Why does stainless steel for valves also rust?

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When brown rust spots appear on the surface of stainless steel pipes, people are very surprised; they think that \"stainless steel doesn’t rust, and if it does rust, then it’s no longer stainless steel – there must be a problem with the quality of the steel.\" In fact, this is a one-sided and incorrect view resulting from a lack of understanding of stainless steel. Stainless steel can also rust under certain conditions. Stainless steel has the ability to resist oxidation in the atmosphere—that is, its rust resistance—and it also possesses the ability to resist corrosion in media containing acids, alkalis, and salts—that is, its corrosion resistance. However, its corrosion resistance varies depending on the chemical composition of the steel itself, its microstructure, operating conditions, and the type of environmental medium. For example, 304 steel pipes have excellent corrosion resistance in a dry and clean atmosphere, but when placed in coastal areas exposed to sea fog containing high levels of salt, they rust very quickly ； The 316 steel pipe, on the other hand, performed well. Therefore, not every type of stainless steel is resistant to corrosion and does not rust in any environment. Stainless steel gains its corrosion resistance thanks to an extremely thin, strong, dense, and stable chromium-rich oxide film (protective layer) that forms on its surface, preventing further penetration of oxygen atoms and continued oxidation. Once, for some reason, this film is continuously damaged, oxygen atoms from the air or liquid will keep penetrating in, or iron atoms from the metal will continue to separate, forming loose iron oxide; as a result, the metal surface suffers from continuous rusting. There are many ways in which this protective film on the surface can be damaged; the most common ones in daily life are as follows: 1. Dust containing other metal elements or particles of different metals accumulate on the surface of stainless steel. In humid air, the condensation water between these deposits and the stainless steel forms a micro-battery, triggering an electrochemical reaction that damages the protective film – this phenomenon is known as electrochemical corrosion. 2. Organic substances such as juices from fruits and vegetables, noodle soup, sputum, etc., adhere to the surface of stainless steel. In the presence of water and oxygen, these substances form organic acids, which over time cause corrosion of the metal surface. 3. Acids, alkalis, and salts adhering to the stainless steel surface (such as alkaline solutions or lime water used in wall decoration that splatter onto it) can cause localized corrosion. 4. In polluted air (such as air containing large amounts of sulfides, carbon monoxide, and nitrogen oxides), when it comes into contact with condensed water, sulfuric acid, nitric acid, and acetic acid are formed, leading to chemical corrosion. All of the above situations can damage the protective film on the surface of stainless steel, leading to rusting. Therefore, to ensure that the metal surface remains permanently shiny and free from rust, we recommend: 1. Regularly cleaning and scrubbing the decorative stainless steel surface to remove any deposits and eliminate external factors that could cause damage. 2. Stainless steel of grade 316 should be used in coastal areas, as this grade can resist corrosion caused by seawater. 3. Some stainless steel pipes available on the market do not meet the corresponding **standards in terms of their chemical composition, and thus do not fulfill the requirements of 304 material. This can also lead to rusting; therefore, users need to carefully choose products from reputable manufacturers and pay attention to the proper procedures during installation. To prevent scratches and the accumulation of contaminants during installation, stainless steel should be worked on while a film is in place. However, over time, residues of the adhesive remain. Depending on the shelf life of the film, the surface should be cleaned when removing the film after application, and special stainless steel tools should be used; when cleaning with ordinary steel cleaning tools, it is necessary to clean them to prevent iron shavings from sticking. Care should be taken to prevent highly corrosive magnetic substances and cleaning agents from coming into contact with the stainless steel surface; if contact occurs, it should be washed immediately. After the construction is completed, use a neutral detergent and water to wash away any cement, plaster, and other substances adhering to the surface.