Bildiriler Kitabı
TMMOB Metalurji ve Malzeme Mühendisleri Odası
Selim Taşçı, Reşat Can Özden, Mustafa Anık
Corrosion Properties of Ni-P-W / Al2O3 Electroless
Composite Coatings on AZ91 Magnesium Alloy
Eskişehir Osmangazi University - Türkiye
Abstract
2. Experimental Procedure
Electroless Ni-P-W, Ni-P-W / Al2O3 composite
coatings were applied on the hot-rolled, strip casted
AZ91(%9 Al, %1 Zn) magnesium plates and the
effect of applied coating on the corrosion resistance
was observed in wt. %3,5 NaCl chloride solution
using potentiodynamic and EIS (Electrochemical
Impedance Spectroscopy) techniques. Ni-P-W
coatings with different tungsten content were
obtained by adding the various amounts of NaWO4 to
the coating bath. Composite coatings were also
obtained by adding 2 gr/lt of Al2O3 to selected
containing
baths.
The
corrosion
NaWO4
characteristics of Ni-P-W electroless and Ni-P-W /
Al2O3 composite coatings on AZ91 magnesium alloy
is discussed in this article.
Twin roll strip casted AZ91 magnesium alloy sheets
were supplied from VIG Metal ± .WDK\D were used
as substrate. 30mmx15mmx 2mm sized sample
preparation and pre-treatments procedures were
explained elsewhere. [3]
Electrochemical experiments solutions were prepared
from doubly distilled water. Solutions were purged
with nitrogen before and during the experiments.
Surface area of the sample was 0,785 cm2.
Standard three electrode corrosion cell containing
working electrode, a reference electrode (SCE:
saturated calomel electrode) and platinum as a
counter electrode. Unless stated otherwise, all
measured potential values were versus SCE. [4]
1.Introduction
Magnesium alloys draw attention by their specific
strength and density. But the usage of magnesium
and its alloys are limited due to their high chemical
activities and vulnerability towards certain
environments. [1]
Among several surface modification techniques
Nickel ± Phosphorus electroless coatings come
forward as their high hardness and corrosion
resistance. This highly sufficient surface modification
technique can be homogenically applied to any
geometric surface without post operation. [2]
In this study, corrosion characteristics of twin roll
casted and Ni-P-W (Nickel - Phosphorus - Tungsten)
electroless, Ni-P-W/Al2O3 composite coated AZ91
magnesium alloys were investigated. Corrosion
experiments were carried out using Electrochemical
Impedance Spectroscopy (EIS) and Potentiodynamic
analysis technique in K2HPO4 buffered K2SO4
solution at pH7 with wt. %3,5 NaCl addition.
Changes in corrosion behavior of Ni-P-W electroless
coatings by the addition of Al2O3 were investigated.
Electrochemical Impedance Spectroscopy (EIS)
experiments were practiced by using Gamry
PC4/300mA Potentiostat/Galvanostat in 100 kHz ±
0.01 Hz frequency range and “ P9 potential with
the respect of corrosion potential (Ecorr).
Potentiodynamics polarization experiments were
FDUULHGRXWEHWZHHQWKHSRWHQWLDORI“9UDQJHRI
open circuit potential (Eoc). Experiment rate was 1
mv/sn. All corrosion measurements were analyzed in
wt %3,5 chloride solution.
Ni2+ + 2H2PO2- + 2OH- o Ni + 2 H2PO3- + H2
WO42-+6H2PO2-+4H2OoW+6H2PO3-+3H2+2OH2H2PO2- o 3ò+2 + H2PO3- + OHH2PO2- + H2O o H2 + H2PO3-
(1)
(2)
(3)
(4)
The basic mechanism of electroless Ni-P-W
deposition was explained through reactions (1) to (4).
[5] Three different NaWO4 amount (0,5 ± 2,5 ± 20
gr/lt) was selected to demonstrate the effect of W in
electroless nickel solutions. For investigating the
Al2O3 effect 2 gr/lt 10 micron particle was added to
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the solution. 100 mg/lt Chemguard S-550-100 nonionic fluorosurfactant was used to increase deposition
rate of particles. [6] Experimental codes of samples
were listed in Table 1.
Table 1. Experimental codes of applied coatings
Sample
Coating
NaWO4 ± 0,5 gr/lt
NiPW - 1
NaWO4 ± 2,5 gr/lt
NiPW - 2
NaWO4 ± 20 gr/lt
NiPW - 3
NaWO4 ± 0,5 gr/lt +
NiPW - Al - 1
Al2O3 ± 2gr/lt
NaWO4 ± 0,5 gr/lt +
NiPW - Al - 2
Al2O3 ± 2gr/lt
NaWO4 ± 0,5 gr/lt +
NiPW - Al- 3
Al2O3 ± 2gr/lt
Figure 1. SEM image of NiPW ± 2 coating
In electroless coating bath, basic nickel carbonate
was used as nickel ion source, sodium hypophosphate
as a reducing agent. Citric acid was used as a
complexing agent to prevent nickel salts to
precipitate, decrease the concentration of unattached
nickel ions and buffer the bath to avoid sudden pH
change. Besides, thiourea were used as a stabilizer to
avoid rapid decomposition. pH was set to 9 by using
ammonia. [3]
3.Results and Discussion
SEM images of NiPW ± 2 electroless coating and
NiPW-Al-2 composite coating were can be seen in
Figure 1 and Figure 2. Compositions of electroless
and composite coating was compared in Table 2.
Sample
NiPW - 1
NiPW - 2
NiPW - 3
NiPW ±
Al ± 2
Table 2. EDX data of coatings
Wt.%
Wt.%
Wt.%
Ni
P
W
87,5
9,1
3,4
84,3
9,6
6,1
86,0
4,5
9,5
89,2
5,0
3,2
Wt.%A
l
-
Figure 2. SEM image of NiPW - Al - 2 coating
Co-deposition of W up to a certain amount (Wt. %5)
does not have a negative effect on amorphous
characteristic of electroless coatings. Above this level
corrosion resistance is starting to decrease due to
crystalline structure and possible galvanic interaction
on newly emerged grain boundaries.
Results obtained by potentiodynamic polarization and
EIS also corrected this theory. Results of
potentiodynamic experiment are shown in Figure 3.
Numerical values of this experiment exhibited in
Table 3.
0,67
P element is the key parameter for defining the
character of electroless Ni-P based coatings.
Increasing the Wt.%P level above 7 resulted with an
amorphous coating characterization. This amorphous
behavior can be directly linked to the corrosion
resistance of Ni-P amorphous coatings. [7]
Wt. %. W is the one other thing to consider to define
the corrosion characterization of electroless coatings.
Tungsten is highly corrosion resistant material. [8]
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18 th International Metallurgy & Materials Congress
Figure 3. Potentiodynamic polarization results of
electroless and composite coatings
Bildiriler Kitabı
TMMOB Metalurji ve Malzeme Mühendisleri Odası
Table 3. Numerical values of Potentiodynamic
Polarization Analyze
Sample
ECor (VSCE)
NiPW - 1
NiPW - 2
NiPW - 3
NiPW - Al - 1
NiPW - Al - 2
NiPW - Al- 3
-0,705
-0,648
-0,654
-0,692
-0,715
-0,675
4.Conclusion
As a result, a correlation between W and P can be
stated to define corrosion resistance of Ni-P based
electroless coatings. Wt.%7 phosphorous content is
critical for the amorphous structure.
Icor
(A/cm2)
8,56x10-6
9,94x10-7
1,84x10-5
7,03x10-6
5,22x10-6
7,70x10-5
Co-depositon of W shows a positive effect up to
certain level. (~wt.%6) Exceeding this value resulted
with a decrease in corrosion resistance due to the
decrease in % amorphous structure.
Although potentiodynamic polarization resistance
seems to be close. Corrosion resistance differences
revealed themselves in EIS technique. Nyquist results
can be found in Figure 4.
Numerical values of corrosion resistance calculated
from nyquist data can expose this big difference in
Table 4.
Table 4. Numerical Values of Nyquist Data
Sample
NiPW - 1
NiPW - 2
NiPW - 3
NiPW - Al - 1
NiPW - Al - 2
NiPW - Al- 3
R (ohm)
6523
15032
5739
5511
7842
2869
Figure 4. EIS - Nyquist curves of coatings
Refining these datas which obtained from
potentiodynamic and EIS techniques lead us that
adding Al2O3 as a particle has negative effect on
corrosion properties of coatings. Although SEM
images does not exhibit reasonable difference; EDS
results shows its clear effect of P and W codeposition mechanism. By lowering both their
content, adding Al2O3 particles to bath solution
resulted with a significant decrease in corrosion
resistance of electroless coatings.
Al2O3 composite coatings were also shown relatively
poor corrosion resistance. Coused by galvanic
interactions and larger surface area in crystalline
structure.
References
[1] Song, G. L., & Atrens, A. (1999). Corrosion
Mechanisms of Magnesium Alloys. Advanced
Engineering Materials, 1(1),11±33
[2] Zhang, W. X., Jiang, Z. H., Li, G. Y., Jiang, Q., &
Lian, J. S. (2008). Electroless Ni-P/Ni-B duplex
coatings for improving the hardness and the corrosion
resistance of AZ91D magnesium alloy. Applied
Surface Science, 254(16), 4949±4955.
[3] Anik, M., & K|rpe, E. (2007). Effect of alloy
microstructure on electroless NiP deposition behavior
on Alloy AZ91. Surface and Coatings Technology,
201(8), 4702±4710.
[4] Sankara Narayanan, T.S.N. et al., 2006,
Deposition of electroless Ni±P graded coatings and
evaluation of their corrosion resistance. Surface and
Coatings Technology, 200(11), pp.3438±3445
[5] Liao, Y., Zhang, S.T. and Dryfe, R., 2011, A
study of corrosion performance of electroless Ni-P
and Ni-W-P coatings on AZ91D magnesium alloy.
Materialwissenschaft und Werkstofftechnik, 42(9),
pp.833±837
[6] Liu, D., Yan, Y., Lee, K., & Yu, J. (2009). Effect
of surfactant on the alumina dispersion and corrosion
behavior of electroless Ni-P-Al2O3 composite
coatings. Materials and Corrosion, 60(9), 690±694.
[7] Balaraju, J. N., Narayanan, T. S. N. S., &
Seshadri, S. K. (2006). Structure and phase
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[8] Anik, M. (2010). Anodic reaction characteristics
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Science, 52(9), 3109±3117.
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