Why does stainless steel rust?

Author: jokddy

2023-06-14

When the surface of the stainless steel pipe appears brown rust spots (spots), people are surprised: think that "stainless steel is not rusty, rust is not stainless steel, there may be a problem with the steel." In fact, this is a one-sided wrong 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 corrosion in the medium containing acid, alkali, salt - that is, corrosion resistance. However, the size of its corrosion resistance is changed with the chemical composition of the steel itself, the mixing state, the use conditions and the type of environmental media. Such as 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it is moved to the seaside area, in the sea fog containing a lot of salt, it will soon rust; The 316 steel pipe performed well. Therefore, not any kind of stainless steel, in any environment can resist corrosion, does not rust.

There are many forms of surface film damage, which are commonly seen in daily life as follows:
Stainless steel is formed on its surface by a layer of extremely thin and solid fine stable chromium-rich oxide film (protective film), to prevent the continued infiltration of oxygen atoms, continue to oxidize, and obtain the ability to resist corrosion. Once for some reason, this film is constantly destroyed, the oxygen atoms in the air or liquid will continue to penetrate or the iron atoms in the metal will continue to be resolved out, forming loose iron oxide, and the metal surface will be constantly corroded. There are many forms of damage to this surface film, which are commonly seen in daily life as follows:
1. The surface of stainless steel contains dust or alien metal particles containing other metal elements, in the humid air, the condensate between the attachment and stainless steel, the two are connected into a microbattery, causing an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.
2. The surface of stainless steel adheres to organic juice (such as melons and vegetables, noodle soup, sputum, etc.), which constitutes organic acid in the case of water oxygen, and the organic acid corrods the metal surface for a long time.
3. Stainless steel surface adhesion containing acid, alkali, salt substances (such as alkaline water, lime water splash on the decoration wall), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large number of sulfides, carbon oxide, nitrogen oxide), in case of condensation, sulfuric acid, nitric acid, acetic acid liquid point is formed, causing chemical corrosion.
In order to ensure that the metal surface is permanently bright and not corroded, we recommend:
The above conditions can cause the damage of the stainless steel surface protective film and lead to corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we recommend:
1. The decorative stainless steel surface must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause modification.
2. The seaside area should use 316 stainless steel, 316 material can resist seawater corrosion.
3. The chemical composition of some stainless steel pipes on the market can not meet the corresponding national standards, and can not meet the 304 material requirements. Therefore, it will also cause rust, which requires users to carefully choose the products of reputable manufacturers.

Why does stainless steel also have magnetic?

People often think that the magnet absorbs stainless steel, verify its advantages and disadvantages and authenticity, does not absorb no magnetic, that is good, genuine; If the magnet is magnetic, it is considered to be a fake. In fact, this is an extremely one-sided, impractical and wrong method of identification.
There are many kinds of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
1. Austenitic type: such as 201, 202, 301, 304, 316, etc.
2. Martensitic or ferritic type: such as 430, 420, 410, etc.;
Austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
Usually used as decorative tube plate stainless steel is mostly Austenitic type 304 material, generally speaking, is non-magnetic or weak magnetic, but because of smelting caused by chemical composition fluctuations or processing state may also appear magnetic, but this can not be considered counterfeit or unqualified, what is the reason?
As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic, due to improper segregation or heat treatment during smelting, it will cause a small amount of martensite or ferrite structure in austenitic 304 stainless steel. In this way, 304 stainless steel will have a weak magnetic.
In addition, 304 stainless steel after cold processing, the structure will also be transformed to martensite, the greater the deformation of cold processing, the more martensite transformation, the greater the magnetic steel. Like a batch of steel strips, the production of Φ76 tubes, no obvious magnetic induction, the production of Φ9.5 tubes. Because the bending deformation is larger, the magnetic sense is obvious, and the production of square rectangular tube is larger than the round tube because of the deformation, especially the corner part, the deformation is more intense and the magnetism is more obvious.
In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, the stable austenite structure can be restored by high-temperature solution treatment, so as to eliminate the magnetic properties.
In particular, the magnetic properties of 304 stainless steel caused by the above reasons are not at the same level as the magnetic properties of other materials such as 430 and carbon steel, that is to say, the magnetic properties of 304 steel always show weak magnetism.
This tells us that if the stainless steel has weak magnetism or no magnetism at all, it should be judged as 304 or 316 material; If it is the same as the magnetism of carbon steel, it shows strong magnetism, because it is not 304 material.