THE EFFECT OF MOLYBDENUM ON THE CORROSION BEHAVIOR OF AMORPHOUS FE-CR-MO-P-C ALLOYS IN HYDROCHLORIC-ACID

The active and passive current densities in the low-potential region of amorphous Fe-8Cr-Mo-13P-7C alloys in 1M HCl aqueous solution decrease with increasing molybdenum content in the alloy. Hence, the addition of molybdenum improves the corrosion resistance and passivating ability. X-ray photoelectron spectroscopy (XPS) and electrochemical investigations reveal that the quantity of chromium dissolved during potentiostatic passivation gradually decreases with an increase in molybdenum content in the alloy. As a result, the surface film formed on the molybdenum-containing alloy is rapidly enriched in chromic ions during passivation. By contrast, the enrichment of chromic ions in the surface film on the alloy without molybdenum is relatively slow, although the concentration of chromic ions in the passive film is almost the same as or higher than that in the passive film formed on the molybdenum-containing alloy after prolonged polarization. When a sufficient quantity of molybdenum is added, almost no chromium is dissolved during potentiostatic polarization in the passive region and passivation takes place by transformation of the air-formed film to the passive film as a result of preferential dissolution of a small quantity of iron.