adsorption isotherm has been selected, the corrosion rate measurements can be used to calculate the thermodynamic data pertaining to adsorption of the inhibitors tested. The standard free energy of adsorption (DG0ads ) can be evaluated with the following expression:

DG0ads ¼ RT InðK ad Þ

ð10Þ

θ

(a)

3.4. Inhibition mechanism Corrosion inhibition features of several substances are directly associated to adsorption phenomena, which can follow different types of isotherms such as those of Temkin, Langmuir, Freundlich and Frumkin, that have been employed to classify adsorption phenomena over steel electrodes [7,32]. The adsorption of organic inhibitors at an electrode/electrolyte interface may take place through displacement of adsorbed water molecules at the inner Helmholtz plane of the electrode, likely in agreement with the following reaction scheme [7]:

(b)

h¼

RctðinhÞ  RctðblankÞ RctðinhÞ

ð5Þ

As mentioned before, four isotherm adsorption models can be employed to determine empirically which adsorption isotherm fits best the surface coverage data. The equations pertaining to the adsorption models cited are:

θ

If it is assumed that the steel electrode is corroding uniformly, then the corrosion rate in the absence of inhibitors is representative of the total number of corroding sites. Consequently, the corrosion rate in the presence of inhibitors may be assumed to represent the number of potentially corroding sites that remain after blockage due to inhibitor adsorption [7]. The fractional surface coverage (h) is represented by the following equation:

μ

OrgðaqÞ þ mH2 OðadsÞ ¡ OrgðadsÞ þ mH2 OðaqÞ

μ Fig. 11. (a) Adsorption plot for IC-11 on API 5L X52 steel in 1 M H2SO4 and (b) comparison between the experimental adsorption isotherm (points) for the inhibitor indicated in the graph, in the system API 5L X52 steel/1 M H2SO4 at different inhibitor concentrations and the theoretical model (lines) proposed by Langmuir, Eq. (7).

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Table 10 Estimation of the adsorption constant (Kad) and the standard free energy of adsorption (DG0ads ) of the different inhibitors tested in the system API 5L X52 steel in 1 M H2SO4: the experimental isotherm was fitted with the Langmuir model. Corrosion inhibitor

Linear regression equation

R2

ln (Kad)

DG0ads (kJ mol1)

IC-3 IC-5 IC-7 IC-11 IC-13

C/h(lM) = 1.22 C/h(lM) = 1.20 C/h(lM) = 1.12 C/h(lM) = 1.02 C/h(lM) = 1.20

0.99 0.99 0.99 0.99 0.99

12.48 11.27 10.67 10.90 11.27

30.93 27.92 26.44 27.02 27.92

C C C C C

(lM) + 3.8 (lM) (lM) + 12.7 (lM) (lM) + 23.2 (lM) (lM) + 18.3 (lM) (lM) + 12.7 (lM)

In order to gain insight into the mode of adsorption of IC-11 onto the steel surface, the variation of the surface coverage values, obtained from EIS technique, as a function of the inhibitor concentration, see Fig. 11a, was theoretically fitted into different adsorption isotherms and the values of correlation coefficient (R2) were used to determine the best fit isotherm [27,33,34]. Fig. 11b shows the plots of C/h as a function of C which is typical of Langmuir adsorption isotherm, see Eq. (7) for all the inhibitors tested in this work, except IC-2. An exact linear plot were obtained in all cases with correlation coefficients of 0.99 and slope values close to 1, see Table 10. The linearity of the plots in Fig. 11b suggests that the adsorption of all the thiadiazole type inhibitors follows the Langmuir isotherm. The standard free energy of adsorption, DG0ads , which can characterize the interaction of adsorption molecules with the metal surface, was calculated by Eq. (10) and they are reported in Table 10. The negative values of DG0ads , ensure the spontaneity of the adsorption process and speak of the stability of the adsorbed layer on the steel surface. Generally, the values of around 20 kJ mol1 or lower are consistent with physisorption, while those around 40 kJ mol1 or higher involve chemisorption [34]. From Table 10 it is possible to note that DG0ads , values are around 30 kJ mol1, therefore, in this case, it is difficult to distinguish between chemisorption and physisorption only based on this criterion. The corrosion of the steel in 1 M H2SO4 was retarded by the presence of different concentrations of the 2 amino 5 alkyl 1,3,4 thiadiazole derivatives evaluated. The results indicated that the inhibition mechanism involves blockage of the mild steel surface by the inhibitor molecules through an adsorption process. The synthesized compounds showed three characteristics that facilitate adsorption on metal surface and as result, promoted a good corrosion inhibition: (a) the presence of a prime amino group (–NH2), which is protonated when in contact with an aggressive environment (—NHþ 3 ), (2) an aromatic heterocyclic ring that contains sulfur and nitrogen atoms, which can participate in the process of retro-donation of electrons, and (3) hydrophobic chains that provide the hydrophobic character and help in the formation of adsorbed monolayers over the metal surface. The results clearly show that the alkyl chain length plays an important role in the inhibition efficiency of the synthesized inhibitors. It was observed that inhibition efficiency of the compounds evaluated first increases (up to the undecyl spacer chain) and then decreases with the length of the spacer for tridecyl chain; this dependence can be explained as a result of the flexibility and the possibility that the alkyl chain bends over itself [35]. It is evident that the geometrical structure of the alkyl chains varied from the linear structure for short chains (C2–C11), and then they formed a loop-like structure with one coil for C12–C16 and more than one coil for longer chains (C18–Cn). As the geometric length is increased, which is defined as the net space occupied by the hydrocarbon chain to describe the distance between metal surface and corrosive medium, the isolation between metal–medium interaction is increased [36]. However, long alkyl chains with more than ten carbon atoms can fold the spacer of the tail and so that the tridecyl chain (IC-13) had a smaller geometric length than the

undecyl chain (IC-11), which was reflected in their inhibitory effects. 4. Conclusions In this work, the inhibition properties of 2 amino 5 alkyl 1,3,4 thiadiazole compounds towards steel corrosion in 1 M H2SO4 have been determined using EIS measurements. The results indicated that the inhibition mechanism involves blockage of the mild steel surface by the inhibitor molecules through a Langmuir type adsorption process. Moreover, the results clearly show that alkyl chain length plays an important role in the inhibition efficiency of the synthesized inhibitors. It was observed that inhibition efficiency of the compounds evaluated first increases (up to undecyl spacer chain) and then decreases with the length of the spacer for tridecyl chain; this dependence can be explained as a result of the flexibility and folding of alkyl chain. From SEM analysis of the electrode surface it was found that in the absence of IC-11 both pits and corrosion products having a sulfate-rich composition were formed on the steel sample surface after exposure to the 1 M H2SO4; however in the presence 100 ppm IC-11 neither pits nor products having a sulfate-rich composition were observed. Acknowledgements M.A.A.Q. expresses his gratitude to CONACyT for his Ph.D., studentship (162447). M.R.R. and M.P.P. would like to thank CONACYT for project 22610714 and H.H.H. to ICyTDF for his postdoctoral fellowship. M.R.R., N.L., H.H.H., J.U. and M.P.P. gratefully acknowledge the SNI for the distinction of their membership and the stipend received. The authors also wish to express their gratitude to Departamento de Materiales at UAMA for the support given through Projects 2261203, 2261204, 2261205. This work was done in partial fulfillment of MAAQ’s Ph.D. requirements. References [1] S. Issaadi, T. Douadi, A. Zouaoui, S. Chafaa, M.A. Khan, G. Bouet, Corros. Sci. 53 (2011) 1484–1488. [2] Y. Tang, X. Yang, W. Yang, Y. Chen, R. Wan, Corros. Sci. 52 (2010) 242– 249. [3] X. Li, S. Deng, H. Fu, Corros. Sci. 53 (2011) 3241–3247. [4] X. Li, S. Deng, H. Fu, Corros. Sci. 53 (2011) 302–309. [5] A. Döner, R. Solmaz, M. Özcan, G. Kardasß, Corros. Sci. 53 (2011) 2902–2913. [6] L. Rodríguez-Bravo, M. Romero-Romo, C. Ángeles-Chávez, M. Palomar-Pardavé, in: Manuel Palomar-Pardavé, Mario Romero-Romo (Eds.), Electrochemistry and Materials Engineering, Research Signpost, 2007, pp. 151–161 (Chapter 9). [7] P. Morales-Gil, G. Negrón-Silva, M. Romero-Romo, C. Ángeles-Chávez, M. Palomar-Pardavé, Electrochim. Acta 49 (2004) 4733–4741. [8] R. Álvarez-Bustamante, G. Negrón-Silva, M. Abreu-Quijano, H. HerreraHernández, M. Romero-Romo, A. Cuán, M. Palomar-Pardavé, Electrochim. Acta 54 (2009) 5393–5399. [9] A.V. Espinoza, G. Negrón, M.E. Palomar-Pardavé, M.A. Romero-Romo, I. Rodríguez, H. Herrera-Hernández, ECS Trans. 20 (1) (2009) 543–553. [10] D.Y. Cruz, G. Negrón, M.A. Romero-Romo, H. Herrera-Hernández, M.E. Palomar-Pardavé, ECS Trans. 20 (1) (2009) 519–527. [11] M. Palomar-Pardavé, C.O. Olivares-Xometl, N.V. Likhanova, J.B. PérezNavarrete, J. Surfactants Deterg. 14 (2011) 211–220. [12] J.B. Pérez-Navarrete, C.O. Olivares-Xometl, N.V. Likhanova, J. Appl. Electrochem. 40 (2010) 1605–1617.

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