Synthesis of Selected Vinylimidazolium Ionic Liquids and Their

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Synthesis of Selected Vinylimidazolium Ionic Liquids and Their Effectiveness as Corrosion Inhibitors for Carbon Steel in Aqueous Sulfuric Acid Diego Guzman-Lucero,† Octavio Olivares-Xometl,*,‡ Rafael Martínez-Palou,† Natalya V. Likhanova,† Marco A. Domínguez-Aguilar,† and Vicente Garibay-Febles† †

Programa de Investigacion y Posgrado, Instituto Mexicano del Petroleo, Eje Central Norte Lazaro Cardenas 152, Mexico 07730 D.F., Mexico ‡ Facultad de Ingeniería Química, Benemerita Universidad Autonoma de Puebla, Av. San Claudio Ciudad Universitaria, Col. San Manuel, Puebla 72570, Puebla, Mexico ABSTRACT: Five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations and bromide as anion (IL1IL5), were obtained by conventional and microwave synthesis. Compounds were tested in aqueous 1 M H2SO4 as corrosion inhibitors for carbon steel. Weight loss and polarization curves indicated that inhibition efficiency increased with concentration, which turns out to be dependent on alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5). The relatively high inhibitory properties (8895%) displayed by IL4 within 2540 °C were ascribed to a chemisorption process that involved the following: the adsorption of protonated imidazolium molecules on both the anodic and cathodic sites, the latter in competition with hydrogen ions to mitigate hydrogen evolution; and also the formation of π bond with iron by the CdN group from imidazolium ring (this way inhibitor produced more than one center of adsorption action). Surface analysis indicated a considerable reduction of corrosion products after the addition of IL4.

1. INTRODUCTION Acidic solutions are commonly used in picking operations as part of the steelmaking finishing process in which oxide and scale are removed from metallic surface. A solution of sulfuric acid is normally employed to treat carbon steel products, and inhibitor is provided to lessen acid attack. Diluted sulfuric acid can be applied to carbon steel up to 90 °C 1 in concentrations of 515 wt %.2 Likewise, refinery operations involved the formation of sulfuric acid derived from the dissolution of sulfur compounds (e.g., hydrogen sulfide, thyosulfates, and mercaptans, etc.). It is therefore important to quantify the effect of acidic solution aggressiveness and develop corrosion inhibitors for carbon steel protection. The use of corrosion inhibitors (CIs) comprises one of the most economical ways to mitigate corrosion rate and protect metallic materials against corrosion to preserve industrial facilities,3 especially in acidic media.4 In this context, the treatment of mild steel corrosion in acidic environment through organic compounds has resulted in considerable savings for the oil industry. Several families of organic compounds, i.e., fatty amides,5,6 pyridines,79 imidazolines,1012 and 1,3-azoles,1315 have shown excellent performance as CIs. However, many of these compounds are toxic, and they do not completely fulfill the requirements imposed by the environmental protection standards. This is the reason why in the past few years great efforts have been made by researchers in this area to develop new environmentally friendly CIs.16 Ionic liquids (ILs) have attracted the attention of researchers in the past decades due to their interesting physical and chemical properties. ILs are an excellent alternative to r 2011 American Chemical Society

substitute volatile organic solvents because of their very low vapor pressures, thermal and chemical stability, no flammability, and their ability to act as catalyst.17 Moreover, ILs present a wide electrochemical window so this property has been studied for electrochemical applications in batteries,18 light emitting electrochemical cells,19 and fuel cells.20 There are few studies that involved ionic liquids as CIs for acid environments; these are two ILs containing 1-butyl-3-methylimidazolium as cation, along with either chloride or hydrogen sulfate as anion, which have shown good properties as CIs for mild steel in aqueous 1.0 M HCl.21 In this work, five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations together with bromide as anion (IL1IL5), were synthesized and evaluated as CIs for acid environment (Table 1). Weight loss tests and electrochemical polarization curves were applied to test the inhibitory properties of these compounds in AISI 1018 carbon steel immersed in 1.0 M H2SO4. All of the ionic liquids studied showed inhibitory properties dependent on the chain length linked to N3. The highest efficiency of IL4 was confirmed by the gravimetric and electrochemical tests, which were completed by scanning electron microscopy/energydispersive X-ray spectroscopy (SEM/EDX) and atomic force microscopy (AFM).

Received: January 10, 2011 Accepted: April 29, 2011 Revised: April 27, 2011 Published: April 29, 2011 7129

dx.doi.org/10.1021/ie1024744 | Ind. Eng. Chem. Res. 2011, 50, 7129–7140

Industrial & Engineering Chemistry Research Table 1. Chemical Structure of the ILs Evaluated as Corrosion Inhibitors

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents (Aldrich) were used without previous purification. The experiments were performed on samples of AISI 1018 carbon steel, whose chemical composition is as follows (wt %): 0.10 C, 0.25 Si, 0.45 Mn, 0.03 P, 0.03 S, and balanced with Fe. Specimens were abraded with wet SiC paper number 400, 600, 1000, and 1200, degreased in hexane, and washed in an ultrasonic bath of acetone for 5 min to remove impurities. 2.2. Measurements and Equipments. The synthesized compounds were characterized by 1H and 13C NMR and FTIR 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 using tetramethylsilane (TMS) as internal standard and the solvent indicated in each case at room temperature. Chemical shifts (δ) were reported in parts per million. Microwave-assisted syntheses were carried out in CEM Discover equipment with simultaneous cooling. The morphology on the metallic surface was observed by a SEM microscope model Philips XL30ESEM. Compositional results were obtained by the EDX module attached to the microscope. The AFM images were obtained by a digital instrument model Nanoscope V Tuna D3100 AFM. This equipment was operated in tapping mode at ambient conditions and low operating voltage in order to reduce any damage on the sample surface. The lateral scan size was 2.5 μm with a number of views of 512, which were recorded at a scan rate of 0.5 Hz. A Netzsch STA 409 highresolution simultaneous thermogravimetric analysis, TGA, and differential scanning calorimetry, DSC, device was used for the thermal analysis at a heating rate of 5 °C/min in nitrogen atmosphere. 2.3. General Procedure for IL Synthesis. 1-Vinylimidazole (0.05 mol) was added to the corresponding alkyl bromide (0.055 mol, 10% excess), mixture was then heated at 60 °C and agitated with a magnetic stirrer for 36 h. During this period of time, we observed the formation of another phase or precipitate, dependent on the length of the alkyl chain. The ionic liquid product was separated as a phase under the solution or filtered. The residual starting materials of the liquid products were removed by three successive extractions with ethyl acetate (4 20 mL). The solid products were purified by recrystallization from ethyl acetate,

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and the ionic liquids were dried under vacuum at 60 °C for 24 h. Additionally, compounds were synthesized for the first time under solvent-free microwave irradiation conditions at 50 °C and with simultaneous cooling. In this case, the products were obtained in similar yields but in only 20 min of irradiation. 1-Vinyl-3-butylimidazolium Bromide (IL1). Following the general procedure with 4.7 g of 1-vinylimidazole and 7.5 g of butyl bromide, a yellow viscose liquid (85%) was obtained. 1H NMR (D2O, ppm): δ 0.99 (t, J = 7.4 Hz, 3H): 1.39 (sx, J = 7.4 Hz, 2H), 1.95 (qi, J = 7.4 Hz, 2H), 4.32 (t, J = 7.2 Hz, 2H), 5.50 (dd, J1 = 8.8 Hz, J2 = 2.8 Hz, 1H), 5.87 (dd, J1 = 15.4 Hz, J2 = 2.6 Hz, 1H), 7.23 (dd, J1 = 15.7 Hz, J2 = 8.8 Hz, 1H), 7.67 (d, J = 1.7 Hz, 1H), 7.85 (d, J = 1.9 Hz, 1H), 9.13 (s, 1H). 13C NMR (D2O, ppm): δ 13.0, 19.1, 31.4, 50.0, 109.8, 119.8, 123.3, 128.6, 134.6. IR (5004000 cm1, KBr pellet, cm1): ν 3434, 3056, 2960, 2873, 1652, 1571, 1550, 1463, 1371, 1172, 962, 754, 599. 1-Vinyl-3-octylimidazolium Bromide (IL2). Following the general procedure with 4.7 g of 1-vinylimidazole and 10.6 g of octyl bromide, a yellow viscose liquid (87%) was obtained. 1H NMR (DMSO-d6, ppm): δ 0.86 (t, J = 7.1 Hz, 3H), 1.27 (m, 10H), 1.84 (qi, J = 7.1 Hz, 2H), 4.21 (t, J = 7.1 Hz, 2H), 5.43 (dd, J1 = 8.8 Hz, J2 = 2.2 Hz, 1H), 6.00 (dd, J1 = 15.7 Hz, J2 = 2.4 Hz, 1H), 7.30 (dd, J1 = 15.7 Hz, J2 = 8.8 Hz, 1H), 7.94 (d, J = 1.6 Hz, 1H), 8.21 (d, J = 1.6 Hz, 1H), 9.62 (s, 1H). 13C NMR (DMSOd6, ppm): δ 13.5, 21.7, 25.2, 28.0, 28.1, 28.8, 30.8, 49.0, 105.5, 119.1, 123.0, 128.6, 135.0. IR (5004000 cm1, KBr pellet, cm1): ν 3432, 3052, 2956, 2927, 2856, 1652, 1572, 1548, 1465, 1172, 964, 599. 1-Vinyl-3-dodecylimidazolium Bromide (IL3). Following the general procedure with 4.7 g of 1-vinylimidazole and 13.7 g of dodecyl bromide, a yellow waxlike solid (85%) was obtained, mp 5657 °C. 1H NMR (DMSO-d6, ppm): δ 0.86 (t, J = 6.8 Hz, 3H), 1.25 (m, 18H), 1.84 (m, 2H), 4.23 (t, J = 6.8 Hz, 2H), 5.43 (dd, J1 = 7.7 Hz, J2 = 2.2 Hz, 1H), 5.98 (dd, J1 = 13.5 Hz, J2 = 2.2 Hz, 1H), 7.33 (dd, J1 = 15.4 Hz, J2 = 7.4 Hz, 1H), 7.96 (d, J = 1.4 Hz, 1H), 8.22 (d, J = 1.4 Hz, 1H), 9.69 (s, 1H). 13C NMR (DMSO-d6): δ 13.4, 21.68, 25.2, 28.0, 28.2, 28.4, 28.5, 28.6 (2C), 28.7, 30.9, 49.0, 108.5, 119.1, 122.9, 128.5, 135.0. IR (500 4000 cm1, KBr pellet, cm1): ν 3474, 3396, 3135, 3093, 2913, 2850, 1648, 1552, 1465, 1365, 1170, 960, 817, 624, 593. 1-Vinyl-3-octadecylimidazolium Bromide (IL4). Following the general procedure with 4.7 g of 1-vinylimidazole and 18.3 g of octadecyl bromide, a white powder was obtained (83%), mp 7374 °C. 1H NMR (CDCl3, ppm): δ 0.88 (t, J = 6.9 Hz, 3H), 1.30 (m, 30H), 1.95 (qi, J = 6.9 Hz, 2H), 4.41 (t, J = 7.4 Hz, 2H), 5.40 (dd, J1 = 8.8 Hz, J2 = 3.0 Hz, 1H), 6.00 (dd, J1 = 15.7 Hz, J2 = 3.0 Hz, 1H), 7.51 (dd, J1 = 15.7 Hz, J2 = 8.8 Hz, 1H), 7.61 (s, 1H), 7.96 (s, 1H), 10.77 (s, 1H). 13C NMR (CDCl3, ppm): δ 14.2, 22.7, 26.3, 29.1, 29.46, 29.5, 29.6, 29.8 (8C), 30.3, 32.0, 50.6, 109.9, 119.5, 122.8, 128.4, 136.0. IR (5004000 cm1, KBr pellet, cm1): ν 3475, 3396, 3133, 3087, 2917, 2848, 1648, 1552, 1465, 1172, 815, 719. 1-Vinyl-3-docosylimidazolium Bromide (IL5). Following the general procedure with 2.6 g of 1-vinylimidazole and 11.7 g of docosyl bromide, a white powder was obtained (86%), mp 8990 °C. 1H NMR (CDCl3, ppm): δ 0.81 (t, J = 6.9 Hz, 3H), 1.23 (m, 38H), 1.88 (qi, J = 6.9 Hz, 2H), 4.34 (t, J = 7.4 Hz, 2H), 5.34 (dd, J1 = 8.8 Hz, J2 = 3.0 Hz, 1H), 5.96 (dd, J1 = 15.7 Hz, J2 = 3.0 Hz, 1H), 7.45 (dd, J1 = 15.7 Hz, J2 = 8.8 Hz, 1H), 7.50 (d, J = 1.6 Hz, 1H), 7.85 (d, J = 1.6 Hz, 1H), 10.76 (s, 1H). 13C NMR (CDCl3, ppm): δ 14.2, 22.8, 26.4, 29.1, 29.4, 29.5, 29.6, 29.8 (12C), 30.4, 32.0, 50.6, 109.9, 119.4, 122.7, 128.5, 136.2. IR 7130

dx.doi.org/10.1021/ie1024744 |Ind. Eng. Chem. Res. 2011, 50, 7129–7140

Industrial & Engineering Chemistry Research

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(5004000 cm1, KBr pellet, cm1): ν 3469, 3401, 3133, 3085, 2917, 2850, 1648, 1550, 1465, 1174, 719. 2.4. Weight Loss Measurements. Weight loss tests were developed on rectangular specimens with a size of 2.54 cm 1.25 cm 0.025 cm. The immersion time was of 1, 2, 3, 4, and 6 h at 25 and 40 °C in 1.0 M H2SO4 solution naturally aerated with and without the addition of different concentrations of inhibitors: 10, 25, 50, and 100 ppm. The weight of each specimen was determined before and after testing in the aggressive medium with a digital balance model Sartorius Basic. After the exposure time of immersion, the specimen was removed and washed with double distilled water. The corrosion product on the steel surface was mechanically removed by rubbing it with a brush. The specimens were washed in an ultrasonic bath with acetone, dried in a flux of high-purity nitrogen, and set to dry in a stove at 100 °C for 2 h, and finally its weight loss was recorded. The experiments were done by triplicate and the average weight loss was used to determine corrosion rate (CR) and inhibition efficiency as follows: CR ¼

θ¼

KW ATD

ð1Þ

CR o CR inh o CR o

ð2Þ

IE=% ¼ θ 100

ð3Þ 2

where CR stands for corrosion rate (g/(m h)), K is a constant (1 104D),22 W is the weight loss (g), A is the coupon area (cm2), T is the exposure time (h), D is the steel density (g/cm3), and CRo, CRinh o are the corrosion rates of carbon steel in the absence and presence of inhibitor, respectively. 2.5. Electrochemical Measurements. Potentiodynamic polarization curves were carried out using the conventional threeelectrode cylindrical glass cell. The working electrode (WE) was made of AISI 1018 carbon steel with a contact area of 0.32 cm2, a high-purity platinum foil (99.9%) with a surface area of 4.3 cm2, which was used as the counter electrode (CE), and a saturated calomel electrode (SCE), via a Luggin capillary probe, was used as the reference electrode (RE). The WE was wetabraded with 400, 600, 100, and 1200 emery paper, degreased in AR-grade 2-propanol and acetone, and rinsed with deionized water prior to each experiment. Experiments were performed in the absence (blank) and presence of corrosion inhibitors at 25 ( 1 °C. Electrochemical tests were performed in a potentiostat/galvanostat PGSTAT302N controlled by a PC through the general purpose electrochemical system (GPES). Before recording the polarization curves, the WE specimen was immersed in the test solution for 30 min until steady-state open circuit potential (Eocp) was reached. The cathodic and anodic polarization scans were performed from 250 mV to Eocp to þ250 mV at a rate of 1.6 mV s1. Tafel slopes were constructed, and inhibition efficiencies (IE, %) were determined as follows: " # jcorr jinh corr IE=% ¼ 100 ð4Þ jcorr where jinh corr and j corr are the corrosion current densities with and without the addition of corrosion inhibitor, respectively.

Scheme 1. General Reaction of IL’s Synthesis

3. RESULTS AND DISCUSSION 3.1. Synthesis of Ionic Liquids. We select ILs containing imidazolium cation with long alkyl chain because it is well-known that many 1,3-azole heterocycles have shown good performance as CIs.717 Alkyl chains play an important role in inhibition efficiency and binding energy between inhibitor and metal surface; for example, the binding energies for imidazolines increase from C8 to C18 series.23,24 Likewise, inhibition efficiency also depends on the presence of insaturations in molecular structure;2527 this is the reason why, 1-vinylimidazole was employed as starting material to synthesize the selected ILs for this study. ILs containing halogens as anions present a very low solubility in hydrocarbon environments and a relatively high water solubility. When a long alkyl chain is part of a molecule, they become amphiphilic compounds that act as charged surfactants. ILs containing halogens are capable of being synthesized in a onepot and one-step synthesis reaction to be the most economical of ILs available. Bearing this in mind, five ILs were synthesized under conventional heating and also under microwave dielectric heating. According to NMR spectroscopy, products whose structures are shown in Table 1 were obtained with purity greater than 95%. They were obtained by alkylation reactions of 1-vinylimidazole with alkylbromides, in which aliphatic chains with lengths between C4 and C22 (IL1IL5) were developed to study the effect of carbon length on the inhibitory properties of compounds (Scheme 1). Microwave-assisted synthesis yielded similar products though in times of about 20 min of dielectric heating at 50 °C. This procedure involved simultaneous cooling to avoid the spontaneous radical-free polymerization of 1-vinylimidazole and also solvent-free conditions, which implicate process savings and green environment protection.28,29 3.2. Weight Loss Tests of ILs as CIs. Table 2 summarizes the results obtained after gravimetric tests of carbon steel in 1 M H2SO4 at 25 °C when added ILs developed at concentrations of 10, 25, 50, and 100 ppm for different exposure times (1, 3, and 6 h). The table shows that corrosion rate depends on immersion time and also on the temperature of corrosive environment. For all of the studied compounds, the IE (%) increased when concentration was increased. It was observed that the corrosion rate increased with temperature, while the IE increased with the exposure time at room temperature. It is important to note that IE follows the order IL4 > IL3 > IL1 > IL2 > IL5 at 25 °C within 13 h of exposure time. At 4 and 6 h, the order is similar except that the IE of IL5 is higher than that of IL2. The structural difference among the ILs evaluated is the N3 alkyl chain size; the compound with an alkyl chain of C12C18 (IL3, IL4) showed better IE than both ILs containing short (C4 and C8) and large (C22) alkyl chains. These results are in correspondence with those obtained by several authors for imidazoline type corrosion inhibitors.24,25 It seems that C18 is the optimum alkyl chain size to obtain a suitable binding force between metal and inhibitor molecules. The results of the weight loss test at 40 °C (Table 3) show that, in the case of IL4, 3 and 5, the IEs are higher than those at 25 °C 7131



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Table 2. Corrosion Parameters Obtained from Weight Loss Test at 25 °C for AISI 1018 Carbon Steel in 1.0 M H2SO4 as a Function of IL Concentration and Immersion Time 1h concn, ppm/M blank IL1

IL2

IL3

IL4

IL5

2

CR, g/(m day)

2h IE, %

0.82

2

CR, g/(m day)

3h IE, %

2

CR, g/(m day)

1.12

4h IE, %

1.57

2

CR, g/(m day)

6h IE, %

3.10

2

CR, g/(m day)

IE, %

5.17

10/4.33 105

0.71

16

0.82

27

0.77

50

1.34

57

2.07

60

25/1.08 104 50/2.16 104

0.52 0.43

36 47

0.59 0.50

47 56

0.57 0.43

64 72

0.89 0.66

71 79

1.30 0.68

75 87

100/4.33 104

0.36

56

0.41

64

0.46

71

0.75

76

0.77

85

10/3.48 105

0.73

13

0.93

17

1.21

22

2.26

27

3.42

34

25/8.70 105

0.66

21

0.82

27

1.12

29

2.05

34

2.89

44

50/1.74 104

0.57

30

0.73

35

0.98

38

1.64

47

2.44

53

100/3.48 104

0.46

46

0.57

49

0.71

55

1.12

64

1.66

68

10/2.91 105

0.48

41

0.62

45

0.82

48

1.44

54

2.07

60

25/7.28 105 50/1.46 104

0.46 0.32

46 61

0.57 0.41

50 64

0.73 0.48

54 69

1.23 0.80

60 74

1.34 0.82

74 84

100/2.91 104

0.25

71

0.25

78

0.27

83

0.41

87

0.32

94

10/2.34 105

0.30

64

0.36

67

0.48

69

0.80

74

1.03

80

25/5.85 105

0.27

68

0.34

70

0.43

73

0.66

79

0.77

85

50/1.17 104

0.18

77

0.25

78

0.30

81

0.52

83

0.62

88

100/2.34 104

0.14

83

0.18

83

0.23

86

0.36

88

0.52

90

10/2.07 105

0.77

6

0.98

13

1.28

19

1.53

51

1.55

70

25/5.17 105 50/1.03 104

0.73 0.62

11 25

0.91 0.77

19 31

1.14 1.00

27 36

1.46 1.21

53 61

1.09 0.98

79 81

100/2.07 104

0.55

35

0.66

41

0.82

48

0.84

73

0.89

83

Table 3. Corrosion Parameters Obtained from Weight Loss Test at 40 °C for AISI 1018 Carbon Steel in 1.0 M H2SO4 as a Function of IL Concentration and Immersion Time 1h concn, ppm/M blank IL1

IL2

IL3

IL4

IL5

2

CR, g/(m day)

2h IE, %

1.60

2

CR, g/(m day)

3h IE, %

2

CR, g/(m day)

3.74

4h IE, %

6.18

2

CR, g/(m day)

6h IE, %

6.86

2

CR, g/(m day)

IE, %

7.38

10/4.33 105

1.37

13

2.99

20

4.56

26

4.60

33

4.51

39

25/1.08 104 50/2.16 104

1.09 0.91

31 43

2.21 1.91

41 49

3.33 2.60

46 58

3.35 2.46

51 64

3.17 2.21

57 70

100/4.33 104

0.73

54

1.60

57

2.53

59

2.67

61

2.39

67

10/3.48 105

1.50

5

3.33

11

4.99

19

5.01

27

5.01

32

25/8.70 105

1.41

10

3.10

17

4.63

25

4.74

31

4.72

36

50/1.74 104

1.25

21

2.64

29

3.90

37

3.90

43

3.40

54

100/3.48 104

1.00

37

2.01

46

3.15

49

3.15

54

2.74

63

10/2.91 105

0.91

43

2.12

43

3.26

47

3.42

50

3.62

51

25/7.28 105 50/1.46 104

0.82 0.55

49 65

1.80 1.19

52 68

2.78 1.66

55 73

2.94 1.71

57 75

2.87 1.55

61 79

100/2.91 104

0.39

76

0.71

81

0.98

84

0.68

90

0.43

94

10/2.34 105

0.68

57

1.00

73

1.48

76

1.37

80

1.25

83

25/5.85 105

0.50

68

0.77

79

0.93

85

0.89

87

0.66

91

50/1.17 104

0.36

77

0.57

85

0.68

89

0.68

90

0.52

93

100/2.34 104

0.39

75

0.48

87

0.55

91

0.34

95

0.36

95

10/2.07 105

1.21

23

2.58

31

3.90

37

2.67

61

2.37

68

25/5.17 105 50/1.03 104

1.05 0.89

33 44

1.94 1.53

48 59

3.51 2.60

43 58

2.12 1.85

69 73

1.98 1.37

73 81

100/2.07 104

0.64

60

1.23

67

2.35

62

1.57

77

1.12

85

7132



Figure 1. Inhibition efficiency vs IL4 concentration at (a) 25 and (b) 40 °C as a function of immersion time.

(Table 2) within 13 h of exposure time; furthermore, IL4 reached top efficiency at 40 °C after 6 h. At 40 °C, IEs followed the order IL4 > IL3 > IL5 > IL1 > IL2 for every time tested, while at 25 °C the IL5 compound displayed a lower efficiency in the first 3 h of exposition. As temperature is increased, IL1 and IL2 lose their efficiency in approximately 820%. These results suggest that IL1 and IL2 may be physically adsorbed on the metal surface, so when there is an increase in temperature, they are desorbed from the surface which conveys a decrease in inhibition efficiency. In the case of IL3, -4, and -5, as temperature increases, physical adsorption changes to chemisorption, a process that is common to filming inhibitors.30 It is worth noticing that compounds IL3IL5 have long aliphatic chains that under the effect of temperature become ordered in the solution and near the metallic surface, which may result in a more uniform adsorption and consequently in an increase of IE.31 In every case, IE was enhanced when inhibitor concentration was increased. According to test results, IL4 reached the best performance as CI (Figure 1) with a moderate alkyl chain size of C18. This fact suggests that the compound may present a balanced chain size to meet the hydrophilic/lipophilic equilibrium, which favors the molecular migration from the aqueous media to the metal surface. This behavior has been observed in ammonium surfactants tested as corrosion inhibitors.32 For neutral molecules, studies report that adsorption of organic inhibitors depends mainly on some physicochemical properties of the molecule,

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related to its functional groups, steric effects, and electronic density of donor atoms. Additionally, adsorption also depends on the interaction of π orbitals of inhibitor with d orbitals of iron atoms, which induces great adsorption of inhibitor molecules onto a mild steel surface, leading to the formation of a corrosion protecting film.3336 Little knowledge exists about how these parameters are involving for the case of charged organic species. ILs properties are highly dependent on the structure of cation and anion, and both species can be involved in the interaction between the IL and metal surface. Shang et al.21 proposed that protonated imidazolium molecules are also adsorbed at cathodic sites in competition with hydrogen ions that reduce hydrogen evolution and that the atom of imidazolium ring, the CdN group, can form a π bond. Therefore not only can the π electrons of the imidazolium basis enter unoccupied orbitals of iron, but also the π* orbital can accept the electrons of d orbitals of iron to form retrodonation bonds so they produce more than one center of adsorption action. Surfactants exert their inhibitory action by adsorption on the metal surface in such a way that the polar or ionic group (hydrophilic part) is attached to the metal surface, while its tail (hydrophobic part) is extended to the solution. The adsorption of surfactant on metal surface can markedly change the corrosion-resisting property of metal.38,39 Many ionic surfactants have been described as CIs.4042 In our case, the ILs containing long saturated alkyl chains (IL3, IL4, and IL5) present an amphiphilic character and behave as corrosion inhibitors. 3.3. Potentiodynamic Polarization Curves. The inhibition efficiencies of ILs were determined through electrochemical experiments (Table 4). The polarization curves for AISI 1018 carbon steel obtained at various inhibitor concentrations of IL4 and IL5 are shown in Figures 2 and 3, respectively. Similar polarization curves were obtained for the other ILs. The values of the corrosion current density (jcorr) and corrosion potential (Ecorr), as well as the cathodic and anodic Tafel slopes (βc, βa), were obtained by the linear extrapolation of the Tafel slopes. These figures show the influence of inhibitor; as concentration is increased, the corrosion current density decreased, indicating that the corrosion rate was mitigated. Inhibition was reached by a blocking mechanism on the anodic and cathodic sites as polarization curves are displaced downward on the whole potential range, which suggests that ILs behave as mixed type inhibitors.43 On the other hand, the presence of these compounds produced a positive shift to more noble values of Ecorr when the concentration was increased, which indicates the formation of a protective layer on the metal surface.44,45 The presence of IL compounds affected the Ecorr with respect to Eocp, as it is observed by the displacement of the polarization curves to anodic potentials in the presence of 100 ppm: IL1 (40 mV), IL2 (59 mV), IL3 (102 mV), IL4 (28 mV), and IL5 (12 mV). These data are important in classifying an inhibitor of anodic or cathodic type.41 If the displacement in potential is at least 85 mV to the right, it is anodic; if not; it is cathodic: this way IL1, IL2, IL4, and IL5 behave as mixed type inhibitors, while IL3 appears to be an inhibitor of cathodic type. IL4 displayed the maximum reduction in corrosion current density, and the compounds follow the order IL4 > IL3 > IL1 > IL2 > IL5. There is a good agreement between the gravimetric and electrochemical results at 25 °C during the first 3 h of the gravimetric tests. 3.4. Adsorption Isotherms. The mode and extent of interactions between inhibitor and iron surface can be studied by 7133



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Table 4. Electrochemical Parameters (Ecorr, jcorr, βc, βa, and Rp) Associated with Polarization Measurements of AISI 1018 Carbon Steel in 1.0 M H2SO4 Solution in the Absence and Presence of Different Concentrations of Corrosion Inhibitors at 25 °C inhibitor

concn, ppm/M

blank IL1

IL2

IL3

IL4

IL5

Ecorr, mV/SCE

jcorr, μA cm2

βc, (mV dec1

βa, mV dec1

Rp, Ω cm2

IE, %

539

330

127

106

76

10/4.33 105 25/1.08 104

532 527

249 191

145 116

113 95

111 119

24 42

50/2.16 104

508

158

110

72

119

52

100/4.33 104

499

133

115

73

146

60

10/3.48 105

504

269

130

75

77

19

25/8.70 105

508

246

131

85

91

26

50/1.74 104

499

203

132

73

100

38

100/3.48 104

480

166

154

57

107

50

10/2.91 105 25/7.28 105

499 490

220 196

145 125

69 106

93 127

33 41

50/1.46 104

437

126

127

85

176

62

100/2.91 104

468

102

114

79

200

69

10/2.34 105

539

131

131

112

202

60

25/5.85 105

533

108

129

111

239

67

50/1.17 104

519

94

110

98

241

72

100/2.34 104

511

63

107

105

366

81

10/2.07 105 25/5.17 105

530 540

297 280

122 131

103 116

82 95

10 15

50/1.03 104

530

229

142

114

120

31

100/2.07 104

527

199

124

105

124

40

Figure 2. Polarization curves of AISI 1018 carbon steel in 1.0 M H2SO4 using IL4 as corrosion inhibitor at different concentrations.

Figure 3. Polarization curves of AISI 1018 carbon steel in 1.0 M H2SO4 using IL5 as corrosion inhibitor at different concentrations.

applying adsorption isotherms. Adsorption of inhibitor molecules on metal surface is a substitution process since it is accompanied by an exchange of adsorbed water molecules with organic molecules. The degree of surface covered (θ) on steel was determined from the weight loss tests as a function of inhibitor concentration at constant temperature; this way the adsorption isotherm is evaluated at equilibrium conditions. Several models of isotherms were applied to fit the surface coverage values at different inhibitor concentrations and temperatures. Experimental results of Cinh/θ vs Cinh yielded straight lines, as shown in Figure 4, which are in agreement

with Langmuir's isotherm: Kads Cinh ¼

θ 1θ

ð5Þ

where Cinh is the inhibitor concentration and Kads is the adsorption equilibrium constant. The values of the correlation coefficients and the adsorption equilibrium constants are given in Table 5 for the ILs tested. The high correlation coefficients and slopes (1.0 ( 0.1) indicated a good fitting of experimental data to Langmuir's isotherm. The correlations of Cinh/θ vs Cinh displayed a linear fitting along with slopes close 7134



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Table 5. Summary of Kads and ΔGads° Data Obtained from the ILs Evaluated as Corrosion Inhibitors of AISI 1018 Carbon Steel in 1.0 M H2SO4 25 °C

40 °C

Δ Gads°,

ð6Þ

ΔGads° is the standard free energy of adsorption, and the value of 55.5 is the concentration of water in solution expressed in moles. As it is known, an increase in Kads with temperature indicates an increase in the extent of adsorption.47 The high negative values of ΔGads° ensure the spontaneity of the adsorption process and the stability of the adsorbed layer on the metal surface. Generally, values of ΔGads° up to 20 kJ/mol are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption), while those more negative than 40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinated type of bond (chemical adsorption).36,38 In our case, Kads increased for IL4 and IL5 when temperature was increased. This means that a chemical adsorption process occurred on the metal surface; as chemisorption requires an

kJ mol1

1

5 436

31.3

4 200

32.2

2

10 723

33.0

7 623

33.7

3

45 956

36.6

11 678

34.8

4 6

72 993 51 387

37.7 36.8

20 610 24 938

36.3 36.8

1

3 834

30.4

20 610

36.3

2

5 671

31.4

2 953

31.3

3

6 619

31.8

5 901

33.1

4

8 217

32.3

9 039

34.2

6

12 767

33.4

11 322

34.8

1

19 646

34.5

20 995

36.4

2 3

20 387 22 427

34.6 34.8

20 259 23 354

36.3 36.6

4

27 949

35.3

22 795

36.6

6

41 563

36.3

24 301

36.7

1

67 568

37.5

122 100

41.0

2

83 542

38.0

144 092

41.4

3

89 445

38.2

166 667

41.8

4

121 951

39.0

146 413

41.4

6 1

252 525 3 062

40.8 29.9

263 852 12 882

43.0 35.1

IL4

ΔGads ° ¼ RT lnð55:5Kads Þ

Δ Gads°, Kads

IL1

IL 3

to unit value as indicated by eq 5. Deviations from unity were ascribed to the presence of sulfate and iron sulfate on the metal surface, products which are less soluble in water and hence more likely to be adhered to surface. The formation of corrosion products is owed to the anodic reaction, which destroy the film already formed on the surface, and therefore a lower amount of inhibitor molecules is efficiently adsorbed. When θ tends to ∼1, a more compact film is formed, though this is dependent on the molecular structure of the compounds.46 The adsorption isotherm is useful for estimating important thermodynamic parameters such as the standard free energy of adsorption:

kJ mol1

time, h

IL2

Figure 4. Langmuir adsorption isotherm for IL3 at 25 (a) and 40 °C (b) on the AISI 1018 carbon steel surface in 1.0 M H2SO4.

Kads

compound

IL5

2

6 671

31.8

24 919

36.8

3

10 662

32.9

30 340

37.3

4

38 506

36.1

107 643

40.6

6

194 553

40.1

106 724

40.6

activation energy provided by a higher temperature to happen, whereas physisorption takes place without a change in activation energy. Although the values of ΔGads° for IL4 and 5 are in the range of 4043 kJ mol1, the increase in activation energy with temperature was higher for IL4 than for IL5, suggesting that nonCoulombic forces prevailed over electrostatic forces; besides that, potential and metallic charge are lower than those for compounds IL1, 2 and 3 (ΔGads° < 40 kJ mol1). 3.5. Surface Analysis. Figure 5 shows the micrographs of the carbon steel surface before and after carbon steel immersion in 1.0 M H2SO4 with and without corrosion inhibitor. Figure 5a shows the surface after the immersion in the corrosive medium; it is observed that a uniform surface finishing is produced by the mechanical grinding on the sample. Figure 5b shows the surface of the carbon steel specimen after immersion in 1.0 M H2SO4 solution for 6 h in the absence of inhibitor. Figure 5c shows the surface of the carbon steel specimen after immersion in the corrosive solution for the same period of time though in the presence of 100 ppm IL4 inhibitor. SEM micrograph revealed that the surface morphology was strongly damaged in the absence of the inhibitor, but in the presence of 100 ppm inhibitor damage was considerably diminished, which confirmed the high efficiency of IL4 at this concentration. The elemental analysis obtained from EDX indicated that the coupon after grinding is mainly composed of Fe, as is expected for carbon steel (Figure 6a), while 7135



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Figure 6. EDX analysis of metallic surfaces: (a) after polishing, (b) after 6 h of immersion in the corrosive media without inhibitor, and (c) after 6 h of immersion in the corrosive media with 100 ppm IL4.

Figure 5. SEM images (1000) of metallic surfaces: (a) after polishing, (b) after 6 h of immersion in the corrosive media without inhibitor, and (c) after 6 h of immersion in the corrosive media with 100 ppm IL4.

after immersion in the corrosive media (1.0 M H2SO4) a relatively high concentration of oxygen and sulfur was detected as a result of formed oxyhydroxides and sulfides from iron in diluted sulfuric acid (Figure 6b). Finally, a considerable reduction in the content of oxygen and sulfur was detected on the coupon surface after immersion in the corrosive media containing the inhibitor IL4 (Figure 6c), which showed inhibitor capability to mitigate uniform corrosion.

3.6. Atomic Force Microscopy. AFM is one of the most powerful tools for observing the surface morphology as it provides a useful means of characterizing substrate microstructure.5,45 Three-dimensional images for the coupon surface morphology were obtained with AFM (Figure 7), after immersion in the testing solution in the absence and presence of ILs. The surface after mechanical grinding shows some abrading scratches (Figure 7a), whereas the surface images of steel before and after the addition of IL4 show clear evidence of the good performance of the IL4 corrosion inhibitor, as surface morphology shows much less damage (Figure 7c) with respect to the surface after immersion in uninhibited corrosive media (Figure 7b). Note that image resolution increased from Figure 7a to Figure 7c; the surface roughness considerably increased from Figure 7a to Figure 7b and decreased in Figure 7c. The latter image shows a surface with a lower number of crest heights and shallower valleys on protected surface topography with IL4. This behavior was 7136



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3.7. Inhibition Mechanism. The corrosion of 1018 carbon steel in 1.0 M H2SO4 was delayed by the presence of different concentrations of 1-alkyl-3-vinylimidazolium bromide derivatives (VImCnBr), which were evaluated in this study. The results indicated that the inhibition mechanism (Scheme 2) involved both physical and chemical adsorption of inhibitor on the metal surface; this is influenced by the nature of the inhibitor (chemical structure) and the surface charge of the metal (acidic media). The physical adsorption involves electrostatic forces between ionic charges of the adsorbed species and the electric charge at the metal/solution interface; and the chemical adsorption involves charge transfer from inhibitor molecules to the metal surface to form coordinate bonds.48 All synthesized compounds have an anionic (Br) and a cationic part, which contain an imidazolium ring with vinyl (CH2dCH) and alkyl groups. Some researchers have reported49,50 that organic compounds containing nitrogen are more efficient against mild steel corrosion in hydrochloric acid than in sulfuric acid. A likely explanation is attributable to the synergistic effect between halide anions and organic cations in the inhibitor for steel corrosion in acid media. The bromide ions in acid solution can be adsorbed on the metal surface and form an interconnecting bridge between the metal atoms and organic cations.1 The adsorbed bromide ions facilitate the adsorption of cations of ILs by electric charge attraction on the metal surface;51 then the stabilization of adsorbed bromide ions by means of electrostatic interaction with IL cations leads to a greater surface coverage and thereby to a greater inhibition effect and minor iron dissolution. This process of anodic sites can be described by the following reactions.

in the absence of ILs : Fe þ xH2 O T ½FeðH2 OÞx ads

ð7Þ

½FeðH2 OÞx ads þ SO4 2 T Fe½ðH2 OÞx SO4 2 ads

ð8Þ

Fe½ðH2 OÞx SO4 2 ads f ½FeðH2 OÞx SO4 ads þ 2e

ð9Þ

½FeðH2 OÞx SO4 ads f Fe2þ þ OH þ SO4 2 þ Hþ

ð10Þ

Fe2þ þ OH þ 1 =2 O2 f FeOOH þ e whereas in the presence of ILs : Fe þ xH2 O T ½FeðH2 OÞx ads ½FeðH2 OÞx ads þ SO4 2 T Fe½ðH2 OÞx SO4 2 ads

ð11Þ ð12Þ ð13Þ

Fe½ðH2 OÞx SO4 2 ads þ VImCn þ f Fe½ðH2 OÞx SO4 2 VImCn þ ads

f ½FeðH2 OÞx SO4 ads VImCn þ þ e f ½ð½FeðH2 OÞx SO4 VImCn Þ ads Figure 7. Three-dimensional AFM images of carbon steel: (a) after mechanical grinding (data scale 50 nm), (b) after 6 h of immersion in the corrosive media without inhibitor (700 nm), and (c) after 6 h of immersion in the corrosive media containing 100 ppm of IL4 (1.0 μm).

ð14Þ

½ð½FeðH2 OÞx SO4 VImCn Þ ads þ VImCn þ þ SO4 2 f fð½FeðH2 OÞx SO4 ads VImCn Þ VImCn þ SO4 2 =VImCn þ gads ð15Þ Fe þ Br T ðFeBr Þads

ascribed to the presence of a film that inhibits metal dissolution that resulted in lower surface roughness. “Vesicle-like” particles also are observed on the metal surface; these are characteristic of an adsorptive film which prevailed in the presence of IL4.

ðFeBr Þads þ VImCnþ f ðFeBr VImCn þ Þads ðFeVImCn Þads f ðFeVImCn þ Þads þ e 7137

ð16Þ ð17Þ ð18Þ



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Scheme 2. Corrosion Inhibition Mechanism and Adsorption Model of ILs on Steel Surface in Aqueous Sulfuric Acid Medium

Table 6. Thermal Behavior and Melting Points of ILs temperature of weight loss, °C ILs

1%

5%

melting point, °C

IL1

215

240

liquid

IL2 IL3

200 209

230 238

liquid 56

IL4

204

230

73

IL5

216

238

89

In addition, the IL cations compete with hydronium (H3Oþ) for available electrons on the metallic surface, where the inhibitor cation size is much larger than the hydrogen molecule due to the aliphatic flexible chain. After electron assimilation, the inhibitor cation evolved to its neutral form with a CHdN group of the imidazolium ring and the vinyl group having a free electron pair that promoted the chemical adsorption on the metallic surface to be protected from corrosion. Additionally, the vinyl group takes place in the hydrogenation reaction by using hydrogen from the ethylic group (eq 24). This process can be described by the following reactions: in the absence of ILs : Fe þ H3 Oþ T FeðH3 Oþ Þads

ð19Þ

FeðH3 Oþ Þads þ e f ½FeðH3 OÞads

ð20Þ

þ

ðFeH3 OÞads þ H þ e f Fe þ H2 þ H2 O -

whereas in the presence of ILs : Fe þ VImCn þ T FeðVImCn þ Þads ½FeðVImCn þ Þads þ e f ðFeVImCn Þads þ

½FeðVImCn Þads þ 2H f ðFeImEtCn Þads

other hand, the best performance obtained at 40 °C, when compared to 25 °C, suggests that these compounds could be considered for further study as a promising corrosion inhibitor. Investigations about the inhibitory properties for these compounds at moderate temperatures are now in course.

4. CONCLUSIONS The five imidazolium-type ionic liquids (IL1IL5) containing N1 unsatured and N3 long alkyl saturated chains (cation) and bromide (anion) showed corrosion inhibition properties for the protection of carbon steel in aqueous 1.0 M H2SO4, as confirmed by weight loss test and polarization curves. Inhibition efficiency increased with concentration (10100 ppm), and it was dependent on the alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5). The inhibition mechanism of ILs was attributed to the strong adsorption ability of these surfactants to form a protective layer that isolates the mild steel surface from an aggressive environment. ILs behaved as mixed type corrosion inhibitors. Their moderate thermal stability suggests that these compounds may be considered for further study to determine the upper level of application as corrosion inhibitors. The relatively high inhibitory properties (8895%) displayed by IL4 within 2540 °C may support this study. ’ AUTHOR INFORMATION Corresponding Author

ð21Þ

*Tel.: (þ01222) 229-5500. Fax: (þ015255) 9175-8380. E-mail: [email protected].

ð22Þ

’ ACKNOWLEDGMENT O.O.-X. thanks SNI and PROMEP/103.5/08/3343.

ð23Þ ð24Þ

3.8. Thermal Behavior. ILs employed in this study seem to be environmentally benign corrosion inhibitors as they present a very low vapor pressure.52 Furthermore, ILs showed a high thermal stability as 1% of weight loss matter does not occur below 200 °C, and they either are in liquid phase at room temperature or present low melting points (Table 6). On the

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