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Journal of The Electrochemical Society, 151 ͑7͒ B379-B386 ͑2004͒
B379
0013-4651/2004/151͑7͒/B379/8/$7.00 © The Electrochemical Society, Inc.
Microscopic Observations of Voids in Anodic Oxide Films
on Aluminum
R. Huang,a, K. R. Hebert,
*
a,**,z
and L. S. Chumbleyb
b
aDepartment of Chemical Engineering and Ames Laboratory-U.S. Department of Energy, and
Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA
The relationship was explored between nanoscale voids in anodic aluminum oxide films and the surface condition of aluminum
samples prior to anodizing. Transmission electron microscopy ͑TEM͒ detected voids on the order of 10 nm in anodic films. Atomic
force microscopy ͑AFM͒ of these films, obtained after partial oxide dissolution, revealed surface cavities; comparison of TEM and
AFM suggested that the cavities were the oxide voids. AFM images after variable extents of oxide dissolution showed that the
voids were distributed evenly through the inner 60% of the film thickness, indicating that they were formed at the metal-oxide
interface during film growth. Both AFM and TEM showed that the void concentration in the film was sensitive to the extent of
dissolution of the aluminum samples in NaOH prior to anodizing. Positron annihilation spectroscopy had previously detected
voids in samples without anodic films, located in the metal near the oxide-metal interface; the quantity of these interfacial voids
was controlled by NaOH dissolution. The void concentration in the inner part of the anodic films was proportional to the quantity
of these pre-existing interfacial voids. It was inferred that the oxide voids were formed by incorporation, during anodizing, of
interfacial metal voids into the oxide film. The uniform concentration of oxide voids in the inner film suggested that interfacial
metal voids formed continuously during anodizing and that metal voids were generated repeatedly at specific interfacial sites
during film growth.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1753582͔ All rights reserved.
Manuscript submitted March 28, 2003; revised manuscript received January 25, 2004. Available electronically May 19, 2004.
Various surface defects have been proposed to act as sites for
corrosion pit initiation on pure metals, including dislocations, mi-
crosegregated impurities, and flaws in the surface oxide film.1 How-
ever, as yet there is no general agreement on the nature of pit pre-
cursor sites. Recently, positron annihilation spectroscopy ͑PAS͒ has
been used to detect nanometer-scale voids in aluminum metal, lo-
cated within 100 nm of the metal-oxide film interface.2-4 The mea-
surements indicated that the void surfaces were free of oxide, sug-
gesting a high reactivity if exposed during uniform corrosion.
Therefore, the possibility that the interfacial voids act as initiation
sites for pitting corrosion was explored. Atomic force microscopy
͑AFM͒, after chemical stripping of the oxide film in CrO3-H3PO4
solution, revealed surface cavities, the depth and area coverage of
which agreed with PAS measurements of buried voids. Further, the
sites of these cavities corresponded to those of corrosion pits, sug-
gesting that the voids may act as pit precursor sites.3,4 Surface treat-
ments such as NaOH dissolution, which are used in etching appli-
cations to enhance the pit number density, also generated increased
numbers of voids.3 The void number density estimated from AFM
was comparable to that of pits formed during anodic etching.4
Because PAS indicated that the voids are at least of nanometer
size, an attempt was made here to view them by high-resolution
microscopic techniques. The visualization of voids can yield infor-
mation not obtainable with PAS about their size, morphology, and
location relative to surface features such as chemical or structural
inhomogeneities. Direct observations of interfacial voids in the
metal using cross-sectional TEM may be possible.5 However, given
the typical number densities of pitting sites, the relatively small
interface area sampled by these cross sections may not permit pit
precursor defects such as voids to be effectively viewed. However,
one can expect the interfacial voids in the metal to be incorporated
into the oxide during anodic film growth as the metal layer contain-
ing voids is oxidized. Assuming that the volume change accompa-
nying incorporation is minor, inferences about void geometry and
location would be possible from the microscopic observations of
oxide voids. Plan-view microscopic images of oxide films sample a
larger interface area compared to cross-sectional images, increasing
the probability of finding voids. Nanoscale voids formed during an-
odic alumina formation have been found in prior transmission elec-
tron microscopy ͑TEM͒ studies, but were thought to derive from gas
evolution.6,7 In another study, TEM-detected voids were due to local
densification during oxide crystallization after anodizing.8
In this work, microscopic observations of voids in anodic oxide
films were carried out using both TEM and AFM. The experimental
protocol is illustrated in Fig. 1. Aluminum foil samples were used in
either the as-received condition or after various times of NaOH im-
mersion at open circuit. The different samples contained variable
quantities of interfacial voids in the metal, as determined by the
NaOH treatment time.3 Because anodic oxide growth occurs at both
metal-oxide and oxide-solution interfaces,9 the interfacial voids
should be incorporated into the interior of the film, as shown. In
some experiments, the metal underlying the anodic film was dis-
solved, and the remaining oxide was examined in plan view with
TEM. Otherwise, the anodic film was partially dissolved in a hot
acidic solution and its surface viewed with AFM; the uniformity of
oxide dissolution by such procedures was established earlier.10 As
illustrated in Fig. 1, an appropriate depth of dissolution would ex-
pose the voids as surface cavities. AFM images obtained after
different extents of oxide dissolution provided an assessment of
the void concentration as a function of depth. This quantitative in-
formation complemented the nondestructive through-thickness TEM
images.
Recently, a PAS study of anodic oxidation demonstrated that new
interfacial voids in the metal are created during anodizing.11 It was
speculated that void formation involves the agglomeration of inter-
facial metal vacancies formed by oxidation. After the formation of
such a void, continued anodic film growth would likely result in its
incorporation into the oxide. The microscopic observations would
then reveal voids occupying a range of depths, as opposed to the
single layer of voids shown in Fig. 1 resulting from only pre-
existing interfacial voids. It is seen here that microscopic character-
ization of the void depth profiles in anodic films yielded insight into
the process of interfacial void formation during anodic oxide
growth. The void depth profiles obtained for samples with different
NaOH pretreatment times suggested that voids are not formed at
random locations on the metal-oxide interface but instead at certain
favored sites. With regard to the possible function of voids as pitting
sites, their association with other types of surface defects may be
consistent with the empirically known correspondence between pits
and surface impurities12 or topographic asperities.13-15
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z E-mail: krhebert@iastate.edu
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