Citation:

Wasserstrom, L., M. Schock, AND S. Miller. Practical Implications from Observed Lead Pipe Scale Mineralogy in a Blended Phosphate Treated System - slides. Presented at AWWA Annual Conference and Exposition, Anaheim, CA, June 07 - 10, 2015.

Impact/Purpose:

This presentation explains the nature of pipe scales on lead and galvanized pipes in Chicago, IL, that are the product of treating the water for corrosion control with a proprietary blended orthophosphate chemical. The results show no crystalline Pb(II) orthophosphate solids, contradicting accepted corrosion control theory. Lead is bound in a complicated amorphous solid material layer that acts as a diffusion barrier, not a passivating film. The deposit consists mainly of Pb, Ca, P, Al, O and inferentially, H. In the galvanized pipe, Fe and Zn are major scale elements, as well. The nature of this film does not readily fit into the framework of the Lead and Copper Rule for defining and setting Water Quality Parameter ranges, since the impacts of water chemistry variation on lead release from these scales cannot be determined.

Description:

Many water utilities in the United States rely on the addition of phosphate-based corrosion inhibitors to optimize their corrosion control and comply with requirements of the Lead and Copper Rule. Orthophosphate is used on the theory of forming low solubility Pb(II)-orthophosphate compounds on the pipe wall to inhibit Pb release into the water. Characterization of the corrosion scales on plumbing materials can show the relationship between scale formation and water treatment history, and how this may influence Pb release over time. This study characterized the composition of corrosion scales formed on the interior surface of a Pb service line and a galvanized steel pipe from different locations in a distribution system, where a common proprietary blend of ortho- and polyphosphate corrosion inhibitor has been added for over 20 years. Since little is known about the exact mechanism of protection by blended phosphate, and no corrosion scale analyses for pipes in this type of system have been reported, it was of interest to determine if Pb(II)-phosphates were the dominant mineral phases in the passivating film. Scales were harvested by layers that were based on color and textural differences, and analyzed using X-ray diffraction and X-ray fluorescence for their mineralogical and elemental composition. Cross-sectional areas of pipe samples were also analyzed using scanning electron microscopy and energy dispersive spectroscopy. Surprisingly, analysis of the Pb pipe scales revealed no crystalline Pb-phosphate solids. Instead, an amorphous layer rich in Al, Ca, P, and Pb was observed at the scale-water interface. It is therefore likely that the mechanism inhibiting Pb release into the water is not a passivating low-solubility Pb(II)-orthophosphate scale, but rather an amorphous diffusion barrier that appeared to be porous and only weakly-adherent to the pipe wall. Such a scale could potentially slough off with a small hydraulic disturbance, exposing underlying layers high in Pb that could dissolve into the water. The galvanized pipe scales showed relatively well-crystallized Fe and Zn compounds, with additional surface deposition of Al, P, and Ca. Moreover, accumulation of Pb (likely from an upstream Pb pipe) was observed, creating a latent source of particulate Pb release and a potential for long-term Pb exposure, even if the upstream Pb pipe were to be removed. These findings demonstrate the complex nature of Pb pipe scales and emphasize the importance of scale analyses in order to understand corrosion control mechanisms. Further, they confirm the suggestion that setting optimal water quality parameters is problematic for utilities with complex scales that do not follow theoretical expectations.

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