This method of corrosion control is based on the theory that the addition of phosphate to a finished water will result in the formation of low-solubility lead-phosphate complexes on interior pipe surfaces. The protective layer acts as a barrier to corrosion, reducing dissolution of lead into the water. Copper solubility is not significantly affected by phosphate inhibitors at reasonable dosages. Phosphate inhibitor products applied to drinking water treatment can be separated into three categories: (reference)
The pH range at which orthophosphate is most effective for minimizing lead solubility is 7.4 to 7.8. Above pH 7.8, metal phosphate precipitation can become problematic. Orthophosphate can interact with other cations such as calcium or magnesium which may be present in the water. These interactions represent an orthophosphate demand, which reduces the amount of orthophosphate available to complex with lead.
Because phosphates are most effective over a defined pH range, maintaining stable pH throughout the distribution system is critical to success of a phosphate-based corrosion control program. Commercial phosphate-based inhibitors tend to be acidic solutions, and their effect on finished water pH must also be considered.
Silicate inhibitors may be added to finished water to form insoluble silicate coatings on pipe interior walls. This technique has been most commonly applied to minimize corrosion of iron from distribution systems. Some data are available documenting improvements in lead and copper corrosion rates with use of these products. However, the use of silicates is generally associated with the corrosion of iron as the primary metal of concern.
In addition to the corrosion inhibitors listed above, several other proprietary chemicals are currently marketed in the U.S.A. such as Stannous Chloride, Seaquest, etc. However, due to a lack of consistent operational and performance data on similar waters they should only be considered with great care.