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Materials Chemistry and Physics 114 (2009) 533–541 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Evaluation of inhibitive action of some quaternary N-heterocyclic compounds on the corrosion of Al–Cu alloy in hydrochloric acid Ehteram A. Noor ∗ King Abd El-Aziz university, Girls, College of Education, Chemistry Department, Al Maddares Street, Jeddah, KSA, Saudi Arabia a r t i c l e i n f o Article history: Received 19 June 2008 Received in revised form 19 September 2008 Accepted 29 September 2008 Keywords: Corrosion Inhibition Al–Cu alloy Hydrochloric Pyridinium Adsorption a b s t r a c t The inhibitive action of some quaternary N-heterocyclic compounds namely 1-methyl-4[4 (-X)-styryl] pyridinium iodides (X: -H, -Cl and -OH)on the corrosion of Al–Cu alloy in 0.5 M HCl solutions was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss measurements. All the studied compounds showed good inhibitive characteristics against the corrosion of Al–Cu alloy in the tested solutions and their performance increases with inhibitor concentration. Polarization data indicated that the studied compounds are cathodic inhibitors without changing the mechanism of hydrogen evolution reaction.The adsorption of all inhibitors on Al–Cu alloy obeys Langmiur adsorption isotherm. The effect of temperature (30–70 ◦ C) on the inhibition efficiency at certain concentration of the studied compounds was investigated. The data revealed that the studied compounds have good pickling inhibitor’s quality as they perform well even at relatively high temperature. The corrosion activation parameters (Ea , H*, S* and G*) were estimated and discussed. It was found that Ea values for Al–Cu alloy corrosion in the inhibited solutions were higher than that for the uninhibited solution, indicating good inhibitor characteristics with physical adsorption mechanism. The effect of acid, s anion on the performance of the studied inhibitors was studied and discussed. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Aluminium and its alloys have excellent durability and corrosion resistance, but, like most materials, their behaviour can be influenced by the way in which they are used. Aluminium and its alloys are widely used in many industries such as reaction vessels, pipes, machinery and chemical batteries because of their advantages. Hydrochloric acid solutions are used for pickling, chemical and electrochemical etching of Al foil and lithographic panels which substitute metallic zinc [1]. Since the metal dissolution in such solutions is rather large, it is necessary to inhibit it by the addition of inhibitors, which should provide a good quality pickled metal surface. Organic compounds containing polar groups such as nitrogen, sulphur and oxygen as well as heterocyclic containing conjugated double bonds have been reported as good inhibitors for Al and its alloys [1,2–10]. The inhibiting of such compounds is based on the adsorption ability of their molecules, where the resulting adsorption film isolates the metal from the corrosion environment. The inhibitor molecules are bonded to the metal surface by chemisorption, physical adsorption or complexation, with ∗ Tel.: +966 26939746. E-mail address: [email protected]. 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.09.065 the polar groups acting as the reactive centers in the molecules [11]. Generally, the process of inhibitor’s adsorption is influenced by the nature and the surface charge of the metal, the chemical structure of organic inhibitors, the distribution of charge in the molecule, the type of aggressive electrolyte and the type of interaction between the organic molecules and the metallic surface [12]. However, determination of the type corresponding to the adsorption on the metal/electrolyte phase boundary gives much valuable information as to the adsorption process, since it makes possible to determine such quantities as the free energy of adsorption, its dependence on the degree of surface coverage, the character of the adsorption layer on the metal/electrolyte phase boundary, the magnitude and character of interaction between the molecules and the surface atoms of the metal. Therefore, the accurate determination for the type of adsorption isotherm corresponding to the investigated adsorption process is of primary importance. Recently, some quaternary N-heterocyclic compounds have been reported as good corrosion inhibitors for Al–Si alloy in HCl [2,3] and mild steel in HCl [13] and H3 PO4 [14]. The present work is an extension of the earlier works and evaluate the inhibitive action of similar compounds on the corrosion of Al–Cu alloy in 0.5 M HCl using electrochemical (potentiodynamic polarization, PDP, and electrochemical impedance spectroscopy, EIS) and weight loss (WL) measurements. 534 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 2. Experimental 2.1. Materials The experiments were performed with Al–Cu alloy specimens with the following composition: 4.320% Cu, 0.210% Mg, 0,072% Pb, 0.099% Fe, 0.223% Mn, 0.634% Si, and Al is the remainder. The studied inhibitors were synthesized as previously reported in the literature [15]. The molecular formula and nomination of the studied inhibitors is shown in Table 1. 2.2. Solutions The used acid solutions were made from analytical grade hydrochloric acid (HCl), and sulphuric acid (H2 SO4 ). H2 SO4 acid was used for comparative study. Appropriate concentration of acid was prepared using de-ionized water in absence and presence of certain inhibitor’s concentration. The investigated range of inhibitor’s concentration was 1.0 × 10−5 to 5.0 × 10−4 M. 2.3. Electrochemical measurements Electrochemical measurements were carried out in standard three-electrode round glass electrochemical cell with a platinum wire as counter electrode (CE) and a saturated calomel electrode (SCE) as reference electrode (RE). The working electrode (WE) was in the form of a cylindrical rod from Al–Cu alloy inserted in glass tube of suitable diameter and fixed with araldite so that a flat surface was the only surface exposed to the tested solutions. The electrode working surface area was 0.785 cm2 . Prior the electrochemical measurements, the WE was abraded with emery papers (grade 320-400-800-1000), washed with de-ionized water and acetone, dried at room temperature and finally immersed and left for 10 min in the tested solution to maintain the open circuit potential. EIS measurements were carried out over the frequency range of 10 kHz to 0.1 Hz, with a signal amplitude perturbation of 30 mV by using Potentiostate/glvanostate ACM Gill AC instrument model 655. EIS results were analyzed using software program named ZSimDemo 3.20. After each EIS run, the instrument turns on automatically to record the PDP curves with scan rate of 60 mV min−1 from potential of −750 to −500 mV vs. SCE. 3. Results and Discussion 3.1. Potentiodynamic polarization (PDP) Anodic and cathodic polarization curves of Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH at 30 ◦ C are shown in Fig. 1. An analysis of the polarization curves indicates that at low over potential, the Tafel relationship is followed showing that both anodic and cathodic reactions are activation controlled [16]. In general, the presence of increasing amount of the studied compounds leads to dramatically decrease in the cathodic current density associated with limited increase in the anodic current density. However the inhibited systems were shifted towards cathodic potentials, emphasizing that the studied compounds act predominately as cathodic inhibitors. This result is in good agreement with that obtained in previous works [2,3] when similar compounds were studied as inhibitors for the corrosion of Al–Si alloy in 0.25 M HCl. The electrochemical parameters such as corrosion potential (Ecorr ), corrosion current density (icorr ), anodic (ba ) and cathodic (bc ) Tafel slopes are estimated by using Tafel ruler. The IEi % can be given by the following equation: IEi % = 1− icorr o icorr × 100 (2) According to the electrochemical theory, polarization resistance (Rp ) is inversely proportional to the corrosion current density and can be estimated from Stern–Geary equation [17]: 2.4. Weight loss measurements The Al–Cu alloy rod of 5 cm in length and 1 cm in diameter was abraded with emery papers (grade 320-400-800-1000) and then washed with de-ionized water and acetone, dried at room temperature and then weighed. After weighing accurately, the specimen was immersed in 50 ml of 0.5 M HCl in absence and presence of certain concentration of 1-H, 2-Cl and 3-OH. After 90 min, the specimen were taken out, washed, dried and weighed accurately. The corrosion rate (WL , g min−1 cm−2 ) for the studied specimen is determined by using the relation: WL = where W1 is the weight of the specimen before corrosion, W2 is the weight of the specimen after corrosion, A is the surface area of the specimen and t∞ is the final immersion time. It must be pointed out that all measurements were conducted in a stagnant and open-air solution at 30 ◦ C except otherwise stated. W1 − W2 At∞ (1) Rp = B (3) icorr where ˇ = (ba bc )/(2.303(ba + bc )). Table 2 represents the electrochemical parameters (ba , bc , Ecorr , icorr , Rp and IEi %) obtained from PDP measurements for Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of the studied compounds at 30 ◦ C. Table 1 Names, structures and abbreviations of the studied inhibitors. Abbreviation Structure Inhibitor Nomination 1-H 1-Methyl-4[4 (-H)-styryl] pyridinium iodide 2-Cl 1-Methyl-4[4 (-Cl)-styryl] yridinium iodide 3-OH 1-Methyl-4[4 (-OH)-styryl] pyridinium iodide E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 535 Table 2 Electrochemical parameters obtained from PDP measurements of Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH. Cinh (M) ba (mV dec−1 ) −bc (mV dec−1 ) −Ecorr (mV) icorr (mA cm−2 ) Rp ( cm2 ) IEi % 1-H 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 89.75 75.74 43.39 42.79 46.52 96.49 98.32 107.82 92.15 98.28 590.33 593.24 603.64 610.49 615.45 6.70 3.66 2.14 0.67 0.32 3.01 5.08 6.28 18.94 42.84 – 45.37 68.06 90.00 95.22 2-Cl 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 89.75 61.25 49.64 36.25 37.89 96.49 99.31 94.48 93.90 90.10 590.33 592.89 599.88 605.15 630.60 6.70 3.07 1.84 0.39 0.16 3.01 5.36 7.68 29.12 72.39 – 54.18 72.54 94.18 97.61 3-OH 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 89.75 38.34 47.41 36.32 43.38 96.49 98.42 93.08 90.23 90.80 590.33 591.02 600.15 611.01 635.49 6.70 2.85 1.48 0.37 0.13 3.01 4.20 9.22 30.39 98.05 – 57.46 77.91 94.48 98.06 The data in Table 2 can be interpreted as follows: 3.2. Electrochemical impedance spectroscopy (EIS) • For the inhibited systems, the values of bc were somewhat greater than the values of ba , once again the studied inhibited systems are under cathodic control. • The cathodic Tafel slopes (bc ) remain almost unchanged for the uninhibited and inhibited systems, indicted that the inhibitive action of the studied compounds is due to adsorption of inhibitor cations on the cathodic active sites. In this case the hydrogen evolution reaction (cathodic reaction) was suppressed without changing its mechanism (i.e. simple blocking mechanism). • The anodic Tafel slopes remain almost unchanged for the inhibited systems but with values lesser than that for the uninhibited system. It was known that the dissolution of Al in inhibitor-free HCl solution may be occurred by a simple mechanism with the formation of AlCl3 [18]: Al + 1/2O2 ⇔ AL : Oads (4-a) Al : Oads + Cl− ⇔ Al : OCl− ads (4-b) Al : OCl− ads ⇒ Al : OClcomp + e (4-c) Al : OClcomp + 2Cl− + 2H+ ⇒ AlCl3 + H2 O (4-d) r.d.s. fast In the present study the inhibited solutions contain both I− (from inhibitor dissociation) and Cl− ions (from the aggressive acid solution), hence the dissolution process (anodic process) may be proceeded by the formation of both AlCl3 and AlI3 leading to the observed decrease in ba value. • The values of Ecorr were shifted to the cathodic direction with the addition of increasing amount of the studied inhibitors, indicating cathodic control mechanism. • The value of icorr decreases while the value of Rp increases with the increase of inhibitor concentration which associated with an increase in the corresponding IEi % value up to 95.22%, 97.61% and 98.06% for 1-H, 2-Cl and 3-OH, respectively. • According to icorr , Rp and IEi % data, The inhibitive properties of the studied compounds can be given by the following order: 1-H < 2-Cl < 3-OH The impedance spectrum obtained on Al–Cu alloy in 0.5 M HCl at the open circuit potential is presented in Fig. 2 as Nyquist plot at 30 ◦ C. Fig. 2 shows a large semicircular capacitive loop at high frequency which is then followed by a large inductive loop at low frequencies. According to Bessone et al. [19] and de Wit et al. [20], the high frequency capacitive loop could be attributed to the oxide layer on Al. On the other hand, Lenderink et al. [21] have been attributed the low frequency inductive loop to the relaxation of adsorbed species like H+ . Inductive behaviour is also observed for ads the pitted active state and attributed to the surface area modulation or salt film property modulation [22]. The influence of various concentrations of 1-H compound on the impedance spectra of Al–Cu alloy in 0.5 M HCl at the open circuit potential is shown in Fig. 3. Similar spectra were obtained for 2-Cl and 3-OH but are not shown. The increase of both the capacitive loop and the charge transfer resistance with the increase of inhibitor concentration can be observed clearly in the recorded Nyquist plots. As in the previous case, the inductive part of the impedance was mainly determined by the relaxation process of H+ , Cl− adsorption and Al-dissolution, while the high frequency capacitive loop can be correlated with dielectric properties of a surface layer; i.e. [metal-oxide–hydroxide-inhibitor]ads complex [23]. The appearance of the inductive loop for all inhibited systems indicates that the studied inhibitors do not affect the anodic process and this result is in good agreement with that obtained from polarization measurements. However, the obtained semicircles either in absence or in presence of inhibitor were depressed. Deviations of this kind are referred to as frequency dispersion, have been attributed to inhomogeneties of solid surfaces as the Al surface is always [24]. A practical way to represent distributed processes such as corrosion of a rough and inhomogeneous electrode is with an element that follows its distribution. The constant phase element (CPE) meets that requirement. The impedance of CPE can be given in the form [23]: n −1 ZCPE = (A(iω) ) (5) where the coefficient A is a combination of properties related to both the surface and the electroactive species. The exponent n has values between −1 and 1. A value of −1 is characteristic of an inductance, a value of 1 corresponds to a capacitor, a value of 0.5 can be assigned to diffusion phenomena. 536 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 Fig. 2. Nyquist plot for Al–Cu alloy in 0.5 M HCl. Fig. 3. Nyquist plot for Al–Cu alloy in 0.5 M HCl in presence (1) 1.0 × 10−5 M, (2) 5.0 × 10−5 M, (3) 1.0 × 10−4 M, (4) 2.5 × 10−4 M and (5) 5.0 × 10−4 M of 1-H. Table 3 Electrochemical parameters obtained from EIS measurements of Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH. Cinh (M) Fig. 1. Potentiodynamic polarization curves for Al–Cu alloy corrosion in 0.5 M HCl in absence (1) and in presence (2) 1.0 × 10−5 M, (3) 5.0 × 10−5 M, (4) 1.0 × 10−4 M and (5) 2.5 × 10−4 M of 1-H. An equivalent circuit of five elements depicted in Fig. 4a was used to simulate the measured impedance data as shown in Fig. 4b. This consists of CPE in parallel to the parallel resistors Rt and RL , and the later is in series with the inductor L. When an inductive loop is present, the polarization resistance Rp can be calculated from [6]: RL × Rt Rp = RL + Rt (6) Table 3 gives the numerical values of Rp and CPE of Al–Cu alloy in 0.5 M HCl solution in absence and presence of the studied inhibitors Rp ( cm2 ) CPE × 103 (F cm−2 ) IER % 1-H 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4 2.88 4.56 7.83 27.20 52.08 85.46 3.37 1.60 0.98 0.20 0.11 0.10 – 36.84 63.22 89.41 94.47 96.63 2-Cl 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4 2.88 5.44 9.11 43.24 72.18 106.67 3.37 1.56 0.91 0.19 0.06 0.05 – 47.06 68.38 93.34 96.01 97.30 3-OH 0.0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4 2.88 5.86 11.28 45.14 125.76 152.38 3.37 1.29 0.77 0.08 0.06 0.04 – 50.85 74.47 93.62 97.71 98.11 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 537 Fig. 4. The equivalent circuit model used to fit the experimental data. at 30 ◦ C. It must be noted that all the studied systems give n values approximately of unity, indicating the predominance of capacitive behaviour. The IER % values can be calculated from Rp data as follows: IER % = 1− Rp−1 Rpo−1 × 100 (7) where Rpo and Rp are the polarization resistances in absence and presence of inhibitor, respectively. The calculated IER % values were also recorded in Table 3. It was observed that the value of CPE decreases while the value of Rp increases with increasing inhibitor concentration, indicated that the studied compounds inhibit the corrosion of Al–Cu alloy in 0.5 M HCl solution by the adsorption mechanism [25] and the thickness of the adsorbed layer increases with the increase of inhibitor concentration. However, Good consistency between the values of Rp and IE% obtained from both PDP and EIS measurements was observed. Inspection of the impedance diagrams in Fig. 3 and the corresponding values of Rp in Table 3, it was observed that with an increase in inhibitor concentration the capacitive loop increases more strongly than Rp value, indicating that the adsorbed inhibitor’s cations on the electrode surface have no influence on the rate of the anodic process [23]. This result agrees well with the suggested cathodic control mechanism. 3.3. Weight loss measurements 3.3.1. Effect of inhibitor concentration With the calculated corrosion rate (Eq. (1)) in absence and presence of certain inhibitor’s concentration, the percentage of inhibition efficiency (IE%) can be estimated as follows: IEWL % = 1− WL o WL × 100 (8) o and where WL WL are the corrosion rates in absence and presence of inhibitor. The values of WL and IEWL % for Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH at 30 ◦ C are given in Table 4. The obtained data revealed that with increasing the concentration of the studied compounds, a considerable decrease in the corrosion rate was observed which normally associated with an increase in the corresponding values of IEWL %. The performance of the studied compounds as corrosion inhibitors can be written in the following increasing order: 1-H < 2-Cl < 3-OH The above order is in good agreement with that obtained from electrochemical measurements (PDP & EIS). It is also in the same order of the electron donor property of the substituent type [26] except that for chloro substituent. The unexpected behaviour of chloro substituent was reported by some authors [27,28]. 3.3.2. Adsorption isotherms The primary step in the action of inhibitors in acid solutions is generally agreed to be adsorption on to the metal surface. This involves the assumption that the corrosion reactions are prevented from occurring over the area (or the active sites) of the metal surface covered by adsorbed inhibitor species, whereas these corrosion reactions occurred normally on the inhibitor-free area [29]. Accordingly, the fraction of the surface covered with inhibitor species ( = IEWL %/100) can be followed as a function of inhibitor concentration and solution temperature. When the fraction of the surface covered is determined as a function of the concentration at a constant temperature, adsorption isotherm could be evaluated at equilibrium conditions. The variation of surface coverage with concentration of the studied compounds is shown in Fig. 5. These curves have S-shaped adsorption isotherms that are characterized by an initial increase in value with inhibitor concentration up to 2.5 × 10−4 M, after which the value of does not change appreciably Table 4 Corrosion parameters obtained from WL measurements of Al–Cu alloy in 0.5 M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH. Cinh (M) 0 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4 WL × 105 (g cm−2 min−1 ) IEWL % 1-H 2-Cl 3-OH 1-H 2-Cl 3-OH 3.529 2.084 1.455 0.333 0.104 5.512 2.929 2.025 1.401 0.275 0.099 2.817 1.994 1.146 0.275 0.097 35.98 62.2 73.61 94.02 98.12 – 46.86 63.26 74.58 95.01 98.2 48.89 63.82 79.21 95.01 98.24 538 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 Table 5 Langmuir, s adsorption parameters for 1-H, 2-Cl and 3-OH in 0.5 M HCl on Al–Cu alloy. Inhibitor Slope Kads × 10−4 (M−1 ) −Gads (kJ mol−1 ) r2 1-H 2-Cl 3-OH 0.96 0.97 0.98 3.08 4.03 5.08 36.14 36.81 37.4 0.998 0.998 0.999 where CH2 O is the concentration of water in solution expressed in M, R is the universal gas constant and T is the absolute temperature. Table 5 represents the estimated Langmuir’s adsorption parameters and the correlation coefficients (r2 ) of the given straight lines. The data in Table 5 can be interpreted as follows: Fig. 5. Variation of surface coverage () for Al–Cu alloy with inhibitor concentration. with increasing inhibitor concentration, suggesting a formation of a complete monolayer adsorbate film on the Al surface. Accordingly, by far the results were best fitted by Langmuir adsorption isotherm (Fig. 6), which is given by [30]: • The slope of Langmuir’s straight lines was closed to unity meaning that each inhibitor molecule occupies one active site on the metal surface. • The studied inhibitors showed high values for the equilibrium constant of adsorption indicating that they adsorbed strongly onto Al–Cu alloy surface and their adsorption ability can be given in the following increasing order: 1-H < 2-Cl < 3-OH (9) • The high negative values of Gads also indicate strong and spontaneous adsorption of the studied compounds onto Al–Cu alloy surface. where Kads (M−1 ) is the Langmuir constant which is defined as the equilibrium adsorption constant and related to the free energy of adsorption by the following equation: Moreover, the essential characteristic of Langmuir isotherm can be expressed in terms of a dimensionless separation factor, KL [31] which describe the type of isotherm and is defined by Cinh −1 = 1 Kads + Cinh log Kads = − log CH2 O − Gads 2.303RT (10) KL = 1 1 + Kads Cinh Fig. 6. Langmuir, s adsorption isotherm for 1-H, 2-Cl and 3-OH in 0.5 M HCl on Al–Cu alloy. (11) E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 539 Table 6 The values of dimensionless separation factor, KL , for 1-H, 2-Cl and 3-OH at various concentrations. Table 7 The values of IEWL % for 5.0 × 10−4 M of 1-H, 2-Cl and 3-OH in 0.5 M HCl at different temperatures. Cinh (M) KL T (◦ C) IEWL % 2-Cl 3-OH 1.0 × 10−5 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4 0.7645 0.3937 0.2451 0.1149 0.061 0.7128 0.3317 0.1988 0.0903 0.0473 0.6631 0.2825 0.1645 0.073 0.0379 30 40 50 60 70 98.12 97.06 92.59 86.4 82.88 Mean value 0.3158 0.2762 0.2442 1-H If KL > 1 KL = 1 0 < KL < 1 KL = 0 unfavourable; linear favourable; irreversible; Table 6 gives the estimated KL values for 1-H, 2-Cl and 3-OH at different concentrations. It was found that all KL values are less than unity confirming that the adsorption process is favourable. However, according to the mean value of KL for each inhibitor, the inhibitory action of the studied compounds can be written in the following order: 1-H < 2-Cl < 3-OH 3.3.3. Effect of temperature The effect of temperature on the corrosion rate (WL ) of Al–Cu alloy in 0.5 M HCl in absence and presence of 5.0 × 10−4 M of all the investigated inhibitors was determined in the temperature range of 30–70 ◦ C and illustrated in Fig. 7. It was observed that the corrosion rate increases with temperature for all the studied systems and its extent was more pronounced in the uninhibited system, indicating the good inhibitive properties of the studied compounds under the studied conditions. Table 7 gives the variation of inhibitor efficiency (IEWL %) with temperature. Table 7 shows good pickling inhibitors, qualities as the studies compounds retain their protective properties even at relatively high temperature (70 ◦ C). Fig. 7. Variation of corrosion rate, WL , for Al–Cu alloy in 0.5 M HCl in absence and presence 5.0 × 10−4 M of 1-H, 2-Cl and 3-OH compounds at different temperatures. 1-H 2-Cl 3-OH 98.2 97.61 95.18 95.05 94.02 98.24 97.28 92.64 89.29 82.76 The corrosion reaction can be regarded as an Arrhenius-type process. The activation parameters for the studied systems were calculated from Arrhenius equation (Eq. (12)) and transition state equation (Eq. (13)) as follows: log WL = log A − log WL T = Ea 2.303RT log (12) R S∗ hN + 2.303R − H ∗ 2.303RT (13) where A is Arrhenius factor, Ea is the apparent activation corrosion energy, N is the Avogadro’s number, h is the Plank’s constant and, S* and H*are the entropy and the enthalpy changes of activation corrosion energies for the transition state complex. Arrhenius and transition state plots for the corrosion rates (WL ) of Al–Cu alloy in absence and presence of 5.0 × 10−4 M for each inhibitor are given in Fig. 8 and the corresponding activation parameters (Ea , H* and S*) for the corrosion process were estimated and listed in Table 8. The change in the activation free energy (G*) of the corrosion process can be calculated at each experimental temperature by applying the famous equation: G∗ = H ∗ − TS ∗ (14) The obtained G*values was also listed in Table 8. According to the data recorded in Table 8 the following discussion can be written: Fig. 8. Arrhenius plots (solid lines) and transition-state plots (dashed lines) of corrosion rate, WL , for Al–Cu alloy in 0.5 M HCl in the absence and presence 5.0 × 10−4 M of 1-H, 2-Cl and 3-OH. 540 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 Table 8 Corrosion activation parameters for Al–Cu alloy in 0.5 M of HCl in absence and presence 5.0 × 10−4 M of 1-H, 2-Cl and 3-OH. Inhibitor Free acid 1-H 2-Cl 3-OH Ea (kJ mol−1 ) 49.59 101.1 76.97 101.09 H* (kJ mol−1 ) 46.91 98.42 74.29 98.42 S* (J mol−1 K−1 ) G* (kJ mol−1 ) −171.18 −34.46 −113.93 −35.42 • The results showed positive sign for both Ea and H*, reflecting the endothermic nature of corrosion process. It is obviously seen that the activation energy strongly increases in the presence of inhibitor. Some authors [10,32,33] attributed this result to that the inhibitor species are physically adsorbed on the metal surface. In this respect the comparison of the inhibiting action of the investigated compounds in HCl and H2 SO4 will be of definite interest. As observed, the trend of Ea for the studied inhibitors is not the same with that obtained from inhibition efficiency. The lower activation energy for compound 2-Cl as compared to that of compound 1-H may be explained according to Riggs and Hurd [34], as they stated that at higher level of surface coverage the corrosion process may proceed on the adsorbed layer of inhibitor and not on the metal surface leading to a decrease in the apparent activation energy and in some cases becomes less than that obtained in the absence of inhibitor. • The negative values of S* pointed to a greater order produced during the process of activation. This can be achieved by the formation of activated complex represents association or fixation with consequent loss in the degrees of freedom of the system during the process [35]. • The values of G* were positive and showed limited increase with rise in temperature, indicating that the activated complex was not stable and the probability of its formation decreased somewhat with rise in temperature. So, the increase in the rate of corrosion with rise in temperature (Fig. 7) can be attributed to large number of corrosion species passing into an activated state with a less stable configuration [35]. However, G* values for the inhibited systems were more positive than that for the uninhibited systems revealing that in cores of inhibitor addition the activated corrosion complex becomes less stable as compared to its absence. 30 ◦ C 40 ◦ C 50 ◦ C 60 ◦ C 70 ◦ C 98.78 108.86 108.81 109.15 100.49 109.21 109.95 109.51 102.2 109.55 111.09 109.86 103.91 109.9 112.23 110.21 105.62 110.24 113.37 110.57 Fig. 9. H* vs. S* for Al–Cu alloy in 0.5 M HC in absence and presence 5.0 × 10−4 M of 1-H, 2-Cl and 3-OH. Fig. 9 represents the variation of H* with S*. This correlation can be treated as the isokinetic relationship, where the slope  represents the isokinetic temperature [36]. The slope of the straight line is 428 K, which is much higher than the experimental temperature. This indicates that the corrosion reaction is under activation-control, where the addition of the studied inhibitors plays an important role in reducing the corrosion rate but without changing the corrosion mechanism as indicated by the observed parallelism between H* and S* values. of 1-H, 2-Cl and 3-OH. The calculated IEWL % values were also listed in Table 9. It was found that in spite the corrosion rate of the studied alloy in HCl solution is higher than that in H2 SO4 solution, the investigated inhibitors are drastically more effective in the former than in the latter. This result can be interpreted on the basis of three factors, these are the metal charge density, the size of the acid’s anion and the chemical structure of the inhibitor. As known the surface charge of the metal is due to the electrical field which emerges at the interface on the immersion in the electrolyte. It can be determined according to Antropov [37] by comparing the potential of zero charge (PZC) and the rest potential of the metal in the corresponding medium. An aluminium surface in an aerobic environment always is covered with aluminium oxide. It was stated that the pH of zero charge for aluminium oxide is 9–9.1 [38]. Below the isoelectric point (pH < 9) the Al surface has a positive charge [38] that leads to electrostatic attraction of the negatively charged species (Cl− or SO2− 4 ). The studied inhibitors are organic compounds in the salt form in which the organic part being the cation while the inorganic part being the anion (I− ions). So, in view of the above the adsorption mechanism may occur as follows: 3.3.4. Effect of the acid’s anion Table 9 gives the values of corrosion rates (WL ) for Al–Cu alloy in 0.5 M of HCl and H2 SO4 in the absence and presence 1.0 × 10−4 M • Firstly, the acid’s anions (Cl− or SO2− ) adsorb physically on the 4 positively charged metal surface, giving rise for a net negative charge on the metal surface. Table 9 Corrosion rate and inhibition efficiency for Al–Cu alloy in 0.5 M of HCl and H2 SO4 in the absence and presence 1.0 × 10−4 M of 1-H, 2-Cl and 3-OH. The medium HCl H2 SO4 a b 0.0 M. 1.0 × 10−4 M. WL × 10−5 (g cm−2 min−1 )a 5.512 0.072 WL × 10−5 (g cm−2 min−1 )b IEWL %b 1-H 2-Cl 3-OH 1-H 2-Cl 3-OH 1.455 0.058 1.401 0.067 1.146 0.044 73.6 19.44 74.58 6.94 79.21 38.8 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 • Secondly, the organic cations are physically attracted to the anions layer which is formed on the metal surface. According to the ionic volume of the acid’s anions the smallest anion (Cl− ) attracted more faster to the metal surface than the biggest one (SO2− 4 ) leading to good inhibitor performance in HCl solution (Table 9). 4. Conclusion The main conclusions are as follows: • All the studied inhibitors show good inhibitive action against the corrosion of Al–Cu alloy in 0.5 M HCl solution. • The value of IE% increases with increasing inhibitor concentration and after certain concentration it does not change appreciably with inhibitor concentration. • PDP measurements revealed that the studied inhibitors can be classified as cathodic inhibitors without changing the cathodic reaction mechanism. • Good agreement between the data obtained from weight loss and electrochemical measurements. • The adsorption of all inhibitors on Al–Cu alloy obeys Langmiur, s adsorption isotherm. • The values of both Kads and Gads indicated that all the studied inhibitors are strongly adsorbed on the Al–Cu alloy in 0.5 M HCl. • The IE% of the studied compounds decreases slightly with temperature increase, showing good protective properties even at 70 ◦ C (82.9%, 94.0% and 82.2% for 1-H, 2-Cl and 3-OH, respectively). • The Ea values for Al–Cu alloy corrosion in the inhibited solutions are higher than that for the uninhibited solutions indicating good inhibitor characteristics associating with physical mechanism. • The studied inhibitors (1.0 × 10−4 M) show inhibitive properties in HCl more than in H2 SO4 , emphasizing the predominance of physical adsorption mechanism. 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