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Buffering effects on electrograining of aluminium in nitric acid
Corrosion Science 47 (2005) 2225–2239 www.elsevier.com/locate/corsci
Buffering effects on electrograining of aluminium in nitric acid E.V. Koroleva a,*, G.E. Thompson a, P. Skeldon a, G. Hollrigl b, S. Lockwood c, G. Smith c a Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60 1QD, UK Alcan Technology and Management Ltd., Bad. Bahnhofstrasse 16, CH-8212 Neuhausen, Switzerland c Bridgnorth Aluminium Limited, Stourbridge Road, Bridgnorth WV15 6AU, UK
b
Received 12 February 2004; accepted 27 September 2004 Available online 16 December 2004
Abstract Electrograining of a binary Al–Si alloy has been undertaken in nitric acid based electrolytes, with the resultant surfaces examined by scanning and transmission electron microscopies. Depending on electrograining conditions, the pit appearance varies from hemispherical to large lateral pits, with the latter favoured in relatively acidic electrolytes. The conditions prevailing in the pit have been explored through use of aluminium ion additions to the nitric acid electrolyte as well as additions of species which influence the precipitation and dissolution of aluminium hydroxide. These confirm that control of the pit solution pH, through hydroxide generation, as a result of the selected electrograining conditions and consequent anodic and cathodic polarisation, enables tailoring of the resultant electrograined surface appearance. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrograining; Nitric acid; Aluminium–silicon binary alloy
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Corresponding author. Tel.: +44 161 200 5954; fax: +44 161 200 4865. E-mail address: [email protected] (E.V. Koroleva).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.09.013
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1. Introduction The mechanism of electrograining of aluminium alloys, i.e., development of a suitably convoluted surface, by pitting processes, in nitric acid based electrolytes is of fundamental and commercial interest because of its relevance to offset lithography [1–4]. It is established that pit growth proceeds during the anodic half cycle of polarisation and hydrogen evolves, with consequent increase of solution pH near the aluminium surface and deposition of hydrated alumina, during the cathodic half cycle of polarisation [2,4,13]. It is also known that the development of the required, uniform pit morphologies on the alloy surface is influenced by alloy composition [5–7]. Generally, AA1050 alloys, with iron and silicon present at levels of 0.3–0.4 wt% and 0.1–0.2 wt%, respectively, are used for electrograining. The presence of iron in the composition gives rise to Al3Fe second phase particles, which are lost rapidly from the alloy, leaving only their imprints, during electrograining [7–9]. The majority of the silicon is present in solid solution in the alloy, with only limited formation of the a-FeSiAl phase. In this study, the mechanism of pit growth during electrograining in nitric acid based electrolytes is considered, with further understanding of the influence of interfacial pH variation being of major interest. Such understanding will assist tailoring of electrograined structures for commercial requirements. As indicated previously, AA1050 alloys are widely employed for electrograining; however, to remove the influence of local processes at the second phase material, a binary alloy, with similar silicon content to the commercial alloy, has been utilised.
2. Experimental procedure The fabrication of the model aluminium–silicon alloy followed closely that of commercial AA1050 alloys, resulting in a final cold rolled alloy microstructure that was in the fully hard condition, with silicon expected to be present in solid solution. The alloy composition was determined by standard optical emission spectroscopy (OES) of the bulk material, with 24 elements analysed; elements present at levels above 1–2 ppm are Si (2100 ppm), Fe (19 ppm) and Cu (15 ppm). Prior to electrograining, samples were degreased in ethanol, etched in 1.5 M sodium hydroxide solution at 80 C for 60 s, rinsed in deionised water, desmutted in 50 wt% nitric acid for 30 s and finally rinsed in deionised water. Beeswax was used to mask the specimens, leaving an exposed area of 30 · 10 mm. Electrograining, using a 50 Hz sinusoidal waveform, was carried out at current densities of 5, 19 and 55 Arms/dm2 for 190, 70 and 20 s, respectively, in 0.24 M HNO3 electrolyte at 20 C, with corresponding charges in the range of 1100 ± 200 Crms/dm2. Concerning the early stages of pit development, selected samples were electrograined at 66 Arms/dm2 for a limited time of 0.5 s. A Derritron TA600 solid state amplifier, combined with a Philips PM-5133 function generator, was used to apply the voltage between a platinum electrode and the alloy electrode. The voltage
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and resultant current response were monitored using a two-channel storage oscilloscope (Tektronix TDS-210). In order to gain insight into the mechanism of pitting, the effects of electrolyte buffering were explored, using organic compounds at concentrations used typically in the quantitative analysis of aluminium ions in solution. Thus, electrograining was undertaken at 15 Arms/dm2 for 60 s (total charge of 900 Crms/dm2) in a solution of 0.3 M HNO3 without and with additions of 0.02 M or 0.2 M Al(NO3)3 Æ 9H2O. Additionally, selected compounds, namely 0.0036 M alizarin red S mono hydrate (3,4-thihydroxy-2-anthraquinone sulphonic acid sodium salt, CI 58005), saturated 8-hydroxyquinoline, made from 0.19 M 8-hydroxyquinoline and 0.38 M NH4NO3, were added separately to the 0.3 M HNO3 + 0.02 M Al(NO3)3 Æ 9H2O solution in order to influence further its buffering ability. The buffering behaviour of 10 ml volumes of the various solutions was determined by titration with 0.1 M NaOH solution; the pH was monitored using a JENWAY 3030 pH meter. The surface appearance of the electrograined alloys was examined prior to and after desmutting in a mixed chromic/phosphoric acid solution at 90 C for 600 s to remove corrosion products, mainly aluminium hydroxides, and any anodic oxide films formed during electrograining. Following gold coating to avoid charging under the electron beam, the specimens were examined in an ISI DS-130 scanning electron microscope operating at 19 kV; secondary electron imaging was used for topographical information. Electron transparent cross-sections of the alloy, that had been electrograined for 0.5 s, with no subsequent desmutting, were prepared by ultramicrotomy using a Reichert Ultracut E ultramicrotome and examined in a JEOL 2000 FXII transmission electron microscope.
3. Results 3.1. Buffering ability of electrograining solution Aluminium cations, introduced deliberately into the nitric acid solution, or generated through alloy dissolution, buffer in strong acid; the consequent resistance to pH x increase is through generation of AlðOHÞy species and, finally, aluminium hydroxide precipitation. The pH range of buffering is revealed in the titration curves of nitric acid with added aluminium nitrate (Fig. 1a). Two pH ranges of buffering, from 3.5 to 4.2 and from 9 to 10, related to precipitation and dissolution of aluminium hydroxide, respectively, are evident. It is known that precipitation of hydroxide commences at pH 3.3 for a 1 M solution of Al3+ ions and at pH 4.0 for a 0.01 M solution of Al3+ ions [10]; precipitation is completed at pH 5.2, with a residual Al3+ ion concentration of less than 105 M. Further, the precipitate begins to dissolve at pH 7.8, with dissolution completed at pH 10.8. Alizarin red S, 8-hydroxyquinoline and ammonium nitrate were selected to modify the buffering ability of the solution containing Al3+ ions. Alizarin red S is commonly used for quantitative spectrofluorimetric or electrochemical determination of trace amounts of aluminium due to the optical and redox properties of the resultant
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Fig. 1. Titration curves of nitric acid based electrograining solutions with standard 0.1 M NaOH; the 10 ml solutions of (a) 0.3 M nitric acid with aluminium nitrate (j) no addition; 0.02 M of aluminium nitrate; (m) 0.2 M of aluminium nitrate; (b) 0.3 M nitric acid with 0.02 M aluminium nitrate solution no further addition; and with addition of (m) 8-hydroxyquinoline, (j) alizarin red S and (d) ammonium nitrate; (c) as (b), but with expansion of the volume range of added NaOH solution.
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aluminium–anthraquinone complexes, which are sensitive to pH variation. Recent studies of the alizarin red PS variant (1,2,4-thihydroxy 9,10-anthraquinone-3sulphonic acid) [11] have indicated that the aluminium–anthraquinone complexes are formed at about pH 3. As shown by titration, introduction of alizarin red to the solution delays the pH increase (Fig. 1b and c) and hence, precipitation of aluminium hydroxide, due to competition of the anthraquinone complexes with OH ions for Al3+ ions. The introduction of 8-hydroxyquinoline results in a significantly early pH increase (Fig. 1b and c), due to commencement of the precipitation of tris-(8-hydroxyquinoline) aluminium at pH 2.5. It is known that 8-hydroxyquinoline forms a metal chelate precipitate with aluminium in the pH range from 3 to 5 [12]. From the titration curves of Fig. 1b and c, no influence of ammonium ions is evident on the first buffering pH range as expected; a significantly higher buffering ability of the solution is observed at pH 8.5, which results in delayed dissolution of the previously formed aluminium hydroxide precipitates. Ammonium ions, commonly used for buffering at about pH 9, compete with Al3+ ions for OH ions in the second buffering range. 3.2. Electrograining in nitric acid Practically, a minimum total electrical charge of 600–700 Crms/dm2 is required generally for development of a fully convoluted alloy surface during nitric acid electrograining [7]. This can be achieved by manipulation of current density and/or time, with rapid electrograining necessitating high current densities. Here, electrograining was performed at various current densities for different times, delivering electrical charges of 900–1300 Crms/dm2, i.e., well above the required minimum charge. The resultant appearances of the desmutted alloy surfaces are shown in Fig. 2. Numerous hemispherical pits, of 1–5 lm diameter, are observed after electrograining at 55 Arms/dm2 for 20 s (Fig. 2a); the pit walls are relatively smooth. At 19 Arms/dm2 for 70 s, fine pits decorate the pit walls (Fig. 2b). With further reduction of current density to 5 Arms/dm2 for 190 s, fine, hemispherical pits are evident, together with relatively large, lateral pits that are decorated by terrace-like steps (Fig. 2c). Observation of the alloy surfaces without desmutting suggests that the amount of corrosion products covering the pits decreases with reduction in current density (Fig. 2d–f). Additionally, the limited precipitation evident after electrograining at 5 Arms/dm2 is located at the hemispherical pits; conversely, it is not present above the large lateral pits (Fig. 2f). Electrograining at 66 Arms/dm2 for 0.5 s initiated pits on the alloy surface (Fig. 3). Areas of light contrast indicate that the pits are covered with corrosion products (Fig. 3b), similar to the fully convoluted surfaces. About 2 · 1010 pits/dm2 were evident after electrograining, with pit sizes varying from 0.1 to 3 lm (Fig. 3c). With continued electrograining to 1100 Crms/dm2, the initial pits grow and merge to form the fully convoluted surface, with a pit population density of about 3 · 109 pits/dm2; the final pit size varies from 1 to 5 lm (Fig. 3d). Transmission electron microscopy of ultramicrotomed sections of the specimen that had been electrograined at 66 Arms/dm2 for 0.5 s reveals the presence of layers of hydrated alumina within the
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Fig. 2. Scanning electron micrographs of binary aluminium silicon alloy after electrograining in 0.24 M nitric acid: (a) smooth hemispherical pits developed at 55 Arms/dm2; (b) fine hemispherical pits developed at 19 Arms/dm2; (c) mixed pit morphologies, revealing areas of fine hemispherical pits and areas of large lateral pits with terrace-like steps, generated at 5 Arms/dm2; (d), (e), (f) show the surfaces of (a), (b), (c), respectively, prior to corrosion product removal.
hemispherical pits only (Fig. 4a and b). In contrast, a relatively thin, evidently compact anodic film, 10–20 nm thick, is sandwiched between the pit wall and the hydrated alumina (Fig. 4a and c); a similar compact film is also evident on the macroscopic alloy surface that has not undergone pitting. 3.3. Electrograining in nitric acid with additives From the previous results, it is evident that the electrical charge passed determines the extent of surface convolution by pitting. However, variation in the appearance of the pits is evident at different current densities for similar charges passed (Fig. 2). In order to assist understanding of this behaviour, and the associated interfacial condi-
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Fig. 3. Electrograining in 0.24 M nitric acid at 66 Arms/dm2 for 0.5 s: (a) scanning electron micrograph showing the general surface appearance after removal of corrosion products; (b) as (a), but prior to removal of corrosion products; (c) high magnification of (a); (d) high magnification of a sample electrograined for 1100 Crms/dm2.
tions resulting from the electrochemical processes, selected additions of aluminium nitrate were made to the 0.3 M HNO3 solution. A relatively low current density of 15 Arms/dm2 and electrical charge of 900 Crms/dm2 were employed; the previous conditions favour the development of relatively large lateral pits, which are not commercially desirable. As expected, lateral pits, of 10–80 lm diameter, decorated by terrace-like steps of 1–2 lm width, are evident (Fig. 5a). The majority of the large lateral pits have merged to produce a generally roughened surface appearance, with only a limited presence of non-grained areas. Similar pit appearances were evident previously after electrograining at the lowest current density of 5 Arms/dm2 (Fig. 2c); in 0.24 M HNO3 solution, a reduced presence of lateral pits was revealed after electrograining at 19 Arms/dm2 (Fig. 2b). This also suggests that increased acidity of the nitric acid electrolyte (from 0.24 to 0.3 M) encourages the development of the relatively large lateral pits at the low and medium current densities employed. Additionally, the acidity of the electrolyte influences the amount of OH ion generation that is required to achieve a pH increase during the cathodic cycle of electrograining. This is illustrated by titration where, for example, the 0.24 M nitric acid solution requires
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Fig. 4. Transmission electron micrographs of ultramicrotomed section of the binary Al–Si alloy after electrograining in 0.24 M nitric acid at 66 Arms/dm2 for 0.5 s: (a) the general appearance, revealing the macroscopic alloy surface and discrete pits; (b) aluminium hydroxide precipitation within the pit; (c) a thin alumina film formed on the pit walls and the macroscopic alloy surface.
24 ml 0.1 M NaOH solution to achieve the pH increase; on the other hand, 30 ml NaOH solution is consumed during titration of the 0.3 M nitric acid solution (Fig. 1a). Introduction of aluminium nitrate to the solution has little effect on the general surface appearance, although an increase in size of the lateral pits is revealed (Fig. 5a–c). Titration of these solutions reveals that similar volumes of NaOH solution are necessary to commence the pH increase (Fig. 1a). Concerning the specific solution additives, alizarin red has no significant effect on the appearance of the large lateral pits and associated terrace-like steps (Fig. 6a). Conversely, with 8-hydroxyquinoline, numerous hemispherical pits of 1–2 lm diameter, decorated by fine pits, are present on the alloy surfaces after electrograining (Fig. 6b). The population density of hemispherical pits, of average diameter less than 1 lm, increases markedly with addition of ammonium nitrate (Fig. 6c). Titration of the solutions containing alizarin red, 8-hydroxyquinoline and ammonium nitrate indicates that increased, decreased and similar volumes of NaOH solution, respectively, are required for the initial pH increase (Fig. 1c).
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Fig. 5. Scanning electron micrographs of binary aluminium silicon alloy after electrograining at 15 Arms/dm2 for 60 s in 0.3 M nitric acid with different additions of aluminium nitrate: (a) no addition; (b) 0.02 M aluminium nitrate; (c) 0.2 M aluminium nitrate.
4. Discussion 4.1. General considerations A brief description of electrograining is provided here to assist subsequent discussion of the results. During electrograining, utilising a fast sinusoidal voltage,
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Fig. 6. Scanning electron micrographs of binary aluminium silicon alloy after electrograining at 15 Arms/ dm2 for 60 s in 0.3 M nitric acid and 0.02 M aluminium nitrate electrolyte with different electrolyte additions: (a) alizarin red; (b) 8-hydroxyquinoline; (c) ammonium nitrate.
the alloy is subjected to alternating anodic and cathodic polarisation, with sequential electrochemical processes of anodic dissolution of aluminium and oxygen evolution and cathodic hydrogen evolution [2,13]. Significant aluminium dissolution clearly proceeds during pit growth at potentials above the pitting potential; however, loss of aluminium to the solution will also proceed as a result of anodic film formation during polarisation at potentials below the pitting potential in nitric acid. The previous anodic processes result in interfacial pH decrease, due to generation
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of H+ ions as a consequence of oxide/hydroxide formation and oxygen evolution. Conversely, an increased pH results from the cathodic process of hydrogen evolution. For fast cyclic polarisation of thousands of volts per second (non-steady conditions) electrochemical reactions are limited by the migration of species to and away from the electrode surface, i.e., they are generally under diffusion control [14]. Consequently, interfacial pH fluctuations during each cycle are delayed; this has an influence on the resultant pit development. Such effects are now considered in the light of the various surface morphologies generated during electrograining. 4.2. Hemispherical pit growth during electrograining After rapid pit initiation, the pits grow and merge with time of electrograining. For an existing pit, aluminium dissolution and continuing pit growth proceed during anodic polarisation above the pitting potential; highly acidic conditions develop locally within the pit. Consequently, reversal of the anodic polarisation is not expected to terminate pit development until sufficiently negative potentials, that allow hydrogen evolution to modify the pH within the pit, are achieved. With sufficient retained Al3+ ions within the pit and increasing pH through hydrogen evolution, aluminium hydroxide precipitation proceeds. Hydroxide precipitation commences at about pH 4 and, consequently, increase of the pH within the pit beneath the aluminium hydroxide layers is restricted. With further polarisation and pH increase, partial chemical dissolution of the aluminium hydroxide precipitate can proceed with highly alkaline solution conditions now developed near the surface. Thus, during electrograining, in the very early period of anodic polarisation, the interfacial pH can vary from 4 to highly alkaline; however, the latter pH is influenced by the concentration of Al3+ ions within the pit and the extent of prior cathodic polarisation. As stated previously, growth of anodic film commences during anodic polarisation of the alloy in the nitric acid based electrolyte at potentials below the pitting potential, but under pH conditions that are inherited from the previous cathodic polarisation, i.e., the pH can vary from 4 to highly alkaline. Highly alkaline conditions are reactive to anodic alumina and result in development of a finely porous film at low effective current efficiency. This gives rise to Al3+ ions in the solution, which influence aluminium hydroxide precipitation at appropriate pH. The presence of the film formed under various pH conditions also influences the pitting potential, with resultant variation in the pit dimensions. The present authors [15] have shown that the pit size decreases with deliberate increase in cathodic polarisation, which increases the solution alkalinity prior to anodic film formation. Generally, above the pitting potential, aluminium dissolution and oxygen evolution occur, resulting in highly acid conditions within the pit, with limited, if any, anodic film growth. However, the pit walls are eventually passivated by a thin oxide film after sufficient reversal of the polarisation, when the pH is increased to about 4 through hydrogen evolution. The thin oxide film is present on the surface during cathodic polarisation when the pH remains between 4 and 10. Further pH increase
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to highly alkaline levels through cathodic polarisation can lead to further thinning of the oxide film and relatively uniform dissolution of the pit walls in the highly alkaline environment. It is evident that cyclic polarisation during electrograining leads to a significant pH variation locally within the pits through continuing aluminium dissolution, hydrogen evolution and filming of aluminium. Consequently, for conditions that lead to localisation of the electrochemical processes, growth of the existing pits proceeds during continued electrograining with the macroscopic non-grained areas remaining relatively passive. The required surface convolution is achieved thorough the growth and merging of the initially developed pits with time of electrograining. The extents of anodic and cathodic polarisation, i.e., the amplitude of the waveform employed, influence the final size of the hemispherical pits, as reported previously in studies of pit development at various current densities in nitric acid solutions [3]. In the present case, increase in current density transforms the hemispherical pits of average 3 lm diameter with their walls decorated by fine pits (size of 0.1–1 lm) to relatively large, smooth hemispherical pits of average 6 lm diameter (Fig. 2a and b). Increase in pit size suggests that the time for pit growth is extended with increased anodic and cathodic polarisations. Increase of anodic polarisation favours pit growth, generating increased concentrations of Al3+ ions within the pit that are available for precipitation. The more extensive precipitation evident at high current density is associated with availability of Al3+ ions together with maintenance of the local pH between 4 and 10 for increased periods of time during cathodic polarisation (Fig. 2d). Increased cathodic polarisation results in further alkalisation beneath the precipitate and the dissolution of the aluminium alloy. The generation of a highly alkaline solution near the surface may contribute to the smooth appearance of the pit walls after electrograining during high current densities.
4.3. Lateral pit growth during electrograining Development of lateral pits is observed for electrograining at low and medium current densities. The relatively limited anodic and cathodic polarisation acts to reduce the generation of Al3+ and OH ions, consequently restricting the time when the pH within the pit is maintained above 4. Thus, with little aluminium hydroxide precipitation, pit growth continues in the acidic conditions during anodic polarisation and into the region of cathodic polarisation. The absence of precipitation over the lateral pits confirms that the pH does not increase above 4. Such conditions favour the appearance of large lateral pits of 10–80 lm diameter, with terrace-like steps of width 1–2 lm. Their presence is explained readily; during anodic polarisation, an initial pit grows, and continues its development, but at a reduced rate, during cathodic polarisation. The local pH gradient along the surface, e.g., from about 4 within the pit to highly acid within a short distance from the pit, promotes development of further pits adjacent to the original pit during subsequent anodic polarisation. The
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separation of the original pit and the new pit is determined by the thickness of diffusion layer formed during cathodic reaction. Consequently, as electrograining proceeds, further new pits develop adjacent to previously pitted regions; with time, their presence gives rise to lateral pits of characteristic internal appearance. With deliberate increase of Al3+ ion concentration, through addition of aluminium nitrate, hydroxide precipitation commences at a reduced pH of 2.5 (Fig. 1a). However, this does not significantly influence the pH range over which the aluminium surface is passivated; additionally, the pit appearance is little changed, since the final size depends on the time for pit growth at low pH. This time depends on the rate of OH ion formation during cathodic polarisation, which may be equated to the volume of NaOH that must be added to commence precipitation in the solution. This volume is similar for any concentration of Al3+ ions in the solution (Fig. 1a). Increase of the time of pit growth, when the pH is low, though addition of alizarin red, results in lateral pit development during electrograining (Fig. 1c). Alizarin red forms aluminium–anthraquinone complexes with Al3+ ions and delays precipitation of aluminium hydroxide, thus extending the period of pit growth. The possibility of oxidation of aluminium–anthraquinone complexes at 0.42 V (SCE) has been considered [16], which can affect the aluminium hydroxide precipitation under fast cyclic polarisation. Reduction of the time over which a low pH is maintained is achieved by addition of 8-hydroxyquinoline (Fig. 1c), which precipitates tris-(8-hydroxyquinoline) aluminium at pH 2.5. The early pH increase within the pit gives more ready passivation of the pit walls; consequently, the resultant pit morphology changes markedly, revealing fine hemispherical pits together with lateral pits of significantly reduced dimensions. Pit morphologies, comprising fine hemispherical pits, result from electrograining in the presence of ammonium nitrate. The absence of the large hemispherical pits, with smooth or decorated walls, is associated with the increased time over which the pH is in the range 4–8.5. During cathodic polarisation, pH increase above 8.5 and consequent alkaline dissolution of the alloy is limited by the buffering ability of ammonium nitrate, which also restricts dissolution of the hydroxide precipitate within the pit. Further, during growth of the anodic film in the early period of anodic polarisation, pH reduction is expected; however, in the presence of a buffered solution of pH 8.5, a relatively compact anodic film growth proceeds leading to an increase in the pitting potential and a reduced time for pit growth.
5. Conclusions 1. During electrograining of aluminium in nitric acid based electrolytes, comprising rapid alternating cathodic and anodic polarisation, the electrochemical processes of aluminium dissolution, anodic film formation, and oxygen and hydrogen evolution proceed in the pits. The local pH close to the aluminium surface may thus vary from highly acidic to highly alkaline values.
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2. The presence of a passive film on the alloy leads to localisation of the electrochemical processes. Thus, the anodic and cathodic currents pass largely through existing pits, with consequent growth and merging to develop the electrograined appearance. 3. Growth of individual pits continues with time when the pH within the pit is below 4; namely during anodic polarisation above the pitting potential, with reversal of anodic polarisation and also in the early stages of cathodic polarisation. 4. Hydrogen evolution during cathodic polarisation leads to a pH increase and, due to the presence of aluminium ions from previous anodic polarisation, precipitation of aluminium hydroxides within the pits. The pit remains passive when the pH is maintained at values of about 4. Relatively uniform dissolution of the pit walls can result from further hydrogen evolution within the pit beneath the precipitate. 5. The appearance of the pits depends strongly on the time intervals, when the near surface pH, at particular range; specifically, the times when pit grows at pH below 4, aluminium is passivated at pH between 4 and 10, and uniform alkaline dissolution occurs at pH above 10. These time intervals can be controlled by current density used during electrograining and the presence of additions to the solution.
Acknowledgments The authors thank the Engineering and Physical Sciences Research Council (EPSRC) and Bridgnorth Aluminium Ltd., for financial support of the work.
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Buffering effects on electrograining of aluminium in nitric acid E.V. Koroleva a,*, G.E. Thompson a, P. Skeldon a, G. Hollrigl b, S. Lockwood c, G. Smith c a Corrosion and Protection Centre, UMIST, P.O. Box 88, Manchester M60 1QD, UK Alcan Technology and Management Ltd., Bad. Bahnhofstrasse 16, CH-8212 Neuhausen, Switzerland c Bridgnorth Aluminium Limited, Stourbridge Road, Bridgnorth WV15 6AU, UK
b
Received 12 February 2004; accepted 27 September 2004 Available online 16 December 2004
Abstract Electrograining of a binary Al–Si alloy has been undertaken in nitric acid based electrolytes, with the resultant surfaces examined by scanning and transmission electron microscopies. Depending on electrograining conditions, the pit appearance varies from hemispherical to large lateral pits, with the latter favoured in relatively acidic electrolytes. The conditions prevailing in the pit have been explored through use of aluminium ion additions to the nitric acid electrolyte as well as additions of species which influence the precipitation and dissolution of aluminium hydroxide. These confirm that control of the pit solution pH, through hydroxide generation, as a result of the selected electrograining conditions and consequent anodic and cathodic polarisation, enables tailoring of the resultant electrograined surface appearance. 2004 Elsevier Ltd. All rights reserved. Keywords: Electrograining; Nitric acid; Aluminium–silicon binary alloy
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Corresponding author. Tel.: +44 161 200 5954; fax: +44 161 200 4865. E-mail address: [email protected] (E.V. Koroleva).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.09.013
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1. Introduction The mechanism of electrograining of aluminium alloys, i.e., development of a suitably convoluted surface, by pitting processes, in nitric acid based electrolytes is of fundamental and commercial interest because of its relevance to offset lithography [1–4]. It is established that pit growth proceeds during the anodic half cycle of polarisation and hydrogen evolves, with consequent increase of solution pH near the aluminium surface and deposition of hydrated alumina, during the cathodic half cycle of polarisation [2,4,13]. It is also known that the development of the required, uniform pit morphologies on the alloy surface is influenced by alloy composition [5–7]. Generally, AA1050 alloys, with iron and silicon present at levels of 0.3–0.4 wt% and 0.1–0.2 wt%, respectively, are used for electrograining. The presence of iron in the composition gives rise to Al3Fe second phase particles, which are lost rapidly from the alloy, leaving only their imprints, during electrograining [7–9]. The majority of the silicon is present in solid solution in the alloy, with only limited formation of the a-FeSiAl phase. In this study, the mechanism of pit growth during electrograining in nitric acid based electrolytes is considered, with further understanding of the influence of interfacial pH variation being of major interest. Such understanding will assist tailoring of electrograined structures for commercial requirements. As indicated previously, AA1050 alloys are widely employed for electrograining; however, to remove the influence of local processes at the second phase material, a binary alloy, with similar silicon content to the commercial alloy, has been utilised.
2. Experimental procedure The fabrication of the model aluminium–silicon alloy followed closely that of commercial AA1050 alloys, resulting in a final cold rolled alloy microstructure that was in the fully hard condition, with silicon expected to be present in solid solution. The alloy composition was determined by standard optical emission spectroscopy (OES) of the bulk material, with 24 elements analysed; elements present at levels above 1–2 ppm are Si (2100 ppm), Fe (19 ppm) and Cu (15 ppm). Prior to electrograining, samples were degreased in ethanol, etched in 1.5 M sodium hydroxide solution at 80 C for 60 s, rinsed in deionised water, desmutted in 50 wt% nitric acid for 30 s and finally rinsed in deionised water. Beeswax was used to mask the specimens, leaving an exposed area of 30 · 10 mm. Electrograining, using a 50 Hz sinusoidal waveform, was carried out at current densities of 5, 19 and 55 Arms/dm2 for 190, 70 and 20 s, respectively, in 0.24 M HNO3 electrolyte at 20 C, with corresponding charges in the range of 1100 ± 200 Crms/dm2. Concerning the early stages of pit development, selected samples were electrograined at 66 Arms/dm2 for a limited time of 0.5 s. A Derritron TA600 solid state amplifier, combined with a Philips PM-5133 function generator, was used to apply the voltage between a platinum electrode and the alloy electrode. The voltage
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and resultant current response were monitored using a two-channel storage oscilloscope (Tektronix TDS-210). In order to gain insight into the mechanism of pitting, the effects of electrolyte buffering were explored, using organic compounds at concentrations used typically in the quantitative analysis of aluminium ions in solution. Thus, electrograining was undertaken at 15 Arms/dm2 for 60 s (total charge of 900 Crms/dm2) in a solution of 0.3 M HNO3 without and with additions of 0.02 M or 0.2 M Al(NO3)3 Æ 9H2O. Additionally, selected compounds, namely 0.0036 M alizarin red S mono hydrate (3,4-thihydroxy-2-anthraquinone sulphonic acid sodium salt, CI 58005), saturated 8-hydroxyquinoline, made from 0.19 M 8-hydroxyquinoline and 0.38 M NH4NO3, were added separately to the 0.3 M HNO3 + 0.02 M Al(NO3)3 Æ 9H2O solution in order to influence further its buffering ability. The buffering behaviour of 10 ml volumes of the various solutions was determined by titration with 0.1 M NaOH solution; the pH was monitored using a JENWAY 3030 pH meter. The surface appearance of the electrograined alloys was examined prior to and after desmutting in a mixed chromic/phosphoric acid solution at 90 C for 600 s to remove corrosion products, mainly aluminium hydroxides, and any anodic oxide films formed during electrograining. Following gold coating to avoid charging under the electron beam, the specimens were examined in an ISI DS-130 scanning electron microscope operating at 19 kV; secondary electron imaging was used for topographical information. Electron transparent cross-sections of the alloy, that had been electrograined for 0.5 s, with no subsequent desmutting, were prepared by ultramicrotomy using a Reichert Ultracut E ultramicrotome and examined in a JEOL 2000 FXII transmission electron microscope.
3. Results 3.1. Buffering ability of electrograining solution Aluminium cations, introduced deliberately into the nitric acid solution, or generated through alloy dissolution, buffer in strong acid; the consequent resistance to pH x increase is through generation of AlðOHÞy species and, finally, aluminium hydroxide precipitation. The pH range of buffering is revealed in the titration curves of nitric acid with added aluminium nitrate (Fig. 1a). Two pH ranges of buffering, from 3.5 to 4.2 and from 9 to 10, related to precipitation and dissolution of aluminium hydroxide, respectively, are evident. It is known that precipitation of hydroxide commences at pH 3.3 for a 1 M solution of Al3+ ions and at pH 4.0 for a 0.01 M solution of Al3+ ions [10]; precipitation is completed at pH 5.2, with a residual Al3+ ion concentration of less than 105 M. Further, the precipitate begins to dissolve at pH 7.8, with dissolution completed at pH 10.8. Alizarin red S, 8-hydroxyquinoline and ammonium nitrate were selected to modify the buffering ability of the solution containing Al3+ ions. Alizarin red S is commonly used for quantitative spectrofluorimetric or electrochemical determination of trace amounts of aluminium due to the optical and redox properties of the resultant
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Fig. 1. Titration curves of nitric acid based electrograining solutions with standard 0.1 M NaOH; the 10 ml solutions of (a) 0.3 M nitric acid with aluminium nitrate (j) no addition; 0.02 M of aluminium nitrate; (m) 0.2 M of aluminium nitrate; (b) 0.3 M nitric acid with 0.02 M aluminium nitrate solution no further addition; and with addition of (m) 8-hydroxyquinoline, (j) alizarin red S and (d) ammonium nitrate; (c) as (b), but with expansion of the volume range of added NaOH solution.
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aluminium–anthraquinone complexes, which are sensitive to pH variation. Recent studies of the alizarin red PS variant (1,2,4-thihydroxy 9,10-anthraquinone-3sulphonic acid) [11] have indicated that the aluminium–anthraquinone complexes are formed at about pH 3. As shown by titration, introduction of alizarin red to the solution delays the pH increase (Fig. 1b and c) and hence, precipitation of aluminium hydroxide, due to competition of the anthraquinone complexes with OH ions for Al3+ ions. The introduction of 8-hydroxyquinoline results in a significantly early pH increase (Fig. 1b and c), due to commencement of the precipitation of tris-(8-hydroxyquinoline) aluminium at pH 2.5. It is known that 8-hydroxyquinoline forms a metal chelate precipitate with aluminium in the pH range from 3 to 5 [12]. From the titration curves of Fig. 1b and c, no influence of ammonium ions is evident on the first buffering pH range as expected; a significantly higher buffering ability of the solution is observed at pH 8.5, which results in delayed dissolution of the previously formed aluminium hydroxide precipitates. Ammonium ions, commonly used for buffering at about pH 9, compete with Al3+ ions for OH ions in the second buffering range. 3.2. Electrograining in nitric acid Practically, a minimum total electrical charge of 600–700 Crms/dm2 is required generally for development of a fully convoluted alloy surface during nitric acid electrograining [7]. This can be achieved by manipulation of current density and/or time, with rapid electrograining necessitating high current densities. Here, electrograining was performed at various current densities for different times, delivering electrical charges of 900–1300 Crms/dm2, i.e., well above the required minimum charge. The resultant appearances of the desmutted alloy surfaces are shown in Fig. 2. Numerous hemispherical pits, of 1–5 lm diameter, are observed after electrograining at 55 Arms/dm2 for 20 s (Fig. 2a); the pit walls are relatively smooth. At 19 Arms/dm2 for 70 s, fine pits decorate the pit walls (Fig. 2b). With further reduction of current density to 5 Arms/dm2 for 190 s, fine, hemispherical pits are evident, together with relatively large, lateral pits that are decorated by terrace-like steps (Fig. 2c). Observation of the alloy surfaces without desmutting suggests that the amount of corrosion products covering the pits decreases with reduction in current density (Fig. 2d–f). Additionally, the limited precipitation evident after electrograining at 5 Arms/dm2 is located at the hemispherical pits; conversely, it is not present above the large lateral pits (Fig. 2f). Electrograining at 66 Arms/dm2 for 0.5 s initiated pits on the alloy surface (Fig. 3). Areas of light contrast indicate that the pits are covered with corrosion products (Fig. 3b), similar to the fully convoluted surfaces. About 2 · 1010 pits/dm2 were evident after electrograining, with pit sizes varying from 0.1 to 3 lm (Fig. 3c). With continued electrograining to 1100 Crms/dm2, the initial pits grow and merge to form the fully convoluted surface, with a pit population density of about 3 · 109 pits/dm2; the final pit size varies from 1 to 5 lm (Fig. 3d). Transmission electron microscopy of ultramicrotomed sections of the specimen that had been electrograined at 66 Arms/dm2 for 0.5 s reveals the presence of layers of hydrated alumina within the
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Fig. 2. Scanning electron micrographs of binary aluminium silicon alloy after electrograining in 0.24 M nitric acid: (a) smooth hemispherical pits developed at 55 Arms/dm2; (b) fine hemispherical pits developed at 19 Arms/dm2; (c) mixed pit morphologies, revealing areas of fine hemispherical pits and areas of large lateral pits with terrace-like steps, generated at 5 Arms/dm2; (d), (e), (f) show the surfaces of (a), (b), (c), respectively, prior to corrosion product removal.
hemispherical pits only (Fig. 4a and b). In contrast, a relatively thin, evidently compact anodic film, 10–20 nm thick, is sandwiched between the pit wall and the hydrated alumina (Fig. 4a and c); a similar compact film is also evident on the macroscopic alloy surface that has not undergone pitting. 3.3. Electrograining in nitric acid with additives From the previous results, it is evident that the electrical charge passed determines the extent of surface convolution by pitting. However, variation in the appearance of the pits is evident at different current densities for similar charges passed (Fig. 2). In order to assist understanding of this behaviour, and the associated interfacial condi-
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Fig. 3. Electrograining in 0.24 M nitric acid at 66 Arms/dm2 for 0.5 s: (a) scanning electron micrograph showing the general surface appearance after removal of corrosion products; (b) as (a), but prior to removal of corrosion products; (c) high magnification of (a); (d) high magnification of a sample electrograined for 1100 Crms/dm2.
tions resulting from the electrochemical processes, selected additions of aluminium nitrate were made to the 0.3 M HNO3 solution. A relatively low current density of 15 Arms/dm2 and electrical charge of 900 Crms/dm2 were employed; the previous conditions favour the development of relatively large lateral pits, which are not commercially desirable. As expected, lateral pits, of 10–80 lm diameter, decorated by terrace-like steps of 1–2 lm width, are evident (Fig. 5a). The majority of the large lateral pits have merged to produce a generally roughened surface appearance, with only a limited presence of non-grained areas. Similar pit appearances were evident previously after electrograining at the lowest current density of 5 Arms/dm2 (Fig. 2c); in 0.24 M HNO3 solution, a reduced presence of lateral pits was revealed after electrograining at 19 Arms/dm2 (Fig. 2b). This also suggests that increased acidity of the nitric acid electrolyte (from 0.24 to 0.3 M) encourages the development of the relatively large lateral pits at the low and medium current densities employed. Additionally, the acidity of the electrolyte influences the amount of OH ion generation that is required to achieve a pH increase during the cathodic cycle of electrograining. This is illustrated by titration where, for example, the 0.24 M nitric acid solution requires
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Fig. 4. Transmission electron micrographs of ultramicrotomed section of the binary Al–Si alloy after electrograining in 0.24 M nitric acid at 66 Arms/dm2 for 0.5 s: (a) the general appearance, revealing the macroscopic alloy surface and discrete pits; (b) aluminium hydroxide precipitation within the pit; (c) a thin alumina film formed on the pit walls and the macroscopic alloy surface.
24 ml 0.1 M NaOH solution to achieve the pH increase; on the other hand, 30 ml NaOH solution is consumed during titration of the 0.3 M nitric acid solution (Fig. 1a). Introduction of aluminium nitrate to the solution has little effect on the general surface appearance, although an increase in size of the lateral pits is revealed (Fig. 5a–c). Titration of these solutions reveals that similar volumes of NaOH solution are necessary to commence the pH increase (Fig. 1a). Concerning the specific solution additives, alizarin red has no significant effect on the appearance of the large lateral pits and associated terrace-like steps (Fig. 6a). Conversely, with 8-hydroxyquinoline, numerous hemispherical pits of 1–2 lm diameter, decorated by fine pits, are present on the alloy surfaces after electrograining (Fig. 6b). The population density of hemispherical pits, of average diameter less than 1 lm, increases markedly with addition of ammonium nitrate (Fig. 6c). Titration of the solutions containing alizarin red, 8-hydroxyquinoline and ammonium nitrate indicates that increased, decreased and similar volumes of NaOH solution, respectively, are required for the initial pH increase (Fig. 1c).
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Fig. 5. Scanning electron micrographs of binary aluminium silicon alloy after electrograining at 15 Arms/dm2 for 60 s in 0.3 M nitric acid with different additions of aluminium nitrate: (a) no addition; (b) 0.02 M aluminium nitrate; (c) 0.2 M aluminium nitrate.
4. Discussion 4.1. General considerations A brief description of electrograining is provided here to assist subsequent discussion of the results. During electrograining, utilising a fast sinusoidal voltage,
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Fig. 6. Scanning electron micrographs of binary aluminium silicon alloy after electrograining at 15 Arms/ dm2 for 60 s in 0.3 M nitric acid and 0.02 M aluminium nitrate electrolyte with different electrolyte additions: (a) alizarin red; (b) 8-hydroxyquinoline; (c) ammonium nitrate.
the alloy is subjected to alternating anodic and cathodic polarisation, with sequential electrochemical processes of anodic dissolution of aluminium and oxygen evolution and cathodic hydrogen evolution [2,13]. Significant aluminium dissolution clearly proceeds during pit growth at potentials above the pitting potential; however, loss of aluminium to the solution will also proceed as a result of anodic film formation during polarisation at potentials below the pitting potential in nitric acid. The previous anodic processes result in interfacial pH decrease, due to generation
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of H+ ions as a consequence of oxide/hydroxide formation and oxygen evolution. Conversely, an increased pH results from the cathodic process of hydrogen evolution. For fast cyclic polarisation of thousands of volts per second (non-steady conditions) electrochemical reactions are limited by the migration of species to and away from the electrode surface, i.e., they are generally under diffusion control [14]. Consequently, interfacial pH fluctuations during each cycle are delayed; this has an influence on the resultant pit development. Such effects are now considered in the light of the various surface morphologies generated during electrograining. 4.2. Hemispherical pit growth during electrograining After rapid pit initiation, the pits grow and merge with time of electrograining. For an existing pit, aluminium dissolution and continuing pit growth proceed during anodic polarisation above the pitting potential; highly acidic conditions develop locally within the pit. Consequently, reversal of the anodic polarisation is not expected to terminate pit development until sufficiently negative potentials, that allow hydrogen evolution to modify the pH within the pit, are achieved. With sufficient retained Al3+ ions within the pit and increasing pH through hydrogen evolution, aluminium hydroxide precipitation proceeds. Hydroxide precipitation commences at about pH 4 and, consequently, increase of the pH within the pit beneath the aluminium hydroxide layers is restricted. With further polarisation and pH increase, partial chemical dissolution of the aluminium hydroxide precipitate can proceed with highly alkaline solution conditions now developed near the surface. Thus, during electrograining, in the very early period of anodic polarisation, the interfacial pH can vary from 4 to highly alkaline; however, the latter pH is influenced by the concentration of Al3+ ions within the pit and the extent of prior cathodic polarisation. As stated previously, growth of anodic film commences during anodic polarisation of the alloy in the nitric acid based electrolyte at potentials below the pitting potential, but under pH conditions that are inherited from the previous cathodic polarisation, i.e., the pH can vary from 4 to highly alkaline. Highly alkaline conditions are reactive to anodic alumina and result in development of a finely porous film at low effective current efficiency. This gives rise to Al3+ ions in the solution, which influence aluminium hydroxide precipitation at appropriate pH. The presence of the film formed under various pH conditions also influences the pitting potential, with resultant variation in the pit dimensions. The present authors [15] have shown that the pit size decreases with deliberate increase in cathodic polarisation, which increases the solution alkalinity prior to anodic film formation. Generally, above the pitting potential, aluminium dissolution and oxygen evolution occur, resulting in highly acid conditions within the pit, with limited, if any, anodic film growth. However, the pit walls are eventually passivated by a thin oxide film after sufficient reversal of the polarisation, when the pH is increased to about 4 through hydrogen evolution. The thin oxide film is present on the surface during cathodic polarisation when the pH remains between 4 and 10. Further pH increase
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to highly alkaline levels through cathodic polarisation can lead to further thinning of the oxide film and relatively uniform dissolution of the pit walls in the highly alkaline environment. It is evident that cyclic polarisation during electrograining leads to a significant pH variation locally within the pits through continuing aluminium dissolution, hydrogen evolution and filming of aluminium. Consequently, for conditions that lead to localisation of the electrochemical processes, growth of the existing pits proceeds during continued electrograining with the macroscopic non-grained areas remaining relatively passive. The required surface convolution is achieved thorough the growth and merging of the initially developed pits with time of electrograining. The extents of anodic and cathodic polarisation, i.e., the amplitude of the waveform employed, influence the final size of the hemispherical pits, as reported previously in studies of pit development at various current densities in nitric acid solutions [3]. In the present case, increase in current density transforms the hemispherical pits of average 3 lm diameter with their walls decorated by fine pits (size of 0.1–1 lm) to relatively large, smooth hemispherical pits of average 6 lm diameter (Fig. 2a and b). Increase in pit size suggests that the time for pit growth is extended with increased anodic and cathodic polarisations. Increase of anodic polarisation favours pit growth, generating increased concentrations of Al3+ ions within the pit that are available for precipitation. The more extensive precipitation evident at high current density is associated with availability of Al3+ ions together with maintenance of the local pH between 4 and 10 for increased periods of time during cathodic polarisation (Fig. 2d). Increased cathodic polarisation results in further alkalisation beneath the precipitate and the dissolution of the aluminium alloy. The generation of a highly alkaline solution near the surface may contribute to the smooth appearance of the pit walls after electrograining during high current densities.
4.3. Lateral pit growth during electrograining Development of lateral pits is observed for electrograining at low and medium current densities. The relatively limited anodic and cathodic polarisation acts to reduce the generation of Al3+ and OH ions, consequently restricting the time when the pH within the pit is maintained above 4. Thus, with little aluminium hydroxide precipitation, pit growth continues in the acidic conditions during anodic polarisation and into the region of cathodic polarisation. The absence of precipitation over the lateral pits confirms that the pH does not increase above 4. Such conditions favour the appearance of large lateral pits of 10–80 lm diameter, with terrace-like steps of width 1–2 lm. Their presence is explained readily; during anodic polarisation, an initial pit grows, and continues its development, but at a reduced rate, during cathodic polarisation. The local pH gradient along the surface, e.g., from about 4 within the pit to highly acid within a short distance from the pit, promotes development of further pits adjacent to the original pit during subsequent anodic polarisation. The
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separation of the original pit and the new pit is determined by the thickness of diffusion layer formed during cathodic reaction. Consequently, as electrograining proceeds, further new pits develop adjacent to previously pitted regions; with time, their presence gives rise to lateral pits of characteristic internal appearance. With deliberate increase of Al3+ ion concentration, through addition of aluminium nitrate, hydroxide precipitation commences at a reduced pH of 2.5 (Fig. 1a). However, this does not significantly influence the pH range over which the aluminium surface is passivated; additionally, the pit appearance is little changed, since the final size depends on the time for pit growth at low pH. This time depends on the rate of OH ion formation during cathodic polarisation, which may be equated to the volume of NaOH that must be added to commence precipitation in the solution. This volume is similar for any concentration of Al3+ ions in the solution (Fig. 1a). Increase of the time of pit growth, when the pH is low, though addition of alizarin red, results in lateral pit development during electrograining (Fig. 1c). Alizarin red forms aluminium–anthraquinone complexes with Al3+ ions and delays precipitation of aluminium hydroxide, thus extending the period of pit growth. The possibility of oxidation of aluminium–anthraquinone complexes at 0.42 V (SCE) has been considered [16], which can affect the aluminium hydroxide precipitation under fast cyclic polarisation. Reduction of the time over which a low pH is maintained is achieved by addition of 8-hydroxyquinoline (Fig. 1c), which precipitates tris-(8-hydroxyquinoline) aluminium at pH 2.5. The early pH increase within the pit gives more ready passivation of the pit walls; consequently, the resultant pit morphology changes markedly, revealing fine hemispherical pits together with lateral pits of significantly reduced dimensions. Pit morphologies, comprising fine hemispherical pits, result from electrograining in the presence of ammonium nitrate. The absence of the large hemispherical pits, with smooth or decorated walls, is associated with the increased time over which the pH is in the range 4–8.5. During cathodic polarisation, pH increase above 8.5 and consequent alkaline dissolution of the alloy is limited by the buffering ability of ammonium nitrate, which also restricts dissolution of the hydroxide precipitate within the pit. Further, during growth of the anodic film in the early period of anodic polarisation, pH reduction is expected; however, in the presence of a buffered solution of pH 8.5, a relatively compact anodic film growth proceeds leading to an increase in the pitting potential and a reduced time for pit growth.
5. Conclusions 1. During electrograining of aluminium in nitric acid based electrolytes, comprising rapid alternating cathodic and anodic polarisation, the electrochemical processes of aluminium dissolution, anodic film formation, and oxygen and hydrogen evolution proceed in the pits. The local pH close to the aluminium surface may thus vary from highly acidic to highly alkaline values.
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2. The presence of a passive film on the alloy leads to localisation of the electrochemical processes. Thus, the anodic and cathodic currents pass largely through existing pits, with consequent growth and merging to develop the electrograined appearance. 3. Growth of individual pits continues with time when the pH within the pit is below 4; namely during anodic polarisation above the pitting potential, with reversal of anodic polarisation and also in the early stages of cathodic polarisation. 4. Hydrogen evolution during cathodic polarisation leads to a pH increase and, due to the presence of aluminium ions from previous anodic polarisation, precipitation of aluminium hydroxides within the pits. The pit remains passive when the pH is maintained at values of about 4. Relatively uniform dissolution of the pit walls can result from further hydrogen evolution within the pit beneath the precipitate. 5. The appearance of the pits depends strongly on the time intervals, when the near surface pH, at particular range; specifically, the times when pit grows at pH below 4, aluminium is passivated at pH between 4 and 10, and uniform alkaline dissolution occurs at pH above 10. These time intervals can be controlled by current density used during electrograining and the presence of additions to the solution.
Acknowledgments The authors thank the Engineering and Physical Sciences Research Council (EPSRC) and Bridgnorth Aluminium Ltd., for financial support of the work.
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[15] E.V. Koroleva, G.E. Thompson, P. Skeldon, G. Hollrigl, G. Smith, S. Lockwood, Electrochem. Acta. in press. [16] P. Akhtar, H.A. Devereaux, A.J. Downard, B. OSullivan, K.J. Powell, Anal. Chim. Acta 381 (1999) 49.