The synergistic effect of chromium and molybdenum in enhancing the corrosion resistance in the very aggressive acid has been studied by electrochemical measurements and XPS analysis. Amorphous alloys are formed in a wide composition range by the addition of nickel to chromium-molybdenum alloys. The corrosion weight loss decreases with increasing molybdenum content, and Cr-Ni-Mo alloys containing 27 at% or more molybdenum exhibit almost the same corrosion rate as that of molybdenum. Cr-Ni alloys dissolve actively, similarly to chromium, while the molybdenum addition to the Cr-Ni alloys results in spontaneous passivation. The molybdenum addition suppresses the anodic dissolution current because of the formation of tetravalent molybdenum oxyhydroxide having a relatively high activity for both cathodic oxygen and hydrogen reduction with a consequent ennoblement of the open circuit potential. The ennoblement of the open circuit potential leads to the formation of the passive chromium oxyhydroxide film. This film is protective to avoid transpassive dissolution of molybdenum under anodic polarization conditions. The increase in the molybdenum content leads to an increase in the passive current density and to an increase in the unstable Mo6+ content in the film under anodic polarization conditions.