-
ss
um
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eny
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17
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Z9
compared and the bonding strength of the EN plating to the
substrate was also measured. The results showed that the EN plating
could easily take
place on the intermediate catalytic layer, directly on which a
smooth and compact NiP alloy layer without obvious flaws, about 20
mm thickness,
ity, and good solderability [8], as well as that other
metallic
become a self-sustaining process only if the so called Pd-
which could provide a good autocatalytic function for EN
plating and a seal function for anodized film. This
novel process achieved acceptable bonding strength and
Available online at www.sciencedirect.com
Applied Surface Science 254 (industries [1]. This makes
protective surface treatment an
essential and practicable part of the manufacturing process
for Mg alloys. Among various surface treatments, anodic
oxidization, or plasma electrolytic oxidation (PEO), was
widely adopted to enhance the corrosion resistance of Mg
alloys [25]. But a single anodized film generally was not
adequate for protection of magnesium alloys. So electroless
nickel (EN) plating was adopted as a protective top layer on
the anodized film [6,7], due to its special advantages of
EN coatings, such as uniform deposition, good corrosion
and wear resistance, good electrical and thermal conductiv-
strictly prolix operation process and a high material cost
[10].
It was reported that a Pd-activation process was also
adopted
for electroless plating of NiB alloy on TiB2 powders to
improve its spraying performance [11]. However EN plating of
TiB2 powders was found to be an autocatalytic process in our
study. The fact clearly implies that TiB2 may act as a
catalyst
for the EN plating process.
Therefore, this paper dealt with a novel palladium-free
activation process for EN plating on anodized magnesium
alloy with a thin intermediate layer containing TiB2
powders,such as high specific strength and stiffness, but the
poor
corrosion resistance restricted their large-scale application
inactivation technique is adopted [6,7]. However, the Pd-
activation technique was inconvenient in practice due to
theMagnesium alloys have a number of desirable properties,2
of about 11 MPa between the substrate and the catalytic layer.
An obvious passivation range and higher Ecorr (0.323 V) for the EN
plating duringanodic polarization in 3.5 wt.% NaCl solution,
implied a typical character of a compact NiP alloy layer, with an
effective protection for the
substrate.
# 2008 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy; Electroless nickel plating;
Palladium-free activation; Catalytic; Corrosion resistance
1. Introduction plating might further be deposited onto the EN
plating
[9]. Traditionally an EN plating on an anodized film maywas
successfully deposited. The catalytic function was principally from
TiB powder. The adhesive tensile test indicated a good bonding
strengthA novel process for electrole
magnesi
Shuo Sun a,b, Jianguo Liu a,*,a State Key Laboratory for
Corrosion and Protection, In
62 Wencui Road, Shb School of Science, Shenyang Universit
Received 22 November 2007; received in revise
Available online
Abstract
In this paper, a novel palladium-free activation electroless
nickel (E
was used as catalyst, was introduced for anodized magnesium
alloy A* Corresponding author. Tel.: +86 24 23921875; fax: +86 24
23893624.
E-mail address: [email protected] (J. Liu).
0169-4332/$ see front matter # 2008 Elsevier B.V. All rights
reserved.doi:10.1016/j.apsusc.2008.01.169nickel plating on
anodized
alloy
huanwei Yan a, Fuhui Wang a
ute of Metal Research, Chinese Academy of Sciences,
ang 110016, China
f Technology, Shenyang 110023, China
rm 30 January 2008; accepted 30 January 2008
February 2008
plating process, by which a TiB2 powders contained intermediate
film
1D. The corrosion behavior of AZ91D without and with coating
was
www.elsevier.com/locate/apsusc
2008) 50165022corrosion resistance of a composite coating for
the Mg alloy
substrate.
-
2. Experiment
2.1. Specimen preparation
Specimens were cut from die cast magnesium alloy AZ91D
with a size of 25 mm 25 mm 10 mm. The specific compo-sition of
the test materials was listed in Table 1. A hole of 2 mm
in diameter was drilled in the middle of one edge of each
Table 1
Nominal composition of AZ91D (in wt.%)
Al 8.59.5
Zn 0.500.90
Cu
-
environments. The major elements of the anodizing film,
Fig. 1. Schematic illustration of adhesiv
S. Sun et al. / Applied Surface Sc5018integrating the component
of anodized bath, were P, F, Na, Mg,
Cr and O (Fig. 2(b)). A rough surface of the anodized
filmwould
be beneficial for the adhesion with the catalytic film
followed.Fig. 2. SEM image (a) and EDX spectrum (b) for anodized
film on Mg alloy.The average size of TiB2 powders used in the
experiment
was about 5 mm (Fig. 3). The adoption of TiB2 as the mainactive
source was based upon its autocatalysis for EN
plating. In fact, when the powders were put into the EN
plating bath, a quick reaction of NiP deposition was
observed.
e tensile test for bonding strength.
ience 254 (2008) 50165022The intermediate catalytic film
consisted of TiB2 powders
and organic adhesive was designed to apply onto the anodized
film. The catalytic layer presented a rough surface (Fig. 4a
and
b), which was helpful for a good adhesion with NiP layer
followed. The element of C, O and Si were primarily from the
organic adhesive (Fig. 4(c)). Some TiB2 powders, or parts of
the
single TiB2 particle, were exposed on the surface of the
catalytic film to act as actively catalytic sites for nucleation
of
EN plating.
At the initial stage of EN plating for 04 min, a nucleation
process was actually occurred (Fig. 5). Some small granule
particles with Ni and P (Fig. 5(c)) deposited on the
catalytic
film surface after 4 min EN plating (Fig. 5(a)). The
nucleation
process was preferentially initiated on the exposed powders
(Fig. 5(b)). Those regions, with powders enwrapped by epoxy
adhesive, had no grains deposition (bigger squared area B,
Fig. 5(d)).
After 20 min of EN plating, the growth of NiP deposit film
was clearly observed (Fig. 6). The NiP film extended both
vertically and laterally on the whole surface of the
catalytic
intermediate film. Consequently, a compact and defect-free
EN
plating layer without distinct cavities and crevices, was
obtained after 120 min (Fig. 7(a)). The appearance of cauli-
flower-like nodule was typical of amorphous materials [13].
The outer layer was verified as the NiP coating containing
9.6
mass% phosphor (Fig. 7(b)).
-
The cross-sectional morphology of the NiP film/inter-
mediate catalytic film/AZ91D system after 120 min EN plating
was shown in Fig. 8, where no obvious defects could be
observed in the substrate, catalytic film or EN plating as well
as
their interfaces. The thickness of the EN plating was about
20 mm. The deposition rate 10 mm/h of the EN plating wasclose to
that of the traditional EN plating [14], but somewhat
slower than that of the Pd-activation process on PEO film
(about 25 mm/h [6] and 18 mm/h [7]). The reason might be
thatthere was no enough TiB2 powders exposed to the surface of
the
intermediate film in the present case. A teeth-like
interface
between catalytic film and NiP layer suggested a good
adhesion between them. The adhesive tensile test showed a
bonding strength about 11 MPa for the coating to the
substrate
and the detachment was occurred at the interface of anodized
film with the AZ91D substrate. So the bonding strength
between Ni and P layer and intermediate catalytic film was
higher than 11 MPa.
It was reported that the autocatalytic reaction for nickel
deposition on metal substrate (such as Ni) or Pd-active
system
was initiated by catalytic dehydrogenation of the reducing
agent. The reactions taking place were as follows [1316]:
H2PO2 H2O !
catalyst heatH HPO23 2Habs (1)
Ni2 2Habs ! Ni 2H (2)H2PO2
Habs ! H2O OH P (3)Then the NiP layer acted as catalytic sites
to absorb
hydrogen atoms for the subsequent EN plating. Thus the
Fig. 3. SEM image of TiB2 powders.
S. Sun et al. / Applied Surface Science 254 (2008) 50165022
5019Fig. 4. SEM images of showing surface morphology of a specimen
with catalytic film before EN plating (a) and (b) and corresponding
EDX spectrum (c).
-
S. Sun et al. / Applied Surface Sc5020catalysis function of the
TiB2 powders might be similar to that
of Ni- or Pd-active system. TiB2 powders could provide high
density nucleation sites for the subsequent EN plating with
enough exposed powders surface. But the real catalytic
mechanism of TiB2, such as the influence of potential range
or
the inducement of element, is still under study and will be
published in the following paper.
Fig. 6. SEM images of the specimen after 20 min EN plating.
Fig. 5. SEM images of showing morphology of specimen after 4 min
EN plaience 254 (2008) 50165022Fig. 9 showed the potentiodynamic
polarization curves of
the bare AZ91D magnesium alloy and the AZ91D alloy with
anodized film, catalytic layer and EN plating, respectively.
The
parameters of the potentiodynamic polarization were summar-
ized in Table 5. From Fig. 9, it was observed that the
corrosion
current density (Icorr) decreased by one order of magnitude
for
the specimen after anodic oxidization. The corrosion
potential
(Ecorr) of the anodized film showed little shift to the
positive
direction in comparison with that of the bare AZ91D. So the
corrosion resistance of the AZ91D alloy was improved to some
extent by anodic oxidization.
When the anodic oxide film was coated with catalytic layer,
the corrosion potential shifted further to more positive
(0.505 V) and the corrosion current density (Icorr)
decreasedsignificantly. This probably resulted from the formation
of a
dense insulated barrier layer by intermediate catalytic film
treatment. After EN plating, NiP layer showed a much higher
corrosion potential (0.323 V), which was close to that of
thetraditional and Pd-activation process [6,7]. Meanwhile, the
corrosion current density dropped down to about 107 A
cm2,indicating less porosity and better corrosion resistance of
the
composite coating system. This value was also close to that
of
Pd-activation process. The passivation rangewas about 400 mV
ting (a) and (b) and corresponding EDX spectrum (c) area A (d)
area B.
-
S. Sun et al. / Applied Surface Sc(from 0 to 0.4 V), which was
different from the results of
literatures [6,7]. There was no obvious passivation in Ref.
[6]
but an about 800 mV passivation range existed in Ref. [7].
This
difference may be due to the different characteristic of
substrate
PEO film. So, from these parameters, an effective protection
of
Fig. 7. SEM image (a) and corresponding EDX spectrum (b) of
specimen after
120 min EN plating.
Fig. 8. Cross-sectional micrograph of the specimen after EN
plating for
120 min (back scattering electron image).Fig. 9. Potentiodynamic
polarization curves of different specimens in 3.5%
NaCl solution.
Table 5
Parameters of potentiodynamic polarization tests of different
specimens in 3.5%
NaCl solution
Sample Ecorr (V) Icorr (A cm2) Rp (V cm
2)
Bare (AZ91D) 1.564 1.765 104 1.44 102Anodized film 1.530 1.658
105 1.57 103Catalytic layer 0.505 2.13 106 1.22 104EN plating 0.323
2.72 107 9.61 104
ience 254 (2008) 50165022 5021the composite coating system could
be expected for Mg alloy
substrate.
4. Conclusions
A novel palladium-free activation EN plating process was
established on the anodized magnesium alloy. The EN plating
obtained was compact and no obvious flaws. TiB2 powders
showed an autocatalysis function for EN plating. The
deposition rate, 10 mm/h, of the new process was close tothat of
the traditional process but somewhat lower than that of
Pd-activation process on PEO film. The corrosion current
density was about 107 A cm2 and the corrosion potential was0.323
V for the new coating systems. Together with theobserved
passivation range in the potentiodynamic polarization
curves, the electrochemical parameters revealed an effective
protectiveness of the coating system for Mg alloy. The
relatively good bonding strength about 11 MPa between the
substrate and the catalytic layer suggested a further
application
in practical industry. This process also provided a new
method
for EN plating on the surface of high reactive metal as well as
of
inert materials such as plastic.
Acknowledgments
This work supported by the National Basic Research
Program of China and the National High-Tech Research and
-
Development Program of China (No. 2006AA03Z538). Thanks
to Prof. Wu for the revision of the paper.
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S. Sun et al. / Applied Surface Science 254 (2008)
501650225022
A novel process for electroless nickel plating on anodized
magnesium alloyIntroductionExperimentSpecimen
preparationAnodizingCatalytic layer applicationElectroless
platingAnalysis methods
Results and discussionConclusionsAcknowledgmentsReferences