Stainless steel resists rust but is not rust-proof. Its chromium forms a self-repairing passive layer that normally prevents corrosion, but it can still rust in aggressive conditions - chloride-rich or acidic environments, tight crevices, or when contaminated with ordinary carbon steel. Higher-alloy grades such as duplex, super duplex and nickel alloys resist the toughest conditions.
Why is stainless steel usually rust-resistant?
Stainless steel contains at least 10.5% chromium, which forms a thin, self-repairing passive layer of chromium oxide on the surface. This film blocks the reaction that causes ordinary steel to rust, and it reforms if scratched, so the metal keeps protecting itself. Explore our stainless steel range, and for how corrosion resistance is measured, see Understanding PREN.
When does stainless steel rust?
The passive layer can be overwhelmed in aggressive conditions:
- Chlorides. Seawater and de-icing salts can cause localised pitting corrosion.
- Crevices. Tight gaps under gaskets or deposits trap stagnant liquid and starve the passive layer of oxygen.
- Acids. Strong acids can attack the surface directly.
- Contamination. Particles of ordinary carbon steel left on the surface can rust and trigger corrosion.
Contact with a dissimilar metal can also drive corrosion - see how to prevent galvanic corrosion.
Which grades resist the toughest conditions?
Higher-alloy grades resist environments that would corrode standard stainless. Duplex and super duplex steels, and nickel alloys, add chromium, molybdenum and nitrogen for far greater pitting and crevice resistance. Learn more in what is super duplex material?