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Electrochemical behavior and localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651
Corrosion Science 48 (2006) 4202–4215 www.elsevier.com/locate/corsci
Electrochemical behavior and localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651 N. Birbilis *, M.K. Cavanaugh, R.G. Buchheit Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, 477 Watts Hall, 2041 College Road, Columbus, OH 43210, USA Received 22 August 2005; accepted 22 February 2006 Available online 18 April 2006
Abstract Initiation of localized corrosion upon high strength aluminum alloys is often associated with cathodic intermetallic particles within the alloy. Electrochemical measurements and metallurgical characterization have been made to clarify and quantify the physical properties of Al7Cu2Fe particles in AA7075-T651. Prior studies regarding either the stereology or electrochemical properties of Al7Cu2Fe are scarce. Quantitative microscopy revealed a significant population of Al7Cu2Fe in the alloy; comprising up to 65% of the constituent particle population and typically at a size of 1.7 ± 1.0 lm. It was determined that Al7Cu2Fe may serve as a local cathode in the evolution of localized corrosion of AA7075-T651 and is capable of sustaining oxygen reduction reactions at rates of several hundreds of lA/cm2 over a range of potentials typical of the open circuit potential (OCP) of AA7075-T651 in NaCl solution of various concentrations and pH. The presence of Al7Cu2Fe leads to the development of pitting at the particle–matrix interface. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Intermetallics; C. Pitting corrosion; Aluminum alloys; Microscopy
*
Corresponding author. Tel.: +1 614 292 2749; fax: +1 614 292 9857. E-mail address: [email protected] (N. Birbilis).
0010-938X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.02.007
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1. Introduction The 7xxx series aluminum alloys have found widespread use in the aerospace sector owing to their high strength, which is due to heterogeneous microstructures based on the Al–Zn–Mg(–Cu) system. The number and variety of intermetallic particles that can form in such alloys are vast [1,2]. A number of recent studies have identified the presence of Al7Cu2Fe in AA7075 [3,4]. Al7Cu2Fe is formed owing to appreciable levels of iron (viz. 0.5 wt%) found in the alloy. The source of such iron is largely from impurities in alloy production and leads to the formation of so-called ‘constituent’ particles. Such particles are comparatively large and irregularly shaped, with characteristic dimensions ranging from a few tenths of a micrometer up to 10 lm, making them the largest (in size) class of particles. These particles form during alloy solidification and are not appreciably dissolved during subsequent thermo-mechanical processing. Rolling and extrusion tends to break-up and align constituent particles into bands within the alloy. Because these particles are rich in alloying elements, their electrochemical behavior is often significantly different than the surrounding matrix phase [5]. In high strength aluminum alloys, pitting and the development of initial corrosion damage are nominally associated with some fraction of the constituent particles present in the alloy [4–8]. Typical examples may include Al7Cu2Fe, Al3Fe, and Al6Mn. A detailed analysis of the number density and size distribution of Al7Cu2Fe particles is presently largely unknown, partly due to the fact that such intermetallics do not play a pivotal role in the mechanical properties of the alloys that they populate. Furthermore, the precise electrochemical properties of Al7Cu2Fe have not realized individual attention, unlike Al2Cu or Al20Cu2Mn3 [7] in spite of the importance in terms of the development of corrosion damage accumulation; leaving an overall knowledge gap. In terms of understanding the physical metallurgy and evolution of microstructure in high strength aluminum alloys, appreciable advances have been made over the past several decades [1,2,9]. These advances however, have been made largely on the sub-micrometer level and focused on processes leading to the development of mechanical properties (viz. strengthening MgZn2, g phase, particles in AA7075). Fig. 1 shows a typical microstructure of AA7075-T651 viewed with an SEM (in back-scattered electron mode), indicating the existence of constituent type particles. We note that the sub-micron g phase particles, present at a high number density, are not resolved with SEM. Overall however, MgZn2 (not evident in Fig. 1), Al7Cu2Fe, Mg2Si, Al2Cu, Al2CuMg, and Al3Fe, have been noted as populating AA7075 in various volume fractions depending on processing and composition [10]. Other studies based upon aluminum alloy 2024-T3 have also noted a significant presence of Al7Cu2Fe in that alloy [7]. Furthermore, it was posited of being of key interest in the development of corrosion damage accumulation upon AA2024; however, the individual electrochemical characteristics of Al7Cu2Fe were not investigated further in that study [7]. Recent mechanically alloyed materials, such as Al/Al–Cu–Fe composites, are also comprised of a large population of Al7Cu2Fe particles [11]. In this work, we have divided the study into three main sections; firstly we aim to determine the extent and morphology of Al7Cu2Fe present in AA7075-T651; secondly we determine the electrochemical properties of Al7Cu2Fe via electrochemical testing on synthesized Al7Cu2Fe crystals in solutions of varying NaCl concentration and pH; and thirdly we
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Fig. 1. Low magnification SEM image (back-scattered electron mode) of AA7075-T651 showing distribution of coarse intermetallic particles seen here as a combination of light and dark particles depending on individual composition.
extend the first two sections towards a discussion of damage accumulation processes that may occur owing to Al7Cu2Fe. 2. Experimental 2.1. Microstructural characterization Commercial AA7075-T651 supplied by Alcoa was used in this study. Specimens were prepared from about 10 mm below the surface of a 76 mm-thick rolled plate. These specimens were nominally 10 · 10 · 10 mm in size and metallographically mounted in conductive bakelite. SEM and EDXS were carried out using a Philips XL30 FEG-ESEM. In order to positively identify intermetallic particles within the specimen, chemical analysis from EDXS was complemented with the collection of Electron Backscattered Diffraction Patterns (EBDP) for structural characterization [12]. The BEKP collection and analysis was facilitated by OIM software (TSL). Sample preparation for the generation of satisfactory EBDPs requires that the sample surface be meticulously flat. In this work, samples were prepared by metallographic polishing down to 1/4 lm finish and subsequently vibratory polished using a 0.05 lm gamma alumina suspension. This suspension was chosen owing to its ‘inertness’, in an effort to avoid surface dissolution. 2.2. Electrochemical characterization 2.2.1. Intermetallic synthesis Electrochemical testing of Al7Cu2Fe is not readily possible via testing of bulk alloy specimens, since the small size of the particles do not allow them to be feasibly isolated (electrochemically). As a result, intermetallic particles were synthesized to create defect free Al7Cu2Fe analogs. Satisfactory synthesis of such particles requires prior knowledge
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Fig. 2. SEM image (back-scattered electron mode) of synthesized ingot cross-section, revealing extensive population of Al7Cu2Fe (light) particles.
of equilibrium conditions under which the intermetallic in question will form and was covered in [13]. Laboratory ingots of approximately 150 g were cast, incorporating a population of Al7Cu2Fe. A requirement for subsequent electrochemical testing was that intermetallic particles be larger than 50 lm in each of their surface dimensions. Consequently, provided a small number of such particles were obtained, then replicated electrochemical testing could be done. A cross-section of the ingot is seen in Fig. 2. As with the bulk alloy, satisfactory identification of intermetallics requires both chemical and structural information, thus quantitative EDXS and BEKP’s were used to provide positive characterization of the crystals. This approach has been used elsewhere [13]. 2.2.2. Electrochemical testing Electrochemical testing of the synthesized intermetallics was carried out using a microcell method, as outlined previously in [14–16]. In this method, the working electrode area is defined by the area of metal which comes into contact with the opening of a microcapillary. The micro-capillary is filled with electrolyte, whilst containing a small-wire counter electrode and electrolytic contact with a saturated calomel reference electrode. A silicone seal was applied to the open end of the capillary in order to avoid any solution leakage and to allow an interference contact with the working electrode. The capillary opening is generally in the vicinity of 20–60 lm in diameter and will vary with each capillary. The microcell used in these studies was incorporated into a lens-piece of an optical microscope and is outlined in [13]. Potentiodynamic polarization was carried out at a scan rate of 0.01 V s1 using an Autolab PGSTAT 100. Equilibration time for the working electrode was nominally 60 s following contact with the solution. Measurements were performed in 0.01, 0.1, and 0.6 M NaCl at pH 6; and also at pH 2.5, 10, and 12.5 in 0.1 M NaCl.
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3. Results and discussion 3.1. Microstructural characterization The typical morphology of Al7Cu2Fe particles in 7075-T651 can be seen in Fig. 3. Positive identification of such Al7Cu2Fe particles (on a particle by particle basis) can be made by complementary EDXS and EBDP analysis as seen in Fig. 4. Fig. 4 shows the EDXS spectrum and the EBDP collected for an Al7Cu2Fe particle, along with the EBDP for grains either side of the particle. Al7Cu2Fe is nominally tetra˚ , c = 14.87 A ˚ ) space group P4/mnc. The size of Al7Cu2Fe particles is gonal (a = 6.336 A generally in excess of 1 lm in diameter, making them well suited to characterization by the above technique. By repeating the characterization process shown in Fig. 4 over a number of particles, it was possible to identify a sufficient sample, along with quantification of their physical characteristics such as size, shape, location, and number density. The results of such two-dimensional stereological investigations are summarized in Table 1, where d is the mean particle equivalent diameter and NA is the number density per unit area. The equivalent diameter is used, the distribution of which is seen in Fig. 5, since Al7Cu2Fe particles do not have an aspect ratio of 1. These values are presented together with the nearest neighbor distance. A total of about 20 mm2 of material (AA7075-T651) was investigated. Al7Cu2Fe along with other coarse (constituent) intermetallic particles are known to fracture during fabrication, reducing their size and hence causing them to become aligned as stringers in the direction of working [1]. This results in a range of particle morphologies, represented by the distribution of aspect ratios determined for Al7Cu2Fe particles. The aspect ratio was found to vary between 1 and 4.7, with the majority of particles having a value of approximately 1.4. The results presented above are typical of the samples taken 10 mm below the surface of a 76 mm plate. Although the results are taken to represent the general alloy structure, we expect the precise values reported to vary slightly with thickness throughout the plate.
Fig. 3. Back-scattered electron (SEM) image showing Al7Cu2Fe particles (light) aligned within 7075-T651.
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Fig. 4. SEM image showing Al7Cu2Fe particle in 7075-T651 matrix. The EDXS spectrum and associated EDBP of the particle are shown, together with the EBDP of two grains either side of the Al7Cu2Fe particle (SEM image is tilted and therefore particle appears slightly elongated in vertical axis).
Table 1 Characteristics of Al7Cu2Fe particles in AA7075-T651 Parameter d (lm) Aspect ratio NA (#/mm2) Nearest neighbor distance (lm)
Mean
Standard deviation
1.7 1.4 1651 10.3
1.0 0.3 950 9.6
Furthermore, the results are necessarily dependant on sample size, and hence, the measured maxima and minima may also slightly vary if greater amounts of material were studied. We point out that at times, small amounts of Mn were detected in the intermetallic, however at all times the Fe + Mn content amounted to 10 at.% of the intermetallic; with no alteration of the crystal structure. 3.2. Electrochemical characterization Synthesized Al7Cu2Fe crystals were nominally 300 lm in d and readily able to be electrochemically isolated via the now well established microcell method [14–16]; which allowed for the collection of potentiodynamic polarization scans for Al7Cu2Fe of which are shown in Fig. 6. Fig. 6 shows results of testing in 0.1 M NaCl (pH 6). It is seen that the electrochemical response is readily reproducible, and Al7Cu2Fe shows a distinct pitting potential (Epit) where the current density sharply increases following a breakdown process. The results
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Fig. 5. Cumulative distribution for equivalent diameter d of Al7Cu2Fe particles multiplied by 100 to give percentage below a certain d value.
Fig. 6. Potentiodynamic polarization curves for Al7Cu2Fe in 0.1 M NaCl (pH 6). The figure shows replicated curves.
for corrosion potential (Ecorr) and Epit are summarized for tests carried out in 0.01, 0.1 and 0.6 M NaCl in Fig. 7. Epit is separated from Ecorr by about 100 mV, and we note that Al7Cu2Fe is ‘spontaneously passive’ at its OCP in these conditions. Al7Cu2Fe is nominally referred to as ‘cathodic’ relative to the 7075-T651 matrix [4–12], since it displays an Ecorr > Ecorr (7075-T651). The Ecorr versus time behavior for 7075-T651 in quiescent 0.1 M NaCl over 1 week of immersion is seen in Fig. 8. We observe the Ecorr (7075-T651) is relatively stable at approximately 0.75 VSCE following an initial transient period. At such values of potential (0.75 VSCE), Al7Cu2Fe is rather heavily polarized cathodically. This is seen more clearly in Fig. 9 which shows cathodic polarization curves for Al7Cu2Fe in NaCl solution.
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-300
E corr E pit
-400
E mVSCE
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-500 -600 -700 -800 0.01
0.1
1
[NaCl] M Fig. 7. Ecorr and Epit values for Al7Cu2Fe in 0.01, 0.1 and 0.6 M NaCl (pH 6).
Fig. 8. Open circuit potential versus time for 7075-T651 in quiescent 0.1 M NaCl (pH 6).
Fig. 9 indicates the rate (in current density) of oxygen reduction reaction (ORR) that Al7Cu2Fe can sustain in the potential range characteristic of 7075-T651 for similar solution. For example, in the case of 0.1 M NaCl, Al7Cu2Fe can sustain ORR at 200 lA /cm2 (viz. at 0.75 VSCE). It is of significance to note that although both the Ecorr and Epit shift to less noble values with increasing Cl concentration, the ability of Al7Cu2Fe to support oxygen reduction increases. This richness in the electrochemical response could not have therefore been predicted based on potentials alone, and indicates the value of polarization testing. Furthermore, it suggests that the electrochemical response may be complex and non-intuitive, such that complete understanding of corrosion mechanisms and likely galvanic interactions upon bulk alloys necessitates a complete characterization be performed for intermetallics such as Al7Cu2Fe.
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Fig. 9. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe in 0.01 M, 0.1 M, and 0.6 M NaCl (pH 6).
3.3. Electrochemical response compared with Al2Cu and Al20Cu2Mn3 The electrochemical characteristics of Al2Cu and Al20Cu2Mn3 have been reported in some detail previously [7] using synthesized crystals and metallographic ‘masking’ techniques to isolate the working electrode. Both compounds have been noted as functioning as potent local cathodes. In order to interpret the cathodic electrochemical activity of Al7Cu2Fe, we have compared the cathodic polarization scans of the above mentioned compounds in 0.1 M NaCl, and reported the results in Fig. 10. What we observe in Fig. 10, is that over the range of potentials tested, Al7Cu2Fe displays the ability to support ORR at rates up to three times greater than Al2Cu and also at greater rates than Al20Cu2Mn3. This effect is attributed to the presence of both Cu (which is known to support high rates of ORR [17]) and Fe, which also has a high
Fig. 10. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe, Al2Cu, and Al20Cu2Mn3 in 0.1 M NaCl (pH 6).
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efficiency for supporting oxygen reduction; this latter point is emphasized by prior studies on Al3Fe, and Fe containing intermetallics [18,19]. As a result of such a comparative analysis, we are now in a position to be able to comment quantitatively on the cathodic efficiency of positively characterized intermetallics, deviating from qualitative first-order analyses which combine so-called ‘Fe-containing’ intermetallics and rank all cathodic intermetallics of equal potency. 3.4. Electrochemical response as a function of pH In order to better and more wholly characterize the electrochemical behavior of Al7Cu2Fe, the electrochemical response over a range of pH values was also explored. The response to cathodic polarization as a function of pH is seen in Fig. 11 below. The results reveal an increase in the ability to sustain ORR by over an order of magnitude as pH varies from 2.5 to 12.5. At the highest pH tested, ORR values of several mA were observed, which represent very high rates. This result is in contrast with those of Schneider et al. [20] upon Al2Cu, who noted a decrease in ORR kinetics as pH increases from 3 to 10 in weaker Cl solutions containing Na2SO4. In the case here however, ORR rates at high pH are enhanced; posited to be as a result of the presence of Fe, and likely linked to its ability to reduce oxygen. This is significant to note, as this point may also be interpreted as having ramifications in the context of damage accumulation upon bulk AA7075. For example, electrochemical heterogeneity will give rise to the establishment of local anodes and cathodes upon the alloy surface, those cathodes, such as Al7Cu2Fe, will see a concomitant rise in electrolyte pH at their surface owing to hydroxyl ion formation due to oxygen reduction. Therefore in essence, the cathodic activity of Al7Cu2Fe will become more potent as ORR proceeds and gives rise to a local alkaline environment. This is significant in the context of interpreting the mechanistic and kinetic aspects of localized corrosion development in commercial Al alloys. The variation in Ecorr and Epit is also reported as a function of pH in Fig. 12. The results for this compound reveal that as pH increases, Ecorr tends to shift to more negative values,
Fig. 11. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe in 0.1 M NaCl at pH 2.5, 6, 10, and 12.5.
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Fig. 12. Ecorr and Epit values for Al7Cu2Fe in 0.1 M NaCl as a function of pH.
whilst a concomitant rise in the associated Epit is observed. Again, Fig. 12 also highlights the weakness in interpreting corrosion potential as an index to cathodic potency, since the decrease in Ecorr is, as we see, associated with significantly larger current densities sustained over a wide range of potentials. 3.5. Corrosion damage accumulation owing to Al7Cu2Fe Fig. 13 shows the morphology of corrosion damage that occurs upon AA7075-T651 following immersion in quiescent 0.1 M NaCl for 10 h. For this timescale of testing, we observe that damage is typically associated with the presence of coarse intermetallics,
Fig. 13. SEM image revealing morphology of corrosion damage upon 7075-T651 following 10 h immersion in 0.1 M NaCl. Damage is associated with the presence of Al7Cu2Fe (viz. aligned with the direction of working) and takes the form of peripheral pitting.
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which are seen here as white/light particles, namely Al7Cu2Fe. It ought to be restated at this point, that Al7Cu2Fe is present amongst other coarse constituent intermetallics, however Al7Cu2Fe represents on average 65% of the constituent population for AA7075 [21]. A deliberate attempt has been made not to discuss all particles present in AA7075-T6, since a number of other particles are the focus of dedicated papers [5–7], whilst the goal of this work was to specifically present Al7Cu2Fe since it has not been discussed independently elsewhere in detail. Fig. 13 reveals ‘peripheral’ type corrosion, which describes pitting at the particle–matrix interface and is consistent with the notion that cathodic particles supporting appreciable ORR cause dissolution of the adjacent material (matrix) in order to maintain a net current balance upon the alloy surface. We also see that the corrosion damage has a strong directionality associated with the alignment of Al7Cu2Fe. This is consistent with other recent observations of Al7Cu2Fe as a contributor to localized damage accumulation upon AA7075 [4] (and AA2024 [7]). Peripheral pitting is also often termed ‘trenching’, and is seen more clearly in Fig. 14, whereby an Al7Cu2Fe particle was sectioned using a focused ion beam. The form of peripheral pitting is apparent following 10 h immersion in 0.1 M NaCl. We also note at this point, that not all particles observed during post-immersion microscopy led to obvious pitting, with some fraction in the vicinity of up to about 10% appearing to be more or less inert. For testing conducted here, in no circumstances was Al7Cu2Fe seen to selectively or preferentially dissolve. It is widely accepted that aluminum and its solid solutions dissolve readily at elevated (>9) pH values [22]. As a result, the hydroxyl ion formation and associated pH increase previously mentioned (owing to ORR upon Al7Cu2Fe) may also be contributing to the peripheral damage observed in Figs. 13 and 14. This phenomenon is often referred to as ‘cathodic corrosion’ [23]. Furthermore, it has recently been postulated that independent of ‘cathodic corrosion’ local compositional changes owing to preferential dissolution of solute at the particle–matrix interface may also contribute to so-called ‘trenching’ in
Fig. 14. SEM image of Al7Cu2Fe particle following cross-sectioning with a focused ion beam. 7075-T651 sample previously exposed for 10 h in 0.1 M NaCl, peripheral pitting damage evident.
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AA2024 [24,25]. However, the above two mechanisms were not studied in great detail here for 7075-T651. Additional to the mechanistic discussion given above, the physical and electrochemical behavior gained herein may potentially be incorporated into the development of mechanistic based modelling of corrosion upon AA7075. We re-iterate at this point, that the overall corrosion damage accumulation upon AA7075-T651 is not exclusively dictated by the presence of Al7Cu2Fe. For example, at advanced stages of corrosion, it is well known that AA7075 develops intergranular attack together with other likely forms of damage accumulation such as exfoliation [25]. The transition from surface corrosion (viz. pitting) to intergranular corrosion is beyond this paper, and whether these abovementioned processes are in fact independent or related is still largely unknown. We do believe however, that the attack owing to Al7Cu2Fe dominates, at least, at the early stages of the development of corrosion damage in AA7075. This leads to a level of attack that may evolve rather rapidly and rather extensively, possibly with the aide of ‘cathodic corrosion’, resulting in damage depths approaching several hundreds of microns after a number of weeks immersion [21]. Such damage depths may however be significant in their own right, not for the ultimate loss of large sections of material, but as initiation sites for corrosion fatigue and fatigue cracks [26]. 4. Summary 1. Al7Cu2Fe particles appear aligned as stringers in AA7075-T651. Such particles have a mean equivalent diameter of 1.7 lm, and a number density of about 1650/mm2. 2. We have successfully been able to determine the electrochemical characteristics of Al7Cu2Fe, and note it to be cathodic to the matrix of AA7075-T651; since EcorrðAl7 Cu2 FeÞ > Ecorr(AA7075-T651) in similar solutions. 3. Cathodic polarization testing indicated that Al7Cu2Fe can sustain ORR at appreciable rates (viz. between 20 lA/cm2 up to 2 mA/cm2 depending on [Cl] or pH) over a potential range typical of the OCP of (7075-T651) in NaCl solutions. 4. The ability of Al7Cu2Fe to support ORR was increased with increasing pH (from pH 2.5 to 12.5) by up to an order of magnitude, concomitant with a negative shift in Ecorr and an increase in Epit. 5. Al7Cu2Fe was capable of sustaining ORR at rates about three times greater than Al2Cu and up to two times greater than Al20Cu2Mn3. 6. Following immersion in 0.1 M NaCl solution, AA7075-T651 primarily corroded at the periphery of Al7Cu2Fe particles. 7. For the timescale of testing investigated herein, peripheral type pitting dominated the observed corrosion attack.
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Electrochemical behavior and localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651 N. Birbilis *, M.K. Cavanaugh, R.G. Buchheit Fontana Corrosion Center, Department of Materials Science and Engineering, The Ohio State University, 477 Watts Hall, 2041 College Road, Columbus, OH 43210, USA Received 22 August 2005; accepted 22 February 2006 Available online 18 April 2006
Abstract Initiation of localized corrosion upon high strength aluminum alloys is often associated with cathodic intermetallic particles within the alloy. Electrochemical measurements and metallurgical characterization have been made to clarify and quantify the physical properties of Al7Cu2Fe particles in AA7075-T651. Prior studies regarding either the stereology or electrochemical properties of Al7Cu2Fe are scarce. Quantitative microscopy revealed a significant population of Al7Cu2Fe in the alloy; comprising up to 65% of the constituent particle population and typically at a size of 1.7 ± 1.0 lm. It was determined that Al7Cu2Fe may serve as a local cathode in the evolution of localized corrosion of AA7075-T651 and is capable of sustaining oxygen reduction reactions at rates of several hundreds of lA/cm2 over a range of potentials typical of the open circuit potential (OCP) of AA7075-T651 in NaCl solution of various concentrations and pH. The presence of Al7Cu2Fe leads to the development of pitting at the particle–matrix interface. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Intermetallics; C. Pitting corrosion; Aluminum alloys; Microscopy
*
Corresponding author. Tel.: +1 614 292 2749; fax: +1 614 292 9857. E-mail address: [email protected] (N. Birbilis).
0010-938X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.02.007
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1. Introduction The 7xxx series aluminum alloys have found widespread use in the aerospace sector owing to their high strength, which is due to heterogeneous microstructures based on the Al–Zn–Mg(–Cu) system. The number and variety of intermetallic particles that can form in such alloys are vast [1,2]. A number of recent studies have identified the presence of Al7Cu2Fe in AA7075 [3,4]. Al7Cu2Fe is formed owing to appreciable levels of iron (viz. 0.5 wt%) found in the alloy. The source of such iron is largely from impurities in alloy production and leads to the formation of so-called ‘constituent’ particles. Such particles are comparatively large and irregularly shaped, with characteristic dimensions ranging from a few tenths of a micrometer up to 10 lm, making them the largest (in size) class of particles. These particles form during alloy solidification and are not appreciably dissolved during subsequent thermo-mechanical processing. Rolling and extrusion tends to break-up and align constituent particles into bands within the alloy. Because these particles are rich in alloying elements, their electrochemical behavior is often significantly different than the surrounding matrix phase [5]. In high strength aluminum alloys, pitting and the development of initial corrosion damage are nominally associated with some fraction of the constituent particles present in the alloy [4–8]. Typical examples may include Al7Cu2Fe, Al3Fe, and Al6Mn. A detailed analysis of the number density and size distribution of Al7Cu2Fe particles is presently largely unknown, partly due to the fact that such intermetallics do not play a pivotal role in the mechanical properties of the alloys that they populate. Furthermore, the precise electrochemical properties of Al7Cu2Fe have not realized individual attention, unlike Al2Cu or Al20Cu2Mn3 [7] in spite of the importance in terms of the development of corrosion damage accumulation; leaving an overall knowledge gap. In terms of understanding the physical metallurgy and evolution of microstructure in high strength aluminum alloys, appreciable advances have been made over the past several decades [1,2,9]. These advances however, have been made largely on the sub-micrometer level and focused on processes leading to the development of mechanical properties (viz. strengthening MgZn2, g phase, particles in AA7075). Fig. 1 shows a typical microstructure of AA7075-T651 viewed with an SEM (in back-scattered electron mode), indicating the existence of constituent type particles. We note that the sub-micron g phase particles, present at a high number density, are not resolved with SEM. Overall however, MgZn2 (not evident in Fig. 1), Al7Cu2Fe, Mg2Si, Al2Cu, Al2CuMg, and Al3Fe, have been noted as populating AA7075 in various volume fractions depending on processing and composition [10]. Other studies based upon aluminum alloy 2024-T3 have also noted a significant presence of Al7Cu2Fe in that alloy [7]. Furthermore, it was posited of being of key interest in the development of corrosion damage accumulation upon AA2024; however, the individual electrochemical characteristics of Al7Cu2Fe were not investigated further in that study [7]. Recent mechanically alloyed materials, such as Al/Al–Cu–Fe composites, are also comprised of a large population of Al7Cu2Fe particles [11]. In this work, we have divided the study into three main sections; firstly we aim to determine the extent and morphology of Al7Cu2Fe present in AA7075-T651; secondly we determine the electrochemical properties of Al7Cu2Fe via electrochemical testing on synthesized Al7Cu2Fe crystals in solutions of varying NaCl concentration and pH; and thirdly we
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Fig. 1. Low magnification SEM image (back-scattered electron mode) of AA7075-T651 showing distribution of coarse intermetallic particles seen here as a combination of light and dark particles depending on individual composition.
extend the first two sections towards a discussion of damage accumulation processes that may occur owing to Al7Cu2Fe. 2. Experimental 2.1. Microstructural characterization Commercial AA7075-T651 supplied by Alcoa was used in this study. Specimens were prepared from about 10 mm below the surface of a 76 mm-thick rolled plate. These specimens were nominally 10 · 10 · 10 mm in size and metallographically mounted in conductive bakelite. SEM and EDXS were carried out using a Philips XL30 FEG-ESEM. In order to positively identify intermetallic particles within the specimen, chemical analysis from EDXS was complemented with the collection of Electron Backscattered Diffraction Patterns (EBDP) for structural characterization [12]. The BEKP collection and analysis was facilitated by OIM software (TSL). Sample preparation for the generation of satisfactory EBDPs requires that the sample surface be meticulously flat. In this work, samples were prepared by metallographic polishing down to 1/4 lm finish and subsequently vibratory polished using a 0.05 lm gamma alumina suspension. This suspension was chosen owing to its ‘inertness’, in an effort to avoid surface dissolution. 2.2. Electrochemical characterization 2.2.1. Intermetallic synthesis Electrochemical testing of Al7Cu2Fe is not readily possible via testing of bulk alloy specimens, since the small size of the particles do not allow them to be feasibly isolated (electrochemically). As a result, intermetallic particles were synthesized to create defect free Al7Cu2Fe analogs. Satisfactory synthesis of such particles requires prior knowledge
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Fig. 2. SEM image (back-scattered electron mode) of synthesized ingot cross-section, revealing extensive population of Al7Cu2Fe (light) particles.
of equilibrium conditions under which the intermetallic in question will form and was covered in [13]. Laboratory ingots of approximately 150 g were cast, incorporating a population of Al7Cu2Fe. A requirement for subsequent electrochemical testing was that intermetallic particles be larger than 50 lm in each of their surface dimensions. Consequently, provided a small number of such particles were obtained, then replicated electrochemical testing could be done. A cross-section of the ingot is seen in Fig. 2. As with the bulk alloy, satisfactory identification of intermetallics requires both chemical and structural information, thus quantitative EDXS and BEKP’s were used to provide positive characterization of the crystals. This approach has been used elsewhere [13]. 2.2.2. Electrochemical testing Electrochemical testing of the synthesized intermetallics was carried out using a microcell method, as outlined previously in [14–16]. In this method, the working electrode area is defined by the area of metal which comes into contact with the opening of a microcapillary. The micro-capillary is filled with electrolyte, whilst containing a small-wire counter electrode and electrolytic contact with a saturated calomel reference electrode. A silicone seal was applied to the open end of the capillary in order to avoid any solution leakage and to allow an interference contact with the working electrode. The capillary opening is generally in the vicinity of 20–60 lm in diameter and will vary with each capillary. The microcell used in these studies was incorporated into a lens-piece of an optical microscope and is outlined in [13]. Potentiodynamic polarization was carried out at a scan rate of 0.01 V s1 using an Autolab PGSTAT 100. Equilibration time for the working electrode was nominally 60 s following contact with the solution. Measurements were performed in 0.01, 0.1, and 0.6 M NaCl at pH 6; and also at pH 2.5, 10, and 12.5 in 0.1 M NaCl.
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3. Results and discussion 3.1. Microstructural characterization The typical morphology of Al7Cu2Fe particles in 7075-T651 can be seen in Fig. 3. Positive identification of such Al7Cu2Fe particles (on a particle by particle basis) can be made by complementary EDXS and EBDP analysis as seen in Fig. 4. Fig. 4 shows the EDXS spectrum and the EBDP collected for an Al7Cu2Fe particle, along with the EBDP for grains either side of the particle. Al7Cu2Fe is nominally tetra˚ , c = 14.87 A ˚ ) space group P4/mnc. The size of Al7Cu2Fe particles is gonal (a = 6.336 A generally in excess of 1 lm in diameter, making them well suited to characterization by the above technique. By repeating the characterization process shown in Fig. 4 over a number of particles, it was possible to identify a sufficient sample, along with quantification of their physical characteristics such as size, shape, location, and number density. The results of such two-dimensional stereological investigations are summarized in Table 1, where d is the mean particle equivalent diameter and NA is the number density per unit area. The equivalent diameter is used, the distribution of which is seen in Fig. 5, since Al7Cu2Fe particles do not have an aspect ratio of 1. These values are presented together with the nearest neighbor distance. A total of about 20 mm2 of material (AA7075-T651) was investigated. Al7Cu2Fe along with other coarse (constituent) intermetallic particles are known to fracture during fabrication, reducing their size and hence causing them to become aligned as stringers in the direction of working [1]. This results in a range of particle morphologies, represented by the distribution of aspect ratios determined for Al7Cu2Fe particles. The aspect ratio was found to vary between 1 and 4.7, with the majority of particles having a value of approximately 1.4. The results presented above are typical of the samples taken 10 mm below the surface of a 76 mm plate. Although the results are taken to represent the general alloy structure, we expect the precise values reported to vary slightly with thickness throughout the plate.
Fig. 3. Back-scattered electron (SEM) image showing Al7Cu2Fe particles (light) aligned within 7075-T651.
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Fig. 4. SEM image showing Al7Cu2Fe particle in 7075-T651 matrix. The EDXS spectrum and associated EDBP of the particle are shown, together with the EBDP of two grains either side of the Al7Cu2Fe particle (SEM image is tilted and therefore particle appears slightly elongated in vertical axis).
Table 1 Characteristics of Al7Cu2Fe particles in AA7075-T651 Parameter d (lm) Aspect ratio NA (#/mm2) Nearest neighbor distance (lm)
Mean
Standard deviation
1.7 1.4 1651 10.3
1.0 0.3 950 9.6
Furthermore, the results are necessarily dependant on sample size, and hence, the measured maxima and minima may also slightly vary if greater amounts of material were studied. We point out that at times, small amounts of Mn were detected in the intermetallic, however at all times the Fe + Mn content amounted to 10 at.% of the intermetallic; with no alteration of the crystal structure. 3.2. Electrochemical characterization Synthesized Al7Cu2Fe crystals were nominally 300 lm in d and readily able to be electrochemically isolated via the now well established microcell method [14–16]; which allowed for the collection of potentiodynamic polarization scans for Al7Cu2Fe of which are shown in Fig. 6. Fig. 6 shows results of testing in 0.1 M NaCl (pH 6). It is seen that the electrochemical response is readily reproducible, and Al7Cu2Fe shows a distinct pitting potential (Epit) where the current density sharply increases following a breakdown process. The results
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Fig. 5. Cumulative distribution for equivalent diameter d of Al7Cu2Fe particles multiplied by 100 to give percentage below a certain d value.
Fig. 6. Potentiodynamic polarization curves for Al7Cu2Fe in 0.1 M NaCl (pH 6). The figure shows replicated curves.
for corrosion potential (Ecorr) and Epit are summarized for tests carried out in 0.01, 0.1 and 0.6 M NaCl in Fig. 7. Epit is separated from Ecorr by about 100 mV, and we note that Al7Cu2Fe is ‘spontaneously passive’ at its OCP in these conditions. Al7Cu2Fe is nominally referred to as ‘cathodic’ relative to the 7075-T651 matrix [4–12], since it displays an Ecorr > Ecorr (7075-T651). The Ecorr versus time behavior for 7075-T651 in quiescent 0.1 M NaCl over 1 week of immersion is seen in Fig. 8. We observe the Ecorr (7075-T651) is relatively stable at approximately 0.75 VSCE following an initial transient period. At such values of potential (0.75 VSCE), Al7Cu2Fe is rather heavily polarized cathodically. This is seen more clearly in Fig. 9 which shows cathodic polarization curves for Al7Cu2Fe in NaCl solution.
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-300
E corr E pit
-400
E mVSCE
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-500 -600 -700 -800 0.01
0.1
1
[NaCl] M Fig. 7. Ecorr and Epit values for Al7Cu2Fe in 0.01, 0.1 and 0.6 M NaCl (pH 6).
Fig. 8. Open circuit potential versus time for 7075-T651 in quiescent 0.1 M NaCl (pH 6).
Fig. 9 indicates the rate (in current density) of oxygen reduction reaction (ORR) that Al7Cu2Fe can sustain in the potential range characteristic of 7075-T651 for similar solution. For example, in the case of 0.1 M NaCl, Al7Cu2Fe can sustain ORR at 200 lA /cm2 (viz. at 0.75 VSCE). It is of significance to note that although both the Ecorr and Epit shift to less noble values with increasing Cl concentration, the ability of Al7Cu2Fe to support oxygen reduction increases. This richness in the electrochemical response could not have therefore been predicted based on potentials alone, and indicates the value of polarization testing. Furthermore, it suggests that the electrochemical response may be complex and non-intuitive, such that complete understanding of corrosion mechanisms and likely galvanic interactions upon bulk alloys necessitates a complete characterization be performed for intermetallics such as Al7Cu2Fe.
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Fig. 9. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe in 0.01 M, 0.1 M, and 0.6 M NaCl (pH 6).
3.3. Electrochemical response compared with Al2Cu and Al20Cu2Mn3 The electrochemical characteristics of Al2Cu and Al20Cu2Mn3 have been reported in some detail previously [7] using synthesized crystals and metallographic ‘masking’ techniques to isolate the working electrode. Both compounds have been noted as functioning as potent local cathodes. In order to interpret the cathodic electrochemical activity of Al7Cu2Fe, we have compared the cathodic polarization scans of the above mentioned compounds in 0.1 M NaCl, and reported the results in Fig. 10. What we observe in Fig. 10, is that over the range of potentials tested, Al7Cu2Fe displays the ability to support ORR at rates up to three times greater than Al2Cu and also at greater rates than Al20Cu2Mn3. This effect is attributed to the presence of both Cu (which is known to support high rates of ORR [17]) and Fe, which also has a high
Fig. 10. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe, Al2Cu, and Al20Cu2Mn3 in 0.1 M NaCl (pH 6).
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efficiency for supporting oxygen reduction; this latter point is emphasized by prior studies on Al3Fe, and Fe containing intermetallics [18,19]. As a result of such a comparative analysis, we are now in a position to be able to comment quantitatively on the cathodic efficiency of positively characterized intermetallics, deviating from qualitative first-order analyses which combine so-called ‘Fe-containing’ intermetallics and rank all cathodic intermetallics of equal potency. 3.4. Electrochemical response as a function of pH In order to better and more wholly characterize the electrochemical behavior of Al7Cu2Fe, the electrochemical response over a range of pH values was also explored. The response to cathodic polarization as a function of pH is seen in Fig. 11 below. The results reveal an increase in the ability to sustain ORR by over an order of magnitude as pH varies from 2.5 to 12.5. At the highest pH tested, ORR values of several mA were observed, which represent very high rates. This result is in contrast with those of Schneider et al. [20] upon Al2Cu, who noted a decrease in ORR kinetics as pH increases from 3 to 10 in weaker Cl solutions containing Na2SO4. In the case here however, ORR rates at high pH are enhanced; posited to be as a result of the presence of Fe, and likely linked to its ability to reduce oxygen. This is significant to note, as this point may also be interpreted as having ramifications in the context of damage accumulation upon bulk AA7075. For example, electrochemical heterogeneity will give rise to the establishment of local anodes and cathodes upon the alloy surface, those cathodes, such as Al7Cu2Fe, will see a concomitant rise in electrolyte pH at their surface owing to hydroxyl ion formation due to oxygen reduction. Therefore in essence, the cathodic activity of Al7Cu2Fe will become more potent as ORR proceeds and gives rise to a local alkaline environment. This is significant in the context of interpreting the mechanistic and kinetic aspects of localized corrosion development in commercial Al alloys. The variation in Ecorr and Epit is also reported as a function of pH in Fig. 12. The results for this compound reveal that as pH increases, Ecorr tends to shift to more negative values,
Fig. 11. Cathodic polarization (potentiodynamic) curves for Al7Cu2Fe in 0.1 M NaCl at pH 2.5, 6, 10, and 12.5.
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Fig. 12. Ecorr and Epit values for Al7Cu2Fe in 0.1 M NaCl as a function of pH.
whilst a concomitant rise in the associated Epit is observed. Again, Fig. 12 also highlights the weakness in interpreting corrosion potential as an index to cathodic potency, since the decrease in Ecorr is, as we see, associated with significantly larger current densities sustained over a wide range of potentials. 3.5. Corrosion damage accumulation owing to Al7Cu2Fe Fig. 13 shows the morphology of corrosion damage that occurs upon AA7075-T651 following immersion in quiescent 0.1 M NaCl for 10 h. For this timescale of testing, we observe that damage is typically associated with the presence of coarse intermetallics,
Fig. 13. SEM image revealing morphology of corrosion damage upon 7075-T651 following 10 h immersion in 0.1 M NaCl. Damage is associated with the presence of Al7Cu2Fe (viz. aligned with the direction of working) and takes the form of peripheral pitting.
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which are seen here as white/light particles, namely Al7Cu2Fe. It ought to be restated at this point, that Al7Cu2Fe is present amongst other coarse constituent intermetallics, however Al7Cu2Fe represents on average 65% of the constituent population for AA7075 [21]. A deliberate attempt has been made not to discuss all particles present in AA7075-T6, since a number of other particles are the focus of dedicated papers [5–7], whilst the goal of this work was to specifically present Al7Cu2Fe since it has not been discussed independently elsewhere in detail. Fig. 13 reveals ‘peripheral’ type corrosion, which describes pitting at the particle–matrix interface and is consistent with the notion that cathodic particles supporting appreciable ORR cause dissolution of the adjacent material (matrix) in order to maintain a net current balance upon the alloy surface. We also see that the corrosion damage has a strong directionality associated with the alignment of Al7Cu2Fe. This is consistent with other recent observations of Al7Cu2Fe as a contributor to localized damage accumulation upon AA7075 [4] (and AA2024 [7]). Peripheral pitting is also often termed ‘trenching’, and is seen more clearly in Fig. 14, whereby an Al7Cu2Fe particle was sectioned using a focused ion beam. The form of peripheral pitting is apparent following 10 h immersion in 0.1 M NaCl. We also note at this point, that not all particles observed during post-immersion microscopy led to obvious pitting, with some fraction in the vicinity of up to about 10% appearing to be more or less inert. For testing conducted here, in no circumstances was Al7Cu2Fe seen to selectively or preferentially dissolve. It is widely accepted that aluminum and its solid solutions dissolve readily at elevated (>9) pH values [22]. As a result, the hydroxyl ion formation and associated pH increase previously mentioned (owing to ORR upon Al7Cu2Fe) may also be contributing to the peripheral damage observed in Figs. 13 and 14. This phenomenon is often referred to as ‘cathodic corrosion’ [23]. Furthermore, it has recently been postulated that independent of ‘cathodic corrosion’ local compositional changes owing to preferential dissolution of solute at the particle–matrix interface may also contribute to so-called ‘trenching’ in
Fig. 14. SEM image of Al7Cu2Fe particle following cross-sectioning with a focused ion beam. 7075-T651 sample previously exposed for 10 h in 0.1 M NaCl, peripheral pitting damage evident.
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AA2024 [24,25]. However, the above two mechanisms were not studied in great detail here for 7075-T651. Additional to the mechanistic discussion given above, the physical and electrochemical behavior gained herein may potentially be incorporated into the development of mechanistic based modelling of corrosion upon AA7075. We re-iterate at this point, that the overall corrosion damage accumulation upon AA7075-T651 is not exclusively dictated by the presence of Al7Cu2Fe. For example, at advanced stages of corrosion, it is well known that AA7075 develops intergranular attack together with other likely forms of damage accumulation such as exfoliation [25]. The transition from surface corrosion (viz. pitting) to intergranular corrosion is beyond this paper, and whether these abovementioned processes are in fact independent or related is still largely unknown. We do believe however, that the attack owing to Al7Cu2Fe dominates, at least, at the early stages of the development of corrosion damage in AA7075. This leads to a level of attack that may evolve rather rapidly and rather extensively, possibly with the aide of ‘cathodic corrosion’, resulting in damage depths approaching several hundreds of microns after a number of weeks immersion [21]. Such damage depths may however be significant in their own right, not for the ultimate loss of large sections of material, but as initiation sites for corrosion fatigue and fatigue cracks [26]. 4. Summary 1. Al7Cu2Fe particles appear aligned as stringers in AA7075-T651. Such particles have a mean equivalent diameter of 1.7 lm, and a number density of about 1650/mm2. 2. We have successfully been able to determine the electrochemical characteristics of Al7Cu2Fe, and note it to be cathodic to the matrix of AA7075-T651; since EcorrðAl7 Cu2 FeÞ > Ecorr(AA7075-T651) in similar solutions. 3. Cathodic polarization testing indicated that Al7Cu2Fe can sustain ORR at appreciable rates (viz. between 20 lA/cm2 up to 2 mA/cm2 depending on [Cl] or pH) over a potential range typical of the OCP of (7075-T651) in NaCl solutions. 4. The ability of Al7Cu2Fe to support ORR was increased with increasing pH (from pH 2.5 to 12.5) by up to an order of magnitude, concomitant with a negative shift in Ecorr and an increase in Epit. 5. Al7Cu2Fe was capable of sustaining ORR at rates about three times greater than Al2Cu and up to two times greater than Al20Cu2Mn3. 6. Following immersion in 0.1 M NaCl solution, AA7075-T651 primarily corroded at the periphery of Al7Cu2Fe particles. 7. For the timescale of testing investigated herein, peripheral type pitting dominated the observed corrosion attack.
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