Title page for ETD etd-10042010-134402


Type of Document Dissertation
Author Nguyen, Caroline Kimmy
Author's Email Address cknguyen@vt.edu, nguyen.caroline@gmail.com
URN etd-10042010-134402
Title Galvanic Lead Corrosion in Potable Water: Mechanisms, Water Quality Impacts, and Practical Implications
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
Edwards, Marc A. Committee Chair
Boardman, Gregory D. Committee Member
Scardina, Robert P. Committee Member
Sean G. Corcoran Committee Member
Keywords
  • chloride
  • Galvanic corrosion
  • sulfate
  • alkalinity
Date of Defense 2010-08-27
Availability unrestricted
Abstract
As stagnant water contacts copper pipe and lead solder (simulated soldered joints), a corrosion cell is formed between the metals in solder (Pb, Sn) and copper. If the resulting galvanic current exceeds about 2 µA/cm2, a highly corrosive microenvironment can form at the solder surface, with pH <2.5 and chloride concentrations 11 times higher than bulk water levels. Waters with relatively high chloride tend to sustain high galvanic currents, preventing passivation of the solder surface and contributing to lead contamination of potable water. If the concentration of sulfate increased relative to chloride, galvanic currents and associated lead contamination could be greatly reduced, and solder surfaces were readily passivated.

Mechanistically, at the relatively high concentrations of lead and low pH values that might be present at lead surfaces, sulfate forms precipitates while chloride forms soluble complexes with lead. Considering net transport of anions in water, a chloride-to-sulfate mass ratio (CSMR) above 0.77 results in more chloride than sulfate transported to the lead anode surface, whereas the converse occurs below this CSMR. Bicarbonate can compete with chloride transport and buffer the pH, providing benefits to lead corrosion.

Although orthophosphate is often an effective corrosion inhibitor, tests revealed cases in which orthophosphate increased lead and tin release from simulated soldered joints in potable water. Phosphate tended to increase the current between lead-tin and copper when the water contained less than 10 mg/L SO42- or the percentage of the anodic current carried by SO42- ions was less than 30%.

Additionally, nitrate in the potable water range of 0-10 mg/L N dramatically increased lead leaching from simulated soldered pipe joints. Chloramine decay and the associated conversion of ammonia to nitrate during nitrification could create much higher lead contamination of potable water from solder in some cases.

In practical bench-scale studies with water utilities, the CSMR was affected by the coagulant chemical, blending of desalinated seawater, anion exchange, and sodium chloride brine leaks from on-site hypochlorite generators. Consistent with prior experiences, increasing the CSMR in the range of 0.1 to 1.0 produced dramatic increases in lead leaching from lead-tin solder connected to copper.

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