Passivation:

Passivation is the process of making a material "passive", usually by the deposition of a layer of oxide on its surface. Passivation affects the properties of almost all metals in air. In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few nanometers thick. In general "passivation" means to make something chemically passive, as opposed to active. An active surface is susceptible to reactions such as corrosion, passive surfaces are resistant to reactions. Passivation removes contamination from the surface of materials. Surface contaminants can contribute to premature corrosion that may damage the component. Passivation protects through cleaning and forming a thin oxide coating.

FWM Metal Finishing Division provides the following:

Passivation Type II

Passivation Type VI