(Invited) Elucidating the Role of Mo during the Passivation of Ni-Cr-Mo Alloys: Real Time Analysis of Elemental Dissolution and Surface Enrichment

Abstract

Molybdenum is a common alloying element used in Ni-Cr and Fe-Cr alloys to improve their passive properties. Despite its ubiquity, the precise role Mo plays during passivation remains poorly understood. Recently we have investigated the role of Mo in commercial allows during both passive film formation [1] in acidic solution and during transpassive dissolution and repassivation in neutral chloride solution [2] using atomic emission spectroelectrochemistry (AESEC). In this work we refine the conclusions of the previous studies by working with pure binary and ternary alloys, Ni22Cr wt.% and Ni22Cr10Mo wt.%, the simplicity of which permits a clearer picture of the underlying mechanisms. The dissolution rates of Ni, Cr, and Mo and the surface enrichments of Cr and Mo were determined as a function of time during electrochemical experiments and film composition and oxidation state information was obtained by X-ray photoelectron spectroscopy (XPS). For both alloys, following cathodic removal of the passive film (activation), a spontaneous open circuit passivation was observed at ≈-0.25 V vs. SCE which involved the enrichment of both Cr and Mo. Stepping the potential further into the passive domain (+0.3 V vs. SCE) lead to a further enhancement of the Cr enrichment but a partial dissolution of the Mo enrichment.The XPS results suggest that Mo dissolution may be attributed to the oxidation of the less soluble Mo(IV) to more soluble Mo(VI) species as suggested in our previous work with a commercial Ni alloy [1]. The suggestion is that Mo enriches on the surface only during a certain domain of potential and this suggests the hypothesis that Mo facilitates the initial growth of the passive film. “The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles”, X Li, K Ogle, J. Electrochem. Soc. 166 (2019)C3179-C3185.“Molybdenum surface enrichment and release during transpassive dissolution of Ni-based alloys”, JD Henderson, X Li, DW Shoesmith, JJ Noël, K Ogle, Corrosion Science 147(2019)32-40 Figure 1