Why is stainless steel rusty

When the surface of stainless steel pipe appears brown rust (point) when people were surprised: that "stainless steel is not rusty, rust is not stainless steel, steel may be a problem." In fact, this is a lack of understanding of stainless steel a one-sided error view. Stainless steel under certain conditions will be rusty.
Stainless steel has the ability to resist atmospheric oxidation --- that is, stainless steel, but also in the acid, alkali, salt medium is the ability to corrosion - that is, corrosion resistance. But its corrosion resistance is the size of its steel chemical composition, plus the state, the use of conditions and environmental media type and change. Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent resistance to corrosion, but it moved to the beach area, in a large amount of salt containing sea fog, will soon rust; and 316 steel performance good. Therefore, not any kind of stainless steel, in any environment can be resistant to corrosion, no rust.
Surface film is damaged in many forms, more common in daily life have the following:
      Stainless steel is the surface of the formation of a thin layer of thin and strong and stable stable chromium oxide film (protective film), to prevent the continued infiltration of oxygen atoms, continue to oxidize, and the ability to obtain corrosion resistance. Once for some reason, the film has been constantly destroyed, the air or liquid oxygen atoms will continue to infiltrate or metal iron atoms continue to precipitate out to form loose iron oxide, metal surface will continue to rust. This surface film is damaged in the form of many, more common in daily life are the following:
1. The surface of the stainless steel is stored with dust or heterogeneous metal particles containing other metal elements. In the humid air, the condensed water between the attached material and the stainless steel is connected into a micro-cell, which triggers an electrochemical reaction , The protective film is damaged, called electrochemical corrosion.
2. Stainless steel surface adhesion of organic juice (such as melons, soup, sputum, etc.), in the case of water and oxygen, constitute organic acids, long time the organic acid corrosion of metal surface.
3. Stainless steel surface adhesion contains acid, alkali, salt substances (such as decorative walls of alkaline water, lime water splash), causing local corrosion.
4. In contaminated air (such as containing a large amount of sulfide, carbon oxides, nitrogen oxide atmosphere), in case of condensate, the formation of sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion.
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