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What is the cause of the rust in stainless steel?

What caused stainless steel to rust?
When brown rust spots (dot) appear on the surface of stainless steel, people are amazed: "Stainless steel will not
Rusted, rust is not stainless steel, it may be a problem with 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, and also has the ability to resist corrosion in a medium containing acid, alkali, and salt, that is, corrosion resistance. However, the amount of corrosion resistance varies with the chemical composition of the steel itself, the state of each other, the conditions of use, and the type of environmental medium. For example, material 304 has absolutely excellent corrosion resistance in a dry and clean atmosphere, but it is moved to the seaside area and will soon rust in sea fog containing a large amount of salt. Therefore, it is not any kind of stainless steel that can resist corrosion and rust at any time.
Stainless steel is a very thin and strong and stable chromium-rich oxide film (protective film) formed on the surface to prevent oxygen atoms from continuing to infiltrate and continue to oxidize, thereby obtaining corrosion resistance. For some reason, the film is constantly destroyed. The air or liquid neutralizes the oxygen atoms and the iron atoms are continuously separated out to form loose iron oxide. The metal surface is also continuously Corroded. There are many forms of such surface film damage, and the following are common in daily life:
1. The surface of stainless steel accumulates dust or other metal particles attached to other metal elements. In humid air, the condensed water between the attached material and the stainless steel connects the two into a micro battery, triggering an electrochemical reaction. The protective film is destroyed, which is called electrochemical corrosion.
2, the surface of the stainless steel adheres to the organic juice (such as melon, noodle soup, etc.), in the case of water and oxygen, constitutes an organic acid, long-term formation of organic acid corrosion of the metal surface.
3. The surface of the stainless steel adheres to acid, alkali and salt substances (such as alkali water and lime water sprayed on the wall) to cause local corrosion.
4. In the polluted air (the atmosphere containing a large amount of sulfides, oxides, and hydrogen oxide), in the case of condensed water,
Formation of sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion.
All of the above can cause damage to the stainless steel surface protective film and cause corrosion. Therefore, in order 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 deposits and eliminate external factors that cause corrosion.
2 There is a kind of 201 and 202 stainless steel on the market which is easy to rust in the coastal area, suitable for use in the environment without industrial pollution and air corrosion.
3 The waterfront area should be made of 304 stainless steel, which is resistant to seawater corrosion.
Six, one, but how to deal with stainless steel rust spots?
a) Chemical methods:
Re-passivation of the rusted portion with a pickling paste or spray to form a chromium oxide film to restore corrosion resistance
Capability, after pickling, in order to remove all contaminants and acid residues, it is important to rinse properly with clean water. After all treatment, it is re-polished with polishing equipment and sealed with polishing wax. For areas with slight rust, use a 1:1 mixture of gasoline and oil to wipe off the rust with a clean cloth.
b) Mechanical methods:
Sandblasting, blasting with glass or ceramic particles, annihilation, brushing and polishing. Mechanically
It can wipe away the pollution caused by previously removed materials, polishing materials or annihilation materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned under dry conditions. The mechanical method can only be used to clean the surface and does not change the corrosion resistance of the material itself. It is therefore recommended to re-polish with a polishing device after mechanical cleaning and to seal with polishing wax.

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