Electrochemical corrosion behaviour of lead - free solder alloys in 3.5 % Nacl solution

Pb free solders has become increasingly important for industrial applications, since an international effort has been made to develop and characterize lead free solders for replacing traditional Pb-Sn solders as a consequence of governmental regulations and marketing pressure concerning health and environmental hazards of lead. The corrosion behaviour of a set of Pb free solder alloys were investigated in 3.5 % NaCl solution by employing potentiodynamic polarisation techniques. The various Pb free solders used in the present study were Sn-XAg-0.5 cu (X= 1-4 wt %); Sn-8.5Zn-XAg-0.1 Al-0.05 Ga (X=0.1-2 wt %), Sn-8.5Zn-XAl-0.5 Ga (X=0.02-0.5 wt %) and Sn-8.5Zn-XAg-0.1 Al-0.5 Ga where X varies from 0.1 to 2 wt %. Polarisation curves revealed that Sn-XAg-0.5 Cu alloys with higher Ag content (> 2wt %) exhibited a stronger tendency towards passivation and the passivation behaviour was ascribed to the presence of both SnO and SnO2 on the cathode surface. An increase in the Ag content from 1 to 4 wt % decreased the corrosion current density (Icorr) and shifted the corrosion potential (E_corr) towards more negative values. These changes were clearly reflected in the corrosion rate and linear polarization resistance of the solder alloy. Nevertheless for Sn-8.5 Zn-XAg-0.1Al-0.05Ga alloy, an increase in the Ag content from 0.1 to 2 wt % resulted in a progressive increase in the corrosion current density and insignificant improvement in the passivation behaviour of the solder alloy. On the other hand the four element Sn-8.5Zn-XAl-0.5 Ga alloy showed decrease in both the corrosion current density and corrosion rate with increase in Al content from 0.02 to 0.05 wt %. The best results were observed for Sn-8.5Zn-XAg-0.1Al-0.5Ga solder alloy. The corrosion current density underwent a significant decrease and the corrosion potential shifted towards more noble values with increase in Ag content from 0.1 to 1.5 wt %. SE micrographs established that the oxides and hydroxides of zinc were responsible for the formation of passive film and the presence of Ag atoms in the oxide layer contributed towards the passivation behaviour of solders to a certain extent. SEM and EDX analysis also confirmed than SnCl2 was the major corrosion product formed after the electrochemical experiments in all the investigated alloys.