Journal of The Electrochemical Society, 159 (1) C29-C32 (2012)
C29
0
013-4651/2012/159(1)/C29/4/$28.00 © The Electrochemical Society
Electrochemical Deposition of γ-Phase Zinc-Nickel Alloys from
Alkaline Solution
Heidi Conrad, John Corbett, and Teresa D. Golden
∗
z
Department of Chemistry, University of North Texas, Denton, Texas 76203, USA
Zinc-nickel alloys (γ-phase) with 8–15% nickel were electrochemically deposited onto stainless steel for increased corrosion
protection. Alkaline deposition conditions were studied using ammonium hydroxide as the base source and a working pH range
between 9.0–9.5. Sodium acetate was used as a complexing ligand and found to stabilize the metal ions in the electrolytic solution.
Strongly adhering, quality deposits were obtained with room temperature electrodeposition. Uniform grain size and smooth deposits
were observed with scanning electron microscopy, which lead to increased corrosion protection. X-ray diffraction and atomic
absorption spectroscopy confirmed the deposits were γ-phase zinc-nickel with nickel content of 8–15% within the alloy. The
corrosion potential for the γ-phase coatings was improved over that of pure zinc coatings by about 200 mV cathodic with respect to
stainless steel but at a lower corrosion potential, thereby impeding the corrosion rate.
©
2011 The Electrochemical Society. [DOI: 10.1149/2.027201jes] All rights reserved.
Manuscript submitted June 6, 2011; revised manuscript received September 2, 2011. Published December 7, 2011. This was Paper
234 presented at the Las Vegas, Nevada, Meeting of the Society, October 10–15, 2010.
1
In the field of corrosion, there is a constant demand for increased
performance at a reduced cost. Steel is a common frame used in
automobiles, planes, boats and ships but harsh environmental condi-
tions lead to corrosion over time. To lengthen the lifetime of metals,
coatings are added that will protect by sacrificial corrosion.1,2 Sev-
eral zinc alloys have been examined as possible corrosion protective
Experimental
All solutions were prepared from reagent grade chemicals and
deionized water with pH adjusted to 9.0–9.5 using NH OH. Zinc-
nickel (γ-phase) alloy layers were deposited from the following
4
bath solution: ZnSO
The solution was gently heated (45 C) to increase the solubility of
4
· H
2
O, Ni(NH
4
)
2
(SO
4
)
2
· 6H
2 2 3 2
O and NaC H O .
◦
1
–4
coatings. Zinc alloyed with cadmium has shown improved corro-
sion protection, however it raises environmental concerns of toxicity
Ni(NH
4
)
2
(SO
4
)
2
· 6H
2
O. After the solution was prepared, the pH was
adjusted with ammonia hydroxide to 9.0–9.5.
3,4
associated with cadmium. Zinc-nickel alloys were examined as a
replacement since nickel is cheap, easy to work with and the alloys
offer comparable, if not better corrosion resistance than zinc-cadmium
A simple three electrode cell was used for all depositions. The
counter electrode was a coiled chromel wire and the reference elec-
trode was a saturated calomel electrode (SCE, +0.241 V vs. SHE).
The working electrode used throughout all electrochemical experi-
2
alloys. Zinc-nickel alloys have shown superior corrosion resistance
2
compared to pure zinc coatings. The alloys are more electrochem-
2
ments was a stainless steel (ss) disc (area = 1.77 cm ). To prepare the
ically noble than pure zinc, so the coating will sacrificially corrode
to protect the steel, but the corrosion occurs at a slower rate. If the
nickel content in the deposit becomes greater then 15% the coating
becomes anodic in relation to steel. The coating is a good barrier but
once scratched or damaged, will no longer protect the substrate.
There are several known phases for zinc-nickel alloys, η- (1% Ni),
working electrode, the ss was mounted in epoxy and then polished to a
mirror finish using 600 SiC grit to 0.05 μm alumina polish. The elec-
trode was then sonicated in DI water for 5 minutes before deposition.
All electrochemical work was done with an EG&G PAR Potentio-
stat/Galvanostat Model 273A using a pulse sequence of −1.5 V for
6
0 sec and then −1.3 V for 20 sec at room temperature. Chrono-
α and β (30% Ni, known as the nickel rich phases), δ- (Ni
3
Zn22) and
◦
coulometry experiments done at T = 25 C were used to calculate
the diffusion coefficients for the Ni and Zn ions in the deposition
solutions.
5–7
γ- (Ni
5
Zn21) known as the zinc rich phases. For maximum corrosion
protection, the γ phase alloy exhibits the best corrosion protection,
5
with a nickel content between 8–15%. Majority of the researchfor Zn-
X-ray diffraction data was obtained on a Siemens D-500 Diffrac-
tometer using Cu Kα radiation (λ = 0.1541 nm) at 35 kV and 24 mA.
3
,8–10
Ni coatings has been in acidic electrolyte conditions.
However in
acidic baths, γ phase deposition is observed but tends to suffer from
δ phase contamination, therefore deposition in alkaline solutions is
being explored.
◦
The scans were run from 35–100 2θ at a step size of 0.05 degrees
and dwell time of 1 second. The SEM micrographs were obtained
on an Environmental SEM, FEI Quanta 200 using an ETD detec-
tor. The percentages of zinc and nickel in the films were determined
with atomic absorption spectroscopy (AAS). A Perkin Elmer AAna-
lyst AAS Spectrophotometer was used, with hollow cathode lamps of
zinc and nickel for the analysis. The corrosion measurements for the
coatings were obtained using linear polarization resistance measure-
pm o et ne nt st ii an l a( O0 .C1 PM ).NaCl solution scanned to ± 20 mV from open circuit
The advantage of alkaline deposition is that the deposit has a
superior alloy distribution compared to acidic deposition; but the de-
posits tend to be duller in color, not the bright finishes expected from
1
acid baths. Alkaline electrodeposition gives a more uniform deposit,
which offers better corrosion protection to the underlying metal. So
far the work done in alkaline condition has mostly been at higher pH
ranges ≥12 using several complexing ligands to prevent precipitation
3
of metal hydroxides.
This work focuses on the electrochemical deposition of the γ-
phase zinc-nickel alloy under less caustic conditions. Our method uses
ammonium hydroxide as the base source, with a pH range of 9–9.5 at
room temperature deposition. The deposits contain the same qualities
obtained from elevated pH ranges and temperature ranges. Also, the
deposits obtained are exclusively γ phase alloy. Deposition parameters
such as pH, electrolyte composition, electrolyte concentration, and
applied potential were studied for the deposition of the γ phase Zn-Ni
alloys.
Results and Discussion
For this study, nickel ammonium sulfate hexahydrate was used as
the nickel ion source for depositions due to the common ammonium
ions with the base source used. Nickel sulfate hexahydrate is com-
4
monly used in Zn-Ni deposition but in our studies, nickel ammonium
sulfate hexahydrate, which has not previously been examined for
Zn-Ni deposition, was used since it gave a better overall morphology
11
for the Zn-Ni (γ-phase) alloy. While nickel ammonium sulfate
hexahydrate is not as soluble in aqueous solutions as nickel sulfate
hexahydrate, gentle heating was used to dissolve the compound. Once
∗
Electrochemical Society Student Member.
E-mail: tgolden@unt.edu
z