XPS study of the thermal stability of passivated NiCrFeCoMo multi-principal element alloy surfaces

X-ray photoelectron spectroscopy analysis was applied to investigate the thermal stability under ultra-high vacuum environment of the surface oxide film formed by electrochemical passivation of a newly-designed Cr15Fe10Co5Ni60Mo10 (at. %) multi-principal element alloy and providing the alloy superior localized corrosion resistance compared to conventional stainless steels and alloys. A spectral decomposition methodology involving the subtraction of Auger peaks overlapping the Fe 2p and Co 2p core level regions was applied for quantification of the oxide film composition and thickness. The results show that, at 100 °C, the passive oxide film is mainly dehydrated and dehydroxylated. Obvious loss of Ni hydroxide and conversion of Mo(VI) to Mo(IV) species are observed at 200 °C, with further reduction of Mo species to Mo(III) observed at 300 °C. In this temperature range, the total cation quantity in the oxide film remains stable despite the compositional alteration. At 400 °C, Cr(III) oxide forms at the expense of Fe and Mo oxides, resulting in an oxide film essentially consisting of chromium oxide. At 500 °C, Cr(III) oxide is eliminated, making the passive film unstable at this temperature. Possible Cr oxide removal mechanisms are discussed