The effect of molybdenum on the stability of passive films formed on amorphous Fe-Cr-Mo-P-C alloys by potentiostatic polarization in deaerated 1 M HCl

Open circuit immersion immediately after potentiostatic polarization is carried out to investigate the effect of molybdenum addition on the stability of the passive films formed on amorphous Fe-SCr-13P-7C alloys by polarization in de-aerated 1 M HCl at -100 mV(SCE), which is in the passive region of both chromium and molybdenum. The open circuit potentials of both molybdenum-free and -containing alloys decrease with immersion time after polarization, apart from the passive potential region of chromium but in the passive region of molybdenum. The passive film formed on molybdenum-free alloy by polarization for 10 min and 2.5 h is disrupted by open circuit immersion for 30 min, and the alloy was corroded seriously, whereas molybdenum-containing alloys passivated by polarization for 10 min remains in the passive state during subsequent immersion for 5 h. Regardless of polarization time and molybdenum addition, the cationic fraction of chromium in the surface film decreases continuously with immersion. Open circuit immersion leads to an increase in the concentration of molybdenum ion in the film on the alloys polarized previously for 10 min and a remarkable enrichment of molybdenum ion occurs after prolonged immersion, showing the formation and stabilization of molybdenum-enriched passive film. On the other hand, the enrichment of molybdenum ion during immersion is more difficult in the film on the alloys polarized previously for 2.5 h than that on the alloys polarized previously for 10 min.