The oxide films formed under the low oxygen pressure in a particular system containing competing chromium and aluminium Bildung von Oxidfilmen unter niedrigem Sauerstoffdruck in einem bestimmten System mit konkurrierendem Chrom und Aluminium

Abstract

In this paper, pre-oxidation treatment was carried out at 950 °C under oxygen pressure of 10⁻¹⁶ atm for nickel-10 % chromium-x % aluminium-1 % silicon-0.5 % yttrium(x=0 wt.–%, 1 wt.–%, 3 wt.–%, 5 wt.–%) alloys. Morphology and structure of oxide layers on the alloys after pre-oxidation were analyzed by scanning electron microscopy and electron discharge spectroscopy. The results showed that a chromium oxide film was formed on the surface of the alloy without aluminium addition, while the oxide layer on the surface of the alloy with aluminium addition gradually transformed into M₂O₃ (M=aluminium and chromium) oxide film. The oxidation behavior was analyzed by means of chemical thermodynamics, and it was seen that because the interaction coefficient,εijof aluminium and chromium is a positive value, the increase of aluminium addition will strengthen the interaction and increase the diffusion coefficient,Diof aluminium and chromium. The diffusion of chromium and aluminium in each other therefore promotes their interaction, which is beneficial to the formation of the outer oxide film. Thus, with the increase of aluminium addition, the thickness of the outer oxide film gradually increased, and the aluminium content in M₂O₃ (M=aluminium and chromium) increased significantly, and with 5 % aluminium added, the aluminium oxide of the outer oxide film became the main body.