Why does stainless steel corrode and rust? When brown rust spots (dots) appear on the surface of stainless steel, people are very surprised: they think that stainless steel will not corrode and rust, and if it corrodes and rusts, it is not stainless steel, and most of the steel has problems. In fact, this is a one-sided and incorrect view that lacks understanding of stainless steel. Stainless steel will also corrode and rust under certain conditions.
1. Stainless steel will also be corroded under corresponding conditions
The application scenarios of stainless steel are very complex, and a single chromium oxide passivation film cannot meet the needs of high corrosion resistance. Therefore, according to the differences in the scope of application, elements such as molybdenum (Mo), copper (Cu), and nitrogen (N) must be added to the steel to improve the composition of the passivation film and further improve the corrosion resistance of stainless steel. Adding Mo, considering that the corrosion product MoO2- is close to the matrix and violently promotes the passivation of the matrix, preventing corrosion of the matrix; adding Cu makes the passivation film on the surface of stainless steel contain CuCl, because it does not react with corrosive substances and improves corrosion resistance; adding N, considering that the passivation film is enriched with Cr2N, the Cr concentration in the passivation film is increased, so the corrosion resistance of stainless steel is improved.
The corrosion resistance of stainless steel is conditional. A grade of stainless steel is corrosion-resistant in a certain medium, but will 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 will never corrode in all environments.
2. Stainless steel is not impossible to corrode and rust
It will also produce an oxide on the surface. The reason why all stainless steel materials on the market are stainless is due to the presence of Cr element. The main reason (principle) for stainless steel corrosion resistance is the concept of passivation film. To put it simply, the passivation film is a film mainly composed of Cr2O3 covering the surface of the stainless steel material. Due to the appearance of this film, the corrosion of the stainless steel matrix in various media is restricted. This situation is called passivation. There are two situations for the production of this type of passivation film. One is that the stainless steel itself has self-passivation ability. This self-passivation ability accelerates with the increase of chromium content, so it has rust resistance; the other more common production condition is that stainless steel produces a passivation film in the process of being corroded in various aqueous solutions (electrolytes), thereby limiting corrosion. When the passivation film is damaged, a new passivation film can be produced immediately.
There are three reasons why the stainless steel passivation film has the ability to resist corrosion: first, the thickness of the passivation film is very thin, which may be only a few microns in an environment with a chromium content of >10.5%; second, the specific gravity of the passivation film is greater than that of the substrate; these two characteristics indicate that the passivation film is both thin and dense, so it is difficult for the passivation film to be penetrated by the corrosive medium to quickly corrode the substrate; the third characteristic is that the chromium concentration of the passivation film is more than three times higher than that of the substrate; so the passivation film has a high corrosion resistance.
Stainless steel has the ability to resist atmospheric oxidation, that is, stainlessness, and also has the ability to not corrode in media containing acids, alkalis, and salts, that is, corrosion resistance. However, its corrosion resistance varies with the chemical composition of the steel itself, the protection state, the scope of application, and the type of environmental media. For example, 304 steel pipes have relatively good corrosion resistance in dry and clean atmospheres, but when they are moved to coastal areas, they quickly rust in sea fog containing more salt; while 316 steel pipes perform well. Therefore, not all stainless steel is corrosion-resistant and rust-free in all environments.