The effect of carbon dioxide is closely linked with the bicarbonate content. Normal carbonates are rarely found in natural waters but sodium bicarbonate is found in some underground supplies. Calcium bicarbonate is the most important, but magnesium bicarbonate may be present in smaller quantities. In general, it may be regarded as having properties similar to those of the calcium compound except that on decomposition by heat it deposits magnesium hydroxide whereas calcium bicarbonate precipitates as carbonate. The concentrations of carbon dioxide in water can be classified as follows:
The amount required to produce carbonate
The amount required to convert carbonate to bicarbonate
he amount required to keep the calcium bicarbonate in solution
Any excess over that accounted for in types 1, 2 and 3.
With insufficient carbon dioxide of type 3 (and none of type 4) the water will be supersaturated with calcium carbonate and a slight increase in pH (at the local cathodes) will tend to cause its precipitation or scaling. If the deposit is continuous and adherent the metal surface may become isolated from the water and hence protected from corrosion. If type 4 carbon dioxide is present there can be no deposition of calcium carbonate and existing deposits will be dissolved; there cannot therefore be any protection by calcium carbonate scale. (reference)
See also: Calcium carbonate, Carbon dioxide, Chlorination, Dissolved oxygen, Langelier calculation, Langelier index, Larson-Skold index, Oddo-Tomson index, pH, Puckorius index, Ryznar index, Scaling Indices, Stiff-Davis index, Total dissolved solids, Water corrosivity