Corrosion inhibition of mild steel in hydrochloric acid solution by some double Schiff bases

Corrosion Science 52 (2010) 1351–1361

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Corrosion inhibition of mild steel in hydrochloric acid solution by some double Schiff bases N. Soltani a,*, M. Behpour b, S.M. Ghoreishi b, H. Naeimi b a b

Payame Noor University (PNU), Shahin Shahr Branch, Isfahan, Iran Department of Chemistry, Faculty of Science, University of Kashan, Kashan, Iran

a r t i c l e

i n f o

Article history: Received 9 September 2009 Accepted 28 November 2009 Available online 5 December 2009 Keywords: A. Mild steel B. Polarization B. EIS C. Kinetic parameters C. Acid inhibition

a b s t r a c t The inhibition effect of four double Schiff bases on the corrosion of mild steel in 2 M HCl has been studied by polarization, electrochemical impedance spectroscopy (EIS) and weight loss measurements. The inhibitors were adsorbed on the steel surface according to the Langmuir adsorption isotherm model. From the adsorption isotherm, some thermodynamic data for the adsorption process were calculated and discussed. Kinetic parameters activation such as Ea, DH*, DS* were evaluated from the effect of temperature on corrosion and inhibition processes. Quantum chemical calculations have been performed and several quantum chemical indices were calculated and correlated with the corresponding inhibition efficiencies. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Acid solutions are widely used in various industries for the pickling of ferrous alloys and steels. To avoid base metal attack and to ensure the removal of corrosion products/scales alone, inhibitors are extensively used. Inhibitors are compounds that control, reduce, or prevent reactions between a metal and its surroundings when added to the medium in small quantities. Organic compounds are widely used in industry to prevent corrosion in acidic environments [1–8]. The compounds used as inhibitors act through a process of surface adsorption. The efficiency of an inhibitor depends on the characteristics of the environment in which it acts, the nature of the metal surface and electrochemical potential at the interface. The structure of the inhibitor itself, which includes the number of adsorption active centers in the molecule, their charge density, the molecule size, the mode of adsorption, the formation of metallic complexes and the projected area of the inhibitor on the metallic surface, has also effect on the efficiency of inhibitor. Previously, many of synthesized organic compounds, such as hexamethylenetetramine [9], 3-(4-amino-2-methyl-5pyrimidyl methyl)-4-methyl thiazolium chloride [10], 1-dodecyl4-methoxy pyridinium bromide [11], bipyrazolic compounds [12], 2,3-quinoxalinedione [13], triazole derivatives [14,15], Schiff bases [16–18], macrocyclic polyether compounds [19] and so after, have been investigated as corrosion inhibitors against corrosion of mild steel in acidic media. Among these compounds, several Schiff

* Corresponding author. Tel.: +98 9133283128; fax: +98 2122441511. E-mail address: [email protected] (N. Soltani). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.11.045

bases have recently been investigated as corrosion inhibitors for various steels in acid media [16,20]. These substances generally become effective due to the presence of an imine (–CH@N) group and conjugated double bonds. Besides the imine group, substitution of different groups also affects the inhibition properties. This study aims at investigating the inhibitive action of four Schiff base compounds against the corrosion of mild steel in 2 M HCl solution. Furthermore, this study investigates the effects of substitution of methoxy, hydroxyl and chloride groups on the inhibitive effect of N,N0 -bis(salicylidene)-phenylmethanediamine on mild steel corrosion in hydrochloric acid solution. This study is conducted by using gravimetric, polarization and electrochemical impedance spectroscopy (EIS) methods. The nature of inhibitor adsorption process and the effect of temperature are also studied and discussed. 2. Experimental 2.1. General procedure for synthesis of N,N0 -bis(salicylidene)arylmethanediamines The Schiff bases SB1–SB4 (Fig. 1) were synthesized according to the following procedure; NH4OAc was added to a mixture of salicylaldehyde (0.38 g, 3 mmol) and arylaldehyde (1.5 mmol) (0.25 g, 3.27 mmol) in the presence of the NEt3 (0.12 ml) as a base by stirring in one portion. The progress of the reaction was monitored by TLC. After the completion of the reaction, oil pail yellow substance was obtained. Then, by dissolving the oil yellow mixture in 1.5 ml MeOH and cooling for one night, a yellow solid

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

OCH 3 N

N

N

N

HO

OH

OH

(SB1): N, N'- bis (salicylidene)-2-methoxyphenyl methanediamine

HO

OH

(SB2): N, N'- bis (salicylidene)-2-hydroxyphenyl methanediamine

Cl

N N

OH

N

N

HO

(SB3): N, N'- bis (salicylidene)-4-chlorophenyl methanediamine

OH

HO

(SB4): N, N'- bis (salicylidene)-phenyl methanediamine

Fig. 1. The names and structures of the investigated Shiff bases.

was precipitated. The solid product was filtered off and washed with cold MeOH. The crude product was purified by recrystallization in ethanol and the pure Schiff base, N,N0 -bis(salicylidene)-arylmethanediamine was obtained in high to excellent yields. The Schiff base products were identified by physical and spectroscopic data. The results are as followed: 2.1.1. N,N0 -bis(salicylidene)-2-methoxy-phenylmethanediamine (SB1) Yellow solid; mp 145–147 °C; IR (KBr)/m (cm1) 3375–3625 (br, OH), 1625 (s, C@N), 1450, 1510 (Ar); 1H NMR/DMSO/d ppm: 2.3 (3 H, OCH3), 6.2 (s, 1 H, NCHN), 6.7–7.4 (m, 12 H), 8.6 (s, 2 H, HC@N),12.90 (s, 2 OH); 13C NMR/DMSO/d ppm: 60, 90, 117, 119, 119.2, 129.5, 130.2, 130.3, 138, 165, 170; UV (CHCl3)/kmax (nm) 324 (w), 262 (s); MS: m/z = 361 (M+ + 1, 2), 360 (M+, 6), 241 (50), 240 (80), 120 (70), 119 (100), 77 (15); Anal. Calcd. For C. H. N: 73.3 (C), 5.5 (H), 7.8 (N); Found: 72.69 (C), 5.48 (H), 8.07 (N). 2.1.2. N,N0 -bis(salicylidene)-2-hydroxy-phenylmethanediamine (SB2) Yellow solid; mp 164–166 °C; IR (KBr)/m (cm1): 3250–3450 (br, OH), 1622 (s, C@N), 1450, 1550 (Ar); 1H NMR/DMSO/d ppm: 6.1 (s, 1 H, NCHN), 6.6–7.5 (m, 12 H), 8.5 (s, 2 H, HC@N); 9.7 (s, 1 H, OH); 12.96 (s, 2 OH); 13C NMR/DMSO/d ppm: 90, 117, 120, 119.6, 127.5, 130.2, 130.3, 138, 165, 167; UV (CHCl3)/kmax (nm) 322 (w), 261 (s); MS: m/z = 348 (M+ + 1, 2), 347 (M+, 8), 225 (55), 224 (65), 107 (70), 108 (100), 77 (20); Anal. Calcd. For C. H. N: 72.62 (C), 5.46 (H), 8.07 (N); Found: 72.63 (C), 5.48 (H), 8.07 (N). 2.1.3. N,N0 -bis(salicylidene)-4-chloro-phenylmethanediamine (SB3) Pail yellow solid; mp 115–117 °C; IR (KBr)/m (cm1) 3300–3550 (br, OH), 1625 (s, C@N),1486, 1572 (Ar), 790 (C–Cl); 1H NMR/ DMSO/d ppm: 6.2 (s, 1 H, NCHN), 6.9–7.9 (m, 12 H), 8.9 (s, 2 H, HC@N), 12.9 (s, 2 OH); 13C NMR/DMSO/d ppm: 90.2, 117, 119.33, 120.88, 127.5, 130.3, 132.94, 136, 144, 165, 169; UV (CHCl3)/kmax (nm) 320 (w), 261 (s); MS: m/z = 366.5 (M+ + 2, 2), 364.5 (M+, 7), 245.5 (50), 243.5 (74), 125.5 (64), 123.5 (100), 77 (18); Anal. Calcd. For C. H. N: 69.13 (C), 4.67 (H), 7.68 (N); Found: 69.14 (C), 4.69 (H), 7.68 (N).

2.1.4. N,N0 -bis(salicylidene)-phenylmethanediamine (SB4) Pail yellow solid; mp 118–120 °C; IR (KBr)/m (cm1) 3300–3500 (br, OH), 1625 (s, C@N), 1450, 1550 (Ar); 1H NMR/DMSO/d ppm: 6.1 (s, 1 H, NCHN), 6.63–7.35 (m, 13 H), 8.6 (s, 2 H, HC@N), 12.80 (s, 2 OH); 13C NMR/DMSO/d ppm: 90, 118, 119, 119.5, 127.5, 130.1, 130.3, 138, 161, 167; UV (CHCl3)/kmax (nm) 320 (w), 260 (s); MS: m/z = 331 (M+ + 1, 3), 330 (M+, 9), 210 (60), 209 (80), 91 (70), 89 (100), 77 (50); Anal. Calcd. For C. H. N: 76.36 (C), 5.45 (H), 8.48 (N); Found: 76.36 (C), 5.46 (H), 8.49 (N). 2.2. Electrodes and chemicals The employed working electrodes were prepared from a mild steel plate with the chemical composition (wt.%) of: C (0.027), Si (0.0027), P (0.009), Al (0.068), Mn (0.340), S (0.007), Nb (0.003), Cu (0.007), Ni (0.030), Ti (0.003), Cr (0.008), V (0.003) and Fe (balance). The mild steel specimens used in weight loss measurements were cut into 1 cm2 coupons. For polarization and electrochemical impedance studies, the metal was soldered with Cu-wire for electrical connection, and embedded in epoxy resin, to expose a geometrical surface area of 1 cm2 to the electrolyte. Prior to all measurements, the mild steel specimens were ground with different emery papers (grade 400, 800, 1000 and 1200), rinsed with double distilled water, degreased with ethanol before being used and dried at room temperature. The aggressive solution (2 M HCl) was prepared by dilution of analytical grade 37% HCl with double distilled water. The concentration range of employed inhibitor was 0.01–1 mM in 2 M HCl. 2.3. Weight loss measurements Weight loss measurements were carried out in a double walled glass cell equipped with a thermostat-cooling condenser. The immersion times were different, the maximum immersion time was 24 h at 25 °C and 1 h at the other temperatures. Specimens were immersed in 50 ml 2 M HCl solution containing various concentrations of the studied inhibitors. The mass of the specimens before and after immersion was determined using an analytical

N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

balance of 0.01 mg accuracy. Experiments were carried out in triplicate and the average of the triplicate values was used. The corrosion rate (m) was calculated from the following equation [21]:

m¼

ðm1  m2 Þ ðS  tÞ

IE ð%Þ ¼

ðm0  mÞ

m

current (Icorr), obtained by extrapolation of the Tafel lines, are given in Table 1. The inhibition efficiency was calculated from the following equation [23]:

ð1Þ IEP ð%Þ ¼

where m1 is the mass of the specimen before corrosion, m2 the mass of the specimen after corrosion, S, the total area of the specimen, t, corrosion time and m, corrosion rate. With the calculated corrosion rate, the inhibition efficiency of inhibitor for the corrosion of mild steel was obtained by using the following equation [21]:

 100

ð2Þ

where m0 and m are the corrosion rates of the specimen in 2 M hydrochloric acid without and with the addition of inhibitor, respectively.

1353

  I0  I  100 I0

ð3Þ

where I0 and I are the corrosion current densities in absence and presence of inhibitor, respectively. The obtained values of IEP (%) are given in Table 1. These results show that all used Schiff bases act as effective inhibitors. Corrosion inhibition increases when the inhibitor concentration increases. This study indicates that by increasing inhibitor concentration, the corrosion current was decreased, corrosion potential (Ecorr) was shifted slightly to more positive values and inhibition efficiency IEP (%), increased. Moreover, these inhibitors cause no change in the anodic and cathodic Tafel slopes, considering the fact that the inhibitors are first adsorbed onto steel surface and therefore impedes by merely blocking the reaction sites of iron surface without affecting the anodic and cathodic reaction mechanism [24].

2.4. Electrochemical measurements Polarizations and impedance measurements were carried out using AUTOLAB model PGSTAT 35. Electrochemical experiments were performed in a conventional three electrodes electrochemical cell at 25 °C with the mild steel as a working electrode, a platinum counter electrode and saturated silver–silver chloride (Ag/AgCl) electrode as reference electrode. Working electrode was first immersed into the test solution for 30 min to establish a steady state open circuit potential. Electrochemical impedance spectroscopy measurements were performed at corrosion potentials, Ecorr, over a frequency range of 100 kHz–0.1 Hz with a signal amplitude perturbation of 5 mV. Impedance data were analyzed using a Pentium IV computer and FRA software. Polarization studies were performed with a scan rate of 0.5 mV s1 in the potential range from 500 to +500 mV relative to the corrosion potential. Polarization data was analyzed using GPES electrochemical software. Experiments were always repeated at least three times so that good agreement was obtained. 2.5. Quantum chemical calculations The molecular sketches of Schiff bases were drawn using the GaussView 3.0. All the quantum calculations were performed with complete geometry optimization by means of standard Gaussian03 software package [22]. The relationship between inhibition efficiency of the molecules and their electronic properties was investigated at the level of B3LYP with 6-311G**, and 6-311++G** basis sets and by the semi-empirical methods including the Austin Model (AM1) and Parametric Method (PM3) methods. The following quantum chemical indices were taken into consideration: the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy band gap, DE = EHOMO  ELUMO and the dipole moment (l).

3.2. Electrochemical impedance spectroscopy The corrosion behavior of mild steel in 2 M HCl solution in the absence and presence of Schiff bases SB1–SB4 were investigated using EIS. Nyquist plots are given in Fig. 3. It is clear from the plots that the impedance response changes with the addition of the inhibitor additives. In general, all the plots display a single capacitive loop. Impedance parameters and the equivalent circuit diagram are given in Table 2 and Fig. 4, respectively. The circuit consists of a constant phase element (CPE) Q, in parallel with a resistor Rp. The use of CPE-type impedance has been extensively described in accordance to previous reports [25,26].

Z CPE ¼ ½QðjxÞn 1

ð4Þ

The above equation provides information about the degree of non-ideality in capacitance behavior. Its value makes it possible to differentiate between the behavior of an ideal capacitor (n = 1) and of a CPE (n < 1) [27]. Polarization resistance, Rp, is correlated with simple corrosion current density in relatively simple corrosion systems characterized only by a charge transfer controlled process [28]. The results indicate that the Rp values increased with increasing additive concentration. Since Rp is inversely proportional to the corrosion current it can be used to calculate the inhibitor efficiency %IEEIS [29]:

IEEIS ð%Þ ¼

Rp;i  Rp;0  100 Rp;i

ð5Þ

where Rp,i and Rp,0 are the polarization resistances with and without the additive, respectively. Inhibition efficiencies increased with increasing additive concentrations. The order of inhibition efficiency decreases in the order SB1  SB2 > SB3 > SB4. It should be noted that in general the n and Rp values increase with increasing inhibitor concentration, while the value of the constant phase element decreases.

3. Results and discussion 3.3. Weight loss measurements 3.1. Potentiodynamic polarization measurements Fig. 2(a–d) presents polarization curves for mild steel electrode in 2 M HCl, in the presence and absence of Schiff bases at various concentrations. Both cathodic and anodic reactions on mild steel electrode were inhibited in the presence of studied Schiff bases. Electrochemical corrosion parameters, such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (ba and bc) and corrosion

The corrosion of mild steel in 2 M HCl medium containing various concentrations of inhibitors was studied by weight loss measurements and IEW (%) is calculated by applying Eq. (2). Table 3 summarizes the corrosion rates (W) of mild steel and the IEW (%) for the Schiff base studied in the different concentrations. It is obvious from these data that all of these compounds inhibit the corrosion of mild steel in 2 M HCl solution in all concentrations used in

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

-1.8 -2.0

(a)

-2.3

-2.5

-2.8

-3.0

-3.3

-3.5

log I (A cm-2 )

log I (A cm-2 )

(b)

-4.0 -4.5

Blank 0.01 mM 0.05 mM 0.10 mM 0.50 mM 1.00 mM

-5.0 -5.5 -6.0 -6.5 -7.0 -0.52

-0.47

-0.42

-3.8 -4.3

Blank 0.01 mM

-4.8

0.05 mM 0.10 mM 0.50 mM

-5.3 -5.8 -6.3

-0.37

1.00 mM

-6.8 -0.52

-0.32

-0.47

-0.42

-0.37

-0.32

E vs. Ag/AgCl (V)

E vs. Ag/AgCl (V) -1.8 -2.0

(c)

-2.3

(d)

-2.5 -2.8 -3.3

log I (A cm-2 )

log I (A cm-2 )

-3.0 -3.5 -4.0

Blank 0.01 mM 0.05 mM 0.10 mM 0.50 mM 1.00 mM

-4.5 -5.0 -5.5 -6.0 -6.5 -0.52

-0.47

-0.42

-3.8 -4.3 -4.8 -5.3 -5.8 -6.3

-0.37

-0.32

-6.8 -0.52

Blank 0.01 mM 0.05 mM 0.10 mM 0.50 mM 1.00 mM -0.47

-0.42

-0.37

-0.32

E vs. Ag/AgCl (V)

E vs. Ag/AgCl (V)

Fig. 2. Polarization curves for mild steel in 2 M HCl with different Schiff bases: (a) SB1, (b) SB2, (c) SB3, (d) SB4.

Table 1 Electrochemical parameters and inhibition efficiencies of steel corrosion in 2 M HCl solutions in the absence and presence of different concentrations from the four Schiff base compounds at 25 °C. Inhibitor

Concentration (mM)

Ecorr vs. Ag/AgCl (mV)

bc (mV dec1)

ba

Icorr (lA cm2)

Blank SB1

IE (%)

–

400

88

70

557.7

–

0.01 0.05 0.10 0.50 1.00

398 394 378 375 373

88 85 89 87 85

70 73 72 72 71

209.4 86.8 49.8 24.9 17.7

62.5 84.4 91.3 95.4 96.8

SB2

0.01 0.05 0.10 0.50 1.00

398 395 394 390 378

84 87 86 85 87

70 68 69 71 73

300.1 147.5 88.3 51.4 22.0

46.2 73.5 84.1 90.8 96.1

SB3

0.01 0.05 0.10 0.50 1.00

399 398 391 389 379

89 88 89 86 92

72 68 69 75 79

368.1 262.6 143.9 58.1 27.9

34.0 52.9 74.2 89.6 95.0

SB4

0.01 0.05 0.10 0.50 1.00

394 393 392 389 384

89 86 87 88 89

67 67 69 73 75

431.2 188.0 143.9 99.4 58.9

22.7 66.3 74.2 82.2 89.4

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

800

700 600

700

0.05 mM 10

0.10 mM

5

0.50 mM 1.00 mM

0 0

400

10

20

600

-Zim(Ω cm2)

-Zim (Ω cm 2 )

500

Blank

15

(b)

0.01 mM

30

Zre(Ω cm 2 )

300

-Zim(Ω cm2 )

-Zim(Ω cm2 )

(a)

0.01 mM 0.05 mM 0. 10 mM 0. 50 mM 1.00 mM

Blank

15 10 5 0

500

0

10

20

30

Zre(Ω cm 2 )

400 300

200 200

100

100

0 0

100

200

300

400

500

600

700

800

0

900 1000

0

100

200

300

400

Zre(Ω cm 2 ) 450

600

700

800

900 1000

5

0.50 mM 1.00 mM

0 0

250

10

20

30 2

Zre(Ω cm )

200

100

0.01 mM

Blank

15

0.05 mM

10

0.10 mM 5

0.50 mM

0

2

300

10

120

-Zim (Ω cm )

- Zim(Ω cm2 )

350

Blank

15

(d)

0.01 mM 0.05 mM 0.10 mM

- Zim(Ω cm2 )

140

(c)

400

-Zim(Ω cm2 )

500

Zre(Ω cm 2 )

1.00 mM 0

80

10

20

30 2

Z re (Ω cm )

60

150

40 100

20

50 0 0

100

200

300

400

0

500

0

50

100

150

200

2

Zre(Ω cm 2 )

Zre (Ω cm )

Fig. 3. Nyquist plots of mild steel in 2 M HCl with various concentrations of Schiff bases: (a) SB1, (b) SB2, (c) SB3, (d) SB4.

Table 2 Impedance data of mild steel in absence and presence of different concentrations of inhibitors. Inhibitor

Concentration (mM)

Rs

Rct 2

Y0 (lF cm

(X cm )

Cdl (lF cm2)

Q 2

)

IE (%)

n

Blank

–

0.8

29.2

46.9

0.87

231.1

–

SB1

0.01 0.05 0.10 0.50 1.00

0.7 1.8 2.8 0.7 0.3

78.3 183.5 336.0 637.0 917.0

23.7 20.7 20.5 8.75 6.55

0.80 0.77 0.80 0.75 0.81

173.7 153.3 121.0 104.8 90.3

62.7 84.1 91.3 95.4 96.8

SB2

0.01 0.05 0.10 0.50 1.00

0.4 0.4 0.4 0.4 0.5

53.6 107.2 180.5 357.0 829.0

43.7 24.0 17.0 10.2 5.99

0.83 0.84 0.82 0.82 0.85

183.5 161.3 148.0 126.4 110.4

45.5 72.8 83.8 91.8 96.5

SB3

0.01 0.05 0.10 0.50 1.00

0.6 0.5 0.5 0.5 0.4

43.3 63.6 77.6 283.0 497.0

43.3 33.9 32.7 28.4 5.09

0.84 0.84 0.85 0.85 0.85

190.8 189.5 165.1 148.9 120.6

32.5 54.1 62.4 89.7 94.1

SB4

0.01 0.05 0.10 0.50 1.00

0.5 0.5 0.5 0.4 0.2

40.1 88.8 142.0 164.1 271.6

38.0 31.1 19.3 12.3 10.5

0.75 0.79 0.77 0.74 0.72

194.7 189.8 174.2 154.9 128.6

27.2 67.1 79.4 82.2 89.2

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361 Table 4 Thermodynamic parameters for the adsorption of Schiff bases in 2 M HCl on the mild steel at 25 °C.

CPE

K (M1)

Inhibitor

Rs

5

SB1 SB2 SB3 SB4

Rct

1.11  10 5.00  104 2.5  104 3.33  104

R2

DG0ads (kJ mol1)

0.9999 0.9998 0.999 0.9996

38.73 36.75 35.04 35.75

Fig. 4. Equivalent circuit model for the studied inhibitors.

Table 3 Weight loss results of mild steel corrosion in 2 M HCl with addition of various concentrations of inhibitors at 25 °C.

Blank

–

2.95

–

SB1

0.01 0.05 0.10 0.50 1.00

1.18 0.48 0.21 0.14 0.06

0.60 0.84 0.93 0.95 0.98

60.0 83.7 92.9 95.3 98.0

0.01 0.05 0.10 0.50 1.00

1.72 0.78 0.46 0.22 0.11

0.42 0.74 0.84 0.93 0.96

41.7 73.6 84.4 92.5 96.3

0.01 0.05 0.10 0.50 1.00

1.97 1.38 1.08 0.33 0.18

0.33 0.53 0.63 0.89 0.94

33.2 53.2 63.4 88.8 93.9

0.01 0.05 0.10 0.50 1.00

2.17 1.04 0.63 0.43 0.28

0.26 0.65 0.79 0.85 0.91

26.4 64.7 78.6 85.4 90.5

SB3

SB4

Temperature (°C)

–

this study and the corrosion rate was seen to decrease with increasing additive concentration at 25 °C. The IEW% for used Schiff bases was found to be in the following order: SB1SB2 > SB3 > SB4.

35

55

65

75

Blank

m0 (mg cm2 h1)

3.48

6.40

14.00

24.56

44.11

SB1

m (mg cm2 h1)

0.12 96.43

0.32 95.00

0.80 94.29

1.54 93.73

3.48 92.10

0.18 94.74

0.39 93.91

1.08 92.29

2.04 91.71

4.00 90.93

0.31 91.15

0.70 89.06

1.62 88.43

2.86 88.37

6.05 86.28

0.41 88.25

0.84 86.88

2.18 84.41

4.50 81.68

8.53 80.66

IEW (%)

m (mg cm2 h1)

SB2

IEW (%)

m (mg cm2 h1)

SB3

IEW (%)

m (mg cm2 h1)

SB4

IEW (%)

45

0 Blank

-1

S B1 S B2

-2

S B3

-3 -1

h

S B4

-4

-2

Weight loss (mg cm2h1)

SB2

Inhibitor

IE (%)

Concentration (mM)

ln ν (g cm h )

Inhibitor type

Table 5 Effect of temperature on the corrosion rate of mild steel in 2 M HCl and at 1 mM of Schiff bases at 1 h.

-5 -6 -7 -8 -9

3.4. Adsorption isotherm

-10

In order to gain more information about the mode of adsorption of these compounds on the surface of mild steel, the experimental data have been tested with several adsorption isotherms. The val-

SB1 1.0

SB3

C/θ (mM)

2.9

3

3.1 -3

3.2

3.3

-1

10 /T (K ) Fig. 6. Arrhenius plots of mild steel in 2 M HCl in absence and presence of 1 mM of different inhibitors.

Table 6 Activation parameters, Ea, DH*, DS*, of the dissolution of mild steel in 2 M HCl in the absence and presence of 1 mM of investigated Schiff bases.

SB2

0.8

2.8

SB4

0.6

Inhibitor

Ea (kJ mol1)

DH* (kJ mol1)

DS* (J mol1 K1)

Blank SB1 SB2 SB3 SB4

57.28 73.53 69.79 65.67 69.15

54.56 73.52 69.79 65.66 69.15

111.50 79.16 88.39 79.33 88.00

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

C (mM) Fig. 5. Langmuir isotherm for adsorption of Schiff bases on the mild steel surface.

ues of surface coverage (h) for different concentrations at 298 K have been used to explain the best isotherm that determines the adsorption process. The surface coverage values (h) were evaluated using corrosion rate values obtained from the weight loss method. Attempts were made to fit these h values to various isotherms including Langmuir, Frumkin, Freundlich and Temkin isotherms. However, the best fit was obtained from the Langmuir isotherm

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

C 1 þC ¼ h K ads

-7 Blank

-8

S B1

The equilibrium constant for adsorption process is related to the free energy of adsorption, DG0ads , and is expressed by following equation:

S B2

ln ν/T (g cm-2 h-1 K-1)

-9

S B3 S B4

-10

K ads ¼ -11 -12 -13 -14 -15 2.8

ð6Þ

2.9

3

3.1 -3

3.2

3.3

-1

10 /T (K ) Fig. 7. Arrhenius plots of ln (mcorr/T) vs. 1/T in the absence and presence of 1 mM of different inhibitors.

(Fig 5). According to this isotherm, the surface coverage h is related to the equilibrium adsorption constant Kads and concentration of inhibitor C via [9]:

1 DG0 exp  ads 55:5 RT

! ð7Þ

where 55.5 is the molar concentration of water in the solution expressed in M (mol L1), R the gas constant (8.314 J K1 mol1) and T the absolute temperature (K). The thermodynamics parameters derived from Langmuir adsorption isotherms for the studied compounds, are given in Table 4. The negative values of DG0ads along with the high K indicate a spontaneous adsorption process [30]. Generally, the energy values of 20 kJ mol1or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface, physisorption; those of 40 kJ mol1 or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond, chemisorption [31,32]. The values of DG0ads in our measurements range from 35 to 38 kJ mol1 (in Table 4), it is suggested that the adsorption of these Schiff bases involves two types of interaction, chemisorption and physisorption.

Fig. 8. SEM micrographs of mild steel samples after 12 h immersion period (a) before corrosion, (b) 2 M HCl, (c) 2 M HCl + 1 mM SB1, (d) 2 M HCl + 1 mM SB2, (e) 2 M HCl + 1 mM SB3, (f) 2 M HCl + 1 mM SB4.

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Fig. 9. Molecular structure, molecular orbital plots of investigated Schiff bases obtained from B3LYP/6-311G** method. Table 7 Quantum chemical parameters of used Schiff bases obtained from four quantum methods. Method

Parameters

Inhibitor SB1

SB2

SB3

SB4

AM1

EHOMO (eV) ELUMO (eV) DE (eV) l (D)

0.3275 0.0061 0.3214 1.7033

0.3281 0.0070 0.3211 1.6395

0.3353 0.0137 0.3216 1.3071

0.3330 0.0108 0.3222 0.9843

PM3

EHOMO (eV) ELUMO (eV) DE (eV) l (D)

0.3333 0.0177 0.3156 1.5021

0.3268 0.0113 0.3155 1.6524

0.3360 0.0182 0.3178 1.5696

0.3343 0.0161 0.3182 1.1891

B3LYP/6-311G**

EHOMO (eV) ELUMO (eV) DE (eV) l (D)

0.2221 0.0638 0.1583 2.7913

0.2174 0.0585 0.1589 5.0890

0.2297 0.0587 0.1710 6.0563

0.2243 0.0546 0.1697 3.8991

B3LYP/6-311++G **

EHOMO (eV) ELUMO (eV) DE (eV) l (D)

0.2262 0.0704 0.1558 2.8189

0.2228 0.0653 0.1574 5.2174

0.2333 0.0650 0.1683 5.7441

0.2300 0.0610 0.1690 3.8454

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361 Table 8 Coefficients of Eq (11) obtained from quantum chemical parameters for four applied quantum methods. State*

Coefficients A

B

C

D

E

AM1

I II

82161892 11979905

77803345 110427384

239689 9.57397

192760178

26731725 101339068

PM3

I II

38933742 35645971

56286013 52847139

5.3051 23955

 1723033

12169331 11666436

B3LYP/6-311G**

I II

3510304 18968146

12209393 8.3203777

13572 0.0006

 15453099

5.1374904 1378473

B3LYP/6-311++G**

I II

5.4155249 19734712

13087458 4.243896

13243 3.1322

 15559004

1677479 1684363

In state I, the D coefficient is equal zero.

3.5. Effect of temperature Gravimetric measurements were carried out in various temperatures (25–75 °C) in the absence and presence of 1  103 M Schiff bases after 1 h of immersion of the specimens in the solution. The values of EW (%) and corrosion rate with and without 1  103 M Schiff bases at different temperatures are given in Table 5. The activation energy could be determined from Arrhenius plots presented in Fig. 6. The corrosion rate’s dependence on temperature can be expressed by Arrhenius equations:

  E m ¼ k exp  a RT  0  E m0 ¼ k0  a RT

ð8Þ ð9Þ

where k and k0 are the rate constants, Ea and E0a are the activation energies for the corrosion in the presence and absence of the inhibitor, respectively. T is the absolute temperature in Kelvin and R is the universal gas constant. The obtained activation energy values are given in Table 6. The increase in activation energy after the addition of the inhibitor to the 2 M HCl solution indicates that physical adsorption (electrostatic) occurs in the first stage [33]. On the other hand, the adsorption phenomenon of an organic molecule is not considered only as a physical or as chemical adsorption phenomenon. A wide spectrum of conditions, ranging from the dominance of chemisorption or electrostatic effects arises from other adsorption experimental data [34]. The higher Ea value in the inhibited solution can be correlated with the increased thickness of the double layer, which enhances the activation energy of the corrosion process [33]. An alternative formulation of Arrhenius equation is [35]:

m¼

    RT DS DH  exp exp  Nh R RT

of 1  103 M of Schiff base are shown in Fig. 8. It can be observed from Fig. 8(b) that the specimen surface was strongly damaged in the absence of the inhibitor. However, Fig. 8(c–f) shows an appreciable inhibiting ability to corrosion on mild steel surface, it is revealed that there is a good protective film adsorbed on specimens surface which is responsible for the inhibition of corrosion. 3.7. Quantum chemical calculations Quantum chemical methods and molecular modeling techniques enable the definition of a large number of molecular quantities characterizing the reactivity, shape, and binding properties of a complete molecule as well as of molecular fragments and substituents. The geometry of the inhibitor as well as the nature of its frontier molecular orbitals, namely, the HOMO and LUMO is involved in the activity properties of the inhibitors. Therefore, in this study, quantum chemical calculations were performed to investigate the relationship between molecular structure of the Schiff bases and their inhibition effect. The optimized molecular structures and the frontier molecule orbital density distribution of the studied molecules are shown in Fig. 9a, and the calculated quantum chemical indices EHOMO, ELUMO, DE, dipole moment (l) are given in Table 7. High values of EHOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy, empty molecular orbital. Therefore, the energy of the lowest unoccupied molecular orbital, ELUMO indicates the ability of accepting electrons to molecule. The lower the value of ELUMO, the more probable, it is that the molecule would accept

120.0

ð10Þ

where h is Planck’s constant, N Avogadro’s number, R the universal gas constant, DH* the enthalpy of the activation and DS* is the entropy of activation. Fig. 7 shows a plot of ln(m/T) against 1/T. Straight lines are obtained with a slope of (DH*/R) and an intercept of (ln(R/ Nh) + (DS*/R)) from which the values of DH* and DS* are calculated and listed in Table 6. The positive signs of DH* reflect the endothermic nature of the mild steel dissolution process [36]. Large and negative values of entropies (DS*) imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex [36,37]. 3.6. SEM analyses SEM photographs obtained from mild steel surface specimens immersion in 2 M HCl solutions for 12 h in the absence and presence

100.0

% IE (Experimental)

*

Method

y = 0.8541x + 9.4827 2 R = 0.9759

80.0

60.0

40.0

20.0

0.0 0.0

20.0

40.0

60.0

80.0

100.0

120.0

%IE (calculated) Fig. 10. Correlation between experimental inhibition efficiency and calculated inhibition efficiency obtained from B3LYP/6-311G** method.

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N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

electrons [38]. If the organic molecules offer electrons to unoccupied d-orbitals of metals and accept the electrons in the d-orbitals of metals by using antibonding orbitals to form covalent bond, the adsorption progress is chemisorption [39], and there will be an obvious relationship between the inhibition efficiency and quantum chemical indices. However, in our experiment result, the inhibition efficiencies are not correlated to EHOMO, ELUMO, and dipole moment. The difficulty in obtaining a direct link between quantum chemical parameters and inhibition efficiency of the Schiff bases indicated that there is a complex interaction in adsorption progress, both chemisorption and physisorption might take place, the conclusion is in agreement with the results of DG0ads which obtained from the thermodynamics calculations [40]. In order to correlate the quantum chemical indices for studied Schiff bases with their experimental inhibition efficiencies, the non-linear model (LKP), proposed by Lukovits et al. [41], has been used in this part of the study. The LKP model is based on the Langmuir adsorption isotherm, where the surface coverage (h) characterizes the adsorption of molecule. Coverage by inhibitor molecules is one of the primary causes of corrosion inhibition [42–44]. By assuming that hi  Ei, the following proposed relation between inhibition efficiency and quantum chemical index can be obtained:

IEcal % ¼

ðAxj þ BÞC i  100 1 þ ðAxj þ BÞC i

ð11Þ

where IEcal is the inhibition efficiency, A and B are the regression coefficients determined by regression analysis, xj is a characteristic quantum index for the inhibitor molecule and Ci denotes the experiment’s concentration of the inhibitor. In order to validate such correlations between calculated quantum parameters of experimental inhibition efficiencies for the four Schiff bases, the non-linear multiple regressions were obtained by different methods (AM1, PM3, B3LYP/6-311G**, B3LYP/6311++G**). The results of these calculations are shown in Table 8.

IEcal % ¼

ðAEHOMO þ BELUMO þ C l þ DDE þ EÞC i  100 1 þ ðAEHOMO þ BELUMO þ C l þ DDE þ EÞC i

ð12Þ

Calculated efficiencies, IEcal (%), for concentration range of inhibitor 0.01–1.00 mM, illustrate good correlation with experimental efficiencies IEexp (%) (R = 0.97) as it is presented in Fig. 10. This significant correlation, using the non-linear model (LKP), indicated that the variation of the corrosion inhibition with the structure of these compounds may be explained in terms of electronic properties.

Fig. 11. The schematic representation of the adsorption behavior of the Schiff bases on mild steel in 2 M HCl solution.

N. Soltani et al. / Corrosion Science 52 (2010) 1351–1361

3.8. Mechanism of the action of inhibitors Different factors need to be considered for elucidating the orientation of organic molecules on the electrode surface [45]. In the case of the studied Schiff bases, the atoms and groups that may interact with the electrode surface were obtained from quantum results (geometry of the inhibitor as well as the nature of its frontier molecular orbitals). Four investigated Schiff bases have three benzene rings that were presented in Fig. 9a by 1, 2 and 3. All of four Schiff bases are the same in benzene rings 1 and 3 and their only difference is related to the substitutions on the benzene ring 2. The frontier molecular orbitals are analyzed in Fig. 9 and it can be seen that for SB1 and SB2, the highest occupied molecular orbital is in the benzene ring 2 and the substitution is on the benzene ring 2, then this is a favorite site for a nucleophilic attack. On the other hand, for the SB3 and SB4, the HOMO is localized over the benzene ring 1 which makes this a preferred zone of the molecule for interaction with the metal surface. The LUMO of all of the studied Schiff bases, as can be seen from Fig. 9c, is the benzene ring 3 and the double iminic bond N@C linked to the benzene ring 3 has larger electric density. We conclude from frontier molecular orbitals that Schiff bases SB1 and SB2 interact with the metal surface through benzene rings 2 and 3 while Schiff bases SB3 and SB4 interact through 1 and 3 rings (Fig. 11). According to the Fig. 11 two differences between SB1, SB2 and SB3, SB4 are observed which explain the better inhabitation effect of SB1 and SB2 with respect to SB3 and SB4. The first difference is related to C@N group. It is known that presence of the C@N group in the molecules Schiff bases, due to the planarity (p) and lone pairs of electrons present on N atoms, causes that Schiff bases to act as good inhibitors. But, as it is shown in Fig. 11, both C@N groups in investigated Schiff bases have steric hindrance which prevents their flat orientation on the metal surface. However, the circumstance of orientation of SB1 and SB2 (Fig. 11) show that one of C@N groups (N3 and N10 for SB1 and SB2, respectively) has lesser steric hindrance and can interact with the steel surface while in SB3 and SB4 the interaction of both C@N groups is impossible. The secant difference arises from the substitutions of OH and OCH3 in benzene rings 2. It seems that the effect of O atoms of OH or OCH3 groups and benzene rings in interact with the electrode surface is higher than C@N groups. Considering the fact that in SB1 and SB2, the benzene ring 2 interacts with surface thus act better than SB3 and SB4.

4. Conclusion Chemical and electrochemical measurements incorporated with quantum chemical studies were used to study the corrosion inhibition characteristics of some Schiff bases on mild steel in 2 M HCl solutions. The principle conclusions are as follow: (1) All examined Schiff base compounds act as excellent inhibitor for mild steel corrosion in 2 M HCl solution especially in high concentration. Inhibition efficiency of these compounds increases with increase in their concentrations due to the formation of a surface film on the steel surface. (2) The adsorption of these compounds on the mild steel surface obeys the Langmuir adsorption isotherm. (3) The thermodynamic parameters (Kads, DG0ads ) of adsorption for the studied compounds are calculated from their adsorption isotherms. The negative values of DG0ads show the spontaneity of the adsorption.

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