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Why does stainless steel rust, and what causes it?

Update:28-12-2019
Summary:

Stainless steel is widely used in various industries as […]

Stainless steel is widely used in various industries as a corrosion-resistant metal material. But does stainless steel really rust? The answer is no. Stainless steel is a material formed by adding some anti-corrosive elements to ordinary steel. This kind of steel can only be said to have stronger corrosion resistance than ordinary steel, but if it exceeds a certain degree of corrosion, stainless steel will still be damaged. Rusty.

First of all, we need to understand what stainless steel balls are. The definition of stainless steel balls: steel balls that contain more than 12.5% ​​chromium and have high resistance to chemical substances (acids, alkalis, salts).

Why do stainless steel balls rust?

1) When brown rust spots (spots) appeared on the surface of the stainless steel ball, people were surprised: "Stainless steel is not rusty, rust is not stainless steel, it may be a problem with the steel." In fact, this is a one-sided wrong view of lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

2) Why is stainless steel magnetic?

3) People often think that magnets absorb stainless steel, verify its pros and cons and authenticity, do not attract non-magnetic, think it is good, genuine, and genuine; if the sucker is magnetic, consider it to be fake. In fact, this is an extremely one-sided and impractical method of identifying errors.

There are many types of stainless steel balls, which can be divided into several types according to their organizational structure at room temperature:

1) Austenitic steel, typically representing 201, 202, 302, 304, 304HC, 304L, 316, 316L. This type of stainless steel contains nickel and has strong rust and corrosion resistance. This type of stainless steel is non-magnetic, but it is used in work. Can produce magnetism, can be removed by heat treatment.

2) Martensitic steel or ferrite, this type is generally 4 series stainless steel, 410, 420, 430, 420C, 440, 440C This type of stainless steel only contains chromium and does not contain nickel, so the rust and corrosion resistance is not as good as Austen Body type steel ball. This stainless steel ball cannot be demagnetized, but martensitic steel has a unique advantage: it can be increased by heat treatment, so it is more suitable for finishing, and has strong wear resistance

Stainless steel has the ability to resist atmospheric oxidation--that is, stainless steel, but also has the ability to corrode in media containing acids, alkalis, and salts--that is, corrosion resistance. However, the size of its anti-corrosion ability changes with the chemical composition of the steel itself, the mutual state, the conditions of use, and the type of environmental medium.

Such as 304 steel balls, it will not rust in the air, but if used in coastal environments, because there is a lot of salt in the sea, it will destroy a layer of oxide film on the surface of stainless steel balls. So use 316 or 316L steel balls in seawater

Stainless steel is a thin, strong and stable chromium-rich oxide film (protective film) formed on its surface, which prevents the oxygen atoms from infiltrating and continuing to oxidize, and obtains the ability to resist rust.

Once for some reason, the film is constantly destroyed, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will be continuously separated out, forming loose iron oxide, and the metal surface will be continuously rusted.