Oxidation of Inconel 625 superalloy upon treatment with oxygen or hydrogen plasma at high temperature

•Inconel 625 alloy was exposed to plasma at high temperatures up to melting point.•Evolution of morphological and structural changes was determined by SEM, AES and XRD.•Depth profiling revealed different oxides on the surface depending on temperature.•Rich nano-structuring was observed but no nanowires appeared.•Hydrogen plasma treatment caused surface segregation of minor elements. Initial stages of Inconel 625 superalloy (Ni60Cr30Mo10Ni4Nb1) oxidation upon short treatment with gaseous plasma at different temperatures up to about 1600K were studied. Samples were treated for different periods up to a minute by oxygen or hydrogen plasma created with a microwave discharge in the standing-wave mode at a pressure of 40Pa and a power 500W. Simultaneous heating of the samples was realized by focusing concentrated solar radiation from a 5kW solar furnace directly onto the samples. The morphological changes upon treatment were monitored using scanning electron microscopy, compositional depth profiling was performed using Auger electron spectroscopy, while structural changes were determined by X-ray diffraction. The treatment in oxygen plasma caused formation of metal oxide clusters of three dimensional crystallites initially rich in nickel oxide with the increasing chromium oxide content as the temperature was increasing. At about 1100K iron and niobium oxides prevailed on the surface causing a drop of the material emissivity at 5μm. Simultaneously the NiCr2O4 compound started growing at the interface between the oxide film and bulk alloy and the compound persisted up to temperatures close to the Inconel melting point. Intensive migration of minority alloying elements such as Fe and Ti was observed at 1600K forming mixed surface oxides of sub-micrometer dimensions. The treatment in hydrogen plasma with small admixture of water vapor did not cause much modification unless the temperature was close to the melting point. At such conditions aluminum segregated on the surface and formed well-defined Al2O3 crystals.