Boiler in process industries often face serious problems of corrosion and scaling on water side. Many times these lead to tube leakage, tube puncturing , overheating etc. and hence calls for unscheduled shut down of boiler or auxiliary equipment. Corrosion and scaling in boiler must be prevented to avoid tube failure and hazards they represent. A good check over corrosion and scaling also results in reduced replacement and down time cost due to such failures.
Deposition on water side of small and medium pressure boilers comprises of calcium and magnesium salts. More complex scales containing silica, iron and phosphate are also formed depending upon the water quality. The scales formed are very hard and dense. Formation of scales and type of deposition, its physical condition and chemical composition are influenced by the following factors
- Quality of feed water
- Type of boiler
- Heat flux
- Operation and maintenance practice.
Therefore it is essential to use chemicals designed for internal treatment of boiler water to prevent hard scales of hardness salt.
Corrosion of boiler metal is taking place mainly due to attack of dissolved gases like oxygen, carbon dioxide and also due to electrochemical action of water on metal.
Generally due to the presence of dissolved oxygen present in boiler feed water causes the oxygen pitting type of corrosion unless the same is scavanged by catalysed sulphite or hydrazine or by organic amines.
Presence of carbon dioxide in condensate water reduces the pH of condensate water and thereby increases the corrosion rate of condensate line and also reduces the pH of boiler feed water also.
Corrosion leads to leakage, tube failure and therefore costly downtime and replacement costs. Corrosion reaction produce insoluble products like iron hydroxide which leads to deposition which further accelerate under deposit corrosion. The deposition of corrosion product reduces heat transfer to a large extent. Therefore it is essential to use corrosion inhibitors to protect metal against corrosion.