Scale: Scales are hard deposits, firmly sticking to the inner surface of the boiler. They are difficult to remove.
Scales are formed due to:
(i) Decomposition of (calcium bicarbonate: Calcium bicarbonate decomposes to form CaCO3 which is insoluble and forms precipitates.
Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2 ↑
Scale formed by CaC03 is soft and the main cause of scale formation in low pressure boilers. In high pressure boilers, CaC03 becomes soluble due to the formation of calcium hydroxide.
CaCO3 + H2O → Ca(OH)2 + CO2 ↑
(ii) Deposition of Calcium The solubility of calcium in water decreases will increase in temperature. It is soluble in cold water but insoluble in super-heated water. (This may be due to increased ionization and less salvation at high temperatures). gets precipitated in hotter parts of high pressure boilers and is difficult to remove.
(iii) Hydrolysis of Magnesium salts: Dissolved magnesium salts get hydrolyzed (at high temperature) to form magnesium hydroxide which is a soft type of scale.
MgCl2 + 2H2O → Mg(OH)2 ↓ + 2HCl
(iv) Presence of silica: The presence of silica leads to the formation of deposits such as calcium silicate (CaSiO3) and Magnesium silicate (MgSiO3). They form hard type of scale and adhere firmly and are difficult to remove. One of the sources of silica is sand filter used for hot lime-soda purification of hard water.
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