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Corrosion inhibition of AA6060 aluminium alloy by lanthanide salts in chloride solution
Journal of Alloys and Compounds 475 (2009) 300–303
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Review
Corrosion inhibition of AA6060 aluminium alloy by lanthanide salts in chloride solution H. Allachi, F. Chaouket ∗ , K. Draoui Laboratoire de Physico-Chimie des Interfaces et Environnement, Faculté des Sciences, Université Abdel Malek Essaâdi, Tétouan 93000, Morocco
a r t i c l e
i n f o
Article history: Received 31 March 2008 Received in revised form 2 July 2008 Accepted 4 July 2008 Available online 20 August 2008 Keywords: Aluminium alloy Corrosion Inhibition Rare earths
a b s t r a c t The electrochemical behaviour of AA6060 in 3.5% NaCl in the absence and presence of some rare earths has been studied by means of polarisation curves and impedance measurements. We find that the natural protective aluminium oxide layer is damaged after nearly 100 h of immersion time in the aggressive medium. The current–potential curves show that the corrosion resistance of the alloy is greatly enhanced by addition of lanthanide salts especially the cerium ion. This latter forms, on the alloy surface, a passive layer whose protective properties are reinforced by ageing in solution. © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Aluminium and its alloys are widely used because of their appearance, low density and corrosion resistance. Nevertheless, sometimes additional protection is required. Then, aluminium alloys can be easily protected with organic coatings or by electrochemical oxidation. The latter generates oxide passive layers providing excellent corrosion resistance in highly aggressive environments. However, due to the different thermomechanical treatments applied to achieve the mechanical properties, they are liable to suffer from various forms of corrosion, mainly pitting and intergranular attack [1]. Pitting corrosion of aluminium alloys is a complex process that can be affected by diverse experimental factors such as pH, temperature, nature of anion in the solution, and the physical–chemical characteristics of the passive layer [2]. The adsorption of aggressive
∗ Corresponding author. Tel.: +212 39972423; fax: +212 39994500. E-mail address: [email protected] (F. Chaouket). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.017
300 301 301 303 303
ions such as Cl− into the faults of the protective film, their penetration and accumulation in these imperfections is considered as one of the triggering factors of the nucleation pitting process [3,4]. It has also been suggested that pits may develop as a result of a hydrolysis process which gives rise to a local reduction of pH. This impedes the subsequent process of repassivity [5]. Equally, another factor which is associated with the susceptibility of aluminium alloys to localised corrosion is the electrochemical nature of the intermetallic phases [6–8]. Thus, its corrosion behaviour is often correlated with the difference in potential between the matrix and the allied elements [9,10]. According to Nisancioglu [8], the metallic inclusions, more noble than the matrix, act as cathodes and induce an anodic dissolution in the surrounding matrix [11,12]. New environmental restrictions have particularly affected the industry of surface treatment and finishing of metals [13]. Consequently, an intense research effort is being undertaken to replace chromate by more ecological compounds in the anticorrosion aluminium alloys treatments [14–23]. The use of rare earth metals has been proposed in the literature. These compounds have been successfully used as corrosion inhibitors for aluminium [21] and some
H. Allachi et al. / Journal of Alloys and Compounds 475 (2009) 300–303
301
Table 1 Chemical composition (in weight percent) of the AA6060 aluminium alloy Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Pb
% minimum % maximum
0.40 0.45
0.15 0.22
– 0.02
– 0.03
0.40 0.45
– 0.02
– 0.02
– 0.02
– 0.01
Others Each
Total
– 0.03
– 0.15
Fig. 1. Evolution in function of time of the free potential of AA6060 aluminium alloy in NaCl 3.5%. Exposed area: 0.78 cm2 .
aluminium alloys such as 5083[17], 3003, 6063[19], 7075 [18] and 8090 [20]. In most studies, cerium and lanthanide salts were found to be the best inhibitors. The present paper reports on results obtained in the study of corrosion and corrosion inhibition of AA6060 aluminium alloy in NaCl solution by lanthanide salts. 2. Experimental The chemical composition of the AA6060 aluminium alloy is given in Table 1. As the aggressive medium, an aerated solution of NaCl at 3.5% and pH of 6.5 was used. The solutions of the inhibitors were prepared by dissolution, in the medium, of the following compounds: Ce(NO3 )3 , La(NO3 )3 , Gd(NO3 )3 and Tb(NO3 )3 , of a purity superior to 99%. Electrochemical measurements are carried out in a conventional three-electrode electrolytic cell. The working electrode is in the form of a disc cut from aluminium alloy and had a geometric working area of 0.78 cm2 . These samples are prepared using a mechanical process of wet sanding with SiC papers of 400 and 600 grits. Before the tests, the samples were carefully degreased with ethanol of 99% purity and rinsed with abundant distilled water. Saturated calomel electrode (SCE) and platinum electrode are used as reference and auxiliary electrodes, respectively. The polarisation curves are carried out with a Volta lab (PGZ 100), piloted by ordinate. The scan rate is 30 mV/min.
3. Results and discussion One of the techniques used to monitor the corrosion behaviour of alloy AA6060 was the recording of the potential of corrosion in function of time. Initially, samples with the same level of surface finish (600 grits), with an exposure area of 1 cm2 , were tested. The recordings obtained are shown in Fig. 1. This curve shows that after a significant evolution of the free potential towards higher values, it stabilizes around a value of −685 mV/ECS. The strong evolution of the free potential at the beginning of the immersion time is related either to the activation of the cathodic reaction, or with a lowering of the activity of the anodic reaction [24,25].
Fig. 2. Effect of immersion time on the potentiodynamic current–potential curves of AA6060 in aerated NaCl 3.5%.
To connect the results of the electrochemical accelerated tests with the real corrosion phenomena of AA6060, we carried out tests of immersion with a long duration in a 3.5% NaCl solution. Curves I-E obtained are illustrated by Fig. 2. The electrochemical parameters are given in Table 2. The alloy studied exhibits a passive behaviour after 24 h of immersion time with a displacement of the free potential towards negative values and a reduction of the corrosion rate. After 48 h, one notes a diminution of the passive current density ip and a weak reduction of the corrosion rate. The corrosion potential (Ecorr ) becomes more negative while the pitting corrosion potential (Epit ) remains quasi-constant (Table 2). It results a widening in the passive potential domain (Epit – Ecorr ) witch rises from 175 to 490 mV between 24 and 48 h immersion time. This attests of an improvement in the protective properties of the oxide layer without changing in its physical–chemical characteristics. It could be related to a growth of the oxide film thickness with immersion time [26]. However, after 120 h of immersion time, we remark a noticeable restriction in passivity field and a growth in the corrosion rate. This phenomenon may be explained by a break of the passive layer, witch leads to a localised attack of the material. Fig. 3 exhibits Nyquist plots of AA6060 aluminium alloy obtained at various immersion times in 3.5% NaCl. We note that Nyquist plot obtained for 2 h consists of a well-defined capacitive loop related to the charge transfer process. After 1-day immersion, EIS spectrum displays two capacitive arcs, associated with the interfaces metal–oxide (M/O) and oxide/electrolyte (O/E). In agreement with Brett [27], the corrosion process in passive systems is con-
Table 2 Electrochemical parameters of AA6060 aluminium alloy in NaCl 3.5% solution Time of immersion (h)
Ecorr (mV/SCE)
Epit (mV/SCE)
icorr (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV/SCE)
2 24 48 120
−696 −861 −1171 −809
−696 −686 −682 −682
10 8 7.4 28.4
– 21 15 27
– 175 489.4 187
SCE: saturated calomel electrode.
302
H. Allachi et al. / Journal of Alloys and Compounds 475 (2009) 300–303
Fig. 3. Effect of immersion time on Nyquist diagrams corresponding to AA6060 alloy in NaCl 3.5% solution.
trolled by the reactions in the interface O/E. Thus, in such systems the (O/E) response predominates at lower frequencies. Hence, the high frequency arc must be associated with the electron transfer process in the M/O interface. After 48 h of immersion, an increase in the rays of the two arcs occurs. This last fact indicates that the ageing of the specimen in NaCl 3% leads to a growth in the resistance of the two interfaces (M/O) and (O/E) and thus in there thickness [28]. In the diagram recorded after 120 h of immersion time, one observes, initially, the disappearance of the capacitive arc at low frequency and a noticeable restriction of the polarisation resistance. This suggests a complete abolition of the passive layer what leads to an initiation of localised attack. These results corroborate those obtained by the stationary study. Fig. 4 illustrates the polarization curves, recorded for the material studied in 3.5% NaCl in the absence and presence of the inhibitors at 10−3 M. Corresponding electrochemical parameters are summarised in Table 3. The corrosion inhibition efficiency (E%) is defined as:
Fig. 5. Linear polarisation curves of AA6060 in NaCl solution at various concentrations of Ce(NO3 )3 . ◦
icorr and icorr are the uninhibited and inhibited corrosion current densities, respectively, determined by extrapolation of the cathodic Tafel lines to corrosion potential (Ecorr ) (Fig. 5). It is clear that the presence of lanthanides salts, with a concentration of 10−3 M, causes a displacement of the cathodic branch towards the negative values and a reduction in the corrosion current density. One notes simultaneously the appearance of a more or less broad stage of passivity with a weak current in particular for La3+ and Ce3+ . The direction of displacement observed indicates that these inhibitors act by blocking of cathodic sites. These results are in accordance with those obtained by Bethencourt et al. [26] for AA5083 and by Hinton for Al–Zn alloy [29].
◦
E% =
icorr − icorr × 100 ◦ icorr
Fig. 4. Polarization curves of AA 6063 alloy in NaCl 3.5% solution without and with inhibitors at 10−3 M.
Fig. 6. Polarization curves of AA6060 in 3.5% NaCl added 10−3 M Ce3+ at various immersion time.
Table 3 Electrochemical parameters of AA6060 in NaCl 3.5% solution with and without inorganic compounds at 10−3 M and corresponding inhibition efficiencies Solution
Ecorr (mV/SCE)
Epit (mV/SCE)
Témoin Gd3+ Tb3+ La3+ Ce3+
−696 −958 −1035 −1064 −956
−696 −686 −686 −682 −686
SCE: saturated calomel electrode.
icor (A/cm2 ) 10 4.6 3.65 2.5 2
ip (A/cm2 )
Epit – Ecorr (mV)
E%
– 14.6 14.79 3.09 1.6
0 272 349.8 382.5 270
– 54 63 72 80
H. Allachi et al. / Journal of Alloys and Compounds 475 (2009) 300–303
303
Table 4 Electrochemical parameters of AA6060 in NaCl 3.5% solution at various concentrations of Ce(NO3 )3 C (mol/l)
Ecorr (mV/SCE)
Epit (mV/SCE)
icor (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV)
E%
Blank 10−4 5 × 10−4 10−3 10−2
−696 −1132 −1093 −956 −984
−696 −685 −685 −686 −685
10 8.91 5.62 2 3.16
– 17.37 12.5 1.6 6.30
0 686 408 270 299
– 11 44 80 69
SCE: saturated calomel electrode. Table 5 Effect of immersion time on the electrochemical parameters of AA6060 in NaCl 3.5% added 10−3 M Ce3+ Immersion time (h)
Ecorr (mV/SCE)
Epit (mV/SCE)
icorr (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV)
E%
2 24 48 120
−956 −950 −954 −875
−686 −680 −684 –
2 1.6 0.8 0.42
1.6 1 1 0.9
270 282 274 –
80 84 92 98.5
SCE: saturated calomel electrode.
The comparative study of the inhibiting effect of the compounds tested shows that the cerium presents the best inhibiting action in the anodic field as well as in the cathodic field. Indeed, the estimate corrosion rate indicates that the minimal value is obtained in the presence of the Ce3+ with an inhibiting efficiency of 80%. Fig. 6 illustrate the polarization curves, recorded for the alloys studied in 3.5% NaCl solution at various concentrations of Ce(NO3 )3 . Corresponding electrochemical parameters are summarised in Table 4. The results obtained show that the addition of the Ce3+ ions at various concentrations supports the passivity of AA6060. The corrosion current density (icorr ) decreases with the increase in the concentration of Ce3+ ions until 10−3 M. Beyond this concentration, icorr increases slightly. The efficiencies (E%) of the inhibitor increases with the content of Ce3+ ions, and reached a maximum value of 80% at 10−3 M. However, at 10−2 M addition, the efficiency falls to 69%. Similar behaviour has been reported by Mishra and Balasubramaniam [21] for pure aluminium in NaCl 3.5% after addition of CeCl3 and by Aballe et al. [25]. For AA5083 in the same conditions. According to the latest authors, the Ce3+ ions react in a first stage with the OH− ions which are generated in the cathodic zones of alloy, giving place to the formation of the islands rich in cerium. Those involve a blocking of the cathodic sites and a reduction in the corrosion speed. The anomalous result observed for 10−2 M, may be then related to a loss in the inhibitive islands coherence when it reaches a critical thickness. Inhibition qualities of a compound could be deteriorated by several factors, in particular, the immersion time in corrosive medium. Fig. 6 represents the polarization curves of AA6060 in 3.5% NaCl solution in the absence and presence of Ce(NO3 )3 at 10−3 M during different exposure times. Corresponding electrochemical parameters are summarised in Table 5. It is observed that the maintenance of the samples to the abandonment during 24 and 48 h reinforces the capacity of cerium inhibition. Indeed, the curves obtained show a reduction in the cathodic currents with a light widening of the passivity field (Epit – Ecorr ) which measures the tendency to the pitting corrosion nucleation. These characteristics become more important after 120 h of immersion. Thus, one notes a widening of the passivity field and a decrease by 50% of the corrosion current density while passing from 48 to 120 h immersion time. It results an improvement in the inhibition efficiency up to 98%. 4. Conclusion • Ce3+ and La3+ were found to be the best inhibitors of the series of rare earths studied.
• The inhibition efficiency of the Ce3+ reaches 80% at 10−3 M. • The cerium protective film formed on the aluminium alloy surface is stabilized by ageing in corrosive medium. Thus, the inhibitive efficiency raised to 98% after 120 h of immersion time. • The protection factors against corrosion obtained from linear polarisation data in doped solutions with Ce3+ or La3+ are of the same order as those found with classical Cr(VI)-based compounds. References [1] Davo, A. Conde, J.J. de Damborenea, Corros. Sci. 47 (2005) 1227–1237. [2] Z. Szklarska-Smialowska, Pitting Corrosion of Metals, NACE, Houston, 1986, p. 3. [3] J.B. Bessone, D.R. Salinas, C.E. Mayer Ebert, W.J. Loren, Electrochim. Acta 37 (1992) 2283. [4] A.G. ËUnowm, J.B. Bessonem, Corros. Sci. 41 (1999) 1447. [5] J.R. Galvele, J. Electrochem. Soc. 123 (1976) 464. [6] M. Bethencourt, F.J. Botana, J.J. Calvino, M. Marcos, M.A. Rodriguez, Mater. Sci. Forum 289–292 (1998) 567. [7] A. Alavi, R. Cottis, Corros. Sci. 27 (1987) 443. [8] K. Nisancioglu, J. Electrochem. Soc. 137 (1990) 69. [9] M. Zamin, Corrosion 37 (1981) 627. [10] O. Seri, N. Matsuko, J. Electrochem. Soc. 32 (1982) 303. [11] E.V. Koroleva, G.E. Thompson, G. Hollrigl, M. Bloeck, Corros. Sci. 41 (1999) 1475. [12] Z. Szklarska-Smialowska, Corros. Sci. 41 (1999) 1743. [13] Toxicological Profile for Chromium, Agency for Toxic Substances, US Public Health Service, Report no. ATSDR/TP-88/10, 1989. [14] F. Mansfeld, Y. Wang, Br. Corros. J. 29 (3) (1994) 194. [15] M. Bethencourt, F.J. Botana, J.J. Calvino, M. Marcos, Corros. Sci. 40 (11) (1998) 803–1819. [16] F. Serrano, J.J. de Damborenea, Rev. Met. Madrid 34 (1998) 127. [17] K. Aramaki, Corros. Sci. 43 (2001) 1573. [18] M.A. Arenas, M. Bethencourt, F.J. Botana, J.J. de Damborenea, M. Marcos, Corros. Sci. 43 (2001) 157–170. [19] Bazzi, R. Salghi, E. Zine, S. El Issami, S. Kertit, B. Hammouti, Can. J. Chem. 80 (2002) 106–112. [20] B. Davo, J.J. de Damborenea, Electrochim. Acta 49 (2004) 4957–4965. [21] A.K. Mishra, R. Balasubramaniam, Mater. Chem. Phys. 103 (2007) 385–393. [22] K.A. Yasakau, M.L. Zheludkevich, M.G.S. Ferreira, J. Electrochem. Soc. 155 (2008) 169–177. [23] M. Kendig, W. Qafsaoui, H. Takenouti, F. Huet, Corrosion 2008 Conference and Expo, March 16–20, 2008. [24] A. Aballe, M. Bethencourt, F.J. Botana, M. Marcos, R. Osuna, Mater. Corros. 52 (2001) 185–192. [25] A. Aballe, M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, Corros. Sci. 45 (2003) 161–180. [26] M. Bethencourt, Thèse Doctoral, Facultad de Ciencias del mar, Universidad de Cadiz, 1999. [27] C.M. Brett, Corros. Sci. 33 (1992) 203. [28] C. Andrade, P. Merino, X.R. Novoa, M.C. Perez, L. Soler, Mater. Sci. Forum 91 (1995) 192–194. [29] B.R.W. Hinton, J. Alloy Compd. 180 (1992) 15.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Review
Corrosion inhibition of AA6060 aluminium alloy by lanthanide salts in chloride solution H. Allachi, F. Chaouket ∗ , K. Draoui Laboratoire de Physico-Chimie des Interfaces et Environnement, Faculté des Sciences, Université Abdel Malek Essaâdi, Tétouan 93000, Morocco
a r t i c l e
i n f o
Article history: Received 31 March 2008 Received in revised form 2 July 2008 Accepted 4 July 2008 Available online 20 August 2008 Keywords: Aluminium alloy Corrosion Inhibition Rare earths
a b s t r a c t The electrochemical behaviour of AA6060 in 3.5% NaCl in the absence and presence of some rare earths has been studied by means of polarisation curves and impedance measurements. We find that the natural protective aluminium oxide layer is damaged after nearly 100 h of immersion time in the aggressive medium. The current–potential curves show that the corrosion resistance of the alloy is greatly enhanced by addition of lanthanide salts especially the cerium ion. This latter forms, on the alloy surface, a passive layer whose protective properties are reinforced by ageing in solution. © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Aluminium and its alloys are widely used because of their appearance, low density and corrosion resistance. Nevertheless, sometimes additional protection is required. Then, aluminium alloys can be easily protected with organic coatings or by electrochemical oxidation. The latter generates oxide passive layers providing excellent corrosion resistance in highly aggressive environments. However, due to the different thermomechanical treatments applied to achieve the mechanical properties, they are liable to suffer from various forms of corrosion, mainly pitting and intergranular attack [1]. Pitting corrosion of aluminium alloys is a complex process that can be affected by diverse experimental factors such as pH, temperature, nature of anion in the solution, and the physical–chemical characteristics of the passive layer [2]. The adsorption of aggressive
∗ Corresponding author. Tel.: +212 39972423; fax: +212 39994500. E-mail address: [email protected] (F. Chaouket). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.017
300 301 301 303 303
ions such as Cl− into the faults of the protective film, their penetration and accumulation in these imperfections is considered as one of the triggering factors of the nucleation pitting process [3,4]. It has also been suggested that pits may develop as a result of a hydrolysis process which gives rise to a local reduction of pH. This impedes the subsequent process of repassivity [5]. Equally, another factor which is associated with the susceptibility of aluminium alloys to localised corrosion is the electrochemical nature of the intermetallic phases [6–8]. Thus, its corrosion behaviour is often correlated with the difference in potential between the matrix and the allied elements [9,10]. According to Nisancioglu [8], the metallic inclusions, more noble than the matrix, act as cathodes and induce an anodic dissolution in the surrounding matrix [11,12]. New environmental restrictions have particularly affected the industry of surface treatment and finishing of metals [13]. Consequently, an intense research effort is being undertaken to replace chromate by more ecological compounds in the anticorrosion aluminium alloys treatments [14–23]. The use of rare earth metals has been proposed in the literature. These compounds have been successfully used as corrosion inhibitors for aluminium [21] and some
H. Allachi et al. / Journal of Alloys and Compounds 475 (2009) 300–303
301
Table 1 Chemical composition (in weight percent) of the AA6060 aluminium alloy Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Pb
% minimum % maximum
0.40 0.45
0.15 0.22
– 0.02
– 0.03
0.40 0.45
– 0.02
– 0.02
– 0.02
– 0.01
Others Each
Total
– 0.03
– 0.15
Fig. 1. Evolution in function of time of the free potential of AA6060 aluminium alloy in NaCl 3.5%. Exposed area: 0.78 cm2 .
aluminium alloys such as 5083[17], 3003, 6063[19], 7075 [18] and 8090 [20]. In most studies, cerium and lanthanide salts were found to be the best inhibitors. The present paper reports on results obtained in the study of corrosion and corrosion inhibition of AA6060 aluminium alloy in NaCl solution by lanthanide salts. 2. Experimental The chemical composition of the AA6060 aluminium alloy is given in Table 1. As the aggressive medium, an aerated solution of NaCl at 3.5% and pH of 6.5 was used. The solutions of the inhibitors were prepared by dissolution, in the medium, of the following compounds: Ce(NO3 )3 , La(NO3 )3 , Gd(NO3 )3 and Tb(NO3 )3 , of a purity superior to 99%. Electrochemical measurements are carried out in a conventional three-electrode electrolytic cell. The working electrode is in the form of a disc cut from aluminium alloy and had a geometric working area of 0.78 cm2 . These samples are prepared using a mechanical process of wet sanding with SiC papers of 400 and 600 grits. Before the tests, the samples were carefully degreased with ethanol of 99% purity and rinsed with abundant distilled water. Saturated calomel electrode (SCE) and platinum electrode are used as reference and auxiliary electrodes, respectively. The polarisation curves are carried out with a Volta lab (PGZ 100), piloted by ordinate. The scan rate is 30 mV/min.
3. Results and discussion One of the techniques used to monitor the corrosion behaviour of alloy AA6060 was the recording of the potential of corrosion in function of time. Initially, samples with the same level of surface finish (600 grits), with an exposure area of 1 cm2 , were tested. The recordings obtained are shown in Fig. 1. This curve shows that after a significant evolution of the free potential towards higher values, it stabilizes around a value of −685 mV/ECS. The strong evolution of the free potential at the beginning of the immersion time is related either to the activation of the cathodic reaction, or with a lowering of the activity of the anodic reaction [24,25].
Fig. 2. Effect of immersion time on the potentiodynamic current–potential curves of AA6060 in aerated NaCl 3.5%.
To connect the results of the electrochemical accelerated tests with the real corrosion phenomena of AA6060, we carried out tests of immersion with a long duration in a 3.5% NaCl solution. Curves I-E obtained are illustrated by Fig. 2. The electrochemical parameters are given in Table 2. The alloy studied exhibits a passive behaviour after 24 h of immersion time with a displacement of the free potential towards negative values and a reduction of the corrosion rate. After 48 h, one notes a diminution of the passive current density ip and a weak reduction of the corrosion rate. The corrosion potential (Ecorr ) becomes more negative while the pitting corrosion potential (Epit ) remains quasi-constant (Table 2). It results a widening in the passive potential domain (Epit – Ecorr ) witch rises from 175 to 490 mV between 24 and 48 h immersion time. This attests of an improvement in the protective properties of the oxide layer without changing in its physical–chemical characteristics. It could be related to a growth of the oxide film thickness with immersion time [26]. However, after 120 h of immersion time, we remark a noticeable restriction in passivity field and a growth in the corrosion rate. This phenomenon may be explained by a break of the passive layer, witch leads to a localised attack of the material. Fig. 3 exhibits Nyquist plots of AA6060 aluminium alloy obtained at various immersion times in 3.5% NaCl. We note that Nyquist plot obtained for 2 h consists of a well-defined capacitive loop related to the charge transfer process. After 1-day immersion, EIS spectrum displays two capacitive arcs, associated with the interfaces metal–oxide (M/O) and oxide/electrolyte (O/E). In agreement with Brett [27], the corrosion process in passive systems is con-
Table 2 Electrochemical parameters of AA6060 aluminium alloy in NaCl 3.5% solution Time of immersion (h)
Ecorr (mV/SCE)
Epit (mV/SCE)
icorr (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV/SCE)
2 24 48 120
−696 −861 −1171 −809
−696 −686 −682 −682
10 8 7.4 28.4
– 21 15 27
– 175 489.4 187
SCE: saturated calomel electrode.
302
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Fig. 3. Effect of immersion time on Nyquist diagrams corresponding to AA6060 alloy in NaCl 3.5% solution.
trolled by the reactions in the interface O/E. Thus, in such systems the (O/E) response predominates at lower frequencies. Hence, the high frequency arc must be associated with the electron transfer process in the M/O interface. After 48 h of immersion, an increase in the rays of the two arcs occurs. This last fact indicates that the ageing of the specimen in NaCl 3% leads to a growth in the resistance of the two interfaces (M/O) and (O/E) and thus in there thickness [28]. In the diagram recorded after 120 h of immersion time, one observes, initially, the disappearance of the capacitive arc at low frequency and a noticeable restriction of the polarisation resistance. This suggests a complete abolition of the passive layer what leads to an initiation of localised attack. These results corroborate those obtained by the stationary study. Fig. 4 illustrates the polarization curves, recorded for the material studied in 3.5% NaCl in the absence and presence of the inhibitors at 10−3 M. Corresponding electrochemical parameters are summarised in Table 3. The corrosion inhibition efficiency (E%) is defined as:
Fig. 5. Linear polarisation curves of AA6060 in NaCl solution at various concentrations of Ce(NO3 )3 . ◦
icorr and icorr are the uninhibited and inhibited corrosion current densities, respectively, determined by extrapolation of the cathodic Tafel lines to corrosion potential (Ecorr ) (Fig. 5). It is clear that the presence of lanthanides salts, with a concentration of 10−3 M, causes a displacement of the cathodic branch towards the negative values and a reduction in the corrosion current density. One notes simultaneously the appearance of a more or less broad stage of passivity with a weak current in particular for La3+ and Ce3+ . The direction of displacement observed indicates that these inhibitors act by blocking of cathodic sites. These results are in accordance with those obtained by Bethencourt et al. [26] for AA5083 and by Hinton for Al–Zn alloy [29].
◦
E% =
icorr − icorr × 100 ◦ icorr
Fig. 4. Polarization curves of AA 6063 alloy in NaCl 3.5% solution without and with inhibitors at 10−3 M.
Fig. 6. Polarization curves of AA6060 in 3.5% NaCl added 10−3 M Ce3+ at various immersion time.
Table 3 Electrochemical parameters of AA6060 in NaCl 3.5% solution with and without inorganic compounds at 10−3 M and corresponding inhibition efficiencies Solution
Ecorr (mV/SCE)
Epit (mV/SCE)
Témoin Gd3+ Tb3+ La3+ Ce3+
−696 −958 −1035 −1064 −956
−696 −686 −686 −682 −686
SCE: saturated calomel electrode.
icor (A/cm2 ) 10 4.6 3.65 2.5 2
ip (A/cm2 )
Epit – Ecorr (mV)
E%
– 14.6 14.79 3.09 1.6
0 272 349.8 382.5 270
– 54 63 72 80
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303
Table 4 Electrochemical parameters of AA6060 in NaCl 3.5% solution at various concentrations of Ce(NO3 )3 C (mol/l)
Ecorr (mV/SCE)
Epit (mV/SCE)
icor (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV)
E%
Blank 10−4 5 × 10−4 10−3 10−2
−696 −1132 −1093 −956 −984
−696 −685 −685 −686 −685
10 8.91 5.62 2 3.16
– 17.37 12.5 1.6 6.30
0 686 408 270 299
– 11 44 80 69
SCE: saturated calomel electrode. Table 5 Effect of immersion time on the electrochemical parameters of AA6060 in NaCl 3.5% added 10−3 M Ce3+ Immersion time (h)
Ecorr (mV/SCE)
Epit (mV/SCE)
icorr (A/cm2 )
ip (A/cm2 )
Epit – Ecorr (mV)
E%
2 24 48 120
−956 −950 −954 −875
−686 −680 −684 –
2 1.6 0.8 0.42
1.6 1 1 0.9
270 282 274 –
80 84 92 98.5
SCE: saturated calomel electrode.
The comparative study of the inhibiting effect of the compounds tested shows that the cerium presents the best inhibiting action in the anodic field as well as in the cathodic field. Indeed, the estimate corrosion rate indicates that the minimal value is obtained in the presence of the Ce3+ with an inhibiting efficiency of 80%. Fig. 6 illustrate the polarization curves, recorded for the alloys studied in 3.5% NaCl solution at various concentrations of Ce(NO3 )3 . Corresponding electrochemical parameters are summarised in Table 4. The results obtained show that the addition of the Ce3+ ions at various concentrations supports the passivity of AA6060. The corrosion current density (icorr ) decreases with the increase in the concentration of Ce3+ ions until 10−3 M. Beyond this concentration, icorr increases slightly. The efficiencies (E%) of the inhibitor increases with the content of Ce3+ ions, and reached a maximum value of 80% at 10−3 M. However, at 10−2 M addition, the efficiency falls to 69%. Similar behaviour has been reported by Mishra and Balasubramaniam [21] for pure aluminium in NaCl 3.5% after addition of CeCl3 and by Aballe et al. [25]. For AA5083 in the same conditions. According to the latest authors, the Ce3+ ions react in a first stage with the OH− ions which are generated in the cathodic zones of alloy, giving place to the formation of the islands rich in cerium. Those involve a blocking of the cathodic sites and a reduction in the corrosion speed. The anomalous result observed for 10−2 M, may be then related to a loss in the inhibitive islands coherence when it reaches a critical thickness. Inhibition qualities of a compound could be deteriorated by several factors, in particular, the immersion time in corrosive medium. Fig. 6 represents the polarization curves of AA6060 in 3.5% NaCl solution in the absence and presence of Ce(NO3 )3 at 10−3 M during different exposure times. Corresponding electrochemical parameters are summarised in Table 5. It is observed that the maintenance of the samples to the abandonment during 24 and 48 h reinforces the capacity of cerium inhibition. Indeed, the curves obtained show a reduction in the cathodic currents with a light widening of the passivity field (Epit – Ecorr ) which measures the tendency to the pitting corrosion nucleation. These characteristics become more important after 120 h of immersion. Thus, one notes a widening of the passivity field and a decrease by 50% of the corrosion current density while passing from 48 to 120 h immersion time. It results an improvement in the inhibition efficiency up to 98%. 4. Conclusion • Ce3+ and La3+ were found to be the best inhibitors of the series of rare earths studied.
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