sealed AA7050 and AA7075 alloys

Applied Surface Science 351 (2015) 997–1003

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Corrosion resistance and high-cycle fatigue strength of anodized/sealed AA7050 and AA7075 alloys Teng-Shih Shih ∗ , Yi-Wei Chiu 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 31 March 2015 Accepted 6 June 2015 Available online 15 June 2015 Keywords: Anodization Ecorr Icorr Fatigue strength

a b s t r a c t AA7075 and AA7050 alloy bars were heat treated and prepared for different anodization treatments to obtain an anodic aluminum oxide (AAO) film with a thickness in the range 9–11 ␮m. Subsequently, the samples were sealed in hot water. Potentiodynamic tests were conducted to obtain different Ecorr and Icorr values. Factors affecting the corrosion resistance of different samples are discussed in this paper. They include the energy consumed in Steps 2 and 3 during anodization and constituted oxide phases and bubbles formed in the AAO film. The anodized and sealed 7075 samples showed corrosion resistance superior to that of the 7050 samples. However, the 7075 alloy samples absorbed more hydrogen from their matrix compared with the 7050 alloy samples, causing them to develop lower fatigue strength at 1 × 107 life cycles. © 2015 Elsevier B.V. All rights reserved.

1. Introduction AA7075 and AA7050 alloys are high-strength aluminum alloys that are commonly used to fabricate components such as chain rings and spindle tubes in the bicycle industry. Both alloys contain zinc, magnesium, and copper and therefore have high tensile strength and high fracture toughness. Anodic films coated on the aluminum alloys can increase the wear resistance and cosmetic benefits of the alloys. For understanding the basic anodic behavior of aluminum in sulfuric acid solution, Wei et al. plotted a potential–time (V–t) curve and divided it into Steps 1–4 [1]. The energy consumed at each step was computed and used to correlate the quality of the AAO film. When the combined energy consumption of Steps 2 and 3 exceeded 7.4 J/cm2 , the nanopore size increased by approximately 20% [2]. When anodic aluminum oxide (AAO) films are attacked by chloride ions, microsized pits are formed on the surface of the films. Subsequently, these pits then propagate and cause cracking of the films [3]. Alloying elements of Cu and Mg are deliberately added to aluminum alloys to enhance the mechanical properties of the alloys. Furthermore, Fe contamination may occur during the melting process. Cu and Mg species and Fe-containing intermetallic compounds commonly appear in the matrix of aluminum alloys. On the basis of electrochemical testing, these species and compounds have been reported to vary in size and cause localized corrosion

∗ Corresponding author. Tel.: +886 3 4267317; fax: +886 3 4254501. E-mail address: [email protected] (T.-S. Shih). http://dx.doi.org/10.1016/j.apsusc.2015.06.030 0169-4332/© 2015 Elsevier B.V. All rights reserved.

[4,5]. During anodization, these species may either be oxidized at the film/metal (f/m) interface and appear as particles in the AAO film [6] or generate microcrevices and oxygen bubble defects in the film [7,8]. These particles and defects serve as preferential sites for Cl− ion attacks [4,5,9] and affect the pit propagation rate [10]. Mukhopadhyay found that the amount of Si, Mn, and Fe impurities has an appreciable effect on the growth of hard anodized AAO films. Only 7075 alloy with low amounts of Si (0.05 wt%), Mn (0.04 wt%), and Fe (0.18 wt%) enabled the uniform growth of hard anodized AAO films [11]. An increase in the Si and Cu content in aluminum alloy increases the probability that gas-filled cavities form above Si particles and that oxygen is generated above trapped particles in hard anodized AAO films [12]. The formation of gas-filled cavities and oxygen generation result in appreciable degradation of the corrosion resistance of the film when it comes in contact with chloride ions. Fabricating uniform AAO films without defects is necessary for obtaining anodized aluminum alloys with high corrosion resistance. Knowledge of the effect of anodization processes on the corrosion behavior and fatigue strength of Al–Mg–Zn–Cu alloys can enable engineers to successfully fabricate high-strength Al–Mg–Zn–Cu alloys. 2. Experimental procedures 2.1. Material As-extruded 7075 and 7050 alloy bars 25 mm in diameter were obtained from Tzan Wei Aluminum Co., Ltd. (Taiwan). Their chemical compositions are listed in Table 1. Both alloy samples contained

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Table 1 Chemical compositions of 7050 and 7075 alloy (wt%). Item

Mg

Si

Fe

Cu

Ti

Zn

Mn

Al

Al7075 Al7050

2.5 2.1

0.06 0.08

0.1 0.1

1.7 2.3

0.02 0.03

5.8 6.1

0.01 0.03

Bal. Bal.

low amounts of Si, Fe, and Mn (all less than 0.1 wt%), and therefore, hard anodization of these alloys could enable the formation of uniform AAO films. The 7075 alloy sample had lower Cu content (1.7 wt% versus 2.3 wt%) but higher Mg content (2.5 wt% versus 2.1 wt%) compared with the 7050 alloy sample. After the samples were annealed at 688 K for 120 min, they were subjected to solution (T4) treatment at 730 K for 120 min, and quenched in water. The sample bars were stored at room temperature for 48 h. Subsequently, they were subjected to aging treatment at 437 K for 8 h. Specimens were machined from these heat-treated bars according to JIS Z2274 specifications (gage diameter: 8 mm) for use in the rotating-bending fatigue test. 2.2. Anodization process and fatigue-strength test The samples were anodized using a 15 wt% sulfuric acid solution and a current density of 2.15 A/dm2 . The standard procedure has been comprehensively described previously [1]. Hard anodization was conducted at 268 K with current densities of 2.15 and 5.0 A/dm2 in a mixed solution of 22 wt% sulfuric acid, 1.5 wt% oxalic acid, 0.1 wt% aluminum sulfate, and demineralized water (balance) [11]. Samples were coded as 7075-A for anodization and 7075-HAh and 7075-HAl for hard anodization with high and low current densities, respectively. The AAO films on different samples were determined to be 9–11 ␮m thick through scanning electron microscopy (SEM). The anodized samples were sealed in hot water at 368 K for 20 min. The fatigue test was conducted in load-controlled conditions by using an ONO rotating-bending fatigue-test machine (Type H7). After the test, the fractured samples were observed through SEM. Bare and anodized samples were obtained for the hydrogen test by slicing fine (0.2–0.9 g) pieces of samples from sites near the subsurface and core region. The 7075 samples had higher Mg content to ensure that they absorbed more hydrogen from the matrix compared with the 7050 samples (0.33 versus 0.25 cc/100 g of Al). The hydrogen content was determined using a hydrogen analyzer (Horiba, EMGA-521) [13]. 2.3. Potentiodynamic testing An AUTOLAB potentiostat/galvanostat/frequency-response analyzer (PGSTAT30) was used for electrochemical measurements. Details of potentiodynamic testing have been described in a previous paper [2]. Polarization curves for all samples were obtained by automatically changing the electrode potential from −0.25 V in the cathodic direction to 0.5 V in the anodic direction with reference to the open-circuit potential at a scanning rate of 5 mV/s. The corrosion potential (Ecorr , the potential at which the current density changes from cathodic to anodic) and corrosion density (Icorr ) were determined from the polarization curves by using the Tafel extrapolation method. 3. Results and discussion 3.1. V–t curve and energy consumption during anodization Figs. 1a and b shows plots of potential (V) versus processing time (t) for 7075 samples subjected to different anodization processes (A, HAh , and HAl ). The potentials (V1 –V3 ) corresponding to

Fig. 1. Measured potential (V) versus time (t) curves for (a) anodization; (b) hardanodization using high current density and (c) hard-anodization using low current density.

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Table 2 Recorded critical potential (V1 , V2 and V3 ) and critical time (t1 , t2 and t3 ) for different anodized samples; data taking from Fig. 1; energy consumption at each step was also included. Sample

Step 1

7075-A 7075-HA1 7075-HAh 7050-A 7050-HAl 7050-HAh

Step 2

Step 3

V0 (V)

V1 (V)

t1 (s)

E1 (J/cm2 )

V2 (V)

t2 (s)

E2 (J/cm2 )

V3 (V)

t3 (s)

E3 (J/cm2 )

0.001 0.024 0.032 0.007 0.004 0.068

14.6 13.4 11.3 15.5 12.9 12.3

4.24 3.78 1.52 6.91 5.43 1.7

0.66 0.57 0.44 1.18 0.75 0.53

20.2 20.8 23.6 22.0 22.0 26.0

7.11 7.34 3.81 11.19 10.78 3.97

1.11 1.39 2.16 1.8 2.12 2.27

17.4 17.5 19.8 18.8 19.6 20.2

24.56 26.33 16.26 27.54 31.54 17.98

6.8 7.5 12.4 7.2 8.9 15.0

Table 3 Total energy consumption for different anodized samples to produce 10–11 ␮m thick AAO films.

Etotal (KJ/cm ) 3

7075-A

7075-HAl

7075-HAh

7050-A

7050-HAl

7050-HAh

374

301

249

411

320

306

different times (t1 –t3 ) were determined from V–t curves and used to compute the energy consumed at each step and the total energy required for forming AAO films with specific thicknesses on different samples. As explained in a previous paper, Step 1 involves barrier layer formation (t < t1 ), Step 2 corresponds to pore formation and early growth (t1 < t < t2 ), and Step 3 involves pore widening and growth (t2 < t < t3 ); subsequently, the AAO film shows stable growth at anodization times beyond t3 [1]. Table 2 lists the potentials (V1 –V3 ) at different times (t1 –t3 ) for different samples and the energy consumed at each step. In the formation of hard anodized AAO films on samples, more energy was required to complete Steps 2 and 3 compared with the energy required for producing anodized AAO films. The 7050 sample required more energy per unit volume (J/cm2 divided by film thickness) compared with the 7075 sample for obtaining a given AAO film thickness (Table 3). All anodized samples showed higher total energy consumption per unit volume of AAO film compared with hard anodized samples. The 7075 samples had a slightly lower electrical conductivity compared with the 7050 samples (7075 samples: 37.2% IACS; 7050 samples: 38.8% IACS). This small difference can be attributed to differences in the alloy compositions and in the amount of Mg- and Cu-containing precipitates in the matrix of the alloy samples. The 7050 samples had slightly higher Zn content, and their matrix likely had a higher cathodic Al7 Cu2 Fe phase content. The ionization energies for Zn, Fe, Cu, Mg, and Al are 906.4, 762.5, 745.5, 737.7, and 577.5 kJ/mol, respectively. Consequently, the 7050 sample showed high potentials (V2 and V3 ) and increased energy consumption during anodization. Wloka et al. found that the opencircuit potential increased considerably when Mg2 Si and coarse Al7 Cu2 Fe phases were in contact with each other [6]. Table 4 confirms that the 7050 alloy samples contained coarser particles than those of the 7075 alloy samples. Particle with sizes in the range 6–10 ␮m mostly contained Mg–Zn and Mg–Si phases, while those with sizes from 2 to 5 ␮m mainly contained the Al–Cu and Al–Cu–Fe phases [13]. An increase in the energy consumed in Steps 2 and 3 leads to an increase in the pore size of the AAO film on pure Al [2].

The AAO films grown on a 7050 sample could be postulated to have larger nanopore sizes compared with those on a 7075 sample because of the higher energy consumption in Steps 2 and 3. The sealing of a porous AAO film in hot water triggered the reaction AlO+ + H+ + H2 O ⇒ AlO(OH) + H2 , producing partly crystallized pseudoboehmite [14]. Sealing the porous channels in an AAO film with larger nanopores required a longer duration. This is beneficial for activating the aforementioned reaction, which leads to more pseudoboehmite being produced in the sealed AAO film. XPS analyses revealed that the anodized/sealed 7050-A sample showed a lower O/Al ratio at the electrolyte/film (e/f) interface compared with the anodized/sealed 7075-A sample (2.7 versus 2.9; Table 5). A larger amount of partly crystallized AlO(OH) oxides formed at the e/f interface of sealed AAO films on 7050 samples compared with the amount formed at the e/f interface of sealed films on 7075 samples. Among all samples, the hard anodized/sealed samples notably showed the lowest O/Al ratios at the e/f interface (2.4 and 2.5); pseudoboehmite formed in large amounts on the hard anodized/sealed samples. The O/Al ratios measured at f/m interfaces were low (1.3–1.6). Amorphous alumina (Al-O-H phase) was present mainly in areas close to the f/m interface. 3.2. Potentiodynamic polarization test During corrosion testing, Cl− ions that contacted the AAO film penetrated the film to enter the matrix of the metal. A sealed AAO film plays a crucial role in preventing Cl−1 ions from penetrating the AAO film. Figs. 2a and b shows the polarization curves of all anodized/sealed 7075 and 7050 alloys, respectively; the curves for the bare 7075 and 7050 alloy samples are shown in Fig. 2c for comparison. The bare 7075 alloy samples showed slightly higher corrosion resistance against chloride ion attack compared with the 7050 alloy samples (Ecorr : −0.68 versus −0.72 V; Icorr : 2.38 × 10−6 versus 7.12 × 10−6 A/cm2 ; Table 6). Such differences can be attributed to the 7050 samples containing more particles in their matrix, including anodic Mg-containing species and cathodic Cu-containing species. Chloride ion attack led to the dissolution of anodic particles and the matrix surrounding

Table 4 Measured particle sizes and particle counts for 7075 and 7050 alloys samples. Counts/mm2

Samples

Particle counts

Size (␮m)

1–10

11–20

21–40

41–60

60–100

100–200

Al7075-T6 Al7050-T6

496 528

20 54

7 38

14 11

0 22

0 32

Each sample took two observations to get values based on photos by magnification 500×.

537 685

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Table 5 Atomic percentages measured by XPS on AAO films at areas close to the (a) f/m interface and (b) e/f interface on different samples. Specimen

Al

O

Si

Cu

Zn

Mg

Cu

O/Al ratio

Mainly phase

(a) 7075-A 7075-HAl 7075-HAh 7050-A 7050-HAl 7050-HAh

17.33 12.64 9.21 15.37 11.52 12.28

23.82 16.58 14.72 21.52 16.13 18.57

0.94 1.22 1.05 1.34 1.23 1.11

0.34 0.20 0.22 0.23 0.25 0.21

0.24 0.12 0.12 0.18 0.07 0.13

0.17 0.22 0.13 0.20 0.17 0.23

Bal. Bal. Bal. Bal. Bal. Bal.

1.4 1.3 1.6 1.4 1.4 1.5

Al–O–H Al–O–H Al–O–H Al–O–H Al–O–H Al–O–H

Specimen

Al

O

Si

Cu

Zn

Mg

O/Al ratio

Mainly phase

(b) 7075-A 7075-HAl 7075-HAh 7050-A 7050-HAl 7050-HAh

25.11 26.17 28.73 26.44 26.46 27.86

72.52 71.25 69.13 71.38 71.43 69.65

2.03 2.23 1.71 1.82 1.73 2.11

0.08 0.14 0.13 0.11 0.15 0.18

0.15 0.07 0.21 0.17 0.13 0.09

0.11 0.14 0.09 0.08 0.13 0.11

2.9 2.7 2.4 2.7 2.7 2.5

Al(OH)3 Al(OH)3 Al(OH)3 or AlOOH Al(OH)3 Al(OH)3 Al(OH)3 or AlOOH

cathodic particles originating from the sample surface. Micropits were formed during the attack and provided preferential sites for the accumulation of Cl− ions to accelerate the penetration of these ions.

The data listed in Table 6 confirm that the AAO film coated on the surface of samples hindered the penetration of Cl− ions into the metal matrix, thereby enhancing the corrosion resistance considerably by reducing Icorr . The anodized/sealed samples exhibited

Fig. 2. The polarization curves recorded from the test of (a) anodized and hard-anodized 7075; (b) anodized and hard-anodized 7050 alloy samples; (c) bare 7050 and 7075 alloy samples.

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Fig. 3. XPS analysis results showing the O 1s peaks at area close to e/f interface and f/m interface of AAO films on (a) e/f; (b) f/m interface, 7075 A, HAl and HAh samples; (c) e/f, interface, 7050-A, HAl and HAh samples.

higher corrosion resistance against chloride ion attack compared with hard anodized/sealed samples, for both alloys. As explained previously, the hard anodized/sealed samples consumed more energy at Steps 2 and 3 compared with the anodized/sealed samples, leading to the formation of large pores with large volumes. After the samples were sealed, the pore channels were filled with oxides (mainly Al(OH)3 and AlOOH), which increased the fraction of pseudoboehmite. Fig. 3a–c shows the XPS analysis results; they show the O 1s peaks measured at the e/f and f/m interfaces of AAO films on different samples. Each spectrum component was

Table 6 The Ecorr and Icorr for different samples; data taking from Fig. 2. Samples 7075-A 7075-HAl 7075-HAh 7075-base

Ecorr (V) −0.42916 −0.49069 −0.79744 −0.68069

Icorr (A/cm2 ) −9

2.01 × 10 1.53 × 10−9 1.94 × 10−8 2.38 × 10−6

Samples

Ecorr (V)

Icorr (A/cm2 )

7050-A 7050-HAlcd 7050-HAhcd 7050-base

−0.51829 −0.70980 −0.83156 −0.71987

2.32 × 10−9 2.34 × 10−9 3.40 × 10−8 7.12 × 10−6

associated with three primary oxides: hydrate alumina Al(OH)3 , hydrated amorphous alumina, and oxyhydroxide AlOOH. To summarize, the sealed 7075 HAh and 7050 HAh samples showed low O/Al ratios and high fractions of AlOOH at the e/f interface (Table 5). During the potentiodynamic test, the chloride ions were absorbed on the AAO film surface and triggered the reaction Al(OH)3 + 3Cl− ⇔ AlCl3 + 3OH− , which yielded aluminum chloride oxide [15]. Micropits formed at the surface of the AAO film, and Al(OH)3 was highly concentrated at the locations of the micropits. When pseudoboehmite was attacked by Cl− ions, the following reactions occurred: AlOOH + Cl− = AlOCl + OH− , 3AlOCl = AlCl3 + Al2 O3 , and Al2 O3 + 6Cl− + 6H+ = 2AlCl3 + 3H2 O. Because of the presence of pseudoboehmite oxide at the e/f interface, the AAO film became vulnerable to Cl− ion attacks, which occur following the absorption of more Cl− ions. Fig. 4 clearly indicates that a decrease in the energy consumption in Steps 2 and 3 increases Ecorr (V) for all samples. The sealed 7075-A and 7075-HAl samples showed a small pore size and low fractions of pseudoboehmite at the e/f interface, corresponding to high

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Fig. 4. Relation of Ecorr for different samples corresponding to their energy consumed at Steps 2 and 3.

corrosion resistance. For further understanding the effect of the AAO film morphology and constituted oxide phases on Icorr , cross sections of different samples were observed through SEM (Fig. 5). The sealed AAO films on the 7050-HAh and 7075-HAh alloy samples clearly showed bubble defects. These bubbles could be due to dissolution and/or reacted anodic Mg- and cathodic Cu-containing particles during anodization [7,11]. Fig. 6 shows a plot of total energy consumption per unit volume of AAO film versus Icorr . The O/Al ratios measured at the e/f interface of AAO films on different samples are also presented for comparison. The total energy consumed per unit volume of AAO film did not affect the values of Icorr in different anodized/sealed samples. Factors that increased the value of Icorr in (hard) anodized/sealed 7050 and 7075 alloy samples were mainly the trapped bubbles and pseudoboehmite that formed at the e/f interface of the AAO films. Pseudoboehmite formed at the e/f interface tends to generate micropits readily, thereby decreasing Ecorr , and bubbles trapped in the film can provide preferential sites for Cl− ion accumulation and penetration, which increase Icorr .

Fig. 6. Relation of Icorr for different samples corresponding to their total energy consumption.

In this study, we found that when the combined energy consumption of Steps 2 and 3 is less than 9.0 J/cm2 , the 7075-A, 7075-HAl , and 7050-A samples showed high corrosion resistance against Cl− ion attack (Table 2). This finding confirmed that monitoring the quality (corrosion resistance) of anodized/sealed high-strength aluminum alloys by measuring the V–t curve is feasible. 3.3. Effect of anodization on fatigue strength Fig. 7a and b shows the S–N curves for the bare, anodized/sealed, and hard anodized/sealed 7050 and 7075 samples. The 7050 samples showed higher fatigue strength at 107 life cycles compared with the 7075 samples (260 versus 240 MPa). All anodized/sealed samples showed lower fatigue strength than the anodized/sealed samples, and the hard anodized/sealed sample showed even lower fatigue strength (lower than the fatigue strength of anodized/sealed samples by approximately 10–15%).

Fig. 5. SEM observations of the f/m interface: (a) 7075-A, (b) 7075-HAl , (c) 7075-HAh , (d) 7050-A, (e) 7050-HAl , (f) 7050-HAh samples.

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Table 7 Hydrogen concentration for different samples. Samples

7075 7050

Anodization

A

T6

Core

Surface

HAlcd Core

Surface

HAhcd Core

Surface

0.33 (0.01) 0.25 (0.01)

0.29 (0.04) 0.22 (0.02)

0.37 (0.06) 0.28 (0.04)

0.27 (0.02) 0.2 (0.02)

0.4 (0.06) 0.32 (0.04)

0.24 (0.04) 0.16 (0.03)

0.44 (0.03) 0.34 (0.04)

At least three tests to obtain mean and standard deviation in parenthesis.

energy consumed in Steps 2 and 3 was less than 9.0 J/cm2 , the AAO films formed on the anodized/sealed 7075 and 7050 samples exhibited mainly hydrated alumina at the e/f interface without (or with very few) defects of bubbles. As a result, these anodized/sealed samples featured good corrosion resistance to against Cl− ions attack. The 7075 samples contained high Mg contents to also absorb more hydrogen content in the matrix than the 7050 samples did. The (hard-) anodized/sealed 7075 samples decreased their fatigue strengths at 1 × 107 life cycles. Acknowledgements We gratefully acknowledge the financial support from the Ministry of Science and Technology of the Republic of China (MOST 103-2221-E-008-026-MY2). Many thanks also to National Central University for providing the SEM and TEM tests and to National Sun-Yat Sen University for the EBSD analysis. References

Fig. 7. S–N curves for different samples; (a)7075, (b)7050 alloy samples.

Fatigue strength was mainly affected by crack initiation and propagation. Both 7050 and 7075 alloy samples contained (Mgand Cu-species) particles in their matrix. After anodization, more pits are likely to have been generated at the f/m interface, leading to crack initiation; crack initiation in turn leads to lower fatigue strength. The bare and anodized/sealed 7050 samples showed relatively lower hydrogen content compared with the 7075 samples, as shown in Table 7. Reducing the hydrogen content of the aluminum matrix enabled the cross-slip process to continue [16]. Dislocations readily move and accumulate at grain boundaries, leading to the preferential formation of high-angle grain boundaries (HAGBs). Fatigue strength could be enhanced by increasing HAGBs in the deformed matrix [13]. The 7050 alloy samples showed higher particle counts (Table 4), reflecting a higher potential risk for crack initiation at the f/m interface. However, because of the relatively low hydrogen content of the matrix, crack propagation was delayed and the fatigue strength was higher than that of the anodized/sealed 7075 alloy samples. 4. Conclusion An increase in the energy consumed in Steps 2 and 3 led to an increase in forming pseuodohomite at the e/f interface. When the

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