Phosphate-based corrosion inhibitors are commonly used to reduce the corrosion and solubility of lead in drinking water distribution systems. While phosphate is established resource, identifying effective alternative corrosion control strategies would be of benefit to water utilities. Silicate is an alkaline chemical and classified as a pH adjustment chemical. In addition, silica is typically associated with scales found on corroding drinking water distribution system materials (e.g., iron, lead, and copper). Thus, adding silicate to drinking water will increase pH, making the water less aggressive to galvanic attack and reduce metal solubility, and potentially improve the protective nature of corrosion by-product scales. Initial pH and dissolved inorganic carbon (DIC) concentration (or alkalinity) of the water are important factors to consider when establishing an appropriate silicate dosage to increase pH to a desired value. However, beyond pH adjustment, there remains an uncertainty as to whether silicate reduces galvanic corrosion and lead release, and if so, the mechanism(s) by which it functions. The objective of the study was to investigate the impact of silicate (10–120 mg SiO2/L) on the early stages of galvanic corrosion (copper-lead connection) during stagnation in chlorinated water (initial pH 8.2, 2 mg Cl2/L and 10 mg C/L, 50 mg Cl-/L, and 50 mg SO42-/L) with a final pH 9.0 using new copper-lead galvanic coupons (2.5×1.2 cm). Microelectrode techniques were used to measure in situ chemical microprofiles (pH, free chlorine, and oxygen) at 50 µm above the metal surface. Surface characterization was also analyzed using conventional methods (SEM and EDS). For a 5-days test, there was no obvious silicate consumption in the presence of new galvanic coupons. However, higher silicate dosage (120 mg SiO2/L) inhibited the pH drop at the anode by forming silicate compounds on the lead surface. pH and oxygen concentration microprofiles showed no pH changes and oxygen consumption across the metal surface of the copper-lead joints. For 40 mg SiO2/L, we observed local pH increase from pH 3.9 to pH 5.0 on lead surface over time, indicating that silicate corrosion control would be a time-course process. Future research will examine silicate impact on lead release on pre-corroded copper lead galvanic coupons.

Effects of sodium silicate on surface chemistry of fresh copper-lead galvanic joints in a prepared chlorinated water

2019 Spring National Meeting

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