Characteristics and selection of stainless steel

Stainless steel is a metal material widely used in industries such as petroleum, chemical industry, fertilizer, food, national defense, tableware, synthetic fiber, and petroleum refining. Many containers, pipes, valves, pumps, etc. are generally in contact with various corrosive media Corroded and scrapped. According to statistics, the world’s annual steel scrap due to corrosion accounts for about 1/4 of the annual steel output. The output of stainless steel accounts for 1% of the total output of steel. Therefore, the failure of materials due to corrosion is one of the three main problems in the research and development of materials.
Stainless steel refers to a type of steel with corrosion resistance.

Generally speaking, stainless steel is the general term for stainless steel and acid-resistant steel.

Stainless steel is not necessarily acid-resistant, but acid-resistant steel is also stainless steel.

The so-called stainless steel refers to the steel that can resist corrosion by the atmosphere and weak corrosive media. Corrosion rate <0.01 mm/year is fully corrosion-resistant steel, and corrosion rate <0.1 mm/year is corrosion-resistant steel. The so-called acid-resistant steel refers to steel that is resistant to acid in various strong corrosive media. The corrosion rate <0.1mm/year is completely corrosion resistant, and the corrosion rate <1mm/year is corrosion resistant. Therefore, stainless steel is not non-corrosive, but the corrosion rate is relatively slow, and there is no steel that is absolutely non-corrosive.

It is worth noting that in the same medium, the corrosion rate of different types of stainless steel is very different, and the corrosion behavior of the same stainless steel in different media is also very different. For example, the corrosion resistance of Ni-Cr stainless steel in oxidizing media is very good, but the corrosion resistance in non-oxidizing media (such as hydrochloric acid) is not good. Therefore, mastering the characteristics of various types of stainless steel is very important for the correct selection and use of stainless steel.

Stainless steel must not only resist corrosion, but also bear or transmit loads, so it also needs to have good mechanical properties. Stainless steel is generally processed into components or parts with plates, tubes and other profiles, so it must have good cutting performance and good welding performance.

Stainless steel is divided into typical structures: ferritic (F) stainless steel; martensitic (M) stainless steel; austenitic (A) stainless steel; austenitic-ferritic (AF) duplex stainless steel; precipitation Hardened stainless steel.

1. Metal corrosion

(1) The corrosion process of metals

The phenomenon that metals are gradually destroyed under the action of external media is called corrosion. There are basically two forms of corrosion: chemical corrosion and electrochemical corrosion. The corrosion encountered in the actual production is mainly electrochemical corrosion. No current is generated in chemical corrosion, and certain corrosion products are formed during the corrosion process. This kind of corrosion product generally covers the metal surface to form a film to isolate the metal from the medium.

If the chemical product of this layer is stable, dense, complete and firmly combined with the metal surface, it will greatly reduce or even prevent the further development of corrosion and protect the metal. The process of forming a protective film is called passivation. For example, oxide films such as SiO2, Al2O3, Cr2O3, etc. are formed. These oxide films have a dense, complete structure, no looseness, no cracks, and are not easy to peel off, which can protect the base metal and avoid continued oxidation. For example, Fe2O3 formed when iron is oxidized at high temperature. On the contrary, some oxide films are discontinuous or porous. They have no protective effect on the base metal. For example, some metal oxides, such as Mo2O3 and WO3, are volatile at high temperatures and have no protective effect to cover the substrate.

It can be seen that the production of oxide film and the structure and properties of oxide film are important features of chemical corrosion. Therefore, to improve the chemical corrosion resistance of metals is mainly through alloying or other methods to form a stable, complete and dense oxide film on the metal surface, which is also known as a passivation film, electrochemical corrosion It is a more important and common form of metal corrosion. It is produced by different metals or different electrode potentials of different metals. This kind of galvanic cell corrosion is produced between the microstructures, so it is also called microbattery corrosion. The characteristic of electrochemical corrosion is the existence of dielectric, the potential difference between different metals, between metal micro-regions or between phases, and there is a corrosion current.

2. Corrosion type

There are many forms of corrosion failure of metal materials in industrial production. Under the action of different loads and different media environments, different materials mainly have the following types of corrosion:

General corrosion: A large area of ​​relatively uniform corrosion occurs on the exposed metal surface. Although the effective area of ​​the component and its service life are reduced, it is less harmful than local corrosion.

Intergranular corrosion: refers to corrosion along the product boundary, which destroys the connection of crystal grains. This kind of corrosion is the most harmful, it can make the metal brittle or lose its strength, and lose the metal sound when struck, which is easy to cause sudden accidents. Intergranular corrosion is the main form of corrosion of austenitic stainless steel. This is due to the difference in the composition or stress between the grain boundary region and the intragranular, which causes the electrode potential in the grain boundary region to significantly decrease and the electrode potential is caused by the difference.

Stress corrosion: metal cracks under the combined action of corrosive media and tensile stress (external stress or internal stress). The fracture mode is mainly along the grain, and there are also transgranular. This is a dangerous low-stress brittle fracture. Stress corrosion often occurs in chlorinated media and alkaline oxides or other water-soluble media. It accounts for many equipment accidents. A considerable proportion.

Pitting corrosion: Pitting corrosion is a form of corrosion damage that occurs in the local area of ​​the metal surface. After pitting corrosion is formed, it can quickly develop into the depths and finally penetrate the metal. Pitting corrosion is very harmful, especially for various containers. After pitting corrosion occurs, it should be polished or painted in time to avoid deepening corrosion.

The cause of pitting corrosion is caused by partial damage of the passivation film on the metal surface under the action of the medium. Or in a medium containing chloride ions, loose material surface defects and non-metallic inclusions can cause pitting corrosion.

Corrosion fatigue: the destruction of metals under the action of corrosive media and alternating stress, which is characterized by corrosion pits and a large number of cracks. Significantly reduce the fatigue strength of steel, leading to premature fracture. Corrosion fatigue is different from mechanical fatigue. It does not have a certain fatigue limit. As the number of cycles increases, the fatigue strength always decreases.

In addition to the various forms of corrosion mentioned above, there is also corrosion due to the effect of the macroscopic battery. For example, the corrosion caused by the difference in electrode potential, such as the different rivets and riveting materials in the metal components, the welding of dissimilar metals, and the different materials of the hull and the propeller.

It can be seen from the above corrosion mechanism that the focus of preventing corrosion should be: reduce the number of galvanic cells as much as possible, so that a stable and complete passivation film that is firmly bonded to the steel matrix is ​​formed on the surface of the steel; in the formation of galvanic cells In the case of, minimize the electrode potential difference between the two poles.