As was just discussed, chromate is sometimes combined with zinc for corrosion control in open recirculating cooling systems. Zinc is a cathodic corrosion inhibitor that inhibits corrosion through the formation of zinc hydroxide or zinc bicarbonate at the cathode of metal surfaces. This barrier film inhibits the passage of oxygen at the cathode, thereby, reducing the rate of corrosion.

Because the barrier films formed are not very durable, zinc is not usually used alone and is most often combined with chromate, phosphonate, and/or aromatic azoles to form a synergistic blend. Because zinc tends to precipitate at pH's above 7.8, most of the synergistic programs are designed to operate at a pH below this point. However, when combined with specific enhancers, the solubility of zinc can be extended, thereby, allowing program operation at pH values up to 8.5 or 9.0.

Zinc, like chromate, is a heavy metal that can pose toxicity problems when discharged to the environment. However, it must be emphasized that the regulations governing the discharge of zinc vary significantly throughout the country. For example, some cities still permit the use of zinc in domestic water systems.

properties of metal zink