English metallugist Harry Brearly see this condition, he work on the project to improve rifle barrels, accidentlly discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. It is the addition of minimum 12% chromium to the steel that makes it resist rust, or stain less than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome containing oxide, called the passive film.
The sizes of chromium atom and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. The passive film requires oxygen to self-repair, so stainless steel have poor corrosion resistance in low oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment.
After of some experiment then merge many kind of stainless steel that mixed with different kind composition of metals:
- Austentic: Alloys containing chromium and nickel (sometimes manganese and nitrogen), there are two type of these alloy, 302 and 304 that different on those metal contained.
- Ferritic: Alloys contain of iron and chromium, based on type 430 that contains of 17% chromium.
- Martensitic: This alloy contains of Iron, 12% chromium, and 0,12% carbon. The steel type is 410.
