a1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
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 (Icorr) and linear polarization resistance (LPR) of the alloy. Nevertheless, the corrosion potential (Ecorr) 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 (EBR) 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.
(Received February 3 2007)
(Accepted May 8 2007)