Why does stainless steel rust?

When brown rust spots appeared on the surface of stainless steel pipes, people were surprised: “Stainless steel is not rusty, rust is not stainless steel, it may be the problem with the steel.”In fact, this is a one-sided wrong view of lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation–that is, stainless steel, and also has the ability to resist corrosion in media containing acid, alkali, and salt–that is, corrosion resistance. However,its anti-corrosion ability changes with the chemical composition of the steel itself, the mutual state, the conditions of use, and the type of environmental medium. For example, SS304 pipe has absolutely excellent rust resistance in a dry and clean atmosphere, but it will be rusted quickly when it is moved to the seashore area in sea fog containing a large amount of salt; while SS316 pipe is good.

Therefore, Therefore, not any stainless steel is resistant to corrosion and rust under any environment. Stainless steel is an extremely thin, strong and stable chromium-rich oxide film (protective film) formed on its surface, which prevents the oxygen atoms from infiltrating and continuing to oxidize, and obtains the ability to resist rust.Once for some reason, this film is constantly destroyed, oxygen atoms in the air or liquid will continue to infiltrate, or iron atoms in the metal will be continuously separated out to form loose iron oxide, and the metal surface will be continuously rusted.

There are many forms of damage to this surface film. The following are common in daily life:

Dust or other foreign metal particles containing deposits of other metal elements are deposited on the surface of stainless steel. In humid air, the condensate between the deposits and stainless steel connects the two into a micro battery, which triggers an electrochemical reaction The protective film is damaged, which is called electrochemical corrosion.

Organic matter juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, they form organic acids. For a long time, organic acids corrode the metal surface.

The surface of the stainless steel contains acids, alkalis, and salts (such as alkali water and lime water splashing on the wall), which may cause local corrosion.

In contaminated air (such as an atmosphere containing a large amount of sulfide, carbon oxide, nitrogen oxide), when it meets condensed water, it forms sulfuric acid, nitric acid, and acetic acid liquid points, causing chemical corrosion. The above conditions can cause the damage of the stainless steel surface protective film to cause corrosion.

In order to ensure that the metal surface is permanently bright and not rusted, it must be cleaned and passivated.