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  • Shaanxi Tonghui Steel Co., Ltd.

  • E-mail: sxthsteel@sxth-group.com

  • Tel: +86-29-89636557

  • Add: 1st Building Of Jinqiao International, Xi'an, Shaanxi,China

  • Why does stainless steel also rust?
    Mar 15, 2018

    Why does stainless steel also rust?

    When the brown rust (spot) appears on the surface of the stainless steel pipe, people are greatly surprised: "Stainless steel does not rust, rust is not stainless steel, and there may be a problem with the steel." In fact, this is a one-sided misconception about 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, stainless, but also has the ability to resist corrosion in the medium containing acid, alkali, salt, corrosion resistance. However, the corrosion resistance of the steel varies with the chemical composition of the steel itself, the interaction state, the conditions of use, and the type of environmental media. Such as 304 steel pipe, in the dry clean atmosphere, there is absolutely excellent resistance to corrosion, but it will be moved to the sea area, in the sea fog containing a lot of salt, will quickly rust; and 316 steel performance good. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.

    There are many forms of surface film that are damaged. Commonly there are the following:

    Stainless steel is formed on the surface of a thin, strong and stable chromium-rich oxide film (protective film), to prevent the continued penetration of oxygen atoms, continue to oxidize, and get the ability to resist corrosion. Once for some reason, this kind of film is destroyed constantly, oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continue to separate out to form loose iron oxides, and the metal surface will be continuously rusted. There are many forms of this type of surface film that are damaged. Common ones are as follows:

    1. The surface of stainless steel contains deposits of dust or other dissimilar metal particles containing other metal elements. In moist air, the condensed water between the attachment and stainless steel connects the two into a microbattery, which triggers an electrochemical reaction. The protective film is damaged, which is called electrochemical corrosion.

    2.  The surface of stainless steel adheres to the organic matter juice (such as melon vegetables, noodles, soup, etc.). In the presence of water and oxygen, organic acids are formed, and organic acids attack the metal surface for a long time.

    3. The surface of stainless steel adheres with acids, alkalis and salts (such as alkaline water on the wall of the decoration, lime water splashes), causing localized corrosion.

    4. In contaminated air (such as an atmosphere containing a large amount of sulfides, carbon oxides, and nitrogen oxides), condensate water is formed and sulfuric acid, nitric acid, and acetic acid liquids are formed, causing chemical corrosion.

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    How to ensure that the metal surface is permanently bright and not rusted?

    The above conditions can cause the destruction of the stainless steel surface protective film to cause corrosion. Therefore, to ensure that the metal surface is permanently bright and not rusted, it is recommended that:

    1. The surface of the decorative stainless steel must be cleaned and scrubbed to remove attachments and eliminate the external factors that cause the modification.

    2. the use of 316 stainless steel in the beach area, 316 material can resist seawater corrosion.

    3. Some chemical compositions of stainless steel pipes on the market cannot meet the corresponding national standards and reach the material requirements of 304. It can also cause rust, which requires the user to carefully select products from reputable manufacturers.

    Why is stainless steel magnetic?

    People often think that magnets absorb stainless steel, verify its pros and cons and authenticity, do not absorb non-magnetic, that is good, genuine; suckers have magnetic, it is considered fake counterfeit goods. In fact, this is a very one-sided, imprecise and erroneous method of identification.

    There are many types of stainless steel, which can be divided into several types according to the organizational structure at room temperature:

    1. austenitic type: such as 201,202,301,304,316, etc.;

    2. martensite or ferrite type: such as 430,420,410, etc.;

    3. austenite type is non-magnetic or weak magnetic, martensite or ferrite is magnetic.

    If you interest in our products Please kindly contact us 

    The most commonly used stainless steel for decorative tube sheets is austenitic type 304. Generally speaking, it is non-magnetic or weakly magnetic. However, magnetic properties may occur due to chemical composition fluctuations or processing conditions due to smelting, but this cannot be considered as Counterfeit or unqualified, what is the reason?

    The austenite mentioned above is either non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel may be caused. Body organization. In this way, 304 stainless steel will have a weak magnetic.

    In addition, 304 stainless steel after cold processing, the organization structure will also be transformed to martensite, the greater the cold deformation degree, the more martensite transformation, the greater the magnetic steel. Such as the same batch of steel, production Φ76 tube, no significant sense of magnetic, production Φ9.5 tube. Due to the large bending deformation of the magnetic induction is obvious, the production of rectangular tube due to the deformation of the larger than the round tube, especially the folding part, the deformation is more intense magnetic more obvious.

    In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, the stable austenite structure can be recovered by high-temperature solution treatment, thereby eliminating magnetic properties.

    In particular, the magnetic properties of 304 stainless steel due to the above reasons are not the same as the magnetic properties of stainless steels of other materials such as 430 and carbon steel. That is, the magnetic properties of 304 steel always show weak magnetic properties.

    This tells us that if stainless steel is weakly magnetic or completely magnetic, it should be judged to be 304 or 316; if it is the same as carbon steel, it shows strong magnetism, because it is not 304 material.