Why does stainless steel rust?

When brown rust spots appeared on the surface of stainless steel pipes, people were amazed: “Stainless steel is not rusted. If it rusts, it is not stainless steel. It may be a problem with steel materials.” In fact, this is because of the lack of understanding of stainless steel to produce the wrong view. Stainless steel will also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation – rust resistance, and also has the ability to resist corrosion in acid, alkali, and salt-containing media – corrosion resistance. However, the magnitude of its corrosion resistance varies with the chemical composition, the inter-state, the conditions of use, and the environmental media type of the steel materials. For example, 304 steel pipe has absolutely excellent rust resistance in a dry and clean atmosphere, but if it is moved to a coastal area, it will rust quickly in a sea fog containing a large amount of salt; the 316 steel pipe performed well. Therefore, no stainless steel can resist corrosion and rust in any environment.

Stainless steel relies on a very thin, sturdy and fine chromium-rich oxide film (protective film) formed on its surface to prevent the oxygen atoms from continuing to infiltrate and continue to oxidize, thereby obtaining the ability to resist rust. Once for some reason, the film is continually destroyed, oxygen atoms in the air or liquid will continue to infiltrate or the iron atoms in the metal will continuously separate out, forming loose iron oxide, and the metal surface will be continuously rusted.

There are many forms of such surface film damage, and the following are common in daily life:
The surface of stainless steel contains deposits of foreign metal particles and dust, which may contain other metal elements. In the humid air, the condensed water between the attached material and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction and the protective film is destroyed, which is called electrochemical corrosion.
The surface of the stainless steel adheres to the organic juice (such as melon, noodle soup, glutinous rice, etc.), which forms an organic acid in the presence of water and oxygen. After a period of time, the organic acid will corrode the metal surface.
The surface of the stainless steel adheres to acid, alkali and salt substances (such as alkali water and lime water used to decorate the wall), causing local corrosion.
Contaminated air (such as the atmosphere containing a large amount of sulfide, carbon oxide, nitrogen oxide) encounters condensed water, forming sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion.
All of the above can cause the corrosion of the stainless steel surface protective film to cause rust. Therefore, in order to ensure that the metal surface is permanently bright and not rusted, we recommend:

The stainless steel surface must be cleaned and scrubbed frequently to remove deposits and eliminate external factors that cause corrosion.
It is best to use 316 stainless steel on the waterfront because 316 is resistant to sea water corrosion.
Some stainless steel pipes on the market do not meet the corresponding national standards and do not meet the requirements of 304 materials, so they will also rust. This requires the user to carefully select the products of reputable manufacturers.