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The removal of hardness with an ion exchange water softener does not affect the factors which cause or accelerate corrosion. Softening does not change the pH or carbon dioxide concentration, the dissolved oxygen concentration or the total chemical concentration of minerals. A softener may reduce the amounts of solid particles in the water but, obviously cannot change other physical factors such as temperature, flow rates through pipes or volumes of water used. Thus, ion exchange softening neither causes nor controls corrosion.

Unfortunately, certain methods of calculating the prObable corrosive potential of natural waters have been misapplied to softened waters, with misleading results. The Langelier Index and some of its modifications may be used to indicate whether or not a particular water will precipitate calcium carbonate scale at a given temperature. This information certainly is useful to those responsible for operating many systems.

Further, when applied to natural waters, these methods of calculation may indicate that such an excess of carbon dioxide over alkalinity exists in the water that the precipitation of calcium carbonate would be impossible. Such excesses of carbon dioxide clearly make waters strongly corrosive as demonstrated by the low pH waters found in some areas of Canada.

These methods, however, should not be applied to waters softened by ion exchange since there is no real chemical similarity with the naturally soft acid waters. The removal of calcium by a softener obviously prevents scale formation. Yet as indicated above, it does not change either the carbon dioxide content of the water or the natural alkalinity which tends to neutralize the carbon dioxide. Thus, softening will not make a water more acid or affect the other corrosion accelerating factors.

Some persons argue that the precipitation of calcium carbonate scale will protect the metals from corrosion. While some scales are capable of such protection, other scales are porous or soft and thus, non-protective. Further, it is rare that scale formation is uniform, for the heaviest scale usually forms at points of 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 may show little or no scale. Thus even in hard scale forming waters, thousands of water heaters fail every year because of corrosion. Examinations of these heaters usually shows that corrosion occurred under or through the scale or in locations where protective scale has not formed. Thus, it is clear that corrosion protection is not assured simply because a water will precipitate calcium carbonate as indicated by various calculation or test methods. Further, none of these methods take into account the effects of dissolved oxygen, water low velocities, the presence or absence of solid particles, the volume of water through the system or other environmental factors which affect the rate of corrosion.

"It is a commonly accepted belief that the corrosive behaviour or a natural water is influenced predominately by pH and calcium carbonate saturation (frequently expressed by the Langelier Saturation Index). The results of extensive studies involving both field and laboratory investigations indicate that this concept presents an oversimplified picture of the problem. As corrosion in natural waters depends on so many interdependent variables, no simple equation or index is capable of describing adequately the corrosive potential of a water and no generally applicable recipe for appropriate corrective treatment can be given.