Misunderstanding 1: "Stainless steel will not rust, rust is not stainless steel"

We know that ordinary broken steel carbon steel reacts with oxygen in the atmosphere to form an oxide film on the surface of the metal, and then continue to oxidize, so that the rust is continuously expanded to form a "thousand-layer cake" type of corrosive material until it is rotten. Of course, people can use paint or oxidation-resistant metals (such as zinc, nickel and chromium) to protect the carbon steel surface. However, the protective layer is only a film. If the protective layer is damaged, the steel below. It began to rust again. However, unlike stainless steel, when its chromium content reaches about 12%, it is in contact with the atmosphere, and a passivation film (Cr2O3) is formed on the surface of the steel. It is a dense chromium-rich oxide. It effectively protects the surface of stainless steel. In particular, it prevents further oxidation. This oxide film is extremely thin, through which the natural luster of the steel surface can be seen, giving the stainless steel a unique surface characteristic. If the surface oxide film is destroyed, the exposed steel surface reacts with oxygen in the atmosphere to reform the passivation film. Continue to protect. Therefore, under the guidance of this theory, many people believe that stainless steel will not rust.

However, the customers who use stainless steel always report that the stainless steel they used has been corroded. Did they buy inferior stainless steel? In fact, the exclusion of some of the factors that really bought inferior stainless steel, the rest is due to the lack of understanding of stainless steel. misunderstanding. We know that stainless steel is a stainless steel acid-resistant steel, which consists of two parts: ordinary stainless steel and acid-resistant steel. Steel that resists erosion in air or weak media is called stainless steel, or ordinary stainless steel: steel that resists erosion in certain highly corrosive media is called acid-resistant steel. In general, a stainless steel may have good corrosion resistance in some media, but in other media, it may be corroded due to low chemical stability. So, "rust" is a relative concept, a stainless steel is not resistant to all media. In fact, if the surface of the stainless steel is contaminated with impurities, dirt, and damage to the passivation film at the defect site, it will cause the stainless steel to rust. Studies have shown that the main corrosion forms of stainless steel are uniform corrosion, pitting corrosion, crevice corrosion, intergranular corrosion and stress corrosion. For example, pitting corrosion is an electrochemical corrosion, which means that localized etched holes are formed on the surface of the metal material, and the depth is different. Some penetrating passivation films develop in depth, and even severe perforations are formed. Especially in media containing chloride ions, pitting corrosion is most likely to occur.

Therefore, stainless steel rust is a relatively complicated situation, and it has a relationship with its own corrosion resistance, use methods, and use of media. Stainless steel does not rust, and there is no metal in the world that never rusts.

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Label: hardware; stainless steel

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