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Boiler Scale Formation




Deposits on heating surfaces results by exceeding the solubility limits of the impurities dissolved in the feed water.
When feed water is heated or concentrated the impurities become insoluble & form deposits.
The greater the evaporation rate the greater the deposits.
Scale formation can lead to reduced efficiency of boiler and boiler tube catastrophic failure due to thermal stress.

Calcium Carbonate
Formed by thermal decomposition of Calcium bicarbonate at heat transfer surface.
Ca(HCO3)2 + Heat      ⟶    CaCO3 + H2O + CO2

Magnesium Silicate
Formed when there is sufficient amount of Magnesium and adequate amount of Silicate ions and OH Alkalinity is deficient.                                                  
MgOH+  +  HSiO3-   ⟶  MgSiO3  +  H2SO4    Which causes Acidic corrosion too.

Silica (SiO2)
Smooth glass like deposit.
Strongly Adherent.
Insulating.
Can only be removed by acid washing with Hydrofluoric Acid.
In High Pressure boilers silica can evaporate from the boiler as Silicic acid & cause deposits on turbine blades.

Copper
Copper deposits initiates Galvanic corrosion.   
Appears as bright red/orange.
Copper comes to the boiler by corrosion and/or erosion of condensate lines, feed pump impellers, tracing lines & etc.
Erosion of copper in condensate system can occur when the boiler is priming or foaming.

Sodium Chloride
Presence of NaCl causes foaming and Priming

Magnesium Chloride
Will break down into Mg(OH)2 HCl which will cause Acidic Corrosion.

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