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Why does stainless steel rust? Understanding this will help you inform your customers!

Category: Industry News Publish Time：2018-10-23

Why does stainless steel rust? When brown rust spots appear on the surface of stainless steel pipes, people are surprised: they think that "stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be a problem with the steel quality." In fact, this is a one-sided and wrong understanding of stainless steel. Stainless steel can also rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation - that is, inoxidizability, and also has the ability to resist corrosion in media containing acids, alkalis, and salts - that is, corrosion resistance. However, the magnitude of its corrosion resistance depends on its own chemical composition, mutual state, use conditions, and environment

　　Why does stainless steel rust?

　　When brown rust spots appear on the surface of stainless steel pipes, people are surprised, thinking that "stainless steel does not rust, and if it rusts, it is not stainless steel, there may be a problem with the steel quality." In fact, this is a one-sided and wrong understanding of stainless steel. Stainless steel can also rust under certain conditions.

　　Stainless steel has the ability to resist atmospheric oxidation—that is, non-rusting, and also has the ability to resist corrosion in media containing acids, alkalis, and salts—that is, corrosion resistance. However, the magnitude of its corrosion resistance varies with the chemical composition of the steel itself, the addition of mutual states, the conditions of use, and the type of environmental medium. For example, 304 steel pipes have excellent rust resistance in dry and clean atmospheres, but if they are moved to coastal areas and in sea fog containing a large amount of salt, they will rust quickly; while 316 steel pipes perform well. Therefore, not all stainless steel is corrosion-resistant and rust-free in any environment.

　　There are many ways in which the surface film can be damaged. The following are some common ones in daily life:

　　Stainless steel relies on an extremely thin, strong, dense, and stable chromium-rich oxide film (protective film) formed on its surface to prevent the continued penetration of oxygen atoms and continued oxidation, thus obtaining the ability to resist rust. Once there is some reason, this thin film is continuously damaged, oxygen atoms in the air or liquid will continuously penetrate or iron atoms in the metal will continuously separate out, forming loose iron oxide, and the metal surface will be continuously corroded. There are many forms of damage to this surface film, and the following are some common ones in daily life:

　　1. The stainless steel surface accumulates dust or attachments of different metal particles containing other metal elements. In humid air, the condensate water between the attachments and the stainless steel connects the two into a micro-battery, causing an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.

　　2. Organic juice (such as melon and vegetable juice, noodle soup, phlegm, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, organic acids are formed, and long-term organic acids corrode the metal surface.

　　3. Acid, alkali, and salt substances (such as alkaline water and lime water splashing on the walls during decoration) adhere to the surface of stainless steel, causing local corrosion.

　　4. In polluted air (such as air containing a large amount of sulfides, carbon oxides, and nitrogen oxides), when encountering condensate water, sulfuric acid, nitric acid, and acetic acid droplets are formed, causing chemical corrosion.

　　To ensure that the metal surface is permanently bright and does not rust, we recommend:

　　All of the above situations can cause damage to the protective film on the surface of stainless steel, causing rust. Therefore, to ensure that the metal surface is permanently bright and does not rust, we recommend:

　　1. The decorative stainless steel surface must be cleaned and wiped frequently to remove attachments and eliminate external factors that cause rust.

　　2. 316 stainless steel should be used in coastal areas, as 316 material can resist seawater corrosion.

　　3. Some stainless steel pipes on the market do not meet the corresponding national standards in terms of chemical composition and do not meet the requirements of 304 materials. Therefore, rust may also occur, so users need to carefully select products from reputable manufacturers.

　　Why is stainless steel sometimes magnetic?

　　People often think that whether a magnet can attract stainless steel verifies its quality and authenticity. If it is not attracted (non-magnetic), it is considered good and genuine; if it is attracted (magnetic), it is considered counterfeit. In fact, this is an extremely one-sided and unrealistic wrong identification method.

　　There are many types of stainless steel. At room temperature, they can be divided into several categories according to their microstructure:

　　1. Austenitic: such as 201, 202, 301, 304, 316, etc.;

　　2. Martensitic or Ferritic: such as 430, 420, 410, etc.;

　　Austenitic is non-magnetic or weakly magnetic, while martensitic or ferritic is magnetic.

　　The stainless steel commonly used for decorative panels is mostly austenitic 304 material, which is generally non-magnetic or weakly magnetic. However, due to fluctuations in chemical composition caused by smelting or different processing states, magnetism may also appear. This cannot be considered counterfeit or unqualified. What is the reason for this?

　　As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to segregation of components during smelting or improper heat treatment, a small amount of martensite or ferrite organization may be formed in austenitic 304 stainless steel. In this way, 304 stainless steel will have weak magnetism.

　　In addition, 304 stainless steel will undergo a structural transformation toward martensite after cold working. The greater the cold working deformation, the more martensite transformation, and the greater the magnetism of the steel. For example, for the same batch of steel strip, when producing Φ76 pipes, there is no obvious magnetic induction, but when producing Φ9.5 pipes, due to the larger cold bending deformation, the magnetic induction is more obvious. For rectangular pipes, the deformation is greater than that of round pipes, especially in the corners, where the deformation is more intense and the magnetism is more obvious.

　　To completely eliminate the magnetism of 304 steel caused by the above reasons, high-temperature solution treatment can be used to restore the stable austenite structure and thus eliminate the magnetism.

　　It is particularly important to note that the magnetism of 304 stainless steel caused by the above reasons is not of the same level as that of other stainless steel materials, such as 430 and carbon steel. In other words, the magnetism of 304 steel always shows weak magnetism.

　　This tells us that if stainless steel has weak magnetism or is completely non-magnetic, it should be judged as 304 or 316 material; if it has the same magnetism as carbon steel, showing strong magnetism, it should be judged as not 304 material.

Keywords: Why does stainless steel rust? Understanding this will help you inform your customers!

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