Stainless steel does 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 for 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.

Passivation of stainless steel

Stainless steel does 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 for 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) to hinder corrosion. When the passivation film is damaged, a new passivation film can be formed immediately.

Characteristics of passivation films

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 indicate 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.

Corrosion conditions of stainless steel

Stainless steel will also 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), nitrogen (N), etc. 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 absolutely non-corrosive in all environments.

Sensitization phenomenon

Stainless steel contains Cr and forms a chromium oxide film on the surface, which loses chemical activity and is called a passivated state. However, if the austenitic system passes through the temperature range of 475~850℃, C will combine with Cr to form chromium carbide (Cr23C6) and precipitate in the crystal. Therefore, the Cr content near the grain boundary is greatly reduced, becoming a Cr-poor region. At this time, its corrosion resistance will be reduced, and it is particularly sensitive to corrosive environments, so it is called sensitization. Sensitization is most likely to corrode in the use environment of oxidizing acid. In addition, there are welding heat-affected zones and hot bending processing zones.

So under what circumstances will stainless steel corrode? 

In fact, stainless steel is not necessarily free of rust, but its corrosion rate is much lower than other steels under the same environment, and sometimes it can be ignored.