Selection for Cooling Water

Some circulating water systems are maintained in a closed loop called a tempered water system. Since the water does not travel through a cooling tower, oxygen, which could cause corrosion, does not enter the system. Still, biocide treatment may be necessary since many corrosion-causing bacteria are anaerobic. — 

Petroleum Refining Process Corrosion, Crude Fractionation Units, and Utilities 

The use of a constant rate of corrosion for design purposes is a conservatism that most materials engineers use. Actually, in most aqueous environments/ the corrosion rate decreases with time unless erosion removes the protective film. The decrease in cbrrosion rate with time can be estimated by the following equation  

For example, the equation predicts that the corrosion rate after 20 y is one-half the rate after 5 y. 

As mentioned previously, oxygen controls the corrosion rate in the neutral pH range, if the oxygen is less than 1 ppm, the penetration rate in carbon steel or cast iron will be less than 1 mpy (0.025 mm/y) at room temperature, provided no corrosive bacteria are present. If corrosion-inducing bacteria are present, treatment with a biocide such as chlorine is imperative. In theory, because freshwater can be treated, carbon steel exchanger tubes can be used. However, control of water treating equipment is sufficiently difficult and 

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