Help page for chipped or flaking coated beater.
Usually the coating flakes off for one of two reasons. If the coating flakes from the tip then it is likely the beater is touching the bottom of the bowl. This can easily be fixed by adjusting the head height. The other main cause is use of a dishwasher. The KitchenAid beaters are hand-wash only. If you put them in a dishwasher a lot of dishwasher detergents will react with the aluminium over time and destroy the bond. This will then lead to the coating flaking off.
Physical damage to the beater can also cause the coating to flake off. If the beater has been dropped or had something drop on it then the soft aluminium which the beater is made from can distort and cause the coating to come away from the beater or dough hook.
Most professional kitchens will use the burnished (uncoated) versions of the dough hook and beater. This is to eliminate any chance of coating entering the food chain. The uncoated versions are hand wash only and should never be placed in a dishwasher as they will react with most dishwashing detergents and become covered in a black greasy coating.