Corrosion inhibition open systems

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

In the case of IRB, the results of a piece of research concluded that the static or nonstatic regime of the environment could have a profound effect on acceleration or inhibition of corrosion. As may be inferred from this research, if the system is closed so that dilution of ferrous ions is not happening, corrosion may be slowed down otherwise, in an open system where dilution of ferrous ions is possible, corrosion acceleration can be expected. 

They are effective inhibitors for iron and a number of metals in a wide variety of waters. Like chromates, nitrites are anodic inhibitors and they inhibit the system by forming a passive film with ferric oxide. These are environmentally-ffiendly inhibitors. Besides steel, nitrites also inhibit the corrosion of copper, tin and nickel alloys at pH levels 9-10. Chromate is an extremely effective inhibitor for corrosion prevention of aluminum alloys. Nitrites should not be used in open systems as they would oxidize to nitrates in the presence of oxygen. 

Once-through cooling systems require inexpensive corrosion inhibitors. In open systems, corrosion is more severe and good inhibition is imperative. The situation is similar to that in municipal water supplies, so comparable remedial measures, namely addition of lime or polyphosphates, are used. In waters that are very corrosive due to high chloride concentrations, chromates or nitrites may be required in addition to polyphosphates. 

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