Stabilization of a Magnesium Silicate Scale Providing Corrosion Control in a Potable Water Distribution System

Abstract Analysis of pipes harvested from the Oklahoma City Water Utility Trust distribution system revealed thick (∼1 mm) magnesium- and silicate-containing scale covering pipes in service prior to the 1960s. After discovering extremely low lead levels in homes (≤1.2 ppb) whose service lines were coated with this lining, it was hypothesized to be a novel and effective means of corrosion control. To mitigate the risk of destabilizing the scale, we (1) characterized the solid, (2) examined its solubility, (3) investigated the impact of phosphate inhibitors, and (4) considered an operational strategy for maintaining the coating. Experimental results indicated that this coating did not form below pH 10 without the presence of a pre-existing solid or “seed”. However, with a seed, the scale was in equilibrium at pH 8.5–9 in one water source (3 mg/L Mg and 1.5 mg/L Si) and 7.5–8 in another source (32.5 mg/L Mg and 3.5 mg/L Si). The scale dissolved below these pH values and precipitated above it. Phosphate inhibitors had little effect on solubility at pH 8.5 and 10, although hexametaphosphate and Zn-orthophosphate slightly reduced the dissolution rate at pH 7. Practical testing with the seed could predict the optimal pH to maintain the coating.