Like all steel, stainless steel starts out with a mixture of iron and carbon. What sets this family of alloys apart is that stainless also has a minimum of 10.5% chromium. This element gives stainless steel its characteristic resistance to oxidation. When stainless steel is exposed to the atmosphere, the chromium combines with oxygen to form a thin, stable passivation layer of chromium (III) oxide (Cr2O3). The passivation layer protects the interior steel from oxidization, and quickly reforms if the surface is scratched.
This passivation layer is different than plating. Some metals are plated with zinc, chromium, or nickel, for surface protection. In those cases, the benefits of the coating are lost once a scratch penetrates the plating. The chromium inside stainless steel provides more than this surface protection. It creates its passive film whenever it is exposed to air. Therefore, even if stainless is deeply scratched, the passivation layer will self-heal.
