This paper presents an investigation on the corrosion behavior of five solders by means of polarization and electrochemical impedance spectroscopy (EIS) measurements. The Sn–9Zn and Sn–8Zn–3Bi solder, in comparison with the Sn–3.5Ag–0.5Cu and Sn–3.5Ag–0.5Cu–9In solder, were tested in 3.5 wt% NaCl solution and 0.1 wt% adipic acid solution, respectively. The Sn–37Pb solder was for reference in this work. The polarization curves indicated that the Sn–9Zn and Sn–8Zn–3Bi solder showed the worst corrosion resistance both in the salt and acid solutions, in terms of corrosion-current density, corrosion potential, linear polarization resistance, and passivation-current density. Meanwhile, the Sn–3.5Ag–0.5Cu solder remained the best corrosion characteristics in both solutions. It was found that due to microstructure alteration, Bi additive to the Sn–9Zn solder improved the corrosion behavior in the salt solution, whereas decreased that in the acid solution. However, the additive of In degraded the Sn–3.5Ag–0.5Cu solder in both solutions. The EIS results agreed well with the noble sequence of the five solders subjected to the two solutions with polarization. The equivalent circuits were also determined. Nevertheless, the four Pb-free solders exhibited acceptable corrosion properties since there was not much difference of key corrosion parameters between them and the Sn–37Pb solder.