Corrosion and corrosion inhibition of Corrosion and Corrosion Inhibition of α-Brass by Thiourea Wisal Abdil Aziz Isa Awf A.R. Ahmed Dept. of Chemistry/College of Education For Pure of Sciences(Ibn Al- Haitham) / Universty of Baghdad Received in : 29 May 2013, Accepted in : 4 December 2013 Abstract The corrosion behavior and corrosion inhibition of α-brass (65.3% Cu, 34.4% Zn and others 0.3%) in 0.6 mol.dm-3 NaCl solution have been investigated using potentiostatic polarization technique, the main results obtained were expressed in terms of corrosion (Ec) and corrosion current (ic). The research was performed in neutral and slightly acidic media [pH=7 and pH=4] over the temperature range (288-318)K. It was found that the rate of corrosion increases with the increase of acidity and the increase of temperature. The rate of corrosion increased with the increase of temperature in conformity with Arrhenius equation. Values of activation energy (Ea*), pre-exponential factor (A) and entropy of activation (∆S*) have been derived for the corrosion process. Also the thermodynamic quantities (∆G, ∆S and ∆H) have been determined for the process. The inhibition effect of thiourea on the corrosion of α-brass in chloride solution was studied, and it was found that the addition of thiourea to the chloride solution caused a decrease in the values of corrosion current density and changed to some extent the values of kinetic parameters. The values of (Ea) increases in the presence of thiourea, this means that the decrease of the concentration of thiourea. Key words: α-brass, thiourea, corrosion inhibition, kinetic and thermodynamic parameters. 212 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Introduction copper and its alloys are of considerable importance as they form the backbone of modern industries due to their excellent electrical and thermal conductivity. [1, 2] brass has been widely used as tubing material for condensers and heat exchangers in various cooling water system. [3] Brass is susceptible to a corrosion process known as dezincification in chloride-containing solutions leading to structural failure. [4,5], and this tendency of dezincification increases with the increas zinc content of the brass. [6] When brass undergoes corrosion, a zinc oxide layer is initially formed which passivites the brass surface, when brass is dipped into a media containing chloride ion, an insoluble film of cuprous chloride is adsorbed on the brass surface. The copper ions can pass into the solution by disproportionation reaction or it can dissolve with the formation of complexes with CuCl2 - [7]. The formation of stable is possible inside the pores of CuCl2- layer. As a result the brass surface became enriched with copper while being impoverished by zinc during corrosion. [8]. These changes make brass surface less resistant to corrosion than copper in chloride containing media. Since, brass is not completely resistant to corrosion, especially in oxygen- containing electrolytes, the use of inhibitor to prevent or minimize corrosion is a standard practice, despite a wide-spread use of inhibitors to control corrosion, still there is a need for more comprehensive and systematic study to understand their mode of action. Precise understanding has become important. Thiocarbamides [9] were well known for its corrosion inhibition efficiency, especially on transition metals mainly due to the presence of sulphur atom, which has a high electron density and can therefore easily bind to the metal surface. In thiocarbamide, the presence of - group will further contribute stronger binding and enhance inhibition [10]. The effectiveness of any corrosion inhibitor is dependent on the type of the metal, properties of corrosive enviroments as well as the state of inhibitor molecule. The aim of the present work is to study the effect of thiourea as corrosion inhibitor for dissolution of α-brass in 0.6mol. dm-3 solution in neutral and slightly acidic media using potentiostatic polarization measurements. The effect of temperature on the dissolution of α- brass in the absence and presence of the inhibitor was also investigated through the calculations of the kinetic and thermodynamic parameters such as activation energy Ea*, activation entropy and pre-exponential factor A. Experimental Part 1.Materials a- Sodium chloride (analar grade) was used for the preparation of the electrolyte solution of 0.6mol.dm-3 (3.5% w/w). b- Commercial α-brass with the following composition as wt% [65.3% Cu, 34.4% Zn], the remainder being trace amounts of Fe, Sn, Pb, Ni and Al. c- Thioura (sigma-Aldrich 98%) was used as received. 2.Potentiostatic Polarization Studies The potentiostatic polarization studies were carried out with apiece of α-brass, which was cut in the form of a disk [2cm diameter and 0.2cm thickness] having an exposed surface area of 1cm2 to corrosive medium. The working electrode (α-brass) was abraded mechanically and successively with different grades of emery paper [200, 400,800, 1200 and 2000] and washed with double distilled water. Further, the samples were degreased with acetone and thoroughly washed with double distilled water then dried in air and kept in a desicator until use. 213 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 An electrochemical cell with a three electrodes assembly was used to study the electrochemical measurements. Brass specimens with exposed area of 1cm2, a platinum electrode and silver-silver chloride in staturated were used as working, auxiliary and reference electrodes respectively. The polarization experiments were carried out using the M lab potentiostat / Galvanostat 200 Germany obtained from bank electronic intelligent controls . M lab was connected to personal computer by a RS 232 serial cable and controlled by computer desktop. The M lab software cares for controlling the potentiostat, recording and processing data. It is provided with electrochemical calculations like tafel line evaluation, re-scaling of the potential and integration. The experiments were performed in the electrolyte solution of 0.6mol. dm-3 at two different values of pH in the absence and presence of different concentrations of the inhibitor (thiourea) over the temperature range mentioned before, the operation program involved sending a set of commands from the computer to the potentiostat, and the polarization experiments were carried for brass specimen at a scan rate of 10mV/s. corrosion current density icorr and corrosion potential Ecorr . Were determind from the polarization curve in addition to other infromations such as tafel. Slopes, weight loss and penetration loss. In order to test the reproducibility of the results, the experiments were performed in triplicate. Results and Discussion 1- Polarization Behavior Figs. 1 and 2 show the anodic and cathodic polarization curves of -brass in 0.6mol.dm- 3 NaCl solution at four temperatures in the range of (288-318) K and two pH values (pH= 7 and pH =4). Tafel extrapolation method was used to calculate the corrosion parameters from polarization curves. The resulting data are displayed in Table (1) and these data show that corrosion current density (icorr) increases with the increase of temperature and nearly corrosion potential (Ecorr) follows similar manner with the increase of temperature. Also it was noticed that all values of icorr at pH=4 are more than those in pH=7 at all temperatures of study which indicate that α- brass has more tendency to corrode in acidic medium, and this result is enhanced by the values of pentration and weight loss. (Table.1). Anodic and cathodic tafel slopes show variation in their values which can be attributed to the variation of the rate determing step (r.d.s.) of the metal dissolution reaction (Andoic) and the charge transfer process (desorption or electrochemical desorption) (cathodic) [11]. 2- Temperature Dependence of The Corrosion Current Density The rate of α-brass corrosion (r) (which is expressed by icorr) at a given concentration increased considerably with the rise of temperature. The dependence of the corrosion current density (icorr) on temperature followed Arrhenius equation [12]. r =icorr = Aexp (-Ea/ RT)…………(1) which can be expressed in logarithmic form: log icorr = log A - RT3.2 E a …………(2) Where A and are respectively the pre-exponential factor and the activation energy of corrosion. A typical linear plot relating values of log to the reciprocal of temperature (1/T) is shown in Figs.3 and 4. The value of could be derived from the slope of the line, and when the linear plot of Figs. 3 and 4 was extrapolated to log value at 1/T=0, the value of A could be obtained. 214 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Table 2 presents the values of and the pre-exponential factor A for α-brass corrosion in the two pH values 7and 4. It was found that there is direct relation between the values of and A i.e. smiltataneous increase or decrease in and log A for particular system which can be ascribed to the compensation effect which describes the kinetics of catalytic and tarnishing reactions on the metal. [13, 14] Entropy of activation (∆S*) was calculated from the value of A using the relationship. A = h kT exp(∆S*/R)…………….(3) Where k is boltzman constant, h is plank constant, R is the universal gas constant, and T is the temperature of the solution. The negative values of the entropy of activation (∆S*) for α-brass corrosion implies a less in the over-all degrees of freedom throughout the formation of the activated complex for the reaction of α-brass constituent with the negative species (Cl- & OH- ) leading to the formation of corrosion product [15], when the activated complex result only after considerable arrangements of the structure of reactant molecules, making the complex aless probable structure, ∆S* is negative, and the reaction will be slower. [16]. 3- Thermodynamics of Corrosion of α-brass The change in Gibbs free energy (∆G) for the corrosion of the metal specimens in a given pH at given temperature can be estimated from the equation[17]: ∆G = - nFEcorr …………..(4) Where n (considered to be equal 2) is the number of electrons involved in the anodic process. From the values of at four different temperatures in the range (288-318)K at the two values of pH (7 and 4), the change in the entropy () of corrosion process could be derived using the well-known thermodynamic relation. -d(∆G)/ dT = ∆S…………..(5) Utilizing the values of and , it was possible to calculate the values of the change in the enthalpy for the corrosion process from the relation: ∆G = ∆H - T∆S……………(6) The thermodynamic quantities ∆G, ∆S and ∆H for α-brass corrosion in 0.6mol.dm-3 NaCl solution and in the two pH values at four different temperatures (288-318)K are given in Table (3). The obtained results indicate negative values of ∆G that means spontaneous reactions occur. The enthalpy changes (∆H) for α-brass corrosion have negative values indicating exothermic reaction while the positive values of ∆S for this corrosion confirm that the corrosion process is entropically favorable. 4- Corrosion Inhibition of α-brass by Thiocarbamide Fig. 5 (A,B,C,D,E,F) shows the typical polarization curves of α-brass in 0.6mol. dm-3 NaCl solution containing three different concentrations of thiourea over the temperature (288- 318)K. Table 4 presents the polarization data (Ecorr. and icorr.) and from these data, it can be noticed that the addition of thiourea caused a decrease in corrosion current densities of α-brass, but this inhibition effect of thiaurea decrease as the concentration of thiourea increased in the range (10-2-10-4) mol.dm-3 at all temperatures of study. Thiourea has high inhibition efficiency at low concentration and loses its efficiency at high concentrations. [17] the acceleration of corrosion at higher concentration of thiourea is explained in several different papers [18-20], but no satisfactory explanation is given. The inhibition efficiency of thiourea is controlled by the adsorption of molecular species which increases the inhibition, and the protonated species, accelerating the rate of corrosion. The protonation process is controlled by the charge density 215 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 on the sulphur atom, lengthening and weakening of the C-S bond and the reactivity of the molecule. Table 5 shows the values of protection efficiencies (p%) which are calculated from equ.(7), icorr(1) and icorr(2) are corrosion current densities in the absence and presence of the inhibitor respectively. P% = (icorr(1) – icorr(2) / icorr(1) × 100 ……………(7) 5- Effect of Temperature on The Inhibition of α-brass The effect of temperature on the corrosion rate of α-brass in 0.6mol.dm-3 NaCl solutions at (two pH values) in the presence of different concentrations of inhibitor was studied in the temperature range (288-318)K. As the temperature increases, the rate of corrosion increases and hence the inhibition efficiency of the inhibitor decreases. This is due to the desorption which is aided by the increase of the temperature. This behave proves that the adsorption of the inhibitor on α-brass surface occurs through the physical adsorption. The activation energy Ea and A and entropy of activation ∆S* for corrosion of α-brass in the presence of different concentrations of inhibitor were calculated from Arrhenius type equation. r= Aexp (-Ea/RT) ……………..(8) Where the values of (r) was taken to be proportional to the corrosion current density (icorr.), A is the pre-exponential factor, Ea is the activation energy and R is the gas constant. The Arrhenius law is presented as a straight line when log icorr values are plotted against 1/T values Fig. 5 (A.B.C.D.E.F) are estimated from the slopes and intercepts of the plots respectively, and these values are collected in Table 6. The variation of activation energy in the presence of different concentrations of thiourea would be illustrated as follows; [21] lower values were found in the presence of inhibitor than those without inhibitors i.e. this type of inhibitor retard corrosion at high temperatures but inhibition is diminished at ordinary temperatures. The decrease of Ea with an increase of Thiourea concentration suggest that the energy barrier of corrosion reaction decreases with inhibitors concentration. The higher Ea values in the presence of inhibitor supports the proposed physisorption mechanism, (the system with 10-4 mol.dm-3 thiourea) unchanged or lower values of Ea in inhibited systems indicate chemisorption mechanism[22]. Conclusion 1. The (α–brass) corrodes in sodium chloride solution by selective corrosion reaction which increases with the increase of acidity and temperature. 2. Thiourea acts as inhibitor for α-brass corrosion, and the inhibition efficiency decreases as concentration of the inhibitor increases. References 1. Zaky A. M., (2001), Electrochemical behaviare of copper-silver alloy in sodium carbonate aqueous solution, Bri. Corros. J 36, 59. 2. Deen K.M.; Ahmad R.; Younas M. A.; Khan M. A. and Khan M.U., (2009), corrosion inhibition Evaluation of Alpha-Beta Brass in tap water, Journal of Pakistan institute of chemical Engineers, 24 (69-71). 3. Gasparac R.; Martin C.R. and Stupnisek-lisac E., (2000), in situ studies of imidazole and its derivatives of copper corrosion inhibitors. I. Activation energies and thermodynamics of adsorption., J. Electro chem. Soc. 147, 548. 216 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 4. Gad-Allah A.G.; Abou-Romia M. M.; Badaway W.A. and Rehan H.H. (1991) Passivity of α-brass (Cu, Zn 67/33) and its break-down neutral and alkaline solution containing halide ions., J. Appl. Electrochem. 21,829. 5. Raj X. Joseph and Rajendran N. (2011) Corrosion inhibition effect of substituted thiadizoles on brass, Int. J. Elctrochem. Sci. 6, 348-366. 6. Quraishi M.A.; Farooqi I.H. and Saini P.A. (2000) Technical note inhibition of dezincification of (70-30) brass by aminoalkyl mercaptotriazoles, Br. Corros. J., 35, 78-80. 7. Abdallah M.; Al-Agez M. and Fouda A.S. (2009) phenylhydrozone Derivatives as Corrosion Inhibitors for α-brass in hydrochloric acid solutions, Int. J. Electrochem. Sci, 4, 336-352. 8. Antonijevic M. Milan; Milic M. Snezana, and Marija B. Petrovi, (2009) films formation copper surface in chloride media in the presence of azoles, corros. Sci. 51, 1228. 9. Taha K.K.; Sheshadri B.S.; Ahmed M.F. and Muralidharan V.S. (2006) corrosion inhibition of brass thiocarbamides, Indian Journal of chemical technology, 13 PP., 128-134. 10. Nathan C.C. (1973) corrosion inhibitors National Association of corrosion Engineers, inhibitors, Houston texas, P. (11-12). 11. Bokris J.O.M. and Reddy A.K.N., (1970) Modern Electrochemistry, phenum press, New York. 2.p. 883. 12. Murguleseu I.G. and Radoviei O., Int., Congr. Metal corrosion, 10-15, April (1961) London, P. 202-205. 13. Fouda A.s. and Mahfouz H. (2009) Inhibition of corrosion of α-brass (Cu-Zn, 67/33) in solutions by some arylazo indole derivatives., Journal of the Chilean Chemical Soc. 54 No.3, PP (302-308). 14. Al-Haidari Y.K. and Saleh J.M. (1988) Studies of Adsorption and corrosion Inhibition of some organic sulphur compounds on some metals and alloys, J. of chem. Soc., 54, 189. 15. Scully J.C., (1978) the fundamentals of corrosion, International series on materials and technology, anodic reaction, 17, P. 220. 16. Sinko P.J. (2000) Physical, Chemical and Biopharmaceutical principles in pharmaceutical sciences, Fifth Ed. USA; P. 413. 17. Sherir L.L. (1976) "Corrosion Metal Environment Reactions", 2nd Ed., 1.pp. 13.164. 18. Loto R.T.; Loto, C.A. and Popovla, A.P. I. (2012) corrosion inhibition of thiourea and thiadiazole derivatives; A Review, J. Mater Environ. Sci. 3(5), 885-894. 19. Tang, Y.; Yang, X.; Chen, W. and Wan, R. (2010) A preliminary. Investigation of corrosion inhibition of mild steel in 0.5M H2SO4 by 2-amino-5- (n-pyridl), 1,3,5 – thiadiazole, polarization EIS and molecular dynamics solution, corrosion, Sci, 52(242): 1801-1808 20. Shen, Wang, and Yang, Long (2006) The adsorption stability and inhibited by allyl- thiourea of bulk nanocrystalline iron in dilute HCl , Applied surface science, Elsevier 2118- 2122. 21. Bentiss, Lebrini, Lagrence (2007) low toxicity copper corrosion inhibitors, corrosion (713-720). 22. Zarroukl A.; Warad, I. and Hsmmouti, B. (2010) Kinetic parameters of Activation, Int. J. Electrochem. Sci., 5:1516-1526. 23. Hammouti, E.B.; Aouniti, A.; Ramli, Y.; Azougagh ,M.; Essassi, E.M. and Bouachrine, M. (2010) Thermodynamic characterization of steel corrosion in in the presence of 2-phenyl thieno (3,2- b) Quinoxaline, J. Mater Enviro, Sci., 121. 217 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Table (1): Data of polarization curve for corrosion of α-brass in 0.6 mol.dm-3 NaCl solution at three pH values (4 and 7) over the temperature range (288-318)K. pH T/K icorr/µA.cm - 2 -Ecorr/mV ba/mV. decade-1 -bc/mV. decade-1 weight loss/ g.m-2. day-1 Penetration loss/mm.year-1 4 288 1.26 238.9 36.5 23.2 0.358 0.0146 298 1.76 240.3 52.4 185.4 0.551 0.0225 308 2.91 268.2 51.5 133.4 0.763 0.0312 318 9.69 264.0 56.4 159.1 1.820 0.0744 7 288 1.22 248.8 57.4 119.5 0.348 0.0142 298 1.63 257.2 53.9 124.5 0.522 0.0213 308 2.17 258.9 48.7 120.2 0.618 0.0253 318 5.85 257.6 43.0 143.2 1.520 0.0622 Table (2): Activation energy (Ea), pre-exponential factor (A) and entropy of activation (∆S*) for α-brass corrosion in 0.6 mol.dm-3 NaCl solution. pH Ea/kj. mol-1 -∆S*/J.K-1.mol-1 A/ molecule. cm-2.s-1 4 47.442 81.835 22.019×1031 7 50.935 74.803 73.703×1031 Table (3): The thermodynamic functions for corrosion of α-brass in 0.6 mol.dm-3 NaCl solution over the temperature range (288-318)K in three pH values (4 and 7). pH T/K -∆G/kj.mol-1 ∆H/Kj.mol-1 ∆S/j.K-1.mol-1 4 288 46.107 3.169 171.1 298 46.377 4.610 308 51.762 0.936 318 50.952 3.457 7 288 48.018 -32.408 54.2 298 49.639 -33.487 308 49.967 -33.273 318 49.716 -32.480 Table (4): Values of (-Ecorr., icorr. and inhibitor effecincy percent) with different concentrations of thiourea at temperature rang (288-318)K in pH=4and 7 pH T/K Inhibitor conc. mol.dm-3 -Ecorr/mV -icorr. µA.cm -2 IE% 4 288 0 196.5 1.26 0 1×10-2 413.3 1.53 - 1×10-3 320.0 0.85 32.5 1×10-4 283.0 0.78 38.0 298 0 248.6 1.76 0 1×10-2 408.4 1.6 9.0 1×10-3 326.3 0.9 48.8 1×10-4 285.0 0.80 54.5 308 0 268.2 2.91 0 1×10-2 403.4 2.76 5.1 1×10-3 335.3 1.72 40.8 1×10-4 314.5 1.38 52.5 218 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 318 0 264.0 9.69 0 1×10-2 392.0 5.8 40.1 1×10-3 339.8 3.42 64.4 1×10-4 272.1 3.1 68.0 7 288 0 248.8 1.22 0 1×10-2 417.0 1.02 16.3 1×10-3 325.7 0.589 51.17 1×10-4 287.4 0.50 59.0 298 0 257.2 1.63 0 1×10-2 403.2 1.3 20.24 1×10-3 327.6 0.675 58.5 1×10-4 295.6 0.31 80.98 308 0 258.9 2.17 0 1×10-2 407.3 2.21 - 1×10-3 331.9 1.22 43.7 1×10-4 311.7 1.12 48.3 318 0 257.6 5.85 0 1×10-2 404.6 4.8 17.9 1×10-3 332.2 2.73 53.33 1×10-4 298.7 1.68 71.28 Table (5): Values of protection efficies calculated from icorr. pH Conc. of thiourea mol.dm-3 T/K P% form icorr. 4 1×10-2 288 -21.4 298 29.8 308 -8.0 318 16.4 1×10-3 288 7.9 298 31.4 308 35.8 318 62.1 1×10-4 288 37.6 298 58.7 308 48.5 318 23.1 7 1×10-2 288 16.3 298 11.4 308 -1.8 318 31.9 1×10-3 288 34.0 298 52.9 308 43.7 318 67.6 1×10-4 288 58.4 298 55.6 308 48.3 318 68.5 219 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Table (6): Activation energy (Ea), pre-exponamtial factor (A) and entropy of activation (∆S*) for the corrosion of α-brass in the pH values 4 and 7 in 0.6mol.dm-3 NaCl solution and different concentrations of thiourea. pH Conce. of thiourea mol.dm-3 Ea/ KJ.mol-1 A/ molecule. cm- 2.S-1 -∆S*/ J.k-1. mol-1 4 1×10-2 51.008 81.411×1031 73.977 1×10-3 52.765 95.699×1031 72.634 1×10-4 53.679 118.279×1031 70.875 7 1×10-2 51.765 89.062×1031 73.231 1×10-3 55.331 196.114×1031 66.677 1×10-4 66.092 8.41×1034 35.467 Fig. (1): Polarization curves for the corrosion of α-brass in pH=7 with 0.6mol.dm-3 solution at different temperatures in the range of (288-318)K. Fig. (2): Polarization curves for the corrosion of α-brass in pH=4 with 0.6mol.dm-3 solution at different temperatures in the range of (288-318)K. 0.00001 0.0001 0.001 0.01 0.1 1 10 Log current (micro A/cm^2) -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 Eco rr. ( mV ) 288K 298K 308K 318K Po te nt ia l / m V v s. SC E Fig. (3): Arrhenius plots relating log icorr. Vs.1/T for the corrosion of α-brass in 0.6mol.dm-3 NaCl in pH=4 Fig. (4): Arrhenius plots relating log icorr. Vs.1/T for the corrosion of α-brass in 0.6mol.dm-3 NaCl in pH=7 221 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Figure (5 A,B and C): Arrhenius plot, Log icorr as a function of the reciprocal temperature (1/T) for corrosion of α-brass in 0.6 mol.dm-3 NaCl solution over the temperature range (288-318)K in pH=7 with three concentration values of thiourea (10- 2, 10-3 and 10-4) mol.dm-3. A B C 222 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 Figure (5 D,E and F): Arrhenius plot, Log icorr as a function of the reciprocal temperature (1/T) for corrosion of α-brass in 0.6 mol.dm-3 NaCl solution over the temperature range (288-318)K in pH=4 with three concentration values of thiourea (10- 2, 10-3 and 10-4) mol.dm-3. D E F 223 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014 براص بواسطة الثایویوریا -سبیكة الفاتآكل وتثبیط تآكل وصال عبد العزیز عیسى عوف عبد الرحمن احمد جامعة بغداد/ )ابن الھیثم(ربیة للعلوم الصرفة كلیة الت /قسم الكیمیاء 2013كانون األول 4، قبل البحث في : 2013آیار 29استلم البحث في : الخالصة )α-brassكھروكیمیائیة لتأكل وتثبیط تاكل سبیكة الفا براص (یتناول موضوع البحث دراسة Cu 65.3 ، %Zn 34.4 باستعمال تقنیة المجھاد 3-مول. دسم 0,6% في محلول ملحي لكلورید الصودیوم بتركیز . درس سلوك التآكل في الوسط المتعادل والقلیل (ic)الساكن، وتم التعبیر عن النتائج من خالل تسجیل قیم جھد التآكل كلفن. لوحظ ان سرعة التآكل تزداد بزیادة 318الى 288وعلى مدى من درجات الحرارة من 7و pH 4الحامضیة الحامضیة وزیادة درجة الحرارة، اذ خضعت حركیة تفاعل التآكل لمعادلة ارینیوس التي من خاللھا حسبت قیم طاقة كما امكن حساب الكمیات الثرمودینامیكیة لتفاعل (*S∆)وانثروبي التنشیط (A)ومسبوق المقدار االسي (*Ea)التنشیط كما اشتمل البحث على دراسة استعمال مادة الثایویوریا مثبطاً لعملیة تآكل سبیكة الفا براص في (G, ∆S, ∆H∆)التآكل تضح ان اضافة الثایویوریا عملت على تقلیل سرعة التآكل من محلول كلورید الصودیوم بالتركیز المشار الیھ اعاله، وا خالل قیم تیار التآكل المتناقصة، ولوحظ زیادة قیم طاقات التنشیط عند اضافة الثایویوریا وھذا یعني ازدیاد الحاضر الطاقي لعملیة التآكل وكانت ھذه الزیادة في عالقة عكسیة مع تركیز الثایویوریا. المفتاحیة: الفا براص، ثایویوریا، تثبیط تآكل، معلمات حركیة وثرمودینامیكیة.الكلمات 224 | Chemistry @a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹127@@ÖÜ»€a@I1@‚b«@H2014 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (1) 2014