Why does stainless steel rust?
When brown rust spots appeared on the surface of stainless steel, people were surprised: “Stainless steel does not rust, and rust is not stainless steel. It may be a problem with the steel.” In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is non-rust, and it also has the ability to resist corrosion in media containing acid, alkali, and salt, that is corrosion resistance. But the size of its anti-corrosion ability is changed with the chemical composition of the steel itself, mutual status, use conditions and environmental media types. Such as 304 material, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it is moved to the beach area, in the sea fog containing a lot of salt, it will quickly rust. Therefore, it is not any kind of stainless steel that is corrosion-resistant and does not rust at all times.
Stainless steel relies on a very thin, strong, dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to penetrate and continue to oxidize, thereby obtaining corrosion resistance. Once for some reason, the film is continuously destroyed, the oxygen atoms in the air or liquid will continue to infiltrate or the iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will also be continuously affected. Rust. There are many forms of damage to this surface film, and the following are more common in daily life:
1. The surface of stainless steel accumulates dust containing other metal elements or attachments of dissimilar metal particles. In humid air, the condensed water between attachments and stainless steel connects the two to form a micro battery, which initiates an electrochemical reaction , The protective film is damaged, which is called electrochemical corrosion.
2. Organic juice adheres to the surface of stainless steel. In the presence of water and oxygen, it forms organic acid, which will corrode the metal surface for a long time.
3. The surface of stainless steel is adhered to contain acid, alkali and salt substances (such as alkali water and lime water spray test on the decoration wall) causing local corrosion.
4. In polluted air (atmosphere containing a lot of sulfides, oxides, and hydrogen oxides), contact with condensed water will form sulfuric acid, nitric acid, and acetic acid liquid spots, causing chemical corrosion.
The above conditions can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, to ensure that the metal surface is permanently bright and not rusted, we recommend:
1. The decorative stainless steel surface must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause corrosion.
2. There is a kind of stainless steel made of 201 and 202 on the market that is easy to rust in coastal areas and is suitable for use in an environment without industrial pollution and air corrosion.
3. Stainless steel 304 should be used in coastal areas, which can resist seawater corrosion.