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Common knowledge of stainless steel

What is stainless steel?

Stainless steel is a kind of steel added with a mass fraction of chromium (wCr) of more than 12% alloy elements on the basis of ordinary carbon steel. It can keep metallic luster under the action of air, that is, it has the characteristics of non-rusting. This is because there is a certain amount of chromium alloy element in this kind of steel, which can form a solid oxide film (passivation film) on the surface of the steel, which can not dissolve in some media, so that the metal can be isolated from the outside without chemical action. Some of these steels, besides containing more chromium (Cr), are matched with other alloying elements, such as nickel (Ni), so that they have good chemical stability in air, water and steam. They also have enough stability in many kinds of acid, alkali and salt aqueous solutions, and can maintain their corrosion resistance even in high or low temperature environments.

Labeling Method of Stainless Steel

Numbering and Representation of Steel

(1) International chemical element symbols and national symbols are used to represent chemical composition, and Arabic letters are used to represent the content of chemical composition.

For example: China, Russia 12CrNi3A

(2) The steel series or numbers are represented by fixed digit numbers, such as the United States, Japan, 300 series, 400 series and 200 series.

(3) Use Latin letters and order to form serial numbers, indicating only the purpose.

International Stainless Steel Marking Method

The American Iron and Steel Society uses three digits to mark various standard grades of malleable stainless steel. Among them:

(1) Austenitic stainless steel is numerically marked with series 200 and 300.

(2) Ferritic and martensitic stainless steels are represented by 400 series numbers.  For example, some of the more common austenitic stainless steels

Marked 201, 304, 316 and 310,

(3) Ferritic stainless steel is marked 430 and 446, martensitic stainless steel is marked 410, 420 and 440C.

Note, biphasic (austenite-ferrite),

(4) Stainless steel, precipitation hardening stainless steel and high alloys with iron content less than 50% are usually named by patent or trademark.

Is the harder the same size profile, the thicker it is?

Not always. At present, some of the remanufactured materials on the market have high carbon content and a small amount of nitrogen due to production technology constraints, which leads to higher hardness. In addition, if the content of Cr and Ni decreases, the hardness will also increase. The toughness and corrosion resistance of these two materials will not decrease much. Then, it is incorrect to pinch profiles by hand, to check their thickness by feeling their hardness, or to verify the hardness and load-carrying capacity of profiles by only requiring the thickness of profiles.

Are stainless steel sure not to rust? Under what conditions does stainless steel rust easily?

Stainless steel is not rusty. In addition to its own defects, stainless steel corrosion is closely related to the environment (e.g. humidity of air, temperature of air, flow velocity, flow direction, composition and quantity of impurities in air, etc.). In atmospheric and weak corrosive media, the corrosion rate is less than 0.01 mm/year, which is considered as "complete corrosion resistance". However, in CL-medium (salt, etc.), sulphuric acid, oxalic acid (often used for external wall cleaning), hydrochloric acid, ceramic detergent (internal decoration cleaning supplies) will rust, in the maintenance process should use neutral substances (water, etc.) often clean, so as to reduce the corrosion media adherence and rust. In addition, scratches of sharp objects (such as knives, wire brushes, etc.) should be avoided to avoid rusting by affecting the appearance and destroying the intergranular structure.

Why does stainless steel rust?

When the stainless steel surface appears brown rust spots, people may be surprised -- "stainless steel is not rusty", "rust is not stainless steel", "may be steel problems"... In fact, this is a misconception about the lack of understanding of stainless steel. Stainless steel will rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation, i.e. stainless steel; at the same time, it also has the ability to resist corrosion in medium containing acid, alkali and salt, i.e. corrosion resistance; however, its corrosion resistance varies with the chemical composition of the steel itself, the state of interaction, the use conditions and the type of environmental media. For example, 304 steel has absolutely excellent corrosion resistance in dry and clean atmosphere, but it will soon rust when it is moved to the coastal area and in the sea fog containing a lot of salt. Therefore, not any stainless steel, in any environment can resist corrosion and rust.

In addition, stainless steel is formed on its surface by a very thin, solid and compact, stable chromium-rich oxide film (protective film), to prevent the continuous infiltration of oxygen atoms, oxidation, and get the ability to resist corrosion. Once for some reason, the film is constantly destroyed, oxygen atoms in air or liquid will continuously infiltrate or iron atoms in metal will continuously separate out, forming loose iron oxide, metal surface will continue to rust. There are many forms of surface film destruction, and the most common ones are:

1) The surface of stainless steel is deposited with dust containing other metal elements or adhesives of different metal particles. In humid air, the condensate water between the adhesives and stainless steel connects them into a micro-battery, which leads to electrochemical reaction, and the protective film is destroyed. It is called electrochemical corrosion.

2) The surface of stainless steel is adhered with organic juice (fruit, vegetable, noodle soup, etc.). Under the condition of water oxygen, organic acid is formed, and for a long time, organic acid corrodes the metal surface.

3) Adhesion of stainless steel surface containing acid, alkali and salts (such as alkali water and lime water for decorative walls) causes local corrosion.

4) In polluted air (such as atmosphere containing large amounts of sulfide, carbon dioxide and nitric oxide), sulfuric acid, nitric acid and acetic acid are formed in condensate water, which causes chemical corrosion.

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