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Electroluminescence of aluminium during electropolishing in nitric acid
Electrochemistry Communications 2 Ž2000. 591–594 www.elsevier.nlrlocaterelecom
Electroluminescence of aluminium during electropolishing in nitric acid L.C. Marsland, G.T. Burstein ) Department of Materials Science and Metallurgy, UniÕersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 12 May 2000; accepted 22 May 2000
Abstract Electropolishing of aluminium in 10 mol dmy3 aqueous nitric acid shows the emission of electroluminescence. The very high anodic current densities involved during electropolishing preclude the presence of all but the thinnest of surface oxide films, and this raises the question of the origins of the luminescence. The electroluminescence during electropolishing is compared with that occurring during anodising in phosphoric acid. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Electropolishing; Electroluminescence; Aluminium; Nitric acid
1. Introduction Electroluminescence is displayed during electrolytic oxidation of metals which produce electronic semiconductors as their oxides w1x. Aluminium shows particularly strong electroluminescence, a phenomenon which has been observed by many authors w1–3x. It is hitherto believed that electroluminescence from reacting electrode surfaces arises from transitions in the surface oxide films w3x. In this paper we demonstrate that aluminium emits electroluminescence during electropolishing in nitric acid, a process generally believed to occur in the absence of surface oxide w4x, implying that the oxide film is not necessary for electroluminescent emission from aluminium. Electropolishing is characterised by smoothing of the surface of a metal to a mirror-finish by making it anodic in an appropriate electrolyte solution w5x. The process is used industrially to polish many metals using a variety of electrolytes w6–8x. Normally, the electrolytes are highly concentrated acidic solutions of low water activity w8x. The role of the small water component in these electrolytes is not known. We have shown, that 10 mol dmy3 nitric acid induces electropolishing of samples of aluminium less than 8 mm in diameter w9x. Electropolishing occurs at a high rate with high current densities. It is characterised electrochemically by a current density approximately independent
) Corresponding author. Tel.: q44-1223-334361; fax: q44-1223334567. E-mail address: [email protected] ŽG.T. Burstein..
of potential, essentially resembling the passive region in a polarisation curve, but reacting anodically at a very high current density w7x. It is thought to occur through a ‘Jacquet layer’ which contains a viscous, saturated solution of metal salt w3,6x; in the case of aluminium in nitric acid, this must be aluminium nitrate. The roughness of the surface is smoothed away to a mirror-like finish by appropriate variations of the concentration gradients established by the Jacquet layer and the potential gradients over the surface. We show below that aluminium emits electroluminescence during electropolishing in nitric acid solution.
2. Experimental Aluminium of 99.998% purity ŽAlfa. was used in the form of 6 mm diameter rod. Short lengths of the rod were mounted in polymethylmethacrylate electrode holders using epoxy resin ŽAraldite. so that only a circular end-surface was exposed, thereby forming a disc electrode. Electrical contact was made to the rear surface of the rod. The exposed surface was prepared for each experiment by grinding to 1200 grit finish. The electrode surface was rinsed in twice-distilled water and cleaned ultrasonically prior to the use. Potentiodynamic sweeps between 0 and 10 VŽSCE. were carried out at a rate of 50 mV sy1 in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode. The potential was imposed via a Solartron 1286 potentiostat. The current and
1388-2481r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 Ž 0 0 . 0 0 0 8 4 - 9
592
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
potential were recorded at the rate of 5 Hz by a computercontrolled digital transient recorder ŽIotech, ADC488. from the potentiostat output. The reference electrode was external to the main cell, connected by means of a tube of length 10 cm, filled with the working electrolyte, and fitted with a Luggin capillary probe tip. The aluminium working electrode was mounted into the cell from the side, so that its axis was horizontal. Directly opposite this was a quartz optic window, approximately 4 cm from the electrode surface. A photomultiplier tube ŽHamamatsu type H570150. was placed against the outside of the optic window. The PM tube was connected to an HP 7150 digital voltmeter and the output recorded on a computer. This assembly was used to record the light emitted from the electrode surface as a function of time. The photomultiplier tube gave a voltage signal proportional to the light intensity. The light intensity was not calibrated in terms of luminous intensity, but is presented below in arbitrary units Ža.u..; these arbitrary units are in fact the voltage signal from the photomultiplier tube expressed in millivolts when a control voltage of 1.200 V was applied to the instrument. The lowest intensity which could be recorded accurately above the background noise was 0.05 a.u. Ž50 mV output from the digital voltmeter.. Optical data were acquired at a rate of 2 Hz and smoothed to reduce the background noise. The light-measurement equipment did not permit wavelength resolution. The experimental cell and optical components were enclosed within a blackened box with light-tight seals. The background light in the box was measured by running a preliminary test of the photomultiplier tube while no electrochemical experiment was running. The smoothed background value was subtracted from the optical data. It was noted that to operate at the required sensitivity, the photomultiplier tube needed to acclimatise to the dark conditions. To accommodate this, a period of at least 5000 seconds was allowed between sealing the light-tight box and starting the experiments. The electrolytes used in these experiments were made from analytical grade reagents ŽMerck. prepared to the required concentrations with twice-distilled water. For the electropolishing solutions 10 mol dmy3 aqueous nitric acid was used. Comparison was also made with a solution in which the metal anodises to produce an oxide film; for this purpose aqueous phosphoric acid of concentration 1 mol dmy3 was used.
3. Results and discussion The current density and electroluminescence from the surface of the aluminium electrode during electropolishing in 10 M nitric acid are shown in Figs. 1Ža. and Žb. respectively. Fig. 1Ža. shows that the surface is initially ‘pseudopassive’ with current densities around 11 A my2 up to a potential of 1.3 VŽSCE.. The surface is termed
Fig. 1. Ža. Anodic polarisation curve for aluminium in 10 M nitric acid measured at a potential sweep rate of 50 mV sy1 . Žb. Electroluminescence of aluminium in 10 M nitric acid during polarisation as described in Ža..
‘pseudopassive’ because it displays all the qualitative electrochemical characteristics of a passive surface, but the current density is too high for the surface to be regarded as passive w9x. The current density then rises strongly to 4 kA my2 , reaching a short plateau before rising further to a peak at 2 VŽSCE.. The peak current density is very large at 10 kA my2 . It then falls to an approximate plateau of 2.5 to 3 kA my2 through the extensive potential range 3 to 10 VŽSCE.. The resulting electrode surface was polished. This is electropolishing as defined by Hoar w4x. The form of Fig. 1Ža. is that ascribed classically to electropolishing, as illustrated by Gabe w7x. We believe that the current peak with a maximum at 2 VŽSCE. is due to the formation of the Jacquet layer by precipitation of the salt ŽAlŽNO 3 . 3 .. The current density above 2.5 VŽSCE. is approximately independent of potential, but it is very high, characteristic of electropolishing. Because the anodic current density is so high, it is improbable that it could be controlled by the presence of an oxide film, or even that it passes through an oxide film. The current must instead be controlled in this region by mass transport of dissolving Al 3q ions away from the electrode surface through the salt-saturated Jacquet layer, as is generally regarded to occur in electropolishing w4,10x. Of course, we cannot preclude the existence of a
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
very thin oxide film, perhaps of monolayer dimensions, for which conventional field-assisted ion transport kinetics, such as the classical kinetics of Verwey w11x and Cabrera and Mott w12x would not be easily applicable. However, this thin film cannot control such a high current density; neither could it be of significant thickness, since it otherwise would indeed control the current density. From Fig. 1Žb., it can be seen that electroluminescence shows similar trends to the current. Electroluminscent emissions are observed at all potentials at which electropolishing occurs. When the current is low Žbelow 1.4 VŽSCE., for which electropolishing does not occur. the electroluminescence is very low, such that measurement is difficult within the background noise. Electroluminescence then follows a curve similar to that of the current density with a double peak reaching a maximum of 1 a.u. at 1.7 VŽSCE. where the current shows a plateau followed by a second peak at 2.2 VŽSCE.. Above 2.2 VŽSCE. the electroluminescence falls to an approximate plateau in a manner similar to the current density. The plateau value of the electroluminescence, for potentials greater than ca. 3 VŽSCE., is low, at 0.14 a.u.; it is however, well above the background level, and above the value recorded at low potentials Ž- 1.4 VŽSCE.. The emission is clearly and unambiguously associated with electropolishing. The electroluminescence is related to the anodic current density during electropolishing: the intensity is high when the current density is high and it is low when the current density is low, although there must also be other factors affecting the electroluminescence intensity. The significant noise on the electroluminescence signal is due to the fairly weak overall intensity of the signal. The results are compared with polarisation and associated electroluminescence during anodising of aluminium in phosphoric acid, shown respectively in Figs. 2Ža. and Žb.. In this solution the metal is covered with a thickening oxide film whose structure consists of a porous layer overlying a dense barrier layer w13x. The polarisation curve, prepared at the same sweep rate as that of Fig. 1Ža., shows features associated with anodising. The current rises to an approximate plateau value of ca. 15 A my2 , and the potential dependence is small. This current density is in fact similar in value to the pre-polishing current in nitric acid Ž11 A my2 . measured at potentials less than ca. 1.4 VŽSCE. Žsee Fig. 1.. This confirms that the current density in the pre-polishing regime of Fig. 1Ža. Ži.e. at potentials - 1.4 VŽSCE.. is indeed controlled by the presence of an oxide film, in exactly the same way as that observed throughout the anodic potential range in phosphoric acid. Note that the similarity in these current densities from the filmed surface in the two acids cannot be ascribed to conductivity of the electrolytes, since the conductivities are quite different Ž0.655 S cmy1 for 10 M HNO 3 and 0.055 S cmy1 for 1 M H 3 PO4 .. Electroluminescence was difficult to detect at potentials less than ca. 2 VŽSCE. during anodisation in phosphoric
593
Fig. 2. Ža. Anodic polarisation curve for aluminium in 1 M phosphoric acid measured at a potential sweep rate of 50 mV sy1 . Žb. Electroluminescence of aluminium in 1 M phosphoric acid during polarisation as described in Ža..
acid. However, as the potential rose to values higher than 2 VŽSCE. electroluminescent emission was detected as shown in Fig. 2Žb.. The highest intensity detected at 10 VŽSCE. Žthe maximum potential used. was ca. 2 a.u., and comparable with the peak value observed in Fig. 1Žb.. However, unlike the data described for electropolishing in nitric acid, the luminescence in Fig. 2Žb. does not appear to relate to the anodising current. We believe that electroluminescence occurs throughout the anodising range, even below 2 VŽSCE., but its magnitude is too low to detect at the lower potentials. It therefore also seems likely that the pre-polishing regime in nitric acid, where the reaction rate of the metal is controlled by the presence of the oxide film Ž- 1.4 VŽSCE.., also emits light, but the intensity is too small to detect with the present experimental procedure. We have thus shown that electroluminescence occurs from aluminium both under conditions of anodic electropolishing and of anodic oxide film growth. From the description by Ikonopisov w2x it is clear that the electroluminescence observed in Fig. 2b is similar to that observed by other authors, namely it is produced by light-emitting reactions occurring at the metalroxide interface. Van Geel w1x proposes that luminescence occurs through electron transitions in the oxide film. Both of these notions imply the necessity for the presence of the oxide film. However
594
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
we have observed electroluminescence from the electropolishing surface, with peaks in the electroluminescence clearly resulting from the formation of the Jacquet layer. We therefore propose that the surface oxide film is not a requirement for electroluminescence to occur; electroluminescence arises also from the actively dissolving metal. Some have proposed that an oxide is indeed present during electropolishing. For example, Tajira et al. w14x observed that a dense, chemically stable oxide layer of approximately 200 nm thickness is formed during electropolishing. Baizuldin w15x found porous films after electropolishing and stripping. Even if these layers did exist whilst electropolishing, their presence cannot control the current during electropolishing, since the current density is far too high. Thus one cannot expect electron current through any possible oxide during electropolishing; neither can electron transfer occur at the metalroxide interface. The electrochemical reaction would necessarily occur from those regions not covered by the oxide film, since these are the paths of least resistance to current flow. The conclusion must inexorably be reached that the actively dissolving metal surface emits electroluminescence, in the absence of the oxide film. It is conceivable that a monolayer of oxide is formed on the metal surface during electropolishing, a process proposed for active dissolution of other metals. Even if this were the case, the film would not be thick enough to support electroluminescence in the conventional sense as described by Ikonopisov w2x or van Geel w1x. Whether the saturated salt layer, the Jacquet layer w4,10x, present on the surface during electropolishing can itself give rise to electroluminescence is not yet known. Even if this layer were capable of supporting electroluminescence, the peaks in the electroluminescence intensity due to actual formation of the Jacquet layer ŽFig. 1b at 1.7 and 2.2 VŽSCE.. would still remain unexplained.
4. Conclusions Electropolishing aluminium in moderately concentrated nitric acid solutions causes electroluminescence. The pro-
cess cannot be ascribed to any mechanism of light emission occurring within an oxide film. We ascribe the process tentatively to the oxidation reactions occurring at the metalrelectrolyte interface.
Acknowledgements We are grateful to Alcan International Ltd for the award of a research grant without which this research could not have been carried out. We are also grateful to Dr. J. Ball and Dr. J.D.B. Sharman for many stimulating and fruitful discussions.
References w1x W.C. van Geel, C.A. Pistorius, B.C. Bouma, Philips Research Reports 12 Ž1957. 465. w2x S. Ikonopisov, Electrochim. Acta 20 Ž1975. 783. w3x E.A. Meulenkamp, J.J. Kelly, G. Blasse, J. Electrochem. Soc. 140 Ž1993. 84. w4x T.P. Hoar, in: J.O’M. Bockris ŽEd.., Modern Aspects of Electrochemistry, Vol. 2, Butterworths Scientific Publications, London, 1959, p. 262. w5x Annual book of ASTM standards, ASTM, Philidelphia, 1979, p. 173. w6x P.A. Jacquet, Metallurgical Reviews 1 Ž1956. 157. w7x D.R. Gabe, in: L.L. Shrier, R.A. Jarman, G.T. Burstein ŽEds.. Corrosion, Butterworth-Heinemann, Oxford, 1994, p. 11.28. w8x E.A. Brandes, Smithells Metals Reference Book, 6th ed., Butterworth-Heinemann, Oxford, 1983, p. 10-10. w9x G.T. Burstein, V.C. Salter, J. Ball, J.D. Sharman, in: M. Datta, B.R. MacDougall, J.M. Fenton ŽEds.., High Rate Metal Dissolution Processes, Vol. 95-19, Electrochemical Society Inc., Princeton, NJ, 1995, p. 131. w10x D. Landolt, Electrochim. Acta 32 Ž1987. 1. w11x E.L.W. Verwey, Physica 2 Ž1935. 1059. w12x N. Cabrera, N.F. Mott, Rept. Prog. Phys. 12 Ž1949. 163. w13x G.C. Wood, in: J.W. Diggle ŽEd.., Oxides and Oxide Films, Marcel Dekker Inc., New York, 1973, p. 402. w14x K. Tajiri, Y. Saito, Y. Yamanaka, Z. Kabeya, J. Vac. Sci. Technol. A 16 Ž1998. 1196. w15x B.M. Baizuldin, Metal Finishing, 91.12 Ž1993. 27.
Electroluminescence of aluminium during electropolishing in nitric acid L.C. Marsland, G.T. Burstein ) Department of Materials Science and Metallurgy, UniÕersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 12 May 2000; accepted 22 May 2000
Abstract Electropolishing of aluminium in 10 mol dmy3 aqueous nitric acid shows the emission of electroluminescence. The very high anodic current densities involved during electropolishing preclude the presence of all but the thinnest of surface oxide films, and this raises the question of the origins of the luminescence. The electroluminescence during electropolishing is compared with that occurring during anodising in phosphoric acid. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Electropolishing; Electroluminescence; Aluminium; Nitric acid
1. Introduction Electroluminescence is displayed during electrolytic oxidation of metals which produce electronic semiconductors as their oxides w1x. Aluminium shows particularly strong electroluminescence, a phenomenon which has been observed by many authors w1–3x. It is hitherto believed that electroluminescence from reacting electrode surfaces arises from transitions in the surface oxide films w3x. In this paper we demonstrate that aluminium emits electroluminescence during electropolishing in nitric acid, a process generally believed to occur in the absence of surface oxide w4x, implying that the oxide film is not necessary for electroluminescent emission from aluminium. Electropolishing is characterised by smoothing of the surface of a metal to a mirror-finish by making it anodic in an appropriate electrolyte solution w5x. The process is used industrially to polish many metals using a variety of electrolytes w6–8x. Normally, the electrolytes are highly concentrated acidic solutions of low water activity w8x. The role of the small water component in these electrolytes is not known. We have shown, that 10 mol dmy3 nitric acid induces electropolishing of samples of aluminium less than 8 mm in diameter w9x. Electropolishing occurs at a high rate with high current densities. It is characterised electrochemically by a current density approximately independent
) Corresponding author. Tel.: q44-1223-334361; fax: q44-1223334567. E-mail address: [email protected] ŽG.T. Burstein..
of potential, essentially resembling the passive region in a polarisation curve, but reacting anodically at a very high current density w7x. It is thought to occur through a ‘Jacquet layer’ which contains a viscous, saturated solution of metal salt w3,6x; in the case of aluminium in nitric acid, this must be aluminium nitrate. The roughness of the surface is smoothed away to a mirror-like finish by appropriate variations of the concentration gradients established by the Jacquet layer and the potential gradients over the surface. We show below that aluminium emits electroluminescence during electropolishing in nitric acid solution.
2. Experimental Aluminium of 99.998% purity ŽAlfa. was used in the form of 6 mm diameter rod. Short lengths of the rod were mounted in polymethylmethacrylate electrode holders using epoxy resin ŽAraldite. so that only a circular end-surface was exposed, thereby forming a disc electrode. Electrical contact was made to the rear surface of the rod. The exposed surface was prepared for each experiment by grinding to 1200 grit finish. The electrode surface was rinsed in twice-distilled water and cleaned ultrasonically prior to the use. Potentiodynamic sweeps between 0 and 10 VŽSCE. were carried out at a rate of 50 mV sy1 in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode. The potential was imposed via a Solartron 1286 potentiostat. The current and
1388-2481r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 Ž 0 0 . 0 0 0 8 4 - 9
592
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
potential were recorded at the rate of 5 Hz by a computercontrolled digital transient recorder ŽIotech, ADC488. from the potentiostat output. The reference electrode was external to the main cell, connected by means of a tube of length 10 cm, filled with the working electrolyte, and fitted with a Luggin capillary probe tip. The aluminium working electrode was mounted into the cell from the side, so that its axis was horizontal. Directly opposite this was a quartz optic window, approximately 4 cm from the electrode surface. A photomultiplier tube ŽHamamatsu type H570150. was placed against the outside of the optic window. The PM tube was connected to an HP 7150 digital voltmeter and the output recorded on a computer. This assembly was used to record the light emitted from the electrode surface as a function of time. The photomultiplier tube gave a voltage signal proportional to the light intensity. The light intensity was not calibrated in terms of luminous intensity, but is presented below in arbitrary units Ža.u..; these arbitrary units are in fact the voltage signal from the photomultiplier tube expressed in millivolts when a control voltage of 1.200 V was applied to the instrument. The lowest intensity which could be recorded accurately above the background noise was 0.05 a.u. Ž50 mV output from the digital voltmeter.. Optical data were acquired at a rate of 2 Hz and smoothed to reduce the background noise. The light-measurement equipment did not permit wavelength resolution. The experimental cell and optical components were enclosed within a blackened box with light-tight seals. The background light in the box was measured by running a preliminary test of the photomultiplier tube while no electrochemical experiment was running. The smoothed background value was subtracted from the optical data. It was noted that to operate at the required sensitivity, the photomultiplier tube needed to acclimatise to the dark conditions. To accommodate this, a period of at least 5000 seconds was allowed between sealing the light-tight box and starting the experiments. The electrolytes used in these experiments were made from analytical grade reagents ŽMerck. prepared to the required concentrations with twice-distilled water. For the electropolishing solutions 10 mol dmy3 aqueous nitric acid was used. Comparison was also made with a solution in which the metal anodises to produce an oxide film; for this purpose aqueous phosphoric acid of concentration 1 mol dmy3 was used.
3. Results and discussion The current density and electroluminescence from the surface of the aluminium electrode during electropolishing in 10 M nitric acid are shown in Figs. 1Ža. and Žb. respectively. Fig. 1Ža. shows that the surface is initially ‘pseudopassive’ with current densities around 11 A my2 up to a potential of 1.3 VŽSCE.. The surface is termed
Fig. 1. Ža. Anodic polarisation curve for aluminium in 10 M nitric acid measured at a potential sweep rate of 50 mV sy1 . Žb. Electroluminescence of aluminium in 10 M nitric acid during polarisation as described in Ža..
‘pseudopassive’ because it displays all the qualitative electrochemical characteristics of a passive surface, but the current density is too high for the surface to be regarded as passive w9x. The current density then rises strongly to 4 kA my2 , reaching a short plateau before rising further to a peak at 2 VŽSCE.. The peak current density is very large at 10 kA my2 . It then falls to an approximate plateau of 2.5 to 3 kA my2 through the extensive potential range 3 to 10 VŽSCE.. The resulting electrode surface was polished. This is electropolishing as defined by Hoar w4x. The form of Fig. 1Ža. is that ascribed classically to electropolishing, as illustrated by Gabe w7x. We believe that the current peak with a maximum at 2 VŽSCE. is due to the formation of the Jacquet layer by precipitation of the salt ŽAlŽNO 3 . 3 .. The current density above 2.5 VŽSCE. is approximately independent of potential, but it is very high, characteristic of electropolishing. Because the anodic current density is so high, it is improbable that it could be controlled by the presence of an oxide film, or even that it passes through an oxide film. The current must instead be controlled in this region by mass transport of dissolving Al 3q ions away from the electrode surface through the salt-saturated Jacquet layer, as is generally regarded to occur in electropolishing w4,10x. Of course, we cannot preclude the existence of a
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
very thin oxide film, perhaps of monolayer dimensions, for which conventional field-assisted ion transport kinetics, such as the classical kinetics of Verwey w11x and Cabrera and Mott w12x would not be easily applicable. However, this thin film cannot control such a high current density; neither could it be of significant thickness, since it otherwise would indeed control the current density. From Fig. 1Žb., it can be seen that electroluminescence shows similar trends to the current. Electroluminscent emissions are observed at all potentials at which electropolishing occurs. When the current is low Žbelow 1.4 VŽSCE., for which electropolishing does not occur. the electroluminescence is very low, such that measurement is difficult within the background noise. Electroluminescence then follows a curve similar to that of the current density with a double peak reaching a maximum of 1 a.u. at 1.7 VŽSCE. where the current shows a plateau followed by a second peak at 2.2 VŽSCE.. Above 2.2 VŽSCE. the electroluminescence falls to an approximate plateau in a manner similar to the current density. The plateau value of the electroluminescence, for potentials greater than ca. 3 VŽSCE., is low, at 0.14 a.u.; it is however, well above the background level, and above the value recorded at low potentials Ž- 1.4 VŽSCE.. The emission is clearly and unambiguously associated with electropolishing. The electroluminescence is related to the anodic current density during electropolishing: the intensity is high when the current density is high and it is low when the current density is low, although there must also be other factors affecting the electroluminescence intensity. The significant noise on the electroluminescence signal is due to the fairly weak overall intensity of the signal. The results are compared with polarisation and associated electroluminescence during anodising of aluminium in phosphoric acid, shown respectively in Figs. 2Ža. and Žb.. In this solution the metal is covered with a thickening oxide film whose structure consists of a porous layer overlying a dense barrier layer w13x. The polarisation curve, prepared at the same sweep rate as that of Fig. 1Ža., shows features associated with anodising. The current rises to an approximate plateau value of ca. 15 A my2 , and the potential dependence is small. This current density is in fact similar in value to the pre-polishing current in nitric acid Ž11 A my2 . measured at potentials less than ca. 1.4 VŽSCE. Žsee Fig. 1.. This confirms that the current density in the pre-polishing regime of Fig. 1Ža. Ži.e. at potentials - 1.4 VŽSCE.. is indeed controlled by the presence of an oxide film, in exactly the same way as that observed throughout the anodic potential range in phosphoric acid. Note that the similarity in these current densities from the filmed surface in the two acids cannot be ascribed to conductivity of the electrolytes, since the conductivities are quite different Ž0.655 S cmy1 for 10 M HNO 3 and 0.055 S cmy1 for 1 M H 3 PO4 .. Electroluminescence was difficult to detect at potentials less than ca. 2 VŽSCE. during anodisation in phosphoric
593
Fig. 2. Ža. Anodic polarisation curve for aluminium in 1 M phosphoric acid measured at a potential sweep rate of 50 mV sy1 . Žb. Electroluminescence of aluminium in 1 M phosphoric acid during polarisation as described in Ža..
acid. However, as the potential rose to values higher than 2 VŽSCE. electroluminescent emission was detected as shown in Fig. 2Žb.. The highest intensity detected at 10 VŽSCE. Žthe maximum potential used. was ca. 2 a.u., and comparable with the peak value observed in Fig. 1Žb.. However, unlike the data described for electropolishing in nitric acid, the luminescence in Fig. 2Žb. does not appear to relate to the anodising current. We believe that electroluminescence occurs throughout the anodising range, even below 2 VŽSCE., but its magnitude is too low to detect at the lower potentials. It therefore also seems likely that the pre-polishing regime in nitric acid, where the reaction rate of the metal is controlled by the presence of the oxide film Ž- 1.4 VŽSCE.., also emits light, but the intensity is too small to detect with the present experimental procedure. We have thus shown that electroluminescence occurs from aluminium both under conditions of anodic electropolishing and of anodic oxide film growth. From the description by Ikonopisov w2x it is clear that the electroluminescence observed in Fig. 2b is similar to that observed by other authors, namely it is produced by light-emitting reactions occurring at the metalroxide interface. Van Geel w1x proposes that luminescence occurs through electron transitions in the oxide film. Both of these notions imply the necessity for the presence of the oxide film. However
594
L.C. Marsland, G.T. Bursteinr Electrochemistry Communications 2 (2000) 591–594
we have observed electroluminescence from the electropolishing surface, with peaks in the electroluminescence clearly resulting from the formation of the Jacquet layer. We therefore propose that the surface oxide film is not a requirement for electroluminescence to occur; electroluminescence arises also from the actively dissolving metal. Some have proposed that an oxide is indeed present during electropolishing. For example, Tajira et al. w14x observed that a dense, chemically stable oxide layer of approximately 200 nm thickness is formed during electropolishing. Baizuldin w15x found porous films after electropolishing and stripping. Even if these layers did exist whilst electropolishing, their presence cannot control the current during electropolishing, since the current density is far too high. Thus one cannot expect electron current through any possible oxide during electropolishing; neither can electron transfer occur at the metalroxide interface. The electrochemical reaction would necessarily occur from those regions not covered by the oxide film, since these are the paths of least resistance to current flow. The conclusion must inexorably be reached that the actively dissolving metal surface emits electroluminescence, in the absence of the oxide film. It is conceivable that a monolayer of oxide is formed on the metal surface during electropolishing, a process proposed for active dissolution of other metals. Even if this were the case, the film would not be thick enough to support electroluminescence in the conventional sense as described by Ikonopisov w2x or van Geel w1x. Whether the saturated salt layer, the Jacquet layer w4,10x, present on the surface during electropolishing can itself give rise to electroluminescence is not yet known. Even if this layer were capable of supporting electroluminescence, the peaks in the electroluminescence intensity due to actual formation of the Jacquet layer ŽFig. 1b at 1.7 and 2.2 VŽSCE.. would still remain unexplained.
4. Conclusions Electropolishing aluminium in moderately concentrated nitric acid solutions causes electroluminescence. The pro-
cess cannot be ascribed to any mechanism of light emission occurring within an oxide film. We ascribe the process tentatively to the oxidation reactions occurring at the metalrelectrolyte interface.
Acknowledgements We are grateful to Alcan International Ltd for the award of a research grant without which this research could not have been carried out. We are also grateful to Dr. J. Ball and Dr. J.D.B. Sharman for many stimulating and fruitful discussions.
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