Improvement of the high temperature oxidation resistance of Ti-50Al at 1000 °C by anodizing in ethylene glycol/BmimPF6 solution

A compact Al-enriched and fluoride-containing surface layer can be fabricated on Ti-50Al by anodizing in ethylene glycol (EG) solution containing 1.8 vol.% ionic liquid named 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6). After high temperature oxidation, an Al2O3 layer is formed on the thin anodic oxide film (AOF) coated Ti-50Al, which efficiently inhibits the diffusion of oxygen and leads to excellent high temperature oxidation resistance. As a result, after oxidation in air at 1000 °C for 100 h, the anodized Ti-50Al shows a very low weight gain of 0.54 mg/cm2, while the weight gain of bare Ti-50Al is as high as 82.40 mg/cm2. The surface morphology and composition of AOF coated Ti-50Al before and after high temperature oxidation were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and glow discharge optical spectroscopy (GDOS). The results show that the excellent high temperature oxidation resistance of the AOF coated Ti-50Al can be ascribed to two aspects. One is the Al-enriched structure formed initially during anodization, which can inhibit the diffusion of oxygen into the substrate. The other one is the halogen effect. The fluorides introduced into the AOF during anodization promote a selective transport of aluminum from the substrate to the alumina scale/substrate interface through pores or micro-cracks due to the formation of gaseous aluminum fluorides. These aluminum fluorides are then oxidized to Al2O3 at the interface and within the alumina scale. Finally, a protective alumina scale is formed and provides excellent high temperature oxidation resistance.