Polarization Study of Sn-9Zn Lead-free Solder in KOH Solutions
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Int. J. Electroactive Mater. 2 (2014) 34 – 39
www.electroactmater.com
Polarization Study of Sn-9Zn Lead-free Solder in KOH Solutions
Muhammad Firdaus Mohd Nazeri1, 2, # and Ahmad Azmin Mohamad1,*
1
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang,
Malaysia
2
School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Arau, Perlis, Malaysia
Email: firdausnazeri@unimap.edu.my#; aam@usm.my*
ABSTRACT: The corrosion behavior of Sn-9Zn solder was compared by means of potentiodynamic polarization in
different concentrations of potassium hydroxide (KOH). The preferential dissolution of Zn from Sn-9Zn solder dictates
the initial corrosion process in all concentrations of KOH solution tested. The effect of KOH concentrations on the
corrosion potential was shown to be relatively insignificant. In contrast, increasing the concentrations from 6 to 9 M
increased the current density by over 200 % compared to 0 M. Post-corrosion analyses show that the light and dark-
contrast corrosion products dominate the surface of the solder after polarization that removes the characteristic
anomalous eutectic structure of Sn-9Zn. Morphological examination revealed that the corrosion products were poorly
distributed after polarization. This result demonstrates that protection offered by the presence of corrosion product from
further corrosion attack is insignificant.
Keywords: Corrosion; Lead-free solders; Sn-9Zn; Potentiodynamic polarization; Potassium hydroxide
1. Introduction Potassium hydroxide (KOH) is one of the common
Soldering technology provides a major contribution in electrolytes for alkaline batteries that have high ionic
supporting the growth of the electronic materials industry, conductivity [5, 6]. In addition, this solution also has been
in which high demand is coming from industrial and reported to cause extreme corrosive effect on solder joints.
public consumers as electronic devices play important role For example, the β-Sn phase of a Sn−3Ag−0.5Cu solder
in daily applications. Until recently, the well-known tin- joint was seriously consumed during a potentiodynamic
lead (Sn-Pb) alloys, such as the eutectic Sn-37Pb and the polarization analysis in 6 M KOH [7]. On the other hand,
near eutectic Sn-40Pb, have been widely used [1-3]. selective leaching of Zn from Sn-9Zn alloy was reported
However, Pb poses a great threat to human health and to in 6 M KOH solution [8, 9]. However, corrosion
the environment. This fact has resulted in the banning of information of Sn-9Zn alloy in different KOH
Pb-containing solders in all applications [3]. concentrations is very limited and should be
Thus, the search for the new Pb-free solder has systematically investigated.
occupied researchers around the globe. As a result, a vast In this work, the corrosion properties of Sn-9Zn in
number of Pb-free solder alloys have been proposed to various concentrations of KOH environment are
date. Among them, tin-based alloys, such as tin-indium presented. The corrosion properties were characterized in
(Sn-In), tin-silver (Sn-Ag), tin-gold (Sn-Au) and tin-zinc terms of its corrosion behavior in various concentrations
(Sn-9Zn) have been extensively studied [3]. The eutectic of KOH solution, including corrosion potential (Ecorr) and
Sn-9Zn alloy has become an attractive alternatives to corrosion current density (icorr). Structural, morphological
replace Sn-Pb due to its low melting temperature and and elemental studies were also implemented to further
excellent mechanical properties [3]. These characteristics investigate the effect of corrosion on the solder.
have driven several multinational companies such as
2. Experimental
Matsushita, AT & T and Hitachi to investigate the
The solder alloy of Sn-9Zn was prepared by melting
possibility of using Sn−9Zn for low-cost electronic home raw materials of Sn (Malaysia Smelting) and Zn (Sigma-
appliances [2]. Aldrich). The weighed metals were cleaned and co-melted
The corrosion properties of Sn-9Zn were investigated using an induction furnace with the presence of nitrogen
in various corrosive environments because one of the gas at 600 °C. The molten alloys were stirred to ensure the
characteristics of a good Pb-based solder replacement is to homogeneity. Once air-cooled to room temperature, the
have good corrosion resistance. Mori et al. [4] studied the solidified alloy was pressed and mechanically punched to
corrosion potential behavior of Sn-9Zn and noticed that produce pellets with a diameter of 5 mm each, at the
this alloy experienced two levels of corrosion potential thickness of 3 mm. Then, the produced pellets were
during immersion in sulfuric acid solution. Meanwhile, ground and degreased.
from potentiodynamic analysis, Lin and Liu [1] proved X-ray diffraction (XRD) analysis was performed using
that Sn-9Zn-based solder possessed better corrosion a Bruker AXS D9 diffractometer at 2θ values of 10-90°
resistance compared to Sn-Pb alloy due to the fact that Sn- with Cu Kα radiation to determine the corresponding
9Zn-based solder showed a lower corrosion current and phases of the solder alloy. The corresponding peaks
passivation current values in sodium chloride solution. attained were matched with the standards from the
Int. J. Electroactive Mater., Vol. 2, 2014 35
International Committee of Diffraction Data (ICDD) X-ray structure of Zn-rich phase was produced in Sn-9Zn alloy.
data file using EVA software. The microstructure and Thus, this type of structural formation is generally known
elemental analyses of the solder alloys were investigated as anomalous eutectic structure [10].
prior and post corrosion measurement by using a field
emission scanning electron microscope (FESEM) and
energy dispersive X-ray spectroscopy (EDX). The
microscope model used was a Zeiss Supra 35 VP equipped
with an EDX system.
The Sn-9Zn solder pellets were each attached to a
copper (Cu) wire measuring 1 mm in diameter to provide
electrical connection. Then, the pellets were cold-mounted
with epoxy resin prior to be subjected for corrosion study.
All the mounted samples were polished and cleaned to
obtain a mirror finish surface. The potentiodynamic
polarization analysis was carried out in a single
compartment cell. A three-electrode system was used with
the mounted sample as the working electrode having an
exposure surface area of 0.196 cm2; a platinum rod and
Hg/HgO were used as counter electrode and reference Figure 1: FESEM image for the as-prepared Sn-9Zn
electrode in 6 M KOH electrolyte, respectively. The
Hg/HgO electrode was used as it owns excellent stability The phases present for the as-prepared Sn−9Zn solder
in alkaline solutions. are confirmed to be the body-centered tetragonal β-Sn
The potentiodynamic polarization analysis was (ICDD 00-004-0673) and the hexagonal Zn (00-004-
performed using an AUTOLAB PGSTAT 30, coupled 0831), as shown in Figure 2. Given the solubility of Zn in
with General Purpose Electrochemical System (GPES) Sn is highly limited; thus, no new intermediate phase is
interface software controlled by personal computer. The expected [11]. Only these two phases are generally
scanning rate for potentiodynamic polarization was 100 detected for the Sn−9Zn solder and have been previously
mV s-1 after allowing the steady-state potential to develop. reported by other researchers [6, 12].
The scan potential range used was -2.00 to 0.00 VHg/HgO (-
1.342 to 0.658 VSCE). Four different concentrations of 3.2 Potentiodynamic Polarization Analysis
KOH were used, which are 0 (distilled water), 3, 6 and 9 The potentiodynamic polarization curves for Sn-9Zn
M. The electrochemical characterization was carried out polarized in distilled water started in the cathodic region at
three times to ensure reproducibility of the results. a potential of −2.0 VHg/HgO (-1.342 VSCE, point A1), as
shown in Figure 3. Rapid reduction in current density was
3. Results and Discussion observed as the applied potential increased up to −1.041 ±
3.1 Solder Characterizations 0.053 VHg/HgO (-0.383 VSCE, point B1). Beyond this
The morphology of the as-prepared Sn-9Zn alloy is potential, the current started to increase swiftly to mark the
represented by the aligned and dark-contrast of the Zn-rich start of anodic region. Further increases in the applied
phase that distributed throughout the entire β-Sn matrix potential caused the current density to linearly increase
(Figure 1). The low solubility limit of Zn in Sn hinders the from 1×10 8 to 1×10 6 A cm-2 (point C1). This linear region
− −
formation of continuous Zn-rich structure. Hence, unlike represents the start of the dissolution of the active material
other eutectic compositions, the broken needle-like and is thus called the primary activation region.
aaaaaaa 1200
Sn (211)
Sn (101)
1000
800
Intensity (a.u.)
Sn (112)
Sn (321)
600
Sn (312)
Sn (200)
Sn (301)
Sn (220)
400
Sn (411)
Sn (420)
Zn (0110)
Zn (002)
Zn (100)
Zn (101)
200
0
10 20 30 40 50 60 70 80 90
2θ
(°)
Figure 2: XRD pattern for the as-prepared Sn-9Zn
Int. J. Electroactive Mater., Vol. 2, 2014 36
0.5
D
9M 6M
0
3M 0M
-0.5
Potential (VHg/HgO)
C1
-1 B1
C2
B2
-1.5 icorr
3M
icorr
0M
-2 icorr icorr A2
9M 6M A1
-2.5
0.0001 0.001 0.01 0.1 1 10 100
Log current density (µA cm-2)
Figure 3: Potentiodynamic polarization curves of Sn-9Zn in various KOH concentrations
As the most active material in the Sn−Zn system, Zn Although ZnO is expected to form at the surface, it can
dissolved in the primary activation region. The specific be deduced that complete protection from further
removal of Zn from the Sn−9Zn solder can be described as corrosion is not provided from this corrosion product. This
a de-alloying process and has also been observed by other is verified by the continuous rise of current density with
researchers using Sn−Zn alloys [4]. The slope intercepts of the increase of applied potential. The continuous rise of
the linear regions from the anodic and cathodic scans the current density also hinders the determination of the
represents the corresponding Ecorr and icorr of the Sn−9Zn passivation current density (ip) in this solution.
solder. In distilled water, the existence of dissolved The whole polarization curves for the Sn-9Zn solder in
oxygen may have taken part and react with the dissolute 3, 6 and 9 M KOH were almost identical and shifted
Zn2+ as Zn is the most electrochemically active material in towards more negative potential compared with the curve
this work. According to Kitano et al. [13], the dissolution for Sn-9Zn in distilled water. Cathodic polarization starts
reaction: at point A2. Further scanning towards the positive
Zn → Zn2+ + 2e- (1) potential direction, reduction of current was observed up
From −0.700 VHg/HgO (-0.042 VSCE) until the end of the to point B2 in all three concentrations.
scan (point D), the current density increased at much lower In these solutions, the primary activation region starts
rate compared with the primary activation region. This from ~ -1.3 VHg/HgO (-0.642 VSCE, point B2). According to
may be attributed to the primary passivation process on the Nazeri and Mohamad [9], dissolution process normally
solder surface, as follows [13]: associated with the corresponding potential is resulted
2e- + ½O2 + Zn2+ → ZnO (2) from the preferential dissolution of Zn in KOH solution.
Table 1: Electrochemical parameters obtained from the polarization curves of Sn−9Zn and solder in different
concentrations of KOH solution
Concentration Ecorr Ecorr icorr ip
(M) (VHg/HgO) (VSCE) (µA cm-2) (µA cm-2)
0 -1.041 ± 0.053 -0.383 ± 0.053 0.223 ± 0.040 -
3 -1.315 ± 0.006 -0.657 ± 0.006 0.253 ± 0.050 -
6 -1.323 ± 0.008 -0.665 ± 0.008 0.606 ± 0.075 3.659 ± 0.068
9 -1.344 ± 0.003 -0.686 ± 0.003 0.573 ± 0.023 -
Int. J. Electroactive Mater., Vol. 2, 2014 37
0 0.8
-0.2 0.7
Corrosion current density, icorr (µA cm-2)
-0.4 0.6
Corrosion potential (VHg/HgO)
-0.6 0.5
-0.8 0.4
-1 0.3
-1.2 0.2
-1.4 0.1
-1.6 0
0 1 2 3 4 5 6 7 8 9 10
KOH concentrations (M)
Figure 4: Corrosion potentials and current densities of Sn-9Zn in various KOH concentrations
The rate of dissolution also increased with the increase 3.659 µA cm-2 (Table 1). The Ecorr decreased to -1.315 ±
of potential, noted by the rapid amplification of the current 0.006 VHg/HgO (-0.657 ± 0.006 VSCE) for the Sn-9Zn/3 M
from point B2 to C2. Meanwhile, beyond point C2, the rise KOH system, as compared with -1.041 ± 0.053 VHg/HgO (-
of current density was very small regardless of the 0.383 ± 0.053 VSCE) for Sn-9Zn/distilled water system
increase in applied potential. This highlighted that (Figure 4). Reduction in Ecorr revealed that the corrosion
passivation process took place on the surface of the solder process was more aggressive in 3 M KOH. However, there
alloy and limit the continuous significant increase of icorr. were no significant changes of Ecorr when KOH
In Sn-9Zn/KOH system, cathodic part of polarization concentrations further increased higher than 3 M KOH.
involves the reaction of water producing hydroxyl ions This indicates that further increase in concentrations of
(OH-) ions as presented by [9]: KOH plays insignificant effect on the Ecorr.
O2 + 2H2O + 4e- → 4 OH- (3) Meanwhile, the icorr slighty increased to 0.253 ± 0.050
In the anodic region, Zn dissolved with increasing µA from 0.223 ± 0.040 µA as the solution changed to 3 M
potential until zincate [Zn(OH)2] reached its critical KOH from distilled water. The highest icorr of 0.606 ±
concentration. As a result, Zn(OH)2 covered parts of the 0.075 µA was recorded at 6 M KOH. A slight decrease to
surface which led to the formation of the primary 0.573 ± 0.023 µA was noted at higher concentration of 9
passivation region as witnessed by the apparent reduction M KOH. Although the difference is small, changes in icorr
in the current with increasing potential. Hu et al. [14] with the increase of concentration highlighted that the
explained that the Zn(OH)2 on the surface is changed into amount of OH- charge carrier plays major role in dictating
ZnO according to: the kinetics of the corrosion of Sn−9Zn.
Zn → Zn2+ + 2e- (4) In distilled water, the amount of OH- charge carrier is
Zn2+ + 2OH- → Zn(OH)2 (5) relatively limited compared to the abundant supplies of
2Zn(OH)2→2ZnO + 2H2O (6) OH- as free charge carriers in 3 M KOH. This increases
The low amount of Zn in the Sn−9Zn solder resulted in the kinetics of the corrosion of Sn−9Zn in 3 M KOH. The
the rapid depletion of this metal at the surface. This highest icorr produced for the Sn-9Zn/6 M KOH system
allowed Sn as the major solder component to dissolve and revealed that the free charge carrier amount was at its
improves the passivation. The dissolution and passivation optimum. Iwakura et al. [15] explained that excessive
processes of Sn in alkaline solution are related with the amount of charge carriers limit its movement. In addition,
formation of tin oxides [9]: formation of ion pairs amongst the anions also plays an
Sn + 2OH → Sn(OH)2 +2e
−
(7)−
important role in restricting its movement [16]. This
Sn(OH)2 → SnO + H2O (8) clarifies the reduction of icorr at 9 M KOH. As a result at 6
SnO + H2O + 2OH → Sn(OH)4 + 2e −
(9) −
M, the KOH solution demonstrated its optimum
Sn(OH)2 + 2OH → Sn(OH)4 + 2e −
(10) −
corrosiveness.
Sn(OH)4 → SnO2 + 2H2O (11) The XRD pattern obtained for Sn-9Zn after
Still, complete passivation was not obtained at all polarization in distilled water is strikingly resembled with
concentrations, except for 6 M KOH. In this solution, full the as-prepared Sn-9Zn where all of the peaks obtained
passivation can be seen starting at the potential belong to Sn and Zn, apart from the presence of ZnO
approaching -1.0 VHg/HgO (-0.342 VSCE), at the ip value of (ICDD 01-089-1397), as shown in Figure 5a.
Int. J. Electroactive Mater., Vol. 2, 2014 38
(d) 9 M
Sn (200)
SnO2 (206)
SnO2 (111)
SnO2 (212)
SnO (311)
SnO (200)
SnO (112)
SnO (114)
SnO2 (333)
SnO2 (244)
SnO (241)
ZnO (201)
SnO (224)
SnO (004)
Sn (301)
Sn (321)
ZnO (102)
Sn (211)
Sn (220)
Sn (312)
SnO (220)
SnO (221)
SnO (222)
ZnO (002)
SnO (134)
Sn (112)
(c) 6 M SnO (004)
Sn (211)
Intensity (a.u.)
Sn (200)
SnO (311)
Sn (220)
SnO2 (111)
SnO (200)
SnO (112)
SnO (114)
Sn (301)
ZnO (002)
SnO2 (206)
SnO (241)
SnO (220)
SnO (221)
SnO (222)
Sn (321)
ZnO (102)
ZnO (201)
SnO2 (333)
SnO2 (244)
SnO (224)
Sn (312)
SnO (134)
Sn (112)
Sn (220)
(b) 3 M
SnO2 (206)
SnO (020)
SnO (311)
Sn (211)
SnO2 (111)
SnO (200)
SnO (112)
SnO (114)
SnO2 (212)
SnO (220)
SnO (221)
SnO (222)
SnO (241)
Sn (321)
ZnO (002)
Sn (200)
ZnO (102)
Sn (301)
ZnO (201)
SnO (134)
SnO (224)
Sn (312)
Sn (112)
(a) 0 M
Sn (211)
Sn (101)
Sn (112)
Sn (321)
ZnO (002)
Sn (312)
Sn (200)
Sn (411)
Sn (301)
Sn (220)
Sn (420)
ZnO (110)
ZnO (102)
Zn (101)
25 35 45 55 65 75 85
2θ (°)
Figure 5: XRD pattern for the polarized Sn-9Zn solder in (a) 0, (b) 3, (c) 6 and (d) 9 M KOH
Meanwhile, formations of new phases of SnO, SnO2 The presence of various oxides further validates the fact
and ZnO were identified after the polarizations were done that both Sn and Zn dissolved during polarization.
in 3, 6 and 9 M KOH (Figure 5b-d). These new phases The surface of Sn-9Zn solder significantly changed
were exactly matched well with ICDD File No. 00-024- where the anomalous eutectic structure was no longer seen
1342, 00-002-1340, and 01-089-1397, respectively. after polarization in all concentrations (Figure 6). The
Given that β-Sn is the major constituent of Sn−9Zn surface was covered by agglomerated, light-contrast
solder, the formation of SnO and SnO2 was expected. corrosion product and rough, dark-contrast corrosion
Moreover, Chang et al. [17] explained that ZnO is product after polarization in distilled water (Figure 6a).
generally detected as a corrosion by-product because it is The EDX analysis on the dark-contrast corrosion product
one of the most stable products in the Zn2+/H2O system. (point A) revealed that the corrosion was richer in Sn.
aaa aaaaa
Figure 6: FESEM images of the polarized Sn-9Zn solder in (a) 0, (b) 3, (c) 6 and (d) 9 M KOH
Int. J. Electroactive Mater., Vol. 2, 2014 39
On the other hand, light-contrast corrosion product (point Acknowledgements
C) was found to be richer in Sn. This result shows that the M.F.M.N. would like to thank USM PRGS (8044035).
surface of the solder was covered by Zn-rich and Sn-rich A.A.M. would like to thank the USM-RUI grant
corrosion products (1001/PBahan/814112) for financial support.
Comparable observations were seen after the Sn-9Zn
solder was polarized in 3, 6 and 9 M KOH electrolytes References
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171-176.
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9. M.F.M. Nazeri, A.A. Mohamad, Measurement 47
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of grooves may become the weak spots under loading in 1483.
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15. C. Iwakura, S. Nohara, N. Furukawa, H. Inoue,
similar pattern in all concentrations although the scan was
Solid State Ionics 148 (2002) 487-492.
completely shifted in active direction in 3, 6 and 9 M. The
16. A. Jamaludin, Z. Ahmad, Z. Ahmad, A. Mohamad,
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compared to 0 and 3 M of which revealed that the Int. J. Hydrogen Energ. 35 (2010) 11229-11236.
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Microstructure and elemental analyses show that corrosion
products were made of Sn-rich and Zn-rich oxides. Received: August 15, 2014 / Accepted: November 26, 2014
Meanwhile, phase analysis proves that these products were © 2014 by Nazeri et al; Licensee Electroactive Materials
SnO, SnO2, and ZnO. Society
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