2,2′-Dithiobis(3-cyano-4,6-dimethylpyridine): A new class of acid corrosion inhibitors for mild steel

Corrosion Science 48 (2006) 3398–3412 www.elsevier.com/locate/corsci

2,2 0 -Dithiobis(3-cyano-4,6-dimethylpyridine): A new class of acid corrosion inhibitors for mild steel M.S. Morad *, A.M. Kamal El-Dean Electrochemistry Research Laboratory, Department of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt Received 15 December 2004; accepted 20 December 2005 Available online 17 April 2006

Abstract The title compound (PyS)2 has been synthesized and its inhibiting action on the corrosion of mild steel in 1–5 M H2SO4 solutions at 35–50 C has been investigated by polarization resistance (Rp), polarization curves and electrochemical impedance spectroscopy (EIS). (PyS)2 showed excellent performance and its efficiency did not affect either by increasing the acid concentration or rise of temperature. Polarization curves indicated that (PyS)2 behaves mainly as anodic inhibitor in 1 M H2SO4 solutions and as a mixed-type inhibitor in 3 and 5 M H2SO4 solutions at different temperatures. Adsorption of (PyS)2 on the steel surface followed Temkin’s adsorption isotherm with a very high negative value of the free energy of adsorption ðDG0ads Þ. The activation parameters of the corrosion process were calculated. EIS showed that the charge transfer controls the corrosion process in the uninhibited and inhibited solutions.  2006 Published by Elsevier Ltd. Keywords: C. Acid corrosion inhibitors; C. Adsorption; B. EIS; A. Mild steel

1. Introduction Acid inhibitors find wide applications in the industrial field as a component in pre-treatment composition, in cleaning solutions for industrial equipments and in acidization of oil *

Corresponding author. Tel.: +20 88 411 475; fax: +20 88 312 564. E-mail addresses: [email protected], [email protected] (M.S. Morad).

0010-938X/$ - see front matter  2006 Published by Elsevier Ltd. doi:10.1016/j.corsci.2005.12.006

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wells. Most of the efficient inhibitors used in industry are organic compounds that mainly contain nitrogen; sulfur atoms and multiple bonds in the molecules through which they are adsorbed on the metal surface. In spite of the large number of organic compounds, the choice of the appropriate inhibitor for a particular system is very limited due to the specificity of the inhibitor and the great variety of corrosion systems [1]. There always exists a need for developing new corrosion inhibitors. Because heterocyclic compounds represent a potential class of corrosion inhibitors, there is a wide range of studies in the literature regarding corrosion inhibition by heterocyclic compounds containing both nitrogen and sulfur atoms [2–11]. In the present study, 2,2 0 -dithiobis(3-cyano-4,6-dimethylpyridine), abbreviated as (PyS)2, was synthesized and investigated as inhibitor for the corrosion of mild steel in 1, 3 and 5 M H2SO4 solutions at 35–50 C using polarization resistance, potentiostatic and electrochemical impedance (EIS) techniques. The choice of the investigated compound is based on three considerations: first, it could be synthesized easily with high yield, second, it could be prepared from relatively cheap materials and third, it possesses multiadsorption centers. These centers are: the electron cloud of the pyridine rings and that of the triple bond of the cyano groups, the lone pairs of electrons on the hetero-atom N and the S–S linkage. The presence of two-methyl groups in the pyridine ring may enhance the electron density of the molecule. It is worth mentioned that (PyS)2 must be handled with care, as its toxicity is not yet known. 2. Experimental 2.1. Synthesis of (PyS)2 The organic compound tested as a corrosion inhibitor was synthesized according to the following procedure. To a solution of 4,6-dimethyl-2-thioxo-1,2-dihydro-pyridine-3-carbonitrile 3.28 g (0.02 mol) in acetic acid (30 ml), a solution of sodium nitrite (2.76 g, 0.04 mol in 5 ml H2O) was added dropwise with stirring during 15 min. The reaction mixture allowed to stand for 3 h, then poured into cold water (100 ml). The solid product was collected and recrystallized from ethanol as white crystals in 88% yield, m.p. 185 C. The key compound 4,6-dimethyl-2-thioxo-1,2-dihydro-pyridine-3-carbonitrile was prepared according to Schmidt and Kubitzek [12]. C N

SH

N

C

N H

S

N

NaNO2 C

AcOH N

S

N N

ð1Þ

C S

N

Elemental analysis calculated for C16H14N4S2 (M. Wt. 326.44): C: 58.87, H: 4.32, N: 17.16, S: 19.64%. IR: mmax/cm1 2220 (CN). 1 H NMR (CDCl3): d = 2.4, 2.7 (2S, 12H, 4CH3), 6.9 (S, 2H, 2H-pyridine). MS; EI: m/z = 326 (M+).

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2.2. Working electrode (WE) The samples were mild steel of the following composition in wt.%: C: 0.15, S: 0.021, P: 0.005, Si: 0.01, Mn: 0.33 and Fe: rest. Before the electrochemical measurements, the steel sample was polished using different grits of wet emery papers, washed with bi-distilled water, degreased with acetone and finally washed with a stream of bi-distilled water. 2.3. Electrolyte The corrosive medium was H2SO4 solution of concentrations of 1, 3 and 5 M. The solutions were prepared using concentrated H2SO4 (Merck) and double distilled water. In each experiment, 200 ml of the acid solution was used and deaerated with purified nitrogen for 1 h. All tests were performed in magnetically stirred solutions at 35, 40, 45 and 50 C. Solutions of 1, 3 and 5 M H2SO4 with 1 · 104 M (PyS)2 are termed as solution A, B and C respectively. 2.4. Electrochemical measurements The experiments were carried out using a standard three-electrode cell. A steel cylinder passed into a Teflon holder served as a working electrode. Its working area of 0.2 cm2 remained precisely fixed. A platinum sheet of a large surface area was used as a counter electrode while a saturated calomel electrode (SCE) acted as a reference electrode. EG&G instruments were used. It includes a PAR Model 273 potentiostat/galvanostat and a 5210 two-phase Lock-in amplifier, which enables impedance measurements from 100 lHz to 100 kHz. They were monitored by an IBM-AT personal computer via GBIP-IIA interface. Two softwares, M352 and M398 were used for running and evaluating DC and AC experiments respectively. In polarization resistance technique, the WE was polarized in the range ±20 mV vs. corrosion potential (Ecorr) at a scan rate of 1 mV s1. The polarization curves in the range from 200 to +120 mV vs. Ecorr at a scan rate of 0.5 mV s1 were recorded. Impedance spectra were recorded in the frequency range from 100 kHz to 0.1 Hz with five points per decade and peak to peak AC amplitude of 5 mV. EQUIVCRT.PAS commercial software [13] was used to fit the impedance results. 3. Results and discussion 3.1. Behaviour at 35 C 3.1.1. Polarization curves and polarization resistance The results obtained for the electrochemical and corrosion behaviour of mild steel in 1 M H2SO4 in the absence and the presence of 5 · 109–1 · 104 M (PyS)2 at 35 C are shown in Fig. 1 and Table 1. Values of anodic b+ and cathodic b Tafel constants and corrosion current density (Icorr) were calculated from the intersection of the anodic and cathodic Tafel lines of the polarization curves at Ecorr. Although the Tafel region of the anodic branch of the polarization curve of the inhibitor-free solution is relatively narrow,

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-1

log I (A/cm2)

-2

-3 1M H2 SO4 5x 10-9 M (PyS)2

-4

1x 10-8 M 1x 10-6 M 1x 10-5 M

-5

1x 10-4 M

-0.6

-0.5

-0.4

-0.3

E (V/SCE) Fig. 1. Potentiostatic polarization curves obtained for the system mild steel/1 M H2SO4 without and with (PyS)2 at 35 C.

Table 1 Electrochemical parameters obtained for the system mild steel/1 M H2SO4 without and with (PyS)2 at 35 C [(PyS)2] (mol l1)

Ecorr (mV/SCE)

b+ (mV/decade)

b (mV/decade)

Rp (X/cm2)

Icorr (lA/cm2)

IE% Tafel

Rp

0 5 · 109 1 · 108 1 · 107 1 · 106 1 · 105 1 · 104

416 390 383 381 381 379 365

44 18 19 17 21 26 34

124 91 95 93 87 79 90

9.0 32.8 39.3 33.0 44.3 69.7 94.6

1135 291 247 216 193 118 84

– 74.4 77.4 81.0 83.0 89.6 92.6

– 72.5 77.1 79.5 81.0 83.0 92.5

corrosion rates measured by the Tafel extrapolation method are valid. Validation of this method was discussed by McCafferty [14] and the following conditions must be fulfilled: (1) At least one of the branches of the polarization curves is under activation control. (2) The anodic and cathodic reactions, which occur at the corrosion potential, are the only reactions, which occur during determination of the polarization curves. (3) Well-defined anodic or cathodic Tafel regions exist. (4) Corrosion is general i.e. uniform. (5) The polarization curves are for the steady-state. Inspection of the results given in Table 1 and Fig. 1 indicates that the investigated system fulfil the above requirements. However, the results of Fig. 1 and Table 1 indicate that the additive affects largely the anodic reaction while the cathodic one is slightly shifted toward lower current densities only at 1 · 104 M concentration. It should be mentioned

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Table 2 Impedance parameters obtained for the corrosion of mild steel in 1 M H2SO4 solution without and with (PyS)2 at 35 C [(PyS)2] (mol l1)

Rct (X cm2)

Cdl (lF/cm2)

IE% Rct

Cdl

0 1 · 106 1 · 105 1 · 104

9.95 34.2 41.7 81.1

1600 466 382 311

– 70.9 76.1 87.7

– 70.9 76.1 80.6

that the anodic branch of the polarization curve of the pure medium has a relatively small linear Tafel region. This region becomes more and more wide as the concentration of (PyS)2 increases. Similar results were found in the literature [16–18]. Based on the large positive shift in the corrosion potential and the displacement of the anodic part of the polarization curve of the uninhibited solution, (PyS)2 is considered as an inhibitor of predominant anodic effect. Values of the corrosion rate in Table 1 indicate corrosion inhibition even at the lowest examined concentration (IE is 74.4% at 5 nM) and good agreement between values of IE% obtained from polarization resistance and extrapolation of polarization curves is noticed. The high values of IE (74–92%) can be ascribed to the strong adsorption of the compound on the steel surface. The polarization resistance method gave approximately the same inhibition efficiency (Table 2). As a new class of acid corrosion inhibitors, (PyS)2 is better than many inhibitors used for corrosion of iron and steel in sulfuric acid solution [15–19]. 3.1.2. Study of the adsorption phenomenon Two main types of interaction can describe adsorption of the organic compounds on the electrode surface: physical adsorption and chemisorption. Both types of adsorption are influenced by the nature and charge (potential of zero charge Eq=0) of the metal, the chemical structure of the inhibitor as well as the type of the corrosive electrolyte. However, basic information on the interaction between the inhibitor and the metal surface can be provided by the adsorption isotherm, which, in turns depends on the degree of electrode surface coverage (h). This quantity can was calculated from values of corrosion current density of steel in the uninhibited (I1) and the inhibited (I2) solutions according to the equation h ¼ 1  ðI 2 =I 1 Þ

ð2Þ

Attempts were made to fit these h values to various isotherms. By far, the best fit was obtained with Timken’s adsorption isotherm (Fig. 2) which characterizes the chemisorption of uncharged molecules on heterogeneous surface [20]. The strong correlation (r2 > 0.9) obtained for Temkin’s isotherm plot for (PyS)2 at 35 C confirms the validity of this approach. For such isotherm h is a linear function of log C according to the equation h ¼ ð1=f Þ lnðK ads CÞ

ð3Þ

where Kads is the adsorption equilibrium constant and C is the bulk concentration of the adsorbate. The value of Kads was determined and used for obtaining values of the free energy of adsorption DG0ads using the following equation:

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0.95 Θ = 1.1 + 0.043 logC

0.90

2

R = 0.986, SD = 0.014

0.85

0.80

0.75

0.70 -9

-8

-7

-6

-5

-4

log C (M) Fig. 2. Temkin adsorption isotherm of (PyS)2 on mild steel surface in 1 M H2SO4 solution at 35 C.

K ads ¼ ð1=55:5Þ expðDG0ads =RT Þ

ð4Þ

Values of Kads and DG0ads were found to be 5.72 · 1025 M1 and 162.08 kJ mol1 respectively. The high negative value of DG0ads indicates chemisorption of (PyS)2 on steel surface and at the same time reveals the spontaneity of adsorption process and the stability of the adsorbed layer [21]. The preceding conclusion was based on the following: although the range of the calculated coverage of Fig. 2 is relatively narrow, it was subjected to Frumkin, Temkin and Langmuir isotherms. Frumkin’s isotherm is not valid and Langmuir isotherm in its simple form (where C/h is plotted vs. C) is also not valid. The modified form of Langmuir, that is: log[h/(1  h)] = log K + log C fits our data (log[h/(1  h)] vs. log C) gave a straight line, but the value of DG0ads calculated from the value of K is very small (5 kJ/mol) which is characteristic for physical adsorption. This type of adsorption contradicts the approximate constancy of IE% at different temperatures (see below). However, values of DG0ads until 20 kJ/mol are consistent with physical adsorption while those higher than 40 kJ/mol indicate chemisorption [22]. Although their results fit Langmuir isotherm, some authors [23–25] attributed corrosion inhibition to physisorption while the results of others [26] were interpreted as chemisorption. Adsorption of some Schiff bases on carbon steel surface in HCl solution followed Temkin’s isotherm and adsorption was considered as chemisorption [27]. 3.1.3. EIS results at the open-circuit potential The aim of this technique is to obtain more information concerning the electrochemical processes which occurs at the mild steel/1 M H2SO4 interface in absence and presence of 1 · 106–1 · 104 M (PyS)2. The impedance response of these systems is presented in Fig. 3. For the uninhibited solution, Nyquist plot is composed of a capacitive loop covers most of the frequency range, followed by a small inductive loop below 1 Hz. The inductive loop is commonly associated with a relaxation process of adsorbed species at the

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1M H2 SO4

50

1x 10-6 M( PyS)2

-Z'' (ohm cm2)

1x 10-5 M

40

1x 10-4 M

6. 3Hz

30 10 Hz

20

63 Hz

10

39 .8Hz

0.63 Hz

1H z

0 0

20

40

60

80

100

Z' (ohm cm2) Fig. 3. Nyquist plots of mild steel corrosion obtained at Ecorr in 1 M H2SO4 without and with (PyS)2 at 35 C.

metal/solution interface. The capacitive loop can be ascribed to the charge transfer process. In presence of 1 · 106–1 · 104 M (PyS)2, only one semicircle is observed. The impedance parameters relevant to the capacitive semicircle of the mild steel/1 M H2SO4/(PyS)2 system are given in Table 2. These parameters were calculated from the NLSF of the equivalent circuit [28]. The results of Table 2 indicate that value of the charge transfer resistance Rct obtained in the pure solution is increased with the increase of the (PyS)2 concentration indicating the increase of the corrosion resistance. As it can be seen from this table, when the concentration of the inhibitor increases, values of double layer’s capacity Cdl tend to decrease. Decrease in Cdl values can result from an increase in the thickness of the electrical double layer. This behaviour suggests that (PyS)2 molecules function by adsorption at the metal solution interface. Another evidence for the effective adsorption of (PyS)2 on the steel surface can be given from the observation that the maximum frequency (fmax) of the capacitive loop of the uninhibited solution is decreased with increasing of the inhibitor concentration. Similar results were obtained for corrosion inhibition of ASTM A606-4 steel in 16% HCl solution by copper phthalocyanine [29]. Values of IE% deduced from values of both Rct and Cdl are also included in Table 2. These values are in agreement with each other and with those obtained from the polarization measurements. 3.2. Effects of temperature and acid’s concentration To ensure the effectiveness of (PyS)2 under various conditions, tests were performed in more concentrated H2SO4 solutions (3 and 5 M) at different temperatures (35–50 C). The concentration of (PyS)2 which gave the highest IE% value in 1 M H2SO4 solution at 35 C (1 · 104 M) was chosen for further study. 3.2.1. Polarization curves and polarization resistance The results deduced from the polarization curves obtained for the corrosion of mild steel in 3 M and 5 M H2SO4 without and with 1 · 104 M (PyS)2 at different temperatures

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0

-1

-2

-3 5M H2 SO4 1x 10-4 M (P yS)2

-4 -0.6

-0.5

-0.4

-0.3

-0.2

E (V / SCE) Fig. 4. Potentiostatic polarization curves obtained for the system: mild steel/5 M H2SO4 without and with 1 · 104 M (PyS)2 at 50 C.

indicated that values of Icorr are increased with increasing both the acid concentration and the temperature. Representative polarization curves obtained in 5 M H2SO4 without and with 1 · 104 M (PyS)2 are shown in Fig. 4. It is clearly shown that the investigated compound affects both cathodic and anodic branches with approximate equal values of both b+ and b. These observations indicate that (PyS)2 is a mixed-type inhibitor and its presence has no influence on the mechanism of hydrogen evolution reaction or iron dissolution. The results obtained from Rp method are shown graphically in Fig. 5a and b for the corrosion of mild steel in 1–5 M H2SO4 solutions without and with 1 · 104 M (PyS)2 respectively. In both cases, values of Rp are decreased and the pronounced decrease is observed in 1 M H2SO4 solutions. On the other hand, Fig. 6a shows the relationship between values of IE% calculated from Rp method and the temperature where the former quantity is slightly increased or remains unchanged with the raise of temperature. This behaviour reflects chemisorption of (PyS)2 on the steel surface. The same behaviour can be observed from Fig. 6b (IE% calculated from the polarization curves vs. temperature). In order to calculate the activation parameters of the corrosion process, Arrhenius equation (5) and transition state equation (6) were used I corr ¼ A exp½Ea =RT 

ð5Þ 



I corr ¼ ðRT =NhÞ expðDS =RÞ expðDH =RT Þ

ð6Þ

where A is the pre-exponential factor; N, h are Avogadro’s number and Plank’s constant respectively and Ea, DH* and DS* are the activation energy, change in enthalpy and change in the entropy of the corrosion process. Fig. 7a and b show typical plots of log Icorr vs. 1/T for the uninhibited and inhibited solutions while Fig. 8 shows a representative plot for the transition state in 5 M H2SO4 solutions without and with 1 · 104 M (PyS)2. Activation parameters obtained from these graphs are given in Table 3. Inspection of Table 3 shows that values of both Ea, DH* obtained in presence of (PyS)2 are lower than those obtained in the inhibitor-free

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10

1M H2 SO4 3M 5M

Rp (ohm/cm2)

8 6 4 2 (a)

35

40

45

50

45

50

120

Rp (ohm/cm2)

100 Solution A Solution B Solution C

80 60 40 20 35

(b)

40 o

t /C

Fig. 5. Variation of the polarization resistance with temperature. (a) In pure H2SO4 solutions and (b) in H2SO4 solutions containing 1 · 104 M (PyS)2. Solution A = 1 M H2SO4 + 1 · 104 M (PyS)2. Solution B = 3 M H2SO4 + 1 · 104 M (PyS)2. Solution C = 5 M H2SO4 + 1 · 104 M (PyS)2.

solutions. This observation further supports the proposed chemisorption mechanism. Unchanged or lower values of Ea in inhibited systems compared to the blank has been reported [22,30,31] to be indicative of chemisorption mechanism, while higher values of Ea suggests a physical adsorption mechanism. On the other hand, values of DH* are positive while DS* values are more positive in H2SO4 solutions containing (PyS)2 than those obtained in the uninhibited solutions. This behaviour can be explained as a result of the replacement process of water molecules during adsorption of (PyS)2 on steel surface. This observation is in agreement with the findings of other workers [32,33]. 3.2.2. Impedance measurements The effect of solution temperature on the impedance behaviour of mild steel in H2SO4 solutions without and with 1 · 104 M (PyS)2 has been studied and the results are given in Fig. 9a and b respectively. The impedance parameters obtained in 1 M H2SO4 solutions are given in Table 4. These results were obtained from NLSF for the previously proposed

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96 94 Solution A Solution B Solution C

IE %

92 90 88 86 (a)

35

40

45

50

98

IE %

97

96 Solution A Solution B Solution C

95

94 (b)

35

40

45

50

o

t /C

Fig. 6. Dependence of the inhibition efficiency of (PyS)2 on temperature. (a) Values obtained from Rp measurements. (b) Values obtained from the polarization curves. Solutions A, B, C as in Fig. 5.

equivalent circuit with the difference that the Cdl is replaced by a constant phase element, CPE, (Q) to overcome the problem of surface heterogeneity [34]. The results demonstrate that values of Rct obtained in the uninhibited medium are decreased while those of Q are increased indicating the increase in the corrosion rate. The decrease in values of the CPE’s exponent, n, with the rise of temperature is an indication for the increase of electrode surface roughness. In the inhibited solution, the reverse behaviour is observed. The increase of n values with temperature reflects that the electrode surface becomes more homogeneous as a result of adsorption of (PyS)2. Similar results were obtained in both 3 and 5 M H2SO4 solutions. Values of IE% deduced from Rct values are also included in Table 4 and it is clearly seen that the compound becomes more efficient at the highest temperature. To obtain the activation parameters from the impedance technique, values of Rct and those of b+ and b obtained at different temperature were used to calculate values of Icorr according to Stern–Geary [35] equation. Fig. 10 shows the Arrhenius plot for the results obtained in 1 M H2SO4 solutions. The activation parameters Ea, DH* and DS* are also

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1M H2 SO4 3M 5M

log I (µA/cm2)

4.5

4.0

3.5

3.0 (a)

3.10

3.15

3.20

Solution A Solution B Solution C

3.0

log I (µA/cm2)

3.25

2.7

2.4

2.1

1.8 3.10 (b)

3.15

(1000/T) /

3.20

3.25

K-1

Fig. 7. Arrhenius plots for mild steel corrosion. (a) In pure H2SO4 solutions. (b) In H2SO4 solutions containing 1 · 104 M (PyS)2. Solutions A, B, C as in Fig. 5.

included in Table 3. The results are in good agreement with those obtained from polarization studies and show the same trend. 3.3. Mechanism of corrosion inhibition by (PyS)2 As far as the inhibition process is concerned, it is generally assumed that the adsorption of the inhibitor at the metal/solution interface is the first step in the action mechanism of the inhibitor in the aggressive medium. Inhibition of mild steel corrosion in H2SO4 solution by (PyS)2 can be explained on the basis of adsorption. The results obtained by different electrochemical techniques indicates that adsorption of (PyS)2 on the steel surface could occur directly on the basis of donor–acceptor interaction between the extensively delocalized p-electrons of the nitrilo groups and the pyridine

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2.4 R2 = 0.9978, SD = 0.054

log (I/T)

1.8

1.2

5M H2SO4 Solution C

R2 = 0.9945, SD = 0.057

0.6

0.0 3.12

3.16

3.20

3.24

-1

(1000/T) / K

Fig. 8. Plot of log(corrosion rate/T) against 1/T for mild steel in 5 M H2SO4 without and with 1 · 104 M (PyS)2.

Table 3 Activation parameters of the corrosion of mild steel in H2SO4 solutions in absence and presence of 1 · 104 M (PyS)2 from polarization and EIS measurements Corrosive medium

1 M H2SO4 Solution A 3 M H2SO4 Solution B 5 M H2SO4 Solution C

From polarization curves

From EIS measurements

Ea (kJ/mol)

DH* (kJ/mol)

DS* (J/mol)

Ea (kJ/mol)

DH* (kJ/mol)

DS* (J/mol)

62.0 51.0 129.8 78.4 142.3 97.5

47.7 42.6 127.7 75.8 139.7 93.9

45.9 55.5 235 41.5 279.3 105.2

51.2 47.7 99.3 68.9 117.1 115.8

48.3 44.6 97.0 66.6 115.7 112.9

27.7 59.2 133.0 15.0 200.3 156.5

moieties and the vacant d-orbitals of iron surface atoms [36]. Moreover, the presence of the disulfide linkage in the organic structure makes the formation of dp–dp bond resulting from the overlap of 3d-electrons from Fe atom to the 3d vacant orbitals of the sulfur atom is possible [37] and thus adsorption of (PyS)2 on the metal surface is enhanced. The investigated compound inhibits the corrosion of mild steel by controlling both the anodic and cathodic reactions. In the acidic solution, (PyS)2 molecules could exist as protonated species. Under cathodic polarization conditions, the protonated molecules may be adsorbed on the cathodic sites and inhibit the hydrogen evolution reaction (specially in 3 and 5 M H2SO4 solutions at different temperatures). On the other hand, the (PyS)2 molecules are able to be adsorbed on the anodic sites through the N and S atoms, the p clouds of the two cyano groups and the pyridine rings (the electron density of the pyridine rings is enhanced by the –CH3 group which is electron-donating group). Adsorption of the compound on the anodic sites results in a decrease of the anodic dissolution of iron. The high performance of (PyS)2 could also be attributed to the large size of the molecule which screen wide areas of the metal surface and thus preventing the general

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1M H2 SO4 3M H2 SO4 5M H2 SO4

-Z'' (ohm cm2)

2.4

1.8

1.2

0.6

63 Hz

1k Hz

100 Hz

(a)

0.63 Hz

10 Hz

0.0 0

1

2

3

20

-Z'' (ohm cm2)

16

4

Solution A Solution B Solution C

15.8 Hz

12 10 Hz

8 1.58Hz

4 15.8 Hz 0.1H z

0 0

10

20

30

40

Z' (ohm cm2)

(b)

Fig. 9. Nyquist plots obtained at Ecorr for mild steel corrosion at 50 C. (a) In pure H2SO4 solutions. (b) In H2SO4 solutions containing 1 · 104 M (PyS)2. Solutions A, B, C as in Fig. 5.

Table 4 Effect of temperature on the impedance parameters obtained for the corrosion of mild steel in 1 M H2SO4 solution without and with (PyS)2 Medium

t (C)

Rct (X cm2)

Q · 104 (X1 cm2 sn)

n

IE%

1 M H2SO4

35 40 45 50 35 40 45 50

9.9 7.0 5.6 3.54 79.4 65.5 52.0 33.9

4.83 5.62 6.71 7.25 4.40 5.07 5.10 5.80

0.91 0.90 0.92 0.89 0.92 0.95 0.96 0.98

– – – – 87.5 89.3 89.2 89.7

1 · 104 M (PyS)2

M.S. Morad, A.M. Kamal El-Dean / Corrosion Science 48 (2006) 3398–3412

R2 = 0.986, SD= 0.026

3.50

log I (µA/cm2)

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3.15 1M H2SO4 Solution A 2.80 R2 = 0.987, SD= 0.022

2.45

2.10 3.10

3.15

(1000/T) /

3.20

3.25

K-1

Fig. 10. Arrhenius plots deduced from the impedance results for mild steel corrosion in 1 M H2SO4 without and with 1 · 104 M (PyS)2.

corrosion [38]. The foregoing discussion may explain why (PyS)2 is better as corrosion inhibitor than others (see Section 3.1.1). 4. Conclusions 2,2 0 -Dithiobis(3-cyano-4,6-dimethylpyridine) was found to be an efficient inhibitor for the corrosion of mild steel in 1–5 M H2SO4 solutions at 35–50 C. The inhibition efficiency of (PyS)2 is slightly increased or remains approximately unchanged irrespective of the acid concentration or the solution temperature. The inhibition is attributed to chemisorption of the heterocyclic compound on the steel surface and blocking its active sites. The inhibitor acts as predominant anodic inhibitor in 1 M H2SO4 solutions and as a mixed-type inhibitor in 3 and 5 M H2SO4 at 40–50 C. The data obtained fit well Temkin’s adsorption isotherm. The activation parameters of corrosion process such as activation energy, Ea, activation enthalpy, DH* and activation entropy, DS*, were calculated from the dependence of corrosion rates on the temperature. Impedance results showed that the charge transfer controls the corrosion process in the uninhibited and inhibited solutions. References [1] [2] [3] [4] [5] [6] [7] [8]

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