Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 2135–2143
High corrosion resistance of electroless composite
plating coatings on AZ91D magnesium alloys
Y.W. Song, D.Y. Shan, E.H. Han ∗
Environmental Corrosion Center, Institute of Metal Research, The Chinese Academy
of Sciences, Shenyang 110016, China
Received 9 July 2007; received in revised form 11 September 2007; accepted 11 September 2007
Available online 22 September 2007
Abstract
The process of electroless plating Ni–P on AZ91D magnesium alloys was improved. The Ni–P–ZrO2 composite coatings and multilayer coatings
were investigated based on the new electroless plating process. The coatings surface and cross-section morphologies were observed with scanning
electron microscopy (SEM). The chemical compositions were analyzed by EDXS. The corrosion behaviors were evaluated by immersion, salt spray
and electrochemical tests. The experimental results indicated that the Ni–P–ZrO2 composite coatings suffered attack in NaCl solution but displayed
passivation characteristics in NaOH and Na2 SO4 solutions. The corrosion resistance of Ni–P–ZrO2 coatings was superior to Ni–P coatings due
to the effect of ZrO2 nano-particle. The multilayer coatings consisting of Ni–P–ZrO2 /electroplating nickel/Ni–P (from substrate to surface) can
protect magnesium alloys from corroding more than 1000 h for the salt spray test.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Corrosion behaviors; Electroless plating; Composite coatings; Multilayer coatings; Magnesium alloys
1. Introduction
The ultra light metal of magnesium and magnesium alloys
are widely applied to the fields of automobile, electronic products, aerospace, etc. However, the surface of magnesium and
its alloys needs to be protected due to the poor corrosion
resistance and wear resistance. The effective protective measurements include chemical conversion, anodization, electroless
plating, PVD, laser cladding and so on [1–5]. Compared with
other protective measurements, the electroless plating technology can obtain coatings with excellent corrosion resistance,
wear resistance, conductibility and electromagnetic shielding
[6,7]. Electroless plating on magnesium alloys originated from
Dow company in the 1940s [8]. Recently, many improvements
had already been made [9–11]. Magnesium alloys are “difficult to plate metal” based on the following reasons [12]:
(1) MgO can be formed on the surface of magnesium alloys
to deteriorate the coatings adhesion; (2) the inhomogenous
microstructure consisting of the primary phase and the sec-
∗
Corresponding author. Tel.: +86 24 23915772; fax: +86 24 23894149.
E-mail address: [email protected] (E.H. Han).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.09.026
ond phase will result in the uneven coatings; (3) magnesium
alloys are susceptible to be corroded in the electroless plating solution of not containing chromate and fluoride and
(4) the coatings has to be pore-free due to the considerable potential difference between magnesium alloys substrate
and metal coatings. Thus, electroless plating on magnesium
alloys confronts serious challenge. Environmental friendly
process and perfect corrosion resistance are absolutely necessary.
Electroless composite plating technology was developed
from electroless plating. It refers to co-depositing the solid
particles together with the metal coatings to improve certain
properties [13,14]. Some researches [15,16] about the coatings
on steel substrates indicated that the corrosion resistance and
wear resistance of composite coatings were better than that of
electroless plating coatings owing to the dual properties of metal
coatings and solid particles. Therefore, the electroless composite coatings can be more competent for protecting magnesium
alloys in corrosive mediums.
However, in severe corrosive environments, single composite
coatings on magnesium alloys surface still meet some troubles. The magnesium alloys substrate will suffer attack by
galvanic corrosion when some pores penetrate the coatings. If the
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Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
Table 1
Chemical compositions of AZ91D magnesium alloys (wt%)
Al
Zn
Mn
Cu
Ni
Fe
Mg
9.14
0.46
0.26
0.002
<0.001
0.002
Balance
magnesium alloys are protected with multilayer coatings, it can
keep longer life time in strong corrosive mediums.
In this paper, Environmental friendly electroless composite
plating Ni–P–ZrO2 process was developed. Then, the multilayer
coatings consisting of composite coatings Ni–P–ZrO2 , electroplating nickel coatings and electroless plating Ni–P coatings
were investigated. Finally, the corrosion mechanisms were discussed.
2. Experimental
The experimental materials used for the investigation are
AZ91D magnesium alloys. The chemical compositions are given
in Table 1.
The surface of substrate materials was ground with 1000
grit SiC paper to ensure the same surface roughness, and then
ultrasonically cleaned in acetone.
The process of electroless composite plating consisted of
alkaline cleaning, activating, electroless plating Ni–P for 30 min
and electroless composite plating Ni–P–ZrO2 . The compositions and operation conditions of electroless plating Ni–P were
as follows: basic nickel carbonate 11 g/l, sodium hypophosphite 23 g/l, sodium citrate 12 g/l, potassium fluoride 8 g/l, buffer
agent 12.5 g/l, pH 6.0, temperature 80 ◦ C. Based on the above
electroless plating solution, an extra of 5 g/l ZrO2 (20 nm) nanoparticle and 2 mg/l stabilizer were added to the electroless
composite plating solution. The ZrO2 nano-particle was prepared as concentrated slurries by milling the ZrO2 mixed with
anion surfactants for 4–6 h using a planetary ball mill machine.
Then the ZrO2 concentrated slurries were mixed with plating
solution and ultrasonically stirred for 0.5–1 h. The plating solution was ultrasonically stirred during composite plating. The
most ultrasonic frequency is 40 kHz, and the percent of the most
frequency can adjust from 40% to 100%. The weight ratio of
ZrO2 nano-particle in the composite coatings was measured
using energy dispersive X-ray spectroscopy (EDXS). The final
result was the average value of five measurements.
The multilayer coatings were composed of Ni–P–ZrO2
(15 ␮m)/electroplating Ni (20 ␮m)/Ni–P (5 ␮m) (from substrate
to surface). The electroplating Ni coatings were obtained from
Watts solution. The compositions and operation conditions of
Watts solution were nickel sulphate 220 g/l, sodium chloride
12 g/l, boric acid 33 g/l, sodium sulphate 25 g/l, magnesium sulphate 35 g/l, room temperature, pH 5.0, cathodic current density
1.0 A/dm2 .
The surface and cross-section morphologies of coatings were
observed using Phillips XL30 scanning electron microscopy
(SEM). The chemical compositions were probed with energy
dispersive X-ray spectroscopy (EDXS).
The coating adhesion was evaluated according to
ASTMB571 standard (heat quenching test, 220 ◦ C aging
treatment for 1 h, then water quenching at room temperature).
Corrosion resistance was studied by immersion, salt spray
and electrochemical tests. For all of the tests, the thickness of
Ni–P, Ni–P–ZrO2 and multilayer coatings was 15, 15 and 40 ␮m,
respectively. The coating thickness was measured using SEM
observation. The required coatings thickness can be obtained by
adjusting the deposition time.
Electrochemical tests were carried out using a classical three
electrodes cell with platinum as counter electrode, saturated
calomel electrode SCE (+0.242 V versus SHE) as reference
electrode, and the samples with an exposed area of 1 cm2 as
working electrode. The potentiodynamic polarization curves
were obtained using a EG&G potentiostat model 273 at a constant voltage scan rate of 0.3 mV/s. Electrochemical impedance
spectroscopy (EIS) measurements were performed using a
model 5210 Lock in amplifier coupled with potentiostat model
273. The scan frequency ranged from 100,000 to 0.01 Hz, and
the perturbation amplitude was 5 mV. The corrosive mediums
of 3.5 wt% (be equivalent to 0.6 M) NaCl, 0.6 M Na2 SO4 and
0.6 M NaOH were used for the electrochemical tests.
Salt spray test was conducted according to ASTMB117 standard (5 wt% NaCl spray, 35 ◦ C). The time interval from the
beginning of tests to the presence of the first corrosion pit
(observed with eyes) was considered as the appraisement standard. In each case, three samples were tested, and the average
values were the final results.
Immersion test was done in accordance with China GB10124-88 standard (the ratio of sample surface (cm2 ) to the
volume of the solution (ml) was set to 1/20, 3.5 wt% NaCl, pH
7, room temperature).
3. Results and discussion
3.1. Electroless composite plating process
The microstructure of AZ91D magnesium alloys consists of
primary ␣ phase, eutectic ␣ phase and ␤ phase as shown in
Fig. 1. The ␤ phase contains higher percentage of aluminum
than that in ␣ phase. The aluminum concentration decreases
along the direction of away from the ␤ phase. As a result, the
electrochemical potential is different for the ␣ phase (−1.71 V
versus SHE) and ␤ phase (−1.0 V versus SHE) [17]. The inhomogeneous microstructure of AZ91D magnesium alloys has an
adverse effect on the electroless plating process. It can result in
the uneven coatings with poor corrosion resistance.
A new environmental friendly pretreatment and electroless
plating process were developed. Fig. 2(a) shows the surface morphology of AZ91D magnesium alloys after alkaline cleaning.
The substrate surface was smooth and flat. The ␣ phase and ␤
phase can be observed clearly. The process of alkaline cleaning can degrease and remove the corrosion products away from
the magnesium alloys surface, which is helpful to improve the
coating adhesion.
Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
Fig. 1. Microstructure of AZ91D magnesium alloys.
The process of activating has an important effect on the electroless plating. It can prevent magnesium alloys substrate from
corroding in the acidic plating solution. For the traditional activating process, hydrofluoric acid was used. The fluoride formed
on the ␣ phase and ␤ phase was inhomogeneous [18]. During
electroless plating, the nickel particles cannot be uniformly
deposited on the ␣ phase and ␤ phase. For the new activating
process, hydrofluoric acid was not used. A porous activating
film was formed as shown in Fig. 2(b). The ␣ phase and ␤ phase
cannot be found. The film made the surface of AZ91D magnesium alloys more uniform. The activating process can remove
the oxides away from the substrate surface. It can also decrease
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the electrochemical potential difference between ␣ phase and ␤
phase. Thus, nickel can be uniformly nucleated on the substrate
surface. The possible nucleation mechanism was described as
follows [19,20]: firstly, the surface film formed at the activating
process was porous and discrete. There were metal magnesium,
aluminum and zinc in the pores. However, these metals have not
catalysis properties as Fe, Co, Ni, etc. The reduction of nickel
by hypophosphite cannot be carried out. Thus, replacement
reactions can happen to produce metal nickel particles, such
as Mg + Ni2+ → Mg2+ + Ni, 2Al + 3Ni2+ → 2Al3+ + 3Ni and
Zn + Ni2+ → Zn2+ + Ni. Secondly, the precursor nickel particles
exhibited catalysis property. They can act as nucleating centers
to form Ni–P coatings in terms of autocatalysis reaction:
Ni2+ + 4H2 PO2 − + H2 O → Ni + 3H2 PO3 − + P + H+ + 3/2H2
[21]. The nickel particles grew in the direction of vertical and
level simultaneously. Finally, all of the nickel particles were
linked to cover the substrate surface. The nucleating centers
were uniformly dispersed in the activating films. Thus, the
Ni–P coatings were compact and continuous.
The surface morphology of electroless plating Ni–P for
30 min is shown in Fig. 2(c). The surface was covered with
regular round nodules. The dimension of these nodules was
smaller than 10 ␮m. With increasing of electroless plating time,
the diameter of nodules will increase.
Electroless composite plating Ni–P–ZrO2 was carried out
after electroless plating Ni–P for 30 min. Fig. 2(d) shows the
surface morphology of Ni–P–ZrO2 coatings. The weight ratio
of ZrO2 in the coatings was approximate 5 wt%. The surface of
Ni–P–ZrO2 composite coatings was rough with different dimensions of nodules. The boundaries of every nodule were tortuous.
Fig. 2. Surface morphologies of AZ91D magnesium alloys treated with different processes: (a) alkaline cleaning; (b) activating; (c) electroless plating Ni–P for
30 min; (d) electroless composite plating Ni–P–ZrO2 .
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Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
During electroless composite plating, nano-particle adsorbing
on the surface of magnesium alloys acted as nucleating centers, and then the deposition of nickel phosphor alloy will wrap
the nano-particle. Numerous nucleating centers were helpful to
decrease the dimension of nodules. Some of the nano-particle
happened to absorb at the nodules boundaries. Parts of the
boundaries will be covered with new nodules. As a result, the
nodules boundaries in Ni–P–ZrO2 coatings were tortuous [22].
The tortuous nodules boundaries were not the weak sites of the
coatings.
Electroless plating Ni–P for 30 min before composite plating
Ni–P–ZrO2 was necessary for improving the corrosion resistance. Fig. 3 shows the potentiodynamic curves of Ni–P–ZrO2
coatings with and without Ni–P as bottom layer. It was found
that the Ni–P bottom layer improved the corrosion potential
(Ecorr ) for approximate 40 mV and also reduced the corrosion
current density (icorr) . This result indicated that the corrosion
resistance of composite coatings was significantly enhanced
due to the existence of Ni–P bottom layer. If the electroless
composite plating was carried out following the activating treatment, some of nano-particle can absorb in the pores of the
activating film. The nucleating sites of metal Mg, Al and Zn
in the pores will be covered. Nickel particles cannot originate from these sites, and pore was easy to be formed in
there. Additional, the nano-particle preferentially absorbing on
the activating film sandwiched the magnesium alloys substrate
and Ni–P–ZrO2 coatings to deteriorate the adhesion. The high
porosity and bad adhesion were vital limitation to the corrosion resistance of Ni–P–ZrO2 coatings. Thus, the electroless
plating Ni–P coatings were applied to act as bottom layer.
After electroless plating for 30 min, magnesium alloys substrate
was completely covered with Ni–P coatings. The Ni–P coatings grew in light of autocatalysis reaction mechanism. Any
sites of the coatings can both act as nucleating centers. The
nano-particle absorbing on the surface of Ni–P coatings cannot
shield these nucleating centers. The Ni–P alloy will wrap the
nano-particle gradually. The nano-particle was sandwiched in
Ni–P alloy, which cannot deteriorate coating adhesion. Thus,
Fig. 3. Potentiodynamic curves of different composite coatings in 3.5 wt% NaCl.
Fig. 4. Effects of stirring methods on the weight ratio of ZrO2 in the coatings.
the improved composite coatings displayed perfect corrosion
resistance.
Stirring methods for composite plating had an important
effect on the coatings properties. Firstly, the nano-particle can
be uniformly suspended in the plating solution and adsorb on the
plated samples. Secondly, the hydrogen bubbles can be removed
away from the samples surface to decrease the porosity. Mechanical stirring and ultrasonic stirring were compared. Fig. 4 shows
the effects of stirring methods on the weight ratio of ZrO2 in the
composite coatings. The weight ratio of ZrO2 exceed at 5 wt% by
ultrasonic stirring, which was twice more than that of mechanical
stirring. The higher ZrO2 weight ratio was useful to improve the
performance of composite coatings including hardness, corrosion resistance and wear resistance [23]. Thus, ultrasonic stirring
was used for the electroless composite plating. However, the
ultrasonic intensity needs to be chosen cautiously as shown in
Fig. 5. The weight ratio of ZrO2 increased with increasing the
percent of the most ultrasonic frequency. It reached the maximum value at 60% of the most ultrasonic frequency. Then, the
weight ratio of ZrO2 decreased at higher ultrasonic intensity.
Fig. 5. Effects of ultrasonic intensity on the weight ratio of ZrO2 in the coatings.
Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
The mechanisms of ultrasonic stirring mainly base on that a
larger number of bubbles developing from “cavitation effect”
can form strong shock wave to break the “soft conglobation” of
nano-particle [24]. With higher ultrasonic intensity, more nanoparticle can be transported to the plated samples surface. Thus,
the weight ratio of ZrO2 in the composite coatings increased.
If the stirring was too intense, the nano-particle was not susceptible to adsorb at the plated samples surface stably. Under
severe condition, plating solution impacted the samples surface strongly. Some nano-particle without firmly embedded into
Ni–P alloy will fall off again. Fig. 6 shows the surface morphology of composite coatings with very strong ultrasonic stirring.
Many holes were observed on the coatings surface. As a result,
the higher ultrasonic intensity can increase the coatings porosity and decline the corrosion resistance. Therefore, ultrasonic
intensity is one of key factors to the coatings properties. Proper
ultrasonic intensity needs to be chosen according to the special
requirements.
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Fig. 7. Potentiodynamic curves of Ni–P–ZrO2 coatings in different corrosive
mediums.
3.2. Corrosion behaviors
The corrosion behaviors of Ni–P–ZrO2 coatings were
investigated by potentiodynamic curves. Fig. 7 shows the potentiodynamic curves of Ni–P–ZrO2 coatings in 3.5 wt% NaCl,
0.6 M NaOH and 0.6 M Na2 SO4 solutions. It was found that the
cathodic reaction was oxygen reduction under diffusion control
in three corrosive mediums. The anodic sides can provide more
information about the corrosion behaviors of Ni–P–ZrO2 coatings. In NaCl solution, the anodic side was controlled by active
dissolution reaction and the corrosion current density quickly
increased with the increasing of anodic potential. In NaOH and
Na2 SO4 solutions, passivation phenomena happened. The maintaining passivation current density (ip ) values were similar in
both solutions, but the passivation potential regions were different. In NaOH solution, the coatings can keep passivation state in
wider potential regions and the corrosion current density (icorr )
was lower. This result indicated that Na2 SO4 solution showed
severer attack to the composite coatings than the NaOH solution. Thus, it can conclude that the attack property of corrosive
Fig. 8. Potentiodynamic curves of AZ91D, Ni–P and Ni–P–ZrO2 coatings in
3.5 wt% NaCl.
mediums to the Ni–P–ZrO2 coatings increase in the sequence
of NaOH < Na2 SO4 < NaCl.
The corrosion behaviors of AZ91D magnesium alloys, Ni–P
and Ni–P–ZrO2 coatings in 3.5 wt% NaCl solution were studied
as shown in Fig. 8. The fitting results of potentiodynamic curves
are listed in Table 2. It needed to explain that the thickness of
Ni–P and Ni–P–ZrO2 coatings was almost at the same of 15 ␮m
and the Ni–P–ZrO2 coatings contained Ni–P as bottom layer. It
was found that the shapes of potentiodynamic curves were similar for both coatings—active reaction dominating the anodic
sides and oxygen reduction dominating the cathodic sides. But
Table 2
Fitting results of potentiodynamic curves in 3.5 wt% NaCl solution
Fig. 6. Surface morphology of composite coatings with intense ultrasonic stirring.
Ni–P
Ni–P–ZrO2
AZ91D
icorr (␮A/cm2 )
Ecorr (V)
Ba (mV)
Bc (mV)
Rp ( cm2 )
1.412
0.9674
133.7
−0.404
−0.361
−1.557
267.15
362.23
45.66
75.16
80.04
120.37
37397
61593
232.3
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Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
the Ni–P–ZrO2 coatings displayed slight more positive corrosion potential (Ecorr ) and lower corrosion current density (icorr )
than Ni–P coatings. It implied that the anodic dissolution reaction of Ni–P–ZrO2 coatings was restrained, which could be
correlated to the reduction of the active surface due to the
presence of inert ZrO2 particles. This result indicated that the
corrosion resistance of Ni–P–ZrO2 coatings was superior to
Ni–P coatings. Compared with the AZ91D substrate, the corrosion potential (Ecorr ) of both coatings positively shifted about
1200 mV and the corrosion current density (icorr ) lowered nearly
100 times. Thus, the Ni–P and Ni–P–ZrO2 coatings can provide
adequate protection to the magnesium alloys in NaCl corrosive
mediums.
The corrosion behaviors of Ni–P and Ni–P–ZrO2 coatings
were evaluated by immersion test and salt spray test again.
The time interval of the first corrosion pit on the coatings surface observed with eyes was as the evaluation standard. The
longer time interval indicated better corrosion resistance. For
the immersion test, the Ni–P coatings and Ni–P–ZrO2 coatings
can protect magnesium alloys from corroding for more than 200
and 400 h, respectively, which was twice more than the salt spray
test results.
Fig. 9 shows the corrosion morphologies of Ni–P coatings
immersed in 3.5 wt% NaCl solution for 215 h. The surface of
Ni–P coatings was covered with black corrosion products. The
corrosion products mainly concentrated at the boundaries of the
nodules. Fig. 10 shows the chemical compositions of the corrosion products analyzed using EDXS. The elements of nickel,
phosphorous, chlorine, oxide, sodium and magnesium were
included. It needed to emphasize that magnesium element was
found in the corrosion products. It indicated that the corrosion
pits already penetrated the Ni–P coatings.
The nodule boundaries were the weak sites of Ni–P coatings.
Cl− was a kind of strong adsorption anion and was easy to preferentially adsorb at the nodule boundaries. Then the dynamic
balance of Ni ↔ Ni2+ + 2e was broken and the soluble NiCl2
(Ni2+ + 2Cl− ↔ NiCl2 ) was produced. The corrosion of Ni–P
coatings initiated from the nodule boundaries. Blocked corrosion cells were formed under the corrosion products. These
Fig. 9. Corrosion morphology of Ni–P coatings immersed in 3.5 wt% NaCl for
215 h.
Fig. 10. Chemical compositions of the corrosion products on the Ni–P coatings.
corrosion pits grew in the light of autocatalysis reaction [25]
and can penetrate the coatings quickly.
However, there was completely different corrosion morphology for the Ni–P–ZrO2 composite coatings (Fig. 11). The
coatings surface appeared a large number of uniform corrosion
pits. There were not visible corrosion products around the corrosion pits. According to the EDXS results, only P, Ni, Zr and
O elements were detected around the corrosion pits. Magnesium element was not found. It indicated that the corrosion pits
did not reach the magnesium substrate. In order to know more
about the corrosion pits, the EIS plots of Ni–P–ZrO2 coatings
immersing in NaCl solution for different time intervals were
measured as shown in Fig. 12. It comprised only one capacitance loop for all of the scan frequency regions. With increasing
of immersion time, the diameter of capacitance loop shrank,
which indicated that the corrosion resistance of composite coatings became worse. After immersion for 15 days, the Nyquist
plot still displayed one capacitance loop. The signal of pitting
corrosion was not found according to the EIS plots.
To investigate the corrosion mechanisms further, the equivalent circuit for EIS tests is shown in Fig. 13. The capacitance loop
can be described with Rs , Rc and CPEc (Q). The plots were fitted
Fig. 11. Corrosion morphology of Ni–P–ZrO2 coatings immersed in 3.5 wt%
NaCl for 846 h.
Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
2141
Fig. 12. EIS plots of Ni–P–ZrO2 coatings with different immersion time.
Fig. 13. Equivalent circuit for EIS plots.
Table 3
Fitting results of EIS plots with different immersion time
1 day
2 days
4 days
7 days
10 days
15 days
Rs ( cm2 )
Q-Y0 (␮F cm−2 )
Q-n
Rc (k cm2 )
8.165
11.29
6.961
9.665
6.390
8.384
20.93
21.66
27.33
43.40
43.94
44.16
0.9346
0.9506
0.9537
0.9530
0.9689
0.9615
92.12
74.88
54.69
44.21
36.72
27.95
using zsimpwin software as listed in Table 3. Then the values
of capacitance (Q-Y0 ) and Rc with different immersion time are
illustrated in Fig. 14. With increasing of immersion time, the
Rc values reduced gradually. On the contrary, the capacitance
values increased at the beginning, and then trended to be stable.
When the Ni–P–ZrO2 composite coatings were immersed
in NaCl solution, Cl− tended to preferentially absorb on the
special sites of the coatings. The tortuous nodule boundaries
were not the weak sites of the coatings based on the description
of Fig. 2(d). Some nano-particle on the surface of composite
coatings was not covered by the Ni–P alloy completely. Cl−
will preferentially absorb at these special sites. Then the Ni–P
alloy around the nano-particle was corroded to form soluble
NiCl2 . As a result, the adhesion between nano-particle and Ni–P
alloy became worse. Some nano-particle can remove away from
the coatings surface, resulting in the presence of a large number of corrosion pits. These corrosion pits were flat and broad.
With increasing of immersion time, more and more nickel was
dissolved and the dimension of corrosion pits increased. The corrosion resistance of composite coatings reduced. Thus, the Rc
value decreased with increasing of immersion time. According
to the capacitance (Q-Y0 ) curve, the immersion of Ni–P–ZrO2
composite coatings included two phases [26]. Firstly, the region
with increasing of capacitance value was named as the early
phase of immersion. Then, the region with stable capacitance
value was named as the middle phase of immersion. The visible corrosion pits observed with eyes was the final phase of
immersion. Once the corrosion pits penetrate the coatings, serious galvanic corrosion will destroy the substrate quickly. The
EIS tests cannot be carried out again. Thus, the final phase of
immersion cannot be shown by the EIS plots. Taking account
of the middle phase of immersion, only one time constant was
found in the EIS plots. If the corrosive mediums already reached
the interface of coatings and substrate, corrosion micro-cell can
be formed on the interface. Correspondingly, two time constants
will be observed. Thus, it indicated that the corrosive mediums
did not reach the interface of coatings and substrate and the
corrosion pits did not penetrate the composite coatings.
3.3. Multilayer coating
Fig. 14. Changes of Rc and capacitance with different immersion time.
In strong corrosive environment, the single Ni–P–ZrO2
coatings cannot provide enough protection to the magnesium alloys substrate. The multilayer coatings were necessary.
Various multilayer coatings were compared. The multilayer
coatings consisting of Ni–P–ZrO2 (15 ␮m)/electroplating Ni
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Y.W. Song et al. / Electrochimica Acta 53 (2008) 2135–2143
Fig. 15. Surface and cross-section morphologies of multilayer coatings.
(20 ␮m)/Ni–P (5 ␮m) from substrate to surface displayed the
optimum protection property to the AZ91D. Fig. 15 shows
the surface and cross-section morphologies of multilayer coatings. The coatings appeared cauliflower surface with countless
smooth nodules. From the cross-section morphology, the coatings were compact and uniform. It was well adhered to the
magnesium alloys substrate. There were not obvious boundaries among three layers. According to the adhesion tests,
the phenomena of blisters and cracks were not found on
the coatings. Thus, the adhesion of multilayer coatings was
good.
The corrosion resistance of multilayer coatings was evaluated
by salt spray test. The magnesium alloys protected with multilayer coatings can keep undamaged for more than 1000 h. The
salt spray test results of Ni–P, Ni–P–ZrO2 and multilayer coatings are compared in Table 4. It was obvious that the multilayer
coatings showed the best corrosion protection property to the
magnesium alloys. The possible mechanisms were explained as
follows. The electrochemical potential of electroplating nickel
middle layer was higher approximate 100 mV than that of Ni–P
upper layer based on the potentiodynamic tests in Fig. 16. Thus,
the Ni–P layer was anodic in comparison with the electroplating
nickel layer. When the upper Ni–P layer suffered from attack,
some pores can penetrate the Ni–P layer and reach the electroplating nickel layer. Then the Ni–P layer went on to be corroded
as a sacrificial coating due to its lower corrosion potential (Ecorr ),
and the electroplating nickel layer was not attacked as the cathode. In some regions, the Ni–P upper layer was corroded out.
Then the electroplating nickel layer will act as the upper layer to
be exposed in the corrosive mediums. The electroplating nickel
layer continued to serve as the protective coating. If some corrosion pits penetrated the electroplating nickel layer again, the
compact Ni–P–ZrO2 bottom layer still can protect magnesium
alloys from corroding. Until the pores penetrated three protective screens and reached the magnesium alloys substrate, the
material was destroyed completely.
Table 4
Comparison of salt spray tests for different coatings
Coatings thickness (␮m)
Salt spray tests results (h)
Ni–P
Ni–P–ZrO2
Multilayer coatings
15
100
15
200
40
1000
Fig. 16. Potentiodynamic curves of Ni–P and electroplating Ni coatings in
3.5 wt% NaCl.
4. Conclusions
(1) The surface of AZ91D magnesium alloys was homogeneous
and porous after activating treatment. As a result, nickel can
uniformly nucleate in the activating film.
(2) The corrosion resistance of Ni–P–ZrO2 composite coatings
containing Ni–P bottom layer was improved due to the better
adhesion and lower porosity.
(3) More nano-particle can be co-deposited with Ni–P alloy
by ultrasonic stirring, but the ultrasonic intensity need be
chosen carefully.
(4) The Ni–P–ZrO2 composite coatings suffered attack in NaCl
solution, and displayed passivation characteristics in NaOH
and Na2 SO4 solutions.
(5) The corrosion resistance of Ni–P–ZrO2 composite coatings
was superior to Ni–P coatings, which was correlative with
the co-deposition of ZrO2 nano-particle.
(6) The multilayer coatings can protect magnesium alloys from
corroding more than 1000 h for the salt spray test.
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