Abstract
Tin-antimony (Sn-Sb) solder is used as a substitute for tin-lead solder, which has been banned from plumbing applications. In the few published studies of Sn-Sb solder corrosion, very little Sb release was found. The highest measured Sb concentration, 7.3 μg/L, was for an exposure period of 90 days. For exposures of 24 hours or less, the maximum Sb concentration was 3.7 μg/L, which is less than the USEPA maximum contaminant level (MCL) of 6 μg/L. A passivating layer of tin oxide was hypothesized to explain the low Sb concentrations. The published studies all used pH values of 8.4 or less. However, higher pH values are common in potable water distribution systems, particularly those that use lime softening. Because the solubility of tin oxide increases with pH, we hypothesized that Sb release from Sn-Sb solder may also increase with pH because of the decreasing degree of protection from tin oxide. We performed two preliminary experiments in which Sn-Sb soldered Cu coupons were exposed to tap water (pH 8.7 and 9.6) and bicarbonate/carbonate buffers (pH 8.3 to 10.4) to test this hypothesis. The Sb concentrations after 24-hour exposure in the first experiment were as high as 643.0 μg/L, and reached a maximum of 122.2 μg/L in the second.
