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1. Corrosion mechanism of steel at room temperature (below 100°C): The corrosion of steel at room temperature is mainly electrochemical corrosion. When steel structures are used in normal temperature atmospheric environments, the steel is corroded by moisture, oxygen, and other pollutants in the atmosphere (such as unremoved weld slag, rust layers, and surface contaminants). When the relative humidity of the atmosphere is below 60%, corrosion of steel is very mild ； However, when the relative humidity increases to a certain value, the corrosion rate of steel rises sharply; this value is known as the critical humidity. At room temperature, the critical humidity for ordinary steel is 60%–70%. When the air is polluted or contains salt in coastal areas, the critical humidity is low, making it easy for a water film to form on the surface of steel. At this time, the slag and the uncleaned rust layer (iron oxide scale) act as the cathode, while the steel structural components (the base material) serve as the anode, resulting in electrochemical corrosion within the water film. The water film formed on the surface of steel by the adsorption of moisture in the atmosphere is the determining factor for steel corrosion ； The relative humidity of the atmosphere and the levels of pollutants are important factors affecting the degree of atmospheric corrosion. 2. Corrosion mechanism of steel at high temperatures (above 100°C): The corrosion of steel at high temperatures is primarily chemical corrosion. At high temperatures, water exists in gaseous form, and electrochemical effects are minimal, becoming a secondary factor. When metals come into contact with dry gases such as O2, H2S, SO2, Cl2, etc., corresponding compounds (chlorides, sulfides, oxides) are formed on the surface, resulting in chemical corrosion of the steel.