Why does stainless steel also rust?

by:Ipromo     2021-06-29
When brown rust spots (spots) appeared on the surface of stainless steel pipes, people were surprised: 'Stainless steel does not rust, and rust is not stainless steel. It may be that there is a problem with the steel.' In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation-that is, rust resistance, and it also has the ability to resist corrosion in media containing acid, alkali and salt, that is, corrosion resistance. However, the size of its corrosion resistance varies with the chemical composition of the steel itself, the state of addition, the conditions of use and the type of environmental media. Such as 304 steel pipe, in a dry and clean atmosphere, there is absolutely excellent anti-corrosion ability, but it is moved to the beach area, in the sea fog containing a lot of salt, it will quickly rust; while the 316 steel pipe performance good. Therefore, it is not any kind of stainless steel, which is corrosion-resistant and does not rust in any environment.

There are many forms of damage to the surface film, the common ones are as follows:

Stainless steel relies on a very thin, strong, dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to penetrate and continue to oxidize, thereby obtaining the ability to resist rust. Once for some reason, the film is continuously destroyed, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will continue to be rusted. There are many forms of damage to this surface film, and the common ones are as follows:

1. The surface of stainless steel has accumulated dust containing other metal elements or attachments of foreign metal particles. In humid air, the condensed water between the attachments and stainless steel connects the two to form a micro battery, which causes Electrochemical reaction destroys the protective film, which is called electrochemical corrosion.

2. The surface of stainless steel adheres to the organic juice (such as melon, vegetable, noodle soup, sputum, etc.), which forms organic acid in the presence of water and oxygen, and the organic acid will corrode the metal surface for a long time.

3. The surface of stainless steel adheres to containing acid, alkali, and salt substances (such as alkaline water and lime water splashing on the decoration wall), causing local corrosion.

4. In polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), it will form sulfuric acid, nitric acid, and acetic acid liquid spots when encountering condensed water, causing chemical corrosion.

How to ensure that the metal surface is permanently bright and not corroded?

The above conditions can cause damage to the protective film on the stainless steel surface and cause rust. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, it is recommended:

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

2. 316 stainless steel should be used in seaside areas, which can resist seawater corrosion.

3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the material requirements of 304. Therefore, rust will also be caused, which requires users to carefully choose products from reputable manufacturers.

Gaogang Hardware——Professional custom production of high-end quality hardware accessories, 304/316 series stainless steel, sincerely waiting for your inquiries
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