The passivation and the stability of passive films of amorphous FeCr(B, P, C) alloys has been investigated by polarization measurements, impedance spectroscopy and potential decay measurements. The results show that phosphorus facilitates the active-passive transition by forming a porous iron phosphate pre-passive layer on the alloy surface in the active range of the dissolution. This layer blocks the active sites of the surface and accelerates the cathodic H2 evolution reaction. The formation of the passivating chromium oxide layer takes place in the pores of this layer. Phosphorus subsequently improves the resistance of the passive film against localized attack in Cl−-containing solutions. The effect of the incorporated phosphates in the passive film is discussed with respect to the bipolar fixed-charge-induced passivity model. The phosphates make the passive film cation selective and thus hinder the penetration of the chlorides into the film. The oxidized boron species cannot change the ion selectivity of the film; instead of this they deleteriously affect the stability of the passive film.
Effect of metalloids on the passivity of amorphous Fe-Cr alloys
ELSENER, BERNHARD;
1988-01-01
Abstract
The passivation and the stability of passive films of amorphous FeCr(B, P, C) alloys has been investigated by polarization measurements, impedance spectroscopy and potential decay measurements. The results show that phosphorus facilitates the active-passive transition by forming a porous iron phosphate pre-passive layer on the alloy surface in the active range of the dissolution. This layer blocks the active sites of the surface and accelerates the cathodic H2 evolution reaction. The formation of the passivating chromium oxide layer takes place in the pores of this layer. Phosphorus subsequently improves the resistance of the passive film against localized attack in Cl−-containing solutions. The effect of the incorporated phosphates in the passive film is discussed with respect to the bipolar fixed-charge-induced passivity model. The phosphates make the passive film cation selective and thus hinder the penetration of the chlorides into the film. The oxidized boron species cannot change the ion selectivity of the film; instead of this they deleteriously affect the stability of the passive film.
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