An x-ray photo-electron spectroscopy study of the passivity of amorphous Fe-Mo alloys

The increase in molybdenum content in amorphous FeMoPC alloys facilitates the passivation in 1 N HCl in contrast to crystalline iron base alloys without chromium. An attempt was made to clarify the effect of molybdenum through the composition of surface film analyzed by XPS. The passive film consists mainly of ferric oxy-hydroxide, while a large amount of hexavalent molybdenum species is found in the surface film formed in the active region. The effect of molybdenum is interpreted as follows: the active dissolution necessary for proceeding the passive film formation leads to the precipitation of hexavalent molybdenum species on the alloy surface and as a result, the active dissolution is stifled. This fact leads to the passivation with the aid of iron ions which are already accumulated in the alloy/solution interface during the active dissolution.