Effect of alloy types on the anodizing process of aluminum

Abstract
<strong>Effect</strong> <strong>of</strong> <strong>alloy</strong> <strong>types</strong> on the anodizing process <strong>of</strong> aluminum
I. Tsangaraki-Kaplanoglou a, *, S. Theohari a , Th. Dimogerontakis a ,
Yar-Ming Wang b , Hong-Hsiang (Harry) Kuo b , Sheila Kia c
a Department <strong>of</strong> Chemistry, University <strong>of</strong> Athens, Panepistimiopolis Zografou Athens 157 71, Greece
b General Motors Research and Development Center, Warren, MI, USA
c General Motors Manufacturing Engineering, Warren, MI, USA
Received 18 June 2004; accepted in revised form 4 July 2005
Available online 15 August 2005
Specimens <strong>of</strong> AA 5083 and AA 6111 (unheat- and heat-treated) were investigated in comparison with the pure aluminum during
anodizing in sulfuric acid bath using electrochemical techniques, SEM/EDS and XRF. The <strong>alloy</strong> type affects the kinetics <strong>of</strong> anodizing but
certain qualitative characteristics and the basic aspects <strong>of</strong> the anodizing mechanism are similar for the <strong>alloy</strong>s and pure aluminum. We
found that for AA 6111 <strong>alloy</strong>, the stage <strong>of</strong> rearrangement <strong>of</strong> the pores during anodizing before the current reaches a steady-state value is
missing due to enrichment <strong>of</strong> the <strong>alloy</strong>ing elements at the oxide/metal interface. For <strong>alloy</strong>s under certain anodizing conditions, formation
<strong>of</strong> a resistive film at the bottom <strong>of</strong> the pores results in decreasing anodizing current. This phenomenon is more prevalent for AA 5083
<strong>alloy</strong> than for AA 6111. For AA 6111 <strong>alloy</strong>, the heat treatment affects anodizing kinetics and improves the anodizing efficiency.
Understanding the effect <strong>of</strong> <strong>alloy</strong> <strong>types</strong> on anodizing process under various operating conditions (voltage, temperature and sulfuric acid
concentration) will enable us to optimize porous oxide structures for improved coating performance in color ability and corrosion
resistance.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Anodizing; Aluminum; Aluminum <strong>alloy</strong>
1. Introduction
Surface & Coatings Technology 200 (2006) 2634 – 2641
Porous anodic films <strong>of</strong> aluminum have received great
attention for their practical uses and morphology [1,2]. The
oxide film, which is formed by anodization in acidic
solutions, consists <strong>of</strong> a porous layer with hexagonally
shaped cells with a central pore perpendicular to the metal
surface and a thin compact barrier oxide layer, which is
present between the metal and the porous layer.
Earlier work [3–7] describes the effect <strong>of</strong> anodizing conditions
(voltage, temperature, composition and acid concentration
<strong>of</strong> the bath) on the anodizing current density, the
* Corresponding author. Tel.: +30 210 7274331; fax: +30 210 7221800.
E-mail address: kaplanoglou@chem.uoa.gr
(I. Tsangaraki-Kaplanoglou).
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2005.07.065
www.elsevier.com/locate/surfcoat
porosity and the diameter <strong>of</strong> the pores <strong>of</strong> the anodic oxide as
follows:
˝ Increasing the temperature at a given anodizing voltage
increases both the current density and the coating
porosity due to thermal enhanced field assisted dissolution.
Additionally, the dissolution <strong>of</strong> the outer oxide
surface is enhanced.
˝ Increasing the sulfuric acid concentration at a given
anodizing voltage increases the current density due to
higher solubility <strong>of</strong> the oxide film. This higher film
dissolution is also responsible for the increase <strong>of</strong> the pore
diameter and the coating porosity.
˝ Increasing the anodizing voltage at a given temperature
increases the current density, the barrier layer thickness
and the diameter <strong>of</strong> the pores, while the coating porosity
decreases.

I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641 2635
Valuable works on the behavior <strong>of</strong> several intermetallic
compounds during anodizing were carried out about 60
years ago by Keller et al. and by Fischer et al [2]. About 30
years ago Spooner, Brace and other researchers (Alcan
Laboratories) [2] tried to establish the electrochemical
reactivity <strong>of</strong> several <strong>alloy</strong>ing elements and the behavior <strong>of</strong>
several intermetallic compounds in aluminum <strong>alloy</strong>s during
sulfuric acid anodizing under constant voltage. In the
meantime the anodic oxidation <strong>of</strong> aluminum <strong>alloy</strong>s was
extensively investigated [3,8]. Attention was given primarily
to the oxidation <strong>of</strong> <strong>alloy</strong>ing elements at the <strong>alloy</strong>/film
interface during the formation <strong>of</strong> barrier type anodic films.
However, the findings are also considered to be applicable
to the formation <strong>of</strong> porous anodic films.
The various <strong>alloy</strong>ing elements are present in the
aluminum substrate as solid solutions, or second phases,
such as zones, precipitates, or intermetallic compounds <strong>of</strong>
various shapes, sizes and compositions. They cause
modifications in porous film growth, film composition and
microstructure during anodization [9–15]. These modifications
are caused through changes in (a) the ionic transport
mechanism in the oxide film, (b) the ejection <strong>of</strong> ionic
species to the electrolyte and (c) the solubility <strong>of</strong> the film
material both in the presence and in the absence <strong>of</strong> the
electric field. For example, according to Dasquet et al. [16],
the <strong>alloy</strong>ing elements seem to cause morphological differences
between the oxide films <strong>of</strong> AA1050 and AA2024 T3.
The latter has a spongy structure with non-unidirectional
pores.
It has been found [17,18] that for the cases where the
Gibbs free energy per equivalent for the formation <strong>of</strong> the
<strong>alloy</strong>ing element oxides are greater than that <strong>of</strong> alumina, the
initial and preferential oxidation <strong>of</strong> aluminum takes place at
the beginning <strong>of</strong> anodizing. This results in the formation <strong>of</strong>
thin layers (1–5 nm into the substrate) enriched with the
respective <strong>alloy</strong>ing element beneath the anodic film.
Preferential oxidation <strong>of</strong> aluminum and enrichment <strong>of</strong> <strong>alloy</strong>
element continue until a critical concentration (20–40%) is
reached in the enriched <strong>alloy</strong> layer so that oxidation <strong>of</strong> the
<strong>alloy</strong> element will occur. The critical level <strong>of</strong> enrichment
increases linearly with the Gibbs free energy per equivalent
for the formation <strong>of</strong> the <strong>alloy</strong>ing element oxide. The time
needed to reach this critical concentration is within the span
<strong>of</strong> normal anodization for forming a relatively thick porous
film. Direct oxidation <strong>of</strong> <strong>alloy</strong>ing elements and incorporation
oxides into the film take place when their Gibbs free
energy is lower than that <strong>of</strong> alumina as in the case <strong>of</strong> Al–
Mg <strong>alloy</strong>s. The different transport numbers <strong>of</strong> Al 3+ and
Mg 2+ through the film result in the formation <strong>of</strong> an
intermediate layer. A non-uniform flow <strong>of</strong> ionic current
through this layer may be related to channeling the current
due to different ionic resistivity <strong>of</strong> alumina and magnesia.
For certain ternary and higher <strong>alloy</strong>s co-enrichments <strong>of</strong>
<strong>alloy</strong>ing elements through anodic oxidation are possible
leading to anodic films <strong>of</strong> relatively complex composition
associated with different stages <strong>of</strong> oxidation <strong>of</strong> the
individual <strong>alloy</strong>ing elements and different migration rates
<strong>of</strong> the oxidized species within the films. For two- and multiphase
<strong>alloy</strong>s the anodic films composition and the composition
<strong>of</strong> the enriched <strong>alloy</strong> layer may vary among different
locations at the <strong>alloy</strong> surface. Highly localized processes,
such as film formation, dissolution or both proceed over the
second phases and there is a complex interplay between the
local film formation over particular particles and the general
film formation over the surrounding matrix regions. A nonuniform
flow <strong>of</strong> ionic current through the barrier film may
be related to channeling the current due to different ionic
resistivities <strong>of</strong> different metal ions, intermetallics and metals
such as Si, etc. [19]. During the anodizing <strong>of</strong> a dilute Al–Si
<strong>alloy</strong> [20], the barrier layer is expected to have an increased
content <strong>of</strong> silicon species relative to the <strong>alloy</strong>. Besides
silicon particles is expected to present in the aluminum
matrix as second phase material, which may partly anodically
oxidized and occluded in the film. Further, when
copper was added to this system in 3% [21] a new
intermetallic phase was formed (Al2Cu) whose low resistance
favors the oxidation and dissolution <strong>of</strong> these particles,
increasing the porosity <strong>of</strong> the oxide. The presence <strong>of</strong> iron
impurities influences also the anodizing process. Particles <strong>of</strong>
Al–Fe and Al–Fe–Si type present in the aluminum
substrate, are either inert or undergo oxidation in lower rate
compared with the adjacent aluminum. They were occluded
in the oxide layer [21]. Iron species were also detected at the
film–electrolyte interface in certain cases [22]. Iron rich
phases locally inhibit the nucleation and growth <strong>of</strong> the
anodic oxide film [23]. The effect <strong>of</strong> the second phase
particles incorporation in the porous film will modify the
pore morphology, produce void and cracks, affect oxygen
gassing rate [24–27], and the chemical solubility <strong>of</strong> the film
material, and roughen the <strong>alloy</strong>/film interface [10].
The purpose <strong>of</strong> this research was to investigate the effect
<strong>of</strong> <strong>alloy</strong> <strong>types</strong> on the sulfuric acid anodizing process under
various operating conditions (voltage, temperature, concentration
<strong>of</strong> sulfuric acid). In the present study, the anodizing
behaviors <strong>of</strong> the aluminum <strong>alloy</strong>s AA5083 and 6111 were
compared with pure aluminum in sulfuric acid solutions
under different conditions. Specimens <strong>of</strong> AA6111 heattreated
were also investigated in order to study the influence
<strong>of</strong> temper on the examined electrochemical process.
Electrochemical techniques were used for studying the
anodizing process. The composition <strong>of</strong> the <strong>alloy</strong>s was
determined by X-ray Fluorescence (XRF) and the in-depth
pr<strong>of</strong>ile <strong>of</strong> the formed oxide films was investigated by
Scanning Electron Microscopy and Energy Dispersive
Spectroscopy (SEM/EDS).
2. Experimental
Specimens <strong>of</strong> pure aluminum (Merck 99.96%) as well as
AA5083 and 6111 (unheat- and heat-treated at 177 -C for 30
min) aluminum <strong>alloy</strong>s were used. The specimens were in the

2636
foil form with dimensions 30 75 mm 2 . The specimens had
undergone the following pretreatments: degreasing in
acetone, etching for 1 min in a solution containing 100 g/l
NaOH at 60 -C, rinsing thoroughly in deionized water and
immersing for 1 min in 1:1 v/v HNO3 at room temperature.
After rinsing in deionized water and drying in a stream <strong>of</strong> air
at room temperature, the specimens were stored in a
desiccator.
The anodizing was carried out in two litters <strong>of</strong> sulfuric
acid solution containing 175 g/l or 250 g/l H2SO4 and 1 g/l
Al 2(SO 4) 3I18H 2O at 20T0.5 -C or otherwise mentioned,
under agitation with a magnetic stirrer. For anodizing, an
aluminum sheet was used as the counter electrode and a
constant voltage <strong>of</strong> 15 or 18 V DC was applied by a
computer-controlled DC power supply (Delta Elektronika
SM3004-D), while the current–time transients were
recorded by a computer via a multimeter (KEITHLEY
2000, T0.025%). After anodizing, the specimens were
rinsed thoroughly with deionized water for 1 min and dried
in a stream <strong>of</strong> air at room temperature. A Permascope EC8
(Fischer Technology, Inc. Norwalk, CT, accuracy <strong>of</strong> T2
Am) was used for the measurement <strong>of</strong> the oxide film
thickness.
The composition <strong>of</strong> <strong>alloy</strong>s was determined by X-ray
Fluorescence (XRF) measurements. In this technique a
silver X-ray tube (50 kV, 20 mA) with two Si (Li) detectors
(resolution 155 eV at 5.9 keV) were used. The signals were
amplified by a TC-244 Tennelec Spectroscopy Amplifier
(including pile up rejection) and collected by a PCA-II
multichannel analyzer card (including ADC converter)
[28,29]. The relative standard deviation (%) for this method
using five experimental measurements was 5%.
The specimens’ surfaces and cross sections were
examined by Scanning Electron Microscope SEM/EDS
(Philips 515). The film was made conductive by deposition
with sputtering <strong>of</strong> an extremely thin layer <strong>of</strong> gold (200 A˚ )
on its surface.
3. Results
3.1. Composition <strong>of</strong> the aluminum <strong>alloy</strong>s
I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641
Table 1
Alloy composition (wt.%)
Alloy Mg Mn Si Cu Fe Zn Cr Ti Al
5083 4.9 0.1–1.0 0.40 0.10 0.40 0.25 0.25 0.15 bal.
6111 0.5–1.0 0.15–0.45 0.7–1.1 0.5–0.9 0.40 0.10 0.10 0.10 bal.
The composition limits for the examined aluminum
<strong>alloy</strong>s AA5083 and 6111 in wt.% are listed in Table 1
[30,31].
It must be noticed the presence <strong>of</strong> Fe and Mn in both
<strong>alloy</strong>s, the high content <strong>of</strong> magnesium in the <strong>alloy</strong> AA5083
and the increased amount <strong>of</strong> Si, Mg and Cu in <strong>alloy</strong>
AA6111.
3.2. Growth rate <strong>of</strong> porous oxide
The thickness <strong>of</strong> the anodic oxide film was plotted as a
function <strong>of</strong> anodizing time (Fig. 1) and it was obvious that
the oxide thickness increased linearly with time during
anodizing under standard conditions (15 V, 20 -C, 175 g/l
H2SO4, 1 g/l Al2(SO4)3I18H2O). The slopes <strong>of</strong> these curves
were 0.29 Am/min for the pure aluminum, 0.35 Am/min for
the <strong>alloy</strong> 5083 and 0.28 Am/min for the <strong>alloy</strong> 6111 unheatand
heat-treated. The AA 5083 <strong>alloy</strong> has the highest slope,
followed by pure aluminum, and the AA 6111 unheat- and
the heat-treated one have about the same slope, and its value
is the lowest.
3.3. Kinetic <strong>of</strong> porous oxide growth—influence <strong>of</strong> anodizing
conditions
The four stages <strong>of</strong> the porous structure development
under anodizing voltages 15 and 18 V and sulfuric acid
concentrations <strong>of</strong> 175 and 250 g/l are shown from the
current density–time curves in Figs. 2–4. A high electric
field at the commencement <strong>of</strong> anodizing at constant voltage
leads to a relatively large current surge. This increasing
current falls to a low value (imin) when the barrier layer is
formed. Pores have been considered [32] to initiate in this
decreasing current stage and their growth is believed to take
place in the next stage, i.e. by the time the current has risen
and steadied out.
The following are evident from Figs. 2–4: imin reaches
earlier and has a higher value for AA 5083 <strong>alloy</strong>, which
seems to permit a greater current to flow through the barrier
layer at this decreasing current stage. Also the imin <strong>of</strong> the
AA 6111 <strong>alloy</strong> unheat-treated is similar to that <strong>of</strong> AA 5083
<strong>alloy</strong> but seems to be influenced by the temper <strong>of</strong> this <strong>alloy</strong>,
because the heat-treated one shows lower values which
approach those <strong>of</strong> pure aluminum especially in the case <strong>of</strong>
anodizing at 18 V.
Oxide Thickness / µm
32
28
24
20
16
12
8
4
pure
6111 H.T.
6111 U.T.
5083
0
0 10 20 30 40 50 60 70 80 90 100 110
Anodizing Time / min
Fig. 1. Thickness <strong>of</strong> the anodic oxide film <strong>of</strong> the examined materials as a
function <strong>of</strong> the anodizing time at standard conditions (15 V, 20 -C, 175 g/l
H2SO4, 1 g/l Al2(SO4)3I18 H2O).

Current Density / A dm -2
The subsequent initiation <strong>of</strong> the current rise seems to be
facilitated in the <strong>alloy</strong> AA 5083 and the steady-state current
density reaches higher values than the pure aluminum. On
the contrary the steady-state value (iss) for AA 6111 is
almost identical with that <strong>of</strong> pure aluminum at 15 V and in
175 g/l H2SO4, while this is not the case in 250 g/l H2SO4
(Fig. 4). Tempering <strong>of</strong> AA 6111 reduces the steady-state
current density during anodizing at standard conditions (Fig.
2). Considering that oxide growth rates were practically the
same for both unheat-treated and tempered samples (Fig. 1),
and the anodizing current is lower for tempered sample, then
a higher anodizing current efficiency must exist for the heattreated
<strong>alloy</strong> to obtain the same coating thickness. This
confirms the importance <strong>of</strong> the role <strong>of</strong> intermetallics on the
anodizing process. Besides, Fig. 3 shows that during all
stages <strong>of</strong> anodizing at 18 V, the heat-treated AA6111
samples show lower current densities than the unheattreated
samples. These differences are probably related to
the role <strong>of</strong> intermetallics on the anodizing process and more
specifically, on the formation <strong>of</strong> composite oxide products.
AA 6111 (both unheat- and heat-treated) exhibits no
current peak in current transient as compared with AA 5083
Current Density / A dm -2
2
1
0
3
2
1
0
I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641 2637
Pure
5083
6111 U.T.
6111 H.T.
0 100 200 300 400
Anodizing Time / s
Pure
5083
6111 U.T.
6111 H.T.
0 100 200
Anodizing Time / s
300 400
Fig. 2. Anodizing current density–time transients at 15 V in 175 g/l H2SO4
at 20 -C.
Fig. 3. Anodizing current density–time transients at 18 V in 175 g/l H 2SO 4
at 20 -C.
Current Density / A dm -2
3
2
1
0
Pure
5083
6111 U.T.
250 g/l H 2 SO 4
250 g/l H 2 SO 4
250 g/l H 2 SO 4
175 g/l H 2 SO 4
175 g/l H 2 SO 4
175 g/l H 2 SO 4
0 10 20 30 40 50 60 70
Anodizing Time / s
Fig. 4. Anodizing current density–time transients at 15 V in 175 and 250 g/
lH2SO4 at 20 -C.
and pure aluminum. This implies that the stage <strong>of</strong> pore
initiation where field-assisted dissolution takes place and the
subsequent rearrangement <strong>of</strong> the pores before the current
reaches its steady-state value is different for AA 6111 <strong>alloy</strong>.
The electrical properties or microstructure <strong>of</strong> the barrier film
for AA 6111 could be quite different as a result <strong>of</strong> its
<strong>alloy</strong>ing elements. It is well known that the <strong>alloy</strong>ing
elements may affect the initial population density <strong>of</strong> pores
by promoting or hindering pore nucleation and the current
density at the pore initiation and growth stage [18].
As it is expected, at 15 V anodizing voltage, the steadystate
current density decreases with decreasing acid concentration
(Fig. 4). However a gradual decline <strong>of</strong> the current
with time is observed for AA 5083 <strong>alloy</strong>.
Fig. 5 derives from the respective current density
transient curves at different temperatures and it shows the
steady-state current density as a function <strong>of</strong> anodizing
Current Density / A dm -2
3,0
2,5
2,0
1,5
1,0
0,5
Pure, 15 V
Pure, 18 V
5083, 15 V
5083, 18 V
6111 U.T., 15 V
6111 U.T., 18 V
6111 H.T., 15 V
6111 H.T., 18 V
12 14 16 18 20 22 24
Anodizing Temperature / oC Fig. 5. The anodizing at 15 and 18 V in 175 g/l sulfuric acid steady-state
current density as a function <strong>of</strong> the anodizing temperature.
80

2638
temperatures. Usually, the current density remains almost
stable when the formation <strong>of</strong> pores has been established.
However, in certain cases, a gradual decline <strong>of</strong> the current
density from its steady-state value was observed with the
progress <strong>of</strong> anodizing. In our study, ‘‘steady-state current
density’’ is noticed the value <strong>of</strong> the current density at 1000 s
<strong>of</strong> anodizing. The increase <strong>of</strong> these ‘‘steady-state anodizing
current densities’’ with increasing temperature and voltages
seems to depend on the kind <strong>of</strong> the <strong>alloy</strong>.
After prolonged anodizing (coating thickness 17–25
Am), an increased amount <strong>of</strong> Fe and Si and in a smaller
extent <strong>of</strong> Cu, is observed by SEM/EDS in certain sites at the
metal/oxide interface <strong>of</strong> AA 6111 specimens anodized at
standard conditions (i.e. Cu 1.2, Fe 4.3, Si 4.6 Mn 0.9
wt.%). For AA 5083 <strong>alloy</strong> the enrichment <strong>of</strong> Mn and Fe is
also observed.
4. Discussion
I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641
At a given anodizing condition, the current transient
curves during anodizing <strong>of</strong> the four examined materials are
clearly distinguishable, although their general characteristics
are similar.
Figs. 2–4 show that samples all exhibit basic stages <strong>of</strong>
anodizing namely, barrier layer formation, pore initiation,
pore rearrangement (except <strong>of</strong> AA 6111), and steady-state
film growth. The initial drop in current corresponding to the
barrier layer formation stage is nearly identical for all
samples (Figs. 2–4). However, the imin and the final steadycurrent
density are affected by <strong>alloy</strong>ing elements, the temper
treatment and operating parameters (anodizing voltage, and
sulfuric acid concentration). Higher oxide growth rate,
higher i min (easier H + entering the oxide) and i ss (steadystate
current density) values were observed for AA 5083
<strong>alloy</strong> in comparing with the pure aluminum. These results
are consistent with the well-known conclusion that increasing
magnesium content substantially increases the rate <strong>of</strong>
oxidation at a given anodizing voltage, at least initially,
though the coating may decrease in thickness after a certain
time [2,15,33]. For AA 5083 <strong>alloy</strong>, direct oxidation <strong>of</strong>
magnesium and oxide incorporation into the film takes place
at the commencement <strong>of</strong> anodizing because its Gibbs free
energy per equivalent <strong>of</strong> formation is lower than that <strong>of</strong>
alumina. This gives rise to high imin. The fact that the
steady-state anodizing current for AA 5083 is the highest
indicates a higher conductance <strong>of</strong> the oxide film for this
<strong>alloy</strong>, which is related to the presence <strong>of</strong> magnesium oxides
that have higher bond energies. In other words, the above
results <strong>of</strong> the experimental measurements are expected
because the anodic film contains incorporated magnesium
oxides, which increase the defect density in the barrier layer,
the channeling <strong>of</strong> the current, the H + entering the oxide [34],
the roughness [35] and the rms electric charge during
electrolytic coloring in comparison with the pure aluminum
and AA 6111 [36].
Anodizing characteristics for AA 6111 <strong>alloy</strong> are similar
to that <strong>of</strong> the pure aluminum under standard anodizing
conditions, except for the disappearance <strong>of</strong> current peak in
current transients. The pore rearrangement stage is missing
in the case <strong>of</strong> AA 6111 unheat- and heat-treated samples.
This could be caused by considerable modification <strong>of</strong> the
film microstructure and by reduced availability <strong>of</strong> aluminum
for oxidation and pores to be rearranged. Besides, AA 6111
responds to the changes <strong>of</strong> anodizing conditions in a little
different manner than the pure aluminum. For example it
shows a smaller increase <strong>of</strong> current density with the increase
<strong>of</strong> anodizing voltage from 15 to 18 V than the pure
aluminum and especially the heat-treated one.
According to the literature [11] the faster migration <strong>of</strong>
manganese species in comparison with aluminum species
results in the incorporation <strong>of</strong> these species into the film.
The nm/V ratio increases slightly indicating a reduced
ionic resistivity <strong>of</strong> the film. According also to the literature
[18,20,21,27,37–39] concerning <strong>alloy</strong>s which have Si, Cu
and Fe as <strong>alloy</strong>ing elements, the main porous film material
<strong>of</strong> these <strong>alloy</strong>s contains both the <strong>alloy</strong>ing element oxides
and alumina and its morphology is modified. The <strong>alloy</strong>ing
elements (Si, Mg, Cu) influence the cooperative ion
migration through the anodic film [34] and the presence
<strong>of</strong> foreign species (Fe, Mn) in the film also affects the
field assisted dissolution and ionic resistivity <strong>of</strong> the anodic
film. Besides, different rates <strong>of</strong> oxidation <strong>of</strong> second phase
and matrix region <strong>of</strong> this <strong>alloy</strong> result in differences in
morphology <strong>of</strong> the film. More specific according to the
above literature when Al2Cu particles, Si and Al–Fe–Si
adjacent particles are present at the substrate/oxide interface
the oxide film advances fastest towards to the copper
intermetallics than to the surrounding aluminum whereas
Si and Al–Fe–Si particles block the growth <strong>of</strong> the oxide.
As a result changes in current distribution between
aluminum matrix, silicon, iron and copper in the substrate
and the overlaying oxide layers result in iron and silicon
bearing phases and in certain cases iron, silicon and
copper bearing phases in the oxide. The entrapment <strong>of</strong>
silicon particles in the growing layers [21,37] is associated
with a non-uniform oxide thickness, increased roughness
at the interface substrate/oxide, surface cracks formation
and appearance <strong>of</strong> flaws and branching and deflection <strong>of</strong>
pores which are developed around occluded silicon
particles.
Further, oxidation <strong>of</strong> the silicon appears to be associated
with oxygen filled voids develop above the oxidizing
particles as a result <strong>of</strong> the electron conducting nature <strong>of</strong>
the Si–O bond. The anodic SiO 2 film and the presence <strong>of</strong>
oxygen gas filled voids represent pathways <strong>of</strong> high effective
resistance for continued film growth [20,21].
Another evidence from the literature [27] is the behavior
<strong>of</strong> Al–0.9 Cu <strong>alloy</strong> which has been related to clustering <strong>of</strong>
copper atoms, resulting in the limited availability <strong>of</strong>
aluminum for oxidation during the anodizing process. The
copper atoms, associated with vacancies created during the

I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641 2639
consumption <strong>of</strong> aluminum at the <strong>alloy</strong>/film interface, tend to
form clusters. At this stage, the extent <strong>of</strong> aluminum
diffusion through such clusters is unclear, with time, their
size and copper content increases such that they can be
considered as obstacles to alumina film formation. It is also
referred [24–26] that oxygen production takes place above
oxidized clusters. The production <strong>of</strong> oxygen is suggested to
be related to the electron energy levels <strong>of</strong> the film material,
which in the case <strong>of</strong> alumina films are modified by the
incorporation <strong>of</strong> copper species. Oxygen evolution also
takes place at the copper intermetallics during the anodizing
process. The production <strong>of</strong> oxygen within the alumina at
sites remote from <strong>alloy</strong>/oxide interface, where the electronic
properties <strong>of</strong> the alumina are altered by incorporated
transition metal species (i.e. Cu, Fe, Cr), is also referred
by other researchers [20,21,23,40,41]. Oxygen evolution
contributes to increasing resistance, film cracking, partial
dissolution and reanodizing as well as local repair <strong>of</strong> the
film [20].
Certain observations for the AA 6111 based on the
results <strong>of</strong> this paper and <strong>of</strong> our previous one [36] are
summarized as follows:
& the higher resistivity (lower iss) <strong>of</strong> the oxide film <strong>of</strong> AA
6111 relative to that <strong>of</strong> pure aluminum,
& the eventual increase <strong>of</strong> the resistivity with the increase
<strong>of</strong> voltage, temperature and sulfuric acid concentration,
& the appearance <strong>of</strong> cracks at the surface <strong>of</strong> electrolytic
coloring <strong>of</strong> AA 6111 specimens [36],
& the observed, by SEM/EDS measurements, enrichment
locally at the surface/oxide interface in Fe, Si and Cu or
Fe, Cu [36] and most important
& the disappearance <strong>of</strong> the stages <strong>of</strong> rearrangement <strong>of</strong> the
pores at the early stages <strong>of</strong> anodizing <strong>of</strong> AA 6111
specimens.
In accordance with the literature referred in this paper
about the microstructure <strong>of</strong> oxide film <strong>of</strong> the <strong>alloy</strong>s
containing the <strong>alloy</strong>ing elements <strong>of</strong> interest in the present
study, the above observations would be explained by the
increased content <strong>of</strong> silicon species in the barrier layer
during anodizing relative to the <strong>alloy</strong>, as well as by the
increasing volume fraction <strong>of</strong> partly oxidized silicon
particles occluded in the film material, or <strong>of</strong> iron–silicon,
copper, iron bearing phases particles and <strong>of</strong> oxygen filled
cavities or other defects.
However, a more detailed investigation by surface
analysis methods such as electron microscopy (SEM,
TEM) or depth pr<strong>of</strong>iling (AES, GDOES, RBS) would be
beneficial, in order to correlate in a better way the
observations from electrochemical measurements with
changes <strong>of</strong> the microstructure <strong>of</strong> these <strong>alloy</strong>s during the
anodizing and other electrolytic processes.
The anodizing current density increases with increasing
voltage at a given temperature, and at a given voltage the
current also increases with increasing temperature, both
due mainly to thermal enhanced field-assisted dissolution.
The observed differences <strong>of</strong> the steady-state current
densities <strong>of</strong> the <strong>alloy</strong>s at high voltages, acid concentrations
and elevated temperatures (Figs. 3–5) could be explained
as follows: the <strong>alloy</strong>ing elements and the temper influence
the ionic transport properties and composition <strong>of</strong> the oxide
film. It could be suggested that for AA 5083 the decline <strong>of</strong>
the steady-state current density probably derives from
building up <strong>of</strong> dissolution products at the bottom <strong>of</strong> the
pores under high anodizing temperature and current
densities (Fig. 5). For example, it is referred [33] that
under strong alkaline conditions the faster migration <strong>of</strong> the
Mg species relative to that <strong>of</strong> Al 3+ ions and the stability <strong>of</strong>
an outer region <strong>of</strong> Mg rich film material at high pH (MgO/
Mg(OH)2) is a barrier to chemical attack <strong>of</strong> the film and to
field-assisted dissolution <strong>of</strong> Al 3+ ions from the film to the
electrolyte. For AA 6111 <strong>alloy</strong>, the decline <strong>of</strong> the steadystate
current density (Fig. 5) probably derives by the fact
that the limited availability <strong>of</strong> aluminum for oxidation and
formation <strong>of</strong> dissolution products [19,26] would be more
obvious at elevated temperature and/or voltages [42]. As
the anodizing <strong>of</strong> AA 6111 is carried out at more active
conditions, an increased resistance to current flow and thus
a decline <strong>of</strong> almost steady-state current density would
probably be associated with local changes in composition
and morphology <strong>of</strong> the oxide film due as it was previously
mentioned to the increased content <strong>of</strong> silicon species in the
barrier layer during anodizing relative to the <strong>alloy</strong>, to the
formation <strong>of</strong> the oxygen filled voids in the oxide, to the
occluded silicon particles in the oxide, which partially
have been oxidized, to iron and copper bearing second
phase particles or other defects in the oxide [20,21]. It
must be noticed that the i min value <strong>of</strong> AA 6111 specimens
remains always almost closed to that <strong>of</strong> AA 5083 under
the anodizing conditions used in the present study. This
could be explained by the hypothesis that this value
depends on the presence <strong>of</strong> magnesium as <strong>alloy</strong>ing
element, which is oxidized first at the commencement <strong>of</strong>
anodizing. However, imin decreases and approaches that <strong>of</strong>
pure aluminum after heat treatment <strong>of</strong> the AA 6111
specimens. Lower electric conductivity characterizes these
specimens after heat treatment. Thus, the AA 6111 heattreated
shows slightly lower current densities than the
unheat-treated one during all stages <strong>of</strong> anodizing at
standard conditions. The differentiation <strong>of</strong> topography <strong>of</strong>
this <strong>alloy</strong> substrate, as a result <strong>of</strong> the heat treatment, seems
to improve slightly the anodizing efficiency.
It has been referred [43] that after heat treatment <strong>of</strong> AA
6111 <strong>alloy</strong> two precipitated phases hµ (monoclinic) and Q
(hexagonal) coexist. hµ phase is the precursor <strong>of</strong> the h phase
whose composition has been well established to be Mg2Si
[43]. Q phase has an Mg/Si ratio in AA 6111 <strong>alloy</strong> HT (180
-C for 0.5 h) about 1.0 to 1.1 and its composition seems to
depend on the relative concentration <strong>of</strong> the <strong>alloy</strong>ing
elements Mg, Si, Cu and the contaminating elements Fe
and Mn [43,44].

2640
The precipitated phases after heat treatment <strong>of</strong> AA 6111
are possibly modified during sample preparation and the
anodizing process. Selective dissolution <strong>of</strong> Mg in the Mg2Si
intermetallics in water has been referred [45,46] and
formation, after oxidation <strong>of</strong> the remaining silicon, <strong>of</strong> an
insulating coating consisting <strong>of</strong> silicon oxide, which encapsulates
the Q-phase. This results to increased passivation <strong>of</strong>
the <strong>alloy</strong> and probably could explain the lower electric
conductivity <strong>of</strong> heat-treated <strong>alloy</strong> than the unheat-treated one.
Another possible hypothesis for the lowering <strong>of</strong> anodizing
current density (imin, iss) and <strong>of</strong> side reactions, by the
heat treatment <strong>of</strong> the <strong>alloy</strong>, is the Mg depletion <strong>of</strong> the
adjacent zones <strong>of</strong> AA 6111 specimens by the precipitation
<strong>of</strong> hµ and Q strengthening phases.
5. Conclusions
Specimens <strong>of</strong> AA 5083 <strong>alloy</strong> and AA 6111 UT and HT
<strong>alloy</strong>s were anodized in sulfuric acid baths in different
conditions and were compared with those <strong>of</strong> pure aluminum
by the use mainly <strong>of</strong> electrochemical measurements. The
following were concluded:
& For AA 5083 <strong>alloy</strong>, the stages <strong>of</strong> porous structure
development are substantially identical with that <strong>of</strong> pure
aluminum, although an increase in oxide growth rate and
high conductance <strong>of</strong> the oxide film were observed.
& For AA 5083 <strong>alloy</strong>, increasing anodizing temperature or
increasing anodizing voltage results in a decrease in
anodizing current with the time and relative to that <strong>of</strong>
pure aluminum.
& For AA 6111 <strong>alloy</strong>, the stage <strong>of</strong> rearrangement <strong>of</strong> the
pores during anodizing before the current reaches a
steady-state value is missing. This is an evidence <strong>of</strong> a
considerable modification <strong>of</strong> the composition and morphology
<strong>of</strong> the film, and it is independent <strong>of</strong> the temper
<strong>of</strong> the <strong>alloy</strong>.
& Under standard anodizing conditions, AA 6111 <strong>alloy</strong> has
an oxide growth rate similar to that <strong>of</strong> pure aluminum.
Increasing anodizing temperature and/or voltage increase
the oxide dissolution rate in a smaller manner to that <strong>of</strong>
pure aluminum. Enrichment <strong>of</strong> Cu, Fe and Si at the
oxide/metal interface has been observed.
& For AA 6111 <strong>alloy</strong>, the heat treatment affects anodizing
kinetics and improves slightly the anodizing current
efficiency.
Acknowledgments
I. Tsangaraki-Kaplanoglou et al. / Surface & Coatings Technology 200 (2006) 2634–2641
The authors wish to acknowledge General Motors and
the Research Committee Secretariat <strong>of</strong> the University <strong>of</strong>
Athens for their financial support and Associate Pr<strong>of</strong>. N.
Kallithrakas-Kontos (Technical University <strong>of</strong> Crete) for Xray
fluorescence studies.
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