Stainless Steel is a generic term for a family of corrosion resistant iron (FE) based alloys containing 10.5% or more chromium (Cr). Chromium produces a thin layer of oxide on the surface of the steel known as a passive layer. This prevents further corrosion of the surface. Increasing the amount of chromium strengthens the passive film, hence increasing the level of corrosion resistance.
Other elements such as molybdenum (MO) and nitrogen (N) further strengthen the passive film and improve corrosion resistance. If the passive film is removed or damaged, it will spontaneously re-form in the presence of air or water.
For further information refer to ASSDA