An XPS study of the passive and transpassive behavior of molybdenum in deaerated 0.1 M HCl

Abstract Pure molybdenum was polarized in deaerated 0.1 M HCl solution at 22°C. Molybdenum was found to be passive in the range between −396 mV to +125 mV(SCE). Variable angle XPS was employed to detect the nature of passive and transpassive films formed at specified potentials on Mo. The passive film was found to be comprised of MoO 2 and MoO(OH) 2 . In the early stage of passive film formation, lower valence species, probably MO 2 O 3 , were observed. In the lower potential range of the transpassive region, Mo 2 O 3 was formed, which coincided with a loss of protection from MoO 2 and MoO(OH) 2 , and the formation of a thick oxidation product. At higher potentials, in the transpassive range, the film was found to consist of trivalent, tetravalent, pentavalent and hexavalent species. HMoO 4 was detected in the anolyte of the transpassive region.