Carboxymethylchitosan + Cu2+ mixture as an inhibitor used for mild steel in 1 M HCl

Electrochimica Acta 52 (2007) 5932–5938

Carboxymethylchitosan + Cu2+ mixture as an inhibitor used for mild steel in 1 M HCl Sha Cheng a , Shougang Chen a , Tao Liu a , Xueting Chang b , Yansheng Yin a,∗ a b

Institute of Materials Science and Technology, Ocean University of China, Qingdao 266100, China College of Materials Science and Technology, Shandong University of China, Jinan 250061, China Received 11 December 2006; received in revised form 10 March 2007; accepted 15 March 2007 Available online 20 March 2007

Abstract The inhibition effect of Carboxymethylchitosan (CMCT), Cu2+ , and CMCT + Cu2+ mixture on the corrosion of mild steel in 1 M HCl has been investigated using gravimetric and electrochemical techniques. CMCT + Cu2+ mixture acts much more effectively than the inhibiting action of each additive separately. In addition, higher efficiency is achieved for the mixture of 20 mg L−1 CMCT + 10−4 M Cu2+ . The efficiency of the optimal mixture increases with the temperature in the range 298–353 K. Activation energy of corrosion reaction in the presence of the optimal mixture of the inhibitors is much lower than that exhibited in 1 M HCl solution. The inhibition mechanism proposed in this paper is based on the results of conductometric investigations. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mild steel; 1 M HCl; Corrosion inhibitor; Carboxymethylchitosan (CMCT); Complex

1. Introduction Due to the wide use of acidic media in many industrial fields, several researchers devoted their attention to develop more effective and non-toxic inhibitors to reduce both acid attack and protection aspects. Carboxymethylchitosan (CMCT) is a derivative of natural products of chitin, commonly applied in medicine, cosmetics, textile, paper food and many others industrial branches [1], owing to its excellent biodegrability. Nitrogen-containing compounds act more effectively in HCl solutions [2,3]. CMCT is a water-soluble derivative rich in hydroxyl group, amino group and carboxyl group, thus it is likely to be a good inhibitor. Results of previous studies [4] show, however, that it does not act excellently at low concentration; hence, other modes of action are needed. Literatures [5,6] report that the presence of many metal cations such as Cu2+ , Zn2+ , Mn2+ can cooperate with many organic inhibitors. Meanwhile, metal cations could change the physical and electrical structure of the electrode/electrolyte interfacial region, which was proposed as the possible mechanism. After forming

∗

Corresponding author. E-mail address: [email protected] (Y. Yin).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.03.038

complex with copper, the binding force between phthalocyanine and metallic surface becomes much stronger and then the inhibition efficiency is increased greatly [7]. From this point, the aim of this work is to evaluate the inhibition effect of CMCT or Cu2+ cation as well as the mixture on mild steel in 1 M HCl, using weight loss measurements and electrochemical methods. 2. Experimental details 2.1. Materials Chemical preparation of water-soluble Carboxymethylchitosan (CMCT) was according to the experimental procedure described in the literature [8]. The average molecular weight has been found to be 3 × 104 and the structure is shown in Fig. 1. Prior to all measurements, the steel specimens (0.045% P; 0.3% Si; 0.3% Cr; 0.3–0.65% Mn; 0.14–0.22% C; 0.05% S; 0.3% Ni; 0.3% Cu and the remainder Fe) were ground with different emery papers (grade 400, 600, 800, 1000 and 1200), rinsed with bidistillded water, degreased ultrasonically in ethanol and dried at room temperature in vacuum dryer before use. The aggressive solutions used were made of AR grade 37% HCl. CuSO4 was used as a source for Cu2+ cation. Appropriate

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Table 1 Gravimetric results of mild steel in 1 M HCl with and without addition of inhibitors at 298 K for 24 h

Fig. 1. Molecular structure of CMCT.

concentrations of acid solutions were prepared using bidistilled water. 2.2. Electrochemical measurements Electrochemical experiments were performed in a conventional three-electrode cell consisting of steel working electrode (WE) with an exposure surface of 1.0 cm2 , a 1.5 cm × 1.5 cm platinum counter electrode (CE) and a saturated calomel reference electrode (SCE). The latter was connected through a Luggin’s capillary to the cell. The EIS measurements were carried out at open-circuit potential (OCP) in the frequency range of 10 KHz–0.01 Hz, by using the ZAHNER IM6 Electrochemical Workstation (Germany) with sine wave voltages (10 mV) peak to peak. The potentiodynamic polarization curves were carried out from −250 to +250 mV versus OCP with a scan rate of 0.5 mV s−1 . The linear polarization curves were obtained between ±10 mV versus OCP with a scan rate of 0.125 mV s−1 . 2.3. Weight loss measurements The weight loss of steel specimens of size 2 cm × 1 cm × 0.5 cm in 1 M HCl with and without addition of different concentrations of inhibitors at different immersion times (a maximum immersion time was 24 h) by weighing the cleaned samples before and after hanging the specimen into 100 cm3 of the corrosive solution without bubbling (in open air). 2.4. Conductometric titration Some conductometric investigations for titration of 3.4 × 10−3 M CMCT dissolved in 1 M HCl solution using 1 M HCl containing 5.76 × 10−3 M Cu2+ ions as a titrant were performed. Conductivity measurements were carried out using DDS-307 conductivity meter of cell constant equal to 9.83 unit. 3. Results and discussion 3.1. Weight loss measurements The corrosion rate (Wcorr ) of mild steel in 1 M HCl at various concentrations of CMCT or Cu2+ cation as well as the mixture was determined. Their values of Ew (%) and Wcorr at different concentrations of CMCT are given in Table 1. Ew (%) can be calculated by well-known relationship: W0 − Wcorr Ew (%) = 100 × W0

(1)

Solution

Wcorr (␮g cm−2 h−1 )

Surface coverage (θ)

Blank

14.33

–

CMCT 10 20 30 40 50

Ew (%) –

(mg L−1 )

Cu2+ (M) 10−3 10−4 10−5

3.91 3.59 2.59 2.46 2.07

0.73 0.75 0.82 0.83 0.85

72.71 74.95 81.87 82.77 85.50

17.09 13.31 12.66

– 0.07 0.12

– 7.20 11.65

0.83 0.92 0.90

82.90 91.97 89.60

CMCT (mg L−1 ) + Cu2+ (M) 2.45 20 + 10−3 20 + 10−4 1.15 1.49 20 + 10−5

where Wcorr and W0 are the values of mild steel corrosion rate with and without the inhibitor, respectively. Gravimetric measurements show that the corrosion rate decreases in the presence of CMCT, the inhibition efficiency increases with CMCT concentration and the surface coverage θ defined by Ew %/100 reaches the maximum of 0.85 at 50 mg L−1 . The inhibition can be explained by the adsorption of CMCT [4]. When the solution only contains Cu2+ ions, the inhibition efficiency is very low and at higher concentration (10−3 M) Cu deposit can be observed on the metallic surface, thus the corrosion process is accelerated by galvanic corrosion. In the presence of the mixtures, the efficiency increases and reaches the maximum at 20 mg L−1 CMCT + 10−4 M Cu2+ which is the critical concentration as shown by the results in Table 1. The efficiency of the additives is arranged in the following order: CMCT + Cu2+ > CMCT > Cu2+ . The values illustrate the synergistic effects of the Cu2+ ions and CMCT. For instance, a concentration of 10−4 M Cu2+ ions (in the absence of CMCT) corresponds to a value of Ew (%) of 7.20%. On the other hand, with a concentration of 20 mg L−1 CMCT (in the absence of Cu2+ ions) Ew (%) of 74.95% is obtained. However, with a concentration of 20 mg L−1 CMCT + 10−4 M Cu2+ mixture, a value of Ew (%) of 91.97% is obtained. This remarkable increase in Ew (%), which is >1 order of magnitude above the values obtained in the presence of the two additives, is an illustration of the synergistic effect. The same phenomenon is borne out by various other sets of numbers in Table 1 and those obtained from electrochemical methods. 3.2. Potentiodynamic polarization The inhibition effect of CMCT and CMCT + Cu2+ mixture on mild steel in 1 M HCl at 298 K was investigated by the potentiodynamic polarization. The obtained polarization curves

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S. Cheng et al. / Electrochimica Acta 52 (2007) 5932–5938

Fig. 2. Potentiodynamic polarization curves for mild steel with and without addition of inhibitors in 1 M HCl solution.

are shown in Fig. 2a–c. Meaningful parameters such as corrosion potential (Ecorr ), cathodic Tafel slop (bc ), and corrosion current density (Icorr ) are listed in Table 2. The percentage inhibition efficiency, EI (%) is calculated from the values of the corrosion current density and Ep (%) is calculated from

the linear polarization resistance according to Eqs. (2) and (3): EI (%) =

Icorr − Icorr(inh) × 100 Icorr

(2)

Table 2 Electrochemical parameters with and without addition of inhibitors in 1 M HCl bc (mV dec−1 )

Icorr (␮A cm−2 )

Ecorr (V)

Blank

−0.470

−136

327.0

–

47.7

–

−0.461 −0.462 −0.464 −0.464 −0.483

−142 −143 −152 −153 −156

103.0 90.2 63.9 49.1 31.5

68.5 72.4 80.4 85.0 90.4

174.5 191.3 197.1 204.2 327.0

72.6 75.0 75.8 76.6 85.4

−0.436 −0.445 −0.450

−160 −142 −142

1370.0 300.8 291.0

−20.1 8.0 11.0

– 51.3 52.9

7.1 9.8

CMCT (mg L−1 ) + Cu2+ (M) 20 + 10−3 −0.462 20 + 10−4 −0.466 20 + 10−5 −0.452

−128 −132 −149

51.0 20.2 29.2

84.4 93.8 91.0

372.9 510.7 476.9

87.2 90.7 89.9

CMCT 10 20 30 40 50

EI (%)

Rp ( cm2 )

Concentration

Ep (%)

(mg L−1 )

Cu2+ (M) 10−3 10−4 10−5

S. Cheng et al. / Electrochimica Acta 52 (2007) 5932–5938

where Icorr and Icorr(inh) are the values of corrosion current density of uninhibited and inhibited specimens, respectively, determined by extrapolation of the cathodic Tafel lines to the corrosion potential (Ecorr ). EP (%) =

RP(inh) − RP × 100 RP(inh)

(3)

where RP and RP(inh) are the values of linear polarization resistance of uninhibited and inhibited specimens (Fig. 3a–c), respectively. As shown in Fig. 2b, it is obvious that polarization curves in the presence of inhibitor have changed notably compared to that of the blank solution and the corrosion current density, Icorr , decreases with the increase of CMCT concentration. The presence of CMCT does not change the current versus potential characteristics at potentials more negative than −300 mV (V versus SCE) from Ecorr . This is probably caused by the dissolution of mild steel and desorption of the inhibitors leading to the increase in surface area. This phenomenon is also reported in document [9]. However, CMCT influences anodic reaction at potentials lower than −300 mV (V versus SCE). This result shows clearly that the inhibition of the steel corrosion is under cathodic and anodic control. Therefore, CMCT can be classified

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as a mixed-type inhibitor [10]. Based on these results, it seems that CMCT forms a protective film to obstruct the available metallic surface. The potentiodynamic polarization curves in the presence of CMCT + Cu2+ are shown in Fig. 2a. The inhibition efficiency reaches the maximum (90.7%) when the solution contains 20 mgL−1 CMCT + 10−4 M Cu2+ , which value is much higher than the simple sum of 20 mg L−1 CMCT (75.0%) and 10−4 M Cu2+ (7.1%). The values of Ecorr in the presence of CMCT + Cu2+ are close to those without Cu2+ , but compared with those without CMCT, they are more negative. The shape and characteristics of anodic curves of CMCT + Cu2+ mixture and CMCT are the same but quite different from those of Cu2+ (Fig. 2c). The above results convince the synergistic effect again. 3.3. Electrochemical impedance spectroscopy (EIS) The corrosion behavior of mild steel in 1 M HCl at various concentrations of CMCT, Cu2+ and CMCT + Cu2+ mixture was investigated by EIS technique at 298 K. Nyquist plots are shown in Fig. 4a–c and electrochemical parameters are summarized in Table 3. Percentage inhibition efficiency ER (%) is calculated

Fig. 3. The i–E plots for resistance polarization determination.

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Fig. 4. Nyquist plots for mild steel with and without addition of inhibitors in 1 M HCl.

through the charge transfer resistance values: ER (%) =

Rt(inh) − Rt × 100 Rt(inh)

(4)

Table 3 Electrochemical parameters from EIS with and without addition of inhibitors in 1 M HCl Concentration

Ecorr (V vs. SCE)

Blank

−0.460

CMCT 10 20 30 40 50

Rt ( cm2 )

Cdl (␮F cm−2 )

ER (%)

36.5

212.9

–

−0.461 −0.462 −0.459 −0.461 −0.464

186.0 199.1 208.1 212.4 260.9

55.1 53.3 45.1 43.8 40.4

80.4 81.7 82.5 82.8 86.0

−0.433 −0.445 −0.461

25.5 40.1 42.6

265.0 150.7 144.1

– 9.0 14.3

111.5 606.9 478.9

48.1 13.2 23.4

73.4 94.0 92.4

(mg L−1 )

Cu2+ (M) 10−3 10−4 10−5

CMCT (mg L−1 ) + Cu2+ (M) 20 + 10−3 −0.465 −0.462 20 + 10−4 −0.469 20 + 10−5

where Rt and Rt(inh) are the values of charge-transfer resistance in the absence and presence of the inhibitor, respectively. From the plots in Fig. 4, it is obvious that the impedance response of mild steel in 1 M HCl has been significantly changed after the addition of CMCT, Cu2+ and CMCT + Cu2+ mixture. In order to analyze the impedance spectra containing one capacitive loop, the equivalent circuit given in Fig. 5 is used [11]. The capacitive loops are not perfect semicircles, because the Nyquist plots obtained in the real system represent a general behavior where the double layer at the interface of metal/solution does not behave as an ideal capacitor [12]. As shown in Table 3, it can be found that the values of Cdl decrease with the concentration of the inhibitor CMCT. In the

Fig. 5. Equivalent circuit of the studied system.

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Table 4 Corrosion parameters for mild steel in 1 M HCl without and with 20 mg L−1 CMCT + 10−4 M Cu2+ at different temperatures Temperature (K)

298 303 313 323 333 343

W (␮g cm−2 h−1 ) 1 M HCl

20 mg L−1 CMCT + 10−4 M Cu2+

10.22 13.21 28.49 31.20 83.54 122.72

5.89 6.08 7.07 7.79 6.91 10.94

Ew (%)

42.33 53.94 75.19 75.01 91.73 91.08

presence of CMCT + Cu2+ mixture the values of Cdl are much lower than those in the presence of Cu2+ . Theoretically, the decrease of Cdl results from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules act by adsorption at the metal/solution interface [13,14]. 3.4. Effect of temperature Gravimetric measurements were carried out various temperatures (298–343 K) in the absence and presence of 20 mg L−1 CMCT + 10−4 M Cu2+ mixture after 1 h of immersion of the specimens. The values of Ew (%) and corrosion rate with and without 20 mg L−1 CMCT + 10−4 M Cu2+ mixture at different temperatures are given in Table 4. It is distinct that with the increasing experiment temperature the corrosion rate of the uninhibited specimen increases dramatically, and the inhibitor mixture protects mild steel effectively even at high temperature. Currently, the inhibition efficiency decreases with the rise of temperature [15]. But it is interesting that the trend is reverse in this study. Inhibition efficiency increases with the rise of temperature. This result could be explained by the characteristics of the cathodic process of hydrogen evolution in acidic solutions. The hydrogen evolution overvoltage decreases with the rising temperature that leads to the increase in the cathodic reaction rate. On the other hand, the increase of temperature accelerates the chemisorption of the inhibitor on the metallic surface. When the latter effect is predominant, the final result is an increase of the inhibition efficiency. Similar results were also found in other documents [16,17]. 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:   Ea Wcorr = k exp − (5) RT   Ea   (6) Wcorr = k exp − RT where Ea and Ea are the values of the activation energy for the corrosion reaction in the absence and presence of the inhibitor mixture, respectively. Ea = 47.71 kJ mol−1 which is

Fig. 6. Arrhenius plot (ln W vs. 1/T) obtained for mild steel without and with 20 mg L−1 + 10−4 M Cu2+ mixed inhibitor in 1 M HCl.

close to the value 50.7 kJ mol−1 from the literature [18]. Ea = 9.85 kJ mol−1 compared with Ea the reduction of the activation energy in the presence of the inhibitor mixture may be attributed to the chemisorption on mild steel surface [19,20]. Singh et al. [21] consider that, with the increase in temperature, some chemical changes occur in the inhibitor molecules, leading to an increment in the electron density at the adsorption centers of the molecule, causing an improvement in inhibition efficiency. 3.5. Conductometric titration In order to learn more about the mechanism of the inhibition, conductometric titration was carried out. If Cu2+ and CMCT formed complex in the solution, associated with the results from weight loss and electrochemical measurements, the mechanism of the inhibition would be explained by the adsorption of the complex. The conductometric titration curve obtained is shown in Fig. 7. It can be found that a intersection point in the conductometric curve, which reflects a molar ratio of 2 Cu2+ :3 CMCT rings (0.667) to form a complex compound between the two additives. The ratio equals to 109.5,

Fig. 7. The conductance titration curve obtained for CMCT with Cu2+ cation as a titrant.

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4. Conclusions 1. The corrosion of mild steel in 1 M HCl is significantly reduced upon the addition of CMCT and the mixture of Cu2+ + CMCT. The results obtained suggest a complex formation between Cu2+ ion and CMCT, which has much more inhibition effect than that of each additive separately. 2. Close agreement has been found between the results from weight loss measurements and electrochemical methods with reference to the ability of the additives to impede corrosion of mild steel in 1 M HCl. 3. Generally the inhibition efficiency of CMCT + Cu2+ mixture increases with the increase of the temperature. It is seen that the inhibitor mixture shows inhibiting properties at all the studied temperatures. Fig. 8. The structure of CMCT ring: blue ball for N atom, red ball for O atom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 5 Mulliken charge data of the protoned compound 1C 2C 3C 5O 6C

0.098879 0.073582 −0.068674 −0.479706 0.350860

7C 11 C 12 O 13 C 18 O

0.075599 −0.637582 −0.462028 −0.085462 −0.667273

19 O 20 C 26 N 29 O 30 O

−0.620877 0.560746 −0.736668 0.550188 −0.454938

10.95, 1.095, 0.1095 when the concentration of Cu2+ is 10−2 , 10−3 , 10−4 and 10−5 M, respectively. Consequently, the ratio of 20 mg L−1 CMCT + 10−4 M Cu2+ mixture is the closest ratio, which conforms the fact that the mixture exhibits the best inhibiting performance (Fig. 8). The possible chemical reactivity was also analyzed by the Mulliken charges. From Table 5, N (26) and O (18) offer greater electron density, it can be indicated that these two sites play important roles in the reaction with the anodic sites and vacant d-orbital of the metal surface. CMCT could be adsorbed through lone pairs of electrons of nitrogen and oxygen, forming donor–acceptor bond between unpaired electrons of the nitrogen/oxygen and the positive active centers of the metal surface. The inhibitor can exist as cationic species, which may be adsorbed on the cathodic sites of the mild steel on specifically absorbed chloride ions. In the inhibited solution with CMCT + Cu2+ mixture, Cu2+ tends to react with the two sites to form a complex working as the bridge to connect more CMCT rings, forming more compact protective film and leading to much higher inhibition efficiency.

Acknowledgements This work was sponsored by the Doctoral Foundation of Shandong Province (2006BS04021), National Natural Science Foundation (50672090) and Technological Generalship Project of Qingdao (05-2-JC-76). References [1] J.M. Wasikiewicz, N. Nagasawa, M. Tamada, Nucl. Instrum. Methods Phys. Res., Sect. B 236 (2005) 617. [2] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2000) 127. [3] S.A. Abd El-Maksoud, Appl. Surf. Sci. 206 (2003) 129. [4] S. Cheng, S.G. Chen, T. Liu, X.T. Chang, Y.S. Yin, Mater. Lett. (2006). doi:10.1016/j.matlet.2006.11.102. [5] M.H. Wahdan, G.K. Gomma, Mater. Chem. Phys. 47 (1997) 176. [6] M. Abdallah, M.M. El-Naggar, Mater. Chem. Phys. 71 (2001) 291. [7] P. Zhao, Q. Liang, Y. Li, Appl. Surf. Sci. 252 (2005) 1596. [8] X.G. Chen, H.J. Park, Carbohydr. Polym. 53 (2003) 355. [9] L. Elkadia, B. Mernaria, M. Traisnelb, Corros. Sci. 42 (2000) 712. [10] L. Larabi, Y. Harek, O. Benali, Prog. Org. Coat. 54 (2005) 256. [11] M.El. Azhar, B. Mernari, M. Traisnel, F. Bentiss, M. Lagrenee, Corros. Sci. 43 (2001) 2229. [12] M. Ozcan, J. Dehri, M. Erbil, Appl. Surf. Sci. 236 (2004) 155. [13] H. Ashassi-Sorkhabi, B. Shaabani, D. Seifzadeh, Appl. Surf. Sci. 239 (2005) 154. [14] E. McCafferty, N. Hackerman, J. Electrochem. Soc. 119 (1972) 146. [15] A. Chetouani, A. Aouniti, B. Hammouti, Corros. Sci. 45 (2003) 1664. [16] A.E. Stoyanova, E.I. Sokolova, S.N. Raicheva, Corros. Sci. 39 (1997) 1595. [17] M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci. 44 (2002) 573. [18] F. Bentiss, M. Traisnel, N. Chaibi, Corros. Sci. 44 (2002) 2271. [19] T. Szauer, A. Brandr, Electrochim. Acta 26 (1981) 1209. [20] S. Sankarapapavinasam, F. Pushpanaden, M.F. Ahmed, Corros. Sci. 32 (1991) 193. [21] D.D.N. Singh, R.S. Chaudhary, B. Prakash, C.V. Agarwal, Br. Corros. J. 14 (1979) 235.