A new concept of oxidation protection of Ni-base alloys by using the halogen effect

Abstract

The Ni-base alloys with Al-contents of less than 10 wt% are widely used in high temperature technology due to their beneficial mechanical properties. However, their oxidation resistance may be insufficient at temperatures above 1000°C. Oxidation of these Ni-base alloys does not form a pure continuous Al2O3 protective scale on the surface, but rather a complex layer structure. This structure is characterized by inward growing oxides showing a discontinuous alumina scale. A new method for the formation of a dense protective alumina scale on the surface is now presented. The method is based on the halogen effect, which was successfully applied for TiAl-alloys. Thermodynamic calculations show the preferred formation of gaseous Al-halogenides within a certain region of fluorine partial pressures. The fluorine treatment is performed by ion implantation. The implantation parameters are defined by using Monte Carlo simulations. Following these results fluorine implantations of the Ni-base alloy IN 738 are performed to meet the required fluorine content near the surface. After oxidation at 1050°C a thin continuous external alumina scale is formed on the surface, whereas the untreated alloy shows a mixed oxide scale with significant inward growing oxides. The results offer a new and innovative way to protect the Ni-base alloys against oxidation.