Scale is a dense coating of predominantly inorganic materials and results from supersaturation of water soluble minerals. Scaling is a crystalline growth of adherent layer of insoluble salt or oxide on surface where heat is exchanged. Even a very small scale thickness (,0.25 cm) can hinder heat transfer. Most common type of scale occurring in cooling water system are .
Calcium salts are inversely soluble in water. This means that solubility of calcium carbonate and other calcium salts decreases when temperature increases. Increase in temperature also decomposes bicarbonate into carbonate ions and thus encouraging Calcium carbonate formation on heat transfer surface. This is the reason that calcium carbonate scale occurs most in cooling water system. If a film of calcium carbonate is allowed to form on a heat transfer surface and controlled properly, will help in preventing corrosion to some extent.