■ First let’s find out why is iron rusting?
It is generally known that in the nature precious metals like gold, platinum exist in the unbound state, meanwhile general metals like iron (Fe) in the nature exist in magnetite(Fe3O4) and hematite(Fe2O3·xH2O), which have good chemical stability.
When rusting happens the iron and oxygen in the atmosphere are having chemical reaction, which will generate Fe2+, Fe3+ in the unprotected, loose and spalling states of iron rich oxide or hydroxide, and the iron turns back to the “iron ore” again.
Iron rusting is an ionization process that iron turns into Fe3+, a typical corrosion. To avoid rust (corrosion), people have to make surface coating to protect iron from atmosphere contact. But iron would still be rusting when the coating is damaged.
■ Why is stainless steel NOT rusting?
The stainless steel doesn’t rust, it was once regarded as magic. But after certain period of time of research and practice, people have fairly well known that the rustless of stainless steel and rust of iron are among the same natural phenomenon.
It was found out that the increase of chromium content in the steel increases the corrosion resistance. As can be seen from the following curve, when the chromium content is greater than 11-12%, the corrosion resistance of the steel changed greatly. From rust to rustless, it is because the chromium(Cr) in the steel reacted to the oxygen in the atmosphere, and generated a smooth transparent and very thin layer of Chromium-rich oxide film, or passive film, which protects the steel from rusting, and gives it a brand-new look.
■ Why does chromium oxide protect steel against rust, while iron oxide rusts?
The reason is that a chromium atom is around the same size of the molecule of a chromium oxide, and they could form a-couple-of-atom-thin and stable layer, which lie orderly on the surface of metal. While an iron atom is much smaller than the molecule of an iron oxide, and the formed oxide has a loose chemical constitution, therefore it is spalling.
The invisible passive film on the surface of stainless steel, even thin (about 2-5 nm), attaches to the surface consecutively and firmly. It is stable and not easy to spall. As long as it is exposed to oxygen, the passive film generates spontaneously, and even damaged part will be repaired, too.
If the chromium content is higher, and the passive film will be stronger. When the chromium content of steel increased to ≥19%, the steel passive film will turn from crystalline film to amorphous film. Because the amorphous film has less defects, and a uniform surface structure, and it has better concentration of chromium. Therefore an amorphous film has greater durability and corrosion resistance.
■ Conclusion: Why is stainless steel not rusting？
The rustless property of stainless steel is decided by its chromium content. Without chromium we cannot produce stainless steel. A large number of studies and experiments locally and worldwide show that chromium is the only chemical element of certain industrial value, which is able to deactivate steel, and to make it resistant to corrosion. And there is no stainless steel without chromium.
Source: 中国特钢企业协会不锈钢分会
https://mp.weixin.qq.com/s/8lVLiJk7VJXgEs0OyPMNKA
