Copper and Lead in Water
Both lead and copper are ordinarily pretty resistant to corrosion. Nevertheless water is the universal solvent, and so long as these two metals continue to be used in water supplies there will be some lead and copper in the water exposed to them.

EPA's Lead and Copper Rule (LCR) doesn't directly address corrosion, rather it addresses the presence of lead and copper in tap water. Controlling the appearance of lead and copper in solution isn't just a question of controlling corrosion. In fact, often, the corrosion of copper and lead does not result in the introduction of metal ions directly in solution, rather, it results in the formation of oxide, carbonate, or hydroxycarbonate scales on the metal surface. Thus the presence of lead or copper in solution is not just a question of corrosion, but also the solubility of these scales. Figure 1 illustrates many of the processes that influence the concentration of a corroding metal in solution.

Where lead is concerned small particulates of scale are also sometimes released, particularly if something occurs to disturb the surface of lead pipe.

Water Treatment to Reduce Lead and Copper Levels
Because the plumbing in the homes of many consumers contains components made of lead and copper and it is not practical to remove them, the LCR requires water utilities to optimize corrosion control treatment to minimize the problem as much as possible. Treatment alternatives listed in the LCR include pH adjustment, adjustment of calcium carbonate saturation, and the addition of orthophosphate or silicate inhibitors. Of these, the adjustment of pH and the addition of orthophosphate have proven the most successful.

Where the adjustment of pH and alkalinity is concerned, the solubility of the scales on the surface of the metal is of particular importance. The solubility is controlled by the pH and the dissolved inorganic carbon (DIC). The DIC is the sum of the components of the carbonate system in the water (CO₂ + HCO₃^- + CO₃⁼), expressed as carbon (mg-C/L). The diagram below illustrates the zones of solubility that are generally observed for both copper and lead. Solubility increases with decreasing pH or increasing DIC.

As a result, treatment strategies generally involve adjusting water quality to regions of lower solubility on this diagram. The addition of sodium hydroxide increases the pH without changing the DIC. If a groundwater has excessive CO₂, air stripping reduces the DIC while also increasing the pH. The addition of carbonate and bicarbonate will increase the DIC and the pH at the same time.

Phosphate addition is commonly used as the treatment of last resort. It appears to perform both by reducing the solubility of lead and copper, but also by behaving as an anodic inhibitor and reducing the underlying corrosion rate.

Trussell Technologies, Inc. would like to acknowledge Prof. Marc Edwards in the Civil and Environmental Engineering Department at Virginia Polytechnic Institute and State University for the corrosion photos.