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What is the composition of stainless steel and why does it not rust?


Stainless steel is a type of steel. In the smelting process of steel, alloy elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added, thereby improving the properties of the steel and making the steel have Corrosion resistance (ie not easy to rust). This is what we often say about stainless steel. The classification of stainless steel is classified according to the structure at room temperature, that is, the arrangement of atoms in stainless steel at room temperature. It can be divided into: martensite type, austenite type, ferrite type, austenite-ferrite duplex stainless steel 5, five categories of precipitation hardening stainless steel. Classified by grade, stainless steel pipes can be divided into: 201, 202, 301, 304 and so on. In the smelting process of stainless steel, due to the variety of alloying elements added, the different types of different amounts of addition, its characteristics are not the same, in order to distinguish, they are given different steel numbers. Reasons why stainless steel does not rust Stainless steel has good corrosion resistance. It is caused by adding chromium to the iron-carbon alloy, forming a dense chromium-rich oxide (Cr203) film on the surface to prevent corrosion and rust.

The passivation film of stainless steel is a compound of oxyhydrocarbon group, water and chromium. Once the surface is scratched, the protective film will fall off, but the surrounding water and oxygen will regenerate it and maintain corrosion resistance, so that the surface of stainless steel maintains its own repairing power. Some people go to the market to buy stainless steel tableware, and they bring a small magnet. What they can't suck is good stainless steel, so they won't rust. This is actually a misunderstanding. Whether stainless steel is magnetic or not is determined by the structure. During the solidification process of molten steel, stainless steels of different structures such as ferrite, austenite, and martensite will be formed due to different solidification temperatures. Among them, ferrite and martensitic stainless steels are magnetic. In terms of corrosion resistance alone, magnetic "ferritic" stainless steels are stronger than "austenitic" stainless steels. At the same time, the austenitic stainless steel undergoes micro-strip magnetic properties after stretching, annealing, polishing, casting and other processes, or changes in magnetic properties due to the shape of the product. Therefore, it is not reliable to judge whether the stainless steel is good or bad and whether it is rusty or not by the magnetic properties of the stainless steel.

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