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.
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.