We know that ordinary 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 continues to expand, forming a "thousand-layer cake" type of corrosion, until it wears out.
Of course, one can use paint or oxidation-resistant metals (such as zinc, nickel, and chromium) for electroplating to protect the surface of carbon steel. However, this protective layer is only a thin film. If the protective layer is damaged, the steel below They began to rust again.
However, stainless steel is different. When its chromium content reaches about 12%, it will produce a passivation film (Cr2O3) on the surface of the steel when it is in contact with the atmosphere. It is a dense chromium-rich oxide that effectively protects the surface of the stainless steel. , Especially to prevent further oxidation.
This oxide film is extremely thin, through which you can see the natural luster of the steel surface, giving stainless steel unique surface characteristics. If the surface oxide film is destroyed, the exposed steel surface will react with oxygen in the atmosphere to reform the passivation film and continue to play a protective role.
Therefore, under the guidance of this theory, many people believe that stainless steel will not rust.
However, customers who use stainless steel always report that the stainless steel they use is corroded. Did they purchase inferior stainless steel?
In fact, excluding some of the factors that actually bought inferior stainless steel, the rest is a misunderstanding caused by insufficient understanding of stainless steel.
We know that stainless steel is the abbreviation of stainless acid-resistant steel, which is composed of ordinary stainless steel and acid-resistant steel;
The steel that can resist erosion in air or weak media is called stainless steel, that is, ordinary stainless steel;
The steel that can resist erosion in some strong corrosive media is called acid-resistant steel.
Generally, a stainless steel may have good corrosion resistance in some media, but in other media it may be corroded due to low chemical stability.
Therefore, "stainless" is a relative concept. A stainless steel cannot be resistant to all media. In fact, if there are impurities, dirt and passivation film damage on the surface of the stainless steel, it will lead to stainless steel 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 a kind of electrochemical corrosion, which refers to localized corrosion holes on the surface of metal materials, varying in depth, and some penetrate through the passivation film to develop in depth, and even form perforations in severe cases, especially in media containing chloride ions Among them, pitting corrosion is most likely to occur.
Therefore, rusting of stainless steel is a relatively complicated situation, which is related to its own corrosion ability, method of use, and medium used. Stainless steel does not rust, there is no metal in the world that will never rust.
![Stainless steel will not rust, but not rusty?]()