Water vapor side corrosion of boiler water wall tubes

The types of water vapor side corrosion include acid corrosion, alkali corrosion, oxygen corrosion, hydrogen damage, stress corrosion cracking, corrosion under deposits and corrosion fatigue.

①Hydrogen corrosion. Due to the poor quality of soda and other reasons, the inner wall is easy to form a scale layer mainly composed of iron oxide. The heat transfer characteristics of the scale are poor, which increases the temperature of the metal tube wall under the scale, and the furnace water that penetrates under the scale will evaporate rapidly and cannot be mixed with the furnace water in the furnace tube.As a result, the concentration of various impurities in the boiler water under the scale becomes very high, resulting in free NaOH, and the concentrated NaOH solution under the scale is highly corrosive, which dissolves the protective film on the inner wall surface of the boiler tube. This part of the steel reacts with free NaOH to generate hydrogen atoms and sodium ferrite, which are hydrolyzed to Fe2O3 and hydrogen atoms. When the hydrogen atoms cannot be taken away by the water flow, they begin to penetrate into the metal, resulting in hydrogen corrosion.

② steam corrosion. The temperature of the pipe wall in the area where the water-cooled wall pipe frequently bursts is above 400°C. When a steam-water layer occurs in the tube or the circulation stops, steam corrosion may occur, and the reaction generates methane. The diffusion rate of methane in steel is very small, and it is easy to accumulate in the original microscopic voids of grain boundaries. As the corrosion reaction continues, the amount of methane on the intergranular continues to accumulate and increase, and it cannot diffuse in the steel, so a very high local internal pressure is generated between the grains, and intergranular cracks are formed along the grain boundaries, and then produce micro-cracks, so that the performance of the steel is drastically reduced, and it cannot withstand the working pressure during operation, which eventually leads to the leakage of the water-cooled wall burst pipe.

③Alkaline corrosion. When the water quality does not meet the standard, the free OH in the boiler water is wrapped under the porous sediment mainly composed of iron oxide scale, silicate scale and calcium and magnesium scale. When the furnace water evaporates, it is easy to form a high concentration of OH, which causes the furnace tube metal to undergo alkaline corrosion under scale. Due to insufficient water treatment measures or lax quality supervision of feed water and boiler water, it is easy to cause scaling on the pipe wall, affecting heat transfer, resulting in local overheating of the pipe wall and reduced strength; the macroscopic performance is that the corrosion pits on the inner wall of the fire side are uneven and shell-shaped, there are no cracks in the pits, and the wall thickness of the leakage port is obviously reduced; the entire pipe has no pipe wall swelling, and the inner wall has obvious scaling. As the corrosion pit deepens, the pipe wall gradually becomes thinner, and the stress that the pipe can withstand will gradually decrease, which will eventually lead to pipe bursting. Figure 1 shows the macroscopic appearance of the burst tube.

Water vapor side corrosion of boiler water wall tubes
Figure 1 – Macroscopic appearance of the burst tube