CORTEN steel, a weathering steel, is made by adding a small amount of copper, chromium, phosphorus, vanadium and other elements to ordinary steel, and made by special treatment. Its weathering and corrosion resistance is four to four times higher than ordinary steel Eight times.
CORTEN steel anti-corrosion principle
The special elements contained in CORTEN form an oxide film after contacting the atmosphere. This strong film isolates CORTEN from the outside world and is no longer affected by the weather. It achieves the purpose of weather resistance and corrosion resistance. When CORTEN or ordinary steel is exposed to the atmosphere, the following chemical reactions will occur after contact with moisture and oxygen:
Fe + 0.5O2 + H2O → Fe (OH) 2
2Fe (OH) 2 + 0.5O2 + H2O → 2Fe (OH) 3
CORTEN steel has a sturdy film outside, which prevents moisture, oxygen and steel from reacting again. As for why CORTEN contains a small amount of copper, chromium, phosphorus, etc., a protective film can be formed. Until now, So far, it is inconclusive, but we can get some inspiration from the following points.
(1) The base layer of CORTEN has almost no FeOOH (experimental formula of Fe2O3.H2O) formed, and a strong oxide film is formed between the FeOOH layer and the base layer. However, this is not the case for ordinary steel materials. FeOOH is even formed directly on the substrate layer, which causes cracks and then peels.
(2) The oxide film between the FeOOH layer and the base layer is radio graphically amorphous.
(3) Copper, chromium, and phosphorus are concentrated on the amorphous layer, and sulfur is uniformly distributed on this layer of oxide film in the form of sulfate, but for ordinary steel materials, sulfur is mostly concentrated on the oxide on the outer layer. .
Difference between CORTEN steel and general steel
The corrosion rate comparison between CORTEN steel and general steel can be expressed by two exposure tests. The first experiment was conducted in Kearney, New Jersey, USA, and the second experiment was conducted in Hyogo, Japan (both experimental locations are industrial areas). It can be seen from the experiment that the corrosion of CORTEN stopped after three to four years of experiment, while other experimental steel materials continued to rust.
The formation of oxide films is related to climatic factors. Generally speaking, climatic factors include: temperature, sunshine duration, humidity, rainfall, wind speed, carbon dioxide and salt content in the atmosphere. Usually, these factors are directly proportional to the corrosion rate. According to experiments conducted at the Shizukuri Cape of Japan, the temperature, humidity, and salinity of the place are high. The corrosion rate of general steel materials reaches 0.75 mm per year, and the corrosion rate of CORTEN Only 0.25 mm per year.
