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Why Do Stainless Steel Tubes Rust?

When brown rust spots (spots) appeared on the surface of stainless steel pipes, people were surprised: "Stainless steel does not rust, and rust is not stainless steel. It may be that there is a problem with the steel." In fact, this is a one-sided misconception about the stainless steel due to the lack of understanding about the stainless steel. Stainless steel will rust under certain conditions.


Stainless steel has the ability to resist atmospheric oxidation - that is, rust resistance, and it also has the ability to resist corrosion in media containing acid, alkali and salt - that is, corrosion resistance. However, the size of its corrosion resistance varies with the chemical composition of the steel itself, the state of mutual addition, the use conditions and the type of environmental media. For example, 304 stainless steel pipe has absolutely excellent anti-corrosion ability in a dry and clean atmosphere. However, if it is moved to the beach area, it will soon rust in the sea fog containing a lot of salt; while 316 stainless steel pipe's performance is good. Therefore, it is not that any kind of stainless steel is corrosion-resistant and does not rust in any environment.


Stainless steel pipe fittings rely on an extremely thin, strong, dense and stable chromium-rich oxide film (protective film) formed on the surface to prevent oxygen atoms from continuing to penetrate and continue to oxidize, thereby obtaining the ability to resist corrosion. Once this film is continuously destroyed for some reason, oxygen atoms in the air or liquid will continuously infiltrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will continue to be rusted.