How Much Do You Know About Stainless Steel?
Steel is a collective term for iron-carbon alloys with a carbon content between 0.02% and 2.11%. Anything above 2.11% is considered iron.
The chemical composition of steel can vary greatly. Steel that contains only carbon is called carbon steel or ordinary steel. During the smelting process, alloy elements such as chromium, nickel, manganese, silicon, titanium, and molybdenum can be added to improve the properties of steel.
Stainless steel is characterized by its resistance to rust and corrosion, with a minimum chromium content of 10.5% and a maximum carbon content of no more than 1.2%.
Stainless Steel and Rusting
When people see brown rust spots on the surface of stainless steel, they are often surprised, thinking that stainless steel should not rust. They may believe that if it rusts, it is not stainless steel, and there might be a problem with the steel quality. However, this is a one-sided and mistaken view due to a lack of understanding of stainless steel. Stainless steel can rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, which is its rust resistance, and it also has corrosion resistance in media containing acids, bases, and salts. However, the degree of its corrosion resistance varies with the chemical composition of the steel itself, its condition, the usage conditions, and the type of environmental medium. For example, material 304 has excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to a coastal area with a lot of salt in the sea fog, it will rust quickly. Therefore, not all stainless steels are corrosion-resistant and rust-free at all times.
Stainless steel relies on a very thin, yet dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent the continued penetration of oxygen atoms and further oxidation, thus gaining its corrosion resistance. If this film is continuously damaged for some reason, oxygen atoms from the air or liquids will continuously penetrate, and iron atoms from the metal will continuously separate, forming loose iron oxide and causing continuous corrosion of the metal surface.
What Kind of Stainless Steel is Less Prone to Rust?
The main factors affecting the corrosion of stainless steel are:
The content of alloying elements Generally speaking, steel with a chromium content of 10.5% or more is less prone to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, 304 material contains 8-10% nickel and 18-20% chromium, and such stainless steel will not rust under normal circumstances.
The smelting process of the manufacturing enterprise The smelting process of the manufacturing enterprise also affects the corrosion resistance of stainless steel. Large stainless steel factories with good smelting technology, advanced equipment, and advanced processes can ensure the control of alloying elements, the removal of impurities, and the control of the cooling temperature of the steel ingot, thus ensuring stable and reliable product quality, good internal quality, and less prone to rust. On the contrary, some small steel mills with outdated equipment and processes cannot remove impurities during the smelting process, and the products they produce are inevitably prone to rust.
External environment Stainless steel is less prone to rust in dry and well-ventilated environments. However, in environments with high humidity, continuous rainy weather, or high levels of acid and alkali in the air, stainless steel is more prone to rust. Even 304 material stainless steel can rust if the surrounding environment is too poor.
How to Deal with Rust Spots on Stainless Steel?
Chemical methods Use pickling paste or spray to assist in the re-passivation of the rusted areas to form a chromium oxide film, restoring their corrosion resistance. After pickling, it is very important to properly rinse with clean water to remove all contaminants and acid residues. After all treatments, re-polish with polishing equipment and seal with polishing wax. For local minor rust spots, a mixture of gasoline and machine oil in a 1:1 ratio can be used with a clean cloth to remove the rust spots.
Mechanical methods Sandblasting cleaning, glass or ceramic micro-particle shot peening, tumbling, brushing, and polishing. Mechanical methods may remove pollution caused by previously removed materials, polishing materials, or tumbling materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, it is best to carry out regular cleaning in dry conditions when mechanically cleaning the surface. Mechanical methods can only clean the surface and cannot change the material's inherent corrosion resistance. Therefore, it is recommended to re-polish with polishing equipment and seal with polishing wax after mechanical cleaning.
Can Magnets Be Used to Judge Stainless Steel?
Many people bring a small magnet with them when they buy stainless steel or stainless steel products. They use the magnet to test the products, believing that those that do not stick are good stainless steel. They think that if it does not stick, it will not rust. However, this is a misunderstanding.
Whether stainless steel is magnetic or not is determined by its microstructure. During the solidification process of steel, different microstructures such as "ferrite," "austenite," and "martensite" are formed at different solidification temperatures. Ferrite and martensite stainless steels are magnetic, while austenitic stainless steel has better comprehensive mechanical properties, processability, and weldability. However, in terms of corrosion resistance, magnetic ferrite stainless steel is stronger than austenitic stainless steel.
Currently, the so-called 200 series and 300 series stainless steels, which are high in manganese and low in nickel, are also non-magnetic, but their performance is much different from that of high-nickel 304. Moreover, 304 stainless steel can also have a slight magnetism after processes such as stretching, annealing, polishing, and casting. Therefore, using whether stainless steel is magnetic or not to judge its quality is a misunderstanding and unscientific.
Common Stainless Steel Grades
201: Manganese-substituted nickel stainless steel with certain acid and alkali resistance, high density, and no bubbles when polished. Used in watch cases, decorative tubes, industrial tubes, and other light stretching products.
202: A low-nickel, high-manganese stainless steel with about 8% nickel and manganese content. It can replace 304 under weak corrosive conditions and has a high cost-performance ratio. 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 good mechanical properties. It has high toughness and is used in the food industry, medical industry, industry, chemical industry, and home decoration industry.
304L: Low-carbon 304 stainless steel used in equipment and machinery parts that require corrosion resistance and formability.
316: Added molybdenum, with excellent high-temperature corrosion resistance. Used in seawater equipment, chemical, food industry, and papermaking.
321: Has excellent high-temperature stress rupture performance and high-temperature anti-creep performance.
430: Resistant to thermal fatigue, with a smaller coefficient of thermal expansion than austenite. Used in household appliances and building decoration.
410: High hardness, good toughness, corrosion resistance, large heat conduction coefficient, small expansion coefficient, and good oxidation resistance. Used to manufacture parts resistant to corrosion by atmosphere, steam, water, and oxidizing acids.
