Influence of Al7Cu2Fe intermetallic particles on the localized corrosion of high strength aluminum alloys

Materials and Design 53 (2014) 118–123

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Influence of Al7Cu2Fe intermetallic particles on the localized corrosion of high strength aluminum alloys Aline Chemin a,⇑, Denys Marques a, Leandro Bisanha b, Artur de Jesus Motheo b, Waldek Wladimir Bose Filho a, Cassius Olivio Figueiredo Ruchert a a b

Department of Materials, Engineering School of São Carlos, University of São Paulo, Av. Trabalhador São-Carlense, 400, Parque Arnold Schmidt, São Carlos, SP, Brazil Department of Physical Chemistry, São Carlos Institute of Chemistry, University of São Paulo, Av. Trabalhador São-Carlense, 400, Parque Arnold Schmidt, São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 25 March 2013 Accepted 1 July 2013 Available online 10 July 2013 Keywords: Aluminum alloy Pitting corrosion Aluminum alloys

a b s t r a c t The development of aluminum alloys of the Al–Zn–Mg–Cu system is the primary factor that enabled the evolution of aircraft. However, it has been shown that these alloys tend to undergo pitting corrosion due to the presence of elements such as iron, copper and silicon. Thus, the purpose of this study is to evaluate the behavior of the Al7Cu2Fe precipitate in 7475-T7351 and 7081-T73511 alloys based on microstructural characterization and polarization tests. The corrosion and pitting potentials were found to be very similar, and matrix dissolution occurred around the Al7Cu2Fe precipitate in both alloys, revealing the anodic behavior of the matrix. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Alloys of the 7xxx series are widely used in the aviation industry because of their high strength produced by precipitation hardening [1–4]. The development of the Al–Zn–Mg–Cu system enabled the evolution of aircraft in terms of size and structural configuration [5]. The 7475-T7351 and 7081-T73511 alloys, which belong to the Al–Zn–Mg–Cu family, were developed to combine mechanical strength, low density and good fracture toughness, allied to stress corrosion resistance [5,6]. The 7081-T73511 alloy was developed recently, which is why studies relating to this alloy have not yet been published. However, one of the main problems of these materials is the occurrence of pitting corrosion. This degradation mechanism impairs the integrity of structures, thus limiting their application [7,8]. Pitting corrosion occurs as a consequence of the rupture of the passive film that forms naturally on the surface. Moisture on metal surfaces favors the growth of oxide films with semiconducting characteristics [7]. The structural characteristics of the passive film formed on the surface of aluminum alloys, as well as the intensity of etching, depend on the chemical composition of the alloys, the presence and distribution of defects and the characteristics of the electrolyte, such as temperature and composition [8].

⇑ Corresponding author. Tel.: +55 16 33738215. E-mail address: [email protected] (A. Chemin). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.07.003

The interaction between the passivated surface and ionic species such as chloride, leads to the rupture of the passive film because the chloride ions penetrate through the film, occupying the voids of anions and cations and increasing their size, and when these ions accumulate at the interface with the metal, the passive film breaks [7–11]. In studies of aluminum alloys typically used in aeronautical applications sodium chloride is used as electrolyte, since chloride ions are abundant in the environment and they are the main cause of pitting corrosion [3–10]. Another agent that contributes to a corrosive environment is SO2, which has been observed in marine environments in countries such as Saudi Arabia and is caused by the use of fossil fuels and the decomposition of seaweed [12]. Thus, once a passive film has been ruptured, the underlying metal surface begins to dissolve. Pits begin to be formed when the potential exceeds the critical value observed in polarization curves. This phenomenon, which depends not only on the material but also on the type of electrolyte, occurs in four stages: (1) phenomena that break the passive film; (2) rapid growth of metastable pits; (3) stable pit growth over a longer period; and (4) repassivation [8,13,14]. For this reason, aircraft structures undergo this type of corrosion, especially when they have been in service for long periods [7,13–15]. It should be noted that the dissolution of metals depends on the intermetallic compounds that are present in alloys. Some researchers have found that Fe and Si-rich particles are cathodic towards the aluminum matrix [9–20]. The 7475-T7351 and 7081-T73511

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Fig. 1. Electrochemical cell for corrosion tests.

Table 1 Chemical composition limits (wt.%) of 7475–T7351 and 7081–T73511 aluminum alloys. Alloy

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

P

V

B

Al

7475 7081

0.03 0.02

0.08 0.04

1.67 1.69

0.01 –

2.156 1.935

0.23 –

5.47 7.24

0.043 –

0.001 –

0.01 –

0.011 –

Base Base

alloys contain Al7Cu2Fe precipitate, which results from the solidification process. The purpose of this study was to evaluate the corrosion potential of both aforementioned alloys, as well as the pitting morphology, in order to correlate their microstructure and corrosion properties. 2. Experimental details 2.1. Metal samples The 7475-T7351 and 7081-T73511 alloys were received in the form of hot-rolled and extruded shapes, respectively, from which 10  10  3 mm samples were removed. The surfaces of these samples were sanded and polished with 1=4 lm diamond paste for analysis by scanning electron microscopy (SEM), followed by open-circuit potential and potentiodynamic polarization tests. 2.2. Scanning electron microscope The AL7Cu2Fe precipitate was analyzed in 7475-T7351 and 7081-T73511 aluminum alloys using a Zeiss LEO scanning electron microscope (SEM, Cambridge, England) equipped with an Oxford Instruments detector, operating with an electron beam of 20 kV. The composition of the constituent phases was analyzed using an energy dispersive X-ray (EDX) spectrometer equipped with a SiLi Pentafet detector, ultrathin ATW II window, resolution of 133 eV– 5.9 keV, coupled to the SEM. The analysis was performed using the Co standard for calibration, 20 kV electron beam, 25 mm focal length, dead-time of 30%, electrical current 2.82 A, and a 950 pA I probe. 2.3. Open circuit potential and potentiodynamic polarization testing The variation in open circuit potential was determined to establish the potential range to be used in the potentiodynamic polarization experiments, according to the ASTM: G69-2012. The open circuit potential and potentiodynamic polarization curves at 0.5 mV/s, with a potential range of 1.3 V to 0.3 V, were obtained

Fig. 2. Microstructure of 7475 T7351 alloy by scanning electron microscopy (a) Al7Cu2Fe precipitated observed on the alloy surface; (b) EDS spectrum of the precipitated.

in aerated 3.5 wt.% NaCl solution. An EG&G/PAR 273-A potentiostat/galvanostat controlled by EG&G/PAR M352 software was used for the electrochemical tests. Measurements were taken in a

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Fig. 4. Open circuit potential curve of the alloys (a) 7475–T7351 and (b) 7081 T73511 in aerated 3.5% NaCl aqueous solution.

dimensions as that of the 7475-T7351 alloy. The microstructure of both alloys, analyzed by SEM, revealed the presence of Fe-rich precipitates. Figs. 2 and 3 depict the morphology and chemical composition of Al7Cu2Fe in the 7475-T7351 alloy. The Al7Cu2Fe shown in Fig. 2 has elongated morphology, which is generally formed adjacent to voids during solidification, as illustrated in Fig. 2(a). The quantity of these intermetallic particles varies with the Fe content in the alloy [15]. Birbilis et al. [16] and Andreatta et al. [20] observed that precipitates rich in alloying

Fig. 3. Microstructure of 7081 T73511 alloy by scanning electron microscopy (a) Al7Cu2Fe precipitated on the alloy surface; (b) EDS spectrum of the precipitated.

single-compartment electrochemical cell. A three-electrode flat corrosion cell was used, containing the 7475-T7351 and 7081T73511 aluminum alloy specimens as working electrode, a Pt foil with an area of 2 cm2 as counter electrode, and a saturated calomel electrode (SCE) as reference. All the electrochemical experiments were performed at 25 ± 2 °C. The tests were carried out in triplicate. Fig. 1 shows the single-compartment electrochemical cell used for the measurements. After the tests, the specimens were analyzed by SEM to verify the effect of the precipitates on pitting corrosion. 3. Results and discussion 3.1. Chemical composition The 7475 alloy was received as a 3 mm thick hot-rolled plate treated to T7351 condition. Its chemical composition was analyzed by optical spectroscopy and is described in Table 1. The 7081 alloy was received as a 20 mm thick extruded L profile treated to T73511 condition. Its chemical composition was analyzed by optical spectroscopy and is also described in Table 1. The chemical composition of the studied alloys lies within the range established by the SAE AMS 2355 standard. 3.2. Microstructural analysis by SEM–EDS The test specimens for the characterization and corrosion test were removed from the 7081-T73511 alloy with the same

Fig. 5. Potentiodynamic curves for the (a) Al 7475 T7351 and (b) 7081 T73511 alloys in aerated 3.5% NaCl aqueous solution at scan rate of 0.5 mV s 1.

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A. Chemin et al. / Materials and Design 53 (2014) 118–123 Table 2 Average values of the measured parameters of samples tested in triplicate: corrosion current density (icorr), corrosion potential (Ecorr), pit potential (Epit) and corrosion rate. Alloy

icorr (lA cm

7475-T7351 7081-T73511

2.767 1.884

2

)

Ecorr (V)

Epit (V)

0.741 0.747

0.731 0.734

elements, such as Fe, behave differently from the base metal. The morphology of the Al7Cu2Fe precipitate in the 7081-T7351 alloy, Fig. 3(a), is more elongated and finer than in the 7475-T7351 alloy, probably because the 7081-T7351 alloy is extruded, and thus underwent more severe stretch, possibly causing the fracture of this structure [13–20]. Birbilis et al. [16] reported that Al7Cu2Fe precipitates fractured during the manufacturing process were reduced in size, resulting in the alignment of these particles to the direction of manufacture [16]. Cvijovic et al. [17,18] performed a fractographic analysis of aluminum alloys and observed the formation of voids around these precipitates. 3.3. Open circuit potential and potentiodynamic polarization measurements The open-circuit potential test was carried out to check the potentiodynamic polarization scan window. Fig. 4 shows the behavior of the two alloys. The measured potential of alloy 7475-T7351 was 0.748 V while that of alloy 7081-T73511 was 0.799 V. In other alloys of

Fig. 6. Images of 7475 T7351 surface alloy by (a) optical microscopy of surface without etching, before the polarization test; (b) scanning electron microscopy after the polarization test.

Corrosion rate (mm year

1

)

0.0298 0.0203

the 7000 series, such as alloy AA7075 treated to the T76 condition, this potential was found to be 0.770 V [19], and this alloy treated in the T651 condition was reported to be 0.750 V [16]. Fig. 5 illustrates the potentiodynamic polarization curves of the two alloys under study. When the films formed on the surfaces of aluminum alloys are exposed to Cl ions, fractures occur at specific points, leading to pits and thus increasing the corrosion potential [7]. The Table 2 describes their measured electrochemical parameters. As can be seen in Table 2, the corrosion (Ecorr) and pit (Epit) potential values are very similar, indicating that neither of the alloys presented passivation behavior, i.e., passive film fracture and pitting corrosion occurred almost simultaneously. The corrosion potential observed in the alloys is similar to that reported by Birbilis et al. [16] for 7050 T651 alloy, whose corrosion potential is also stable at about 0.750 V [16]. Andreatta et al. [19] concluded that Al7Cu2Fe precipitate in 7075 T6 alloy causes a strong galvanic connection with the matrix, indicating that in the matrix of aluminum alloys of the 7000 series, this precipitate causes a similar corrosion behavior [19].

Fig. 7. Images of 7081 T73511 surface alloy by (a) optical microscopy of surface without etching, before the polarization test; (b) scanning electron microscopy after the polarization test.

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Fig. 8. Images of 7475 T7351 surface alloy by scanning electron microscopy after the polarization tests in (a) and (b) Al7Cu2Fe precipitated adjacent to pits formed.

3.4. Surface attack on the alloys Figs. 6 and 7 depict the surfaces of the alloys before and after potentiodynamic polarization testing. Images 6(a) and 7(a) reveal the presence of inclusions in the two alloys. In Figs. 6(b) and 7(b), note the pitting corrosion sites formed by fracture of the oxide film during the potentiodynamic polarization test. The pitting corrosion morphology in Figs. 6(b) and 7(b) shows an elliptical shape, as well as some circular pits in Fig. 7 (b). Pit morphology depends on the nature of the solid solution in the alloy. Ezuber et al. [8] and Ives [11] reported that intermetallic Al3Fe is cathodic towards the matrix and that the nature of this reaction generates hydroxyl ions. The presence of this ion, in turn, increases the pH around the intermetallic phase, dissolving the aluminum matrix and resulting in the formation of pits. The morphology of the pits therefore depends on the nature of the solid solution in the alloy [8,11]. As can be seen, the Al7Cu2Fe particles have different morphologies, according to the forming process to which each one was subjected. The 7475-T7351 alloy was hot-rolled and therefore contains larger and coarser Al7Cu2Fe precipitates than those in the 7081-T73511 alloy, which was extruded and therefore shows finer precipitates. Figs. 8 and 9 show details of corrosion pits analyzed by SEM on the surfaces of the 7475-T7351 and 7081T73511 alloys after the potentiodynamic polarization test. The matrix underwent dissolution around the Al7Cu2Fe in both the 7475-T7351 and 7081-T73511 alloys, characterizing the

cathodic behavior of this intermetallic compound. The difference in the morphology of the precipitates, in the same particle, which shows a cathodic behavior towards the matrix, may be the reason for the difference between the sites formed by pitting corrosion observed in the 7475-T7351 and 7081-T73511 alloys.

4. Conclusions The SEM analysis revealed the presence of intermetallic Al7Cu2Fe particles in the 7475-T7351 and 7081-T73511 alloys, which were also characterized by EDX. The corrosion behavior, which was evaluated by open-circuit potential and potentiodynamic polarization tests, indicated that the corrosion potential of the two alloys was very similar, and also within the range of the other alloys of the 7000 series. However, the corrosion current density value, icorr, indicated that although the alloys have similar potentials, the 7475-T7351 alloy corrodes faster once the process begins. Corrosion pits were observed around Al7Cu2Fe precipitates, which explain the cathodic behavior towards the matrix. However, these alloys are known to contain Mg-rich precipitates which are difficult to detect by SEM because of their small size. Nevertheless, these precipitates were detected in the 7475-T7351 alloy and showed an anodic behavior towards the matrix, i.e., they may have become dissolved and also contributed to pitting.

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Fig. 9. Images of 7081 T73511 surface alloy by scanning electron microscopy after the polarization tests in (a) and (b) Al7Cu2Fe precipitated adjacent to pits formed.

Acknowledgements The authors wish to thank the Brazilian research funding agencies CNPq (National Council for Scientific and Technological Development), CAPES (Federal Agency for the Support and Improvement of Higher Education), and FAPESP (São Paulo Research Foundation) for granting them research scholarships, and the Brazilian aircraft manufacturer EMBRAER for its donation of the metal alloys used in this study. References [1] Su J-Q, Nelson T, Mishra R, Mahoney M. Microstructural investigation of friction stir welded 7050–T651 aluminium. Acta Mater 2003;51:713–29. [2] Wanhill R, Thart W, Schra L. Flight simulation fatigue crack propagation in 7010 and 7075 aluminium plate. Int J Fatigue 1979;1:205–9. [3] Holdoyd NJH, Hardie D. Factors controlling crack velocity in 7000 series aluminium alloys during fatigue aggressive environment. Corros Sci 1983;23:527–46. [4] Cardoso KR, Rodrigues CAD, Botta-Filho WJ. Processing of aluminium alloys containing titanium addition by mechanical alloying. Mat Sci Eng A 2004;375– 377:1201–5. [5] Hunsicker HY. Development of Al–Zn–Mg–Cu Alloys for aircraft. Phil Trans R Soc A 1976;282:359–76. [6] Alrubaie K, Barroso E, Godefroid L. Fatigue crack growth analysis of prestrained 7475–T7351 aluminum alloy. Int J Fatigue 2006;28:934–42. [7] Szklarska-Smialowska Z. Pitting corrosion of aluminum. Corros Sci 1999;41:1743–67.

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