How does stainless steel rust and corrode?

Why does stainless steel also rust? When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: “Stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be a problem with the steel.” In fact, this is just a one-sided misconception about the lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.

1. Stainless steel does not rust

An oxide will also be formed on the surface. The rust mechanism of all stainless steels currently on the market is due to the presence of the Cr elements. The fundamental reason for the corrosion resistance of stainless steel is the passive film theory. The so-called passive film is a thin film mainly composed of Cr2O3 on the surface of stainless steel. Due to the existence of this film, the corrosion of the stainless steel substrate in various media is hindered, and this phenomenon is called passivation. There are two cases for the formation of this passivation film. One is that stainless steel itself has the ability of self-passivation, and this self-passivation ability is accelerated with the increase of chromium content, so it has rust resistance; A more extensive formation condition is that stainless steel forms a passive film in the process of being corroded in various aqueous solutions (electrolytes), which hinders corrosion. When the passivation film is damaged, a new passivation film can be formed immediately.

The reason why the passive film of stainless steel has the ability to resist corrosion has three characteristics: one is that the thickness of the passive film is extremely thin, generally, only a few microns when the chromium content is >10.5%; the other is the specific gravity of the passive film is greater than the specific gravity of the substrate; these two characteristics indicate that the passivation film is both thin and dense, so it is difficult for the passive film to be broken down by the corrosive medium to rapidly corrode the substrate; the third characteristic is that the chromium concentration ratio of the passive film The substrate is more than three times higher; therefore, the passive film has high corrosion resistance.

2. Under certain conditions, stainless steel will also be corroded

The application environment of stainless steel is extremely complex, and the simple chromium oxide passive film cannot meet the requirements of high corrosion resistance. Therefore, it is necessary to add molybdenum (Mo), copper (Cu), nitrogen (N), and other elements to the steel according to the different use conditions to improve the composition of the passivation film and further improve the corrosion resistance of stainless steel. The addition of Mo strongly promotes the passivation of the matrix because the corroded product MoO2- is close to the matrix, preventing the corrosion of the matrix; the addition of Cu makes the passivation film on the stainless steel surface contain CuCl, which does not interact with the corrosive medium. Corrosion resistance; adding N, because the passivation film is enriched with Cr2N, the Cr concentration in the passivation film is increased, thus improving the corrosion resistance of stainless steel.

The corrosion resistance of stainless steel is conditional. A grade of stainless steel is corrosion-resistant in a certain medium but may be damaged in another medium. At the same time, the corrosion resistance of stainless steel is also relative. So far, there is no stainless steel that is absolutely non-corroding in all environments.

Stainless steel has the ability to resist atmospheric oxidation—that is, rust resistance, and also has the ability to resist corrosion in media containing acids, alkalis, and salts—that is corrosion resistance. However, the size of its anti-corrosion ability is changed with the chemical composition of its steel itself, the state of protection, the conditions of use, and the type of environmental media. For example, 304 steel pipe has absolutely excellent anti-corrosion ability in a dry and clean atmosphere, but if it is moved to the seaside area, it will rust soon in the sea fog containing a lot of salt; while the 316 steel pipe performs well. Therefore, it is not any kind of stainless steel that is resistant to corrosion and rust in any environment.