Enhanced Passivity and Resistance to Pitting of New Cr-Fe-Co-Ni-Mo Multi-Principal Element Single-Phase Alloys

Extensive thermodynamic calculations exploring the large composition space of the Cr-Fe-Co-Ni-Mo system were performed in order to design new multi-principal element single-phase fcc alloys with enhanced passivity and resistance to localised corrosion. This presentation will cover several aspects from design to production, passivation, surface analysis and pitting resistance of alloys with up to 15 at.% Mo . The newly designed and produced quinary alloys, Cr25Fe25Co5Ni40Mo5 and Cr15Fe10Co5Ni60Mo10 (at.%) show superior resistance to localised corrosion in chloride solution. The chemical states, composition and stratification of passive films were investigated by X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The resistance to pitting was measured after pre-passivation of the alloy surfaces in Cl--free acidic solution. Stratification of the passive films was observed, with Cr, Fe, Ni,Mo oxides and hydroxides in the outer layer and Cr oxide in the inner layer.The amounts and distribution of chloride ions that penetrated into the passive films were also investigated. In-depth profiles show that, on the Mo-bearing alloys, chloride ions do not penetrate the Mo-enriched outer layer of the oxide film. A direct link between the structural and chemical nature of passive films and the resistance to pitting in chloride solution was observed. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (advanced grant agreement # 741123)