Anodizing is a process of coating a metal with a protective oxide layer on the surface to resist the corrosion of the metal. This can be done by either chemical or electrolytic means. In this regard, mainly aluminium or magnesium is anodized. However, metals like beryllium, tantalum, titanium and zinc can also be anodized. 

               The natural oxide film on aluminium is thin. Anodizing makes a thicker oxide layer. This protects the aluminium from corrosion and makes it last longer.

               In anodizing, aluminium is used as the positively charged electrode of an electrolytic cell. Electrolytes such as sulphuric acid or chromic acid are used as a solution. The oxide layer forms from the metal surface outside. This makes the outside layer slightly rough and porous. After anodizing, the pores on the metal are sealed by hot water or steam treatment which causes dehydration and results in the expansion of volume of the oxide. This further prevents corrosive substances from attacking the metal.

               Anodizing with sulphuric acid makes a clear oxide film. With chromic acid, a dull film is produced. Even coloured films can be produced by using dyes. Chromic acid is also used for anodizing zinc.