Why does stainless steel get rusty? When brown rust spots (spots) appear on the surface of stainless steel pipes, people are greatly surprised: they think that "stainless steel is rustless, and if it gets rusty, it is not stainless steel. It may be that there is a problem with the quality of the steel." In fact, this is just a one-sided misconception about the lack of understanding of stainless steel. Stainless steel also can get rusty under certain conditions.
1. Stainless steel will not rust
An oxide will also be formed on the surface. The stainless mechanism of all stainless steels currently on the market is due to the presence of Cr elements. The fundamental reason for the corrosion resistance of stainless steel is the passive film theory. The so-called passivation film is a layer of Cr2O3-based thin film 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 in 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 anti-rust property; A wider formation condition is that stainless steel forms a passivation 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 stainless steel passivation film has the ability to resist corrosion has 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%; second, the specific gravity of the passivation film greater than the specific gravity of the substrate; these two characteristics indicate that the passivation film is thin and dense, so it is difficult for the passivation film to be broken down by the corrosive medium to quickly corrode the substrate; the third characteristic is that the chromium concentration ratio of the passivation film is The substrate is more than three times higher; therefore the passivation 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 a simple chromium oxide passivation film cannot meet the requirements of high corrosion resistance. Therefore, depending on the conditions of use, elements such as molybdenum (Mo), copper (Cu), and nitrogen (N) need to be added to the steel 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, strongly promotes the passivation of the substrate and prevents the corrosion of the substrate; adding Cu makes the passivation film on the stainless steel surface contain CuCl because it does not interact with the corrosive medium and improves the corrosion resistance of the substrate.
Corrosion resistance: Adding N, because the passivation film is enriched with Cr2N, increases the Cr concentration in the passivation film, 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 relative. So far, there is no stainless steel that is absolutely non-corrosive in all environments.
Stainless steel has the ability to resist atmospheric oxidation - that is, stainless, 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 the steel itself, the state of protection, the conditions of use, and the type of environmental medium. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to tarnishing ability is arranged, but it is moved on to riviera, containing the sea fog of a large number of salts, will soon get rusty; 316 steel pipes then show well. . Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.
Contact
TEL: +8618992731201
FAX: 0917-3873009
EMAIL: zhangjixia@bjygti.com
ADD: 1502, Block A, Chuang Yi Building No. 195, Gaoxin Avenue, High-tech Development Zone, Baoji City, Shaanxi, China
