INVESTIGATING THE EFFECTS OF ORTHOPHOSPHATE, SODIUM SILICATE, FREE CHLORINE, AND NITRATE ON THE CORROSION OF TIN-LEAD SOLDER IN PREMISES PLUMBING

Abstract

Lead is released into drinking water sources from lead-containing materials in plumbing. Even with the implementation of lead service line replacements, residents may still be at risk of lead exposure from old premises plumbing, including lead solder. Lead solder creates a galvanic corrosion scenario and is more affected by changes in source water quality, including nitrate contamination and chemical agents.

This long-term study evaluates the effects of various water quality conditions on tin-lead solder corrosion by (1) investigating the effects of orthophosphate, sodium silicate, and free chlorine, and (2) assessing the impacts of nitrate levels and interactions with orthophosphate and free chlorine on lead solder. These experiments utilized simulated copper pipe joints, galvanically connected to 50:50 tin-lead solder, using a dump-and-fill design. The first experiment was conducted over 85 weeks with 207 water changes, and the second experiment for 46 weeks with 123 water changes.

The results of this research demonstrated that free chlorine lowered total lead (according to rigorous statistical analysis), which coincided with robust scale formation by scanning electron microscopy. Silicate dramatically increased dissolved and total lead corrosion, and the interaction with orthophosphate and silicate increased total lead. Orthophosphate had no effect on total lead corrosion, yet decreased dissolved lead, indicating that lead phosphates formed but did not always remain as scale. This was confirmed by low phosphate detected by energy dispersive X-ray spectroscopy (EDS) mapping, pitting regions and regions depleted of lead. Silicate and orthophosphate were ineffective on their own, yet with chlorine addition, the combinations had lower median total lead levels.

Lastly, this study found that nitrate at 12.5 and 25 mg L-1 (nitrate as nitrogen) induced high conductivity, galvanic current, and corrosion of tin and lead. A statistically significant increase was found in lead release with the addition of orthophosphate at high nitrate levels, indicating it did not aid in protection, but rather worsened lead release.