Corrosion of lead-bearing pipes, plumbing fittings, fixtures, and solder can release lead to drinking water. To minimize the lead release, EPA recommends three options for corrosion control: pH and alkalinity adjustment, addition of orthophosphate, or addition of silicates. However, there has been only a limited application of sodium silicates for lead corrosion control in drinking water. Despite being identified in early 1920s as a potential corrosion inhibitor, studies on the use of sodium silicates was not pursued until the late 1970s. Therefore, to understand the mechanism and effectiveness of sodium silicates for corrosion control, a series of experiments are being conducted in lead-soldered copper pipes and harvested lead pipes. For these experiments, six identical pipe assemblies made from copper pipes soldered with 50 Pb/50 Sn solder and six harvested lead pipes with existing corrosion scales were used. These two systems (with six pipes each) carried water with pH 7.7 and alkalinity of 93 mg/L as CaCO3 that are typical of many utilities in the Midwest and Great Lakes region. The experiments started with a conditioning period of several months that was continued until lead and copper concentrations had dropped to consistent levels. On achieving consistent lead and copper in the water, three pipes from each system were treated with a passivation dose of 20 mg/L as SiO2 of sodium silicate for 8 weeks followed by a maintenance dose of 12 mg/L as SiO2. Sodium silicate may inhibit corrosion by increasing the pH of the water followed by formation of a protective film on the inner surface of the pipe. This underlying mechanism was studied by analyzing dissolved and suspended concentrations of lead and copper via inductively coupled plasma mass spectrometry. The differences in the concentrations of lead and copper between the control assemblies that did not receive sodium silicate and the assemblies that did receive it were compared to assess the effectiveness of the inhibitor. After the treatment, the inner surfaces of the pipes were examined to determine the changes in the morphology and composition of the scales following sodium silicate addition.

Effects of sodium silicate corrosion inhibitors on lead release from pipe materials

2019 Spring National Meeting

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