What is the primary mechanism by which polyphosphate corrosion inhibitors reduce corrosion in distribution systems?

The primary mechanism by which polyphosphate corrosion inhibitors reduce corrosion in distribution systems is through the formation of a protective film on the internal surfaces of pipes. Corrosion in water distribution systems is an electrochemical process where metal from the pipe material dissolves into the water. Polyphosphates are a class of chemical compounds that contain multiple phosphate units linked together. When added to water, polyphosphates slowly hydrolyze (break down) into orthophosphates. Both polyphosphates and orthophosphates contribute to corrosion inhibition, but through different mechanisms. Polyphosphates primarily act by forming a thin, protective film on the metal surface of the pipes. This film acts as a physical barrier, preventing corrosive agents in the water, such as dissolved oxygen and aggressive ions, from reaching the metal surface and causing corrosion. The polyphosphate molecules adsorb onto the metal surface, reacting with the metal ions to form a complex layer that inhibits further corrosion. Orthophosphates, produced from the hydrolysis of polyphosphates, also contribute to the protective film formation. They react with metal ions, particularly iron and calcium, to form a stable, insoluble phosphate layer on the pipe surface. This layer helps to passivate the metal surface and reduce the rate of corrosion. Polyphosphates can also help to sequester (bind) calcium and iron ions in the water, preventing them from precipitating and forming scale. Scale formation can create localized corrosion cells and reduce the effectiveness of corrosion inhibitors. Therefore, polyphosphates provide multiple benefits in corrosion control, including protective film formation, passivation, and scale inhibition. For example, a polyphosphate inhibitor can be added to the water to reduce lead levels by forming a protective film on lead service lines, preventing lead from dissolving into the drinking water.