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Corrosion
Corrosion of Cooling System Materials
The effects of corrosion on cooling tower components are easily recognized and can have a negative impact on the environment.
Cooling systems are constructed from a wide variety of metallic and non-metallic materials. Metals such as stainless steel, carbon steel, brass, admiralty brass, titanium, copper and zinc are commonly found in most cooling water systems. Some metal choices, such as carbon steel, are extremely susceptible to corrosion. Copper is commonly used in chiller tubes because of its superior heat conductive properties; however it, too, is susceptible to corrosion. Corrosion of metals can result in reduced efficiency, costly replacement and plant downtime, and eventually destroy a cooling system.

Coating exposed metal surfaces, a cheaper alternative than purchasing less corrosive metals, has long been a common practice that can prevent corrosion and prolong equipment life. Some facilities have gone a step farther to address corrosion at the design stage by selecting plastic, wood or concrete as the material choice for their cooling towers. Most facilities use a copper corrosion inhibitor to protect their chiller tubes.

Slowing Corrosion
Metal corrosion is a natural phenomenon that can be slowed down but not entirely halted. In cooling tower systems, corrosion inhibitors are used to delay the corrosion rate to an acceptable degree. Unlike most potable water, recycled water contains a nominal amount of phosphate, which acts as a built-in corrosion inhibitor.

As the table shows, SBWR’s water quality as it relates to corrosion is generally not a concern because the impacts to equipment can be effectively managed using simple existing treatment technologies. In some cases, even with treatment, corrosion may limit the number of cycles recycled water is effective. For example, ammonia levels may limit cycles of concentration for cooling systems with mild steel components. Table 4 below describes Best Management Practices to deal with corrosion.

Water Quality Parameter(1)

Level of Concern

Operating Range(2)

SBWR (2010 avg)

Impact

Method of Control

Ammonia (N) 2 0-2 <0.7 mg/L
  • Promotes biofilm
  • Corrosive to copper alloys. 
  • Combines with chloride to negate disinfecting effect of chlorine.
  • Use azoles to control copper corrosion
  • Zinc can reduce mild steel corrosion in combination with nitrates and phosphates
  • Use bromine as a biocide if ammonia is present
Chloride (Cl) >1,000 <1000 183 mg/L Corrosive to most metals. Corrosion occurs at 300 for stainless steel Use phosphate-based inhibitor combined with calcium phosphate scale inhibitor
Nitrites (NO2)
Nitrates (NO3)
300 <300 10.3 mg/L
0.28 mg/L
Corrodes mild steel Use corrosion inhibitors
Total Suspended
Solids (TSS)
2 2-10 <1.0 mg/L Adheres to biofilms causing underdeposit corrosion Control through pretreatment, sidestream filtration or use of deposit control agents