STUDIES OF PASSIVE FILMS ON AMORPHOUS FE-CR-(B,P,C) ALLOYS

The corrosion resistance of amorphous Fe-Cr-X alloys has been shown to be strongly affected by the metalloids. Phosphorus facilitates the passivation and leads to the formation of a highly stable passive film. Photoelectrochemical measurements indicate that the passive films formed on Fe-10Cr-13B-7C and Fe-10Cr-13P-7C in neutral Na2SO4 have a similar composition (Cr-content) and degree of order in the films. The high pitting resistance of the P-containing alloy in neutral solutions is thus a direct effect of the alloyed phosphorus. XPS measurements and photocurrent spectra clearly show that the passive film on the P-containing alloy repels electrolyte anions. This is probably due to the incorporation of oxidized phosphorus as PO43- in the passive film. Photoelectrochemical measurements further indicate that the surface of the alloy Fe-10Cr-13P-7C becomes enriched in chromium during active dissolution in acidic solutions. This facilitates the self-passivation of this alloy. A stronger Cr-enrichment of the passive film takes place, when the film is formed in a more acidic solution. This is due to a selective dissolution of iron and phosphorus, which is enforced in the more acidic solutions.