Stainless steel is deeply loved by people for its beautiful appearance, corrosion resistance and resistance to damage.
But when brown rust spots appear on the surface of stainless steel pipes, people will be surprised: Why does "stainless steel" also rust? If it's rusty, is it still considered "stainless steel"? Is there something wrong with the material?
In fact, this is a one-sided misconception about stainless steel, because stainless steel will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation--that is, stainless steel--it also has the ability to corrode in media containing acid, alkali and salt--that is, corrosion resistance. However, its corrosion resistance changes with the chemical composition of the steel itself, its additive state, usage conditions and environmental medium types.
For example, 304 stainless steel has absolutely excellent rust resistance in a dry and clean atmosphere. However, if it is moved to a coastal area, it will soon get rusty in the sea fog containing a large amount of salt; while 316 stainless steel It performs well. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.
The stainless steel protective film will also be damaged
Stainless steel relies on the formation of an extremely thin, strong, fine, and stable chromium-rich oxide film (protective film) on its surface to prevent the continued penetration and oxidation of oxygen atoms, thereby obtaining the ability to resist rust. Once this film is continuously damaged for some reason, oxygen atoms in the air or liquid will continue to penetrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be continuously corroded. There are many forms of damage to this surface film.
Common ones in daily life include the following:
1. Dust containing other metal elements or attachments of heterogeneous metal particles accumulate on the surface of stainless steel. In humid air, the condensed water between the attachments and stainless steel connects the two into a micro-battery, triggering an electrochemical reaction. , the protective film is damaged, which is called electrochemical corrosion.
2. Organic juice adheres to the surface of stainless steel. In the presence of water and oxygen, it forms organic acid. For a long time, the organic acid corrodes the metal surface.
3. The surface of stainless steel contains acids, alkalis, and salts (such as alkaline water and lime water splashed on walls), causing local corrosion.
4. In polluted air (such as an atmosphere containing a large amount of sulfides, carbon oxides, and nitrogen oxides), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid liquid points will form, causing chemical corrosion.
All of the above situations can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, to ensure that the metal surface is permanently bright and protected from rust, we recommend:
1. The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause modification.
2. 316 stainless steel should be used in seaside areas. 316 material can resist seawater corrosion.
3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the 304 material requirements. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.
Why is stainless steel also magnetic?
People often think that magnets absorb stainless steel to verify its quality and authenticity. If it does not attract non-magnetic products, it is considered to be good and genuine; if it attracts magnets, it is considered to be a fake. In fact, this is a wrong identification method.
There are many types of stainless steel, which can be divided into several categories according to their 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.;
Austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic.
Most of the stainless steel usually used for decorative tube sheets is austenitic 304 material. Generally speaking, it is non-magnetic or weakly magnetic. However, due to fluctuations in chemical composition due to smelting or different processing conditions, magnetism may also occur, but this cannot 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 component segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel will be caused. body tissue. In this way, 304 stainless steel will have weak magnetism.
In addition, after cold working of 304 stainless steel, the organizational structure will also transform to martensite. The greater the degree of cold working deformation, the more martensitic transformation, and the greater the magnetism of the steel. Just like a batch number of steel strips, φ76 pipes are produced without obvious magnetic induction, and φ9.5 pipes are produced. Because the bending deformation is larger, the magnetic induction will be more obvious. The deformation of the square rectangular tube is larger than that of the round tube, especially the corner part, the deformation is more intense and the magnetism is more obvious.
In order to completely eliminate the magnetism of 304 steel caused by the above reasons, the stable austenite structure can be restored through high-temperature solution treatment, thereby eliminating the magnetism.
In particular, it should be pointed out that the magnetism of 304 stainless steel due to the above reasons is not at the same level as the magnetism of other stainless steel materials, such as 430 and carbon steel. In other words, the magnetism of 304 steel always shows weak magnetism.
This tells us that if stainless steel has weak magnetism or no magnetism at all, it should be identified as 304 or 316 material; if it has the same magnetism as carbon steel and shows strong magnetism, it should be identified as not 304 material.
