What’s Stainless Steel?
‘A spoon and a fork in the restaurant’
‘A railing on the street’
The common things of these products are made of stainless steel. Do you know why?
Because the stainless steel is literally ‘Stainless’.
Different with general iron(Fe), stainless steel is alloyed metal so that it’s contained many components
especially Chromium(Cr). Cr helps stainless steel not only stainless but also make beautiful surface.
Stainless steel is largely classed as 18Cr-18Ni steel (Austenite-based), 18Cr steel (Ferrite-based) and
13Cr steel (Martensite-based). The main component Cr forms a very thin layer of chromium oxide on the surface of the stainless
steel by the role of passive film to prevent oxygen from entering into the base.
If you look the picture below, you can know better the principle of passive film.
· While the surface is in contact with the external oxygen, the passive film is created as chrome chloride oxide
· The film is too thin and elaborate to penetrate the external oxygen
· However, the film could be destroyed on the harsh environment such as acid, high temperature and radiation
· Since the porous film is thick, it’s easy to be penetrated by external oxygen
· In normal ambient environment it’s easily rust so that it can’t prevent rust fundamentally