Effect of the Fe Content on the Oxide Film Composition and Corrosion Resistance of Inconel 625: An XPS and Cyclic Potentiodynamic Polarisation Study

The present work focuses on investigating the effect of increasing Inconel 625 Fe content on the oxide film composition and corrosion behaviour through XPS and cyclic potentiodynamic polarisation (CPP) studies. Different proportions of ASTM A182 F22 steel and Inconel 625 were arc‐melted to achieve a range of Fe contents (2–15 wt.%) in the samples. XPS analysis showed a progressive increase in the proportion of protective oxides, namely, NiO, Cr2O3 and Nb2O5, in the native oxide film, as the Fe content of the alloy increased, whereas MoO2 did not change across the studied compositions. The oxide thickness was also observed to increase from 1.8 nm for Inconel 625 to 3.5 nm for the sample with 15 wt.% Fe. CPP tests in artificial seawater revealed that the pitting potential progressively increases with Fe content, reaching a value of 1.12 V/SCE for the alloy with 15 wt.% Fe. These results show that increased concentrations of Fe (up to 15 wt.%) in Inconel 625 do not reduce the protective nature of the oxide and actually improve the pitting corrosion resistance, which may pave the way towards using more Fe in this alloy.