International Journal of Corrosion. Corrosion Inhibitors

Transcription

1 International Journal of Corrosion Corrosion Inhibitors

2 Corrosion Inhibitors

3 International Journal of Corrosion Corrosion Inhibitors

4 Copyright 211 Hindawi Publishing Corporation. All rights reserved. This is a focus issue published in International Journal of Corrosion. All articles are open access articles distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

5 Editorial Board Raman Singh, Australia Carmen Andrade, Spain Ksenija Babic, USA José Maria Bastidas, Spain Pier Luigi Bonora, Italy Marek Danielewski, Poland Flavio Deflorian, Italy Omar S. Es-Said, USA Sebastian Feliu, Spain Wei Gao, New Zealand Karl Ulrich Kainer, Germany W. Ke, China H. K. Kwon, Japan Dongyang Y. Li, Canada Chang-Jian Lin, China Efstathios I. Meletis, USA Vesna Mišković-Stanković, Serbia Rokuro Nishimura, Japan Michael I. Ojovan, UK F. J. M. Pérez, Spain Ramana M. Pidaparti, USA Willem J. Quadakkers, Germany Aravamudhan Raman, USA Michael J. Schütze, Germany Yanjing Su, China Jerzy A. Szpunar, Canada Yu Zuo, China

6 Contents Study of Temperature Effect on the Corrosion Inhibition of C38 Carbon Steel Using Amino-tris(Methylenephosphonic) Acid in Hydrochloric Acid Solution, NajouaLabjar,FouadBentiss, Mounim Lebrini, Charafeddine Jama, and Souad El hajjaji Volume 211, Article ID , 8 pages A Comparative Study of the Inhibitory Effect of the Extracts of Ocimumsanctum, Aegle marmelos, and Solanum trilobatum on the Corrosion of Mild Steel in Hydrochloric Acid Medium, M. Shyamala and P. K. Kasthuri Volume 211, Article ID , 11 pages Corrosion Inhibition of the Galvanic Couple Copper-Carbon Steel in Reverse Osmosis Water, Irene Carrillo, Benjamín Valdez, Roumen Zlatev, Margarita Stoycheva, Michael Schorr, and Mónica Carrillo Volume 211, Article ID , 7 pages Inhibition Effect of 1-Butyl-4-Methylpyridinium Tetrafluoroborate on the Corrosion of Copper in Phosphate Solutions, M. Scendo and J. Uznanska Volume 211, Article ID , 12 pages The Effect of Ionic Liquids on the Corrosion Inhibition of Copper in Acidic Chloride Solutions, M. Scendo and J. Uznanska Volume 211, Article ID , 13 pages

7 Hindawi Publishing Corporation International Journal of Corrosion Volume 211, Article ID , 8 pages doi:1.1155/211/ Research Article Study of Temperature Effect on the Corrosion Inhibition of C38 Carbon Steel Using Amino-tris(Methylenephosphonic) Acid in Hydrochloric Acid Solution Najoua Labjar, 1, 2 Fouad Bentiss, 3 Mounim Lebrini, 2 Charafeddine Jama, 2 and Souad El hajjaji 1 1 Laboratoire de Spectroscopie Infrarouge, Faculté des Sciences, University Med V Agdal, avenue Ibn Battouta, BP 114, Rabat 1, Morocco 2 Unité Matériaux et Transformations (UMET), Ingénierie des Systèmes Polymères CNRS UMR 827, ENSCL, BP 918, Villeneuve d Ascq Cedex, France 3 Laboratoire de Chimie de Coordination et d Analytique, Faculté des Sciences, Université Chouaib Doukkali, BP 2, El Jadida 24, Morocco Correspondence should be addressed to Souad El hajjaji, selhajjaji@hotmail.com Received 13 March 211; Revised 17 July 211; Accepted 12 August 211 Academic Editor: Carmen Andrade Copyright 211 Najoua Labjar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tafel polarization method was used to assess the corrosion inhibitive and adsorption behaviours of aminotris(methylenephosphonic) acid (ATMP) for C38 carbon steel in 1 M HCl solution in the temperature range from 3 to 6 C. It was shown that the corrosion inhibition efficiency was found to increase with increase in ATMP concentration but decreased with temperature, which is suggestive of physical adsorption mechanism. The adsorption of the ATMP onto the C38 steel surface was found to follow Langmuir adsorption isotherm model. The corrosion inhibition mechanism was further corroborated by the values of kinetic and thermodynamic parameters obtained from the experimental data. 1. Introduction Corrosion inhibition of steel in acid solutions by different types of inhibitors has been extensively studied. The use of environmentally acceptable inhibitors is favoured. Phosphonates are known to be environmentally friendly corrosion inhibitors, which form adsorbed layers on oxide- or hydroxide-covered metal surfaces [1 4]. Many works can be found in the literature about the interactions between phosphonates and iron or steels. In particular, Ochoa and al. [2, 4] studied the interaction between phosphonocarboxylic acid salts (monophosphonates) and carbon steel. Their environmental impact at usual concentrations for corrosion inhibition is negligible [5, 6]. Moreover, in contrast to inorganic phosphorous compounds, they do not cause eutrophication. Their high stability to hydrolysis and resistance to degradation is also beneficial. It was found that few inhibitors with acid-metal systems have specific reactions that are still effective at high temperatures as (or more) they are at low temperatures [7, 8]. A large number of investigations have studied the temperature effects on acidic corrosion and corrosion inhibition of iron and steel in HCl and H 2 SO 4 solutions [9 17]. In previous work [1], the improving of the corrosion resistance of C38 carbon steel in 1 M HCl solution using ATMP has been investigated at 3 C by means of gravimetric and electrochemical (ac impedance and Tafel polarisation) methods. We have found that this compound is efficient inhibitor in 1 M HCl and the corrosion inhibition is mainly controlled by a physisorption process. The antibacterial activity investigations have been shown that the ATMP has an antibacterial effect against both Gram-positive and Gramnegative bacteria [1]. A great limitation of the inhibitor application is the fall down of their efficiencies at high temperatures. The effect of temperature on the inhibited acidmetal reaction is highly complex because many charges

8 2 International Journal of Corrosion occur on the metal surface such as rapid etching and desorption of the inhibitor, and the inhibitor itself, in some cases, may undergo decomposition and/or rearrangement [18]. However, it provides the ability of calculating many thermodynamic functions for the inhibition and/or the adsorption processes which contribute in determining the type of adsorption of the studied inhibitor. The aim of this work is then to study the effect of temperature on C38 carbon steel corrosion process in 1 M HCl both in the absence and in the presence of amino-tris(methylenephosphonic) acid- (ATMP) using Tafel polarisation method. The thermodynamic parameters for both activation and adsorption processes were calculated and discussed. 2. Experimental Details The material used in this study is a C38 carbon steel with a chemical composition (in wt %) of.37% C,.23% Si,.68% Mn,.16% S,.77% Cr,.11% Ti,.59% Ni,.9% Co,.16% Cu, and the remainder iron (Fe). The C38 carbon samples were pretreated prior to the experiments by grinding with emery paper SiC (12, 6, and 12), rinsed with distilled water, degreased in acetone in an ultrasonic bath immersion for 5 min, washed again with bidistilled water, and then dried at room temperature before use. The tested compound, namely amino-tris(methylenephosphonic) acid (N[CH 2 P(O)(OH) 2 ] 3 ), (ATMP), obtained from Sigma-Aldrich (5 wt.% in H 2 O), was tested without further purification. The molecular structure of the ATMP is shown in Figure 1. The acid solutions (1 M HCl) were prepared by dilution of an analytical reagent grade 37% HCl with doubly distilled water. Polarisation curves were conducted using an electrochemical measurement system Tacussel-Radiometer model PGZ 31 potentiostat controlled by a PC and supported by Voltamaster 4. software. Electrochemical measurements were carried out in a conventional three-electrode cylindrical Pyrex glass cell. The temperature is thermostatically controlled. The working electrode (WE) in the form of disccutfromsteelhasageometricareaof1cm 2 and is embedded in polytetrafluoroethylene (PTFE). A saturated calomel electrode (SCE) and a platinum electrode were used, as reference and auxiliary electrodes, respectively. A fine Luggin capillary was placed close to the working electrode to mini-mize IR drop. All test solutions were deaerated in the cell by using pure nitrogen for 1 min prior to the experiment. During each experiment, the test solution was mixed with a magnetic stirrer, and the gas bubbling was maintained. The mild steel electrode was maintained at corrosion potential for 3 min and thereafter prepolarised at 8 mv SCE for 1 min. The potentiodynamic current potential curves were obtained by changing the electrode potential automatically from 8 to 2 mv SCE with a scan rate of.5 mv s Results and Discussion 3.1. Corrosion Kinetic Study. In order to gain more information about the type of adsorption and the effectiveness HO HO O HO OH P P N O Figure 1: Molecular structure of the amino-tris(methylenephosphonic) acid (ATMP). of the ATMP inhibitor at higher temperature, polarisation experiment was conducted in the range of 3 6 C without and with selected concentrations of the inhibitor. Representative Tafel polarisation curves for C38 steel electrode in 1 M HCl without and with.1 M of ATMP at different temperatures are shown in Figure 2. Similar polarisation curves were obtained in the case of the other concentrations of ATMP (not given). The analysis of these figures reveals that raising the temperature increases both anodic and cathodic current densities, and consequently the corrosion rate of C38 steel increases. Electrochemical kinetic parameters (corrosion potential (E corr ), corrosion current density (I corr ), and cathodic Tafel slope (b c )), determined from these experiments by extrapolation method [19 23], are reported in Table 1.TheI corr was determined by Tafel extrapolation of only the cathodic polarization curve alone, which usually produces a longer and better defined Tafel region[24]. The inhibition efficiencies, E(%), are calculated from I corr values as described elsewhere [18]. The surface coverage θ was calculated from the following equation [25]: θ = I corr I corr(inh), (1) I corr I sat where I corr, I corr(inh),andi sat are the corrosion current density values in the absence, the presence of ATMP, and in an entirely covered surface, respectively, (I sat = I corr for the most elevated concentration of inhibitor). As I sat I corr,thus P OH O OH θ = I corr I corr(inh) I corr. (2) Analyse of the results in Table 1 indicates that in the presence of ATMP molecules, the I corr of C38 steel decreases at any given temperature as inhibitor concentration increases compared to the uninhibited solution, due to the increase of the surface coverage degree. In contrast, at constant ATMP concentration, the I corr increases as temperature rises, but this increase is more pronounced for the blank solu-tion. Hence we can note that the E(%) depends on the temperature and decreases with the rise of temperature from 3 to

9 International Journal of Corrosion 3 Table 1: Electrochemical parameters and the corresponding inhibition efficiencies at various temperatures studied of C38 steel in 1 M HCl containing different concentrations of ATMP. Temperature (C ) Conc.(M) E corr versus SCE mv I corr (μa cm 2 ) b c (mv dec 1 ) E (%) θ 1MHCl MHCl MHCl MHCl C. This can be explained by the decrease of the strength of the adsorption process at elevated temperature and would suggest a physical adsorption mode. The activation parameters for the corrosion reaction can be regarded as an Arrhenius-type process, according to the following equation: ( I corr = A exp E ) a, (3) RT where E a is the apparent activation corrosion energy, R is the universal gas constant, and A is the Arrhenius preexponential factor. The apparent activation energies (E a ) in the absence and in the presence of various concentrations of ATMP are calculated by linear regression between ln (I corr )and1/t (Figure 3), and the results are given in Table 2. All the linear regression coefficients are close to 1, indicating that the steel corrosion in hydrochloric acid can be elucidated using the kinetic model. As observed from Table 2, the E a increased with increasing concentration of ATMP, but all values of E a in the range of the studied concentration were higher than that of the uninhibited solution. The increase in E a in the presence of ATMP may be interpreted as physical adsorption. Indeed, a higher energy barrier for the corrosion process in the inhibited solution is associated with physical adsorption or weak chemical bonding between the inhibitor species and the steel surface [14, 26]. Szauer and Brand. explained that the increase in activation energy can be attributed to an appreciable decrease in the adsorption of the inhibitor on the carbon steel surface with the increase in temperature. A corresponding increase in the corrosion rate occurs because of Table 2: Corrosion kinetic parameters for C38 steel in 1 M HCl in absence and presence of different concentrations of ATMP. Concentration (M) E a (kj mol 1 ) ΔH a (kj mol 1 ) ΔS a (J mol 1 k 1 ) E a ΔH a (kj mol 1 ) Blank the greater area of metal that is consequently exposed to the acid environment [27]. The enthalpy of activation (ΔH a )andtheentropyofactivation (ΔS a ) for the intermediate complex in the transition state for the corrosion of C38 steel in 1 M HCl in the absence and in the presence of different concentrations of ATMP were obtained by applying the alternative formulation of Arrhenius equation [28]: I corr = RT ( ) ( Nh exp ΔSa exp R ΔH a RT ), (4) where h is the Plank s constant and N is the Avogadro s number. Figure 4 shows a plot of ln(i corr /T) versus1/t. A straight lines are obtained with a slope of ( ΔH a /R) and an intercept of (ln R/Nh + ΔS a /R) from which the values of ΔH a and ΔS a were calculated (Table 2). The positive values of ΔH a in the absence and the presence of ATMP reflect the endothermic nature of the C38 steel dissolution process. One can also notice that E a and ΔH a values vary in the same way

10 4 International Journal of Corrosion Log I (A cm 2 ) Log I (A cm 2 ) d c Blank E versus SCE (mv) (a) a: 3 C b: 4 C a MofATMP a E versus SCE (mv) (b) b d c c: 5 C d: 6 C Figure 2: Effect of temperature on the cathodic and anodic responsesforc38steelin1mhcland1mhcl +.1MofATMP. as shown in Table 2, indicating that the corrosion process is a unimolecular reaction [29]. This result permits verifying the known thermodynamic equation between the E a and ΔH a [29] b E a ΔH a = RT. (5) Thevaluesofactivationentropy(ΔS a ) are higher for inhibited solutions than that for the uninhibited solution and increase gradually with increasing ATMP concentrations (Table 2). The positive increment of ΔS a suggests that an increase in randomness occurred on going from reactants to the activated complex [3]. This observation is in agreement with the findings of other workers [3, 31] Adsorption Isotherm and Thermodynamic Parameters. The values of surface coverage θ corresponding to different concentrations of AMTP in the temperature range from 3 to 6 C have been used to explain the best isotherm to determine the adsorption process. As it is known that the adsorption of an organic adsorbate onto metal-solution interface can be presented as a substitutional adsorption process between the organic molecules in the aqueous solution Org (sol) and the water molecules on the metallic surface H 2 O (ads), Org (sol) + n H 2 O (ads) Org (ads) + n H 2 O (sol), (6) where Org (sol) and Org (ads) are the organic molecules in the aqueous solution and adsorbed on the metallic surface, respectively, H 2 O (ads) is the water molecules on the metallic surface, and n is the size ratio representing the number of water molecules replaced by one molecule of organic adsorbate. When the equilibrium of the process described in this equation is reached, it is possible to obtain different expressions of the adsorption isotherm plots, and thus the surfacecoveragedegree(θ) can be plotted as a function of the concentration of the inhibitor under test [32]. The Langmuir adsorption isotherm was found to give the best description of the adsorption behaviour of ATMP. In this case, the surface coverage (θ) of the inhibitor on the steel surface is related to the concentration of inhibitor in the solution according to the following equation: θ 1 θ = K adsc inh. (7) Rearranging this equation gives C inh θ = 1 K ads + C inh, (8) where θ isthesurfacecoveragedegree,c inh is the inhibitor concentration in the electrolyte, and K ads is the equilibrium constant of the adsorption process. The K ads values may be taken as a measure of the strength of the adsorption forces between the inhibitor molecules and the metal surface [33]. To calculate the adsorption parameters, the straight lines were drawn using the least squares method. The experimental (points) and calculated isotherms (lines) are plotted in Figure 5. The results are presented in Table 3. A very good fit is observed with a regression coefficient (R 2 )up to.99 and the obtained lines have slopes very close to unity, which suggests that the experimental data are well described by Langmuir isotherm and exhibit single-layer adsorption characteristic [18]. This kind of isotherm involves the assumption of no interaction between the adsorbed species and the electrode surface. From the intercepts of the straight lines C inh /θ-axis, the K ads values were calculated and given in Table 3.AscanbeseenfromTable 3, K ads values decrease with increasing temperature from 3 to 6 C. Such behaviour can be interpreted on the basis that the increase in temperature results in desorption of some adsorbed inhibitor molecules from the metal surface [18]. The well-known thermodynamic adsorption parameters are the free energy of adsorption (ΔG o ads ), the standard enthalpy of adsorption (ΔHads o ), and the entropy of adsorption (ΔS o ads ). These quantities can be calculated depending on the estimated values of K ads from adsorption isotherms, at different temperatures. The constant of adsorption, K ads, is related to the standard free energy of adsorption, ΔG o ads, with the following equation [34]: K ads = 1 ( ΔG o ) 55.5 exp ads, (9) RT

11 International Journal of Corrosion 5 ln Icorr (μa cm 2 ) /T (K 1 ) a: Blank b: M c: M d: M e: M Figure 3: Arrhenius plots for C38 steel corrosion rates ln I corr versus 1/T in 1 M HCl in absence and in presence of different concentrations of ATMP. ln Icorr/T (μacm 2 K 1 ) a b.5 1 c d e a: Blank b: M c: M 1/T (K 1 ) d: M e: M Figure 4: Transition-state plots for C38 steel corrosion rates ln I corr versus 1/T in 1 M HCl in absence and in presence of different concentrations of ATMP. Cinh /θ (M) a b c d e 6 C 5 C 4 C 3 C C inh (M) Figure 5: Langmuir s isotherm adsorption model of ATMP on the C38 steel surface in 1 M HCl at different temperatures. where R is the universal gas constant, T is the thermodynamic temperature, and the value of 55.5 is the concentration of water in the solution in mol/l. The calculated ΔG o ads values, at all studied temperatures, are given in Table 3. ThenegativevaluesofΔG o ads indicate the spontaneity of the adsorption process and the stability of the adsorbed layer on the C38 steel surface [16]. Generally, the adsorption type is regarded as physisorption if the absolute value of ΔG o ads is in the range of 2 kj mol 1 or lower. The inhibition behaviour is attributed to the electrostatic interaction between the organic molecules and steel surface. When the absolute value of ΔG o ads is in the order of 4 kj mol 1 or higher, the adsorption could be seen as chemisorption. In this process, the covalent bond is formed by the charge sharing or transferring from the inhibitor molecules to the metal surface [35, 36]. The obtained ΔG o ads values in the studied temperature domain are in the range of 23.5 to 26.5 kj mol 1, indicating, therefore, that the adsorption mechanism of the ATMP onto C38 steel in 1 M HCl solution is mainly due to physisorption (Table 3). This behaviour is in good agreement with that obtained at 3 C using ac impedance technique [1]. On the other hand, the obtained values of ΔG o ads show a regular dependence on temperature, indicating a good correlation among thermodynamic parameters. However, a limited decrease in the absolute value of ΔG o ads with the increase in temperature values is observed. This behaviour is explained by the fact that the adsorption is somewhat unfavourable with increasing experimental temperature, indicating that the physisorption has the major contribution while the chemisorption has a minor contribution in the corrosion inhibition mechanism [37]. The otherthermodynamic functions (ΔHads o and ΔSo ads ) can be calculated from the following equation: ΔG o ads = ΔHo ads TΔSo ads. (1) Figure 6 shows the plot of ΔG o ads versus T which gives straight lines with slopes of ΔS o ads and intercepts of ΔHads o.the obtained values of ΔHads o and ΔS o ads are given in Table 3. The obtained value of ΔHads o is negative, reflecting the exothermic nature of the adsorption process on C38 steel surface. The value of ΔHads o can also provide valuable information about the type of inhibitor adsorption. While an endothermic adsorption process (ΔHads o > ) is attributed unequivocally to chemisorption [38], an exothermic adsorption process (ΔHads o < ) may involve either physisorption or chemisorption or a mixture of both the processes. In an exothermic process, chemisorption is distinguished from physisorption by considering the absolute value of ΔHads o. For the chemisorption process, ΔHo ads approaches 1 kj mol 1, while for the physisorption process, it is less than 4 kj mol 1 [37]. In the case of ATMP, the calculated value of ΔHads o ( kj mol 1 ) is larger than the common physical adsorption enthalpy, but smaller than the commonchemical adsorption enthalpy, confirming that the adsorptionmechanismofatmponcarbonsteelsurfaceprobably involves two types of interactions, predominant physisorption (ionic), and weak chemisorption (molecular). The value of ΔS o ads is negative (Table 3), meaning that the inhibitor

12 6 International Journal of Corrosion ΔG ads (KJ moi 1 ) Table 3: Thermodynamic parameters for the adsorption of ATMP on the C38 steel in 1 M HCl at different temperatures. Temperature (K) R 2 K ads (M 1 ) ΔG o ads (kj mol 1 ) ΔH o ads (kj mol 1 ) ΔS o ads (J mol 1 K 1 ) T (K) Figure 6: Variation of ΔG o ads versus T on C38 steel in 1 M HCl containing ATMP. In Kads (M 1 ) /T (K 1 ) Figure 7: Vant t Hoff plot for the C38 steel/atmp/1 M HCl. molecules move freely in the bulk solution (are chaotic) before adsorption, while as adsorption progresses, the inhibitor molecules adsorbed onto the mild steel surface become more orderly, resulting in a decrease in entropy [39]. ΔHads o and ΔSo ads can be also deduced from the integrated version of the Van t Hoff equation expressed by [4] ln K ads = ΔHo ads RT + constant. (11) Figure 7 shows the plot of ln K ads versus 1/T which gives straight lines with slopes of ( ΔHads o /R) and intercepts of (ΔS o ads /R +ln1/55.5). The calculated ΔHo ads using the Van t Hoff equation is kj mol 1 for ATMP, confirming the physisorption process and the exothermic behaviour of the adsorption of the ATMP molecule on the steel surface. Values of ΔHads o obtained by both methods are in good agreement. Moreover, the deduced ΔS o ads value of J mol 1 K 1 for ATMP is very close to that obtained in Table Conclusion We studied the inhibitor action of ATMP on corrosion of C38 steel in 1 M HCl depending on effect of temperature. We obtained the following conclusion. (1) Based on the Tafel polarization results, the E (%) of ATMP is found to decrease with increasing temperature, and its addition to 1 M HCl leads to an increase of apparent activation energy (E a ) of the corrosion process. (2) The corrosion process is inhibited by the adsorption of ATMP on C38 steel surface. This adsorption fits a Langmuir isotherm model. Thermodynamic adsorption parameters show that ATMP is adsorbed on steel surface by an exothermic and spontaneous process. (3) The calculated values of ΔG o ads and ΔHo ads corroborate that the adsorption mechanism of ATMP on steel surface in 1 M HCl solution is mainly due to physisorption. (4) At temperatures higher than 3 C, this inhibitor is not efficient to control the corrosion of steel in 1 M HCl at the concentration range studied. References [1] N. Labjar, M. Lebrini, F. Bentiss, N. E. Chihib, S. E. Hajjaji, and C. Jama, Corrosion inhibition of carbon steel and antibacterial properties of aminotris-(methylenephosphonic) acid, Materials Chemistry and Physics, vol. 119, no. 1-2, pp , 21. [2] N. Ochoa, G. Baril, F. Moran, and N. Pébère, Study of the properties of a multi-component inhibitor used for water treatment in cooling circuits, Journal of Applied Electrochemistry, vol. 32, no. 5, pp , 22. [3] A. Pilbáth, I. Bertóti, I. Sajó, L. Nyikos, and E. Kálmán, Diphosphonate thin films on zinc: preparation, structure characterization and corrosion protection effects, Applied Surface Science, vol. 255, no. 5, pp , 28. [4] N. Ochoa, F. Moran, and N. Pébère, The synergistic effect between phosphonocarboxylic acid salts and fatty amines for the corrosion protection of a carbon steel, Journal of Applied Electrochemistry, vol. 34, no. 5, pp , 24. [5] H. S. Awad and S. Turgoose, Influence of hardness salts on the effectiveness of xinc-1 hydroxyethylidene 1,1 diphosphonic acid (HEDP) mixtures in inhibiting the corrosion of mild steel in neutral oxygen-containing solutions, Corrosion, vol. 6, no. 12, pp , 24. [6] J. Jaworska, H. van Genderen-Takken, A. Hanstveit, E. van de Plassche, and T. Feijtel, Environmental risk assessment of phosphonates, used in domestic laundry and cleaning agents in the Netherlands, Chemosphere, vol. 47, no. 6, pp , 22.

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15 Hindawi Publishing Corporation International Journal of Corrosion Volume 211, Article ID , 11 pages doi:1.1155/211/ Research Article A Comparative Study of the Inhibitory Effect of the Extracts of Ocimumsanctum, Aegle marmelos, andsolanum trilobatum on the Corrosion of Mild Steel in Hydrochloric Acid Medium M. Shyamala 1 and P. K. Kasthuri 2 1 Department of Chemistry, Government College of Technology, Tamil Nadu Coimbatore 64113, India 2 Department of Chemistry, L.R.G. Government Arts College for Women, Tamil Nadu Tirupur 63864, India Correspondence should be addressed to M. Shyamala, shyam @rediffmail.com Received 1 April 211; Accepted 24 June 211 AcademicEditor:F.J.M.Pérez Copyright 211 M. Shyamala and P. K. Kasthuri. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A comparative study of the inhibitory effect of plant extracts, Ocimum sanctum, Aegle marmelos, and Solanum trilobatum, on the corrosion of mild steel in 1N HCl medium was investigated using weightloss method, electrochemical methods, and hydrogen permeation method. Polarization method indicates plant extracts behave as mixed-type inhibitor. The impedance method reveals that charge-transfer process mainly controls the corrosion of mild steel. On comparison, maximum inhibition efficiency was found in Ocimum sanctum with 99.6% inhibition efficiency at 6.% v/v concentration of the extract. The plant extracts obey Langmuir adsorption isotherm. The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the high performance of inhibitive effect of the plant extracts. From hydrogen permeation method, all the plant extracts were able to reduce the permeation current. The reason for the reduced permeation currents in presence of the inhibitors may be attributed to the slow discharge step followed by fast electrolytic desorption step. Results obtained in all three methods were very much in good agreement in the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum. 1. Introduction Mild steel is a structural material widely used in automobiles, pipes and used in most of the chemical industries. Mild steel suffers from severe corrosion in aggressive medium of acids and pickling processes. Hydrochloric acid is widely used for pickling, descaling, and chemical cleaning processes of mild steel. 9% of pickling problems can be solved by introducing appropriate pickling inhibitor to the medium. Generally, organic compounds containing O, N, and S atoms are normally used as inhibitors to reduce the corrosion of mild steel in hydrochloric acid medium [1, 2]. Environmental concerns worldwide are increasing and are likely to influence the choice of corrosion inhibitors in the present and in future. Environmental requirements are still being developed, but some elements have been established. One of the methods to protect metals against corrosion is addition of species to the solution in contact with the surface in order to inhibit the corrosion rate. Unfortunately, many of the inhibitors used are inorganic salts or organic compounds with toxic properties or limited solubility. Increasing awareness of health and ecological risks has drawn attention to find more suitable inhibitors which are nontoxic. Accordingly, greater research efforts have been directed towards formulating environmentally acceptable inhibitors. Due to the diversity of their structures, many extracts of common plants have been used as corrosion inhibitors for materials in pickling and cleaning processes. Plant materials contain proteins, polysaccharides, polycarboxylic acids, tannin, alkaloids, and so forth. These compounds are potential acid corrosion inhibitors for many metals [3]. The cost of using green inhibitors is very low when compared to that of organic inhibitors which require a lot of chemicals and also time for its preparation. Chemical inhibitors are more expensive and cause more hazard effects. Nowadays due to strict environmental legislation, emphasis is being focused

16 2 International Journal of Corrosion on usage of natural products that are corrosion inhibitor. The recent and growing trend is using plant extracts as corrosion inhibitor. Recently, many plant extracts have been reported as effective corrosion inhibitors within India and outside India [4 2]. In this study, leaf extracts of three medicinal plants, namely, Ocimum sanctum (Tulasi),Aegle marmelos (Vilvam), and Solanum trilobatum (Thuthuvalai), have been selected to study the inhibition effect on the corrosion of mild steel in 1N hydrochloric acid medium using weight loss method, the potentiodynamic polarization method, electrochemical impedance method, and hydrogen permeation method. 2. Experimental Procedure 2.1. Preparation of Mild Steel Specimen. Mild steel strips were mechanically cut into strips of size 4.5 cm 2cm.2 cm containing the composition of.14% C,.35% Mn,.17% Si,.25% S,.3% P, and the remainder Fe and provided with a hole of uniform diameter to facilitate suspension of the strips in the test solution for weight loss method. For electrochemical studies, mild steel strips of the same composition but with an exposed area of 1 cm 2 were used. Mild steel strips were polished mechanically with emery papers of 1/ to 4/ grades, subsequently degreased with trichloroethylene or acetone and finally with deionised water, and stored in the desiccator. Accurate weight of the samples was taken using electronic balance Preparation of the Plant Extract. The leaves of the plants Ocimum sanctum, Aegle marmelos, andsolanum trilobatum were taken and cut into small pieces, and they were dried in an air oven at 8 Cfor2handgroundwellintopowder. From this, 1 g of the sample was refluxed in 1 ml distilled water for 1 h. The refluxed solution was then filtered carefully, the filtrate volume was made up to 1 ml using double distilled water which is the stock solution, and the concentration of the stock solution is expressed in terms of % (v/v). From the stock solution, 2 1% concentration of the extract was prepared using 1N hydrochloric acid. Similar kind of preparation has been reported in studies using aqueous plant extracts in the recent years [21 3] Weight Loss Method. The pretreated specimens initial weights were noted and were immersed in the experimental solution with the help of glass hooks at 3 Cforaperiod of 3h. The experimental solution used was 1N HCl in the absence and presence of various concentrations of the inhibitors. After three hours, the specimens were taken out, washed thoroughly with distilled water, and dried completely, and their final weights were noted. From the initial and final weights of the specimen, the loss in weight was calculated and tabulated. From the weight loss, the corrosion rate (mmpy), inhibition efficiency (%), and surface coverage (θ) of plant extracts were calculated using the formula Corrosion rate ( mmpy ) = KW AtD, (1) where K = (constant), W is weight loss in g, A is area in cmm 2, t is time in hours, and D is density in gm/cmm 3 (7.86): Inhibition efficiency (%) = (CR B CR I ) CR B 1, Surface coverage (θ) = CR B CR I CR B, (2) where CR B and CR I are corrosion rates in the absence and presence of the inhibitors Potentiodynamic Polarization Method. Potentiodynamic polarization measurements were carried out using electrochemical analyzer. The polarization measurements were made to evaluate the corrosion current, corrosion potential, and Tafel slopes. Experiments were carried out in a conventional three-electrode cell assembly with working electrode as mild steel specimen of 1 sq. cm area which was exposed and the rest being covered with red lacquer, a rectangular Pt foil as the counter electrode, and the reference electrode as SCE. Instead of salt bridge, a luggin capillary arrangement was used to keep SCE close to the working electrode to avoid the ohmic contribution. A time interval of 1 15 minutes was given for each experiment to attain the steady state open circuit potential. The polarization was carried from a cathodic potential of 8 mv (vs SCE) to an anodic potential of 2 mv (vs SCE) at a sweep rate of 1 mv per second. From the polarization curves, Tafel slopes, corrosion potential, and corrosion current were calculated. The inhibitor efficiency was calculated using the formula IE (%) = I Corr I Corr I Corr 1, (3) where I Corr and I Corr are corrosion current in the absence and presence of inhibitors Electrochemical Impedance Method. The electrochemical AC-impedance measurements were also performed using electrochemical analyzer. Experiments were carried out in a conventional three-electrode cell assembly as that used for potentiodynamic polarization studies. A sine wave with amplitude of 1 mv was superimposed on the steady open circuit potential. The real part (Z ) and the imaginary part (Z ) were measured at various frequencies in the range of 1 KHz to 1 MHz. A plot of Z versus Z was made. From the plot, the charge transfer resistance (R t ) was calculated, and the double layer capacitance was then calculated using C dl = 1 2πf max R t, (4) where R t is charge transfer resistance, and C dl is double layer capacitance. The experiments were carried out in the absence and presence of different concentrations of inhibitors. The percentage of inhibition efficiency was calculated using

17 International Journal of Corrosion 3 Table 1: Corrosion parameters obtained from weight loss measurements for mild steel in 1N HCl containing various concentrations of the plant extracts. Name of the plant extract Conc. of the extract (% in v/v) Corrosion rate (mmpy) Inhibition efficiency (%) Surface coverage (θ) Blank Ocimum sanctum Blank Aegle marmelos Blank Solanum trilobatum IE (%) = R t R t R 1, (5) t where R t and R t are the charge transfer resistance in the presence and absence of inhibitors Hydrogen Permeation Method. When metals are in contact with acids, atomic hydrogen is produced. Before they combine to produce hydrogen molecules, a fraction may diffuse into the metal. Inside the metal, the hydrogen atoms may combine to form molecular hydrogen. Thus, a very high internal pressure is built up. This leads to heavy damage of the metal. This is known as hydrogen embrittlement. This phenomenon of hydrogen entry into the metals can occur in industrial processes like pickling, plating, phosphating, and so forth. An inhibitor can be considered as completely effective only if it simultaneously inhibits metal dissolution and hydrogen penetration into the metal [31]. Hydrogen permeation study has been taken up with an idea of screening the inhibitors with regard to their effectiveness on the reduction of hydrogen uptake. Hence, the hydrogen permeation study was carried out using an adaptation of the modified Devanathan-Stachurski twocompartment cell assembly [32, 33] in 1N HCl medium in the absence and presence of optimum concentration of the extracts. Similar kind of study is reported in the works of Quraishi and Rawat [34] Surface Examination Studies. Surface examination of mild steel specimens in the absence and presence of the optimum concentration of the extracts immersed for 3 h at 3 C was studied using JEOL-Scanning electron microscope (SEM) with the magnification of 1x specimens. 3. Results and Discussion 3.1. Weight Loss Studies. Theweightlossstudiesweredone in 1N hydrochloric acid in the absence and presence of various concentrations of the plant extracts ranging from 2% to 1% v/v. Using the weight loss data, the corrosion rate, inhibition efficiency, surfacecoverage,andtheoptimum concentration of the extract have been calculated. The corrosion parameters obtained in the weight loss method are listed in Table 1. From Table 1, it was found that with the addition of the plant extract to 1N hydrochloric acid, the weight loss of mild steel decreased, the corrosion rate also decreased, while the inhibition efficiency increased. The optimum concentration for Ocimum sanctum was found to be 6% v/v with maximum inhibition efficiency of 99.6%, Aegle marmelos at 8% v/v with maximum inhibition efficiency of 97.5%, and Solanum trilobatum at 1% v/v with maximum inhibition efficiency of 9.2% for a period of 3 hours of immersion time. This result indicated that the plant extracts could act as effective corrosion inhibitors for mild steel in 1N HCl. The effect of immersion time studied for a period of 3 h to 24 h as given in Table 2 reveals that the plant extracts showed maximum efficiency at 3 h of immersion time which is sufficient for pickling process. The order of inhibition effect among the three plant extracts on mild steel in 1N HCl is found to be Ocimum sanctum > Aegle marmelos > Solanum trilobatum Potentiodynamic Polarization Studies. The potentiodynamic polarization parameters for different concentrations of the plant extracts are given in Table 3, and the polarization curves are given in Figure 1. Potentiodynamic polarization studies revealed that the corrosion current density (I corr )

18 4 International Journal of Corrosion Table 2: Effect of immersion time on percentage inhibition efficiency of mild steel in 1N HCl at 3 C in the presence of optimum concentration of the plant extracts. Inhibition efficiency (%) Name of the plant extract with optimum conc. Time (h) % v/v of Ocimum sanctum % v/v of Aegle marmelos % v/v of Solanum trilobatum Table 3: Potentiodynamic polarization parameters for mild steel in 1N HCl containing various concentrations of the plant extracts. Name of the plant extractconc. of extract (% in v/v) E corr (V) I corr (ma/cm 2 ) Tafel slope mv/decade b a b c Inhibition efficiency (%) Blank Ocimum sanctum Aegle marmelos Solanum trilobatum Table 4: Impedance parameters for the corrosion of mild steel in 1N HCl in the absence and presence of various concentrations of the plant extracts at 3 C. Name of the plant extract Conc. of extract (% in v/v) R t (Ω cm 2 ) C dl (μf/cm 2 ) Inhibition efficiency (%) Blank Ocimum sanctum Aegle marmelos Solanum trilobatum

19 International Journal of Corrosion E (Volts).5 E (Volts) I (amps/cm 2 ) I (amps/cm 2 ) (1) Blank (2) 2 (% v/v) (3) 4 (% v/v) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) (1) Blank (2) 2 (% v/v) (3)4(%v/v) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) (a) (b) E (Volts) I (amps/cm 2 ) (1) Blank (2) 2 (% v/v) (3) 4 (% v/v) (c) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) Figure 1: Potentiodynamic polarization curves for mild steel in 1N HCl solution in the absence and presence of various concentrations of the plant extracts (a) Ocimum sanctum,(b)aegle marmelos, and (c) Solanum trilobatum. markedly decreased with the addition of the extract and the corrosion potential shifts to less negative values upon addition of the plant extract. Moreover, the values of anodic and cathodic Tafel slopes (b a and b c ) are slightly changed indicating that this behavior reflects the plant extracts ability to inhibit the corrosion of mild steel in 1N HCl solution via the adsorption of its molecules on both anodic and cathodic sites, and, consequently, the extracts act through mixed mode of inhibition [15, 16]. It was observed that with increase in concentration of the plant extract from 2% to 1%, the maximum inhibition efficiency of 99.7% was observed for Ocimum sanctum extractat6%v/v,foraegle marmelos with 97.5% at 8% v/v, and for Solanum trilobatum with 9.8% at 1% v/v of the extract.

20 6 International Journal of Corrosion Z (ohms) 2 Z (ohms) Z (ohms) Z (ohms) (1) Blank (2) 2 (% v/v) (3) 4 (% v/v) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) (1) Blank (2) 2 (% v/v) (3) 4 (% v/v) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) (a) (b) 1 75 Z (ohms) Z (ohms) (1) Blank (2) 2 (% v/v) (3) 4 (% v/v) (c) (4) 6 (% v/v) (5) 8 (% v/v) (6) 1 (% v/v) Figure 2: Impedance diagrams for mild steel in 1N HCl solution in the absence and presence of various concentrations of the plant extract (a) Ocimum sanctum,(b)aegle marmelos, and (c) Solanum trilobatum Electrochemical Impedance Studies. Impedance measurements were studied to evaluate the charge transfer resistance (R t ) and double layer capacitance (C dl ), and through these parameters, the inhibition efficiency was calculated. Figure 2 shows the impedance diagrams for mild steel in 1N HCl with different concentrations of the plant extract, and the impedance parameters derived from these investigations are given in Table 4. As noticed from Figure 2, the obtained impedance diagrams are almost in a semicircular appearance, indicating

21 International Journal of Corrosion 7 HO Figure 3: Structure of α-bisabolene. Figure 4: Structure of β-bisabolene. Figure 5: Structure of β-caryophyllene. CH CH 2 NH CO CH=CH C 6 H 5 OH Meo Figure 6: Structure of aegelin. H CH 3 N H 3 C H O CH 3 H 3 C H H H H Figure 7: Structure of solasodine. that the charge-transfer process mainly controls the corrosion of mild steel. Deviations of perfect circular shape are often referred to the frequency dispersion of interfacial impedance. This anomalous phenomenon may be attributed to the inhomogeneity of the electrode surface arising from surface roughness or interfacial phenomena. In fact, in the presence of the plant extracts, the values of R t have enhanced and the values of double-layer capacitance are also brought down to the maximum extent. The decrease in C dl shows that the adsorption of the inhibitors takes place on the metal surface in acidic solution. For Ocimum sanctum extract, the maximum R t value of Ω cm 2 and minimum C dl value of 6. μf/cm 2 are obtained at an optimum concentration of 6% in v/v with a maximum inhibition efficiency of 97.9%. For Aegle marmelos extract, the maximum R t value of Ω cm 2 and minimum C dl value of 9.62 μf/cm 2 are obtained at an optimum concentration of 8% in v/v with a maximum inhibition efficiency of 96.6%. For Solanum trilobatum extract, the maximum R t value of Ω cm 2 and minimum C dl value of μf/cm 2 are obtained at an optimum concentration of 1% in v/v with a maximum inhibition efficiency of 91.4%. A good agreement is observed between the results of weight loss method and electrochemical methods (potentiodynamic polarization method and impedance method) Kinetics and Reason for the Corrosion Inhibition. The major phytochemical constituents present in Ocimum sanctum are β-bisabolene ( %), α-bisabolene ( %), and eugenol ( %) as given in Figures 3, 4, and 5, and the other phytochemical constituents present are 1,8-cineole (5.6 11%), E-β-ocimene (4. 4.7%), β-caryophyllene ( %), α-humulene (2. 3.5%), methylchavicol ( %), and germacrene-d ( %). The major phytochemical constituent present in Aegle marmelos is Aegelin (Figure 6), and the major phytochemical constituent present in Solanum trilobatum is Solasodine as shown in Figure 7 [35 37]. Inspection of the chemical structures of the phytochemical constituents reveals that these compounds are easily hydrolysable and the compounds can adsorb on the metal surface via the lone pair of electrons present on their oxygen atoms and make a barrier for charge and mass transfer leading to decrease the interaction of the metal with the corrosive environment. As a result, the corrosion rate of the metal was decreased. The formation of film layer essentially blocks discharge of H + and dissolution of metal ions. Acid pickling inhibitors containing organic N, S, and OH groups behave similarly to inhibit corrosion [38, 39]. It follows that inhibition efficiency (IE) is directly proportional to the fraction of the surface covered by the adsorbed molecules (θ). Therefore, (θ) with the extract concentration specifies the adsorption isotherm that describes the system. Adsorption isotherm gives the relationship between the coverage of an interface with the adsorbed species and the concentration of species in solution. The use of adsorption isotherms provides useful insight into the corrosion inhibition mechanism. The values of the degree

22 8 International Journal of Corrosion 12 1 R 2 =.9996 SD = R 2 =.9986 SD = C/θ 6 C/θ 4 4 Solanum trilobatum 2 Ocimum sanctum C (% v/v) C (% v/v) (a) (b) 12 1 R 2 =.996 SD = C/θ Aegle marmelos C (% v/v) (c) Figure 8: Langmuir adsorption isotherm plot for the adsorption of various concentrations of the plant extracts on the surface of mild steel in 1N HCl solution. 15 kv WD 15 mm 1 Figure 9: SEM Photograph of mild steel immersed in 1N HCl solution (blank). of surface coverage (θ) were evaluated at different concentrations of the inhibitors in 1N HCl solution. Attempts were made to fit θ values to various adsorption isotherm. An inhibitor is found to obey Langmuir, if a plot of log θ/1 θ versus log C is linear. Similarly, for Temkin plot θ versus log C, for BDM plot (log C logθ/1 θ) versusθ 3/2, and for Frumkin plot log θ/(1 θ)c versus θ will be linear. On examining, the adsorption of different concentrations of Ocimum sanctum, Aegle marmelos, andsolanum trilobatum extracts on the surface of mild steel in 1N hydrochloric acid was found to obey Langmuir adsorption isotherm. The Langmuir adsorption isotherm plot for the adsorption of various concentrations of the plant extracts is given in Figure Surface Examination Studies. Surface examination of the mild steel specimens was made using JEOL-Scanning electron microscope (SEM) with the magnification of 1x. The mild steel specimens after immersion in 1N HCl solution for three hours at 3 C in the absence and presence of optimum concentration of the plant extracts were taken out, dried, and kept in a dessicator. The SEM images of mild steel immersed in 1N HCl in the absence and presence of the optimum concentration of the plant extracts are shown in Figures 9, 1, 11, and12. The protective film formed on the surface of the mild steel was confirmed by SEM studies. From the SEM images, it was found that more grains were found in

23 International Journal of Corrosion 9 15 kv WD 15 mm 1 15 kv WD 15 mm 1 Figure 1: SEM Photograph of mild steel immersed in 1N HCl solution containing an optimum conc. (6% v/v) of Ocimum sanctum. 15 kv WD 15 mm 1 Figure 11: SEM Photograph of mild steel immersed in 1N HCl solution containing an optimum conc. (8% v/v) of Aegle marmelos. SEM image of mild steel immersed in 1N HCl solution in the absence of the inhibitor, whereas no grains were found in the SEM image of mild steel immersed in 1N HCl solution in the presence of the plant extracts, which shows the presence of a protective film over the surface of the mild steel in the presence of the inhibitors, and the protective film is uniform in the order: Ocimum sanctum > Aegle marmelos > Solanum trilobatum. The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the high performance of inhibitive effect of the plant extracts Hydrogen Permeation Studies. The behaviour of the inhibitors with regard to hydrogen permeation can be understood by measuring the permeation current with and without inhibitors. Those inhibitors which reduce the permeation current are good at inhibiting the entry of hydrogen into the metal concerned [31]. There are basically two reaction schemes. Common to both schemes, the first step is the diffusion of few hydrogen atoms that get onto the electrode surface. Hydrated protons are reduced to form neutral hydrogen atoms upon those areas of the surface, which are unoccupied. One can say protons are discharged on to free sites on the electrode to form adsorbed hydrogen atoms M(e) +H 3 O + MH ads +H 2 O, (6) where M is the cathodic metal surface. The second step is the desorption step. The two basic reaction paths are (i) discharge D, followed by chemical desorption, CD, MH ads +MH ads 2M + H 2 (7) Figure 12: SEM Photograph of mild steel immersed in 1N HCl solution containing an optimum conc. (1% v/v) of Solanum trilobatum. Permeation current (μa) Time (min) (1) Blank (2) Solanum trilobatum (1% v/v) (3) Aegle marmelos (8% v/v) (4) Ocimum sanctum (6% v/v) Figure 13: Hydrogen permeation current versus time plots for mild steel in 1N HCl solution in the absence and presence of an optimum concentration of the inhibitors. (ii) discharge D, followed by electrolytic desorption, ED, MH ads +H 3 O + +M(e) 2M + H 2 O+H 2. (8) For transition metals, it has been reported that the electrolytic desorption is the rate determining step. A part of the atomic hydrogen liberated during these processes enters the metal, when the remainder is evolved as hydrogen gas [4]. Permeation current versus time curves for mild steel in 1N HCl in the absence and presence of inhibitors are shown in Figure 13, and their corresponding permeation are given in Table 5. From the hydrogen permeation studies on mild steel in 1N HCl in the absence and presence of inhibitors, it was observed that all the prepared extracts were able to reduce the permeation current compared to the control. The decrease in the permeation current follows the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum. The reason for the reduced permeation currents in presence of the inhibitors can be attributed to the slow discharge step followed by fast electrolytic desorption step M(e) +H 3 O + slow MH ads +H 2 O, MH + H 3 O + +M(e) fast 2M + H 2 O+H (9)

24 1 International Journal of Corrosion Table 5: Values of hydrogen permeation current for the corrosion of mild steel in 1N HCl alone and in the presence of inhibitors. Inhibitor Conc. of the extract (% in v/v) Permeation current (μa) Reduction in permeation current (%) Blank 23. Ocimum sanctum Aegle marmelos Solanum trilobatum The reduction of hydrogen uptake could be attributed to adsorption of the phytochemical constituents present in the plant extracts on the mild steel surface, which prevented permeation of hydrogen into metal. 4. Conclusion (i) The leaf extracts of Ocimum sanctum, Aegle marmelos, andsolanum trilobatum act as good and efficient inhibitors for corrosion of mild steel in 1N hydrochloric acid. (ii) Potentiodynamic polarization studies revealed that the extracts act through mixed mode of inhibition. (iii) The Nyquist diagrams obtained in impedance method revealed that charge-transfer process mainly controls the corrosion of mild steel. (iv) The mechanism involved in this study is the phytochemical constituents in the plant extracts that have adsorbed on the mild steel surface forming a protective thin film layer preventing the discharge of H + ions and dissolution of metal ions and has prevented the small corrosion on the surface of the metal. (v) The plant extracts obey Langmuir adsorption isotherm. (vi) The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the high performance of inhibitive effect of the plant extracts. (vii) From hydrogen permeation method, it was observed that all the plant extracts were able to reduce the permeation current compared to the control. (viii) The reduction of hydrogen uptake in hydrogen permeation method could be attributed to adsorption of the phytochemical constituents present in the plant extracts on the mild steel surface, which prevented permeation of hydrogen into metal. (ix) Results obtained in weight loss method were very much in good agreement with the electrochemical methods (potentiodynamic polarization method and impedance method) and hydrogen permeation method in the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum. (x) Among the three plant extracts studied, the maximum inhibition efficiency was found in Ocimum sanctum which showed 99.6% inhibition efficiency at 6.% v/v concentration of the extract. References [1] M. Ajmal, A. S. Mideen, and M. A. Quraishi, 2-hydrazino- 6-methyl-benzothiazole as an effective inhibitor for the corrosion of mild steel in acidic solutions, Corrosion Science, vol. 36, no. 1, pp , [2] A. A. Hosary, R. M. Saleh, and A. M. S. Eldin, Corrosion inhibition by naturally occurring substances-1. 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26 Hindawi Publishing Corporation International Journal of Corrosion Volume 211, Article ID , 7 pages doi:1.1155/211/ Research Article Corrosion Inhibition of the Galvanic Couple Copper-Carbon Steel in Reverse Osmosis Water Irene Carrillo, Benjamín Valdez, Roumen Zlatev, Margarita Stoycheva, Michael Schorr, and Mónica Carrillo Instituto de Ingeniería, Universidad Autónoma de Baja California, Boulevard, Benito Juárez y Calle a la Normal S/N, 218 Mexicali, BCN, Mexico Correspondence should be addressed to Benjamín Valdez, benval@iing.mxl.uabc.mx Received 9 March 211; Revised 24 May 211; Accepted 24 May 211 Academic Editor: Jerzy A. Szpunar Copyright 211 Irene Carrillo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The purpose of this paper is to evaluate the electrochemical behaviour of corrosion inhibition of the copper-carbon steel galvanic couple (Cu-CS), exposed to reverse osmosis water (RO) used for rinsing of heat exchangers for heavy duty machinery, during manufacture. Molybdate and nitrite salts were utilized to evaluate the inhibition behaviour under galvanic couple conditions. Cu-CS couple was used as working electrodes to measure open circuit potential (OCP), potentiodynamic polarization (PP), and electrochemical impedance spectroscopy (EIS). The surface conditions were characterized by scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDS). The most effective concentration ratio between molybdate and nitrite corrosion inhibitors was determined. The morphological study indicated molybdate deposition on the anodic sites of the galvanic couple. The design of molybdate-based corrosion inhibitor developed in the present work should be applied to control galvanic corrosion of the Cu-CS couple during cleaning in the manufacture of heat exchangers. 1. Introduction The study of inhibition mechanism, electrochemical, and kinetic behaviour of inorganic inhibitors such as molybdates and other salts applied to protect Cu-CS galvanic couple in aqueous media, contributes to the prevention of corrosion, particularly in industrial equipment which inevitably requires joining pieces of different metals for its construction [1 6]. Heat exchangers are often constructed by dissimilar metals such as copper fins that cool the fluid by convection, internal copper tubes and carbon steel shells (Figure 1). The conditions of the cleaning process during the manufacture of heavy duty heat exchangers promote the dissolution of the anodic metal in a galvanic couple when it is rinsed with RO water, especially with an unfavourable cathode-anode area ratio of 2.4 to 1.. The use molybdate-based corrosion inhibitors represents an environmentally friendly alternative, since sodium molybdate is considered a nontoxic inhibitor used for corrosion protection of cooling systems handling softened water. Sodium molybdate has a good performance as anodic inhibitor, that is incorporated into the metal surface forming a protective film. In combination with other chemical agents it may promote or inhibit the corrosion process. These results are directly dependent on the concentration, temperature, ph, and the oxidizing agent. These studies have a relevant importance to find an efficient and economical solution to control the galvanic corrosion in industrial processes. 2. Methodology 2.1. Materials. Specimens of carbon steel UNS G12 and copper UNS C13 were used to prepare the galvanic couples. All the experiments were done with an area ratio 2.4 to 1. cathode-anode in order to simulate the real dimensions of heat exchangers for heavy machinery Specimen and Solutions Preparation. The working electrode was constructed joining metal coupons with a Cu area of 4. cm 2 and 1.7 cm 2 for CS, connected to an insulated

27 2 International Journal of Corrosion OCP (V versus Ag/AgCl) (1) (2) (3) Figure 1: Heat exchanger for heavy duty machinery of copper UNS C13 and carbon steel G Time (hr) Cu wire. The surfaces were sanded to 4 grit and rinsed with distilled water and acetone before coupling. The corrosive environment was RO water at ph 5.5 and the applied temperature was 77 C to simulate the real rinse process conditions. The corrosion inhibitor solutions were prepared by adding the solid salts to the RO water: molybdate and nitrite Electrochemical Measurements. The OCP variation along time was recorded to analyze the effect of the sodium molybdate Na 2 Mo 2 O 4 in different concentrations, adding sodium nitrite NaNO 2 as oxidizing agent in RO water. The copper and steel coupons were separated by 7 cm and connected by an insulated Cu alligator clip which in turn connects to a multimeter Digital Protek Model B-45 to obtain a response in mv. Electrochemical polarization plots were produced using a three electrode cell: a Cu-CS working electrode, an Ag/AgCl reference electrode and a high density graphite electrode as counter electrode. The potential-current plots were obtained as a function of sodium molybdate and sodium nitrite concentrations for each working electrode applying a scan rate of 5 mv/s in a potential range from.5 to.5 V versus. a Ag/AgCl electrode. In order to evaluate the mass transfer process and the film formed under molybdate effect, electrochemical impedance analysis were carried out after achieve the steady state potential, in the frequency range from.1 Hz to 1 5 Hz with 1 points/decade. The Nyquist and Bode plots were obtained under potentiostatic conditions Morphology Analysis. Surface analyses of specimens tested in RO water inhibited with the most efficient formulations were performed without any previous treatment by scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDS). The model SEM used was Jeol JSM Results and Discussion 3.1. Open Circuit Potential Measurements. The OCP values with a potential shift to positive values in the presence of Figure 2: Variation of the open circuit potential with time: (1) Na 2 MoO 4 :NaNO 2 18 : 75 ppm; (2) Na 2 MoO 4 18 ppm; (3) RO water. corrosion inhibitors obtained under different conditions for the galvanic couples tested are shown in Figure 2. At the moment of immersion in 18 ppm MoO 2 4 a potential of 14 mv was recorded. This potential moves fast toward negative values around 4 mv as a result of the breakdown of the molybdate layer formed on the steel surface. Simplistically, when iron corrodes, ions, in conjunction with other anions adsorb to form a nonprotective complex with Fe 2+ ions [3, 7].Theresultisasolubleand shallow protective film due to the poor oxidation ability of Na 2 MoO 4. Because of dissolved oxygen or other oxidizers in the water, some of the Fe 2+ ions are oxidized to the ferric (Fe 3+ ) state, and the ferrous molybdate is transformed to ferric molybdate, which is both insoluble and protective in neutral and alkaline waters [3]. An OCP value of 2 mv was obtained when NaNO 2 was added, due to its oxidant properties that improve the corrosion protection of CS Potentiodynamic Polarization. The effect of the ratio between the two inhibitors: Na 2 MoO 4 ;NaNO 2, was studied by its electrochemical behaviour recording potentiodynamic polarization (PP) plots. The plots in Figure 3 were obtained in RO water at 77 C in the presence of the sodium molybdate; they clearly show a significant change in the displacement of the potential to electropositive values, implying a potential shift driven by kinetic changes in the cathodic process. As the sodium molybdate concentration increases without sodium nitrite (Figure 3), the current density in the anodic curve tends to decrease and the performance inhibition process improves. Nevertheless, the inhibition only by sodium molybdate is not enough to avoid the corrosion damage of the CS in the short term due to the galvanic effect. The addition of sodium nitrite improves the efficiency of the corrosion inhibition process. The presence of an oxide layer is essential for the corrosion inhibition action of

28 International Journal of Corrosion Electrode potential (V versus Ag/AgCl) Corrosion rate (mm/y) Blank M:N 18:75 M:N 28 : 12 Concentration (ppm) M:N 15 : 5 M:N 2 : Current density (μa/cm 2 ) Blank 15, Na 2 MoO 4 2, Na 2 MoO 4 25, Na 2 MoO 4 12 : 5, Na 2 MoO 4 : NaNO 2 18 : 75, Na 2 MoO 4 : NaNO 2 2 : 133, Na 2 MoO 4 : NaNO 2 28 : 12, Na 2 MoO 4 : NaNO 2 Figure 3: Current-Potential plots varying the concentration ratio Na 2 MoO 4 :NaNO 2 in ppm. Table 1: Current potential data varying the concentration ratio Na 2 MoO 4 :NaNO 2. Concentration (ppm) Current (μa/cm 2 ) E (mv) Blank MoO MoO MoO : 15 MoO 2 4 :NO : 75 MoO 2 4 :NO : 133 MoO 2 4 :NO : 12 MoO 2 4 :NO molybdate [3, 7 9] and in order to accelerate and stabilize the oxidized surface an oxidizing compound such as sodium nitrite is necessary. Finally, it was noted that in all the cases, the potential shift to positive values and the current density decreases, due to the electrostatic attraction force between the MoO 4 2 anions and the metal electrons and its oxide promoting the formation of a polyoxomolybdate protective layer [3, 7, 1]. Since nitrite and dissolved oxygen (DO) or other anions seem to promote competitive adsorption over the anodic surface, the molybdate and nitrite ratio is very important to find the most efficient and economical solution. The current and potential values of Table 1 were calculated from potentiodynamic tests by the Tafel polarization. The results of the corrosion rates are showed in Figure 4.The results showed that the best inhibitor concentration ratio was Figure 4: Corrosion rates of Cu-CS as a function of MoO 4 :NO 2 (M : N) ratio concentration. Electrode potential (V versus Ag/AgCl) Current density (μa/cm 2 ) Blank 5 : 33, MoO 2 :NO 2 1 : 66, MoO 2 :NO 2 15 : 1, MoO 2 :NO 2 2 : 133, MoO 2 :NO 2 25 : 166, MoO 2 :NO 2 3 : 2, MoO 2 :NO 2 Figure 5: Current potential polarization plots for the Cu-CS galvanic couple as a function of Na 2 MoO 4 :NaNO 2 concentration, in a 1.5 : 1 ratio. 2 : 133 ppm MoO 4 :NO 2. The electrochemical behaviour of the proportion 1.5 to 1 for MoO 4 and NO 2,respectively, was studied varying their concentration. The effect of both compounds under this ratio concentration is shown in Figure 5, which reveals a continuous ennobling of the potential that drives an acceleration of the anodic reaction. Table 2 shows the potential and current values from Tafel polarization. On the other hand, the ratio ppm leads to a potential of.146 volts with a lower

29 4 International Journal of Corrosion Table 2: Current potential data as a function of Na 2 MoO 4 :NaNO 2 concentration in a 1.5 : 1 ratio. Concentration (ppm) Current (μa/cm 2 ) E (mv) Blank : 33 Na 2 MoO 2 4 :NO : 66 Na 2 MoO 2 4 :NO : 1 Na 2 MoO 2 4 :NO : 1 Na 2 MoO 2 4 :NO : 166 Na 2 MoO 2 4 :NO : 2 Na 2 MoO 2 4 :NO Corrosion rate (mm/y) Blank M 2 M 15 M 25 M:N 15 : 1 M:N 1 : 66 Concentration (ppm) M:N 3 : 2 M:N 2 : 133 M:N 25 : 166 Figure 6: Corrosion rate of Cu-CS galvanic couple in function of the molybdate-nitrite ratio in RO water solutions. current density that indicates the presence of CS corrosion protection. When carbon steel corrodes, the MoO 4 2 ions and other anions are fixed to the surface by adsorption to form a molybdate-fe 2+ complex, which sometimes is nonprotective because the ferrous compounds are soluble. Due to dissolved oxygen and another oxidants such as sodium nitrite in the RO water, the Fe 2+ ions are oxidized to the ferric state (Fe 3+ ) and ferrous molybdate complex is transformed to ferric molybdate complex forming an insoluble and protective layer [3] inneutral,alkaline,androwater. Figure 6 shows a corrosion rate decrease up to a 94% of inhibition efficiency for the galvanic couple regarding the blank. The excess concentration of Na 2 MoO 4 affects the inhibition process, given by the adsorption competitiveness of other anions. The potentiodynamic polarization is an accelerated technique for corrosion measurements, therefore was possible to decrease the concentration inhibitors in real manufacture lines that involves cleaning process where the exposition time is short. The formulation Na 2 MoO 4 :NaNO 2 was applied in a real industrial process and the corrosion inhibitors were added at the rinsing where the galvanic corrosion occurs during the heat exchanges cleaning process. In Figure 7 it is possible to observe an application of these corrosion inhibitors combination. The surface appearance (a) Figure 7: Heat exchangers manufacturing process. (a) without corrosion inhibitor in cleaning process. (b) Molybdate and nitrite were applied 18 : 12 ppm to inhibit galvanic corrosion. of the heat exchanger after the cleaning process adding corrosion inhibitors is really good Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy assays for the systems: blank, 18 : 12 and 25 : 166 Na 2 MoO 4 :NaNO 2 were performed to confirm the inhibition process and study the electrochemical corrosion inhibition of molybdate. An initial process of charge transfer was recorded in the Nyquist diagram (Figure 8) for the blank represented by the small semicircle of low impedance at high frequencies followed by a greater semicircle at medium frequencies, that dominate the process where the carbon steel loss electrons to reach an equilibrium potential with the copper. Finally diffusion at very low frequencies contributes to the rapid formation of a porous layer which was observed at the end of the test. It is very notable that mass transfer by diffusion controls the process for the systems containing Na 2 MoO 4 and NaNO 2 since medium frequencies, the projection of centres from the experimental data points is also characterized by the relatively large angle of tilt which sometimes reaches values close to 45 and is feature of Warburg impedance. At high frequencies a small charge transfer semicircle was recorded in these systems, attributed at the oxidizing species that promote the oxidation followed for a diffusion process due the adsorbed specie where a combination is adjusted between the double layer capacitance value represented by a constant phase element (CPE) and the transfer charge resistance value (Rct) when the molybdate concentration increases over the surface and the result is a stable layer formation of great charge transfer resistance. An equivalent circuit was proposed in order to analyze the Bode and Nyquist plot showed in Figure 9. The fit line of Figure 8 shows the accuracy of the proposed circuit. Table 3 shows the impedance data of this arrangement. The adjustment of these data shows a capacitive behaviour, dominated by the CPE element and an increasing in the impedance at high frequencies due to the corrosion inhibition process. Every interface has one capacitance and a charger transfer resistance related at the ions migration, the oxidation process, and the oxide film, therefore the equivalent circuit is constituted by two parts: the first elements represent the solution resistance and the carbon (b)

30 International Journal of Corrosion Zimag (Ω cm 2 ) Z real (Ω cm 2 ) Blank 25 : 166 MoO 4 :NO 2 18 : 12 MoO 4 :NO 2 Fit (a) KCnt O 373 O Fe C Fe (a) Fe Z (Ω cm 2 ) Zphz (grad) 186 C Fe Na Mo (b) 3 Figure 1: SEM and EDS for CS exposed to RO water without corrosion inhibitors Frequency (Hz) 1 2 Blank 25 : 166 MoO 4 :NO 2 18 : 12 MoO 4 :NO 2 Fit (b) Figure 8: Nyquist and Bode diagram for Cu-CS galvanic couple in RO water with molybdate sodium and nitrite sodium. steel surface portion in contact with the electrolyte and its respective polarization resistance increase under the effect of molybdate concentration increment and the oxidation of the Fe 2+ to Fe 3+ promoted by the nitrite and DO. The second part represents the double layer that contains the Rct element (R3) which increases significantly because the film is protecting CS and Cu remains stable; the subsequent studies of surface analysis by EDS show the molybdenum presence in the surface, nevertheless is not a uniform layer. R 1 CPE1 R 2 CPE2 R 3 Ws1 s Figure 9: Electrical equivalent circuit proposed to simulate the impedance behaviour of Cu-CS couple in RO water. w 4. Morphological Study The anodic surface was analyzed to evaluate the molybdate incorporated by adsorption to form a polyoxomolybdate ferric complex [3, 7, 9] that protect the carbon steel even under galvanic conditions. The microphotography in Figure 1(a) shows an oxide porous thick layer in the carbon steel surface that was in contact with Cu. The porous of the iron oxide black layer was also observed visually The amount in weight recorded was of 87% wt iron and 8.9% wt oxygen. An area zoom of Figure 1(a) was done on a surface defect (b). High oxygen

31 6 International Journal of Corrosion Table 3: Parameters obtained from the fitting process with the equivalent circuit in Figure 9. [Na 2 Mo 4 ](Ppm) [NaNO 2 ](Ppm) R 1 (Ω cm 2 ) R 2 (Ω cm 2 ) Y 2,s N R 3 (Ω cm 2 ) Y 2, S n WS e e e e Fe.9 Fe KCnt.8.5 O KCnt C Fe Na Mo (a) C Fe Na Mo (a) KCnt Fe KCnt Fe.3 O C Na Mo Fe (b) Figure 11: SEM and EDS of carbon steel under galvanic conditions in inhibited RO water by 18 : 12 Na 2 MoO 4 and NaNO 2..2 O C Na Mo (b) Figure 12: SEM and EDS of carbon steel under galvanic conditions in inhibited RO water by 25 : 166 Na 2 MoO 4 and NaNO 2. Fe contents (14% wt) and a decreasing of Fe composition (8% wt) was recorded by EDS test as consequence of the galvanic corrosion and the precipitation by soluble compounds formation. The film deposited on CS surface was characterized by EDS and is shown in Figure 11. The EDS test Figure 11(a) recorded small amounts of molybdenum (.26.45% weight) by a reduction reaction of MoO 4 2 anion to join it with the ferric oxide layer [1, 3, 6 8]. The homogeneous surface improves with the molybdate concentration increment. There was a 3% decreasing of oxygen with respect to the blank and the Fe content is above 92%. The area zoom Figure 11(b) 18 : 12 ppm Na 2 MoO 4 :NaNO 2 shows a small accumulation of molybdate in a surface defect which seems as restorative effect. This behaviour was reported from alloy with molybdenum can repair the defect of the iron passive in borate buffer solution and inhibit pit growth [11]. It is possible to observe in low quantity the same behaviour in a zoom area of part Figure 12(b) 25 : 166 ppm Na 2 MoO 4 :NaNO 2 when the surface has defects, no homogeneous zones or susceptible zones by copper contact, the adsorption of the MoO 4 2 anions increases, taking account there is an electrostatic attraction between the Fe cations and the MoO 4 2,[12, 13] this is deposited in the anodic zones (.45.9%), the most susceptible area portions in contact with the copper and the solution. The electrochemical tests showed the effect by increasing the concentration of sodium molybdate (limited to 25 ppm in combination with the sodium nitrite) [6, 8, 14, 15] to decrease the corrosion rate and the results by EDS confirm

32 International Journal of Corrosion 7 a deposition of molybdenum compounds joined to the oxide layer and oxide reduction in all the cases. The combination 25 : 166 MoO 4 :NO 2 was totally confirmed as the optimal formulation for copper and carbon galvanic couple. 5. Conclusions Good performance of the corrosion inhibitor requires the polyoxomolybdate complex layer formation which is more stable, highly protective and a great charge transfer with ferric compounds, necessary to ensure the oxidation of Fe 2+ to Fe 3+ promoted by the nitrite. The optimal inhibitor concentration was 25 ppm of sodium molybdate and 166 ppm of sodium nitrite obtaining 94% inhibition efficiency. The economical application for the heat exchanger cleaning was 18 : 12 molybdate : nitrite and it takes 24 minutes. Molybdate excess concentration affects the inhibition related to the nitrite and DO molecules and possible other anions which promote the adsorption competitiveness influenced by the concentration ratio between molybdate and nitrite hence the importance of the efficient combination. The impedance test shows the diffusion control for the formation of resistant molybdate film. Molybdate can be physically adsorbed on the metal surface or on the hydroxide layer available on the surface and act in anodic zones by electrostatic attraction. mild steel corrosion inhibition in simulated cooling water, Corrosion Science, vol. 48, no. 6, pp , 26. [9] V. S. Sastri, Corrosion Inhibitors Principles and Applications, John Wiley & Sons, Ontario, Canada, [1] K. C. EmregülandA.A.Aksüt, The effectofsodiummolybdate on the pitting corrosion of aluminum, Corrosion Science, vol. 45, no. 11, pp , 23. [11] E. Fujioka, H. Nishihara, and K. Aramaki, The inhibition of pit nucleation and growth on the passive surface of iron in a borate buffer solution containing Cl- by oxidizing inhibitors, Corrosion Science, vol. 38, no. 11, pp , [12] M. Shams and L. Wang, Mechanism of corrosion inhibition by sodium molybdate, in Proceedings of the Material Testing Laboratory, Government of Abu Dhabi Water, vol. 5, pp , Electricity Department, Umm Al Nar Station, Abu Dhabi, UAE, [13] M. Meziane, F. Kermiche, and C. Fiaud, Effect of molybdate ions as corrosion inhibitors of iron in neutral aqueous solutions, British Corrosion Journal, vol. 33, no. 4, pp , [14] D. G. Kolman and S. R. Taylor, Sodium molybdate as a corrosion inhibitor of mild steel in natural waters part 2: molybdate concentration effects, Corrosion,vol.49,no.8,pp , [15] D. G. Kolman and S. R. Taylor, Sodium molybdate as a corrosion inhibitor of mild steel in natural waters. Part 1: flow rate effects, Corrosion, vol. 49, no. 8, pp , Acknowledgments The authors thank Honeywell Thermal Systems of Mexicali, for support of the study development and to implement the corrosion inhibitor in their cleaning process for heat exchangers during the manufacture. Thanks are also due to CONACYT for the Scholarship support to I. Carrillo. References [1] C. M. Mustafa and S. M. S. I. Dulal, Molybdate and nitrite as corrosion Iinhibitors for copper-coupled steel in simulated cooling water, Corrosion, vol. 52, no. 1, pp , [2] E. J. Talbot and D. R. Talbot, Corrosion Science and Technology, CRC Press, New York, NY, USA, 27. [3] J. R. Davis, Corrosion fundamentals, testing and protection, ASM International and The materials Information Society, Ohio, EE.UU, 2, [4] R. Francis, Galvanic corrosion of high alloy stainless steels in sea water, British Corrosion Journal, vol. 29, no. 1, pp , [5] P. R. Roberge, Corrosion Engineering: Principles and Practice, McGraw Hill, New York, NY, USA, 28. [6] I. Carrillo, Inhibition of the corrosion in galvanic couples copper and carbon steel of Heat Exchangers for heavy machinery industry, M.S. thesis, Instituto de Ingenieria de la Universidad Autonoma de Baja California for Engineering Master Degree, 29. [7] M.R.Ali,C.M.Mustafa,andM.Habib, Effect of molybdate, nitrite, zinc ions on the corrosion inhibition of Mild steel in aqueous chloride media containing cupric ions, Journal of Scientific Research, vol. 1, pp , 29. [8] M. Saremi, C. Dehghanian, and M. M. Sabet, The effect of molybdate concentration and hydrodynamic effect on

33 Hindawi Publishing Corporation International Journal of Corrosion Volume 211, Article ID , 12 pages doi:1.1155/211/ Research Article Inhibition Effect of 1-Butyl-4-Methylpyridinium Tetrafluoroborate on the Corrosion of Copper in Phosphate Solutions M. Scendo and J. Uznanska Institute of Chemistry, UJK Kielce, Swietokrzyska Street 15G, 2546 Kielce, Poland Correspondence should be addressed to M. Scendo, Received 19 November 21; Accepted 1 February 211 Academic Editor: Flavio Deflorian Copyright 211 M. Scendo and J. Uznanska. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The influence of the concentration of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) as ionic liquid (IL) on the corrosion of copper in.5 M PO 3 4 solutions of ph 2 and 4 was studied. The research involved electrochemical polarization method, and scanning electron microscopy (SEM) technique. The results obtained showed that the inhibition efficiency of corrosion of copper increases with an increase in the concentration of 4MBPBF 4 but decreases with increasing temperature. The thermodynamic functions of corrosion analysis and adsorptive behavior of 4MBPBF 4 were carried out. During the test, the adsorption of the inhibitor on the copper surface in the phosphate solutions was found to obey the Langmuir adsorption isotherm and had a physical mechanism. 1. Introduction Copper is used as a construction metal in the central heating installations, car industry, energetics, oil refineries, sugar factories, marine environment, to name only a few of its various applications. This extensive use of copper is due to its mechanical and electric properties as well as the behaviour of its passivation layer. Acidic solutions are widely used in various industries for the cleaning of copper. The behaviour of copper in acidic media is extensively investigated, and several ideas have been presented for the dissolution process [1, 2]. To avoid the base metal attack and to ensure the removal of corrosion products/scales alone, inhibitors are extensively used. The most well-known acid inhibitors are organic compounds containing nitrogen, phosphor, sulfur, and oxygen atoms. The surfactant inhibitors have many advantages such as high inhibition efficiency (IE), low price, low toxicity, and easy production [3 5]. The interactions between the inhibitor molecules and the metal surfaces should by all means be explained and understood in detail. In examining of these interactions, theoretical approaches applied can be very useful [6 1]. Many N-heterocyclic compounds have been used for the corrosion inhibition of metals, such as imidazoline [11], triazole [12 14], tetrazole [15], pyrrole [16], pyridine [17], pyrazole and bipyrazole [18, 19], pyrimidine [2], pyridazine [21], and some derivatives. Some heterocyclic compounds containing a mercapto group have been developed as copper corrosion inhibitors. These compounds include: 2-mercaptobenzothiazole [22], 2,4- dimercaptopyrimidine [23], 2-amino-5-mercaptothiadzole, 2-mercaptothiazoline [24], potassium ethyl xanthate [25 28] and indole and derivatives [29]. Among the numerous organic compounds tested and industrially applied as corrosion inhibitors, nontoxic ones are far more strategic now than in the recent past. These compounds include such amino acids [3 32] andderivativesascysteine[33]. In the past two decades, the research in the field of green corrosion inhibitors has been addressed towards the goal of using cheap effective molecules at low or zero environmental impact. These compounds include purine and adenine, which have been tested for copper corrosion in chloride [34, 35], sulfate [36], and nitrate solutions [37].

34 2 International Journal of Corrosion Ionic liquids (ILs) are molten salts with melting points at/or below ambient temperature, which are composed of organic cations and various anions. Configuration of ILs consists of an amphiphilic group with a long chain, hydrophobic tail, and a hydrophilic polar head. Usually, ILs have nitrogen, sulphur, and phosphorus as the central atoms of cations, such as imidazolium, pyrrolidinium, quaternay ammonium, pyridinium, piperidinium, sulfonium and quaternary phosphonium. Currently, funtionalized IL is a very noticeable topic in the field of IL research. Introducing different functional groups into cations provides a great deal of ILs with new structures that can markedly change the physicochemical properties of ILs, and it also affords more choices for applications of ILs in electrochemical devices. Imidazolium compounds are reported to show corrosion resistant behavior on mild steel [38], copper [39, 4], and aluminium [41]. It was found that the action of such inhibitors depends on the specific interaction between the functional groups and the metal surface, due to the presence of the C = N group and electronegative nitrogen in the molecule. Ionic liquids and different types of surfactans base inhibitors are well known to have a high activity in acid medium [42, 43] and therefore are used in an oil field to minimize carbon-dioxide-induced corrosion [44, 45]. Among many kinds of functionalized ionic liquids ether-functionalized ILs have been investigated intensively, and ether groups have been successfully introduced in to imidazolium cations [46 52]. However, no substantial information is available on pyridinium ionic liquids being used as corrosion inhibitors of copper. The present work describes a study of the corrosion of copper in.5 M PO 3 4 solutions of ph 2 and 4 without and with different concentrations of 1-Butyl-4- methylpyridinium tetrafluoroborate (4MBPBF 4 ), based on copper stationary disc electrode voltammetry measurements and scanning electron microscope. Moreover, the thermodynamic functions were appointed for the adsorption process and to gain more information about the mode of adsorption of the inhibitor on the surface of copper. 2. Experimental Methods 2.1. Solutions. 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 )(>99.8%) was purchased from Fluka. The molecular structures of compound are shown in Figure 1. It is worth to notice that 4MBPBF 4 is not flat molecule. The 4MBPBF 4 is stable in air, water, and in majority organic solvents. However, this compound is well enough solvable in water. All the solutions were prepared using analytical grade reagent and triple distilled water (resistivity 13 MΩ cm). The 4MBPBF 4 was dissolved at concentrations in the range of mm in.5 M PO 3 4 solutions of ph 2 and 4. During the measurements, the solution was not stirred or deaerated Electrodes and Apparatus. The working electrode was a home-made stationary disk electrode (SDE) of Specpure copper (Johnson Matthey Chemicals Ltd.) with r =.24 cm N Figure 1: Molecular structure ionic liquid: 1-Butyl-4- methylpyridinium tetrafluoroborate (4MBPBF 4 ). and A =.181 cm 2. Prior to each experiment the working electrode was mechanically polished to mirror gloss by using 1 and 2-grade emery papers. Then the electrode was washed several times interchangeably with bidistilled water and ethanol. Finally, SDE was dried using a stream of air. Such pretreatment of the disk was repeated after each voltammetric measurement. Other details were published in [53 56]. All the surface-area-dependant values are normalized with respect to the geometric surface area of the working electrode. Electrode potentials were measured and reported against the external saturated calomel electrode with NaCl solution (SCE(NaCl)) coupled with a fine Luggin capillary. To minimize the ohmic contribution, the capillary was kept close to the working electrode. A platinum (purity 99.99%) wire was used as an auxiliary electrode. Auxiliary electrode was individually isolated from the test solution by a glass frit. All voltammetric experiments were performed using a Model EA9C electrochemical analyzer, controlled via Pentium computer using the software Eagraph V Scanning Electron Microscope. A scanning electron microscope (SEM) PHILIPS XL 3 was used to study the morphology of the copper surface in the absence and presence of the inhibitor. Samples were attached on top of an aluminum stopper by means of 3 M carbon conductive adhesive tape (SPI) Potentiodynamic Polarization Measurements. Electrochemical experiments were carried out in a classical threeelectrode glass cell. The cell was open to air. The degreased SDE was quickly inserted into the solution and immediately cathodically polarized at 11 mv (SCE(NaCl)) for 3 min to reduce any oxide on the copper surface. The polarization curves were obtained using the linear potential sweep (LSV) B F F F F

35 International Journal of Corrosion (b) (c) (d) (a) j (ma cm 2 ) (d) (c) (b) (a) E (mv) versus SCE(NaCl) Figure 2: Some chosen polarization curves of the copper electrode in.5 M PO 3 4 solutions containing different concentrations of 1- Butyl-4-methylpyridinium tetrafluoroborate: (a), (b) 1., (c) 1., and (d) 5. mm, ph 4, de/dt 1 mv s 1. j (ma cm 2 ) E (mv) versus SCE(NaCl) Figure 3: Some chosen Tafel plots of the copper electrode in.5 M PO 3 4 solutions containing different concentrations of 1-Butyl-4- methylpyridinium tetrafluoroborate: (a), (b) 1., (c) 1. and (d) 5. mm, ph 2, de/dt 1 mv s 1. technique. The scan started from the cathodic ( 11 mv) to the anodic direction with the scan rate of 1 mv s 1. Electrochemical experiments were repeated many times, and the average values of the current were used. All experiments were carried out using an air thermostat with the forced air circulation. 3. Experimental Results and Discussion 3.1. Polarization Behaviour of Copper. The effect of 1-Butyl- 4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) on the corrosion reactions of copper was determined by polarization measurements at 2 C. Figure 2 shows example of polarization curves for the copper electrode in.5 M PO 3 4 solutions of ph 4 without and with different concentrations of 4MBPBF 4. Similar curves were recorded for solution of ph 2. It is clear that the presence of different concentrations of the inhibitor decreases the current densities and reduces both of the cathodic and anodic current densities in comparison to those recorded in the additive-free solution. However, in case of more acid solutions (ph 2) were observed smaller changes in the cathodic and anodic current densities. The decrease in current densities could be attributed to the decrease in the phosphate ions attack on the copper surface due to the adsorption of the inhibitor molecules at the copper/solution interface Corrosion Parameters. The corrosion kinetic parameters were calculated on the basis of cathodic and anodic potential versus current characteristics in the Tafel potential region (Figure 3). The corrosion parameters such as corrosion potential (E corr ), corrosion current density (j corr ), and cathodic (b c )andanodic(b a ) Tafel slope are listed in Table 1. It is worth noticing that addition of the 4MBPBF 4 causes more negative shift in corrosion potential values independently from ph solutions. Hence small changes in potentials can be a result of the competition of the cathodic and the anodic inhibiting reactions. The corrosion current density (Table 1) decreased when the concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate were increased for both solutions of ph 2 and 4. This indicates the inhibiting effect of 4MBPBF 4 on corrosion of copper. The decrease in cathodic (b c )andanodic (b a ) or the increase in (b a ) only in case of solutions of ph 4 Tafel slopes (Table 1) indicated that the 1-Butyl-4- methylpyridinium tetrafluoroborate molecules are adsorbed on both the anodic and cathodic sites resulting in an inhibition of both anodic dissolution of copper and cathodic reduction reactions. Moreover, these inhibitors cause small change in the cathodic and anodic Tafel slopes, indicating that 4MBPBF 4 is first adsorbed onto copper surface and therefore impedes the reaction by merely blocking the reaction sites of copper surface without affecting the cathodic and anodic reaction mechanism [57] Polarization Resistance. The polarization resistance (R p ) values are related to the corrosion current density ( j corr ), which can be calculated from the equation: [ R p = b a b c 2.33(b a + b c ) ] [ 1 j corr ]. (1) The R p values listed in Table 1 are used to estimate the corrosion inhibition effect of the inhibitor. The addition of 1-Butyl-4-methylpyridinium tetrafluoroborate to the phosphate solutions produced higher R p values than the blank solution indicating the formation of a protective layer on the electrode surface. Hence, the polarization resistance values increase with an increase in the concentration of 4MBPBF 4

36 4 International Journal of Corrosion Table 1: Corrosion parameters and polarization resistance of copper electrode in.5 M PO 3 4 solutions in the absence or presence of different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) of ph 2 and 4 at 2 C. Inhibitor ph Concentration inhibitor (mm) E corr (mv) j corr (μa cm 2 ) b c (mv dec 1 ) b a (mv dec 1 ) R P (Ω cm 2 ) Blank MBPBF Blank MBPBF for both solutions of ph 2 and 4. It seems that protective layer created on surface of copper is the most tight in of less acid solution about the largest concentration of 1-Butyl-4- methylpyridinium tetrafluoroborate Inhibition Efficiency. Inhibition efficiency (IE) can also be calculated from polarization tests by using the following equation [58, 59]: ( ) jo j corr IE(%) = 1, (2) where j o and j corr are the corrosion current densities in the absence and presence of inhibitor, respectively. The inhibition efficiency depends on both the nature and the concentration of the investigated compounds. The calculated inhibition efficiencies are presented in Figure 4. In the presence of 1-Butyl-4-methylpyridinium tetrafluoroborate solution of ph 2 and 4, the inhibition efficiency increases with an increase in the concentration of inhibitor. This confirms the inhibiting character of 1-Butyl- 4-methylpyridinium tetrafluoroborate. However, IE is higher in case of solution of ph 4 than 2. It is obvious that in the presence of 1-Butyl-4-methylpyridinium tetrafluoroborate solution of ph 2 the film on copper does not cover tightly the surface and hence does not protect it prior to corrosion of Cu in an adequate degree Corrosion Rate. The corrosion current density (j corr ) was converted into the corrosion rate (k r ) by using the expression [6]: k r ( mm year ) j o ( ) = jcorr M Cu, (3) nρ where M Cu is the molecular weight of copper, n is the number of electrons transferred in the corrosion reaction, and ρ is the density of Cu (g cm 3 ). IE (%) c (mm) ph 2 ph 4 Figure 4: Inhibition efficiency of corrosion of copper in.5 M PO 3 4 solution with different concentrations of 1-Butyl-4- methylpyridinium tetrafluoroborate of ph 2 and 4. The values of the copper corrosion rate in the absence and the presence of inhibitor for solution of both ph Values are presented in Table 2. The corrosion rate of copper is significantly reduced as a result of the reduction in the corrosion current densities. The protective layer on surface of metal causes that the corrosion rate to be more diminishes in case of less acid solution of phosphates Scanning Electron Microscopy Studies. The surface morphology of copper samples immersed in.5 M PO 3 4 (ph 2 and 4) for 24 hours in the absence and in the presence of 5. mm of 1-Butyl-4-methylpyridinium tetrafluoroborate

37 International Journal of Corrosion 5 Table 2: Corrosion rate of copper in.5 M PO 3 4 solutions in the absence or presence of different concentrations of 1-Butyl-4- methylpyridinium tetrafluoroborate (4MBPBF 4 )ofph2and4. Concentration of 1-Butyl- 4-methylpyridinium k r (mm/year) tetrafluoroborate (mm) ph 2 ph was studied by scanning electron microscopy (SEM). The solutions were not degassed. Figure 5 show the surface morphology of copper specimens (a) before and (b) after being immersed in corrosive solution (ph 2). The photograph (b) revels that the surface was strongly damaged in absence of the inhibitor. Figures 5(c) and 5(d) show SEM images of the surface copper specimens after immersion (for the same time interval) in corrosive solution containing additionally 5. mm of 1- Butyl-4-methylpyridinium tetrafluoroborate of ph 2 and 4, respectively. In the presence of the inhibitor the film precipitates on the surface of copper. The SEM photographs show that protective layer does not cover tightly the surface, and, hence does not protect the Cu surface to an adequate degree especially in case of solution of ph 2. Phosphate ions, oxygen and water penetrate the protective film through pores, flaws or other weak spots what results in the further corrosion of copper. In order to check the results of action by aggressive solution, the protective layer was removed from surface of copper. The layer was well adhered to the surface of the metal, and the removal of it was really difficult. Therefore ultrasonic water bath was used. The sample was shaken in diluted acetic acid and rinsed in propanol. Figure 5 presents samples after the removal of the inhibiting film for ph 4 (Figure 5(e)) and2(figure 5(f)). However. However, received results indicated that more tight protective layer was forming in solution of ph 4 (Figure 5(e)). Moreover, in phosphate solution the 4MBPBF 4 acts better as the inhibitor in less acidic environment Mechanism of Corrosion Inhibition. Regarding the mechanism the oxygen reduction reaction on copper in acidic solutions a lot of work has been carried out [61 67]. The cathodic global reaction in an aerated aqueous phosphate solution could be described as follows: O 2 +4H + +4e 2H 2 O. However, the first cathodic wave is attributed to reaction: O 2 +2H + +2e H 2 O 2. (4a) (4b) In the more negative potential at the electrode, surface occurs the next reaction: H 2 O 2 +2H + +2e 2H 2 O. (4c) Furthermore, reaction (4a) is strongly influenced by potential [66]. The dissolution process of copper (anodic corrosion reaction) at low overpotentials runs according to the following steps [68 7]: Cu e Cu + ads, Cu + ads e Cu 2+, (5a) (5b) where the Cu + ads is an adsorbed monovalent species of copper at the electrode surface. The inhibition effect of 1-Butyl-4-methylpyridinium tetrafluoroborate on the copper surface could be explained as follows The inhibitor of 4MBPBF 4 can be protonated in acidic solutions: 4MBPBF 4 +H + [4MBPBHF 4 ] +. (5c) Then the inhibitor molecules adsorb through electrostatic interactions between the negatively charged copper surface and positively charged [4MBPBHF 4 ] +. However, the electrode carried the negative charge, therefore [4MBPBHF 4 ] + ions should be first adsorbed directly on copper to probably form a protective layer at active sites: Cu + [4MBPBHF4] + [Cu 4MBPBF 4 ] ads +H +, (5d) and blocks the further oxidation reaction of Cu + ads to Cu 2+ (reaction (5b)). Moreover, the inhibitor molecules lead to the blocking of the transfer of oxygen from the bulk solution to the copper/solution interface that is going to reduce the cathodic reaction of oxygen (reaction (4a)). This indicates that the presence in phosphate solution of 4MBPBF 4 affects both the cathodic and anodic reactions, therefore the, compound acts as a mixed-type inhibitor. The proposed mechanism of corrosion inhibition of copper by 4MBPBF 4 in phosphate solutions (reactions (4a)-(5a)) requires the confirmation through making additional research. However, exhausting information regarding mechanism of corrosion inhibition can be obtained on the basis of thermodynamic measurements Effect of Temperature. The effect of temperature on the corrosion of copper in.5 M PO 3 4 solution in the absence and presence of 1. mm of 1-Butyl-4-methylpyridinium tetrafluoroborate of ph 2 and 4 at temperature ranging from 33 to 343 K was investigated by potentiodynamic polarization measurements. The corrosion parameters and the inhibition efficiency are presented in Table 3.Thecor- rosion potential and cathodic and anodic Tafel slope change similarly in case of low temperature of solutions (Table 1). Therefore, the growth of temperature of solutions does not influence the change of inhibition mechanism. Worth

38 6 International Journal of Corrosion (a) (b) (c) (d) 3 kv 75x (e) 1 μm 3 kv 75x (f) 1 μm Figure 5: SEM micrographs of the surface of copper: (a) before, (b) after being immersed in.5 M PO 3 4 (ph 2) for 24 hours, (c), (d) corrosive solution contained additionally 5. mm of 1-Butyl-4-methylpyridinium tetrafluoroborate of ph 4 and 2, respectively, after removal of the inhibiting film for ph: (e) 4 and (f) 2 (magnification 75x). noticing is that the corrosion current density increases and inhibition efficiency decreases with increasing temperature, which indicates desorption of the inhibitor molecules from the surface of copper with rising temperature of solutions Thermodynamic Activation Parameters. Thermodynamic activation parameters are important to study the inhibitive mechanism. The mechanism of the inhibitor action can be deduced by comparing the apparent activation energies, E a, in the presence and absence of the corrosion inhibitor. Activation parameters such as E a,theenthalpyof activation, ΔH a, and the entropy of activation, ΔS a,were calculated from an Arrhenius-type plot [71, 72]: ( ) Ea j corr = A exp, (6) RT where A is the Arrhenius constant, E a is the apparent activation energy, R is the universal gas constant, and T is the absolute temperature. An alternative formula of the Arrhenius equation is the transition state equation [73]: j corr = ( ) RT exp Nh ( ΔSa R ) exp ( ΔHa RT ), (7) where N is the Avogadro s constant, h is the Planck s constant, ΔS a is the change of entropy for activation, and ΔH a is the change of enthalpy for activation. Plots of ln(j corr ) versuss 1/T,andln(j corr /T)versus1/T give straight lines with slopes of E a /R and 1/TΔH a /R, respectively. The intercepts, which can then be calculated, will be [ln(r/nh)+ (ΔS a /R)] for the Arrhenius and transition-state equations, respectively. Figures 6 and 7 represent the data plots in the absence and presence of 4MBPBF 4 of ph 2 and 4. The calculated thermodynamic activation parameters are listed in Table 4. The values of E a and ΔH a in the presence of 1. mm 4MBPBF 4 are higher than those in black solutions, indicating that more energy barrier for the reaction in the presence of 4MBPBF 4 is attained, especially in case of ph 4. This shows that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution of copper [74]. The entropy of activation, ΔS a, in the absence and presence of 4MBPBF 4 is large and negative (especially with ph 4), implying that the rate-determining step for the activated complex is the association rather than the dissociation step, which means that a decrease in disordering takes place by going from reactants to the activated complex [75] Adsorption Isotherm. It has been assumed that inhibitor molecules establish their inhibition action via the adsorption of the inhibitor onto the metal surface. The adsorption processes of inhibitors are influenced by the chemical

39 International Journal of Corrosion 7 Table 3: Corrosion parameters and inhibition efficiency of copper electrode in.5 M PO 3 4 solutions in the absence or presence of 1. mm of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) of ph 2 and 4 at different temperatures. Inhibitor ph Temperature (K) E corr (mv) j corr (μacm 2 ) b c (mv dec 1 ) b a (mv dec 1 ) IE (%) Blank MBPBF Blank MBPBF ln jcorr (A cm 2 ) (a) (c) (b) (d) ln(jcorr/t)(acm 2 K 1 ) (a) (c) (b) (d) /T (K 1 ) /T (K 1 ) Figure 6: Arrhenius plots for copper in.5 M PO 3 4 solutions containing: (a), (c) and (b), (d) 1. mm of 1-Butyl-4- methylpyridinium tetrafluoroborate. The ph of solutions was the following: (a), (b) 2 and (c), (d) 4. Figure 7: Transition state plots for copper in.5 M PO 3 4 solutions containing: (a), (c) and (b), (d) 1. mm of 1-Butyl-4- methylpyridinium tetrafluoroborate. The ph of solutions was the following: (a), (b) 2 and (c), (d) 4. structure of organic compounds, the nature and surface change of metal, the distribution of charge in molecule and the type of aggressive media [76]. The adsorption isotherm can provide the basic information on the interaction between the inhibitor and the metal surface, which depends on the degree of surface coverage, Θ [77]. The values of surface coverage for different concentrations of inhibitor in.5 M PO 3 4 solutions of ph 2 and 4 were evaluated from polarization curves according equation Θ = 1 j corr. (8) j o

40 8 International Journal of Corrosion Table 4: Thermodynamic activation parameters for copper in.5 M PO 3 4 solutions in the absence or presence of 1. mm of 1- Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 )forph2 and 4. Inhibitor ph E a (kj mol 1 ) ΔH a (kj mol 1 ) ΔS a (J mol 1 K 1 ) Blank MBPBF Blank MBPBF c/θ (mm) (a) (b) Table 5: Surface coverage of copper electrode in.5 M PO 3 4 solutions for different concentrations of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 )forph2and4. Concentration ph (mm), 1-Butyl-4-methylpyridinium tetrafluoroborate Table 6: Slope (b), linear correlation coefficient (R 2 ), equilibrium constant (K), and standard free energy of adsorption (ΔG ads) in.5 M PO 3 4 of 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) solutions of ph 2 and 4. ph b R 2 K(M 1 ) ΔG ads (kj mol 1 ) The values of the degree of surface coverage are listed in Table 5. It can be seen that the values of Θ increased with an increase in the concentration of 4MBPBF 4. It is also worth to notice that the degree of surface coverage is higher in case of solutions of ph 4 (Table 5). Using these values of Θ, different adsorption isotherms can be used to deal with the experimental data. The Langmuir adsorption isotherm [78, 79] was applied to investigate the adsorption of 4MBPBF 4 on copper surface given by the following equation: c Θ = 1 + c, (9) K where K is the adsorption equilibrium constant and c is the concentration of inhibitor. Figure 8 represents the adsorption plots of 1-Butyl-4- methylpyridinium tetrafluoroborate on copper. It should be explained that other adsorption ishotherms (Frumkin and Temkin) were checked. The linear correlation coefficient was used to choose the isotherm that best fits the experimental data. It should be noted that the data fits the straight line with a slope nearly equal unity with linear correlation coefficient higher than.999 (Table 6) indicating that these inhibitors adsorb according to the Langmuir adsorption isotherm c (mm) Figure 8: Adsorption isotherm of 1-Butyl-4-methylpyridinium tetrafluoroborate on the copper surface in.5 M PO 3 4 solutions of ph: (a) 2 and (b) 4. The nature of corrosion inhibition has been deduced in terms of the adsorption characteristics of the inhibitor. The metal surface in aqueous solution is always covered with adsorbed water dipoles. The adsorption of inhibitor molecules from aqueous solution is a quasi-substitution process between the organic compounds in the aqueous phase and water molecules at the electrode surface [8]. The Langmuir isotherm is based on the assumption that each site of metal surface holds one adsorbed species: 4MBPBF 4(sol) +H 2 O (ads) 4MBPBF 4(ads) +H 2 O (sol). (1) In this situation, the adsorption of of one molecule of 4MBPBF 4 is accompanied by desorption one molecule of H 2 O from the surface of copper. This kind of isotherm involves the assumption of no interaction between the adsorbed species on the metal surface. A graph c/θ against c leads to values of K, as the equilibrium constant of the adsorption process (Figure 8). Thefreeenergiesofadsorption,ΔG ads were calculated from the adsorption equilibrium constant using the equation [81]: ΔG ads = RT ln(55.5 K), (11) where value 55.5 is the molar concentration of water in the solution. The adsorption equilibrium constant and the standard free energy of adsorption of 4BMPBF 4 for solutions of ph 2 and 4 on copper are presented in Table 6. The values of K are relatively low, meaning that interactions between 1-Butyl-4- methylpyridinium tetrafluoroborate and the metal surface are weaker. The negative values of ΔG ads mean that the adsorption of 4MBPBF 4 on copper surface is a spontaneous process, and indicates the strong interaction between the inhibitor molecules and the copper surface [82]. Generally, values of ΔG ads around 4 kj mol 1 or lower are consistent with the electrostatic interaction between the

41 International Journal of Corrosion 9 Table 7: Thermodynamic adsorption parameters for copper in.5 M PO 3 4 solutions in the presence of 1. mm of 1-Butyl-4- methylpyridinium tetrafluoroborate (4MBPBF 4 )forph2and4at different temperatures. ph Temperature (K) 2 4 K 1 2 (M 1 ) ΔG ads (kj mol 1 ) ΔH ads (kj mol 1 ) ΔS ads (J mol 1 K 1 ) charged molecules and the charged metal surface (physisorption) [83 85]. For investigated inhibitor the values of ΔG ads equal 23.6 and 24.7 kj mol 1 for solutions of ph 2 and 4, respectively (Table 6). The results indicate the 4MBPBF 4 to be physically adsorbed on the copper surface. The adsorption of the inhibitor at the metal surface is the first step in the action mechanism of inhibitors in aggressive acid media. The adsorption of 1-Butyl-4-methylpyridinium tetrafluoroborate on the copper surface makes a barrier for mass and charge transfers. This situation leads to the protection of the copper surface against the attack of aggressive solution Thermodynamic Adsorption Parameters. Thermodynamically, the free energy of adsorption, ΔG ads, is related to the standard enthalpy, ΔH ads and entropy, ΔS ads of the adsorption process as follows [86, 87]: ΔG ads = ΔH ads TΔS ads. (12) Moreover, the standard enthalpy of adsorption could be calculated according to the Van t Hoff equation ln K = ΔH ads +const. (13) RT The adsorption equilibrium constant is related to the degree of surface coverage by: Θ K = c(1 Θ). (14) It should be noted that thek decreases with increasing temperature, (Table 7). This confirms earlier made admission that the molecules of 4MBPBF 4 are physically adsorbed on surface of copper. However, desorption process of inhibitor enhances with raising of the temperature of the solution. The free energies of adsorption of 1-Butyl-4-methylpyridinium tetrafluoroborate were calculated at different temperatures (11)andaregiveninTable 7. The values of ΔG ads are around 2 kj mol 1 indicating that the adsorption mechanism ln K (M 1 ) (b) /T (K 1 ) Figure 9: The Van t Hoff plots for the copper in.5 M PO 3 4 solutions containing 1. mm of 1-Butyl-4-methylpyridinium tetrafluoroborate. The ph of solutions was as the following: (a) 2 and (b) 4. of 4MBPBF 4 in.5 M PO 3 4 solution of ph 2 or 4 is physisorption at the studied temperatures. A plot ln K versus 1/T gives of straight lines, as shown in Figure 9. The slope of the straight line is ΔH ads/r. The values of the standard enthalpy are given also in Table 7. The ΔH ads values are negative, for that reason adsorption of 1-Butyl-4-methylpyridinium tetrafluoroborate molecules onto the Cu surface is an exothermic process. Moreover, the values of ΔH ads are less than 4 kj mol 1 [88], therefore, once again implying that in investigated solutions, physical adsorption is taking place. The standard entropy of inhibitor adsorption, ΔS ads can be calculated from (12). The values of ΔS ads, are recorded in Table 7. The positive values of ΔS ads mean that the increase in disordering takes place by going from reactants to the Cu/solution interface, which is the driving force for the adsorption of 1-Butyl-4-methylpyridinium tetrafluoroborate onto the copper surface [89]. 4. Conclusion The following results can be drawn from this study. (1) The investigated 1-Butyl-4-methylpyridinium tetrafluoroborate (4MBPBF 4 ) exhibits inhibiting properties for the corrosion of copper in.5 M PO 3 4 solutions of ph 2 and 4. (2) The inhibition efficiency (IE(%)) increased with the increase in inhibitor concentration but decreases with increasing temperature. IE at all concentrations of 4MBPBF 4 followed the order of ph: 4 > 2. (3) The of 1-Butyl-4-methylpyridinium tetrafluoroborate acts as a mixed-type inhibitor, independently from ph solutions. (a)

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45 Hindawi Publishing Corporation International Journal of Corrosion Volume 211, Article ID , 13 pages doi:1.1155/211/ Research Article The Effect of Ionic Liquids on the Corrosion Inhibition of Copper in Acidic Chloride Solutions M. Scendo and J. Uznanska Institute of Chemistry, UJK Kielce, ul. Swietokrzyska 15G, 2546 Kielce, Poland Correspondence should be addressed to M. Scendo, Received 19 August 21; Accepted 21 December 21 Academic Editor: Willem J. Quadakkers Copyright 211 M. Scendo and J. Uznanska. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The influence of the concentration of the 1-Butyl-3-methylimidazolium chloride (BMIMCl) and 1-Butyl-3-methylimidazolium bromide (BMIMBr) as ionic liquids (ILs) on the corrosion inhibition of copper in 1. M Cl solutions of ph 1. was studied. The investigation involved electrochemical polarization methods as well as electrochemical quartz crystal microbalance (EQCM) technique and scanning electron microscopy (SEM). The inhibition efficiency increases with an increase in the concentration of BMIMCl and BMIMBr. Adherent layers of inhibitors were postulated to account for the protective effect. Both of the compounds act as a mixed-type inhibitor. The values of standard free energy of adsorption suggest the chemical adsorption BMIMCl and BMIMBr on the copper surface. 1. Introduction Copper and its alloys are used extensively in many kinds of chemical equipment. Several corrosion inhibitors for copper and its alloys have been known and applied for corrosion protection. The inhibition properties of triazole, imidazole, and thiazole derivatives in the corrosion of copper have been studied [1 4]. Some heterocyclic compounds containing a mercapto group have been developed as copper corrosion inhibitors. These compounds include 2-mercaptobenzothiazole [5], 2,4-dimercaptopyrimidine [6], 2-amino-5-mercaptothiadzole, 2-mercaptothiazoline [7], potassium ethyl xanthate [8 11], and indole and derivatives [12]. Among the numerous organic compounds tested and industrially applied as corrosion inhibitors, nontoxic are far more strategic now than in the recent past. These compounds include such amino acids [13 15] andderivativesascysteine[16]. In the past two decades, the research in the field of green corrosion inhibitors has been addressed toward the goal of using cheap effective molecules at low or zero environmental impact. These compounds include purine and adenine, which have been tested for copper corrosion in chloride [17, 18], sulfate [19], and nitrate solutions [2]. Ionic liquids (ILs) are molten salts with melting points at or below ambient room temperature, which are composed of organic cations and various anions. Ionic liquids possess a large number of physicochemical properties [21 24], mainly, good electrical conductivity, solvent transport, and a relatively wide electrochemical window [24]. Configuration of ILs consists of an amphiphilic group with a long chain, hydrophobic tail, and a hydrophilic polar head. Usually, ILs have nitrogen, sulphur, and phosphorus as the central atoms of cations, such as imidazolium, pyrrolidinium, quaternary ammonium, pyridinium, piperidinium, sulfonium and quaternary phosphonium. Currently, functionalized IL is a very noticable topic in the field of IL research. Introducing different functional groups into cations, which provides a great deal of ILs with new structures, can markedly change the physicochemical properties of ILs, and it also affords more choices for applications of ILs in electrochemical devices. Imidazolium compounds are reported to show corrosionresistant behavior on mild steel [25], copper [26, 27], and aluminium [28]. It was found that the action of such inhibitors depends on the specific interaction between the functional groups and the metal surface, due to the presence

46 2 International Journal of Corrosion Cl H H H H H C C 1 N H H C C H H H C N + C H H C C H H H 3 2 H H 1 H H C H N C + C N H C H C H C H H Br H H C H H H 3 2 (a) (b) Figure 1: Molecular structures ionic liquids: (a) 1-Butyl-3-methylimidazolium chloride (BMIMCl), (b) 1-Butyl-3-methylimidazolium bromide (BMIMBr). j (μa cm 2 ) (a) (b) (c) (d) E (mv) versus SCE(NaCl) Figure 2: Some chosen polarization curves of copper in 1. M Cl solutions containing different concentrations of 1-Butyl-3- methylimidazolium bromide: (a), (b) 1., (c) 1., and (d) 5. mm, ph 1.. The scan rate of 1 mv s 1. Arrows indicate potential for chronoamperometric measurements. 4 η (%) BMIMCI BMIMBr 1 5 c (mm) Figure 3: Inhibition efficiency of copper in 1. M Cl solutions with different concentrations of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide. 1 5 of the C=N group and electronegative nitrogen in the molecule. Ionic liquids and different types of surfactants base inhibitors are well known to have a high activity in acid medium [29, 3] and therefore are used in oil field to minimize carbon dioxide-induced corrosion [31, 32]. Among many kinds of functionalized ionic liquids ether-functionalized ILs have been investigated intensively, and ether groups have been successfully introduced into imidazolium cations [33 39].Shi et al. [4] have synthesized a series of new imidazolium ionic liquids with highly pure naphthenic acids and investigated the relationship between the alkyl connecting with N 3 of imidazolium ring and corrosion inhibition performance in acidic solution. The inhibition efficiency (η) was found to increase with increasing the carbon chain length of the alkyl connecting with N 3 of imidazolium ring. However, no substantial information is available on imidazolium ionic liquids using as corrosion inhibitors in acidic chloride solutions.

47 International Journal of Corrosion 3 j (μa cm 2 ) (a) (b) (e) (c) (d) 9 12 t (s) E = 35 mv E = 1 mv E = 25 mv Figure 4: Chronoamperometric curves for copper in 1. M Cl solutions: (a) and(d) without,(b), (c) and(e) with 5. mm 1- Butyl-3-methylimidazolium chloride. Potential electrode: (a), (b) +35, (c), (d) 25, and (e) +1 mv. Δm (μgcm 2 ) (b) (a) E (mv) versus SCE(NaCl) Figure 5: Mass change of copper electrode with potential in 1. M Cl solutions: (a) withoutand(b) with 5. mm 1-Butyl- 3-methylimidazolium bromide. The scan rate of 1 mv s 1.Arrows indicate of direction of potential scanning. The present work describes a study of the corrosion of copper in 1. M Cl solutions of ph 1. without and with different concentrations of 1-Butyl-3-methylimidazolium chloride (BMIMCl) or 1-Butyl-3-methylimidazolium bromide (BMIMBr), based on copper stationary disk electrode (SDE) voltammetry measurements as well as quartz crystal microbalance (EQCM) and scanning electron microscopy (SEM) Experimental 2.1. Solutions. 1-Butyl-3-methylimidazolium chloride, C 8 H 15 ClN 2 (BMIMCl) and 1-Butyl-3-methylimidazolium bromide, C 8 H 15 BrN 2 (BMIMBr) (>99.8%) were purchased from Fluka. The molecular structures of compounds are shown in Figure 1. It is worth to notice that both BMIMCl and BMIMBr are not flat molecules. BMIMCl and BMIMBr are stable in air, water, and in the majority of organic solvents. Both compounds are enough well solvable in water. The solutions were prepared using analytical grade reagents and bidistilled water (resistivity 12 MΩ cm). BMIMCl and BMIMBr were dissolved at concentrations in the range of.1 5. mm. All studied solutions contained 1. M Cl of ph 1.. The solutions were prepared through the mixing up of suitable quantities of NaCl and HCl for all experiments were used a naturally aerated solutions Electrochemical Measurements. Electrochemical experiments were carried out in a classical three-electrode glass cell. The cell was open to air. The working electrode (W)was home made stationary disk electrode of the Specpure copper (Johnson Matthey Chemicals Ltd.) with r =.24 cm, A =.181 cm 2. Prior to each experiment, the W was mechanically abraded to mirror gloss using in this aim 1 and 2 grade emery papers. Then electrode was washed several times on change bidistilled water and ethanol. Finally, SDE was dried using a stream of air. Such pretreatment of the disk was repeated after each voltammetric measurement. Other details were published in [12, 41 44]. All the surface area-dependent values are normalized with respect to the geometric surface area of the working electrode. Electrode potentials were measured and reported against the external saturated calomel electrode with NaCl solution (SCE(NaCl)) coupled to a fine Luggin capillary. To minimize the ohmic contribution, the capillary was kept close to the working electrode. A platinum (purity 99.99%) wire was used as an auxiliary electrode. Auxiliary electrode was individually isolated from the test solution by glass frit. All voltammetric experiments were performed using a Model EA9C electrochemical analyzer, controlled via Pentium computer using the software Eagraph v. 4.. The polarization curves were obtained using the linear potential sweep (LSV) technique. Before each run, the clean copper SDE was quickly inserted into the solution and immediately cathodically polarized at 8 mv (SCE(NaCl)) for 3 min to reduce any oxide on the copper surface. The scan started from the cathodic to the anodic direction with the scan rate of 1 mv s 1. The chronoamperometric curves were recorded at the different potentials electrode in solutions without and with inhibitors. The potentials applied for the copper electrode were chosen on basis of polarization curves. During the measurements, the solution was not stirred. Electrochemical experiments were repeated many times, until reproducible curves were received Electrochemical Quartz Crystal Microbalance. Electrochemical quartz crystal microbalance (EQCM) experiments

48 4 International Journal of Corrosion (a) (b) (c) (d) 3 kv 75x 1 μm (e) Figure 6: SEM micrographs of the surface of copper: (a) before, (b) after being immersed for 24 hours in 1. M Cl ph 1., (c) corrosive solution contained additionally 5. mm of 1-Butyl-3-methylimidazolium chloride and after the removal of the inhibiting film: (d) 1-Butyl- 3-methylimidazolium chloride, (e) 1-Butyl-3-methylimidazolium bromide (magnification 75x). were performed with an apparatus constructed in the Institute of Physical Chemistry, Warsaw. The quartz crystal had a geometric area of.432 cm 2 and was operated at the fundamental frequency of 5 MHz (refers to air). The sensitivity of the EQCM can be a few nanograms per square centimeter, which makes it an ideal equipment for corrosion studies. Other details of the EQCM system used in this study were similar to those previously described [11, 12, 44]. Copper was galvanostatically deposited onto one surface of the crystal (resonator). The freshly deposited copper electrode was thoroughly washed with bidistilled water. Then the aggressive solution was immediately added to the cell Scanning Electron Microscope. A scanning electron microscope PHILIPS (XL 3) was used to study the morphology of the copper surface in the absence and presence of the inhibitor. Samples were attached on the top of an aluminum stopper by means of 3 M carbon conductive adhesive tape (SPI). All experiments were carried out at 25. ±.2 Cusingan air thermostat with the forced air circulation. 3. Results and Discussion 3.1. Polarization Behaviour of Copper. The effect of 1-Butyl- 3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide on the corrosion reactions of copper was determined by polarization measurements. Figure 2 shows an example of polarization curves for the copper electrode in 1. M Cl solutions of ph 1. without and with different concentrations of BMIMBr. Similar curves were recorded for BMIMCl. Regarding the mechanism of the oxygen reduction reaction on copper in acidic solutions a lot of work has been done [45 48]. The cathodic polarization curve (a) maybe attributed to the diffusion-controlled reduction of oxygen. It is also worth emphasizing that the curve (a) includes two faintly visible waves, which do not appear for deoxidized solutions [17, 18]. The cathodic global reaction in an aerated acidic aqueous solution could be described as follow [49 51]: O 2 +4H + +4e 2H 2 O. (1a) However, the first cathodic wave is attributed to the reaction O 2 +2H + +2e H 2 O 2. (1b)

49 International Journal of Corrosion 5 (c/θ) (mm) (b) (a) 3 c (mm) Figure 7: Langmuir s adsorption plots for copper in 1. M Cl solutions containing different concentrations of inhibitors: (a) 1-Butyl-3-methylimidazolium chloride and (b) 1-Butyl-3- methylimidazolium bromide. In the more negative potential of the electrode surface the next reaction occurs: 4 H 2 O 2 +2H + +2e 2H 2 O. 5 6 (1c) Furthermore, the reaction (1a) is strongly influenced by potential [5]. The cathodic parts, curves (b) (d), show that in the presence of 1-Butyl-3-methylimidazolium bromide (similarly as in the case of 1-Butyl-3-methylimidazolium chloride) the cathodic currents decrease as the BMIMCl or BMIMBr concentrations increase. However, the curves (b) (d) split up into two waves, which correspond to reactions (1b) and(1c). As it is discussed below, this shoulder may concern to the adsorption of the BMIMCl or BMIMBr on copper surface. After crossing potential at about the 3 mv, (SCE(NaCl)) was observed of the growth of density of current as a result of the formation of gas hydrogen on the electrode surface. The dissolution process of copper (anodic corrosion reaction) at low overpotentials runs according to the following steps [15, 52]: Cu e Cu + ads Cu + ads e Cu 2+, (2a) (2b) where the Cu + ads is an adsorbed monovalent species of copper on the electrode surface. In corrosive medium in presence of complexing ions such as Cl the dissolution process of Cu proceeds via a two-step reaction mechanism [17, 18]. During the first step, copper is ionized under the influence of Cl ion, yielding CuCl adsorbed at the electrode Cu + Cl e CuCl ads. (3) This adsorbed compound dissolves by combining with another Cl ion according to reactions or CuCl ads +Cl e CuCl 2,sol CuCl ads +Cl CuCl 2,sol. (4a) (4b) Products as a result of reactions (4a)and(4b)movetobulkof solution. However, in 1. M, the Cl concentration range of CuCl 2 is the dominant cuprous species [1 12, 53], while at higher concentrations the reaction is proportional to [Cl ] x, where x>2[54]. Figure 2 shows that the cathodic and anodic currents decrease with the increase of the concentration of BMIMBr (curves (b) (d)). Probably the protonated 1-Butyl-3- methylimidazolium chloride [BMIMClH + ] and 1-Butyl-3- methylimidazolium bromide [BMIMBrH + ] molecules are electrostatically adsorbed on the cathodic sites of Cu. Cations of ILs are large; in addition, the alkyl chain covers a wide part of the copper surface [55]. However, the hydrophobic chain may be oriented horizontally or vertically to the electrode plane. The adsorption or desorption of Cl and Br ions occurs on the anodic sites. The adsorbed species such as CuCl ads interact with the cations of ILs to form molecular layers as a complexonthecoppersurface [56]. This indicates that the addition of BMIMCl and BMIMBr affects both the cathodic and anodic reactions; therefore, the compounds act as mixed-type inhibitors Corrosion Parameters. Electrochemical corrosion kinetic parameters were calculated on the basis of cathodic and anodic potential versus current characteristics in the Tafel potential region (Figure 2). The corrosion parameters such as corrosion potential (E corr ), corrosion current density (j corr ), cathodic (b c ), and anodic (b a ) Tafel slope are listed in Table 1. It is worth noticing that no definite trend was observed in the shift of the E corr values in the presence of various concentrations of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide. This confirms earlier advanced admission that both inhibitors belong to mixed-type inhibitors. The current density (Table 1) decreased when the concentrations of BMIMCl and BMIMBr were increased. This indicates the inhibiting effect of 1-Butyl-3-methylimidazolium chloride and 1- Butyl-3-methylimidazolium bromide. An increase in cathodic (b c ) and a decrease in anodic (b a ) Tafel slopes (Table 1) indicated a mixed cathodic and anodic effect of the inhibition on the copper corrosion mechanism. Moreover, these inhibitors cause small change in the cathodic and anodic Tafel slopes, indicating that BMIMCl and BMIMBr are first adsorbed onto copper surface and therefore impeded by merely blocking the reaction sites of copper surface without affecting the cathodic and anodic reaction mechanism [57]. Generally speaking, the inhibitor molecule blocks the active corrosion sites on the copper surface.

50 6 International Journal of Corrosion Charge Z N HOMO N Y X (a) (b) LUMO (c) Figure 8: Molecular structure of 1-Butyl-3-methylimidazolium ion (the charges density distribution on nitrogen: N 1,N 3 and carbon: C 2, C 4,C 5 atoms), and molecular orbital density distribution of BMIM + ; highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) Inhibition Efficiency. The inhibition efficiency (η) can also be calculated from polarization tests by using the following equation [58, 59]: ( ) jo j corr η (%) = 1, (5) where j o and j corr are the corrosion current densities in the absence and presence of inhibitor, respectively. The inhibition efficiency dependson both the nature and the concentration of the investigated compounds. The calculated inhibition efficiencies are presented in Figure 3. In the presence of 1-Butyl-3-methylimidazolium chloride and 1-Butyl-3-methylimidazolium bromide, the inhibition efficiency increases with an increase in for the concentration of inhibitors. However, for concentration 5. mm both inhibitors η are the highest for 1-Butyl-3-methylimidazolium j o chloride. It is obvious that for higher concentration of 1- Butyl-3-methylimidazolium bromide we will get a film which considerably better protects the surface of copper Corrosion Rate. The corrosion current density (j corr ) was converted into the corrosion rate (k r ) by using the expression [6] k r ( mm year ) ( ) = jcorr M Cu, (6) nρ where the corrosion current density (j corr ) should be in μacm 2, M Cu is the molecular weight of copper, n is the number of electrons transferred in the corrosion reaction, and ρ is the density of Cu (g cm 3 ). The values of the copper corrosion rate in the absence and presence of inhibitors were calculated from (6) and