The so-called stainless steel refers to the steel that can resist the corrosion of the atmosphere and weak corrosive medium. Corrosion rate < 0.01 mm/year is complete corrosion-resistant steel, and corrosion rate < 0.1 mm/year is corrosion-resistant steel. The so-called acid-resistant steel refers to the steel that can resist acid in various strong corrosive media. The corrosion rate is less than 0.1mm/year for complete corrosion resistance, and the corrosion rate is less than 1mm/year for corrosion resistance. Therefore, stainless steel is not non-corroding, but the corrosion rate is slower, and there is no steel that is not corroded.
It is worth noting that in the same medium. The corrosion rate of different varieties of stainless steel is very different, and the corrosion behavior of the same stainless steel in different media is also very different. E.g. The corrosion resistance of Ni-Cr stainless steel in oxidizing medium is very good. However, the corrosion resistance in non-oxidizing media (such as hydrochloric acid) is not good. Therefore, it is very important to control the characteristics of various types of stainless steel and to correctly select and use stainless steel.
Stainless steel not only has corrosion resistance, but also accepts or transmits loads, so it also needs to have better mechanical properties. Stainless steel is generally processed into components or parts with plates, tubes and other profiles, so. Have good machinability and good welding performance.
Stainless steel is divided into: ferritic (F) type stainless steel; martensitic (M) type stainless steel; austenitic (A) type stainless steel; austenite-ferritic (A-F) duplex stainless steel; precipitation Hardened stainless steel.
1. Metal corrosion
(1) Corrosion process of metal
The phenomenon that metal is gradually destroyed under the action of external medium is called corrosion. There are basically two ways to corrode. Chemical corrosion and electrochemical corrosion. The corrosion encountered in production practice is mainly electrochemical corrosion, no current is generated in chemical corrosion, and a certain corrosion product is formed in the corrosion process. This corrosion product is usually covered up on the surface of the metal to form a film, which separates the metal from the medium.
If this layer of chemical products is stable, dense, intact and solidly separated from the metal surface, it will greatly reduce or even prevent the further development of corrosion and play a protective role on the metal. The process of forming a maintenance film is called passivation. For example, oxide films such as SiO2, Al2O3, and Cr2O3 are formed. These oxide films are dense, intact, free of loose, crack-free and not easy to peel off, which can protect the base metal and prevent continued oxidation. For example, Fe2O3 is formed when iron is oxidized at high temperature. On the contrary, some oxide films are discontinuous or porous. There is no maintenance effect on the base metal. E.g. Some metal oxides, such as Mo2O3 and WO3, are volatile at high temperatures, which do not completely cover the maintenance effect of the matrix.
It can be seen that the production of oxide film and the structure and properties of oxide film are important characteristics of chemical corrosion. Therefore, to improve the ability of metal to resist chemical corrosion, it is mainly through alloying or other methods to form a stable, complete and dense oxide film on the surface of the metal that is separated from the substrate and solid, also known as passivation film, electrochemical corrosion. It is a more important and common way of metal corrosion, which is generated by galvanic cells composed of different metals or different electrode potentials of metals. This galvanic cell corrosion is generated between the microstructures, so it is also called micro-battery corrosion. The characteristics of electrochemical corrosion are that there is a dielectric, there is potential difference connection or contact between different metals, between metal micro-domains or between phases, and corrosion current is generated at the same time.