Deposition and corrosion of the boiler
Deposition and corrosion of steam boilers
How deposits and corrosion of steam boiler are formed.
The water entering the boiler should not be hard. Sometimes, due to the following reasons, some hardness enters the feed water of the boiler.
Improper use of the water softener
Lack of timely regeneration of resins
Since the increase in water temperature in the steam boiler reduces the solubility of the solutes, the water near the hot surfaces is saturated and sedimentation conditions are provided.
Sediment is a heat insulator, as a result, the thermal efficiency of the boiler decreases.
By reducing heat absorption, energy is wasted and steam production is also reduced.
As a result, to prevent the reduction of steam production, the fuel and heat load of the boiler are increased, the pipes are melted and lose their resistance.
Investigating boiler deposits and corrosion
One of the most important problems of boiler deposits and corrosion that steam boilers face is exposure to oxygen.
Since iron oxides are naturally stable, if favorable thermodynamic conditions are provided, steels will turn into these oxides.
In general, if the steel is not covered by iron oxide, favorable conditions are provided for the steel to be exposed to oxygen and the reaction occurs:
2Fe + H2O + O2 Fe 2 O3 + 2H
Rust is a reddish-brown flaky coat present on the metal. Technically, rust is a Hydrated Iron (III) Oxide whose chemical formula is Fe2O3 x H2O. The rusting of the iron formula is represented by
4Fe + 3O2 + 6H2O → 4Fe(OH)3
This reaction is the basis for chemical and mechanical aeration methods.
These methods are usually successful. In fact, oxygen corrosion in steam boilers is generally limited to steam boilers out of service.
For example, moisture accumulation on the pipe wall of a super heater out of service will dissolve atmospheric oxygen in itself.
The fracture of the protective layer of magnetite occurs due to the contraction stresses of the super heater during cooling.
The broken points prepare the anodic areas so that the oxygen in the moisture can react with the unprotected and bare parts of the metal.
Steam boiler corrosion identification method
If the destroyed surface is available, a simple visual inspection is enough to identify it.
Non-destructive testing methods such as ultrasonic should be performed at the unreachable surface.
tip:
The three factors that are effective in corrosion caused by oxygen are: moisture or water, oxygen and non-protective metal surface.
Water is usually present in working boilers. Also, the protective cover of magnetite is constantly being broken and repaired.
As far as the corrosion caused by oxygen is concerned, in
Boiler full of water is out of service, the conditions are the same as the working boiler.
The successful maintenance of this type of boiler depends on the removal of moisture or oxygen.
The boiler protection method can be by using dryers and nitrogen coating, or continuous circulation of dry and moisture-free air (30% relative humidity).
