Cooling Water Corrosion Control
Rust never sleeps
Corrosion is a natural process of life. The major reason corrosion takes place in a cooling water system is the water coming in contact with metal. Water is often thought of as just H2O. It is more correctly stated as H2O ==> ( H^+) + (OH^-) . The hydrogen ion (H^+) is responsible for corrosion because it seeks an electron to become balanced. Electrons are readily available from iron atoms since they are loosely held in the outside shell. Losing the outermost electrons makes iron atoms water soluble as the iron goes from elemental to ferrous +2 oxidation state.
Maximizing corrosion control in cooling water systems requires keeping the surfaces clean. As the cooling water concentrates the impurities, they reach a point where they are no longer soluble. Corrosion is caused by these deposits, which can be of a microbiological origin or an inanimate origin such as silt scale or hydrocarbon. Most deposits are composed of both organic and inorganic components, with the microbiological growth often serving as a binder for the inorganic portion.
Most cooling towers are operated with microbiological treatments to reduce bacterial growth. These can either be an oxidizing agent such as chlorine or bromine, or non-oxidizing type such as gluteraldehyde or isothiazolin .
Other Corrosion Inhibitors
There are traditional types of corrosion inhibitors which include orthophosphate, zinc, phosphonates, molybdate, and silicate.
Orthophosphate reacts with calcium in the water to form calcium phosphate, which then deposits on the metal surface. The difficulty with this form of treatment is the ability to control the deposit formation. In areas where the water chemistry has very low hardness, phosphate is fed at high levels to get enough calcium phosphate to form. In other areas where the hardness is high, too much calcium phosphate can form and result in impaired heat transfer. High levels of phosphate accelerate the microbiological growth.
Adding zinc to phosphate is a strategy used by many programs to get better performance from the corrosion control program. Zinc has negative impacts on the environment and is banned from use in some places.
Phosphonates are used to inhibit corrosion with some effectiveness, but they degrade over time to form orthophosphate.
Molybdate is often used as a tracer for organic treatment programs such as phosphonate. Molybdate can be analyzed at low levels, 1-2 PPM. To use molybdate as a corrosion inhibitor, concentrations of 40-50 PPM are required, which is very expensive.
Using silicate is rarely done in industrial cooling water treatment programs, as the dosage levels required are high (40 + PPM), and overfeed can cause glassy deposits.
Stannous Chloride Corrosion Inhibitor
Our tin technology based corrosion control programs represent the best alternative to other treatments. Tin reduces corrosion by passivating metal surfaces and it helps to keep the heat exchange surfaces cleaner by eliminating most of the nutrient from the treatment program. This extends equipment life and reduces time and labor during routine maintenance.
Tin is environmentally superior to phosphate since the latter is a vital nutrient for microbiological growth. If you don’t feed the bugs, you won’t need to kill them later and keep the dead ones from becoming a deposit. Tin is environmentally friendly compared to the other inhibitors and is an excellent enhancement for your current corrosion control program.
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