Effects of cathodic hydrogen evolution on electrodeposited Au-Cu-Cd alloys

Article abstract of DOI:10.1149/1.1350673

Hydrogen can be incorporated into electrodeposited Au alloys either during electrodeposition or by cathodic charging. Incorporated hydrogen in these alloys generally impairs their mechanical properties. In this work, we present results of a study of electrochemical, structural, surface composition, and morphological effects of cathodic charging of Au-Cu-Cd alloys electrodeposited from a cyanoalkaline bath. Different kinds of linear sweep voltammetric behaviors are observed for alloys with different compositions and for repeated scans, which is related to a loss of electrocatalytic activity for high Cu content alloys and prolonged cathodic charging. The surface composition of alloys is altered by cathodic charging, resulting in a decrease in Au content. The surface morphology, as observed by scanning electron and atomic force microscopies, is modified by cathodic charging: large crystallites are attacked and globular features appear on flat deposits. The crystallographic structure of the alloys is affected by prolonged hydrogen evolution; the original preferred orientation is disrupted and a Cu-rich phase appears.

Full text of DOI:10.1149/1.1350673

Journal of The Electrochemical Society, 148 ͑3͒ C231-C235 ͑2001͒
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0
013-4651/2001/148͑3͒/C231/5/$7.00 © The Electrochemical Society, Inc.
Effects of Cathodic Hydrogen Evolution on Electrodeposited
Au-Cu-Cd Alloys
a, ,z
b,
B. Bozzini * and P. L. Cavallotti *
a
INFM, Dipartimento di Ingegneria dell’Innovazione, Universit a` di Lecce, I-73100 Lecce, Italy
Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, I-20131, Milano, Italy
b
Hydrogen can be incorporated into electrodeposited Au alloys either during electrodeposition or by cathodic charging. Incorpo-
rated hydrogen in these alloys generally impairs their mechanical properties. In this work, we present results of a study of
electrochemical, structural, surface composition, and morphological effects of cathodic charging of Au-Cu-Cd alloys electrode-
posited from a cyanoalkaline bath. Different kinds of linear sweep voltammetric behaviors are observed for alloys with different
compositions and for repeated scans, which is related to a loss of electrocatalytic activity for high Cu content alloys and prolonged
cathodic charging. The surface composition of alloys is altered by cathodic charging, resulting in a decrease in Au content. The
surface morphology, as observed by scanning electron and atomic force microscopies, is modified by cathodic charging: large
crystallites are attacked and globular features appear on flat deposits. The crystallographic structure of the alloys is affected by
prolonged hydrogen evolution; the original preferred orientation is disrupted and a Cu-rich phase appears.
©
2001 The Electrochemical Society. ͓DOI: 10.1149/1.1350673͔ All rights reserved.
Manuscript submitted February 17, 2000; revised manuscript received October 2, 2000.
Electrodeposited Au-Cu-Cd has been in consistent use for deco-
rative purposes ͑practically all of the electroforming of 14 to 18
carat Au hollow jewelry͒. The electronics industry is also interested
in this material ͑high-performance electric contacts, sliding contacts,
slip rings, connectors͒. The requirements for both electronic and
decorative fields strongly stress the mechanical properties of the Au
alloy layers ͑wear resistance, ductility, toughness͒. It has been
proved in previous work1 that hydrogen embrittlement is a major
cause of mechanical failure of electroplated Au-based alloys. In this
work, we report on compositional, structural, and morphological
alterations brought about by hydrogen evolution on electrodeposited
Au-Cu-Cd alloys of various compositions.
tions. The electrokinetic behavior of these baths and the composi-
tional and structural properties of the alloys will be presented in a
separate paper.
Structural and compositional investigations.—Electrochemical
measurements were complemented by structural and compositional
studies. The crystallographic structure of the alloys before and after
galvanostatic hydrogen evolution was studied by X-ray diffractom-
etry with powder and thin film goniometers. The surface morphol-
ogy of the cathodes before and after HER was studied by scanning
electron microscopy ͑SEM͒, atomic force microscopy ͑AFM͒, and
laser interferometric profilometry, and the composition was mea-
sured by energy dispersive spectroscopy ͑EDS͒ ͑EDS measurements
,2
Accurate but phenomenological studies of Au-Cu-Cd electrode-
were replicated four times for each sample͒. Hydrogen release from
3
-5
posited alloys were proposed in the 1970s and early 1980s. More
recent work is either focused on very specific problems such as
cathodically charged samples was studied by the LECO technique.7
6
pulse plating or is markedly application-oriented. No description is
Results and Discussion
found in the literature concerning hydrogen evolution effects on
3
,4,6
HER linear sweep voltammetry experiments.—Cathodic linear
these alloys, apart from indirect data on cathodic efficiency.
sweep voltammetry experiments were carried out in the phosphate
Structural and morphological data are reported for ϳ18 carat
2
deposits.4,5 All authors agree on the fact that at least one Au-based
solution described above with Au-Cu-Cd ͑2, 5, 10, 20, 50 mA/cm ,
5
ϳ60 ␮m͒ cathodes. Typical polarization curves are shown in Fig.
disordered solid solution is observed. Steinmann et al. report one
1-3. The peculiarities of the shape of each curve ͑waves and pla-
such phase with strong ͑111͒ preferred orientation, while Dettke
4
teaus͒ can be interpreted in terms of the scheme proposed in Ref. 7
for the case of Au, Cu, and Au-Cu. We do not go into further
mechanistic details here; rather, we are interested in relative effects
et al. report somewhat doubtful evidence of a polyphasic structure
with reflections form four superlattice structures.
Materials and Methods
Baths and electrochemical experiments.—The investigated al-
loys were electrodeposited with the following bath and operating
Ϫ1
conditions: Au ͑as KAu͑CN͒ ) 5 g L , Cu ͑as KCu͑CN͒ ) 50 g
2
3
Ϫ1
Ϫ1
Ϫ1
Ϫ1
L
5
, CdCO 1.5 g L , KCN 20 g L , KHCO 7 g L , and K CO
3 3 2 3
Ϫ1
g L , pH 10.5, temp 70°C. Hydrogen evolution experiments were
carried out in a 0.125 M aqueous H PO solution at pH 6.0 ͑by
3
4
NaOH͒ at 25°C. Electrodeposition of Au-Cu-Cd alloys was carried
out galvanostatically ͑current densities ͑cd͒: 2, 5, 10, 20, 50 mA
Ϫ2
cm ͒ in a prismatic cell equipped with a magnetic stirrer. Hydro-
gen evolution reaction ͑HER͒ on Au-Cu-Cd cathodes was studied by
Ϫ1
linear sweep voltammetry ͑scan rate 0.5 mV s , a Ag/AgCl refer-
ence electrode connected to the working electrode via a lateral-
channel Piontelli probe and voltages reported vs. Ag/AgCl͒, and by
Ϫ2
galvanostatic experiments at 10 mA cm for 3 h. The electrochemi-
cal experiments were carried out under natural convection condi-
Figure 1. Linear sweep voltammograms for HER on Au-Cu-Cd alloys elec-
trodeposited at various current densities. The curves were obtained at the first
scan of a series of four successive scans.
*
Electrochemical Society Active Member.
E-mail: bozzini.benebetto@unile.it
z

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Journal of The Electrochemical Society, 148 ͑3͒ C231-C235 ͑2001͒
Figure 4. EDS compositional analyses of Au-Cu-Cd alloys electrodeposited
at various current densities before and after HER ͑error bars refer to one
sample standard deviation evaluated on four replicated analyses͒.
Figure 2. Linear sweep voltammograms for HER on Au-Cu-Cd alloys elec-
trodeposited at various current densities. The curves were obtained at the last
scan of a series of four successive scans.
brought about by the composition of the tested alloys and by the
effects of HER. As far as limiting cds and wave potentials are con-
cerned, approximately the same values were observed as those for
Au-Cu alloys, and the same comments apply in this case. The most
obvious plateau present in the polarization curves ͑Ϫ400 to Ϫ800
mV͒ is clearly affected by the composition of the cathode ͑Fig. 1
and 2͒. In addition, the shape of the curves changes with the number
of successive scans ͑Fig. 1-3, particularly Fig. 3͒.
ues, before and after HER, are reported in Fig. 4. After the HER, the
surfaces ͑EDS analyses are sensitive to the depth of ϳ2 ␮m in the
relevant alloys͒ of Au-rich deposits tend to be slightly depleted of
Au, while those of Cu-rich deposits tend to be rich in Au.
Morphological effects of HER.—The surface morphology of
Ϫ2
single phase Au-rich deposits ͑2 mA cm , Fig. 5a͒ consists of a
granular structure with sharp ridges, very similar to those observed
Observe in Fig. 1 that, except for the alloy grown at 50 mA
Ϫ2
cm , there is an anticorrelation between plateau cd and elec-
trodeposition cd, i.e., Au content. This result indicates that the cata-
lytic activity is higher for the alloys with higher Au content. This
view is further confirmed by the high voltage quasi-Tafel slope,
which is much higher for Cu-rich cathodes. The exception of the
highest cd sample can be attributed to its much higher specific sur-
face area resulting from unstable crystal growth at the current close
to the limiting cd. Figure 3 indicates that the catalytic activity of
high Au samples, as evaluated from the plateau cd, seems to be
degraded by potentiodynamic cycling. This behavior is probably
attributable to Au impoverishment of Au atoms at the surface as
determined by EDS after the HER experiments. The high voltage
quasi-Tafel behavior is unaffected.
Compositional effects of HER.—The results of compositional
analyses performed by EDS of samples obtained at various cd val-
Figure 3. Linear sweep voltammograms for HER on an Au-Cu-Cd alloy
Ϫ2
electrodeposited at 2 mA cm ͑highest Au content͒. The curves relate to a
Figure 5. SEM micrographs of Au-Cu-Cd alloys electrodeposited at various
current densities ͑mA cm ͒: ͑a͒ 2, ͑b͒ 5, ͑c͒ 10, ͑d͒ 20.
Ϫ2
series of four successive scans.

Journal of The Electrochemical Society, 148 ͑3͒ C231-C235 ͑2001͒
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Figure 6. SEM micrographs of Au-Cu-Cd alloys electrodeposited at various
current densities after HER, at electrodeposition current densities ͑mA
Ϫ2
cm ͒: ͑a͒ 2, ͑b͒ 5, ͑c͒ 10, ͑d͒ 20.
7
with Au-Cu alloys. With larger amounts of codeposited Cu, a very
smooth morphology is obtained ͑5 mA cmϪ2, Fig. 5b͒, which tends
Ϫ2
first to develop a very slight waviness ͑10 mA cm , Fig. 5c͒ and
then to grow unstable through the formation of granular ͑20 mA
Figure 7. AFM images of a Au-Cu-Cd alloy electrodeposited at 10 mA
cm , as as-plated sample. Images at two different magnifications are shown
in parts a and b.
Ϫ2
Ϫ2
cm , Fig. 5d͒ and powdery features at the highest cd values inves-
Ϫ2
tigated ͑50 mA cm ͒.
The SEM morphology is altered profoundly by HER. In general,
a submicrometric granular structure appears on top of the as-plated
morphology ͑Fig. 6͒. In particular, the large crystals of low Cu
2
2
mA/cm ͒. At 10 mA/cm , three highly ͑111͒-oriented phases can be
observed with different reticular parameters. In high Cu deposits
͑20, 50 mA/cm ͒, two highly ͑111͒-oriented, disordered face-
Ϫ2
2
samples grown at 2 mA cm are attacked and flattened ͑Fig. 6a͒
and this alteration correlates with the disruption of crystallographic
preferred orientation, described in the subsection on structural ef-
fects of HER. Such an HER effect was not observed on high Cu
centered cubic phases are obtained with the Cu-rich phase prevail-
ing.
HER affects the XR crystallographic structure markedly. The
͑111͒-PO of the as-plated deposits is disrupted. In addition, traces of
a Cu-rich phase appear in high Au samples, while a Au-rich phase
tends to separate in high Cu deposits, which is in agreement with the
EDS data. It is worth noting that no sign of ordering is seen after
HER.
Ϫ2
samples grown at 50 mA cm . This morphological alteration is
seen even more clearly by AFM observation of smooth samples ͑10
Ϫ2
mA cm ͒. Isolated submicrometer humps can be resolved by AFM
in as-plated samples, protruding from a flat surface ͑Fig. 7͒. After
HER, these humps are enlarged and take the shape of growth crys-
tallites. The humps are no longer isolated, but occupy the entire
surface of the cathode ͑Fig. 8͒.
SEM observations can be quantified by laser interferometric pro-
filometry, which yielded the average roughness, R, and root mean
square roughness, Rrms , data shown in Fig. 9. The averaging effects
of HER can be observed.
LECO analyses.—LECO thermal desorption measurements of
as-plated samples show that virtually no hydrogen is incorporated
during the electrodeposition from the bath described in this paper.
Spectra of cathodically charged alloys show that hydrogen is incor-
7
porated only in the low energy trap at 450°C. This behavior is
general for all the investigated Au-Cu-Cd alloys, irrespective of
composition, structure, and morphology. Typical amounts of hydro-
Structural effects of HER.—XRD work was performed for the
relevant alloys before ͑Fig. 10͒ and after ͑Fig. 11͒ HER. Highly
gen incorporated under the above-described conditions are in the
Ϫ1
͑
111͒-ordered Au-rich phases are obtained at low cd values ͑2, 5
range of 0.3 to 0.7 mL g .

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Journal of The Electrochemical Society, 148 ͑3͒ C231-C235 ͑2001͒
Figure 9. Laser interferometric measurements of RaRrms of Au-Cu-Cd alloy
electrodeposited at various current densities before and after HER.
Figure 10. X-ray diffractograms of Au-Cu-Cd alloys as electrodeposited at
various current densities.
Figure 8. AFM images of a Au-Cu-Cd alloy electrodeposited at 10 mA
Ϫ2
cm . The sample was subjected to HER. Images at two different magnifi-
cations are shown in parts a and b.
Conclusions
The electrode kinetics, as measured by linear sweep voltammetry
of HER from a slightly acidic solution, is affected by alloy compo-
sition and scan repetition. A reduced electrocatalytic activity of Cu-
rich alloys was observed. Subsequent scans gave rise to a relative
reduction of catalytic activity. Prolonged galvanostatic HER gives
rise to a series of alterations in Au-Cu-Cd cathodes with a range of
compositions. The Au content of the alloys, as measured by EDS, is
systematically reduced. As far as SEM surface morphology is con-
cerned, the relatively large crystallites of Au-rich deposits are at-
tacked and flattened; globular dispersed features grow on the flat
surfaces of Cu-rich deposits. AFM reveals finer details of these mor-
phological alterations for flat deposits: initially tiny and separated
humps, which cannot be resolved by SEM, tend to grow and then
collapse. The morphological evolution can be followed also with
laser-interferometric rugosimetry, but the low lateral resolution of
Figure 11. X-ray diffractograms of Au-Cu-Cd alloys electrodeposited at
various current densities after HER.

Journal of The Electrochemical Society, 148 ͑3͒ C231-C235 ͑2001͒
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the technique makes it sensitive only to the average aspects of the
relevant variations. The crystal structure, as observed by XRD, is
affected by prolonged HER: the originally ͑111͒-preferentially ori-
ented layers tend to become disoriented and the appearance of a new
Cu-rich disordered solid solution is observed.
2. B. Bozzini, G. Giovannelli, and P. L. Cavallotti, Inst. Chem. Eng. Symp. Ser., 145,
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Universit a` di Lecce assisted in meeting the publication costs of this
article.
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