A Novel Mechanism of Lead–Tin Solder Spallation in the Presence of Nitrate

Elevated 90th percentile water lead levels associated with the release of large chunks of lead–tin solder to drinking water were recently attributed to corrosion by nitrate. A new mechanism of water lead contamination via nitrate-induced spallation of solder was elucidated through surface analyses of new solder coupons and harvested pipe samples, bench-scale testing with lead–tin alloys of varying tin content, and electrochemical tests. Nitrate attack was associated with the formation of ammonia and other reduced nitrogen compounds, causing detinning of solder alloys with more than 40% tin by weight and eventually complete detachment of solder from the point of adhesion to copper pipe via the formation of a corrosive microenvironment. Solder spallation was observed in as little as 6 weeks and resulted in 1.3 times more metal weight loss than could be explained by Faraday’s law for both tin and 50/50 lead–tin solder in 32 h electrochemical tests. Understanding this mechanism of attack can help explain and predict water lead contamination events and inform the development of optimal corrosion control strategies for potable water supplies.