Atmospheric corrosion resistant steel (i.e. weathering steel) refers to a low-alloy steel with good atmospheric corrosion resistance made by adding a certain amount of Cu, P, C or Ni, Mo, Nb, Ti and other alloy elements to the steel. In industrial and rural atmospheric environments, weathering steel has excellent resistance to atmospheric corrosion due to the formation of a dense and stable oxide protective film on its substrate surface, which hinders the entry of corrosive media. However, the rust layer structure formed by corrosion on the surface of ordinary carbon steel substrate is loose and has microcracks, which cannot truly protect the substrate steel.
Corrosion resistance mechanism
From the influence of corrosion products on the corrosion process, atmospheric corrosion of steel is a process in which oxygen in the air undergoes electrochemical reactions through the rust layer in the presence of a water film. The rust layer is composed of a loose outer rust layer and a dense inner rust layer, and the alloying elements in steel mainly act through the influence of the inner rust layer. In the corrosion products of weathering steel, dense internal rust layers of Cu, P, and Cr can be observed to be enriched. The high corrosion resistance of weathering steel is not only related to the high impedance of the internal rust layer, but also to the dense and fine grain size of the internal rust layer and the enrichment of Cu and P. The presence of this dense internal rust layer is reflected in the electrochemical behavior of the steel, which is hindered by the anodic process. The process of gradually improving its density during the corrosion process precisely illustrates the characteristic of longer time and stronger corrosion resistance.
Atmospheric corrosion
From a global perspective, the main components of the atmosphere remain largely unchanged. Through experimental measurements, the composition of air by volume is approximately 78% nitrogen, 21% oxygen, 0.94% rare gases, 0.03% carbon dioxide, and 0.03% other gases and impurities. Due to natural location, climate, and human environmental pollution, the composition of the atmosphere has become complex and diverse, with some harmful components showing a gradual increase trend. Especially the global acid rain caused by sulfur dioxide and nitrogen dioxide in the air exacerbates the corrosion of steel materials, despite the absence of naturally occurring sulfur dioxide and nitrogen dioxide. However, the vast majority of these basic air pollutants are generated by human activities. Once the sulfur dioxide and nitrogen dioxide pollutants emitted by humans enter the atmosphere, they may be converted into secondary pollutants such as 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, acid snow, etc.
Different types of impurities in the atmosphere have varying effects on the corrosion rate of steel. The impact of sulfur dioxide in industrial atmosphere and salt particles in oceanic atmosphere on the corrosion rate of steel is the greatest, and the corrosion rate of steel is very low in pure rural atmospheric environment. The atmospheric corrosion of steel is a complex system, and in addition to human environmental pollution, the corrosion rate is also related to wind speed and direction, temperature and rainfall, dew period, solar radiation, seasonal changes, and even natural dust in the atmosphere. Even under the same external conditions, the corrosion rate on the back of the steel is significantly higher than that on the sunny side due to the accumulation of rainwater.
Use
Weathering steel has been widely used in steel structures such as buildings, vehicles, bridges, and towers due to its excellent corrosion resistance. There are three main ways to use weathering steel: exposed use, coating use, and use after rust stabilization treatment.
(1) Bare use
The most common use of weathering steel is exposed. After 3 to 10 years of use, the rust layer on the surface of weathering steel gradually stabilizes, corrosion development slows down, and the appearance takes on a beautiful chocolate color.
Due to the influence of factors such as the chemical composition of the steel, usage environment, water retention and dust accumulation in structural details, and mechanical wear, the stabilization process of the rust layer in weathering steel is affected. Therefore, if used improperly and the formation conditions of the stable rust layer are disrupted, weathering steel will also experience severe corrosion.
(2) Painting application
In many departments such as construction, bridges, and vehicles, weathering steel, like ordinary steel, is mostly used for coating. Compared with ordinary steel, coated weathering steel exhibits extremely superior corrosion resistance. However, due to increased usage costs and operational procedures, it is difficult to widely use painting on large components.
(3) Used after stabilization treatment
At the beginning, a treatment is applied to the surface of the component to shorten the formation process of the stabilized rust layer of weathering steel. This can not only avoid the phenomenon of yellow rust liquid hanging in the early stage of weathering steel use, prevent pollution, but also form a stable rust layer. Although exposed use is an economical and unique method of using weathering steel, it takes a considerable amount of time to complete the stabilization process of the rust layer in the natural environment. Before forming the stabilized rust layer, early rust liquid hanging and flying pollution of the surrounding environment often occur.
