Inorganic coatings

Anodizing and phosphating are the examples of organic coatings. When we make aluminium as the cathode, it allows the passage of electrons but withheld passage when aqueous solutions of chromic acid and sulphuric acid as anode. This anodic passivity of aluminium is mainly because of the formation of a thin and compact layer of aluminium oxide over the metal. This phenomenon is known as anodizing or anodic oxidation. Phosphating is the process of depositing a coating of metal phosphate on metal surface. This improves corrosion resistance and also provides the surface a better paint adhesion quality. Phosphating bath solutions contain a mixture of free phosphoric acid, a metal phosphate and an accelerator. Nitrites are the commonly used accelerators which speed up the coating rate and helps to reduce the crystal size.