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Anodizing of aluminium under applied electrode temperature: Process evaluation and elimination of burning at high current densities
Surface & Coatings Technology 204 (2010) 2754–2760
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Anodizing of aluminium under applied electrode temperature: Process evaluation and elimination of burning at high current densities Tim Aerts ⁎, Iris De Graeve, Hernan Terryn Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B–1050 Brussels, Belgium
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Article history: Received 1 October 2009 Accepted in revised form 9 February 2010 Available online 16 February 2010 Keywords: Aluminium Anodising Electrode temperature Scanning electron microscopy (SEM) Defects
a b s t r a c t Using a temperature controlling electrode holder anodizing of aluminium is studied under conditions of applied electrode temperature. At different current densities the influence of the electrode temperature on the anode potential and oxide morphology is evaluated. Additionally, the results are compared to those of experiments performed under conditions of uncontrolled electrode temperatures at various electrolyte temperatures. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. The stationary, as well as the initial maximum potential decline; though, at high electrode temperatures the latter is more sensitive to temperature variations. Due to the observed large decline of the maximum potential at high electrode temperature, the possibility to grow oxide layers at very high current densities without encountering burning is evaluated at low electrolyte temperatures. At high electrode temperatures of 65 °C the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25 °C anodizing can be performed up to 25 A/dm2 without observing burning. By increasing the electrolyte temperature up to 45 °C, even at 30 A/dm2 burning does not occur during anodizing at applied electrode temperature of 65 °C. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Porous anodizing of aluminium comprises the use of aggressive electrolytes and therefore involves oxide growth as well as oxide dissolution. The latter is mainly controlled by the electrolyte concentration, temperature and anodizing time [1]. Among these three parameters the anodizing time is actually chosen based on the rate of oxide growth, which is controlled by the anodic current density. Therefore, for a fixed electrolyte concentration the temperature and current density are the main factors determining the ratio between oxide growth and dissolution. The ‘temperature’ referred to in literature generally is the electrolyte temperature; the electrode temperature is rarely considered. In a limited number of studies the electrode temperature is measured during anodizing and, these studies clearly indicate the influence of this parameter on local oxide growth [2–6]. In other papers the temperature of the aluminium is actively influenced during anodizing, either to evaluate the effect on local oxide growth [7], either to improve the temperature control of the process [8–10]. In a previous publication the authors have introduced the approach of anodizing under conditions of applied electrode temperature [11,12]. In this study the separate
⁎ Correponding author. Tel.: + 32 2 629 35 35; fax: + 32 2 629 32 00. E-mail address: [email protected] (T. Aerts). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.031
influences of the electrolyte temperature and of the electrode temperature on the process were evaluated. The important effect of the electrode temperature, dominating that of the electrolyte temperature, was clearly demonstrated. The electrochemical behaviour of the anode, the morphology and thickness of the formed oxide layer, as well as the formation ratio, all displayed larger variations with increasing electrode temperatures than with increasing electrolyte temperatures. The previous study only considered experiments performed at a relatively low current density of 1 A/dm2. Depending on the particular process and applications, porous anodic oxide growth on aluminium is performed at current densities varying from low values of about 1 A/dm2 during anodizing for bright trim applications, to very high values of 30 A/dm2 in continuous coil anodizing [1,13]. The low current densities, up to say 5 A/dm2, are typically considered for batch anodizing processes, such as anodizing for architectural purposes. Due to the reduced growth rates they involve longer anodizing times for the production of the anodic layers. To avoid advanced oxide dissolution during prolonged exposure to the electrolyte, low temperatures (up to about 25 °C), and/or reduced concentrations are considered. Hard anodizing, for example, carries this to the extreme by anodizing at very low temperatures (generally 0 to 5 °C) in dilute electrolyte to grow oxide layers with thicknesses up to several hundred μm [1]. The above mentioned conditions are not favourable for anodizing at higher current densities as this might lead to local anomalous oxide
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growth referred to as ‘burning’. It gives rise to the formation of locally thicker but highly degraded oxide layers teSheasby2001, Aerts2008 and is often encountered during (batch) anodizing experiments performed at higher current densities [2–4,9,14–19]. This issue, however, is overcome during anodizing processes which involve high current densities, such as continuous or coil anodizing processes, by increased temperatures (commonly up to about 60 °C, or higher), in combination with conditions of intense electrolyte convection. The high current densities and resulting high rates of oxide formation enable avoiding excessive film attack by significantly reducing the necessary anodizing time. In the present paper the combined influences of temperature and current density on porous anodizing of aluminium are discussed. Using the developed temperature controlling electrode holder anodizing is performed under conditions of applied electrode temperatures. Hence, at different current densities the influence of the electrode temperature is studied, considering the anodic potentials and corresponding oxide morphology. Special attention is given to the relation between the initial maximum potential and the occurrence of burning. In the second part of the paper oxide growth at high current densities is evaluated at high electrode temperatures. 2. Experimental conditions Anodizing under conditions of applied and controlled electrode temperature (TAl) is performed using an in-house developed electrode holder. The design and configuration of this temperature controlling electrode holder is elaborately described in a previous work [11]. Due to the implementation of a thermo electronic component the holder is capable of either transferring heat towards, or removing heat from the aluminium electrode. Hence, ‘heated’, as well as ‘cooled’ anodes, i.e. with controlled temperatures respectively higher or lower than the electrolyte, can be considered. During the experiments this holder is immersed into a 50 l reservoir of a 145 g/l H2SO4 + 5 g/l Al2(SO4)3.18H2O solution, used as electrolyte. The electrolyte temperature (TH2SO4) is controlled up to ± 0.1 °C by a Lauda RP845 thermostat. The latter controls the flow of H2O through a glass heat exchanger immersed in the 50 l electrolyte reservoir and is equipped with an immersed PT-100 temperature probe. A mechanical stirrer provides intense electrolyte agitation. Due to the high rotation speed of the stirrer the anode potential is no longer affected by a further increase in rotation speed. Two electrolyte temperatures (TH2SO4) are considered: 25 and 45 °C. At both temperatures the influence of the electrode temperature is evaluated by applying temperatures between 5 and 65 °C in 10 °C steps. The anode material is AA1050 aluminium (99.5% Al sheet 0.3 mm), the diameter of the active surface limited to 10 mm. Prior to anodizing samples are alkaline etched in 60 g/l NaOH solution at 60 °C for 60s, followed by a desmutting treatment in a 1:1 concentrated HNO3:H2O solution at room temperature for 90s. The anodizing cell consists of a three electrode configuration, involving a large aluminium sheet as counter electrode and a Ag/AgCl reference electrode (+ 222 mV vs. SHE at 25 °C). Anodizing is performed under galvanostatic conditions, the current being applied by a computer controlled Delta Elektronica SM 300–20 power source. The first part of the paper focuses on the influence of the moderate current densities 1, 2, 4 and 8 A/dm2. In the second part the influence of high current densities from 12 up to 50 A/dm2 is evaluated. For all considered current densities a total charge density of 512 C/dm2 is considered. The resulting potential evolutions are recorded with a National Instruments M-6220 DAQ card and using National Instruments VILogger software. The presented electrode potentials are corrected for the Ohmic drop between the anode and reference electrode. To check reproducibility at least three separate experiments are performed for each considered condition.
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FE-SEM analyses are performed on a Jeol JSM-7000F FE-SEM. Charging effects due to the non-conductive properties of the oxide are avoided by covering the samples for FE-SEM analysis with a 1.5 nm Pt/ Pd layer, applied by a Cressington 208 HR sputter coater equipped with a Cressington MTM-20 thickness controller. 3. Results and discussion 3.1. Influence of TAl at current densities from 1 up to 8 A/dm2 In Fig. 1 the influence of the electrode temperature on the evolution of the electrode potential Uwe is presented for anodizing experiments at 2 A/dm2 in the electrolyte at 25 °C. The different curves in this plot, which are characteristic for these conditions, correspond to different applied electrode temperatures. These curves display typical evolutions of the electrode potential Uwe as recorded during galvanostatic anodizing [20,21]: a steep initial rise until a maximum potential is reached, followed by a decline to a quasi stationary value. This variation of the electrode potential is closely linked to different stages of the growth of the porous oxide, which is elaborately described in the latter two references. As can be observed, increasing the electrode temperature results in lower electrode potentials, despite the constant TH2SO4. Anodizing under conditions of applied electrode temperature therefore enables reducing the anodic potentials, even at low electrolyte temperatures. At TAl = 5 °C an average initial maximum potential (indicated by the arrow) and stationary potential of respectively 27.1 V and 21.6 V are recorded, whereas they are respectively only 4.9 V and 5.6 V when the electrode temperature is 65 °C. Of these two parameters the initial maximum potential Uwe,max displays the largest variation with increasing TAl. This observation is more clearly presented in Fig 2, in which the variations of Uwe,max (full lines) and Uwe,stat (dashed lines) with TAl are plotted for current densities of 1 up to 8 A/dm2. The subfigures (a) and (b) respectively correspond to electrolyte temperatures of 25 and 45 °C. Note that no data is provided for an electrode anodized at 8 A/dm2 under conditions of TAl = 5 °C and TH2SO4 = 25 °C. Due to intense burning under these conditions this electrode will not be considered in the following discussion. Uwe,max and Uwe,stat both decline at increasing TAl, with at low electrode temperatures Uwe,max being higher than Uwe,stat for all considered current densities and at both electrolyte temperatures. Up to a certain electrode temperature, depending on the current density and electrolyte temperature, they decrease with temperature
Fig. 1. Evolutions of the anode potential Uwe recorded at a current density of 2 A/dm2 at different applied electrode temperatures for an electrolyte temperature of 25 °C.
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By increasing the temperature of the electrolyte from 25 to 45 °C the values of Uwe,max at corresponding electrode temperatures slightly decrease — cf. Fig. 1 at 45 °C compared to Fig. 2(a) at 25 °C. Though, the influence of the increased electrolyte temperature is mainly reflected in the different characteristics of the evolution of Uwe,max with TAl. At TH2SO4 = 45 °C a similar evolution of Uwe,max vs. TAl is observed as at 25 °C, but at 45 °C the steep decline of Uwe,max starts at lower electrode temperatures. For example, at an applied current density of 8 A/dm2 Uwe,max already steeply declines from 25 °C on; at 1 A/dm2 this evolution even starts at 5 °C. The importance of the observed decrease of Uwe,max as a function of TAl is demonstrated by considering the morphology of the oxide films formed at 8 A/dm2. When applying this high current density under conditions of low TAl burning is encountered. In this case ‘low TAl’ is relative, as at 8 A/dm2 burning already occurs during anodizing at an applied electrode temperature of 25 °C. This is illustrated by Fig. 3, which shows a cross section of an oxide film at TAl = TH2SO4 = 25 °C. A cracked and undulating oxide surface is visible, characterised by protruding oxide hillocks [14]. The locally significantly increased oxide thickness at these hillocks is clearly visible. Not only at 8 A/dm2, but also at the second highest current density of 4 A/dm2 burning is evoked at low TAl. Under galvanostatic conditions this breakdown phenomenon typically arises during the initial transient phase after the attainment of Uwe,max and is due to the high values of the latter
Fig. 2. Evolution of the initial maximum electrode potential Uwe,max (full lines) and the stationary anode potential Uwe,stat (dashed lines) as a function of the applied electrode temperature T Al , recorded at different current densities. (a) T H2SO4 = 25 °C, (b) TH2SO4 = 45 °C.
at approximately the same rate; the full and dashed curves at corresponding current densities remain quasi parallel at low TAl. At 8 A/dm2 this parallel evolution is observed up to TAl = 35 °C, whereas at 1 A/dm2 it only lasts up to 15 °C. At electrode temperatures beyond this parallel evolution of both characteristic potentials, Uwe,max steeply declines and decreases to the value of stationary potential, as indicated in the graphs by circles. At high electrode temperatures Uwe,max is therefore more sensitive to variations of TAl than the stationary potential, displaying a steeper decline with TAl in this region than Uwe,stat. This decrease of Uwe,max indicates that the initial potential rise is more limited at high electrode temperatures, corresponding to a reduced thickening of the initially present barrier-like oxide [23]. The development of the porous oxide structure from the initial barrier-like film is apparently strongly enhanced at higher electrode temperatures. Another important observation is that this steep decline of Uwe,max at elevated TAl is not limited to the lower current densities but is also recorded at higher values; e.g. at a current density of 8 A/dm2 the initial maximum potential steeply decreases from TAl = 35 °C on, even at a relatively low electrolyte temperature of 25 °C.
Fig. 3. FE-SEM cross section images of oxide layer formed at 8 A/dm2 and at (a) TAl = TH2SO4 = 25 °C, (b) TAl = 25 °C in combination with TH2SO4 = 45 °C.
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[14]. In a previous study [14] the authors reported a critical value of Uwe,max, above which burning is observed, of approximately 27 V. At 8 A/dm2 increasing TH2SO4 up to 45 °C, at a constant electrode temperature of 25 °C, does not allow to overcome the occurrence of burning at 8 A/dm2. As shown by Fig. 2 burning still arises at the higher electrolyte temperature if TAl is kept at 25 °C. This observation is in line with the high value of Uwe,max, recorded under these conditions. As during anodizing at applied electrode temperatures the electrode potentials are mainly determined by TAl, rather than by TH2SO4 [12], Uwe,max will not significantly decrease by increasing TH2SO4 at constant TAl. At low values of the latter burning will therefore persist during anodizing at high current densities. On the other hand, burning can even be avoided at lower electrolyte temperatures by increasing the electrode temperature resulting in the observed decrease of Uwe,max. This is illustrated by the normal morphology of the oxide layer presented in Fig. 4, which is formed at 8 A/dm2 in the electrolyte at 25 °C, but at an increased electrode temperature of 45 °C. Considering anodizing under conditions of uncontrolled electrode temperature, i.e. the ‘conventional’ approach, the recorded variations of Uwe,max as a function of the electrolyte temperature are displayed in Fig. 5. During these experiments TAl is not applied and the influence of temperature is therefore evaluated by changing the electrolyte temperature. A similar behaviour as in Fig. 2 is encountered; at low electrolyte temperatures a moderate decrease in Uwe,max is observed, followed by a steep decline at higher TH2SO4. The electrolyte temperature at which this steep decline starts, increases with current density, as is clearly visible in Fig. 5. However, by increasing TH2SO4 under conditions of uncontrolled electrode temperature Uwe,max does not decline to the same extent as by increasing the applied TAl. At uncontrolled electrode temperatures Uwe,max maintains high values up to higher electrolyte temperatures, an effect which is enhanced at higher current densities. For example, at uncontrolled electrode temperature anodizing at 8 A/dm2 a value of Uwe,max = 27.0 ± 0.2 V is still recorded at TH2SO4 = 45 °C — cf. Fig. 5. As a result, under the conventional approach burning is not limited to low electrolyte temperatures and is still encountered at higher TH2SO4. As shown in Fig. 6, burning is observed during anodizing at 8 A/dm2 under conditions of uncontrolled electrode temperature up to TH2SO4 = 45 °C. Hence, under the conventional approach increasing the electrolyte temperature to enable anodizing at high current densities without observing burning, is only efficient to a lesser degree; very high electrolyte temperatures need to be considered at
Fig 4. FE-SEM cross section images of oxide layer formed at 8 A/dm2 under conditions of TH2SO4 = 25 °C and TAl = 45 °C.
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Fig. 5. Evolution of the initial maximum electrode potential Uwe,max as a function of the electrolyte temperature, recorded at different current densities under conditions of uncontrolled electrode temperature.
increasing current densities. For example, at 8 A/dm2 the required electrolyte temperature to avoid burning corresponds to 55 °C. This is in contrast to anodizing under applied electrode temperatures, where applying high TAl enables anodizing at high current densities without observing burning, even at low electrolyte temperatures. 3.2. High applied current densities The observed significant decrease in Uwe,max at high electrode temperatures is further evaluated by experiments at TAl = 65 °C, while applying very high current densities. The characteristic potential evolutions recorded during these experiments are presented in Fig. 7 for electrolyte temperatures of 25 °C (Fig 7(a)) and 45 °C (Fig 7(b)). Note that Fig. 7 does not display the anode potentials versus time but versus passed charge density. The ripple present in the potential evolutions in Fig. 6 is not caused by anomalous oxide growth but is due to the significant temperature difference between TAl and TH2SO4. Using the temperature controlling
Fig. 6. FE-SEM cross section images of oxide layer formed at 8 A/dm2 under conditions of TH2SO4 = 45 °C and uncontrolled TAl.
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Fig. 7. Evolutions of the anode potential Uwe recorded at different high current densities at TAl = 65 °C. (a) TH2SO4 = 25 °C, (b) TH2SO4 = 45 °C.
electrode holder it is possible to apply electrode temperature which significantly differ from the electrolyte temperature, but with increasing difference between TAl and TH2SO4 fluctuations in the anode potential occur and become more pronounced. In Fig. 7(b), corresponding to TH2SO4 = 45 °C, for example, the ripple in the potential evolutions logged at TAl = 65 °C is reduced compared to Fig. 7(a). These variations in the electrode potentials are induced by fluctuations of the actual electrode temperature. A measure for the ripple on the electrode potentials is obtained by considering the standard deviation of the potential values recorded under stationary conditions (i.e. after passing the initial potential transient). For the potential evolutions, presented in Fig. 7(a), these standard deviations vary between 0.2 V and 0.25 V. An estimation of the corresponding fluctuations of the actual electrode temperature, inducing these potential variations, can be obtained by considering a previous study by the authors [22]. This study considers anodizing experiments performed under conditions of controlled convection in an electrochemical reactor with a wall-jet geometry. Similar to the presented results, variations in the local electrode temperatures were considered in [22]. However, in the latter case they were deliberately evoked during the anodizing process by changing the electrolyte convection (and thus influencing the convective heat transfer). Based on the
recorded variation of the potential, combined with the corresponding variation of the in-situ recorded electrode temperature, an influence of variations in the electrode temperature on the stationary electrode potential could be determined. For the anodizing experiments at 8 A/dm2 in the same electrolyte as considered in the submitted paper, at a temperature of 45 °C, an influence of 0.25 V/°C is observed. By applying this value to the potentials evolutions presented in Fig. 7(a) the encountered fluctuations of the actual electrode temperatures can thus be estimated to equal ±1 °C. Despite the relatively low electrolyte temperature of 25 °C anodizing is performed up to very high current densities like 30 A/dm2 without observing very high anode potentials (see Fig. 6). By increasing the temperature of the electrolyte to 45 °C at TAl = 65 °C the electrode potentials are even further reduced, as presented in Fig. 7(b). These figures confirm that by increasing the electrode temperature up to high values the anodic potential scan significantly be reduced. Concerning Uwe,max, its values recorded at the three highest current densities are summarized in Table 1 for both electrolyte temperatures. At an applied electrode temperature of 65 °C the value of Uwe,max, measured at a very high current density of 25 A/dm2 only equals 23.0± 0.4 V at TH2SO4 = 25 °C. In comparison, if anodizing is performed under conditions of uncontrolled TAl in this electrolyte, applying a current density of only 2 A/dm2 already leads to Uwe,max = 24.1 ± 0.3 V. Under controlled conditions even at an applied current density of 30 A/dm2 the initial maximum potential equals merely 25.4 ± 0.6 V. When the electrolyte temperature is increased up to 45 °C the recorded value of Uwe,max at 30 A/dm2 further decreases to 22.5 ± 0.4 V, whereas even at a very high current density of 50 A/dm2 Uwe,max is limited to 27.4 ± 0.2 V. Similar to the results at the lower current densities presented in Section 3.1, the strongly reduced values of Uwe,max at high applied electrode temperatures enable growing oxide layers at very high current densities without encountering burning. At an electrolyte temperature of 25 °C, an oxide with a normal porous structure is still formed at an applied current density of 25 A/dm2. As presented in Fig. 8(a) the corresponding anodic film displays straight, wellaligned pores, and no features indicating the occurrence of burning are observed on these electrodes. At TH2SO4 = 45 °C, anodizing without encountering burning is even possible up to a current density of 30 A/dm2 at an electrode temperature of 65 °C. This is illustrated by the cross-section image in Fig. 8(b), which also shows an oxide layer with straight and well-aligned pores formed under these conditions. For both electrolyte temperatures these very high densities lead to very high growth rates. By anodizing at 25 A/dm2 at TH2SO4 = 25 °C an average oxide thickness of 2.6 ± 0.2 μm is obtained, corresponding to a growth rate of 7.4 μm/min. The highercurrent density of 30 A/dm2, which could be applied at TH2SO4 = 45 °C yields an oxide layer with a comparable average thickness of 2.7 ± 0.2 μm. Due to the smaller anodizing time in the latter case, this results in a higher further increased growth rate of 9.5 μm/min. These values are significantly higher than some of the ‘high growth rates’ of e.g. 0.85 up to 1.2 μm/ min [8] reported in literature. The comparable morphology of the oxides displayed in Fig. 8, with their straight and well-aligned pores, is in line with the literature. A high current density, and therefore high electric field strength, is
Table 1 Summary of the Uwe,max recorded at the three highest considered current densities j during anodizing at TAl = 65 °C, for electrolyte temperatures of 25 and 45 °C. j (A/dm2)
TH2SO4 = 25 °C
j (A/dm2)
TH2SO4 = 45 °C
20 25 30
21.9 ± 0.6 V 23.0 ± 0.4 V 25.4 ± 0.6 V
30 40 50
22.5 ± 0.4 V 25.4 ± 0.3 V 27.4 ± 0.2 V
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example, the occurrence of burning is reported to be avoided at the beginning of the anodizing process by considering a gradual increase of the applied current or potential [8,9,24], and/or by considering a pre-formed porous oxide layer [25]. Furthermore, due to the low temperatures considered in the literature [15,19] the reported current densities at applied sub-critical potentials are also lower than the 30 A/dm2 recorded in the present work. The two current densities 25 and 30 A/dm2, at respectively TH2SO4 = 25 °C and 45 °C, still allow normal oxide growth, whereas by increasing these current densities respectively to 30 and 40 A/dm2, burning is observed. Images of the oxides, formed at the latter conditions, are displayed in Figs 9a and b. Anodic layers with protruding oxide hillocks and a cracked oxide surface, characteristic for burning, are present. In both cases it does not involve intense burning, though, the phenomenon is nonetheless encountered. The recorded values of Uwe,max, corresponding to these conditions, both exceed 25 V (cf. Table 1), whereas this is not the case for the high current density conditions which do not lead to burning (i.e. 25 A/dm2 at TH2SO4 = 25 °C, and 30 A/dm2 at TH2SO4 = 45 °C). Moreover, the observation of this critical limit of 25 V for Uwe,max is registered for all anodizing conditions considered in the present study. For example, during anodizing at 8 A/dm2 at temperatures TAl = 45 °C and TH2SO4 = 25 us a value of Uwe,max just below 25 V is recorded, and burning is not observed. On the other hand, anodizing
Fig. 8. FE-SEM cross section images of oxide layer formed at TAl = 65 °C while applying a current density of (a) 25 A/dm2 at TH2SO4 = 25 °C–(b) 30 A/dm2 at TH2SO4 = 45 °C.
acknowledged to be a key factor determining the self-ordering behaviour during the growth of porous anodic alumina films [8,15– 17,19,24,25]. Anodizing experiments performed at very high current densities, corresponding to conditions just below the critical condition leading to burning, are observed to produce self-ordered structures [15,17,19]. Although the level of self-ordering of the films displayed in Fig. 8 is not investigated in detail, the observed morphologies are indicative for the tendency to self-ordering under the considered high current density conditions. The cited studies on the growth of self-ordered anodic alumina films [8,15–17,19,24,25] generally consider low temperatures, often in combination with low electrolyte concentrations. These conditions are sometimes explicitly referred to as ‘hard anodizing conditions’ [8,25]. They are considered for the intended growth of very thick oxide layers (N100 μm), which is enabled by reducing the oxide dissolution due to the reduced electrolyte concentration and temperature. To this purpose even a passive cooling of the electrode is considered [8,25]. The present work, on the other hand, considers high temperatures, i.e. high applied electrode temperatures more precisely. Although this approach is not suited for the production of the previously mentioned very thick oxide films, it enables immediately (i.e. from the onset of anodizing on) applying very high current densities and obtaining very high growth rates without encountering burning. This is not possible under hard anodizing conditions. In studies on the growth of self-ordered anodic alumina films, for
Fig. 9. FE-SEM cross section images of oxide layer formed at TAl = 65 °C while applying a current density of (a) 30 A/dm2 at TH2SO4 = 25 °C–(b) 40 A/dm2 at TH2SO4 = 45 °C.
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at this current density considering temperatures of TAl = 25 °C and TH2SO4 = 25 °C, or of TAl = 25 °C and TH2SO4 = 45 °C, leads to values of Uwe,max exceeding 25 V and burning is encountered. Hence, the critical value of the initial maximum potential above which burning is observed, previously reported to approximately equal 27 V [14], needs to be adjusted. By elaborate evaluation at different current densities and temperatures a more accurate value of 25 V (versus an Ag/AgCl reference electrode) is determined for the considered aluminium electrodes and galvanostatic conditions.
Acknowledgements
4. Conclusions
References
Anodizing of aluminium is performed at different current densities under conditions of applied and controlled electrode temperature. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. This holds for the stationary, as well as for the initial maximum potentials, though, at high electrode temperatures the initial maximum potential is more sensitive to temperature variations and declines at a higher rate. Furthermore, increasing the electrolyte temperature, at controlled or uncontrolled electrode temperature, does not affect the maximum potential to the same extent. The large decline of the maximum potential, obtained by increasing the electrode temperature, enables anodizing at high current densities without the occurrence of burning, even at low electrolyte temperatures. At high electrode temperatures of 65 s the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25 °C anodizing can be performed up to 25 A/dm2 without observing burning. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning. An elaborate evaluation at different anodizing conditions indicates a critical value of the initial maximum anode potential equal to 25 V under galvanostatic conditions. At different current densities and temperatures considered in this study burning is encountered during anodizing experiments characterised by an initial maximum potential higher than this value.
The authors acknowledge the support from the Instituut voor de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract number SBO 040092). Raf Claessens and Marnix Depauw are greatly acknowledged for the design and development of the temperature controlling electrode holder.
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Anodizing of aluminium under applied electrode temperature: Process evaluation and elimination of burning at high current densities Tim Aerts ⁎, Iris De Graeve, Hernan Terryn Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B–1050 Brussels, Belgium
a r t i c l e
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Article history: Received 1 October 2009 Accepted in revised form 9 February 2010 Available online 16 February 2010 Keywords: Aluminium Anodising Electrode temperature Scanning electron microscopy (SEM) Defects
a b s t r a c t Using a temperature controlling electrode holder anodizing of aluminium is studied under conditions of applied electrode temperature. At different current densities the influence of the electrode temperature on the anode potential and oxide morphology is evaluated. Additionally, the results are compared to those of experiments performed under conditions of uncontrolled electrode temperatures at various electrolyte temperatures. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. The stationary, as well as the initial maximum potential decline; though, at high electrode temperatures the latter is more sensitive to temperature variations. Due to the observed large decline of the maximum potential at high electrode temperature, the possibility to grow oxide layers at very high current densities without encountering burning is evaluated at low electrolyte temperatures. At high electrode temperatures of 65 °C the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25 °C anodizing can be performed up to 25 A/dm2 without observing burning. By increasing the electrolyte temperature up to 45 °C, even at 30 A/dm2 burning does not occur during anodizing at applied electrode temperature of 65 °C. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Porous anodizing of aluminium comprises the use of aggressive electrolytes and therefore involves oxide growth as well as oxide dissolution. The latter is mainly controlled by the electrolyte concentration, temperature and anodizing time [1]. Among these three parameters the anodizing time is actually chosen based on the rate of oxide growth, which is controlled by the anodic current density. Therefore, for a fixed electrolyte concentration the temperature and current density are the main factors determining the ratio between oxide growth and dissolution. The ‘temperature’ referred to in literature generally is the electrolyte temperature; the electrode temperature is rarely considered. In a limited number of studies the electrode temperature is measured during anodizing and, these studies clearly indicate the influence of this parameter on local oxide growth [2–6]. In other papers the temperature of the aluminium is actively influenced during anodizing, either to evaluate the effect on local oxide growth [7], either to improve the temperature control of the process [8–10]. In a previous publication the authors have introduced the approach of anodizing under conditions of applied electrode temperature [11,12]. In this study the separate
⁎ Correponding author. Tel.: + 32 2 629 35 35; fax: + 32 2 629 32 00. E-mail address: [email protected] (T. Aerts). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.031
influences of the electrolyte temperature and of the electrode temperature on the process were evaluated. The important effect of the electrode temperature, dominating that of the electrolyte temperature, was clearly demonstrated. The electrochemical behaviour of the anode, the morphology and thickness of the formed oxide layer, as well as the formation ratio, all displayed larger variations with increasing electrode temperatures than with increasing electrolyte temperatures. The previous study only considered experiments performed at a relatively low current density of 1 A/dm2. Depending on the particular process and applications, porous anodic oxide growth on aluminium is performed at current densities varying from low values of about 1 A/dm2 during anodizing for bright trim applications, to very high values of 30 A/dm2 in continuous coil anodizing [1,13]. The low current densities, up to say 5 A/dm2, are typically considered for batch anodizing processes, such as anodizing for architectural purposes. Due to the reduced growth rates they involve longer anodizing times for the production of the anodic layers. To avoid advanced oxide dissolution during prolonged exposure to the electrolyte, low temperatures (up to about 25 °C), and/or reduced concentrations are considered. Hard anodizing, for example, carries this to the extreme by anodizing at very low temperatures (generally 0 to 5 °C) in dilute electrolyte to grow oxide layers with thicknesses up to several hundred μm [1]. The above mentioned conditions are not favourable for anodizing at higher current densities as this might lead to local anomalous oxide
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growth referred to as ‘burning’. It gives rise to the formation of locally thicker but highly degraded oxide layers teSheasby2001, Aerts2008 and is often encountered during (batch) anodizing experiments performed at higher current densities [2–4,9,14–19]. This issue, however, is overcome during anodizing processes which involve high current densities, such as continuous or coil anodizing processes, by increased temperatures (commonly up to about 60 °C, or higher), in combination with conditions of intense electrolyte convection. The high current densities and resulting high rates of oxide formation enable avoiding excessive film attack by significantly reducing the necessary anodizing time. In the present paper the combined influences of temperature and current density on porous anodizing of aluminium are discussed. Using the developed temperature controlling electrode holder anodizing is performed under conditions of applied electrode temperatures. Hence, at different current densities the influence of the electrode temperature is studied, considering the anodic potentials and corresponding oxide morphology. Special attention is given to the relation between the initial maximum potential and the occurrence of burning. In the second part of the paper oxide growth at high current densities is evaluated at high electrode temperatures. 2. Experimental conditions Anodizing under conditions of applied and controlled electrode temperature (TAl) is performed using an in-house developed electrode holder. The design and configuration of this temperature controlling electrode holder is elaborately described in a previous work [11]. Due to the implementation of a thermo electronic component the holder is capable of either transferring heat towards, or removing heat from the aluminium electrode. Hence, ‘heated’, as well as ‘cooled’ anodes, i.e. with controlled temperatures respectively higher or lower than the electrolyte, can be considered. During the experiments this holder is immersed into a 50 l reservoir of a 145 g/l H2SO4 + 5 g/l Al2(SO4)3.18H2O solution, used as electrolyte. The electrolyte temperature (TH2SO4) is controlled up to ± 0.1 °C by a Lauda RP845 thermostat. The latter controls the flow of H2O through a glass heat exchanger immersed in the 50 l electrolyte reservoir and is equipped with an immersed PT-100 temperature probe. A mechanical stirrer provides intense electrolyte agitation. Due to the high rotation speed of the stirrer the anode potential is no longer affected by a further increase in rotation speed. Two electrolyte temperatures (TH2SO4) are considered: 25 and 45 °C. At both temperatures the influence of the electrode temperature is evaluated by applying temperatures between 5 and 65 °C in 10 °C steps. The anode material is AA1050 aluminium (99.5% Al sheet 0.3 mm), the diameter of the active surface limited to 10 mm. Prior to anodizing samples are alkaline etched in 60 g/l NaOH solution at 60 °C for 60s, followed by a desmutting treatment in a 1:1 concentrated HNO3:H2O solution at room temperature for 90s. The anodizing cell consists of a three electrode configuration, involving a large aluminium sheet as counter electrode and a Ag/AgCl reference electrode (+ 222 mV vs. SHE at 25 °C). Anodizing is performed under galvanostatic conditions, the current being applied by a computer controlled Delta Elektronica SM 300–20 power source. The first part of the paper focuses on the influence of the moderate current densities 1, 2, 4 and 8 A/dm2. In the second part the influence of high current densities from 12 up to 50 A/dm2 is evaluated. For all considered current densities a total charge density of 512 C/dm2 is considered. The resulting potential evolutions are recorded with a National Instruments M-6220 DAQ card and using National Instruments VILogger software. The presented electrode potentials are corrected for the Ohmic drop between the anode and reference electrode. To check reproducibility at least three separate experiments are performed for each considered condition.
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FE-SEM analyses are performed on a Jeol JSM-7000F FE-SEM. Charging effects due to the non-conductive properties of the oxide are avoided by covering the samples for FE-SEM analysis with a 1.5 nm Pt/ Pd layer, applied by a Cressington 208 HR sputter coater equipped with a Cressington MTM-20 thickness controller. 3. Results and discussion 3.1. Influence of TAl at current densities from 1 up to 8 A/dm2 In Fig. 1 the influence of the electrode temperature on the evolution of the electrode potential Uwe is presented for anodizing experiments at 2 A/dm2 in the electrolyte at 25 °C. The different curves in this plot, which are characteristic for these conditions, correspond to different applied electrode temperatures. These curves display typical evolutions of the electrode potential Uwe as recorded during galvanostatic anodizing [20,21]: a steep initial rise until a maximum potential is reached, followed by a decline to a quasi stationary value. This variation of the electrode potential is closely linked to different stages of the growth of the porous oxide, which is elaborately described in the latter two references. As can be observed, increasing the electrode temperature results in lower electrode potentials, despite the constant TH2SO4. Anodizing under conditions of applied electrode temperature therefore enables reducing the anodic potentials, even at low electrolyte temperatures. At TAl = 5 °C an average initial maximum potential (indicated by the arrow) and stationary potential of respectively 27.1 V and 21.6 V are recorded, whereas they are respectively only 4.9 V and 5.6 V when the electrode temperature is 65 °C. Of these two parameters the initial maximum potential Uwe,max displays the largest variation with increasing TAl. This observation is more clearly presented in Fig 2, in which the variations of Uwe,max (full lines) and Uwe,stat (dashed lines) with TAl are plotted for current densities of 1 up to 8 A/dm2. The subfigures (a) and (b) respectively correspond to electrolyte temperatures of 25 and 45 °C. Note that no data is provided for an electrode anodized at 8 A/dm2 under conditions of TAl = 5 °C and TH2SO4 = 25 °C. Due to intense burning under these conditions this electrode will not be considered in the following discussion. Uwe,max and Uwe,stat both decline at increasing TAl, with at low electrode temperatures Uwe,max being higher than Uwe,stat for all considered current densities and at both electrolyte temperatures. Up to a certain electrode temperature, depending on the current density and electrolyte temperature, they decrease with temperature
Fig. 1. Evolutions of the anode potential Uwe recorded at a current density of 2 A/dm2 at different applied electrode temperatures for an electrolyte temperature of 25 °C.
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By increasing the temperature of the electrolyte from 25 to 45 °C the values of Uwe,max at corresponding electrode temperatures slightly decrease — cf. Fig. 1 at 45 °C compared to Fig. 2(a) at 25 °C. Though, the influence of the increased electrolyte temperature is mainly reflected in the different characteristics of the evolution of Uwe,max with TAl. At TH2SO4 = 45 °C a similar evolution of Uwe,max vs. TAl is observed as at 25 °C, but at 45 °C the steep decline of Uwe,max starts at lower electrode temperatures. For example, at an applied current density of 8 A/dm2 Uwe,max already steeply declines from 25 °C on; at 1 A/dm2 this evolution even starts at 5 °C. The importance of the observed decrease of Uwe,max as a function of TAl is demonstrated by considering the morphology of the oxide films formed at 8 A/dm2. When applying this high current density under conditions of low TAl burning is encountered. In this case ‘low TAl’ is relative, as at 8 A/dm2 burning already occurs during anodizing at an applied electrode temperature of 25 °C. This is illustrated by Fig. 3, which shows a cross section of an oxide film at TAl = TH2SO4 = 25 °C. A cracked and undulating oxide surface is visible, characterised by protruding oxide hillocks [14]. The locally significantly increased oxide thickness at these hillocks is clearly visible. Not only at 8 A/dm2, but also at the second highest current density of 4 A/dm2 burning is evoked at low TAl. Under galvanostatic conditions this breakdown phenomenon typically arises during the initial transient phase after the attainment of Uwe,max and is due to the high values of the latter
Fig. 2. Evolution of the initial maximum electrode potential Uwe,max (full lines) and the stationary anode potential Uwe,stat (dashed lines) as a function of the applied electrode temperature T Al , recorded at different current densities. (a) T H2SO4 = 25 °C, (b) TH2SO4 = 45 °C.
at approximately the same rate; the full and dashed curves at corresponding current densities remain quasi parallel at low TAl. At 8 A/dm2 this parallel evolution is observed up to TAl = 35 °C, whereas at 1 A/dm2 it only lasts up to 15 °C. At electrode temperatures beyond this parallel evolution of both characteristic potentials, Uwe,max steeply declines and decreases to the value of stationary potential, as indicated in the graphs by circles. At high electrode temperatures Uwe,max is therefore more sensitive to variations of TAl than the stationary potential, displaying a steeper decline with TAl in this region than Uwe,stat. This decrease of Uwe,max indicates that the initial potential rise is more limited at high electrode temperatures, corresponding to a reduced thickening of the initially present barrier-like oxide [23]. The development of the porous oxide structure from the initial barrier-like film is apparently strongly enhanced at higher electrode temperatures. Another important observation is that this steep decline of Uwe,max at elevated TAl is not limited to the lower current densities but is also recorded at higher values; e.g. at a current density of 8 A/dm2 the initial maximum potential steeply decreases from TAl = 35 °C on, even at a relatively low electrolyte temperature of 25 °C.
Fig. 3. FE-SEM cross section images of oxide layer formed at 8 A/dm2 and at (a) TAl = TH2SO4 = 25 °C, (b) TAl = 25 °C in combination with TH2SO4 = 45 °C.
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[14]. In a previous study [14] the authors reported a critical value of Uwe,max, above which burning is observed, of approximately 27 V. At 8 A/dm2 increasing TH2SO4 up to 45 °C, at a constant electrode temperature of 25 °C, does not allow to overcome the occurrence of burning at 8 A/dm2. As shown by Fig. 2 burning still arises at the higher electrolyte temperature if TAl is kept at 25 °C. This observation is in line with the high value of Uwe,max, recorded under these conditions. As during anodizing at applied electrode temperatures the electrode potentials are mainly determined by TAl, rather than by TH2SO4 [12], Uwe,max will not significantly decrease by increasing TH2SO4 at constant TAl. At low values of the latter burning will therefore persist during anodizing at high current densities. On the other hand, burning can even be avoided at lower electrolyte temperatures by increasing the electrode temperature resulting in the observed decrease of Uwe,max. This is illustrated by the normal morphology of the oxide layer presented in Fig. 4, which is formed at 8 A/dm2 in the electrolyte at 25 °C, but at an increased electrode temperature of 45 °C. Considering anodizing under conditions of uncontrolled electrode temperature, i.e. the ‘conventional’ approach, the recorded variations of Uwe,max as a function of the electrolyte temperature are displayed in Fig. 5. During these experiments TAl is not applied and the influence of temperature is therefore evaluated by changing the electrolyte temperature. A similar behaviour as in Fig. 2 is encountered; at low electrolyte temperatures a moderate decrease in Uwe,max is observed, followed by a steep decline at higher TH2SO4. The electrolyte temperature at which this steep decline starts, increases with current density, as is clearly visible in Fig. 5. However, by increasing TH2SO4 under conditions of uncontrolled electrode temperature Uwe,max does not decline to the same extent as by increasing the applied TAl. At uncontrolled electrode temperatures Uwe,max maintains high values up to higher electrolyte temperatures, an effect which is enhanced at higher current densities. For example, at uncontrolled electrode temperature anodizing at 8 A/dm2 a value of Uwe,max = 27.0 ± 0.2 V is still recorded at TH2SO4 = 45 °C — cf. Fig. 5. As a result, under the conventional approach burning is not limited to low electrolyte temperatures and is still encountered at higher TH2SO4. As shown in Fig. 6, burning is observed during anodizing at 8 A/dm2 under conditions of uncontrolled electrode temperature up to TH2SO4 = 45 °C. Hence, under the conventional approach increasing the electrolyte temperature to enable anodizing at high current densities without observing burning, is only efficient to a lesser degree; very high electrolyte temperatures need to be considered at
Fig 4. FE-SEM cross section images of oxide layer formed at 8 A/dm2 under conditions of TH2SO4 = 25 °C and TAl = 45 °C.
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Fig. 5. Evolution of the initial maximum electrode potential Uwe,max as a function of the electrolyte temperature, recorded at different current densities under conditions of uncontrolled electrode temperature.
increasing current densities. For example, at 8 A/dm2 the required electrolyte temperature to avoid burning corresponds to 55 °C. This is in contrast to anodizing under applied electrode temperatures, where applying high TAl enables anodizing at high current densities without observing burning, even at low electrolyte temperatures. 3.2. High applied current densities The observed significant decrease in Uwe,max at high electrode temperatures is further evaluated by experiments at TAl = 65 °C, while applying very high current densities. The characteristic potential evolutions recorded during these experiments are presented in Fig. 7 for electrolyte temperatures of 25 °C (Fig 7(a)) and 45 °C (Fig 7(b)). Note that Fig. 7 does not display the anode potentials versus time but versus passed charge density. The ripple present in the potential evolutions in Fig. 6 is not caused by anomalous oxide growth but is due to the significant temperature difference between TAl and TH2SO4. Using the temperature controlling
Fig. 6. FE-SEM cross section images of oxide layer formed at 8 A/dm2 under conditions of TH2SO4 = 45 °C and uncontrolled TAl.
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Fig. 7. Evolutions of the anode potential Uwe recorded at different high current densities at TAl = 65 °C. (a) TH2SO4 = 25 °C, (b) TH2SO4 = 45 °C.
electrode holder it is possible to apply electrode temperature which significantly differ from the electrolyte temperature, but with increasing difference between TAl and TH2SO4 fluctuations in the anode potential occur and become more pronounced. In Fig. 7(b), corresponding to TH2SO4 = 45 °C, for example, the ripple in the potential evolutions logged at TAl = 65 °C is reduced compared to Fig. 7(a). These variations in the electrode potentials are induced by fluctuations of the actual electrode temperature. A measure for the ripple on the electrode potentials is obtained by considering the standard deviation of the potential values recorded under stationary conditions (i.e. after passing the initial potential transient). For the potential evolutions, presented in Fig. 7(a), these standard deviations vary between 0.2 V and 0.25 V. An estimation of the corresponding fluctuations of the actual electrode temperature, inducing these potential variations, can be obtained by considering a previous study by the authors [22]. This study considers anodizing experiments performed under conditions of controlled convection in an electrochemical reactor with a wall-jet geometry. Similar to the presented results, variations in the local electrode temperatures were considered in [22]. However, in the latter case they were deliberately evoked during the anodizing process by changing the electrolyte convection (and thus influencing the convective heat transfer). Based on the
recorded variation of the potential, combined with the corresponding variation of the in-situ recorded electrode temperature, an influence of variations in the electrode temperature on the stationary electrode potential could be determined. For the anodizing experiments at 8 A/dm2 in the same electrolyte as considered in the submitted paper, at a temperature of 45 °C, an influence of 0.25 V/°C is observed. By applying this value to the potentials evolutions presented in Fig. 7(a) the encountered fluctuations of the actual electrode temperatures can thus be estimated to equal ±1 °C. Despite the relatively low electrolyte temperature of 25 °C anodizing is performed up to very high current densities like 30 A/dm2 without observing very high anode potentials (see Fig. 6). By increasing the temperature of the electrolyte to 45 °C at TAl = 65 °C the electrode potentials are even further reduced, as presented in Fig. 7(b). These figures confirm that by increasing the electrode temperature up to high values the anodic potential scan significantly be reduced. Concerning Uwe,max, its values recorded at the three highest current densities are summarized in Table 1 for both electrolyte temperatures. At an applied electrode temperature of 65 °C the value of Uwe,max, measured at a very high current density of 25 A/dm2 only equals 23.0± 0.4 V at TH2SO4 = 25 °C. In comparison, if anodizing is performed under conditions of uncontrolled TAl in this electrolyte, applying a current density of only 2 A/dm2 already leads to Uwe,max = 24.1 ± 0.3 V. Under controlled conditions even at an applied current density of 30 A/dm2 the initial maximum potential equals merely 25.4 ± 0.6 V. When the electrolyte temperature is increased up to 45 °C the recorded value of Uwe,max at 30 A/dm2 further decreases to 22.5 ± 0.4 V, whereas even at a very high current density of 50 A/dm2 Uwe,max is limited to 27.4 ± 0.2 V. Similar to the results at the lower current densities presented in Section 3.1, the strongly reduced values of Uwe,max at high applied electrode temperatures enable growing oxide layers at very high current densities without encountering burning. At an electrolyte temperature of 25 °C, an oxide with a normal porous structure is still formed at an applied current density of 25 A/dm2. As presented in Fig. 8(a) the corresponding anodic film displays straight, wellaligned pores, and no features indicating the occurrence of burning are observed on these electrodes. At TH2SO4 = 45 °C, anodizing without encountering burning is even possible up to a current density of 30 A/dm2 at an electrode temperature of 65 °C. This is illustrated by the cross-section image in Fig. 8(b), which also shows an oxide layer with straight and well-aligned pores formed under these conditions. For both electrolyte temperatures these very high densities lead to very high growth rates. By anodizing at 25 A/dm2 at TH2SO4 = 25 °C an average oxide thickness of 2.6 ± 0.2 μm is obtained, corresponding to a growth rate of 7.4 μm/min. The highercurrent density of 30 A/dm2, which could be applied at TH2SO4 = 45 °C yields an oxide layer with a comparable average thickness of 2.7 ± 0.2 μm. Due to the smaller anodizing time in the latter case, this results in a higher further increased growth rate of 9.5 μm/min. These values are significantly higher than some of the ‘high growth rates’ of e.g. 0.85 up to 1.2 μm/ min [8] reported in literature. The comparable morphology of the oxides displayed in Fig. 8, with their straight and well-aligned pores, is in line with the literature. A high current density, and therefore high electric field strength, is
Table 1 Summary of the Uwe,max recorded at the three highest considered current densities j during anodizing at TAl = 65 °C, for electrolyte temperatures of 25 and 45 °C. j (A/dm2)
TH2SO4 = 25 °C
j (A/dm2)
TH2SO4 = 45 °C
20 25 30
21.9 ± 0.6 V 23.0 ± 0.4 V 25.4 ± 0.6 V
30 40 50
22.5 ± 0.4 V 25.4 ± 0.3 V 27.4 ± 0.2 V
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example, the occurrence of burning is reported to be avoided at the beginning of the anodizing process by considering a gradual increase of the applied current or potential [8,9,24], and/or by considering a pre-formed porous oxide layer [25]. Furthermore, due to the low temperatures considered in the literature [15,19] the reported current densities at applied sub-critical potentials are also lower than the 30 A/dm2 recorded in the present work. The two current densities 25 and 30 A/dm2, at respectively TH2SO4 = 25 °C and 45 °C, still allow normal oxide growth, whereas by increasing these current densities respectively to 30 and 40 A/dm2, burning is observed. Images of the oxides, formed at the latter conditions, are displayed in Figs 9a and b. Anodic layers with protruding oxide hillocks and a cracked oxide surface, characteristic for burning, are present. In both cases it does not involve intense burning, though, the phenomenon is nonetheless encountered. The recorded values of Uwe,max, corresponding to these conditions, both exceed 25 V (cf. Table 1), whereas this is not the case for the high current density conditions which do not lead to burning (i.e. 25 A/dm2 at TH2SO4 = 25 °C, and 30 A/dm2 at TH2SO4 = 45 °C). Moreover, the observation of this critical limit of 25 V for Uwe,max is registered for all anodizing conditions considered in the present study. For example, during anodizing at 8 A/dm2 at temperatures TAl = 45 °C and TH2SO4 = 25 us a value of Uwe,max just below 25 V is recorded, and burning is not observed. On the other hand, anodizing
Fig. 8. FE-SEM cross section images of oxide layer formed at TAl = 65 °C while applying a current density of (a) 25 A/dm2 at TH2SO4 = 25 °C–(b) 30 A/dm2 at TH2SO4 = 45 °C.
acknowledged to be a key factor determining the self-ordering behaviour during the growth of porous anodic alumina films [8,15– 17,19,24,25]. Anodizing experiments performed at very high current densities, corresponding to conditions just below the critical condition leading to burning, are observed to produce self-ordered structures [15,17,19]. Although the level of self-ordering of the films displayed in Fig. 8 is not investigated in detail, the observed morphologies are indicative for the tendency to self-ordering under the considered high current density conditions. The cited studies on the growth of self-ordered anodic alumina films [8,15–17,19,24,25] generally consider low temperatures, often in combination with low electrolyte concentrations. These conditions are sometimes explicitly referred to as ‘hard anodizing conditions’ [8,25]. They are considered for the intended growth of very thick oxide layers (N100 μm), which is enabled by reducing the oxide dissolution due to the reduced electrolyte concentration and temperature. To this purpose even a passive cooling of the electrode is considered [8,25]. The present work, on the other hand, considers high temperatures, i.e. high applied electrode temperatures more precisely. Although this approach is not suited for the production of the previously mentioned very thick oxide films, it enables immediately (i.e. from the onset of anodizing on) applying very high current densities and obtaining very high growth rates without encountering burning. This is not possible under hard anodizing conditions. In studies on the growth of self-ordered anodic alumina films, for
Fig. 9. FE-SEM cross section images of oxide layer formed at TAl = 65 °C while applying a current density of (a) 30 A/dm2 at TH2SO4 = 25 °C–(b) 40 A/dm2 at TH2SO4 = 45 °C.
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at this current density considering temperatures of TAl = 25 °C and TH2SO4 = 25 °C, or of TAl = 25 °C and TH2SO4 = 45 °C, leads to values of Uwe,max exceeding 25 V and burning is encountered. Hence, the critical value of the initial maximum potential above which burning is observed, previously reported to approximately equal 27 V [14], needs to be adjusted. By elaborate evaluation at different current densities and temperatures a more accurate value of 25 V (versus an Ag/AgCl reference electrode) is determined for the considered aluminium electrodes and galvanostatic conditions.
Acknowledgements
4. Conclusions
References
Anodizing of aluminium is performed at different current densities under conditions of applied and controlled electrode temperature. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. This holds for the stationary, as well as for the initial maximum potentials, though, at high electrode temperatures the initial maximum potential is more sensitive to temperature variations and declines at a higher rate. Furthermore, increasing the electrolyte temperature, at controlled or uncontrolled electrode temperature, does not affect the maximum potential to the same extent. The large decline of the maximum potential, obtained by increasing the electrode temperature, enables anodizing at high current densities without the occurrence of burning, even at low electrolyte temperatures. At high electrode temperatures of 65 s the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25 °C anodizing can be performed up to 25 A/dm2 without observing burning. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning. An elaborate evaluation at different anodizing conditions indicates a critical value of the initial maximum anode potential equal to 25 V under galvanostatic conditions. At different current densities and temperatures considered in this study burning is encountered during anodizing experiments characterised by an initial maximum potential higher than this value.
The authors acknowledge the support from the Instituut voor de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract number SBO 040092). Raf Claessens and Marnix Depauw are greatly acknowledged for the design and development of the temperature controlling electrode holder.
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