Electrochemical behavior of anodized AA7075-T73 alloys as affected by the matrix structure

Applied Surface Science 283 (2013) 249–257

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Electrochemical behavior of anodized AA7075-T73 alloys as affected by the matrix structure Yung-Sen Huang, Teng-Shih Shih ∗ , Jun-Hung Chou Department of Mechanical Engineering, National Central University, Chung-Li, 32001, Taiwan

a r t i c l e

i n f o

Article history: Received 26 December 2012 Received in revised form 4 May 2013 Accepted 13 June 2013 Available online 28 June 2013 Keywords: Intermetallic compound Precipitates AAO films Potentiodynamic polarization test

a b s t r a c t A set of standard 7075-T73 alloy samples was prepared for comparison with samples that had been coldrolled before the T73 treatment. One set of samples was subjected to cold rolling at room temperature and another deep-cooled in liquid nitrogen prior to rolling. Both sets of samples were then subjected to a T73 treatment. The microstructure of the different samples was observed and their micro-hardness was tested and recorded. The samples that had first been subjected to the deep-cooling treatment prior to rolling and T73 treatment (CRST73) showed few subgrains and smaller amounts of second phase particles in the matrix than was the case with the other two sets of samples. The experimental results also indicated that the matrix of the CRST73 samples mostly displayed disk-like precipitates of Mg2 Zn and Al2 Cu. After anodization, this batch of samples demonstrated superior corrosion resistance and the lowest passive current density during potentiodynamic polarization testing. © 2013 Elsevier B.V. All rights reserved.

1. Introduction AA 7075 aluminum alloys are widely used in industry due to their excellent mechanical properties. This type of alloy contains many alloying elements such as zinc, magnesium and copper that form complex intermetallic compounds within the aluminum matrix. The precipitates and/or intermetallic compounds acquire a specific morphology due to the application of a heat treatment and manufacture process. These intermetallic compounds and fine precipitates existing in the aluminum matrix are greatly influence the anodizing performance and mechanical properties of AA7075 aluminum alloys [1–4]. Anodic coatings of aluminum are used for protection from corrosion of parts that applied to severely corrosive environments. However, localized dissolution tends to occur around intermetallic particles and precipitates during anodization. Researchers have determined that the oxidation rate is faster with large intermetallic particles such as the Mg2 Si, Al2 Cu, Al7 Cr, ␤-AlMg and AlZnMg compounds than the aluminum matrix [5,6], meaning that the particles could dissolve into the electrolyte during the anodizing treatment, whereas immobile elements such as silicon would form a silicon containing particle that will be trapped in the AAO film [7]. The morphology of the precipitates and texture of the matrix can be affected by the application of a cold working process which in turn affects the mechanical properties of the resultant aluminum

∗ Corresponding author. Tel.: +886 3 4227151x37300; fax: +886 3 4254501. E-mail addresses: [email protected], [email protected] (T.-S. Shih). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.094

alloy. The mechanical properties can be strengthened depending on the degree of cold working and the resultant dislocation density remaining in the matrix. Tajally et al. [8] investigates significant increase in the yield strength of 7075-O aluminum alloy after 120% cold-rolling. Panigrahi and Jayaganthan prepared AA7075 aluminum alloys under different rolling temperatures after solution treatment [9]. They indicated that a higher Vickers hardness, UTS and yield strength was obtained from the samples held in liquid nitrogen then sequentially moved for rolling treatment (cryorolled) than those cold rolled at room temperature. Dislocations that accumulated in the cryo-rolled 7075 samples acted to retard the dynamic recovery and accelerate the precipitation of MgZn2 during the aging treatment. The strengthening of the properties of the deformed sample was mainly due to formation of ultra-fine grains that led to yield a higher driving force for the formation of precipitates during further aging treatment [10,11]. Precipitates and intermetallic compound particles tend to be trapped in anodic aluminum oxide (AAO) films during anodization of the aluminum alloy. Pitting that preferentially occurs at the interface between the silicon/iron particles and AAO film reduces the corrosion resistance of the films. The influence of the anodization process on the fatigue life of 7050-T7451 alloys has been discussed by Sharzad et al. [12]. The inclusion particles trapped in the anodized film had a significant effect on the formation of corrosion pits which accelerated fatigue crack initiation. As a result, the fatigue life of the aluminum alloy was reduced. FratilaApachitei et al. [13] utilized transmission electron microscopy (TEM) to observe the particles trapped in anodized alumina oxides on an Al–Si alloy. The alumina film was encroached upon by

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Table 1 Chemical composition for 7075 aluminum alloy evaluated in this study (wt%). Alloy

Zn

Mg

Cu

Fe

Si

Cr

Mn

Al

7075

5.44

2.43

1.57

0.1

0.07

0.2

0.1

Bal.

silicon (oxide) particles in association with a non-uniform porosity. In addition, gas-filled voids above the silicon particles were also observed to influence the morphology of the enveloping porous AAO film, pore termination above the particle and pore branching and deflection around and beneath the particles. Mukhopadhyay et al. used electron probe micro analysis (EPMA) to examine the constitution of intermetallic particles in the matrix. As indicated in the experimental results, the particles containing an iron rich phase could be trapped in hard anodic coating films to locally inhibit the formation of anodic oxide films [14]. The formation of the AAO film is affected by the field-assisted dissolution [15,16]. After formation of a barrier layer, nano-pores are initiated and after early develop into fully-developed pores [17,18]. According to the oxygen bubbles mold effect (OBME) recently discussed by Zhu et al., the formation oxygen gas at the pore base caused variation in the field potential [19,20]. The selfordering of pores, volume expansion and viscous flow of anodic oxide [21–24], flow modulated of Al3+ ion and O2− ion migration [25], and water molecular dissociation with field enhancement conditions during anodization [26] have also been discussed by many researchers. The sealing behavior and mechanism of AAO film have been examined in many studies [27–32]. The basic reaction in a normal hydrothermal sealing process can be expressed as follow: Al2 O3 + H2 O → 2AlO(OH)

(1) 80 ◦ C,

When the temperature reaches this reaction leads to an expansion in the volume of AAO films and then a final blocking of the nano-pores, as in crystalline boehmite. The four steps of the sealing process have been described by Lopez et al. [31], starting with the filling of the pores by the sealing solution, plugging the pore mouths, the formation of acicular pseudoboehmite crystals and a compact intermediate layer on the surface and finally the growth of hydrated alumina crystals during the sealing process. The entrapped intermetallic compound particles and/or precipitates in the AAO film are affected by the sealing process which alters the quality of the resultant AAO films. In this study, we discuss the varying matrix structures produced in AA7075-T73 alloy samples with/without deformation before solution treatment. The second phase particles and precipitates were counted and observed for comparison. The effects of Cryorolling on improving the mechanical properties of Al–Zn–Mg (Cu) alloys have been extensively examined in many studies [8–11]. Anodization can significantly improve the corrosion resistance of aluminum alloy. However, no research has been conducted to investigate the effect of matrix structure on the quality of AAO films on the Al–Zn–Mg (Cu) alloys. We provide experimental results to show that such matrix effect is significant and would be confirmed by corrosion testing. 2. Experimental procedure The chemical composition of As-received AA7075 extruded plates, 7 mm in thickness, purchased from the Dubai Aluminum Co., Ltd., is listed in Table 1. The plates were cut into 80 mm × 20 mm pieces. Some of the plates were removed for machining to 4.2 mm in thickness. After cleaning, the different sets of samples (7 and 4.2 mm in thickness) were heated to 688 K for 120 min then furnace cooled to room temperature. After annealing, one set of samples

with a plate thickness of 7 mm was subjected to cold rolling at room temperature to obtain a 40% reduction in thickness to 4.2 mm. Another set of specimens was subjected to cooling in liquid nitrogen for 60 s prior to the rolling process. The rolling process was carried out to obtain a 20% reduction in thickness per pass until the total reduction reached 40% (for the two sets of deformed samples). The three sets of samples were then prepared for the T73 treatment which involved immersion in a solution at 748 K for 90 min, followed by rapid quenching in water and finally two-step age hardening at 383 K and 448 K for 480 min, respectively. The sample without rolling was coded ST73; that cooled by liquid nitrogen then rolling was coded CRST73; and that rolled at room temperature was coded RRST73. The samples were first polished with #2000 grid abrasive sand papers followed by further polishing with an alumina powder slurry 1 micron meter (␮m) and then silica gel (0.03 ␮m) prior to optical microscopic (OM) observation and intermetallic particle count measurement. Vickers hardness testing (Hv: AKASHI, MVK-G1, 50 g load) was utilized to measure the samples’ hardness. After polishing, the samples were etched by Keller’s regent to show the grain boundaries. At least ten locations on each sample were tested to get the average and standard deviation of micro-hardness. The particle sizes and their fractions were measured and calculated according to the OM photos. At least 5 photos were taken at 200× magnifications for each sample by using the OPTIMAS6 software. The microstructures of different samples were also observed by a field emission scanning electron microscopy (FE-SEM) and intermetallic compound were analyzed by energy dispersive spectrometer (EDS). Before transmission electron microscope (TEM) observation, the samples were sliced then thinned by mechanical polishing until the thickness was reduced to 10 ␮m, this was followed the application of a precision ion polishing system (PIPS), which was held on a cold stage, operated at 5.5 kV, 1 mA with incident angles of 15◦ , 12◦ and 7◦ , respectively. A TEM (JEOL-2000 FX II) operated at 160 kV was used to observe the morphology of the precipitates within the matrix of the T73 treated samples. Before anodization, all samples were polished with abrasive sand paper (from grid #120 to #2000) followed by cleaning by being dipped into methanol and ultrasonic vibration. The final samples had a surface roughness of about (Ra) 160 nm. The standard procedure for anodization has been described in reference [17] and the anodization time was 15 min. Parts of the samples were removed for sealing treatment by immersion in boiling water for 20 min. After anodization, the samples were prepared for measurement of the AAO film thickness and pore observation by using FE-SEM images and electron probe X-ray micro analyzer (EPMA). Cross-sectional SEM observation and EPMA chemical analysis were carried out on the polished samples without etching. The AAO film thickness was measured from the SEM images counting from the cross section of the observed AAO films. A quantitative wavelengthdispersive X-ray spectroscopy (WDS) system attached to the EPMA was used to analyze the chemistry of the constituent particles. The roughness of the AAO films was measured by an atomic force microscope (AFM). A digital instrument NS3a controller with a D3100 stage in the tapping mode equipped with silicon nitride probe (tip head: 10 nm) with as sharpened pyramidal tip applied. Electrochemical tests were conducted and results recorded by using an AUTOLAB PGSTAT30 Potentiostat. An Ag/AgCl (3 M KCl) reference electrode was used in this study for corrosion testing, and the counter electrode was a high density graphite plate. The polarization curves for the anodized samples, which had been immersed in an unstirred 1 M NaCl solution, were measured at room temperature. Before electrochemical testing, the anodized samples were cleaned with methanol and dried in warm air then immediately immersed in the electrolyte solution. The polarized potential of the tested sample was returned to its rest potential, and the anodic

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Table 2 Measured intermetallic particle counts distribution and Vickers hardness of tested samples. Sample

ST73 CRST73 RRST73

Particle counts fractions (%) 1–5 ␮m

6–10 ␮m

11–15 ␮m

79 93 87.3

18 5.5 8.5

3 1.5 4.1

Counts/mm2

Hardness (Hv/Dev)

363 251 292

183.4/2.0 188.6/1.1 181.5/1.6

polarization curve was obtained by stepping up the potential (with a scan rate 0.01 V/s) toward the noble direction until a of 2 V was reached for the unsealed sample and 2–3 V for sealed film. All of the polarization curves indicate the average value of five time repeated measurements. The average value was obtained with AUTOLAB software. 3. Results and discussion 3.1. Microstructure observation OM images of the microstructure of different samples (ST73, CRST73 and RRST73) are shown in Fig. 1(a)–(c). The intermetallic particles located at the grain boundaries are aligned at the longitudinal direction of the ST73 sample plates as shown in Fig. 1(a). Fine subgrains, as marked by the arrows, are visible within some grains; see Fig. 1(b) and (c). Both the CRST73 and RRST73 samples, which had undergone cold-rolling, showed a lower fraction of intermetallic particles and fewer subgrains within the matrix than did the ST73 samples. The subgrains that appear in the matrix of the ST73 sample likely formed due to thermal-activated recrystallization which was affected by soluble Mg and Zn atoms in the Al matrix [33,34]. This difference can be attributed to the effects of dislocations and recrystallization that occurred in the matrix of the cold-rolled samples. It has been pointed out in several studies that heterogeneous segregation and nucleation at dislocations and grain boundaries is affected by the atomic movement generated during the deformation process [35,36]. After T73 treatment, mostly coarse grains with some subgrains were obtained in the CRST73 and RRST73 samples; compare with Fig. 1. Table 2 shows the intermetallic particle counts and fractions obtained from different samples as measured by OM observations: 363, 251 and 292 counts/mm2 for the ST73, RRST73 and CRST73 samples, respectively. At least four observations were carried out

Fig. 1. Optical microscopic observations showing top views of the microstructure of the: (a) ST73, (b) CRST73 and (c) RRST73 samples with particles trapped in the matrix.

Fig. 2. TEM micrographs of the matrix structure of the: (a) ST73, (b) CRST73 and (c) RRST73 samples.

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for each specific set of two samples. According to the OM observation, the main particle size ranged from 2 to 10 ␮m, with low fraction of particles reaching about 15 ␮m. Shear stress introduced by cold working prior to solution treatment acted to decrease the particle size in the matrix of the 7075-T73 alloy samples. According to the EDS analysis, we identified intermetallic particles that ranged in size between 6 and 10 ␮m mostly contained Mg–Si or Mg–Zn, while those that ranged in size from 2 to 5 ␮m were mainly composed of Al–Si, Al–Cu or Al–Cu–Fe. The compound particle size and count were decreased by the rolling process. This was especially apparent for the CRST73 sample. Fig. 2(a)–(c) shows TEM observation of the morphologies of fine precipitates in the different samples. Some visible needle-like precipitates are marked by arrows in Fig. 2(a) with fine disk-like

Fig. 3. FE-SEM observations of the AAO surface morphology from the: (a) ST73, (b) CRST73 and (c) RRST73 samples after anodization in a 15 wt% sulfuric acid solution at 293 K.

precipitates showing up at a higher resolution in the matrix of the ST73 samples. It can be seen from the TEM images that the substrates of both the CRST73 and RRST73 samples are occupied by mostly fine disk-like precipitates coexisting with and a few short rod-like MgZn2 precipitates (Á and Á -phase), see Fig. 2(b) and (c). The needle-like precipitates are likely Al2 CuMg (S phase) and disklike precipitates have been reported to be Al2 Cu (␪ -phase) and Al2 Mg3 Zn3 precipitates (T-phase) [37–39]. The needle-like precipitates were noticeably absent in the matrix of the CRST73 and RRST73 samples. Cold rolling increased the storage of energy in the aluminum matrix due to the formation of high density dislocations [8] which acted to preferentially increase low angle grain boundaries within the grains [9]. The dislocations blocked the movement of Cu and Mg atoms at the sites of the dislocation cells. Copper and magnesium added into the aluminum alloy could

Fig. 4. SEM micrographs of the AAO films that formed on: (a) ST73, (b) CRST73 and (c) RRST73 substrates about 10 ␮m in thickness.

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significantly reduce the alloy’s stacking fault energy (SFE) [40]. For the alloys with low SFE such as Al–Cu [40], Al–Mg [41] and Al–Zn [34] alloys, planar slip is readily occurred during deformation. Dislocations tended to be blocked at grain boundaries. Thus the thermal effect from solution treatment could enhance recrystallization of the matrix at where dislocations are heavily tangled at the grain boundaries. Optical observations indicated apparent crystallization in that the CRST73 sample with fine grains at the grain boundaries, as shown in Fig. 1. Therefore, the substrates of the CRST73 and RRST73 samples showed grains with few (or no) subgrains, and the trapped Mg and Cu atoms readily formed fine disk-like MgZn2 , Al2 Mg3 Zn3 and Al2 Cu precipitates in situ instead of the needle-like Al2 CuMg precipitates 500–1200 nm in size that formed in the ST73 sample, as shown in Fig. 2(a). Table 2 also lists the measured micro-hardness for the three samples. The CRST73 sample had the highest hardness and the lowest deviation among all the samples. As pointed out by Panigrahi et al., such a difference is mainly to the fact that cryo-rolling is a more effective means of resisting dynamic recovery than the room temperature rolling process [9]. As a result, the above-mentioned fine Al2 Mg3 Zn3 and Al2 Cu precipitates were more uniformly distributed, which acts to decrease the chance for the growth of needle-like precipitate and/or growth of the pre-existing compound particles.

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3.2. AAO film quality after anodization and sealing FE-SEM observations of the surface morphologies of anodized films for different samples before sealing are shown in Fig. 3(a)–(c). Aligned particles or voids present on the surface of the anodized ST73 sample are marked by arrows in Fig. 3(a). These aligned particles (voids) coincided with particles that were present in the substrate, as shown in Fig. 1(a). The surface morphology of the anodized CRST73 sample was more uniform than that of the RRST73 or ST73 samples. The trapped particles or voids in the AAO film could have originated from compound particles located on the sample’s substrate. Cross-sectional views of the anodized films on different samples are shown Fig. 4(a)–(c). An examination of these figures further confirms that the CRST73 sample had a robust AAO film with fewer visible particles and/or voids than the other two sets of samples. Prior to assessing the performance of the AAO films that formed on the different samples, it is worth first discussing more information about the characteristics of these films. The AAO films on the anodized ST73, CRST73 and RRST73 samples were 10.35 (0.6), 9.34 (0.2) and 10.12 (0.4) ␮m (deviation) in thickness, respectively (according to the SEM observations), and their surface roughness (Ra) after anodization were about 109 (4.4), 59 (1.2) and 103 (2.1) nm (by AFM measurement), respectively. The CRST73 sample had the thinnest and smoothest of the AAO films among the

Fig. 5. SEM image of a cross section of the AAO film that formed on ST73. The elemental X-ray maps of the area shown in the SEM image reveal the association of Al, Mg, Si, Cu, and Zn with the constituent particles.

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Table 3 The EDS analyses of intermetallic particles trapped in the AAO films (from Fig. 4) on different samples (wt%). No.

Cu

Fe

Mg

Zn

Si

Size (␮m)

1 2 3 4 5 6

21.2 – 25.8 30.1 – –

7.9 – 11.4 1.9 – –

– 1.7 – – 0.8 1.1

– 3.5 – – 1.4 2

– 30.5 – – 27.5 30

1×1 2×1 2×1 3×1 2×2 1×1

three. The particles trapped in the AAO films on the different samples were further characterized by EDS analysis; see Table 3 for the results. The intermetallic particles trapped in the film included particles containing Al–Cu–Fe, Al–Mg–Zn–Si and Al–Si and the sizes of the intermetallic compound being mostly less than 5 ␮m in the AAO film. For a more detailed understanding of the metal elements in the trapped particles, EPMA mappings were recorded for Al, Mg, Si, Cu and Zn, as shown in Fig. 5. We can see a Si-containing particle (about 12 ␮m) trapped above the film/metal interface and a lath-like

Al–Cu particle located at the film/metal interface. A Si-containing particle trapped above the film/metal interface accompanied by a concave film/metal interface nearby. The irregularity of film/metal interface was strongly affected by the high electrical resistivity of Si compared with other metals; Si (0.1–60 ohm m), other metals: Al(2.826 × 10−8 ohm m), Cu(1.724 × 10−8 ohm m), Mg(4.467 × 10−8 ohm m), Zn (6–6.8 × 10−8 ohm m) [42,43]. As explained previously, there were some large particles in the sample substrate (>5 ␮m) related to the Mg–Si containing particles. When the AAO film was encroached upon by an Mg–Si-containing particles, the lower ionization energy of Mg than Si (Mg (738 kJ/mol), Si (786 kJ/mol) [44]) mean that the Mg atoms would tend to be ionized and migrate into the AAO film to form magnesium oxide and/or be eventually ejected into the electrolyte [45]. The Mg–Si intermetallic particles encroached upon the film they gradually reduce to form a silicon containing particle. As a result, Si remained in situ in the particle as shown in Fig. 5. This induced an intense current density nearby which accelerated ionization of the neighboring Al matrix to form a concave interface. This result was also observed in a previous study with an anodized Al–Mg–Si alloy sample where Si-containing particles trapped in the film also caused a roughening

Fig. 6. SEM images of the cross sections of the AAO films that formed on the ST73, CRST73 and RRST73 samples: (a), (c) and (e), respectively, after anodization; (b), (d) and (f), respectively, after the sealing treatment.

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of the AAO film’s surface [46]. Shearing introduced by rolling leads to a significant decrease in particle size (most apparent for coarse Mg–Si containing particles) which therefore improves the AAO film’s surface roughness. During anodization, the O2− ions migrated inward to react with copper ions to form a CuO oxide at the f/m interface. Consequently, the electrons could pass across the barrier layer to oxidize O2− ions just above the coppercontaining particles to form an oxygen gas pocket [47]. According to our observations, the Al–Cu particles in the sample substrates tended to be 2–5 ␮m in size. During anodization, the Al–Cu particles released Al ions into the AAO film, and high amounts of oxygen ions migrated inward the film/metal interface to interact with the Al–Cu particles. As a result, a new AAO film would form atop of the Al–Cu particle [47]. The metal (Mg, Zn, Cu, etc.) ions have a faster outward migration rate than the Al3+ ions so will possibly be ejected into the electrolyte solution given the field potential during anodization. The oxygen ions that moved toward the particles would be transferred to oxygen atoms due to the release of electrons to the film/metal interface. Subsequently, the oxygen atoms reacted with copper atoms to develop oxide and oxygen bubbles above the Al–Cu particles in the AAO films. Lowering the metal–oxygen bond energy makes it easier to form a metal oxide such as Mg O: 166 kJ/mol, Zn O: 184 kJ/mol and Cu O: 186 kJ/mol [48]. The metal oxide could concurrently be dissolved in the anodizing solution due to its high solubility in an acidic solution. The process of forming and dissolving the Mg O and Cu O oxides persisted within the AAO film during anodization. A higher resolution SEM photo does indeed show the existence of many tiny voids within the films on different samples; see Fig. 6(a)–(f). These tiny voids are the products of the fine Al–Cu precipitates that were trapped in the film during anodization. Fig. 7(a)–(d) shows TEM images of AAO films before and after sealing treatment at different magnifications. Before sealing, the AAO film deposited on the ST73 sample displays apparent needle-like voids and tiny gas-pockets. However, these gas-pockets become partly sealed and reduced in size to create a vague pocket/film interface. As explained by Thompson [49], the reactions that occur during sealing treatment act to form hydrated alumina and crystallized boehmite within the film.

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To sum up, the fine precipitates and coarse compound particles in the 7075-T73 alloy samples produced tiny voids, gas-pockets and Si-containing particles in the AAO films after anodization. These tiny voids and gas-pockets would greatly influence the corrosion resistance of the AAO films. The Si O bond energy is 466 kJ/mol that significantly higher than the 281 kJ/mol for Al O [48], meaning that the silicon containing particles would be more immobile and remain in the AAO films during anodization. They became oxidized to form aluminum silicate in situ. The resultant quality of the AAO film in terms of corrosion resistance was greatly affected by the voids, gas-pockets and Si-containing particles in the film. 3.3. Potentialdynamic polarization test The polarization curves of anodized and sealed 7075 samples are shown in Fig. 8(a) and (b), respectively. During the polarization testing, the potential started from −2.3 V. The current density initially decreased with increasing potential then was converted at the corrosion potential (Ecorr ). The current density increased slowly and oscillated with increasing potential until lifting rapidly when the pitting potential (Epit ) was reached. As can be seem from the measured curves in Fig. 8(a), the varying matrix structure has a minor effect on improving the corrosion resistance of the anodized samples; however, after sealing, there was a significant improvement in the corrosion resistance, as shown in Fig. 8(b). The corrosion resistance of the CRST73 sample showed remarkable improvement that was higher than the other samples. Detailed data can be extracted from Fig. 8 and are discussed below. After anodization, a similar corrosion potential (ST73: −1.34 V; CRST73: −1.31 V; RRST73: −1.29 V) and pitting potential (ST73: −0.675 V; CRST73: −0.705 V; RRST73: −0.708 V) was obtained for two deformed samples. However, after sealing, the anodized CRST73 sample demonstrated the lowest passive current density, lower than for the RRST73 and ST73 samples; 2.5–3.8 × 10−8 , 2–4.3 × 10−7 and 3–4.3 × 10−5 A/cm2 , and no significant pitting formation. During corrosion testing, Cl− ions could penetrate through voids and/or gas-pockets to form channels (pitting). The tiny voids and gas-pockets that existed in the AAO films provided an easy path for chloride ions to pass through the film to reach film/matrix interface during corrosion testing. As a result, pitting or erosion of the specific AAO film occurred. In this study, the sealed CRST73 sample

Fig. 7. TEM images of stripped AAO films that formed on the ST73 sample experiencing an anodizatiom time of about 30 s; (a) before and (b) after the sealing process.

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and leading toward a more uniform dispersion of fine precipitates in the alloy samples. There was a significant reduction in intermetallic particles in the CRST73 and RRST73 samples; their precipitates were mostly short rod-like (Al2 Mg3 Zn3 ) and disklike (MgZn2 , Al2 Cu) with high macro-hardness in the matrix (see Table 2). After anodization and sealing, the CRST73 samples demonstrated higher corrosion resistance, yielding the lowest passive current density among the three samples. For comparison, the ST73 sample contained some needle-like precipitates (Al2 CuMg) about 500–1200 nm in size. The AAO films yield correspondingly high amounts of needle-like voids and air-pockets 400–500 nm in size. As a result, the ST73 alloy demonstrated the lowest corrosion resistance among three samples.

Acknowledgements The authors gratefully acknowledge the financial support of this study from the National Science Council of the Republic of China (100-2221-E-008-040). We also extend our thanks to National Central University for providing the TEM equipment (JEOL-2000 FX II), and National Taiwan University for the FESEM and EPMA results.

References

Fig. 8. Polarization curves for the anodized ST73, CRST73 and RRST73 samples immersed in a 1 M NaCl solution. The results from test samples with: (a) unsealed films and (b) sealed films, respectively.

to be more passive than other two sealed samples which enabled it to resist the attack of chloride ions. After sealing, tiny voids and gaspockets in the AAO films were partly closed. The decrease in size leading to a significant improvement and reduction in the formation of corrosion channels. As a result, the Si-containing particles became the dominant factor to influence the corrosion resistance of AAO films. High amounts of coarse Si-containing macroscopic particles and microscopic needle-like precipitates were obtained in the ST73 samples. The latter precipitates (Al2 CuMg) tended to incubate elongated submicron-sized gas-pockets (see Fig. 3(a)). After sealing, both the Si-containing particles and (partly sealed) elongated gas-pockets offered potential sites for forming corrosion channels that would induce pitting. With the CRST73 sample, there was a great decrease in the size and amount of Si-containing particles. Furthermore, no needle-like precipitates were observed in the matrix. The voids in the film had been partly sealed leaving few sites available for the initiation of pitting. This resulted in a passive current density about 1.5–2 orders lower than that of the RRST73 sample and 3 orders lower than that of the ST73 sample. 4. Conclusion Cold rolling (especially cryo-rolling) changed the structure of the 7075-T73 alloy, increasing the hardness of the substrate,

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