- Email: [email protected]
Corrosion properties of anodized aluminum: Effects of equal channel angular pressing prior to anodization
Accepted Manuscript Corrosion properties of anodized aluminium: effects of equal channel angular pressing prior to anodization Oussama Jilani, Nabil Njah, Pierre Ponthiaux PII: DOI: Reference:
S0010-938X(14)00409-0 http://dx.doi.org/10.1016/j.corsci.2014.08.020 CS 6003
To appear in:
Corrosion Science
Received Date: Accepted Date:
27 May 2014 16 August 2014
Please cite this article as: O. Jilani, N. Njah, P. Ponthiaux, Corrosion properties of anodized aluminium: effects of equal channel angular pressing prior to anodization, Corrosion Science (2014), doi: http://dx.doi.org/10.1016/ j.corsci.2014.08.020
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Corrosion Properties of Anodized Aluminium: Effects of Equal Channel Angular Pressing prior to Anodization. Oussama Jilania, Nabil Njaha*, Pierre Ponthiauxb a
GEOMAT-Laboratoire Géoressource, Environnement, Matériaux et Changements Globaux,
Faculté des Sciences de Sfax, Université de Sfax, Tunisia. b
LGPM-Laboratoire Génie des Procédés et Matériaux, École Centrale de Paris, Grande voie des Vignes, 92290 Châtenay-Malabry, France. Abstract Many aluminium alloys are subjected to mechanical treatments before anodization. We have studied the effects of Equal Channel Angular Pressing (ECAP) on the properties of the passive film of aluminium 99.1%. Electrochemical measurements and micrography were used for characterisation. In 3% NaCl solution, anodization after ECAP leads to higher free potential values than that without deformation. Grain refinement and high dislocation density obtained by ECAP enhance the formation of a more continuous film after anodization. The destruction of inclusions increases pitting site density and leads to low-depth pits. The role of inclusions and pit sizes in the repassivation is illustrated Keyword: A: aluminium; B: potentiostatic; B: polarization C: anodic films; C: pitting corrosion Introduction Aluminium and its alloys show a good corrosion resistance due to the oxide film formed on the surface over a neutral range of pH. However, the average natural oxide film thickness, which is below 10nm, is not sufficient for protection [1, 2]. Therefore, an improvement of alloy protection is obtained by increasing the film thickness through anodization process. In practice, anodization is a widely studied and accepted process for protection against corrosion and abrasion [3]. The most widely used electrolyte for anodizing is sulphuric acid [4-7]. Industrially, sulphuric anodization is traditionally followed by hydrothermal sealing (HTS) [8]. Generally, together with the important increase in corrosion protection [6, 7, 9-12], negative effects on other properties are sometimes observed. Thus, Hemmouche et al. [13] have shown in aluminium-copper alloy that anodizing results in a decrease in fatigue life. They ascribed this to two features: In addition to the intrinsic brittleness of oxide, there was an important effect of precipitates that constituted favorable sites of pores and discontinuities in the oxide layer. Zaraska et al. [14] experiments revealed the impurity effect on film morphology in anodized industrial and high purity aluminiums. They remarked that the presence of impurities affects not only the rate of oxide growth but also the film behavior. In order to obtain a sufficiently continuous film, the substrate must be as pure as possible which may not be the case in industrial aluminium. In fact, in such materials, inclusions cannot be easily avoided due to the presence, among others, of iron and silicon but an improvement of corrosion properties is possible by reducing the size of these inclusions. During the few last decades, Equal Channel Angular Pressing (ECAP) has been widely used as a method of grain refinement and of hardness increase. It consists in introducing a high amount of deformation without change in the shape of the material [15]. It is well known that oxide properties like adherence and grain size are related to the surface properties of *
Corresponding author. Tel.: (+216)98656472.
E-mail address: [email protected].
2
substrates especially if the latter was subjected to mechanical treatments. The complex microstructure of ECAPed alloys, particularly multiphased ones (fine grains, high defect density mainly dislocations and grain boundaries, finer precipitates,...) is often the origin of result discrepancies: In AZ61 magnesium alloy containing B precipitates, grain refinement using high-ratio differential speed rolling (HRDSR) improves corrosion resistance due to the continuous film formed [16]. Furthermore, a substantial refinement of B precipitates reduces their galvanic effect [16]. In Al-Zn-Mg alloy, the resolution of Zn and Mg in the matrix by cryorolling was found to lower the resistance of the alloy to general corrosion as compared with the non deformed alloy. A further annealing shifts the OCP curve to noble potentials with a clear tendency to pitting; this was ascribed to the galvanic effect of precipitates [17]. On the other hand, Song et al. [18] have shown, that refinement of grains and precipitates, and the high density of defects lower corrosion resistance of AZ91D alloy together with an increase in the sensitivity to pitting. In fine grained aluminium-copper alloy obtained by ECAP, Son et al. [19] have shown that pitting resistance after anodization increased with increasing the number of passes, i.e. the amount of deformation. In fact they observed that the time required before the initiation of pitting in AlCl3 solution increased by ECAP. They attributed this to a decrease in the size of precipitates which were not coated by the oxide film and which constitute favorable sites for pitting. On the other hand, in Al-Mg alloys, Brunner et al. [20] did not note any change in corrosion resistance when increasing the number of ECAP passes. They suggested that passive layer properties are independent of the volume fraction of grain boundaries and of dislocation density in the substrate surface. Due to severe plastic deformation, the microstructure is deeply changed. Thus, Terwilliger et al. [21] have shown that impurity redistribution depended on grain size [21]. Also, in a previous work, Rebhi et al. [22] have shown in industrial aluminium of the present investigation, that there was a substantial crushing of foundry-formed Al8Fe2Si inclusions by ECAP. The effects of ECAP on the corrosion properties of the same material have been presented previously [23]: We have shown that severe plastic deformation leads to a substantial increase in pitting sites through a refinement and a redistribution of inclusions. The aim of the present work is to study the effects of ECAP before anodization on the properties of the passive film. The latter was obtained by anodization and hydrothermal sealing (HTS). For this purpose, electrochemical, microscopic and topographic methods were used. Experimental methods The experiments were carried out on an aluminum (99.1%) received in the form of cast ingot. It was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES). The main impurities are given in table 1. The ingot was annealed at 500 °C for 24 h for homogenization. Samples 10×10×90 mm3 were machined for ECAP; they were heated after at 550°C for one hour and quenched in iced water. ECAP was performed at room temperature. Prior to pressing, the samples were coated with a lubricant containing MoS2. The die used was made up from two tool steel blocks bolted together. Two square channels of 10×10 mm2 cross sectional area intersecting at an angle & = 90° were machined in one of the two blocks. The arc of curvature at the outer point of the intersection of the two channels delineated an angle ࣛ = 90°. For such angles, ECAP creates an equivalent strain of about 0.906N [24], where N is the number of passes. Repetitive pressings of each sample were conducted to achieve an equivalent strain ranging between 1 and 5 without rotation (route A [24, 25]). Electrochemical tests were performed in a 3 wt% NaCl solution using a Radiometer analytical potentiometer PGZ301. The pH was fixed to 6.7; a classic three-electrode cell was utilised in which the working electrode was the test material, the counter and reference electrodes were platinum grid and a Silver-Silver chloride Electrode (Ag/AgClsat) respectively. In this paper, all potentials are given with respect to Ag/AgClsat. Anodization
3
experiments were performed at room temperatures for one hour in 150 g/l sulphuric acid medium under an applied voltage of 15 V. It was followed by boiling water immersion for one hour. OCP and Potentiodynamic curves were recorded on the X plane which is perpendicular to the longitudinal axis of the sample [26]. A scan rate of 0.1 mV/s was chosen. Electrochemical impedance spectroscopy was performed after anodization, between 100 mHz and 100 kHz at free potential. The amplitude of the sinusoidal voltage signal was 10 mV. Before each EIS measurement, a “Radiometer analytical RCB200” Resistor Capacitor Box is utilised to verify the potentiometer precision, An EIS Spectrum is obtained. An automatic R1R2C fitting is established using “Voltamaster 4” and “Zview 2” computer programs; same programs are used to determine specific polarization resistance from EIS measurement. For surface characterisation, Scanning Electron Microscopy (SEM) was performed using a JEOL T220A microscope and Topographic images were recorded using a “STIL CHR 150” Microtopograph. Results and discussion Figure 1 Figure 1 shows the microstructure of the material. Before ECAP, the microstructure is formed by grains irregulars in shape and in size (fig 1a). The latter ranges between 70 and 230 µm. Grain boundaries are coated with inclusions which were defined previously to be Al8Fe2Si of the hexagonal structure [27]. This microstructure is similar to that before tempering and seems to be defined during casting. After ECAP (Figure 1b), grains are flatten progressively and inclusions are broken up together with grain boundaries [22, 27-29]. Figure 2 Figure 2 shows a set of optical micrographs of the surfaces of anodized samples with and without ECAP. Bright zones on the micrographs denote the coated area of the surface and the dark zones represent the uncoated part. As it is commonly observed, oxide formation is located mainly within grains rather than in grain boundaries or around the inclusions. The effect of ECAP on the distribution of the oxide is related to a change in the grain shape. Oxide film thickness seems to be homogenous, except for few regions where oxide layer looks thinner or absent. The results of Son et al. in aluminium-copper alloy [19] showed that the discontinuities of the film are mainly located around inclusions. The latter are trapped in oxide films and they are not oxidized. They showed also that the anodic oxide film was absent in the boundary between the natural oxide films and these inclusions. The behaviour of anodized materials with respect to pitting in 3% NaCl solution can be illustrated from the polarization curves of figure 3: In the cathodic area, current is retained negative indicating aluminium protection. When the potential increases, a rapid increase in current is observed at certain potentials indicating the initiation of pitting corrosion. Figure 3 Pitting starts at a potential Epit which is mostly considered to merge with corrosion potential Ecorr [23, 30]. In the reverse scans, polarization curves show a repassivation potential Erep which is more cathodic than Epit. The effect of ECAP before anodization is clearly revealed: For N = 0, Epit appears to merge with Erep whereas larger hysteresis loops are observed in ECAP processed specimens (figure 3). The substantial increase of current for N = 1 is due to the formation of relatively stable pits of low density and important depth. Moreover, the microstructure of the processed alloys for N = 1 is usually considered to be heterogeneous; the morphology of passive film should then vary from one zone to another and so does the pitting behaviour. For N = 5, only a slight increase in current is observed at Epit then it keeps a near-constant value in a relatively wide range of potential (between -0.65 and 0.60V/ Ag/AgClsat); this is due to the formation of a high density of unstable pits that
4
repassivate instantaneously in this range of potential. The formation of more stable deep pits is observed however, only at sufficiently higher potentials. Pitting potential seems to be the same with or without anodization. This may be explained by the fact that pitting areas in the vicinity of particles are not influenced by anodization. Similar results were obtained by Son et al. [21] who attributed this to the absence of oxide film on heterogeneities. Nakato et al.[31] have shown in aluminium and AlMg alloy that pitting potentials are shifted to noble values by ECAP. The reason is that pitting occurs near impurity precipitates; the latter are refined by ECAP. It has been shown that pitting potential characterizes the difficulty of pit starting and it was used to compare the pitting resistance of stainless steels. However, in aluminium, the determining step is rather pit propagation so that Epit cannot be considered as a criterion of pitting resistance [32]. Furthermore, it is difficult to separate the effects of different features on pitting potential since the latter depends on both volume and surface properties of the material. The difference between Epit and Erep is rather the criterion of susceptibility to pitting corrosion of aluminium alloys [33]. In fact, it can give an idea about the density and the depth of the pits: the larger the difference between Epit and Erep, i.e. the larger the area of the hysteresis loop, the higher the susceptibility of the material to pitting corrosion. Figure 4 shows the dependence of ǻ(=Epit-Erep on the pass number N. Except for N=1, ǻ( values indicate an increase in the susceptibility to repassivation as the number of ECAP increases; this is related to an enhanced stability of oxide film. Figure 4 In order to study the pitting sensitivity of anodized samples, OCP tests were carried out under the condition of zero current in 3% NaCl solution up to two hours of immersion. The OCP curves with and without ECAP are shown in Figure 5; horizontal lines corresponding to pitting and repassivation potentials of each sample are also plotted for comparison. In all cases, the materials show an increase in potential to reach rapidly a constant value. It is clear that the corrosion potential in the non deformed specimen is significantly lower than in ECAP processed ones (figure 5). Moreover, for N=0, there is a tendency to a decrease for Ecorr value. In the range of pH investigated, the oxide layer is sufficiently stable due to its low solubility. The higher values of Ecorr in ECAP processed materials are related to more continuous films rather to higher film thickness since the latter was found to be the same in different specimens. The results of Son et al [34] showed also that the rate of formation of oxide film increased by ECAP even though a same final thickness can be obtained through different kinetics. In the present work, the observed decrease in Ecorr in the non deformed specimen indicates a poor adherence of the oxide film. By contrast, the curves of deformed materials show rather a tendency to stabilization indicating a better adherence of the oxide film. Figure 5 Another important feature is that OCP curves of ECAP processed materials exhibit a noise that becomes stronger as the number of passes increases. The noise was related to pitting [22, 23]. The absence of pitting for N=0 is due to the reached value of Ecorr which is much lower than the value -750mV/ Ag/AgClsat obtained from the polarisation curves of figure 3. The strong noise observed in strongly-deformed material (N=5) is due to the reached values of Ecorr which exceed the pitting potential of figure 3. SEM micrographs of figure 6 show actually a higher number of pits (covered with corrosion products) for N=5 sample but not than for N=0. For N=1 to 4, Ecorr is retained below Epit; for this reason, a weaker noise is observed. Figure 6 Microtopographic observations were used to characterize surface morphology of anodized materials after immersion tests. Typical images are shown in figure 7. In figures 7a and 7b, blue zones represent the corrosion sites. In figure 7c the red zones represent the corrosion
5
products. Figure 7d is the same as figure 7c in which we have represented the oxide thickness profile. For N=0 and N=1, attacked areas have same dispersion as the non anodized areas of figure 2; no pitting was observed and corrosion is mainly intergranular. The absence of pitting agrees with the OCP results indicating lower values than Epit, no pitting was observed and corrosion is mainly intergranular. For N=5, a relatively large number of cones of corrosion products is observed mainly on precipitates and the pre-existing grain boundaries indicating an important number of pits; this explains the high OCP values that reached Epit. In fact, during immersion, the precipitate areas are not protected and become prone to spontaneous local corrosion; this can be illustrated by the steps suggested in figure 8. In this figure a model is given to explain the role of inclusions in the discontinuity filling during immersion. Figure 7 Figure 8 As was noted above, due to a splitting of precipitates, pits in ECAP processed specimens are dense and small . The narrow discontinuities or pits within the film are then easily filled by corrosion products (figure 8c); for this reason, the phenomenon is observed mainly for N = 5 (fig.7c-d). Furthermore, at Erep potential, pits stop growing and repassivate; once the potential reaches Epit again, these coated pits will not be reinitiated again and pitting will start then on fresh sites leading to more corrosion product on the surface [35]. In many areas, corrosion product thickness reaches 25 µm (fig.7d) which is sufficient to provide a good separation from the corrosive medium. Figure 9 gives the impedance spectra on Nyquist plots of the anodized specimens after immersion; the extracted values of specific polarisation resistance Rp are also plotted (fig 10). On the same figure, Rp values before anodization are given for comparison [23]. We note that without anodization, Rp values are around 5~6 k7.cm² and seem to be independent of ECAP passes; these low values should be the proper solution resistance. After anodization, Rp increases even for N = 0 but the positive effect of ECAP is clearly revealed; a value as high as 300 k7.cm² is achieved after four passes through the die. This improvement of specific polarization resistance of anodized alloy by the ECAP process is due to a more continuous film and to the role of corrosion products described above. Figure 9 Figure 10 Conclusion Aluminium alloys, mainly the 1000 family exhibit good corrosion resistance after anodization. The increase of mechanical properties via the refinement of grain size has been widely investigated. Equal Channel Angular Pressing (ECAP) has been extensively used to achieve sub-micron grained alloys. It is then important to enhance mechanical properties without loss of corrosion resistance. The aim of the present work was to study the effects of ECAP on the efficiency of the passive film obtained by anodization. An aluminium (99.1% in title) was chosen and ECAP was performed up to 5 passes. As it is the case in industrial alloys, the material contains a certain fraction of inclusions. ECAP was shown to reduce inclusion size. After anodization, polarization curves do not show any effect of ECAP on the pitting potential. However, repassivation potential is shifted to noble values as the number of passes increases, indicating that repassivation is easier in ECAP processed materials. OCP value increases when increasing the number of passages N and a pitting value is reached for N=5. Micrographic observations show that the oxide film is more continuous in ECAP processed specimens due to a refinement of grains and inclusions. Corrosion products play a determinate role in high deformed aluminium. For N=5, pits are so small that they can be completely filled by the corrosion products. This improves the
6
protection properties of the passive film; a value as high as 300 kohm.cm² is obtained for the polarization resistance Rp. Refinement of grains and partial dissolution or splitting of cathodic precipitates enhance corrosion resistance by the formation of continuous passive layer.
Acknowledgements The Authors are grateful to M.Charters and J. El Bekri (ECP) for SEM observations and to Q.Credling and N.Bouadjaja (ECP) for performing microtopography. References [1] M. T. A. Saif, S. Zhang, A. Haque and K. J. Hsia. Effect of native Al2O3 on the elastic response of nanoscale Al films, Acta Mater. 50 (2002) 2779–2786. [2] Timothy Campbell, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta. Dynamics of Oxidation of Aluminum Nanoclusters using Variable Charge Molecular-Dynamics Simulations on Parallel Computers, Phys. Rev. Lett., 82 (1999) 4866-4869. [3] O. Zubillaga, F.J. Cano, I. Azkarate, I.S. Molchan, G.E. Thompson, A.M. Cabral, P.J. Morais. Corrosion performance of anodic ¿OPVFRQWDLQLQJSRO\DQLOLQHDQG7L2 2 nanoparticles on AA3105 aluminium alloy, Surf. Coat. Technol. 202 (2008) 5936. [4] J. Hou, D.D.L. Chung. Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization, J. Mater. Sci. 32 (1997) 3113-3121. [5] S. Wernick, R. Pinner,The Surface Treatment and Finishing of Aluminium and its Alloys; E. Eyrolles: Paris (1962),228-260. [6] W. Bensalah, M. Feki, M. Wery, H.F. Ayedi, Thick and Dense Anodic Oxide Layers Formed on Aluminum in Sulphuric Acid Bath, J. Mater. Sci. Technol, 26 ( 2010) 113-118, [7] W. Bensalah, M. Feki, M. Wery, H.F. Ayedi, Chemical dissolution resistance of anodic oxide layers formed on aluminum, Trans. Nonferrous. Met. Soc. China, 21 (2011) 1673-1679. [8] J. A. González, V. López, A. Bautista, E. Otero, X. R. Nóvoa. Characterization of porous aluminium oxide films from a.c. impedance measurements, J. Appl. Electrochem., 29 (1999) 229-238. [9] Yufei Jia, Haihui Zhou, Peng Luo, Shenglian Luo, Jinhua Chen, Yufei Kuang, Preparation and characteristics of well-aligned macroporous films on aluminum by high voltage anodization in mixed acid, Surf. Coat. Technol., 201 (2006) 513-518. [10] R. Kötz, B. Schnyder, C. Barbero, Anodic oxidation of aluminium in sulphuric acid monitored by ex-situ and in-situ spectroscopic ellipsometry, Thin Solid Films, 233, (1993) 63-68, [11] B. Schnyder, R. Kötz, Spectroscopic ellipsometry and XPS studies of anodic aluminum oxide formation in sulfuric acid, J. Electroanal. Chem., 339 (1992) 167-185. [12] A. Belwalkar, E. Grasing, W. Van Geertruyden, Z. Huang, W.Z. Misiolek, Effect of processing parameters on pore structure and thickness of anodic aluminum oxide (AAO) tubular membranes, J. Membrane. Sci, 319 (2008), 192-198, [13] L. Hemmouche, C. Fares, M.A. Belouchrani, Influence of heat treatments and anodization on fatigue life of 2017A alloy, Eng. Fail. Anal., 35( 2013), 554-561, [14] Leszek Zaraska, GrzHJRU]'6XOND-DQXV]6]HUHPHWD0DULDQ-DVNXáD3RURXVDQRGLF alumina formed by anodization of aluminum alloy (AA1050) and high purity aluminum, Electrochim. Acta., 55 (2010) 4377-4386. [15] V.M. Segal, Materials processing by simple shear. Mater Sci.Eng A197 (1995) 157–64. [16] H.S. Kim, W.J. Kim Enhanced corrosion resistance of ultrafine-grained AZ61 alloy containing very fine particles of Mg17Al12 phase. Corros. Sci, 75 (2013) 228–238. [17] K. Gopala Krishna, K. Sivaprasad , T.S.N. Sankara Narayanan , K.C. Hari Kumar, Localized corrosion of an ultrafine grained Al–4Zn–2Mg alloy produced by
7
Cryorolling. Corros. Sci, 60 (2012) 82–89. [18] D. Song, A.B. Ma, J.H. Jiang, P.H. Lin, D.H. Yang, J.F. Fan, Corrosion behaviour of bulk ultra-fine grained AZ91D magnesium alloy fabricated by equal-channel angular pressing, Corros. Sci, 53 (2011) 362–373. [19] In-Joon Son, Hiroaki Nakano, Satoshi Oue, Shigeo Kobayashi, Hisaaki Fukushima, Zenji Horita, Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy, Trans. Nonferrous. Met. Soc. China, 19 (2009) 904-908, [20] J.G. Brunner, J. May, H.W. Höppel, M.Göken, S. Virtanen, Localized corrosion of ultrafine-grained Al–Mg model alloys, Electrochim. Acta., 55 (2010) 1966–1970. [21] C.D. Terwilliger, Y.M. Chiang, size-dependant solute segregation and total solubility in ultrafine polycrystals: Ca in TiO2, Acta. Metall. Mater. 43 (1995) 319–328. [22] A. Rebhi, T. Makhlouf, N. Njah, Y. Champion, J.P. Couzinie, Characterization of aluminum processed by equal channel angular extrusion: Effect of processing route, Mater. Charac. 60 (2009) 1489-1495. [23] O.Jilani, N.Njah and P. Ponthiaux, Transition from Intergranular to Pitting Corrosion in Fine Grained Aluminum Processed by Equal Channel Angular Pressing, Corros. Sci, 87 (2014) 259-264. [24] Yoshinori Iwahashi, Jingtao Wang, Zenji Horita, Minoru Nemoto, Terence G. Langdon, Principle of equal-channel angular pressing for the processing of ultra-fine grained materials, Scripta. Mater. 35 (1996) 143-146 [25] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, An investigation of microstructural evolution during equal-channel angular pressing, Acta Mater., 45 ( 1997), 4733-4741, [26] R.Z Valiev, R.K Islamgaliev, I.V Alexandrov, Bulk nanostructured materials from severe plastic deformation, Prog. Mater. Sci. 45 ( 2000), 103-189. [27] A. Korchef, N, Njah, Y. Champion, S. Guérin, C. Leroux, J. Masmoudi, A. Kolsi. Material Flow During Equal Channel Angular Pressing of Aluminum Containing Al8Fe2Si Precipitates, Adv. Eng. Mater. 6 (2004) 222–228. [28] Atef Korchef, Yannick Champion, Nabil Njah, X-ray diffraction analysis of aluminium containing Al8Fe2Si processed by equal channel angular pressing, J. Alloys. Compd, 427, (2007), 176-182, [29] V.V. Stolyarov, R Lapovok, I.G Brodova, P.F Thomson, Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure, Mater. Sci. Eng., A357 ( 2003), 159-167. [30] B. Zaid, D. Saidi, A. Benzaid, S. Hadji, Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy, Corros. Sci.,50 (2008), 1841-1847 [31] Hiroaki Nakano, In-Joon Son, Satoshi Oue, Shigeo Kobayashi, Hisaaki Fukushima, Zengi Horita, Effect of equal channel angular pressing on the pitting corrosion resistance of the aliminum alloys with/without anodization, Mater. Sci. Forum 638-642 (2010) 1989-1994. [32] Max Reboul, Corrosion des alliages d’aluminum, Tech. Ing, TI-éditions, cor 325, 2005. [33] Monica Trueba, Stefano P. Trasatti, Study of Al alloy corrosion in neutral NaCl by the pitting scan technique, Mater. Chem. Phys., 121 (2010) 523-533, [34] In-Joon Son, Hiroaki Nakano, Satoshi Oue, Shigeo Kobayashi,Hisaaki Fukushima and Zenji Horita. Pitting Corrosion Resistance of Ultrafine-Grained Aluminum Processed by Severe Plastic Deformation, Mater. Trans. JIM., 47 (2006) 1163 to 1169. [35] Christian Vargel, Michel Jacques and Martin P. Schmidt, Chapter B.2-Types of Corrosion on Aluminium, In Corrosion of Aluminium, edited by Christian Vargel, Michel Jacques and Martin P. Schmidt, Elsevier, Amsterdam, 2004, Pages 113-146.
8
Figure captions Figure 1: Optical micrographs showing the grain structure of aluminium: (a) before ECAP (N=0); (b) after ECAP (N=5). Figure 2: Optical micrographs of anodized aluminium in different magnifications: a, b and c: before ECAP (N=0); d,e and f: after ECAP (N=5). Dark dots represent the uncoated zones. Figure 3: Polarization curves of aluminium after anodization in 3% NaCl solution treatment without ECAP (N=0) and with ECAP (N=1 to N=5). Arrows indicates backward sweeps. Figure 4: susceptibility to pitting E=Epit-Erep of anodized aluminium as a function of the number of ECAP passes N. Figure 5: Open circuit potential of aluminium after anodization with and without ECAP. Horizontal lines correspond to pitting and repassivation potentials. Figure 6: SEM micrographs of aluminium after anodization and immersion tests in 3% NaCl: (a) N=0, (b): N=5. Note that corrosion products are clearer and finer after ECAP (micrograph B). Figure 7: Microtopographic images of aluminium after anodization and immersion tests in 3%NaCl: (a) N=0, (b) N=1, (c) N=5. Note that passive film is finer for N=5. (d) line scans of the surface of figure 7c . Figure 8: A model explaining the effect of inclusion size on the formation of passive film: (a) before anodization, (b) after anodization, (c) after anodization and immersion. Figure 9: Nyquist plots after two hour of immersion in 3% NaCl solution for different ECAP passes (N) and anodization. Figure 10: Specific polarization resistance as a function of N calculated from the curves of figure 9. Open circles: Before anodization, full circles: after anodization.
9
10
11
12
13
14
15
16
17
Wt%
Fe 0.25
Mg 0.22
Mn 0.10
Si 0.10
Zn 0.16
Al Balance
Table 1: Chemical compositions of the investigated material (wt%). Highlights Industrial aluminium passivates with and without ECAP. Grain and inclusion refinement leads to more continuous and efficient passive film. Pits are finer after ECAP due to a refinement of inclusions. Fine pits in the film are easily filled by corrosion product.
S0010-938X(14)00409-0 http://dx.doi.org/10.1016/j.corsci.2014.08.020 CS 6003
To appear in:
Corrosion Science
Received Date: Accepted Date:
27 May 2014 16 August 2014
Please cite this article as: O. Jilani, N. Njah, P. Ponthiaux, Corrosion properties of anodized aluminium: effects of equal channel angular pressing prior to anodization, Corrosion Science (2014), doi: http://dx.doi.org/10.1016/ j.corsci.2014.08.020
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Corrosion Properties of Anodized Aluminium: Effects of Equal Channel Angular Pressing prior to Anodization. Oussama Jilania, Nabil Njaha*, Pierre Ponthiauxb a
GEOMAT-Laboratoire Géoressource, Environnement, Matériaux et Changements Globaux,
Faculté des Sciences de Sfax, Université de Sfax, Tunisia. b
LGPM-Laboratoire Génie des Procédés et Matériaux, École Centrale de Paris, Grande voie des Vignes, 92290 Châtenay-Malabry, France. Abstract Many aluminium alloys are subjected to mechanical treatments before anodization. We have studied the effects of Equal Channel Angular Pressing (ECAP) on the properties of the passive film of aluminium 99.1%. Electrochemical measurements and micrography were used for characterisation. In 3% NaCl solution, anodization after ECAP leads to higher free potential values than that without deformation. Grain refinement and high dislocation density obtained by ECAP enhance the formation of a more continuous film after anodization. The destruction of inclusions increases pitting site density and leads to low-depth pits. The role of inclusions and pit sizes in the repassivation is illustrated Keyword: A: aluminium; B: potentiostatic; B: polarization C: anodic films; C: pitting corrosion Introduction Aluminium and its alloys show a good corrosion resistance due to the oxide film formed on the surface over a neutral range of pH. However, the average natural oxide film thickness, which is below 10nm, is not sufficient for protection [1, 2]. Therefore, an improvement of alloy protection is obtained by increasing the film thickness through anodization process. In practice, anodization is a widely studied and accepted process for protection against corrosion and abrasion [3]. The most widely used electrolyte for anodizing is sulphuric acid [4-7]. Industrially, sulphuric anodization is traditionally followed by hydrothermal sealing (HTS) [8]. Generally, together with the important increase in corrosion protection [6, 7, 9-12], negative effects on other properties are sometimes observed. Thus, Hemmouche et al. [13] have shown in aluminium-copper alloy that anodizing results in a decrease in fatigue life. They ascribed this to two features: In addition to the intrinsic brittleness of oxide, there was an important effect of precipitates that constituted favorable sites of pores and discontinuities in the oxide layer. Zaraska et al. [14] experiments revealed the impurity effect on film morphology in anodized industrial and high purity aluminiums. They remarked that the presence of impurities affects not only the rate of oxide growth but also the film behavior. In order to obtain a sufficiently continuous film, the substrate must be as pure as possible which may not be the case in industrial aluminium. In fact, in such materials, inclusions cannot be easily avoided due to the presence, among others, of iron and silicon but an improvement of corrosion properties is possible by reducing the size of these inclusions. During the few last decades, Equal Channel Angular Pressing (ECAP) has been widely used as a method of grain refinement and of hardness increase. It consists in introducing a high amount of deformation without change in the shape of the material [15]. It is well known that oxide properties like adherence and grain size are related to the surface properties of *
Corresponding author. Tel.: (+216)98656472.
E-mail address: [email protected].
2
substrates especially if the latter was subjected to mechanical treatments. The complex microstructure of ECAPed alloys, particularly multiphased ones (fine grains, high defect density mainly dislocations and grain boundaries, finer precipitates,...) is often the origin of result discrepancies: In AZ61 magnesium alloy containing B precipitates, grain refinement using high-ratio differential speed rolling (HRDSR) improves corrosion resistance due to the continuous film formed [16]. Furthermore, a substantial refinement of B precipitates reduces their galvanic effect [16]. In Al-Zn-Mg alloy, the resolution of Zn and Mg in the matrix by cryorolling was found to lower the resistance of the alloy to general corrosion as compared with the non deformed alloy. A further annealing shifts the OCP curve to noble potentials with a clear tendency to pitting; this was ascribed to the galvanic effect of precipitates [17]. On the other hand, Song et al. [18] have shown, that refinement of grains and precipitates, and the high density of defects lower corrosion resistance of AZ91D alloy together with an increase in the sensitivity to pitting. In fine grained aluminium-copper alloy obtained by ECAP, Son et al. [19] have shown that pitting resistance after anodization increased with increasing the number of passes, i.e. the amount of deformation. In fact they observed that the time required before the initiation of pitting in AlCl3 solution increased by ECAP. They attributed this to a decrease in the size of precipitates which were not coated by the oxide film and which constitute favorable sites for pitting. On the other hand, in Al-Mg alloys, Brunner et al. [20] did not note any change in corrosion resistance when increasing the number of ECAP passes. They suggested that passive layer properties are independent of the volume fraction of grain boundaries and of dislocation density in the substrate surface. Due to severe plastic deformation, the microstructure is deeply changed. Thus, Terwilliger et al. [21] have shown that impurity redistribution depended on grain size [21]. Also, in a previous work, Rebhi et al. [22] have shown in industrial aluminium of the present investigation, that there was a substantial crushing of foundry-formed Al8Fe2Si inclusions by ECAP. The effects of ECAP on the corrosion properties of the same material have been presented previously [23]: We have shown that severe plastic deformation leads to a substantial increase in pitting sites through a refinement and a redistribution of inclusions. The aim of the present work is to study the effects of ECAP before anodization on the properties of the passive film. The latter was obtained by anodization and hydrothermal sealing (HTS). For this purpose, electrochemical, microscopic and topographic methods were used. Experimental methods The experiments were carried out on an aluminum (99.1%) received in the form of cast ingot. It was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES). The main impurities are given in table 1. The ingot was annealed at 500 °C for 24 h for homogenization. Samples 10×10×90 mm3 were machined for ECAP; they were heated after at 550°C for one hour and quenched in iced water. ECAP was performed at room temperature. Prior to pressing, the samples were coated with a lubricant containing MoS2. The die used was made up from two tool steel blocks bolted together. Two square channels of 10×10 mm2 cross sectional area intersecting at an angle & = 90° were machined in one of the two blocks. The arc of curvature at the outer point of the intersection of the two channels delineated an angle ࣛ = 90°. For such angles, ECAP creates an equivalent strain of about 0.906N [24], where N is the number of passes. Repetitive pressings of each sample were conducted to achieve an equivalent strain ranging between 1 and 5 without rotation (route A [24, 25]). Electrochemical tests were performed in a 3 wt% NaCl solution using a Radiometer analytical potentiometer PGZ301. The pH was fixed to 6.7; a classic three-electrode cell was utilised in which the working electrode was the test material, the counter and reference electrodes were platinum grid and a Silver-Silver chloride Electrode (Ag/AgClsat) respectively. In this paper, all potentials are given with respect to Ag/AgClsat. Anodization
3
experiments were performed at room temperatures for one hour in 150 g/l sulphuric acid medium under an applied voltage of 15 V. It was followed by boiling water immersion for one hour. OCP and Potentiodynamic curves were recorded on the X plane which is perpendicular to the longitudinal axis of the sample [26]. A scan rate of 0.1 mV/s was chosen. Electrochemical impedance spectroscopy was performed after anodization, between 100 mHz and 100 kHz at free potential. The amplitude of the sinusoidal voltage signal was 10 mV. Before each EIS measurement, a “Radiometer analytical RCB200” Resistor Capacitor Box is utilised to verify the potentiometer precision, An EIS Spectrum is obtained. An automatic R1R2C fitting is established using “Voltamaster 4” and “Zview 2” computer programs; same programs are used to determine specific polarization resistance from EIS measurement. For surface characterisation, Scanning Electron Microscopy (SEM) was performed using a JEOL T220A microscope and Topographic images were recorded using a “STIL CHR 150” Microtopograph. Results and discussion Figure 1 Figure 1 shows the microstructure of the material. Before ECAP, the microstructure is formed by grains irregulars in shape and in size (fig 1a). The latter ranges between 70 and 230 µm. Grain boundaries are coated with inclusions which were defined previously to be Al8Fe2Si of the hexagonal structure [27]. This microstructure is similar to that before tempering and seems to be defined during casting. After ECAP (Figure 1b), grains are flatten progressively and inclusions are broken up together with grain boundaries [22, 27-29]. Figure 2 Figure 2 shows a set of optical micrographs of the surfaces of anodized samples with and without ECAP. Bright zones on the micrographs denote the coated area of the surface and the dark zones represent the uncoated part. As it is commonly observed, oxide formation is located mainly within grains rather than in grain boundaries or around the inclusions. The effect of ECAP on the distribution of the oxide is related to a change in the grain shape. Oxide film thickness seems to be homogenous, except for few regions where oxide layer looks thinner or absent. The results of Son et al. in aluminium-copper alloy [19] showed that the discontinuities of the film are mainly located around inclusions. The latter are trapped in oxide films and they are not oxidized. They showed also that the anodic oxide film was absent in the boundary between the natural oxide films and these inclusions. The behaviour of anodized materials with respect to pitting in 3% NaCl solution can be illustrated from the polarization curves of figure 3: In the cathodic area, current is retained negative indicating aluminium protection. When the potential increases, a rapid increase in current is observed at certain potentials indicating the initiation of pitting corrosion. Figure 3 Pitting starts at a potential Epit which is mostly considered to merge with corrosion potential Ecorr [23, 30]. In the reverse scans, polarization curves show a repassivation potential Erep which is more cathodic than Epit. The effect of ECAP before anodization is clearly revealed: For N = 0, Epit appears to merge with Erep whereas larger hysteresis loops are observed in ECAP processed specimens (figure 3). The substantial increase of current for N = 1 is due to the formation of relatively stable pits of low density and important depth. Moreover, the microstructure of the processed alloys for N = 1 is usually considered to be heterogeneous; the morphology of passive film should then vary from one zone to another and so does the pitting behaviour. For N = 5, only a slight increase in current is observed at Epit then it keeps a near-constant value in a relatively wide range of potential (between -0.65 and 0.60V/ Ag/AgClsat); this is due to the formation of a high density of unstable pits that
4
repassivate instantaneously in this range of potential. The formation of more stable deep pits is observed however, only at sufficiently higher potentials. Pitting potential seems to be the same with or without anodization. This may be explained by the fact that pitting areas in the vicinity of particles are not influenced by anodization. Similar results were obtained by Son et al. [21] who attributed this to the absence of oxide film on heterogeneities. Nakato et al.[31] have shown in aluminium and AlMg alloy that pitting potentials are shifted to noble values by ECAP. The reason is that pitting occurs near impurity precipitates; the latter are refined by ECAP. It has been shown that pitting potential characterizes the difficulty of pit starting and it was used to compare the pitting resistance of stainless steels. However, in aluminium, the determining step is rather pit propagation so that Epit cannot be considered as a criterion of pitting resistance [32]. Furthermore, it is difficult to separate the effects of different features on pitting potential since the latter depends on both volume and surface properties of the material. The difference between Epit and Erep is rather the criterion of susceptibility to pitting corrosion of aluminium alloys [33]. In fact, it can give an idea about the density and the depth of the pits: the larger the difference between Epit and Erep, i.e. the larger the area of the hysteresis loop, the higher the susceptibility of the material to pitting corrosion. Figure 4 shows the dependence of ǻ(=Epit-Erep on the pass number N. Except for N=1, ǻ( values indicate an increase in the susceptibility to repassivation as the number of ECAP increases; this is related to an enhanced stability of oxide film. Figure 4 In order to study the pitting sensitivity of anodized samples, OCP tests were carried out under the condition of zero current in 3% NaCl solution up to two hours of immersion. The OCP curves with and without ECAP are shown in Figure 5; horizontal lines corresponding to pitting and repassivation potentials of each sample are also plotted for comparison. In all cases, the materials show an increase in potential to reach rapidly a constant value. It is clear that the corrosion potential in the non deformed specimen is significantly lower than in ECAP processed ones (figure 5). Moreover, for N=0, there is a tendency to a decrease for Ecorr value. In the range of pH investigated, the oxide layer is sufficiently stable due to its low solubility. The higher values of Ecorr in ECAP processed materials are related to more continuous films rather to higher film thickness since the latter was found to be the same in different specimens. The results of Son et al [34] showed also that the rate of formation of oxide film increased by ECAP even though a same final thickness can be obtained through different kinetics. In the present work, the observed decrease in Ecorr in the non deformed specimen indicates a poor adherence of the oxide film. By contrast, the curves of deformed materials show rather a tendency to stabilization indicating a better adherence of the oxide film. Figure 5 Another important feature is that OCP curves of ECAP processed materials exhibit a noise that becomes stronger as the number of passes increases. The noise was related to pitting [22, 23]. The absence of pitting for N=0 is due to the reached value of Ecorr which is much lower than the value -750mV/ Ag/AgClsat obtained from the polarisation curves of figure 3. The strong noise observed in strongly-deformed material (N=5) is due to the reached values of Ecorr which exceed the pitting potential of figure 3. SEM micrographs of figure 6 show actually a higher number of pits (covered with corrosion products) for N=5 sample but not than for N=0. For N=1 to 4, Ecorr is retained below Epit; for this reason, a weaker noise is observed. Figure 6 Microtopographic observations were used to characterize surface morphology of anodized materials after immersion tests. Typical images are shown in figure 7. In figures 7a and 7b, blue zones represent the corrosion sites. In figure 7c the red zones represent the corrosion
5
products. Figure 7d is the same as figure 7c in which we have represented the oxide thickness profile. For N=0 and N=1, attacked areas have same dispersion as the non anodized areas of figure 2; no pitting was observed and corrosion is mainly intergranular. The absence of pitting agrees with the OCP results indicating lower values than Epit, no pitting was observed and corrosion is mainly intergranular. For N=5, a relatively large number of cones of corrosion products is observed mainly on precipitates and the pre-existing grain boundaries indicating an important number of pits; this explains the high OCP values that reached Epit. In fact, during immersion, the precipitate areas are not protected and become prone to spontaneous local corrosion; this can be illustrated by the steps suggested in figure 8. In this figure a model is given to explain the role of inclusions in the discontinuity filling during immersion. Figure 7 Figure 8 As was noted above, due to a splitting of precipitates, pits in ECAP processed specimens are dense and small . The narrow discontinuities or pits within the film are then easily filled by corrosion products (figure 8c); for this reason, the phenomenon is observed mainly for N = 5 (fig.7c-d). Furthermore, at Erep potential, pits stop growing and repassivate; once the potential reaches Epit again, these coated pits will not be reinitiated again and pitting will start then on fresh sites leading to more corrosion product on the surface [35]. In many areas, corrosion product thickness reaches 25 µm (fig.7d) which is sufficient to provide a good separation from the corrosive medium. Figure 9 gives the impedance spectra on Nyquist plots of the anodized specimens after immersion; the extracted values of specific polarisation resistance Rp are also plotted (fig 10). On the same figure, Rp values before anodization are given for comparison [23]. We note that without anodization, Rp values are around 5~6 k7.cm² and seem to be independent of ECAP passes; these low values should be the proper solution resistance. After anodization, Rp increases even for N = 0 but the positive effect of ECAP is clearly revealed; a value as high as 300 k7.cm² is achieved after four passes through the die. This improvement of specific polarization resistance of anodized alloy by the ECAP process is due to a more continuous film and to the role of corrosion products described above. Figure 9 Figure 10 Conclusion Aluminium alloys, mainly the 1000 family exhibit good corrosion resistance after anodization. The increase of mechanical properties via the refinement of grain size has been widely investigated. Equal Channel Angular Pressing (ECAP) has been extensively used to achieve sub-micron grained alloys. It is then important to enhance mechanical properties without loss of corrosion resistance. The aim of the present work was to study the effects of ECAP on the efficiency of the passive film obtained by anodization. An aluminium (99.1% in title) was chosen and ECAP was performed up to 5 passes. As it is the case in industrial alloys, the material contains a certain fraction of inclusions. ECAP was shown to reduce inclusion size. After anodization, polarization curves do not show any effect of ECAP on the pitting potential. However, repassivation potential is shifted to noble values as the number of passes increases, indicating that repassivation is easier in ECAP processed materials. OCP value increases when increasing the number of passages N and a pitting value is reached for N=5. Micrographic observations show that the oxide film is more continuous in ECAP processed specimens due to a refinement of grains and inclusions. Corrosion products play a determinate role in high deformed aluminium. For N=5, pits are so small that they can be completely filled by the corrosion products. This improves the
6
protection properties of the passive film; a value as high as 300 kohm.cm² is obtained for the polarization resistance Rp. Refinement of grains and partial dissolution or splitting of cathodic precipitates enhance corrosion resistance by the formation of continuous passive layer.
Acknowledgements The Authors are grateful to M.Charters and J. El Bekri (ECP) for SEM observations and to Q.Credling and N.Bouadjaja (ECP) for performing microtopography. References [1] M. T. A. Saif, S. Zhang, A. Haque and K. J. Hsia. Effect of native Al2O3 on the elastic response of nanoscale Al films, Acta Mater. 50 (2002) 2779–2786. [2] Timothy Campbell, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta. Dynamics of Oxidation of Aluminum Nanoclusters using Variable Charge Molecular-Dynamics Simulations on Parallel Computers, Phys. Rev. Lett., 82 (1999) 4866-4869. [3] O. Zubillaga, F.J. Cano, I. Azkarate, I.S. Molchan, G.E. Thompson, A.M. Cabral, P.J. Morais. Corrosion performance of anodic ¿OPVFRQWDLQLQJSRO\DQLOLQHDQG7L2 2 nanoparticles on AA3105 aluminium alloy, Surf. Coat. Technol. 202 (2008) 5936. [4] J. Hou, D.D.L. Chung. Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization, J. Mater. Sci. 32 (1997) 3113-3121. [5] S. Wernick, R. Pinner,The Surface Treatment and Finishing of Aluminium and its Alloys; E. Eyrolles: Paris (1962),228-260. [6] W. Bensalah, M. Feki, M. Wery, H.F. Ayedi, Thick and Dense Anodic Oxide Layers Formed on Aluminum in Sulphuric Acid Bath, J. Mater. Sci. Technol, 26 ( 2010) 113-118, [7] W. Bensalah, M. Feki, M. Wery, H.F. Ayedi, Chemical dissolution resistance of anodic oxide layers formed on aluminum, Trans. Nonferrous. Met. Soc. China, 21 (2011) 1673-1679. [8] J. A. González, V. López, A. Bautista, E. Otero, X. R. Nóvoa. Characterization of porous aluminium oxide films from a.c. impedance measurements, J. Appl. Electrochem., 29 (1999) 229-238. [9] Yufei Jia, Haihui Zhou, Peng Luo, Shenglian Luo, Jinhua Chen, Yufei Kuang, Preparation and characteristics of well-aligned macroporous films on aluminum by high voltage anodization in mixed acid, Surf. Coat. Technol., 201 (2006) 513-518. [10] R. Kötz, B. Schnyder, C. Barbero, Anodic oxidation of aluminium in sulphuric acid monitored by ex-situ and in-situ spectroscopic ellipsometry, Thin Solid Films, 233, (1993) 63-68, [11] B. Schnyder, R. Kötz, Spectroscopic ellipsometry and XPS studies of anodic aluminum oxide formation in sulfuric acid, J. Electroanal. Chem., 339 (1992) 167-185. [12] A. Belwalkar, E. Grasing, W. Van Geertruyden, Z. Huang, W.Z. Misiolek, Effect of processing parameters on pore structure and thickness of anodic aluminum oxide (AAO) tubular membranes, J. Membrane. Sci, 319 (2008), 192-198, [13] L. Hemmouche, C. Fares, M.A. Belouchrani, Influence of heat treatments and anodization on fatigue life of 2017A alloy, Eng. Fail. Anal., 35( 2013), 554-561, [14] Leszek Zaraska, GrzHJRU]'6XOND-DQXV]6]HUHPHWD0DULDQ-DVNXáD3RURXVDQRGLF alumina formed by anodization of aluminum alloy (AA1050) and high purity aluminum, Electrochim. Acta., 55 (2010) 4377-4386. [15] V.M. Segal, Materials processing by simple shear. Mater Sci.Eng A197 (1995) 157–64. [16] H.S. Kim, W.J. Kim Enhanced corrosion resistance of ultrafine-grained AZ61 alloy containing very fine particles of Mg17Al12 phase. Corros. Sci, 75 (2013) 228–238. [17] K. Gopala Krishna, K. Sivaprasad , T.S.N. Sankara Narayanan , K.C. Hari Kumar, Localized corrosion of an ultrafine grained Al–4Zn–2Mg alloy produced by
7
Cryorolling. Corros. Sci, 60 (2012) 82–89. [18] D. Song, A.B. Ma, J.H. Jiang, P.H. Lin, D.H. Yang, J.F. Fan, Corrosion behaviour of bulk ultra-fine grained AZ91D magnesium alloy fabricated by equal-channel angular pressing, Corros. Sci, 53 (2011) 362–373. [19] In-Joon Son, Hiroaki Nakano, Satoshi Oue, Shigeo Kobayashi, Hisaaki Fukushima, Zenji Horita, Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy, Trans. Nonferrous. Met. Soc. China, 19 (2009) 904-908, [20] J.G. Brunner, J. May, H.W. Höppel, M.Göken, S. Virtanen, Localized corrosion of ultrafine-grained Al–Mg model alloys, Electrochim. Acta., 55 (2010) 1966–1970. [21] C.D. Terwilliger, Y.M. Chiang, size-dependant solute segregation and total solubility in ultrafine polycrystals: Ca in TiO2, Acta. Metall. Mater. 43 (1995) 319–328. [22] A. Rebhi, T. Makhlouf, N. Njah, Y. Champion, J.P. Couzinie, Characterization of aluminum processed by equal channel angular extrusion: Effect of processing route, Mater. Charac. 60 (2009) 1489-1495. [23] O.Jilani, N.Njah and P. Ponthiaux, Transition from Intergranular to Pitting Corrosion in Fine Grained Aluminum Processed by Equal Channel Angular Pressing, Corros. Sci, 87 (2014) 259-264. [24] Yoshinori Iwahashi, Jingtao Wang, Zenji Horita, Minoru Nemoto, Terence G. Langdon, Principle of equal-channel angular pressing for the processing of ultra-fine grained materials, Scripta. Mater. 35 (1996) 143-146 [25] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, An investigation of microstructural evolution during equal-channel angular pressing, Acta Mater., 45 ( 1997), 4733-4741, [26] R.Z Valiev, R.K Islamgaliev, I.V Alexandrov, Bulk nanostructured materials from severe plastic deformation, Prog. Mater. Sci. 45 ( 2000), 103-189. [27] A. Korchef, N, Njah, Y. Champion, S. Guérin, C. Leroux, J. Masmoudi, A. Kolsi. Material Flow During Equal Channel Angular Pressing of Aluminum Containing Al8Fe2Si Precipitates, Adv. Eng. Mater. 6 (2004) 222–228. [28] Atef Korchef, Yannick Champion, Nabil Njah, X-ray diffraction analysis of aluminium containing Al8Fe2Si processed by equal channel angular pressing, J. Alloys. Compd, 427, (2007), 176-182, [29] V.V. Stolyarov, R Lapovok, I.G Brodova, P.F Thomson, Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure, Mater. Sci. Eng., A357 ( 2003), 159-167. [30] B. Zaid, D. Saidi, A. Benzaid, S. Hadji, Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy, Corros. Sci.,50 (2008), 1841-1847 [31] Hiroaki Nakano, In-Joon Son, Satoshi Oue, Shigeo Kobayashi, Hisaaki Fukushima, Zengi Horita, Effect of equal channel angular pressing on the pitting corrosion resistance of the aliminum alloys with/without anodization, Mater. Sci. Forum 638-642 (2010) 1989-1994. [32] Max Reboul, Corrosion des alliages d’aluminum, Tech. Ing, TI-éditions, cor 325, 2005. [33] Monica Trueba, Stefano P. Trasatti, Study of Al alloy corrosion in neutral NaCl by the pitting scan technique, Mater. Chem. Phys., 121 (2010) 523-533, [34] In-Joon Son, Hiroaki Nakano, Satoshi Oue, Shigeo Kobayashi,Hisaaki Fukushima and Zenji Horita. Pitting Corrosion Resistance of Ultrafine-Grained Aluminum Processed by Severe Plastic Deformation, Mater. Trans. JIM., 47 (2006) 1163 to 1169. [35] Christian Vargel, Michel Jacques and Martin P. Schmidt, Chapter B.2-Types of Corrosion on Aluminium, In Corrosion of Aluminium, edited by Christian Vargel, Michel Jacques and Martin P. Schmidt, Elsevier, Amsterdam, 2004, Pages 113-146.
8
Figure captions Figure 1: Optical micrographs showing the grain structure of aluminium: (a) before ECAP (N=0); (b) after ECAP (N=5). Figure 2: Optical micrographs of anodized aluminium in different magnifications: a, b and c: before ECAP (N=0); d,e and f: after ECAP (N=5). Dark dots represent the uncoated zones. Figure 3: Polarization curves of aluminium after anodization in 3% NaCl solution treatment without ECAP (N=0) and with ECAP (N=1 to N=5). Arrows indicates backward sweeps. Figure 4: susceptibility to pitting E=Epit-Erep of anodized aluminium as a function of the number of ECAP passes N. Figure 5: Open circuit potential of aluminium after anodization with and without ECAP. Horizontal lines correspond to pitting and repassivation potentials. Figure 6: SEM micrographs of aluminium after anodization and immersion tests in 3% NaCl: (a) N=0, (b): N=5. Note that corrosion products are clearer and finer after ECAP (micrograph B). Figure 7: Microtopographic images of aluminium after anodization and immersion tests in 3%NaCl: (a) N=0, (b) N=1, (c) N=5. Note that passive film is finer for N=5. (d) line scans of the surface of figure 7c . Figure 8: A model explaining the effect of inclusion size on the formation of passive film: (a) before anodization, (b) after anodization, (c) after anodization and immersion. Figure 9: Nyquist plots after two hour of immersion in 3% NaCl solution for different ECAP passes (N) and anodization. Figure 10: Specific polarization resistance as a function of N calculated from the curves of figure 9. Open circles: Before anodization, full circles: after anodization.
9
10
11
12
13
14
15
16
17
Wt%
Fe 0.25
Mg 0.22
Mn 0.10
Si 0.10
Zn 0.16
Al Balance
Table 1: Chemical compositions of the investigated material (wt%). Highlights Industrial aluminium passivates with and without ECAP. Grain and inclusion refinement leads to more continuous and efficient passive film. Pits are finer after ECAP due to a refinement of inclusions. Fine pits in the film are easily filled by corrosion product.