Basic knowledge of stainless steel

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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 stemming 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 has 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 protective condition, 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 rusting in any environment. Stainless steel gains its rust-resistant properties 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 begin to penetrate, or iron atoms from the metal separate out, forming loose iron oxide; as a result, the metal surface suffers from continuous rusting. There are many ways in which this surface film can be damaged. In daily life, the following types are commonly seen: 1. Dust containing other metal elements or particles of dissimilar metals accumulate on the surface of stainless steel. In humid air, the condensed 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 adhere to the stainless steel surface (such as alkaline solutions or lime water used in wall decoration that splash onto it), causing localized corrosion. 4. In polluted air (such as air containing large amounts of sulfides, carbon monoxide, and nitrogen oxides), when condensation water forms, it creates liquid phases of sulfuric acid, nitric acid, and acetic acid, leading to chemical corrosion. Tip: To ensure that the metal surface remains shiny permanently and is 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 on the market do not meet the corresponding **standards in terms of chemical composition, and fail to satisfy the requirements for 304 material. This can also lead to rusting, which requires users to carefully choose products from reputable manufacturers. Here is another comprehensive set of information on stainless steel for everyone. Stainless steel is short for rust-resistant and acid-resistant steel; steel grades that can resist mild corrosive agents such as air, steam, and water, or that possess rust resistance, are referred to as stainless steel ； Steel grades that are resistant to corrosion by chemically aggressive substances such as acids, bases, and salts are known as acid-resistant steel. Common classifications: Generally, they are classified based on their microstructure. Typically, ordinary stainless steels are divided into three categories according to their microstructure: austenitic stainless steels, ferritic stainless steels, and martensitic stainless steels. Based on these three basic microstructural types, duplex steels, precipitation-hardening stainless steels, and high-alloy steels with an iron content of less than 50% have been developed to meet specific requirements and purposes. 1. Austenitic stainless steel. A stainless steel in which the matrix is primarily composed of austenite structure (CY phase) with a face-centered cubic crystal structure, is non-magnetic; it is strengthened mainly through cold working (which may also result in some magnetism). The American Iron and Steel Institute uses numbers from the 200 and 300 series to designate them, such as 304. 2. Ferritic stainless steel. A stainless steel in which the matrix is primarily composed of ferrite with a body-centered cubic crystal structure ((phase a)); it is magnetic, generally cannot be hardened through heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute uses 430 and 446 as designations. 3. Martensitic stainless steel. Stainless steel with a martensitic structure (body-centered cubic or cubic), which is magnetic and whose mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute designates them with the numbers 410, 420, and 440. Martensite has an austenitic structure at high temperatures; when cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., hardening). 4. Austenite-ferrite (duplex) stainless steels. A stainless steel whose matrix consists of both austenite and ferrite phases, in which the amount of the latter phase is generally greater than 15%; it is magnetic and can be strengthened through cold working. 329 is a typical duplex stainless steel. Compared to austenitic stainless steels, duplex steels have higher strength, as well as significantly improved resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion. 5. Precipitation-hardening stainless steel. Stainless steel with an austenitic or martensitic structure that can be hardened through precipitation hardening. The American Iron and Steel Institute uses numbers from the 600 series to identify them, such as 630, which corresponds to 17-4PH. Generally speaking, aside from alloys, austenitic stainless steels exhibit relatively excellent corrosion resistance. In environments with low corrosivity, ferritic stainless steels can be used; in mildly corrosive environments, if high strength or high hardness are required from the material, martensitic stainless steels and precipitation-hardening stainless steels can be employed.