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Corrosion of Al–Cu–Fe quasicrystals and related crystalline phases
Journal of Non-Crystalline Solids 250±252 (1999) 898±902
www.elsevier.com/locate/jnoncrysol
Corrosion of Al±Cu±Fe quasicrystals and related crystalline phases A. R udiger *, U. K oster Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany
Abstract Corrosion of quasicrystalline and related crystalline phases in the Al±Cu±Fe system was investigated by anodic polarization in acid and alkaline solutions at room temperature. Corrosion products as well as surface morphology were measured by means of X-ray diraction, scanning and transmission electron microscopy. During polarization of icosahedral quasicrystals in a strong alkaline solution a two-layered ®lm is formed: an inner layer with reduced Al content which consists of quasicrystals and a bcc phase and an nanocrystalline oxide layer with a needle-like morphology. At low pH (<2) dissolution of the quasicrystalline phases was observed to precede deposition of a porous Cu ®lm. In a strong alkaline medium icosahedral quasicrystals have a smaller corrosion resistance than neighboring crystalline phases, e.g. the b-phase (Al50 Cu30 Fe20 ); in strong acid solution other crystalline phases (for example Al2 Cu) show less corrosion. The open circuit potentials have been observed to vary in a similar manner. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Quasicrystals are already used as coatings due to their low coecient of friction, good wear and oxidation resistance [1]. For many applications corrosion resistance is also of importance. We are aware of the fact that corrosion of bulk material diers from the corrosion of plasma sprayed coatings. Usually corrosion depends on the chemical composition and microstructure. An in¯uence of structure on the corrosion is known for example for crystalline phases in Cu±Al alloys [2]. In an early investigation Massiani et al. [3] studied the electrochemical properties of icosahedral quasicrystals in the Al±Cu±Fe system in comparison with a number of related crystalline
*
Corresponding author. E-mail: [email protected]
phases, in particular Al7 Cu2 Fe: During anodic polarization at pH 13 the open circuit potential (OCP) was reported to be the same for icosahedral Al64 Cu24 Fe12 and crystalline Al7 Cu2 Fe; the current density as a measure of the corrosion resistance, however, was about one order of magnitude larger for the crystalline phase. At pH 2 the icosahedral phase had a only slightly nobler OCP, but also larger current density. As a result the authors concluded that the quasicrystalline state improves corrosion resistance because of particular compositions and not as consequence of the icosahedral structure. The aim of this paper is to contribute to the understanding of corrosion of bulk icosahedral Al64 Cu24 Fe12 . Results from anodic polarization will be combined with microstructural investigations of surface morphology and corrosion products. The comparison with related crystalline phases in the Al±Cu±Fe system should give addi-
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 2 0 1 - X
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
899
tional information on the in¯uence of the quasicrystalline structure on corrosion. 2. Experimental procedure The casting and preparation of icosahedral Al63 Cu25 Fe12 alloys and related crystalline phases in the Al±Cu±Fe system as well as their metallographic preparation were described elsewhere [4]. Common measuring equipment for polarization curves was used, where the reference electrode (Ag/AgCl in acid, Hg/HgO in alkali) is separated from the solution by a electrolyte bridge through a Haber±Luggin capillary. Samples with a freshly ground and polished surface (down to 1 lm diamond paste) were polarized in alkaline solutions (pH 13, pH 9) as well as in acidic solutions (pH 0, pH 5). The samples were kept at the free corrosion potential for 30 min and then anodic polarized at a scan rate of 0.2 mV/s. Other samples were held for 1 h at a potential of lower corrosion current density to build an oxide layer at the surface. For each material and condition the polarization curves are measured 2 or 3 times. The microstructure was studied by ¯at-on as well as cross sectional TEM (Philips CM200 operating at 200 kV), X-ray diraction as well as scanning electron microscopy (SEM), Hitachi S4500, operating between 1 and 30 kV. The low-kV operation allows the microscopical study of oxide scales without a conducting surface coating. 3. Results Figs. 1 and 2 show anodic polarization curves of dierent single phase materials in strong alkaline (0.1N NaOH, pH 13) and strong acid solution (1N H2 SO4 , pH 0) at room temperature. Depending on concentration and electrochemical conditions three typical surface morphologies were observed after polarization of quasicrystalline specimen at a given potential: · a clean surface after homogeneous dissolution in strong alkaline as well as strong acidic solutions at potentials close to the OCP;
Fig. 1. Anodic polarization curves in alkaline solutions (0.1N NaOH).
Fig. 2. Anodic polarization curves in acid solutions (1N H2 SO4 ).
· a porous Cu layer in acid solutions at potentials around 700 mVSHE ; · an oxide layer in alkaline solutions in a potential range around 400 mVSHE ; The passive ®lm formed on quasicrystals in strong alkaline solution after 1 h polarization at 360 mVSHE is shown in Fig. 3. The ®lm consists of a two-layered structure: an intermediate layer exhibits a two-phase microstructure (see Fig. 4) of quasicrystals with crystalline precipitates of a bccphase (a 0.308 nm). An analysis by EDX indicates a de®ciency of Al in this layer as compared to the matrix. This layer is covered by a surface layer with a large number of needles, which possess
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A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
Fig. 3. Two layered structure after polarization of quasicrystalline Al63 Cu25 Fe12 in a strong alkaline solution (SEM ± 1 h in 0.1N NaOH at 360 mV).
Fig. 5. Mixture of Al2 O3 needles and a Cu oxide after polarization of Al2 Cu in a strong alkaline solution (SEM ± 1 h in 0.1N NaOH at 480 mV).
Fig. 6. Porous Cu layer after polarization of quasicrystalline Al63 Cu25 Fe12 in a strong acid solution (SEM ± 1 h in 1N H2 SO4 at 700 mV).
4. Discussion Fig. 4. TEM diraction image of the intermediate layer formed after polarization of quasicrystals in a strong alkaline solution (1 h in 0.1N NaOH at 360 mV).
a textured nanocrystalline microstructure. The oxide ®lm formed under similar conditions on Al2 Cu (see Fig. 5) has also a two-layered structure, the surface layer contains a mixture of needles and ¯akes. Fig. 6 shows a porous Cu ®lm formed during polarization at a potential of 700 mVSHE .
The b-phases, AlFe and Al50 Cu30 Fe20 , possess the same bcc structure. Our results on anodic polarization in alkaline solution (Fig. 1) indicate that partial substitution of Fe by Cu leads to a more noble OCP but a greater dissolution rate; the passive current density increases by about one order of magnitude. Therefore, we conclude that Cu disturbs the formation of a passivating oxide layer. The icosahedral quasicrystalline phase possesses a larger Al content at the expense of Cu and Fe
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
as compared to Al50 Cu30 Fe20 . The polarization curve of the quasicrystalline phase has several peaks resulting from as yet unknown reactions, but has on OCP even less noble than AlFe and current densities of at least one order of magnitude larger than in Al50 Cu30 Fe20 . Al2 Cu also follows this trend. The shifts in the OCP might be due to the less noble property of the Al; the larger current density probably re¯ects the formation of a less passivating oxide layer. In acid solutions (Fig. 2) Al50 Cu30 Fe20 again shows a nobler OCP than AlFe due to the in¯uence of the noble component, Cu, and ± similar to the behavior in alkaline solution ± a larger passive current density over a large potential range. In the icosahedral as well as Al2 Cu phase we recognized an in¯uence of Cu on the OCP as well. The icosahedral alloy shows a less noble potential close to that observed for AlFe, Al2 Cu, however, has a potential similar to that of the b-phase Al50 Cu30 Fe20 . These shifts can be understood at least qualitatively due to the nobler potentials with increasing Cu or decreasing Al content. At intermediate pHs the dierences in the corrosion of the phases studied is less pronounced; the current densities, however, are usually about one order of magnitude less than in strong electrolytes. The observed corrosion of the quasicrystalline alloy is consistent with the change of properties of related crystalline phases due to composition. From these experiments and in comparison with Massiani's results [3] it is not known whether the observed polarization curves of the quasicrystalline phase can be explained by composition or if the icosahedral structure has an additional in¯uence (for example due to the formation of a less protective oxide layer or due to a dierent diusivity in the icosahedral phase which might in¯uence the selective oxidation). The layered microstructure after polarization on the quasicrystalline surface probably results from selective oxidation of Al leading to an Al depleted zone, i.e. a destabilized icosahedral structure, which undergoes precipitation of a bcc phase. From the lattice parameter and a de®ciency in Al we assume that the precipitates are b-phase. The
901
formation of the b-phase might also be an eect of ion-beam milling as described by Shen et al. [5]. An explanation for the formation of the needles at the surface is that this scale could be a `precipitated scale', formed by a two step mechanism: dissolution of Al into a hydroxide followed by precipitation of oxide crystals (e.g. Al2 O3 á xH2 O or NaAlO2 ) when the solubility limit is reached. The formation of a porous Cu layer on quasicrystals in acid solution (Fig. 6) can proceed either by redeposition of Cu or by selective corrosion of Al and Fe combined with volume diffusion of Cu. In our case, the mechanism has not been determined. The observed morphology, however, indicates homogeneous dissolution of thequasicrystalline material followed by redeposition of small facetted Cu crystals. The observed color of the surface is in accordance with these microscopical results. Similar microscopical observation on related crystalline phases are under way. Such investigations should allow a more de®nitive answer on the question of the in¯uence of the icosahedral structure on the corrosion behaviour. 5. Summary The corrosion of quasicrystals and related crystalline phases in the Al±Cu±Fe system was studied by means of anodic polarization and can be understood qualitatively from the electrochemical property of the components. Of particular interest is the two layered structure formed on quasicrystals in strong alkaline solution in a potential range around 400 mV, indicating a selective corrosion process, which leads to a phase transformation of the quasicrystalline alloy. Acknowledgements The authors are indebted to Dr H. Alves for fruitful discussion. This work was supported by the Deutsche Forschungsgemeinschaft DFG (Ko 668/22-1).
902
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
References [1] J.-M. Dubois, Bulk and surface properties of quasicrystalline materials and their potential application, in: Introduction to Quasicrystals, Springer, Berlin, in press. [2] R. Langer, H. Kaiser, H. Kaesche, Werkstoe 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] H. Liebertz, Hochtemperaturverforming von Al±Cu±Fe Quasikristallen und kristallinen Phasen ahnlicher Zusammensetzung (Ph.D. Dortmund, 1996) Fortschrittsberichte VDI, Reihe 5: Grundund Werkstoe, Nr. 471, VDI-Verlag D usseldorf, 1997. [5] Z. Shen, M.J. Kramer, C.J. Jenks, A.I. Goldman, T. Lograsso, D. Delaney, M. Heinzig, W. Raberg, P.A. Thiel, Phys. Rev. B 58 (1998) 9961.
www.elsevier.com/locate/jnoncrysol
Corrosion of Al±Cu±Fe quasicrystals and related crystalline phases A. R udiger *, U. K oster Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany
Abstract Corrosion of quasicrystalline and related crystalline phases in the Al±Cu±Fe system was investigated by anodic polarization in acid and alkaline solutions at room temperature. Corrosion products as well as surface morphology were measured by means of X-ray diraction, scanning and transmission electron microscopy. During polarization of icosahedral quasicrystals in a strong alkaline solution a two-layered ®lm is formed: an inner layer with reduced Al content which consists of quasicrystals and a bcc phase and an nanocrystalline oxide layer with a needle-like morphology. At low pH (<2) dissolution of the quasicrystalline phases was observed to precede deposition of a porous Cu ®lm. In a strong alkaline medium icosahedral quasicrystals have a smaller corrosion resistance than neighboring crystalline phases, e.g. the b-phase (Al50 Cu30 Fe20 ); in strong acid solution other crystalline phases (for example Al2 Cu) show less corrosion. The open circuit potentials have been observed to vary in a similar manner. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Quasicrystals are already used as coatings due to their low coecient of friction, good wear and oxidation resistance [1]. For many applications corrosion resistance is also of importance. We are aware of the fact that corrosion of bulk material diers from the corrosion of plasma sprayed coatings. Usually corrosion depends on the chemical composition and microstructure. An in¯uence of structure on the corrosion is known for example for crystalline phases in Cu±Al alloys [2]. In an early investigation Massiani et al. [3] studied the electrochemical properties of icosahedral quasicrystals in the Al±Cu±Fe system in comparison with a number of related crystalline
*
Corresponding author. E-mail: [email protected]
phases, in particular Al7 Cu2 Fe: During anodic polarization at pH 13 the open circuit potential (OCP) was reported to be the same for icosahedral Al64 Cu24 Fe12 and crystalline Al7 Cu2 Fe; the current density as a measure of the corrosion resistance, however, was about one order of magnitude larger for the crystalline phase. At pH 2 the icosahedral phase had a only slightly nobler OCP, but also larger current density. As a result the authors concluded that the quasicrystalline state improves corrosion resistance because of particular compositions and not as consequence of the icosahedral structure. The aim of this paper is to contribute to the understanding of corrosion of bulk icosahedral Al64 Cu24 Fe12 . Results from anodic polarization will be combined with microstructural investigations of surface morphology and corrosion products. The comparison with related crystalline phases in the Al±Cu±Fe system should give addi-
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 2 0 1 - X
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
899
tional information on the in¯uence of the quasicrystalline structure on corrosion. 2. Experimental procedure The casting and preparation of icosahedral Al63 Cu25 Fe12 alloys and related crystalline phases in the Al±Cu±Fe system as well as their metallographic preparation were described elsewhere [4]. Common measuring equipment for polarization curves was used, where the reference electrode (Ag/AgCl in acid, Hg/HgO in alkali) is separated from the solution by a electrolyte bridge through a Haber±Luggin capillary. Samples with a freshly ground and polished surface (down to 1 lm diamond paste) were polarized in alkaline solutions (pH 13, pH 9) as well as in acidic solutions (pH 0, pH 5). The samples were kept at the free corrosion potential for 30 min and then anodic polarized at a scan rate of 0.2 mV/s. Other samples were held for 1 h at a potential of lower corrosion current density to build an oxide layer at the surface. For each material and condition the polarization curves are measured 2 or 3 times. The microstructure was studied by ¯at-on as well as cross sectional TEM (Philips CM200 operating at 200 kV), X-ray diraction as well as scanning electron microscopy (SEM), Hitachi S4500, operating between 1 and 30 kV. The low-kV operation allows the microscopical study of oxide scales without a conducting surface coating. 3. Results Figs. 1 and 2 show anodic polarization curves of dierent single phase materials in strong alkaline (0.1N NaOH, pH 13) and strong acid solution (1N H2 SO4 , pH 0) at room temperature. Depending on concentration and electrochemical conditions three typical surface morphologies were observed after polarization of quasicrystalline specimen at a given potential: · a clean surface after homogeneous dissolution in strong alkaline as well as strong acidic solutions at potentials close to the OCP;
Fig. 1. Anodic polarization curves in alkaline solutions (0.1N NaOH).
Fig. 2. Anodic polarization curves in acid solutions (1N H2 SO4 ).
· a porous Cu layer in acid solutions at potentials around 700 mVSHE ; · an oxide layer in alkaline solutions in a potential range around 400 mVSHE ; The passive ®lm formed on quasicrystals in strong alkaline solution after 1 h polarization at 360 mVSHE is shown in Fig. 3. The ®lm consists of a two-layered structure: an intermediate layer exhibits a two-phase microstructure (see Fig. 4) of quasicrystals with crystalline precipitates of a bccphase (a 0.308 nm). An analysis by EDX indicates a de®ciency of Al in this layer as compared to the matrix. This layer is covered by a surface layer with a large number of needles, which possess
900
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
Fig. 3. Two layered structure after polarization of quasicrystalline Al63 Cu25 Fe12 in a strong alkaline solution (SEM ± 1 h in 0.1N NaOH at 360 mV).
Fig. 5. Mixture of Al2 O3 needles and a Cu oxide after polarization of Al2 Cu in a strong alkaline solution (SEM ± 1 h in 0.1N NaOH at 480 mV).
Fig. 6. Porous Cu layer after polarization of quasicrystalline Al63 Cu25 Fe12 in a strong acid solution (SEM ± 1 h in 1N H2 SO4 at 700 mV).
4. Discussion Fig. 4. TEM diraction image of the intermediate layer formed after polarization of quasicrystals in a strong alkaline solution (1 h in 0.1N NaOH at 360 mV).
a textured nanocrystalline microstructure. The oxide ®lm formed under similar conditions on Al2 Cu (see Fig. 5) has also a two-layered structure, the surface layer contains a mixture of needles and ¯akes. Fig. 6 shows a porous Cu ®lm formed during polarization at a potential of 700 mVSHE .
The b-phases, AlFe and Al50 Cu30 Fe20 , possess the same bcc structure. Our results on anodic polarization in alkaline solution (Fig. 1) indicate that partial substitution of Fe by Cu leads to a more noble OCP but a greater dissolution rate; the passive current density increases by about one order of magnitude. Therefore, we conclude that Cu disturbs the formation of a passivating oxide layer. The icosahedral quasicrystalline phase possesses a larger Al content at the expense of Cu and Fe
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
as compared to Al50 Cu30 Fe20 . The polarization curve of the quasicrystalline phase has several peaks resulting from as yet unknown reactions, but has on OCP even less noble than AlFe and current densities of at least one order of magnitude larger than in Al50 Cu30 Fe20 . Al2 Cu also follows this trend. The shifts in the OCP might be due to the less noble property of the Al; the larger current density probably re¯ects the formation of a less passivating oxide layer. In acid solutions (Fig. 2) Al50 Cu30 Fe20 again shows a nobler OCP than AlFe due to the in¯uence of the noble component, Cu, and ± similar to the behavior in alkaline solution ± a larger passive current density over a large potential range. In the icosahedral as well as Al2 Cu phase we recognized an in¯uence of Cu on the OCP as well. The icosahedral alloy shows a less noble potential close to that observed for AlFe, Al2 Cu, however, has a potential similar to that of the b-phase Al50 Cu30 Fe20 . These shifts can be understood at least qualitatively due to the nobler potentials with increasing Cu or decreasing Al content. At intermediate pHs the dierences in the corrosion of the phases studied is less pronounced; the current densities, however, are usually about one order of magnitude less than in strong electrolytes. The observed corrosion of the quasicrystalline alloy is consistent with the change of properties of related crystalline phases due to composition. From these experiments and in comparison with Massiani's results [3] it is not known whether the observed polarization curves of the quasicrystalline phase can be explained by composition or if the icosahedral structure has an additional in¯uence (for example due to the formation of a less protective oxide layer or due to a dierent diusivity in the icosahedral phase which might in¯uence the selective oxidation). The layered microstructure after polarization on the quasicrystalline surface probably results from selective oxidation of Al leading to an Al depleted zone, i.e. a destabilized icosahedral structure, which undergoes precipitation of a bcc phase. From the lattice parameter and a de®ciency in Al we assume that the precipitates are b-phase. The
901
formation of the b-phase might also be an eect of ion-beam milling as described by Shen et al. [5]. An explanation for the formation of the needles at the surface is that this scale could be a `precipitated scale', formed by a two step mechanism: dissolution of Al into a hydroxide followed by precipitation of oxide crystals (e.g. Al2 O3 á xH2 O or NaAlO2 ) when the solubility limit is reached. The formation of a porous Cu layer on quasicrystals in acid solution (Fig. 6) can proceed either by redeposition of Cu or by selective corrosion of Al and Fe combined with volume diffusion of Cu. In our case, the mechanism has not been determined. The observed morphology, however, indicates homogeneous dissolution of thequasicrystalline material followed by redeposition of small facetted Cu crystals. The observed color of the surface is in accordance with these microscopical results. Similar microscopical observation on related crystalline phases are under way. Such investigations should allow a more de®nitive answer on the question of the in¯uence of the icosahedral structure on the corrosion behaviour. 5. Summary The corrosion of quasicrystals and related crystalline phases in the Al±Cu±Fe system was studied by means of anodic polarization and can be understood qualitatively from the electrochemical property of the components. Of particular interest is the two layered structure formed on quasicrystals in strong alkaline solution in a potential range around 400 mV, indicating a selective corrosion process, which leads to a phase transformation of the quasicrystalline alloy. Acknowledgements The authors are indebted to Dr H. Alves for fruitful discussion. This work was supported by the Deutsche Forschungsgemeinschaft DFG (Ko 668/22-1).
902
A. R udiger, U. K oster / Journal of Non-Crystalline Solids 250±252 (1999) 898±902
References [1] J.-M. Dubois, Bulk and surface properties of quasicrystalline materials and their potential application, in: Introduction to Quasicrystals, Springer, Berlin, in press. [2] R. Langer, H. Kaiser, H. Kaesche, Werkstoe 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] H. Liebertz, Hochtemperaturverforming von Al±Cu±Fe Quasikristallen und kristallinen Phasen ahnlicher Zusammensetzung (Ph.D. Dortmund, 1996) Fortschrittsberichte VDI, Reihe 5: Grundund Werkstoe, Nr. 471, VDI-Verlag D usseldorf, 1997. [5] Z. Shen, M.J. Kramer, C.J. Jenks, A.I. Goldman, T. Lograsso, D. Delaney, M. Heinzig, W. Raberg, P.A. Thiel, Phys. Rev. B 58 (1998) 9961.