Iron and aluminum passivation produces dense ferric oxide and alumina, which will not react further with concentrated sulfuric acid at room temperature, but will react with concentrated sulfuric acid under heating, thus destroying the oxide layer and making concentrated sulfuric acid react further with iron and aluminum.
First of all, the reaction with alumina reflects the acidity of concentrated sulfuric acid, Al2O3 3H2SO4=Al2(SO4)3 3H2O (to be heated before the reaction), and further reaction with Al reflects the oxidation of concentrated sulfuric acid, but in the case of heating is not passivated into alumina, but: 2Al 6H2SO4=Al2(SO4)3 6H2O 3SO2