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1. Relative humidity of air: The relative humidity of air refers to the percentage of absolute humidity (the amount of water vapor actually present in the air, in grams per cubic meter) compared to saturated humidity (the amount of water vapor at which the air is saturated; beyond this limit, water vapor condenses into droplets that fall). 2. Changes in air temperature and humidity: Low temperatures and high humidity lead to the condensation of water vapor, resulting in dew formation on metal surfaces. This triggers electrochemical corrosion, thereby accelerating the corrosion process. Dew formation can occur when there is a large temperature difference between day and night, as well as between indoors and outdoors. Rusting is most severe when dew forms periodically. Metal corrosion occurs most easily in humid and rainy conditions, especially in areas or during seasons with high relative humidity, such as hot and rainy periods or foggy times. 3. Acidity and alkalinity of the solution: Chlorides in the solution have a significant impact on the rate of corrosion. Salt spray, sweat from hands, residual salts from heat treatment, and leftover welding flux all contain chloride ions; if these are not thoroughly removed, rusting can occur very easily. 4. Corrosive gas pollution in the air: The most harmful substance is sulfur dioxide (SO2) in the atmosphere. When SO2 dissolves in water, it turns into sulfurous acid, H2SO3. If SO2 is oxidized to form sulfur trioxide (SO3), its hygroscopicity increases, and the corrosiveness of its aqueous solution becomes greater. In particular, when there is pollution from chemical plants such as hydrogen sulfide (H2S), ammonia (NH3), and hydrochloric acid (HCl), it accelerates the electrochemical corrosion of metals. 5. Oxygen in the atmosphere: Metals rust in the atmosphere, with moisture and oxygen being the main factors involved. There is a large amount of oxygen in the atmosphere, accounting for about 1/5 of the air’s volume. The water film layer adsorbed on the metal surface in the atmosphere is very thin, allowing oxygen to dissolve and penetrate easily. This facilitates the depolarization process of oxygen and plays a key role in atmospheric corrosion. 6. Air pollutants: Pollutants in the air, such as soot, coal ash and other carbon and carbide compounds, metal oxides, sand, sodium chloride, sulfuric acid, and other salt particles, can easily penetrate the water film and passivation layer on the metal surface, especially when combined with water vapor. They displace and replace the oxygen ions in the passivation layer, turning them into soluble chlorides, thereby causing and accelerating the anodic dissolution corrosion of the metal. 2SO2 + O2 → 2SO3; SO3 + H2O → H2SO4; H2SO4 + Fe → FeSO4 + H2; 4FeSO4 + O2 + 2H2SO4 → 2Fe2(SO4)3 + 2H2O; Fe2(SO4)3 + 6H2O → 3H2SO4 + 2Fe(OH)3. According to available data, one molecule of sulfuric acid is capable of corroding at least 100 iron atoms, which illustrates the severity of its corrosive effect. Furthermore, salt mist and dust possess water absorption and corrosive properties; when they fall on metal surfaces, they accelerate the corrosion of that metal.