Electrochemical corrosion behaviour of lead-free Sn-8.5 Zn-X Ag-0.1 Al-0.5 Ga solder in 3.5% NaCl solution

The electrochemical corrosion behaviour of Pb-free Sn-8.5 Zn-X Ag-0.1 Al-0.5 Ga solder in 3.5% NaCl solution was investigated by using potentiodynamic polarization methods, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis. The results obtained from polarization studies showed that an increase in the Ag content from 0.1 to 1.5 wt% decreased the corrosion current density (Icorr) and shifted the corrosion potential (Ecorr) towards more noble values. These changes were also reflected in the linear polarization resistance (LPR), corrosion rate, anodic Tafel slope (bA) and the cathodic Tafel slope (bc) values, respectively. Passivation behaviour was noted in the Sn-Zn-X Ag-Al-Ga solders with Ag content > 0.1 wt%. The oxides and hydroxides of zinc were responsible for the formation of passive film. Presence of Ag atoms in the oxide layer also improved the passivation behaviour of solders to a certain extent. X-ray photoelectron spectroscopy revealed that two different oxygen species were formed on the surface films, one was assigned to OH− in Zn(OH)2 and the other to O2− in ZnO. XPS depth profile results revealed that the two species had different depth distribution in the films. SEM and EDX analyses confirmed SnCl2 as the major corrosion product formed after the electrochemical experiments.