A study on the self-sealing process of anodic films on aluminum by EIS

Surface & Coatings Technology 200 (2006) 6846 – 6853 www.elsevier.com/locate/surfcoat

A study on the self-sealing process of anodic films on aluminum by EIS Xu-hui Zhao, Yu Zuo ⁎, Jing-mao Zhao, Jin-ping Xiong, Yu-ming Tang Faculty of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Received 19 May 2005; accepted in revised form 19 October 2005 Available online 1 December 2005

Abstract The electrochemical behaviors of anodized aluminum in neutral NaCl and Na2SO4 solutions were studied using electrochemical impedance spectroscopy (EIS). The results reveal that there is a self-sealing process for unsealed anodic film in neutral NaCl and Na2SO4 solutions. The resistance of the porous layer (Rp) and the capacitance of the barrier layer (CPEb) increase and the capacitance of the porous layer (CPEp) decreases with immersion time in the initial stage. Corrosion resistance provided by the anodic film is improved by the self-sealing process of the porous layer. However, chloride ions have an opposite effect. The improving effect of the self-sealing process on film resistance is decreased with the increase of chloride concentration of the solution. © 2005 Elsevier B.V. All rights reserved. Keywords: Anodic film; Aluminum; Self-sealing; EIS; Corrosion

1. Introduction The corrosion resistance of aluminum is strongly increased by anodizing, which produces a porous array of columnar hexagonal cells normal to the substrate surface and separated from it by a barrier layer [1–4]. The porous structure of the anodic film provides aluminum with absorbent characteristics that increase its applications (for example anodized aluminum can be colored by different techniques in order to be used in ornamental and household applications) [3,5]. Generally, the corrosion resistance provided by anodic films depends on the properties, integrity and thickness of the films, and sealing quality is one of the factors affecting the film properties. Sealing process decreases the porosity of the anodic layers and improves the resultant corrosion resistance [6–11]. In natural environments, the unsealed or poorly sealed anodized films are subjected to two opposing effects because of their absorbing properties, namely: self-sealing, which results in gradual protection of the metal substrate; and deterioration, also a gradual process that arises from fouling and corrosion. Depending on the particular environment, one phenomenon prevails over the other [12,13]. Aging of unsealed or poorly sealed anodic films has been known to improve sealing quality ⁎ Corresponding author. Tel.: +86 10 64434818; fax: +86 10 64423089. E-mail address: [email protected] (Y. Zuo). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.10.031

[13,14]. Atmospheric exposure of anodized aluminum produces significant changes in the anodic layer (due to the selfsealing process), which improves the corrosion resistance to some extent [15–17]. Suay et al. [18] reported a self-sealing phenomenon of anodic film on aluminum exposed to the ambient atmosphere, and found that the quality of the anodic film was improved with exposure time. However, the self-sealing process of anodic film in electrolytic solutions has been seldom reported. As an effective testing method, electrochemical impedance spectroscopy (EIS) has been used for investigation of the properties of porous aluminum oxide films prepared under different conditions [15,18–25]. In this paper, the self-sealing process of porous anodic films on aluminum in different solutions was studied by means of electrochemical impedance spectroscopy (EIS). The experimental impedance results obtained were simulated using equivalent electrical circuits, and the corresponding electrical parameters of the anodic film were analyzed to elucidate the electrochemical behavior of the anodic films. 2. Material and methods 2.1. Materials The material studied was commercial pure aluminum. The composition of the material is shown in Table 1.

X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

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2.5

Table 1 Composition of the material

2.0

0.01 M NaCl

0.1 M NaCl

Composition (mass %) Si

Cu

Al

≤0.25

≤0.20

≤0.015

≥99.5

2.2. Preparation of anodic films The preparation process of anodic films was as follows. The surfaces of the alloy samples were finished using SiC abrasive paper up to 600#, and cleaned with water and acetone. A degreasing treatment was carried out for the samples in 50 g/L NaOH solution for 3 min at room temperature. After water cleaning, the samples were immersed in 200 g/L HNO3 solution for 1 min at room temperature. Anodizing was carried out in a solution of 200 g/L H2SO4 (98%) + 20 g/L C2O4H2·2H2O + 15 g/L C3H5(OH)3 at a current density of 1.5 A/dm2 (voltages of 15–20 V) for 60 min at about 20 °C. Some of the specimens were sealed in boiling deionized water (pH 6–7.5) for 30 min. Then the samples were cleaned with water and dried. The thickness of the anodic films obtained was approximately 20 μm.

1.5

E / V (vs.SCE)

Fe

1.0 1 M NaCl

0.5 0.0 -0.5 -1.0 10-10

10-9

10-8

10-7

10-6

10-5

i / A/cm2 Fig. 2. Polarization curves of the unsealed anodic films in 0.01 M, 0.1 M and 1 M NaCl solutions.

solutions at 20 °C, over the frequency range from 100 kHz to 0.01 Hz (three frequencies per decade) under controlled potential conditions, with an AC potential signal of 10 mV varied about the open-circuit potential.

2.3. Test methods

2.4. The simulation of the barrier and porous layers by equivalent circuits

Electrochemical tests were carried out using a model 273A potentiostat and a model 5210 lock in amplifier. A saturated calomel electrode (SCE) was used as the reference electrode, and the counter electrode was platinum. The sample surface area exposed to the electrolyte was about 1 cm2. Polarization curves of the unsealed specimens were measured in neutral NaCl (0.01 M, 0.1 M, and 1 M) solutions (pH = 6.5–7.5) at 20 °C. The potential scanning rate was 0.66 mV/s. The impedance measurements were made in unstirred, neutral NaCl (0.01 M, 0.1 M, and 1 M) and Na2SO4 (0.1 M)

As mentioned above, the anodic film on aluminum consists of a very thin compact layer and a thick porous layer, and the later is composed of pores and walls of approximately hexagonal cells. In order to describe the electrochemical behavior of this system, the thin barrier layer and the porous layer are considered to be independent with each other. According to previous studies [26–29], the properties of each layer may be characterized by resistances and capacitances, in parallel and in series, describing their electronic and dielectric behaviors. Since the pioneering work of Hoar and Wood [1], various equivalent circuits have been proposed to model the response

(a) Rel

i)

Rel : electrolyte resistance Rw : hexagonal cells resistance Cw : hexagonal cells capacitance Rp : electrolyte resistance through pores Cp : electrolyte capacitance through pores Rb : barrier layer resistance Cb : barrier layer capacitance

ii)

Cw

Rw Cp

Rp

Cb

Rb

iii)

(b) Rel

Cp

Cb

Rp

Rb

i) wall of hexagonal cells ii) porous layer: pores iii) barrier layer

(c) Rel

CPEp

CPEb

Rp

Rb

(d)

CPEb Rel

Rb

Fig. 1. Equivalent circuits (EC) for modeling the behavior of anodic films. (a) General model, (b, c) simplified model, (d) simplified model of an unsealed anodic film.

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X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

8.0x106

6

6.0x10

6

represents the capacitance of the barrier layer. CPE is defined as [3]:

1x104

CPE ¼ 1=ðj2pfCÞn

0

4.0x10

Thus, parameters np and nb are affected by CPEp and CPEb, respectively, due to the non-ideal capacitive behaviors [28]. Parameter n is the frequency dispersion factor and varies from 1 to 0. Only when n = 1 can CPEp and CPEb be considered as real capacitances. For unsealed anodic films, the solution may penetrate to the barrier layer through the pores. Thus the EC is reduced to a very simple model shown in Fig. 1d.

3

0

1x10

Real part / Ω.cm2 Zoom

2.0x106

0.0 0.0

2.0x106

4.0x106

6.0x106

Real part /

8.0x106

ð1Þ

1.0x107

Ω.cm2

Fig. 3. Impedance diagrams for the unsealed anodic films in the neutral 0.1 M NaCl solution (immersion time: 10 min).

of porous anodic films. It is accepted that the porous anodic film may be modeled by the equivalent circuit proposed by Hitzig et al. [27], which is presented in Fig. 1a. This model is successfully applied to explain the properties of the barrier and porous layers [2,9]. In this model, Rel is the resistance of the solution between the reference electrode and the anodic film. Rel is usually very small, i.e. in NaCl solutions. One of the parallel branches in the circuit is formed by resistance Rw and the associate capacitance Cw. They represent the walls of the cells. Rw and Cw are generally omitted since they are extremely high and extremely low, respectively. Indeed, the walls of hexagonal cells prevent the passage of current. In this way, equivalent circuit can be reduced to that illustrated in Fig. 1b. The porous layer properties are characterized by capacitance Cp and resistance Rp. Barrier layer properties are described by capacitance Cb and resistance Rb, respectively. However, porous and barrier layers have heterogeneities that make their capacitive behavior better simulated by constant phase elements (CPE) than by simple capacitances (C). Hence the EC can be simplified to Fig. 1c, where CPEp represents the capacitance of the porous layer and CPEb

3. Experimental results Fig. 2 is the polarization curves of the unsealed anodic films in neutral NaCl solution. The anodized specimens showed stable passivity, and no pitting occurred in the studied potential range (up to 2 V). The lower the concentration of the electrolyte, the smaller was the passive current density. Since the porous layer was not sealed, the barrier layer played an important role against corrosion. Fig. 3 shows Nyquist plot of the unsealed anodic film in neutral 0.1 M NaCl solution when the immersion time is very short, for example, 10 min. Obviously, only one capacitive arc is observed in the testing frequency range. Previous studies have shown that this corresponds to the characteristics of barrier layers [1,26,27]. Fig. 4 shows Nyquist and Bode plots (impedance and phase angle) of the unsealed anodic film in 0.1 M NaCl solution (pH = 6.5–7.5) for periods of 1, 30, 60 and 150 days. It can be seen from the Nyquist plots that there is a high frequency semicircle and a low frequency arc. But the high frequency semicircle is very small in the first day, and the diameter of the high frequency semicircle increases as immersing time increases in the initial stage (about a month). However, as the immersion time further increases, the impedance gradually decreases. The Bode plots show that the impedance modulus decreases at lower frequencies as immersion time increases, but Z module

108 50.0k

(a)

107

6.0x105

4.0x105

0.0

0

0.1 M NaCl 1 day 30 days 60 days 150 days

om Zo

2.0x105

0.0 0

100k

1x106

Real Part / Ω.cm2

2x106

Z module / Ω.cm2

Imaginary Part / Ω.cm2

8.0x105

Phase

1 days 30 days 60 days 150 days

100 90 80

106

70

105

60 50

104

40 30

3

10

20

102 101 10-2

(b) 10-1

100

101

102

103

104

Phase angle / degree

Imaginary part / Ω.cm2

Imaginary part / Ω.cm2

1.0x107

10

0 105

Frequency / Hz

Fig. 4. Impedance diagrams for anodic films immersed in the neutral 0.1 M NaCl solution for 1 day, 30 days, 60 days and 150 days. (a) Nyquist plot, (b) Bode plot (impedance module data: solid symbol, phase angle data: open symbol).

X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

90 80

6

10

105

70

104

60

3

10

50

102

40

101 -2 10

10

-1

0

10

1

10

10

2

10

3

4

10

100

0. 01 M 0. 1 M 1M

30 days

107

NaCl

90 80

106

70

5

60

10

50

104

40

3

30

10

20

102

10

101 -2 10

5

10

(b)

Phase angle / degree

Z module / Ω cm2

107

108

NaCl

0.01 M 0.1 M 1 M

Z module / Ω cm2

(a) 1 day

Phase angle / degree

108

6849

-1

10

Frequency / Hz

0

1

10

10

2

10

3

10

0

4

5

10

10

Frequency / Hz

Fig. 5. Bode plots (impedance and phase angle) of the anodic films in 0.01 M, 0.1 M and 1 M NaCl solution after (a) 1 day and (b) 30 days (impedance module data: full symbol, phase angle data: open symbol).

the impedance modulus increase at higher frequencies as immersion time increases. The impedance modulus of the horizontal part in the Bode plots also increases in the early stage then gradually decreases as the immersion time prolongs. Fig. 5 shows Bode plots (impedance and phase angle) of the unsealed anodic films in 0.01 M, 0.1 M, and 1 M NaCl solution for 1 day and 30 days. Each impedance curve in the Bode diagram shows three straight-line sections, especially for 30 days. It is a typical impedance plot of sealed anodic films. From Fig. 5, it is seen that for 1 day of immersion the impedance curves are very close in NaCl solutions with different concentrations, but after 30 days of immersion, at intermediate and high frequencies, the impedance values of the unsealed films in 1 M NaCl solution are obviously smaller than that of the unsealed films in 0.1 M and 0.01 M NaCl solution. Fig. 6 shows Bode plots (impedance and phase angle) of the unsealed anodic films in 0.1 M NaCl and 0.1 M Na2SO4 solutions, respectively. The Bode plots of a sealed film, which was sealed in deionized water, in 0.1 M NaCl solution are also shown in Fig. 6 as comparison. At intermediate and high frequencies, the impedance value of the sealed film is obviously larger than that of the unsealed films after 1 day of immersion in 0.1 M NaCl solutions. While the impedance values of the unsealed films in 0.1 M NaCl and 0.1 M Na2SO4 solution is very similar.

60 50

104

40 30

103

20

(a)

10 10-2

10-1

100

101

102

Frequency / Hz

103

104

10

0 105

100

0.1 M NaCl unsealed 0.1 M NaCl sealed in water 0.1 M Na2SO4 unsealed

107

90 80

106

70 60

105

50

104 103

40 30 30 days

102 10-2

10-1

(b) 100

101

102

103

104

Phase angle / degree

105

1 day

108

Z module / Ω cm2

Z module / Ω cm2

80 70

1

The result shown in Fig. 3 indicates that the porous layer is not protective due to the high conductivity of the electrolytic solution inside the pores [1,26,27]. So, the equivalent circuit can be reduced to the simple model with Rel, Rb and CPEb (Fig. 1d). Considering that for unsealed anodic films the electrolyte can easily penetrate through the porous layer to attack barrier layer, a modified model with these new parameters is presented in Fig. 7 [23,27,30]. Parameter θ represents the fraction of the aluminum surface covered by the anodic film; parameter Rcorr represents corrosion resistance of the aluminum substrate and parameter Cdl corresponds to the double layer capacitance of the substrate. For θ = 1, the porous and barrier layers are not damaged. The aluminum surface is entirely covered by the anodic film. Parameters Rcorr and Cdl do not exist. When θ is less than 1, the porous layer and the barrier layer are not intact. However,

90

106

102

4.1. For short immersion time (10 min)

100

0.1 M NaCl unsealed 0.1 M NaCl sealed in water 0.1 M Na2SO4 unsealed

107

As has already been noted, the slope and situation of the Bode diagram depends on the characteristics of the porous and barrier layers, and the effects of different factors on the integrity and quality of the layers may be reflected by the impedance parameters [9,10].

Phase angle / degree

108

4. Discussion

20

10 105

Frequency / Hz

Fig. 6. Impedance diagrams for unsealed anodic films immersed in the neutral 0.1 M NaCl and 0.1 M Na2SO4 solution, for the anodic films sealed in boiling deionized water immersed in the neutral 0.1 M NaCl solution, for (a) 1 day and (b) 30 days (impedance module data: full symbol, phase angle data: open symbol).

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X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

Rel Rel : electrolyte resistance Rb θ

θ ·CPEb

Rb : barrier layer resistance CPEb : barrier layer capacitance

Rcorr (1– θ )

θ : fraction of substrate covered by anodic film Rcorr : corrosion resistance of substrate Cdl : double layer capacitance of substrate

(1– θ )·Cdl

Fig. 7. Equivalent circuit to model the electrolyte penetration through the anodic film.

parameters Rcorr and Cdl are very small and it is difficult to distinguish Rcorr from Rb and Cdl from Cb. When θ is close to 0, only Rcorr and Cdl could be detected. When the immersion time is relatively short, the porous and barrier layers keep intact, and θ ≈ 1. Fig. 8 shows the experimental and simulated complex plane plots for unsealed anodic films in 0.1 M NaCl solution for 10 min, and the simulated results are very similar to the experimental data. The obtained Rb value is 4.046 × 107 Ω cm2, CPEb value is 5.79 × 10− 7 F/ cm2, and nb value is 0.97. The parameter nb is very close to 1, indicating that the barrier layer is relatively homogeneous and few defects are present. It is known that capacitance can be connected with the barrier layer thickness by the relation as follows: C¼

ee0 S d

ð2Þ

where ε0 = 8.85 × 10− 14 F/cm is the dielectric constant in vacuum, ε = 10 is the relative constant for alumina, S is the electrode surface area and d the dielectric layer thickness. When the nb value is close to 1, the CPEb is close to an ideal capacitance. The CPEb value is connected to barrier layer thickness (d). The thickness of the barrier layer is calculated to be approximately 150 Å. This is quite consistent with the expected thickness for the present operating conditions (the films formed are widely accepted to be 10–14 Å/V thick) [19].

Imaginary part / Ω.cm2

Imaginary part / Ω.cm2

1.0x107

5.0x106

1x104

0 0.0

3.0x103 Real part / Ω.cm2

om

experimental

Zo

simulated

0.0 0.0

5.0x106

1.0x107

Real part / Ω.cm2 Fig. 8. Experimental and simulated complex plane plots for the unsealed anodic films in the neutral 0.1 M NaCl solution (immersion time: 10 min).

Because the characteristics frequency is in the nHz range, which is far from the lowest frequency used in the tests (10 mHz), estimated Rb values will be uncertain. However, the analytical result (Rb = 4.046 × 107 Ω cm2) is approximately consistent with the values proposed in the literature (Rb = 107 Ω cm2 [9], Rb = 109 Ω cm2 [3,18]). 4.2. For longer immersion time From Figs. 4–6, for longer immersion time, two characteristical arcs are seen in the high frequency range and low frequency range of EIS spectra respectively, which correspondingly represent the information of the porous and barrier layers [1,18,26,27]. According to the model shown in Fig. 7, the parameters θ, Rcorr and Cdl are introduced to symbolise the electrolyte penetration through the porous layer and the barrier layer [26]. A modified model with these new parameters is presented in Fig. 9 [23,27,30]. The parameter θ corresponds to the fraction of the substrate surface covered by the anodic film. Fig. 10 shows experimental and simulated complex plane plots for unsealed anodic films immersed in neutral 0.1 M NaCl solutions for 1 day, 30 days, 60 days and 150 days, respectively. Fig. 11 shows the change of Rp with exposure time. Fig. 12 shows the variation of CPEp and np as a function of immersion time. From Figs. 11 and 12, it can be seen that the resistance of the porous layer (Rp) obviously increases while capacitance of the porous layer (CPEp) obviously decreases with immersion time in initial stage, then Rp value slowly decreases with immersion time. This behavior indicates that the porous layers become more resistant in initial stage. Besides, the values of the exponent np are close to 0.7 (the value of an ideal capacitor is 1), indicating that the porous layer becomes relatively homogeneous with immersion time. When anodized samples are immersed in test solution, the electrolyte may be absorbed into the pores in the anodic film. The films are subjected to two opposing effects (self-sealing and deterioration). Depending on the particular environment, one phenomenon prevails over the other [12,13]. The anhydrous alumina reacts with absorbed water, which leads to voluminous hydrated alumina and resulting in a selfsealing effect. Thus, the resistance of the porous layer increases. The self-sealing mechanism involves degradation,

X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

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Rel

Rp θ

θ ·CPEp

(1– θ )·Cdl

Rcorr (1– θ )

Rb θ

θ ·CPEb

Rel : electrolyte resistance Rp : electrolyte resistance through pores CPEp : electrolyte capacitance through pores Rb : barrier layer resistance CPEb : barrier layer capacitance θ : fraction of substrate covered by anodic film Rcorr : corrosion resistance of substrate Cdl : double layer capacitance of substrate

Fig. 9. Equivalent circuit to model the electrolyte penetration through the unsealed anodic film.

reaches the barrier layer because the porous layer is in a “relatively open” state. For longer immersion time, CPEb value slowly decreases. The homogeneity of the barrier layer seems to decrease as the hydration process progresses, as the decrease of nb value indicates. After about 30 days, the nb value is close to 0.6. This shows the parameter θ decreases (less than 1) and the defects in the barrier layer increase. Parameter Rcorr and Cdl are also important. However, it is difficult to distinguish Rb from Rcorr and Cb from Cdl. 106 0.1 M NaCl 20 μm

Rp / (Ω cm2)

gelling, agglomeration and a precipitation process [18]. As there is a reduction of free water content inside the pores because of the formation of hydrated alumina, and the permittivity of the hydrated alumina is lower than that of free water, the CPEp value obviously decreases. At the same time, the anodic film may also be attacked by aggressive ions from the electrolyte to some extent. The conversion of alumina to hydrated alumina may be retarded because of the aggressive ions in the electrolyte. Thus, the resistance of the porous layer decreases and capacitance of the porous layer increases. In the initial stage, there is a remarkable increase in the impedance value and the corrosion resistance provided by the film increases because the self-sealing process prevails over the deterioration. When the two opposite effects arrive at a balanced state, Rp and CPEp values reach to their extremes. As the immersion time further increases, the effect of the aggressive ions (e.g. Cl−) is gradually enhanced, consequently the Rp value slightly decreases and the CPEp value increases. Fig. 13 shows the variations of CPEb and nb with exposure time. In the initial stage of immersion, the capacitance of the barrier layer (CPEb) obviously increases with immersion time. A reasonable explanation is that the electrolytic solution easily

105

104

103

0

20

40

60

80

100 120 140 160 180 200

Time / day Fig. 11. The changes in the Rp parameter of unsealed anodic films in 0.1 M NaCl solution with immersion time.

5

8.0x10

10-4

6.0x105

CPEp np

4.0x10

om

2.0x105

200.0k 0.1 M NaCl experimental modelized 1 day 30 days 60 days 150 days

0.0 0.0

5

5.0x10

6

1.0x10

6

1.5x10

1.0 0.8

10-6

0.6

10-7

0.4

10-8

0.2

np

0 0.0

5

CPEp / (F cm-2)

10-5

Zo

Imaginary Part / Ω.cm2

100k

6

2.0x10

Real Part / Ω.cm2

10-9

0

20

40

60

0.0 80 100 120 140 160 180 200

Time / day Fig. 10. Experimental and simulated complex plane plots for the unsealed anodic films immersed in the neutral 0.1 M NaCl solution for 1 day, 30 days, 60 days and 150 days.

Fig. 12. The changes in the CPEp and np parameter of unsealed anodic films in 0.1 M NaCl solution with immersion time.

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X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853 1.0

10-5

0.8 0.7

nb

CPEb / (F cm-2)

0.9

10-6

0.6 CPEb nb

10-7

0

50

100

0.5 0.4 0.3 200

150

Time / day Fig. 13. The changes in the CPEb and nb parameter of the unsealed anodic films in 0.1 M NaCl solution with immersion time.

In short, it is clear that the Rp value increases and the CPEp value decreases with immersion time in the initial stage because of a self-sealing process of the porous layer, and the corrosion resistance provided by the unsealed anodic films is improved by the self-sealing process. 4.3. The effects of electrolyte on the self-sealing process Fig. 14 shows Rp and CPEp values for anodic film in NaCl solutions with different concentrations. It is seen that

10-6

(a) CPEp / ( F. cm-2)

Rp / (Ω . cm2)

106

105

104 20μm NaCl

103

in 0.01 M and 0.1 M NaCl solutions the Rp and CPEp values of anodic film are very close. However, the values are different in 1 M NaCl. In 1 M NaCl solution the Rp value is smaller and the CPEp value is larger than those in 0.1 M and 0.01 M NaCl solutions. This result shows that the self-sealing effect is smaller when the concentration of chloride is higher. Obviously in solutions with higher chloride concentration, the harmful effect of aggressive ions on the anodic film prevails. Fig. 15 shows the measured Rp and CPEp values in different electrolytic solutions. It is seen that in 0.1 M Na2SO4 solution, the Rp value is higher and the CPEp value is lower than those in 0.1 M NaCl solution, which again indicates that in the solution without aggressive ions the self-sealing effect is more evident. In 0.1 M NaCl solution, the sealed film shows higher Rp value than the unsealed samples, especially for the first month. As a comparison, during the first month the Rp values of unsealed films show gradual increase due to the self-sealing effect. The Rp value of the sealed film slowly decreases with immersion time, while the CPEp value of the sealed film slowly increases. The results shown above reveal the fact that the self-sealing process exerts a similar effect on unsealed anodic films in 0.1 M NaCl and 0.1 M Na2SO4 solutions. But for sealed anodic films no self-sealing effect is observed.

0

20

40

60

(b)

10-7 NaCl 20 μm 0.01 M 0.1 M 1M

0.01M 0.1 M 1M

10-8

80 100 120 140 160 180 200

0

20

40

60

Times / day

80 100 120 140 160 180 200

Times / day

Fig. 14. The changes in the Rp (a) and CPEp (b) parameter of unsealed anodic films in 0.01 M, 0.1 M and 1 M NaCl solution with immersion time.

(a)

105

104

10-5

0.1 M NaCl 0.1 M Na2SO4 0.1 M NaCl sealed in water

CPEp / (F cm-2)

Rp / (Ω cm2)

106

0

20

40

60

80 100 120 140 160 180 200

Time / day

(b)

0.1 M NaCl 0.1 M Na2SO4 0.1 M NaCl sealed in water

10-6

10-7

10-8

10-9

0

20

40

60

80 100 120 140 160 180 200

Time / day

Fig. 15. The changes in the Rp (a) and CPEp (b) parameter of the anodic films with immersion time under different conditions.

X. Zhao et al. / Surface & Coatings Technology 200 (2006) 6846–6853

5. Conclusions (1) EIS is an ideal tool for obtaining detailed information about the characteristics of the anodized aluminum. It can quantify the changes of the barrier layer and porous layers with immersion time by analyzing the equivalent circuit. (2) A self-sealing process is observed for unsealed anodic films on aluminum in NaCl and Na2SO4 solutions. When anodized samples are immersed in test solution, the films are subjected to two opposing effects (self-sealing and deterioration). In initial stage of immersion, the resistance of the porous layer (Rp) and the capacitance of the porous layer (CPEp) decrease with immersion time because the self-sealing process prevails over the deterioration. As the immersion time further increases, Rp values slightly decrease, and CPEp value increases. (3) The corrosion resistance provided by the anodic films is improved by the self-sealing process of the porous layer. However, chloride ions have an opposite effect. The improving effect of the self-sealing process on film resistance is decreased with the increase of chloride concentration of the solution. References [1] T.P. Hoar, G.C. Wood, Electrochim. Acta 7 (1962) 333. [2] J.P.C. Dasquet, D. Caillard, E. Conforto, J.P. Bonino, R. Bes, Thin Solid Films 371 (2000) 183. [3] G.E. Thompson, Thin Solid Films 297 (1997) 192. [4] F. Keller, M.S. Hunter, O.L. Robinson, J. Electrochem. Soc. 100 (1953) 411. [5] G. Patermarakis, N. Papandreadis, Electrochim. Acta 38 (1993) 1413.

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