Most of the indices that have been developed to characterize the corrosion potential
of waters are based on the assumption that water with a tendency to deposit a calcium
carbonate scale on metal surfaces will be less corrosive. The Langelier index is the difference
between the actual pH of a water and its “saturation pH”, this being the pH at
which a water of the same alkalinity and calcium hardness would be at equilibrium
with solid calcium carbonate. Waters with a positive Langelier index are capable of
depositing calcium carbonate scale from solution