Course: BFM 203 Water Treatment and Control

Lecture

(4/7)

Corrosion Control

Increased water analysis in the distribution system is required to ensure that chemicals are continually dosed correctly. In treating for calcium carbonate saturation, chemicals should be fed after filtration. Chemicals used include quicklime (calcium oxide), hydrated lime (calcium hydroxide), caustic soda (sodium hydroxide), and soda ash (sodium carbonate. Quick lime and hydrated lime are added to waters with low alkalinity and hardness. Caustic soda and soda ash are added to waters with high alkalinity and hardness, but pH must be increased to reach saturation.

Zinc, silica, and polyphosphate compounds may be used to prevent corrosion. Zinc compounds form cathodic films to control corrosion. Sodium silicate can be used by individual customers to treat corrosive waters. Zinc is a common sacrificial anode that is designed to corrode instead of the metal parts of the system. The anodes must be inspected and replaced when necessary. Like Zinc, polyphosphate compounds form protective films to control corrosion. Because they react with calcium and reduce calcium concentration, they may increase the corrosion rate.

Cathodic protection may be used on metal tanks. This type of protection introduces into the natural corrosion cycle an external D.C. electric current strong enough to offset the corrosion-producing action. It is important to jar test polyphosphates before use to determine the effectiveness, dose range, and any negative effects that may occur.