Does stainless steel really rust?

When brown rust spots (spots) appeared on the surface of stainless steel pipes, people were surprised: "Stainless steel does not rust, and rust is not stainless steel. It may be that there is a problem with the steel." In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

1. Stainless steel will not rust

It also generates an oxide on the surface. The rust-free mechanism of all stainless steels currently on the market is due to the presence of Cr. The fundamental reason (mechanism) of the corrosion resistance of stainless steel is the passive film theory. The so-called passivation 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 situations for the formation of this kind of passivation film. One is that stainless steel itself has the ability of self-passivation. This self-passivation ability increases with the increase of chromium content, so it has rust resistance; the other 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 stainless steel passivation film has the ability to resist corrosion, there are three characteristics: first, the thickness of the passivation film is extremely thin, generally only a few microns under the condition of chromium content> 10.5%; the second is the specific gravity of the passivation film It is greater than the specific gravity of the substrate; these two characteristics show that the passivation film is thin and dense, therefore, the passivation film is difficult to be penetrated by the corrosive medium to quickly corrode the substrate; the third feature is the chromium concentration ratio of the passivation film The substrate is more than three times higher; therefore, the passivation film has high corrosion resistance.

2. Stainless steel will be corroded under certain conditions

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

The corrosion resistance of stainless steel is conditional. A brand 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 non-corrosive in all environments.

Stainless steel has the ability to resist atmospheric oxidation—that is, non-rust property, but also has the ability to corrode in media containing acid, alkali, and salt—that is, corrosion resistance. However, the size of its corrosion resistance varies with the chemical composition of the steel itself, the state of protection, the conditions of use and the type of environmental media. For example, 304 steel pipe has excellent anti-corrosion ability in a dry and clean atmosphere, but when it is moved to a coastal area, it will quickly rust in sea fog containing a lot of salt; while 316 steel pipe performs well. . Therefore, it is not any kind of stainless steel, which can resist corrosion and rust in any environment.