Microsoft Word - 86-102 86 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Corrosion Inhibition of Aluminium in Acidic Medium using Amino Acid (Methionine) Wisal A. Isa Zainab W. Ahmed Dept. Of chemistry /College of Education for Pure Sciences (Ibn-Al-Haitham)/ University of Baghdad Received in: 26/march/2015, Accepted: 7/June/2015 Abstract Corrosion behavior of aluminium in 0.6 mol. dm-3 NaCl solution in acidic medium 0.7 was investigated in the absence and presence of different concentrations of amino acid, methionine, as environmentally – friendly corrosion inhibitor over temperature range (293-308)K. The investigation involved electrochemical polarization method using potentiostatic technique and optical microscopy, the inhibition efficiency increased with an increase in inhibitor concentration but decreased with increase in temperature. Results showed that the inhibition occurs through adsorption of the inhibitor molecules on the metal surface and it was found to obey Langmuir adsorption isotherm. Some thermodynamic parameters ∆ . and activation energy were calculated to elaborate the mechanism of corrosion inhibition. The polarization measurements indicated that methionine is of mixed type. The surface characteristic of the inhibited and uninhibited metal samples were investigated by optical microscopy. Keywords: Corrosion inhibition, Aluminium, Methionine, Adsorption Mechanism Thermodynamic parameters. 87 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Introduction Aluminium and its alloys have a remarkable economic and attractive materials for engineering applications owing to its low density, high thermal and electrical conductivity, attractive appearance and relatively good corrosion resistance. Alumininum is actually a very active metal, meaning that its nature to oxidize is very quickly, while a weakness for most metals; this quality is in fact the key to its ability to resist corrosion. The combination of these properties makes it a preferred choice for many industrial applications such as automobiles, aviation, aerospace, food handling, containers, electronic devices, buildings, etc[1]. Corrosion is the deterioration of metal by chemical attack or reaction with its environment. It is constant and has continuous problem, often difficult to eliminate completely. Prevention would be more practical and achievable than complete elimination[2]. Corrosion of metal is caused by electrochemical reaction between a metal (or an alloy) and aqueous phase. It proceeds according to a complex electrochemical process that is related to the atomic structure of matter. The corrosion of metal is the result of two simultaneous reactions that are in electrical equilibrium, i.e. oxidation of metal to ions and reduction of hydrogen to hydrogen gas [3]. Hydrochloric acid and sulphuric acid solutions are used for pickling of aluminium or for its chemical or electrochemical etching. It is very important to add corrosion inhibitors to decrease the rate of metal dissolution using organic compounds. It is known that the organic compounds are effective corrosion inhibitors due to their ability to form an adsorbed protective film at the metal surface[3,4]. Unfortunately, most of the inhibitors available in the markets are not only expensive but also toxic to living beings. Again, the safety of environmental issues of corrosion inhibitors which may be arise industry has always been a global concern[5]. To avoid toxicity of numerous compounds tested as inhibitors[6,7], natural compounds, regarded as environmentally friendly inhibitors. Amino acids have been tested as inhibitors of corrosion of a variety of metals, for example copper, aluminium and steel corrosion in acid media. They are relatively easy to produce with high purity at low cost, non-toxic and soluble in aqueous media[6,7]. In the present work, the inhibitive effectiveness of amino acid (methionine), have been studied in retarding corrosion of aluminium in 0.2M by use of electrochemical polarization measurements. Furthermore, this study aim to throw some light on the role of the functional groups of the inhibitors used in the adsorption process as well as the influence of the nature of the medium on the corrosion mechanism. Optical microscopy was employed to observe the surface morphology of the aluminium corroded in test solution. 88 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 2. Experimental part 2.1 Materials The experiments were performed with aluminium sheets (99.8% purity) of 2mm thickness, which was mechanically press-cut into circular form 2cm diameter. All chemicals and reagents used are with analytical grade and use without further purification, they include: a. Sodium chloride, was used for preparation of the aggressive solution of 0.6 . b. Sulphuric acid. c. The amino acid (L-Methionine) tested as inhibitor (>99% purity). Its structure is shown below. H2N CH COOH S 2.2. Solution The aggressive solution used was made of A.R. Sodium chloride to prepare 0.6 concentration in 0.7 . Doubled distilled water was used for the preparation. For each experiment a freshly solution was made. Three concentrations of the inhibitor (methionine) were used 10 ,5 10 10 . . 2.3 Potentiostatic polarization measurements The polentiostatic polarization measurements were carried out with aluminium sheets (explained before) having an exposed surface area of 1 to corrosive medium [in absence and presence the inhibitor]. The open side of the working electrode (aluminium sheet) was grinded and polished mechanically using wheel machine. All samples were grinded via a series grades of emery papers [100, 200,400,800,1200 and 2000] and polished with smooth cloth to a mirror finish. The specimen were degreased with acetone and thoroughly washed with distilled water then dried and kept in a desicator until use. Polarization measurements were performed in a three electrodes cell with aluminium specimen of 1 exposed area, a platinum electrode and silver-silver chloride in saturated KCl solution were used as working, auxiliary (counter) and reference electrodes respectively. The measurements were carried out using M lab potentionstat / Galvanostat 200 Germany obtained from Bank Electronic Intelligent Controls Gnb H. It was connected to personal computer desktop the Mlab software cares for controlling the potentiostat, recording and processing data. It provides electrochemical calculations like Tafel line evalution, re-scalling the potential and integrating. The experiments were preformed in 0.6 solution of 0.7 in absence and presence of three different concentrations of the inhibitor (methionine) over the temperature range (293-308)K. The experiments were carried out at a scan rate of 2 / . Corrosion current density and corrosion potential were determined from the polarization curve, in addition other informations were obtained such as Tafel slopes weight loss and penetration values. In order to test the reproducibility of the results, the experiments were done in triplicate. 3.Results and Discussion 3.1 Tafel polarization measurements The effect of 0.6 mol. dm-3 NaCl solution in acidic medium on the corrosion of aluminium sample was studied using Tafel polarization technique. Figure 1 represents the potentiostatic 89 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 polarization curves of aluminium at four temperature in range of (293-308)K. Corrosion parameters such as corrosion potential , corrosion current density , anodic slope and cathodic slope are obtained from the Tafel polarization curves. Results are tabulated in Table 2. The results indicate the increase in the corrosion current density with an increase in temperature. The negative shift in the corrosion potential with the increase in temperature indicates that the anodic process is much more affected than the cathodic process, this observation is in accordance with other published results[1,2] which proposed the dependence of and on solution parameters. The change in and reflects the mechanism of corrosion process. The electrochemical cathodic and anodic reactions of aluminium in acidic solution can be described as follows[8]. → 1 5 → 6 2 2 → 3 → 4 Thus soluble complex ion formed leads to the dissolution of the metal. 3.2 Effect of Temperature The effect of temperature on the corrosion rate of aluminium (expressed by ) was studied by measuring the corrosion at the temperature range of (293-308)K. The dependence of the corrosion current density on temperature followed Arrhenius equation[9]. ≡ / (5) which can be expressed in logarithmic form: ≡ ln (6) Where A and are repectively the pre-exponential factor and the activation energy of corrosion reaction. A typical linear plot relating values of ln to the reciprocal of temperature as shown in Figure 2 was obtained . The values of could be derived from the slope of the line, and when the linear plot of Figure 2 was extrapolated to ln value at 0, the value of A could be obtained. Table 3 represents the values of ,∆ ∗ and the pre-exponential factor for aluminium sample in 0.7 . Entropy of activation ∆ ∗ was calculated from the value of using the relationship: exp ∆ ∗ (7) were is boltzman constant, is Plank constant, is the universal gas constant and the absolute temperature of the solution[10,11]. 90 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 The entropy of activation is negative, this implies that activated complex in the rate determining step represents association than dissociation, indicating that a decrease in disorder takes place, in going from reactants to the activated complex. Figure 3 shows the typical polarization curves of aluminium in 0.6 solution containing three different concentrations of amino acid (Methionine) as a green inhibitor over the temperature range (293-308)K. Table (4) presents the polarization data and from these data it can be noticed that the addition of methionine caused a decrease in corrosion current densities of aluminium, the inhibition effect of methionine increases as the concentration of the inhibitor increases in the range 10 10 at all temperatures of study. Table (5) shows the values of inhibition efficiencies % which were calculated from equation(8): ≡ (8) Where and are corrosion current densities in absence and presence of the inhibitor respectively. 3.3: Corrosion Inhibition of Aluminum\\ by Methionine The essential effect of corrosion inhibition is because of the presence of electron donor groups (S,N,O) in the molecular structure of the Methionine. It is well known that the presence of heteroatoms, for nitrogen, oxygen and sulfur in the inhibitor structure results in better inhibition[12]. Also the most organic substance employed as corrosion inhibitors can adsorb on the metal surface through heteroatoms. Its inhibition efficiency should follow the sequence [13] . Electron pairs on the heteroatoms can be shared with metal orbitals, forming an insoluble complex that protect the surface from the aggressive ions by blocking of its corrosion sites and hence decreasing the corrosion rate [14]. The presence of pairs of free electrons on sulfur, nitrogen and oxygen atoms favours adsorption of methionine and, thus , the best efficiency of this inhibitor. The activation parameters such as activation energy ∗ and the entropy of activation ∆ ∗ in the range of the studied temperatures (293-308)K for corrosion inhibition of aluminium in 0.6 solution in the presence of various concentrations of methionine were calculated using Arrhenius equation: ≡ (9) The calculated values ∗ and ∆ ∗ are summarized in Table (6). In the presence of the inhibitor, an increase in ∗ values were observed, the variation of activation energy ∗ in the presence of different concentrations of methionine can be illustrated as follows[15]: higher values of ∗ were found in presence of the inhibitor than those without inhibitor, i.e., the addition of methionine raise the energy barrier for the corrosion process of aluminium in chloride acid solution[16]. It was found that there is a direct relation between the values of and A, i.e. simultaneous increase or decrease in and ln for a particular system which can be ascribed to the compensation effect which describes the kinetics of catalytic and tarnishing reaction on the metal. The entropy values of activation, ∆ ∗, in the presence of methionine are negative implying the rate determining step for the activated complex which is the association rather than 91 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 dissociation step, in addition the adsorption process is accompanied by an increase in entropy which is the driving force for the adsorption of methionine on to the aluminium surface[17]. 3.4: Thermodynamic Parameters of The Adsorption Isotherm The efficiency of organic compounds as good corrosion inhibitors depends mainly on their adsorption ability on the metal surface. Basic information about the interaction between inhibitor and metal can be provided by the adsorption isotherm. The investigation of the relation between corrosion inhibition and adsorption of inhibitor is of great importance. The surface coverage Table (7) and the concentration of methionine solution were tested by fitting to various isotherms like: Langmuir, Tempkin and freundlich. However, the fit was obtaind with Langmuir isotherm as shown in Figure (4) which is given by the following equation[18]. 1 (10) Where is the equilibrium constant of the adsorption / desorption process, and it reflects the affinity of the inhibitor molecules towards surface adsorption sites. From the intercepts of the straight lines on the ⁄ axis Fig (4) leads to the equilibrium constant for the adsorption / desorption of methionine process, Table (8). The high value of reveals that the methionine molecule possesses strong adsorption ability on to the aluminium. However, decreased with an increase of temperature indicating that adsorption of methionine on to the metal surface was favorable at lower temperatures. The equilibrium constant of the adsorption / desorption was related to the standard free energy of adsorption according to the equation. ∆ 55.5 (11) Where R is the universal gas constant, T is the absolute temperature, and 55.5 is concentration of water in solution. The standard free energy of adsorption was calculated and is given in Table (8). Generally the standard free energy of adsorption values 25 . or less negative are associated with an electrostatic interaction between charged metal surface or more negative involves charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond. So it can be concluded that adsorption of methionine on to aluminium surface takes place through both physical and chemical adsorption[19]. From Table (4) it was clearly seen that the presence of methionine reduces the corrosion rate of aluminium in sulphuric acid solution. As its well known that different substituents on the organic molecules polarize the functional group in a different manner[11]. This indicates that the protective effect of methionine is not solely due to their reactivity with sulphuric acid in acidic media, of amino acid molecule is readily protonated to form . Also the addition of amino acids with aliphatic radical R leads to decrease corrosion current density but when the radical contains heteroatom as N,O or S a decrease of corrosion current densities in the presence of methionine at 10 ,5 10 and 10 . was noted. shifts to little more negative values in the presence of methionine tested-as shown in Table (7). This result indicates that the inhibitors have been adsorbed to both cathodic and anodic areas. That means methionine is mixed type inhibitor. The same results have been reported by other authers[20,21]. 92 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Then the addition of amino acids molecules in sulphuric acid solution seems that no effect on anodic behavior is observed. This result shows that the inhibitory action of these compounds depends on the potential for higher potential inhibitory character disappears. It is generally accepted that the first step in the adsorption of methionine on the metal surface usually involves the replacement of one or more water molecules adsorbed at the metal surface[22]. ⇌ 12 The inhibitor methionine may then combine with freshly generated ions on the aluminium surface, forming metal inhibitor complex[23] ⟶ 3 13 ⟶ 14 Valuable information about the mechanism of corrosion inhibition can be provided by the values of thermodynamic parameters for the adsorption of inhibitor. Thermodynamically, ∆ were related to the standard enthalpy, ∆ and standard entropy, ∆ according to[8]. ∆ ∆ ∆ (15) And the standard enthalpy of adsorption can be calculated on basis of the Van't Hoff formula: ln ∆ (16) A plot of ln .1⁄ gives a straight line as shown in Fig (5). The slope of straight line is ∆ . The negative sign of ∆ reveals that the adsorption of inhibitor molecules is an exothermic process. Generally, an exothermic adsorption process suggests either physisorption or chemisorption while endothermic process is attributed to chemisorption. The unshared electron pairs in investigated molecules may interact with P-orbitals of aluminium to provide a protective chemisorbed film[16]. The values of ∆ in the presence of inhibitor are negative that is accompanied with exothermic adsorption process. ∆ of inhibitor can be calculated from equation (8) according to[8] following formula . ∆ ∆ ∆ 17 3.5: Optical Microscopy Observation The surface morphology of Aluminium specimens exposed to acid chloride solution in absence and presence of methionine for two hours was examined by optical microscope technique. Figure (6) showed the image of Aluminium surface which immersed in the aggressive solution with and without the addition of 10 mol. dm-3 of methionine. It can be observed that the specimen surface was strongly damaged in absence of inhibitor due to metal dissolution in the aggressive solution. A number of pits distributed over the surface are seen Figure (6c). However, no pits and cracks were observed in the micrograph after the addition of inhibitor to the aggressive solution Figure (6b). Inhibitor molecules adsorbed on active 93 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 sites of Al and a smoother surface was observed when compared to the surface treated with uninhibited acid chloride solution. References 1. Nasim ziaifer, Jila talatmehrabad and farzad arjomendirad (2011). "Aluminium corrosion inhibitors in Acidic Media", J.Basic Appl. Sci. Res, 1(12) 2886-2888. 2. Sahaya. A. Raja, Rajendean, S.; Sathiyabama, J. and Angel P. (2014). "Corrosion control by Aminoactic acid (Glycine) an over view, international Journal of Innovatine Research in science, Engineering and Technology. 3, 4, 11455-11467. 3. Panchal, V.A.; Patel, A.S.; Trivedi, P.T. and Shah, N.K. (2012). "Corrosion Inhibition of AL-Mg alloy in hydrochloric acid using Benzylamine N. (P-methoxy benzylidene" J. Mater. Environ. Sci 3(2) (360-373). 4. Musa, A.Y.; Mohamad, A.B.; Kadhumand, A.A.H., and Tabal, Y.B.A. (2011) "Inhibition of Aluminium Alloy corrosion in 0.5 nitric acid solution by 4-4-Dimethyloxalidine-2- thione Jounrnal of mater. Engin. and per for, 20(3), 294. 5. Ingrid Milosev, Jasminka Pavlinac, Milan Hodosceek And Antonija Lesar (2013). "Amino acids as corrosion inhibitors for copper in acid medium: Experimental and theoretical study" J. Serb. Chem. Soci. 78(12) 2069-2086. 6. Ashassi-Sorkhabi, H.; Ghasemi, Z., and Scifzadeh, D. (2005). "The inhibition effect of some amino acids towards the corrosion of aluminium in 1M HCl+1M H2SO4 solution", Applied surface science 249, 408-418. 7. Zhang, D.Q.; Cai, Q.R.; He, X.M.; Gao, L.X. and Kim, G.S. (2009). "Corrosion inhibition and adsorption behavior of methionine on copper in HCl and Synergistic effect of zinc ions", materials chemistry and physics, 114 (612-617). 8. Deepa Prabhu and Padmalatha RaO (2014). "Corrosion behavior of 6063 aluminium alloy in acidic and in alkaline media" Arabian Journal of chemistry, (1-9). 9. Murgulesen, L.G. and Rdovi, O. (1961). "metal corrosion", Int. congr. 10-5 April, London, 202-205. 10. Zarrouki, A.; warad. I. and Hamouti. B. (2012), "Kinetic Parameters of Activation, int, J. Electrochem. Sci, 5, 1516-1526. 11. Trowsdale, A. J.; Nabel, B.; Harris, S.J.; Gibbins, I.S.R.; Thompson ,G.E. and Wood, G.C., (1996). "The influence of silicon carbide reinforcement on the pitting behavior of aluminium" corros. Sci. 2,. 177-191. 12. Mihit., M.; Bazzi, L.; Salghi. R.; Hammouti B.; elissami S. and Ait adds., E. (2008). "Some tetrazolic compounds as corrosion inhibitors for copper in nitric acid medium" int. Sci. Althern. Energ. Ecology 62, 173 . 13. Moretti, G.; Guidi ,F. (2002) '' Tryptophan as copper corrosion inhibitor in 0.5 M aerated sulfuric acid' Corros. Sci. 44, 1995. 14. khaled, K.F.; fadl-Allah., S.A. and Hammouti., B. (2009) ""some benzotriazole , drivatives as corrosion inhibitors for copper in acidic medium : experimental and quantum chemical molecular dynamics approach" Materials chemistry and physics 117 148-155. 15. Barouni, K.; Kassale, A.; ALbourine,A.; Jbara, O.; Hammouti, B. and Bazzi L. (2014). "Amino acids as corrosion inhibitors for copper in nitric acid medium: Experimental and theoretical study" J.Mater. Environ. Sci. 5(2),. 456-463. 16. Aplya,S. ;Fakrudeen, P.; Ananda murthyh, C. and Bheema Ratu. V., (2012). "Corrosion Inhibition of AA 6061 and 6063 Alloys in Hydrochloric Acid Medium by Schiff base compound", Journal of the Chilean chemical Society, 57, 4, 1364-1370. 17. Fouda, A. S.; Shalabi, K. and Mohamed. N.H. (2014). "Corrosion Inhibition of aluminium in Hydrochloric Acid solutions using some chalcone derivatives" international J. of innovative research in science, engineering and technology, vol 3, 9861-9875. 18. Kliskic, M.; Radosevic, J. and Gridic. S.(1997)."Pyridine and its derivatives as inhibitors of aluminium corrosion in chloride solution". J. Appl. Electrochem.. 27, 947-952. 94 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 19. Ghongli, X.; Deng., Sh. and Huifu (2011). "Sodium molbydate as corrosion inhibitor for aluminium in solution J. corrosion science. 53 (2748-2753). 20. Achutha kini, U.; Prakash Shctty, Divakara Shetty, S. and Arun Isloor., M. (2011). "Corrosion inhibition of 6061 AA/SiCp composite in hydrochloric acid medium using. 3- chloro-1-benzothiophene-2-carbohydrazide". 18, 439-445. 21. Sorkhabi, A.; Majidi, H. and Seyyedi., M.R., K. (2004). "investigation of inhibition effect of some aminoacids against steel corrosion in HCl solution", Applied surface science,.225 (1-4), 176-185. 22. Lebe Nnanna, A.; Onyinyechi, Nwadiuko, C.; Nneka ekekwe., D.; Chibneze ukpadi., F., Solomon udensi., C., Kelechukwu okema., B., Beniah onwugba., N. and Ihebrodike mejeha M. (2011). "Adsorption and inhibitive properties of leaf exract of new bouldia leavis as Green inhibitor for Aluminium Alloy in ", American Journal of materials science 1(2), 143-148. 23. Shrier., L.L. (1976). "Over potential, corrosion Metal/ Environments Reactions", Newnes, Butterworths, Boston,. 1, 35. Table (1): The chemical composition of pure aluminium Si Fe Cu Mn Mg Cr Ni Zn Ga V 0.07 0.2 0.05 0.05 0.001 - - 0.009 - 0.005 Table (2): Data of polarization curve for corrosion of pure aluminium in . . solution at . over the temperature range (293-308)K. T/K .⁄ ⁄ ⁄ decade-1 ⁄ decade-1 Weight loss/ . . Penetration loss/ .year-1 293 129 634 528.3 47.1 10.3 1.4 298 163 678 777.6 44.2 13.1 1.78 303 203 696 303.1 44.5 16.3 2.21 308 326 739 195.9 64.7 26.2 3.55 Table (3): Activation energy , pre-exponential and entropy of activation ∆ ∗ for pure aluminium corrosion in . . solution in . . .⁄ ∆ ∗ . . A/molecule . 44.86 57 602300000001023 95 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Table (4): Values of , with different concentrations of methionine at temperature range (293-308) K in . T/K Inhibitor . . ⁄ .⁄ 293 0 634 129 1 10 667.9 59.98 5 10 630 54 1 10 624 43.73 298 0 678 163 1 10 654 82.44 5 10 654 74.75 1 10 622 58 303 0 696 203 1 10 645.9 103.66 5 10 685 94 1 10 654 76 308 0 739 326 1 10 674 179.96 5 10 688 171 1 10 678 131.7 96 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Table (5): Values of inhibitor efficiencies (IE%) calculated from . For pure aluminium in . Conc. of methionine T/K IE% from . 1 10 293 53.6 298 50 303 49 308 45 5 10 293 58.2 298 54.2 303 53.7 308 48 1 10 293 66.2 298 64.5 303 62.6 308 60 Table (6): Activation energy ,pre-exponential factor and entropy of activation ∆ ∗ for the corrosion of pure aluminium in the value 0.7 in . . solution and different concentrations of L-methionine. Conc.of L-methionine . .⁄ ∆ ∗ . . A/molecule . 1 10 45.96 52.25 954581169801023 5 10 48.83 48.45 1697512040001023 1 10 51.7 37.62 60230000001023 Table (7): Chosen corrosion parameter, degree of surface coverage and corrosion inhibition efficiency for pure aluminium in . . solution in . with different concentrations of methionine at various temperature. T/K . Meth. . ⁄ .⁄ IE% 293 0 634 129 - 1 10 667.9 59.98 0.53 53.6 5 10 630 54 0.58 58.2 1 10 624 43.73 0.66 66.2 298 0 678 163 - - 1 10 654 82.44 0.50 50 5 10 654 74.75 0.54 54.2 1 10 622 58 0.64 64.5 303 0 696 203 - - 1 10 645.9 103.66 0.49 49 5 10 685 94 0.53 53.7 1 10 654 76 0.62 62 308 0 739 326 - - 1 10 674 179.96 0.45 45 5 10 688 171 0.48 48 1 10 678 131.7 0.60 60 97 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Table (8): Equilibrium constant adsorption / desorption, standard free energy, enthalpy and entropy of adsorption onto pure aluminium in . . solution in . in the presence of methionine at various concentrations. T/K k ∆ . ∆ . ∆ . . 293 1666.7 26.75 26 180.03 298 1428 27.20 178.83 303 1111 27.70 177.23 308 1000 28.12 175.7 98 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Fig. (1): Polarization curves of pure Al corrosion in pH=0.7 with (0.6 mol.dm-3) NaCl solution at four temperatures in the range of (293-308)K. Fig. (2): Arrhenius Plot relating Log (icorr) to 1/T for the corrosion of pure Al in the pH = 0.7 in presence of (0.6 mol.dm-3) NaCl over the temperature range (293 – 308) K. 3.2E-3 3.2E-3 3.3E-3 3.3E-3 3.4E-3 3.4E-3 3.5E-3 1/T 2.0 2.1 2.2 2.3 2.4 2.5 2.6 L o g ( ic o rr ) Y = -2057.14 * X + 9.09428 R-squared = 0.999691 99 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 (a) (b) (c) Fig. (3): The typical polarization curves of pure Al in with (0.6 mol.dm-3) NaCl solution containing of L-methionine as inhibitor (a= 110-3, b = 510-3, c =110-2 mol.dm-3) over the temperature rang (293-308)K. 100 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 Fig. (4): Langmuir adsorption plots of L-methionine on the pure Al in 0.6 mol.dm-3 NaCl solution at various temperature. Fig. (5):Van't Hoff plot pure Al in 0.6 mol. dm-3 NaCl solution containing L-methionine in pH = 0.7 3.2E-3 3.2E-3 3.3E-3 3.3E-3 3.4E-3 3.4E-3 3.5E-3 1/T 6.6 6.8 7.0 7.2 7.4 7.6 L n ( K a d s .) 101 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 (a)Before any treatment (b)With inhibitor 110-2 . . (c) 0.6 . solution at 0.7 Fig. (6): Typical microstructure of pure aluminium in the corroded medium at . , magnification power 400 102 | Chemistry 2015) عام 3العدد ( 28مجلة إبن الھيثم للعلوم الصرفة و التطبيقية المجلد Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 28 (3) 2015 اعاقة تآكل األلمنيوم في المحيط الحامضي باستعمال الحامض االميني المثيونين وصال عبد العزيز عيسى زينب وجدي احمد الكيمياء / كلية التربية للعلوم الصرفة (ابن الھيثم) / جامعة بغداد قسم 2015/حزيران/7قبل البحث في:، 2015/نيسان/26استلم البحث في: الخالصة لكلوريد 0.7يتناول موضوع البحث دراسة كھروكيميائية لسلوك تآكل االلمنيوم في محلول حامضي 0.6الصوديوم بتركيز بغياب ووجود تراكيز مختلفة من الحامض االميني المثيونين كمثبط صديق للبيئة . ) كما اشتملت الدراسة على طريقة االستقطاب الكھروكيميائي باستعمال جھاز 308K-293على مدى درجات الحرارة ( المجھاد الساكن والمجھر الضوئي. كيز المثبط لكنھا تقل بزيادة درجة الحرارة. اظھرت النتائج ان التثبيط يحدث من خالل وجد ان كفاءة التثبيط تزداد بزيادة تر Langmuirامتزاز جزيئات المثبط على سطح الفلز (االلمنيوم) ووجد بأنھا تتبع مسار درجة الحرارة لالمتزاز لالنكمير ( Isotherm for Adsorption وطاقة التنشيط ∆). وقد تم حساب بعض المعلمات الثرموديناميكية∆ mixed type)لتوضيح ميكانيكة تثبيط التآكل. قياسات االستقطاب اوضحت بأن المثيونين مثبط من النوع المختلط inhibitor) ) وقد تم تمييز سطح الفلز قبل وبعد التثبيط بواسطة المجھر الضوئيoptical microscope.( المثيونين، ميكانيكية تثبيط التآكل ، المعلمات الثرموديناميكية.: االلمنيوم، الكلمات المفتاحية