How to Distinguish 304 Stainless Steel （SUS304）and 202 Stainless Steel（SUS202)

Explanation and analysis of stainless steel

1、 What kind of steel is stainless steel?

Stainless steel is a kind of steel. Among them, iron-containing less than 2% carbon (C)is called steel, and more than 2% carbon (C) is called iron. In the smelting process of steel, it is necessary to add chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo), and other alloy elements in order to improve the performance of steel, so that the steel has corrosion resistance (i.e., no rust), which is commonly known as stainless steel.

2. Why are there different models of stainless steel?

Therefore, in order to distinguish these different types of stainless steel, they are crowned with varying grades of steel. The following is the content table of “alloy elements” of different grades of common decorative stainless steel. The following contents are 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 8-10301 ≤0.15 ≤1.00 ≤2.00 ≤0.045 ≤0.03 16-18 6-8202 ≤0.15 ≤1.00 7.5-10 ≤0.05 ≤0.03 17-19 4-6201 ≤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 the corrosion of stainless steel

Firstly, the content of alloying elements. Generally speaking, steel containing 10.5% chromium is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance of steel. For example, the content of chromium and nickel in 304 stainless steel is 8-10%, and the content of chromium reaches 18-20%. In general, such stainless steel will not rust.

Secondly, the smelting process of the manufacturer will also affect the corrosion resistance of stainless steel. If the stainless steel smelting technology is good, the equipment is advanced, and it is smelted in a large-scale stainless steel factory with advanced technology, then the stainless steel can be guaranteed in terms of alloy element control, impurity removal, and billet cooling temperature control. Therefore, the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. On the contrary, if smelting in some small steel plants, equipment, and technology is backward, then in the smelting process, impurities can not be removed, and the products produced will inevitably rust.

Thirdly: in the external environment, dry climate, and good ventilation environment, stainless steelares not easy to rust. And in the air humidity, continuous rainy weather, or the air containsa high pH environme, and stainlessss steel is easy to rust. Even 304 stainless steel, if the surrounding environment is too bad, it will rust.

4. Stainless steel is steel without magnetism. Is steel without magnetism good stainless steel? If the microstrip magnetic, then it is not 304 stainless steel?

Many customers will take a small magnet with them when they go to the market to buy stainless steel. When they look at the goods, they will use the magnet to suck it. They think that the stainless steel that can’t be absorbed is good stainless steel. In fact, it is a wrong understanding that there is no rust without magnetic field. In fact, the structure of stainless steel strip without magnetic field is determined by the structure. During the solidification process of molten steel, due to the different solidification temperature, stainless steel with different micro structure such as “ferrite”, “austenite” and “martensite” will be formed. Among them, “ferrite” and “martensite” stainless steel are magnetic. However, “austenitic” stainless steel has good weldability due to its comprehensive mechanical properties. But in terms of corrosion resistance, “ferrite” stainless steel with magnetic is better than “austenitic” stainless steel. At present, the so-called 200 series stainless steel and 300 series stainless steel with high manganese content and low nickel content in the market are also non magnetic, but their performance is far from that of 304 with high nickel content. In fact, 304 stainless steel after stretching, annealing, polishing, casting, and other processes will also be micromagnetic. Therefore, it is a misunderstanding and unscientific to judge the quality of stainless steel by a stainless steel belt without a magnetic field.

5、 Why does stainless steel rust?

When there are brown rust spots (spots) on the surface of stainless steel, people will be very surprised, because in people’s cognition, “stainless steel will not rust, rust is not stainless steel, maybe there is a problem with steel quality”. In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel will rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, stainless steel has the ability of corrosion resistance in the medium containing acid, alkali and salt. However, its corrosion resistance is determined by the chemical composition of steel itself, mutual state, service conditions and environmental medium type. For example, for 304 material, when it is in a dry and clean atmosphere, it has absolutely excellent corrosion resistance. Still, when it is moved to a coastal area, it will quickly rust in the sea fog containing a lot of salt. of. Therefore, not any kind of stainless steel is corrosion-resistant and does not rust at all times.

Stainless steel is formed on its surface by a layer of extremely thin and firm and stable chromium-rich oxide film (protective film), so as to prevent oxygen atoms from penetrating and oxidizing continuously, thus obtaining the ability of corrosion resistance. However, once the film is constantly damaged for some reason, the oxygen atoms in the air or liquid will continuously penetrate, or the iron atoms in the metal will continuously separate out to form loose iron oxide, which will lead to the continuous corrosion of the metal surface. There are many forms of destruction of the surface film.

1. Dust containing other metal elements or attachments of dissimilar metal particles are accumulated on the surface of stainless steel. When the stainless steel is in humid air, the condensed water between the attachment and stainless steel will connect them to form a microcell, causing electrochemical reactions and damaging the protective film. This process is called electrochemical corrosion.
2. On the surface of stainless steel, organic substances (such as melons and vegetables, noodle soup, etc.) are adhered to the surface of stainless steel, and organic acids are formed under the condition of water and oxygen. Under the action of a long time, the formed organic acids will corrode the metal surface.
3. On the surface of stainless steel, the adhesion contains acid, alkali, and salt substances (such as alkali water and lime water spray test for decoration wall), which will cause local corrosion.
4. In the polluted air (containing a large number of sulfide, oxide, hydrogen oxide atmosphere), when meeting condensate water, sulfuric acid, nitric acid, acetic acid, the liquid point will be formed, thus causing chemical corrosion.

When encountering the above conditions, the surface protective film of stainless steel can be damaged, and corrosion can be caused. Therefore, in order to ensure that the metal surface is permanently bright and not rusted, we put forward the following suggestions:

① The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove the attachments on the surface and eliminate the external factors causing corrosion.

② At present, there is a kind of 201 and 202 stainless steel on the market, which is easy to rust in coastal areas. This material is suitable for use in the environment without industrial pollution and air corrosion.

③ If you are in the coastal area, you need to use 304 stainless steel, because 304 material can resist seawater corrosion.

6、 How to deal with rust spot of stainless steel?

A) chemical method: using acid pickling paste or spray to assist the rust part to be passivated to form a chromium oxide film, so as to restore its corrosion resistance. After pickling, in order to remove all contaminants and acid residues, rinse properly with clean water. When everything is finished, Polish again with polishing equipment, and then seal with polishing wax. If there are slight rust spots on the part, the rust spots can also be wiped off with a clean rag with a mixture of gasoline and engine oil of 1:1.

B) Mechanical method: blast cleaning. Cleaning, annihilation, brushing, and polishing with glass or ceramic particles. If mechanical methods are used, it is possible to wipe off contamination caused by previously removed, polished, or annihilated materials. All kinds of pollution, especially foreign iron particles, can be the source of corrosion, especially in a humid environment. Therefore, when using mechanical methods to clean the surface, it is better to carry out regular cleaning in dry conditions. The mechanical method can only clean the surface, but can not change the corrosion resistance of the material itself. Therefore, it is recommended to use polishing equipment to re Polish after mechanical cleaning and seal with polishing wax.