1. Analysis of
stainless steel
Stainless steel is a type of steel. Steel refers to steel that contains less than 2% carbon (C), and iron if it contains more than 2%. During the smelting process of steel, alloy elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the performance of the steel and make the steel corrosion-resistant. (That is, it does not rust) It is what we often call stainless steel.
2. Why do stainless steel have different steel grades?
During the smelting process of stainless steel, due to the different types of alloying elements added, the adding amounts of different types are different. Their characteristics are also different. In order to distinguish them, they are labeled with different steel grades. The following is the "alloy element" content table of different steel grades of common decorative stainless steel for reference only: Steel Grade Carbon (C) Silicon (Si) Manganese (Mn) ) Phosphorus (P) Sulfur (S) Chromium (Cr) Nickel (Ni) Molybdenum (Mo) Copper (Cu)
304 ≤0.08 ≤1.00 ≤2.00 ≤0.045 ≤0.03 18-20 8-10
301 ≤0.15 ≤1.00 ≤2.00 ≤0.045 ≤0.03 16-18 6-8
201 ≤0.15 ≤1.00 5.5-7.5 ≤0.05 ≤0.03 16-18 3.5-5.5
3. What kind of stainless steel is not easy to rust?
There are three main factors affecting stainless steel corrosion:
One: the content of alloying elements. Generally speaking, steel will not rust easily if the chromium content is 10.5%. The higher the chromium and nickel content, the better the corrosion resistance. For example, the nickel content of 304 material is 8-10%, and the chromium content reaches 18-20%. Such stainless steel will not rust under normal circumstances.
Second: The smelting process of the production company will also affect the corrosion resistance of stainless steel. Large stainless steel plants with good smelting technology, advanced equipment and advanced processes can ensure stable and reliable product quality, good intrinsic quality and not easy to rust, whether it is the control of alloy elements, the removal of impurities, or the control of billet cooling temperature. On the contrary, some small steel plants have backward equipment and backward processes. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.
Third: External environment, a dry and ventilated environment is not easy to rust. However, areas with high air humidity, continuous rainy weather, or environments with high pH in the air are prone to rust. Made of 304 stainless steel, it will rust if the surrounding environment is too poor.
4. Stainless steel is non-magnetic. Is non-magnetic stainless steel good?
Many customers go to the market to buy stainless steel and bring a small magnet with them. They use the magnet to suck it when looking at the goods, thinking that the stainless steel that cannot be sucked is good stainless steel. If there is no magnet, it will not rust. In fact, this is a wrong understanding. Whether a stainless steel strip is magnetic or not is determined by its organizational structure. During the solidification process of molten steel, stainless steel with different organizational structures such as "ferrite", "austenite" and "martensite" will be formed due to different solidification temperatures. Among them, "ferrite" Both "bulk" and "martensitic" stainless steels are magnetic. "Austenitic" stainless steel has good comprehensive mechanical properties, process performance and weldability, but in terms of corrosion resistance alone, magnetic "ferritic" stainless steel is stronger than "austenitic" stainless steel. The so-called 200 series and 300 series stainless steel currently circulating in the market, which contain high manganese content and low nickel content, are not magnetic, but their performance is very different from that of 304, which contains high nickel content. On the contrary, 304 has been stretched, annealed, polished, cast, etc. Technological treatment will also cause micro-magnetism. Therefore, it is a misunderstanding and unscientific to judge the quality of stainless steel by whether the stainless steel belt is magnetic or not.
5. Why does stainless steel also rust?
When brown rust spots (dots) appear on the surface of stainless steel, people are surprised and think that "stainless steel will not rust. If it is rusty, it is no longer stainless steel. There may be something wrong with the steel." In fact, this is a one-sided wrong view due to lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation-that is, it is rustless. It also has the ability to resist corrosion 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 mutual state, usage conditions and environmental medium types. Materials such as 304 have absolutely excellent corrosion 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. Therefore, not any kind of stainless steel can be corrosion-resistant and rust-free at any time.
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 oxygen atoms from continuing to penetrate and oxidize, thereby gaining the ability to resist corrosion. 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 constantly damaged. rust. There are many ways in which this surface film is damaged, 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 the 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 (such as vegetables, noodle soup, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, it forms organic acid. For a long time, organic acid corrodes the metal surface.
3. Substances containing acids, alkalis, and salts adhering to the surface of stainless steel (such as alkaline water and lime water sprayed when decorating walls) cause local corrosion.
4. In polluted air (an atmosphere containing a large amount of sulfide, oxide, and hydrogen oxide), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid liquid points will form, causing chemical corrosion.
