Question

why iron is painted while aluminium is not?

Answer 1

iron comes more quickly in contact with air

Answer 2

Iron is painted to prevent it from rusting. while aluminium does not rust because it forms an inert layer of aluminium oxide on disposal with oxygen which prevent it from rusting
4Al  +    3O2 ------> 2Al2O3 (balanced equation)
Al is aluminium
O is oxygen
Al2O3 is aluminium oxide