- Email: [email protected]
Corrosion behavior of Al-Cu-Fe quasicrystals
Materials Science and Engineering 294–296 (2000) 890–893
Corrosion behavior of Al-Cu-Fe quasicrystals A. Rüdiger∗ , U. Köster Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany Received 19 September 1999; accepted 22 December 1999
Abstract For any application of quasicrystals or quasicrystalline coatings, e.g. for extruders, turbine blades or frying pans, corrosion resistance is of utmost importance. Therefore, corrosion of the quasicrystalline as well as related crystalline phases in the system Al-Cu-Fe was studied in particular at intermediate pH values in order to clarify the influence of the quasicrystalline structure. Salt spray tests were performed at 35◦ C with several single phase materials in the Al-Cu-Fe system. Highest corrosion resistance was observed in crystalline aluminides AlFe as well as Al5 Fe2 where a protective oxide film had developed. The quasicrystalline alloy as well as crystalline Al7 Cu2 Fe exhibit severe corrosion accompanied with the formation of Cu, Cu2 O and Al(OH)3 . Anodic polarization of Al63 Cu25 Fe12 icosahedral quasicrystals reveal after a potentiodynamic scan in 0.1N NaOH the formation of an oxide layer consisting of FeO and Cu2 O; there is only very little evidence for a phase transformation of the quasicrystal due to Al loss by selective corrosion. During further polarization in the passive region the oxide layer was observed to transform into probably higher oxidized states. The overall thickness of the oxide layer depends on the scan time, however, not on further polarization time at a distinct potential. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Quasicrystal; Salt spray test; Corrosion; Polarization; Al-Cu-Fe
1. Introduction
2. Experimental procedure
For any application [1] of quasicrystals or quasicrystalline coatings, e.g. for frying pans, extruders, or turbine blades, corrosion resistance is of utmost importance. The corrosion resistance of an intermetallic phase or alloy usually depends strongly on its chemical composition and microstructure. An influence of the atomic structure is known for the crystalline phases in Cu-Al alloys [2]. Massiani et al. [3] studied corrosion of crystalline and quasicrystalline phases in the system Al-Cu-Fe(-Cr) by polarization and impedance measurements in strong acid and alkaline solutions. They concluded that the corrosion resistance depends on the alloy composition, not on a specific structure. Rüdiger et al. [4] observed that the corrosion of quasicrystals and related crystalline phases in the system Al-Cu-Fe can be understood qualitatively from the electrochemical property of the components. The aim of this paper is to verify the observed results, in particular at intermediate pH values. In addition, corrosion resistance of the quasicrystalline alloy and other crystalline phases in the Al-Cu-Fe systems will be tested in a salt spray test which allows an easy comparison of corrosion resistance under conditions comparable to application.
Bulk icosahedral quasicrystals and related crystalline alloys (polycrystalline single-phased) phases in Al-Cu-Fe were produced by casting and annealing as described in detail elsewhere [5]. The microstructure was studied by flat-on as well as cross-sectional transmission electron microscope,TEM (Philips CM200 operating at 200 kV), X-ray diffraction as well as scanning electron microscopy (SEM: Hitachi S-4500, operating between 1 and 30 kV). Operation below 5 kV allowed the microscopical study of oxide scales without an additional conducting surface coating. Salt spray tests for 120 h with a 5% NaCl solution at 35◦ C were performed with the different single phase material. The specimen were polished (down to 3 m diamond paste), then fixed on a flat polymer plate and tested in a climate chamber (SKA-400, Liebisch) under an angle of 70◦ to the horizontal. Testing for 120 h under these conditions is assumed to be equivalent to a 3-year application outdoors. Common measuring equipment for polarization curves was used, where the reference electrode (Ag/AgCl in acid, Hg/HgO in alkali) is separated from the bulk solution by an electrolyte bridge through a Haber–Luggin capillary. Samples with a freshly ground and polished surface (down to 1 m diamond paste) were polarized in NaOH or H2 SO4 solutions of different concentrations.
∗ Corresponding author. Tel.: +49-231-7554323; fax: +49-231-7555978. E-mail address: [email protected] (A. Rüdiger).
0921-5093/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 3 7 - 6
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
891
by a thick (about 16 m) cracked oxide film (see Fig. 2b). Similar surface morphologies were observed on Al13 Fe4 as well as Al7 Cu2 Fe. The lines in the X-ray pattern (shown in Fig. 3) of the corroded quasicrystalline surface can be assigned to crystalline Cu, Cu2 O and Al(OH)3 . The same products are observed on corroded Al7 Cu2 Fe. The film on Al2 Cu consists of Cu and Al(OH)3 . There was no evidence for an improvement in corrosion resistance due to the quasicrystalline structure observed. 3.2. Electrochemical polarization Fig. 1. Surfaces of different phases in the Al-Cu-Fe system after a salt spray test for 120 h at 35◦ C.
Polarization was started at 300 mV cathodic to free corrosion potential at a scan rate of 0.25 mV/s and in one case 0.05 mV/s. Some samples were kept for certain time at a potential of low corrosion current density in order to build a stable oxide layer at the surface. For each material and condition the polarization curves were measured 2–3 times. The potentials are calculated from reference to standard hydrogen electrode (SHE).
3. Results and discussion 3.1. Salt spray test Fig. 1 shows the surfaces of the different phases after a salt spray test for 120 h at 35◦ C. Soft smooth layers were formed on Al and Cu; colored but still shiny surfaces were observed on AlFe, Al5 Fe2 , Al50 Cu30 Fe20 . Al2 Cu shows also a shiny Cu surface after the test, but became very brittle. The quasicrystalline phase Al63 Cu25 Fe12 , however, as well as Fe, Al13 Fe4 and Al7 Cu2 Fe were heavily attacked and covered with thick brown layers. SEM reveals a very thin (about 0.2 m), but continuos oxide layer on top of AlFe (see Fig. 2a) consisting of small oxide crystals with a diameter less than 0.2 m. The surface of the quasicrystalline specimen is at least partially covered
Figs. 4 and 5 show anodic polarization curves for the icosahedral Al63 Cu25 Fe12 in solutions with different pH. Whereas in alkaline solutions (NaOH) the free corrosion potential is shifted to more noble values with decreasing pH, in particular between pH 11 and pH 7, in acid solutions (H2 SO4 ) no significant influence of the pH was observed. Depending on the pH of the solution the polarization at the free corrosion potential were found to lead to the following reactions: homogeneous dissolution of all components and redeposition of Cu (and a small amount of Cu2 O) at pH0 (Fig. 6) as well as in other acid solutions; in pH 13: formation of a nanocrystalline layer consisting of Cu2 O (cp, a=0.422 nm) and probably FeO (fcc, a=0.433 nm), see Fig. 7. In alkaline solutions between pH 11 and pH 7 the layers formed are too thin to be analyzed by X-ray diffraction. The observed shift of the corrosion potential can be understood as due to these surface layers formed. Whereas in acid solutions the current density in the anodic regime was found to drop only by less than one order of magnitude between pH 0 and pH 5, in alkaline solutions the current density changes by about two orders of magnitude between pH 13 and pH 7. At pH 13 the influence of the scan rate on the anodic polarization was studied. Reducing the scan rate by a factor of five one observes a slightly higher first anodic peak; during the further scan the current densities are always lower by about one order of magnitude. Fig. 8 exhibits a SEM cross section of the quasicrystal after potentiodynamic scan and potentiostatic polarization for 0.5 h at 360 mV in 0.1N NaOH. The quasicrystal is covered
Fig. 2. Surfaces after a salt spray test for 120 h at 35◦ C: (a) thin oxide layer on top of AlFe; (b) thick cracked oxide layer on i-Al63 Cu25 Fe12 .
892
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
Fig. 3. X-ray diffraction pattern of quasicrystalline i-Al63 Cu25 Fe12 before and after a salt spray test with 5% NaCl solution for 120 h at 35◦ C.
by a layered structure consisting of an inner nanocrystalline layer of FeO and Cu2 O followed by a layer showing higher charging in SEM which contains probably Fe2 O3 and CuO. The surface exhibits a needle-like morphology of 2-Al2 O3 . The thickness of the FeO/Cu2 O layer was found to be increased with a reduced scan rate which goes along with the reduced current density, but did not depend on the length of time at the final potential. There is, however, some evidence for a further transformation of these oxides towards higher oxidation states. The precipitation of a bcc phase in the quasicrystalline alloy underneath, mentioned earlier [4] as a result of selective corrosion of Al, could not be verified under the conditions used in this study. Fig. 5. Polarization curves for i-Al63 Cu25 Fe12 in acid solutions (H2 SO4 ).
Fig. 4. Polarization curves for i-Al63 Cu25 Fe12 in alkaline solutions (NaOH).
Fig. 6. SEM: porous Cu layer on i-Al63 Cu25 Fe12 after polarization at the free corrosion potential in 1N H2 SO4 (pH 0).
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
893
structure. The surface of the icosahedral Al63 Cu25 Fe12 is covered by a thick cracked layer consisting mainly of Cu, Cu2 O and Al(OH)3 very similar to crystalline Al7 Cu2 Fe. The results from anodic polarization of quasicrystals in dependence of pH confirm this behavior. In alkaline solutions with decreasing pH the current density decreases and the free corrosion potential is shifted to more noble values; in acid solution with increasing pH the current density decreases, but the free corrosion potential does not change. Fig. 7. SEM: layer of Cu2 O and FeO on i-Al63 Cu25 Fe12 after polarization at the free corrosion potential in 0.1N NaOH (pH 13).
Acknowledgements This work was supported partially by the GIF (I-531046.10/97) and DFG (Ko 668/22). The authors are indebted to Dr. H. Alves for fruitful discussions. References
Fig. 8. SEM: layered structure after polarization scan and 0.5 h at 360 mV in 0.1N NaOH (pH 13).
4. Summary A salt spray test does not indicate any evidence for an improvement in corrosion resistance due to the quasicrystalline
[1] J.M. Dubois, Bulk and surface properties of quasicrystalline materials and their potential application, in: J.B. Suck, M. Schreiber, P. Haussler (Eds.), Quasicrystals: An introduction to structure, physical properties and application of quasicrystalline alloys, Springer, Berlin, in press. [2] R. Langer, H. Kaiser, H. Kaesche, Werkstoffe und Korrosion 29 (1978) 409. [3] Y. Massiani, S. Ait Yaazza, J.P. Crousier, J.M. Dubois, J. Non-Cryst. Solids 159 (1993) 92. [4] A. Rüdiger, U. Köster, J. Non-Cryst. Solids 898 (1999) 250–252. [5] H. Liebertz, Hochtemperaturverformung von Al-Cu-Fe Quasikristallen und kristallinen Phasen ähnlicher Zusammensetzung, Fortschrittsberichte VDI, Reihe 5: Grund und Werkstoffe, Nr. 471, VDI-Verlag, Düsseldorf, 1997.
Corrosion behavior of Al-Cu-Fe quasicrystals A. Rüdiger∗ , U. Köster Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany Received 19 September 1999; accepted 22 December 1999
Abstract For any application of quasicrystals or quasicrystalline coatings, e.g. for extruders, turbine blades or frying pans, corrosion resistance is of utmost importance. Therefore, corrosion of the quasicrystalline as well as related crystalline phases in the system Al-Cu-Fe was studied in particular at intermediate pH values in order to clarify the influence of the quasicrystalline structure. Salt spray tests were performed at 35◦ C with several single phase materials in the Al-Cu-Fe system. Highest corrosion resistance was observed in crystalline aluminides AlFe as well as Al5 Fe2 where a protective oxide film had developed. The quasicrystalline alloy as well as crystalline Al7 Cu2 Fe exhibit severe corrosion accompanied with the formation of Cu, Cu2 O and Al(OH)3 . Anodic polarization of Al63 Cu25 Fe12 icosahedral quasicrystals reveal after a potentiodynamic scan in 0.1N NaOH the formation of an oxide layer consisting of FeO and Cu2 O; there is only very little evidence for a phase transformation of the quasicrystal due to Al loss by selective corrosion. During further polarization in the passive region the oxide layer was observed to transform into probably higher oxidized states. The overall thickness of the oxide layer depends on the scan time, however, not on further polarization time at a distinct potential. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Quasicrystal; Salt spray test; Corrosion; Polarization; Al-Cu-Fe
1. Introduction
2. Experimental procedure
For any application [1] of quasicrystals or quasicrystalline coatings, e.g. for frying pans, extruders, or turbine blades, corrosion resistance is of utmost importance. The corrosion resistance of an intermetallic phase or alloy usually depends strongly on its chemical composition and microstructure. An influence of the atomic structure is known for the crystalline phases in Cu-Al alloys [2]. Massiani et al. [3] studied corrosion of crystalline and quasicrystalline phases in the system Al-Cu-Fe(-Cr) by polarization and impedance measurements in strong acid and alkaline solutions. They concluded that the corrosion resistance depends on the alloy composition, not on a specific structure. Rüdiger et al. [4] observed that the corrosion of quasicrystals and related crystalline phases in the system Al-Cu-Fe can be understood qualitatively from the electrochemical property of the components. The aim of this paper is to verify the observed results, in particular at intermediate pH values. In addition, corrosion resistance of the quasicrystalline alloy and other crystalline phases in the Al-Cu-Fe systems will be tested in a salt spray test which allows an easy comparison of corrosion resistance under conditions comparable to application.
Bulk icosahedral quasicrystals and related crystalline alloys (polycrystalline single-phased) phases in Al-Cu-Fe were produced by casting and annealing as described in detail elsewhere [5]. The microstructure was studied by flat-on as well as cross-sectional transmission electron microscope,TEM (Philips CM200 operating at 200 kV), X-ray diffraction as well as scanning electron microscopy (SEM: Hitachi S-4500, operating between 1 and 30 kV). Operation below 5 kV allowed the microscopical study of oxide scales without an additional conducting surface coating. Salt spray tests for 120 h with a 5% NaCl solution at 35◦ C were performed with the different single phase material. The specimen were polished (down to 3 m diamond paste), then fixed on a flat polymer plate and tested in a climate chamber (SKA-400, Liebisch) under an angle of 70◦ to the horizontal. Testing for 120 h under these conditions is assumed to be equivalent to a 3-year application outdoors. Common measuring equipment for polarization curves was used, where the reference electrode (Ag/AgCl in acid, Hg/HgO in alkali) is separated from the bulk solution by an electrolyte bridge through a Haber–Luggin capillary. Samples with a freshly ground and polished surface (down to 1 m diamond paste) were polarized in NaOH or H2 SO4 solutions of different concentrations.
∗ Corresponding author. Tel.: +49-231-7554323; fax: +49-231-7555978. E-mail address: [email protected] (A. Rüdiger).
0921-5093/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 3 7 - 6
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
891
by a thick (about 16 m) cracked oxide film (see Fig. 2b). Similar surface morphologies were observed on Al13 Fe4 as well as Al7 Cu2 Fe. The lines in the X-ray pattern (shown in Fig. 3) of the corroded quasicrystalline surface can be assigned to crystalline Cu, Cu2 O and Al(OH)3 . The same products are observed on corroded Al7 Cu2 Fe. The film on Al2 Cu consists of Cu and Al(OH)3 . There was no evidence for an improvement in corrosion resistance due to the quasicrystalline structure observed. 3.2. Electrochemical polarization Fig. 1. Surfaces of different phases in the Al-Cu-Fe system after a salt spray test for 120 h at 35◦ C.
Polarization was started at 300 mV cathodic to free corrosion potential at a scan rate of 0.25 mV/s and in one case 0.05 mV/s. Some samples were kept for certain time at a potential of low corrosion current density in order to build a stable oxide layer at the surface. For each material and condition the polarization curves were measured 2–3 times. The potentials are calculated from reference to standard hydrogen electrode (SHE).
3. Results and discussion 3.1. Salt spray test Fig. 1 shows the surfaces of the different phases after a salt spray test for 120 h at 35◦ C. Soft smooth layers were formed on Al and Cu; colored but still shiny surfaces were observed on AlFe, Al5 Fe2 , Al50 Cu30 Fe20 . Al2 Cu shows also a shiny Cu surface after the test, but became very brittle. The quasicrystalline phase Al63 Cu25 Fe12 , however, as well as Fe, Al13 Fe4 and Al7 Cu2 Fe were heavily attacked and covered with thick brown layers. SEM reveals a very thin (about 0.2 m), but continuos oxide layer on top of AlFe (see Fig. 2a) consisting of small oxide crystals with a diameter less than 0.2 m. The surface of the quasicrystalline specimen is at least partially covered
Figs. 4 and 5 show anodic polarization curves for the icosahedral Al63 Cu25 Fe12 in solutions with different pH. Whereas in alkaline solutions (NaOH) the free corrosion potential is shifted to more noble values with decreasing pH, in particular between pH 11 and pH 7, in acid solutions (H2 SO4 ) no significant influence of the pH was observed. Depending on the pH of the solution the polarization at the free corrosion potential were found to lead to the following reactions: homogeneous dissolution of all components and redeposition of Cu (and a small amount of Cu2 O) at pH0 (Fig. 6) as well as in other acid solutions; in pH 13: formation of a nanocrystalline layer consisting of Cu2 O (cp, a=0.422 nm) and probably FeO (fcc, a=0.433 nm), see Fig. 7. In alkaline solutions between pH 11 and pH 7 the layers formed are too thin to be analyzed by X-ray diffraction. The observed shift of the corrosion potential can be understood as due to these surface layers formed. Whereas in acid solutions the current density in the anodic regime was found to drop only by less than one order of magnitude between pH 0 and pH 5, in alkaline solutions the current density changes by about two orders of magnitude between pH 13 and pH 7. At pH 13 the influence of the scan rate on the anodic polarization was studied. Reducing the scan rate by a factor of five one observes a slightly higher first anodic peak; during the further scan the current densities are always lower by about one order of magnitude. Fig. 8 exhibits a SEM cross section of the quasicrystal after potentiodynamic scan and potentiostatic polarization for 0.5 h at 360 mV in 0.1N NaOH. The quasicrystal is covered
Fig. 2. Surfaces after a salt spray test for 120 h at 35◦ C: (a) thin oxide layer on top of AlFe; (b) thick cracked oxide layer on i-Al63 Cu25 Fe12 .
892
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
Fig. 3. X-ray diffraction pattern of quasicrystalline i-Al63 Cu25 Fe12 before and after a salt spray test with 5% NaCl solution for 120 h at 35◦ C.
by a layered structure consisting of an inner nanocrystalline layer of FeO and Cu2 O followed by a layer showing higher charging in SEM which contains probably Fe2 O3 and CuO. The surface exhibits a needle-like morphology of 2-Al2 O3 . The thickness of the FeO/Cu2 O layer was found to be increased with a reduced scan rate which goes along with the reduced current density, but did not depend on the length of time at the final potential. There is, however, some evidence for a further transformation of these oxides towards higher oxidation states. The precipitation of a bcc phase in the quasicrystalline alloy underneath, mentioned earlier [4] as a result of selective corrosion of Al, could not be verified under the conditions used in this study. Fig. 5. Polarization curves for i-Al63 Cu25 Fe12 in acid solutions (H2 SO4 ).
Fig. 4. Polarization curves for i-Al63 Cu25 Fe12 in alkaline solutions (NaOH).
Fig. 6. SEM: porous Cu layer on i-Al63 Cu25 Fe12 after polarization at the free corrosion potential in 1N H2 SO4 (pH 0).
A. Rüdiger, U. Köster / Materials Science and Engineering 294–296 (2000) 890–893
893
structure. The surface of the icosahedral Al63 Cu25 Fe12 is covered by a thick cracked layer consisting mainly of Cu, Cu2 O and Al(OH)3 very similar to crystalline Al7 Cu2 Fe. The results from anodic polarization of quasicrystals in dependence of pH confirm this behavior. In alkaline solutions with decreasing pH the current density decreases and the free corrosion potential is shifted to more noble values; in acid solution with increasing pH the current density decreases, but the free corrosion potential does not change. Fig. 7. SEM: layer of Cu2 O and FeO on i-Al63 Cu25 Fe12 after polarization at the free corrosion potential in 0.1N NaOH (pH 13).
Acknowledgements This work was supported partially by the GIF (I-531046.10/97) and DFG (Ko 668/22). The authors are indebted to Dr. H. Alves for fruitful discussions. References
Fig. 8. SEM: layered structure after polarization scan and 0.5 h at 360 mV in 0.1N NaOH (pH 13).
4. Summary A salt spray test does not indicate any evidence for an improvement in corrosion resistance due to the quasicrystalline
[1] J.M. Dubois, Bulk and surface properties of quasicrystalline materials and their potential application, in: J.B. Suck, M. Schreiber, P. Haussler (Eds.), Quasicrystals: An introduction to structure, physical properties and application of quasicrystalline alloys, Springer, Berlin, in press. [2] R. Langer, H. Kaiser, H. Kaesche, Werkstoffe und Korrosion 29 (1978) 409. [3] Y. Massiani, S. Ait Yaazza, J.P. Crousier, J.M. Dubois, J. Non-Cryst. Solids 159 (1993) 92. [4] A. Rüdiger, U. Köster, J. Non-Cryst. Solids 898 (1999) 250–252. [5] H. Liebertz, Hochtemperaturverformung von Al-Cu-Fe Quasikristallen und kristallinen Phasen ähnlicher Zusammensetzung, Fortschrittsberichte VDI, Reihe 5: Grund und Werkstoffe, Nr. 471, VDI-Verlag, Düsseldorf, 1997.