A Bipolar Model of the Passivity of Stainless Steel: The Role of Mo Addition

The passive films formed on the surface of and alloys in have been characterized by AES and variable angle XPS. It is found that additions of Mo to results in the formation of a passive film with a more highly developed interfacial barrier film composed mainly of , containing in solid solution which forms a glassy phase. In addition, the passive films were found to contain more Fe. Molybdenum appears to be present as Mo 4+ and Mo 6+ identified as hydrated and , apparently as . It is proposed that anions are formed in the solid state along with , which together are responsible for producing, in , a bipolar film consisting of a cation selective outer layer containing and and an intrinsically anion selective inner layer. The ion selective property of this duplex film is considered to be largely responsible for the development of the barrier layer and resisting Cl − and OH − ingress. Both of these properties provide greater resistance to break down of passivity in Cl − ion media.