When there are brown rust spots (spots) on the surface of stainless steel pipes, people are surprised: "stainless steel is not rusty, and rust is not stainless steel, it may be that there is a problem with the steel".
In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel will rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, it is not rusty, but also has the ability to corrode in the medium containing acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of the steel varies with its chemical composition, protection status, service conditions and types of environmental media.
For example, 304 steel pipe has absolutely excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to the coastal area, it will rust soon in the sea fog containing a lot of salt; The 316 steel pipe performs well. Therefore, not any kind of stainless steel can resist corrosion and rust under any environment.
Stainless steel is resistant to corrosion by a thin, firm, fine and stable chromium rich oxide film (protective film) formed on its surface to prevent oxygen atoms from penetrating and oxidizing continuously. Once for some reason, this kind of film is constantly destroyed, oxygen atoms in air or liquid will continuously penetrate or iron atoms in metal will continuously separate, forming loose iron oxide, and the metal surface will be continuously corroded.
This surface facial mask is damaged in many ways.
The following are common in daily life:
1. There are dust containing other metal elements or attachments of dissimilar metal particles on the surface of stainless steel. In the humid air, the condensate between the attachment and stainless steel connects them into a micro cell, which causes electrochemical reaction and damages the protective film, which is called electrochemical corrosion.
2. Organic juice (such as melon, vegetable, noodle soup, sputum, etc.) is adhered to the surface of stainless steel to form an organic acid in the presence of water and oxygen. For a long time, the organic acid will corrode the metal surface.
3. There are acid, alkali and salt substances on the surface of stainless steel (such as alkali water and lime water splashing on the wall for decoration), causing local corrosion.
4. In the polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), sulfuric acid, nitric acid and acetic acid will be formed in the presence of condensate, causing chemical corrosion.
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