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

The passive films formed on the surface of Fel9Cr9Ni and Fel9Cr9Ni2.5Mo alloys in 0.1M HCl have been characterized by AES and variable angle XPS. It is found that additions of Mo to Fel9Cr9Ni results in the formation of a passive film with a more highly developed interfacial barrier film composed mainly of Cr₂O₃, containing CrO₃ 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 Mo⁴⁺ and Mo⁶⁺ identified as hydrated MoO₂ and MoO₄ ^2-, apparently as FeMoO₄. It is proposed that MoO₄ ^2-anions are formed in the solid state along with CrO₄ ^2-, which together are responsible for producing, in 0.1M HC1, a bipolar film consisting of a cation selective outer layer containing CrO₄ ^2- and MoO₄ ^2- 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 XCr₂O_3. YCrO₃ barrier layer and resisting Cl^- and OH^- ingress. Both of these properties provide greater resistance to break down of passivity in Cl^- ion media.