Electrodeposition of bright Al-Zr alloy coatings from dimethylsulfone-based baths

Article abstract of DOI:10.1149/2.079204jes

Electrodeposition of Al coatings from dimethylsulfone (DMSO ₂)-AlCl₃ baths with the addition of ZrCl₄ was studied. Although pure Al coatings electrodeposited from the bath without ZrCl₄ are lusterless, bright and smooth coatings were obtained when the ZrCl₄ content in the baths was 0.005-0.015 mol per 10 mol DMSO₂. The Zr content in the coatings varied up to 3.5 at% in proportion to the ZrCl₄ content in the baths. The bright Al-Zr alloy coating showed high reflectance of 50-80% in the wavelength range of 450-1000 nm, whereas that of the matte pure Al coating was 10-20%. Morphological observations confirmed a reduction in the grain size of Al and surface leveling caused by the addition of ZrCl₄ to the baths. Moreover, a strong 100 preferential orientation of Al crystals was observed for the bright coatings. The bright coating containing ～3.5 at% Zr had a higher corrosion potential by 0.1 V than the pure Al coating.

Full text of DOI:10.1149/2.079204jes

Electrodeposition of Bright Al-Zr Alloy Coatings from
Dimethylsulfone-Based Baths
Suguru Shiomi, Masao Miyake and Tetsuji Hirato
J. Electrochem. Soc. 2012, Volume 159, Issue 4, Pages D225-D229.
doi: 10.1149/2.079204jes
Receive free email alerts when new articles cite this article - sign up
in the box at the top right corner of the article or click here
Email alerting
service
To subscribe to Journal of The Electrochemical Society go to:
http://jes.ecsdl.org/subscriptions
©2012 ECS - The Electrochemical Society

Journal of The Electrochemical Society, 159 (4) D225-D229 (2012)
D225
0013-4651/2012/159(4)/D225/5/$28.00 © The Electrochemical Society
Electrodeposition of Bright Al-Zr Alloy Coatings from
Dimethylsulfone-Based Baths
Suguru Shiomi, Masao Miyake, and Tetsuji Hirato∗,z
Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
Electrodeposition of Al coatings from dimethylsulfone (DMSO2)-AlCl3 baths with the addition of ZrCl4 was studied. Although pure
Al coatings electrodeposited from the bath without ZrCl4 are lusterless, bright and smooth coatings were obtained when the ZrCl4
content in the baths was 0.005–0.015 mol per 10 mol DMSO2. The Zr content in the coatings varied up to 3.5 at% in proportion
to the ZrCl4 content in the baths. The bright Al-Zr alloy coating showed high reflectance of 50–80% in the wavelength range of
450–1000 nm, whereas that of the matte pure Al coating was 10–20%. Morphological observations confirmed a reduction in the
grain size of Al and surface leveling caused by the addition of ZrCl4 to the baths. Moreover, a strong ꢀ100ꢁ preferential orientation
of Al crystals was observed for the bright coatings. The bright coating containing ∼3.5 at% Zr had a higher corrosion potential by
0.1 V than the pure Al coating.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.079204jes] All rights reserved.
Manuscript submitted October 7, 2011; revised manuscript received December 15, 2011. Published February 1, 2012.
Aluminum offers good corrosion resistance owing to the natural
oxide layer formed on its surface and thus can be used as a corrosion-
resistant coating for metallic materials. In general, electrodeposition is
the preferred method for the fabrication of such coatings because it is
simple and cost-effective compared to other common processes such
as hot dipping,1 thermal spraying2 and chemical vapor deposition.3
Moreover, electrodeposition has merits that complex-shaped compo-
nents can be coated and the thickness of the coatings can be easily
controlled.
Experimental
Electrodeposition.— Copper plates (1.5 cm × 3.0 cm) were used
as substrates for the electrodeposition of Al and Al-Zr alloys. Prior to
the electrodeposition, the substrates were polished with a SiC paper,
and then cleaned by sonication in ethanol. After the cleaning, a part
of each substrate was covered with PTFE tape so that a square area
(1 cm × 1 cm) would be exposed. The anode was an aluminum plate
(2.5 cm × 3 cm), which was polished and rinsed in water and ethanol
before the electrodeposition. The Al plate works as a sacrificial anode
to maintain the concentration of Al species in the bath.13
The plating bath was prepared by mixing DMSO2 (Tokyo Chem-
ical Industry, >99.0%) and AlCl3 (Fluka, anhydrous ≥99.0%) at a
mol ratio of 10 : 2, and then ZrCl4 (Wako Pure Chemical) was added
to the bath. The ZrCl4 content in the bath was varied from 0 mol to
0.015 mol per 10 mol DMSO2 (All the values of ZrCl4 content in this
paper describe the amount of ZrCl4 per 10 mol DMSO2). Prior to use,
DMSO2 had been dried in a vacuum at 60◦C for more than one day.
No further purification was conducted. AlCl3 and ZrCl4 were stored in
an Ar-filled glove box with a circulation system and used as received.
After mixing, they were melted at 110◦C.
A conventional two-electrode cell was employed for the electrode-
position. Al-Zr alloys were electrodeposited at constant current den-
sity of 60 mA cm−2 for 10 min using an electrochemical analyzer
(ALS, model 660C). Assuming 100% current efficiency, an Al layer
with a thickness of about 12 μm was obtained under the deposi-
tion conditions. During the electrodeposition, the bath was stirred at
400 rpm and the temperature of the bath was kept at 110˚C with a ther-
mostat. The preparation of the baths and the electrodeposition were
carried out in the Ar-filled glove box.
However, it is well known that metallic Al cannot be electrode-
posited from commonly-used aqueous solutions, and hence a num-
ber of non-aqueous media including aromatic hydrocarbons,4 etheric
solvents4 and inorganic molten salts5 have been studied to date. How-
ever, they have some drawbacks such as combustibility, high vapor
pressure and dendritic growth of deposit. In recent years, ionic liq-
uids, also known as room temperature molten salts, have been ex-
tensively explored for the electrodeposition of pure Al as well as Al
alloys.6–12 The ionic liquids are attractive media since they have low
vapor pressure, high electrical conductivity and a wide electrochemi-
cal window.6 Dimethylsulfone (DMSO2)-AlCl3 electrolyte is also an
attractive medium for the electrodeposition of Al because it is more
stable and therefore easier to handle than the conventional media. In
this electrolyte, AlCl3 undegoes a solvolysis reaction and forms two
−
3+
soluble species, AlCl4 and Al(DMSO2)3 and electrodeposition
of Al occurs from the Al(DMSO2)33+complex.17 Smooth, dense Al
coatings are reportedly obtained from DMSO2-AlCl3 baths.13–16,18–20
Although electrodeposition of Al-Ti,9,10 Al-Mo,11 Al-Zr12 alloys
has been studied in ionic liquid systems, electrodeposition of Al alloys
from the DMSO2 system has not been studied extensively. To the best
of our knowledge, the attempted electrodeposition of Al-Ti alloy by
Legrand and co-workers is the only report on electrodeposition of Al
alloys from DMSO2 based baths available in the literature.21,22 Thus,
little is known about the effects of secondary metal-elements on the
electrodeposition of Al from the DMSO2-AlCl3 electrolytes.
In this paper, we focus on Zr, which is one of the elements known
to improve the corrosion resistance of Al.23,24 Our preliminary exper-
iments confirmed that Zr can be co-deposited with Al from a DMSO2
based bath containing ZrCl4. Furthermore, we found that the bright-
ness of the electrodeposited coatings was drastically enhanced by the
addition of ZrCl4. The electrodeposition of bright Al coatings from the
DMSO2 system has not been reported so far, although it has been stud-
ied in ionic liquid systems.25 In the present study, the effect of ZrCl4
addition to DMSO2 baths on the brightness, surface morphologies
and corrosion resistance of electrodeposited coatings was examined
in detail.
Characterization.— Normal-incidence specular reflectance spec-
tra for the electrodeposited coatings were measured using a mul-
tichannel photodetector (Otsuka electronics, MCPD-7700) coupled
with an optical microscope (Nikon, Eclipse LV100). Spectra were
taken from a 20 μm diameter spot using a 10x objective lens with
a numerical aperture of 0.3 with reference to an Al mirror with a
50 nm MgF2 coating (Sigma Koki Co., Ltd., TFA-25C05-20). The
measured data were converted to absolute reflectance with the use of
the simulated reflectance spectrum for the mirror. The composition of
the coatings was detemined by EDX coupled with an SEM (Hitachi
S-3500). XRD patterns were taken by employing a diffractometer
(Panalytical, X’Pert PRO-MPD) with Cu-Kα radiation. An FE-SEM
(Hitachi, SU6600) was used to observe the surface morphology of the
coatings. The roughness was measured by a surface texture measuring
instrument (Surfcom 1400D, Tokyo Seimitsu). The parameters for the
measurement were cutoff length of 0.8 mm, and cutoff ratio of 300.
∗
Electrochemical Society Active Member.
z E-mail: hirato.tetsuji.2n@kyoto-u.ac.jp
The scanned length was 3.0 mm and the scan rate was 0.15 mm s−1
.

D226
Journal of The Electrochemical Society, 159 (4) D225-D229 (2012)
The roughness was calculated based on ISO ‘97. The corrosion re-
sistance of the coatings was evaluated by recording potentiodynamic
anodic polarization curves in deaerated 0.1 M NaCl solution at am-
bient temperature, using a potentiostat (Hokuto Denko, HZ-5000) at
a scan rate of 0.5 mV s−1. The conventional three-electrode cell was
employed for the measurement. The measurement was performed for
a circular area with a diameter of 5.8 mm on the surface of the coat-
ings. A Pt coil was used as the counter electrode and an Ag/AgCl
electrode (SSE) in a KCl saturated aqueous solution was used as the
reference electrode.
Results and Discussion
The electrodeposition of Al from the DMSO2-AlCl3 bath with-
out any additives produces dense, uniform pure Al coatings, but the
coatings are lusterless. In contrast, a bright coating was electrode-
posited when 0.01 mol ZrCl4 was added to the plating bath. Figure 1
presents photographs comparing the appearance of the coatings elec-
trodeposited from baths without and with 0.01 mol ZrCl4. Whereas the
coating obtained in the absence of ZrCl4 is dull-white, that from the
ZrCl4-containing bath has a silvery, mirror-like appearance. Normal-
incidence reflectance spectra for these coatings show more quanti-
tatively that the coating obtained with incorporation of ZrCl4 has a
bright, reflective surface (Figure 2). The reflectance of the matte Al
coating without ZrCl4 is as small as 10-20% over the visible and near-
infrared region (450–1000 nm). As for the bright coating, even the
minimum reflectance is as high as 50% at 450 nm. The reflectance
increases at longer wavelengths and approaches 80% in the near-
infrared region (800–1000 nm), showing that the bright coating from
the ZrCl4-containing bath has over 4-times higher reflectance than the
pure Al coating.
In order to examine the effect of ZrCl4 in more detail, electrode-
position was conducted from DMSO2-AlCl3 baths containing various
amounts of ZrCl4 at a constant current density of 60 mA cm−2. When
the ZrCl4 content was less than 0.005 mol, the electrodeposited coat-
ings were matte. Bright coatings were obtained from the baths with
ZrCl4 contents from 0.005 to 0.015 mol. When the ZrCl4 content
was higher than 0.015 mol, the color of the coatings turned to dark,
and adhesion of the coatings degraded. Such coatings are not suit-
able for corrosion protection and thus we did not carry out further
characterization for the dark coatings.
Figure 2. Normal-incidence reflectance spectra of (a) matte Al coating and (b)
bright Al-Zr coating electrodeposited on Cu substrates from DMSO2-AlCl3
bath containing no and 0.01 mol ZrCl4, respectively.
(0.5–3.5 at% Zr) exceeded the maximum solubility of Zr in Al phase
(<0.1 at%), no evidence for the formation of Al-Zr intermetallics or
metallic Zr was found in the XRD patterns. If Zr is dissolved in the
Al phase to form an oversaturated solid-solution, corresponding peak
shift should be observed in the XRD pattern of the Al phase. However,
no peak shift was observed. This is possibly because most of the Zr
deposited is present separately from the Al phase. Secondary phase
cannot be detected by XRD because the Zr amounts in the coatings are
too small. The diffraction patterns of the pure Al (Figure 4a) and the
Al-Zr alloy electrodeposited with the addition of 0.0025 mol ZrCl4
(Figure 4b), which exhibit a matte appearance, are similar to that of
an Al powder, indicating that the coatings are composed of randomly
orientated Al crystal grains. The diffraction pattern significantly
changes when the ZrCl4 content in the bath is raised to 0.005 mol;
the intensity of Al 200 diffraction becomes about 40 times greater
than that for the pure Al coating (Figure 4c), while the intensity of
Al 111 diffraction remains as low as that for the pure Al coating. The
strong Al 200 diffraction clearly shows that the Al-Zr alloy coating
obtained with 0.005 mol ZrCl4 has a preferrential orientation where
(100) planes are parallel to the substrate. Similar XRD patterns are
The relationship between the ZrCl4 content in the plating baths
and Zr content in the coatings is shown in Figure 3. The amount
of Zr in the coatings varied from 0 at% to 3.5 at% in proportion
to the ZrCl4 content in the plating bath, suggesting that the depo-
sition of Zr is diffusion-controlled. The bright coatings contained
1.2–3.5 at% Zr. The EDX analysis also revealed that the coatings
contained 0.2 ∼ 1.4 at% of Cl and S as impurities.
The coatings were also examined by XRD (Figure 4). Diffraction
peaks corresponding to Al and Cu substrates were confirmed for
all the samples. Although the Zr content of the Al-Zr alloy coatings
Figure 1. Photographs of (a) matte Al coating and (b) bright Al-Zr coating
electrodeposited on Cu substrates from DMSO2-AlCl3 bath containing no and
0.01 mol ZrCl4, respectively.
Figure 3. Relationship between ZrCl4 content in the plating bath and Zr
content in electrodeposited coating.

Journal of The Electrochemical Society, 159 (4) D225-D229 (2012)
D227
Figure 4. XRD patterns of pure Al and Al-Zr alloy coatings electrode-
posited from DMSO2 based baths containing various amounts of ZrCl4:
(a) 0 mol (pure aluminum), (b) 0.0025 mol, (c) 0.005 mol, (d) 0.010 mol, and
(d) 0.015 mol. Diffraction peaks of Cu substrate are denoted by •.
observed for the coatings obtained from the baths containing 0.01
and 0.015 mol ZrCl4 (Figures 4d and 4e) except that the Al 111
diffraction completely disappears and the Al 200 diffraction becomes
more intense. This set of XRD patterns indicates that the Al crystals
in the bright coatings have a strong ꢀ100ꢁ preferrential orientation,
while those in the matte coatings have no preferred orientation.
Figure 5 presents SEM images showing typical surface mor-
phologies of the coatings. Randomly-scattered angular grains of
Al approximately 5 μm in size can be seen on the surface of the
pure Al coating (Figure 5a). A similar morphology, except with the
grain size decreased to around 1 μm, can be found on the sample
obtained from the bath containing 0.0025 mol ZrCl4 (Figure 5b). In
contrast, the grain size of the bright coatings obtained by the addition
of of 0.005-0.015 mol ZrCl4 diminished to approximately 20 nm
(Figures 5c–5e). As a result, the surfaces of these coatings became
smoother than that of the matte pure Al coating. Although the SEM
images of the surface of the bright coatings indicated the drastic
decrease in the grain size, the XRD peak of the bright coatings was
not broadened. This suggests that the bright coatings consist of thin
columnar grains grown perpendicularly to the substrate.
The smooth surface of the coatings was also evidenced by rough-
ness measurement (Figure 6). The mean roughness (Ra) of the matte
pure Al coating was approximately 0.1μm. The roughness of the
coating did not change when 0.0025 mol ZrCl4 was added to the plat-
ing bath, whereas the Ra dramatically decreased to 0.01 μm when
Figure 5. SEM images of the surfaces of pure Al and Al-Zr alloy coat-
ings electrodeposited from DMSO2 based baths containing various amounts
of ZrCl4: (a) 0 mol (pure aluminum), (b) 0.0025 mol, (c) 0.005 mol,
(d) 0.010 mol, and (d) 0.015 mol.

D228
Journal of The Electrochemical Society, 159 (4) D225-D229 (2012)
Figure 7. Anodic polarization curves recorded in 0.1 M NaCl aqueous solution
for (a) pure aluminum coating and (b) Al-3.5% Zr alloy coating. The Al-3.5%
Zr alloy coating was obtained from a bath containing 0.015 mol ZrCl4.
in particular crystal directions at the peaks of deposit, leading to the
production of preferentially oriented fine crystal grains and a leveled
surface with small irregularities. The resulting surface with smaller
irregularities than the wavelength of the visible light looks bright and
lustrous, because light incident to the surface is reflected without being
diffused.
Al-Zr alloys prepared by the electrodeposition from AlCl3-1-ethyl-
3-methylimidazolium chloride ionic liquid are reported to have supe-
rior corrosion resistance to that of pure Al.12 In order to examine
whether the corrosion resistance of the Al-Zr alloy coatings obtained
from the DMSO2 bath is also improved, the corrosion resistance of
the pure Al and the Al-3.5% Zr alloy coatings electrodeposited from a
DMSO2 bath containing 0.015 mol ZrCl4 was compared by potentio-
dynamic anodic polarization in a NaCl aqueous solution (Figure 7).
The passive region of the pure Al coating lies below −0.6 V vs. SSE
followed by a sudden rise in anodic current. The rise in the anodic
current is explained by pitting corrosion of Al induced by chloride
ions.23,24 The curve for the Al-Zr alloy displays a wider passive re-
gion up to −0.5 V vs. SSE, indicating that the addition of Zr to Al
by the electrodeposition from the DMSO2 bath is effective for the
enhancement of the corrosion resistance of Al.The shift in the cor-
rosion potential of 0.1 V is comparable to those for the Al-Zr alloys
electrodeposited from the ionic liquid bath.12
Figure 6. Surface roughness of coatings electrodeposited from DMSO2-AlCl3
baths as a function of ZrCl4 content in the bath: (a) arithmetical mean roughness
(b) maximum roughness height (ISO ‘97).
0.005 mol or more ZrCl4 was added to the bath. The maximum height
(Rz) varied similarly to Ra and was 1.2-1.7 μm and 0.3 μm for the
matte coatings and bright coatings, respectively (Figure 6b). The vari-
ations in the surface roughness are in agreement with the surface
morphologies of the coatings revealed by the SEM images (Figure 5).
The Ra and Rz for the bright coatings obtained in this study are much
smaller than the reported values for the bright Al coatings electrode-
posited from an ionic liquid bath containing 1,10-phenanthroline as a
brightener (Ra = 0.12 μm, Rz = 1.0 μm),25 thus showing the better
brightening ability of ZrCl4 in the DMSO2 based bath.
The SEM images (Figure 5) and the XRD patterns (Figure 4) show
that the grain size and the crystal orientation simultaneously undergo
significant changes when 0.005 mol or more ZrCl4 is added to the
bath. The grain refinement and preferential crystal orientation are
typical phenomena in bright coatings electrodeposited with the aid
of brighteners.26 Additionaly, we carried out cyclic voltammetry in
the DMSO2-AlCl3 electrolyte with and without ZrCl4 and confirmed
that the reduction current is suppressed when ZrCl4 is present in the
electrolyte. For these reasons, we presume that ZrCl4 produces bright
coatings by a similar mechanism to the commonly-used brighteners.
The proposed mechanism is as follows: Zr(IV) species in a plating
bath are preferentially adsorbed onto peaks of Al deposit and particular
crystal planes. The adsorbed species suppress the crystal growth of Al
Conclusions
In this paper, we presented the electrodeposition of Al-Zr alloy
coatings fromDMSO2-basedbaths. Al-Zr alloy coatings containing up
to 3.5 at% Zr were obtained in the presence of ZrCl4 in the baths. The
addition of ZrCl4 had the effects of grain refining and surface leveling,
resulting in the formation of bright coatings with a silvery, mirror like
appearance. The bright Al-Zr alloy coating showed high reflectance
of 50–80% in the visible and near-infrared region. Moreover, the
addition of Zr to the coating from the DMSO2 bath was confirmed
to be effective for the enhancement of corrosion resistance. These
results expand the potential applications of electrodeposited Al-Zr
coatings to decorative coatings and coatings for optical devices as
well as corrosion resistant coatings.
Acknowledgments
The authors thank Kenji Kazumi and Teruyoshi Unesaki for their
assistance in SEM observation.

Journal of The Electrochemical Society, 159 (4) D225-D229 (2012)
D229
References
13. L. Legrand, A. Tranchant, and R. Messina, Electrochim. Acta, 39, 1427 (1994).
14. L. Legrand, A. Tranchant, and R. Messina, J. Electrochemi. Soc., 141, 378 (1994).
15. L. Legrand, M. Heintz, A. Tranchant, and R. Messina, Electrochim. Acta, 40, 1711
(1995).
16. L. Legrand, A. Tranchant, and R. Messina, Electrochim. Acta, 41, 2715 (1996).
17. L. Legrand, A. Tranchant, R. Messina, F. Romain, and A. Lautie, Inorg. Chem., 35,
1310 (1996).
1. W.-J. Cheng and C.-J. Wang, Surf. Coat. Tech., 205, 4726 (2011).
2. M. Campo, M. Carboneras, M. D. Lo´pez, B. Torres, P. Rodrigo, E. Otero, and
J. Rams, Surf. Coat. Tech., 203, 3224 (2009).
3. Y. Liu, L. J. Overzet, and M. J. Goeckner, Thin Solid Films, 510, 48 (2006).
4. M. Ga´lova´, Surf. Technol., 11, 357 (1980).
5. L. Qingfeng, H. A. Hjuler, R. W. Berg, and N. J. Bjerrum, J. Electrochem. Soc., 137,
593 (1990).
6. T. Jiang, M. J. C. Brym, G. Dube´, A. Lasia, and G. M. Brisard, Surf. Coat. Tech.,
18. T. Hirato, J. Fransaer, and J.-P. Celis, J. Electrochem. Soc., 148, C280 (2001).
19. J. Fransaer, E. Leunis, T. Hirato, and J-P. Celis, J. Appl. Electrochem., 32, 123 (2002).
20. T. Jiang, M. J. C. Brym, G. Dube´, A. Lasia, and G. M. Brisard, Surf. Coat. Tech.,
201, 6309 (2007).
201, 1 (2006).
7. Q. X. Liu, S. Z. E. Abedin, and F. Endres, Surf. Coat. Tech., 201, 1352 (2006).
8. H. C. De Long, J. Fuller, and P. C. Trulove, J. Electrochem. Soc., 145, 1598 (1998).
9. T. Tsuda, C. L. Hussey, G. R. Stafford, and J. E. Bonevich, J. Electrochem. Soc., 150,
C234 (2003).
21. L. Legrand, A. Chausse´, and R. Messina, J. Electrochem. Soc., 145, 110 (1998).
22. L. Legrand, A. Chausse, and R. Messina, Electrochim. Acta, 46, 2407 (2001).
23. G. D. Davis, W. C. Moshier, T. L. Frits, and G. O. Cote, J. Electrochem. Soc., 137,
422 (1990).
10. D. Pradhan, R. G. Reddy, and A. Lahiri, Metall. Mater. Tans. B, 40B, 114 (2009).
11. T. Tsuda, C. L. Hussey, and G. R. Stafford, J. Electrochem. Soc., 151, C379 (2004).
12. T. Tsuda, C. L. Hussey, G. R. Stafford, and O. Kongstein, J. Electrochem. Soc., 151,
C447 (2004).
24. Z. Szklarska-Smialowska, Corros. Sci., 41, 1743 (1999).
25. L. Barchi, U. Bardi, S. Caporali, M. Fantini, Alessandro Scrivani, and Andrea
Scrivani, Prog. Org. Coat., 67, 146 (2010).
26. L. Oniciu and L. Mures¸an, J. Appl. Electrochem., 21, 565 (1991).