Why does stainless steel rust?
When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: they think that "stainless steel is never rusty, and if it gets rusty, it is not stainless steel. It may be that there is a problem with the quality of the steel." In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel also can get rusty under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation -- that is, stainless, and also has the ability to be corroded in media containing acids, alkalis, and salts -- that is, corrosion resistance. But the size of its resistance to corrosion with the chemical constitution of its steel itself, add mutual state, working conditions and surrounding medium type and change. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to tarnishing ability is arranged, but it is moved on to riviera, in containing the sea fog of a large amount of salts, will soon get rusty; good. Therefore, not any kind of stainless steel can resist corrosion and rust in any environment.
There are many forms of damage to the surface film, and the most common ones in daily life are as follows:
Stainless steel relies on a layer of extremely thin, firm, dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to infiltrate and oxidize, thereby obtaining the ability to resist corrosion. Once for some reason, the film is continuously damaged, the oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continuously separate out to form loose iron oxide, and the metal surface will be continuously rusted. There are many forms of damage to this surface film, and the most common ones in daily life are as follows:
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 juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, organic acids will be formed. For a long time, organic acids will corrode the metal surface.
3. The surface of stainless steel adheres to acid, alkali and salt substances (such as alkaline water and lime water splashed on the wall for decoration), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), when it encounters condensed water, it will form liquid points of sulfuric acid, nitric acid, and acetic acid, causing chemical corrosion.
In order to ensure that the metal surface is permanently bright and not corroded, we recommend:
The above conditions can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we recommend:
1. The surface of the decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause modification.
2. The seaside area should use 316 material stainless steel, 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 requirements of 304 materials. 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, and verify its quality and authenticity. If it does not attract and has no magnetism, it is considered good and genuine; if it attracts magnetism, it is considered a counterfeit. In fact, this is an extremely one-sided, impractical and wrong way to distinguish.
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. Martensite or ferrite type: such as 430, 420, 410, etc.;
Austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
Most of the stainless steel used for decorative tube sheets is austenitic 304 material, generally speaking, it is non-magnetic or weakly magnetic, but it may also appear magnetic due to chemical composition fluctuations or different processing conditions caused by smelting, but this cannot be considered as 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 composition segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel will be formed. body tissue. In this way, 304 stainless steel will have weak magnetism.
In addition, after cold working of 304 stainless steel, the microstructure will also transform to martensite. The greater the degree of cold working deformation, the more martensite transformation, and the greater the magnetic properties of the steel. Like a batch of steel strips, Φ76 tubes are produced, without obvious magnetic induction, and Φ9.5 tubes are produced. The magnetic induction is more obvious due to the larger cold bending deformation. The deformation of square and rectangular pipes is larger than that of round pipes, especially at the corners, and 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 recovered by high temperature solution treatment, so as to eliminate the magnetism.
In particular, the magnetism of 304 stainless steel caused by the above reasons is not at the same level as that of other stainless steels, such as 430 and carbon steel, that is to say, 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 is the same as carbon steel, it shows strong magnetism, because it is not 304 material.
