[ Corrosion in Stainless Steel ]

The passive layer is a extremely thin, continuous, stable and resistant. It is formed on the stainless steel surface by the combination of the oxygen from the air with the chromium of the steel. That layer protects the stainless steel against the corrosion of the environment.

Formation and Characteristics of the Passive Layer: it appears spontaneously when there is the presence of chromium and oxygen. The formation is extremely fast, that means, instantaneous (chrome and oxygen have a lot of affinity). It is very stable (it doesn't come off) and it is present in the whole steel surface. It is not porous (it blocks the action of the aggressive environment). It is practically invisible.

Source: Núcleo Inox