What are the causes of under-scale corrosion in tubular heating furnaces?

Are there corrosive substances in the water used for the boiler?

All heating furnaces suffer from coking problems. Different process conditions result in varying degrees of coking. Under conditions of high component weight, high temperature, and low flow rate, coking tends to be more severe. Coking can cause overheating and carburization of the material. The wall temperature of the coke bed with an average thickness of 1 mm increases by 30–50°C. It exacerbates high-temperature oxidation on the surface of the furnace tubes, accelerating the thinning of the tube walls and causing the tubes to bulge or rupture. Coking in the furnace tubes accelerates the carburization of the material. The carburization process alters the microstructural properties of the steel. Carburization: During the production process, highly active carbon atoms are generated. The process of gradually penetrating and diffusing from the inner surface of the furnace tube into the interior of the substrate is called carburization. In the coking inside the furnace tube, carbon atoms penetrate and diffuse into the matrix metal of the inner wall, where they form carbides of Cr with the Cr present in the matrix. Oxidation occurs in the presence of oxygen, which **reduces** the bonding strength between the matrix grains, leading to the formation of microcracks. Following the cycle of carburization, oxidation, creep, and cracking, the cracks continue to expand, leading to leakage and damage of the furnace tube. The corrosion beneath the fouling in the convection chamber is mainly caused by acidic substances.

Reasons for under-scale corrosion in the furnace tubes of heating furnaces: On the side of the furnace tubes covered with scale, heat gradients exist. The reduced flow rate of the working fluid leads to a significant increase in temperature. At high temperatures, water and iron hydrolyze to form ferric oxide and hydrogen. Hydrogen reacts with the carbon in the pipe to form methane at high temperatures. Methane polymerizes between the grains of the pipe material, causing cracks.