Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
C557
0013-4651/2008/155共12兲/C557/8/$23.00 © The Electrochemical Society
Anodic Film Formation on Aluminum in Nitric Acid
E. V. Koroleva,z T. Hashimoto, G. E. Thompson,* and P. Skeldon**
Corrosion and Protection Centre, School of Materials, The University of Manchester,
Manchester, M60 1QD, United Kingdom
Anodic film formation on aluminum and AA1050 aluminum alloy has been examined during anodic polarization at constant
current density and by cyclic potentiodynamic polarization in nitric acid. The resultant filmed substrates have been observed by
scanning and transmission electron microscopies. Film formation under potentiodynamic conditions results in nonuniform film
thickening that is limited by the onset of pitting and oxygen evolution at appropriate potential. For anodic polarization at constant
current density, transformation of the nonuniform film to a porous anodic film with increase of polarization time is revealed. The
relatively high chemical reactivity of nitric acid to the alumina film material is displayed by potentiodynamic polarization at low
sweep rates, when the chemical dissolution rate, ⬃0.6 nm min−1, is similar to the film growth rate. Addition of cathodic activity
to aluminum and the AA1050 alloy and enhanced localized aluminum dissolution above the pitting potential do not significantly
influence the kinetics of alumina growth and dissolution. Consistently high cathodic activity of the alloy during cyclic extended
polarization is associated with the exposure of the second-phase particles within the pits at the surface of the AA1050 alloy that
contrasts with the reducing cathodic activity of 99.99% aluminum.
© 2008 The Electrochemical Society. 关DOI: 10.1149/1.2987747兴 All rights reserved.
Manuscript submitted March 11, 2008; revised manuscript received July 21, 2008. Published October 7, 2008.
Oxide film growth on aluminum in nitric acid during anodic
polarization has been reported previously.1-4 Such studies have suggested that mainly amorphous alumina films result, containing fine
clusters of ␥-Al2O3;1,2 furthermore, film growth proceeds at a current efficiency of about 10–20%.2 From consideration of the voltage
共potential兲–time behavior during polarization at a constant current
density in nitric acid, porous anodic films were considered to develop over the aluminum surface.3,4 Furthermore, during anodic film
formation the aluminum is lost to solution through film dissolution,
which is a prerequisite for porous anodic film formation, and
through pitting.2,3 The relative contributions of pitting and film formation to the anodic current under galvanostatic anodic polarization
have been examined previously over a wide range of current densities in solutions of different nitrate concentrations and pH values.4
Film formation during anodic polarization of aluminum in nitric
acid is important in the electrograining of aluminum alloys for lithographic plate production. Electrograining, or fast cyclic potentiodynamic polarization 共anodic and cathodic兲, is used to develop a
highly convoluted surface of AA1050 alloy surface through pitting
in nitric or hydrochloric acids. Previous studies have shown that the
pit morphologies developed in nitric acid differed from those formed
in hydrochloric acid; thus, hemispherical pits of well-defined shape
and with relatively smooth walls were generated in nitric acid compared to pit walls that are faceted by fine cubic pits through electrograining in hydrochloric acid.5-8 Because it is well known that the
pitting potential of aluminum in nitric acid is relatively high compared to that of hydrochloric acid, the different appearance of the
pits has been explained partly by repassivation of aluminum prior to
achieving the pitting potential during treatment in nitric acid unlike
that in hydrochloric acid.9,10 The presence of the passive anodic film
on the aluminum surface and its influence on pit initiation sites
during electrograining in nitric acid has been first reported by Marshall and Ward.11 It was suggested that the dislocation structure in
the cold-worked alloy controls the defect distribution in the oxide
film; such defects provide the pit initiation sites during electrograining in nitric acid. Defects are also provided by second-phase particles in the commercial AA1050 alloy that exhibit preferential dissolution during electrograining in nitric acid, with the filming of the
macroscopic aluminum surface.3 Additionally, the contribution of
the filming behavior of aluminum on pitting and corrosion product
formation was examined during biased electrograining, where the
anodic or cathodic polarization was deliberately extended.12
The present study of anodic oxide formation on pure aluminum
and AA1050 aluminum alloy discloses the highly reactive nature of
nitric acid to the alumina film that contributes to pit development on
aluminum. The morphology of the film and the mechanism of film
formation during galvanostatic anodic polarization have been
probed directly using transmission electron microscopy 共TEM兲, enabling correlation with film formation/dissolution and pitting during
potentiodynamic polarization; the latter is of direct relevance to offset printing plate manufacture.
Experimental
Spade electrodes of dimensions 10 ⫻ 20 mm were prepared
from superpure aluminum 共99.99 wt % aluminum with 0.0006%
iron, 0.0004% silicon, 0.0003% copper, and 0.0015% magnesium as
impurities兲 and AA1050 alloy 共0.32 wt % iron, 0.04% silicon, and
0.02% zinc兲, supplied in the fully hard condition. The presence of
iron in the AA1050 alloy results in the formation of Al3Fe secondphase particles of average size about 2 ␮m. Specimens were etched
in 0.5 M sodium hydroxide solution at 80°C for 60 s, rinsed in
deionized water, desmutted in 50% HNO3 for 30 s, rinsed again and
dried in a cool airstream. This procedure leads to the exposure of
second-phase particles on the surface of AA1050 alloy because the
dissolution rate of the particles is reduced compared to the rate of
aluminum dissolution. Subsequently, anodic polarization of individual specimens was undertaken at a constant current density of
10 A m−2 in 0.24 M HNO3 solution at ambient temperature for 300
and 600 s in a two-electrode cell, with a cylindrical aluminum cathode. Additionally, cyclic voltammetry 共CV兲 in nitric acid was performed at sweep rates in the range from 0.01 to 0.35 V s−1 in a
three-electrode cell that incorporated a platinum counter electrode
and a saturated calomel reference electrode. All polarization experiments were carried out using a Solartron 1280 Electrochemical System, with the data analyzed by CorrWare software.
The morphologies of the films present on superpure aluminum
and the alloy after alkaline etching and anodic polarization at constant current density were examined by TEM of ultramicrotomed
sections. Sections of nominal thickness of 15 nm comprising the
aluminum substrate and attached film, were prepared using an RMC
6000 XL ultramicrotome. The sections were examined in a JEOL
2000 FX II transmission microscope at 120 kV. Additionally, the
surfaces of aluminum and the alloy after etching and anodic polarization were examined using secondary electrons in an ISI DS 130
scanning electron microscope 共SEM兲 operating at 20 kV.
Results
* Electrochemical Society Fellow.
** Electrochemical Society Active Member.
z
E-mail: [email protected]
Voltage-time response during anodic polarization.— The typical
voltage-time responses during anodic polarization of etched aluminum and the AA1050 alloy at a constant current density of
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
Figure 1. Voltage-time behavior during anodic polarization at a constant
current density of 10 A m−2 in nitric acid solution: 䊏–aluminum; 䊊–the
AA1050 alloy.
10 A m−2 are presented in Fig. 1; triplicate specimens were employed for reproducibility, with the average behavior shown. For
superpure aluminum, after an initial voltage surge to 1.47 ⫾ 0.13 V,
the voltage increased at a progressively decreasing rate to a maximum of 2.72 ⫾ 0.02 V; thereafter, the voltage decreased from the
maximum over the following 6 s to a steady value of
2.45 ⫾ 0.01 V. The slope of the initial increase in voltage from the
commencement of anodizing was 0.25 ⫾ 0.02 V s−1. For the
AA1050 alloy, an initial voltage surge of 1.24 ⫾ 0.13 V was evident that was followed by an increase of voltage to a maximum of
2.62 ⫾ 0.03 V and subsequent decrease to a steady voltage of
2.40 ⫾ 0.01 V after a further 8 s. The initial rate of voltage rise was
0.17 ⫾ 0.02 V s−1; the reduced value of the initial slope of voltagetime behavior is associated with the presence of the second-phase
particles on the alloy surface.
The voltage-time behavior revealed is reminiscent of porous anodic film formation on aluminum during anodizing in acid electrolytes, which is usually described as initial nonuniform film growth
in the region of approximately linear voltage rise at the commencement of polarization and thickening of the regular porous anodic
film in the steady voltage region.13 Furthermore, the magnitudes of
the initial voltage surges indicate slight differences in thicknesses of
the air-formed films on the etched superpure aluminum and the alloy; additionally, the lower steady voltage for the alloy suggests a
slightly reduced barrier layer thickness beneath the outer porous
region. In the presence of the additional anodic processes on the
alloy 共i.e., at Al3Fe intermetallic particles兲, the effective current density for film growth over the macroscopic surface is reduced, with a
consequent reduction in the steady cell voltage.
Characterization of etched and anodically polarized specimens.— A TEM image of an ultramicrotomed section of the etched
AA1050 alloy is shown in Fig. 2a; a film of thickness ⬃3 nm is
present over the macroscopic surface. After anodic polarization under the conditions of Fig. 1 for 300 and 600 s, films of increased
thicknesses, 18 ⫾ 2 and 42 ⫾ 5 nm, respectively, developed 共Fig.
2c and e兲. Anodic polarization of aluminum for similar times results
in formation of films of thicknesses of 30 ⫾ 4 and 62 ⫾ 5 nm, respectively 共Fig. 2b and d兲. The films appear amorphous, with no
evidence of crystalline alumina islands. Because of the fine features
of the porous anodic film compared to the section thickness parallel
to the electron beam, the detailed appearance is not readily discerned. However, scrutiny of aluminum after anodic polarization for
300 s reveals distinct porous film morphology. The average pore
diameter measured at the pore base is 7 ⫾ 2 nm, and the pores are
separated by cell walls of 4 ⫾ 1 nm width. The pore base is separated from the aluminum substrate by a barrier layer of ⬃3 nm
thickness 共Fig. 2b兲. After anodic polarization for 600 s, thickening
of the anodic film resulted in a less regular morphology 共Fig. 2e兲.
Figure 2. TEM images of the ultramicrotomed sections of the variously
treated aluminum and the AA1050 alloy: 共a兲 etched AA1050 alloy; 共b兲
etched aluminum, and 共c兲 the etched AA1050 alloy after anodic polarization
under the conditions of Fig. 1 for 300 s; and 共d兲 etched aluminum and 共e兲 the
etched AA1050 alloy after anodic polarization under the conditions of Fig. 1
for 600 s.
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
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Figure 3. SEM images of 共a, c兲 the etched aluminum surface after anodic polarization under the conditions of Fig. 1 for 300 s; 共b, d兲 the etched AA1050 alloy
surface after anodic polarization under the conditions of Fig. 1 for 600 s.
Barrier layers of about 3–4 nm thickness are evident at the pore
bases on the aluminum and AA1050 alloy surfaces 共Fig. 2b-e兲.
SEM images of aluminum and the alloy after anodic polarization
for 300 and 600 s, respectively, are presented in Fig. 3; occasional
pits are evident on the surfaces of the specimens, with dimensions
ranging from 5 to 50 ␮m. Additionally, through undermining of the
substrate, the film developed over the macroscopic aluminum and
alloy surfaces is evident adjacent to the pits 共Fig. 3c and d兲.
CV in anodic region of polarization.— In order to gain further
insight into film development on the aluminum and alloy surfaces,
CV has been employed. In particular, imposition of two cycles has
been used to examine the anodic film development and the consequences of such film formation on reimposing the polarization.14
The two cycles of polarization, from 0.2 V below the open-circuit
potential 共OCP兲 to 1.6 V, followed by reversal of the polarization to
the OCP, were performed at different sweep rates 共Fig. 4兲. Prior to
polarization, the OPCs of aluminum and the alloy were
−0.52 ⫾ 0.07 and −0.36 ⫾ 0.03 V, respectively 共five specimens of
each material were used兲, with the increased potential in the latter
case reflecting the increased presence of cathodically active secondphase material.
During the first cycle of polarization at a sweep rate of
0.010 V s−1, the initial increase of anodic current density was delayed for 0.43 and 0.56 V for aluminum and the AA1050 alloy,
respectively 共Fig. 4a and b兲. After the initial increase, the anodic
current density rose progressively from 2 A m−2 to a maximum
value of 4.5 A m−2 for aluminum and the alloy specimens. With
reversal of polarization, the current density decreased rapidly to
4 A m−2, which was followed by a decrease in current density at a
rate of 0.018 A m−2 s−1 for both specimens 共Fig. 4a and b兲. During
the second cycle, the delay in initial increase of anodic current density was reduced to 0.12 and 0.06 V for aluminum and the AA1050
alloy, respectively. After the initial increase, the anodic current density generally followed the trends observed in the first cycle.
With an increased potential sweep rate of 0.050 V s−1, potential
delays of 0.56 and 0.62 V before an increase in anodic current density were evident for aluminum and the AA1050 alloy, respectively
共Fig. 4c and d兲. These potential delays increased slightly with further increase in the potential sweep rate and, for polarization at
potential sweep rates of ⬎0.17 V s−1, it remained constant at the
maximum values of 0.64 and 0.75 V for aluminum and the alloy,
respectively 共Fig. 4e-j兲. During the second cycle of polarization, the
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
Figure 4. Two cycles of potentiodynamic polarization in nitric acid solution of 共a兲 etched aluminum and 共b兲 the etched AA1050 alloy at a potential sweep rate
of 0.01 V s−1; 共c兲 etched aluminum and 共d兲 the etched AA1050 alloy at potential sweep rate of 0.05 V s−1; 共e兲 etched aluminum and 共f兲 the AA1050 alloy at
potential sweep rate of 0.17 V s−1; 共g兲 etched aluminum and 共h兲 the AA1050 alloy at potential sweep rate of 0.25 V s−1; and 共i兲 etched aluminum and 共j兲 the
AA1050 alloy at potential sweep rate of 0.35 V s−1.
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
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Figure 5. Variation of the dissolution of the anodic oxide in nitric acid
solution with time, calculated from the difference between the values of the
maximum delay and the delay at a given sweep rate.
increase in current density was delayed further with increase of potential sweep rate and, for potential sweep rates above 0.25 and
0.17 V s−1 for aluminum and alloy, respectively, the delays revealed
little further changes. The maximum potential delay values were
2.12 and 1.98 V for aluminum and alloy, respectively 共Fig. 4e-j兲.
The sweep rates at which the potential delays reached their maximum values are similar to the values of initial voltage increase observed during anodic polarization at constant current density. Furthermore, the potential delays observed during cyclic polarization
are directly related to the thickness of the anodic oxide film resident
on the surface. A decrease in the potential delay with reduction of
the potential sweep rate, or with an increase of time of immersion, is
consistent with chemical dissolution of the anodic oxide film in the
reactive nitric acid solution. The dissolution rate of the anodic oxide
can be calculated from the difference between the values of the
maximum delay 共2.12 and 1.98 V for aluminum and alloy, respectively兲 and the delay at a given sweep rate plotted against the immersion time of the specimen 关i.e., the time of reverse polarization
during the first cycle plus the time of second cycle polarization prior
to current increase 共Fig. 5兲兴. The potential decays corresponded to
the oxide dissolution rates were 8.4 and 8.6 mV s−1 for aluminum
and the alloy, respectively; using a value of 1.2 nm V−1 for the
formation rate of barrier layer in phosphoric acid solutions at
50 A m−2,13 the respective dissolution rates are 0.60 and
0.62 nm min−1. The oxide dissolution rate in nitric acid is comparable to the chemical dissolution rate of barrier-type anodic alumina
films in phosphoric acid solutions 共0.4 nm min−1兲 and significantly
higher than that in sulfuric acid 共0.07 nm min−1兲.15
Additionally, during polarization at increased potential sweep
rates, the anodic current density increased from 5.7 to 14 A m−2,
with a value of 10 A m−2 reached at a sweep rate 0.17 V s−1 and
above. Generally, the anodic current density was higher for aluminum compared to that for the AA1050 alloy for all sweep rates.
Reversal of the polarization resulted in a sharp decrease in the current density to 2 A m−2, which was followed by a rapid fall at a rate
of 0.2 A m−2 s−1 共Fig. 4e-j兲. This confirms the presence of the anodic oxide film over the macroscopic surface.
CV that includes the cathodic region of polarization.— In addition to the previous polarization, two cycles of extended polarization, from the OCP to −1.8 V, with reverse of the scan to 1.9 V and
final reversal to the OCP, were performed at 0.05 V s−1 for aluminum and the alloy 共Fig. 6兲. This extended polarization now included
potential regions above the pitting potential in nitric acid and below
the potential for hydrogen evolution. During such polarization, the
aluminum initially experienced cathodic activity at a maximum cathodic current density of 400 A m−2; then, a fast increase of anodic
current density was observed from the corrosion potential of
−0.62 to 0.15 V of anodic polarization. This was followed by a region of nearly constant current density 共9.8 A m−2 at 1.5 V兲 with
Figure 6. Two cycles of potentiodynamic polarization extended to a potential above the pitting potential and below the potential of hydrogen evolution
in nitric acid solution for 共a兲 etched aluminum and 共b兲 the etched AA1050
alloy.
increasing polarization to the pitting potential of 1.74 V. The pitting
potential was indicated by the sharp increase of current density to
150 A m−2 共Fig. 6a兲. During reverse polarization, the repassivation
potential was evident at 1.61 V and the corrosion potential was at
0.35 V. The second cycle of polarization revealed a significant reduction in the cathodic activity on the surface of aluminum, with a
cathodic current density of 60 A m−2 on reversal of the cycle. This
is associated with the presence of the anodic oxide film of increased
thickness compared to that of the initial air-formed oxide. An increase in anodic current density was only observed from 0.73 V,
which was delayed by 1.14 V compared to the first cycle 共Fig. 6a兲.
This delay is similar to that observed, 1.04 V 共1.6–0.56 V兲, during
anodic polarization below the pitting potential 共Fig. 4c兲, indicating
that enhanced cathodic activity and additional aluminum dissolution
within the pits have insignificant effects on the anodic film growth
or dissolution. A similar constant current density of 9.8 A m−2 was
evident from 1.34 V to the pitting potential of 1.84 V, with the
maximum anodic current density falling from 150 to 40 A m−2. The
reverse polarization was similar to that of the first cycle 共Fig. 6a兲.
Increase in the pitting potential and reduction in the maximum anodic current density observed during the second cycle of polarization indicate that the anodic film has been reformed on localized
sites, where pitting and cathodic activity proceed under appropriate
polarization.
Generally, the extended polarization behavior of the AA1050 alloy was similar to that of aluminum 共Fig. 6b兲. However, the maximum values of cathodic current density of 670 A m−2 were similar
for both cycles, which are attributed to an enhanced cathodic activity on second-phase particles. A noticeable increase in anodic current density was observed from 0.09 V for the first cycle and 0.74 V
for the second cycle, indicating the presence of the oxide film of an
increased thickness. The previous values are displaced significantly
from the corrosion potential of −0.42 V 共the delay in current density
increase is 1.16 V兲. The anodic current density reached a nearly
constant value of 7.7 A m−2 at 1.5 V. The pitting potentials were
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
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1.77 and 1.85 V for the first and second cycles, respectively, with
similar repassivation potentials of 1.64 V and corrosion potentials of
0.25 V 共Fig. 6b兲.
Discussion
Anodic film growth in nitric acid.— Polarization at constant current density in nitric acid and associated scanning and transmission
electron microscopies confirmed the development of anodic films
over the macroscopic aluminum surface that has been reported
previously.1-4 The film formation is also confirmed by the cyclic
anodic polarization behavior, with film growth on the first cycle
hindering further film growth in the subsequent cycle until sufficiently high potentials are achieved to support ionic migration under
the high electric field. Such cyclic anodic polarization has been employed previously for characterization of anodic oxide film growth
and its dissolution in borate, sulfate, and chromate electrolytes of
various pH values.14,16 In electrolytes that support anodic film
growth at 100% efficiency 共for example, borax兲, the anodic current
density increases rapidly to a value that remains constant with increasing anodic polarization. Reverse of the polarization results in a
rapid fall in current density to values approaching zero; immediate
application of the second cycle of anodic polarization is associated
with little change in current density until sufficiently high potentials
are achieved to stimulate ionic migration across the previously
formed barrier film.
Potentiodynamic polarization at a fixed potential sweep rate results in anodic oxide film formation at a constant current density,
provided the field strength, E, across the thickening film remains
constant. This is readily shown from manipulation of Faraday’s law
to give the expression
⌬V ⌬V ⌬t
⌬V ␳nF ⌬t
⌬V ␳nF 1
E=
=
=
=
⌬d
⌬t ⌬d
⌬t M ⌬Q
⌬t M i
关1兴
where ⌬V/⌬t is the potential sweep rate, Q is the charge passed to
form a film of thickness d at a current density i for time t, ␳ is the
density of anodic alumina 共3.1 ⫻ 103 kg m−3兲, M is the molecular
weight of alumina 共0.102 kg mol−1兲, n is the number of electrons
共six兲 transferred in the oxidation of aluminum, and the Faraday constant has the usual value.
The simplified high field ion conduction model for film growth
reveals the exponential dependence of current density i with field
strength
i = i° exp
冉 冊
qaE
kT
关2兴
where q is the elemental charge of Al3+ ions 共3 ⫻ 1.60219
k
is
the
Boltzmann
constant
共1.3806
⫻ 10−19 C兲,
⫻ 10−23 m2 kg s−2 K−1兲, T is the temperature, i° is the current density produced by the elemental charge 共with values in the range
3–6 ⫻ 10−14 A m−2兲, and a is the jump distance of the ions
共0.26–0.5 nm兲. Values of i° = 2.47 ⫻ 10−11 A m−217 and a = 0.87
⫻ 10−9 m were used in calculations.
Manipulation of Eq. 2 gives the field strength as
E=
kT
i
ln
qa i°
关3兴
For film growth at 100% efficiency at a current density of 10 A m−2,
a field strength of 7.85 ⫻ 108 V m−1 and a rate of voltage rise 共polarization sweep rate兲 of 0.45 V s−1 are calculated 共Eq. 1 and 3兲;
such films form at a rate of 0.57 nm s−1. In reality, during anodic
polarization at 10 A m−2, the voltage increases at a rate of
0.25 V s−1 over the first 6.5 s for aluminum and at a rate of
0.17 V s−1 during the first 12 s for the alloy 共Fig. 1兲; these correspond to nonuniform film growth at efficiencies of 56 and 38% or
the alumina film forms at the rates of 0.32 and 0.22 nm s−1 for
aluminum and the alloy, respectively. Thus, the total voltage increases of 1.25 and 1.38 V for aluminum and the AA1050 alloy,
respectively, result in formation of nonuniform films of thickness
that increased by 1.6 and 1.8 nm in comparison to an initial oxide
film present on the surface prior to polarization.
Generally, the voltage surge at the commencement of anodic
polarization is proportional to the thickness of the air-formed
oxide present on the surface prior to polarization. At the onset
of anodic polarization, the voltage surges were 1.47 ⫾ 0.13 and
1.24 ⫾ 0.13 V for aluminum and the alloy, respectively; therefore,
the initial film of thickness ⬃2 nm was slightly thicker on aluminum, if the same electrical field of 7.85 ⫻ 108 V m−1 is required for
anodic film formation. The initial oxide thickness corresponds to
that of ⬃3 nm, measured directly for the alkaline etched AA1050
alloy 共Fig. 2a兲. In summary, during the early stages of anodic polarization at constant current density 共first 6.5–12 s兲 a nonuniform film
develops on the surfaces of aluminum and the alloy leading to a film
thickness increase of ⬃1 nm prior to formation of the porous anodic
film.
Porous anodic film growth.— The later stage of anodic polarization at constant current density, when the voltage remains constant
with time 共Fig. 1兲, results in the growth of a porous anodic film. The
thickness of the film increases with polarization time, being 30 ⫾ 2
and 18 ⫾ 2 nm for 300 s of polarization and 62 ⫾ 5 and
42 ⫾ 5 nm for 600 s of polarization for aluminum and the alloy,
respectively 共Fig. 2b-e兲. The thicknesses 共dp兲 enable calculation of
the current efficiency of porous anodic oxide film growth using
Faraday’s law
efficiency =
Q共real兲
␳nFdp/M
=
Q共100%兲
it
关4兴
where ␳ is the density of porous anodic alumina 共0.65 ⫻ 3.1
⫻ 103 kg m−3兲 and dp thickness of the porous film.
The porous films on aluminum and the AA1050 alloys grow
at current efficiencies of 12 and 8%, respectively. The remaining
88–92% of the current density is consumed through aluminum dissolution and oxygen evolution processes. The majority of aluminum
is lost during dissolution of the alumina film over the macroscopic
surface with some enhanced local aluminum anodic oxidation accompanied by oxygen evolution within occasional pits 共Fig. 3兲.
From the voltage-time response during polarization of highpurity aluminum at constant current density, the gradient in the linear period of voltage increase 共0.25 ⫾ 0.02 V s−1兲, i.e., the rate of
voltage rise with time, is consistent with all of the current being
used only for nonuniform barrier layer thickening and for through
film dissolution of aluminum. During the period of relatively constant voltage that corresponds to porous film growth, the porous film
undergoes relatively little reduction in thickness under the low rate
of alumina dissolution 共0.60 nm min−1兲. Porous alumina films
formed in acids are typically 1–1.5 times thicker than the layer of
oxidized aluminum. Hence, the film thickness of 30 nm on aluminum after 5 min of polarization suggests oxidation of 20–30 nm of
aluminum, with an average oxidation rate of 4–6 nm min−1, compared to 0.25 V s−1 /7.85 ⫻ 108 V m−1 = 20 nm min−1 during
barrier-layer thickening. The findings indicate that the proportion of
the current used in oxide film growth dramatically reduces at the
transition from nonuniform barrier-layer thickening to porous film
growth. The reduction in the current used in oxidation of aluminum
can be attributed to oxygen evolution and pitting that commences at
the peak values of the voltage-time response.
Film growth during potentiodynamic polarization.— The formation of the anodic film under potentiodynamic polarization in
nitric acid is influenced by the potential sweep rate, with effects of
chemical dissolution in the reactive electrolyte particularly evident
at low sweep rates. For sweep rates of 0.17 V s−1 and above, which
are similar to the rates of initial voltage rise from the commencement of anodic polarization at constant current density, it is considered that the film develops initially under broadly similar conditions.
Subsequently, for film growth at constant current density, a steady
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Journal of The Electrochemical Society, 155 共12兲 C557-C564 共2008兲
voltage region is evident where the relatively regular porous anodic
film develops. For potentiodynamic polarization, nonuniform film
growth proceeds with barrier layer thickening as the potential increases with time; such thickening is limited by local dissolution of
aluminum at potentials above the pitting potential.
Interestingly, the electrical equivalents of the total barrier layer
thicknesses achieved by aluminum and the AA1050 alloy at the
maximum voltage of the voltage-time behavior for anodizing at constant current density, 2.72 ⫾ 0.02 and 2.62 ⫾ 0.03 V, respectively,
are close to the potential increases from the OCP to the pitting
potential, 2.26 ⫾ 0.07 and 2.13 ⫾ 0.03 V, respectively, evident during cyclic polarization. Furthermore, the second cycle of polarization indicates that local dissolution of aluminum at pit locations and
an increased cathodic activity during extended cathodic polarization
do not significantly influence subsequent anodic oxide growth.
From consideration of the polarization behavior, it is thus evident
that the growth of the anodic film during potentiodynamic polarization is limited to nonuniform film growth or barrier-layer thickening;
the barrier-layer thickens to about 3–4 nm thick at efficiencies of
⬍53%. At such a low efficiency, the anodic film develops at an
effective Pilling–Bedworth ratio ⬍1 共1.72 ⫻ 0.53兲, suggesting the
development of tensile stresses within the oxide film, which leads to
localized cracking/healing.13 Cracking of the relatively thin oxide
film at preferred locations 共i.e., geometrical heterogeneities at the
metal/film interface兲 under polarization close to the pitting and oxygen evolution potentials, provides sites for pit development. The
similar values of the pitting potentials for superpure aluminum and
the AA1050 alloy, despite the presence of second-phase particles in
the alloy, indicates that pitting initiates on the filmed aluminum
surface. Furthermore, the nature of the anodic oxide film, largely
covering the macroscopic aluminum surface, may govern the nature
of preferred defects or flaws that are activated at sufficient potentials. Of further interest, anodic polarization through the potential
region that corresponds to oxide film formation during the second
cycle results in an increase of the pitting potential by ⬃100 mV.
Thus, preferred sites are repaired or healed, making subsequent pit
development more difficult, probably though reduced population
and dimensions of the now favorable sites.
During potentiodynamic polarization at the reduced potential
sweep rate of 0.05 V s−1, the efficiency of film growth falls to 22%,
as the average anodic current density of film formation drops from
10 to 5 A m−2 共Fig. 4c and d兲. Reduction in the efficiency is associated with an increased dissolution of the film in the chemically
reactive nitric acid, because the rate of polarization or film formation becomes comparable to the rate of alumina dissolution 共in the
range of 0.60–0.62 nm min−1兲. Therefore, during the second cycle
of polarization, the additional growth of the film is evident for
1.6–1.0 = 0.6 V of polarization. When the potential sweep rate was
further reduced to 0.01 V s−1, the efficiency of film growth was
⬃6%. Film growth is observed during the second cycle and reverse
polarization 共Fig. 4a and b兲. In this case, the rate of alumina dissolution is close to the rate of film formation. The reduced potential
delay in the current density increase at the beginning of polarization
共Fig. 4a and b兲, compared to initial potential delay at the higher
sweep rates, is a further conformation of oxide film dissolution. The
initial delay corresponds to the voltage drop across the original oxide film present on the surface prior to immersion into nitric acid
solution.
During extended polarization at 0.05 V s−1, when the rates of
alumina growth and dissolution are comparable, addition of cathodic
activity to aluminum and the AA1050 alloy, which results in a pH
increase near the surface prior to anodic film growth does not influence significantly the kinetics of alumina growth and dissolution
共Fig. 6兲. The enhanced localized aluminum dissolution above the
pitting potential does not have any effect on regrowth of the film
over the macroscopic surface during the second cycle of polarization. However, the reduction in the anodic current density, associated with pit propagation above the pitting potential, and an increase
in the value of pitting potential during the second cycle of polariza-
C563
tion indicate that pits, developed during the first cycle, have been
passivated by the anodic film formed during the second cycle.
Cathodic behavior of aluminum and AA1050 alloy.— The presence of second-phase particles at the surface of the AA1050 alloy
results in consistently high cathodic activity of the alloy during both
cycles of polarization that contrasts with the cathodic activity of
99.99% aluminum, which reduces during the second cycle of polarization 共Fig. 6a and b兲. As mentioned previously, this is associated
with filming of the aluminum surface. The formation of the oxide
film during the first direct polarization reduces the limiting values of
the cathodic current density during reverse polarization for aluminum. Consequently, during reverse polarization, the initial cathodic
current density reaches limiting values in the range of 4–60 A m−2.
The current density arrests are associated with the limiting current
density for nitrate reduction, which varies and depends on mass
transport of nitrate anions, the pH near the surface and nature of the
cathodic sites. The presence of iron-containing, second-phase particles on the surface of the AA1050 alloy can promote nitrate reduction. Nitrate reduction on the surface of iron is a commonly used
procedure in groundwater treatment.18 A maximum current density
of nitrate reduction of 67 A m−2 is calculated from the equation
ilim = nFDa␦−1, where D is the diffusion coefficient of nitrate
共9.311 ⫻ 105 cm2 s−1兲, a is the activity of nitrates in 0.24 M solution 共0.18 mol l−1兲, and ␦ is the thickness of electrical double layer
共10−4 m兲.
Conclusions
Anodic polarization of aluminum and the AA1050 alloy in nitric
acid solutions reveals growth of anodic film. During potentiodynamic polarization, growth of a nonuniform barrier type anodic film
of about 3–4 nm thick is evident, with thickening restricted by the
onset of pitting and oxygen evolution at appropriate potential. Conversely, for anodic polarization at constant current density, the nonuniform film growth progresses with time to develop a porous anodic film over the macroscopic aluminum surface.
The relatively high chemical reactivity of nitric acid to the alumina film, ⬃0.6 nm min−1, is evident from cyclic potentiodynamic
polarization at low potential sweep rates, when the chemical dissolution rate is similar to the film growth rate. Thus, the onset potential
for film growth during a second cycle of anodic polarization is significantly reduced compared to that of polarization at increased
sweep rates. The localized aluminum dissolution accompanied by
oxygen evolution within occasional pits contributes to low efficiencies of porous anodic film formation, ⬍12% in addition to high
chemical reactivity of the nitric acid.
Addition of cathodic activity to aluminum and the AA1050 alloy
and enhanced localized aluminum dissolution above the pitting potential do not significantly influence the kinetics of alumina growth
and dissolution. Consistently high cathodic activity of the alloy during cyclic extended polarization is associated with the exposure of
the second-phase particles within the pits at the surface of the
AA1050 alloy that contrasts with the reducing cathodic activity of
99.99% aluminum.
Acknowledgment
The authors acknowledge the financial support provided by the
Engineering and Physical Sciences Research Council Portfolio
Award, LATEST.
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