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Local corrosion outside the pipe is mainly atmospheric corrosion, occurring in areas where the protective layer is damaged and where moisture exists along with a tendency for dust to accumulate. Atmospheric corrosion is an electrochemical corrosion that occurs when a water film is present on the metal surface, primarily involving the oxygen reduction cathodic process. The main factors affecting the corrosion of pipelines in the atmosphere include relative humidity, temperature, and atmospheric composition. At a certain temperature, the corrosion rate of steel by dry air is very low; it is only when the relative humidity of the air increases to a certain level that the corrosion rate rises sharply. This range of relative atmospheric humidity is known as the critical humidity, and it has been determined to be approximately 50% to 70%. When the pipe surface is exposed to air at a temperature higher than its own, dew formation may occur on the metal surface. Whether dew formation occurs near the critical humidity is related to temperature changes; the atmospheric corrosion rate is higher in areas with higher average temperatures. Industrial air contains pollutants such as SO2, H2S, CO2, and ash. Among them, SO2 is the most harmful pollutant, and its corrosion mechanism can be explained by the \"acid regeneration cycle\": SO2 is first adsorbed on the surface of pipes where it reacts with oxygen to form FeSO4; then FeSO4 hydrolyzes to produce free sulfuric acid, which in turn accelerates the corrosion of steel. The newly formed FeSO4 then hydrolyzes to yield more free acid, and this cycle repeats itself, accelerating the corrosion of steel. Solid dust particles falling on the pipe surface serve as aggregation centers for water molecules.