1-7 Al-Khwarizmi Engineering Journal,Vol. 11, No. Effect of Acetic Acid AA2319-T3 Al-Alloys Department (Received Abstract The present study investigates the effect of sealed anodized AA2319-Al-alloys. Anodizing treatment was performed in without addition of acetic acid. Hydrothermal sealing was carried out in boiling water for each anodized specimen. open circuit potential of the unsealed and sealed anodized measurement for the purpose of starting in scanning polarization diagrams. measurements were performed to assess and sealed anodized aluminum. It was aluminum was sensitive to degree of seal phosphoric acid electrolyte mainly due to their lower anodic and cathodic currents obtained on the sealed anodized aluminum surface corrosion improvement. The present study clearly validates the beneficial role of anodizing baths for the corrosion protection of AA2319 Keywords: Phosphoric Acid, Acetic Acid 1. Introduction Aluminum-Copper (Al-Cu) alloys have a low density and high electrical and thermal conductivity [1]. AA2319 (~ 6% Cu), composed of an Al2Cu phase and Al-rich areas, is particularly promising due to its high strength and thus it is currently used in aerospace applications [1, 2]. Unfortunately, these alloys suffer from poor corrosion resistance, due to their high Cu content, and are highly susceptible to water initiated pitting corrosion, potentially leading to fatigue failure [2]. The widely using of Chromic acid (C anodizing in the aeronautic industry corrosion resistance of aluminum alloys [ incorporation of Cr (VI) and Cr (III) into the oxide gives higher corrosion resistance than that produced by other anodizing processes. Since the beginning of the 1990s, the uses of Khwarizmi Engineering Journal,Vol. 11, No. 4, P.P. 1- 7 (2015) Acid on Electrochemical Behavior of Sealed lloys Anodized in Phosphoric Acid Khalid H. Rashid Department of Chemical Engineering / University of Technology Email: alhadidikhalid@yahoo.com (Received 26 January 2015 ; accepted 21 June 2015) The present study investigates the effect of acetic acid on corrosion behavior and its potential . Anodizing treatment was performed in stagnant phosphoric . Hydrothermal sealing was carried out in boiling water for each anodized specimen. open circuit potential of the unsealed and sealed anodized samples was examined using open circuit potential measurement for the purpose of starting in scanning polarization diagrams. The potentiostatic polarization performed to assess corrosion behavior and sealing quality (i.e., degree of It was shown that the potentiostatic anodic and cathodic polarization of anodized aluminum was sensitive to degree of seal, which was more efficient in the case of anodized aluminum acid electrolyte mainly due to their higher corrosion potential and lower corrosion current obtained at sealing exposure time 60 min. This suggests a specific role of the species surface coating, which contributes to the increasing of the performance in terms of . The present study clearly validates the beneficial role of acetic acid protection of AA2319-Al-alloys. Acetic Acid, Aluminum Alloy 2319, Anodizing, Polarization Technique Cu) alloys have a low density and high electrical and thermal 6% Cu), composed rich areas, is promising due to its high strength and thus it is currently used in aerospace applications ]. Unfortunately, these alloys suffer from poor corrosion resistance, due to their high Cu content, and are highly susceptible to water- ion, potentially leading to Chromic acid (CrO3) anodizing in the aeronautic industry is to improve corrosion resistance of aluminum alloys [3]. The incorporation of Cr (VI) and Cr (III) into the oxide gives higher corrosion resistance than that produced by other anodizing processes. Since the uses of Cr (VI) have been restricted in industrial applications their high toxicity. As a consequence, numerous attempts have been made to find less toxic alternatives, but with limited success [ During the last decade, interesting acid (H3PO4) anodizing and acid (CH3COOH-H3PO4) anodiz to replace CrO3 anodizing has increased [ Anodizing with phosphoric introduced to obtain thin anodic films approximately (1-5.4 µ m resistance for specific structural materials [ Even though the fatigue specimens anodized in increased, the corrosion resistance is lower than that of specimens anodized in [10] have proposed a new anodizing procedure involving the addition of acetic acid in phosphoric acid electrolyte, called acetic (CH3COOH-H3PO4) anodizing. sometimes used as anodizing electrolyte for Al-Khwarizmi Engineering Journal (2015) on Electrochemical Behavior of Sealed Acid Electrolytes potential of hydrothermally phosphoric acid electrolyte with or . Hydrothermal sealing was carried out in boiling water for each anodized specimen. The was examined using open circuit potential otentiostatic polarization technique (i.e., degree of sealing) of the unsealed the potentiostatic anodic and cathodic polarization of anodized anodized aluminum formed in acetic- lower corrosion current, generally passes This suggests a specific role of the species of the performance in terms of acetic acid in phosphoric acid Technique. in industrial applications due to . As a consequence, numerous attempts have been made to find less toxic alternatives, but with limited success [4, 5]. interesting to phosphoric anodizing and acetic-phosphoric anodizing as alternatives anodizing has increased [6, 7, 8]. phosphoric acid (H3PO4) has been introduced to obtain thin anodic films µ m) that increase fatigue resistance for specific structural materials [9]. Even though the fatigue resistance of the specimens anodized in phosphoric acid is increased, the corrosion resistance is lower than that of specimens anodized in CrO3. Nazree, et al. proposed a new anodizing procedure involving the addition of acetic acid in phosphoric acid electrolyte, called acetic-phosphoric acid ) anodizing. Acetic acid is sometimes used as anodizing electrolyte for Khalid H. Rashid Al-Khwarizmi Engineering Journal, Vol. 11, No. 4, P.P. 1- 7 (2015) 2 aluminum alloys due to its ability to form anodic films [11, 12]. However, the use of acetic - phosphoric acid mixtures as anodizing electrolytes is poorly reported in the literature. Ma Song-Jiang et al. [13] has been reported that mixed acid solution as anodizing electrolytes can decrease the film dissolution rate, then increase the film formation efficiency and improve the film properties. Wahab and Derman [14] have investigated the influence of the addition of acetic acid to phosphoric acid electrolyte on anodizing of pure aluminum (99%). Yaro and Al-Asadi [18] have investigated the effect of hydrothermal sealing time on the anodic polarization of anodized aluminum alloy AA5052 in different conditions. The authors showed that the addition of acetic acid reduced by ~20% the steady-state current during film growth at constant voltage. Neither of the research groups attempted to investigate corrosion behavior of the anodic films formed in the two solutions [10, 14]. The role of acetic acid in the anodizing bath still remains unclear. The present study aimed for a better understanding of the effect of acetic acid on the corrosion behavior and its potential of hydrothermally sealed anodized AA2319-Al- alloys. 2. Experimental Work 2.1. Materials and Conditions The substance used was a 2319 T3 aluminum alloys. The average chemical composition of the alloy is given in Table (1). The dimensions of samples are 125 × 8 × 1.6 mm plates machined from an AA2319 T3 rolled plate. Aluminum samples before anodizing and corrosion tests were abraded in sequence on 220, 320, 400 and 600 grades of emery paper under running tap water on a Metasern hand grinder tray washed with tap water followed by distilled water, dried with a clean tissue, degreased by immersing them in acetone & benzene, then dried with paper tissue and kept in a dissector over a silica gel bed. The treatment of anodizing was performed in a 40 g L−1 (0.41 M) phosphoric acid (H3PO4) solution in the presence or absence of 80 g L−1 (1.33 M) acetic acid (CH3COOH). Experiments of anodizing were carried out at a constant cell voltage of 15 V for 30 min. The operating temperature was maintained within 35 ± 1 °C by a thermometer. Sealing was carried out in (DI) water at a temperature of 100 °C. Table 1, Chemical Composition of 2319 T3 Aluminum Alloy (% wt). Si Fe Cu Mn Mg Zn Ti Al 0.20 0.30 6.3 0.30 0.02 0.10 0.15 92.63 min 2.2. Polarization Measurements The polarization measurements were carried out in a corrosion cell (three-electrode electrochemical cell) which is contain the auxiliary electrode, reference electrode and working electrode. It contained a platinum foil type (Tacussel Pt-F05/30) as auxiliary electrode (area of ~ 30 cm2), a saturated calomel reference electrode (SCE) type (Tacussel type C4) bridged by a laggin-Haber probe. The reference electrode, which was bridged by a luggin Haber capillary was set at about the optimum value of 1 mm to minimize the experimental error. The AA2319 T3 samples as working electrode (exposed area of 10 cm2 and other side and thickness of the samples were coated with the epoxy to prevent exposure to the solution), held by aluminum jigs in order to hold the samples, it was coated with rubber tube to hold it stable in the cell and prevent the short circuit contact. Open circuit potential (OCP) measurments were carried out and measured from off-circuit potential-immersion time measurments [17]. The corrosion cell parts were joined to each other, and then connected to potentiostat, ammeter and voltmeter as described in Figure 1 and polarization was carried out in electrochemical cell with electrical circuit shown in Figure 2. The auxiliary electrode was a platinum electrode of (2.7×11 cm), held by plastic plate set directly opposite to the working electrode. The potentiostat device using in this article is modern computerize type Wenking M Lab device of Bank Electronic, Germany. The scan rate is 10 mV/min. Reference electrode calibration was carried out by 30 % KCl solution for activation. The polarization figures were plotted under potentiostatic conditions at the actual corrosion potential. The electrochemical cell was maintained at 25 °C and open to surrounding. The open circuit potential (OCP) of the unsealed and sealed anodized aluminum samples were measured after 1.5 h of immersion at room temperature in a 0.5 M Na2SO4 solution and 3.5 M NaCl solution respectively. Sodium sulfate was chosen toward aluminum and aluminum alloys Khalid H. Rashid Al-Khwarizmi Engineering Journal, Vol. 11, No. 4, P.P. 1- 7 (2015) 3 due to its property (low corrosiveness). The polarization behavior of sealed and unsealed anodized samples was investigated in 3.5 M NaCl solution, at different sealing times. Anodizing treatment was performed in stagnant phosphoric acid electrolyte with acetic - phosphoric acid or without (H3PO4) addition of acetic acid. Hydrothermal sealing was carried out in boiling deionized (DI) water for each anodized sample. The corrosion behavior of the hydrothermally sealed and unsealed anodic films was carried out using potentiostatic polarization technique. The sensitivity of potentiostatic polarization makes it a successful technique for assessing the sealing quality and corrosion parameters of the anodic films from polarization curves [7, 15 and 16]. Stagnate solutions of chloride containing solutions at 25ºC have been investigated using polarization technique was carried out to characterize the properties of the unsealed and sealed anodic films. The electrochemical parameters obtained from a fitting procedure of the polarization diagrams allowed the corrosion behavior difference of the two samples to be shown and discussed. The corrosion potential results were correlated with the open circuit potential (OCP). Fig. 1. Wenking M Lab device. Fig. 2. Schematic diagram of corrosion cell used in polarization. 3. Results and Discussion The corrosion potential diagrams obtained after different sealing times (5, 15, 30, 45 and 60 min.) for the hydrothermally sealed anodized samples formed in phosphoric acid have the different shape as those obtained for the samples formed in acetic-phosphoric acid. Figure 3 compares the corrosion potential (Ecorr) values for both systems. It appears that for the anodic films formed in acetic-phosphoric acid, the variation is linear, whereas for the films in phosphoric acid it is not, and the Ecorr values always remain lower than those obtained with acetic-phosphoric acid. For the shortest sealing time, Ecorr was < - 850 mV, which reveal the low level of the anodic film sealing. Sealing times of > 30 min. are necessary to obtain acceptable sealing of the anodic film. These results show that the lower part of the porous structure of the anodic film formed in phosphoric acid was not perfectly sealed after 30 min. of reaction, corroborating the trends deduced from electrochemical potential results. The corrosion potential results stress that film sealing differs, depending on the initial anodic film. A better sealing efficiency (higher Ecorr) was obtained for the anodic films formed in the presence of acetic acid. Then, to investigate the influence of sealing quality on the corrosion behavior of unsealed and sealed anodized AA2319 samples, the open circuit potential of the unsealed and sealed anodic films formed in phosphoric acid and acetic-phosphoric acid was investigated by plotting the open circuit potential diagrams versus exposure time (from 0 to 1.5 h) in 0.5 M Na2SO4 and 3.5 M NaCl solutions respectively is presented in Figures 4, 5 & 7. Fig. 3. Corrosion potential values vs. sealing time for the sealed anodized aluminum AA2319 in phosphoric acid and acetic-phosphoric acid solutions in 3.5 M NaCl solution at 25 ºC. Thermometer A.E: Auxiliary Electrode R.E: Reference Electrode SCE W.E: Working Electrode Potentiosta Hot Plate A.E R.E W.E V A V R W A A 0 10 20 30 40 50 60 70 Sea ling time (min.) -900 -850 -800 -750 -700 -650 -600 -550 -500 -450 C o rr o si o n P o te n ti a l (m V ) v s. S C E Phosphoric acid Acetic-phosphoric acid Khalid H. Rashid Al-Khwarizmi Engineering Journal, Vol. 11, No. 4, P.P. 1- 7 (2015) 4 0 10 20 30 40 50 60 70 80 90 100 Immersion time (min.) -1600 -1500 -1400 -1300 -1200 -1100 -1000 -900 -800 P o te n ti a l ( m V ) v s. S C E Phosphoric acid Acetic-phosphoric acid Fig. 4. Open circuit potential of unsealed anodized aluminum AA2319 in phosphoric acid and acetic- phosphoric acid solutions in 0.5 M Na2SO4 solution at 25 ºC as a function of immersion time. -20 0 20 40 60 80 100 Immersion time (min.) -1150 -1100 -1050 -1000 -950 -900 -850 -800 -750 P ot en ti al ( m V ) vs . S C E 5 min. Sealing time 15 min.Sealing time 30 min.Sealing time 45 min.Sealing time 60 min.Sealing time Fig. 5. Open circuit potential of sealed anodized aluminum AA2319 in phosphoric acid solution after different sealing times in 3.5 M NaCl solution at 25 ºC as a function of immersion time. -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Log Current Density (mA/cm2 ) -1150 -1100 -1050 -1000 -950 -900 -850 -800 -750 -700 P ot en ti a l (m V ) v s. S C E 5 m in. Sealing tim e 15 m in. Sea ling tim e 30 m in. Sea ling tim e 45 m in. Sea ling tim e 60 m in. Sea ling tim e Unsealed Fig. 6. Potentiostatic polarization curves of unsealed and sealed anodized aluminum AA2319 in phosphoric acid solution after different sealing times in 3.5 M NaCl solution at 25 ºC. Fig. 7. Open circuit potential of sealed anodized aluminum AA2319 in acetic-phosphoric acid solution after different sealing times in 3.5 M NaCl solution at 25 ºC as a function of immersion time. -20 0 20 40 60 80 100 Immersion time (min.) -1100 -1000 -900 -800 -700 -600 -500 -400 P ot en ti al ( m V ) vs .S C E 5 m in.Sealing tim e 15 m in.Sealing tim e 30 m in.Sealing tim e 45 m in.Sealing tim e 60 m in.Sealing tim e Khalid H. Rashid Al-Khwarizmi Engineering Journal, Vol. 11, No. 4, P.P. 1- 7 (2015) 5 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Log Curr ent Density ( mA/cm2 ) -1100 -1000 -900 -800 -700 -600 -500 -400 -300 P ot en ti al ( m V ) vs . S C E 5 m in. Sealing time 15 m in. Sealing time 30 m in. Sealing time 45 m in. Sealing time 60 m in. Sealing time Unsealed Fig. 8. Potentiostatic polarization curves of unsealed and sealed anodized aluminum AA2319 in acetic- phosphoric acid solution after different sealing times in 3.5 M NaCl solution at 25 ºC. 3.1. Potentiostatic Polarization Measurements The potentiostatic polarization behavior of unsealed and hydrothermally sealed aluminum AA2319 anodized in phosphoric acid and acetic- phosphoric acid in 3.5 M NaCl solution at 25 ºC is presented in Figure 6 and Figure 8 respectively. Figures 6 and 8 show the effect of acetic acid on corrosion behavior of sealed AA2319 anodized in phosphoric acid electrolyte and acetic- phosphoric acid electrolyte after different sealing times in 3.5 M NaCl solution at 25 ºC, on the relationship between current density and potential of aluminum alloy AA2319. In these figures it can be seen that: 1. The obtained corrosion potential (Ecorr) values for sealed anodized aluminum alloy 2319 are quite sensitive to the sealing of the anodized aluminum after different sealing exposure times shift to more positive (more noble) direction compared with unsealed samples due to sharp anodic and cathodic kinetic change accompanied reduction of corrosion current densities (Icorr) because the initial principle purpose of sealing is to further improve the corrosion resistance of anodized aluminum. This is also in close agreement with the research done by Yaro and Al-Asadi [18]. 2. The cathodic reactions behavior of the polarization curves shows over a wide potential range that in the case of sealed anodized samples, in phosphoric acid the sealed anodized surface is more diffusion (mass transfer) to the predominant oxygen reduction reaction (as the principle rate limiting reaction) than the unsealed surface and in acetic-phosphoric acid the sealed anodized surface is more activation to the predominant hydrogen evolution reaction than the unsealed surface, i.e., the sealed anodized surface has an electronic resistivity of order [19]: H3PO4 sealed anodized > Unsealed one CH3COOH-H3PO4 sealed anodized > Unsealed one CH3COOH-H3PO4 sealed anodized > H3PO4 sealed anodized 3. At anodic and cathodic sites on the sealed anodized surface, suppressing the kinetic of anodic and cathodic polarization curves is more pronounced aluminum alloy 2319 anodized in acetic-phosphoric acids electrolyte and the anodic and cathodic current densities for sealed anodized samples reduced high value particularly at sealing exposure time 60 min than unsealed anodized samples. 4. Conclusions The sealing quality (i.e., degree of sealing) and the corrosion protection of AA2319 T3 anodized in phosphoric acid in absence and presence of acetic acid were investigated by potentiostatic polarization technique. The following conclusions might be withdrown from this investigation: 1. Electrochemical polarization measurements are sensitive to the presence of anodized aluminum surface films. 2. Anodizing of AA2319 T3 in presence of acetic acid led to the formation of anodic films with higher corrosion potential (i.e., improvement of the corrosion resistance of anodized aluminum surface) and lower corrosion current density than those obtained in phosphoric acid. 3. The sealing quality was better for the anodized samples formed in acetic-phosphoric acid than in phosphoric acid in agreement with the higher corrosion potential with reduction of anodic and cathodic currents of the anodized samples formed in acetic-phosphoric acid. 4. The sealed anodized samples formed in acetic- phosphoric acid are significantly more potential to corrosion than the sealed anodized samples formed in phosphoric acid. 5. The corrosion current densities for unsealed samples are found to be higher than Khalid H. Rashid Al-Khwarizmi Engineering Journal, Vol. 11, No. 4, P.P. 1- 7 (2015) 6 hydrothermally sealed anodized samples for both systems and the corrosion potential for hydrothermally sealed anodized samples are shifted more positive than unsealed samples for both systems 5. References [1] F. King, ″Aluminum and its Alloys″, 1st ed. New York: Ellis Horwood Series in Metals and Associated Materials, 1987, p.18. [2] J.R. Davis, ″Corrosion of Aluminum and Aluminum Alloys″, ASM International, The Material Information Society, 1999. [3] E. Eichinger, J. Osborne and T. Van Cleave, Met. Finish, J., Vol.95, No.36, 1997. [4] V. Moutarlier, M. P. Gigandet, L. Ricq and J. Pagetti, Appl. Surf. Sci.,J., Vol. 183, No.1, 2001. [5] V.Moutarlier, M, P. Gigandet, J. Pagetti and B. Normand, Surf. Coat. Technol., J., Vol. 161, No.267, 2002. [6] G. E. Thompson, L. Zhang, C.J. E. Smith and P. Skeldon, Corrosion, (Houston), J., Vol. 55, No.1052, 1999. [7] L. Domingues, J.C.S. Fernandes, M. Da Cunha Belo, M. G. 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Derman, "Characterization of Porous Anodic Aluminum Oxide Film on Aluminum Templates Formed in Anodizing Process", (http://www.seientific.net/AMR.173.55.htm l), December, 2011. [15] V. Marzocchi, L. Iglesias-Rubianes, G. E. Thompson and F. Bellucci, Corrosion Reviews, J., Vol. 25, No. 461, 2007. [16] V. Lopez, M. J. Bartolome, E. Escudero, E. Otero and J.A. Gonzalez, Electrochem. Soc., J., Vol. 153, No. 75, 2006. [17] R. D. James and J. A. Frank, "Effect of Measurement and Instrumentation Errors on Potential Readings", Corrosion, Vol. 11, No. 1294, pp.1-10, NACE, 2001. [18] A. S. Yaro and A. M. A. Al-Asadi, "Anodic Polarazation of Anodized Aluminum Alloy 5052", M.Sc. Thesis, University of Baghdad, Iraq, 2004. [19] N.L. Sukiman, X. Zhou, N. Birbilis and A. E. 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