The Online Journal on Mathematics and Statistics (OJMS)
Vol. (1) – No. (1)
Study of Electroless Ni-P Plating
on Stainless Steel
I. E. Ayoub
Hot Laboratory Center, Atomic Energy Authority, P. O. 13759, Cairo, Egypt.
Abstract- Electroless Ni-P plating treatment was applied
to stainless steel as the substrate for improving its
corrosion resistance and micro-hardness. In this research
a layer of Ni-P was deposited on stainless steel by using
sodium hypophosphite as reducing agent. In order to
determine the properties of the deposited layers, corrosion
resistance of the Ni-P coatings was evaluated by
potentiodynamic polarization through immersing in 3 %
NaCl solution. A microhardness tester was used for
microhardness measurement, XRD for microstructure
analysis, and a scanning electron microscope equipped
with EDAX for determining morphology and analyzing
the deposits. The results showed that corrosion resistance
and hardness of the stainless steel were improved after
electroless plating.
Keywordselectroless,
plating,
microstructure, Ni-P, stainless steel.
corrosion,
I. INTRODUCTION
Normally, stainless steel is resistant to surface attack in
mildly corrosive environments. When corrosion does occur,
pits forms on the surface or within details of the component.
What can be done to prevent this? Electroless nickel plating is
the process of applying film on stainless steel to offers
superior protection from corrosion and oxidation. Electroless
nickel plating also provides moderate abrasion resistance,
high wear and hardness resistance [1-4]. Electroless nickel
plating produces uniformly plating specific areas which has
correct tolerance specification and do not require any
machining rework after plating [5,6].
Electroless deposition technique of Ni-P [7-9] alloy
coatings has been a well- known commercial process that has
found numerous applications in many fields due to excellent
properties of coatings, such as high corrosion resistance, high
wear-resistant, good lubricity, high hardness and acceptable
ductility [10,11].
Low-phosphorous deposits (1 to 3 % P) are crystalline and
exhibit good wear resistance, but relatively poor corrosion
resistance in a chloride environment. Medium-phosphorus
deposits (5 to 8 % P) have a smaller crystalline size and tend
to be semi-amorphous, whereas high- phosphorus deposits
(more than 10 percent P) exist mainly as a metallic glass.
Medium- phosphorus deposits usually have properties lying
between the low and high - phosphorus deposits, but the
solution offer enhanced deposition rates. High - phosphorus
deposits typically exhibit the best corrosion properties, but
suffer from slow rates of deposition [12]. Moreover, in order
to improve further the properties (mechanical and chemicals)
Ref er en ce Num b e r: W10-0028
of EN coatings, numerous EN composite coatings [13] are
investigated.
II. EXPERIMENTAL DETAILS
Coating on stainless steel is very difficult because the
problem in activation of that surface. The pre-treat mental
processes play a very important role in getting a good
protective coating on stainless steel. Successful deposition
depends on the removal of thin, passivating surface oxides
layer on the stainless steel. This was accomplished by an
initial cleaning [14].
Samples of stainless steel were used as the substrate
material for deposition of electroless Ni-P. First the surface of
the samples were mechanically cleaned from corrosion
products and then degreased with a detergent solution were
then rinsed in distilled water, and then etching in 10 ml
HNO3, 20 ml HCL and 30 ml H2O at room temperature for 2
min. After these operations, they were rinsed in distilled
water. The samples were activated in 0.6 g/l PdCl2 +5 ml
HCL at room temperature for 10 min. After these operations,
they were rinsed in distilled water in ultrasonic cleaning for 1
min and finally the samples dealing with 50 g/l NaH2PO2 for
10 min and then wash with distilled water and then
introduced in electroless plating Ni-P bath. The chemical
composition of electroless Ni-P coating bath is given in Table
1. A scanning electron microscope (SEM, JEOL Model JSM5400) was employed for the observations of the Ni-P alloy
coating and an EDAX attachment was used for qualitative
elemental chemical analysis and crystallographic structure of
the sample was studied by the X-ray difractometer (XRD).
The electroless Ni-P plating was heat-treatment film from
100 to 500 °C in an electric convection. The specimens were
maintained for 1 hr at the setting temperature and allowed to
be coaled.
The corrosion resistance of the deposited was measured by
using computerized auto lab system PG state 30 (potentiostatgalvanostat) electrochemical tests, in electrolyte used was 3
% sodium chloride. The working electrode was electroless
nickel Ni-P deposited on stainless steel with 3 cm2 surface
area. The auxiliary electrode was a platinum electrode and a
saturated calomel electrode (SCE) used as a reference
electrode and the scan rate 1mV/s.
III. RESULTS AND DISCUSSION
3.1 Surface morphology and elemental analysis
The properties of electroless Ni-P deposits are attributed to
their microstructure characteristics. The details of structure of
EN deposits are not well understood but as plated EN films
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The Online Journal on Mathematics and Statistics (OJMS)
has been reported to be either crystalline, amorphous or a
coexistence of both. Studying the microstructure of the
deposits helps us to understand the mechanism of deposition
and evaluate the properties of EN deposits. The electroless
Ni-P coatings were practically identical, with an average
phosphorus content of 8.57 % determined by using EDX
analysis as shown in Table 2 and thickness 13 μm for 1hr.
The scanning electron micrograph of electroless Ni-P
coating is shown in fig.1. It seems that the surface of the as
deposited samples was uniformly and continuously covered
by precipitates the electroless Ni–P deposition, and there were
no obvious flaws or apertures on the coating surface.
3.2 X-ray diffraction analysis
Figure 2 shows the X-ray diffraction (XRD) patterns of
electroless Ni-P coatings of as plated. The as plated have one
abroad peak of Ni at 2Ө = 44.5255 and exhibited more
diffused peaks revealing that the plate contains mixture of
microcrystalline and amorphous phases du to the plate have
medium phosphorous and the phosphorus was dissolved in
the nickel matrix [15]and exhibited a [1 1 1] preferred
orientation. After annealing at 400 °C for 1h, as shown in fig.
3 the X- ray diffraction, indicate high crystalline structure of
free nickel at 2Ө = 43.6453 with preferred orientation of [1 1
1] and d- spacing 2.07217 (A°), good crystalline structure of
Ni3P and Ni12P5 which have low crystalline.
3.3 Corrosion resistance test
NaCl is one of the most common corrosion medium for the
electrochemical tests. It can significantly accelerate the pitting
corrosion of Ni-P coatings in the artificial solution. The Cl
easily replace oxygen in water molecules to prefer initially
adsorb on the Ni-P coatings surface and form soluble NiCl2
(Ni +2 + 2Cl2 ←→ NiCl2) [14].
Electroless Ni–P coating could exhibit some voids.
Chloride ions can penetrate through these voids causing
pitting corrosion. Therefore the Ni–P coating should be
expected much more compact and flawless [16].
Figure 3,4 shows the potentiodynamic polarization curves
for electroless Ni-P coating of as plated and heat-treated at
400°C in 3% NaCl solution respectively. The electrochemical
corrosion data could be obtained based on the
potentiodynamic polarization curves, which are showed in
Table 1.
For the heat-treatment sample the corrosion potential Ecorr
shifts positively 28 mV than that of the as plated – 409 mV.
And the corrosion current density Icorr after plating is much
lower than that of the substrate. These results explain that the
heat-treated samples have better corrosion resistance and
protection characterization than the as plated.
3-4 Effect of heat-treatment on hardness resistance
The effect of heat treatment over the temperature range of
100 to 500°C for 1 hour on the microhardness of the coating
is shown in fig. 6. It seems that heat treatment at temperature
range from 100 to 200°C improve the adhesion of the
deposits on the substrate only, and a few changes on their
Ref er en ce Num b e r: W10-0028
Vol. (1) – No. (1)
hardness but heat treatment at temperature above 200°C
enhanced the microhardness of the coating up to 400°C, as a
result of hard compounds of Ni3P and Ni12P5 phase formation
in deposit [17] then microhardness decrease with increasing
heat treatment temperature up to 500°C.
Table 1. Chemical composition and deposition condition for
Ni-P coatings.
Composition
NiSo4-6H2O
Na3C6H5O7
NaOOCCH3 - 3H2O
NaH2PO2-H2O`
pH
Temp
Condition
35
g/l
15
g/l
5
g/l
20
g/l
4-5
80-90 °C
Table 2. Chemical composition of electroless Ni-P deposits.
Bath
Ni-P
Composition
Ni
P
Wt%
91.25
8.57
Atomic%
86.8
13.2
Table 3 Corrosion characteristics of the electroless Ni-P
alloys in 3 % NaCl.
Results
I corrosion
Rp ( Polarization
resistance )
E corr.
Corrosion rate
Sample area
Condition
7.069E-6 A/cm2
2.07E+4 Ohm
-0.381
V
8.919E-2 mm/year
3
cm2
Figure (1). Scanning electron micrographs of the electroless
Ni-P as plated
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The Online Journal on Mathematics and Statistics (OJMS)
Vol. (1) – No. (1)
800
700
Figure (2) X-ray diffraction pattern of Ni-P coating
As plated
Microhardness HV
600
500
400
300
200
100
0
0
100 200 300 400 500 600
o
Temperature ( C)
Figure (6). Microhardness of electroless Ni-P coating
vs. heat-treated temperature.
IV. CONCLUSION
Figure (3) X-ray diffraction pattern of electroless Ni-P in
heat-treated (400°).
Figure (4). Polarization curve obtained from electroless
Ni-P coating as plated.
1- Ni-P coating can be successfully plated on the surface of
stainless steel. The coating was compact, uniform and
showed mixture microstructure of amorphous and
microcrystalline with phosphorus contents of 8.57 wt.%.
2- The corrosion resistance of Ni-P on stainless is greatly
improved by heat-treatment which were tested by the
immersion experiment and the potentiodynamic
polarization experiment in 3 % NaCl solution.
3- The microhardness of the coating was increasing with
increased the heat treatment temperature for a constant
time (1hr), maximum hardness was achieved at 400°C.
Increase in microhardness after heat treatment is related to
formation of hard compound Ni3P and Ni12P5.
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