Why does stainless steel rust?

Published on:

2022-08-23 00:00

When brown rust spots (spots) appear on the surface of stainless steel pipes, people are amazed: "Stainless steel will not rust, it is not stainless steel if it rusts, it may be a problem with the steel quality", in fact, this is a lack of understanding of stainless steel a one-sided misconception. Stainless steel can also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, no rust, and also has the ability to 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, the 304 pipe has absolutely excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to the seaside area, it will rust quickly in the sea fog with a large amount of salt, while the 316 pipe is good performance. Therefore, not any kind of stainless steel is corrosion-resistant and rust-free at all times.

Stainless steel is a very thin, firm and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to infiltrate, continue to oxidize, and obtain the ability to resist corrosion. When the film encounters continuous damage, the oxygen atoms in the air or liquid will continue to infiltrate or the iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be continuously corroded. 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 heterogeneous metal particles. In humid air, the condensed water between the attachments and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction. The damage to the protective film is called electrochemical corrosion.

2. The surface of stainless steel adheres to organic juices (such as vegetables, noodle soup, sputum, etc.), which form organic acids in the presence of water and oxygen, and form organic acids to corrode the metal surface for a long time.

3. The surface of stainless steel adheres to contain acids, alkalis, and salts (such as alkali water and lime water spray for decorative walls), causing local corrosion.

4. In polluted air (atmosphere containing a large amount of sulfides, oxides, and hydrogen oxide), in the presence of condensed water, it will form 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, in order to ensure that the metal surface is permanently bright and not rusted.

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