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This paper updates the data presented in the American Water Works Association Research Foundation publication International Corrosion of Water Distribution Systems. The research concludes the following: 1) Corrosion problems and damage in water systems can be reduced by increasing pH. The pH of hard, mineralized waters can be increased by aeration without increasing and alkalinity. The pH of soft, low mineralized acid waters can be increased by addition of an alkaline solution such as lime, soda ash, or caustic soda. Another alternative, often used in small systems, is granular limestone bed filtration. This increases alkalinity and calcium content in a most effective and least expensive manner. It is essentially a self-regulating chemical feed system. The EPA has sponsored the development of nomographs and computer programs that assist in determining size and operation time of limestone contactors. 2) If water quality cannot be improved sufficiently by pH adjustment alone, or if the quality of materials requires further corrosion reduction, the introduction of an inhibitor can be helpful. 3) Restrictive regulations on the addition of chemicals to drinking water allow phosphate and silicate dosing. In the U.S., the sodium, potassium, and calcium salts of these acids are used inhibitors, and zinc compounds are used as bimetallic inhibitors. The use of zinc is not permitted by European regulations. Its use is now being restricted by new wastewater environmental regulation in the U.S. and Canada. 4) Controlled laboratory tests have shown that the corrosion inhibiting effects of orthophosphate, blended ortho- polyphosphates, silicates, and their mixtures on different metallic materials vary considerably. 5) It is necessary to evaluate and optimize the inhibitors of choice by testing them on the water to be treated and on material to be protected. 6) Generally, polyphosphates have no positive effect on either corrosion inhibition or the reduction of metal uptake. Increased corrosion and metal uptake rates often occur because of the complexing properties of polyphosphates, which lead to either a higher solubility of metal salts or to the formation of less protective layers. 7) Orthophosphates normally have an inhibiting effect if the corrosion products or the corroding metal can form insoluble phosphates and improve the layer quality by increasing its impermeability and adherence. Corrosion produces have a low solubility and reduce the metal uptake of the water. 8) Silicates are generally less effective corrosion inhibitors except for hot water, very low TOS, pH water, or in blends. The observed effects are enhanced by the pH increase caused by the alkaline character of the silicate compounds.

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Published: 01/01/1990 ISBN(s): 0898675537 Number of Pages: 75File Size: 1 file , 2.4 MB