The electrochemical behavior of Sn-Ag binary alloys in sulfate solutions

Abstract

The corrosion and passivation behavior of four binary xSn-Ag (x=26, 50, 70, and 96.5 wt%) alloys and their pure metal components was investigated in sodium sulfate solutions using electrochemical techniques. The influence of two different electrolytic parameters, namely, concentration (0.01-1.0 M) and pH (3.0-11.0) were studied while the specimens were potentiodynamically polarized between -1000 and 500mV versus saturated calomel electrode (SCE). The results showed that the corrosion current density (icorr) increases with increasing either the tin content in the alloy or the SO2-4 ions concentration in the electrolyte. Increasing the pH value of a constant composition sulfate solution was found to improve the corrosion resistance of the tin-richer alloys (x=50-96.5 wt%), and decreases a little the stability of the silver-rich one (74wt% Ag). Impedance measurements at the free corrosion potential (Ecorr) give good support for these results, where a small addition of tin to silver up to the intermetallic ratio (x=26 wt%) gives an alloy with better corrosion resistance to the aggressive sulfate medium. In terms of the tin ratio, the order of surface film stability on the tested samples, generally follows the ranking: 26>96. 5>50>Ag>70>Sn.The good corrosion resistance of the tin-rich alloy (x=96.5 wt%), surpassing those for the pure constituents tin and silver can be attributed to the homogenous phase structure of this eutectic mixture. © 2010 WILEY-VCH Verlag GmbH & Co.