Why does stainless steel also rust?
When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: “Stainless steel does not rust, and rusted is not stainless steel, and there may be a problem with the steel.”
In fact, this is a one-sided misconception about the 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, rust resistance, and it also has the ability to corrode in media containing acids, alkalis, and salts—that is, corrosion resistance.
However, the size of its anti-corrosion ability varies with the chemical composition of its steel itself, the state of mutual addition, the conditions of use and the type of environmental media.
For example, 304 steel pipe has excellent rust resistance in a dry and clean atmosphere, but if it is moved to a coastal area, it will rust soon in the sea fog containing a lot of salt; while 316 steel pipe performs well . Therefore, it is not any kind of stainless steel that can resist corrosion and rust in any environment.
Stainless steel is a very thin, firm, fine and stable chromium-rich oxide film (protective film) formed on its surface to prevent the continuous infiltration and oxidation of oxygen atoms, and obtain the ability to resist rust. Once the film is damaged continuously for some reason, 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 be continuously corroded.
There are many forms of damage to this surface film, the most common in daily life are the following:
1. On the surface of stainless steel, there are deposits of dust or heterogeneous metal particles containing other metal elements. In humid air, the condensed water between the deposits and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction. , the protective film is damaged, called electrochemical corrosion.
2. Organic juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, organic acids are formed, and organic acids will corrode the metal surface for a long time.
3. The surface of stainless steel adheres to substances containing acids, alkalis and salts (such as alkali water and lime water splashing from the decoration walls), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide, nitrogen oxide), when encountering condensed water, it will form sulfuric acid, nitric acid, and acetic acid liquid points, causing chemical corrosion. The above conditions can cause damage to the protective film on the stainless steel surface and cause rust.
Therefore, to ensure that the metal surface is bright and not rusted, we recommend:
1. The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause modification.
2. 316 stainless steel should be used in seaside areas, and 316 material can resist seawater corrosion.
3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding standards and cannot meet the material requirements. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.
