Invar alloy oxidation in air

The invar (64% Fe and 36% Ni), oxidizes in air from 580 degrees C. At the b eginning (Delta m/S < 2 mg/cm(2)) the oxidation follows an isothermal parab olic kinetic law between 760 and 860 degrees C, associated to an apparent e nergy of activation of 221 + 11 kJ/mol. The oxide layer, very porous, is pr actically composed by iron oxides, mainly Fe2O3, with some Fe3O4 and FeO ne ar the alloy. An important internal oxidation zone is shown inside the allo y whose composition becomes apparently rich in nickel because of the centri fugal iron diffusion. The reaction mechanism is very complex and could not be entirely determined.