Ferromagnetic stainless steels (SS) have been investigated as potential candidates for dental prosthesis applications in replacement of magnetic attachments made of noble and expensive alloys. In order to be used as biomaterials, their corrosion resistance has to be appropriate. The corrosion resistance of passive materials is related to the characteristics of the passive film formed and this property might be investigated by Mott-Schottky procedure. The capacitance of passive films formed during 2 days of immersion in a Phosphate Buffered Solution (PBS), at room temperature on PM2000, DIN 1.4575 and 17-4 PH ferritic stainless steels (SS) was studied by Mott-Schottky approach. A commercial alloy was also analyzed for comparison reasons. The results showed that films on tested materials behave as both, n-type and p-type semiconductors and the PM2000 presented the highest corrosion resistance among all materials studied.