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Surface and Coatings Technology 165 (2003) 232–240
AlSi(Cu) anodic oxide layers formed in H2SO4 at low temperature using
different current waveforms
L.E. Fratila-Apachitei*, J. Duszczyk, L. Katgerman
Faculty of Applied Sciences, Department of Materials Science and Technology, Delft University of Technology,
Section of Advanced Materials and Solidification Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
Received 21 June 2002; accepted in revised form 18 September 2002
Abstract
Anodic oxidation of Al, AlSi10 and AlSi10Cu3 permanent mold cast substrates in 2.25 M H2 SO4 , at 0 8C for 50 min using
different current waveforms (i.e. square, ramp-square, ramp-down and ramp-down spike) was performed, in an attempt to evaluate
the effects of pulsed current on layer growth and properties. The pulses were unipolar and superimposed (amplitude ratio 4:1),
applied with a frequency of 0.0125 Hz and a duty cycle of 75%. The same average current densities (i.e. 3.0 and 4.2 A dmy2)
were imposed for all waveforms. The results on voltage transients, layer thickness, morphology, microhardness (HV0.025 ) and
surface roughness (Ra) have been compared to the results obtained when direct and ramped current were applied. Voltage
transients followed the current waveforms to a certain extent depending on the waveform shape and substrate composition. The
differences obtained in layer properties were not statistically different relative to the direct current experiments and remained
dependent mainly on substrate composition.
䊚 2002 Elsevier Science B.V. All rights reserved.
Keywords: Anodic oxide layers; Aluminum; Current waveforms; Microhardness; Surface roughness
1. Introduction
Whereas hard anodizing of aluminum has been
applied since 1950s, the use of pulsed current for the
process started almost 30 years later, in the 1980s w1,2x.
Since then, the main benefit of using pulsed current is
believed to be related more with the possibility of
anodizing at higher current densities andyor temperatures with lower chances of ‘burning’ (i.e. severe dissolution of the specimen). The effect is explained
through enhanced heat dissipation during the base current, thus avoiding local overheating w3x. This is mainly
important for the high strength aluminum alloys that are
prone to burning before reaching the required layer
thickness.
The application of pulsed current in (hard) anodizing
for enhancing layer properties remains a subject for
research due to the lack of experimental and theoretical
data. In general, the results obtained on aluminum or
*Corresponding author. Tel.: q31-15-278-9083; fax: q31-15-2786730.
E-mail address: [email protected]
(L.E. Fratila-Apachitei).
dilute aluminum alloys w3–12x showed a certain
improvement in some of the characteristics of the oxide
layers formed (e.g. thickness, coating ratio, density,
hardness, corrosion resistance, coloring) relative to
direct current (d.c.) processes, provided that all the other
conditions are similar. However, the lack of a statistical
analysis of the results makes difficult the evaluation of
the real benefits of the process or the translation of the
results to a broader range of conditions (e.g. alloy
compositions, microstructures). Furthermore, pulse characteristics such as type and shape, amplitude and frequency become additional process variables that have
to be established, for each substrate composition and set
of anodizing conditions, based on their impact on the
oxide growth mechanismykinetics and layer properties.
These relationships still await elucidation.
The research so far w3–12x was focused on square
pulses, sulfuric acid (with or without additives) and
oxalic acid electrolytes, and aluminum or dilute aluminum alloys used as substrates. Sulfuric acid hard anodizing of permanent mold cast aluminum alloys (i.e. Al,
AlSi10 and AlSi10Cu3) using (i) direct current, (ii)
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 3 3 - 8
L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
ramped current, (iii) square pulses, (iv) ramp-square
pulses, (v) ramp-down pulses, and (vi) ramp-down
spike pulses was performed in this study. The purpose
was to evaluate the potential benefits of pulsed current
(applied in new, different waveforms) when highly
alloyed substrates are hard anodized. The resultant voltage transients, layer thickness, morphology, microhardness and surface roughness were compared with the d.c.
and ramped current results.
2. Experimental
2.1. Anodic oxidation
The substrates (i.e. Al, AlSi10, AlSi10Cu3) have
been obtained by permanent mold casting as described
elsewhere w13x. Prior to anodizing, the disk-shaped
specimens (with a diameter of 45 mm and a thickness
of 7 mm) have been automatically polished w13x, ultrasonically cleaned for 15 min in isopropanol and oven
dried for 30 min at 50 8C. The anodic oxidation was
performed in 2.25 M H2SO4, for 50 min at a temperature
of 0.0"0.5 8C. A description of the equipment used
and more experimental details are included in Ref. w13x.
The experiments were carried out at constant average
current densities (3.0 and 4.2 A dmy2) using four
different shapes of current waveforms: square (sq),
ramp-square (rsq), ramp-down (rd) and ramp-down
spike (rds) (Fig. 1). The pulses were unipolar and
superimposed. Their shapes were designed using the
Windows-based, computer-aided, pulse-plating system
(WinCapp) consisting of the WinCapp PC software, a
power supply and a function generator. The current and
233
Table 1
Average current density and pulse characteristics
Average current density
Pulse characteristics
j1t1qj2t2
t1qt2
0.33t1Ž2j1qj2.qj2t2
jrsqs
t1qt2
0.625j1t1qj2t2
jrds
t1qt2
B 721j1t1
E
1
jrdss
=C
qj2t2F
G
t1qt2 D 1960
t1s60 s, t2s20 s
1
j 2s j 1
4
1
Frequencys
s0.0125 Hz
t1qt2
t1
Duty cycles
100s75%
t1qt2
jsqs
voltage transients were recorded on-line using a sampling time interval of 3 s. The main pulse characteristics
are shown in Table 1. The results obtained for the pulsed
oxide layers were compared to the oxide layers produced
using d.c. w13x and ramped (r) current.
After anodizing, the specimens were rinsed in demineralized water for approximately 3 min, oven dried for
30 min at 50 8C and kept in desiccator until further
testing.
2.2. Surface roughness
Surface roughness, Ra for the substrates and anodized
specimens was evaluated with a surface texture meter
type Surtronic 3q. The Ra value indicates the arithmetic
mean of the departures of the surface profile from the
mean line (i.e. the line that bisects the profile such that
the area below and above it is equal). The sampling
distance was 2.5 cm. Six measurements were performed
for each sample, towards the outer region of the diskshaped specimens in order to keep the same underlying
Fig. 1. Schematic representation of the current waveforms: (a) direct current, d.c.; (b) ramped current, r; (c) square pulses, sq; (d) ramp-square
pulses, rsq; (e) ramp-down pulses, rd; (f) ramp-down spike pulses, rds. The values for times t1, t2 (s) are included in Table 1 whereas j1s3.0
and 4.2 A dmy2, and j2s1y4 j1.
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L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
microstructure and avoid the errors introduced by the
pores from the casting process, present mainly in the
center of the specimens.
2.3. Surface topography of the oxide layers
The laser scanning confocal microscopy (LSCM)
technique has been used for analysis of surface topography of the oxide layers w13x. The two dimensional
(2D) and three dimensional (3D) images were produced
using a Leica TCS SP multi band confocal imaging
spectrophotometer interfaced with a computer equipped
with the software Leica (50=objective, 80 optical sections in a z-series, argon-ion laser, ls458 nm). The
images were captured from the outer zone of the diskshaped specimens, similar to surface roughness
measurements.
comprising about six sampling points located at a
distance of 50–100 mm. Three such profiles were
measured for each sample. Since different current waveforms are expected to affect the structure of the aluminum oxide with possible influence on layer
microhardness, the measurements were performed selectively on the Al2O3 matrix and not on the second-phase
particles (e.g. Si, Al2Cu, AlFeSi) or their zone of
influence. It is believed that at the latter locations, the
potential differences in microhardness due to different
matrix structure would be obscured by the more pronounced effect expected from the presence of second
phases. The same tests have been performed for the
substrates. The results represent the average and standard
deviation of approximately 20 measurements for each
sample.
2.4. Morphology of the oxide cross-section
3. Results and discussion
Layer thickness measurements and morphological
investigations have been performed by optical microscopy (cross-section) using (i) an Olympus BX60M
microscope equipped with a digital camera type DP 10
and (ii) the software analysis. A section from each
anodized disk was embedded in a transparent acrylic
thermoplastic resin (to improve the contrast between the
oxide layer and the resin) and was automatically polished following the next steps: grinding with 220-grade
SiC abrasive paper (0.5 min, 10 N); polishing with 9mm (1 min, 20 N), 6-mm (4 min, 30 N) and 3-mm (3
min, 25 N) diamond suspensions, and final polishing
with colloidal silica suspension (1 min, 10 N). Ultrasonic cleaning in ethanol was performed in between two
successive steps for both the samples and their holder.
The data on layer thickness represent the average and
standard deviation of 60–80 readings for each sample.
Anodic oxidation of permanent mold cast aluminum
substrates (i.e. Al, AlSi10 and AlSi10Cu3) was performed in 2.25 M H2SO4, at 0 8C for 50 min using
different current waveforms. Pulse characteristics, i.e.
type (unipolar and superimposed), frequency (0.0125
Hz), duty cycle (75%) and amplitude ratio (4:1) were
established based on existing studies w3–12,14x performed on anodic oxidation of aluminum substrates in
sulfuric acid electrolytes (with or without additives) at
temperatures ranging from 5 to 40 8C and using square
pulses. The same average current densities were imposed
for all the waveforms, i.e. 3.0 and 4.2 A dmy2.
Next to the square pulses, the selected current waveforms included three new ones (i.e. ramp-square, rampdown and ramp-down spike). Some of the latter have
been tested in electrodeposition of nickel w15–17x. The
results on voltage transients, layer thickness, morphology, microhardness and roughness have been compared
with the results obtained when d.c. and ramped current
were applied, the most used current waveforms in the
industrial practice.
2.5. Cell structure investigation by scanning electron
microscopy
Samples anodized at 4.2 A dmy2 using square pulses
were fractured under liquid nitrogen and the resultant
sections were coated with a thin gold layer (f1 nm)
prior to the scanning electron microscopy (SEM) analysis performed using a JEOL JSM 840A microscope,
under a voltage of 15 kV.
2.6. Microhardness
Vickers microhardness of the oxide layers was determined using an automatic Buehler Omnimet microhardness tester and a 25-g load (i.e. HV0.025). The
measurements have been performed on the oxide crosssection using an indent profile parallel with the substrateyoxide interface at half thickness of the layer,
3.1. Voltage transients
Typical voltage transients obtained for the three different compositions when anodized under the selected
current waveforms, are presented in Fig. 2.
The general trend of the voltage transients for each
composition is similar with that obtained using d.c. w13x.
Secondary transients are present for the binary and
ternary systems associated with an increase in voltage
rate for AlSi10 and a decrease in dVydt for the
AlSi10Cu3 substrate. The final voltages increased in the
order Al-AlSi10Cu3-AlSi10 with no particular trend
related to the shape of current waveform.
L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
235
Fig. 2. Voltage transients for different current waveforms (r—ramped current; sq—square pulses; rsq—ramp-square pulses; rd—ramp-down pulses;
rds—ramp-down spike pulses). Anodic oxidation was performed in 2.25 M H2SO4, 0 8C, 50 min at 3.0 and 4.2 A dmy2.
The sampling interval time for the current and voltage
data was 3 s thus following well the current waveforms
imposed at a frequency of 0.0125 Hz. It is, however,
observed that the voltage waveforms differ to some
extent from the current waveforms. Firstly, the peak to
base voltage ratio increases with anodizing time for all
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L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
types of waveforms, the effect being more pronounced
for the AlSi10 composition. This effect can be determined by the increasing resistance of the thickening
oxide layer.
Secondly, in the case of AlSi10 composition, it is
observed that out of the four current waveforms, the
ramp-down (mainly during the first transient) and rampdown spike current shapes seem to be best translated in
the voltage transients. The square pulses look more like
ramp-up pulses whereas ramp-square pulses translate
into saw-tooth shape. For the AlSi10Cu3 substrate,
square and ramp-down spike current shapes were better
translated in the voltage transients. The ramp-square
pulses become more like saw-tooth pulses and the rampdown pulses show a pattern that resembles the rampdown spike shape. These effects are probably caused by
the recovery effects in response to the predetermined
changes in current.
Thirdly, in the case of square pulses, the voltage
during the base current showed a different evolution
with anodizing time for the three substrates. Thus, for
Al and AlSi10 substrates, initially the voltage during
the base current increased (i.e. dVydt)0), then it
showed a constant value (i.e. dVydts0) and decreased
(i.e. dVydt-0) towards the end of the experiment. In
the case of AlSi10Cu3 substrate, the voltage during the
base current remains relatively stable (i.e. dVydtf0).
3.2. Layer thickness and morphology
The results on layer thickness (h) at 3.0 and 4.2
A dmy2 are presented in Fig. 3 and the oxide growth
rates (dhydt) are included in Table 2.
The large standard deviations for the layer thickness
make the differences obtained using different current
waveforms not statistically significant relative to the d.c.
results. In other words, none of the waveforms could
increase significantly the thickness of the oxides, regardless of the substrate composition. Large standard deviations indicate a non-uniform layer thickness that is
determined by the presence of second-phase particles
that affects local current distribution and consequently
oxide morphology. For the ternary substrate, the results
indicate that the ramp-down spike pulses may have a
favorable effect for the growth of the oxide layer as
maximum layer thickness was obtained (i.e. 77.2"17
mm at 4.2 A dmy2 relative to 68.9"17 mm when d.c.
was used). Since the same average current density was
applied, this increase may be determined by the effects
of pulses on local current distribution. However, the
improvement was not significant relative to the d.c.
results indicating that further optimization of the pulse
parameters may be required.
The oxide morphology, as determined by optical
microscopy (Fig. 4), was similar to that obtained during
the d.c. experiments w13x. Entrapment of certain particles
Fig. 3. Thickness of the oxide layers formed at 3.0 and 4.2 A dmy2
using different current waveforms (2.25 M H2SO4, 0 8C, 50 min).
in the oxide (e.g. Si, AlFeSi) associated with roughening
of the substrateyoxide interface, flaws around eutectic
silicon and defects associated with the oxidation of
Al2Cu intermetallic were the main features observed.
The cell structure of the oxide layers formed at 4.2
A dmy2 using square pulses was revealed by SEM
analysis of the cryofractures (Fig. 5). The images were
selected from areas with no defects generated by the
second phases. The stairs-like pattern may have been
determined by the layered structure of the oxide formed
Table 2
Oxide growth rates for the three different substrates after 50 min anodizing at 3.0 and 4.2 A dmy2 applied in different waveforms
Current
wave
form
dhydt (mm miny1)
Al
AlSi10
3.0
4.2
3.0
4.2
3.0
4.2
r
sq
rsq
rd
rds
–
1.1
–
–
–
1.46
1.69
–
–
–
–
1.01
1.11
1.13
1.48
1.48
1.47
–
–
–
0.90
–
0.95
0.94
1.38
1.46
1.42
1.47
1.54
d.c.
1.1
1.52
0.97
1.42
0.90
1.38
AlSi10Cu3
L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
237
Following a significant increase relative to the substrates, the trend indicates minor differences among
different current waveforms at both current densities and
for the three compositions. In the case of AlSi10Cu3
substrate anodized at 4.2 A dmy2 (Fig. 6b), the rampsquare waveform appears to result in a decrease in
microhardness whereas the ramp-down and ramp-down
spike seem to have a relatively beneficial effect. The
values obtained using d.c. lie in between those obtained
using pulsed current, making the differences statistically
not significant. This may indicate that the envisaged
structural changes (i.e. cell geometry), due to the use
Fig. 4. Optical micrographs of oxide cross-sections: (a) Al; (b)
AlSi10; (c) AlSi10Cu3 (4.2 A dmy2, square pulses, 2.25 M H2SO4,
0 8C, 50 min). Inclusion of certain second phases in the oxide layers,
i.e. Fe bearing particles (a) and Si particles (b), and defects generated
by Al2Cu (c) are observed.
during the base and peak currents. At the transition
zones, merging or branching of the pores may represent
points of minimum resistance during layer fracture.
Similar images of the d.c. oxides did not show this
pattern regularly.
3.3. Microhardness
As already mentioned, the Vickers microhardness was
determined for the Al2O3 matrices to avoid the more
pronounced effects of second phases. The results are
presented in Fig. 6.
Fig. 5. SEM micrographs of the oxide cryofractures: (a) Al; (b)
AlSi10; (c) AlSi10Cu3 (4.2 A dmy2, square pulses, 2.25 M H2SO4,
0 8C, 50 min).
238
L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
of microhardness, the effect of pulsed current on the
oxide surface roughness for a certain substrate composition is not significantly different when compared with
the d.c. results. Among different current waveforms,
square pulses and ramp-down spike pulses seem to favor
the increase in roughness for the AlSi10 and AlSi10Cu3
substrates whereas ramped current seems to decrease it.
However, the effects of pulsed current on the oxide
layers roughness could not determine modifications of
their surface topography, as shown by the laser scanning
confocal micrographs of the AlSi10Cu3 samples anodized at 4.2 A dmy2 using square pulses and rampdown pulses (Fig. 8). It appears that the different current
waveforms could not surmount the effects of large
second-phase particles (especially silicon particles) on
the current distribution over the macroscopic substratey
oxide interface.
While no (statistically) significant changes were
observed for the different characteristics of the oxides
formed under different current waveforms, the general
Fig. 6. Vickers microhardness (HV0.025 ) of the Al2O3 matrix formed
at 3.0 and 4.2 A dmy2 using different current waveforms (2.25 M
H2SO4, 0 8C, 50 min). ‘Sub’ indicates substrates.
of different current waveforms, did not result in
improved microhardness of the alumina matrix. In addition, the presence of alloying elements in the solid
solution (i.e. Si, Cu) seems to have little influence on
the microhardness of the alumina matrix probably due
to the low solubility of silicon in aluminum (e.g. -0.05
at.% at room temperature w18x) and higher migration
rate of the incorporated copper ions relative to aluminum
ions (f3= w19x) favoring their ejection into the
electrolyte.
3.4. Surface roughness
After anodizing, surface roughness increased significantly (i.e. 10–30 times) in the order Al-AlSi10AlSi10Cu3 (Fig. 7). Surface roughness results reflect
the effect of multiphase alloy composition on the growth
of anodic oxide layers.
The presence of silicon particles and of copper containing intermetallics with a lower and respectively
higher oxidation rate relative to the aluminum matrix
leads to scalloped oxide surfaces originating from a
rough substrateyoxide interface (Fig. 4). As in the case
Fig. 7. Surface roughness of the oxide layers formed at 3.0 and 4.2
A dmy2 using different current waveforms (2.25 M H2 SO4 , 0 8C, 50
min). ‘Sub’ indicates substrates.
L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
239
Fig. 8. 2D and 3D LSCM micrographs revealing the surface topography of the AlSi10Cu3 oxide layers formed at 4.2 A dmy2 using square pulses
(a and b) and ramp-down pulses (c and d) (2.25 M H2SO4, 0 8C, 50 min).
trend of these screening experiments indicates (at least
for the ternary composition at 4.2 A dmy2) that rampdown spike current may help in slightly improving oxide
layer thickness and microhardness, at the expense of
surface roughness, whereas ramped current without
spikes can be an alternative to decrease surface roughness (for binary and ternary substrates). Therefore,
refining pulse characteristics for these current waveforms that showed a beneficial trend should not be
excluded in a further attempt to significantly improve
such layer properties. In parallel however, efforts should
be focused on understanding the effects of pulsed current
on oxide growth kinetics for different substrate compositions as a prerequisite for the selection of certain type
and shape of current waveform.
In addition, the results of this study indicate that it is
not sufficient and sometimes not reliable to judge the
effects of pulse anodizing process, especially when
multiphase alloys are used as substrates, based only on
scarce results without being referenced to the d.c. results
and without a statistical processing of the data, as
frequently encountered in literature.
4. Conclusions
The paper presented the results on the effects of
different current waveforms on the growth of anodic
oxide layers on permanent mold cast Al, AlSi10 and
AlSi10Cu3 substrates in 2.25 M H2SO4 at 0 8C for 50
min. The current waveforms used were square, rampsquare, ramp-down and ramp-down spike. The same
average current densities were imposed (i.e. 3.0 and 4.2
A dmy2) for all current waveforms and the results were
referenced to the d.c. and ramped current, the most used
waveforms in practice. The main conclusions can be
summarized as follows.
(i) The general trend of the voltage transients was
similar to that of the d.c. experiments. However, certain
current waveforms (e.g. square, ramp-square) were not
fully translated into the voltage transients.
(ii) For a certain substrate composition, layer properties investigated, i.e. thickness, microhardness, surface
roughness and morphology, showed no significant
changes at different current waveforms relative to the
d.c. experiments. Among the different current waveforms, the ramp-down spike current had slightly beneficial effects on layer thickness and microhardness at
the expense of surface roughness (for the ternary substrate) whereas ramped currents without spikes may be
an alternative to lower the surface roughness (for binary
and ternary substrates). Refining pulse characteristics
for these waveforms should be considered, with a
parallel focus on their effects on oxide growth kinetics
for different substrate compositions.
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L.E. Fratila-Apachitei et al. / Surface and Coatings Technology 165 (2003) 232–240
(iii) The significant differences in layer properties
remained determined by substrate composition.
(iv) The effects of pulsed current on hard anodizing
of multiphase aluminum alloys should be evaluated
based on statistical data processing and extensive experimental and theoretical investigations.
Acknowledgments
The research is financially supported by the Innovatiegerichte Onderzoekprogramma (IOP) Oppervlaktetechnologie (project IOT 99002), The Netherlands.
Many thanks are due to Mr Tommy C. Dorge from TCD
Teknology ApS, Denmark for his technical assistance
with the WinCapp system.
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