Flow modulated ionic migration during porous oxide growth on aluminium

Article abstract of DOI:10.1016/j.electacta.2010.06.088

The ability to tailor the morphology of porous anodic alumina (PAA) drives applications in the important architectural, aerospace, packaging, lithography and nanotechnology sectors with the mechanisms regulating oxide growth being pursued vigorously. Here, we have probed the barrier layer of PAA during film growth, using high voltage, in situ, electrochemical impedance spectroscopy. Our new findings account for aluminium oxidation at the metal/oxide interface, ionic migration and interaction of charge carriers through the layer, and oxide viscous flow under the field. Separation of these processes by appropriate modelling discloses that ionic migration, being driven by the electrical field only, is independent of the electrolyte. The incorporated electrolyte-derived species modify the mechanical properties of the oxide, by regulating the oxide displacement from the barrier layer. The approach developed provides the vital foundations for an integrated theory of porous film growth on a range of metals in various film-forming electrolytes.

Full text of DOI:10.1016/j.electacta.2010.06.088

Electrochimica Acta 55 (2010) 7044–7049
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Flow modulated ionic migration during porous oxide growth on aluminium
M. Curioni∗, E.V. Koroleva, P. Skeldon, G.E. Thompson
Corrosion and Protection Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The ability to tailor the morphology of porous anodic alumina (PAA) drives applications in the important
architectural, aerospace, packaging, lithography and nanotechnology sectors with the mechanisms regu-
lating oxide growth being pursued vigorously. Here, we have probed the barrier layer of PAA during film
growth, using high voltage, in situ, electrochemical impedance spectroscopy. Our new findings account
for aluminium oxidation at the metal/oxide interface, ionic migration and interaction of charge carriers
through the layer, and oxide viscous flow under the field. Separation of these processes by appropriate
modelling discloses that ionic migration, being driven by the electrical field only, is independent of the
electrolyte. The incorporated electrolyte-derived species modify the mechanical properties of the oxide,
by regulating the oxide displacement from the barrier layer. The approach developed provides the vital
foundations for an integrated theory of porous film growth on a range of metals in various film-forming
electrolytes.
Received 21 April 2010
Received in revised form 28 June 2010
Accepted 29 June 2010
Available online 6 July 2010
Keywords:
Electrochemical impedance spectroscopy
Anodic oxides
Ionic migration
Anodizing
Flow
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
revealed a capacitive arch and an inductive loop, which were asso-
ciated respectively with the dielectric properties of the oxide and
Porous anodic alumina (PAA) films are generated by anodic
polarization of aluminium in aqueous electrolytes and their proper-
ties can be tailored by selecting the anodizing conditions to obtain
ordered templates [1–5], corrosion [6–9] or wear resistant layers
and coloured surfaces [10–13].
During porous film growth, new material is generated within
the barrier layer by migration of charged species (aluminium and
oxygen ions) under the electric field. Previous work on barrier-type
oxides [14,15] has revealed that (i) growth proceeds at an effi-
ciency close to 100%, 60% and 40% of the oxide is generated close
to the metal/oxide interface and to the oxide/electrolyte interfaces
respectively [14], (ii) the anodic oxide is amorphous and migra-
tion proceeds by a co-operative transport mechanism [14] and (iii)
ionic transport is the rate-determining step [15]. It has also been
suggested that the oxide may exhibit semiconductive properties
due to the nature and distribution of charge carriers during growth
[15,16].
Electrochemical impedance spectroscopy has been applied
rarely to the study of growing barrier-type films because the oxide
thickness increases with the charge passed and, therefore, the use
of a frequency sweep is inappropriate for such non-stationary sys-
tem. However, De Wit et al. acquired a series of single frequency
impedance measurements during linear voltage sweeps, obtain-
ing potential dependent spectra [17]. The resultant Nyquist plots
the occurrence of relaxation processes during growth. In elec-
trochemical or semiconductive systems that display similarities
with a growing anodic oxide (i.e. supra-linear current–voltage
dependence [18,19], a reaction involving intermediates [20,21], DC
biasing [18,19] or p–n junctions [22,23], inductive contributions
are often revealed and interpreted as relaxation phenomena [17],
change in concentration of adsorbed intermediates [20,21], or neg-
ative capacitance resulting from charge distribution [23].
Information regarding barrier-type films can be transferred to
porous-type films, with the difference that porous film growth pro-
ceeds at an efficiency of about 60% due to ejection of outwardly
mobile aluminium ions into the electrolyte. Further, pore forma-
tion accompanies ionic conduction and, after an initial transient, a
barrier layer with a steady thickness is generated, enabling the use
of a frequency sweep to study the growth.
Concerning pore formation, much research on oxides gener-
ated with specific combinations of anodizing parameters has been
undertaken, but an integrated theory of porous anodic film growth
is currently awaited. O’Sullivan and Wood [24] pioneered the field
by using transmission electron microscopy of fracture sections to
reveal the oxide morphology. They concluded that the morphology
results from a dynamic equilibrium between generation of oxide
and thermally enhanced, field-assisted dissolution at the pore base.
However, recent evidence, based on the ratio of the thickness of
the oxide generated to the aluminium consumed and observation
of the displacement of ion tracers within the barrier layer [25–29],
has suggested that, for sulphuric, oxalic and phosphoric acids, the
contribution to film growth from displacement of newly formed
∗
Corresponding author. Tel.: +44 0 161 306 5971; fax: +44 0 161 306 4865.
E-mail address: michele.curioni@manchester.ac.uk (M. Curioni).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.06.088

M. Curioni et al. / Electrochimica Acta 55 (2010) 7044–7049
7045
oxide material from the barrier layer towards the cell walls domi-
nates dissolution-related effects. Such displacement is possible due
to plasticization of the oxide material under the electric field and
the action of growth and electrostriction stresses within the barrier
layer.
2. Experimental
Anodizing was undertaken on superpure (99.99 wt.%) alu-
minium specimens in stirred 0.3 and 0.6 M oxalic acid, 0.4 and
0.8 M sulphuric acid at 20 and 30 ◦C in a three-electrode cell, with
a saturated calomel electrode (SCE) as reference. Prior to anodiz-
ing, the specimens were degreased in acetone and in ethanol,
rinsed in water and dried with cold air. For anodizing with simul-
taneous acquisition of the electrochemical impedance spectra, a
Solartron Modulab potentiostat was employed. The acquisition of
the impedance spectra at each potential was initiated after a steady
anodizing current was achieved. In order to promote the rapid
establishment of a relatively steady condition for the impedance
measurement, a voltage sweep (1 V/min) was employed to achieve
the potential of interest. Subsequently, a preliminary impedance
spectrum, requiring about 4 min, was acquired at this potential
(over a reduced frequency range), followed by a 2 min of poten-
tiostatic anodizing. After the short potentiostatic anodizing, the
final impedance spectrum was acquired. The stationarity of the
system was confirmed by verifying that the current–time curves
acquired during the final impedance measurement did not display
significant DC drift and by verifying that the spectra acquired dur-
ing the preliminary impedance measurement overlapped with the
spectra acquired during the final impedance measurement over
the frequency range available. The amplitude of the sinusoidal sig-
nal superimposed on the DC anodizing potential (in the range of
4–20 V) was 50 mV, with the frequency varied between 100 kHz
and 0.001 Hz. The acquisition time for an impedance spectrum was
approximately 3 h. Fitting of the impedance data was performed by
using the Modulab software.
3. Results and discussion
3.1. Semiconductive nature of the growing oxide
Fig. 1. The metal–oxide system. Schematic view of the metal–oxide–electrolyte sys-
tem during oxide growth (without accounting for the externally applied potential)
with qualitative plots of ion concentration, resulting space charge, electric field and
potential distribution.
In order to fully interpret the data obtained by electro-
chemical impedance spectroscopy, the intrinsic behavior of
the metal–oxide–solution system during oxide growth is con-
sidered initially. Fig.
1 displays a schematic view of the
metal–oxide–electrolyte system during oxide growth. Oxidation
of aluminium to aluminium ions is thermodynamically favoured;
therefore, an accumulation of aluminium ions is expected within
the oxide close to the metal interface, with a corresponding accu-
mulation of electrons expected within the metal close to the oxide
interface. When DC current is passed, oxide growth proceeds with
additional aluminium atoms oxidized to aluminium ions, which are
continuously injected into the oxide at the metal/oxide interface.
Simultaneously, oxygen anions, generated from water adsorbed
at the oxide/electrolyte interface, are injected into the oxide at
the oxide/solution interface. When a given number of aluminium
atoms (i.e. 10 in Fig. 1), are oxidized, a corresponding number of alu-
minium ions are generated at the metal/oxide interface. Of the 10
ions injected into the oxide at the aluminium/oxide interface, only
6 combine with a corresponding number of oxygen ions within the
oxide (9), to form 3 units of Al2O3. 4 aluminium ions injected at
the metal/oxide interface migrate further under the electric field
with eventual ejection into the electrolyte. From Fig. 1, an excess
of Al3+ positive charge carriers (similar to p-type semiconductor),
resides close to the metal and an excess of O2− negative charge
carriers (similar to n-type semiconductor), is present close to the
film/electrolyte interface. Between the p-type oxide and the n-type
oxide, charge recombination takes place, generating Al2O3 units.
The process of charge recombination requires that the positive
charge carried by the aluminium ions equals the negative charge
carried by the oxygen ions and, therefore, the behavior of this region
is similar to an intrinsic (i-type) semiconductor [15,16]. Conse-
quently, the system can be described as a metal-p–i–n junction,
with additional qualitative consideration of the spatial distribution
of charge, field and potential, assisting selection of the appropri-
ate equivalent circuit (Fig. 2) for fitting of impedance data. From
Fig. 1, it is evident that, as a result of the charge distribution, the
potential is a non-monotonic function of position, increasing at
the metal/film interface and decreasing across the p–i–n oxide.
Through aluminium accumulation at the metal/oxide interface, an
interfacial capacitance, CMO, similar to the double layer capacitance
in aqueous electrolytes, and an associated parallel resistance RMO
,
similar to a charge transfer resistance for aqueous electrolytes,
are expected. During oxide growth, the current passes from the

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M. Curioni et al. / Electrochimica Acta 55 (2010) 7044–7049
Fig. 2. Equivalent circuit. Equivalent circuit representative of the
metal/oxide/electrolyte system.
metal towards the oxide i.e., when crossing the metal/oxide inter-
face, from a region of more negative potential towards a region of
more positive potential. Therefore, since resistance and capacitance
are defined as positive for current passing from a region of more
positive potential towards a region of more negative potential, neg-
ative values of CMO and RMO are expected thereby determining the
inductive behaviour in the Nyquist impedance plots. Similar to RMO
,
accounting for the potential drop (ꢀVMO) due to the metal/p-oxide
interface, RJ relates to the potential drop (ꢀVJ) due to the p–i–n
oxide, resulting from the distribution of charge carriers within the
oxide. The resistor RO accounts for the dissipative nature of ionic
transport across the i-type oxide, and a Warburg element considers
diffusion of the charged species within the oxide. The contribu-
tion of the Warburg element to the overall resistive behavior (see
Additional Table 1) is always below 5% and, for simplicity, it is
not discussed further. The capacitance of the barrier layer oxide,
Fig. 3. Impedance data. Nyquist plots obtained at 4, 8, 12 and 16 V DC in 0.4 M
sulphuric acid at 20 ◦C. In the inset the high frequency region of the spectra is
enlarged.
ity constant close to 1.05 nm/V in agreement with cited values [24].
For the specimens anodized in 0.3 M oxalic acid 30 ◦C at 16 and
20 V, slightly lower values of barrier layer thickness were revealed,
suggesting an increase of electric field during growth under high
voltage and high temperature conditions. The potential drops due
to the metal–oxide interface and the p–i–n junction within the
oxide, calculated by multiplying the DC current by the values of RM
and RJ obtained from the fitting of the impedance data, respectively
decreased and increased linearly with the applied potential, but
irrespective of other experimental parameters (Fig. 6). The value of
the resistance, RO, representative of the process of ionic migration,
was higher for oxides formed in oxalic acid than for sulphuric acid
under similar anodizing conditions and decreased with increasing
applied potential, temperature, or electrolyte concentration (Fig. 7).
RO was generally one order of magnitude higher than RM and RJ (see
Additional Table 1). The oxide resistivity, i.e. the ratio between RO
and the barrier layer thickness, only displayed a dependence on
the DC current, decreasing with increase of current with a log–log
relationship (Fig. 8).
proportional to its thickness, is represented by the capacitor CBL
.
Finally, CTH accounts for an apparent capacitance related to the
dynamic reorganization of the porous oxide under the oscillat-
ing potential associated with impedance probing. Specifically, for a
dynamic condition at sufficiently low frequencies, i.e. below 0.1 Hz,
the variation in potential is followed by a variation in the barrier
layer thickness due to the proportionality between applied poten-
tial and barrier layer thickness [24,30]. Over the frequency range
where cyclic thickening (or thinning) of the barrier layer is evident,
a contribution to the current that is proportional to the time deriva-
tive of the potential appears [30], which is revealed as an apparent
large capacitance.
3.2. Effect of experimental parameters on impedance spectra
4
experimental parameters have been examined, namely
applied DC potential and electrolyte composition, concentra-
tion and temperature. Generally, the appearance of the complex
impedance plots was similar for all the experimental conditions
(Fig. 3). Thus, spectra with 3 time constants, comprising a high fre-
quency capacitive arch, a medium frequency inductive loop and a
low frequency capacitive arch, where always recorded. The low fre-
quency capacitive arch was one order of magnitude larger than the
high frequency capacitive arch, and the diameter of the inductive
loop was approximately one-half of the high frequency capaci-
tive arch. An increase in DC current, achieved either by increasing
potential, temperature or electrolyte concentration, consistently
reduced the diameter of all the 3 arches. Unreported experiments
performed on variously pre-anodized specimens indicated that,
once a steady condition is achieved, the impedance measurement
senses only the barrier layer properties, irrespective of the pre-
vious anodizing history. Examples of experimental and calculated
impedance spectra for various conditions are provided in Fig. 4.
3.4. Flow modulated ionic migration
During porous oxide growth, the migration of charged species
across the barrier layer region of the porous oxide is responsi-
ble for the generation of new oxide material. However, in parallel
with the ionic migration process, the viscous displacement of the
newly formed oxide from the barrier layer towards the cell walls
plays a key role. The overlap of these 2 processes determines the
geometry and the growth rate of the porous oxide films, and the
resulting electrical properties that are measured during growth.
In order to understand further the growth process, the intrinsic
electrical properties of the oxide, regulating only ionic migration,
must be separated from the extrinsic electrical properties that arise
from the dynamic equilibrium between ionic migration and viscous
displacement. If this is achieved, it is then possible to explain the
influence of electrolyte nature, determining incorporation of acid
anion species within the oxide film, on ionic migration and viscous
displacement.
3.3. Modelling and interpretation
Our work unveils that the intrinsic electrical properties, regulat-
ing ionic migration, are completely independent of the electrolyte
nature within the experimental window inspected. Specifically,
the potential drops across the metal/oxide interface and the p–i–n
oxide are affected only by the value of the applied potential, indicat-
The barrier layer thickness calculated from CBL (relative dielec-
tric constant εr = 9, Fig. 5), increased linearly with applied potential,
independent of the experimental conditions, with a proportional-

M. Curioni et al. / Electrochimica Acta 55 (2010) 7044–7049
7047
Fig. 4. Fitting of impedance data. Experimental and calculated impedance spectra for anodizing in (a) 0.6 M oxalic acid at 20 ◦C, and 20 V, (b) 0.4 M sulphuric acid at 30 ◦
C
and 4 V, (c) 0.3 M oxalic acid at 30 ◦C and 8 V, (d) 0.8 M sulphuric acid at 20 ◦C and 8 V.
ing that ion incorporation does not modify the electrical properties
and the distribution of charge. Further, by disclosing the relation
between RO and the DC current, and revealing that RO is one order
of magnitude greater than RJ and RMO, it is confirmed, from a
purely electrical viewpoint, that the rate-determining process dur-
ing oxide growth is ionic migration across the barrier layer oxide.
However, the variation of RO with the applied potential, show-
ing a dependence on all the experimental parameters, suggests
that RO contains contributions from the overlapping processes of
ionic migration and mechanical displacement. Therefore, RO cannot
be considered strictly an intrinsic electrical property of the oxide,
but is the result of the combined electrical and the mechanical
behaviour. However, by normalizing RO to the barrier layer thick-
ness, the oxide resistivity can be calculated. As shown, the value of
oxide resistivity only relates to the DC current and, consequently,
it can be considered an intrinsic property that is unaffected by the
electrolyte nature or flow regime. The log–log dependence of the
oxide resistivity on the DC current is identical to the dependence
observed in p–i–n diodes [31]. In p–i–n diodes, generally used as
current-controlled resistors, a layer of intrinsically semiconductive
silicon is placed between the p-type and n-type layers. Due to the
increased resistivity of the i-layer with respect to the p and n layers,
the potential drop is largely localized across the i-layer, and charge
Fig. 6. Potential drops within the oxide. Potential drops due to the metal/oxide
junction (RM × IDC) and to the p–i–n junction (RJ × IDC) as a function of the applied
potential.
Fig. 5. Barrier layer thickness. Variation of the barrier layer thickness with applied
potential for all the selected conditions.

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M. Curioni et al. / Electrochimica Acta 55 (2010) 7044–7049
of current generates new oxide, which contributes to an increase
of barrier layer thickness. Therefore, since the intrinsic electrical
properties of the oxides are not affected by the electrolyte compo-
sition from the electrical viewpoint, it can be concluded that the
barrier layer oxide can rapidly approach Thmax, determining the
porous oxide morphology, irrespectively of the electrolyte.
When electrolyte ions are incorporated into the oxide, viscous
displacement of the oxide from the barrier layer region become
possible [25–28]. As shown in our work, this process does not affect
the intrinsic electrical properties of the oxide and, for a given poten-
tial, an oxide with a barrier layer thickness equal to Thmax, is always
obtained. As a direct consequence, the compressive stresses act-
ing on the barrier layer are independent of the electrolyte, since
they are related only to the applied potential and to the barrier
layer geometry. Under this condition, the oxide viscosity becomes
the rate-determining property and the mechanical displacement of
oxide material from the barrier layer towards the cell walls can be
considered to be an attempt to reduce the barrier layer thickness.
However, as detailed previously, the electrical system can promptly
respond by lowering the resistivity and increasing the growth rate,
to achieve Thmax. The sulphate-containing oxides have reduced vis-
cosity compared with the oxalate-containing oxides and flow faster
under the electrostriction force, as evident from the reduced values
of RO.
Fig. 7. Oxide resistance. Resistance to ionic migration, RO, as a function of the applied
potential.
transport takes place under the resulting electric field. In the case
of growing aluminium oxide, the thickness of the i-type region is
variable, since the barrier layer oxide thickness is proportional to
the applied potential.
Depending on the experimental conditions and, therefore, on
the combined ionic transport-mechanical flow regime, the growth
of the oxide occurs at a fixed value of current for a given DC
potential. From the log–log dependence of the resistivity on the
current, the system is stable from the electrical viewpoint; the bar-
Overall, the electrical properties of the oxide are independent
of the electrolyte and determine the barrier layer thickness and
the resulting pore geometry, while the oxide mechanical properties
modulate the ionic migration across the barrier layer, determining
the anodizing current and, ultimately, the growth rate.
4. Conclusions
The mechanism of porous oxide growth has been investigated
in situ by high voltage electrochemical impedance spectroscopy to
disclose the relation between experimental parameters, electrical
properties of the growing oxide and flow regime. Manipulation
of the electrochemical impedance data enables separation of the
intrinsic electrical properties of the growing oxide, due only to the
oxide nature and independent of electrolyte composition, from the
extrinsic properties, resulting from the overlapping of the processes
of ionic migration and mechanical displacement. It is disclosed that
the anodic oxide can be represented as a system described by a
metal-p–i–n junction, with characteristic intrinsic potential drops,
revealed readily by the impedance measurement. From the elec-
trical viewpoint, it is revealed that the process of ionic transport
across the oxide is the rate-determining step during the growth.
Further, the oxide intrinsic electrical properties are independent of
the electrolyte, providing the reason for the observed the potential
dependent porous film morphology. Importantly, the electrolyte
nature modifies the mechanical properties of the oxide by regu-
lating the oxide displacement from the barrier layer towards the
cell walls. The rate of displacement modulates ionic transport by
determining the anodizing current and the film growth rate.
rier layer thickness increases rapidly to a maximum value, Thmax
,
corresponding to a negligible growth rate in the absence of mechan-
ical flow. When Thmax is attained, the resistivity and, therefore, the
resistance are very high and ionic migration is negligible. Simulta-
neously, the porous film morphology, if any, is determined, since
the pore diameter and curvature directly correlate with the barrier
layer thickness. If for some reason the barrier layer becomes locally
thinner, the electric field increases and, consequently, the current
also increases. This produces a substantial drop in oxide resistivity,
which promotes a further passage of current. Clearly, the passage
Acknowledgement
The authors acknowledge the financial support provided by the
Engineering and Physical Sciences Research Council (U.K.) Portfolio
Award, Light Alloys Towards Environmentally Sustainable Trans-
portation.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2010.06.088.
Fig. 8. Oxide resistivity. Oxide resistivity, ꢁo = RO/barrier layer thickness, as a func-
tion of the DC current.

M. Curioni et al. / Electrochimica Acta 55 (2010) 7044–7049
7049
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