Materials Science and Engineering A 445–446 (2007) 663–668
Study on the microstructure, mechanical properties, tarnish and corrosion
resistance of sterling silver alloyed with manganese
E. Nisaratanaporn a , S. Wongsriruksa b , S. Pongsukitwat a , G. Lothongkum a,∗
a
Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Phyathai Rd., Patumwan, Bangkok 10330, Thailand
b Department of Material Science, Faculty of Science, Srinakharinwirot University, Sukhumvit, Bangkok 10110, Thailand
Received 8 June 2006; received in revised form 26 September 2006; accepted 29 September 2006
Abstract
The objective of this research was to investigate the effect of manganese in as-cast sterling silver alloys used in jewellery industry on microstructure,
mechanical properties, and tarnish and corrosion resistance. In order to determine the suitable manganese content, manganese in the specimens
varied from 0, 0.38, 0.76, 1.3, 1.7, 2.1, 2.6 and 3.0 wt%. The microstructure consisted of both silver-rich solid solution, as a matrix, and eutectic
structure. The amount of manganese examined in the eutectic region was higher than those in the matrix. By increasing manganese content, the
amount of eutectic structure decreased, and therefore, tensile and yield strengths and hardness of the alloys also decreased. It was obvious that the
hardness of 3.0% Mn alloys (52.1 HV) was lower than that of manganese-free alloys (66.8 HV). The tarnish resistance improved when the amount
of manganese increased. The tarnish films were studied quantitatively using a spectrophotometer. For 3-h tarnishing test time, the colour difference,
DE* , of the manganese-free alloy was 24.98 and higher than that of the 3.0% Mn alloy, which was 5.19. The anodic polarization curves, from which
the corrosion potentials (Ecorr ), the primary passive potentials (Epp ), the pitting potentials (Ep ) and passive current density (Ip ) were evaluated,
of the test alloys were measured in 1% NaCl and H2 S-saturated 1% NaCl solutions. In 1% NaCl solution, increasing of manganese content not
only promoted the noble shift in corrosion potentials but also reduced the average passive current density. In H2 S-saturated 1% NaCl solution,
the passive region was not observed and the effect of manganese in sterling silver on corrosion potential was not clear, but the corrosion current
decreased with the increase of the manganese content. The tarnish and corrosion resistance of sterling silver were improved with manganese. This
may be due to the decrease of eutectic structure, of which the presence of copper content was higher than that of matrix.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Sterling silver; Microstructure; Mechanical properties; Tarnish and corrosion resistance
1. Introduction
Sterling silver is most commonly used in jewellery industries
to produce silver jewellery articles. According to the international standard, silver content in sterling silver must be at least
92.5 wt% [1,2]. The rest of the content (7.5 wt%) is usually copper in order to improve the strength and hardness of the alloy via
precipitation hardening, because pure silver is too soft. During
melting and casting, copper in the sterling silver can be oxidized
resulting in copper oxides, which cause a decrease in mechanical properties and poor surface finish. These oxides provide red
rose or brown-black color due to cuprous oxide, cupric oxide
[1]. Sterling silver is also often tarnished or blemished after a
∗
Corresponding author. Tel.: +66 2 2186938; fax: +66 2 2186842.
E-mail addresses: Gobboon.L@chula.ac.th, fmtglt@eng.chula.ac.th
(G. Lothongkum).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2006.09.106
long time use. This is due to the chemical reaction between silver
or copper with environment containing sulphur or sulphur compound as hydrogen sulphide to produce silver sulphide, which
is brown-green-black, or copper sulphide, which is black. To
avoid the oxidation and tarnish of sterling silver, the alloying
elements such as aluminum, germanium, silicon, zinc, cadmium
and phosphorus are added. Aluminum builds up the transparent
aluminum oxide film, which prevents the tarnish [3]. The addition of 2% of Aluminium to sterling silver alloys is sufficient to
reduce the fire stain (grey-red oxide film) [4], but the addition
of 1.5% or more of this element causes an undesirable color and
too high hardness for forming [5]. Germanium also builds up
the transparent germanium oxide film which reduces the tarnish
and fire stain during welding or annealing, but causes deterioration of ductility [5] when it is alloyed with 7.5% germanium.
Silicon was reported to be both anti-tarnish element and acted
as a deoxidizer, which decreased oxygen and produced shiny
surface but promoted a large grain size resulting in the decrease
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of tensile strength and elongation [6]. Zinc and cadmium reduce
not only the oxygen content but also the strength [7]. Phosphorus decreases the oxygen content but the surface does not reflect
shine [7]. Manganese is a well-known reducing element used in
the iron and steel making industry. However, there has not been
the investigation of sterling silver alloyed with manganese. In
this work, it is suitable for investigating the effect of manganese
on microstructures, mechanical properties, tarnish and corrosion
resistance of the sterling silver alloys.
2. Experimental
The samples, size of 9 mm × 26 mm × 1.5 mm, were prepared by melting pure silver (99.99%) and copper (99.99%)
or copper–manganese (50%) alloys in a vacuum induction furnace and casted into multisampling mould, which was formed
by the lost-wax technique [8]. The vacuum level of 10−1 Torr
was used for degassing in melting chamber and subsequently
high purity argon gas (99.95%) was flown in the chamber as
a protective gas. Casting temperature of sterling silver alloyed
with copper was 990 ◦ C and that of sterling silver alloyed with
copper and manganese was 1030 ◦ C. After casting, the surfaces
of the samples were cleaned by immersing in an acid solution
to eliminate the plastic mould and oxides. The chemical compositions of the samples were analyzed by atomic absorption
spectroscopy as shown in Table 1. Because of the manganese
loss during melting, the analyzed chemical compositions were
lower than the designed compositions. All the samples have
silver contents more than 92.5 wt%. To examine tarnish and
corrosion resistance, the samples were ground down to grit
no. 1200. The specimens were also polished with diamond
paste down to 1/4 ␮m for hardness test and etched by 50%
H2 O2 + 25% NH4 OH + 25% H2 O solution for investigating the
as-cast microstructure. Microstructure was examined by optical
microscope and scanning electron microscope equipped with
EDX. The hardness was determined with Vickers hardness tester
using 1 kg weight for 10 s following ASTM E92-82. For tensile
test, following ASTM E 8M-96, the tension speed of 0.5 mm/s
and the sample size of 4 mm in diameter with gauge length of
20 mm were used. In tarnish test, the surface color difference
(DE* ) was calculated by Eq. (1) according to the Commission
International d’ Eclairage (CIELAB). The samples were tested
in sulphur vapor atmosphere produced by the saturated sodium
Table 1
The chemical composition of samples analyzed by atomic absorption technique
Alloy number
1
2
3
4
5
6
7
8
Composition (wt%)
Ag
Cu
Mn
92.35
92.65
92.61
92.56
92.90
93.00
93.00
93.00
7.65
6.97
6.63
6.14
5.40
4.90
4.40
4.00
–
0.38
0.76
1.30
1.70
2.10
2.60
3.00
sulphide nanohydrate (Na2 S·9H2 O) aqueous solution for 0.5, 1,
2, 3 and 4 h. The Macbeth color-eye® 7000 spectrophotometer
was used with artificial daylight D65 at angle 2◦ .
DE∗ = (L∗2 − L∗1 )2 + (a2∗ − a1∗ )2 + (b2∗ − b1∗ )2 ]1/2
(1)
where L∗1 and L∗2 are the shiny values before and after the test,
respectively; a1∗ and a2∗ the red-green colour values before and
after test, respectively; and b1∗ and b2∗ are the yellow-blue colour
values before and after the test, respectively.
In corrosion test, the polarization curves were determined in
1% NaCl and H2 S-saturated 1% NaCl solution at 25 ◦ C. The
counter electrode was Pt, and reference electrode was Ag/AgCl.
From the polarization curves, the corrosion potential (Ecorr ), corrosion rate (CR), primary passive potential (Epp ), passive current
density (Ip ), and pitting potential (Ep ) were obtained.
3. Results and discussion
3.1. As-cast microstructure
Fig. 1 shows the micrograph of the as-cast microstructures
of some sterling silvers alloyed with copper and manganese.
It was found that dendrites existed in the cast structure. The
eutectic structure existed between dendrite arms (interdendritic
regions). The results of SEM observation on the eutectic structures of those four alloys in Fig. 1 are shown in Fig. 2. For
sterling silver alloyed with only copper (Fig. 2a), the chemical compositions, determined by EDX, in the matrix is 95.53%
Ag–4.47% Cu (Ag-rich ␣-phase) whereas the composition in
eutectic region (A in Fig. 2a) is 69.53% Ag–30.47% Cu (Cu-rich
phase). This composition is almost near the eutectic composition
(28.1%) in binary Ag–Cu phase diagram [9]. The composition
of black phase (B in Fig. 2a) in eutectic structure is 22.09%
Ag–77.91% Cu (Cu base phase). Then, the eutectic reflects a
finger print structure which is composed of Ag-rich ␣-phase and
Cu-base phases. The matrix and eutectic structures observed in
sterling silver alloyed with copper and manganese (Fig. 2b–d)
are similar to those in sterling silver alloyed with only copper (Fig. 2a). In the sterling silver alloyed with copper and
manganese, the matrix (␣-phase) and eutectic compositions are
still rich in silver and copper, respectively. However, it can be
clearly seen that the addition of manganese in alloy increases
in manganese contents of both matrix and eutectic structure. In
addition, manganese tends to be present in eutectic structure at
higher level than that in matrix. For example, in sterling silver alloy having 4.90% Cu–2.10% Mn, the matrix (␣-phase)
had its composition of 93.80% Ag–4.14% Cu–2.05% Mn, and
the eutectic structure had composition of 57.41% Ag–39.20%
Cu–3.39%Mn. Table 2 summarizes the compositions in each
matrix (␣-phase), eutectic structure and black phase in eutectic structure of all specimens analyzed by EDX technique. By
SEM observation of the surrounding area of eutectic structure
at high magnification (Fig. 2c), the interdendritic shrinkage was
also found.
It is clear in Fig. 1 that when the manganese content of alloys
increased, i.e. the copper content of alloys decreased, the amount
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665
Fig. 1. Micrographs of as-cast microstructure of sterling silver alloyed with copper and manganese: (a) 7.65% Cu–0% Mn, (b) 6.14% Cu–1.30% Mn, (c) 4.90%
Cu–2.10% Mn and (d) 4.00% Cu–3.00% Mn.
Fig. 2. SEM images on eutectic phase in as-cast microstructures of sterling silver alloyed with copper and manganese: (a) 7.65% Cu–0% Mn, (b) 6.14% Cu–1.30%
Mn, (c) 4.90% Cu–2.10% Mn and (d) 4.00% Cu–3.00% Mn.
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Table 2
Summary of the composition (wt%) of matrix (␣-phase), eutectic structure and black phase in eutectic structure in the tested sterling silver alloys analyzed by EDX
technique
Sterling silver alloys
(Cu–Mn, wt%)
7.65–0
6.97–0.38
6.63–0.76
6.14–1.30
5.40–1.70
4.90–2.10
4.40–2.60
4.00–3.00
*
Composition (Ag–Cu–Mn in wt%)
Difference (M − E)
Matrix (M)
Eutectic (E)
Black phase in eutectic (E)
+Ag
−Cu
−Mn
95.53–4.47–0
92.46–7.23–0.31
93.11–6.22–0.67
69.53–30.47–0
70.90–28.62–0.48
71.74–27.10–1.17
65.19–32.92–1.88
60.84–36.54–2.61
57.41–39.20–3.39
66.58–29.36–4.06
53.99–40.41–5.60
22.09–77.91–0
29.29–70.17–0.54
26
21.56
21.37
–
33.32
36.39
26.23
37.43
26
21.39
20.88
–
32.32
35.06
24.74
35.08
0
0.17
0.50
–
0.99
1.34
1.49
2.35
*
94.16–4.22–1.62
93.80–4.14–2.05
92.81–4.62–2.57
91.42–5.33–3.25
*
20.96–76.69–2.35
*
*
15.76–78.52–5.71
*
Not determined.
of eutectic structure decreased. From references [9–12] and the
SEM analysis results, the composition of eutectic structure was
rich in copper content. Even though the manganese content in
eutectic structure was higher than that in matrix (␣-phase), its
amount was so small compared with the copper content. Therefore, the lower copper content in alloy resulted in reducing the
amount of the eutectic structure.
strength is. It was obvious that the increase in copper content
increased both yield strength and ultimate tensile strength. The
eutectic structure is also responsible for this result.
3.3. Tarnish and corrosion test
Three mechanical properties, hardness, yield strength (YS)
and ultimate tensile strength (UTS) were discussed. Fig. 3 shows
the effects of copper and manganese on the Vickers hardness.
It was found that increasing the manganese content (Cu-content
decreased) decreased the Vickers hardness. This may be related
with the eutectic structure which is harder than the matrix. As
mentioned in the previous section, when the copper content
decreased, the amount of eutectic structure decreased. Therefore, this provides a reduction in hardness.
Fig. 4 shows the yield and ultimate tensile strengths of some
tested sterling silvers. This result was in good agreement with
the harness in Fig. 3, i.e. the higher the hardness, the higher the
Fig. 5 shows effects of copper and manganese on the color
difference (DE* ) of the tested sterling silver alloys. When the
test times were long enough (at least 60 min), it was clear that
increase in copper content (decreasing in manganese content)
increased the color difference. This means a reduction in the tarnish resistance. The corrosion test results in 1% NaCl solution at
25 ◦ C, as shown in Figs. 6 and 7, agreed with this result. From the
pitting potential and primary passive potential results, it could
not be seen obviously any effects of copper and manganese. Nevertheless, from the corrosion potential result, it seemed to have
some effects. An increase of manganese content (decreasing in
copper content) results in increasing corrosion potential, and
thus corrosion resistance is increased. Considering the corrosion
current density (Icorr ) and passive current density (Ip ) shown in
Fig. 7, it is also clear that when the manganese content increases
and the copper content decreases, the reduction in the corrosion
Fig. 3. Effects of copper and manganese on hardness of sterling silver alloyed
with copper and manganese.
Fig. 4. Effects of copper and manganese on yield and ultimate tensile strength
of sterling silver alloyed with copper and manganese.
3.2. Mechanical properties
E. Nisaratanaporn et al. / Materials Science and Engineering A 445–446 (2007) 663–668
667
Fig. 5. Effects of copper and manganese on colour difference (DE* ) of sterling
silver alloyed with copper and manganese.
Fig. 7. Effects of copper and manganese on corrosion current density and passive
current density of sterling silver alloyed with manganese in 1% NaCl solution,
25 ◦ C.
Fig. 6. Effects of copper and manganese on corrosion potential (Ecorr ), primary
passive potential (Epp ), and pitting potential (Ep ) of sterling silver alloyed with
copper and manganese in 1% NaCl solution, 25 ◦ C.
current density and passive current density can be attained. This
indicated that the corrosion rate at corrosion potential decreased
and the passive film became more stable. From the results in
Figs. 5–7, the suitable range of manganese contents to increase
the tarnish and corrosion resistance should be 1–3 wt%. The corrosion test results in H2 S-saturated 1% NaCl solution at 25 ◦ C
as shown in Figs. 8 and 9. The corrosion potentials in H2 Ssaturated 1% NaCl solution were much lower than those in 1%
NaCl solution. The passive potential range also did not appeared.
This may be because H2 S-saturated 1% NaCl solution was too
strong corrosive for the tested alloys. From Fig. 8, it seems that
manganese had no significant effect on the corrosion potential.
However, it is obvious that the corrosion currents reduce (Fig. 9)
with increase in manganese content.
Manganese has a detrimental effect on general corrosion
resistance of Fe–Mn–Al alloy because of the unstable man-
Fig. 8. Effects of copper and manganese on corrosion potential of sterling silver alloyed with copper and manganese in H2 S-saturated 1% NaCl solution, 25 ◦ C.
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alloyed with manganese. The following conclusions could be
drawn:
1. Manganese in sterling silver alloy existed in eutectic structure
(Cu-rich phase) more than in the matrix (Ag-rich ␣-phase).
2. An increase in manganese content in sterling silver alloy
resulted in decrease in the amount of eutectic structure.
3. The hardness and strength of sterling silver decreased with
decreasing the copper content via an increase in manganese
content.
4. Increasing the manganese content and decreasing copper
content of sterling silver alloy improved the tarnish and corrosion resistance.
5. The recommended manganese content in sterling silver for
good tarnish and general corrosion resistance should be
1–3 wt%.
Fig. 9. Effects of copper and manganese on corrosion current density of sterling
silver alloyed with manganese in H2 S-saturated 1% NaCl solution, 25 ◦ C.
ganese oxide film [13]. However, the positive effect of manganese on tarnish and corrosion resistance of sterling silver was
found in this work. As shown in Fig. 1, increase in the manganese
content of sterling silver resulted in decreasing eutectic structure. The different distributions of silver, copper and manganese
between matrix and eutectic structure may have an effect on the
different corrosion resistance of matrix and eutectic structure
or may cause galvanic corrosion between matrix and eutectic
structure as observed in duplex stainless steel [14]. The matrix
has higher silver but lower copper as well as manganese contents
than the eutectic structure. From Table 2, the different contents
of silver and copper between matrix and eutectic structure are
nearly the same for each alloy and greater than those of manganese. Those different contents of silver, copper and manganese
tend to increase when manganese content of alloys increases.
The standard reduction potentials of silver (Ag+ /Ag), copper
(Cu2+ /Cu) and manganese (Mn2+ /Mn) are +0.799, +0.340 and
−1.18 V (SHE), respectively [15]. This says that the corrosion
resistance order of those three elements is Ag > Cu ≫ Mn. Silver
is then assumed to mainly have effect on corrosion resistance of
both matrix and eutectic region. Therefore, reduction of silver
content and amount of eutectic structure when manganese content of alloys increases may be responsible for the increase in
tarnish and corrosion resistance of sterling silver.
4. Conclusions
This work studied the as-cast microstructure, mechanical
properties and tarnish and corrosion resistance of sterling silver
Acknowledgement
The financial support by the Thailand Research Fund (Project
No. RDG5/0018/2544) is gratefully acknowledged.
References
[1] R.V. Carrano, The Santa Fe Symposium on Jewellery Manufacturing Technology, 1997, pp. 162–163.
[2] A.M. Reti, The Santa Fe Symposium on Jewellery Manufacturing Technology, 1997, p. 339.
[3] L.E. Price, G.J. Thomas, Inst. Metals (January) (1938) 357.
[4] G.E. Gardam, Sterling silver containing aluminium, Metallurgia 47 (279)
(1953) 29–33.
[5] P. Johns, The Santa Fe Symposium on Jewellery Manufacturing Technology, 1997, pp. 38–42.
[6] J.C. Mccloskey, P.R. Welch, S. Aithal, Gold Technol. (30 (Winter)) (2000)
4–7.
[7] A.M. Reti, The Santa Fe Symposium on Jewellery Manufacturing Technology, 1997, p. 353.
[8] S. Wongsriruksa, Mechanical properties and tarnish resistance of 92.5%
Ag–Cu–Mn alloys, Master Thesis in Metallurgical Engineering, Chulalongkorn University, 2002.
[9] A.M. Reti, The Santa Fe Symposium on Jewellery Manufacturing Technology, 1997, p. 342.
[10] Hugh Baker (Ed.), ASM Handbook. Alloy Phase Diagrams, vol. 3, ASM
International, 1992, pp. 2–32.
[11] Hugh Baker (Ed.), ASM Handbook. Alloy Phase Diagrams, vol. 3, ASM
International, 1992, pp. 2–172.
[12] P. Villars, A. Prince, H. Okamoto, Handbook of Ternary Alloy Phase Diagrams, vol. 3, ASM International, 1995, p. 2303.
[13] Y.S. Zhang, X.M. Zhu, Corros. Sci. 41 (9) (1999) 1817.
[14] H. Hoffmeister, G. Lothongkum, Weld. Res. Abroad 42 (12) (1996)
18–23.
[15] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and
Application, second ed., John Wiley & Sons, Inc., 2001, pp. 808–809.