How much knowledge do you know about stainless steel?

Steel is a general term for iron carbon alloys with a carbon content between 0.02% and 2.11%. More than 2.11% is iron.
The chemical composition of steel can vary greatly, and steel containing only carbon elements is called carbon steel or ordinary steel. During the smelting process, alloy elements such as chromium, nickel, manganese, silicon, titanium, and molybdenum can also be added to improve the performance of steel.
Stainless steel is characterized by its stainless steel and corrosion resistance, with a chromium content of at least 10.5% and a carbon content of no more than 1.2%.
01
Stainless steel will definitely not rust? When brown rust spots (spots) appear on the surface of stainless steel, people are amazed and believe that stainless steel will not rust. Rust is not stainless steel anymore, it may be due to a problem with the steel quality. In fact, this is a one-sided erroneous view that lacks understanding of stainless steel. Stainless steel can also rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation - rust resistance, and also has the ability to resist corrosion in media containing acid, alkali, and salt, that is, corrosion resistance. But the magnitude of its corrosion resistance varies with the chemical composition, mutual state, usage conditions, and type of environmental medium of the steel itself. Like 304 material, it has absolutely excellent corrosion resistance in a dry and clean atmosphere, but when moved to coastal areas, it will quickly rust in sea mist containing a large amount of salt. Therefore, not any type of stainless steel can withstand corrosion and rust at all times. Stainless steel is a stable chromium rich oxide film (protective film) formed on its surface, which is extremely thin and sturdy, preventing oxygen atoms from continuing to infiltrate and oxidize, thus obtaining the ability to resist corrosion. Once for some reason, this thin film is constantly damaged, and oxygen atoms in air or liquid will continuously infiltrate or iron atoms in the metal will continuously separate, forming loose iron oxide, and the metal surface will also be constantly corroded.
02
What type of stainless steel is less prone to rusting?
The main factors affecting stainless steel corrosion are as follows.
1) The content of alloying elements
Generally speaking, steel with a chromium content of 10.5% is less prone to rusting. The higher the content of chromium and nickel, the better the corrosion resistance. For example, 304 material requires a nickel content of 8% to 10%, and a chromium content of 18% to 20%. Such stainless steel will not rust under normal circumstances.
2) Smelting process of production enterprises
The smelting process of production enterprises can also affect the corrosion resistance of stainless steel. Large stainless steel plants with good smelting technology, advanced equipment, and advanced processes can ensure the control of alloy elements, removal of impurities, and control of cooling temperature of steel billets. Therefore, the product quality is stable and reliable, with good internal quality and less prone to rusting. On the contrary, some small steel mills have outdated equipment and processes. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.
3) External environment
A dry and well ventilated environment is less prone to rusting. Areas with high air humidity, continuous rainy weather, or high acidity and alkalinity in the air are prone to rusting. 304 stainless steel can also rust if the surrounding environment is too poor.
03
How to deal with rust spots on stainless steel?
1) Chemical methods
Use acid cleaning paste or spray to assist the rusted parts to passivate again to form chromium oxide film to restore their corrosion resistance. After acid cleaning, in order to remove all pollutants and acid residues, it is very important to properly rinse with clean water. After all processing, use polishing equipment to re polish and seal with polishing wax. For areas with slight rust spots, a 1:1 mixture of gasoline and engine oil can be used to wipe off the rust spots with a clean cloth.
2) Mechanical methods
Sandblasting, shot blasting with glass or ceramic particles, immersion, brushing, and polishing. It is possible to use mechanical methods to wipe away contamination caused by previously removed materials, polishing materials, or annihilated materials. Various types of pollution, especially foreign iron particles, can become sources of corrosion, especially in humid environments. Therefore, mechanical cleaning surfaces should be properly cleaned under dry conditions. The use of mechanical methods can only clean the surface and cannot alter the corrosion resistance of the material itself. Therefore, it is recommended to re polish with polishing equipment after mechanical cleaning and seal with polishing wax.
04
Can a magnet be used to determine stainless steel?
Many people buy stainless steel or stainless steel products and bring a small magnet with them. When they look at the goods, they suck on it and think that what they can't suck on is good stainless steel. Without magnetism, rust will not occur, which is actually a misconception.
Stainless steel with or without magnetism is determined by its microstructure. During the solidification process of molten steel, different microstructures such as "ferrite", "austenite", and "martensite" are formed due to different solidification temperatures. Among them, "ferrite" and "martensite" stainless steel are all magnetic. Austenitic stainless steel has good comprehensive mechanical properties, processability, and weldability, but in terms of corrosion resistance, magnetic ferritic stainless steel is stronger than austenitic stainless steel.
The so-called 200 series and 300 series stainless steels with high manganese content and low nickel content currently circulating in the market do not have magnetism, but their performance differs greatly from that of 304 with high nickel content. On the contrary, 304 can also have slight magnetism after being processed by stretching, annealing, polishing, casting, and other processes. Therefore, using stainless steel strips without magnetism to judge the quality of stainless steel is a misunderstanding and unscientific.
05
What are the commonly used stainless steel grades?
201: Replacing nickel stainless steel with manganese, it has certain acid and alkali resistance, high density, and is polished without bubbles. It is used in shallow stretched products such as watch cases, decorative tubes, and industrial tubes.
202: It belongs to low nickel high manganese stainless steel with a nickel and manganese content of about 8%. Under weak corrosion conditions, it can replace 304 and has high cost-effectiveness. It is mainly used in building decoration, highway guardrails, municipal engineering, glass handrails, highway facilities, etc.
304: Universal stainless steel, with good corrosion resistance, heat resistance, low temperature strength and mechanical properties, high toughness, applied in the food industry, medical industry, industrial, chemical industry, and home decoration industry.
304L: Low carbon 304 stainless steel, used for equipment components with corrosion resistance and formability.
316: With the addition of Mo, it has excellent high-temperature corrosion resistance and is applied in seawater equipment, chemistry, food industry, and papermaking fields.
321: It has excellent high-temperature stress fracture performance and high-temperature creep resistance.
430: Heat resistant to fatigue, with a lower coefficient of thermal expansion than austenite, suitable for household appliances and building decoration.
410: High hardness, toughness, good corrosion resistance, high thermal conductivity, low expansion coefficient, good oxidation resistance, used for manufacturing components with atmospheric, steam, water, and oxidative acid corrosion.