The influence of heat treatment on the corrosion behaviour of Fe69.5Cu1Nb3B9Si13.5Cr4 FINEMET type alloy

The influence of the annealing treatment on the passivating ability of the Fe65.5Cu1Nb3B9Si13,5Cr4 nanocrystalline alloy in 2 M HCl solution was studied by differential scanning calorimetry (DSC), atomic force microscopy (AFM), scanning electron microscopy (SEM) and conventional electrochemical techniques. The samples were isothermally annealed at 525 C (onset of nanocrystallization) and 600 C during different times and the fraction of transformation was determined by DSC. The initial stages of crystallization could be well characterized by AFM and the images have showed hemispherical features related to the first stage of nanocrystallization. These features, interpreted as α-Fe(Si) nanocrystallites embedded in the residual amorphous matrix, increased in number depending on the time or temperature of the thermal treatment, in agreement with the DSC studies. The results indicate that the nanocrystallization of the alloy produced an improvement in their passivating ability. The current density in the active-passive transition have a minimum value when the fraction crystallized reached a maximum and the nanocrystallites formed have not started to coalesce.