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 (I_corr) and shifted the corrosion potential (E_corr) towards more noble values. These changes were also reflected in the linear polarization resistance (LPR), corrosion rate, anodic Tafel slope (b_A) and the cathodic Tafel slope (b_c) 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.