A thin layer of magnetite is sometimes observed in the scales of hot-rolled sheets and wire rod at the scale-metal interface. The results of this study show that this layer of magnetite is produced, during the cooling of the products, by the wustite decomposition. The time/temperature field in which it appears has been defined. This inner layer is composed of pure magnetite, in epitaxy with the neighboring wustite, and the metal surface is not essential to its nucleation. The proeutectoīd reaction which, during cooling, produces an increase in the oxygen concentration of the wustite in contact with the metal favors the subsequent growth of the magnetite layer.