Causes & Influencing Factors of Boiler Shutdown Corrosion

EPCB Boiler provides you with professional solutions to boiler problems. Understand the causes influencing factors boiler shutdown corrosion

The corrosion damage occurred during the cold standby, hot standby or overhaul of the boiler is called shutdown corrosion. Boiler shutdown corrosion is mainly due to the metal inner surface is not completely dry during the stoppage and oxygen in the atmosphere continues to leak into. Although the internal and external corrosion of the boiler heating surface occurs simultaneously during the shutdown, the internal corrosion is more serious than the external corrosion, so the shutdown corrosion mainly refers to the internal corrosion of the heating surface.

After stopping the boiler, with the decrease of pressure and temperature, the water in the boiler vapor condenses into water, so there will be a vacuum inside the boiler. If the external air leaks into the furnace, Oxygen can easily corrode the metal when with water and steam. Because of the boiler structure, if not use anyway the shutdown corrosion is inevitable. For example, the vertical superheater can not drain off the storage water in it.

When the water-soluble salt scale is deposited inside the heating surface of the metal, it will absorb water and form a high concentration of the salt solution, which will aggravate the stop-use corrosion and cause ulcer corrosion. This happens frequently at the inlet of the superheater. Because the separated saturated steam always carries some of the boiler water when it enters the superheater. When the saturated steam enters the superheater, it absorbs heat. The boiler water evaporates into steam, and the salt in the boiler water deposits on the inner wall of the inlet tube of the superheater.

The main factors affecting the boiler shutdown corrosion are as follows:

For the shutdown boiler which drains water, the moisture on the metal surface has a great influence on the corrosion rate. When the air humidity is big, it is easy to dew on the metal surface and form a water film, causing the corrosion to aggravate. When the metal surface is dry and the relative humidity inside the boiler is less than 30%, the corrosion of iron can be completely prevented.

2.      The salt content in water

The increasing salt content in boiler water or metal surface water film will accelerate corrosion, especially when the content of oxide and sulfate increases, the corrosion rate will increase obviously.

3.      The cleanliness of the metal surface.

When there is sediment or water slag on the metal surface, it is easy to accumulate residual water, which makes the metal surface under the scale slag always in a wet state. At the same time, the scale slag hinders the diffusion of oxygen, while the metal surface around the scale slag has more oxygen. In this way, due to the difference in oxygen concentration, a concentration corrosion battery is formed. The cathode is the place where the oxygen concentration around the scale is high, and the anode is the place where the oxygen concentration under the scale is low so that the metal under the scale is corroded.

It can be seen from the above that in order to prevent corrosion of shut-down boilers, the metal surface of boilers should be kept dry and clean, and scale or accumulated slag should be removed in time. For wet protection, salt content in water should be reduced as far as possible to prevent accelerated oxygen corrosion.

EPCB Boiler provides you with professional solutions to boiler problems. Understand the causes influencing factors boiler shutdown corrosion