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Passivation refers to the covering of an active metal with an inert layer of oxide in the presence of oxygen. For example, metallic aluminium, when exposed to oxygen, becomes covered in a thin layer of aluminium oxide that prevents further oxidation. Passivation layers can be produced naturally though contact with air, or by anodization (as is often done with silver, aluminium, tantalum and niobium) or contact with nitric acid (as can be done with chromium) or chromic acid (as may be done with zinc).