Corrosion Resistance Properties of Cu-Sn Electrodeposits from Cyanide-Free Bath
Authors: Nakamura Toshihiro , Konno Yoshiki , Yamamoto Takayo , Nagayama Tomio 
Published: February 2020

Cu-Sn alloy is widely known and frequently used to avoid nickel in many decorative, and electronic applications such as connector and terminal. Electrodeposition of Cu-Sn alloys from electrolytes containing uncomplexed, divalent metal ions is difficult because of the large difference in the standard electrode potentials of Cu, 0.340 V and Sn, -0.138 V. At present, Cu-Sn alloy is still plated commercially from cyanide baths, which causes environmental problems in the use and disposal of toxic cyanide.
Recently, we demonstrated that silvery white Cu-Sn alloy electrodeposits (40-55mass%Sn), called “speculum alloy,” or “white bronze”. It has been investigated as an promising alternative to an allergenic nickel coating, can be obtained from environmental friendly cyanide-free sulfosuccinate bath [1]. Cu-Sn alloy deposits as an under layer restrained degradation of contact resistance, in particular, thin gold overlay on the Cu-Sn alloy deposits containing 40 to 55 mass% Sn, called “speculum alloy,” maintained a lower contact resistance during the salt spray test for prolonged periods [2]. In addition, gold thickness can be reduced by using the Cu-Sn alloy deposits as an undercoat has been reported [2]. In this study, crystalline structure and anodic property of Cu-Sn alloy electrodeposits were examined. These characteristic features might relate to the corrosion resistance and be important surface properties when using the Cu-Sn layer for the underplating. The Cu-Sn alloy deposits, speculum alloy(40 to 55 mass% Sn) exhibited better corrosion resistance than those of pure Cu, Sn and Ni.
The Cu-Sn alloy electrodeposition carried out on copper substrate using sulfosuccinate electrolytes with additives at 1A/dm2. Bath composition were as follows: CuSO4 (0.15mol/L), SnSO4 (0.05mol/L), and HOOCCH2CH(SO3H)COOH (sulfosuccinic acid, 1.0 mol/L), L-methionine (0.4mol/L), and polyoxyethylene-α-naphthol (3g/L). The bath temperature was 298K and the bath pH was adjusted to pH0.5. The anode was Sn sheet.
Fig1 shows XRD patterns of the Cu-Sn deposits obtained from the baths containing different concentration ratio of metal ions. According to the Cu-Sn phase diagram, the alloy of 40–55 mass% Sn generally consists of two phases, Cu6Sn5 and Cu3Sn. On the other hand the diffraction peaks of Cu-40mass%Sn and Cu-47mass%Sn electrodeposits were assigned to only Cu6Sn5, and unknown peak(labeled* in Fig.1) near 42° was observed except the peaks derived from copper substrate. All the diffraction peaks of Cu-55mass%Sn electrodeposits was assigned to Cu6Sn5 single-phase. The diffraction peaks of Cu-62mass%Sn or above were assigned to coexistence Cu6Sn5 and β-Sn. Consequently, phase structure of the Cu-Sn alloy deposits containing 40 to 55 mass% Sn, called “speculum alloy,” which exhibited excellent corrosion resistance were identified mainly of Cu6Sn5. It might be related to the corrosion resistance.
Figure 2 shows anodic polarization curves for the Cu-40mass%Sn layer obtained in diluted sulfuric acid (50 mmol/L H2SO4) at 25°C. Electrode potential was measured with a Ag/AgCl (sat. KCl) reference electrode. Corrosion potential of Cu-40mass%Sn layer was less noble, -310mV vs. Ag/AgCl, than that of pure Cu and Ni. When polarized positively from the corrosion potentials, the Cu-40mass%Sn layer (almost Cu6Sn5phase) thoroughly passivated, whereas pure Cu, Sn and Ni readily dissolved anodically.　Consequently, excellent corrosion resistance of thin gold overlay on the Cu-Sn speculum electrodeposits might be achieved by passivated film formed instantaneously on the Cu-Sn deposits.

References

[1] T. Nakamura, T. Nagayama, T. Yamamoto, Y. Mizutani, H.Nawafune, Mater. Sci. Forum., 654-656, 1912(2010).
[2] T. Nakamura, T. Yamamoto, T. Nagayama, Abst. 66th Ann. Meet. of ISE, s07-P-005 (2015).

Figure 1