In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy

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In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance

spectroscopy

F. Bertram, F. Zhang, J. Evertsson, F. Carla, J. Pan, M. E. Messing, A.

Mikkelsen, J-O. Nilsson, E. Lundgren

To cite this version:

F. Bertram, F. Zhang, J. Evertsson, F. Carla, J. Pan, et al.. In situ anodization of aluminum surfaces

studied by x-ray reflectivity and electrochemical impedance spectroscopy. Journal of Applied Physics,

American Institute of Physics, 2014, 116 (3), 6 p. �10.1063/1.4890318�. �hal-01572978�

In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy

F. Bertram, F. Zhang, J. Evertsson, F. Carlà, J. Pan, M. E. Messing, A. Mikkelsen, J.-O. Nilsson, and E.

Lundgren

Citation: Journal of Applied Physics 116, 034902 (2014); doi: 10.1063/1.4890318 View online: http://dx.doi.org/10.1063/1.4890318

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/3?ver=pdfcov Published by the AIP Publishing

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In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy

F. Bertram,

^1,a)

F. Zhang,

²

J. Evertsson,

¹

F. Carl a,

³

J. Pan,

²

M. E. Messing,

¹

A. Mikkelsen,

¹

J.-O. Nilsson,

⁴

and E. Lundgren

¹

1

Division of Synchrotron Radiation Research, Lund University, Box 118, 221 00 Lund, Sweden

2

KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Division of Surface and Corrosion Science, Drottning Kristinas v€ ag 51, 10044 Stockholm, Sweden

3

ESRF, B. P. 220, 38043 Grenoble, France

4

Sapa Technology, Kanalgatan 1, 612 31 Finspa˚ng, Sweden

(Received 21 March 2014; accepted 3 July 2014; published online 16 July 2014)

We present results from the anodization of an aluminum single crystal [Al(111)] and an aluminum alloy [Al 6060] studied by in situ x-ray reflectivity, in situ electrochemical impedance spectroscopy and ex situ scanning electron microscopy. For both samples, a linear increase of oxide film thickness with increasing anodization voltage was found. However, the slope is much higher in the single crystal case, and the break-up of the oxide film grown on the alloy occurs at a lower anodization potential than on the single crystal. The reasons for these observations are discussed as are the measured differences observed for x-ray reflectivity and electrochemical impedance spectroscopy.

V

^C

2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4890318]

I. INTRODUCTION

Aluminum and its alloys are used in a broad range of everyday commercial products as well as of interest in future microelectronics. As a result of the attractive properties of Al, extensive research on Al has been made for a long time, fo- cusing both on applied as well as on basic research, with the ultimate aim on improved Al products. For instance, it has since long been realized that the corrosion resistance of Al is dependent upon a protective oxide film

¹

formed spontane- ously in air at room temperature. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly self-repaired by re-oxidation. The growth and properties of this oxide film are crucial for the corrosion protection and other functions, in par- ticular in aggressive environments, and the oxide film forma- tion has therefore received enormous attention.

²

Surface science studies have provided detailed informa- tion on the initial formation of the protective oxide at low or ambient temperature using highly controlled ultra high vac- uum (UHV) conditions and well-prepared single crystal surfaces

^3–11

as well as by using theoretical means.

¹²

Many applications for Al require increased corrosion protection for better durability. This can be obtained by increasing the thickness of the protective oxide by electro- chemical means using anodization. Anodization is the elec- trochemical growth of an oxide by applying an anodic potential to the aluminum in an electrolyte. However, much less is known about the atomic scale structures and chemical processes occurring on Al surfaces under these conditions, since it is necessary to perform measurements during the electrochemical reaction and the process of forming a thick anodic oxide. Traditional surface science methods based on

electrons fail in such environments due to the short mean free path of the electrons, although recent technical develop- ments appear promising.

^13–15

Furthermore, in industrial applications, different kinds of polycrystalline Al alloys are used to provide various desir- able materials properties depending on the final application, making fundamental studies of the anodization processes even more complicated.

It is, however, known that the structure and properties of anodized Al depend on the anodizing electrolyte as well as on the quality of the natural oxide, which can be modified by alloying elements in the Al, resulting in dilute Al alloys. For instance, addition of Mg, Ti, and Ni has an effect on the quality and appearance of the anodic alumina film,

¹⁶

and fur- thermore such oxide films have been found to be more stable at high pH environments. It is also known that in an aqueous environment, pure aluminum forms a thin passive film (sur- face oxide layer) with a typical thickness of 2–4.3 nm, simi- lar to that formed in air.

¹⁷

In contrast to electron based techniques hard x-ray scat- tering techniques such as x-ray diffraction (XRD) and x-ray reflectivity (XRR) are only slightly affected by gases or liq- uid. A number of studies applying x-ray scattering techni- ques to study catalytic

^18,19

or electrochemical

^20,21

systems under realistic or semi-realistic conditions can be found in the literature.

In electrochemistry, electrochemical impedance spec- troscopy (EIS) has been the method of choice for studying interfacial processes occurring at various metal-electrolyte interfaces and for characterization of properties of surface films, such as corrosion protection of natural passive films and anodic oxides.

²²

In the present report, we report on in situ studies of the anodization of an Al(111) single crystal and an Al 6060 industrial alloy using XRR and EIS. Our goal with the

a)

florian.bertram@sljus.lu.se

0021-8979/2014/116(3)/034902/6 116, 034902-1 VCAuthor(s) 2014

JOURNAL OF APPLIED PHYSICS 116, 034902 (2014)

present study is two-fold: The first goal is to detect differen- ces in anodization properties between a single crystal alumi- num and an industrial alloy. The second goal is to compare the experimental structural capabilities of XRR and EIS with the future aim to simultaneously measure XRR (or other x-ray techniques suitable for liquids) at synchrotrons. Our study demonstrates that there is a significant difference between the anodization of the single crystal and the indus- trial alloy. Although, we find differences, XRR and EIS pro- vide consistent and complementary information and are suitable to combine in an experimental setup at a synchrotron.

II. EXPERIMENTAL SETUP

To perform the anodization experiments, two different electrochemical setups have been used. One setup for in situ x-ray reflectivity measurements and one setup for electro- chemical impedance measurements.

Prior to anodization, the sample surfaces were polished to 0.03 lm roughness and cleaned by rinsing with acetone and purified water.

The in situ XRR experiments were performed at the ESRF beamline ID03 (Ref. 23) using a photon energy of 24 keV (k ¼ 0.516 A ˚ ). For these experiments, an electro- chemical cell as described by Foresti et al.

²⁴

was used. The counter electrode was a glassy carbon rod and a saturated Ag/AgCl electrode was used as reference electrode.

The EIS spectra were obtained by a Multi Autolab (Metrohm Autolab B.V. Netherlands) instrument, using a three-electrode electrochemical cell with a saturated Ag/

AgCl electrode as reference electrode, a Pt mesh as counter electrode, and the sample with an exposed area of 0.3 cm

²

as working electrode. The software Nova 1.9 was employed for the spectra collection and analysis. All measurements were performed at room temperature.

We studied the anodization behavior of two different samples while stepwise increasing the anodization voltage from 1 V up to 8 V in 2M Na

₂

SO

₄

solution. The first sample studied is an Al(111) single crystal. The second sample is an Al 6060 type alloy, with the main alloy components being 0.49% Si, 0.2% Fe, and 0.37% Mg. Fig. 4(b) shows the typi- cal current response after changing the voltage. The current increases rapidly and then decreases exponentially reaching a constant value after a few seconds. This current transient is typical for the formation of a stable anodic oxide on an alu- minum surface. This illustrates that the oxide growth occurs mainly within the first few seconds after the voltage was increased since the oxide growth rate depends on the current density.

III. RESULTS

A. In situ x-ray reflectivity (XRR)

To study the evolution of the thickness of the anodic ox- ide film, we performed x-ray reflectivity (XRR) measure- ments. Fig. 1 shows data from the both samples before anodization measured in argon. The experimental data have been fitted using the Parratt-algorithm

²⁵

with a N evot-Corce

roughness model.

²⁶

The Parratt algorithm is a recursive algo- rithm calculating the reflectivity of a layer system by consid- ering the Fresnel reflectivity of each interface. In the Parratt model, each layer is characterized by a certain thickness and electron density. To consider interface roughness using the Parratt algorithm, the reflectivity of each interface is modi- fied, in case of the N evot-Corce roughness model by a damp- ening factor assuming an error-function profile for the electron density.

Before anodization (see Fig. 1), we observe clear thick- ness oscillations due to a native oxide. Our measurements yield a thickness of 2.7 nm of the native oxide for the Al(111) sample, in good agreement with previous studies.

¹⁷

In case of the Al 6060 alloy, we calculate the thickness of the oxide film to be 4.1 nm.

To anodize the samples voltage steps used are 1, 2, 4, and 8 V, without interruption of the anodization. Figs. 2(a) and 2(b) show the experimental data from XRR measure- ments performed at various anodization steps.

Starting with the Al(111), the XRR data at different anodization potentials are shown in Fig. 2(a). At 1 V, the dis- tance between the oscillations decreases compared to the native oxide, directly indicating the anodic growth of a thicker aluminum oxide film, since the width of these oscilla- tions is inversely proportional to the film thickness. As the anodization potential is increasing in steps to 8 V, the width is continually decreasing, directly showing the growth of an increasingly thick aluminum oxide film.

As we anodize the Al 6060 alloy with increasing poten- tial up to 8 V, the width between the oscillations decrease with increasing potential, as shown in Fig. 2(b), indicating the growth of an increasingly thick oxide film, similar to the behavior observed for the Al(111) single crystal.

Thus, we conclude from Figs. 2(a) and 2(b) that in both the Al(111) single crystal and Al 6060 alloy cases the thick- ness of the aluminum oxide films on both surfaces increases with increasing anodization potential.

Fig. 2(c) shows the film thickness obtained by fitting the reflectivity curves as a function of anodization voltage. For both samples, we observe a linear increase of film thickness.

However, the slope differs for single crystal and alloy, and clearly the oxide thickness is increasing faster for the

FIG. 1. XRR measurements of the native oxide films for Al(111) and Al 6060 alloy.

034902-2 Bertramet al. J. Appl. Phys.116, 034902 (2014)

Al(111) single crystal as compared to for the Al 6060 alloy with increasing anodization potential. For the Al(111) single crystal, we obtain an increase in thickness of Dd ¼ 9.3 nm/V and for the Al 6060 alloy we obtain Dd ¼ 1.0 nm/V.

The roughness values obtained by data fitting show an increase with increasing film thickness. No clear difference can be observed between the two samples. The electron den- sities obtained by fitting show no clear trends upon anodiz- ing. However, they are in general lower than the electron density of crystalline bulk Al

₂

O

₃

.

B. Electrochemical impedance spectroscopy (EIS) EIS measurements as a function of anodization potential were performed to evaluate the resistance of the oxide layers formed on the Al surfaces. The EIS measurements were car- ried out at the open-circuit potential (OCP) with perturbation amplitude of 10 mV over the frequency range from 10

⁴

to 10

²

Hz. In this experiment, the application of the anodic potential was stopped after each anodization step in order to perform EIS measurement at the OCP. So there is some small difference in the experimental procedures between the two electrochemical setups.

EIS spectra for the Al(111) single crystal and Al 6060 alloy before and after anodization at 1, 2, 4, and 8 V were

recorded. Spectra are plotted in Nyquist format as shown in Fig. 3.

All the spectra show essentially only one time constant feature. In such cases, the surface layer behaves like a single layer and can be fitted by a simple equivalent circuit. The simplest equivalent circuit describing the metal-electrolyte interface consists of a polarization resistance (R

p

) and a ca- pacitance in parallel, and a solution resistance (R

s

) connected in series. In practice, a capacitive response of an interface is not ideal, therefore a constant phase element (CPE) is often used for spectra fitting instead of a capacitance (see Fig. 4(a)). The impedance function of a CPE is given by:

Z

CPE

ð Þ ¼ x 1

Y

0

ð ix Þ

ⁿ

: (1) Therefore, we used an equivalent circuit as described above but with a CPE instead of a capacitance for spectra fitting.

The capacitance (C) of such a system can be derived from fitted data Y

0

, n, and R

p

(Ref. 27) and is given by

C ¼ ð Y

0

R

p

Þ

¹⁼ⁿ

R

p

: (2)

Fig. 5(a) shows the values for R

p

obtained by fitting the EIS spectra. For all the EIS measurements, the value of n obtained by fitting is close to 1. Thus, the CPE behaves

FIG. 2. XRR measurements during at anodization steps of 1, 2, 4, and 8 V for (a) the Al(111) single crystal and (b) the Al 6060 alloy. Red dots show the exper- imental data and blue lines show simulated data. The thickness obtained from XRR analysis is shown in (c). Solid lines show linear fits to the data.

FIG. 3. EIS measurements (Nyquist representation) performed before anod- ization and after the anodization steps of 1, 2, 4, and 8 V for the Al(111) single crystal (a) and the Al 6060 alloy (b).

Dashed lines show the fitted curves.

034902-3 Bertramet al. J. Appl. Phys.116, 034902 (2014)

almost as an ideal capacitor. The film thickness shown in Fig. 5(b) was derived from the capacitance assuming that the oxide films act as a parallel-plate capacitor.

In the calculation, we assumed a dielectric constant

_r

¼ 10 for the aluminum oxide films formed on both the samples. We like to emphasize that a wide range of possible values

_r

is reported in the literature.

¹⁷

The Al(111) single crystal shows high R

p

values indicat- ing the formation of a compact and protective oxide layer on the surface. From OCP to 4 V, both the R

p

and oxide layer thickness increases with the increasing of anodization poten- tial, which demonstrates that a higher potential induces a thicker oxide layer and a corresponding higher resistance.

However, from 4 V to 8 V, the oxide layer became even thicker but a drop of R

p

was observed.

For the Al 6060 sample, the R

p

increased largely after anodization at 1 V, which is associated with the formation of

a protective oxide with a high resistance. However, the R

p

decreased with the increasing of potential from 1 V to 8 V, while the anodic oxide layer became thicker and thicker.

Fig. 5(c) shows the thickness of the anodic oxide as a function of anodization potential. For the Al(111) single crystal, we obtain an increase in thickness of Dd ¼ 3.3 nm/V and for the Al 6060 alloy we obtain Dd ¼ 2.0 nm/V.

C. Scanning electron microscopy

Fig. 6 shows ex situ SEM images from the single crystal and the alloy samples recorded after anodization up to 8 V.

Images from the single crystal show significant charging effects due to the high oxide resistance. On the alloy sample, we observe a surface structure with a large amount of fis- sures, which are uniformly distributed over the whole sample.

In the case of the Al(111) single crystal, we observe large flat areas with only slight and less distinct fissures.

However, on some stripes on the sample a similar fissures structure like on the alloy sample is observed, however, still smaller than the fissures observed on the alloy sample.

IV. DISCUSSION

The results in the present report show that the trends in anodic oxide growth measured by XRR or EIS are similar, the thickness of the anodic oxide film increases linearly with increasing anodization voltage. In fact, in the case of the Al 6060 alloy, the results are even qualitatively similar.

Further, the thickness increase observed for the alloy sample using both techniques is within a range of values reported in the literature.

¹⁷

On the other hand, the value obtained by XRR for the single crystal sample is much higher than values usually reported for aluminum polycrystalline alloys.

With both methods, it is clear that the oxide film growth on the single crystal Al(111) surface is considerably faster than on the Al 6060 alloy. One reason for this difference could be the presence of the alloy components in the poly- crystalline alloy, which may affect the anodization behavior.

For example, the aluminum diffusion could be hindered by a steric effect by the alloying elements effectively lowering the degree of anodization in the alloy compared to pure alu- minum. Another reason could be the effect of surface orien- tation. Al(111) is the most close-packed aluminum surface having the highest density of aluminum atoms. The (111) surface also has a higher atomic density than a polycrystal- line surface since a polycrystalline surface consists only partly of (111) surfaces and partly of surfaces with a lower

FIG. 4. (a) Sketch of the equivalent circuit describing the metal electrolyte interface. The electrolyte is represented by the solution resistance (R

s

) and the aluminum oxide layer is modeled by an electrical resistance (R

p

) and a constant phase element (CPE) in parallel. In this case, the measured polar- ization resistance at the metal-electrolyte interface is dominated by the oxide resistance. (b) The current response after a voltage change.

FIG. 5. Results from the EIS analysis.

(a) The

Rp

value as a function of anod- ization voltage. Note that the

Rp

values for the alloy are multiplied by a factor of 25 for better visibility. (b) Oxide film thickness (left axis) calculated from the capacitance (right axis)) for the Al(111) and Al 6060 alloy samples.

For thickness calculations a parallel- plate capacitor like behavior was assumed.

034902-4 Bertramet al. J. Appl. Phys.116, 034902 (2014)

density, such as the (100) and the (110). A higher atomic density facilitates the growth of a thicker oxide simply because of the availability of more atoms for oxide formation.

From the present study, it is also clear that the change in the R

p

value provides information on the properties of the oxide formed at different anodization voltages. In the case of the Al(111) single crystal, the R

p

value increases up to 4 V and decreases at 8 V. One possible explanation for the decrease in the R

p

at high potentials is due to the doping effect of some ions from the electrolyte incorporated into the oxide film. Another interpretation is that the morphol- ogy of the growing oxide is changing from compact to a compact film with small fissures, indicating the initial for- mation of a porous oxide. This interpretation is corrobo- rated by the SEM images after anodization as shown in Figs. 6(a) and 6(c). In the case of the Al 6060 alloy, the decrease of the R

p

value is observed already at 2 V, a nota- bly lower voltage compared to the single crystal. In this case, the decrease in the R

p

at high potentials also could be due to the doping effect of both the alloying elements and the ions from the electrolyte. Moreover, above a certain anodization potential, the barrier oxide starts to break (forming fissures) and some porosity is generated in the ox- ide film, which could also cause the decrease in the meas- ured resistance. The decreasing R

p

at a lower voltage indicate that the oxide film on the alloy forms fissures even though it is thinner than the oxide on the Al(111). As a con- sequence, the fissures appear more prominent in the SEM image in the case of the alloy oxide film as was shown in Figs. 6(b) and 6(d). The interpretation of the experimental results leads to the tentative model of the oxide growth dur- ing anodization as shown in Figure 7. In this model, the ini- tial compact and closed oxide film formed on the single crystal grows thicker and has less tendency to crack as com- pared to the alloy oxide film. The reason for the earlier cracking could possibly be due to a larger number of initial

nucleation sites for the oxide formation on the alloy as com- pared to the single crystal surface.

Finally, it is also clear that the estimated thickness of the oxide films as obtained by XRR and EIS differ from each other, in particular in case of the single crystal Al(111). In both techniques, the estimation relies on a fitting procedure, relying on fit parameters. Further, a major uncertainty in the XRR fitting is the estimation of the roughness fit parameter which may introduce an error of 5%. In particular, the fis- sures and porosity of the film will affect the roughness parameter.

An important uncertainty in the thickness calculation from the EIS results is the exact value of the dielectric con- stant of the oxides. In our calculation, we used a fixed value of

_r

¼ 10 for all the films. In the literature values ranging from 7.5 up to 15 are reported,

¹⁷

which is not surprising con- sidering the many different aluminum oxide films that can be produced. However, even assuming these extreme values for the dielectric constant, we still obtain growth rates (Dd

_r¼7.5

¼ 1.5 nm/V and Dd

_r¼15

¼ 3.0 nm/V for the alloy sample) different from the XRR results, especially for the Al(111) crystal (Dd

_r¼7.5

¼ 2.5 nm/V and Dd

_r¼15

¼ 5.0 nm/V).

Therefore, it is obvious that we cannot obtain reasonable dielectric constants from the capacitance using the film thicknesses as measured by XRR. In our study, the change of the oxide films at different anodization potential could in principle affect the dielectric constant due to incorporation of other ionic species into the oxides from the electrolyte and also from the alloying elements in the case of the alloy sample. Moreover, the formation of cracks in the oxide films may also give rise to an error in the estimation of the thickness.

Another uncertainty lies in the different experimental setups used for the present in situ XRR and EIS measure- ments. This may also introduce some uncertainties in the exact chemical and electrochemical conditions at the metal- electrolyte interfaces, which may affect the oxide growth during anodization. Furthermore, in the case of the alloy, even if the samples are cut from the same extrusion profile, the surface properties of the different samples may not be exactly the same since the surface presence of the alloy com- ponents may still differ.

Despite all these uncertainties, excellent agreement between the XRR and EIS measurements are observed for the alloy sample. The present report shows the feasibility to combine the XRR (physical technique) and EIS (electro- chemical technique) for in situ study of anodization of

FIG. 7. Tentative models of the morphology of the oxide film after anodiza- tion at 8 V for the Al(111) and the Al 6060 alloy.

FIG. 6. SEM images recorded after stepwise anodization up to 8 V from the single crystal (a) and (c) and the alloy (b) and (d) sample.

034902-5 Bertramet al. J. Appl. Phys.116, 034902 (2014)

metals, which can provide detailed information of the oxide growth and properties of the formed oxide films. The above considerations clearly suggest that it would be extremely favorable to measure the XRR and the EIS at the same setup, preferably at a synchrotron.

In summary, we have studied the anodization of a Al(111) single crystal and an industrial Al 6060 polycrystal- line alloy sample by using XRR and EIS. With both techni- ques, we find a much faster growth of the oxide on the Al(111) as compared to the alloy and attribute this difference to the alloy components and/or the surface orientation. We also observe that the oxide grown on the alloy forms fissures at a lower anodization potential and thinner oxide film thick- ness as compared to the Al(111). We argue that this differ- ence may be due to differences in the initial oxide nucleation for the single crystal and the alloy. Finally, we critically compare the XRR and EIS results and propose to perform anodization experiments using XRR and EIS simultaneously in the same electrochemical setup.

ACKNOWLEDGMENTS

The in situ anodization and XRR experiments were performed on the ID03 beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France.

This work was financially supported by the foundation for strategic research (SSF).

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