Phosphate inhibition of soluble copper corrosion by-product release

Copper corrosion by-product release to potable water is a complex function of pipe age, water quality, stagnation time, and type of phosphate inhibitor. Moderate (approximate to1 mg/l as P) doses of polyphosphate or orthophosphate generally decreased copper release in pipe rig tests of >3 years duration. The exception was pH 7.2 and alkalinity 300 mg/l as CaCO3, in which phosphate dosing significantly increased copper release over long time periods by hindering the formation of relatively insoluble malachite scale. Dosing of polyphosphates. which is invariably a blend of orthophosphate and polyphosphate due to reversion, was not as beneficial as orthophosphate. These relative effects are consistent with expectations based on simplistic solubility models, in which copper complexation by polyphosphate tends to increase soluble copper release, whereas higher doses of orthophosphate tend to decrease copper solubility. (C) 2002 Elsevier Science Ltd. All rights reserved.