- Email: [email protected]
The influence of the anodization temperature and voltage on the porosity of the anodization layer on aluminium
146
Materials
Chemistry
and Physics,
36 (1993)
146-149
The influence of the anodization temperature and voltage on the porosity of the anodization layer on aluminium F. Debuyck, Laboratorium
(Received
M. Moors
voor Non-Ferro
December
and A.P. Van Peteghem
Metallurgic,
11, 1992; accepted
Universiteit
Gent,
Grote
Steenweg
Noord
6, B-9052
Zwijnaarde
(Belgium)
March 22, 1993)
Abstract The pore-filling method is a known process for measuring the porosity of porous anodic oxide layers on aluminium. This method has been used to study the relationship between the anodizing conditions and the porosity of the porous anodic film. In a series of experiments, the influence of the anodization voltage and temperature on the porosity of an anodization layer formed in 15 wt.% sulphuric acid was investigated. From these results a mathematical equation was deduced that gives the porosity as a function of the anodization voltage and temperature. In the second part of the study the height of the voltage jump, on which the thickness of the barrier layer at the start of the re-anodization process depends, was investigated as a function of the anodization voltage. A linear relationship was found between the voltage jump and the anodization voltage.
Introduction If aluminium is anodically polarized in an electrolysis cell, it oxidizes to form aluminium oxide. Depending on the type of electrolyte one can form a porous or a compact oxide layer. In sulphuric, oxalic or phosphoric acid a porous layer forms, with the pores separated from the base metal by a barrier layer [l]. When a boric acid solution is used the anodization forms a compact layer with high electrical resistivity. A porous anodic oxide layer can be anodized a second time (re-anodized) in a boric acid solution; this process is known as the pore-filling method. Dekker and Middelhoek [2] and later Takahashi and Nagayama [3] introduced this method to study the porosity of porous anodic layers. The aim of the present research was to obtain more information about the influence of the anodization parameters on the porosity of porous anodization layers on aluminium.
Experimental The experiments were performed on flat aluminium (99.9%) surfaces attached to hollow Plexiglas@ cylinders to ensure a fixed surface area of 1.54 cm2 and a constant electrolyte volume. Before starting with the anodization and re-anodization process, it was necessary to prepare the samples
0254-0584/93/$6.00
properly. The procedure consists of various steps: polishing of the aluminium surface; cleaning with water; etching in NaOH solution (30 g 1-l) for 10 min; rinsing in distilled water; immersion in HNO, solution (30 wt.%) for 15 set; and rinsing in distilled water. Anodization was carried out in a 15 wt.% sulphuric acid solution at different voltages and temperatures for 30 min. A lead plate served as the cathode. After anodization the samples were rinsed, first in sterile water, then in distilled water and finally in alcohol. They were air dried at 50 “C. The next step is preparing the sample for the reanodization. This is done by sticking a hollow cylinder on the anodized surface, as mentioned before. The re-anodization process was carried out in a 0.05 M Na,B,O,/O.S M H,BO, solution at 23 “C and with a platinum counter electrode. A voltage of up to 2000 V d.c., needed for the reanodization process, was generated by a device constructed in our laboratory. This galvanostat keeps the cell current constant at a preset value. In the first series of experiments samples were anodized at different values of the anodization potential V, and temperature T,. The V, values were 3, 9, 12, 15 and 18 V, while the temperature was set at 0, 6, 12, 16, 20, 24 and 28 “C. After anodization, pore-filling experiments were performed at 23 “C and with a constant current density (0.1 mA cm-‘). During this re-anodization the voltage was recorded as a function of time.
0 1993 - Elsevier
Sequoia.
All rights reserved
147
Dekker and Middelhoek [2] obtained, using an appropriate current density, a k’d curve that consists of two straight lines with different slopes (Fig. 1). At the start of the pore-filling a voltage jump V, is noticed, which is due to the existence of the barrierlayer resistance. The first part of curve a, with slope m,, is due to the filling of the pores, while the second part, with slope m2, represents the growth of the oxide layer over the whole surface area. In the second series of experiments the value of the voltage jump V, was evaluated as a function of the potential. The anodization temperature and time were kept constant at 20 “C and 30 min for these sample preparations. After the anodization process the pores were filled, under the same conditions, during the reanodization process.
Results
and discussion
As a first step, virgin aluminium surfaces were anodized in the boric acid setup. By doing so we could measure a rising voltage line (Fig. 1, curve b) with slope m2. This slope seems to be identical with that of the voltage measured when the pores are filled up (slope m2, curve a). The mean value of slope m2, averaged over at least five experiments, is m,=0.030
V s-’
(1)
The error of the measured slopes ml and m2 for the same set of anodization parameters is a function of the small variations of the geometrical characteristics of the pores, the effective surface area and also of electrochemical phenomena such as polarization effects, etc. The equation for the porosity (Yis [2]
(2)
ff=
.
where TAls+is the transport number for aluminium ion, defined as T.b.,,+=
of new oxide formed in the pore per unit time
weight
total weight of new oxide formed per unit time
According to Takahashi and Nagayama [3], the transport number for aluminium ion can be considered to have a constant value, equal to 0.4, in the stationary period of growth. Substituting the values of the slopes and the transport number TA13+for aluminium ion, the porosity of the porous anodization layer can be obtained as a function of voltage and temperature. The results are presented in Table 1. It follows from the values in the table that the porosity is dependent on the voltage and the temperature. Raising the voltage results in a lower porosity, while increasing the temperature gives a higher porosity of the anodization layer. From previous research in this field, done in our laboratory [4], we deduced the following experimental relationship between the porosity and the anodization potential: 1 - =aa--b ff
where a and b are constants that depend on the slope m2 and the transport number for aluminium ion a= ~ c “%TA,z+ 1 - TAp+ b=
TA,l+
Constant
C is a proportion
factor between the slope voltage. Using the porosity values of Table 1 in eqn. (3) gives the values of a and b, which are given in Table 2 along with the temperature. Using eqns. (4) and (5) the transport number and constant C were calculated as functions of the temm, and the square root of the anodization
TABLE 1. The porosity anodization temperatures
Temperature
T,
a of the porous oxide layer T, and potentials V,
for various
Porosity
a
3V”
9v
12 v
15 v
18 V
0.224 0.277 0.333 0.330 0.341 0.347 0.351
0.108 0.130 0.159 0.144 0.141 0.148 0.165
0.096 0.105 0.123 0.106 0.118 0.112 0.113
0.087 0.093 0.100 0.098 0.110 0.108 0.109
0.073 0.085 0.099 0.097 0.105 0.107 0.112
P-3
10
Fig. acid with
*P
time
1. Voltage vs. time plot during re-anodization in a boric solution (constant current) [2, 31: (a) aluminium covered an anodization layer; (b) aluminium with a virgin surface.
0 6 12 16 20 24 28 “Anodization
potential
va.
148 TABLE
2. Experimental
Temperature
r,
(“C) 0
values of a and b in eqn. (3) Constant (V-In)
a
Constant
3.510 3.134 2.999 3.112 2.708 2.757 2.692
6 12 16 20 24 28
b
1.586 2.124 2.290 2.170 1.410 1.559 1.610
Samples were prepared at a constant temperature of 20 “C for 30 min and at different voltages in the sulphuric acid solution. After anodization, the samples were re-anodized in the boric acid solution at 23 “C and with a constant current density of 0.1 mA cm-‘. The initially measured voltage jump V, is due to the thickness of the barrier layer. The results are presented in Table 4. Curve-fitting of the results gave the following function as the best fit: V, = 0.872 V,
TABLE 3. The valves of constant T*,J+ of the aluminium ion
C and the transport
Temperature
Constant
(“C)
(V “2 s-9
0
0.041 0.030 0.027 0.030 0.034 0.032 0.031
6 12 16 20 24 28
number
C
0.387 0.320 0.304 0.315 0.415 0.391 0.383
perature. The values obtained are presented in Table 3. For the transport number of aluminium ion a mean value of 0.36 was obtained, which is very close to the value proposed by Takahashi and Nagayama [3]. The mean value of constant C (0.032 V1’2 s-l) is approximately equal to slope m2 when an anodization voltage of 1 V is applied. Table 2 shows that constants a and b are voltage and temperature dependent, respectively. Curve-fitting results in the following functions for a and b: a= -0.154
J-+3.51
(6)
b = - 0.005( T, - 11.83)2 + 2.28
(7)
Substitution of eqns. (6) and (7) in eqn. (3) results in a mathematical equation for the porosity as a function of the anodization voltage and temperature: 1 -=_
(Y
0.154ma+3.51
~+0.005(Z-a-11.83)2-2.28 (8)
(9)
This function confirms the conclusion of Dekker and Middelhoek [2]. In the V-t curve recorded (Fig. 2), in addition to the two slopes m, and m2 and the voltage jump found by Dekker and Middelhoek [2] and Takahashi and Nagayama [3], two more phenomena can be detected. First of all, it should be mentioned that in reality the end of the filling of the pores (VP, tP) is not a well-defined twist (Fig. 2), but rather a smooth change in slope. This is due, we suspect, to pores that are not totally cylindrical, whose mouths have a rather conical shape owing to the attack of the sulphuric acid solution [5]. Also, the heterogeneity of the porosity and a small variation of the applied current may cause a smooth change in the slope during the filling of the pores. TABLE 4. The measured anodization potential Anodization
voltage jump V, as a function
Voltage jump V,
voltage V,
(V)
(V)
2 4 6 8 10 12 14 16 18 20
1.60 3.54 5.49 7.29 8.91 10.88 12.30 13.80 15.76 16.93
voltage
,/”
(VI m, _A’
300.-
With eqn. (8) the porosity of porous anodization layers on aluminium can be estimated as a function of the applied voltage and temperature. In the introduction it was mentioned that the pores are separated from the base metal by a barrier layer. Dekker and Middelhoek [2] found that the thickness of the barrier layer is dependent on the anodization voltage. We tried to prove this in the second series of experiments.
0
of the
2000
4000
/” “Ia
6000 time
Fig. 2. Measured V-t curve during the re-anodization anodization layer on aluminium (3 V, 6 “C, 30 min).
(sec1
of an
149
However, extreme caution is necessary, because suppositions were not proved by direct shape surements. In addition, in the first part of the I/-t curve, the voltage jump, a deviation of slope m, (Fig. noticeable. A new slope m, is proposed: mO=
lim
m,
these mea-
Conclusions -
after 3) is
(10)
1-O
Figure 3 shows that m,
With the pore-filling method, i.e., re-anodization in a boric acid solution, it is possible to investigate the porosity change of porous anodic layers on aluminium as a function of the applied voltage and temperature. - The porosity is dependent on the voltage and temperature. Raising the temperature results in an increase in the porosity; raising the voltage has the opposite effect on the porosity. - With the mathematical equation derived it is possible to estimate the porosity for a given potential and temperature. - The barrier-layer voltage jump, which is related to the thickness of this layer, varies linearly with the anodization voltage.
voltage
References
,_,’
“b
/
,. /’ ____= _---
m0
time
10
Fig. 3. Detail
of the voltage
jump
V, in the
V-t curve.
M. Schenk, Werkstoff Aluminium und seine Anodische Oxydation, A. Francke AG, Berne, 1948. A. Dekker and A. Middelhoek, J. Electrochem. Sot., 117 (1970) 440. H. Takahashi and M. Nagayama, Corros. Sci., 18 (1978) 911. L. Lemaitre et aZ., Mater. Chem. Phys., 17 (1987) 285. S. Wernick, R. Pinner and P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and Its Alloys, Finishing Publications, Teddington, UK, 1987. G.C. Wood, J.P. O’Sullivan and B. Vaszko, J. Electrochem. Sot., 115 (1968) 618. G.E. Thompson et al., Trans. Inst. Met. Finish., 56 (1978) 159.
Materials
Chemistry
and Physics,
36 (1993)
146-149
The influence of the anodization temperature and voltage on the porosity of the anodization layer on aluminium F. Debuyck, Laboratorium
(Received
M. Moors
voor Non-Ferro
December
and A.P. Van Peteghem
Metallurgic,
11, 1992; accepted
Universiteit
Gent,
Grote
Steenweg
Noord
6, B-9052
Zwijnaarde
(Belgium)
March 22, 1993)
Abstract The pore-filling method is a known process for measuring the porosity of porous anodic oxide layers on aluminium. This method has been used to study the relationship between the anodizing conditions and the porosity of the porous anodic film. In a series of experiments, the influence of the anodization voltage and temperature on the porosity of an anodization layer formed in 15 wt.% sulphuric acid was investigated. From these results a mathematical equation was deduced that gives the porosity as a function of the anodization voltage and temperature. In the second part of the study the height of the voltage jump, on which the thickness of the barrier layer at the start of the re-anodization process depends, was investigated as a function of the anodization voltage. A linear relationship was found between the voltage jump and the anodization voltage.
Introduction If aluminium is anodically polarized in an electrolysis cell, it oxidizes to form aluminium oxide. Depending on the type of electrolyte one can form a porous or a compact oxide layer. In sulphuric, oxalic or phosphoric acid a porous layer forms, with the pores separated from the base metal by a barrier layer [l]. When a boric acid solution is used the anodization forms a compact layer with high electrical resistivity. A porous anodic oxide layer can be anodized a second time (re-anodized) in a boric acid solution; this process is known as the pore-filling method. Dekker and Middelhoek [2] and later Takahashi and Nagayama [3] introduced this method to study the porosity of porous anodic layers. The aim of the present research was to obtain more information about the influence of the anodization parameters on the porosity of porous anodization layers on aluminium.
Experimental The experiments were performed on flat aluminium (99.9%) surfaces attached to hollow Plexiglas@ cylinders to ensure a fixed surface area of 1.54 cm2 and a constant electrolyte volume. Before starting with the anodization and re-anodization process, it was necessary to prepare the samples
0254-0584/93/$6.00
properly. The procedure consists of various steps: polishing of the aluminium surface; cleaning with water; etching in NaOH solution (30 g 1-l) for 10 min; rinsing in distilled water; immersion in HNO, solution (30 wt.%) for 15 set; and rinsing in distilled water. Anodization was carried out in a 15 wt.% sulphuric acid solution at different voltages and temperatures for 30 min. A lead plate served as the cathode. After anodization the samples were rinsed, first in sterile water, then in distilled water and finally in alcohol. They were air dried at 50 “C. The next step is preparing the sample for the reanodization. This is done by sticking a hollow cylinder on the anodized surface, as mentioned before. The re-anodization process was carried out in a 0.05 M Na,B,O,/O.S M H,BO, solution at 23 “C and with a platinum counter electrode. A voltage of up to 2000 V d.c., needed for the reanodization process, was generated by a device constructed in our laboratory. This galvanostat keeps the cell current constant at a preset value. In the first series of experiments samples were anodized at different values of the anodization potential V, and temperature T,. The V, values were 3, 9, 12, 15 and 18 V, while the temperature was set at 0, 6, 12, 16, 20, 24 and 28 “C. After anodization, pore-filling experiments were performed at 23 “C and with a constant current density (0.1 mA cm-‘). During this re-anodization the voltage was recorded as a function of time.
0 1993 - Elsevier
Sequoia.
All rights reserved
147
Dekker and Middelhoek [2] obtained, using an appropriate current density, a k’d curve that consists of two straight lines with different slopes (Fig. 1). At the start of the pore-filling a voltage jump V, is noticed, which is due to the existence of the barrierlayer resistance. The first part of curve a, with slope m,, is due to the filling of the pores, while the second part, with slope m2, represents the growth of the oxide layer over the whole surface area. In the second series of experiments the value of the voltage jump V, was evaluated as a function of the potential. The anodization temperature and time were kept constant at 20 “C and 30 min for these sample preparations. After the anodization process the pores were filled, under the same conditions, during the reanodization process.
Results
and discussion
As a first step, virgin aluminium surfaces were anodized in the boric acid setup. By doing so we could measure a rising voltage line (Fig. 1, curve b) with slope m2. This slope seems to be identical with that of the voltage measured when the pores are filled up (slope m2, curve a). The mean value of slope m2, averaged over at least five experiments, is m,=0.030
V s-’
(1)
The error of the measured slopes ml and m2 for the same set of anodization parameters is a function of the small variations of the geometrical characteristics of the pores, the effective surface area and also of electrochemical phenomena such as polarization effects, etc. The equation for the porosity (Yis [2]
(2)
ff=
.
where TAls+is the transport number for aluminium ion, defined as T.b.,,+=
of new oxide formed in the pore per unit time
weight
total weight of new oxide formed per unit time
According to Takahashi and Nagayama [3], the transport number for aluminium ion can be considered to have a constant value, equal to 0.4, in the stationary period of growth. Substituting the values of the slopes and the transport number TA13+for aluminium ion, the porosity of the porous anodization layer can be obtained as a function of voltage and temperature. The results are presented in Table 1. It follows from the values in the table that the porosity is dependent on the voltage and the temperature. Raising the voltage results in a lower porosity, while increasing the temperature gives a higher porosity of the anodization layer. From previous research in this field, done in our laboratory [4], we deduced the following experimental relationship between the porosity and the anodization potential: 1 - =aa--b ff
where a and b are constants that depend on the slope m2 and the transport number for aluminium ion a= ~ c “%TA,z+ 1 - TAp+ b=
TA,l+
Constant
C is a proportion
factor between the slope voltage. Using the porosity values of Table 1 in eqn. (3) gives the values of a and b, which are given in Table 2 along with the temperature. Using eqns. (4) and (5) the transport number and constant C were calculated as functions of the temm, and the square root of the anodization
TABLE 1. The porosity anodization temperatures
Temperature
T,
a of the porous oxide layer T, and potentials V,
for various
Porosity
a
3V”
9v
12 v
15 v
18 V
0.224 0.277 0.333 0.330 0.341 0.347 0.351
0.108 0.130 0.159 0.144 0.141 0.148 0.165
0.096 0.105 0.123 0.106 0.118 0.112 0.113
0.087 0.093 0.100 0.098 0.110 0.108 0.109
0.073 0.085 0.099 0.097 0.105 0.107 0.112
P-3
10
Fig. acid with
*P
time
1. Voltage vs. time plot during re-anodization in a boric solution (constant current) [2, 31: (a) aluminium covered an anodization layer; (b) aluminium with a virgin surface.
0 6 12 16 20 24 28 “Anodization
potential
va.
148 TABLE
2. Experimental
Temperature
r,
(“C) 0
values of a and b in eqn. (3) Constant (V-In)
a
Constant
3.510 3.134 2.999 3.112 2.708 2.757 2.692
6 12 16 20 24 28
b
1.586 2.124 2.290 2.170 1.410 1.559 1.610
Samples were prepared at a constant temperature of 20 “C for 30 min and at different voltages in the sulphuric acid solution. After anodization, the samples were re-anodized in the boric acid solution at 23 “C and with a constant current density of 0.1 mA cm-‘. The initially measured voltage jump V, is due to the thickness of the barrier layer. The results are presented in Table 4. Curve-fitting of the results gave the following function as the best fit: V, = 0.872 V,
TABLE 3. The valves of constant T*,J+ of the aluminium ion
C and the transport
Temperature
Constant
(“C)
(V “2 s-9
0
0.041 0.030 0.027 0.030 0.034 0.032 0.031
6 12 16 20 24 28
number
C
0.387 0.320 0.304 0.315 0.415 0.391 0.383
perature. The values obtained are presented in Table 3. For the transport number of aluminium ion a mean value of 0.36 was obtained, which is very close to the value proposed by Takahashi and Nagayama [3]. The mean value of constant C (0.032 V1’2 s-l) is approximately equal to slope m2 when an anodization voltage of 1 V is applied. Table 2 shows that constants a and b are voltage and temperature dependent, respectively. Curve-fitting results in the following functions for a and b: a= -0.154
J-+3.51
(6)
b = - 0.005( T, - 11.83)2 + 2.28
(7)
Substitution of eqns. (6) and (7) in eqn. (3) results in a mathematical equation for the porosity as a function of the anodization voltage and temperature: 1 -=_
(Y
0.154ma+3.51
~+0.005(Z-a-11.83)2-2.28 (8)
(9)
This function confirms the conclusion of Dekker and Middelhoek [2]. In the V-t curve recorded (Fig. 2), in addition to the two slopes m, and m2 and the voltage jump found by Dekker and Middelhoek [2] and Takahashi and Nagayama [3], two more phenomena can be detected. First of all, it should be mentioned that in reality the end of the filling of the pores (VP, tP) is not a well-defined twist (Fig. 2), but rather a smooth change in slope. This is due, we suspect, to pores that are not totally cylindrical, whose mouths have a rather conical shape owing to the attack of the sulphuric acid solution [5]. Also, the heterogeneity of the porosity and a small variation of the applied current may cause a smooth change in the slope during the filling of the pores. TABLE 4. The measured anodization potential Anodization
voltage jump V, as a function
Voltage jump V,
voltage V,
(V)
(V)
2 4 6 8 10 12 14 16 18 20
1.60 3.54 5.49 7.29 8.91 10.88 12.30 13.80 15.76 16.93
voltage
,/”
(VI m, _A’
300.-
With eqn. (8) the porosity of porous anodization layers on aluminium can be estimated as a function of the applied voltage and temperature. In the introduction it was mentioned that the pores are separated from the base metal by a barrier layer. Dekker and Middelhoek [2] found that the thickness of the barrier layer is dependent on the anodization voltage. We tried to prove this in the second series of experiments.
0
of the
2000
4000
/” “Ia
6000 time
Fig. 2. Measured V-t curve during the re-anodization anodization layer on aluminium (3 V, 6 “C, 30 min).
(sec1
of an
149
However, extreme caution is necessary, because suppositions were not proved by direct shape surements. In addition, in the first part of the I/-t curve, the voltage jump, a deviation of slope m, (Fig. noticeable. A new slope m, is proposed: mO=
lim
m,
these mea-
Conclusions -
after 3) is
(10)
1-O
Figure 3 shows that m,
With the pore-filling method, i.e., re-anodization in a boric acid solution, it is possible to investigate the porosity change of porous anodic layers on aluminium as a function of the applied voltage and temperature. - The porosity is dependent on the voltage and temperature. Raising the temperature results in an increase in the porosity; raising the voltage has the opposite effect on the porosity. - With the mathematical equation derived it is possible to estimate the porosity for a given potential and temperature. - The barrier-layer voltage jump, which is related to the thickness of this layer, varies linearly with the anodization voltage.
voltage
References
,_,’
“b
/
,. /’ ____= _---
m0
time
10
Fig. 3. Detail
of the voltage
jump
V, in the
V-t curve.
M. Schenk, Werkstoff Aluminium und seine Anodische Oxydation, A. Francke AG, Berne, 1948. A. Dekker and A. Middelhoek, J. Electrochem. Sot., 117 (1970) 440. H. Takahashi and M. Nagayama, Corros. Sci., 18 (1978) 911. L. Lemaitre et aZ., Mater. Chem. Phys., 17 (1987) 285. S. Wernick, R. Pinner and P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and Its Alloys, Finishing Publications, Teddington, UK, 1987. G.C. Wood, J.P. O’Sullivan and B. Vaszko, J. Electrochem. Sot., 115 (1968) 618. G.E. Thompson et al., Trans. Inst. Met. Finish., 56 (1978) 159.