Using of Surfactant Nanostructures as Green Compounds in Corrosion Inhibition

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1 International Journal of Environmental Science and Development, Vol. 5, No. 1, February 214 Using of Surfactant Nanostructures as Green Compounds in Corrosion Inhibition A. Yousefi and S. Javadian II. MATERIAL AND METHODS Abstract The corrosion inhibition of some common surfactants and biosurfactant on mild steel has been investigated in hydrochloric acid at different concentrations of the inhibitor using mass loss, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The interest in these surfactants stems from their potential as green compounds because of chemical and thermal stability, nonflammability, high ionic conductivity, and a wide electrochemical potential window, which makes them as good inhibitors. A significant decrease in the corrosion rate was observed in presence of the investigated inhibitor. The polarization curves showed that, the inhibitor behaves as mixed type but the cathodic effect is more pronounced. The surface tension at 298K was measured; the critical micelle concentration (CMC) and some surface active parameters were calculated. The inhibition efficiency (η %) of this surfactant has been studied by both chemical and electrochemical techniques at 25 C. It was shown that an excess of cationic and anionic surfactant resulted in the formation of nanostructures. The observed corrosion data indicate that, the inhibition of mild steel corrosion is due to the adsorption of these molecules on the surface. The adsorption mechanism onto steel surface in acid medium in the absence and presence of surfactant was also discussed. Materials: cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and hydrochloric acid (HCl) was obtained from Merck Company and rhamnolipid was synthesized. The mild steel sheets were abraded with a series of emery paper (grade ) and then washed with bidistilled water and acetone. Methods: Electrochemical experiments were carried out in a conventional three-electrode cell with a platinum counter electrode (CE) and an Ag/AgCl reference electrode. The potential of potentiodynamic polarization curves was done from a potential of -25 mv vs. OCP, to 25 mv vs. OCP at a sweep rate of.1 mv s-1. Scanning electron microscopy (SEM), the specimens shown in SEM micrographs were exposed to the hydrochloric acid for a desired period of immersion under optimum conditions in the absence and presence of inhibitors. III. RESULTS AND DISCUSSION The CMC values of the binary CTAB/SDS mixtures in aqueous mixtures of EG have been determined from surface tension and conductivity plots (Fig. 1). For pure CTAB and SDS and 98:2 CTAB/SDS systems, the surface tension and conductance gave the same value of CMC at 298 K in water, but for the 92:8 CTAB/SDS system, the CMC value obtained by conductance was much higher. This may be due to the second state of aggregation, which arises of changing in micelle shape. Index Terms Corrosion, surfactants, green inhibitors, interaction. I. INTRODUCTION Acid solutions are widely used for removal of undesirable scale and rust in many industrial processes. Inhibitors are generally used in these processes to control metal dissolution as well as the consumption of acid [1], [2]. Today, surfactants perform this role well. The choice of the inhibitor is based on two considerations. Firstly, it can be synthesized conveniently from relatively cheap raw materials. Secondly, the presence of nitrogen, oxygen, sulfur and multiple bonds in the inhibitor molecule is needed. This leads to increase the adsorption of the compound on the steel surface and to promote effective inhibition. Surfactants exert the inhibition action by adsorption on the metal surfaces such that chemical structure of the ionic head (hydrophilic part) attacks the metal surface while its tail (hydrophobic part) extends to the solution face [3]-[5]. In the present work, the inhibition of the mild steel corrosion by cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and their mixtures in acidic solution was investigated. Fig. 1. Effect of the total concentration on the specific conductivity and surface tension (CTAB/SDS 92:8). TEM images confirmed the results of surface tension and conductance studies (Fig. 2). The inhibition effect of common surfactants and biosurfactant on the corrosion mild steel in 2. M HCl solution was studied by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. According to the Fig. 3, the results show that Manuscript received August 7, 213; revised September 6, 213. A. Yousefi and S. Javadian are with Department of chemistry, Faculty of sciences, Tarbiat Modares University, Tehran, Iran ( a.yousefi84@yahoo.com). DOI: /IJESD.214.V

2 International Journal of Environmental Science and Development, Vol. 5, No. 1, February 214 the surfactants are very good inhibitor at little concentrations, and the inhibition efficiencies obtained from all methods employed are in good agreement with each other. Generally, the inhibition efficiency increased with increase of the inhibitor concentration. The obtained results show that studied inhibitors are the best inhibitor at 1 1-4, and M additive concentration, respectively. type of inhibition. According to the polarization measurements (Fig. 5), the efficiency of mixed surfactants at low concentration is higher than alone surfactants in both cationic-rich and anionic-rich while at higher concentrations, the efficiency of mixed systems is higher than CTAB and SDS in cationic-rich only. The corrosion potential did not obviously change before and after addition of CTAB, SDS, and their mixtures; suggesting that the compounds act as a mixed-type inhibitor and the inhibition action is caused by the geometric blocking effect. The surface parameters of surfactant at HCl media was calculated from its surface tension including the critical micelle concentration (CMC). Decrease in surface tension values is observed as the concentration increases. The CMC was determined from the intersection points in the γ log C curves. An effective measure of the adsorption of surfactant at the air liquid interface is usually obtained by the surface excess concentration, Γmax, which can be determined by the Gibbs equation for dilute solution [6]: Fig. 2. TEM photography of CTAB/SDS mixture at 92:8 (a), after 12h and (b), after 1 month. 1 -Zimg (Ώ cm2) HCl CTAB.5mM CTAB.5mM CTAB.1mM Zreal(Ώ cm2) mg BIO 35 -Z img (Ώ cm2) , 1 Г where NA is Avogadro s number. The results show that the values of Γmax for CTAB/SDS mixtures change significantly and its decrease is due to the attraction interaction between the surfactants. The free energy of micellization (ΔGmic) was calculated. The tendency of surfactants to form micelles can be established on the basis of the standard Gibbs energy of aggregation (ΔGmic). In this approach, we determined the values of the standard Gibbs energy of micellization (aggregation) of CTAB and SDS and their mixtures from equation Here R and T are respectively the gas constant and temperature, γ is the surface tension, C is the concentration of surfactant, and n is the number of species formed in solution considering the dissociation per monomer. The minimum area per head group (Amin) for surfactant molecules at the CMC at the saturated interface was obtained by the following Г 15 Zre(Ώ cm2) Fig. 3. Nyquist plots for mild steel in 2 M HCl solution without and with different concentrations of cationic surfactant and rhamnolipid at 298 K. (2 Changes in impedance parameters (charge transfer resistance, Rct, and double-layer capacitance, Cdl) were indicative of adsorption of CTAB, SDS and their mixtures on the metal surface, leading to the formation of a protective film. The decrease in capacitance and increase of residence can be attributed to the decrease in local dielectric constant and/or the increase in the thickness of the electrical double layer; signifying that the molecule acts by adsorption at the interface of metal/solution. Adsorption of mixtures of cationic-anionic exhibited synergy at solid/liquid interface. We showed that CTAB/SDS mixtures formed nano spherical and cylindrical aggregates in low concentrations; because of attractive electrostatic interactions between their two oppositely charged polar groups. Hence, in 1:9 SDS/CTAB more hemisphere and hemi spherical cylinder micelles can be also absorbed on the metal surface and enhance the inhibition efficiency (Fig. 4). The potentiodynamic polarization measurements indicated the where α is degree of counterion dissociation. The obtained values are order of -2 kj mol-1. 5 C(9)+S(1).1mM S(9)+C(1).15mM S(9)+C(1).1mM C(9)+S(1).15mM 45 -Zimg /ohm cm Zreal /ohm cm2 Fig. 4. Nyquist plots for mild steel in 2 M HCl solution without and with different concentrations of mixed surfactants at 298 K. 1

3 International Journal of Environmental Science and Development, Vol. 5, No. 1, February charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate covalent bond (chemical adsorption). The values of ΔGads are between -2 and -4 kj mol-1 suggests that physical adsorption (electrostatic) occurs in the first stage. On the other hand, the adsorption phenomenon of the surfactants are not considered only as a physical or as chemical adsorption phenomenon. The adsorption mechanism of surfactants on the steel surface from HCl solution is more physical adsorption than chemical one. HCl CTAB.5mM CTAB.1mM E /V vs Ag\AgCl CTAB.5mM CTAB.1mM C/Θ (mm) C(9)/S(1) C(1)/S(9).8 log i/a cm-2 Fig. 5. Anodic and cathodic polarization curves obtained at 298 K in 2 M HCl in different concentrations of CTAB It has been assumed that surfactants and their mixtures establish their inhibition action via adsorption of the inhibitor onto the metal surface. The adsorption processes of the inhibitor are influenced by the chemical structure of surfactant, nature and surface charge of the metal, the distribution of the charge in the molecule and type of aggressive media. The adsorption of surfactants on metal surface can markedly change the corrosion resisting properties of the metals. So, the investigation of relation between the adsorption and corrosion inhibition is important. The efficiency of surfactants as a successful corrosion inhibitor mainly depends on their adsorption ability at the metal/solution interface which takes place through the replacement of water molecules by inhibitors. So, it is essential to know the mode of adsorption and the adsorption isotherm. In order to get a better understanding for the adsorption mechanism, the Langmuir adsorption isotherm equation was employed as the following equation: C (mm) Fig. 6. Langmiur adsorption plots for mild steel in 2. M HCl in different concentrations of mixed surfactants. The temperature range from 298 to 338K in the presence of CTAB, SDS and mixed CTAB (9)/SDS (1) solutions was studied. The effect of temperature on the corrosion parameter can be deduced by comparing the activation energy in the presence and absence of the inhibitor. The Arrhenius plot was used to determine the activation energy (Ea), activation enthalpy (ΔHa), and activation entropy (ΔSa) for the corrosion of mild steel in 2. M HCl. The activation energy can be obtained by the Arrhenius equation and Arrhenius plot [7]: + where icorr is corrosion current, A is the constant, Ea is the activation energy of the metal dissolution reaction, R is the gas constant and T is the temperature. The Ea value can be determined from the slopes of the plot of ln (icorr) against 1/T (Fig. 7). Moreover, the Arrhenius equation can be converted an alternative equation as follows: where θ is the surface coverage, which was determined from the EIS data, Cinh is the molar concentration of the inhibitor. Kads is the standard adsorption equilibrium constant, related to the standard free energy of adsorption (ΔGads) by the following equation: ) where R is the universal gas constant and T is the absolute temperature. The plot of Cinh/θ against Cinh yields a straight line (Fig. 6), and this supports the assumption that the adsorption of surfactants on the mild steel surface in 2. M HCl solution obeys the Langmuir adsorption isotherm. The negative value of ΔGads means that the adsorption of the surfactants on steel surface is a spontaneous process and also shows a strong interaction of the inhibitor molecule onto the carbon steel surface. It is accepted that the values of ΔGads in the order of -2 kj mol-1 or less negative are associated with an electrostatic interaction between the charged inhibitor molecules and the charged metal surface (physical adsorption); those of -4 kj mol-1 or more negative involves where N is Avogadro s constant, h is the Plank s constant, ΔSa is the entropy of activation and ΔHa is the enthalpy of activation. A plot of ln (icorr/t) against 1/T should give a straight line with a slope of (ΔHa /R) and intercept of [ln (R/Nh) + (ΔSa/R)]. The increasing activation energy in the presence of inhibitor indicates that physical adsorption (electrostatic) occurs in the first stage. On the other hand, the adsorption phenomenons of surfactants are 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 11

4 International Journal of Environmental Science and Development, Vol. 5, No. 1, February 214 adsorption experimental data. The Ea value was greater than 2 kj mol-1 in both the presence and absence of the inhibitor, which reveals that the entire process is controlled by the surface reaction. The values of ΔHa and Ea are nearly the same and are higher in the presence of the inhibitor. This indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution. The positive values of ΔHa for both corrosion processes with and without the inhibitor reveal the endothermic nature of the steel dissolution process and indicate that the dissolution of steel is difficult. The entropy of activation ΔSa in the absence and presence of inhibitor is negative and tends to become positive at higher concentration. The increase of ΔSa is generally interpreted by increase in disorder taking place on going from reactants to the activated complex. Fig. 8. SEM images for the mild steel surface in 2 M HCl (a) without surfactant, (b) with.5mm of CTAB, (c) with.5mm of SDS and (d) with.5mm of 9:1 CTAB/SDS. -3 ln (icorr ) (A cm-2 ) -4 IV. CONCLUSION -5 CTAB -6 A binary composition of SDS/CTAB with an excess of cationic surfactant produced two CMC values. From the obtained corrosion data, it is clear that CTAB, SDS and their mixtures act as good inhibitors for the corrosion of mild steel in 2. M HCl solution. The surfactants exert the inhibition action by adsorption on the metal surfaces such that chemical structure of the ionic head (hydrophilic part) attacks the metal surface while its tail (hydrophobic part) extends to the solution face. Mixture REFERENCES 1/T (K-1) Fig. 7. Arrhenius plots for mild steel in 2. M HCl with surfactants. Weight loss tests were carried out by weighing the mild steel specimens before and after immersion in 5 ml acid solutions without and with surfactants for 27 min at 25 C. The corrosion rate (W) and the percentage protection efficiency IEw (%) were calculated according to the following equations IE (% W W 1 W where Δm(g) is the mass loss, S (cm2) is the area, t (h) is the immersion period, and W(g cm-2h-1) and W (g cm-2h-1) are the corrosion rates of mild steel without and with the inhibitor, respectively. The inhibition efficiency values in the absence and presence of different concentrations of CTAB, SDS and their mixtures which were calculated from weight loss measurements are compared with electrochemical methods. However, the exact values of inhibition efficiency obtained by the two measurements remain different. The surface morphology of steel sample was investigated by scanning electron microscopy (SEM). It can be observed (Fig. 8) that the mild steel surface was damaged in the absence of inhibitor, while the mild steel surface exposed to different kinds of surfactant was smooth. 12 [1] M. A. Hegazy, A novel Schiff base-based cationic gemini surfactants: Synthesis and effect on corrosion inhibition of carbon steel in hydrochloric acid solution, Corr. Sci., vol. 51, pp , Jan 29. [2] M. A. Migahed, E. M. S. Azzam, and A. M. Al-Sabagh, Corrosion inhibition of mild steel in 1 M sulfuric acid solution using anionic surfactant, Materials Chem. Phys., vol. 85 pp , July 24. [3] X. Li, S. Deng, and H. Fu, Inhibition by tetradecylpyridinium bromide of the corrosion of aluminium in hydrochloric acid solution, Corr. Sci., vol. 53 pp , May 211. [4] L. G. Qiu, A. J. Xie, and Y. H. Shen, The adsorption and corrosion inhibition of some cationic gemini surfactants on carbon steel surface in hydrochloric acid, Corros. Sci., vol. 47, pp , Agu 25. [5] R. Fuchs-Godec, Effects of surfactants and their mixtures on inhibition of the corrosion process of ferritic stainless steel, Electrochim. Acta, vol. 54, pp , Jan 29. [6] M. J. Rosen and X. Y. HUA, Surface Concentrations and Molecular Interactions in Binary Mixtures of Surfactants, J. Colloid Interface Sci., vol. 86 pp Agu [7] A. K. Maayta and N. A. F. Al-Rawashdeh, Inhibition of acidic corrosion of pure aluminum by some organic compounds, Corros. Sci., vol. 46 pp Jan. 24. Soheila Javadian is an associate professor in physical chemistry from Tarbiat Modares University, Tehran. She was born in Tabriz, Iran (1974). She studied applied chemistry (BS) at Tabriz University ( ), physical chemistry (MS and PhD) at Tarbiat Modares University ( and 2-26). Her research group members are interested in investigation of different surfactants systems which have a tremendous impact on modern research. In her group, they study the physicochemical properties of mixed surfactants. They also investigate the effete of additives such as salt, different organic compounds and miscellaneous solvents on surfactant systems. In her research, some students deal with in computational chemistry as the theoretical investigation of structural and properties of water confined inside nanotube structures. In summary, her research is

5 International Journal of Environmental Science and Development, Vol. 5, No. 1, February 214 divided into: studying interaction and synergistic effects of mixed surfactants/ studying interaction between surfactants and ionic liquids, dyes, polymers and macromolecules/ studying colloidal systems as corrosion inhibitors/ studying colloidal systems in dispersion of nanotubes/ electrochemical synthesis of nanoparticles for use in lithium - ion batteries/ computational chemistry. Her research group have published many articles in different journals. She is also one of the members of Electrochemical Society of Iran. Dr. Soheila Javadian Farzaneh is with Department of Chemistry, Tarbiat Modares University papers and also submitted some others to be published in the near future. His research experiences are: studying interaction and synergistic effects of mixed surfactants/ Studying interaction between surfactants and ionic liquids, dyes, polymers and macromolecules/ studying colloidal systems as corrosion inhibitors/ studying of colloidal systems contribution in dispersion of nanotubes Electrochemical synthesis of nanoparticles for use in lithium - ion batteries. Ali Yousefi is a Ph.D. student in physical chemistry from Tarbiat Modares University, Tehran. I was born in Rasht, Iran (1984). I studied pure chemistry (B.Sc.) at Guilan University (23-27) and physical chemistry (M.S) at Tarbiat Modares University (27-29). He has worked in multidisciplinary fields of research in nanoscience and physical chemistry. He has worked on Modifying the nanostructures of surfactants using additives and studying their efficiency as corrosion inhibitor. In addition, in parallel with his thesis, He worked on mixed surfactant with different molecules such as ionic liquid and biosurfactants. As for his research activities and publications in this period, He has published some 13