Novel Insights into Lead Corrosion Control: Safely Changing Source Waters, Effective Use of Zinc Phosphate Inhibitors, Electrochemical Reversal of Solder, and Significance of Lead-Polyphosphate Complexation

Abstract

Recent high profile lead contamination events arising from lead solder corrosion have drawn attention to our alarmingly inadequate understanding of corrosion control chemistry. This dissertation provides novel contributions to an improved understanding of lead corrosion control in four sequential chapters that: 1) develop a new framework to proactively evaluate the impact of source water changes on lead solder corrosion, 2) demonstrate that in certain corrosive waters zinc orthophosphate can dramatically reduce lead contamination from solder at pHs around 7.5-9.5, but is ineffective in relatively non-corrosive waters or at pHs outside this range, 3) reveal that prolonged exposure to free chlorine can dramatically reduce lead contamination from solder by causing "electrochemical reversal," protecting solder from galvanic corrosion, and 4) apply a method for measuring problematic lead complexation by polyphosphate, which reveals a significant health concern in some water chemistries but insignificant problems in others. Chapter 2 describes how water utilities are increasingly changing source waters in response to groundwater contamination, sustainability efforts, new regulations, and consolidation. We worked with three communities who unexpectedly experienced very severe lead contamination from lead-tin solder following a seemingly innocuous change in source water. We demonstrate how existing frameworks did not anticipate lead solder corrosion problems because: 1) prolonged exposure to non-corrosive water can preserve lead solder in relatively pristine condition, which is more susceptible to severe corrosion following changes in water chemistry, and 2) prolonged exposure to corrosive water can be self-limiting because all exposed lead in copper pipes could have corroded away within a period of decades, counter-intuitively producing low present day lead contamination in a highly corrosive water. A failure to understand the implications of these scenarios resulted in erroneous logic predicting that a source water change would be safe when it was not. We develop a proactive bench scale testing protocol and improved framework that can allow utilities to avoid similar mistakes in the future. Chapter 3 addresses divergent results regarding the effectiveness of zinc orthophosphate corrosion inhibitor in controlling lead solder corrosion; that is, some research reported remarkable advantages compared to adjusting pH/alkalinity or dosing orthophosphate alone, whereas other research found no benefits. We examined relative performance of zinc orthophosphate at a wide range of pHs (6.5, 7.5, 8.5, 9.5, 10.5) in a low alkalinity, low conductivity water with corrosivity increased by addition of extra chloride or nitrate, and demonstrate that the results depend on water chemistry. Specifically, zinc orthophosphate has little benefit in water with low chloride or nitrate, in which corrosivity is low, at any pH tested. But at higher corrosivity zinc orthophosphate significantly reduced lead release at pH 7.5-9.5, including 40X reductions in lead release at pH 8.5. Yet no relative advantage occurred at pH 6.5 and pH 10.5. Results are consistent with passivation from zinc phosphate precipitation from pH 7.5 to 9.5 as predicted by equilibrium modeling and supported by scale analysis. Similar methods can be used to predict relative performance of inhibitors for controlling lead solder corrosion in other source waters and determine if these results are generalizable. Chapter 4 explores a 40-year-old mystery with profound present-day implications. A study in Portland, Oregon had revealed that free chlorine had orders of magnitude less lead contamination when compared to chloramine, but the result was discounted because chlorine was always believed to be more corrosive than chloramine. Here, we resolve the controversy by demonstrating "electrochemical reversal" of the copper-solder galvanic couple in synthesized water similar to Portland's. That is, short-term exposure to chlorine did not reduce lead contamination, but prolonged exposure to free chlorine caused the normally anodic solder to become cathodic via formation of a Pb(IV) scale. This eventually caused free-chlorine treated water to have 10-100X less lead contamination than chloramine-treated water after several months of exposure. This discovery has major implications for water treatment and can also help explain why some waters dosed with free chlorine have anomalously low lead contamination. Chapter 5 examines fears that utilities adding polyphosphates to drinking water to sequester iron and manganese, or to prevent calcium carbonate scaling, will invariably suffer from higher lead solubility and a higher risk of lead contamination. Here, we develop a simple semi-quantitative method using cation exchange resins to evaluate the significance of lead-polyphosphate complexation. Applying this test in a range of waters, we reveal the dependency of lead-polyphosphate complexation on water hardness, polyphosphate dose, and the ratio of orthophosphate to polyphosphate. This approach can be used to predict the impact of a given polyphosphate product and dose on lead complexation, allowing utilities to predict the magnitude of the problem for a given water in about a day. We find a strong linear relationship between polyphosphate dose and complexed lead. Additionally, when >200 mg/L as CaCO3 of calcium is present, >3X less lead-polyphosphate complexation can occur versus when no calcium is present. Our work also demonstrates that testing artifacts probably caused past research to overestimate the danger of the lead complexation problem. This work has significant implications for public health and is timely as water lead contamination is increasingly scrutinized. The simple tests and corrosion control methods described herein can help drinking water utilities make informed water treatment decisions to reduce lead contamination and protect public health.