Why does stainless steel rust?

Update: 21-01-2020

All metals react with oxygen in the atmosphere, forming […]

All metals react with oxygen in the atmosphere, forming an oxide film on their surface. However, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the rust to swell and eventually form holes. Carbon steel surfaces can be plated with paint or anti-oxidant metals such as zinc, nickel, and chromium, but it is known that this protection is only a thin film.

Stainless steel is a thin, strong and stable chromium-rich oxide film (protective film) formed on its surface, which prevents further oxygen penetration. Continuous oxidation. And get anti-rust ability. For some reason, its protective film is often damaged. Oxygen atoms in the air or liquid will continue to penetrate, or iron atoms in the metal will be separated out to form loose iron oxide.

If the protective layer breaks, the base steel begins to rust. The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection method is different. When the added amount of chromium reaches 10.5%, the atmospheric corrosion resistance of the steel is significantly improved, but when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that when steel is alloyed with chromium, the type of surface oxide becomes similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface from further oxidation. The oxide layer is very thin, and it can provide a natural gloss to the steel surface, giving stainless steel a unique surface.

Therefore, all stainless steel elements have the common feature that the chromium content is above 10.5%.
Why does stainless steel rust? In some cases, stainless steel can also rust. Stainless steel has the ability to resist atmospheric oxidation-that is, stainless steel-and also the ability to prevent corrosion in media containing acids, alkalis and salts-that is, corrosion resistance. However, the size of its corrosion resistance will vary with the chemical composition of the steel itself, the mutual state, the conditions of use, and the type of environmental medium. For example, 304 steel pipe has absolutely excellent rust resistance in a dry and clean environment, but will quickly rust when it is moved to the seashore in sea fog containing a large amount of salt. The 316 steel pipe performed well.