A study on the atmospheric corrosion mechanism of carbon steel exposed to an industrial environment

摘要

Steels as structural materials are widely used in outdoor atmospheric environment without paint or other protective coatings. Thus, they are inevitably in contact with the atmosphere when deployed in service, leading to the occurrence of corrosion. Atmospheric corrosion of steels is an interaction between steel and its surrounding environment. However, since the environmental conditions vary from one place to another, the factors that directly or indirectly influence the mechanism of atmospheric corrosion of steels also vary in different environments. Shenyang is an important industrial city with a dry climate as is typical in northern China. Its environmental condition possesses relative higher concentrations of sulfur dioxide and chloride ion as well as contaminants owing to industrial development. Therefore, the study on atmospheric corrosion mechanism of steels in industrial environment is useful in terms of selecting construction materials, adopting the appropriate corrosion protection methods and predicting the life of metallic structures under service. In present work, the corrosion behavior of carbon steel subjected to Shenyang industrial atmosphere has been investigated. Also, the outdoor accelerated wet-dry cyclic corrosion testing has been adopted to study the correlation between the natural environment and the accelerated corrosion condition. The experimental results reveal that the corrosion kinetics of the initial corrosion of carbon steel follows empirical equation D=At~n, and there is a transition from corrosion acceleration to deceleration. The initial corrosion on carbon steel exposed to outdoor wet-dry cyclic condition and natural environment has found to originate from localized corrosion: the former is characterized by pitting corrosion, and the latter by pitting corrosion and filiform corrosion. Outdoor wet-dry cyclic exposure significantly promoted the initiation but rate of corrosion was about three times as fast. The rust layer formed on steels exposed to natural environment was bi-layered, comprised of an inner and outer layer. The outer layer was formed within the first 245 days and had lower iron content compared to the inner layer. However, the outer layer disappeared after 307 days of exposure, which is considered to be associated with the depletion of Fe_3O_4. The long-term corrosion kinetics of carbon steel can be divided into four stages, corresponding to different corrosion kinetics models, and the corrosion rate varies during different corrosion stages. The corrosion rate fluctuation during the deceleration process is attributed to the change in the relative amount of corrosion products. The evolution of the rust layer formed on the carbon steel has also been discussed.