Why Does Stainless Steel Also Rust?

Why does stainless steel also rust? When brown rust spots appear on the surface of stainless steel pipes, people are surprised: they think "stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be something wrong with the steel."  In fact, this is a one-sided wrong view due to the lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation---that is, stainlessness, and also has the ability to resist corrosion in media containing acids, alkalis, and salts---that is, corrosion resistance. However, the magnitude of its corrosion resistance varies with the chemical composition of the steel itself, the state of annealing, the conditions of use, and the type of environmental media. For example, 304 stainless steel have absolutely excellent corrosion resistance in dry and clean atmosphere, but if they are moved to coastal areas, they will soon rust in sea fog containing a lot of salt; while 316 stainless steel performs well. Therefore, not all kinds of stainless steel can resist corrosion and rust in any environment.

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

The surface of stainless steel accumulates dust containing other metal elements or alien metal particles. In humid air, the condensed water between the attachments and the stainless steel connects the two into a micro-battery, triggering an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.

Organic juice (such as vegetables, noodle soup, sputum, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, it forms organic acid, which forms organic acid corrosion on the metal surface over a long period of time.

Substances containing acids, alkalis, and salts (such as alkaline water and lime water splashing for wall decoration) adhere to the surface of stainless steel, causing local corrosion.

In polluted air (such as an atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), condensed water forms sulfuric acid, nitric acid, and acetic acid spots, causing chemical corrosion.

All of the above situations can cause the destruction of the protective film on the surface of stainless steel and cause rust. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we recommend:

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

316 stainless steel should be used in coastal areas, because 316 material can resist seawater corrosion.

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

Since we know that stainless steel may also rust, what should we do once rust appears on stainless steel? Juye Metal has ways to help you.

Chemical method:

Use pickling paste or spray to assist the rusted parts to re-passivate to form a chromium oxide film to restore its corrosion resistance. After pickling, it is very important to rinse with clean water to remove all pollutants and acid residues. After all treatments, re-polish with polishing equipment and seal with polishing wax. For minor local rust, you can also use a 1:1 gasoline and oil mixture with a clean cloth to wipe off the rust.

Mechanical methods:

Sandblasting, glass or ceramic particle shot blasting, annihilation, brushing and polishing. Mechanical methods may wipe off the pollution caused by previously removed materials, polished materials or annihilated materials. All kinds of pollution, especially foreign iron particles, may become a source of corrosion, especially in humid environments. Therefore, the mechanically cleaned surface should preferably be cleaned regularly under 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.

 Juye Metal is one of the premium stainless steel manufacturers, focusing on the manufacturing, processing and distribution of stainless steel products. Any questions with stainless steel, please feel free to contact us at any time.