Carboxymenthylchitosan as an ecofriendly inhibitor for mild steel in 1 M HCl

Materials Letters 61 (2007) 3276 – 3280 www.elsevier.com/locate/matlet

Carboxymenthylchitosan as an ecofriendly inhibitor 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 25 April 2006; accepted 12 November 2006 Available online 8 December 2006

Abstract The inhibiting influence of Carboxymenthylchitosan (CM-chitosan) on the corrosion of mild steel in 1 M HCl solution was studied by weight loss measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. Polarization measurements show that the CM-chitosan acts essentially as a mixed-type inhibitor. The protection efficiency of this inhibitor increases with the inhibitor concentration to reach 93% at 200 mg/L but decreases slightly with the rise of temperature. The adsorption of used compound on the steel surface obeys modified Langmuir's isotherm. The efficient inhibition is also characterized by a series of greater activation energies of corrosion reaction in the presence of CM-chitosan at various concentrations. © 2006 Elsevier B.V. All rights reserved. Keywords: Carboxymenthylchitosan (CM-chitosan); Corrosion inhibitor; Potentiodynamic polarization; Electrochemical impedance spectroscopy (EIS)

1. Introduction Acid solutions are commonly used in the chemical industry to remove mill scales from the metallic surface. The addition of inhibitors effectively secures the metal against an acid attack. And many studies using organic inhibitors have been reported [1–7]. The inhibitor adsorption mode is strictly affected by the inhibitor structure. Most acid inhibitors are organic compounds containing oxygen, nitrogen and/or sulphur. These compounds are adsorbed on the metallic surface blocking the active corrosion sites. Although the most effective and efficient organic inhibitors are compounds that have π bonds, the biological toxicity of these products, especially organic phosphate, is documented especially about their environmental harmful characteristics [8,9]. From the standpoint of safety, the development of non-toxic and effective inhibitors is considered more important and desirable. Chitin is a natural polysaccharide found particularly in the shells of crustaceans such as crab and shrimp, the cuticles of insects and the cell walls of fungi. Carboxymenthylchitosan ⁎ Corresponding author. E-mail address: [email protected] (Y. Yin). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.102

(CM-chitosan) is the derivative of chitin. Their main advantages are occurrence in large amounts in nature, as well as their biodegradability. These organic compounds have wide applications in medicine, cosmetics, textile, paper food and many other industrial branches [10]. Based on the chemical structure shown in Fig. 1, rich in hydroxyl and carboxyl, so CM-chitosan is a good potential inhibitor but little has been reported about its inhibition behavior. The aim of this study is to investigate the inhibition of CMchitosan on the mild steel surface in 1 M HCl solution, using weight loss measurements and electrochemical measurements. Chemical preparation of water-soluble CM-chitosan was according to experimental procedure [11]. 2. Experimental section 2.1. Materials Prior to all measurements, the mild 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 bidistilled water, degreased ultrasonically in ethanol

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Table 1 Gravimetric results of steel in 1 M HCl with and without CM-chitosan at 298 K and 24 h

Fig. 1. Molecular structure of CM-chitosan.

before use and dried at room temperature. The aggressive solutions used were made of AR grade 37% HCl. Appropriate concentrations of acid were prepared using bidistilled water. The concentration range of inhibitor employed was 50 to 250 mg/L in acid. 2.2. Weight loss measurements The weight loss of steel specimens (2 cm × 1 cm × 0.5 cm) in 1 M HCl with and without addition of different concentrations of CM-chitosan was determined in air without bubbling. 2.3. Electrochemical measurements Electrochemical experiments were performed in a conventional three-electrode cell consisting of a steel working electrode (WE) with exposure surface of 1.0 cm2, a 1.5 cm × 1.5 cm platinum counter electrode (CE) and a saturated calomel reference electrode (SCE) was used for measurements. The electrochemical impedance spectroscopy (EIS) measurements were carried out at Ecorr with the ZAHNER IM6 Electrochemical Workstation (Germany). After the determination of steadystate current at a given potential, sine wave voltages (10 mV), peak to peak, were at frequencies between 100 kHz and 10 mHz. In the case of potentiodynamic polarization curves, the potential sweep rate was 0.5 mV/s. 3. Results and discussion

Concentration (mg/L)

W (μg/cm2 h)

Ew (%)

0 50 100 150 200 250

8.125 1.212 1.158 0.825 0.550 0.804

– 85 86 90 93 90

Values of the electrochemical corrosion parameters, such as corrosion potential (Ecorr), cathodic Tafel slop (bc), anodic Tafel slop (ba), corrosion current density (Icorr) are presented in Table 2. The values of inhibition efficiency (EI%) are calculated from the following equation: EI % ¼

Icorr −IcorrðinhÞ  100 Icorr

ð2Þ

where Icorr and Icorr(inh) are the values of corrosion current density of uninhibited and inhibited specimens, respectively, determined by extrapolation of the cathodic and anodic Tafel lines to the corrosion potential Ecorr. The results of Fig. 2 and Table 2 show that the corrosion current density Icorr decreases considerably with the increase of CMchitosan concentration. The change of cathodic and anodic Tafel slopes alters unremarkably in the presence of the inhibitor. It can be seen that values of Ecorr shift to the negative direction compared to the uninhibited specimen but change within 30 mV for all these concentrations, these results indicate that the tested CM-chitosan is of the mixed-type but acts as a cathodic inhibitor [12] for mild steel in 1 M HCl. The corrosion behavior of mild steel in 1 M HCl in the absence and presence of various concentrations of CM-chitosan was also investigated by EIS technique at 298 K. Nyquist plots are shown in Fig. 3. As we have noticed, the impedance diagrams consist of one large capacitive loop and they are not perfect semicircles that are generally attributed to the frequency dispersion [13]. The impedance response of mild steel is significantly changed after the addition of CM-chitosan.

3.1. Effect of concentration Based on weight loss measurements, the corrosion rate (Wcorr) and the values of inhibition efficiency (Ew%) for various concentrations of CM-chitosan after 24 h of immersion at 298 K are given in Table 1. The relation equation determines the inhibition efficiency (Ew%): Ew % ¼ 100 

W0 −Wcorr W0

ð1Þ

where Wcorr and W0 are the corrosion rates of steel with and without the inhibitor, respectively. Gravimetric measurements show that Wcorr decreases in the presence of CM-chitosan and Ew increases with inhibitor concentration to reach 93% at 200 mg/L. However, when the concentration is 250 mg/L, the efficiency declines, designating 200 mg/L as the critical concentration. The inhibition may be explained by the adsorption of CM-chitosan on the steel surface and the surface coverage θ defined by Ew% / 100 reaches the maximum at 200 mg/L. Fig. 2 shows the influence of CM-chitosan concentration on the cathodic and anodic polarization curves of steel in 1 M HCl.

Fig. 2. Polarization curves of mild steel in 1 M HCl for various concentrations of CM-chitosan.

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Table 2 Electrochemical parameters of mild steel in 1 M HCl for various concentrations of CM-chitosan

Table 3 Data from electrochemical impedance measurements of mild steel in 1 M HCl for various concentrations of CM-chitosan

Concentration (mg/L)

Ecorr (mV)

Icorr (μA/cm2)

bc (mV/dec)

ba (mV/dec)

EI (%)

Concentration (mg/L)

Rt (Ω cm2)

Cdl (μF/cm2)

θ

ER (%)

0 50 100 150 200 250

−456 −450 −460 −468 −470 −460

291.0 45.1 40.3 37.6 21.0 31.4

99 101 102 104 134 112

63.9 65.8 70.3 71.7 84.3 76.7

– 84.5 85.6 87.1 92.8 89.2

0 50 100 150 200 250

39.8 279.0 321.0 340.0 475.5 363.5

388.4 53.3 46.7 45.1 27.2 38.4

– 0.862 0.879 0.884 0.930 0.901

– 86.2 87.9 88.4 93.0 90.1

Table 3 collects various parameters such as charge-transfer resistance (Rt), double layer capacitance (Cdl) and percentage inhibition efficiency (ER%) calculated from the following equation: ER % ¼

RtðinhÞ −Rt  100 RtðinhÞ

ð3Þ

where Rt and Rt(inh) are charge-transfer resistance values in the absence and presence of the inhibitor, respectively. The values of Cdl are obtained at the frequency fmax, at which the imaginary component of the impedance is maximal −Zim(max), using the following equation: Cdl ¼

1 2pfmax Rt

ð4Þ

The inhibition efficiencies, calculated from impedance results, show the same trend as those obtained from polarization and weight loss measurements. In fact, the presence of CM-chitosan is accompanied by the increase of the value of Rt in an acidic solution indicating a charge-transfer process mainly controlling the corrosion of steel. The decrease of Cdl is due to the adsorption of the inhibitor on the metal surface leading to the formation of a film.

The degree of surface coverage θ for different concentrations of the inhibitor in acidic media has been evaluated from weight loss using the equation: h¼

Table 4 Effect of temperature on the steel corrosion for various concentrations of CMchitosan Temperature (K)

Concentration (mg/L)

W (μg/cm2 h)

Ew (%)

θ

303

1 M HCl 50 100 150 200 250 1 M HCl 50 100 150 200 250 1 M HCl 50 100 150 200 250 1 M HCl 50 100 150 200 250 1 M HCl 50 100 150 200 250

8.43 1.71 1.50 1.40 1.27 1.48 9.71 2.62 2.45 2.15 1.65 1.72 13.70 3.07 2.98 2.93 2.65 2.76 35.72 11.30 10.51 9.09 7.02 7.52 67.90 22.61 19.73 18.30 16.64 21.40

– 79.67 82.19 83.36 84.90 82.40 – 73.03 74.74 77.90 83.03 82.27 – 77.65 78.32 78.68 80.76 79.90 – 68.38 70.74 74.53 80.33 78.94 – 66.74 71.00 73.03 78.27 74.74

– 0.7967 0.8219 0.8336 0.8490 0.8240 – 0.7303 0.7474 0.7790 0.8303 0.8227 – 0.7765 0.7832 0.7868 0.8076 0.7990 – 0.6838 0.7074 0.7453 0.8033 0.7894 – 0.6674 0.7100 0.7303 0.7827 0.7474

313

323

333

343

Fig. 3. Nyquist plots for mild steel in 1 M HCl for various concentrations of CMchitosan.

ð5Þ

where Wm is the smallest corrosion rate. From Table 4, the increase of W0 is more pronounced with the rise of temperature for the blank solution. In the presence of CM-chitosan θ decreases slightly with increasing experimental temperature, which could be caused by, desorption of the inhibitor from the steel surface. The slight decrease of θ suggests that the efficiency of CM-chitosan is independent of temperature. The result shows that CM-chitosan effectively protects the steel even at high temperature.

3.2. Effect of temperature Generally speaking, the corrosion increases with the rise of temperature. Gravimetric measurements were taken at various temperatures (303–343 K) in the absence (W0) and presence of the inhibitor CM-chitosan (W′) after 1 h of immersion. The corresponding efficiency (Ew%) is summarized in Table 4.

W0 −W V W0 −Wm

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Table 5 Thermodynamic parameters for the adsorption of CM-chitosan in 1 M HCl on mild steel at different temperatures

Fig. 4. Arrhenius plots of corrosion LogW versus 1 / T at different concentrations of CM-chitosan.

Temperature (K)

n

K/n (1/M)

R2

ΔG 0ads (kJ/mol)

ΔH 0ads (kJ/mol)

ΔS 0ads (J/mol K)

303 313 323 333 343

1.19 1.22 1.23 1.23 1.26

2.5 × 104 5.0 × 104 1.1 × 105 1.7 × 105 2.5 × 105

0.999 0.995 0.999 0.996 0.996

− 35.63 − 38.61 − 41.98 − 44.40 − 46.89

50.03

− 283.40

sion reaction increases with the concentration of CM-chitosan. This phenomenon is often interpreted by physical adsorption leading to the formation of an adsorptive film of electrostatic character [14]. 3.3. Adsorption isotherm

Fig. 4 shows that the logarithm of the corrosion rate Wcorr can be represented as a straight-line function of 1000 / T, where T is the temperature in Kelvin. The activation energies could be determined from Arrhenius plots for the steel corrosion rate presented in Fig. 4 by the following equations:   Ea Wcorr ¼ kexp − ð6Þ RT

The adsorption of the inhibitor is influenced by the nature and the charge of the metal, the chemical structure of the inhibitor, distribution of the charge in the molecule, and the type of electrolyte [15–19]. Important information about the interaction between the inhibitor and steel surface can be provided by the adsorption isotherm. In the above work, it could be concluded that θ increases with the inhibitor concentration, this is attributed to more adsorption of inhibitor molecules onto the steel surface. As it is known, the adsorption of inhibitor is always a displacement reaction involving removal of absorbed water molecules from the metal surface:

  EV Wcorr ¼ k Vexp − a RT

OrgðsolÞ þ nH2 OðadsÞ⇔OrgðadsÞ þ nH2 OðsolÞ

ð7Þ

where Ea and Ea′ are the activation energies for the corrosion in the absence and presence of CM-chitosan, respectively. The apparent activation energies at different concentrations of the inhibitor are calculated by a linear regression between LogW and 1000 / T. Ea = 47.64 kJ mol− 1, when the concentration is 50, 100, 150, 200, 250 mg/L, the corresponding Ea′ is 61.36, 64.21, 64.35, 64.61, 64.38 kJ/mol. It can be found that the energy barrier of corro-

Fig. 5. Langmuir adsorption plots for mild steel in 1 M HCl at different temperatures.

where Org(sol) and Org(ads) are the organic molecules in the aqueous solution and adsorbed on the steel surface, respectively. H2O(ads) is the water molecule on the steel surface; n is the size ratio representing the number of water molecules replaced by one unit of CM-chitosan. Now, assuming that the adsorption of CM-chitosan belonged to the

Fig. 6. Optimized structure of CM-chitosan and the modes of the adsorption onto the mild steel surface.

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monolayer adsorption, then the Langmuir adsorption isotherm is applied to investigate the mechanism by the following equation: C 1 ¼ þC h K

ð8Þ

where C is calculated by the molecular weight of the monomer. K is the adsorption coefficient. The free energy of adsorption (ΔG0ads) values of CM-chitosan have been obtained by using the following equation: K¼

  1 DG0 exp − 55:5 RT

ð9Þ

Three representative Langmuir adsorption plots at different temperatures are shown in Fig. 5. Linear plots are obtained with slopes equal to 1.19, 1.23 and 1.26 for the experimental temperature at 303, 323 and 343 K, respectively. These results indicate that each CMchitosan unit occupies more than one adsorption site on the steel surface. A modified Langmuir adsorption isotherm [20,21] could be applied to this phenomenon, which is given by the corrected equation: C n ¼ þ nC h K

ð10Þ

The heat of adsorption ΔH0ads and the standard adsorption entropy ΔS0ads are obtained according to the thermodynamic basic equation: ΔG0ads = ΔH0ads − TΔS0ads from the plot of ΔG0ads versus T (omitted), more detailed results including the correlation coefficient (R2) are mentioned in Table 5. The negative values of ΔG0ads at different temperatures are all around 40 kJ/mol, this phenomenon indicates that the inhibitor is spontaneously and strongly adsorbed onto the mild steel surface. Survey of literature reveals that negative values of ΔG0ads around 20 kJ/ mol or lower are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption); those around 40 kJ/mol or higher involve charge sharing or transfer from organic molecules to the metal surface to form a coordinate type of bond (chemisorption) [22,23]. In the author's view, the adsorption of the inhibitor is not considered only as a physical or a chemical adsorption phenomenon in this case. At lower temperatures and when the specimen is immersed into the solution, electrostatic interactions (Fig. 6a) between the positively charged nitrogen atom and the negative charged metal surface contribute to the adsorption at first [24]. Then according to the hard and soft acid-base theory [25,26] inhibitors are chemisorbed at the surface of steel by sharing an electron pair of its nitrogen and oxygen atoms with the d-orbital of iron forming covalent bonding, leading to the positive value of ΔH0ads. With the temperature rising, the increase in the negative values of ΔG0ads suggests that the inhibitor adsorbed more strongly onto the steel surface at higher temperatures, however, more tempestuously the inhibitors vibrate, in this experiment the latter is dominant, thus the integrated result is that the inhibition efficiency declines slightly with increasing temperature. Chemisorption can be favored by the CMchitosan planarity (Fig. 6b). It has been well documented that chelates enhance and strengthen the chemisorption of known inhibitors through the formation of stable five and six-membered chelate rings [27]. As to CM-chitosan, it preferably forms five-membered chelate rings with iron (Fig. 6c). Each CM-chitosan unit has more than one active group, that is to say, each unit can form more than one covalent bonding with iron, resulting in that one CM-chitosan unit can replace more than one water molecule.

4. Conclusions • The CM-chitosan studied is a good ecofriendly inhibitor for mild steel in 1 M HCl. The protection efficiency of this inhibitor increases when the inhibitor concentration reaches 93% at 200 mg/L but decreases slightly with the rise of temperature. • Polarization measurements show that CM-chitosan acts as a mixed-type inhibitor. • CM-chitosan adsorbs on the steel surface according to the modified Langmuir adsorption isotherm. • The weight loss, electrochemical impedance spectroscopy and polarization curves were in good agreement. Acknowledgement The authors would like to thank the financial support from the National Natural Science Foundation of China (No: 50672090). References [1] M. El Achouri, M.R. Infante, F. Izquierdo, S. Kertit, H.M. Gouttoya, B. Nciri, Corros. Sci. 43 (2001) 19–35. [2] N.E. Hamner, in: C.C. Nathan (Ed.), Corrosion Inhibitors, Nace Houston, Texas, USA, 1973, p. 1. [3] S.S. Abd El Rehim, M.A.M. Ibrahim, K.F. Khalid, J. Appl. Electrochem. 29 (1999) 593. [4] J.M. Sykes, Br. Corros. J. 25 (1990) 175. [5] S. Rengamani, S. Muralidharan, M. Anbu Kulamdainathan, S. Venkatakrishna Iyer, J. Appl. Electrochem. 24 (1994) 355. [6] M. Ajmal, A.S. Mideen, M.A. Quraishi, Corros. Sci. 36 (1994) 79. [7] A. El-Sayed, J. Appl. Electrochem. 27 (1997) 193. [8] J. Sinko, Prog. Org. Coat. 42 (2001) 267. [9] S.E. Manahan, Environmental Chemistry, sixth ed.Lewis, Boca Raton, 1996. [10] Jaroslaw M. Wasikiewicz, Naotsugu Nagasawa, Masao Tamada, Hiroshi Mitomo, Fumio Yoshii, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 236 (2005) 617. [11] Xi-Guang Chen, Hyun-Jin Park, Carbohydr. Polym. 53 (2003) 355–359. [12] O.L. Riggs Jr., in: C.C. Nathan (Ed.), Corrosion Inhibitors, Houston, TX, USA, 1973, p. 151. [13] F. Bentiss, M. Lagrenée, M. Traisnel, J.C. Hornez, Corros. Sci. 41 (1999) 789. [14] A. Popova, E. Sokolova, S. Raicheva, M. Chritov, Corros. Sci. 45 (2003) 33. [15] A.P. Yadav, A. Nishikata, T. Tsurn, Corros. Sci. 56 (1957) 104. [16] I.L. Rozenfeld, Corrosion Inhibitors, McGraw-Hill Inc., New York, 1981, p. 182. [17] T. Mimani, S.M. Mayanna, Munichandraiah, J. Appl. Electrochem. 23 (1993) 339. [18] J. Ueara, K. Aramaki, J. Electrochem. Soc. 138 (1991) 3245. [19] I. Granese, Corrosion 44 (1988) 322. [20] R.F.V. Villamil, P. Corio, J.C. Rubin, S.M.L. Agostinho, J. Electroanal. Chem. 535 (2002) 75. [21] R.F.V. Villamil, P. Corio, J.C. Rubin, S.M.L. Agostinho, J. Electroanal. Chem. 472 (1999) 112. [22] F.M. Donahue, K. Nobe, J. Electrochem. Soc. 112 (1965) 886. [23] E. Kamis, F. Bellucci, R.M. Latanision, E.S.H. El-r, Corrosion 47 (1991) 677. [24] G. Moretti, F. Guidi, G. Grion, Corros. Sci. 46 (2004) 394. [25] K. Aramaki, Y. Node, H. Nishihara, J. Electrochem. Soc. 137 (1990) 1354. [26] F.H. Walters, J. Chem. Educ. 68 (1991) 29. [27] D.C. Zecher, Mater. Perform. 15 (4) (1976) 33–37.