The chemical composition of steel can vary greatly. Steel containing only carbon is called carbon steel or ordinary steel. Alloy elements such as chromium, nickel, manganese, silicon, titanium and molybdenum can also be added to improve the properties of steel during smelting.
Stainless steel is a steel with stainless and corrosion resistance as its main characteristics, and the chromium content is at least 10.5% and the carbon content is not more than 1.2%.
Stainless steel must not rust?
When brown rust spots (spots) appear on the surface of stainless steel, people are greatly surprised, thinking that stainless steel will not rust, and rust is not stainless steel, which may be a problem with steel. In fact, this is a one-sided misconception about stainless steel. Stainless steel will rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, rust, and also has the ability to resist corrosion in the medium containing acid, alkali and salt, that is, corrosion resistance. However, its corrosion resistance varies with the chemical composition, mutual state, service conditions and environmental medium types of its steel. For example, material 304 has absolutely excellent corrosion resistance in a dry and clean atmosphere, but it will soon rust if it is moved to the coastal area in the sea fog containing a lot of salt. Therefore, not any kind of stainless steel can resist corrosion and rust at any time.
Stainless steel is a very thin, firm and stable chromium-rich oxide film (protective film) formed on its surface, which prevents oxygen atoms from continuing to infiltrate and continue to oxidize, thus gaining corrosion resistance. However, for some reason, this kind of film is constantly destroyed, and the neutralized oxygen atoms in air or liquid will continue to infiltrate or the iron atoms in metal will continue to separate out, forming loose iron oxide, and the metal surface will be constantly corroded.
What kind of stainless steel is not easy to rust?
The main factors affecting stainless steel corrosion are as follows.
1) Content of alloying elements
Generally speaking, steel with chromium content of 10.5% is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the content of nickel in 304 material is 8-10%, and the content of chromium reaches 18-20%. Under normal circumstances, such stainless steel will not rust.
2) Smelting process of production enterprises
The smelting process of production enterprises will also affect the corrosion resistance of stainless steel. Large stainless steel plants with good smelting technology, advanced equipment and advanced technology can ensure the control of alloying elements, impurity removal and billet cooling temperature, so the product quality is stable and reliable, the internal quality is good and it is not easy to rust. On the other hand, some small steel mills are backward in equipment and technology. In the smelting process, impurities cannot be removed, and the products produced will inevitably rust.
3) External environment
A dry and ventilated environment is not easy to rust. However, environmental areas with high air humidity, continuous rainy weather, or high pH in the air are prone to rust. 304 stainless steel will rust if the surrounding environment is too bad.
How to deal with rust spots on stainless steel?
1) Chemical methods
Using pickling paste or spray to assist the re-passivation of the corroded part to form a chromium oxide film to restore its corrosion resistance. After pickling, it is very important to wash it with clear water in order to remove all pollutants and acid residues. After all treatment, re-polish with polishing equipment and seal with polishing wax. For those with slight rust spots locally, 1: 1 mixture of gasoline and engine oil can also be used to wipe off the rust spots with a clean rag.
2) Mechanical method
Blast cleaning, blasting cleaning with glass or ceramic particles, annihilation, brushing and polishing. It is possible to wipe off the pollution caused by previously removed materials, polished materials or annihilation materials by mechanical methods. All kinds of pollution, especially foreign iron particles, may be the source of corrosion, especially in humid environment. Therefore, it is best to clean the surface mechanically under dry conditions. Using mechanical method can only clean its surface, but can't change the corrosion resistance of the material itself. Therefore, it is suggested to re-polish with polishing equipment and seal with polishing wax after mechanical cleaning.
Can stainless steel be judged by magnets?
Many people go to buy stainless steel or stainless steel products, bring a small magnet with them, and suck it when they look at the goods, thinking that what they can't suck is good stainless steel. Without magnetism, it will not rust. In fact, this is a wrong understanding.
Whether the stainless steel strip is magnetic or not is determined by the structure. During the solidification process, molten steel will form stainless steels with different structures such as ferrite, austenite and martensite due to different solidification temperatures, among which ferrite and martensite stainless steels are all magnetic. Austenite stainless steel has good comprehensive mechanical properties and weldability, but ferrite stainless steel with magnetism is stronger than austenite stainless steel only in terms of corrosion resistance.
At present, the so-called 200-series and 300-series stainless steels with high manganese content and low nickel content in the market are also nonmagnetic, but their properties are far from those of 304 with high nickel content. On the contrary, 304 will be microstrip magnetic after being stretched, annealed, polished and cast, so it is a misunderstanding and unscientific to judge the quality of stainless steel by using stainless steel belts without magnetism.
What are the commonly used stainless steel brands?
201: Replacing nickel stainless steel with manganese has certain acid and alkali resistance, high density and no bubbles in polishing, and is applied to watch cases, decorative tubes, industrial tubes and other shallow tensile products.
202: belongs to low-nickel and high-manganese stainless steel, with about 8% nickel and manganese content. It can replace 304 under weak corrosion conditions, with high cost performance, and is mainly used in architectural decoration, highway guardrail, municipal engineering, glass handrails, highway facilities, etc.
304: General-purpose stainless steel, with good corrosion resistance, heat resistance, low-temperature strength, good mechanical properties and high toughness, is used in food industry, medical industry, industry, chemical industry and home decoration industry.
304L: Low-carbon 304 stainless steel, used for corrosion-resistant and formable equipment and parts.
316: Mo is added, which has excellent high temperature resistance and corrosion resistance, and is used in seawater equipment, chemistry, food industry and paper making.
321: It has excellent high-temperature stress breaking performance and high-temperature creep resistance.
430: Thermal fatigue resistance, smaller thermal expansion coefficient than austenite, used in household appliances and architectural decoration.
410: High hardness, toughness, good corrosion resistance, high thermal conductivity, small expansion coefficient and good oxidation resistance, used to manufacture parts corrosive to atmosphere, steam, water and oxidizing acids.