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Corrosion and Softened Water
Water Quality Answers
Is softened water more corrosive?
No.
Because the Langlier (calcium saturation) Index is lowered in water that has had calcium removed, skeptics sometimes consider softened water to be more corrosive. But softening of water via cation exchange does not make water more corrosive. In fact, the U.S. Environmental Protection Agency (USEPA) and the American Water Works Association have both recently corrected their enclosed brochures as to the misconception that ion exchange softening has an effect on the corrosivity of water.
WQA has recently completed a research study with Tom Sorg and Mike Schock in the USEPA Drinking Water Research Division in Cincinnati, Ohio to clarify the noncorrosive effect of the ion exchange softening process. The study is on well waters with 23 and 11 grains per gallon (gpg) total hardness and softening to less than one gpg to examine any differences between the hard and softened waters in corrosion rates and leaching from lead, copper, copper tubing with 50/50 solder, galvanized pipe, and brass. Results from the study shown no increased dissolution of lead in the softened water. The final research report is now available from the WQA Publications Department, and states the same conclusion—that ion exchange softened water does not produce higher metal levels than nonsoftened water. Research results, such as these, confirm the minimal effects of hardness minerals on corrosion.
WQA also has available other materials to show that ion exchange softening does not affect any of the factors which contribute to water corrosivity. Neither the water pH, dissolved oxygen content, TDS concentration, electrical conductivity, ammonia, chloride, or sulfide amounts, temperature, nor flow velocity are significantly altered by home water softening. While it is true that soft water will deposit less scale coating on metal surfaces, the softening itself will not change a water's corrosivity or lack of it. Corrosion of copper pipe, for example, is most often caused by oxygen concentrations in the water. Oxygen corrosion is usually found with surface water supplies and in deep well supplies in arid regions.
Municipal water systems often use calcium carbonate saturation indices to help control precipitation in city water mains. This information is useful where utilities try to lay down a protective film in hopes of retarding the rate of corrosion in municipal distribution systems. The Langlier Index (LI) is such a calcium carbonate saturation index that measures the potential of a water to deposit calcium carbonate scale. Water with an LI greater than zero tends to be of higher hardness and alkalinity and therefore to be scale forming. An LI less than zero represents water that tends to dissolve CaCO3.
However, these calcium carbonate saturation indices do not rate the corrosive tendency of the water itself, nor the effect of scale in household plumbing. While some scales are capable of such protection, scales in a household water system are often porus or soft and thus non-protective. It is rare that hardness scale formation is uniform in household plumbing, for the heaviest scale usually forms at points of greatest heat transfer and at low points in a system. In a water heater, for example, most scale forms at the bottom where heat is applied, while the top of the heater tank may show little or no scale. Thus, even in hard scale-forming water, thousands of water heaters can show that corrosion has occurred under or through the scale, or in locations where protective scale has not formed. Thus, it is clear that corrosion protection in household plumbing is not assured simply because a water heater will precipitate calcium carbonate, as indicated by various scale indices. Further, none of these indexing methods take into account the effects of dissolved oxygen, ammonia, chlorides, hydrogen sulfide and other sulfur compounds, water flow velocities, the presence or absence of solid particles or the volume of water through the system which markedly affect water corrosivity.
The simple replacement of hard water calcium and magnesium with soft water sodium or potassium has no detrimental effect on water contacting materials. In fact, the nonscaling characteristic of soft water is a benefit to such pumping and plumbing appurtenances. Ion exchange water softening neither causes nor controls corrosion. Please call the Water Quality Association if you have any questions or would like to discuss any of these materials.
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Sat, Jul 31, 2010
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Copyright 2010 by the Water Quality Association – All rights reserved.
Water Quality Association
International Headquarters & Laboratory
4151 Naperville Road
Lisle, IL 60532-3696
USA
630 505 0160, Fax 630 505 9637
info@wqa.org, www.wqa.org
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