- Email: [email protected]
The role of aluminum anodizing conditions on the effect of the additive light green
Thin Solid Films 402 (2002) 121–125
The role of aluminum anodizing conditions on the effect of the additive light green Th. Dimogerontakis, I. Tsangaraki-Kaplanoglou* University of Athens, Department of Chemistry, Panepistimiopolis Zografou, 157 71 Athens, Greece Received 22 January 2001; received in revised form 11 July 2001; accepted 6 September 2001
Abstract The influence of operating conditions such as temperature, concentration of electrolyte and applied voltage, on the effectiveness of the additive light green in a phosphoric acid bath, on film porosity and current density during anodizing of pure aluminum has been investigated using electrochemical methods. X-Ray fluorescence was used to determine the incorporated amount of phosphorous in the anodic oxide. The principal factor which affects the action of this additive seems to be the applied voltage, while the other parameters have a much smaller influence. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Anodizing conditions; Light green
1. Introduction The use of phosphoric acid for the anodizing of aluminum is very important for certain applications for example, the adhesive bonding of aluminum in aircraft industry, the pre-treatment for painting or electroplating of aluminum, and the production of lithographic printing plates w1x. It is known w2x that when anodizing is carried out at current densities below the so-called critical value, a porous type anodic aluminum oxide film is formed with the pores separated from the base metal by a barrier layer. When the anodizing reaches an almost steady state, the anodic oxide is formed by O2y yOHy ingress only at the metalyfilm interface, while field-assisted dissolution takes place with the same rate at the filmy electrolyte interface at the bottom. Therefore, the thickness of the barrier layer remains constant, while the total film thickness increases with anodizing time. During anodization of aluminum, ions arise in the electrolyte bulk via three distinctly separate processes. * Corresponding author. Tel.: q30-301-932-7339; fax: q30-301724-9103. E-mail address: [email protected] (I. TsangarakiKaplanoglou).
Chemical dissolution of the film material is the first process; direct ejection of Al3q ions due to the applied electric field during the film growth is the second and major process, and field-assisted dissolution of the anodic film at the pore base is the third. In our previous work w3x, the sulfonic dye light green, added in the anodizing electrolyte, was found to influence the rate of film growth as well as the porosity and diameter of the cells of the porous anodic oxide layer, which was produced in 0.4 M phosphoric acid at constant voltage. This influence was explained by assuming that the additive moves by diffusion and migration toward the pore bottom where it is concentrated and interacts with Al3q ions, thus inhibiting their movement to the electrolyte bulk. This results in a reduction of the field-assisted dissolution and of the direct ejection of Al3q ions to the solution, thus leading to a decrease of the rate of formation of anodic oxide. This is clearly suggested by the decrease of the current density and of the thickness of the produced oxide film. If it is assumed that the above-described effectiveness of the additive results in a decrease of the local electric field at the pore bottom, then the observed decrease in porosity and population density of the pores as well as the increase in diameter of the cells can be explained as
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 2 2 - 4
122
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
Fig. 1. Current density vs. time behavior during anodizing of pure aluminum at 30 V in 0.4 M H3PO4 at 258C in the presence of light green at the following concentrations: 0.025; 0.250; 0.750; 1.000; and 1.250=10y3 M.
follows. According to the geometrical model of O’Sullivan and Wood w7x, a decrease in the local electric field at the pore bottom results in an alteration of the radius of curvature of the pore and cell base. These alterations cause the above-mentioned changes in porosity and diameter of the cells. In this work, the influence of anodizing conditions such as temperature, concentration of electrolyte and applied voltage was investigated in order to further highlight the mode of action of this additive. Earlier investigations w1,4–7x of the influence of the above anodizing conditions on current density, porosity and diameter of the pores of the anodic oxide arrive at certain interesting conclusions. (a) If the temperature is increased, both current density and porosity increase due to thermal enhanced field-assisted dissolution, while the pore diameter has slightly reduced. Additionally, the external surface chemical dissolution is enhanced. (b) If the concentration of phosphoric acid is increased, the current density increases mainly due to chemical dissolution. The diameter of the pores is not practically altered if acid concentration increases up to 1.5 M. (c) If the voltage is increased, the current density, as well as the diameter of the pores also increase, while the porosity decreases. The effect of voltage increase on the unit barrier layer thickness is negligible.
The porosity of the anodized specimens, expressed as the total area of pores per unit surface area, was calculated by the voltageytime (V–t) curves produced by galvanostatic re-anodizations with current density of 1 mA cmy2 at 208C in 0.5 M H3BO3–0.05 M Na2B4O7 bath (pH 7.4) w3,8,9x. A power supply of Delta Elektronika (E0300-0.1, "0.025 mA) was used and the above curves were obtained via a multimeter (Keithley 2000, "0.002%). The square correlation coefficients (R 2) of linear fittings of the V–t curves were ) 0.99 and the relative range (RR) of the calculated porosity was -1.1%. The amount of phosphorous which was incorporated into the anodic film, expressed as surface density, was measured by energy dispersive X-ray fluorescence (Tonnele-Nucleus). This amount can be calculated by comparison of the X-ray results with those of standard materials. 3. Results and discussion 3.1. Anodizing current density Fig. 1 shows the current density–time transient during anodizing of aluminum at elevated temperature (258C), for different concentration of light green in the 0.4 M phosphoric acid electrolyte. Fig. 2 presents similar information for different concentrations of phosphoric acid (0.4 and 1.2 M). It is obvious from these figures that the relative position of the curves depends on the temperature and on acid concentration, while the shape of the curves remains unchanged. It is also obvious from these figures that as the concentration of light green in the bath increases, the duration of the stages of barrier layer formation and pore initiation increase progressively, while at the same time, the respective current density decreases at a declining rate.
2. Experimental Test specimens of pure aluminum (99.96%) were used after the appropriate pre-treatment w3x. Potentiostatic anodizations were performed in phosphoric acid at concentrations of 0.4 and 1.2 M using a thermostated cell at 20 and 258C with a counter electrode of stainless steel and magnetic stir. A Delta Elektronika (SM3004-D) power supply together with a computerized multimeter (Keithley 2000, "0.025%) were used.
Fig. 2. Current density vs. time behavior during anodizing of pure aluminum at 30 V in 0.4 and 1.2 M H3PO4 at 208C in the presence of light green at the following concentrations: 0.025; 0.250; 0.750; 1.000; and 1.250=10y3 M.
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
123
of the pores rather than in the electrolyte bulk due to migration and adsorption phenomena in the case of the additive, and due to field-assisted dissolution and direct ejection in the case of the Al3q ions. An attempt to highlight the role of temperature in this case is based on the fitting results of Fig. 3. The steady state current density at temperatures of 20 and 258C vs. light green concentration is presented by the exponential function: isAqBeˆ ŽycyC.
Fig. 3. Current density during anodizing of pure aluminum at 30 V for 10 min vs. concentration of light green in: (a) 0.4 M H3PO4, 208C; (b) 0.4 M H3PO4, 258C; and (c) 1.2 M H3PO4, 208C.
Fig. 3 shows the steady state current density as a function of light green concentration in two different anodizing temperatures and phosphoric acid concentration. All results of Fig. 3 are given after 10 min of anodizing, assuming implicitly that all current densities have practically reached a steady value at this time, despite the fact that the steady state is reached later in the presence of the additive. As it is obvious from this figure, the steady state current density during anodizing at both temperatures decreases as the concentration of light green in the bath increases until it reaches a limited value (1.250=10y3 M). On the contrary, in the case of elevated phosphoric acid concentration, the current density in the steady state continues to decrease progressively as the concentration of the additive increases (Fig. 3). It can be concluded that an inhibition of the action of light green is observed when the concentration of the acid is increased. This inhibition tends to be minimized as the concentration of light green increases. It is obvious that a difficulty appears in the transport of the organic anions inside the pores, which is probably due to the increased competitive action of phosphoric acid anions and to the increased concentration of Al3q ions in the electrolyte bulk. The latter are derived from enhanced chemical dissolution. The presence of sulfonic groups in the molecule of the additive and its electron donating properties contribute to the formation of aluminum compounds with the additive via the N atoms and the delocalised conjugation of the molecule. The additive anions interact with the Al3q cations in the electrolyte bulk. The same interaction takes place at the pore bottom. However, the extent of the interaction depends on the concentration of the reacting species which is not uniform. This is much greater at the bottom
where i is the current density in A dmy2, c is the concentration of light green in molyl and the constants A, B and C take at 20 and 258C, respectively, the following estimates: As0.0646, Bs0.0366 and Cs 0.3518 (R 2s0.9948); and As0.1051, Bs0.0486 and Cs0.2807 (R 2s0.9931). Comparing the derivatives at both temperatures, it can be concluded that the steady state current density decreased slightly more rapidly at 25 than at 208C, as the concentration of light green increases. Thus, some improvement of the action of the additive takes place by increasing the anodizing temperature, despite the fact that the diameter of the pores is slightly reduced. This means that factors such as diffusion and migration contribute more than the diameter of the pores to a better transport of organic anions to the pore bottom, especially when the latter are in small concentration in the bath. The effect of the anodizing voltage (Fig. 4) in the absence and presence of 1.250=10y3 M light green on the steady state current density, is presented according to the fitting results by the exponential function: isAqBeˆ ŽvyC. where i is the current density in A dmy2, and v is the anodizing voltage in Volts.
Fig. 4. Steady state current density vs. applied voltage for anodization of pure aluminum in 0.4 M H3PO4 at 208C for 40 min in the presence of 1.250=10y3 M light green.
124
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
porosity due to the presence of the additive used in the present study. If the anodizing bath temperature and the phosphoric acid concentration are increased, the calculated difference of the decrease in porosity due to the presence of light green is very small and lies within the error margins justified by the method used. According to the results of our previous work w3x and from the above observations on porosity, it can be concluded that the alteration of the local electric field at the pore bottom in the presence of the additive, is mainly influenced by the applied voltage and slightly, or not at all, by the temperature or concentration of the acid. Fig. 5. Porosity vs. applied voltage of films formed by anodizing for 40 min in 0.4 M H3PO4 at 208C in the presence of 1.250=10y3 M light green.
The estimates of the constants are: As0.0465, Bs 0.0276, and Cs47.7469 (R 2s0.9995) without the presence of additive; and As0.0349, Bs0.0146 and Cs 42.8375 (R 2s0.9979) in the presence of 1.250=10y3 M light green. The derivatives of these functions show that as the anodizing voltage increases, the rate of increase of the steady state current density is reduced considerably in the presence of light green. This indicates a significant positive correlation between the action of the additive and the anodizing voltage. This correlation can be explained primarily by the better migration of the additive anions inside the pores under the influence of the higher electric field and secondarily, by the increase of the diameter of the pores. On the contrary, the transport of the additive due to diffusion w12x does not change because the concentration of the additive and the temperature of the bath remain constant. According to Chao et al. w10x and Kim et al. w11x, the electrochemical dissolution of the barrier layer at the pore bottom is crucially determined by the aluminum ion removal rate, which is assisted by an electrical field across the barrier layeryelectrolyte interface. Thus, the electrochemical dissolution of the barrier layer, which is equal to the rate of formation of the oxide film at the metalyoxide interface, is reduced at a rate dependent on the reaction rate of the additive with the Al3q ions at the oxideysolution interface at the pore bottom. The result of this interaction is the inhibition of Al3q ion removal towards the electrolyte bulk.
3.3. Phosphate content of the film According to our previous work w3x, the incorporated amount of phosphorous in the anodic film per unit layer thickness remains almost unaltered by the presence of light green at 30 V. It is obvious from Fig. 6 that an analogous influence of light green on the incorporated amount of phosphorous takes place at 60 V. This means that although the additive is more active at higher voltages, it does not seem to influence the process of phosphate incorporation into the anodic oxide film. 4. Conclusions From the present work, it can be concluded that during anodization of pure aluminum in phosphoric acid in the presence of the dye light green as additive, the applied voltage and, to a lesser extent, the temperature and concentration of phosphoric acid in the bath, influence its action on the current density and porosity of the anodic film.
3.2. Porosity of the film The porosity of the porous film, calculated by the voltage–time curves obtained during galvanostatic reanodization, is presented in Fig. 5 as a function of the anodizing voltage. It is obvious from this figure that the increase in anodizing voltage further decreases the
Fig. 6. Incorporated amount of phosphorous in anodic film formed by anodizing in 0.4 M H3PO4 at 208C for 40 min at 30 and 60 V vs. light green concentration.
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
The action of the additive at optimum concentration in the bath, seems to depend principally on its migration to the pore bottom under the influence of the applied electric field and, to a lesser extent, on the thermally enhanced transport of the additive anions, the diameter of the pores, the amount of chemically dissolved anodic oxide, and on the concentration of competitive phosphate anions. Thus, although diffusion contributes to the transport process, migration seems to be the most important factor in order for the additive to approach the pore bottom. From the results of this work, it can be concluded that the concentration of both reacting species (additive and Al3q ions) at the pores bottom, seems to play an important role on the extent of the action of the additive and depends mainly on the applied electric field. Acknowledgements The authors wish to acknowledge Assistant Prof. N. Kallithrakas-Kontos (Technical University of Crete) for X-ray fluorescence studies and the Research Committee Secretariat of the University of Athens for their financial support.
125
References w1x S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and its Alloys, vol. I, II, Finishing Publications Ltd, England, 1987. w2x G.E. Thompson, Y. Xu, P. Skeldon, K. Shimizu, S.H. Han, G.C. Wood, Phil. Mag. A 55 (6) (1987) 651. w3x Th. Dimogerontakis, I. Kaplanoglou, Thin Solid Films 385 (2001) 182. w4x J.W. Diggle, Oxide and Oxide Films, vol. II, Marcel Dekker Inc, New York, 1973. w5x J. De Laet, H. Terryn, J. Vereecken, Thin Solid Films 320 (1998) 241. w6x Y.-C. Kim, B. Quint, R.W. Kesler, D. Oelkrug, J. Electroanal. Chem. 468 (1999) 121. w7x J.P. O’Sullivan, G.C. Wood, Proc. R. Soc. Lond. A 317 (1970) 511. w8x H. Takahashi, M. Nagayama, Corros. Sci. 18 (1978) 911. w9x A. Dekker, A. Middelhoek, J. Electrochem. Soc. 117 (1970) 440. w10x C.Y. Chao, L.F. Lin, D.D. Macdonald, J. Electrochem. Soc. 128 (1981) 1187. w11x Y.-S. Kim, S.-I. Pyun, S.-M. Moon, J.-D. Kim, Corros. Sci. 38 (1996) 329. w12x H. Grossmann, Schweizer Alum. Rundschau 12 (1973) 1.
The role of aluminum anodizing conditions on the effect of the additive light green Th. Dimogerontakis, I. Tsangaraki-Kaplanoglou* University of Athens, Department of Chemistry, Panepistimiopolis Zografou, 157 71 Athens, Greece Received 22 January 2001; received in revised form 11 July 2001; accepted 6 September 2001
Abstract The influence of operating conditions such as temperature, concentration of electrolyte and applied voltage, on the effectiveness of the additive light green in a phosphoric acid bath, on film porosity and current density during anodizing of pure aluminum has been investigated using electrochemical methods. X-Ray fluorescence was used to determine the incorporated amount of phosphorous in the anodic oxide. The principal factor which affects the action of this additive seems to be the applied voltage, while the other parameters have a much smaller influence. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Anodizing conditions; Light green
1. Introduction The use of phosphoric acid for the anodizing of aluminum is very important for certain applications for example, the adhesive bonding of aluminum in aircraft industry, the pre-treatment for painting or electroplating of aluminum, and the production of lithographic printing plates w1x. It is known w2x that when anodizing is carried out at current densities below the so-called critical value, a porous type anodic aluminum oxide film is formed with the pores separated from the base metal by a barrier layer. When the anodizing reaches an almost steady state, the anodic oxide is formed by O2y yOHy ingress only at the metalyfilm interface, while field-assisted dissolution takes place with the same rate at the filmy electrolyte interface at the bottom. Therefore, the thickness of the barrier layer remains constant, while the total film thickness increases with anodizing time. During anodization of aluminum, ions arise in the electrolyte bulk via three distinctly separate processes. * Corresponding author. Tel.: q30-301-932-7339; fax: q30-301724-9103. E-mail address: [email protected] (I. TsangarakiKaplanoglou).
Chemical dissolution of the film material is the first process; direct ejection of Al3q ions due to the applied electric field during the film growth is the second and major process, and field-assisted dissolution of the anodic film at the pore base is the third. In our previous work w3x, the sulfonic dye light green, added in the anodizing electrolyte, was found to influence the rate of film growth as well as the porosity and diameter of the cells of the porous anodic oxide layer, which was produced in 0.4 M phosphoric acid at constant voltage. This influence was explained by assuming that the additive moves by diffusion and migration toward the pore bottom where it is concentrated and interacts with Al3q ions, thus inhibiting their movement to the electrolyte bulk. This results in a reduction of the field-assisted dissolution and of the direct ejection of Al3q ions to the solution, thus leading to a decrease of the rate of formation of anodic oxide. This is clearly suggested by the decrease of the current density and of the thickness of the produced oxide film. If it is assumed that the above-described effectiveness of the additive results in a decrease of the local electric field at the pore bottom, then the observed decrease in porosity and population density of the pores as well as the increase in diameter of the cells can be explained as
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 2 2 - 4
122
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
Fig. 1. Current density vs. time behavior during anodizing of pure aluminum at 30 V in 0.4 M H3PO4 at 258C in the presence of light green at the following concentrations: 0.025; 0.250; 0.750; 1.000; and 1.250=10y3 M.
follows. According to the geometrical model of O’Sullivan and Wood w7x, a decrease in the local electric field at the pore bottom results in an alteration of the radius of curvature of the pore and cell base. These alterations cause the above-mentioned changes in porosity and diameter of the cells. In this work, the influence of anodizing conditions such as temperature, concentration of electrolyte and applied voltage was investigated in order to further highlight the mode of action of this additive. Earlier investigations w1,4–7x of the influence of the above anodizing conditions on current density, porosity and diameter of the pores of the anodic oxide arrive at certain interesting conclusions. (a) If the temperature is increased, both current density and porosity increase due to thermal enhanced field-assisted dissolution, while the pore diameter has slightly reduced. Additionally, the external surface chemical dissolution is enhanced. (b) If the concentration of phosphoric acid is increased, the current density increases mainly due to chemical dissolution. The diameter of the pores is not practically altered if acid concentration increases up to 1.5 M. (c) If the voltage is increased, the current density, as well as the diameter of the pores also increase, while the porosity decreases. The effect of voltage increase on the unit barrier layer thickness is negligible.
The porosity of the anodized specimens, expressed as the total area of pores per unit surface area, was calculated by the voltageytime (V–t) curves produced by galvanostatic re-anodizations with current density of 1 mA cmy2 at 208C in 0.5 M H3BO3–0.05 M Na2B4O7 bath (pH 7.4) w3,8,9x. A power supply of Delta Elektronika (E0300-0.1, "0.025 mA) was used and the above curves were obtained via a multimeter (Keithley 2000, "0.002%). The square correlation coefficients (R 2) of linear fittings of the V–t curves were ) 0.99 and the relative range (RR) of the calculated porosity was -1.1%. The amount of phosphorous which was incorporated into the anodic film, expressed as surface density, was measured by energy dispersive X-ray fluorescence (Tonnele-Nucleus). This amount can be calculated by comparison of the X-ray results with those of standard materials. 3. Results and discussion 3.1. Anodizing current density Fig. 1 shows the current density–time transient during anodizing of aluminum at elevated temperature (258C), for different concentration of light green in the 0.4 M phosphoric acid electrolyte. Fig. 2 presents similar information for different concentrations of phosphoric acid (0.4 and 1.2 M). It is obvious from these figures that the relative position of the curves depends on the temperature and on acid concentration, while the shape of the curves remains unchanged. It is also obvious from these figures that as the concentration of light green in the bath increases, the duration of the stages of barrier layer formation and pore initiation increase progressively, while at the same time, the respective current density decreases at a declining rate.
2. Experimental Test specimens of pure aluminum (99.96%) were used after the appropriate pre-treatment w3x. Potentiostatic anodizations were performed in phosphoric acid at concentrations of 0.4 and 1.2 M using a thermostated cell at 20 and 258C with a counter electrode of stainless steel and magnetic stir. A Delta Elektronika (SM3004-D) power supply together with a computerized multimeter (Keithley 2000, "0.025%) were used.
Fig. 2. Current density vs. time behavior during anodizing of pure aluminum at 30 V in 0.4 and 1.2 M H3PO4 at 208C in the presence of light green at the following concentrations: 0.025; 0.250; 0.750; 1.000; and 1.250=10y3 M.
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
123
of the pores rather than in the electrolyte bulk due to migration and adsorption phenomena in the case of the additive, and due to field-assisted dissolution and direct ejection in the case of the Al3q ions. An attempt to highlight the role of temperature in this case is based on the fitting results of Fig. 3. The steady state current density at temperatures of 20 and 258C vs. light green concentration is presented by the exponential function: isAqBeˆ ŽycyC.
Fig. 3. Current density during anodizing of pure aluminum at 30 V for 10 min vs. concentration of light green in: (a) 0.4 M H3PO4, 208C; (b) 0.4 M H3PO4, 258C; and (c) 1.2 M H3PO4, 208C.
Fig. 3 shows the steady state current density as a function of light green concentration in two different anodizing temperatures and phosphoric acid concentration. All results of Fig. 3 are given after 10 min of anodizing, assuming implicitly that all current densities have practically reached a steady value at this time, despite the fact that the steady state is reached later in the presence of the additive. As it is obvious from this figure, the steady state current density during anodizing at both temperatures decreases as the concentration of light green in the bath increases until it reaches a limited value (1.250=10y3 M). On the contrary, in the case of elevated phosphoric acid concentration, the current density in the steady state continues to decrease progressively as the concentration of the additive increases (Fig. 3). It can be concluded that an inhibition of the action of light green is observed when the concentration of the acid is increased. This inhibition tends to be minimized as the concentration of light green increases. It is obvious that a difficulty appears in the transport of the organic anions inside the pores, which is probably due to the increased competitive action of phosphoric acid anions and to the increased concentration of Al3q ions in the electrolyte bulk. The latter are derived from enhanced chemical dissolution. The presence of sulfonic groups in the molecule of the additive and its electron donating properties contribute to the formation of aluminum compounds with the additive via the N atoms and the delocalised conjugation of the molecule. The additive anions interact with the Al3q cations in the electrolyte bulk. The same interaction takes place at the pore bottom. However, the extent of the interaction depends on the concentration of the reacting species which is not uniform. This is much greater at the bottom
where i is the current density in A dmy2, c is the concentration of light green in molyl and the constants A, B and C take at 20 and 258C, respectively, the following estimates: As0.0646, Bs0.0366 and Cs 0.3518 (R 2s0.9948); and As0.1051, Bs0.0486 and Cs0.2807 (R 2s0.9931). Comparing the derivatives at both temperatures, it can be concluded that the steady state current density decreased slightly more rapidly at 25 than at 208C, as the concentration of light green increases. Thus, some improvement of the action of the additive takes place by increasing the anodizing temperature, despite the fact that the diameter of the pores is slightly reduced. This means that factors such as diffusion and migration contribute more than the diameter of the pores to a better transport of organic anions to the pore bottom, especially when the latter are in small concentration in the bath. The effect of the anodizing voltage (Fig. 4) in the absence and presence of 1.250=10y3 M light green on the steady state current density, is presented according to the fitting results by the exponential function: isAqBeˆ ŽvyC. where i is the current density in A dmy2, and v is the anodizing voltage in Volts.
Fig. 4. Steady state current density vs. applied voltage for anodization of pure aluminum in 0.4 M H3PO4 at 208C for 40 min in the presence of 1.250=10y3 M light green.
124
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
porosity due to the presence of the additive used in the present study. If the anodizing bath temperature and the phosphoric acid concentration are increased, the calculated difference of the decrease in porosity due to the presence of light green is very small and lies within the error margins justified by the method used. According to the results of our previous work w3x and from the above observations on porosity, it can be concluded that the alteration of the local electric field at the pore bottom in the presence of the additive, is mainly influenced by the applied voltage and slightly, or not at all, by the temperature or concentration of the acid. Fig. 5. Porosity vs. applied voltage of films formed by anodizing for 40 min in 0.4 M H3PO4 at 208C in the presence of 1.250=10y3 M light green.
The estimates of the constants are: As0.0465, Bs 0.0276, and Cs47.7469 (R 2s0.9995) without the presence of additive; and As0.0349, Bs0.0146 and Cs 42.8375 (R 2s0.9979) in the presence of 1.250=10y3 M light green. The derivatives of these functions show that as the anodizing voltage increases, the rate of increase of the steady state current density is reduced considerably in the presence of light green. This indicates a significant positive correlation between the action of the additive and the anodizing voltage. This correlation can be explained primarily by the better migration of the additive anions inside the pores under the influence of the higher electric field and secondarily, by the increase of the diameter of the pores. On the contrary, the transport of the additive due to diffusion w12x does not change because the concentration of the additive and the temperature of the bath remain constant. According to Chao et al. w10x and Kim et al. w11x, the electrochemical dissolution of the barrier layer at the pore bottom is crucially determined by the aluminum ion removal rate, which is assisted by an electrical field across the barrier layeryelectrolyte interface. Thus, the electrochemical dissolution of the barrier layer, which is equal to the rate of formation of the oxide film at the metalyoxide interface, is reduced at a rate dependent on the reaction rate of the additive with the Al3q ions at the oxideysolution interface at the pore bottom. The result of this interaction is the inhibition of Al3q ion removal towards the electrolyte bulk.
3.3. Phosphate content of the film According to our previous work w3x, the incorporated amount of phosphorous in the anodic film per unit layer thickness remains almost unaltered by the presence of light green at 30 V. It is obvious from Fig. 6 that an analogous influence of light green on the incorporated amount of phosphorous takes place at 60 V. This means that although the additive is more active at higher voltages, it does not seem to influence the process of phosphate incorporation into the anodic oxide film. 4. Conclusions From the present work, it can be concluded that during anodization of pure aluminum in phosphoric acid in the presence of the dye light green as additive, the applied voltage and, to a lesser extent, the temperature and concentration of phosphoric acid in the bath, influence its action on the current density and porosity of the anodic film.
3.2. Porosity of the film The porosity of the porous film, calculated by the voltage–time curves obtained during galvanostatic reanodization, is presented in Fig. 5 as a function of the anodizing voltage. It is obvious from this figure that the increase in anodizing voltage further decreases the
Fig. 6. Incorporated amount of phosphorous in anodic film formed by anodizing in 0.4 M H3PO4 at 208C for 40 min at 30 and 60 V vs. light green concentration.
T. Dimogerontakis, I. Tsangaraki-Kaplanoglou / Thin Solid Films 402 (2002) 121–125
The action of the additive at optimum concentration in the bath, seems to depend principally on its migration to the pore bottom under the influence of the applied electric field and, to a lesser extent, on the thermally enhanced transport of the additive anions, the diameter of the pores, the amount of chemically dissolved anodic oxide, and on the concentration of competitive phosphate anions. Thus, although diffusion contributes to the transport process, migration seems to be the most important factor in order for the additive to approach the pore bottom. From the results of this work, it can be concluded that the concentration of both reacting species (additive and Al3q ions) at the pores bottom, seems to play an important role on the extent of the action of the additive and depends mainly on the applied electric field. Acknowledgements The authors wish to acknowledge Assistant Prof. N. Kallithrakas-Kontos (Technical University of Crete) for X-ray fluorescence studies and the Research Committee Secretariat of the University of Athens for their financial support.
125
References w1x S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and its Alloys, vol. I, II, Finishing Publications Ltd, England, 1987. w2x G.E. Thompson, Y. Xu, P. Skeldon, K. Shimizu, S.H. Han, G.C. Wood, Phil. Mag. A 55 (6) (1987) 651. w3x Th. Dimogerontakis, I. Kaplanoglou, Thin Solid Films 385 (2001) 182. w4x J.W. Diggle, Oxide and Oxide Films, vol. II, Marcel Dekker Inc, New York, 1973. w5x J. De Laet, H. Terryn, J. Vereecken, Thin Solid Films 320 (1998) 241. w6x Y.-C. Kim, B. Quint, R.W. Kesler, D. Oelkrug, J. Electroanal. Chem. 468 (1999) 121. w7x J.P. O’Sullivan, G.C. Wood, Proc. R. Soc. Lond. A 317 (1970) 511. w8x H. Takahashi, M. Nagayama, Corros. Sci. 18 (1978) 911. w9x A. Dekker, A. Middelhoek, J. Electrochem. Soc. 117 (1970) 440. w10x C.Y. Chao, L.F. Lin, D.D. Macdonald, J. Electrochem. Soc. 128 (1981) 1187. w11x Y.-S. Kim, S.-I. Pyun, S.-M. Moon, J.-D. Kim, Corros. Sci. 38 (1996) 329. w12x H. Grossmann, Schweizer Alum. Rundschau 12 (1973) 1.