1. Introduction. 2. Experimental

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1 Cationic surfactant as corrosion inhibitor for aluminum in acidic and basic solutions Chemistry Department, Jordan University of Science & Technology, Irbid, Jordan Abstract Purpose To investigate the inhibiting effect of the cationic surfactant cetyl trimethylammonium chloride (CTAC) on aluminum (Al). Design/methodology/approach Pure aluminum rods were immersed in hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions for weightloss tests and potentiostatic polarization measurements. Findings The inhibition action depends on the concentration of the inhibitor, the concentration of the corrosive media, and the temperature. The inhibition efficiency in NaOH was higher than that in HCl solutions. In both acidic and basic media, the increase in temperature resulted in a decrease of the inhibition efficiency and a decrease in the degree of surface coverage. The results were indicative of increased aluminum dissolution with increasing temperature. It was found that adsorption of CTAC on the aluminum surface follows Temkin s isotherm in HCl and Langmuir s isotherm in NaOH. Originality/value Clarifies the effects of concentration and temperature on the inhibition efficiency of a cationic surfactant on aluminum. Keywords Surfactants, Corrosion inhibitors, Non-ferrous metals Paper type Research paper 1. Introduction Aluminum has a remarkable economic and industrial importance due to its low cost, lightweight, high thermal and electrical conductivity. Evaluation of corrosion inhibitors for aluminum in acidic and basic media is important because of the widespread use of aluminum in contact with the number of corrosive environments. Many researches are devoted to study the corrosion of aluminum in different aqueous solutions (Brett, 1992; Beck, 1988). Hydrochloric acid solutions are generally used as pickling acid for aluminum or for chemical and electrochemical etching of aluminum foils. Its function is to remove undesirable oxide coatings and corrosion products. To prevent the attack of acid, it is very important to add a corrosion inhibitor to decrease the rate of aluminum dissolution in such solutions. Thus, numerous studies concerning the inhibition of aluminum corrosion using organic substances are conducted in acidic and basic solutions (Maayta and Al-Rawashdeh, 2004; Ebenso et al., 2003; Bereket et al., 2004; Hassan et al., 1990; Zhao and Mu, 1999). The major currently applied inhibitor compounds for several metals are quaternary ammonium salts, organic amine derivatives, organic phosphates, succinimides and imidazoline derivatives (Branzoi et al., 2000; Bereket et al., 2004; Mahmoud and El-Mahdy, 1997). The protective action of an inhibitor in metal corrosion is often associated with chemical or physical adsorption involving a variation in the charge of the adsorbed substance The Emerald Research Register for this journal is available at The current issue and full text archive of this journal is available at 52/3 (2005) q Emerald Group Publishing Limited [ISSN ] [DOI / ] and transfer of charge from one phase to the other. Special attention is paid to the electron density on the atom or the group responsible for adsorption. Most of the efficient acid inhibitors are organic compounds that contain mainly nitrogen, sulphur or oxygen atoms in their structure. Owing to the presence of unshared electron pairs, those atoms present at the centre are for chemisorption processes. Despite the large number of organic compounds, there are always needs for developing new organic corrosion inhibitors (Ajmal et al., 1994). Quaternary ammonium salts and cationic surfactants have been used frequently as inhibitors against acid corrosion of iron and steel (Elachouri et al., 1995; Osman, 1998). However, very little work has yet been reported on cationic surfactants as inhibitors for aluminum corrosion in acidic and basic media. The present study aimed to investigate the efficiency of CTAC (CH 3 (CH 2 ) 15 N þ (CH 3 ) 3 Cl 2 ), as a corrosion inhibitor for aluminum in acidic and basic media. An attempt was also made to clarify the effects of concentration and temperature on the inhibition efficiency (%I) of studied inhibitor. 2. Experimental 2.1 Sample and solutions Pure aluminum (99.95 percent) was supplied in the form of rods (Aldrich). The organic inhibitor CTAC (Aldrich) was used as received. All solutions were made up from Analar hydrochloric acid and NaOH pellets in triple distilled water. Before measuring, all tested aluminum samples were mechanically polished using emery paper, degreased with acetone, washed with distilled water, dried in alcohol and ether, and then weighed using Electronic Semimicro Balance Sartorius 2024 MP6 with precision of ^0.001 mg. 2.2 Weight loss measurements The weight loss experiments were carried out using metallic aluminum rods having the dimensions (length ¼ 30 mm; diameter ¼ 3mm; exposed total area ¼ 2:97 cm 2 ), weighed 160

2 and immersed for 1 h in 10 ml of Analar HCl or NaOH solutions (0.5, 1.0, 1.5 M) containing different concentrations of additive ( ppm). The measurements of the corrosion rate were performed in 0.2 M HCl and NaOH aqueous solution with different concentrations of inhibitor ( ppm). After testing, all samples were rinsed thoroughly with triply distilled water, dried, and weighed again. The measurements were carried out at 30, 40, and 508C using a water thermostat controlled to ^0.58C. Figure 1 The weight loss curves of aluminum with the addition of the surfactants in 1.5 M HCl (A) and 1.5 M NaOH (B) at various temperatures 30, 40 and 508C 2.3 Polarization measurements Potentiostatic polarization studies were carried out using EG&G model 264 potentiostat/galvanostat. A conventional three-electrode cell consisting of aluminum as working electrode (a cylindrical rod embedded in araldite with exposed surface area of 0.6 cm 2, was used), saturated calomel and platinum were used as a reference and auxiliary electrodes, respectively. 3. Results 3.1 Weight loss measurements The weight loss curves of the aluminum rods with addition of the inhibitor in 1.5 M HCl and 1.5 M NaOH at various temperatures are shown in Figure 1. In both HCl and NaOH, the weight loss of aluminum decreased with increasing inhibitor concentration. The weight loss of the aluminum rods in the presence of the tested inhibitor (CTAC) at various concentrations ( ppm) was determined in different concentrations of HCl and NaOH and various temperatures. At each concentration of the inhibitor, the percentage inhibition efficiency (%I) is calculated by: %I ¼ W u 2 W i W u 100 percent ð1þ where W i and W u are the weight loss with and without the inhibitor, respectively. Figure 2 shows the variation in inhibition efficiency (%I) of CTAC according to its concentration ( ppm) in various HCl (Figure 2(A)) and NaOH (Figure 2(B)) concentrations at 308C. The results show less inhibition efficiency of CTAC in HCl than in NaOH. The corrosion rate of aluminum is determined by using the relation: W corr ¼ Dm ð2þ St where Dm is the mass loss, S is the area and t is the immersion period. Table I shows the corrosion rate and the percentage inhibition efficiency of the inhibitor (CTAC) at various concentrations ( ppm) in 0.2 M HCl and NaOH solutions in an immersion period 1 h, at 408C. The results showed that inhibition efficiency increased as the concentration of inhibitor increased from 50 to 550 ppm. The maximum inhibition efficiency was observed to be 65 percent in NaOH. The tested compound showed a slight decrease in the inhibition efficiency (%I) with increasing temperature (Figure 3), but the inhibitive effect still persisted even at higher temperature (508C). Figure 4 shows the variation of inhibition efficiency of 200 ppm CTAC in 0.2 M and 0.5 M HCl at different temperatures. This behavior indicates that the adsorbed molecules formed a barrier film on the aluminum surface (Metikoš-Huković et al., 1994). 3.2 Adsorption isotherms The experimental data for the tested inhibitor in HCl have been applied to different adsorption isotherm equations. The best correlation among the experimental results obtained from weight loss measurements from the adsorption of CTAC on aluminum surface fitted Temkin adsorption isotherm, which is given by: u ¼ð1=f Þ lnði 0 CÞ where u is the surface coverage of the metal surface by adsorbate which is defined as: ðw u 2 W i Þ=W u ; C is the bulk concentration of the inhibitor, I 0 is the adsorption equilibrium constant, and f is the heterogeneous factor of the metal surface. Figure 5 shows a typical Temkin adsorption isotherm of CTAC in HCl solution. The parameter f is a heterogeneous factor that gives the variation of adsorption energy with coverage. The values of I 0 and f for CTAC were ð3þ 161

3 Figure 2 Effect of CTAC concentration on inhibition efficiency of Al corrosion in various concentrations of HCl (A) and NaOH (B) at 308C Figure 3 Variation of inhibition efficiency with CTAC concentration in 0.5 M HCl at 30, 40 and 508C Figure 4 Variation of inhibition efficiency of CTAC with temperature in 0.5 M and 0.2 M HCl with 200 ppm inhibitor concentration Figure 5 Variation in surface coverage (u) with the logarithm of CTAC concentration in various concentrations of HCl at 308C Table I Corrosion rate (mg/cm 2 h), percentage inhibition efficiency (I%) of inhibitors, and the I%/[CTAC] ratio at various concentrations of CTAC in 0.2 M of HCl and NaOH at 408C NaOH HCl I%/[CTAC] [CTAC] (ppm) W corr I% W corr I% NaOH HCl Blank

4 calculated to be and 4.0, respectively. This result is an opposite to the order of inhibition efficiency. In all cases the value of f were greater than zero, indicating a Temkin adsorption isotherm. The obtained data from weight loss measurement in NaOH solution fitted Langmuir s adsorption isotherm which is given by: Figure 7 Dependence of surface coverage (u) on temperature at various CTAC concentrations in 0.5 M HCl (A) and 0.5 M NaOH (B) u=ð1 2 uþ ¼KC ð4þ where K is the equilibrium constant of the adsorption process and C the bulk concentration of the inhibitor. Figure 6 shows a typical Langmuir adsorption isotherm for CTAC in various concentrations of NaOH. From the straight line, an equilibrium constant, K, for the adsorption process can be obtained. The equilibrium constant of the adsorption process, K, is related to the free energy of adsorption, DG ads, by (Bereket et al., 2004): K ¼ð1=55:5Þ expð2dg ads =RTÞ ð5þ where 55.5 is the molar concentration of water in the solution. The values of DG ads obtained from Figure 6 in presence of 0.5, 1.0, and 1.5 M of NaOH are , , and kj/mol, respectively. Figure 7 shows the dependence of surface coverage (u) on temperature at various concentrations of CTAC in 0.5 M HCl and 0.5 M NaOH solutions. The surface coverage (u) decreased with increasing temperature as a result of increase in the desorption process of the inhibitor molecules with rising temperatures. 3.3 Potentiostatic polarization Anodic and cathodic polarized potentials were measured in the absence and presence of inhibitors in a current density range of ma/cm 2. Figure 8 shows the anodic and cathodic polarization curves for inhibitors as a representative curve in 0.5 M HCl and in 0.5 M NaOH solutions in the presence and absence of inhibitors at optimized concentration. It is evident from Figure 8 that cathodic curves are much more polarized than anodic curves. Thus the cathodic sites are blocked to a greater extent than the anodic sites by the inhibitor molecules. Figure 6 Variation of logðu=1 2 uþ versus log [CTAC] for the inhibition of aluminum in various concentrations of NaOH at 308C The electrochemical parameters, corrosion potential (E corr ), corrosion rate (I corr ), anodic Tafel constant (b a ), cathodic Tafel constant (b c ), and inhibition efficiency (I%) were calculated from the curves of Figure 8 and summarized in Tables II and III. 4. Discussion 4.1 Weight loss measurements The curves in Figure 1 show that the weight loss values (mg) of aluminum in 1.5 M HCl solution (Figure 1(A)) containing CTAC decreased as the concentration of the inhibitor increased, i.e. the corrosion inhibition strengthened with the increase of the surfactant concentration. The trend of CTAC in 1.5 M NaOH solution (Figure 1(B)) is similar to that observed in HCl, but with more inhibition efficiency. This trend may result from the fact that adsorption amount and the coverage of surfactants on the aluminum surface increases with the increase of the concentration, thus the aluminum surface is efficiently separated from the medium (Zhao and Mu, 1999). Also, the curves in Figure 1 show that when the concentration of the tested surfactant reaches approximately, 163

5 Figure 8 Anodic and cathodic polarization curves of aluminum for CTAC in 0.5 M HCl (A) and in 0.5 M NaOH (B). [Inhibitors] ¼ 150, 250, and 350 ppm Table II Corrosion parameters of aluminum in the presence of different concentration of inhibitor in 0.5 M HCl solution Inhibitor Table III Corrosion parameters of aluminum in the presence of different concentration of inhibitor in 0.5 M NaOH solution Inhibitor Cationic surfactant as corrosion inhibitor for aluminum E corr (mv/sce) E corr (mv/sce) I corr (ma/cm 2 ) I% I corr (ma/cm 2 ) I% b a b a b c Blank CTAC (150 ppm) CTAC (250 ppm) CTAC (350 ppm) b c Blank CTAC (150 ppm) CTAC (250 ppm) CTAC (350 ppm) ppm, the weight loss of aluminum reaches a limiting value that does not change remarkably from it. The critical micelle concentration (CMC) of CTAC is 415 ppm (Evan, 1956; Bujake and Goddard, 1965; Gerschman, 1957). Thus, the abrupt change in the inhibition efficiency of the studied cationic surfactant around and above its CMC value suggests that surface active compounds inhibit corrosion by adsorption, not as monomeric molecules, but in micellar form (El-Dahan et al., 1999). In another word, when the concentration of surfactant reaches its CMC, the adsorption amount and coverage do not change and the weight loss amount of aluminum tends to reach equilibrium. The obtained results reveal that the weight loss of Aluminum in HCl was less than that in NaOH at the same concentration range. Inspection of the data in Table I reveals that CTAC appeared to act as an inhibitor over the studied concentration range. The corrosion rate values indicated that CTAC has a great effect on the dissolution of aluminum in NaOH medium. The reduction in the dissolution of aluminum in the presence of CTAC inhibitor may be attributed to the quaternary ammonium ion present in the functional group. It is obvious that the inhibitor effect of CTAC increases with increasing its concentration and reaches a limiting value. This phenomenon is more likely to take place when the concentration of the tested surfactant is near CMC. The effect of HCl strength on the inhibition efficiency of the additive tested is shown in Figure 2, whereas a decrease in the inhibition efficiency (%I) with increasing HCl concentration is observed (Figure 2(A)). The corresponding curves in NaOH (Figure 2(B)) show the same trend with more inhibition efficiency. A very important criterion to characterize the efficiency of inhibitor is to determine the ratio of their efficiency to concentration. High protection at low inhibitor concentration is required to maintain appropriate inhibitor concentration and avoid insufficient inhibition, and also for economic reasons. This ratio is calculated and summarized in Table I, for HCl and NaOH. When discussing inhibition action of surface-active agents, various factors must be taken into consideration (Fouda et al., 1986). These include the number of functional groups taking part in the adsorption of the inhibitor molecule and their electron charge density, molecular size and geometry, mode of interaction, heats of hydrogenation and of additives. From weight loss results, it is remarkably observed that inhibition efficiency reaches a limiting value near the CMC of the tested inhibitor. This behavior is related to a multilayer adsorption mechanism. Therefore, a multiple layer protective coverage is expected on the entire aluminum surface by the inhibitor for the inhibition to be effective. To emphasize the nature of adsorption, the effect of temperature (30-508C) on the corrosion behavior of aluminum in the presence of 200 ppm inhibitor in 0.2 M and 0.5 M HCl was studied using weight loss method. Results of this study (Figure 4) showed that inhibition efficiency (%I) decreased with increasing temperature as a result of the higher dissolution of aluminum at higher temperature. Also, this trend is suggestive of a multilayer adsorption mechanism of CTAC on aluminum surface. 164

6 4.2 Adsorption behavior Basic information on the interaction between the studied surfactant and aluminum surface can be provided from the adsorption isotherms. The two main types of adsorption of an organic inhibitor on a metal surface are physical and chemical. Chemisorption is probably the most important type of interaction between the metal surfaces and an inhibitor molecule. A covalent type of bond involving electron transfer from the inhibitor to the metal is assumed to take place in the process. In general, organic inhibitors have reactive functional groups, which are the sites for the chemisorption process. The strength of the adsorption bond depends on the electron density on the donor atom of the functioned group, and the polarizability of the group (Sastri, 1998). The values of DG ads of the studied molecule are negative and they increased as the inhibition efficiencies increased. These results illustrate the spontaneity of the adsorption process and the stability of the adsorbed layer on the aluminum surface. As can be seen from the results, the studied cationic surfactant had high DG ads values. This was because adsorption of CTAC molecules on the aluminum surface not only resulted from electrostatic interaction, but also from chemisorption, which take place due to the high electronic charge density on the nitrogen atom in CTAC. 4.3 Potentiostatic polarization Inspection of Tables II and III reveals that the corrosion potential is slightly shifted to more noble direction. Moreover, the corrosion current decreases markedly in the same previous order and the I% increases. The increases in the cathodic Tafel slopes indicate an increase in percentage inhibition efficiency. Furthermore, inspection of Tables II and III reveals that cathodic Tafel slopes increase markedly upon addition of inhibitors, whereas, the anodic Tafel slopes slightly increase. This large change in cathodic Tafel slopes in presence of inhibitors indicates that the inhibitors affect the cathodic reactions. Also, it is obvious that all the calculated absolute values of electrochemical parameters (E corr, I corr, b a, b c, and I%) obtained from polarization method in NaOH are greater than those in HCl. The results obtained from the polarization technique in acidic and basic were in good agreement with those obtained from the weight loss method with a small variation of ^2 percent. 5. Conclusions The cationic surfactant tested was beneficial inhibitor for aluminum corrosion in acidic and basic media. Its ability as inhibitor differs in acidic and basic media. This may be due to differences of mode of adsorption. The inhibition efficiency (%I) increased with increasing inhibitor concentration and decreasing the corrosive media concentration. The inhibition efficiency (%I) decreased with increasing temperature as a result of the higher dissolution of aluminum at higher temperature. The decrease in inhibition efficiency (%I) and decrease in surface coverage (u) with increasing temperature indicating a weak interaction between the inhibitor molecules and the aluminum surface. Also, it is suggestive of a multilayer adsorption mechanism of CTAC on aluminum surface. The adsorption of CTAC on the aluminum surface in NaOH was observed to comply with Langmuir adsorption isotherm behavior. While, the adsorption of CTAC on the aluminum surface in HCl was observed to comply with Temkin adsorption isotherm behavior. The large increase in cathodic Tafel slopes reveals that the inhibitors control the cathodic reaction. References Ajmal, M., Mideen, A.S. and Quraishi, M.A. (1994), 2- Hydrazino-6-methyl-benzothiazole as an effective inhibitor for the corrosion of carbon steel in acidic solutions, Corros. Sci., Vol. 36, p. 79. Beck, T.R. 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