Discuss the characteristics of corrosion in heavy steel structures and its prevention methods

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Characteristics of corrosion in heavy steel structures and protection methods. Large-scale steel structures such as bridges, television towers, large reservoir gates, storage tanks, spherical tanks, chimneys, containers, and offshore platform steel structures are all exposed to corrosive environments caused by marine and industrial atmospheres over extended periods of time. For long-term use, the long-lasting coating protection can last 20–30 years, resulting in lower maintenance costs and significant economic benefits. 1. Corrosion characteristics of steel structures in marine and industrial atmospheres: Steel structures in atmospheric environments are exposed to sunlight, wind and sand, rain and snow, frost, as well as changes in temperature and humidity throughout the year. Oxygen and moisture in the atmosphere are key factors that cause corrosion in outdoor steel structures, leading to electrochemical corrosion. Industrial gases contain substances such as SO2, CO2, NO2, Cl2, H2S, and NH3. Although their concentrations are low, their corrosive effects on steel cannot be ignored; among these, SO2 has the greatest impact, while Cl2 can destroy the passivation layer on the metal surface. When these gases dissolve in water, they become acidic, forming acid rain that corrodes metal structures. The marine atmosphere is characterized by a high content of salts, primarily NaCl. These salt particles settle on metal surfaces; due to their hygroscopic nature and their ability to increase the conductivity of the surface film, coupled with the strong corrosive properties of Cl-, this accelerates the corrosion of the metal surfaces. The closer a steel structure is to the coast, the more severe its corrosion becomes, with its corrosion rate being many times higher than that in inland areas. 2. Common protection methods for large-scale steel structures: Steel structures are often treated with zinc or aluminum spraying, or coated with anti-corrosion coatings to create a long-lasting protective layer; alternatively, specialized heavy-duty anti-corrosion coatings can be used for protection. Metallic zinc and aluminum possess excellent resistance to atmospheric corrosion. Zinc or aluminum is sprayed on steel components; zinc and aluminum have a negative potential, and they act as sacrificial anodes to protect the steel, thereby providing effective protection for it. Currently, aluminum spraying coatings are used to prevent corrosion in industrial and marine atmospheres, and their advantages are as follows: (1) The aluminum spraying coating has a strong bond with the steel substrate, offers a long service life, and provides good economic benefits over the long term ； (2) Flexible process, suitable for long-term protection of important large-scale and difficult-to-maintain steel structures, with on-site construction possible ； (3) Coating with zinc or aluminum spray followed by a protective coating can **extend the service life of the coating. Both theoretically and in practical applications, zinc or aluminum spray coatings serve as the best base layer for protective coatings. The protective lifespan of a composite coating that combines a metal spray coating with an anti-corrosion coating is longer than the sum of the lifespans of the metal spray coating and the anti-corrosion coating alone, and it is several times longer than the lifespan of a coating applied as a single layer. (4) The heavy-duty anti-corrosion long-lasting coating consists of a primer, an intermediate coat, and a topcoat. From the perspective of long-term cost efficiency, aluminum spraying is the most economical option, although it requires a high initial investment; a well-applied coating can remain maintenance-free for up to 10 years. The epoxy zinc-rich primer + epoxy ferrochrome intermediate coat and acrylic polyurethane long-term protection system offer good cost-effectiveness.