Abstract
In a closed system, water circulates in a closed cycle and is subjected to alternate cooling and heating without air contact. Advantages for a closed recirculating system include better control of temperatures in heat-producing equipment, small makeup water requirements simplifying control of potential waterside problem, and reduce corrosion problems since circulating water is not continuously saturated with oxygen.
The three most reliable corrosion inhibitors for closed cooling water systems are chromate, molybdate, and nitrite. As mandated by environmental regulations, the use of chromate was discontinued, thus becoming a molybdate only, nitrite only, or a synergy of molybdate/nitrite. Molybdates are thermally stable and can provide excellent corrosion protection in both soft and hard water. Dissolved oxygen is necessary for the corrosion inhibition when using molybdate chemical treatment only. Nitrite inhibits steel corrosion by enhancing oxide formation on the surface. Air ingress can be a problem with the use of nitrite, since dissolved oxygen can cause nitrite oxidizing to nitrate. The combination of molybdate and nitrite has been shown to be synergistic and was considered by many to be the treatment of choice for closed systems. When using the combination, nitrite takes the place of oxygen in forming the protective oxide film. In all cases, pH of the circulating water should be maintained in the alkaline range. A typical closed loop chemical treatment solution include triazole thus providing the system a high pH solution. At a very high pH above 12, dezincification is a concern. This paper explores the effect of pH on closed loop corrosion inhibitors using linear polarization and cyclic polarization techniques.
