What happens is that pinprick holes in the chrome plate allow in oxidants (oxygen) and the iron turns into brown iron oxide and explodes over the surface of the chrome......"rust", the hope is that the chrome is virtually intact but is covered in "rust"
Attacking the iron oxide (rust) with various chemicals (oxalate, aluminium foil) turns it back into iron which is a silver colour revealing the original chrome with the pinprick holes now a silver iron colour... The aluminium is oxidised and turns into sludge.
A level chemistry stuff
If the chrome plating is flaking you have no hope.
Shaun