Which chemical is used as a corrosion inhibitor in water treatment?

• A. Phosphate
• B. Lime
• C. Silicate
• D. Sodium hydroxide

Answer: (A)

Phosphate-based corrosion inhibitors are added to water to form a protective film on the interior surfaces of metal pipes, slowing the leaching of metals into the water. Orthophosphate ions react with the metal surfaces (like iron, copper, and lead) to form insoluble metal phosphate films, creating a barrier that reduces corrosion. This approach directly targets corrosion control in the distribution system, which is why phosphate is the best choice. Lime and sodium hydroxide mainly serve to raise pH and increase alkalinity, which can influence corrosion indirectly but are not dedicated corrosion inhibitors. Silicate can also form a protective film, but phosphate is the standard and most effective inhibitor in many water treatment programs.