Knowledge of stainless steel

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  1. What is stainless steel?

Stainless steel is a type of steel. Steel refers to steel containing less than 2% of carbon (C), and more than 2% iron. The addition of chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo), and other alloying elements during the smelting process improves the performance of the steel and makes the steel more resistant to corrosion. (that is, not rusted) is what we often call stainless steel.

  1. Why does stainless steel have different grades?

In the smelting process of stainless steel, due to the different varieties of alloying elements added, the amount of addition of different varieties is different. Their characteristics are also different. To distinguish them, they are crowned with different steel grades.

  1. What kind of stainless steel is not easy to rust?

Three main factors affect stainless steel corrosion:

1) The content of alloying elements, generally speaking, when the content of chromium is 10.5%, steel 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 is 18-20%. Such stainless steel will not rust under normal circumstances.

2) The smelting process of the manufacturer will also affect the corrosion resistance of stainless steel. A large stainless steel plant with good smelting technology, advanced equipment, and advanced technology can ensure the control of alloying elements, the removal of impurities, and the control of the cooling temperature of the billet, so the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. On the contrary, some small steel mills have backward equipment and backward technology. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.

3) The external environment, the climate is dry and ventilated, and the environment is not easy to rust. The air humidity is high, and continuous rainy weather or environmental areas with high pH in the air are prone to rust. 304 stainless steel, if the surrounding environment is too bad, it will rust.

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

Many customers go to the market to buy stainless steel and bring a small magnet with them. Without magnetism, it will not rust. This is a wrong understanding.

Whether the stainless steel strip is magnetic or not is determined by its structure. During the solidification process of molten steel, due to the different solidification temperatures, stainless steel with different structures such as “ferrite”, “austenite” and “martensite” will be formed. Among them, is “ferrite”. Both “solid” and “martensitic” stainless steels are magnetic. “Austenitic” stainless steel has good comprehensive mechanical properties and process performance and weldability, but only in terms of corrosion resistance, magnetic “ferritic” stainless steel is stronger than “austenitic” stainless steel. At present, the so-called 200 series and 300 series stainless steels with high manganese content and low nickel content in the market are not magnetic, but their performance is far from that of 304 with high nickel content. On the contrary, 304 has been stretched, annealed, polished, cast, etc. Process treatment will also be microstrip magnetic, so it is a misunderstanding and unscientific to judge the quality of stainless steel by using stainless steel strips without magnetism.

  1. Why does stainless steel also rust?

When brown rust spots (spots) appear on the surface of stainless steel, people are greatly surprised: “Stainless steel will not rust, and if it rusts, it is not stainless steel, and there may be a problem with the steel.” This is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

Stainless steel can resist atmospheric oxidation, that is, rust resistance, and also can resist corrosion in media containing acids, alkalis, and salts, that is corrosion resistance. However, the size of its corrosion resistance is changed with the chemical composition of the steel itself, the mutual state, the conditions of use, and the type of environmental media. For example, 304 material has excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to a coastal area, it will rust quickly 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 relies on a very thin, firm, and fine stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to penetrate and continue to oxidize, to obtain the ability to resist corrosion. Once for some reason, the film is constantly damaged, the oxygen atoms in the air or liquid will continue to penetrate or the iron atoms in the metal will continue to separate, forming loose iron oxide, and the metal surface will be constantly exposed to Rust. There are many forms of damage to this surface film, the most common in daily life are the following:

1) The surface of stainless steel accumulates dust containing other metal elements or attachments of dissimilar metal particles.

In the air, the condensed water between the attachment and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction and damages the protective film, which is called electrochemical corrosion.

2) Organic juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, organic acids are formed, and organic acids formed to corrode the metal surface for a long time.

3) The surface of stainless steel adheres to substances containing acids, alkalis, and salts (such as alkali water and lime water spraying for decoration of walls), causing local corrosion.

4) In polluted air (atmosphere containing a large number of sulfides, oxides, and hydrogen oxides), when it encounters condensed water, it forms liquid spots of sulfuric acid, nitric acid, and acetic acid, causing chemical corrosion.

The above conditions can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, to ensure that the metal surface is permanently bright and does not get rusted, we recommend:

① The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause rust.

② There are stainless steels made of 201 and 202 materials on the market that are easy to rust in coastal areas and are suitable for use in environments without industrial pollution and air corrosion.

③ In seaside areas, use 304 stainless steel, which can resist seawater corrosion.

  1. How to deal with rust spots on stainless steel?

1) Chemical method:

Use pickling paste or spray to assist the re-passivation of the rusted part to form a chromium oxide film to restore the corrosion resistance. After pickling, to remove all contaminants and acid residues, it is very important to rinse properly with clean water. After all treatment, re-polish with polishing equipment, and seal with polishing wax. For local slight rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean rag.

2) Mechanical method:

Sandblast cleaning, blast cleaning with glass or ceramic particles, annihilation, brushing, and polishing. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should preferably be properly cleaned in dry conditions. The use of mechanical methods can only clean the surface, and cannot change 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.

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