Electrochemical corrosion study of Sn-XAg-0.5Cu alloys in 3.5% NaCl solution

The electrochemical corrosion behavior of Sn-XAg-0.5Cu alloys in 3.5% NaCl solution was examined using potentiodynamic polarization techniques. The Ag content in the alloy was varied from 1 to 4 wt%. The polarization curves obtained for the alloys show an active-passive transition followed by a transpassive region. Sn-XAg-0.5Cu alloys with higher Ag content (>2 wt%) show a strong tendency toward passivation. The passivation behavior has been ascribed to the presence of both SnO and SnO₂ on the anode surface. Increase in Ag content from 1 to 4 wt% results in a decrease in the corrosion-current density (I_corr) and linear polarization resistance (LPR) of the alloy. Nevertheless, the corrosion potential (E_corr) shifts toward negative values, and a decrease in corrosion rate is observed. The presence of Cl^- ion initiates pitting and is responsible for the rupture of the passive layer at a certain breakdown potential. The breakdown potential, (E_BR) decreases and shifts toward more noble values with increase in Ag content in the alloy. Surface analyses by x-ray photoelectron spectroscopy (XPS) and Auger depth profile studies confirmed the formation of both Sn(II) and Sn(IV) oxides in the passive layer.