Stainless steel is an iron-based alloy containing at least 10.5% chromium. That chromium forms a thin, self-repairing passive oxide layer that resists corrosion, which is what makes the steel "stainless". Other elements such as nickel, molybdenum, nitrogen and carbon are added in varying amounts to tune strength, corrosion resistance and structure for each grade.
What makes steel stainless?
Steel becomes "stainless" once it contains at least 10.5% chromium. The chromium reacts with oxygen to form a very thin, self-repairing passive layer of chromium oxide across the surface. This film blocks further corrosion and reforms if it is scratched. The more chromium, and other elements such as molybdenum, the more resistant the passive layer becomes.
Which elements are added, and what do they do?
Different grades add different elements to tune structure, strength and corrosion resistance:
| Element | Role |
|---|---|
| Chromium | Forms the passive layer and provides corrosion resistance; minimum 10.5%. |
| Nickel | Promotes an austenitic structure, adding ductility, toughness and high-temperature performance. |
| Molybdenum | Improves resistance to pitting and crevice corrosion, especially in chlorides. |
| Nitrogen | Increases strength and pitting resistance (raises PREN). |
| Carbon | Adds strength, but is kept low in low-carbon "L" grades to protect corrosion resistance. |
| Copper | Improves resistance to acids; for example, Ferralium® 255 contains around 2%. |
For how pitting resistance is quantified, see Understanding PREN.
How does composition define the families?
Composition determines the microstructure, which groups stainless steels into families:
- Austenitic (300-series) - high chromium and nickel, non-magnetic and very ductile.
- Ferritic (400-series) - chromium with little nickel, and magnetic.
- Martensitic - can be hardened by heat treatment for higher strength.
- Duplex and super duplex - a mixed austenitic-ferritic structure with around 22-25% chromium, roughly twice the strength of austenitic grades and strong corrosion resistance.
The higher-alloy end - duplex, super duplex and nickel alloys - is where Langley Alloys specialises. Learn more in what is super duplex material?, or explore our stainless steel range and Ferralium® 255.