Comparative corrosion behaviour of different Sn-based solder alloys

Sn-Pb solders, particularly Sn-37 %Pb eutectic alloy, have been widely used as low temperature joining alloys for some time. However, the restriction of Pb use in industry has been strongly promoted to protect the environment and establishing a Pb-free solder has become a critical issue and an important task for material engineers. New solders must fulfil several requirements; in particular they must be corrosion resistant. In the present work, the corrosion behaviour of five Pb-free solders was studied in a 0.1 M NaCl aqueous solution by means of polarization measurements (corrosion potential measurements, potentiodynamic polarization curves and linear polarization resistance tests), and compare to that of a conventional Sn-37 %Pb solder alloy and pure Sn. The results show that the Sn-3.5 %Ag-0.9 %Cu, Sn-3.5 %Ag and Sn-0.7 %Cu solders have the best resistance to localized as well as to general corrosion, similar to that obtained for the Sn-37 %Pb solder and pure Sn. The Sn-57 %Bi solder has poorer corrosion properties but its behaviour is still acceptable, because it passivates and shows a relatively low corrosion rate. In all these cases the corrosion resistance is good due to the content of noble elements (Ag, Cu, Pb and Bi) in the alloys. On the other hand, the Sn-9 %Zn is definitely the one that exhibits the worst behaviour, not only to localize but also to general corrosion, due to the addition of a less noble material to the Sn matrix.