Globally, the main composition of the atmosphere remains basically unchanged, and the composition of air by volume is approximately 78% nitrogen, 21% oxygen, 0.94% of noble gases, 0.03% of carbon dioxide, and 0.03% of other gases and impurities [4]. Due to the natural location, climate and other conditions, as well as man-made environmental pollution, the composition of the atmosphere has become complex and diverse, and some harmful components have a tendency to gradually increase. In particular, global acid rain caused by airborne sulphur dioxide and nitrogen dioxide exacerbates the corrosion of steel materials, even though there is no naturally occurring sulphur dioxide and nitrogen dioxide. However, the vast majority of these essential air pollutants are produced by human activities. Once the sulfur dioxide and nitrogen dioxide polluting gases emitted by humans enter the atmosphere, they may be converted into secondary pollutants, nitric acid and sulfuric acid, which are easily soluble in water and form acidic water droplets that return to the ground to form acid rain and acid snow.
Different types of impurities in the atmosphere have different effects on the corrosion rate of steel. Sulfur dioxide in the industrial atmosphere and salt particles in the oceanic atmosphere have the greatest impact on the corrosion rate of steel, and the corrosion rate of steel is very low in the pure rural atmosphere. Atmospheric corrosion of steel is a complex system, in addition to man-made environmental pollution, the corrosion rate is also related to wind speed and direction, temperature and rainfall, dew cycle, solar radiation, seasonal changes and even natural dust in the atmosphere. Even under the same external conditions, due to the siltation of rainwater, the corrosion rate of the steel back side is significantly higher than that of the sunny side.
