The influence of pre-immersion on the potentiostatic polarization behavior of amorphous Fe-Cr-Mo-P-C alloys in DE-aerated 1 M HCl

The effect of immersion prior to polarization on the potentiostatic polarization behavior of amorphous Fe-8Cr-xMo-13P-7C (x = 0, 2, 4 and 6 at%) alloys and the composition of the surface film produced by potentiostatic polarization at a low potential in the passive region in de-aerated 1 M HCl is investigated. Passivation of amorphous Fe-8Cr-13P-7C alloy becomes difficult with immersion prior to potentiostatic polarization and the specimen immersed for 10 min in de-aerated 1 M HCl does not passivate even after potentiostatic polarization of several hours. By contrast, the extension of pre-immersion time for molybdenum-containing alloys results in a decrease in the current density necessary for passivation. Although chromium ions accumulate rapidly in the surface film on the molybdenum-free alloy polarized immediately after immersion, the chromium concentration in the film does not increase by prolonged polarization. The chromium concentration in the surface film on the molybdenum-free alloy immersed previously for 10 min increases continuously with polarization time up to a very high value, which is much higher than that in the surface film on molybdenum-bearing alloys. Chromium is enriched in the surface film on molybdenum-bearing alloys throughout the polarization experiment of 5 h. Preliminary immersion of molybdenum-bearing alloys decreases the enrichment of chromium in the passive film formed by polarization. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys by pre-immersion, and is enriched remarkably in the film after prolonged polarization.