Passivation is the process of making a material "passive" in relation to another material prior to using the materials together. For example, prior to storing hydrogen peroxide in an aluminum container, one method to passivate the container is to rinse it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide will oxidize and dissolve any impurities on the inner surface of the container, and the deionized water will rinse away the acid and oxidized impurities.

In the context of corrosion, passivation is the spontaneous formation of a hard surface film which inhibits further corrosion. This layer is usually an oxide or nitride, a few atoms thick. Under normal conditions of pH and oxygen concentration, passivation is seen in such materials as aluminum, stainless steel, titanium, and silicon. Ordinary steel can form a passivating layer in alkali environments, as for example rebar does in concrete. The conditions necessary for passivation are recorded in Pourbaix diagrams.

As an example, you can take 316L stainless steel, which is 18% chromium, and by passivating the surface reach up to 90% chromium in the surface layer. This chromium oxide layer is much more resistant to corrosion than the original surface.