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Why do stainless steel pipes rust?
Jan 29, 2019

Why do stainless steel pipes rust?

When the stainless steel tube surface appears brown rust spot (point), people feel surprised: think that "stainless steel is not rust, rust is not stainless steel, may be steel problems".In fact, this is a one-sided view of the lack of understanding of stainless steel.Stainless steel will rust under certain conditions.Stainless steel has the ability to resist atmospheric oxidation, that is, rust, but also has the ability to contain acid, alkali, salt in the medium is corrosion resistance, that is.However, the corrosion resistance of steel varies with its chemical composition, interaction state, service condition and environmental media type.For example, 304 steel pipe has absolutely excellent anti-rust ability in the dry and clean atmosphere, but it will soon rust if moved to the coastal area in the sea fog containing a large amount of salt.The 316 pipe performed well.

Therefore, not any kind of stainless steel, in any environment can resist corrosion, not rust.Stainless steel is a very thin and solid thin stable chromium rich oxide film (protective film) formed on the surface of *, to prevent the continuous infiltration of oxygen atoms, continue to oxidation, and obtain the ability to resist corrosion.If, for some reason, the film is continually destroyed, the oxygen atoms in the air or liquid will continue to infiltrate or the iron atoms in the metal will continue to separate out, forming loose iron oxide, the metal surface will be constantly corrosion.

This kind of surface film is damaged in many forms, common in daily life are as follows:

  1. The surface of stainless steel is covered with dust containing other metal elements or the attachment of heterogeneous metal particles. In the humid air, the condensation water between the attachment and the stainless steel connects them into a micro battery, triggering an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.

2. Stainless steel surface adhesion organic juice (such as melon and vegetable, noodle soup, phlegm, etc.), in the case of water oxygen, constitute organic acid, organic acid on the metal surface corrosion for a long time.

3 stainless steel surface adhesion contains acid, alkali, salt substances (such as decoration of the wall of alkali, limestone spray), causing local corrosion.

4. In polluted air (such as the atmosphere containing a large number of sulfides, carbon oxides and nitrogen oxides), condensate water will form sulfuric acid, nitric acid and acetic acid solution points, causing chemical corrosion.The above situation can cause the destruction of the stainless steel surface protective film to cause corrosion.

In order to ensure the permanent brightness of the metal surface, not to be corroded, the surface must be cleaned, passivation and other treatment.