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Z. Phys. Chem. 2015; aop
Waheed A. Badawy*, Sahar A. Fadl-Allah, and Ahlam M. Fathi
Influence of Ni–Cu–P Deposits on the
Surface Characteristics of Anodized Al,
Al2014 and Al7075
Abstract: Nickel-copper-phosphorous layers were electro-less deposited on the
surface of anodized aluminum and aluminum alloys. The electrochemical be-
havior of the improved surface was investigated in 0.5 M Na2SO4. The corrosion
resistance of the modified alloy’s surface is ten folds that measured on the an-
odized materials. The surface morphology was examined by scanning electron
microscopy, SCE, and the deposit composition was subjected to x-ray energy dis-
persive analysis, EDAX. The experimental impedance data were fitted to theoret-
ical data according to a proposed equivalent circuit model. The results show that
the Ni–Cu–P layer was deposited homogeneously on the anodized Al surface but
on the anodized-alloy surface the layer had a defective nature.
Keywords: Alloys, Coatings, Electrochemical Techniques, SEM, Electrochemical
Properties.
DOI 10.1515/zpch-2014-0645
Received November 7, 2014; accepted January 20, 2015
1 Introduction
Anodized aluminum or aluminum alloys of definite porous structure represent
good substrates for the fabrication of nanostructures important for many appli-
cations [1–4]. A simple, anodization process in aqueous electrolyte was reported;
where no sophisticated and expensive techniques were needed. The process leads
to the formation of hexagonal cells of nano-porous alumina layer. At the bottom of
the pores, continuous and dielectric oxide layer, called “barrier layer” is built [5].
*Corresponding author: Waheed A. Badawy, Chemistry Department, Faculty of Science, Cairo
University, 12613 Giza, Egypt, e-mail: wbadawy@cu.edu.eg
Sahar A. Fadl-Allah: Chemistry Department, Faculty of Science, Cairo University, 12613 Giza,
Egypt
Ahlam M. Fathi: Department of Physical Chemistry, National Research Centre, Giza, Egypt
2 | W. A. Badawy et al.
Recently, anodic aluminum oxide was investigated as hydrogen sensor [6].
The oxide consists of highly uniform nano-pores with large aspect ratio
(depth/width >1000). Characteristic parameters of porous anodic alumina in-
cluding pore diameter, inter-pore distance, porosity, pore density and thickness
of the oxide layer can be easily controlled by adjusting anodizing conditions such
as type of electrolyte, anodizing potential, current density, temperature and an-
odization time. Most of the anodization processes were carried out in sulfuric
acid [4, 7, 8], oxalic acid [1, 9], and phosphoric acid[11, 12] solutions.
High purity aluminum foil (min 99.99%) is used as a substrate for anodiza-
tion; and only few studies on the anodization of aluminum alloys were re-
ported [13–16]. From the economic point of view, aluminum alloys are interesting
because of their reasonable price and better workability compared to high purity
Al. Thecorrosionprotectionof aluminum alloysis animportantaspectindifferent
industrial applications. The corrosion resistance depends essentially on the al-
loying elements and the alloy surface modification. In the engineering processes
most of alloying elements are added to improve the mechanical and wear proper-
tiesby solid solutionstrengtheningor agehardening [17] withoutregard to thecor-
rosion characteristics of the alloys. Alloy anodization is, generally, not adequate
to protect the alloy surface from corrosion. So, electro-less plating was adopted
to produce a protective top layer on the anodized surfaces, especially nickel[17].
The electro-less deposited Ni-coatings have several advantages such as uniform
deposition, good corrosion and wear resistance, good magnetic properties, good
electrical and thermal conductivity, and good solder ability, as well as the abil-
ity of co-deposition of metallic or non-metallic materials [18–20]. The deposited
layer always contains phosphorous in addition to nickel, as a result of the pres-
ence of sodium hypophosphite [21]. Electrochemical deposition of Ni–P layer on
anodized Al or its alloys is widespread due to the unique physical, chemical and
mechanicalpropertiesof thesurfacelayer. Thepropertiesof thealloy aremanifold
and highly dependent upon the phosphorus content and the plating conditions.
Layers of various thicknesses find application as protective coatings to improve
corrosion resistance, wear resistance and hardness. The amorphous structure and
hardness of the Ni–P deposit is used in making molds for optical applications
and wheel bearings and valves in automotive industry. The presence of copper in
the deposited layer, i.e. the electro-less Ni–Cu–P coatings, exhibit better wetting
property and higher thermal stability than the electro-less Ni–P deposit [22–25].
Moreover, the addition of copper into the electro-less Ni–P matrix improves the
corrosion resistance of the coatings [26, 27]. Deposition by electro-less plating us-
ing the after pretreatment anodization of Al or Alalloys is considered as newly de-
veloped process that needs further research to optimize the corrosion resistance
and to obtain brilliant finishing.
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 3
In this paper, the surface characteristics of Al, Al2014 and Al7075 were im-
proved by the electro-less deposition of Ni–Cu–P layer on the anodized surface.
The surface morphology of the specimens was examined by the scanning elec-
tron microscope, SEM, and the composition of the surface layer was analyzed by
x-ray energy dispersive technique, EDAX. The electrochemical characteristics of
the electrode/electrolyte interface were investigated by polarization techniques
and electrochemical impedance spectroscopy, EIS. It was important to emphasize
the improvement of the alloy surface by the Ni–Cu–P deposit to benefit from its
relatively low price and the extraordinary workability.
2 Experimental
2.1 Surface preparation and electro-less plating
Al, Al2014 and Al7075 specimens were cut as cylindrical electrodes of 0.12 cm2
surface areas to contact the test electrolyte. The mass spectrometric analysis of
the specimens is presented in Table 1. Prior to each experiment the electrode
was abraded by successive grades emery papers down to 2000 girt, then rubbed
against a soft cloth until it acquired a mirror bright surface. The electrode was
then washed with triple distilled water and transferred quickly to the electrolytic
cell. The cell is an all glass double walled three electrode cell with Al or Al al-
loy as working electrode and a Pt spiral as counter electrode. The working elec-
trode potential was measured against and referred to a Hg/Hg2SO4/Na2SO4 ref-
erence electrode [ 𝐸o
= 0.640 V vs the standard hydrogen electrode, SHE]. The an-
odization process was carried out potentiostatically at 20 V for 30 min in a stirred
1.0 M H2SO4 solution with a regulated DC power supply (Sci-tech RDC 3002T) at
25 ∘
C. The anodizing conditions were chosen to produce porous oxide films [16].
The freshly anodized specimen was transferred to the electro-less plating bath,
where the deposition process was carried out for 1 h at 90 ∘
C. The plating bath is
a hypophospite solution of the composition presented in Table 2, which includes
Table 1: The mass spectrometric analysis of the Al 2014 and Al 7075 samples.
Chemical composition (wt. %)
Sample Si Fe Cu Mn Mg Cr Zn Ti Al
Al2014 0.5 0.5 5.20 0.4 0.8 0.10 0.25 0.15 Balance
Al7075 0.4 0.5 1.20 0.3 2.1 0.18 5.10 0.20 Balance
4 | W. A. Badawy et al.
Table 2: Plating bath composition and deposition parameters used for Ni–Cu–P electro-less
deposition.
Constituents of Concentration Deposition conditions
the plating bath (g/L)
NiSO4.7H2O 26.27 Parameters Value
NaH2PO2 38.12 Bath temperature 90 ∘
C
Na3C6H5O7.H2O 38.71 Deposition time 60 min
CH3COONa 27.2 Solution PH 9 ± 0.1
NH2CH2COOH 21
CuSO4.5H2O 0.015
also the deposition parameters. The specimens were then rinsed with triply dis-
tilled water, dried, and then the different investigations were conducted.
2.2 Electrochemical measurements
2.2.1 Polarization tests
The potentiodynamic current-potential curves were recorded by changing the
electrode potential automatically from −800 mV to +200 mV. To achieve quasi-
stationary condition a potential scan rate of 1 mV s−1
was used. All polariza-
tion measurements were accomplished with Autolab Galvanostat/Potentiostat,
(GPES), connected to a computer. The polarization measurements were carried
out in naturally aerated stagnant 0.5 M Na2SO4 solution at room temperature
(25 ± 1 ∘
C). The corrosion parameters i.e. corrosion current density, 𝑖corr, corro-
sion potential, 𝐸corr, and polarization resistance, 𝑅p, were evaluated from the in-
tersection of the linear anodic and cathodic branches of the Tafel plots.
2.2.2 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy, EIS, is a non-destructive sensitive
technique which enables the detection of any changes occurring at the
electrode/electrolyte interface. In our investigations, the impedance data were
presented in the Bode plot format. This format enables the whole impedance
data to be presented and the presence of the phase angle 𝜃, as a sensitive pa-
rameter for any surface changes, as a function of the frequency is beneficial. The
EIS experiments were performed using the IM6d.AMOS system (Zahner Elektrik
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 5
GmbH & Co., Kronach, Germany). The input signal was usually 10 mV peak to
peak in the frequency domain 10−1
–105
Hz. Before each experiment, the work-
ing electrode was immersed in the test solution until a steady-state was reached,
then the impedance data were recorded. The effect of immersion time on the
impedance characteristics of each electrode was also studied. The impedance
data were fitted to theoretical data according to proposed equivalent circuitmodel
using a complex non-linear least squares (CNLS) circuit fitting software [28].
2.3 Surface characterization
The surfacemorphology of the different samples was investigated by the scanning
electron microscopy, SEM, using a JEOL-840 Electron probe micro-analyzer. The
micro-analyzer with accelerating voltage 30 kV enables energy diffraction x-ray
analysis of eachspecimen. Details of all experimental procedures are as described
elsewhere [29–31].
3 Results and discussion
The properties of the anodic films formed on the aluminum alloys are influenced
by the alloying elements. For example, magnesium and manganese elements are
beneficial for aluminum anodization, while copper and silicon have harmful ef-
fects, so the 2000 series aluminum alloys are hard to anodize [15]. The essential
alloying elements in the two alloys under investigation are copper in Al2014 and
zinc in Al7075. It is well known that that the abraded non-anodized surfaces
of the alloys are rough and contain cracks or flawed regions compared to pure
Al [32]. Generally, the surface morphology plays an important role in the anodiza-
tion process and also influences the electrochemical behavior of the metallic ma-
terial [30, 32]. The surface morphology of the anodized Al, Al2014 and Al7075
is presented in Figure 1a, b and c, respectively. For anodized Al, the anodizing
process converts aluminum into aluminum oxide which possesses a unique or-
dered geometrical structure with hexagonally arranged cells containing cylindri-
cal pores in each cell-center (cf. Figure 1a) [16]. The anodized surfaces of the two
alloys i.e. Al2014 and Al7075 are more defective compared to the anodized Al
surface (cf. Figure 1). The SE micrograph of the anodized Al2014 surface (Fig-
ure 1b) represents a porous surface with random distribution of pores with differ-
ent size. Morphology differences between anodized aluminum and anodized alu-
minum alloys can be attributed to the influence of the alloying elements. During
6 | W. A. Badawy et al.
Figure 1: SE micrographs of anodized Al (a), anodized Al2014(b), and anodized Al7075 (c).
anodization, the alloying element enriches the alloy surface and influences the
alloy/anodic film interface [33, 34]. In most cases a disordered film with porous
structure is formed. The nature of the film disorder depends on the anodization
conditions e.g. anodizing bath composition and temperature. The presence of
pores leads to poor corrosion resistance and hence surface instability of the an-
odized alloys. A smooth compact anodic film leads to high corrosion resistance
and stability of metallic material. Such surfaces are not accessible for coatings.
On the other hand, rough anodized surfaces are accessible for coatings, which
improve the surface characteristics of the anodic film, especially its stability in
corrosive solutions. These findings can be confirmed by the electrochemical in-
vestigations. Inthisrespect, potentiodynamic polarizationmeasurementsand EIS
were used.
In the polarization experiments, anodized Al, Al2014 and Al7075 free and
after electro-less deposition of Ni–Cu–P coatings were investigated. Typical
potentiodynamic polarization curves of these measurements are presented in
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 7
Figure 2: Potentiodynamic polarization curves of anodized Al and Ni–Cu–P modified anodized
Al (a), anodized Al2014 and Ni–Cu–P modified anodized Al2014 (b), and anodized Al7075 and
Ni–Cu–P modified Al7075(c) measured in naturally aerated stagnant 0.5 M Na2SO4 aqueous
solution at 25 ∘
C and scan rate 1 mV s−1
.
8 | W. A. Badawy et al.
Figure 2: Continued.
Figure 2 (a, b and c) for the three electrodes, respectively. The values of the corro-
sion parameters are presented in Table 3. These values show clearly that the cor-
rosion potential of pure Al was shifted to more negative value and the corrosion
resistance was decreased. On the other hand, the surface modified anodized al-
Table 3: Corrosion parameters obtained from the polarization curves of anodized and Ni–Cu–P
modified anodized Al and Al-alloys in stagnant naturally aerated 0.5 M Na2SO4 solution at
25 ∘
C.
Type of film 𝑖corr 𝐸corr 𝑅 𝑝 𝑏𝑎 𝑏c Corrosion rate
μA/cm2
mV Ohm mV/dec mV/dec mm/year
Anodized Al 4.19 −508 476 124 59 4.5 × 10−3
Ni–Cu–P/ 4.58 −544 450 29 26 5.0 × 10−3
Anodized Al
Anodized Al2014 1.36 −407 548 15 18 1.5 × 10−3
Ni–Cu–P/ 7.85 −301 460 37 36 8.6 × 10−3
Anodized Al2014
Anodized Al7075 3.18 −605 302 24 15 3.5 × 10−3
Ni–Cu–P/ 0.499 −577 330 26 24 0.5 × 10−3
Anodized Al7075
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 9
loys show a positive shift in the corrosion potential. The corrosion resistance and
the corrosion rate of the surface modified alloys were found to be dependent on
the alloying element. The corrosion rate of Al7075 was decreased by about one
order of magnitude, whereas the corrosion rate of Al2014 was increased by about
the same order of magnitude. This means that the morphology of the surface and
the alloying elements affect the Ni–Cu–P coatings, which depends on three fac-
tors [35]:
1. The amorphous nature of the surface.
2. The extent of internal stresses.
3. The extent of phosphorous in the deposited film.
In general, the corrosion resistance of a surface film depends on the formation
speed of the film and its protective nature. Some alloying elements can decrease
the corrosion current by promoting anodic and/or cathodic reactions during the
corrosion process [36]. Copper improves the corrosion resistance of Ni–P coatings
provided that the coatings thickness has reached a certain range [37]
To confirm the potentiostatic polarization results, EIS investigations of all
specimens were carried out. For comparison between anodized Al and Al-alloys
and the Ni–Cu–P modified anodized electrodes, the impedance spectra were
Figure 3: Bode plots of anodized Al, Al2014 and Al7075 after 3 h immersion in naturally aerated
stagnant 0.5 M Na2SO4 aqueous solution at 25 ∘
C.
10 | W. A. Badawy et al.
Figure 4: Bode plots of Ni–Cu–P modified anodized Al, Al2014 and Al7075 after 3 h immersion
in naturally aerated stagnant 0.5 M Na2SO4 aqueous solution at 25 ∘
C.
recorded after 3 h of electrode immersion in the electrolyte and presented as Bode
plots in Figure 3 for the anodized electrodes and in Figure 4 for the Ni–Cu–P
modified electrodes under the same conditions. In general, anodization of Al
or Al-alloys in sulfuric acid produces anodic films of duplex nature, which con-
sist of a thin non-porous compact layer known as the barrier layer and an outer
porous layer [16, 29]. The presence of porous structures, surface inhomogeneity
and roughness factors leads to frequency dispersion and enables formation of
nano-Ni–Cu–P deposits. The advantage of EIS is that it shows relaxation phe-
nomena over a wide frequency range. The Bode plots of Figures 3 and 4 show one
clear phase maximum at high frequency and an indication of another phase max-
imum at lower frequency. This means that the electrode/electrolyte interface is
controlled by two time constants. The impedance data were fitted to theoretical
data according to the equivalent circuit model presented in Figure 5 in which two
parallel combination terms are used, the first is the charge transfer resistance,
𝑅ct, and the double layer capacitance, 𝐶dl, and the second is the barrier film resis-
tance, 𝑅b, and the barrier film capacitance, 𝐶b, in series to the solution resistance,
𝑅s [38]. The experimental impedance data are consistent with the data obtained
according to the proposed model. The impedance behavior of the systems under
investigation can be represented by the dispersion formula, which is given by the
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 11
Figure 5: Equivalent circuit model used for fitting of impedance data of anodized and modified
anodized Al, Al2014 and Al7075. 𝑅s = solution resistance, 𝑅ct = charge transfer resistance,
𝐶dl = double layer capacitance, 𝑅b = barrier film resistance and 𝐶b = barrier film capacitance.
following equation:
𝑍 = 𝑅s + [𝑅ct/{1 + (2𝜋𝑓𝑅ct 𝐶dl) 𝛼
}] (1)
where 𝑍 is the total impedance and 𝛼 is an empirical parameter (0 < 𝛼 < 1) and f is
the frequency in Hz. The above relation takes into account the deviation from the
ideal RC-behavior in terms of distribution of time constants due to surface inho-
mogeneity, roughness effects and variations in properties or compositions of sur-
face film [39, 40]. The calculated impedance parameters of the modified anodized
materials after 3.0 h immersion in stagnant naturally aerated 0.5 M Na2SO4 at
25 ∘
C according to the fitting procedure are presented in Table 4. These values
show clearly that the corrosion resistance of the modified anodized Al surface is
more than 10 times that of the modified anodized alloys. This can be attributed
to the galvanic interactions between the micro-structural constituents of the al-
loy [41, 42]. Corrosion resistance of the anodized films of duplex nature is repre-
sented by the sum of the resistances of the inner and outer layers and also the
solution resistance. In most cases the barrier film resistance is much larger than
the others and hence it controls the corrosion process.
The impedance data of the Ni–Cu–P modified anodized alloy elec-
trodes show that the phase angle has two maxima, which means that the
Table 4: Equivalent circuit parameters after 3 h of immersion of the modified anodized Al and
Al-alloys in 0.5 M Na2SO4 at 25 ∘
C.
Modified anodized 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/
sample Ω μF cm−2
Ω cm2
μF cm−2
Ω cm2
Al 62.6 52.5 0.85 72.0 8.8 0.99 318
Al2014 75 47.5 0.61 42.2 82.5 0.71 31
Al7075 101 39.7 0.79 24.2 40.8 0.58 300
12 | W. A. Badawy et al.
Table 5: Equivalent circuit parameters at different time intervals of immersion in stagnant
naturally aerated 0.5 M Na2SO4 at 25 ∘
C for:
(a) Ni–Cu–P/ anodized Al
(a) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/
Time/min Ω μF cm−2
Ω cm2
μF cm−2
kΩ cm2
5 62.0 64.2 0.81 63.8 35.8 0.87 0.91
30 62.0 56.7 0.79 80 20.0 0.99 1.26
60 62.5 54.2 0.84 78.1 18.9 0.99 1.90
120 61.7 51.6 0.81 66.5 12.5 0.99 2.50
180 62.6 52.5 0.85 72.0 8.8 0.99 3.20
(b) Ni–Cu–P/ anodized Al2014
(b) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/
Time/min Ω μF cm−2
Ω cm2
μF cm−2
kΩ cm2
5 76.5 51.7 0.75 33.4 150 0.59 0.12
30 77 50.8 0.59 29.4 125 0.74 0.37
60 79 56.7 0.61 40.3 84.2 0.72 0.37
120 75 53.3 0.61 41.9 82.5 0.71 0.32
180 75 47.5 0.61 42.2 82.5 0.71 0.31
(c) Ni–Cu–P/ anodized Al7075
(c) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/
Time/min Ω μF cm−2
Ω cm2
μF cm−2
kΩ cm2
5 93 50.8 0.95 14.3 91.7 0.68 0.20
30 91 55.8 0.93 11.6 66.7 0.58 0.17
60 95 54.1 0.81 18.2 53.3 0.53 0.18
120 101 48.3 0.81 15.1 39.2 0.50 0.2
180 101 39.7 0.79 24.2 40.8 0.58 0.3
electrode/electrolyte interface is controlled by two time constants. For modified
anodized Al, the single phase maximum means that the porous layer proper-
ties has high conductivity due to the inclusion of the electrolytic solution inside
the pores [43]. The capacitance of the modified anodized alloys is higher than
those recorded for modified anodized Al. This indicates the presence of thicker
and rougher porous layer on the modified anodized alloy surfaces. The calcu-
lated values of the empirical parameter 𝛼, for the modified anodized Al elec-
trode approach1 indicating that the film is homogeneous and behaves like ideal
capacitor. On the other hand, all 𝛼 values of the modified anodized alloys devi-
ate from 1, which means that the formed film is defective and deviates from ideal
RC behavior.
The obtained impedance values are in good agreement with those calculated
from polarization measurements. The values obtained for the corrosion resistance
from both impedance and polarization measurements for Al2014 show that the
deposition of Ni–Cu–P did not improve the surface protection, whereas the mod-
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 13
Figure 6: SE micrograph of Ni–Cu–P modified anodized Al surface (a) and EDAX analysis of
Ni–Cu–P modified anodized Al electrode (b).
14 | W. A. Badawy et al.
Table 6: The composition of the Ni–Cu–P modified anodized Al after 3 h of electrode immersion
in 0.5 M Na2SO4 at 25 ∘
C as obtained from EDAX analysis.
Composition
Ni K P K Cu K
Atomic % 72.99 25.52 1.48
Weight % 82.89 15.29 1.82
ified anodized Al7075 was improved. This difference can be attributed to the al-
loying elements, such as Zn which is used for activating the coating on the sur-
face [21].
To investigate the stability of the deposited film, the effect of immersion time
on the different Ni–Cu–P modified anodized samples in sulfate solution was
studied by EIS. The values of the fitting parameters calculated at different time
intervals from electrode immersion in the electrolyte according to the equivalent
circuitmodelof Figure 5arepresented inTable 5. Itisclear thatthecorrosionresis-
tance of the anodized surface increases with increasing the immersion time. This
can be attributed to an improvement in the protective properties of the barrier film
with ageing [4].
The difference in behavior between Al, Al2014 and Al7075 can be attributed
to the difference in the surface morphology presented in Figure 1. While the SE
micrograph of modified anodized Al is homogeneous and smooth, the anodized
alloy surfaces are porous and contain voids and flawed regions (cf. Figure 1). In
either case the deposition of the nano-Ni–Cu–P deposit takes place successfully.
A compact defect–free Ni–Cu–P deposit on anodized Al without distinct cavities
and crevices, was recorded and presented in the SE micrograph of Figure 6a. The
appearance of cauliflower–like nodule is typical of amorphous materials [44]. The
composition of surface film was confirmed by EDAX and presented in Figure 6b.
The EDAX data analysis is presented in Table 6.
Figure 6b and Table 6 indicate that the anodic film is completely covered by
Ni, P and Cu and there is no appearance for any Al on the surface. The presence
of copper in the plating bath enhances the full surface coverage by Ni–Cu–P de-
posit. The good dispersion of Ni–Cu–P deposit over the anodized surfaces may
account for its high corrosion resistance.
Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 15
4 Conclusions
1. The corrosion resistance of the Ni–Cu–P modified anodized Al or Al-alloy
surfaces increases with ageing in sulfate solutions.
2. Modified anodized Al surface behaves like ideal RC, whereas the modified an-
odized alloy surfaces are defective and deviate from ideal capacitivebehavior.
3. The presence of copper in the plating bath resulted in the formation of more
dispersed film.
4. The improved surfaces of the Ni–Cu–Palloys suggest their application as cor-
rosion resistant materials.
Acknowledgement: The authors are grateful to the AvH foundation and the Cairo
University for providing the electrochemical work station and continuous sup-
port.
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zpch-2014-0645 (1)

  • 1. Z. Phys. Chem. 2015; aop Waheed A. Badawy*, Sahar A. Fadl-Allah, and Ahlam M. Fathi Influence of Ni–Cu–P Deposits on the Surface Characteristics of Anodized Al, Al2014 and Al7075 Abstract: Nickel-copper-phosphorous layers were electro-less deposited on the surface of anodized aluminum and aluminum alloys. The electrochemical be- havior of the improved surface was investigated in 0.5 M Na2SO4. The corrosion resistance of the modified alloy’s surface is ten folds that measured on the an- odized materials. The surface morphology was examined by scanning electron microscopy, SCE, and the deposit composition was subjected to x-ray energy dis- persive analysis, EDAX. The experimental impedance data were fitted to theoret- ical data according to a proposed equivalent circuit model. The results show that the Ni–Cu–P layer was deposited homogeneously on the anodized Al surface but on the anodized-alloy surface the layer had a defective nature. Keywords: Alloys, Coatings, Electrochemical Techniques, SEM, Electrochemical Properties. DOI 10.1515/zpch-2014-0645 Received November 7, 2014; accepted January 20, 2015 1 Introduction Anodized aluminum or aluminum alloys of definite porous structure represent good substrates for the fabrication of nanostructures important for many appli- cations [1–4]. A simple, anodization process in aqueous electrolyte was reported; where no sophisticated and expensive techniques were needed. The process leads to the formation of hexagonal cells of nano-porous alumina layer. At the bottom of the pores, continuous and dielectric oxide layer, called “barrier layer” is built [5]. *Corresponding author: Waheed A. Badawy, Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt, e-mail: wbadawy@cu.edu.eg Sahar A. Fadl-Allah: Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt Ahlam M. Fathi: Department of Physical Chemistry, National Research Centre, Giza, Egypt
  • 2. 2 | W. A. Badawy et al. Recently, anodic aluminum oxide was investigated as hydrogen sensor [6]. The oxide consists of highly uniform nano-pores with large aspect ratio (depth/width >1000). Characteristic parameters of porous anodic alumina in- cluding pore diameter, inter-pore distance, porosity, pore density and thickness of the oxide layer can be easily controlled by adjusting anodizing conditions such as type of electrolyte, anodizing potential, current density, temperature and an- odization time. Most of the anodization processes were carried out in sulfuric acid [4, 7, 8], oxalic acid [1, 9], and phosphoric acid[11, 12] solutions. High purity aluminum foil (min 99.99%) is used as a substrate for anodiza- tion; and only few studies on the anodization of aluminum alloys were re- ported [13–16]. From the economic point of view, aluminum alloys are interesting because of their reasonable price and better workability compared to high purity Al. Thecorrosionprotectionof aluminum alloysis animportantaspectindifferent industrial applications. The corrosion resistance depends essentially on the al- loying elements and the alloy surface modification. In the engineering processes most of alloying elements are added to improve the mechanical and wear proper- tiesby solid solutionstrengtheningor agehardening [17] withoutregard to thecor- rosion characteristics of the alloys. Alloy anodization is, generally, not adequate to protect the alloy surface from corrosion. So, electro-less plating was adopted to produce a protective top layer on the anodized surfaces, especially nickel[17]. The electro-less deposited Ni-coatings have several advantages such as uniform deposition, good corrosion and wear resistance, good magnetic properties, good electrical and thermal conductivity, and good solder ability, as well as the abil- ity of co-deposition of metallic or non-metallic materials [18–20]. The deposited layer always contains phosphorous in addition to nickel, as a result of the pres- ence of sodium hypophosphite [21]. Electrochemical deposition of Ni–P layer on anodized Al or its alloys is widespread due to the unique physical, chemical and mechanicalpropertiesof thesurfacelayer. Thepropertiesof thealloy aremanifold and highly dependent upon the phosphorus content and the plating conditions. Layers of various thicknesses find application as protective coatings to improve corrosion resistance, wear resistance and hardness. The amorphous structure and hardness of the Ni–P deposit is used in making molds for optical applications and wheel bearings and valves in automotive industry. The presence of copper in the deposited layer, i.e. the electro-less Ni–Cu–P coatings, exhibit better wetting property and higher thermal stability than the electro-less Ni–P deposit [22–25]. Moreover, the addition of copper into the electro-less Ni–P matrix improves the corrosion resistance of the coatings [26, 27]. Deposition by electro-less plating us- ing the after pretreatment anodization of Al or Alalloys is considered as newly de- veloped process that needs further research to optimize the corrosion resistance and to obtain brilliant finishing.
  • 3. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 3 In this paper, the surface characteristics of Al, Al2014 and Al7075 were im- proved by the electro-less deposition of Ni–Cu–P layer on the anodized surface. The surface morphology of the specimens was examined by the scanning elec- tron microscope, SEM, and the composition of the surface layer was analyzed by x-ray energy dispersive technique, EDAX. The electrochemical characteristics of the electrode/electrolyte interface were investigated by polarization techniques and electrochemical impedance spectroscopy, EIS. It was important to emphasize the improvement of the alloy surface by the Ni–Cu–P deposit to benefit from its relatively low price and the extraordinary workability. 2 Experimental 2.1 Surface preparation and electro-less plating Al, Al2014 and Al7075 specimens were cut as cylindrical electrodes of 0.12 cm2 surface areas to contact the test electrolyte. The mass spectrometric analysis of the specimens is presented in Table 1. Prior to each experiment the electrode was abraded by successive grades emery papers down to 2000 girt, then rubbed against a soft cloth until it acquired a mirror bright surface. The electrode was then washed with triple distilled water and transferred quickly to the electrolytic cell. The cell is an all glass double walled three electrode cell with Al or Al al- loy as working electrode and a Pt spiral as counter electrode. The working elec- trode potential was measured against and referred to a Hg/Hg2SO4/Na2SO4 ref- erence electrode [ 𝐸o = 0.640 V vs the standard hydrogen electrode, SHE]. The an- odization process was carried out potentiostatically at 20 V for 30 min in a stirred 1.0 M H2SO4 solution with a regulated DC power supply (Sci-tech RDC 3002T) at 25 ∘ C. The anodizing conditions were chosen to produce porous oxide films [16]. The freshly anodized specimen was transferred to the electro-less plating bath, where the deposition process was carried out for 1 h at 90 ∘ C. The plating bath is a hypophospite solution of the composition presented in Table 2, which includes Table 1: The mass spectrometric analysis of the Al 2014 and Al 7075 samples. Chemical composition (wt. %) Sample Si Fe Cu Mn Mg Cr Zn Ti Al Al2014 0.5 0.5 5.20 0.4 0.8 0.10 0.25 0.15 Balance Al7075 0.4 0.5 1.20 0.3 2.1 0.18 5.10 0.20 Balance
  • 4. 4 | W. A. Badawy et al. Table 2: Plating bath composition and deposition parameters used for Ni–Cu–P electro-less deposition. Constituents of Concentration Deposition conditions the plating bath (g/L) NiSO4.7H2O 26.27 Parameters Value NaH2PO2 38.12 Bath temperature 90 ∘ C Na3C6H5O7.H2O 38.71 Deposition time 60 min CH3COONa 27.2 Solution PH 9 ± 0.1 NH2CH2COOH 21 CuSO4.5H2O 0.015 also the deposition parameters. The specimens were then rinsed with triply dis- tilled water, dried, and then the different investigations were conducted. 2.2 Electrochemical measurements 2.2.1 Polarization tests The potentiodynamic current-potential curves were recorded by changing the electrode potential automatically from −800 mV to +200 mV. To achieve quasi- stationary condition a potential scan rate of 1 mV s−1 was used. All polariza- tion measurements were accomplished with Autolab Galvanostat/Potentiostat, (GPES), connected to a computer. The polarization measurements were carried out in naturally aerated stagnant 0.5 M Na2SO4 solution at room temperature (25 ± 1 ∘ C). The corrosion parameters i.e. corrosion current density, 𝑖corr, corro- sion potential, 𝐸corr, and polarization resistance, 𝑅p, were evaluated from the in- tersection of the linear anodic and cathodic branches of the Tafel plots. 2.2.2 Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy, EIS, is a non-destructive sensitive technique which enables the detection of any changes occurring at the electrode/electrolyte interface. In our investigations, the impedance data were presented in the Bode plot format. This format enables the whole impedance data to be presented and the presence of the phase angle 𝜃, as a sensitive pa- rameter for any surface changes, as a function of the frequency is beneficial. The EIS experiments were performed using the IM6d.AMOS system (Zahner Elektrik
  • 5. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 5 GmbH & Co., Kronach, Germany). The input signal was usually 10 mV peak to peak in the frequency domain 10−1 –105 Hz. Before each experiment, the work- ing electrode was immersed in the test solution until a steady-state was reached, then the impedance data were recorded. The effect of immersion time on the impedance characteristics of each electrode was also studied. The impedance data were fitted to theoretical data according to proposed equivalent circuitmodel using a complex non-linear least squares (CNLS) circuit fitting software [28]. 2.3 Surface characterization The surfacemorphology of the different samples was investigated by the scanning electron microscopy, SEM, using a JEOL-840 Electron probe micro-analyzer. The micro-analyzer with accelerating voltage 30 kV enables energy diffraction x-ray analysis of eachspecimen. Details of all experimental procedures are as described elsewhere [29–31]. 3 Results and discussion The properties of the anodic films formed on the aluminum alloys are influenced by the alloying elements. For example, magnesium and manganese elements are beneficial for aluminum anodization, while copper and silicon have harmful ef- fects, so the 2000 series aluminum alloys are hard to anodize [15]. The essential alloying elements in the two alloys under investigation are copper in Al2014 and zinc in Al7075. It is well known that that the abraded non-anodized surfaces of the alloys are rough and contain cracks or flawed regions compared to pure Al [32]. Generally, the surface morphology plays an important role in the anodiza- tion process and also influences the electrochemical behavior of the metallic ma- terial [30, 32]. The surface morphology of the anodized Al, Al2014 and Al7075 is presented in Figure 1a, b and c, respectively. For anodized Al, the anodizing process converts aluminum into aluminum oxide which possesses a unique or- dered geometrical structure with hexagonally arranged cells containing cylindri- cal pores in each cell-center (cf. Figure 1a) [16]. The anodized surfaces of the two alloys i.e. Al2014 and Al7075 are more defective compared to the anodized Al surface (cf. Figure 1). The SE micrograph of the anodized Al2014 surface (Fig- ure 1b) represents a porous surface with random distribution of pores with differ- ent size. Morphology differences between anodized aluminum and anodized alu- minum alloys can be attributed to the influence of the alloying elements. During
  • 6. 6 | W. A. Badawy et al. Figure 1: SE micrographs of anodized Al (a), anodized Al2014(b), and anodized Al7075 (c). anodization, the alloying element enriches the alloy surface and influences the alloy/anodic film interface [33, 34]. In most cases a disordered film with porous structure is formed. The nature of the film disorder depends on the anodization conditions e.g. anodizing bath composition and temperature. The presence of pores leads to poor corrosion resistance and hence surface instability of the an- odized alloys. A smooth compact anodic film leads to high corrosion resistance and stability of metallic material. Such surfaces are not accessible for coatings. On the other hand, rough anodized surfaces are accessible for coatings, which improve the surface characteristics of the anodic film, especially its stability in corrosive solutions. These findings can be confirmed by the electrochemical in- vestigations. Inthisrespect, potentiodynamic polarizationmeasurementsand EIS were used. In the polarization experiments, anodized Al, Al2014 and Al7075 free and after electro-less deposition of Ni–Cu–P coatings were investigated. Typical potentiodynamic polarization curves of these measurements are presented in
  • 7. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 7 Figure 2: Potentiodynamic polarization curves of anodized Al and Ni–Cu–P modified anodized Al (a), anodized Al2014 and Ni–Cu–P modified anodized Al2014 (b), and anodized Al7075 and Ni–Cu–P modified Al7075(c) measured in naturally aerated stagnant 0.5 M Na2SO4 aqueous solution at 25 ∘ C and scan rate 1 mV s−1 .
  • 8. 8 | W. A. Badawy et al. Figure 2: Continued. Figure 2 (a, b and c) for the three electrodes, respectively. The values of the corro- sion parameters are presented in Table 3. These values show clearly that the cor- rosion potential of pure Al was shifted to more negative value and the corrosion resistance was decreased. On the other hand, the surface modified anodized al- Table 3: Corrosion parameters obtained from the polarization curves of anodized and Ni–Cu–P modified anodized Al and Al-alloys in stagnant naturally aerated 0.5 M Na2SO4 solution at 25 ∘ C. Type of film 𝑖corr 𝐸corr 𝑅 𝑝 𝑏𝑎 𝑏c Corrosion rate μA/cm2 mV Ohm mV/dec mV/dec mm/year Anodized Al 4.19 −508 476 124 59 4.5 × 10−3 Ni–Cu–P/ 4.58 −544 450 29 26 5.0 × 10−3 Anodized Al Anodized Al2014 1.36 −407 548 15 18 1.5 × 10−3 Ni–Cu–P/ 7.85 −301 460 37 36 8.6 × 10−3 Anodized Al2014 Anodized Al7075 3.18 −605 302 24 15 3.5 × 10−3 Ni–Cu–P/ 0.499 −577 330 26 24 0.5 × 10−3 Anodized Al7075
  • 9. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 9 loys show a positive shift in the corrosion potential. The corrosion resistance and the corrosion rate of the surface modified alloys were found to be dependent on the alloying element. The corrosion rate of Al7075 was decreased by about one order of magnitude, whereas the corrosion rate of Al2014 was increased by about the same order of magnitude. This means that the morphology of the surface and the alloying elements affect the Ni–Cu–P coatings, which depends on three fac- tors [35]: 1. The amorphous nature of the surface. 2. The extent of internal stresses. 3. The extent of phosphorous in the deposited film. In general, the corrosion resistance of a surface film depends on the formation speed of the film and its protective nature. Some alloying elements can decrease the corrosion current by promoting anodic and/or cathodic reactions during the corrosion process [36]. Copper improves the corrosion resistance of Ni–P coatings provided that the coatings thickness has reached a certain range [37] To confirm the potentiostatic polarization results, EIS investigations of all specimens were carried out. For comparison between anodized Al and Al-alloys and the Ni–Cu–P modified anodized electrodes, the impedance spectra were Figure 3: Bode plots of anodized Al, Al2014 and Al7075 after 3 h immersion in naturally aerated stagnant 0.5 M Na2SO4 aqueous solution at 25 ∘ C.
  • 10. 10 | W. A. Badawy et al. Figure 4: Bode plots of Ni–Cu–P modified anodized Al, Al2014 and Al7075 after 3 h immersion in naturally aerated stagnant 0.5 M Na2SO4 aqueous solution at 25 ∘ C. recorded after 3 h of electrode immersion in the electrolyte and presented as Bode plots in Figure 3 for the anodized electrodes and in Figure 4 for the Ni–Cu–P modified electrodes under the same conditions. In general, anodization of Al or Al-alloys in sulfuric acid produces anodic films of duplex nature, which con- sist of a thin non-porous compact layer known as the barrier layer and an outer porous layer [16, 29]. The presence of porous structures, surface inhomogeneity and roughness factors leads to frequency dispersion and enables formation of nano-Ni–Cu–P deposits. The advantage of EIS is that it shows relaxation phe- nomena over a wide frequency range. The Bode plots of Figures 3 and 4 show one clear phase maximum at high frequency and an indication of another phase max- imum at lower frequency. This means that the electrode/electrolyte interface is controlled by two time constants. The impedance data were fitted to theoretical data according to the equivalent circuit model presented in Figure 5 in which two parallel combination terms are used, the first is the charge transfer resistance, 𝑅ct, and the double layer capacitance, 𝐶dl, and the second is the barrier film resis- tance, 𝑅b, and the barrier film capacitance, 𝐶b, in series to the solution resistance, 𝑅s [38]. The experimental impedance data are consistent with the data obtained according to the proposed model. The impedance behavior of the systems under investigation can be represented by the dispersion formula, which is given by the
  • 11. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 11 Figure 5: Equivalent circuit model used for fitting of impedance data of anodized and modified anodized Al, Al2014 and Al7075. 𝑅s = solution resistance, 𝑅ct = charge transfer resistance, 𝐶dl = double layer capacitance, 𝑅b = barrier film resistance and 𝐶b = barrier film capacitance. following equation: 𝑍 = 𝑅s + [𝑅ct/{1 + (2𝜋𝑓𝑅ct 𝐶dl) 𝛼 }] (1) where 𝑍 is the total impedance and 𝛼 is an empirical parameter (0 < 𝛼 < 1) and f is the frequency in Hz. The above relation takes into account the deviation from the ideal RC-behavior in terms of distribution of time constants due to surface inho- mogeneity, roughness effects and variations in properties or compositions of sur- face film [39, 40]. The calculated impedance parameters of the modified anodized materials after 3.0 h immersion in stagnant naturally aerated 0.5 M Na2SO4 at 25 ∘ C according to the fitting procedure are presented in Table 4. These values show clearly that the corrosion resistance of the modified anodized Al surface is more than 10 times that of the modified anodized alloys. This can be attributed to the galvanic interactions between the micro-structural constituents of the al- loy [41, 42]. Corrosion resistance of the anodized films of duplex nature is repre- sented by the sum of the resistances of the inner and outer layers and also the solution resistance. In most cases the barrier film resistance is much larger than the others and hence it controls the corrosion process. The impedance data of the Ni–Cu–P modified anodized alloy elec- trodes show that the phase angle has two maxima, which means that the Table 4: Equivalent circuit parameters after 3 h of immersion of the modified anodized Al and Al-alloys in 0.5 M Na2SO4 at 25 ∘ C. Modified anodized 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/ sample Ω μF cm−2 Ω cm2 μF cm−2 Ω cm2 Al 62.6 52.5 0.85 72.0 8.8 0.99 318 Al2014 75 47.5 0.61 42.2 82.5 0.71 31 Al7075 101 39.7 0.79 24.2 40.8 0.58 300
  • 12. 12 | W. A. Badawy et al. Table 5: Equivalent circuit parameters at different time intervals of immersion in stagnant naturally aerated 0.5 M Na2SO4 at 25 ∘ C for: (a) Ni–Cu–P/ anodized Al (a) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/ Time/min Ω μF cm−2 Ω cm2 μF cm−2 kΩ cm2 5 62.0 64.2 0.81 63.8 35.8 0.87 0.91 30 62.0 56.7 0.79 80 20.0 0.99 1.26 60 62.5 54.2 0.84 78.1 18.9 0.99 1.90 120 61.7 51.6 0.81 66.5 12.5 0.99 2.50 180 62.6 52.5 0.85 72.0 8.8 0.99 3.20 (b) Ni–Cu–P/ anodized Al2014 (b) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/ Time/min Ω μF cm−2 Ω cm2 μF cm−2 kΩ cm2 5 76.5 51.7 0.75 33.4 150 0.59 0.12 30 77 50.8 0.59 29.4 125 0.74 0.37 60 79 56.7 0.61 40.3 84.2 0.72 0.37 120 75 53.3 0.61 41.9 82.5 0.71 0.32 180 75 47.5 0.61 42.2 82.5 0.71 0.31 (c) Ni–Cu–P/ anodized Al7075 (c) 𝑅s/ 𝐶b/ 𝛼1 𝑅b/ 𝐶p/ 𝛼2 𝑅p/ Time/min Ω μF cm−2 Ω cm2 μF cm−2 kΩ cm2 5 93 50.8 0.95 14.3 91.7 0.68 0.20 30 91 55.8 0.93 11.6 66.7 0.58 0.17 60 95 54.1 0.81 18.2 53.3 0.53 0.18 120 101 48.3 0.81 15.1 39.2 0.50 0.2 180 101 39.7 0.79 24.2 40.8 0.58 0.3 electrode/electrolyte interface is controlled by two time constants. For modified anodized Al, the single phase maximum means that the porous layer proper- ties has high conductivity due to the inclusion of the electrolytic solution inside the pores [43]. The capacitance of the modified anodized alloys is higher than those recorded for modified anodized Al. This indicates the presence of thicker and rougher porous layer on the modified anodized alloy surfaces. The calcu- lated values of the empirical parameter 𝛼, for the modified anodized Al elec- trode approach1 indicating that the film is homogeneous and behaves like ideal capacitor. On the other hand, all 𝛼 values of the modified anodized alloys devi- ate from 1, which means that the formed film is defective and deviates from ideal RC behavior. The obtained impedance values are in good agreement with those calculated from polarization measurements. The values obtained for the corrosion resistance from both impedance and polarization measurements for Al2014 show that the deposition of Ni–Cu–P did not improve the surface protection, whereas the mod-
  • 13. Ni-Cu-P deposits on anodized Al, Al2014 and Al7075 | 13 Figure 6: SE micrograph of Ni–Cu–P modified anodized Al surface (a) and EDAX analysis of Ni–Cu–P modified anodized Al electrode (b).
  • 14. 14 | W. A. Badawy et al. Table 6: The composition of the Ni–Cu–P modified anodized Al after 3 h of electrode immersion in 0.5 M Na2SO4 at 25 ∘ C as obtained from EDAX analysis. Composition Ni K P K Cu K Atomic % 72.99 25.52 1.48 Weight % 82.89 15.29 1.82 ified anodized Al7075 was improved. This difference can be attributed to the al- loying elements, such as Zn which is used for activating the coating on the sur- face [21]. To investigate the stability of the deposited film, the effect of immersion time on the different Ni–Cu–P modified anodized samples in sulfate solution was studied by EIS. The values of the fitting parameters calculated at different time intervals from electrode immersion in the electrolyte according to the equivalent circuitmodelof Figure 5arepresented inTable 5. Itisclear thatthecorrosionresis- tance of the anodized surface increases with increasing the immersion time. This can be attributed to an improvement in the protective properties of the barrier film with ageing [4]. The difference in behavior between Al, Al2014 and Al7075 can be attributed to the difference in the surface morphology presented in Figure 1. While the SE micrograph of modified anodized Al is homogeneous and smooth, the anodized alloy surfaces are porous and contain voids and flawed regions (cf. Figure 1). In either case the deposition of the nano-Ni–Cu–P deposit takes place successfully. A compact defect–free Ni–Cu–P deposit on anodized Al without distinct cavities and crevices, was recorded and presented in the SE micrograph of Figure 6a. The appearance of cauliflower–like nodule is typical of amorphous materials [44]. The composition of surface film was confirmed by EDAX and presented in Figure 6b. The EDAX data analysis is presented in Table 6. Figure 6b and Table 6 indicate that the anodic film is completely covered by Ni, P and Cu and there is no appearance for any Al on the surface. The presence of copper in the plating bath enhances the full surface coverage by Ni–Cu–P de- posit. The good dispersion of Ni–Cu–P deposit over the anodized surfaces may account for its high corrosion resistance.
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