Inorganic protective layers - also called conversion layers - are non-metallic, very thin coatings on a metal surface and serve to protect against corrosive attacks.
Inorganic protective coatings are usually produced by the targeted chemical reaction of an aqueous electrolyte solution with the metallic substrate. This is also known as passivation. In addition, spontaneous passivation is also possible - for example in the formation of metal oxides. In the elemental state, aluminium immediately forms aluminium oxide on the surface and thus prevents further oxidation as a dense layer. Anodising processes can intensify this phenomenon.
Further examples of inorganic protective coatings are phosphating, alkali passivation and chromating.